OIL SPILL SCIENCE chapter 4 – measurement of oil physical properties

OIL SPILL SCIENCE chapter 4 – measurement of oil physical properties OIL SPILL SCIENCE chapter 4 – measurement of oil physical properties OIL SPILL SCIENCE chapter 4 – measurement of oil physical properties OIL SPILL SCIENCE chapter 4 – measurement of oil physical properties OIL SPILL SCIENCE chapter 4 – measurement of oil physical properties OIL SPILL SCIENCE chapter 4 – measurement of oil physical properties

Measurement of Oil Physical Properties

Bruce Hollebone

Chapter Outline

4.2 Bulk Properties of Crude

Oil and Fuel Products

634.3 Hydrocarbon Groups 73

4.4 Quality Assurance and

Control

77

4.5 Effects of EvaporativeWeathering on Oil BulkProperties

l the physical properties of the oil and how these change over time

l how the compositional and bulk property changes affect an oil’s behaviorand fate

l whether emulsions will form

l whether the oil is likely to submerge

l the hazard to on-site personnel during cleanup

l the oil toxicity to marine or aquatic organisms

4.2 BULK PROPERTIES OF CRUDE OIL AND FUEL PRODUCTS

The physical properties of the almost limitless variety of crude oils aregenerally correlated with aspects of chemical composition Some of these keyOil Spill Science and Technology DOI: 10.1016/B978-1-85617-943-0.10004-8

properties for determining the fate and behavior of oil and petroleum products

in the environment are viscosity, density, specific gravity (density relative towater), flash point, pour point, distillation, and interfacial tension Theseproperties for the oils are listed inTable 4.1

Viscosity is the resistance to flow in a liquid The lower the viscosity, the morereadily the liquid flows The viscosity of an oil is a function of its composition;therefore, crude oil has a wide range of viscosities For example, the viscosity ofFederated oil from Alberta is 5 mPa$s, while a Sockeye oil from California is 45mPa$s at 15
C In general, the greater the fraction of saturates and aromatics andthe lower the amount of asphaltenes and resins, the lower the viscosity As oilweathers, the evaporation of the lighter components leads to increased viscosity

As with other physical properties, viscosity is affected by temperature,lower temperatures giving higher viscosities For most oils, the viscosity variesapproximately exponentially with temperature Oils that flow readily at hightemperature can become a slow-moving, viscous mass at low temperature Interms of oil spill cleanup, viscous oils do not spread rapidly, do not penetratesoils readily, and affect the ability of pumps and skimmers to handle the oil Thedynamic viscosity of an oil can be measured by a viscometer using a variety ofstandard cup-and-spindle sensors at controlled temperatures

Density is the mass of a unit volume of oil, usually expressed as grams permillilitre (g/mL) or, equivalently, as kilograms per cubic metre (kg/m3) It isused by the petroleum industry to grade light or heavy crude oils Density isalso important because it indicates whether a particular oil will float or sink inwater As the density of water is 1.0 g/mL at 15
C and the density of most oilsranges from 0.7 to 0.99 g/mL, oils typically float on water As the density ofseawater is 1.03 g/mL, even heavier oils will usually float on it Only a fewbitumens have densities greater than water at higher temperatures However, aswater has a minimum density at 4
C and oils will continue to contract astemperature decreases, heavier oils, including heavy crudes and residual fueloils, may sink in freezing waters Furthermore, as density increases as the lightends of the oil evaporate off, a heavily weathered oil, long after a spill event,may sink or be prone to overwashing, where the fresh oil, immediately after thespill, may have floated readily

A related measure is specific gravity, an oil’s density relative to that of water

As the densities of both water and oil vary differently with temperature, thisquantity can be highly variable The American Petroleum Institute (API) uses thespecific gravity of petroleum at 50
F (15.56
C) as a quality indicator for oil Purewater has an API gravity of 10 Oils with progressively lower specific gravitieshave higher API gravities Heavy, inexpensive oils have less than 25 API; mediumoils are 25 to 35
API; and light commercially valuable oils are 35 to 45
API APIgravities generally vary inversely with viscosity and asphaltene content.Interfacial tensions are the net stresses at the boundaries between differentsubstances They are expressed as the increased energy per unit area (relative tothe bulk materials), or equivalently as force per unit length The ‘Standard

TABLE 4.1 Typical Oil and Fuel Properties at 15
C

International (SI)’ units for interfacial tension are milliNewtons per meter (mN/m) Surface tension is thought to be related to the final size of a slick The lowerthe interfacial tension of oil with water, the greater the extent of spreading andthinner terminal thickness of oil In actual practice, the interfacial tension alonedoes not apparently account for spreading behavior; environmental effects andother effects seem to be dominant.

