The well-known first double-piston pumps of Ktesebios during Archimedes" time, water supply pipes in the ancient world together with Roman pump developments, as well as Agricola's (see: Twelve Books of Mining 1596) wooden "high pressure pumps" for the drainage of mines (100 m depth - 10 bar pressure) during the Middle Ages show early applications of high pressure.
James Watt's steam engine (around 1785) working with several bar steam pressure only, innovated the world's energy supply and induced an industrial revolution. This steam engine represented one of earliest high pressure processes for power generation.
Starting in the Middle Ages, from the development of firearms and guns based on explosives emerged the problem of designing safe containments (gun barrels) against the high detonation pressure (today, several thousand bar).
As an early milestone of high pressure chemical processing should be mentioned the synthesis of ammonia by Haber and Bosch (Nobel prize 1918). This typical high pressure ( 3 0 0 - 700 bar) process already shows all the characteristics of the similar ones of today. It should be regarded as the initiation of the very successful development of the high pressure chemistry during the last century, including the still up-to-date super-pressure polymerisation of ethylene (3000 bar). Since the mid-20 th century diamonds have been synthesized by transforming graphite into diamond at pressures above 120000 bar (3000~ with a solid-state process and special apparatus.
High pressure is a proven tool for a number of industrial processes and promising ones in the future. The following effects of high pressure should be distinguished.
The chemical effect of high pressure is to stimulate the selectivity and the rate of reaction together with better product properties and quality as well as improved economy. This is based on better physico-chemical and thermodynamic reaction conditions such as density, activation volume, chemical equilibria, concentration and phase situation. Many successful reactions are basically enhanced by catalysis.
The physico-chemical effect of high pressure, especially in the supercritical state, to enhance the solubility and phase conditions of the components involved. Supercritical hydrogenation, or enzymatic syntheses are offer new steps with high pressure. Supercritical water oxidation at high pressure represents an efficient method for the decontamination of wastes.
From the application of high pressure liquid or supercritical carbon dioxide as a solvent have emerged a number of promising or successful production processes such as supercritical extraction, fractionation, dyeing, cleaning, degreasing and micronisation (rapid expansion, crystallization, anti-solvent recrystallization). New material properties can be achieved by foam expansion, aerogel drying, polymer processing, impregnation and cell-cracking with high pressure supercritical CO2 [1, 2].
The physico-bio-chemical effect of the high pressure treatment predominantly of foodstuffs and cosmetics, is now emerging. For the sterilization (pasteurisation, pascalisation) high pressure offers an alternative to high temperature. Furthermore, treatment with static high pressure gives a promising improvement of certain organic natural products by advantageous swelling, gelation, coagulation and auto-oxidation effects in combination with fats or proteins. This selection of high pressure effects actually is however only under increasing research however only and successful practical applications have not been achieved yet [3].
The physico-hydrodynamical effect of high pressure is based on the conversion of the potential (pressure) into kinetic energy (high speed fluid jetting: 100 - 1000 m/s). The main applications are the homogenisation of fluid mixtures by expanding them through very narrow clearances, water-jet cutting and water-jet cleaning, and the generation of sprays with fine droplets for efficient combustion or spray-drying of fine particles.
The physico-hydraulic effect of high pressure is involved during the conveying of fluids against large differential pressures, for example the filtration of polymer melts, or pipeline transport over long distances. The hydrostatic energy is applied for hydroforming of complex metal parts, isostatic pressing for sintered products, or the autofrettage treatment of high pressure components in order to generate beneficial residual stresses [4, 5].
Table 1.4-1 gives a summary about high pressure applications, exhibiting the methods, the pressure levels applied, and the products or results of the processes involved. The survey is not complete, as the development is changing and progressing permanently.
It should be pointed out at this stage that the application of high pressure as a beneficial tool for production procedures, from the experience of the past decades, is increasing and decreasing all the time. High pressure equipment and plants are expensive in their development, investment, operation, and safety aspects. So there is the general tendency to reduce the pressures as soon as the process development offers the chances (e.g. by the introduction of new catalysts) to do so.
