Chapter 2

Crude Assay and Physical Properties

Around the Earth, different living organisms perished and drifted to the bottom of soil
or bodies of water, along with other inorganic materials. Over hundreds of millions of
years, under the influence of high pressure and temperature, some carbon-containing
portions of the dead organisms escaped full oxidation and ended up as organic matter
in sedimentary rocks, as kerogen, oil, or gas. Due to differing climates, organisms, and
depositional environments, the compositions of oil depend drastically with region
and can vary very significantly. Hence, it is important to determine physical and
chemical characteristics of crude oil through a crude oil assay (crude assay).

2.1 Crude Assay

There are hundreds of different crude oils produced in the world with large variations
in properties and composition. Crudes typically are named for their source country
and reservoir. Both physical and chemical properties of crude oils vary with location,
depth, and the age of the oil field.
Characteristics of crude oil that can indicate these differences include: boiling
point, calorific value (heating value), API gravity (density), pour point, cloud point,
freeze point, aniline point, smoke point, flash point, viscosity, color and fluorescence,
Reid vapor pressure, refractive index, sulfur content, nitrogen content, metal content,
salt content, micro carbon residue, optical activities, and total acid number [1]. These
crude oil bulk properties measured by testing laboratories, along with distillation and
product fractionation data at pilot plants, are compiled into a crude oil assay (or crude
assay) to characterize a specific crude oil.
A crude assay can be an inspection assay or a comprehensive assay (full assay).
Testing can include characterization of the whole crude oil and/or its various boiling
fractions produced from fractional distillation. High quality assay data are important
to refinery planners and oil traders, who select crudes by comparing their properties
with refinery specifications and constraints as well as meeting environmental and
other standards. Sulfur content and total acid number (TAN) are common constraints,
© Springer Nature Switzerland AG 2019 19
C. S. Hsu and P. R. Robinson, Petroleum Science and Technology,
https://doi.org/10.1007/978-3-030-16275-7_2
20 2 Crude Assay and Physical Properties

because they relate to corrosion in piping and process equipment. The refiners make
changes in plant operations based on the crude assay data, to meet process and
product requirements. Hence, crude assays provide the basis upon which companies
and traders negotiate contracts, which include pricing and possible penalties due to
impurities and other undesired properties. However, traders might purchase off-spec
crudes for subsequent trading.
The assays determine selected chemical and physical properties of whole crudes
and several distilled fractions. The fractions correspond to boiling ranges for com-
mon fuels. Full assays are extensive, and especially important for new crudes. For
inspection assays, just a few tests are conducted. In a whole crude assay, analyses
are done by combining atmospheric and vacuum distillation runs to provide a true
boiling-point (TBP) distillation data. The extent of an assay depends on customer
need and affordability. At minimum, the assay should contain a distillation curve,
typically a TBP curve, and a specific gravity curve. The common assay inspections
are discussed below.

2.2 Distillation

Distillation is the primary method of separating crude oil into useful products. It
utilizes differences in boiling points (volatility) of fractions. Boiling point distribu-
tion of the components, determined by fractional distillation, is the most important
characteristic of petroleum. Large scale and continuous distillation is also the most
basic process in any refinery to separate crude oil into fractions of different boiling
ranges, shown in Fig. 2.1. Table 2.1 lists the major distillation products (fractions)
with their boiling ranges.
Naphtha of high octane-number is a main component of gasoline. It is composed of
hydrocarbons mainly from C5 to C10 , with some C11 and C12 . The carbon numbers of
heavy naphtha extend to mid-teens, overlapping with those of kerosene. Kerosene is
a middle distillate fraction of crude oil with C10 to C16 . It is mainly used for heating
oil, jet fuel, and diesel. It is the first fraction to show an appreciable increase in
the cyclic hydrocarbons that dominate the heavier fractions. Aromatics in kerosene
range from 10 to 40%. Light gas oils, C14 to C18 , are used in both jet fuels and
diesel fuels. The C20 to C50 range contains diesel fuels, heating oils, lubricating oils,
paraffin waxes and some asphalts. Residuum (or resid) includes resins, asphaltenes
and waxes, and is the most complex and least understood fraction of petroleum. The
wax fraction in most residua is about half that in the lube oil fraction. Asphaltenes are
dark brown to black amorphous solids. The resins may be light to dark colored, thick,
viscous substances to amorphous solids. The resins and the asphaltenes contain about
half of the total nitrogen and sulfur in crude oil. Heavy crude oils invariably have
more nitrogen and sulfur. Residua frequently contain over 5% oxygen. Resins are
the highest of the fractions in oxygenated compounds, while asphaltenes are highest
in sulfur compounds.
2.2 Distillation 21