The flash point of an oil is the temperature at which the vapor over the liquidcan be ignited A liquid is considered to be flammable if its flash point is lessthan 60
C Flash point is an important consideration for the safety of spillcleanup operations Gasoline and other light fuels can ignite under mostambient conditions and therefore are a serious hazard when spilled Manyfreshly spilled crude oils also have low flash points until the lighter componentshave evaporated or dispersed On the other hand, Bunker C and heavy crude oilsgenerally are not flammable when spilled

The pour point of an oil is the temperature at which no flow of the oil isvisible over a period of 5 seconds from a standard measuring vessel The pourpoint of crude oils ranges from60
C to 30
C Lighter oils with low viscos-ities generally have lower pour points As oils are made up of hundreds ofcompounds, some of which may still be liquid at the pour point, the pour point

is not the temperature at which an oil will no longer pour The pour pointrepresents a consistent temperature at which an oil will pour very slowly andtherefore has limited use as an indicator of the state of the oil For example,waxy oils can have a very low pour point, but may continue to spread slowly atthat temperature and can evaporate to a significant degree

4.2.1 Density and API Gravity

The density of an oil sample, in g/mL, is best measured using a digital densitymeter following American Society for Testing and Materials (ASTM) method

D 5002.1The instrument is calibrated using air and distilled, deionized water.Acoustically measured densities must be corrected for sample viscosity, asspecified by the instrument manufacturer

API gravity (API 82) is calculated using the specific gravity of an oil at 60
F(15.56
C).2

The oil density at 15.56
C can be estimated by exponentialextrapolation from the higher (THi) and lower (TLo) data points, if necessary.This is converted to specific gravity by division by the density of water at15.5
C, using the following equation:

rT andrT are the measured oil densities at T and T , respectively, and

r(H2O)15.56is the density of water at 15.56
C The API gravity is then mined using the formula (API 82):

From a qualitative observation of the oil, either the NV or the SV1 sensor ischosen to measure the sample The NV sensor is used for oils with viscositiesbelow 100 mPa$s, and the SV1 sensor, for oils above 70 mPa$s to 10,000mPa$s For oils with higher viscosity, measurements must be made on cone andplate or parallel plate instruments (see below)

For both cases using the rotary viscometer, the measurement cup is filledwith a sample to the edge or the rotating surface The sensor is mounted ontothe instrument, and the sample volume is adjusted to the proper level Thesample is allowed to equilibrate until the sample temperature probe stabilizes atthe measurement temperature and remains stable for 5 minutes Samples andsensors are kept chilled at the appropriate temperature prior to use

For the NV sensor, the rotational shear rate is set at 1,000/s, the SV1 sensor

at 50/s If the oil is observed to be non-Newtonian, single samples are run atshear rates of 1/s, 10/s, and 100/s In all cases, the sensors are ramped up tospeed over a period of 5 minutes The viscosity is measured for a subsequent 5minutes, sampled once per second The viscosity reported is that at time zero ofthe second, constant-shear rate interval This may be obtained by the mean ofthe constant-shear rate interval data or by linear fit to the time-viscosity series iffriction-heating has occurred during the measurement For Newtonian samples,triplicate measurements are averaged and the mean is reported as the absolute

or dynamic viscosity For non-Newtonian samples, viscosities are reported foreach of the three shear rates

Viscosities above 50,000 mPa$s are measured on a parallel plate rheometerwith an air bearing Measurement for most oils can be performed with a 35 mmplate/plate geometry at a gap of 2 mm between plates A stress sweep in forcedoscillation mode at 1 Hz performed over an appropriate range will determinethe stress independent regions A creep test can then be performed at a stressvalue selected in the stable “sol” range of flow response for the material Thisprovides the zero shear viscosity value

