Subproject E: Stirred Vessel Fouling Tests The objectives of subproject E are i to design and oper-ate a small high-pressure, high-temperature stirred cell ∼1 L oil volume based on the E
Trang 2CopyrightTaylor and Francis Group, LLC
ISSN: 0145-7632 print / 1521-0537 online
DOI: 10.1080/01457632.2010.503108
editorial
Heat Exchanger Fouling: Mitigation and Cleaning Strategies
H M ¨ ULLER-STEINHAGEN,1,2 M R MALAYERI,2and A P WATKINSON3
1Institute of Technical Thermodynamics, German Aerospace Centre (DLR), Stuttgart, Germany
2Institute of Thermodynamics and Thermal Engineering, University of Stuttgart, Stuttgart, Germany
3Department of Chemical & Biological Engineering, University of British Columbia, Vancouver, Canada
Heat exchangers are the workhorse of most chemical,
petro-chemical, food-processing, and power-generating processes
The global heat exchanger market is estimated to top a total
of $12.7 billion by 2012, with an increase of 3–5% per annum
[1] Despite this very positive market outlook, manufacturers
are under increasing pressure to produce heat exchangers that
are more efficient in terms of heat recovery and use of material,
while at the same time being faced with fluids that are
increas-ingly difficult to process One major problem directly related
to these requirements is the deposition of unwanted materials
on the heat transfer surfaces, which occurs in the majority of
heat exchangers [2] Conservative studies estimated that heat
ex-changer fouling leads to additional costs in the order of 0.25%
of the gross domestic product (GDP) of industrialized
coun-tries, and that it is responsible for 2.5% of the total equivalent
anthropogenic emissions of carbon dioxide [2, 3]
Therefore, efficient mitigation and cleaning methods must
be available to safeguard the operation of heat exchangers Two
basic approaches are possible to combat heat exchanger
foul-ing, namely, mitigation (including on-line cleaning) and off-line
cleaning techniques The general criteria for the selection of any
of these strategies are:
• Dominant fouling mechanism
• Severity of fouling
• Type of heat exchanger
Address correspondence to Dr Mohammad Reza Malayeri, Institute of
Ther-modynamics and Thermal Engineering, University of Stuttgart, Pfaffenwaldring
6, D-70550, Stuttgart, Germany E-mail: m.malayeri@itw.uni-stuttgart.de
• Extent of required cleanliness
• Mitigation and cleaning costs
• Time intervals between cleaning cycles
In the following, various mitigation and cleaning techniquesare discussed and areas for further developments are identified
MITIGATION APPROACHES
Figure 1 lists the main methodologies for the mitigation offouling in industrial heat exchangers It is understood that mit-igation techniques are not limited to those given in Figure 1,which have been selected because they are widely used andknown to be successful in a number of applications The generalpreference is to mitigate fouling firstly through proper design
of heat exchangers, then by on-line mitigation techniques Inreality, a combination of these methods may be necessary tocombat fouling
Mitigation of Heat Exchanger Fouling by Design
Heat exchanger fouling may effectively be mitigated at thedesign stage of the heat exchanger through the following steps:
• Selection of a suitable heat exchanger type and geometry
• Omission of operating conditions that promote fouling
• Optimum design with adequate velocities in the heat changer and that avoids hot spots, bypass flow, or dead zones
ex-• Design for easy cleaning
189
Trang 3Fouling mitigation techniques
Design of heat
Change of
operating conditions
Mechanical
Projectiles
Physical
Surface coating
magnetic Ultrasound
Chemical inhibitors
Feed filtration
Feed
dilution
Shear stress increase
Reverse flow or pulsation
Gas rumbling
Thermal shock
Wire brushes Scrappers
Figure 1 Various fouling mitigation methodologies.
M¨uller-Steinhagen [4, 5] extensively discussed how these
various options can be implemented in the design stage of heat
exchangers to reduce fouling
Compact heat exchangers, such as plate-and-frame heat
exchangers, spiral flow heat exchangers, and fin-tube heat
exchangers, have been found to experience reduced fouling in
many (but certainly not all) applications due to increased level
of turbulence, reduced surface temperatures, and homogeneous
flow distribution [5] While this has only been known for
con-vective heat transfer to liquids, Esawy et al [6] recently showed
that the buildup of deposits during pool boiling of CaSO4
solu-tions can be substantially reduced by the presence of fins on the
tube outside (Figure 2)
Scraped-surface heat exchangers [7] where rotating
instal-lations continuously keep the pipe internal surfaces free from
deposits have been used in industry for many years Their
invest-ment, operation, and maintenance cost, as well as the complex
geometry and maintenance, limit this technique to applications
where very severe fouling occurs
Fluidized-bed heat exchangers are a very effective
technol-ogy to reduce or even eliminate scale formation in many types
of applications Particles of different materials and shapes are
transported upward through the vertical heat exchanger tubes
together with the fouling liquid [8] They are then separated
from the liquid and returned in an external downcomer In
ad-dition to having a slightly abrasive effect on the heat transfer
surface, the particles will also improve the tube-side heat
trans-fer coefficients Typical applications include, e.g., desalination,
processing of aqueous solutions, and processing of
hydrocar-bons
On-Line Mitigation of Heat Exchanger Fouling
The purpose of on-line mitigation is to keep the heat transfersurfaces in an acceptable state of cleanliness in order to main-tain high operating efficiency and plant availability On-linemitigation includes many different methodologies that can bedivided, in order of their applicability, into (i) changing operat-ing conditions, (ii) chemical, (iii) mechanical, and (iv) physicalapproaches A breakdown of these techniques has already beenpresented in Figure 1, and this subsection briefly outlines each
Figure 2 Comparison of fouling resistances for smooth and finned tubes at a heat flux of 200 kW/m 2 and a CaSO 4 concentration of 1.6 g/L [6].
heat transfer engineering vol 32 nos 3–4 2011
Trang 4individual technique and its limitations More details are
pro-vided in ref [5]
Filtration and/or washing Removal of contaminants can
substantially reduce fouling For example, removal of
mate-rials such as sodium, sulfur, or vanadium from fuels prior to
combustion and contaminant removal from combustion gases
are two approaches to mitigate gas-side fouling Water washing
has helped to overcome some of the fouling problems
experi-enced with crude oils and with residual oils in marine
applica-tions by removing sodium and sediment
Feed dilution or blending Refineries are increasingly
be-coming more complex as heavier crude oils need to be
pro-cessed Accordingly, more severe fouling is expected,
partic-ularly in the refinery preheat heat exchanger trains Among
various mitigation techniques, diluting feed by blending light
and denser crude oils may be considered to ease the problem
However, a thorough and careful chemical analysis of the crude
is necessary, since this procedure may sometimes lead to even
harsher fouling [9]
Thermal shock Short-time under- or overheating of the heat
transfer surfaces may cause brittle deposit layers to crack due to
the different thermal expansion of tubes and deposits Figure 3
shows the impact of sudden reduction of heat flux during pool
boiling of CaSO4solutions when the thickness of deposit layer
had reached about 2 mm The whole deposit layer spalled off
the surface instantaneously [10]
Intermittent changes in flow direction or velocity Regular
reversal of flow direction or short-time increase of the flow
velocity (flow pulsation) has been used to mitigate the formation
of weakly adhering deposits Generally, better performance is
achieved by continuously operating at a higher flow velocity
However, this technique may be effective if applied right from
the beginning of operation, before the deposit layer starts to
harden
Gas rumbling Deposits with moderate stickability to the
heat transfer surfaces (e.g., particulate and some biological
de-posits) can be dislodged and washed out by periodically
in-creasing the fluid shear forces for a short time by introducing
compressed air or nitrogen into the liquid system The resulting
Table 1 Categorization of chemical inhibitor agents for different fouling mechanisms [4]
Fouling mechanism Foulant Inhibitor agent Crystallization,
precipitation
Ca2+, Mg2 + Ion exchange
CaCO 3 Ph control CaSO 4 Scale inhibitors (e.g.,
ethylenediamine tetraacetic acid [EDTA]) Soft and hard scalants Adsorption agents (e.g.,
polyphosphates) Soft and hard scalants Crystalline weakening agents
(e.g., polycarboxylic acid) Particulate Particulate matter Surfactants or dispersants Chemical reaction Oxygen (polymerization) Antioxidants
Metals (reaction catalyst) Metal deactivators Insoluble hydrocarbon
particles
Dispersants Biofouling Micro- and
Chemical fouling mitigation methods The most widespread
mitigation strategy during on-line operation of heat exchangers
is the use of chemical agents or inhibitors, which is particularlyuseful for heat exchangers with complex geometries where noother cleaning methods are possible Commercial antifoulantsare usually polyfunctional and hence more versatile and effec-tive, as they can be designed to combat various types of pre-cursors that may be present in any given system For instance,for crude oil fouling, various precursors such as oxygen, metals,salts, and asphaltenes may lead to different forms of depositformation Antifoulants are designed to prevent equipment sur-faces from fouling but are usually not effective in removingalready formed deposits Therefore, antifoulant addition should
Figure 3 Breakage and removal of a deposit layer formed during pool boiling of a CaSO 4 solution with 1.6 g/L and a heat flux of 300 kW/m 2 : (a) during steady-state heating and (b) immediately after switching off the heater [10].
heat transfer engineering vol 32 nos 3–4 2011
Trang 5Figure 4 (a) Typical spiral insert (SPIRELF system) [15] and (b) hiTRAN wire matrix insert [16].
be started immediately after equipment is cleaned The usage
and dosage of antifoulants depend strongly on fouling
mecha-nisms and anticipated deposit hardness Thus, information about
the prevailing fouling mechanism and the influence of
operat-ing conditions such as dominant precursors, temperature and
velocity are important Table 1 lists typical inhibitor agents for
different fouling mechanisms and foulants
On-line chemical fouling mitigation is effective but the
chem-ical agents may contain substances that are potentially harmful
to the environment, such as chlorine, hypochlorite,
polyphos-phate, coagulants, etc The use of many of these chemical
in-hibitors has to be reduced and eventually phased out due to the
implementation of restrictive environmental legislations such
as the Water Framework Directive 60/2000/EC of the European
Union Furthermore, compatibility of the chemistry of the
in-hibitors with the metallurgy of the equipment has to be checked,
to avoid corrosion or cracking
Increased efforts are dedicated to the monitoring of fouling,
development of less toxic substitute additives, and optimization
of inhibitor dosage For instance, chlorine can be replaced by
other chemicals such as methylene thiocyanate or
chlorophe-noles [11] To reduce the dosage of treatment chemicals,
Ferreira et al [12] reported that antimicrobials can be
trans-ported on micro-sized particles in much lower concentrations to
target only microorganisms on the surface
On-line mechanical mitigation techniques If applicable,
mechanical mitigation may have some advantages over
chemi-cal methods, which often involve materials that are difficult to
handle and control Their applicability is usually determined by
the type of fouled heat exchanger, deposit intensity and growth
rate, operating conditions, and cleaning costs The utilization
of on-line mechanical fouling mitigation may lead to
signifi-cantly reduced maintenance downtime, avoidance of
antifoul-ing chemicals, and more efficient plant operation These need
to be balanced against investment and operating costs, e.g., for
replacement of devices due to wear or increased pressure drop
due to flow resistance
The previous conference proceedings on heat exchanger
foul-ing and cleanfoul-ing [13] provide an extensive source of information
on mitigation and cleaning Hence these techniques are only
briefly addressed in this editorial
Cleaning Projectiles
Projectiles of different shapes, e.g., sponge balls and wire
brushes, can be propelled through the heat exchanger tubes to
remove deposits already during the early stage of formation Thefrequency and duration of application depends on the severity offouling and the strength of interaction between cleaning projec-tile and deposit Typically, projectile on-line cleaning techniquesare limited to aqueous systems at temperatures below about
120◦C, due to the stability of the projectile material There mayalso be some limitations due to chemical incompatibility If theapplication of cleaning projectiles to individual tubes occurs atrandom (i.e., in sponge ball systems), this may lead to over-and undercleaning of tubes depending on their location in thetube bundle The “CleanEx” project [14] that has recently beenfunded by the European Community endeavors to address some
of these drawbacks The installation of mechanical systems forthe continuous propulsion of cleaning devices requires modifi-cations of the flow system and is, therefore, best implemented
in the design stage of heat exchangers
Tube Inserts
Tube inserts, such as twisted tapes, coils, and wire matrix serts, can significantly increase the heat transfer coefficients byacting as turbulence promoters (see Figure 4 [15, 16]) As depo-sition rates for most fouling mechanisms are inversely dependent
in-on fluid wall shear stress and heat transfer surface temperature,reduction of the viscous and thermal sublayer thickness mayalso considerably reduce fouling These inserts work best forflow in the laminar or transitional flow regime In combinationwith further reduction of flow velocity (i.e., tube passes), designvariations may be possible where significant improvements ofheat transfer can be achieved with no or little increase in pressuredrop
The selection of a particular type of insert and insert geometrydepends on the type of fouling and the availability of suitablestrainers or filters that may trap particulate or fibrous mattersbefore these enter the heat exchanger
Physical Mitigation Techniques
Physical fouling mitigation methods attempt to reduce/avoidfouling without changing heat exchanger layout, operation, orchemical additives, by modifying the interaction of depositforming precursors and heat transfer surface
Surface modification Of several fouling mitigation
tech-niques, surface modification is gaining increased attention due
to its environmentally friendly features Surface coatings withheat transfer engineering vol 32 nos 3–4 2011
Trang 6On-line mitigation systems
Possible corrosion impacts
Lack of effective control and timing Ineffective distribution Increased pressure drop Limited to certain chemicals Abrasive impacts Require modification of heat exchanger
Figure 5 Limitations of various chemical and mechanical mitigation systems.
organic materials such as polytetrafluoroethylene (PTFE) and
S¨akaphen have been shown to reduce fouling from various
fluids, for example, during seawater evaporation and heat
transfer to Kraft black liquor The main reason why such
mate-rials/coatings are not more widely used is that they are poor heat
conductors and form an additional resistance to heat transfer
that is comparable to the TEMA fouling resistance for cooling
water Another drawback is the poor stickiness of coatings
to the substrate If very thin coatings are used, the resistance
against erosion or other mechanical stress is greatly diminished
These problems may be avoided with several novel coating
methods, such as ion beam implantation, magnetron sputtering,
multi-arc ion plating, filtered cathodic vacuum arc plating, or
electroless Ni-P-PTFE plating, which have been investigated in
recent years [17–19] These thin and stable coatings have been
found to reduce scale formation during convective and boiling
heat transfer and to reduce the adhesion of bacteria
Sonic technologies High- and low-frequency sound has
suc-cessfully been used in heat exchangers for gases to dislodge and
weaken particulate deposits, which can subsequently be carried
away by the process gas stream In suitable cases, this can be a
very cost-effective option As for the application for liquid-side
fouling, several laboratory investigations have shown promising
effects It is, however, questionable whether sound or vibration
generators can sensibly be installed in industrial heat
exchang-ers, and whether their effects will extend over the typically large
heat transfer surfaces
Magnetic, electronic, or catalytic means When it comes to
commercial mitigation of scale formation, one of the most quently and emotionally discussed topics is devices that claim
fre-to reduce scaling by magnetic, electronic, or catalytic means
To date, no conclusive scientific proof or theory for the nisms that may be responsible for the beneficial effects of suchtechnologies has been found A considerable number of inves-tigations have been reported in the literature; many of themclaim some sort of success with the applied technology Ger-man Industry Standards (DIN) have been formulated for per-formance evaluation of physical water conditioners Pilot-plantand laboratory-scale investigations have provided contradictingresults For example, references [20], [21], and [22] report thatthe installation of magnets considerably reduced cooling waterfouling, whereas [23] and [24] found no effect of the water con-ditioner Even the mechanisms of scale inhibition are highly dis-puted Systematic investigations (e.g., [22]) indicate that the ef-fectiveness of electromagnetic fouling mitigation methods may
mecha-be limited to a certain window of operation
COMPARISON OF CHEMICAL AND MECHANICAL MITIGATION TECHNIQUES
In general, heat transfer engineers rely on chemical andmechanical approaches, as the physical systems are still intheir early development Figure 5 summarizes the limitations
of chemical and mechanical mitigation techniques All systemswork best if applied to an initially clean heat exchanger Some
of the mechanical systems are less dependent on the type offouling, while chemical systems are always specific to the com-position of the process fluid
OFF-LINE CLEANING OF HEAT EXCHANGERS
Periodical cleaning of heat exchangers will be necessary,even if the heat exchanger is well designed and the fluid treat-ment is effective Additionally, conditions in the heat exchangermay deviate from the design conditions due to changes in flowrates and temperatures, plant failures, ingress of air and bac-teria, changes in the fluid composition, or upstream corrosion,which all may promote fouling It is, therefore, advantageous
Off-line cleaning
Figure 6 Categorization of various off-line cleaning systems.
heat transfer engineering vol 32 nos 3–4 2011
Trang 7to remove nonprotective deposits soon after the onset of their
formation Heat exchangers may be cleaned by various off-line
methods as categorized in Figure 6
Intense mechanical and chemical cleaning may remove not
only the deposit but also part of the protective oxide layer on
the pipe surfaces Under certain circumstances, this may create a
corrosion problem On the other hand, regular cleaning removes
deposit and avoids flow conditions that promote corrosion due
to chemical reaction or stagnant flow For very severe fouling
problems, a combination of chemical and mechanical cleaning
may be recommended
Off-line cleaning is most prevalent in petroleum, minerals,
and chemicals processing industries and mainly involves
man-ual or semiautomatic cleaning at predetermined maintenance
intervals [5] Although generally effective, these techniques do
not mitigate the gradual performance degradation (due to
foul-ing) between physical cleaning intervals As a result, most heat
exchangers will operate at significantly less than peak efficiency
Some of the most promising off-line cleaning systems are
briefly discussed next
Blasting and Jetting Techniques
High-pressure water or steam blasting up to 1,500 bar can
be an effective way of removing unwanted deposits Of those,
water jetting is probably the most effective and technologically
advanced Delivery may be through multijet sprays or through
high-pressure water lances The addition of wetting agents or
detergents may improve the washing process If deposits are
very tenacious, abrasive particles such as sand may be added to
the pressurized water to increase the cleaning efficiency
Simi-larly, air blasting with sand or solid CO2particles is frequently
used Air, steam, and hydro blasting are labor-intensive and keep
the exchanger off-line for a considerable time, even though
semiautomatic cleaning devices have been developed and are
commonly used [5] Blasting may not completely eliminate all
deposits and some significant roughness can remain The shell
side of tube bundles can only be cleaned completely if the tubes
are arranged in-line The particular geometry of twisted tubes
provides flow lanes for pressurized water or steam which
facil-itates cleaning
Ice pigging has also been reported as a successful technique to
remove moderately adhering deposits, since the shear forces are
increased by a factor of 4–5 due to the presence of the ice slurry
[25] Such a system can be applied for complex geometries and
is reported to have a reduced cleaning downtime
Mechanical Cleaning Techniques
Several cleaning methods can be used for the inside of
straight tubes For example, the on-line fouling mitigation
sponge ball system can also be used as a transportable,
off-line cleaning system, particularly if used with corundum-coated
Figure 7 Conco tube cleaner in operation [26].