Table 1.4-1.
Applications of high pressure
Method Pressure (bar) Product, application
Solid state reaction > 125 000 synthetic diamonds Polymerisation of ethylene 1300 - 3000 low density polyethylene
Synthesis 100 - 700 ammonia
propionic and acetic acid urea (fertilizers)
butanediol methanol
Fischer-Tropsch synthesis hydrocarbons
Hydrogenation 100 - 300
Hydroformylation
edible oils hydrogasification hydrocracking desulfurization catalytic cracking naphtha hydroforming coal liquefaction fatty alcohols 1-6-hexanediol 1-4-butanediol hexamethylenediamine C4 to C15 products Wet (air) oxidation 1 0 0 - 400 organic waste elimination Extraction with supercritical fluids 8 0 - 3 0 0
(e.g., CO2)
decaffeinated coffee (tea) spices, hops
colours drugs
oils, lecithine and fats tobacco (nicotine) perfumes
Micronization with supercritical fluids
- Crystallization
- Rapid expansion
- Gas anti-solvent Recrystallization
- Precipitation with compressed anti-solvent - Solution-enhanced dispersion - Particles from gas-saturated
solutions
8 0 - 300
fine particles and powders from various products and of designed properties
Dyeing with supercritical fluids Cell structure treatment with supercritical fluids (e.g., CO2) Leaching of ores
Cryo processing Oil/gas production
8 0 - 300 100 - 300
100 - 400
Separation of isotopes Fluid conveying (transport) Polymer processing
300 100 - 200 100 - 400
High performance 1 0 0 - 700
liquid chromatography
Kinetic fluid (jet) energy with water up to 4000
Kinetic fluid energy Homogenisation Emulsification Dispersing Cell-cracking
Potential (pressure) fluid energy
up to 2000 up to 600 up to 1500
up to 10000 up to 5000 up to 4000
Spray drying 5 0 - 200
(up to 1000)
dyeing of fabrics tobacco impregnation aluminium (from bauxite) technical gases (N2, 02, H2, He ...) gas liquefaction
drying inhibition
desulfurization, odorization secondary and tertiary production methods drilling support heavy water
pipeline transport of ores and coal polymer spinning
polymer filtration polymer extrusion analytical chemistry chemical production jet cutting
jet cleaning
jet treatment of fabrics foodstuffs
cosmetics
pharmaceutical products chemical products bio-products
autofrettage (residual stresses) hydroforming
isostatic pressing (sintered parts)
fine powders of various products
Fuel injection 1000- 2000 diesel motors
(improved combustion) Thermal power generation 100 - 250 steam power plants Potential (pressure) energy effects up to 5000
on organic products
sterilization pascalisation coagulation
gelation of various foodstuffs and other bio-products
The following examples of successful and well developed high pressure processes concentrate mainly on the general aspects and a consideration of the high pressure machinery involved. The explanations will discuss primarily the general aspects and benefits of high pressure as a tool, and will not address details of the methodology.
Example 1: Production of Polyethylene (PE)
The different available high pressure polymerisation processes of polyethylene (PE) yield LDPE (low density PE), LLDPE (linear low density PE) and copolymer features of the same.
The various process variations have been developed during recent decades and introduced a number of well developed steps and devices to achieve safe and economical operating conditions at the very high reaction pressures of 1500 to 3000 bar.
I m p 3 00o: :i; r como~mer k G
~ 5 0 0 ba r ~ : ~ L -
.... l__.i + h
@
product (LDPE)
Fig. 1.4-1. Production of LDPE
need for the types of LPDE, new and scaled-up plants are under erection.
The process (Fig. 1.4-1) makes heavy demands on the pumps, compressors, reactors, piping, fittings and valves, as well as for other devices at the pressure range mentioned. The monomer ethylene (storage tank, a) is compressed by a primary reciprocating compressor with several stages (b), up to around 300 bar, and then by a two-stage "hyper" reciprocating compressor (c) up to around 3000 bar. Between the two piston-type compressors (b and c) is the main location for injecting modifiers, especially co-monomers, in order to achieve certain modifications of the polymer properties. As these additives mainly represent solvents or liquified gases high pressure diaphragm pumps (m) must normally be applied.