Fig. 2.1 Relationship

between boiling points under
atmospheric pressure and
under 40 mmHg vacuum [3]

Table 2.1 Boiling range and its corresponding distillation product

Boiling range Distillation product
°F °C
<85 <29 Petroleum Gas (butane and lighter)
90–200 30–90 Light Naphtha (Gasoline, Petrochemicals)
200–400 90–200 Heavy Naphtha (Gasoline)
300–525 150–275 Kerosene (Jet fuel, Heating oil)
350–650 180–345 Distillate (Diesel, Heating oil), Straight-run gas oil
650–1050 345–565 Vacuum gas oil (Lubricant oil)
>1050 >566 Vacuum residuum (Residual fuel oil, Asphalt,
Coke)

The amount of valuable products, or the yields of fuels and lubricant oils that can
be produced from a crude oil simply by distillation is a major factor in determining
the value of the oil.
Smaller scale or laboratory scale distillation methods are described in the Annual
Book of ASTM Standards [2]. ASTM stands for the American Society for Testing
Materials, now called ASTM International. The methods include ASTM D2892
for atmospheric distillation and ASTM D5236 for vacuum distillation. The ASTM
D2892 (15/5 distillation) method uses a 15 theoretical plate fractionation column
with a 5:1 reflux ratio. It determines accurate boiling points and yields of distillation
fractions. The highest temperature is limited to ~350 °C (650 °F) to avoid thermal
22 2 Crude Assay and Physical Properties

decomposition. In order to extend the boiling range, distillation is operated under

~40 mmHg vacuum for higher boiling components. Hence, the ASTM D5236 (Hivac
distillation) method is done in a vacuum potstill to produce vacuum gas oil, which
boils in the traditional lubricant oil range, as well as vacuum resid. A true boiling
point (TBP) curve for the distribution of boiling point versus accumulated weight
percent distilled is created by combining atmospheric (ASTM D2892) and vacuum
(ASTM D5236) distillation runs. Since the components boil at lower temperatures
under vacuum, their boiling points need to be adjusted for the difference in pressure
to give atmospheric equivalent boiling points (AEBP). Figure 2.1 shows the linear
relationship between the AEBP and the boiling points under 40 mmHg vacuum. The
correction enables the extension of TBP curves from 350 °C (~650 °F) to above
565 °C (1050 °F). The determination of TBP curves is very time-consuming (>8 h)
and requires a large volume of sample, for example, 10 L.
Figure 2.2 shows the TBP curve obtained from distillation. Atmospheric distilla-
tion ends at 350 °C and vacuum distillation ends at atmospheric equivalent tempera-
ture at 565 °C. The curve is extrapolated beyond 565 °C to estimate final boiling point
(FBP) of the whole crude; the extrapolation can be rather arbitrary, sometimes using
molecular modeling based on hypothetical structures postulated from constituents
measured by nuclear resonance spectroscopy (NMR) and vibrational spectroscopy
(infrared and Raman) of the whole oil or vacuum residuum.
More accurate extended TBP curves could be obtained by molecular distillation,
or short-path vacuum distillation. The sample, such as vacuum residue (boiling point

Fig. 2.2 TBP curve

obtained from atmospheric
and vacuum distillation [5]
2.2 Distillation 23

>565 °C), is exposed at increasingly higher temperatures in a distillation column