sponge balls Using air pressure or hydro pressure, rubber plugs
or metal scrapers can be shot through the tubes Rubber plugsmay fail for hard deposits In general, water pressure systemsare safer than air pressure systems, due to the compressibilityand subsequent rapid expansion of gases Advanced systems,such as the one shown in Figure 7 [26], are rather fast and allowcleaning of up to 15,000 tubes within 24 hours
Very dirty and plugged tubes must be cleaned with drillsequipped with drill bits, brushes, or bit–brush combinations
Chemical Cleaning Methods
Chemical cleaning methods have a number of advantages,namely:
• They are relatively quick
• Surfaces do not experience mechanical damage
• Chemical solutions reach normally inaccessible areas
• They are less labor-intensive than mechanical cleaning
• Cleaning can be performed in situ
Problems may arise due to the danger of handling (burns,toxicity), due to elevated application temperatures, due to thecosts of cleaning agents, due to the chemical attack on the heatexchanger material (overcleaning, uneven cleaning, corrosion),and due to disposal problems Acids and alkalis must be neu-tralized, organic materials may be burned, and fluorides must
be reacted to inactive solid residues Some of the organic acids,such as citric acid and gluconic acid, are biodegradable.Research on the mechanisms of chemical cleaning of heattransfer surfaces is far less developed than research on foulingmechanisms, even though similar approaches may be used Nev-ertheless, some first modeling has been attempted, assuming thatthe cleaning process is a reversed fouling process Understand-ing of the interactions of cleaning and fouling is less advanced
in the process industry than in the food industry, to which aseries of conferences have been dedicated [27, 28]
SUMMARY
While significant progress has been made in the mitigation
of heat exchanger fouling, the challenge to reduce its impact onheat exchanger performance is still enormous Many mitigationand cleaning techniques that have found their way into regularplant operation have been developed by an empirical trial-and-error approach These antifouling strategies have few or even nolinks to academic research findings, since industry and academicheat transfer engineering vol 32 nos 3–4 2011
Trang 8research institutions have traditionally approached the problem
of fouling from different aspects To optimize the effectiveness
of mitigation methods, which highly depends on the dominant
fouling mechanisms and influential operating conditions, and to
develop new approaches for fouling mitigation, closer
collabo-ration between the two communities is essential
For the past 15 years, conferences on heat exchanger
foul-ing have been held at bi-yearly intervals to facilitate
innova-tive thinking and to explore new theoretical and practical
ap-proaches These conferences have successfully provided a forum
for experts from industry, academia, and government research
centers from around the world to present their latest research and
technological developments in the areas of fouling mitigation
and cleaning technologies The meetings in San Luis Obispo
(1995), Lucca (1997), Banff (1999), Davos (2001), Santa Fe
(2003), Kloster Irsee (2005), Portugal and Tomar (2007) were
organized by Engineering Conferences International The 8th
conference in this series was organized by the present authors
under the auspices of EUROTHERM in Schladming, Austria,
in June 2009 In total, 100 participants attended this meeting,
presenting 81 papers/posters, which were the highest numbers
in any meeting of this series to date
The following papers in this special issue of Heat Transfer
Engineering have been selected from the contributions to the
2009 Fouling Conference in Schladming after a careful
ref-ereeing and revision process The full e-proceedings of the
2009 conference as well as those from the previous
confer-ences from 2003 until 2007 can be obtained free of charge from
http://heatexchanger-fouling.com
They cover various aspects of heat exchanger fouling, along
with updated state-of-the-art fouling mitigation and cleaning
strategies Their content is of significant value for researchers,
plant operators, equipment manufacturers, chemical suppliers,
and heat exchanger cleaning companies
This website also contains the actual information about the
next conference in this series (Heat Exchanger Fouling and
Cleaning IX), which is scheduled for June 5–10, 2011 on the
beautiful island of Crete, Greece
REFERENCES
[1] Heat Exchanger: A Global Strategic Business Report,
Global Industry Analysts, Inc., San Jose, CA, 2008
[2] Steinhagen, R., M¨uller-Steinhagen, H., and Maani, K.,
Problems and Costs Due to Heat Exchanger Fouling in
New Zealand Industries, Heat Transfer Engineering, vol.
14, no 1, pp 19–30, 1992
[3] M¨uller-Steinhagen, H., Malayeri, M R., and Watkinson, A
P., Heat Exchanger Fouling: Environmental Impacts, Heat
Transfer Engineering, vol 30 no 10–11, pp 773–776,
2009
[4] M¨uller-Steinhagen, H., Fouling of Heat Exchanger
Sur-faces, VDI Heat Atlas, Section C4, VDI Gesellschaft
Verfahrenstechnik und Chemieingenieurwesen, Verlag Berlin Heidelberg, 2010
Mitigation and Cleaning Technologies, Publico
Publica-tions Essen, Germany, 2000
[6] Esawy, M., Malayeri, M R., and M¨uller-Steinhagen, H.,Crystallization Fouling of Finned Tubes During Pool Boil-
ing: Effect of Fin Density, J Heat and Mass Transfer, vol.
[9] Wiehe, I A, The Oil Compatibility Model and Crude
Oil Compatibility, Energy & Fuels, vol 14, pp 56–59,
2000
[10] Evangelidou, M., Crystallization Fouling of Structured Tubes During Pool Boiling Heat Transfer, Diploma the-sis, University of Stuttgart, Stuttgart, Germany, 2010.[11] Waite, T D., and Fagan, J R., Summary of Biofouling
Control Alternatives, in Condenser Biofouling Control, ed.
J Garey, Ann Arbor Science, Ann Arbor, MI, pp 441–462,1980
[12] Ferreira, C., Sim˜oes, M., Pereira, M C., Bastos, M M
S M., Nunes, O C., Coelho, M., and Melo, L F., trol of Biofouling of Industrial Surfaces Using Micropar-
Con-ticles Carrying a Biocide, Proceedings of EUROTHERM International Conference on Heat Exchanger Fouling and Cleaning VIII–2009, eds H M¨uller-Steinhagen, M R.
Malayeri, and A P Watkinson, Schladming, Austria, June14–19, pp 378–383, 2009
fouling and cleaning, http://heatexchanger-fouling.com/proceedings.htm
[14] EU Project: A Method for On-Line Cleaning of Heat changers to Significantly Increase Energy Efficiency in theOil, Gas, Power & Chemical Process Sectors (CleanEx),2009–2013, http://www.pro-cleanex.org
Ex-[15] Pouponnot, F., and Kreuger, A W., Heat Exchanger TubeInserts: An Update in New Applications With TroubleShooting Aspects in Crude Units, Residue Service, Re-boilers, U-Tubes “SPIRELF, TURBOTAL, and FIXOTALSystems” Application Examples in Chemical Plants and
Refineries, Proceedings of 6th International Conference on Heat Exchanger Fouling and Cleaning—Challenges and Opportunities, eds H M¨uller-Steinhagen, M R Malayeri
and A P Watkinson, ECI Symposium Series, vol RP2,
pp 221–230, 2005
heat transfer engineering vol 32 nos 3–4 2011
Trang 9[16] Ritchie, J M., Droegemueller, P., and Simmons, M J.
H., hiTRAN Wire Matrix Inserts in Fouling
Applica-tions, Heat Transfer Engineering, vol 30, no 10–11, pp.
876–884, 2009
[17] Bornhorst, A., Zhao, Q., and M¨uller-Steinhagen, H.,
Re-duction of Scale Formation by Ion Implantation and
Mag-netron Sputtering on Heat Transfer Surfaces, Heat Transfer
Engineering, vol 20, no 2, pp 6–14, 1999.
[18] M¨uller-Steinhagen, H., Zhao, Q., Helalizadeh, A., and
Ren, X G., The Effect of Surface Properties on CaSO4
Scale Formation During Convective Heat Transfer and
Subcooled Flow Boiling, Canadian Journal of Chemical
Enginneering, vol 78, pp 12–20, 2000.
[19] F¨orster, M., Augustin, W., and Bohnet, M., Influence of the
Adhesion Force Crystal/Heat Exchanger Surface on
Foul-ing Mitigation, Chemical EngineerFoul-ing and ProcessFoul-ing, vol.
38, pp 449–461, 1999
[20] Parkinson, G., and Price, W., Getting the Most Out of
Cooling Water, Chemical Engineering, vol 91, no 1, pp.
22–25, 1984
[21] Donaldson, J., and Grimes, S., Lifting the Scale From Our
Pipes, New Scientist, vol 18, pp 43–46, 1988.
[22] Cho, Y I., Lee, S H., Kim, W., and Suh, S.,
Physi-cal Water Treatment for the Mitigation of Mineral
Foul-ing in CoolFoul-ing-Tower Water Applications, ProceedFoul-ings of
5th International Conference on Heat Exchanger Fouling
and Cleaning—Fundamentals and Applications, eds A.
P Watkinson, H M¨uller-Steinhagen, and M R Malayeri,
ECI Symposium Series, vol RP1, pp 107–114, 2003
[23] Hasson, D., and Bramson, D., Effectiveness of Magnetic
Water Treatment in Suppressing CaCO3Scale Deposition,
Industrial & Engineering Chemistry Process Design and
Development, vol 24, pp 588–592, 1985.
[24] S¨ohnel, O., and Mullin, J., Some Comments on the
Influ-ence of a Magnetic Field on Crystalline Scale Formation,
Chemistry and Industry, vol 6, pp 356–358, 1988.
[25] Ainslie, E A., Quarini, G L., Ash, D G., Deans, T
J., Herbert, M., and Rhys, T D L., Heat Exchanger
Cleaning Using Ice Pigging, Proceedings of 6th
Inter-national Conference on Heat Exchanger Fouling and
Cleaning—Challenges and Opportunities, eds H
M¨uller-Steinhagen, M R Malayeri and A P Watkinson,
Schlad-ming, Austria, June 14–19, pp 433–438, 2009
[26] Saxon, G E., Jr., and Putman, R E., The Practical
Ap-plication and Innovation of Cleaning Technology for Heat
Exchangers, Proceedings of 5th International Conference
on Heat Exchanger Fouling and Cleaning—Fundamentals and Applications, eds A P Watkinson, H M¨uller-
Steinhagen, and M R Malayeri, ECI Symposium Series,vol RP1, pp 294–301, 2003
[27] Fryer, P., ed., Fouling and Cleaning in Food Processing,
Special Topic Issue, Food and Bioproducts Processing, vol.
77, issue 2, p 71, 1999
[28] Wilson, D I., and Chew, Y M J., eds., Fouling and
Clean-ing in Food ProcessClean-ing, ProceedClean-ings of Conference at Jesus College, University of Cambridge, 22–24 March 2010.
H M ¨uller-Steinhagen is the director of the
Insti-tute of Technical Thermodynamics of the German Aerospace Centre and the director of the Institute for Thermodynamics and Thermal Engineering of the University of Stuttgart His research work cov- ers a wide range of topics related to heat and mass transfer, multiphase flow, fuel cells, solar technol- ogy, and process thermodynamics He is the author
of more than 550 articles, and was awarded the 1992 and 1993 TMS Bauxite Processing Awards, the 1994 Light Metals Award, the Beilby Medal and Prize, the UK Heat Transfer Society
Mike Akrill Trophy, the Best Paper 2000 in the Canadian Journal of Chemical
Engineering, and the 2008 AIChE D Q Kern Award He is a fellow of the
Royal Academy of Engineering, president of EUROTHERM, member of the Executive Boards of EUREC and ICHMT, member of the Innovation Council
of the Prime Minister of Baden-W¨urttemberg, chairman of the Advisory Board
of the DESERTEC Industrial Initiative, and associate editor of Heat Transfer
Engineering.
M R Malayeri is the head of the heat exchanger
fouling and mitigation research group at the tute of Thermodynamics and Thermal Engineering (ITW), University of Stuttgart, Germany He grad- uated from the Department of Chemical Engineer- ing, Amirkabir University of Technology, Iran He received his Ph.D from the University of Surrey, UK His research interests include enhanced heat transfer, heat exchanger fouling and mitigation, multiphase flows, and numerical modeling.
Insti-A P Watkinson is a professor of chemical
engineer-ing in the Department of Chemical and Biological gineering at the University of British Columbia, Van- couver, Canada He is involved in research on foul- ing of organic fluids, asphaltene precipitation, coke formation, scaling in aqueous systems, and on the pyrolysis, gasification, and combustion of biomass fuels.
En-heat transfer engineering vol 32 nos 3–4 2011
Trang 10CopyrightTaylor and Francis Group, LLC
ISSN: 0145-7632 print / 1521-0537 online
DOI: 10.1080/01457632.2010.495579
Fouling in Crude Oil Preheat Trains:
A Systematic Solution to an Old
Problem
S MACCHIETTO,1 G F HEWITT,1 F COLETTI,1 B D CRITTENDEN,2 D.
R DUGWELL,1 A GALINDO,1 G JACKSON,1 R KANDIYOTI,1
S G KAZARIAN,1 P F LUCKHAM,1 O K MATAR,1 M MILLAN-AGORIO,1
E A M ¨ ULLER,1 W PATERSON,3 S J PUGH,4 S M RICHARDSON,1and
D I WILSON3
1Department of Chemical Engineering and Chemical Technology, South Kensington Campus, Imperial College London,
United Kingdom
2Department of Chemical Engineering, University of Bath, United Kingdom
3Department of Chemical Engineering and Biotechnology, University of Cambridge, United Kingdom
4IHS ESDU, London, United Kingdom
A major cause of refinery energy inefficiency is fouling in preheat trains This has been a most challenging problem for
decades, due to limited fundamental understanding of its causes, deposition mechanisms, deposit composition, and impacts
on design/operations Current heat exchanger design methodologies mostly just allow for fouling, rather than fundamentally
preventing it To address this problem in a systematic way, a large-scale interdisciplinary research project, CROF (crude
oil fouling), brought together leading experts from the University of Bath, University of Cambridge, and Imperial College
London and, through IHS ESDU, industry The research, coordinated in eight subprojects blending theory, experiments, and
modeling work, tackles fouling issues across all scales, from molecular to the process unit to the overall heat exchanger
network, in an integrated way To make the outcomes of the project relevant and transferable to industry, the research team
is working closely with experts from many world leading oil companies The systematic approach of the CROF project is
presented Individual subprojects are outlined, together with how they work together Initial results are presented, indicating
that a quantum progress can be achieved from such a fundamental, integrated approach Some preliminary indications with
respect to impact on industrial practice are discussed.
INTRODUCTION
About 6% of the energy content of each crude barrel
pro-cessed in an oil refinery is used in the refinery itself With
The authors gratefully acknowledge EPSRC (grants EP/D503051/1,
EP/D506131/1, EP/D50306X/1) for financial support of the project and the
industrial partners of the CROF consortium for valuable inputs, data, materials,
and discussions Special thanks go to the CROF researchers, M Abubakar, E.
Al-Muhareb, M Behrouzi, C Berrueco, J Chew, S Dong, T Gu, C Hale, A.
Haslam, E Ishiyama, J Jover, T J Morgan, J K Pental, K Rostani, K C.
Sahu, H Shumba, D Sileri, F H Tay, S Venditti, and A Young.
Address correspondence to Prof Sandro Macchietto, Department of
Chemical Engineering, Roderic Hill Building, South Kensington Campus,
Imperial College London, London SW7 2AZ, United Kingdom E-mail:
s.macchietto@imperial.ac.uk
a global production of about 82–85 million barrels per day(bbl/day), this is roughly equivalent to the entire production ofExxon or Shell to operate the world’s 720 refineries Crude oildistillation, where the incoming crude is first heated up andsplit into its main fractions, accounts for a large fraction ofthis energy Thus, strenuous attempts are made to recover asmuch as possible of the energy from the product streams of thecrude distillation column (and other refinery units) by means of
a network of heat exchangers, often called the “preheat train”(PHT) A typical crude preheat train is illustrated in Figure
1 Unfortunately, crude oil contains a variety of substances,which tend to deposit as fouling layers in the heat exchangerswhen heated The material deposited ranges from gel-like tosolid-like and may change its properties with time The foulingdeposit growth results over time in decreased energy recovery197
Trang 11Storage
Heavy gas oil
Light gas oil Kerosene
Light gas oil
Bottom pump around Residue
Kerosene
Top pump
around
Bottom pump around
Top pump around
Figure 1 Schematic diagram of typical crude distillation unit (after Panchal
and Huangfu [61]).
and thus increased energy demand (via the furnace prior to the
column in Figure 1), with extra cost of fuel and CO2emissions
Occlusion of tubes in the exchangers due to fouling requires
extra pumping power to overcome the pressure drops When
the furnace hits its maximum capacity (firing limit) the crude
oil throughput must be throttled back, with serious economic
impact
Periodically, individual exchangers must then be taken out of
service and cleaned, again with high impact on production and
considerable health and safety issues Fouling by asphaltenes,
waxes, and hydrates also has serious implications in areas other
than refining For example, it has caused blockages of several
pipelines under the North Sea The economic cost of crude oil
fouling in refinery preheat trains is huge In the United States
alone it was estimated at around US$1.2 billion per annum (circa
1992) [1], at a time when extra CO2emissions were not costed
The cost of preheat train fouling in one 160,000-bbl/day
To-tal refinery was estimated in 2003 to be US$1.5 million in a
3-month period [2] One 200,000-bbl/day UK refinery reported
recently (April 2009) to the authors that 1◦C of loss of preheat
(that is, in the oil temperature at the furnace inlet) cost the
oper-ator some £250,000 per year (per annum, p.a.) Other estimates
state that the energy equivalent of some 0.25% of all oil
pro-duction is lost to fouling in the preheat train This translates to
close to 1 day’s production lost per annum (85 million barrels
on a worldwide basis) The principal benefit to the refiners of
reducing fouling is increased capacity Increasing effective
on-stream time due to reduced fouling/cleaning can lead to massive
savings in some refineries G T Polley (personal
communica-tion, 2009) reported that the profit loss through reduced
pro-duction (last quarter of 2008) ranged from around US$2/bbl in
some Texas refineries to up to US$10/bbl in a refinery
process-ing heavy “opportunity” crudes and that 1 day lost production
on a 200,000-bbl/day refinery is worth between US$0.4M and
US$2M Some older analysis [3] for a then typical
100,000-bbl/day refinery attributes almost 40% of the costs of fouling
to reduced throughput The preceding estimates must be taken
with some care, as they span a period of oil prices ranging from
Fouling mechanism
Figure 2 The 5 × 5 fouling matrix (after Epstein [11]) Dark shading indicates areas of high research levels: pale areas indicate where little research has been done.