The polymerisation reaction takes place in tubular or stirred vessel reactors (d) under careful control of pressure and temperature, enhanced or initiated by the injection of initiator- solvents (e) (as well as co-monomers 1) which are frequently based on organic peroxides. The typical injection pumps for this metering problem are of the two-cylinder amplifier types. The further process comprises a number of further steps such as heat exchange (f, h), separation (g, j), gas recycling (k), and polymer discharge (i). The art of producing high pressure PE is based on an excellent understanding of the process and skill in designing and operating the high pressure equipment required.
Example 2: Production of unsaturated fatty alcohols
This hydrogenation process (Fig. 1.4-2) is, among others, the basis for the production of washing detergents.
I,,_
..Q 1.0 O4
i ~ 1 ~ 2 5 0 ba P2K1
..Q C:)
fatty acids catalyst /
fatty alcohols hydrogen
Fig. 1.4-2. Production of unsaturated fatty alcohols
(new) J
unsaturated fatty alcohols
P1 and P2) reacts with hydrogen (staged dry-running piston compressor, K1) to unsaturated fatty alcohols (reactor, a). Several high pressure steps such as heat exchange, separation, recycling catalyst feed (b to f) together with proper high pressure components, are required.
The dry hydrogen compression is avoids any contamination of the product with lubricants.
The diaphragm feed pumps offer the best service with respect to endurance and wear protection, with the lowest life-cycle costs.
E x a m p l e 3: D e c a f f e i n a t i o n of coffee b e a n s
Of the various extraction processes the decaffeination with supercritical C 0 2 exhibits the most commercial advantages for bulk production. The process is a discontinuous one.
Fig. 1.4-3 shows a number of serially arranged extractors (5) charged with the supercritical CO2 feed by the centrifugal circulation pump (1).
Q
7
I
i
I
I
4
!
N
I
I
Fig. 1.4-3. Decaffeination of coffee beans
The supercritical solvent is expanded with the throttling valve (9) in order to remove the caffeine (separator 8) and to bring the solvent back to the liquid state (condenser 10). The gas- recycling (dry running) reciprocating compressor (7), the CO2 and the co-solvent feed (2, 3;
diaphragm pumps) represent variable process components if required. Heat exchangers (4) maintain the suitable thermodynamic conditions.
periods (6). The decaffeination process is providing the decaffeinated coffee beans as well as the caffeine as valuable products.
The supercritical extraction of hops, tea, and other foodstuffs can be performed in similar plants. The challenge of the discontinuous extraction of bulk materials is in the design and automatic operation of high pressure extractors which can easily be opened and closed for the filling and discharging procedure.
Example 4: Homogenisation of milk and other foodstuffs
Liquid foodstuffs, for example milk products must be submitted to homogenisation treatment in order to improve their long-term physical stability ("shelf life"). The liquid is pumped at very high pressure by a multiplex reciprocating piston pump through the narrow clearances of a hydraulically controlled homogenisation valve (Fig. 1.4-4, C, bottom).
Fig. 1.4-4. Homogenisation of milk and other foodstuffs
By the action of hydraulic shear forces, cavitation, turbulence and impact owing to the very high flow velocity (several 100 m/s) or high differential pressure (low viscosity liquids, 300 to 400 bar, or more viscous liquids, up to 1500 bar) the liquid is turned into a very fine (homogeneous) dispersion.
The homogenisation process is only one step (or sometimes two stages, see Fig. 1.4-4, top) within the production line. The feed (raw product) is adjusted in temperature by heat exchange (HE), passed through the homogeniser (H septic), then treated by ultra-high- temperature (UHT), homogenized a second time (H aseptic) and UHT-treated, ending with an aseptic final product.
Homogenisation processes now extend up to 1500 bar differential pressures. As the materials to be homogenized exhibit varying properties with respect to viscosity, corrosiveness and abrasiveness the high pressure components, such as homogenising pumps and valves, need very careful design and choice of materials.