operated in high vacuum (around 10−4 mmHg) and a short distance, about 2 cm,
between evaporator and collector. The hot plate covered by a thin film of sample is
typically suspending next to a cold plate with a line of sight in between. In molecular
distillation, fluids are in molecular flow regime, i.e., the free path of the molecules
is comparable to the size of the equipment. The final temperature for heavy oils,
however, cannot be reached before the sample turns into char that is completely
involatile. An example of extending TBP curve from 565 to 700 °C (1290 °F) has
been demonstrated by Lopes et al [4].
No viable refining technology for vacuum residues based on distillation has been
developed. Extraction would become a preferable method if additional fractionation
is desired.
In many plant operations, material balances, physical property correlations, and
computer process simulations are based on TBP of the streams of interest.
Single plate distillation or analytical distillation was also developed by
ASTM—Method D86 for atmospheric distillation and Method D1160 for vacuum
distillation at 40 mmHg [6]. The data were then converted to TBP by mathematical
correlation. Since the distillations are carried out using Hempel columns, the com-
bination is also referred to as Hempel distillation. A rather large volume of sample
is also required.
An alternative quick and robust method with a minute amount of sample was
developed. Whole crude simulated distillation (SimDist) is described by method
ASTM D2887 (also D7900 and D7169). It uses high temperature gas chromatography
(HTGC) with a thermally stable aluminum-clad nonpolar GC column and flame
ionization detector (FID) to determine true distillation curve and predict distillate
yields. The GC oven temperatures can be as high as 538 °C, thus making possible
the analysis of up to C120 hydrocarbons present in petroleum and related materials.
Figure 2.3 illustrates a SimDist result. The GC oven is temperature programmed
from room temperature to a maximum temperature under the flow of a carrier gas,
typically helium. The vapor of components diffuses in and out of the stationary phase
coated inside the GC column. A nonpolar GC column is essentially a boiling point
column to separate components by their boiling points. The series of normal paraffins
(alkanes) serve as boiling point reference based on their individual boiling points and
retention times. Since FID measures the weight percent of the component, the area
under a specific boiling range represents the yield of that range. The data are compared
with a crude assay True Boiling Point (TBP) curve, shown in Fig. 2.4. It should be
noted that boiling points of multiple-ring aromatic components as determined by
simulated distillation can differ substantially (20–100 °F or 10–50 °C) from the
true boiling points of the pure components. This is partially due to the difference
of the vapor-pressure-temperature equilibrium of aromatic compounds compared to
other hydrocarbon types. Hence, the deviation of SimDist data is greater in heavy
fractions where high concentrations of aromatics are present. In practice, the yields
of distillation fractions determined by SimDist need to be correlated or calibrated
with the yields from atmospheric and/or vacuum distillation.
24 2 Crude Assay and Physical Properties

Fig. 2.3 Simulated distillation by gas chromatography (the numbers on top of the peaks are carbon
numbers of normal paraffins)

Fig. 2.4 Comparison of SimDist data with TBP curve

2.3 Physical Testing 25

2.3 Physical Testing

The following lists some of the most important tests for physical properties of crude
oil or its fractions.
• Specific gravity and API gravity
Specific gravity, or relative density, is the ratio of the mass of a given volume of the
oil to the mass of an equal volume of water at a specific temperature, which is often
60 °F. A commonly used tool for measuring specific gravity includes a temperature
bath heated to the desired temperature, a metal tube to fill with the test sample, and a
hydrometer which indicates the specific gravity. This test determines specific gravity
which is then converted to API gravity, a measurement that has become a standard
in the petroleum industry.
API gravity is another notation of the density of the oil, which was established
by the American Petroleum Institute (API) as a measure of how heavy or light a
petroleum liquid is compared to water. It is defined by ASTM D287/1298 as

◦
API = (141.5/specific gravity at 60◦ F) − 131.5

The specific gravity of water at 60 °F is 1.0; hence, its °API is 10. The °API
of crude oils ranges from <10 for asphaltic crude to >50°API for condensate. Most
crudes are in the 20–45°API range. Condensates range between 50 and 70°API. The
API gravity of heavy oils falls in the range of 10–15° or <20°API, and for bitumen it
falls in the range of 5–10°. In 2010, the World Energy Council defined “extra heavy
oil” as crude oil having a gravity less than 10° API and reservoir viscosity no more
than 10,000 cP, with a lower API limit of 4°, so it sinks in water rather than floating
on it. API gravity is designed so that its value is more or less proportional to the
commercial value of the crude oil. A denser oil has a lower °API, and hence, the
commercial value of the oil is lower.
• Viscosity
Viscosity is a measurement of fluid resistance to shearing flow using a small cap-
illary tube (viscometer) for the sample to flow through. This measures the kinematic
viscosity, that is, the ratio of dynamic (shear) viscosity to the density of the fluid, in
centistoke (cSt) at a given temperature. It is a very important oil property.
In oil production, viscosity determines the flow of oil and gas through the reservoir,
thus, the amount and rate at which oil and gas can be produced.
In engines, viscosity determines how easily the oil is pumped to the working
components, how easily it passes through filters, and how quickly it drains back to the
engine. The lower the viscosity, the easier all this will happen. That is why cold starts
are so critical to an engine: because the oil is cold and so relatively thick that it loses
its lubricity. A fluid’s viscosity is directly related to its load-carrying capabilities. The
greater the viscosity, the greater the loads it can withstand. The viscosity must be
adequate to separate moving parts under normal operating conditions (temperature
26 2 Crude Assay and Physical Properties