US$10 to US$150/bbl Fouling-related losses are, however, portant at all oil prices, as they affect directly refining marginsand revenue losses
im-The impact of crude oil fouling is increasing for all oil panies Crudes are generally becoming heavier and more com-plex, yet refineries were generally designed to process the lightercrudes that are today becoming scarcer The worldwide short-age of middle distillates is also a driver to the processing ofheavier, dirtier crudes that have a higher yield of these valu-able components Fouling problems are therefore increasing inseverity
com-The importance of the crude oil fouling problem evidenced inthe figures just cited led to a number of industrial and academicstudies over several decades, focusing on mechanisms and pa-rameters that govern rates of fouling, namely, temperature, pres-sure, oil composition, salinity, and fluid velocity A number ofexcellent reviews are available (see, for example, Taborek et
al [4], Watkinson [5], Watkinson and Wilson [6], Taborek [7],Bott, [8], M¨uller-Steinhagen [9], and M¨uller-Steinhagen et al.[10]) This research is summarized in Figure 2 (after Epstein[11]) Work has been done in a number of major centers Inthe United States, groups include those at Heat Transfer Re-search, Inc (HTRI) (Bennet et al [12]), where a pilot-scalefacility has been constructed, and at the Argonne National Lab-oratory (ANL) [13, 14] Weihe [15–17] has devised methods
of identifying the fouling propensity for self-incompatible andmixed crudes In Canada, work is proceeding at the University
of British Columbia [18, 19] In the United Kingdom, ESDU(now IHS ESDU) has set up an international Oil Industry Foul-ing Working Party of oil companies and their suppliers; a majorguidance document on crude oil fouling [1] has been producedunder the aegis of this working party Work at Bath [20–22] orig-inally focused on modeling and on the use of model fluids assubstitutes for crude oil More recently, a major test facility wasbuilt and a first program of work completed Experiments haveheat transfer engineering vol 32 nos 3–4 2011
Trang 12determined the effects not only of major processing parameters
(surface temperature, velocity, and heat flux) but also the
ef-fectiveness of in-tube inserts [23] At Cambridge, the focus has
been on developing physical measurement methods [24, 25] and
on modeling the influence of fouling in heat exchangers and heat
exchanger networks [26] The (then) European Community
re-search and development (R&D) strategy for and projects on heat
exchanger fouling in general are briefly reviewed in Pilavachi
and Isdale [27]
Although good progress has been made in experimental
stud-ies of crude oil fouling, it appears that an asymptotic state of
knowledge has been reached Data-led mitigation is an active
area, but this provides a response rather than a cure While
the results are useful, the mechanisms by which the fouling
proceeds are still not fully understood For example, it is
gener-ally assumed that materials depositing on heat transfer surfaces
are asphaltene derived These are typically complex mixtures
of polynuclear aromatic ring structures, with high heteroatom
content; they carry most of the trace element content of the
oil However, it is apparent that only small fractions of the
as-phaltene content in crudes actually deposit on heat-exchange
surfaces and that this process can be initiated by other species
There are complex, ill-understood interactions between oil
prop-erties, phase stability, rheology, chemical reactions, heat
trans-fer, interfacial and adhesion properties, surface properties, and
exchanger geometry, all of which affect all the mechanisms and
subprocesses in the fouling matrix of Figure 2 What is clearly
needed is a much more fundamental approach to the problem
Recognizing that it is unlikely that this will emerge from the
type of studies carried out by and for industry, a fundamental
and integrated project was initiated that would address the
ba-sic issues but also provide a route for exploiting the findings in
industry
THE CROF PROJECT
To meet the challenges just outlined, a 3-year crude oil ing (CROF) project was initiated in June 2006 with substan-tial funding from the UK Engineering and Physical SciencesResearch Council (EPSRC) and industry The project broughttogether a multidisciplinary team of experimentalists (carryingout both small- and large-scale experiments), theoreticians (de-veloping new analysis, theory, and software in each of the un-derlying activities), systems engineers (carrying out modeling,component integration and scaling up, and analysis, design, andoptimization studies at plant scale), and industrial researchers(providing data, materials, advice, assessment, case studies, andfeedback)
foul-The CROF team comprises academic researchers at rial College London, the University of Bath, and the University
Impe-of Cambridge (15 academic staff supervising 10 PhD studentsand 8 postdoctoral research associates directly funded on thisproject, augmented by short-term undergraduate and postgrad-uate projects) From inception, the CROF project also involved
a large consortium of multinational companies from the and-gas sector, participating through the Oil Industry FoulingWorking Party operated by IHS ESDU, an industrial consul-tancy based in London Oil companies in the Fouling WorkingParty include BP, Chevron, ConocoPhillips, ExxonMobil, Petro-bras, PETRONAS, Shell, and Total and represent about 70% ofthe world’s refining capacity Also represented are heat transferequipment manufacturers (e.g., Alfa Laval, Calgavin) and chem-ical additive suppliers (e.g., Nalco) Each of the companies isrepresented by its top internal heat exchange and fouling experts(typically from a central specialized R&D function supportingthe worldwide operations of the respective organizations) IHSESDU coordinates the development of close links with the oil
oil-PlantUnit
Pilot plantMolecular
ACharacterisation
of deposits
DThermodynamics
& molecular modelling
EStirred vessel fouling tests
FAnnulus flow tests
GControl &
mitigationC
Fundamental transfer processes
BInterfacial &
rheological properties
HTechnology transfer
Figure 3 CROF subprojects and some key interrelations.
heat transfer engineering vol 32 nos 3–4 2011
Trang 13refining industry and their suppliers and ensures the industrial
relevance of the proposed work
The work program represents a new approach, as needed
to advance the subject The overall problem is tackled via a
closely linked set of subprojects, bringing together a wide range
of skills and techniques to produce a coordinated attack on this
important problem (Figure 3) Understanding the fouling
pro-cess requires investigating the temperature dependence of the
link between feedstocks and deposits, the advanced
character-ization of these materials, and examining the relationship
be-tween fouling rates and asphaltene (and other species) contents
(subproject A)
A fundamental understanding of mechanisms by which
de-posits adhere to surfaces is developed in subproject B and of the
transfer processes that take them there in subproject C Crucial to
all of this is the investigation of the underlying thermodynamic
interrelationships
Significant advances have been made in understanding the
thermodynamics of complex compounds at the molecular level,
and there is scope for improving the understanding of
asphal-tene behavior using these advances (subproject D)
Experimen-tal studies of fouling are carried out in two facilities to establish
rates of deposition and provide deposit samples for physical
and chemical characterization The first is a stirred vessel test
facility developed at Bath University (subproject E) The second
involves continuous flow tests in an electrically heated annulus
test facility at Imperial College, with novel sensing equipment
developed at Cambridge (subproject F) These experiments are
highly complementary The use of a stirred vessel allows
re-peated low-inventory tests to be done quickly and economically,
enabling the systematic study of a variety of crude oils, flow
modifiers, and surface conditions It is also easy to extract
sam-ples for analysis under subproject A The flow tests represent a
realistic representation of the situation in a heat exchanger and
allow investigating, among other phenomena, the effect of
foul-ing deposits on pressure gradients The flow tests are designed to
make continuous measurements of fouling layer thickness and
heat transfer coefficients (and thus fouling factor) and to study
the dynamics of the fouling process Understanding the physics,
chemistry, thermodynamics, and associated transfer processes
would be of little avail if it did not lead to means of control
and mitigation of the fouling problem Subproject G is a joint
Cambridge/Imperial College effort on control and mitigation
involving advanced modeling of heat exchangers (individually
and in network systems) at industrial scale and of cleaning and
control strategies Finally, there is little point in making any
advances on this important subject if the technology cannot be
transferred efficiently to industry The bridge to industry is
pro-vided by the role of IHS ESDU in the program As noted, IHS
ESDU is charged with the task of interfacing with the oil refining
companies and their suppliers and transferring the technology
by various means (subproject H)
An important feature of the overall programme is its
multi-scale nature, with inputs ranging from molecular multi-scale to plant
scale By adopting such an approach, new insights and
method-ologies can be developed through understanding the nisms that link the different scales It was recognized that closeintegration of the activities is essential in order to reap the max-imum benefits, and the structure and management of the projectand team were designed accordingly
mecha-PROGRESS AND ACHIEVEMENTS TO DATE
To date, substantial experimental results have been obtained
in subprojects A and B Molecular- and continuum-scale eling developments in subprojects C and D, respectively, arecontinuing apace The two pilot plant rigs (subprojects E andF) have been designed and built Some interesting data havealready been collected from the batch rig, while the continuousrig is presently being commissioned and tested, with data to fol-low by the end of the year Good progress has been made in thedevelopment of plant-scale models of heat exchangers underfouling and exchanger network applications The interactionswithin the team and with industry were developed in a num-ber of ways and are growing well For presentational purposes,progress and achievements in this section are described accord-ing to the subprojects detailed in Figure 3 This also reflects the
mod-“bottom-up” nature of the implementation work However, able emphasis is made on the strong links between subprojectsand the highly beneficial interplay between them
suit-Subproject A: Characterization of Asphaltenes and Deposited Foulants
The main focus of research in this subproject is to explorewhether and how molecular masses and structures of the de-posits actually relate to components of the crude oil feed materi-als and to identify the chemical transformation leading to depositformation It focuses on the study of changes in the crude oilleading to fouling, as well as in the chemical characterisation ofdeposits, both laboratory-generated and of industrial origin, andliquid phase after the crude oil has been heated Many techniqueshave been used to investigate the molecular masses and chem-ical structures of asphaltenes and other species, but no singletechnique is able to unravel the molecular masses and structures
of complex hydrocarbon liquids as each technique allows only
a limited, specific view of these complicated materials For thisreason an array of multiple approaches is used
A batch micro-bomb reactor (MBR) was used to study theeffect of time–temperature history of crude oil samples on theformation and deposition of a solid phase First, this approachallows us to decouple the transformations induced by exposi-tion of the samples to temperature from flow-related effects;second, it enables measuring the effect of different variables onthe chemical characteristics of the liquid and the deposits undercarefully controlled operating conditions, not available from re-finery operation data Formation of deposits has been studied attemperatures between 280 and 390◦C and residence times up toheat transfer engineering vol 32 nos 3–4 2011
Trang 14Figure 4 Macro ATR-FTIR images of deposits from a refinery PHT exchanger Images show the distribution of (a) CH stretching band, (b) carbonate compounds, (c) oxalate compounds, and (d) sulfate compounds.
24 hours Both the liquid and the solid phase have been analyzed
by a number of techniques, which include size exclusion
chro-matography (SEC) [28], ultraviolet (UV)–fluorescence
spec-troscopy, elemental analysis, solubility tests in various solvents,
and thermogravimetric analysis (TGA) A process of chemical
transformations due to heating was observed to take place in
both the fraction of crude oil remaining in liquid state and in the
solid deposits In the former, results indicate that the crude oil
gets progressively heavier as it is heated up, even at the lowerend of the temperature range where no solid deposits were ob-served in the MBR The molecular weight distribution of thecrude oil was found to increase upon heating, showing the for-mation of larger molecules in the liquid, which may be related
to deposit precursors In the case of the deposits, it was foundthat their solubility properties are also different in several sol-vents in comparison with the original samples, which suggestsheat transfer engineering vol 32 nos 3–4 2011
Trang 15that there are chemical transformations accompanying
deposi-tion The amount of deposits obtained in each run has shown
a strong dependence with temperature and residence time No
deposits were obtained at low residence times (1 hour)
regard-less of the temperature, which reinforces the role that stagnant
zones in the heat exchangers have in the formation of deposits
Relatively high temperatures and long residence times favor
de-position The hypothesis that the only origin of the deposits was
the asphaltene fraction of the crudes was put to test by carrying
out fouling experiments in the MBR with the asphaltene-free
heptane-soluble (HS) fraction of a crude oil [29] Carbonaceous
deposits have been proven to appear not only from the
asphal-tene fraction of the crude, but also from this deasphalted crude
It was observed in some cases that the deasphalted fraction
pro-duced a larger amount of deposits than the crude oil itself under
the same operating conditions Work is ongoing to check these
results with a broader range of crude oils as well as to analyze in
more detail the chemical characteristics of both industrial and
laboratory-scale deposits
A new imaging technique based on infrared spectroscopy
was applied to the characterization of fouling deposits and
asphaltenes This attenuated total reflection–Fourier-transform
infrared (ATR-FTIR) spectroscopic imaging, which relies on
the infrared focal plane array detector to simultaneously obtain
thousands of spectra from different locations in a sample, is a
nondestructive analytical technique and most importantly,
pro-vides both chemical and spatial information of a sample ATR
imaging spectrometers with a focal-plane array (FPA) detector
are patented by Varian [30] The small penetration depth of the
evanescence wave of the ATR approach makes it a convenient
sampling method with little or no sample preparation, and it
can be applied to highly absorbing materials such as
carbona-ceous hydrocarbons The advantages and intrinsic limitations of
ATR-FTIR spectroscopic measurements of high refractive
in-dex materials, such as petroleum deposits, have been addressed
by Tay and Kazarian [31] A laboratory-made aperture,
devel-oped by Chan et al [32], to vary the angle of incidence of
the incoming infrared radiation in the ATR diamond accessory
is used to correct the distortion of spectral bands due to the
dispersion of refractive index This allowed reliable spectral
information to be obtained on high refractive index materials
using ATR-FTIR spectroscopy with a diamond accessory Tay
and Kazarian [31] introduce novel applications of combining
macro and micro ATR modes in FTIR imaging to characterize
deposits from a refinery heat exchanger Using different ATR
ac-cessories for the macro and micro modes, FTIR imaging yields
important information about the spatial distribution of different
components in the deposits The macro ATR imaging approach
provides a larger field of view which can be used to obtain the
overall distribution of different components in the sample
(Fig-ure 4) With the enhanced spatial resolution of the micro ATR
approach, spectra of nearly pure components can be isolated
ATR-FTIR imaging allows the visualisation of different
chem-ical components in a heterogeneous sample Combining macro
ATR and micro ATR-FTIR spectroscopic imaging, the complex
petroleum deposit provides an important tool in chemical acterization of fouling material, which will aid in understandingthe fundamental of crude oil fouling Results demonstrate theviability of this approach; future in situ studies of heating ofcrude oil may reveal the onset of precipitation and deposition
char-Subproject B: Interfacial and Rheological Properties
The persistence of deposits on heat exchanger surfaces pends not only on the transfer of depositing material to theinterface but also on the adhesion of this material to the sur-face The adhesion of asphaltenic material onto surfaces will inturn depend on a number of complex factors including surfaceenergies, surface temperature, the shear acting on the surface,and the nature of previously deposited layers (smooth, rough,dendritic, etc.) Knowledge of the mechanical properties of theadsorbed asphaltene film is crucial if one aims to remove thefilm through mechanical or hydrodynamic forces The aims ofthis subproject are: to characterize the interactions between as-phaltenes and heat exchanger surfaces and to determine whetherthey can be reduced; to determine the interactions between as-phaltene molecules and so provide data for the molecular mod-eling studies described under subproject D; and to determinethe mechanical properties of the adsorbed asphaltene film, anecessary input into the modeling effort in subproject C.Atomic force microscopy (AFM) has been used to study theadsorption and adhesion of asphaltene to metal surfaces Adhe-sion was monitored by attaching an asphaltene coated sphere tothe AFM cantilever, pushing it against the metal surface, leaving
de-it in contact for a certain period of time and then monde-itoring theforce required to separate the metal surface from the asphaltenelayer (Figure 5) The interaction between asphaltene moleculeswas measured similarly, only with an asphaltene-coated glasssurface rather than a bare metal one The local mechanical prop-erties of the rheology of the asphaltene layer were determined
by indenting a sphere into the asphaltene layer (the instrumentwas initially calibrated by indenting a film of cross-linked poly-dimethylsiloxane, PDMS) Data were obtained both by com-pressing the films at a continuous rate (force measurement data)and by performing a stress relaxation experiment (any differencebetween the two methods is a consequence of some viscous flow
of the film) The results are summarized in Table 1 All resultsobtained have been on asphaltene films deposited onto surfaces
at room temperature It is our hope that we shall be able tocarry out similar experiments on asphaltene films deposited atthe temperatures experience by heat exchanges, which will besupplied by subprojects E and F
Subproject C: Fundamental Transfer Processes
The main aim of subproject C is to achieve a detailed scription of (i) the fluid mechanics of the flow in the tubes ofheat exchangers, (ii) the heat and mass transfer associated withheat transfer engineering vol 32 nos 3–4 2011
Trang 16de-Figure 5 DIM simulations of fouling in the absence and presence of deposition (a) and aging (b) The initial, intermediate, and late-time dynamics are shown in the top, middle, and bottom panels, respectively Details can be found in [62].
this flow, and (iii) their change as a function of temperature and
deposition and removal inside the tubes The ultimate goal is
the development of mathematical and numerical tools based on
first principles to predict, evaluate, and minimize fouling in the
tube side of PHT exchangers
The specific objectives of the subproject are to develop
ca-pabilities to simulate accurately, reliably, and efficiently the
spatial–temporal evolution of the fouling process in the heat
exchanger tubes as a function of the chemical, physical, and
thermal characteristics of the system To this end, we have
de-veloped a mathematical model based on the conservation of
mass, momentum, and energy, which is complemented by a
chemical equilibrium model based on the Gibbs free energy [33,
34] The latter is employed to account for the destabilization of
organic components in the oil leading to phase separation We
also account for diffusion of the asphaltene phase to the pipe
wall and its subsequent deposition on the wall in order to form
the fouling deposit The model also accounts for the changes in
the physicochemical, thermal, and rheological characteristics of
the deposit as a function of time due, for instance, to
“aging”-type phenomena Finally, the model accounts for turbulent flow
in the bulk, away from the wall, and can be extended to cover
complex geometries The predictions of the model are limited
by the input into this model in terms of thermodynamic and
physical properties The reliability of the model predictions can
therefore be enhanced with more accurate measurements of the
latter, obtained in the other subprojects of the CROF project
We consider the dynamics of two immiscible materials
under-going pressure-driven flow in a cylindrical pipe or rectangular
channel These materials correspond to the crude oil and the
foulant, located in the bulk and at the walls, respectively The
equations governing the flow in each phase are those of mass,
Table 1 A summary of the mechanical properties of thin films of PDMS of
two different cross link densities, 10:1 and 10:2, and Maya asphaltenes
Young’s modulus (kPa) Material Force measurement Stress relaxation
of the deposit, which is also treated as incompressible ical solutions of the governing equations are obtained subject
Numer-to no slip and no penetration, and temperature conditions atthe solid walls; constant-flow-rate conditions at the inlet; andoutflow conditions at the outlet Two approaches are used tosolve the model equations, depending on the nature of the flowregime in the tube In the unrealistic but useful case where theflow remains laminar throughout, we use direct numerical sim-ulations (DNS) of the governing equations; in cases where theflow is turbulent, we use large eddy simulations (LES) Theseapproaches are described next
Direct Numerical Simulations Here, we use a diffuse
inter-face method (DIM) formulation developed by Ding and Spelt[36] and Ding et al [37], which has been suitably modified toaccount for fouling deposition and the “aging” phenomena de-scribed earlier The DIM formulation allows one to trace theinterface between the two phases (oil and deposit) and can ac-commodate the changes of topology that will accompany thedisplacement and entrainment of the deposit following its inter-action with the bulk flow of oil Use of the DIM formulation (asopposed to the LES approach described later) also affords onethe opportunity to elucidate the physics of the interaction of thetwo phases and the aging phenomena [38] in the absence of thecomplexities associated with the presence of turbulence Theunderstanding gained as part of this approach provides an inputinto the LES studies Sample results are shown in Figure 6 start-ing from an initially thin, fouling layer at the wall of uniformthickness Here, it is clearly seen that the interaction betweenthe oil and deposit phase leads to the development of interfacialwaves In the absence of aging and deposition, this interactionresults in the removal of the deposit (Figure 6a); in the presence
of these effects, however, the deposit persists (Figure 6b).heat transfer engineering vol 32 nos 3–4 2011
Trang 17Figure 6 Determination of the interactions between an asphaltene-coated glass particle and a stainless steel metal surface immersed in water at pH 5.5.