Example 5: High speed water-jetting as an efficient tool for production and other treatment steps
The growing demand for fully automated production processes must take benefit of new steps in order to achieve and secure the quality standards requested. During continuous sheet steel production the permanent descaling of the sheet surfaces (Fig. 1.4-5, S) is realized by high speed water-jetting (Fig. 1.4-5, top) at suitable locations in the rolling-mill train (usually 600 bar water supply to the jetting nozzles, N). The high pressure plunger pumps (HP) should provide a smooth volume flow by multiplex design.
Fig. 1.4-5. Descaling, cleaning and jet-cutting with high pressure
A very similar process is the high speed water-jet cleaning applied during reconstruction of buildings, cleaning procedures in production processes, for ships, and especially in wastewater systems. Depending on the nature of the surface layers to be removed the required water pressure can approach 2500 bar, and thus make outstanding demands on the high pressure pump design and the installation (Fig. 1.4-5, bottom left side).
The prerequisite of the successful application of water-jet cleaning should be a proper understanding of the parameters involved in the jet-cleaning physics requiting profound case studies.
Super-speed water-jets are further applied increasingly for the production steps requiting the cutting of pieces of material which should be kept at low temperature and which appear soft and restrictive towards mechanical tools. The water-jet as a "hydrodynamic cutter"
provides a number of advantages in cases which should be selected by case studies.
Jet-cutting systems need to be compact and suitable for robotic action in automated trains of production. Usually the hyper-pressure plunger pumps for water-jet cutting purposes are based on hydraulic amplifiers, of double-cylinder design, and provide high pressure water of up to 5000 bar.
If very hard materials (e.g., natural stone, or metal sheets) must be cut, the injection of abrasives into the water jet will support and accelerate the cutting procedure (see Fig. 1.4-5, bottom, fight side). The water-jet cutting represents a very flexible production method which can be regarded as supplementary to LASER methods if thermal influences on the materials involved cannot be accepted.
Example 6: Polymer processing
During the production of polymers (e.g., polyolefins, polyamide, polystyrene), very viscous (up to 4.10 6 mPas) polymer melts have to be extracted with high pressure gear pumps (PGP) from the reactors (PR) or degasifiers (DG), then transferred through heat exchangers (HE), static mixers (MI), filters (F) and diverters (DI), depending on the process, onto spinning gear (SP) pumps (Fig. 1.4-6).
M
-I PR
PGP
Fig. 1.4-6. Polymer processing
DG
PGP
MI
SP
~D
-@-.
/---
o.. ...
, " ~ ~ ' " : ' : " .... "" I". ~.1~.
I E X
a) PGP b)
c)
. . . . g . . . ..
.... .... i ~
Fig. 1.4-7. Polymer extrusion a, b foil extrusion c bottle extrusion f co - extrusion
d cable extrusion e blow film extrusion
The viscosity of the transferred fluids increases from the monomer tank (M) to the polymer reactor (PR) and the degasifier (DG). The highest viscosity (occasionally over 106 mPas) is seen in the polymer extraction pump (PGP) behind the vacuum degasifier. As the polymer melt has to pass mixers and filter systems its extreme viscosity requires very high pressures from the polymer gear pumps in order to force the material through the system (up to 400 bar) to the spinningpumps (SP). During extrusion polymer processing the extruder (EX) is responsible for the homogenous melting and the following polymer gear pump (PGP) for generating the high and constant pressure for pressing the material through the extrusion tools (co-extrusion, foil extrusion, cable extrusion etc., Fig. 1.4-7).The gear pumps for extremely viscous polymers must be designed accordingly, with very large inlet nozzles and crescent- shaped clearances in the suction area between the gear wheels and the pump housings.
Example 7: The sterilization of fruit juices with high pressure
This method (ultra-high pressure treatment UHP) for the aseptic processing of food stuffs and other organic products still appears to be some way from extended application.