Fig. 2.5 Viscosity and density of crude oils. cp: centipoise [4]

and speed). Knowing that a fluid’s viscosity is directly related to its ability to carry
a load, one would think that the more viscous a fluid, the better it is. The fact is,
the use of a high-viscosity fluid can be just as detrimental as using too light an oil.
Viscosity index (VI), that is, the change of viscosity with temperature, is a critical
property of lubricant base oil. The higher the VI, the lower is the viscosity change
with temperature. High performance and synthetic lube oils have high VI’s.
For the same homologous series of hydrocarbons, the greater the molecular weight
of the compound, the greater the viscosity. When the molecular weight is similar,
the viscosity of a cyclic molecule is larger than that of the chain-like molecule.
The greater the number of rings is, the greater the viscosity is. Therefore, the “ring
structure is the viscosity carrier”.
Viscosity is a parameter of oil mobility, also an indispensable physical property
for crude oil recovery and refining processes. The viscosity of the oil decreases as
its temperature increases.
The viscosity and density of crude oils are closed related, as shown in Fig. 2.5.
Oil sand (tar sand) bitumen is the most viscous and has the highest density (lowest
API gravity) . On the other hand, conventional crude oils have less viscosity with
lower density (or API gravity) . Oil sand was defined by the United States Congress
in 1976 as rock types that contain extremely viscous hydrocarbons which cannot be
recovered by conventional oil production methods, including enhanced recovery. Oil
sand bitumen and extra heavy crude oils are therefore referred to as unconventional
oils.
Viscosity for conventional crude oils ranges from 10–100 mPa (884–934 kg/m3 ),
and for heavy crude oils from 1000 to 10,000 mPa (966–1000 kg/m3 ). Tar sand
bitumen have viscosities that ranges from 100,000 to 1 million mPa and density over
1000 kg/m3 .
• Total Sulfur (atom) content
The sulfur (atom) content of crude oils is in the range of 0.1–5.0 wt%, measured
by x-ray fluorescence (ASTM D4294 or D5291). The terms “sweet” and “sour” are
historical terms which refer to the taste of crude oil as a function its sulfur content.
Indeed, early prospectors would taste oil to determine its quality. Low sulfur oil
actually tasted sweet. Crude oil is currently considered sweet if it contains less than
0.5% sulfur.
Sulfur compounds can affect the activity of catalysts. As mentioned earlier, com-
bustion converts organic sulfur compounds into sulfur oxides (SOx ), which can react
2.3 Physical Testing 27