4.2.3 Surface and Interfacial Tensions

Surface and interfacial tensions, in mN/m, are normally determined by one of twomethods The de No€uy ring is a common technique, used by many laboratories,

and has been codified as ASTM method D 971.4 It depends on accuratemeasurement of the maximum force that a platinum ring can exert on the surface

of a liquid before detachment A second emerging technique that shows muchpromise for improved speed and accuracy is the pendant/rising drop method,which depends on shape calculations of a droplet of oil in air or water.5,6The values that are important for spill responders include the oil/air, oil/water, and the oil/seawater interfacial tensions The oil/air interfacial tension isoften called surface tension As interfacial tensions are temperature dependent,

it is often convenient to determine these quantities for several temperatures.Two measurements at freezing, 0
C, and at ambient temperature, 25
C, allowfor a wide range of interpolated values Measurement at 50
F/15
C also allowsdetermination of common marine temperatures

De No€uy Ring Determination of Interfacial Tensions

A measurement apparatus specific to the de No€uy ring test is required Manualmachines are common, but automated systems are now available that makemeasurements much quicker and repeatable All measurement equipment,rings, measurement vessels, transfer, and storage containers must be scrupu-lously clean before measurement Surface and interfacial tension measure-ments are very sensitive to contamination by organic chemicals or salts.For sample/air surface tensions, the instrument is zeroed with themeasurement ring in the air A small amount of sample, approximately 15 mL,

is poured into a vessel of sufficient diameter that the wall effects on themeniscus do not affect the area through which the ring will pass The ring isdipped into the sample to a depth of no more than 5 mm and is then pulled upsuch that it is just visible on the surface of the liquid The system is allowed torest for 30 seconds The measurement is initiated, terminating when the upwardpulling force on the ring just balances the downward force exerted by the liquid.The apparent surface tension,sAPP, is recorded

For sample/water and sample/brine interfacial tensions, the ring is zeroed inthe sample at a depth of not more than 5 mm The ring is removed and cleaned

A volume of water or brine is dispensed into the measurement vessel The ring isdipped 5 mm into the aqueous phase A small volume of sample is carefullypoured down the side of the vessel wall, with great care taken so as to disturb theaqueous/oil interface as little as possible The overlying layer should be at least

5 mm thick The ring is then raised to the bottom on the interface, and the system

is allowed to rest for exactly 30 seconds The measurement is started, and theapparent interfacial tension is recorded,sAPP, when the force balance is reached.The apparent surface tension is corrected for mass of the upper phase lifted

by the ring during measurement using the Zuidema and Waters6correction:

wheres is the interfacial tension, sAPP is the instrument scale reading, C isthe ring diameter, D is the density of the lower phase, d is the density of theupper phase, R is the radius of the du No€uy ring, and r is the radius of the ringwire.

As these measurements depend on temperature, samples, aqueous phasesand glassware should be kept at the measurement temperature for a minimum

of 30 minutes before a determination is made

Pendant/Rising Drop Determination of Interfacial Tensions

In this test, the interfacial tension is determined by calculation with comparison

to the shape of a drop hanging from the end of a needle A camera is used tophotograph a picture of a drop hanging from a needle The digital picture isanalyzed by software; then a parameterized curve shape is developed, fromwhich the surface tension is calculated.6

In the case of a liquideliquid interfacial tension, the surrounding fluid must

be clear, so that a good image may be generated For oil in water, this requiresthat the oil be suspended in water However, as most oils are less dense thanwater, the rising oil bubble, rather than the pendant drop, must be measured Inthis case, the image is inverted in software and, instead of the force of gravity,the buoyant force, determined as the fraction of gravity based on the specificgravity of the oil is used:

where b is the buoyant force, g is the acceleration due to gravity, rwater

is the density of water at the measurement temperature, and roil is the oildensity

4.2.4 Flash Point

The flash point of an oil product can be determined by several methods,depending on the oil product and the quantity available Lower viscosityproducts, including light fuel oils and most fresh crudes, are measured by theTag closed-cup method This follows ASTM method D 1310.7Though accu-rate, the Tag method uses a comparatively large volume of oil, 50 to 70 mL.Smaller volumes, 1e2 mL, can be measured by ASTM D6450.8The practicalworking range of these two methods is e10
C to approximately 100
C Withsubambient cooling, using dry ice baths and/or liquid nitrogen baths, muchlower flash point temperatures can be measured, but this is often not necessaryfor emergency response considerations

Heavier products, including intermediate and heavy fuel oils, can bemeasured by a Pensky-Martins analyzer, following ASTM D 93.9As with theTag method, this method uses 50e70 mL of crude oil Smaller volumes can beused with the newer method ASTM D7094, which uses only 2 mL of oil.10Theworking range for these heavier type tests is approximately 50
C to 225
C.