From a number of pilot applications Fig. 1.4-8 shows the quasi-continuous train for the sterilization of fruit juices with pulp contents. The high pressure sterilization offers valuable advantages with respect to the quality of the final product compared to other sterilization procedures, especially if natural fractions of fruit pulp are desired by the consumers.
The fruit juice enters the autoclaves (5) by the pumping action of the floating pistons (4) involved. The drinking-water supply (vessel 1, low pressure pump 2, high pressure pump 3) is capable of submitting the fruit juice to the high pressure required (around 4000 bar), during a definite time period, through the floating piston. Then the juice is discharged by the water hydraulic-control system. At the same time, other parallel autoclaves perform the same steps
with a certain time shift so that quasi-continuous operation of the sterilization process can be achieved.
Fig. 1.4-8. Quasi-continuous sterilization of fruit juice (adopted from Mitsubishi Heavy Ind., Japan 1992)
Example 8: Hydrostatic pressure as an efficient tool for production
The high pressure treatment is growing rapidly for a number of productions steps.
Traditional methods such as hot and cold isostatic pressing (HIP, CIP) for the production of sintered metallic or ceramic parts have been developed further. They are now also applied as a post-treatment for castings in order to eliminate or heal porosity or internal cracks and to improve the quality. Isostatic pressing is a tool to produce intricately shaped parts demanding high density and homogeneity. The process requires suitable presses to generate pressures of up to 6000 bar at high (up to 2000 ~ or ambient temperature.
Hydroforming is a new method to produce hollow components of intricate shape by internal fluid pressure (Fig. 1.4-9, A). The untreated part may represent for example, a piece of pipe which is fixed with appropriate joints in a swage body and closed at both ends. By admitting an appropriate high internal pressure, various intricate geometrics can be achieved (1000 to 4000 bar). Another similar approach for the production of large fiat and curved parts from sheet material is the hydropressing by means of special presses transmitting pressure by appropriate diaphragms.
The autofrettage treatment (Fig. 1.4-9, B) is certainly one of the oldest, but still very useful methods to create beneficial residual stresses in thick-walled components (e.g., pipes).
The autofrettage pressure must be adjusted to a level so that the material in the thick wall is plastically strained within a certain percentage (e.g., 50 %), the rest staying only elastically strained.
Fig. 1.4-9. Hydro forming (A), Autofrettage (B)
1 swage 2 treated part 3 pressurization system 4 closures
After the removal of the autofrettage pressure (typically 3000 to 8000 bar) the plastically over-strained region exhibits compressive residual stresses, especially at the internal "surface (Fig. 1.4-10). When submitting the thick-walled pipe to the desired operational pressure the compressive internal strains will reduce the operational ones effectively at the inner surface so the same pipe then can carry much more pressure before any failure can occur (compare Cyv and CYvA at the inner diameter). Autofrettage treatment, although first used for the gun-barrel reinforcement hundreds of years ago, is used today for high pressure components in the process industries as well as for appropriate components in common rail diesel injection systems for combustion motors. The autofrettage method can be included in automated manufacturing sequences.
I.
I
(3",, \
//"(~tR /
Figure 1.4-10. Stresses in an autofrettaged thick-walled cylinder (plastification 50% through the wall)
cyt = tangential stress without autofrettage (YtA -- tangential stress with autofrettage (YtR =
tangential residual stress after autofrettage
C~v- stress intensity without autofrettage CYvA = stress intensity with autofrettage
References
1. M.B. King, T.R. Bott (editors): Extraction of Natural Products Using Near-Critical Solvents, Blackie Academic & Professional, Glasgow, 1993
2. N.N.: Supercritical Fluids - Material and Natural Products Processing, Nice/France, 1998, Institut National Polytechnique de Lorraine
3. N.N.: International Conference on High Pressure Bioscience & Biotechnology, University of Heidelberg/Germany, Section Physical Chemistry, 1998
4. W.R.D. Manning, S. Labrow: High Pressure Engineering, Leonard Hill, London, 1971 5. I.L. Spain, J. Paauwe (editors): High Pressure Technology, Vol II, Marcel Dekker New
York, 1977