with moisture (H2 O) to form fine sulfuric acid and sulfate particulates (aerosols) in
air. The particulates are transported hundreds of miles, eventually returning to the
earth as wet or dry acid deposition. The deposition, also known as acid rain, harms
buildings, trees and other plants, fish, and land animals. Similarly, organic nitrogen
compounds are converted by combustion into nitrogen oxides (NOx ), which reacts
with volatile organic compounds (VOC) by photochemical reactions to form smog.
Both SOx and NOx form particulate matter (PM) in air. PM with diameters smaller
than 2.5 μm (PM 2.5) are inhalable particles that can reach bronchial tubes to damage
lungs, causing respiratory ailments.
Sweet crude is easier to refine and safer to extract and transport than sour crude.
Because sulfur is corrosive, light crude also causes less damage to refineries and thus
results in lower maintenance costs over time.
Major locations where sweet crude is found include the Appalachian Basin in
Eastern North America, Western Texas, the Bakken Formation of North Dakota and
Saskatchewan, the North Sea of Europe, North Africa, Australia, and the Far East,
including Malaysia and Indonesia. African crudes tend to be relatively sweet: Bonny
Light, the main Nigerian crude, contains about 0.16 wt% sulfur. Brega, the main
Libya crude, contains about 0.2 wt% sulfur.
Sour crude oils have more than 0.5% total organic sulfur not including dissolved
hydrogen sulfide. Sour crude also contains more carbon dioxide. Most sulfur in
crude oil is actually bonded to carbon atoms as sulfur-containing hydrocarbons.
Nevertheless, high quantities of hydrogen sulfide in sour crude can pose serious
health problems or even be fatal.
• Hydrogen sulfide
Hydrogen sulfide is famous for its “rotten egg” smell, which is only noticed at low
concentrations. At moderate concentrations, hydrogen sulfide can cause respiratory
and nerve damage. At high concentrations, it is instantly fatal. Hydrogen sulfide is
so much of a risk that sour crude has to be stabilized via removal of hydrogen sulfide
before it can be transported by pipelines and oil tankers. Sour crude is more common
in the Gulf of Mexico, Mexico, South America, and Canada. Middle Eastern crudes
tend to be relatively sour.
• Mercaptans
Mercaptans (thiols) smell like as garlic or rotten eggs and can be toxic. A trace
amount can be used as an odorant of natural gas for detection; natural gas is odorless in
pure form. Mercaptan sulfur is measured by potentiometric titration of an isopropanol
solution of a hydrocarbon sample containing a small amount of NH4 OH. The solution
is then titrated with a silver nitrate solution. Mercaptans are removed from sour
crudes and oxidized into sulfides through the Merox process or into elemental sulfur
by LOCAT® and other methods, discussed in Sect. 7 of Chapter 9, to reduce odor
prior to transportation.
28 2 Crude Assay and Physical Properties

• Nitrogen content
Nitrogen-containing compounds can cause poisoning of acidic catalysts due to
their basicity. Nitrogen content is normally determined by oxidative combustion and
chemiluminescence detection (ASTM D3228 or D4629).
• Total Acid Number (TAN)
Total Acid Number (TAN) is the amount of KOH in mg that is needed to neutralize
the acid in a gram of oil dissolved in toluene/isopropanol/water (ASTM D664).
Typically, values are 0.05–6.0 mg KOH per gram of the sample. High TAN crudes
are purchased and processed carefully due to possible corrosion problems.
• Metal Content
Metal Content ranges from a few to several hundred ppm. It is measured by induc-
tively coupled plasma atomic emission spectroscopy (ICP-AES) (ASTM D5708) or
x-ray fluorescence (ASTM D5863). Nickel and vanadium are common in crude oils;
they can severely affect catalyst activity.
• Salt Content
Salt Content is measured by conductivity of a crude oil sample dissolved in water
compared to reference salt solutions (ASTM D3230) to determine crude oil corro-
sivity that can lead to shorter life times of pipes and pumps in the refinery. Desalting
is needed when the salt content is greater than 30 ppm to bring it down to 2 ppm.
• Micro Carbon Residue (MCR)
Micro Carbon Residue (MCR) is measured by the Conradson carbon (CCR or
ConCarbon) method ASTM D189.
MCR is a measurement of hydrocarbon mixtures’ tendency to leave carbon
deposits (coke) when burned as fuel or subjected to intense heat in a processing
unit such as a catalytic cracker. The ConCarbon test involves destructive distilla-
tion, i.e., subjection to high temperature, which causes cracking, coking, and drives
off any volatile hydrocarbons produced, and weighing the residue which remains.
A somewhat similar test, Ramsbottom carbon, also measures mixtures tendency to
form coke.
• Calorific Value (Heating Value)
In the fuel fractions of petroleum, the heating value is of importance. Paraffins
have higher calorific values than aromatics. The average value for a crude oil is
2100–2230 kcal/kg, compared with 1170–1670 kcal/kg of bituminous coal.
• Pour Point
The pour point of a liquid is the temperature at which it becomes semi solid and
loses its fluidity. It is the lowest temperature at which the oil no longer moves. The
pour point is determined as 3° above the point at which a sample no longer moves
when inverted (ASTM D97). It is important for pipeline transportation from source
2.3 Physical Testing 29