The standard test material for assuring quality control for a temperature flash point apparatus historically has been para-xylene; however,heavier normal alkane standards, n-decane, n-undecane, n-tetradecane, andn-hexadecane have also been found to be suitable and offer a wider range of testtemperatures.11

lower-4.2.5 Pour Point

The pour point of an oil sample, in degrees Celsius, can only be determined byfollowing ASTM method D 97.12 Sample aliquots are poured into ASTM-approved jars, stopped and fixed with ASTM-certified thermometers Thetemperature regime described in the standard is critical; particularly in waxyoils, with high normal alkane contents, a crust of waxy crystals can form on thesurface of the oil as it cools The ASTM D 97 heating and cooling process foroil is designed to ensure that the formation of these microstructures does notinterfere with reproducible measurement of the pour point

4.2.6 Sulphur Content

The mass fraction of atomic sulphur in oil is conveniently determined usingX-ray fluorescence closely following ASTM method D 4294.13 In brief, themethod is as follows: approximately 3 g of oil is weighed out into standard

31 mm XRF cells The sealed cells are then measured in an XRF spectrometer.The spectrometer response is calibrated using a series of certified referencematerial standards Spectra should be corrected for interference by chlorine bysubtraction, based on a calibration curve established by the certified referencematerials Matrix effects, X-ray absorption by the base oil, can be corrected

by subtraction of a spectrum of an oil free of sulphur, such as a mineral orlubricating oil

4.2.7 Water Content

The mass fraction of water in oil or an emulsion, expressed as a percentage, isbest determined by Karl Fischer titration, using ASTM method D 4377.14TheKarl Fischer reaction is an amine-catalyzed reduction of water in a methanolicsolution:

CH3OHþ SO2þ RN/½RNHþþ ½SO3CH32RNþ H2Oþ I2

þ ½RNHþ½SO3CH3/½RNHþ½SO4CH3þ 2½RNHþI (5)The amine, RN, or mixture of amines is proprietary to each manufacturer

An aliquot of approximately 1 g of oil is accurately weighed, then duced to the reaction vessel of the autotitrator A solution of 1:1:2 (by volume)mixture of methanol:chloroform:toluene is used as a working fluid

intro-4.2.8 Evaluation of the Stability of Emulsions Formed

from Brine and Oils and Oil Products

Water-in-oil emulsions are formed in 2.2-liter fluorinated vessels on an over-end rotary mixer at a rotational speed of 50 RPM.15,16

end-1 600 mL of salt water (3.3% w/v NaCl) is placed in each mixing vessel

2 30 mL of oil is added to each vessel for a 1:20 oil:water ratio

3 The vessels are sealed and placed in the rotary mixer such that the cap ofeach mixing vessel follows, rather than leads, the direction of rotation.The rotary mixer is kept in a temperature-controlled cold room at 15
C.

4 The vessels and their contents are allowed to stand for approximately 4hours before rotation begins, then mixed continuously for 12 hours

5 At the conclusion of the mixing time, the emulsions are collected from thevessels for measurement of water content, viscosity, and the complexmodulus The emulsions are stored at 15
C for one week, then observed

for changes in physical appearance

Water content for the emulsions should be determined The Karl-Fischertitration method works well for all types of emulsion and watereoil mixtures.The complex modulus of the emulsion is measured on a rheometer using a 35

mm plate-plate geometry A stress sweep is performed in the range 100 to10,000 mPa in the oscillation mode at a frequency of 1 Hz The complexmodulus value in the linear viscoelastic region is reported