Large Eddy Simulations In order to carry out large eddy
sim-ulations (LES), the commercially available code ANSYS CFX
was employed This code utilizes the volume-of-fluid method
and allows the inclusion of turbulence models and complex
geometries in the flow simulations; this, then, permits
simula-tion of the realistic case of a heat exchanger tube side In the
present work, we account for deposition and aging processes in
cylindrical pipe geometry The dynamics accompanying the
tur-bulent oil flow are now considerably more complex than those
associated with the laminar flow shown in Figure 6 Simulation
results clearly show the waviness of the interface separating
the two phases, as well as the removal of the deposit and its
entrainment into the bulk oil phase, promoted by the turbulent
flow
For future work, we aim to use the models developed so far
to carry out parametric studies of the fouling process in order to
elucidate its dependence on the relevant system parameters Of
particular interest would be the determination of the variation of
the pressure drop and the total mass of foulant deposited on the
walls as a function of time, and the implementation of measures
that would mitigate against fouling-induced pipe blockage
Subproject D: Thermodynamics and Molecular Modeling
The objectives of subproject D are (i) to develop a molecular
model of asphaltenes in the context of physically-based
equa-tion of state and (ii) to exploit the link between the equaequa-tion
of state and the underlying intermolecular potential to perform
molecular simulations on asphaltene-crude systems We have
taken the task of utilizing macroscopic information regarding
experimental asphaltene deposition boundaries and developed
a thermodynamic model based on a molecular-based equation
of state (EoS) A recent implementation of the statistical
as-sociating fluid theory (SAFT) (Gil-Villegas et al [39]) is used(for a review of SAFT and its applications see M¨uller and Gub-bins [40]), and by appropriately adjusting the intermolecularpotential parameters to capture the data we obtain effectivecoarse-grained parameters that represent in an average way apotential of model asphaltene-like fluids This latter molecu-lar description allows us to perform molecular-dynamics (MD)simulations to obtain the details of the aggregation mechanisms
on a microscopic scale
In this particular application, there is much uncertainty inthe exact molecular description of asphaltenes (see [41] for adiscussion on the archipelago and continental models for as-phaltenes) so we model the system initially as an asymmet-ric binary mixture consisting of a polymer (polystyrene) in oil(hydrocarbon) We have shown, in the context of this project,how this polystyrene asphaltene mapping (PAM) can be used todescribe phase behavior of asphaltene deposition Figure 7, ourcalculated pressure–temperature projection of the mixture phasediagrams for asphaltene+ oils of different molecular weights,exemplifies the method We model the system of Buenrostro
et al [42] using a PAM of molecular weight of 3066 g mol−1
to represent the asphaltene together with a C9 oil The blacksquares represent the asphaltene precipitation data from Buen-rostro et al [42]; the connected symbols represent our calcu-lations Areas under the curves represent states where phaseseparation occurs It is seen that the PAM model is consistentwith the experimental data In addition, we predict the effect onthe phase diagram of altering the molecular weight of the oil(e.g., by adding heavies or lights) As expected, the lighter theoil (solvent), the larger is the region of immiscibility for the as-phaltene Furthermore, the model predicts that at constant pres-sure, one may, given the correct conditions, observe a two-phasesplit of an otherwise homogeneous fluid upon an increase inheat transfer engineering vol 32 nos 3–4 2011
Trang 18Figure 7 Pressure–temperature projection of the phase diagrams of mixtures
of asphaltene (MW = 3066 g mol −1) with oils of increasing molecular weight.
temperature This latter phenomenon is seen in
experimen-tal data for asphaltene precipitation, but is rarely captured by
conventional thermodynamics modeling The PAM model is
straightforward to apply, given the knowledge of some
experi-mental data from which to infer the analog fluid
Although other EoS can be used to model
asphaltene-deposition behavior, the parameters in these EoS cannot be
traced back to intermolecular potentials The SAFT model, on
the other hand, is based on a rigorous statistical–mechanical
theory, so the parameters may be used in molecular-modeling
studies We have performed large-scale MD simulations of
archipelago-like molecules and more conventional
continental-like models with the parameters obtained using PAM models
and have obtained the expected qualitative behavior Having a
well-defined molecular model that represents in a broad sense
the molecular detailed behavior of asphaltenes and crudes will
allow us in the future to explore other fundamental issues
such as aggregation rates and tendencies, and the effects of
interfaces on the precipitation of asphaltenes Furthermore,
molecular modeling allows clear visualization of the underlying
molecular physics leading to the macroscopic behavior
Subproject E: Stirred Vessel Fouling Tests
The objectives of subproject E are (i) to design and
oper-ate a small high-pressure, high-temperature stirred cell (∼1 L
oil volume) based on the Eaton patent [43] and the ensuing
Eaton and Lux paper [44], (ii) to obtain fouling rate data as
a function of both temperature and surface shear stress for
a range of crude oils of interest to the CROF partnership,
Figure 8 Batch stirred vessel based on the Eaton and Lux design [44].
(iii) to interpret the data using established and new models
of fouling behavior, (iv) to characterize deposits, deposit tributions and thicknesses, (v) to supply samples of depositsand crude oil to other partners for further specialist character-ization at Imperial College, (vi) to compare experimental datawith that obtained from the continuous-flow loop apparatus atImperial College, and (vii) to integrate information, data, andmodels from the batch cell experiments into the overall CROFproject
dis-heat transfer engineering vol 32 nos 3–4 2011
Trang 19Figure 9 Typical deposit on a mild steel test finger [45] (finger is vertical in
the apparatus).
Figure 8 shows the general arrangement of the batch stirred
cell, which has been constructed from a steel block The cell
has been designed to operate at a crude oil bulk temperature
and pressure up to 300◦C and 30 bar, respectively The crude oil
is agitated by a rotating concentric cylinder placed around the
central electrically heated test finger, which has been designed
to operate with a surface temperature over 400◦C, if required
The cell is designed principally to obtain experimental data for
operating conditions that are typical of the hottest parts of the
crude oil preheat train The agitator is driven through the top
flange by an electric motor, which is fitted with a magnetic drive
unit The batch cell is fitted with a crude oil bulk temperature
thermocouple and a cooling coil through which a nonfouling
heat transfer oil is passed Heat is ultimately dissipated to the
atmosphere
The test finger is heated internally by a cartridge heater, the
heat flux from which is controlled electrically Three
thermo-couples are inserted at various heights inside the wall of the test
finger Being operated at constant heat flux and constant bulk
crude oil temperature, the rate of fouling becomes equal to the
rate of change of test finger wall temperature with time The test
finger/cartridge heater assembly can be removed from the base
of the batch cell so that the test finger can be removed for
inspec-tion and deposit characterizainspec-tion An example of a deposit on a
test finger is shown in Figure 9 The distribution of the deposit
thickness along and around the test finger is made by a laser and
coherent light scanning procedure (Proscan) Scanning electron
microscopy (SEM) and closeup photography support the
mor-phological analyses The experimental data are supported by
computational fluid dynamics (CFD) analysis using COMSOL
[45] CFD provides not only the flow patterns in the batch stirred
cell but also predictions of the distributions of surface
temper-ature and surface shear stress These predicted distributions are
validated against experimental data CFD has revealed (i) that
the surface shear stress is virtually constant along the axial
length of the test finger, except at its ends, and (ii) that there
is a near-parabolic distribution of surface temperature along the
length of the test finger
Large quantities of experimental data have been generatedwith three different crude oils [46] In virtually all cases thefouling rate has been found to be constant beyond the induc-tion period An induction period is not usually found for animperfectly cleaned test finger, and this is in accordance withinduction-period modeling [47] Experiments can be completedduring 1 day (<10 h) if required, and it has been found that they
can be continued from day to day if necessary with an overnightshutdown revealing no loss of continuity in the fouling curves.Some examples are detailed in Young et al [46]
Achievements to date have included (i) the accumulation ofcrude oil fouling data as a function of surface temperature, bulktemperature, and surface shear stress for three crude oils, (ii) thecalculation of Arrhenius data confirming that apparent activa-tion energies are a function of surface shear stress [48], (iii) thecreation of compensation plots [48] for the new experimentaldata alongside previous data on crude oils, inorganic salts, andproteins, (iv) the extrapolation of data in order to obtain thresh-old fouling temperatures for given surface shear stresses, givingrise to the ability to test existing threshold fouling models (e.g.,Ebert and Panchal [13]), and (v) the creation of an inductionperiod model that has been tested on new and existing crude oildata as well as on inorganic and protein fouling [47]
The batch stirred test cell is relatively fast to use Not onlycan crude oil samples be changed relatively quickly but also
a considerable amount of experimental data relating to theeffects of bulk temperature, surface temperature, and surfaceshear stress can be obtained typically within 1 day The ex situtechniques of surface profiling and CFD [45] add a consider-able amount of complementary and verification data Both thestarting and final chemistries have been studied in order to as-certain whether there have been any significant changes thatmight have an impact on both the fouling propensity and the in-duction period Parameters focused on have been asphalteneprecursor concentrations together with material and surfaceproperties [46] Experimental fouling rate data can be obtaineddirectly from changes in the three test finger thermocouplesknowing the local heat flux Since the fouling rate is constantwith time, the change of the rate of fouling as a function ofsurface temperature can be found at the end of an experimen-tal run when the deposit thickness has been measured usingthe Proscan technique The rate of increase in fouling depositthickness can be correlated against the CFD predicted surfacetemperature to provide a continuous Arrhenius plot along thelength of the heated test finger This means that, for the firsttime for crude oils, a single fouling experiment has been used toobtain the Arrhenius parameters which can then be incorporatedinto the compensation plot analysis [45] A new generic lumpedparameter model of fouling with an induction period has beendeveloped [47] The new model can be used to fit experimentaldata from the start of the induction period through to the foul-ing growth phase Two different types of crude oil have beenstudied
Future work will focus particularly on developing and testingmodels for crude oil fouling as a function of process conditions,heat transfer engineering vol 32 nos 3–4 2011
Trang 20including induction periods, fouling thresholds, and
compen-sation plots Experimental work will continue on providing
data, information, and samples to other partners, broadening
the ranges of crude oils and operating conditions We plan to
determine comparability between fouling data obtained using
the batch stirred cell with those from the continuous flow loop
Finally, it is planned to study the effects of metallurgies,
sur-face finishes, and intensification devices on crude oil fouling
rates
Subproject F: Annulus Flow Tests
The design, construction, and operation of a
high-temperature, high-pressure, crude oil flow loop equipped with
electrically heated test sections on which deposits will form was
undertaken at Imperial College London in collaboration with
the University of Cambridge The loop will be used primarily
to study the effects of fouling on heat transfer and pressure drop
in an annulus test section, simulating flow in a shell-and-tube
heat exchanger The rig will also accommodate analysis within
an actual size exchanger tube and could, in the future, be used
to study fouling in more complex heat exchanger geometries
The system thus permits comparison between tubular, annular,
and possibly other geometries The annulus configuration hasbeen used in previous laboratory investigations, e.g., the HTRIheated rod system, and allows fouling deposits to be recovered
or imaged in situ
The rig configuration will permit the continuous monitoring
of the heat transfer rate between the electrically heated test tion surface and the flowing crude oil, hence inferring the rate ofbuildup of fouling resistance This will be achieved by accuratemonitoring of the inner surface temperature of the heated annu-lus test section, using a radiation equilibrium thermometer Theactual thickness of the foulant layer will be measured simul-taneously using a novel dynamic gauging technique developed
sec-at the University of Cambridge The pressure drop through thetest section will also be measured continuously, as further evi-dence of buildup of the foulant layer Fouling-rate data will becompared with data derived from the stirred vessel tests beingconducted at the University of Bath (SP-E) and from refineryheat exchangers, with the hope of detecting systematic linkagebetween fouling rates Furthermore, samples of deposits will
be recovered from the rig for chemical/rheological sation, as a function of position, time, flow conditions, oil type,etc., in subprojects A and B The rig will also provide the facil-ity, in combination with group G, for testing of fouling controland mitigation strategies, such as surface modification, the use
characteri-Figure 10 Flowsheet for the fouling flow loop.
heat transfer engineering vol 32 nos 3–4 2011
Trang 21of crude oil antifouling additives, and cleaning strategies The
flow loop has been designed to supply crude oil to the heated
test section at a maximum temperature of 300◦C, although initial
operations will be limited to 200–270◦C The maximum loop
operating pressure is 30 barg The annulus test section is 2 m
long, permitting fouling measurements over about 1.5 m, after
allowance for an entry section for flow stabilization The optimal
oil delivery configuration (four injection points) was identified
from CFD studies The inner tube will be subject to direct
elec-trical resistance (Joule) heating Test surface temperatures of
250–350◦C are planned, i.e., oil plus 120◦C maximum, leading
to an imparted heat flux from the surface to the oil in the region
20–100 kW m−2 Fluid hydrodynamics will be representative
of conditions in refinery shell-and-tube heat exchangers, with a
linear velocity in the region 1–3 m/s, and Reynolds number up
to a maximum of 20,000 A standard tubular test section will be
mounted in parallel to the annulus section
A schematic diagram of the complete flow loop is given in
Figure 10 and a photo is shown in Figure 11 The test section,
which is connected to power supplies capable of delivering up
to 20 kW, is equipped with a radiation equilibrium
thermo-couple (Bennett et al [49]) inside the inner tube and a fluid
dynamic gauge within the annular gap The thermocouple,
con-tained within ceramic shields, traverses axially down the test
section and measures the temperature of the inner wall of the
heated tube to a high degree of accuracy The temperature of the
outer wall, on which deposition is occurring, can be calculated
exactly from this measured inner wall temperature, plus the
elec-trical power supply, by consideration of the radial conduction
through the cylindrical tube wall [50] The outer wall
tempera-ture will fluctuate, at a given power input and oil flow rate and
inlet temperature, in response to buildup of the fouling layer on
the tube outer surface Combination of the heat transfer and the
dynamic gauging measurements will permit direct estimation of
the thermal conductivity of the fouling layer for the first time
The pressure gradient will be measured using high-temperature
pressure transducers Since the surface of the deposit will be
rough and the outer tube relatively smooth, the shear stress on
the inner surface will be calculated from the pressure gradient
using a transformation method [51]
Fluid dynamic gauging (FDG), a noncontact technique
de-veloped at Cambridge, allows the thickness of soft solid layers
to be monitored in situ and in real time An account of the FDG
technique is given in Tuladhar et al [24] The device works
by measuring the flow rate of liquid through a nozzle located
close to, but not touching, the surface This separation distance,
or clearance, h, changes as a deposit grows or is removed: It
is tracked over time to yield thickness–time profiles FDG had
not been applied to curved surfaces before, nor at the operating
pressures and temperatures planned for the test loop A cold flow
system was constructed at Cambridge, replicating the test loop
geometry, and demonstrated that FDG can be applied to annular
surfaces across a wide range of flow rates Results show that the
mass flow rate–clearance profiles exhibit the sensitivity to h/dt
(distance between the gauging nozzle tip and the surface being
Figure 11 Side view of the fouling flow loop.
studied/nozzle throat diameter) required to locate the depositsurface under the turbulent flow conditions expected in the foul-ing test section experiments The paper by Gu et al [52] reportsextension of the work to lower Reynolds numbers and to heatedsurfaces, in particular a heated rod system similar to the HTRIprobe They demonstrate the successful application of FDG totrack the growth of a soft simulated milk fouling deposit Theyalso report computational fluid dynamics (CFD) simulations ofthe gauging system at low-annulus Reynolds numbers, whichallows the stresses being imposed on the surface under study to
be quantified This information can be used to characterize thestrength of the fouling layer These studies have enabled an FDGsystem to be designed for service in the Imperial College testloop This incorporates a new mode of FDG operation, intended
to minimize disruption of the flow field and fouling layer duringgauging Thickness measurements are made intermittently, anddisruption of the fouling layer is checked by comparing localmeasurements of the surface temperature using the travellingradiation thermocouple
The rig is designed to be operated remotely and the trol system incorporates automatic shut-down procedures in theheat transfer engineering vol 32 nos 3–4 2011
Trang 22con-Figure 12 Schematic of simulation activities, linking scientific advances to plant operations.