to loading ports. In crude oil a high pour point is generally associated with a high
paraffin content, typically found in crude derived from a larger proportion of plant
material as in Type III kerogen.
• Cloud Point
In the petroleum industry, cloud point refers to the temperature below which wax in
a sample, particularly jet fuel and diesel, forms a cloudy appearance. The presence
of solidified waxes thickens the oil and clogs fuel filters and injectors in engines.
The wax also accumulates on cold surfaces (e.g. causing pipeline or heat exchanger
fouling) and forms an emulsion with water. Therefore, cloud point indicates the
tendency of the oil to plug filters or small orifices at cold operating temperatures.
In crude or heavy oils, cloud point is synonymous with wax appearance temper-
ature (WAT) and wax precipitation temperature (WPT). It is determined by ASTM
D2500 or D5773 method as the temperature at which a haze appears in a sample due
to formation of wax crystals by cooling.
• Freeze Point
Freeze Point is the temperature at which crystals start to form in hydrocarbon
liquid and then disappear when heated (ASTM D2386).
• Aniline Point
Aniline Point is the lowest temperature at which aniline and an oil are completely
miscible. It is measured by ASTM D611. The mixture is heated and stirred until
homogeneous, then it is cooled with stirring until the two liquids separate. The
temperature at which such separation occurs is the aniline point. For clear samples,
the aniline point is that at which the mixture suddenly becomes cloudy. In darker
samples, the apparatus becomes more complicated, as it is less obvious when the
mixture is cloudy. It is used in some specifications as an indication of aromatic
content. The lower the aniline point, the greater is the aromatic content.
• Smoke Point
Smoke Point is performed on jet fuels and kerosene cuts to determine clean burning
by measuring flame height with a standard wick, expressed in mm, in a lamp without
smoke forming (ASTM D1322). To test for smoke point, a sample is burned in a
lamp that is precisely calibrated using hydrocarbons with known smoke points. The
maximum height of the flame without production of smoke is recorded to the nearest
0.5 mm. This is important in determining the composition of crude oil. Samples with
lower smoke points are more aromatic. Smoke point is usually most important in jet
fuel. Smoke can shorten the lifetime of engine parts, therefore higher smoke points
are favorable.
• Flash Point
Flash point is the temperature at which combustion occurs when the hydrocarbon
mixture is exposed to air or oxygen.
30 2 Crude Assay and Physical Properties

• Color and Fluorecence

The colors of crude oils are as varied as light yellow, green, red, brown and black.
Most of the colors come from molecules with aromatic character having large π elec-
tron systems in the molecule. The color depends on the degree of conjugation and
the size of the conjugated system. Some molecules can absorb light at high frequen-
cies (such as ultraviolet) and re-emit it at a lower frequency (in the visible range), a
phenomenon known as fluorescence. In general, the large the aromatic molecules in
the heavier fractions, the stronger is the fluorescence at longer wavelengths.
• Refractive Index
Refractive Index (RI) is the ratio of the velocity of light of a specific wavelength in
vacuum to the velocity of light in the sample tested (ASTM D1218). RI is dependent
upon the density of the oil: the heavier oils have higher reflective indices. It’s been
used to estimate the content of polynuclear aromatics (PNA, or polynuclear aromatic
hydrocarbons, PAH) which generally is higher in heavier oils. PNAs are produced
by thermal cracking in refineries and by the burning of fuels. PNAs are a significant
component of soot. Some PNAs can interact with DNA and prove to be toxic or
carcinogenic. Extended exposure to PNAs can lead to lung, skin, or bladder cancer.
It is important to test for PNAs to be aware of the risks associated with burning crude
oils with these properties.
• Optical Activities
Some saturated carbon atoms have four different substituents attached to them
through single covalent bonds. These asymmetric carbons provide chiral centers to
form non-superimposable isomers which as referred as enantiomers. Enantiomers,
which result from the inversion of all asymmetric carbon centers, show similar chemi-
cal properties, but rotate plane-polarized light in opposite directions. Inversion of less
than all of the available asymmetric carbon centers yields diastereomers or epimers.
Enzymes in living organisms, which are responsible for biosynthesis of cellular mate-
rials, are chiral and generate biomolecules with only one configuration at certain
asymmetrical carbon centers. Since petroleum is derived from ancient living organ-
isms, these specific configurations are carried over in saturated biomarker hydrocar-
bons which are found in immature oils. When an oil reaches maturity, epimers with
different optical activity are formed. An example of an epimer is cholestane (C27
sterane) which has a chiral center at the C-20 position.
• Reid Vapor Pressure (RVP)
Reid Vapor Pressure (RVP) is a measurement of volatility of a liquid hydrocarbon
mixture at 100 °F (ASTM D323). The measurement is taken using an apparatus that
includes a liquid chamber that is filled with a chilled sample and a vapor chamber that
is heated to 100 °F. The chambers are connected and immersed in a temperature bath
at 100 °F until constant pressure is observed. This pressure is the Reid vapor pressure,
measured as kilopascals relative to atmospheric pressure (kPa). The vapor pressure
of petroleum is important for several reasons. In gasolines, high temperatures and
2.3 Physical Testing 31