4.2.9 Evaluation of the Relative Dispersability of Oil

and Oil Products

This method determines the relative ranking of effectiveness for the persibility of an oil sample by to a dispersant test mixture It is used either todetermine the effectiveness of a dispersant product for a standard crude oil or totest the dispersability of a crude oil against a standard dispersant This methodfollows ASTM F 2059 closely.17

dis-A premix of 1:25.0 dispersant:oil is made up by adding oil to 100 mg ofdispersant (approximately 2.50 mL of oil in total) Six ASTM-standardswirling conical flasks modified with side spouts, containing 120 mL of 33&brine, are placed into an incubator-shaker An aliquot of 100mL of premix isadded to the surface of the liquid in each flask, care being taken not to disturbthe bulk brine The flasks are mechanically shaken at 20.0
C with a rotationspeed of 150 rpm for exactly 20 minutes The solutions are allowed to settlefor 10 minutes

Using the side spout, 30 mL of the oil-in-water phase is transferred to

a 250 mL separatory funnel, first clearing the spout by draining 3 mL of liquid.The 30 mL aliquot is extracted with 35 mL of 70:30 (v:v) dichlor-omethane:pentane, collected into a 25 mL graduated cylinder

A Gas Chromatograph-Flame Ionization Detector (GC/FID) is used todetermine the oil concentration in the solvent A 900mL aliquot of the 15 mLsolvent extract is combined with 100mL of internal standard (200 ppm of 5-a-androstane in hexane) in a crimp-top injection vial and shaken well The totalpetroleum hydrocarbon content of the sample is quantified by the internalstandard method using the total resolved peak area and the average hydro-carbon response factor over the entire analytical range:

RPH ¼ ATOTAL=AI:S:=RRF  20  15  120=30=0:9 (6)where RPH is the resolved petroleum hydrocarbon (mg/mL), ATOTALis the totalresolved peak area, AI.S. is the internal standard peak area, and RRF is therelative response factor for a series of alkane standards covering the analyticalrange

The method is calibrated using a series of six oil-in-solvent mixturesprepared from the premix for each oil The volume of premix dispersant/oilsolution for each standard is selected to represent a percentage efficiency of thedispersed oil The volume of the premix is then carefully applied to the surface

of the brine in a shaker flask and shaken exactly as one of the samples, asdescribed previously Upon removal from the shaker however, the entirecontents of the flask is transferred to the separatory funnel This is extractedwith 3 20 mL of 70:30 (v:v) dichloromethane:pentane and made up to 60 mL.Chromatographic quantitation is then performed using the formula:

RPH ¼ ATOTAL=AI:S:=RRF  20  60  120=120=0:9 (7)The RPH values as a function of % effectiveness for the calibration stan-dards are plotted The sample RPH values are then used to determine thepercentage effectiveness of the dispersant

Note that these effectiveness percentages are not expected to correlate toreal-world dispersabilities It is important to remember that these values arerelative rankings only

4.2.10 Adhesion to Stainless Steel

Adhesion to stainless steel is useful to responders in order to judge the

“stickiness” of oil to certain drum skimmer configurations EnvironmentCanada has developed a quantitative test for this purpose.18,19

An analytical balance is prepared by hanging an ASTM method D 6 dard penetrometer needle from the balance hook and allowing the apparatus tostabilize and tare Approximately 80 mL of oil sample is poured into a 100 mLbeaker The beaker is elevated until the oil reaches the top of the stainless steelneedle Care is taken not to coat the brass segment of the needle The needlerests for 30 seconds immersed in the oil The beaker is lowered until the needle

stan-is clear of the oil The system stan-is left undstan-isturbed, closed inside a draft shield.After 30 minutes, the weight of the oil adhering to the needle is recorded The

mass of the oil divided by the surface area of the needle is the adhesion of theoil in g/cm2 Typically, four measurements are taken for each oil sample and themean reported as the final value.

4.3 HYDROCARBON GROUPS

The fate and behavior of crude oils and petroleum products are stronglydetermined by their chemistries The main constituents of oils can be groupedinto four categories: saturated hydrocarbons (including waxes), aromatics,resins, and asphaltenes

Saturates: A group of hydrocarbons composed of only carbon and hydrogenwith no double bonds or aromaticity They are said to be “saturated” withhydrogen They may by straight-chain (normal), branched, or cyclic Typically,however, the group of “saturates” refers to the aliphatics generally includingalkanes, as well as a small amount of alkenes The lighter saturates, those lessthan ~C18, make up the components of an oil most prone to weathering Thelarger saturates, generally those heavier than C18, are termed waxes