event of the detection of any mal-operation Continuous running
periods of 2–5 days are envisaged, based on estimates of fouling
buildup using the well-known Ebert–Panchal correlation
At the time of writing (April 2009), construction of the flow
loop and its containment has been completed and manufacture
of both the annular test section and the dynamic gauge is well
un-der way Commissioning of the system will commence shortly,
using Paratherm in the first instance The first fouling tests were
planned for autumn 2009 and would be conducted using a
post-desalter crude blend supplied by Petronas of Malaysia Small
samples of this blend, plus its constituent crudes, are being
studied elsewhere in the CROF project
Subproject G: Control and Mitigation
Activities here concentrated on developing tools and
tech-niques for scaling up the results and models generated in the
de-tailed investigations for application to refinery exchangers and
units, particularly the networks of exchangers found in crude
dis-tillation unit preheat trains The work was subdivided into two
parts (Figure 12) The main focus of part A was the construction
of a new modeling platform that could stimulative dynamic,
dis-tributed behavior of individual shell and tube heat exchangers
of any configuration (single and multipass, etc.) under fouling
and predict its effects on performance This required the local
fouling rate and deposit thickness to be evaluated over time and
space (i.e., across all points in the exchanger), and designed so
that it could incorporate the thermal, hydralic, deposition, and
ageing models developed by the fundamental investigations The
overall performance of the exchanger could then be evaluated
and expressed in terms readily understood by operators, such as
the overall fouling resistance and the pressure drop across the
unit The importance of parameters in the detailed fundamental
models could thus be established, and those critical for unit
per-formance could be identified Such models, although simpler
than those in subproject C, are substantially more detailed than
those used in current practice but still easily solvable for single
exchanger and network simulations
The platform was constructed using the “threshold fouling”
model of Ebert and Panchal [53] for fouling calculations and
implemented in the gPROMS simulation environment
(Pro-cess Systems Enterprise [54]) The single exchanger model
was validated against actual plant data in a demanding end exchanger [55] showing excellent predictive capabilities.Multiple exchangers in a network could then also be readilysimulated [56] to give a detailed quantitative assessment of thecosts of fouling (Figure 13) for alternative network configuration[65]
hot-Part B considered the effect of fouling on the thermal andhydraulic performance of preheat train networks It was de-cided to focus on existing networks, and thus on the opera-tion and retrofit of existing systems, rather than on the design
of new units, as this was a greater priority for operators sign must include the distillation system, and represents an areafor further work A network simulation tool was constructed
De-in the MATLAB programmDe-ing language combDe-inDe-ing the heat simulation approaches developed by Wilson et al [57] andthe exchanger modeling methods reported by Yeap et al [26].The simulation incorporated crude flow rate variation caused byhigh pressure drop, cleaning, and varying fouling rates in in-dividual exchangers calculated using threshold fouling models.Similarly to part A, the tool was constructed so that more ad-vanced fouling models could be “plugged in” once they becameavailable
pre-In modeling the performance of a network subject to fouling,and including realistic operating features, the simulation toolallows different mitigation options to be compared (and foul-ing managed) on the basis of quantitative indicators The paper
by Ishiyama et al [58] illustrates the importance of ment parameters (cleaning, fuel, and production costs) on op-timal cleaning (and noncleaning) decisions Simpler optimiza-tion methods were employed, deliberately, to calculate cleaningschedules in order to retain the engineering insight to the prob-lem Ishiyama et al [59] reports application of the simulation
manage-to an operating refinery, including control of key internal ables such as desalter temperature Future work will includedevelopment of a refinery software tool and extension to designaspects
vari-Existing models such as the “threshold fouling” approach
do not incorporate deposit aging, which could be important inextending experimental results and models to heat exchangerperformance There has been little quantitative work on thisaspect of chemical reaction fouling, and a new modelingframework for aging was developed as part of this work [60,64]
heat transfer engineering vol 32 nos 3–4 2011
Trang 23Figure 13 Cumulative costs of fouling over 1 year after cleaning (Coletti and
Macchietto [56]).
Subproject H: Technology Transfer
The promotion and coordination of interactions with
indus-trial partners within CROF are carried out by IHS ESDU of
London, represented by its director of process engineering
tech-nology ESDU has a long tradition of brokering industrial
in-novation It produces, under the guidance of independent
ex-pert, industrially based committees, high-quality engineering
data and software for industry and the universities ESDU has
been working on fouling since the mid-1980s, with particular
emphasis in the last 5 years on crude oil fouling
The CROF project evolved from the collaborative efforts of
the oil industry companies that form the IHS ESDU Oil
In-dustry Fouling Working Party This working party,
represent-ing some 70% of the world’s refinrepresent-ing capacity, was formed
in 1999 in recognition of the economic importance of crude
oil fouling and in acceptance that oil companies working with
each other and with leading international researchers in the
field was the best chance to find practical methods of fouling
abatement
In 1999 there was a landscape of very low oil prices—US$10/
bbl—and very low refinery margins Although the economic
landscape has changed considerably in recent years and even
months, refinery margins remain very tight and the importance
of fouling abatement is even more acute For example, the
en-vironmental effects of burning excess fuel are now being
rec-ognized in legislation, with refineries looking to minimize CO2
emissions and operate as efficiently as possible and minimize
use of their carbon emissions credits Oil refining is an extremely
safety-conscious industry, and safety problems associated with
the disposal of toxic fouling waste products and over-frequent
maintenance operations have to be minimized as far as is
prac-tical Tools developed by IHS ESDU with this working party,
and especially the heat exchanger analysis program
EXPRESS-plus, are now being applied by the oil company members of the
working party However, within this group it was recognized
that to make a real step-change in understanding the nature offouling problems it was necessary to have strategic funding forthe coordinated project that is CROF
As technology transfer partner for the CROF project, IHSESDU bridges the gap between industry and research and facil-itates close cooperation between all parties It organized bian-nual research and development meetings taking place at variouslocations in the United Kingdom, including an IHS ESDU of-fice, Imperial College, Cambridge University, and, most recently(April 2009), the Chevron refinery in South Wales As an ex-ample of the commitment of the refiners to the CROF project,engineers travelled from four continents to attend the 3-daymeeting kindly hosted by Chevron, which welcomed over 40engineers and scientists, followed by laboratory visits to Bathand Imperial College
Early in the project, the main concern was exchange of formation and advice on setting the research directions As theexperimental program started, access to representative crude-oilsamples became a key issue and adequate quantities of crudewere supplied from around the world More recently, a num-ber of industrial case studies were set up providing access tovaluable plant data and personnel Some of the case studieshave already been completed and are providing invaluable as-sessment of the techniques under development, feedback fromoperations, and a measure of their potential eventual impact
in-CONCLUSIONS
The project presented represents possibly the largest bined academic/industrial effort toward the systematic under-standing and solution of oil fouling problems in several decades
com-So has the problem been solved yet? The simple answer is, notyet; however, results so far indicate that the interdisciplinary,multiscale, combined experimental and theoretical, industrialand academic approach of CROF is very powerful and that ex-cellent progress has been achieved in a relatively short time
A team has been created that brings together most of theskills required Exchanges between researchers across the vari-ous subprojects are fostered by regular meetings between theresearchers, with regular presentations of respective results,question-and-answer sessions, and by many more informal ses-sions Progress reports from each subgroup are circulated to allresearchers and there is a common repository of data, informa-tion, reports, papers, etc through a project website, with privateand public sections It is very clear to all involved that this hasled to very significant progress in developing an understanding
of the overall problem and the potential of each other’s niques to address its various angles In several cases this newunderstanding has brought about a reorientation of the scientificwork in much more tailored directions, which reinforce andcomplement each other’s work
tech-Excellent experimental facilities have been designed andbuilt that are starting to provide data of quality and range notpreviously available Progress in the characterization of depositsheat transfer engineering vol 32 nos 3–4 2011
Trang 24has also been significant In particular, the ability to generate
and collect fouling deposit samples in controlled conditions and
to deploy a number of complementary analysis techniques,
in-cluding some on-line ones, on the same samples opens a new
window of rich primary information Modeling at the
molecular-scale level of transport and thermodynamics is hard work, but
again significant progress is being made with simplified “model”
systems, which, however, seem to offer good representative
fea-tures At the industrial equipment unit scale, progress has been
made in devising rather more detailed, distributed models than
used in the past for heat exchangers, models that can
incor-porate a variety of deposition, heat transfer, and deposit aging
processes The “threshold fouling” approach (used in this
con-text while awaiting for the new thermodynamics, transport, and
reaction models from the other groups) has been given a
signif-icant extension Work at the exchanger network scale has
pro-duced a new way to incorporate fouling when devising cleaning
scheduling
There is obviously much left to do, and in many ways, things
are just warming up Some future activities have been indicated
in this article in the description of the work of each of the groups
The main future opportunity, however, will lie in exploiting the
interaction between quality experimental data, which are now
becoming available, and theoretical modeling work at all levels,
for the purpose of parameter estimation, testing, and validation
There will also be the need, and opportunity, to more closely
integrate the results of the various groups A specific outcome
of the close exchanges within CROF has been a new awareness
of how the results of a group need to be “packaged” in order
for others to use them, which data are needed and in which
form This bodes well for such integration effort Finally, the
need continues for pilot-scale and plant “case studies” to
val-idate all work, in collaboration with the industrial partners of
the project Supported by such validation efforts, there will then
be the opportunity to use the new machinery to address
mitiga-tion strategies via improved operamitiga-tions and new heat exchanger
design methodology
Feedback on the CROF work and project so far has been
excellent, and the initial 3 years were extended to the end of
2009 There is a strong industry demand and support for the
project to continue and the team to be kept together To this
effect, a proposal is under way (April 2009) to consolidate the
present project into a more permanent interdisciplinary
cen-ter for research on fouling and cleaning It is envisaged the
center’s scope will be extended to cover a number of related
application areas: crude oil preheat trains (building on the work
in CROF); upstream (heavy) oil recovery to include fouling
within reservoirs due to asphaltene and wax deposition during
production; water cooling systems (including scale formation,
biofouling, and particulate fouling), food production systems;
production of biomass-based fuels; and nuclear and
conven-tional power production systems All these problems share
sev-eral generic features that include the poorly understood
me-chanics, thermodynamics, and physical chemistry of deposition
and removal, and will benefit from a combined experimental,
theoretical, mathematical modeling, and systems engineeringapproach
REFERENCES
[1] ESDU, Heat Exchanger Fouling in the Pre-Heat Train of aCrude Oil Distillation Unit, ESDU Data Item 00016, 2000.[2] Bories, M., and Patureaux, T., Preheat Train Crude Dis-tillation Fouling Propensity Evaluation by the Ebert and
Panchal Model, in ECI Conference on Heat Exchanger Fouling and Cleaning: Fundamentals and Applications, Santa Fe, New Mexico (USA), May 18–22, eds P Watkin-
son, H M¨uller-Steinhagen, and M.R Malayeri, BerkeleyElectronic Press, Berkeley, CA, pp 200–210, 2003.[3] Van Nostrand, W L., Leach, S H., and Haluska, J L.,Economic Penalties Associated with the Fouling of Refin-
ery Heat Transfer Equipment, in Fouling of Heat Transfer Equipment, eds E F C Somerscales and J G Knudsen,
Hemisphere, Troy, NY, pp 619–643, 1981
[4] Taborek, J., Aoki, T., Ritter, R B., Palen, J W., and sen, J G., Fouling: The Major Unresolved Problem in Heat
Knud-Transfer, Chemical Engineering Progress, vol 68, no 2,
pp 59–67, 1972
[5] Watkinson, P., Critical Review of Organic Fluid Fouling: Final Report, Argonne National Laboratory, Argonne, IL,
1988
[6] Watkinson, A P., and Wilson, D I., Chemical Reaction
Fouling: A Review, Experimental Thermal and Fluid ence, vol 14, no 4, pp 361–374, 1997.
Sci-[7] Taborek, J., Assessment of Fouling Research on the
De-sign of Heat Exchangers, in Fouling Mitigation of trial Heat-Exchange Equipment, ed C B Panchal, Begell
Indus-House, Redding, CT, pp 27–39, 1995
[8] Bott, T R., Fouling of Heat Exchangers, Chemical
Engi-neering Monographs, vol 26, Elsevier Science, dam, pp 211–211, 1995
Amster-[9] M¨uller-Steinhagen, H., Fouling of Heat Exchangers
Sur-faces, Chemistry & Industry, vol 5, pp 171–175, 1995.
[10] M¨uller-Steinhagen, H., Malayeri, M R., and Watkinson,
A P., Fouling of Heat Exchangers—New Approaches to
Solve an Old Problem, Heat Transfer Engineering, vol 26,
no 1, pp 1–4, 2005
[11] Epstein, N., Thinking About Heat Transfer Fouling: A 5×5
Matrix, Heat Transfer Engineering, vol 4, no 1, pp 43–56,
1983
[12] Bennett, C A., Appleyard, S., Gough, M., Hohmann, R.P., Joshi, H M., King, D C., Lam, T Y., Rudy, T M., andStomierowski, S E., Industry-Recommended Proceduresfor Experimental Crude Oil Preheat Fouling Research,
Heat Transfer Engineering, vol 27, no 9, pp 28–35, 2006.
[13] Ebert, W A., and Panchal, C B., Analysis of Exxon
Crude-Oil-Slip Stream Coking Data, in Fouling Mitigation of Industrial Heat-Exchange Equipment, C B Panchal, T R.
Bott, E F C Somerscales, and S Toyama, Begell House,Redding, CT, pp 451–460, 1997
heat transfer engineering vol 32 nos 3–4 2011
Trang 25[14] Panchal, C B., Kuru, W C., Liao, C F., Ebert, W A., and
Palen, J W., Threshold Conditions for Crude Oil Fouling,
in Understanding Heat Exchanger Fouling and Its
Mitiga-tion, eds T R Bott, L F Melo, C B Panchal, and E F.C.
Somerscales, Begell House, Redding, CT, pp 273–282,
1999
[15] Weihe, I A., and Kennedy, R J The Oil Compatibility
Model and Crude Oil Incompatibility, Energy & Fuels,
vol 14, no 1, pp 56–59, 2000
[16] Weihe, I A., Prevention of Fouling by Incompatible Crudes
With the Oil Compatibility Model, in Proceedings
Inter-national Conference on Petroleum Phase Behaviour and
Fouling, AIChE, pp 353–358, 1999.
[17] Weihe, I A., Mitigation of Plugging of a Hydrotreater With
the Oil Compatibility Model, in Proceedings International
Conference on Petroleum Phase Behaviour and Fouling,
AIChE, pp 401–411, 1999
[18] Watkinson, A P., Comparison of Crude Oil
Foul-ing UsFoul-ing Two Different Probes, in Heat Exchanger
Fouling and Cleaning: Fundamentals and
Applica-tions, Santa Fe, NM, Engineering Foundation/Polytechnic
Conferences, 2003 Available online at: http://services
bepress.com/eci/heatexchanger, paper 32
[19] Srinivasan, M., and Watkinson, A P., Fouling of Some
Canadian Crude Oils, Heat Transfer Engineering, vol 26,
no 1, pp 7–14, 2005
[20] Crittenden, B D., Hout, S A., and Alderman, N J.,
Model Experiments of Chemical Reaction Fouling,
Chem-ical Engineering Research and Design, vol 65, no 2, pp.
165–170, 1987
[21] Crittenden, B D., Kolaczkowski, S T., and Downey, I
L., Fouling of Crude-Oil Preheat Exchangers, Chemical
Engineering Research and Design, vol 70, no A6, pp.
547–557, 1992
[22] Takemoto, T., Crittenden, B D., and Kolaczkowski, S
T., Interpretation of Fouling Data in Industrial Shell
and Tube Heat Exchangers, Chemical Engineering
Re-search and Design, vol 77, no 8, pp 769–778,
1999
[23] Phillips, D Z., Mitigation of Crude Oil Fouling by the Use
of HiTran Inserts, PhD Thesis, University of Bath, Bath,
UK, 1999
[24] Tuladhar, T R., Paterson, W R., and Wilson, D I.,
Inves-tigation of Alkaline Cleaning-in-Place of Whey Protein
Deposits Using Dynamic Gauging, Food and Bioproducts
Processing, vol 80, no 3, pp 199–214, 2002.
[25] Gu, T., Chew, Y.M.J., Paterson, W.R., and Wilson, D.I.,
Experimental and CFD Studies of Fluid Dynamic Gauging
in Annular Flows, AIChE J., vol 55, no 8, pp 1937–1947,
2009
[26] Yeap, B L., Wilson, D I., Polley, G T., and Pugh, S J.,
Mitigation of Crude Oil Refinery Heat Exchanger Fouling
Through Retrofits Based on Thermo-Hydraulic Fouling
Models, Chemical Engineering Research and Design, vol.
82A, pp 53–71 [erratum 84A, 248], 2004
[27] Pilavachi, P A and Isdale, J D., European CommunityR&D Strategy in the Field of Heat Exchanger Fouling:
Projects, Heat Recovery Systems and CHP, vol 13, no 2,
pp 133–138, 1993
[28] Berrueco, C., Venditti, S., Morgan, T J., Alvarez, P.,Millan-Agorio, M., Herod, A A., and Kandiyoti, R., Cali-bration of Size-Exclusion Chromatography Columns With1-Methyl-2-pyrrolidinone (NMP)/Chloroform Mixtures as
Eluent: Applications to Petroleum-Derived Samples, ergy & Fuels, vol 22, no 5, pp 3265–3274, 2008.
En-[29] Venditti, S., Berrueco, C., Alvarez, P P., Morgan, T J.,Millan-Agorio, M., Herod, A A., and Kandiyoti, R., De-position of Solids From a Petroleum Heptane-Soluble
Fraction Upon Heating in a Micro-Bomb Reactor, in ROTHERM Heat Exchangers Fouling and Cleaning, eds.
EU-H M¨uller-Steinhagen, R Malayeri, and P Watkinson,Schladming, Austria, 2009
[30] Burka, E M., and Curbelo, R., Imaging ATR Spectrometer,
US Patent, 6,141,100, 2000
[31] Tay, F H., and Kazarian, S G., Study of Petroleum Exchanger Deposits With ATR-FTIR Spectroscopic Imag-
Heat-ing, Energy & Fuels, vol 23, no 8, pp 4059–4067, 2009.
[32] Chan, K L A., Tay, F H., Poulter, G., and Kazarian, S G.,Chemical Imaging With Variable Angles of Incidence Us-
ing a Diamond Attenuated Total Reflection Accessory, plied Spectroscopy, vol 62, no 10, pp 1102–1107, 2008.