altitudes increase vaporization, which can lead to “vapor lock.” While handling crude
oil, vapor pressure is of high importance for safety reasons.
Note that RVP differs slightly from true vapor pressure (TVP) of a liquid. TVP is
measured in the presence of water vapor and air during sample vaporization in the
confined space of the test equipment. Hence, the RVP is the absolute vapor pressure
and TVP is the partial vapor pressure.

2.4 Crude Assay Template and Report

Table 2.2 exhibits an example assay report template; there is no standard assay testing
grid—each refinery or trading company has its own. Note that not all the tests need
to be done for all samples. The tests are requested and performed depending on the
needed information for processes and products.
Figure 2.6 shows an example of crude assay report of a benchmark crude, Brent
blend, and its fractions [5]. The yields of different fractions (cuts) is expressed by
volume %. It can be seen that paraffins are concentrated in the lowest distillation cuts
as “paraffinic”, with increasing naphthene and aromatic contents in higher distillation
cuts as “asphaltic”.

2.5 Bulk Properties of Crude Oil

Table 2.3 presents selected bulk physical and chemical properties for 21 crude oils.
Athabasca is a heavy oil, with a specific gravity >1.0. In the table, sulfur contents
range from 0.14 to 5.3 wt%, and nitrogen contents range from nil to 0.81 wt%.
Specific gravities range from 0.798 to 1.014. In sweet crudes such as Tapis, the
sulfur content is low. Sour crudes have more sulfur, which gives them a tart taste; in
the old days, prospectors did indeed characterize crude oil by tasting it. The crude
oils in China (Henan, Liaohe, Shengli, Xinjiang, etc.) have relatively higher nitrogen
contents in general compared to the crude oils outside China. The high nitrogen
(relative to sulfur) presents operational challenges in refining, where it is converted
into ammonia. The usual way to remove ammonia is in the form of ammonium
bisulfide (NH4 SH). The NH4 SH dissolves in wash water and is transported to sulfur
plants. If there is more NH3 than H2 S, the NH3 is not completely removed in this
fashion and remains in process gas streams, where it can accelerate corrosion.
Table 2.2 Example crude assay report template
32

Whole crude Light naphtha Medium naphtha Heavy naphtha Kero AGO LVGO HVGO VR AR
True Boiling Initial 10 80 150 200 260 340 450 570 340
Point, °C
True Boiling Final 80 150 200 260 340 450 570 End End
Point, °C
True Boiling Initial 55 175 300 400 500 650 850 1050 650
Point, °F
True Boiling Final 175 300 400 500 650 850 1050
Point, °F
Yield of Cut (wt% x x x x x x x x x
of Crude)
Yield of Cut x x x x x x x x x
(vol.% of Crude)
Gravity, °API x x x x x x x x x x
Specific Gravity x x x x x x x x x x
Sulfur, wt% x x x x x x x x x x
Nitrogen, ppm x x x x x x x x x
Viscosity @ 50°C x x x x x x x x
(122°F), cSt
Viscosity @ x x x x x x x x
135°C (275°F),
cSt
Freeze Point, °C x x x x
Freeze Point, °F x x x x
Pour Point, °C x x x x x x x x
Pour Point, °F x x x x x x x x
2 Crude Assay and Physical Properties