Aromatics: These are cyclic organic compounds that are stabilized by

a delocalized p-electron system They include such compounds as BTEX(benzene, toluene, ethylbenzene, and the three xylene isomers), polycyclicaromatic hydrocarbons (PAHs, such as naphthalene), and some heterocyclicaromatics such as the dibenzothiophenes Benzene and its alkylated derivativescan constitute several percent in crude oils PAHs and their alkylated deriva-tives can also make up as much as a percent in crude oils

Resins: This is the name given to a large group of polar compounds in oil.They include heterosubstituted aromatics (typically oxygen- or nitrogen-containing PAHs), acids, ketones, alcohols, and monoaromatic steroids.Because of their polarity, these compounds are more soluble in polar solventsthan the nonpolar compounds, such as waxes and aromatics, of similarmolecular weight

Asphaltenes: A complex mixture of very large organic compounds thatprecipitate from oils and bitumen by natural processes For the purposes ofthis method, asphaltenes are defined as the fraction that precipitates inn-pentane

The separation of petroleum and its products into these four characteristicgroups is known as fractionation The quantification of the groups is oftenreferred to as SARA analysis, an acronym of the characteristic groups: satu-rates, aromatics, resins, and asphaltenes Historically, many techniques havebeen used to perform this separation, including distillation, solvent precipita-tion (ASTM D6560)20, treatment with strong acids (ASTM D2006)21,adsorption (ASTM D2007 and D4124)22,23, and thin-layer chromatography.24For reviews of the methods, see Speight and Becker.24-26 While excellentmethods for the determination of the SARA groups have been developed usingthin-layer chromatograph (TLC), there has been continuing interest in alternate

test methods based on solvent separation and adsorption techniques.22-24Gravimetric methods are typically based on the solubilities of the groups inn-pentane, hexane/benzene, and methanol.3Such methods can rely on gravi-metric determinations of all components, including the saturate and aromaticgroups However, the drawback of such methods is that they contain significantvolatile components This is particularly true of crude oils and lighter fuels.More sophisticated methods rely on a combination method involvingdetermination of the saturate and aromatic fractions by gas chromatography, anadaptation of total petroleum hydrocarbon methods, while gravimetricallydetermining the nonvolatile resin and asphaltene components.27,28

Resin and Asphaltene Gravimetric Determination

A 100 mL quantity of n-pentane is added to a preweighed sample of imately 5 g of oil The flask is shaken well and allowed to stand for 30minutes.27 The sample is filtered through a 0.45 mm membrane using

approx-a minimum of rinsings of n-pentapprox-ane The precipitapprox-ate is approx-allowed to dry, thenweighed The weight of the precipitate as a fraction of the initial oil sampleweight is reported as the percentage asphaltenes

The filtrate from the precipitation, the “maltene” fraction, is recovered andmade up to 100 mL with n-pentane A 15 g, a 1 cm diameter column of acti-vated silica gel is prepared The top of the column is protected by a 1 cm layer

of sodium sulphate A 5 mL aliquot of the maltene fraction is loaded onto thecolumn A 60 mL volume of 1:1 (v:v) benzene:hexane is eluted through thecolumn and discarded A 60 mL volume of methanol, followed by a 60 mLvolume of dichloromethane, are eluted through the column and combined Themethanol/dichloromethane fractions are reduced by rotary evaporation andblown down to dryness under nitrogen The mass fraction of this dried eluent,compensating for the volume fraction used, is reported as the percentage ofresins in the sample

Resin and Asphaltene Thin-Layer Chromatography DeterminationWhile no standard method for this technique exists, it has the advantages overthe gravimetric methods of being much faster, requiring much less oil orproduct and being more reproducible It has the disadvantage of requiring

a sophisticated instrument, a TLC with a flame ionization detector (FID)

A TLC that quantifies analytes developed on silica gel-coated glass rods,such as the Iatroscan Mark 6, is necessary for this method Briefly, an aliquot ofsample dissolved in dichloromethane at a concentration of 1 mg/mL is spotted

at a point, the origin, near one end of a rod, the foot of the rod The rods are thendeveloped by immersion of the feet into a series of solvents to separate the fourhydrocarbon groups The origin points must remain above the liquid surface,but the feet of the rods must be immersed sufficiently to cause solvent to travel

up the rods by capillary action