Ap-[33] Won, K W., Thermodynamics for Solid Solution–Liquid–Vapor Equilibria–Wax Phase Formation From
Heavy Hydrocarbon Mixtures, Fluid Phase Equilibria, vol.
30, pp 265–279, 1986
[34] Svendsen, J A., Mathematical Modeling of Wax
Deposi-tion in Oil Pipeline Systems, AIChE Journal, vol 39, no.
8, pp 1377–1388, 1993
[35] Coussot, P., Tabuteau, H., Chateau, X., Tocquer, L., andOvarlez, G., Aging and Solid or Liquid Behavior in Pastes,
Journal of Rheology vol 50, no 6, pp 975–994, 2006.
[36] Ding, H., and Spelt, P D M., Wetting Condition in Diffuse
Interface Simulations of Contact Line Motion, Physical Review E vol 75, no 4, pp 046708–1–8, 2007.
[37] Ding, H., Spelt, P D M., and Shu, C., Diffuse InterfaceModel for Incompressible Two-Phase Flows With Large
Density Ratios, Journal of Computational Physics, vol.
226, no 2, pp 2078–2095, 2007
[38] Coussot, P., Rheophysics of Pastes: A Review of
Micro-scopic Modelling Approaches, Soft Matter, vol 3, no 5,
pp 528–540, 2007
[39] Gil-Villegas, A., Galindo, A., Whitehead, P J., Mills, S.J., Jackson, G., and Burgess, A N., Statistical AssociatingFluid Theory for Chain Molecules With Attractive Poten-
tials of Variable Range, Journal of Chemical Physics, vol.
Trang 26[41] Aguilera-Mercado, B., Herdes, C., Murgich, J., and M¨uller,
E A., Mesoscopic Simulation of Aggregation of
Asphal-tene and Resin Molecules in Crude Oils, Energy & Fuels
vol 20, no 1, pp 327–338, 2006
[42] Buenrostro-Gonzalez, E., Lira-Galeana, C., Gil-Villegas,
A., and Wu, J., Asphaltene Precipitation in Crude Oils:
Theory and Experiments, AIChE Journal, vol 50, no 10,
pp 2552–2570, 2004
[43] Eaton, P., Laboratory Fouling Test Apparatus for
Hydro-carbon Feedstocks, US Patent, 4383438, 1983
[44] Eaton, P., and Lux, R., Laboratory Fouling Test Apparatus
for Hydrocarbon Feedstocks, ASME-HTD, New York, pp.
33–42, 1984
[45] Yang, M., Young, A., and Crittenden, B D., Use of CFD to
Correlate Crude Oil Fouling Against Surface Temperature
and Surface Shear Stress in a Stirred Fouling Apparatus,
in International Conference on Heat Exchanger Fouling
and Cleaning VIII, Schladming, Austria, June 14–19, eds.
H M¨uller-Steinhagen, R Malayeri, and P Watkinson, pp
272–280, 2009
[46] Young, A., Venditti, S., Yang, M., and Crittenden, B D.,
Characterisation of Crude Oils and their Fouling Deposits,
in International Conference on Heat Exchanger Fouling
and Cleaning VIII, Schladming, Austria, eds H
M¨uller-Steinhagen, R Malayeri, and P Watkinson, pp 17–26,
2009
[47] Yang, M., Young, A., and Crittenden, B D., Modelling of
Fouling Induction Periods, in International Conference on
Heat Exchanger Fouling and Cleaning VIII, Schladming,
Austria, eds H M¨uller-Steinhagen, R Malayeri, and P
Watkinson, pp 69–75, 2009
[48] Crittenden, B D., Kolaczkowski, S T., Takemoto, T., and
Phillips, D Z., Crude Oil Fouling in a Pilot-Scale Parallel
Tube Apparatus, Heat Transfer Engineering, vol 30, no.
10, pp 777–785, 2009
[49] Bennett, J A R., Collier, J G., Pratt, H R C., and
Thorn-ton, D., Heat Transfer to Two-Phase Gas–Liquid Systems
Steam–Water Mixtures in the Liquid-Dispersed Region in
an Annulus, Transactions of the Institution of Chemical
Engineers, vol 39, pp 113–126, 1961.
[50] Barbosa, J R., Phase Change of Single Component
Flu-ids and Mixtures in Annular Flow, PhD Thesis, Imperial
College London, UK, 2001
[51] Hewitt, G F., Interpretation of Pressure Drop Data
From an Annular Channel, UKAEA Report AERE-R4340,
UKAEA, CulhamScience Center, Abingdon, UK 1964
[52] Gu, T., Albert, F., Augustin, W., Scholl, S., Chew, Y M
J., Paterson, W R., Sheikh, I., Wang, K., and Wilson, D I.,
Fluid Dynamic Gauging for Annular Fouling and
Clean-ing Test Systems, in International Conference on Heat
Ex-changer Fouling and Cleaning VIII, Schladming, Austria,
eds H M¨uller-Steinhagen, R Malayeri, and P Watkinson,
pp 468–475, 2009
[53] Polley, G T., Wilson, D I., Yeap, B L., and Pugh, S
J., Evaluation of Laboratory Crude Oil Threshold Fouling
Data for Application to Refinery Pre-Heat Trains, Applied Thermal Engineering, vol 22, no 7, pp 777–788, 2002.
[54] Process Systems Enterprise, gPROMS, www.psenterprise.com/gproms, 1997–2009
[55] Coletti, F., and Macchietto, S., A Dynamic, DistributedModel of Shell-and-tube Heat Exchangers Undergoing
Crude Oil Fouling, Industrial and Engineering Chemistry Research, submitted.
[56] Coletti, F., and Macchietto, S., Refinery Pre-Heat TrainNetwork Simulation: Assessment of Energy Efficiency
and Carbon Emissions, in EUROTHERM Heat ers Fouling and Cleaning, Schladming, Austria, eds.
Exchang-H M¨uller-Steinhagen, R Malayeri, and P Watkinson,
pp 2009 pp 61–68 Available online at http://www.heatexchanger-fouling.com/proceedings2009.htm, paper12
[57] Wilson, D I., Vassiliadis, V S., and Smaili, F., gating Fouling at the Plant Scale by Strategic Clean-
Miti-ing: Potential, Pitfalls, and Needs, in Heat Exchanger Fouling—foundamental Approaches and Technical Solu- tions, ed H M¨uller-Steinhagen, Davos, Switzerland, Pub-
lico Publications, Germany pp 325–332, 2001
[58] Ishiyama, E., Paterson, W., and Wilson, D., A Platform forTechno-Economic Analysis of Fouling Mitigation Options
in Refinery Preheat Trains, Energy & Fuels, vol 23, no 3,
pp 1323–1337, 2009
[59] Ishiyama, E M., Heins de Petro, A V., Paterson, W R.,Spinelli, L., and Wilson, D I., The Importance of ControlConsiderations for the Preheat Trains of Crude Distilla-
tion Units: A Case Study, in International Conference on Heat Exchanger Fouling and Cleaning VIII, Schladming,
Austria, eds H M¨uller-Steinhagen, R Malayeri, and P.Watkinson, pp 52–60, 2009
[60] Ishiyama, E M., Coletti, F., Macchietto, S., Paterson, W.R., and Wilson, D I., Impact of Deposit Ageing on Thermal
Fouling: Lumped Parameter Model, AIChE Journal, vol.
56, no 2, pp 531–545, 2010
[61] Panchal, C B., and Huangfu, E.-P., Effects of MitigatingFouling on the Energy Efficiency of Crude-Oil Distillation,
Heat Transfer Engineering, vol 21, no 3, pp 3–9, 2000.
[62] Sileri, D., Sahu, K C., Matar, O K., and Ding, H., ematical Modelling of Asphaltenes Deposition and Re-
Math-moval in Crude Distillation Units, in International ference on Heat Exchanger Fouling and Cleaning VIII,
Con-Schladming, Austria, H M¨uller-Steinhagen, R Malayeri,and P Watkinson, pp 245–251, 2009
[63] Coletti, F., Macchietto, S., and Polley, G T., Effects offoulingon performance of retrofitted heat exchanger net-works; a thermo–hydraulic based analysis A thermo-
hydraulic based analysis, in 20th European Symposium
on Computer Aided Process Engineering, eds S Pierucci,
and G Buzzi-Ferraris, Computer Aided Chemical neering Series No 28, 19-24, Elsevier, 2010
Engi-[64] Coletti, F., Ishiyama, E.M., Paterson, W R., Wilson, D I.,and Macchietto, S., Impact of deposit ageing and surfaceheat transfer engineering vol 32 nos 3–4 2011
Trang 27roughness on thermal fouling: distributed mode, AIChE J.,
2010 DOI 10.1002/aic.122221
S Macchietto is a professor of process systems
engi-neering at Imperial College London, which he joined
in 1983, after degrees in Italy and the United States.
His research addresses the development of matical methods and their use to model and optimise the design and operation of processes and sustain- able energy systems He has co-authored over 190 research papers and his work on optimisation and model-based design of experiments is widely used within leading industrial process simulators He co- founded and was a director of the Centre for Process Systems Engineering, led
mathe-the launch of Process Systems Enterprise Ltd., co-founded Imperial’s Energy
Futures Lab, and launched a successful Master in Sustainable Energy Futures.
He was made a Cavaliere of the Order of Merit in Italy and is a co-winner of
the 2008 MacRobert Award, the Royal Academy of Engineering top award for
innovation.
G F Hewitt is a professor emeritus of chemical
en-gineering at Imperial College, London He worked at the UKAEA Harwell Laboratory from 1957 to 1990, becoming a professor at Imperial College in 1985 He has specialized mainly in heat transfer and fluid flow, with particular reference to multiphase systems In
1968, he was founder of the Heat Transfer and Fluid Flow Service (HTFS) (serving the chemical and pro- cess industry) and has participated in mainly collabo- rative research programmes, including the Transient Multiphase Flow (TMF) projects with the oil industry and nuclear thermal hy-
draulics projects He has authored and edited many books and published over
500 papers and reports, in particular on annular flow, nuclear power, heat
ex-changer design, turbulence modeling, and multiphase flow metering He is the
recipient of the AIChE Donald Q Kern Award, the ASME Max Jacob Award,
the Nusselt Reynolds Prize, the Luikov Medal, the IChemE Council and
Arm-strong medals, the Senior Multiphase Flow Award, and the Global Energy Prize.
He has hon doctorates from Louvain, UMIST, and Heriot Watt He is a fellow
of the Royal Academy of Engineering, fellow of the Royal Society, and foreign
associate of the U.S National Academy of Engineering.
F Coletti has just completed a PhD in chemical
en-gineering at Imperial College London He holds a degree in chemical engineering from the University
of Padova, Italy and an MSc in process systems neering from Imperial College London His research interests are in the area of process systems engineer- ing with focus on the development of mathemati- cal models for bio and energy systems He received the Townend Prize 2008 from Imperial College Lon- don for excellence in research in the area of fuel production.
engi-B Crittenden is a professor of chemical engineering
at the University of Bath and leader of its Advanced Materials and Porous Solids research group His re- search interests include ceramic membranes, acti- vated carbon monoliths for environmental control, and hydrocarbon fouling in refinery heat exchangers.
D R Dugwell is a professor of chemical engineering
at Imperial College London and has 40 years of search experience, with over 150 papers and reports co-authored After spending 10 years working on in- dustrial gas utilisation and in the steel industry, he joined Imperial in 1978 He has taught heat transfer for more than 20 years and researched a range of heat transfer and energy topics He has collaborated widely with organizations throughout the European Union in 30 European-funded research projects, plus participation in EPSRC and other UK-funded projects.
re-A Galindo is a reader in physical chemistry in the
Department of Chemical Engineering and in the tre for Process Systems Engineering at Imperial Col- lege London She holds a PhD in chemistry from the University of Sheffield and a BSc in chemistry from the Universidad Complutense of Madrid Her inter- ests are in the development of statistical mechanical approaches for complex fluids and the study of phase behavior and modeling of these systems She has co- authored over 70 papers and book chapters She is the recipient of an EPSRC Advanced Research Fellowship and an ExxonMobil Teaching Fellowship.
Cen-G Jackson is a professor of chemical physics in
the Molecular Systems Engineering Group at the Chemical Engineering Department of Imperial Col- lege London After undertaking his DPhil (PhD) in physical chemistry at Oxford University on model- ing the phase behavior of molecules of large size asymmetry with Sir John Rowlinson FRS, he moved
to the School of Chemical Engineering of Cornell University as a postdoctoral associate with Keith Gubbins During this time he and his collaborators (Walter Chapman, Keith Gubbins, and Mac Radosz) developed the SAFT approach, a highly successful modern equation of state for complex fluids His move to Imperial followed 10 years at the Chemistry Department of the
Sheffield University, where he was one of the youngest ever editors of Molecular
Physics.
R Kandiyoti is distinguished research fellow in the
Department of Chemical Engineering at Imperial College London, following a career as a professor
of chemical engineering there His research interests include pyrolysis, gasification, combustion and liq- uefaction of solid fuels, including biomass, wastes, and coal, catalytic hydrocracking of heavy coal liq- uids and petroleum residues, their molecular mass and structural characterization, and the development
of analytical techniques relating to solid fuels and their liquid products On all these subjects he has published extensively.
S G Kazarian is a professor of physical chemistry at
the Department of Chemical Engineering of Imperial College London Professor Kazarian is a fellow of the Royal Society of Chemistry After obtaining his PhD in physical chemistry from the USSR Academy
of Sciences in Moscow in 1987, he held research sitions in the United Kingdom and the United States.
po-He joined Imperial College London in 1998 sor Kazarian has co-authored more than 140 journal publications and 10 book chapters and he serves on
Profes-heat transfer engineering vol 32 nos 3–4 2011
Trang 28the editorial board of Vibrational Spectroscopy His research encompasses the
fields of vibrational spectroscopy, supercritical fluids, intermolecular
interac-tions and materials A large part of his current research focuses on the
applica-tions of spectroscopic imaging to complex materials, biomedical samples, and
pharmaceuticals.
P F Luckham, BSc, PhD, obtained his PhD in
col-loid science from the University of Bristol in 1980;
he then spent 3 years in the Cavendish Laboratories at Cambridge University, directly measuring the inter- action forces between adsorbed polymer molecules.
He was appointed as lecturer in the Chemical gineering and Chemical Technology Department of Imperial College in 1983, and was appointed a pro- fessor of particle technology in 1997, in the same year he was also awarded the Beilby Medal He is currently the Kodak Professor of Interface Science.
En-O K Matar received an MEng degree in chemical
engineering from Imperial College London (ICL) in
1993 He then undertook postgraduate studies in fluid mechanics at Princeton University, receiving a Ph.D.
in 1998 He subsequently became a lecturer in the partment of Chemical Engineering at ICL in 1998 He was promoted to senior lecturer in 2003 and then to professor in 2007 He is currently an Exxon-Mobil fellow and professor of fluid mechanics, working in the area of multiphase flows with emphasis on the detailed modeling of the dynamics of thin liquid films and slender jets, driven
De-by Marangoni stresses, surface and bulk diffusion, gravitational, capillary,
inter-molecular forces, and parametric forcing His work places particular emphasis
on hydrodynamic instabilities and pattern formation, with applications in
in-tensive processing, coating flow technology, fouling prevention, oil and gas
transportation, cleaning processes, distillation, micro-fluidics, and surfactant
replacement therapy.
M Millan graduated as a chemical engineer from
the University of the Republic of Uruguay in 2001.
In 2005, he completed his PhD on catalytic racking and characterisation of heavy hydrocarbons liquids in the Department of Chemical Engineering
hydroc-at Imperial College, which he joined as a lecturer in
2006 His research interests include upgrading and chemical characterization of heavy oils and coal- derived liquids, gasification of solid fuels, abatement
of trace element emissions from combustion and fication processes, and biomass to liquid processes He has authored more than
gasi-30 publications in refereed international journals.
E A M ¨uller is a reader in thermodynamics at
Im-perial College London He received his Ph.D from Cornell University in 1995 His research interests in- clude molecular modeling and simulation of complex fluids and interfaces and the application of multiscale modeling to industrial problems Research highlights include the prediction of the unique adsorption mech- anism for water confined in nanopores, the first sim- ulations of Joule–Thomson inversion curves of real gases, and the proposal of the use of angle-averaged potentials and new simulation techniques to model complex fluid phase equi-
libria and self-assembly of liquid crystals and amphiphiles Dr Muller has co-authored more than 150 papers and conference publications, besides editing and contributing to a number of books.
W Paterson is a senior lecturer in chemical
engi-neering at Cambridge University His research ests include reaction engineering, process simulation and synthesis, and granular flows He holds a Ph.D from the University of Edinburgh and is a chartered engineer.
inter-S J Pugh is Director of Process Engineering
Tech-nology at IHS ESDU and is based in London, UK His current role includes the management of all ESDU’s heat transfer work, which is undertaken under the guidance of international independent committees of experts from industry and the universities He leads the Technology Transfer Sub-project of CROF, which includes the management of the interaction and col- laboration between the researchers and the oil com- pany members of the IHS ESDU Oil Industry Fouling Working Party He is currently leading a group of engineers working on the de- velopment of a range of design guides to oil industry fouling problems and computer programs for better heat exchanger selection, design, and operation, with particular emphasis on reducing crude fouling in preheat trains He holds
a mechanical engineering degree from Brunel University.
S M Richardson is deputy rector, principal of the
Faculty of Engineering, and professor of chemical engineering at Imperial College He was educated at Imperial College; after working as a research assis- tant at Cambridge University, he was appointed as
a lecturer at Imperial College in 1978 and then moted through the ranks to professor in 1994 His principal research interest is safety, specifically the depressurization of vessels and pipelines, particularly those associated with oil and gas production, and de- velopment of the computer program BLOWDOWN, which has since been used
pro-in the design of well over 200 pro-installations In 1996, he was made a fellow of the Royal Academy Engineering.
D I Wilson is a reader in chemical engineering at
Cambridge He has worked in the area of fouling and cleaning of heat transfer systems since 1988 His research interests include rheology, paste process- ing, and food engineering He is a chartered engi- neer and holds a Ph.D from the University of British Columbia.
heat transfer engineering vol 32 nos 3–4 2011
Trang 29CopyrightTaylor and Francis Group, LLC
ISSN: 0145-7632 print / 1521-0537 online
DOI: 10.1080/01457632.2010.495603
Characterization of Crude Oils and
Their Fouling Deposits Using a Batch Stirred Cell System
ANDREW YOUNG,1 SILVIA VENDITTI,2 CESUR BERRUECO,2
MENGYAN YANG,1 ANDREW WATERS,1 HADDY DAVIES,1 SIMON HILL,1
MARCOS MILLAN,2and BARRY CRITTENDEN1
1Department of Chemical Engineering, University of Bath, Bath, United Kingdom
2Department of Chemical Engineering, Imperial College London, United Kingdom
A small (1 L) batch stirred cell system has been developed to study crude oil fouling at surface temperatures up to 400◦C
and pressures up to 30 bar Fouling resistance–time data are obtained from experiments in which the principal operating
variables are surface shear stress, surface temperature, heat flux, and crude oil type The oils and deposits are characterized
and correlated with the experimental heat transfer fouling data to understand better the effects of process conditions such
as surface temperature and surface shear stress on the fouling process Deposits are subjected to a range of qualitative
and quantitative analyses in order to gain a better insight into the crude oil fouling phenomenon Thermal data that can be
obtained relatively quickly from the batch cell provide fouling rates, Arrhenius plots, and apparent activation energies as a
function of process variables The experimental system, supported by computational fluid dynamics (CFD) studies, allows
fouling threshold conditions of surface temperature and shear stress to be identified relatively quickly in the laboratory The
data also contribute to existing knowledge about the compensation plot.
INTRODUCTION
Various methods have been adopted to study the complex
problem of fouling in crude oil preheat exchangers Research
using actual plant data is slow, subject to a variety of logistical
and operational requirements that do not lend themselves well
to fundamental scientific studies [1] and can create difficulties in
the interpretation of the thermal data [2] A number of laboratory
methods have been developed to study liquid-phase fouling [3,
4] including for crude oils use of the stirred batch cell [5], or
use of a recycle flow loop with either a tubular cross section [6]
or an annular cross section [7, 8]
While laboratory studies can eliminate the principal
prac-tical disadvantages of studying fouling on refinery
exchang-The authors are grateful to the UK’s Engineering and Physical Sciences
Research Council (EPSRC) for the award of a research grant (EP/D506131/1)
to study the role of asphaltenes in crude oil fouling The authors are grateful
also to their project partners at the University of Cambridge, and to ExxonMobil
and Petronas.
Address correspondence to Professor Barry Crittenden, Department of
Chemical Engineering, University of Bath, Bath BA2 7AY, United Kingdom.
E-mail: cesbdc@bath.ac.uk
ers, they, in turn, introduce their own disadvantages, the cipal one being that the crude oil is not exposed to thetime–temperature–flow history of the crude in the oil refin-ery Accordingly, concerns are that crucial fouling precursorsmight become depleted in extended running of both batch cellsand recycle flow loops, and that flow structures are not typi-cal of industrial practice Nevertheless, laboratory-scale studiescan be made with precise operating conditions and can provideexcellent access to the heat transfer surface, its deposits, andthe fluids from which the fouling originates Laboratory-scalemethods also allow study of the effects of the heat transfer sur-face (its material, morphology, enhancement, etc.), as well as
prin-of devices that might be used to simultaneously improve heattransfer and reduce fouling [9]
In the current research a batch stirred cell has been designed
to operate under conditions close to those found in crude oilpreheat trains, namely, a maximum pressure of 30 bar and amaximum surface temperature of around 400◦C The cell de-sign following closely that of Eaton and Lux [5] and Eaton [10]was chosen since it offers extraordinary flexibility Crude oilscan be changed easily, as can the fouling fluid chemistry (e.g.,
by adding asphaltenes, metal salts, etc.) The cell can be sparged216
Trang 30Figure 1 The batch stirred cell.
with various gases, e.g., oxygen and nitrogen, and the heat
trans-fer surface is easily inspected and changed Computational fluid
dynamics (CFD) software also allows the thermal and fluid flow
characteristics of the complex batch stirred cell geometry to be
predicted and validated [11]
APPARATUS AND EXPERIMENTAL METHOD
Apparatus
The general arrangement of the cell is shown in Figure 1 The
cell comprises a pressure vessel made in-house from a block of
304 stainless steel together with a top flange The base of the
vessel houses an upward-pointing test probe heated internally
by a cartridge heater, the heat flux from which is controlled
Figure 2 Heated test probe.
electrically (Figure 2) The crude oil (∼1.0 L) is agitated by
a downward-facing cylindrical stirrer mounted coaxially withthe test probe and driven by an electric motor via a magneticdrive External band heaters are incorporated to provide ini-tial heating to the vessel and its contents An internal coolingcoil uses a nonfouling fluid (Paratherm) to remove heat at therate that it is inputted via the cartridge heater during the foul-ing run The vessel is fitted with a pressure relief valve andthere is a single thermocouple to measure the crude oil bulktemperature
Heated test probes are manufactured in-house from carbon steel with a material specification similar to that of
low-a stlow-andlow-ard A179 helow-at exchlow-anger tube Elow-ach probe fits tightlyover the cartridge heater supplied by Watlow Ltd A coherentlight scanning probe (Scantron Industrial Products, Taunton,
England) is used to obtain the initial surface roughness Ra.
Three test probes have been made (denoted A, B, and C) Probes
A and B have relatively high Ra values of∼10 µm Probe Chad a similar surface characterization to that of a standard mildsteel heat exchanger tube (A179) Three small-diameter type
J thermocouples (supplied by TC Thermocouples Ltd) located
at various heights within the walls of the test probe are used
to follow progress of the fouling at different axial locations
The three thermocouples are labeled twb, twm, and tws, with
positions shown in Figure 2 Pressure is maintained by means
of a nitrogen blanket Air can be admitted to sparge the crudeoil in advance of a fouling run if so desired Heat is ultimatelyheat transfer engineering vol 32 nos 3–4 2011
Trang 31dissipated to atmosphere from the Paratherm by means of an
air-cooled heat exchanger
Experimental Method
The vessel is initially filled with the specimen crude oil For
most experiments, the oil is then sparged with air at 1 bar while
being stirred with the desired experimental agitation speed for 30
min The fully aerated vessel is then purged and blanketed with
nitrogen at 20 bar The vessel and its contents are then brought
up to the bulk operating temperature by means of the external
band heaters The heating ramp is normally 15◦C min−1
At 200◦C the stirrer is started and heating with the band
heaters is continued until the desired bulk temperature (typically
around 260◦C) is reached Once the desired bulk temperature is
reached, power is applied to the cartridge heater to provide the
desired heat flux or desired surface temperature, as appropriate
Also at this point the controlled cooling circuit is established
Figure 3 Deposit on a heated probe.
to develop a steady-state heat transfer rate in the cell Control
of the steady-state heat transfer conditions is achieved with abespoke combination of band heating and cooling coil oper-ation under traditional PID (proportional, integral, derivative)control
A single fouling test run usually lasts for less than 10 hours
At the end of an experiment the power to the cartridge heater isswitched off, the stirrer speed is reduced, and the vessel is al-lowed to cool down by maintaining the flow through the coolingcoil This arrangement should prevent the fouling deposit frombeing removed, so that the fouling run can be resumed the nextday if necessary When a linear fouling rate has been observedfor a sufficiently long period, the fouling run is terminated andthe test probe and cartridge heater assembly are removed fromthe base of the batch cell once it has been drained so that thetest probe can be removed for inspection and deposit character-ization An example of a heated test probe with fouling deposit
is shown in Figure 3 The deposit can be scraped off with analuminum bar for further analysis
Experimental errors are due to a number of variations anduncertainties in the operational conditions, such as the stirrerspeed, heater power input, and errors in thermocouple readings,which are in the range of 1–2◦C In practice, interest centers onthe accuracy of the initial fouling rate and not on the accuracy
of an absolute fouling resistance at any particular time After
an initial induction period, fouling rates are always found to belinear Hence, any local variations that do occur in the foulingresistance–time plot are smoothed out by linear regression ofthe data obtained over a period of several hours Table 1 lists theoperational conditions used in the fouling test runs
If a previously fouled probe is to be used for a subsequent run,
it is thoroughly cleaned by brushing in the following sequence
of solvents: ethanol, kerosene, para-xylene, and acetone.
Fouling Resistance and Fouling Rate Calculations
The fouling resistance Rfis calculated from Eq (1) [8, 12]
R f =
T s − Tb q
t=0
(1)
Here, q is the heat flux and Ts and Tb are the local surface
and bulk temperatures at time t and time zero, respectively.
This equation avoids the need to calculate film heat transfer
Table 1 Operational parameters used in the fouling test runs Operational parameter Range Bulk temperature ( ◦C) 240–280
Average heat flux (kW/m 2 ) 85–122 Surface temperature ( ◦C) 345–420
Stirring speed (rpm) 100–400 Pressure (bar) 24–28
heat transfer engineering vol 32 nos 3–4 2011
Trang 32Table 2 Properties of crude oil blends tested
Crude CrudeB Crude
A B DS API 27.5 24.43 30.92
(2)
Here, Tst and Tso are the surface temperatures at time t and
zero, respectively In the cell, measurements are made of
tem-peratures within the probe wall The locations of these
thermo-couples, denoted twb, twm, and tws, are shown in Figure 2 CFD
is used to convert these temperatures to those at the actual metal
surface/crude oil interface [11] Provided that the film heat
trans-fer coefficient does not alter as fouling proceeds, then the initial
interfacial temperature remains constant throughout a fouling
run [13] This argument is subject to a negligible effect of the
Table 3 Element concentrations (mg/kg) in the crude oils tested
Analysis Crude A Crude B
Before After Before After Silver 4 <2 <2 <2
Crude Oils
Table 2 summarizes the principal properties of three crudeoil blends tested in the cell to date Analyses were provided byLGC Ltd (Teddington, UK) Crudes A and B are untreated rawblends of sour crudes blended in the supplier’s laboratory Crude
DS is a desalted blend
A more complete analysis of metals present in crudes A and
B both before and after fouling experiments were made ing inductively coupled plasma–mass spectroscopy (ICP-MS)
us-by LGC Ltd The comparisons are shown in Table 3 All centrations are in milligrams per kilogram The data show thatthe only significant elements present are nickel, sulfur, iron,and vanadium For both crudes it is not clear that nickel andvanadium take any role in the fouling process The increase iniron concentration for crude A and the high levels of sulfur areaddressed later in the paper
con-Using the standard contact test BS EN828 (1998) the criticalsurface tension was found to be about 30 mN m−1for the surface
of test probe C The data to determine the critical contact angleare shown in Figure 4
Critical Surface Tension Test - 10/7/08
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
Figure 4 Critical surface tension test.
heat transfer engineering vol 32 nos 3–4 2011
Trang 33Figure 5 Example Proscan deposit thickness profile.
Deposit Distribution and Morphology
There are two reasons why the deposit is not uniformly
dis-tributed over the axial length of the probe (Figure 3) First, the
heated section of the cartridge heater does not extend to the ends
of the probe Second, the flow patterns near the end of the probe
are quite complex, especially near the bottom Hence,
distribu-tions of shear stress, surface temperature, and heat flux over the
cell’s probe surface are modeled for each fouling experiment
using the Comsol CFD package [11] For any given agitation
speed, bulk temperature, and heat flux, the surface stress is found
to be virtually constant along the surface of the probe except for
a reduction at the very bottom [11] The CFD simulation results
are validated against the measured temperature data at various
axial positions within the probe [11] The readings from the
three thermocouples are different Indeed, the temperature is
not intended to be constant along the height of the probe Hence
each thermocouple reads the temperature at its own particular
location The thermocouples were calibrated using a standard
calibration method Basically, the CFD-predicted surface
tem-perature reaches its maximum at the middle of the height of the
probe and gradually decreases toward both the top and bottom
This distribution is in good agreement with the readings from
the thermocouples
After a fouling run, the cell is drained of crude oil,
disman-tled, and the probe is removed After drying in a vacuum oven
overnight at 100◦C, the deposit thickness along and around
the probe is measured using a laser and coherent light
scan-ning procedure (Proscan 2000, Scantron Industrial Products Ltd,
Taunton, UK) An example screen shot of a Proscan 2000
sur-face profile is shown in Figure 5, with the peak deposit thickness
being about 15µm
The deposit thickness profile along the heated surface reveals
a maximum near the middle of the heated test probe surface TheCFD-predicted surface temperature profiles are strikingly sim-ilar in shape [11] This resemblance between deposit thicknessand surface temperature profiles is to be expected since the effect
of surface temperature on a crude oil fouling rate is strong [6]
It is now possible to correlate, for a fixed surface shear stress,the local fouling rate against the local surface temperature, andhence to obtain an Arrhenius plot from a single experimentalrun [11] Since the shear stress can be varied by changing thespeed of rotation of the cell’s cylindrical stirrer, the effect ofshear stress on fouling behavior can be studied as well
Fouling Resistance and Rate
In virtually all experiments to date, the fouling resistancehas been found to vary linearly with time (e.g., Figure 6) Aninduction period is usually seen when a well-cleaned probe isused Conversely, an induction period is not normally observedwhen a test probe is left in situ in the cell between runs Arelatively simple lumped parameter model has been developed
to account for the growth in fouling resistance including theinduction period [14] The fouling rate used in all Arrhenius plotcalculations is that after completion of the induction period
Photomicrographs and Scanning Electron Microscopy
Figure 7 shows a photomicrograph of a deposit formed fromcrude DS (not sparged with air) at a surface temperature of
380◦C, bulk temperature of 280◦C, heat flux of 110 kW m−2,heat transfer engineering vol 32 nos 3–4 2011
Trang 34and stirrer speed of 100 rpm, which corresponds to a shear stress
of 0.2 Pa at the probe surface The photograph was taken at the
location of thermocouple twb It is clear that the deposit surface
is rough, in accordance qualitatively with the scan shown in
Figure 5
Figure 8 shows the test probe fitted with a small coupon
removable for inspection by scanning electron microscopy The
scanning electron microscopy (SEM) picture that is also shown
in Figure 8 reveals that the deposit formed on the surface is
striated in a manner very similar to that found previously by
Wilson and Watkinson [7, 15] in their hydrocarbon autoxidation
fouling studies The individual particle sizes seen in Figure 8
seem to be of the order of 100µm
Figure 7 Photomicrograph of deposit from crude DS.
Figure 8 Left: Probe fitted with test coupon held by a thin wire Right: SEM
of test coupon surface (the white line represents 1 mm length).
Thermogravimetric Analysis
Thermogravimetric analysis (TGA) in nitrogen/air was ried out at Imperial College London on a deposit obtained inthe batch stirred cell with crude B Prior to TGA, the depositwas dried overnight in a vacuum oven at 100◦C in an attempt
car-to remove as much free crude oil as possible As can be seen
in Figure 9, the deposit lost appreciable weight (about 27%) asthe temperature was ramped from about 200 to 450◦C Almostcertainly this loss was due to the removal of free crude oil up to
a temperature that was not too far above the maximum surfacetemperature the deposit experienced in the batch cell
A further weight loss (about 4%) occurred as the ture was ramped further from 450 to 870◦C, which was almostcertainly due to pyrolysis of the deposit Finally, air was in-troduced into the apparatus to burn remaining combustible ele-ments (about 13%) and the final weight was that of the ash inthe deposit, namely, about 56%
tempera-Size Exclusion Chromatography and UV-Fluorescence
An initial characterization of deposit samples by raphy did not show significant differences between them and thefeed material This led to the use of different solvents to frac-tionate the sample in order to make the identification easier viathe isolation of different classes of compounds Deposit samples
chromatog-Figure 9 TGA analysis of deposit from crude B.
heat transfer engineering vol 32 nos 3–4 2011
Trang 35Table 4 Solubility test results for crude B
Solubility (%) BPre BD BPost
HS 90.8 16 98
HI 8.61 84 1.8
NMP/CHCl 3 (6:1) — —
were separated into heptane-soluble (HS) and heptane-insoluble
(HI) fractions Solubility in the mixture N-methylpyrrolidone
(NMP)/CHCl3 (6:1) was finally estimated due to the use of
this eluent in the size exclusion chromatography (SEC) system
Only the soluble fraction of the samples is analyzed by this
tech-nique Results are shown in Table 4 The observed changes in
the solubility class of significant amounts of material strongly
suggest that chemical reactions are taking place within these
samples
Overall, Table 4 shows decreasing proportions of HI material
in the BPost sample (1.8%) in comparison with the feed crude,
BPre (8.61%), as the deposit precipitated It is also interesting
to note the high percentage of HI in the deposits recovered
(84%)
Size exclusion chromatography (SEC) and ultraviolet
(UV)-fluorescence (UV-F) tests were carried out with the details
be-ing reported by [16] Figure 10 presents size exclusion
chro-matograms of the deposits using crude oil B, allowing for a
comparison between retention times of the main eluted peaks
and the corresponding samples There are slight differences
be-tween the crude used (BPre) and the deposits obtained (BD)
This fact justifies the use of heptane to fractionate deposits into
heptane-soluble (HS) and heptane-insoluble (HI) fractions (16%
and 84%, respectively) with the aim of isolating heavy material
As expected, there was a shift to shorter elution times (greater
masses) between the deposit (BD) and the HI fraction,
suggest-ing the presence of larger molecules in the HI sample The
maximum intensity of the peak of material resolved by column
porosity was found to have shifted by about 2 minutes, which
corresponds to a difference of about 200 U (daltons) between
the deposit and its HI fraction, as calculated using the polycyclic
aromatic hydrocarbons (PAHs) calibration [17]
Figure 10 Size exclusion chromatograms for crude B.
Table 5 Elemental analyses of deposits (Medac) Element C H N S Fe Crude A 36.12 4.11 <0.1 16.61 13.71
Crude B 53.98 6.11 0.48 13.82 7.53
In addition, the HI fraction of the deposit was compared withasphaltenes (BAs.) extracted from the same crude oil [16], whichare, by definition, heptane-insoluble/toluene-soluble SEC re-sults show similarities in terms of molecular mass distribution.The corresponding UV-F results indicate that the biggestpolynuclear aromatic systems are present in the HI fraction This
is consistent with the larger molecular mass material observed
by SEC In addition, it is relevant to note that the HI fraction ofthe deposits shows the sizes of chromophores broadly similar tothose from asphaltenes extracted from the same crude
Elemental Analyses
Elemental analyses were made by Medac Ltd (Egham, UK)and Imperial College The mass data of Table 5 reveal that de-posits formed from both crude oils contained significant quanti-ties of sulfur and iron, suggesting the formation of iron sulfide,
as discussed by Watkinson [18] The atomic H:C ratios werealmost identical for the two crudes at 1.37 for crude A and 1.36for crude B The H/C atomic ratios of the deposits from crude Aand B were found to be 1.37 and 1.36, respectively These ratiosare within the range of 1.0 to 1.5 for asphaltenes as reported inthe literature [19]
The elemental analyses in Table 6 indicate a compositionmainly based on carbon and minor N content The results alsoindicate a higher content of sulfur (22.22%) and iron (26.87%)for the deposit (BD) in comparison with those of feed material(BPre) Iron is almost certainly due to corrosion reactions, whilesulfur probably originates from the crude oil itself
Corrosion products are confirmed by the high value of ashcontent (56%) Twelve percent of fixed carbon is related to theshort period of exposure at high temperature in comparison withthe heat exchanger deposits This factor depends on the operat-ing conditions at the laboratory scale Further work on elementanalysis may be worthwhile in order to draw a conclusion withstatistical significance
Table 6 Ultimate and proximate analyses (% wt) of deposit from crude B (Imperial College)
Component BPre BD BAs
C/S atomic ratio 0.011 0.28 0.021
Ash — 56 —
heat transfer engineering vol 32 nos 3–4 2011
Trang 36Figure 11 EDS analysis of a deposit formed from crude B.
Energy-Dispersive X-Ray Analysis
The energy-dispersive x-ray spectroscopy (EDS) data shown
in Figure 11 for a deposit formed from crude B reveal the
pres-ence of iron and sulfur in addition to carbon Although
energy-dispersive x-ray analysis data are unable to give any quantitative
information of the elements, the presence of iron and sulfur in
the deposit might suggest a probability of iron sulfide formation
Hence, it is quite possible that the fouling process from this
crude oil is related to iron sulfide corrosion of the heat transfer
surface, which might help to explain why preliminary
experi-mental work on the batch stirred cell failed to produce any
mea-surable fouling even at quite high surface temperatures That is,
the role of sulfur present in a crude oil (Tables 2 and 3) might
be crucial to the first fouling of the test probes
Effect of Shear Stress
The shear stress at the surface of the heated probe was
determined using CFD [11] The stirred cell flow structure
becomes turbulent at relatively low cell Reynolds numbers
[20, 21]
For refinery applications, the relationship between the stirred
cell Reynolds number and the tubular flow Reynolds number
that gives the same surface shear stress is shown in Figure 12
Here, the tube internal diameter was 14.8 mm and the crude
oil viscosity and density were 0.0008 Pa-s and 760 kg m−3,
respectively The stirred cell Reynolds number is defined by
Re = ρND 2 / µ, where N is the rotational speed of the stirrer of
Re (Tube flow) = 0.6316 Re (Swirl flow)
Figure 12 Equivalent Reynolds numbers.
370 372 374 376 378 380 382 384 386 388 390
Figure 13 Effect of stirrer speed and hence of shear stress.
diameter D, andµ and ρ are the viscosity and density of thebulk crude oil, respectively
The effect of stepping up for a short period the stirrer speedand hence the shear stress on the fouling rate of crude B is shown
in Figure 13 This shows that the fouling rate for all the threeprobe thermocouple locations was constant over a 3-hour periodwhen the surface shear stress was kept constant at 0.75 Pa Thestirrer speed was then increased from 200 rpm to 550 rpm suchthat the surface shear stress was increased to 2.5 Pa for a shortperiod of time The stirrer speed was then reduced back to 200rpm such that the surface shear stress was reduced back to itsoriginal value of 0.75 Pa Figure 13 shows that the fouling ratesmore-or-less resumed their original values, albeit at somewhatlower fouling resistances The results from this experimentprovide evidence therefore that an increase in surface shearstress can lead to the removal of a crude oil fouling deposit
Effect of Surface Temperature
The stirred cell can also be used to show how the surface perature affects the fouling rate Preliminary experiments hadshown that the batch stirred cell could be used to obtain a long-time fouling run by shutting down the apparatus overnight andresuming the experiment later on without cleaning the test probesurface No changes in fouling rates were found by operation inthis manner provided that the same stirrer speed and cartridgeheater power were used This property of the system was thenexploited in the study of the effect of surface temperature.For a given stirrer speed the surface temperature can bechanged quickly by changing the power supplied to the car-tridge heater This change manifests itself in a change in thedifference between the probe and bulk temperatures Figure
tem-14 shows such changes for crude B stirred at 200 rpm The
initial values of twb and tws were 390◦C and 380◦C, tively It can be seen that the probe–bulk temperature dif-ference increases with an increase in power supplied It canalso be seen that the fouling rate (which is proportional to theheat transfer engineering vol 32 nos 3–4 2011
Trang 37Figure 14 Effect of changing cartridge power for crude B (500 W from 0 to 6.8 h; 515 W from 6.8 to 11 h; 525 W from 11 to 14.5 h).
slope of the temperature difference plot) increases as well, as
expected
Figure 15 shows the change in probe temperatures (twb, twm,
and tws) as a function of the cumulative run time for crude
A for a fixed stirrer speed of 100 rpm Six runs were carried
out consecutively on the same test probe without cleaning in
between runs
For the run on 11 February 2009 the fouling rate for twm was
2.82× 10−6m2K kJ−1, which compares well with rates found in
other studies [22, 23] Between experiments, the power supplied
to the cartridge heater was reduced, thereby lowering the surface
temperatures This experiment was repeated for stirrer speeds of
200 and 300 rpm The general Arrhenius expression that relates
the initial fouling rate to the surface temperature, preexponential
factor, and apparent activation energy is given by Eq (3)
The Arrhenius plots obtained for the three stirrer speeds are
shown in Figure 16 It is clear that there is an increase in apparent
Figure 15 Effect of surface temperature for crude A.
activation energy EAwith stirrer speed, an effect that has beennoted before with the effect of velocity (i.e., turbulence) onapparent activation energy in a tubular recycle system [6]
no fouling occurs This is the fouling threshold [24, 25] Anexample plot of fouling rate as a function of stirrer speed isshown in Figure 17
The data of Figure 17 have been extrapolated linearly back
to zero fouling rate to find a relationship between surface perature and stirrer speed at which there would be no foul-ing The threshold conditions of temperature and related appar-ent activation energy are shown plotted against the equivalent
Figure 16 Effect of stirrer speed on Arrhenius plot (crude A).
heat transfer engineering vol 32 nos 3–4 2011
Trang 38tubular Reynolds number in Figure 18 As expected, the
thresh-old surface temperature is increased by operation at a higher
shear stress (i.e., at a higher stirrer speed in the batch cell or
a higher flow velocity in the equivalent tubular flow system)
Figure 18 shows also that the relatively high apparent
activa-tion energy increases with the amount of surface shear This
phenomenon has been seen previously in fouling studies with
styrene polymerization and Maya crude oil fouling in tubular
recycle systems [6]
Compensation Plot
The relationship between the logarithm of the preexponential
factor and the apparent activation energy of an Arrhenius plot
forms the basis of the compensation plot [6, 8] The equation of
the compensation plot is given by Eq (4) where a is the gradient
and b is the intercept [6].
Two types of compensation plot are suggested: a “true” one
in which an isokinetic relationship occurs such as that found
by Bennett et al [8], who worked with Kuwaiti crude oils, or
a “false” one that is based on experimental conditions and the
0 50 100 150 200 250 300
Crude A Crude B Maya crude oil [6]
Kuwaiti crude oil [8]
Desalted crude oil [25]
Shell Westhollow crude oil [24]
Exxon refinery crude oil [26]
Shell Wood River crude oil [24]
ln A
Figure 19 Compensation plot for a range of crude oils.
way in which the compensation plot is constructed
Figure 19 shows the compensation plot for eight separatecrude oil systems that include crude A and crude B While itmight seem remarkable that all the data fall virtually on thesame straight line, this plot almost certainly contains data fromsystems with both “true” and “false” types The potential value
of the compensation plot is described elsewhere [6]
CONCLUSIONS
A batch cell stirred cell system has been developed that allowsrelatively fast studies to be made of the effect of key operatingparameters on crude oil fouling and facilitates characterization
of deposits formed and feedstocks used The experimental tem, supported by CFD studies, also allows fouling thresholdconditions of surface temperature and shear stress to be identi-fied relatively quickly in the laboratory Because of the limitednumber of crude oils tested to date, it is not yet possible to reachany conclusions about the relationship between the activationenergy of fouling and fluid properties Nevertheless, the exper-imental system is able to generate further data to be used in theconstruction of a generic compensation plot for all crude oils.The effects of bulk temperature, oxygen concentration, and pres-sure will also be the subject of further study Indeed, analysis ofthe crude oils before and after fouling might reveal whether theautoxidation mechanism is important in the fouling process
sys-NOMENCLATURE
a gradient of linear plot of Eq (4), mol kJ−1
A pre-exponential factor in Arrhenius equation, m2K W−1
b intercept of linear plot of Eq (4)
E A apparent activation energy, kJ mol−1
R gas constant, kJ mol−1K−1
R a probe surface roughness,µmheat transfer engineering vol 32 nos 3–4 2011
Trang 39Re batch stirred cell Reynolds number
R f fouling resistance, m2K W−1
T b bulk temperature, K
T s surface temperature, K
T so initial (clean) surface temperature, K
T st surface temperature at time t, K
twb wall temperature at location shown in Figure 2, K
twm wall temperature at location shown in Figure 2, K
tws wall temperature at location shown in Figure 2, K
[1] Crittenden B D., Kolaczkowski, S T., and Downey, I L.,
Fouling of Crude Oil Preheat Exchangers, Transactions
IChemE Part A, vol 70, pp 547–557, 1992.
[2] Takemoto, T., Crittenden, B D., and Kolaczkowski, S T.,
Interpretation of Fouling Data in Industrial Shell and Tube
Heat Exchangers, Transactions IChemE Part A, vol 77,
pp 769–778, 1999
[3] Epstein, N., Fouling in Heat Exchangers, in Fouling of
Heat Transfer Equipment, eds E F C Somerscales and J.
G Knudsen, Hemisphere, Washington, DC, pp 701–732,
1981
[4] Chenoweth, J M., Liquid Fouling Monitoring Equipment,
in Fouling Science and Technology, eds L F Melo, T R.
Bott, and C A Bernardo, Kluwer Academic Publishers,
Dordrecht, The Netherlands, pp 49–65, 1988
[5] Eaton, P., and Lux, R., Laboratory Fouling Test for
Hy-drocarbon Feed-Stocks, ASME-HTD, vol 35, pp 33–42,
1984
[6] Crittenden, B D., Kolaczkowski., S T., and Phillips, D
Z., Crude Oil Fouling in a Pilot-Scale Parallel Tube
Appa-ratus, Heat Transfer Engineering, vol 30, no 10–11, pp.
777–785, 2009
[7] Wilson, D I., and Watkinson, A P., Model Experiments
of Autoxidation Reaction Fouling, Transactions IChemE
Part A, vol 73, pp 59–68, 1995.
[8] Bennett, C A., Kistler, R S., Nangia, K., Al-Ghawas, W.,
Al-Hajji, N., and Al-Jemaz, A., Observation of an
Isoki-netic Temperature and Compensation Effect for High
Tem-perature Crude Oil Fouling, Heat Transfer Engineering,
vol 30, no 10–11, pp 794–804, 2009
[9] Crittenden, B D., Kolaczkowski, S T., and Takemoto, T.,
Use of In-Tube Inserts to Reduce Fouling From Crude Oils,
AIChE Symposium Series, vol 89, no 295, pp 300–307,
1993
[10] Eaton, P., Fouling Test Apparatus, U.S patent 4383438,
1983
[11] Yang, M., Young, A., and Crittenden, B D., Use of CFD
to Correlate Crude Oil Fouling Against Surface ture and Surface Shear Stress in a Stirred Fouling Appara-
Tempera-tus, Proc of International Conference on Heat Exchanger Fouling and Cleaning VIII–2009, Schladming, Austria,
June 14–19, pp 272–280, 2009
[12] Crittenden, B D., Hout, S A., and Alderman, N J., Model
Experiments of Chemical Reaction Fouling, Transactions IChemE Part A, vol 65, pp 165–170, 1987.
[13] Crittenden, B D., and Alderman, N J., Negative Fouling
Resistances: The Effect of Surface Roughness, Chemical Engineering Science, vol 43, pp 829–838, 1988.
[14] Yang, M., Young, A., and Crittenden, B D., Modelling of
the Induction Period of Crude Oil Fouling, Proc of ternational Conference on Heat Exchanger Fouling and Cleaning VIII–2009, Schladming, Austria, June 14–19,
In-pp 69–75, 2009
[15] Wilson, D I., and Watkinson, A P., A Study of
Autoxi-dation Fouling in Heat Exchangers, Canadian Journal of Chemical Engineering, vol 74, no 2, pp 236–246, 1996.
[16] Venditti, S., Berrueco, C., Alvarez, P., Morgan, T., Millan,M., Herod, A A and Kandiyoti, R., Developing Charac-terisation Methods for Fouling Deposited in Refinery Heat
Exchangers, Proc of International Conference on Heat Exchanger Fouling and Cleaning VIII–2009, Schladming,
Austria, June 14–19, 44–51, 2009
[17] Berrueco, C., Venditti, S., Morgan, T J., Alvarez, P.,Millan-Agorio, M., Herod, A A., and Kandiyoti, R., Cali-bration of Size-Exclusion Chromatography Columns With1-Methyl-2-Pyrrolidinone (NMP)/Chloroform Mixtures as
Eluent: Applications to Petroleum-Derived Samples, ergy and Fuels, vol 30, no 10–11, pp 3265–3274, 2008.
En-[18] Watkinson, A P., Chemical Reaction Fouling of Organic
Fluids, Chemical Engineering and Technology, vol 15, no.
2, pp 82–90, 2004
[19] Speight J G., The Chemistry and Technology of Petroleum,
Marcel Dekker, New York, pp 192–193, 1980
[20] Smith, G P., and Townsend A A., Turbulent CouetteFlow Between Concentric Cylinders at Large Taylor Num-
bers, Journal of Fluid Mechanics, vol 123, pp 187–217,
1982
[21] Mullin T., Lorenzen, A., and Pfister, G., Transition to
Tur-bulence in a Non-Standard Rotating Flow, Physics Letters
A, vol 96, no 5, pp 236–238, 1983.
[22] Asomaning, S., Panchal, C B., and Liao, C F., Correlating
Field and Laboratory Data for Crude Oil, Heat Transfer Engineering, vol 21, no 3, pp 17–23, 2000.
[23] Saleh, Z S., Sheikholeslami, R., and Watkinson, A P.,Fouling Characteristics of a Light Australian Crude Oil,
Proc 2003 ECI Conference on Heat Exchanger Fouling and Cleaning: Fundamentals and Applications, Sante Fe,
NM, pp 226–233, 2004
[24] Panchal, C B., Kuru, W C., Ebert, W A., Liao, C F., andPalen, J., Threshold Conditions for Crude Oil Fouling, in
Proc Int Conference on Understanding Heat Exchanger
heat transfer engineering vol 32 nos 3–4 2011
Trang 40Fouling and its Mitigation, eds T R Bott, L F Melo, C.
B Panchal, and E F C Somerscales, Begell House, New
York, pp 273–279, 1999
[25] Knudsen, J G., Lin, D C., and Ebert, W A., The
Deter-mination of the Threshold Fouling Curve for a Crude Oil,
Proc Int Conference on Understanding Heat Exchanger
Fouling and Its Mitigation, pp 265–271, Castelvecchio
Pas, Italy, 1997
[26] Scarborough, C E., Cherrington, D C., Diener, R., and
Golan, L P., Coking of Crude Oil at High Heat Flux Levels,
Chemical Engineering Progress, vol 75, no 7, pp 41–46,
1979
Andrew Young is a Ph.D student researching crude
oil fouling in the Department of Chemical ing at the University of Bath He obtained his first degree in mechanical engineering from the Univer- sity of Bradford in 1987 and his M.Sc in process analytics and quality technology from the University
Engineer-of Newcastle in 2003 He is an experienced teacher, trainer, and design engineer.
Silvia Venditti is a Ph.D student in the Chemical
Engineering Department at Imperial College London.
Her research interests include the characterization of coal and petroleum derivatives samples, as well as understanding fouling mechanisms She obtained her first degree in chemical environmental engineering in
2005 from University of L‘Aquila (Italy).
Cesar Berrueco has been a research associate in
the Energy Engineering Group of the Department of Chemical Engineering at Imperial College London since 2007 He graduated as a chemical engineer in
2000 and in 2005 completed his Ph.D at the sity of Zaragoza (Spain) His research interests in- clude pyrolysis, gasification and liquefaction of solid fuels, including biomass, wastes, and coal, and up- grading and chemical characterization of heavy oils and coal-derived liquids.
Univer-Mengyan Yang is a senior research officer in the
De-partment of Chemical Engineering at the University
of Bath He received his Ph.D in chemical ing at Bath in 1993, after which he continued with postdoctoral research before joining Davisco Foods International, Minnesota, USA, as a senior scientist.
engineer-He rejoined Bath in 2007 to pursue research in the field of crude oil fouling.
Andrew Waters is an undergraduate student in the
Department of Chemical Engineering at the sity of Bath As part of his M.Eng degree program,
Univer-he completed his research project on tUniver-he subject of crude oil fouling, and he is currently on a year’s in- dustrial placement with MAST Carbon International, Guildford, Surrey.
Haddy Davies is an undergraduate student in the
De-partment of Chemical Engineering at the University
of Bath As part of her M.Eng degree program, she completed her research project on the subject of foul- ing threshold plots, and she is currently on a year’s in- dustrial placement with Dupont-Teijin Films, Wilton, Middlesborough, Teesside.
Simon Hill is an undergraduate student in the
De-partment of Chemical Engineering at the University
of Bath As part of his M.Eng degree program, he completed his research project on the fouling com- pensation plot, and he is currently on a year’s in- dustrial placement with Merck Sharpe and Dohme, Hoddesden, Hertfordshire.
Marcos Millan graduated as a chemical engineer
from the University of the Republic of Uruguay in
2001 On completion of his Ph.D on the catalytic hydrocracking and characterization of heavy hydro- carbon liquids in the Department of Chemical En- gineering at Imperial College London, he joined the Department as a lecturer in 2006 His research inter- ests include upgrading and chemical characterization
of heavy oils and coal-derived liquids, gasification
of solid fuels, abatement of trace element emissions from combustion and gasification processes, and biomass-to-liquid processes.
Barry Crittenden is a professor of chemical
en-gineering at the University of Bath and leader of its Advanced Materials and Porous Solids research group His research interests include ceramic mem- branes, activated carbon monoliths for environmental control, and hydrocarbon fouling in refinery heat ex- changers.
heat transfer engineering vol 32 nos 3–4 2011