(continued)
Table 2.2 (continued)
Whole crude Light naphtha Medium naphtha Heavy naphtha Kero AGO LVGO HVGO VR AR
Smoke Point, mm x x x
Aniline Point, °C x x x x x x
Aniline Point, °F x x x x x x
Cetane Index, x x x
ASTM D976
Diesel Index x x x x x x
Characterization x x x x x x x x x x
2.5 Bulk Properties of Crude Oil

Factor (K)
Research Octane x x x
Number, Clear
Motor Octane x x
Number, Clear
Paraffins, vol.% x x x x x x
Naphthenes, x x x x x x x
vol.%
Aromatics, vol.% x x x x x x x
Heptane x x x
Asphaltenes, wt%
Micro Carbon x x x
Residue, wt%
Ramsbottom x x x
Carbon, wt%
Vanadium, ppm x x x
Nickel, ppm x x x
Iron, ppm x x x
33
34 2 Crude Assay and Physical Properties

Fig. 2.6 Crude assay results of a Louisiana crude oil and its fractions [7]

Among these crude oils, West Texas Intermediate (WTI) and Brent from North
Sea are commonly used in the commercial or merchandise communities around
the globe as benchmark crudes for the crude oil prices which change constantly
depending on supply and demand. Both benchmark crudes have relatively low sulfur
and nitrogen compared to other oils. The price of a specific crude oil can be higher
or lower than these reference crudes. Value is set primarily by the results of crude
assays. In general, the crude oils with low API gravity, high sulfur content and high
acid content are sold at deep discounts compared to the reference oils.
2.5 Bulk Properties of Crude Oil 35

Table 2.3 Bulk properties of 21 selected crude oils

Crude oil API Gravityb Specific Sulfur (wt%) Nitrogen
gravity (wt%)
Alaska North Slope 26.2 0.8973 1.1 0.2
Arabian Light 33.8 0.8560 1.8 0.07
Arabian Medium 30.4 0.8740 2.6 0.09
Arabian Heavy 28.0 0.8871 2.8 0.15
Athabasca (Canada) 8 1.0143 4.8 0.4
Beta (California) 16.2 0.9580 3.6 0.81
Brent (North Sea) 38.3 0.8333 0.37 0.10
Bonny Light (Nigeria) 35.4 0.8478 0.14 0.10
Boscan (Venezuela) 10.2 0.9986 5.3 0.65
Ekofisk (Norway) 37.7 0.8363 0.25 0.10
Henan (China) 16.4 0.9567 0.32 0.74
Hondo Blend (California) 20.8 0.9291 4.3 0.62
Kern (California) 13.6 0.9752 1.1 0.7
Kuwait Export 31.4 0.8686 2.5 0.21
Liaohi (China) 17.9 0.9471 0.26 0.41
Maya (Mexico) 22.2 0.9206 3.4 0.32
Shengli (China) 24.7 0.9058 0.82 0.72
Tapis Blend (Malaysia) 45.9 0.7976 0.03 nil
West Hackberry Sweeta 37.3 0.8383 0.32 0.10
West Texas Intermediate 39.6 0.8270 0.34 0.08
Xinjiang (China) 20.5 0.9309 0.15 0.35
a Produced
from a storage cavern in the U.S. Strategic Petroleum Reserve
b APIGravity is related to specific gravity by the formula
°API = 141.5 ÷ (specific gravity @ 60 °F) − 131.5

The weight percent data of sulfur and nitrogen contents in the crude oils listed
in Table 2.3 can also be presented in graphical form as in Fig. 2.7. Both sulfur
and nitrogen correlate inversely with API gravity, but for this particular dataset, the
correlations are rough due to wide scattering of data points, especially for sulfur.
Table 2.4 exhibits one example of reporting selected assay data for the whole
crude and its different residue cuts in a Mexican crude oil. As expected, API gravity
decreases with cut point; that is, the deeper cut point, the lower the API gravity.
The API of the resid obtained at cut point at 650 °F (lowest cut point) is 17.3
while at 1050 °F (highest cut point) it is 7.1. Sulfur content, nitrogen content, metal
content, viscosity, pour point and Conradson carbon (ConCarbon) residue increase
with increasing boiling point.
36 2 Crude Assay and Physical Properties

Fig. 2.7 Sulfur and nitrogen versus API gravity for selected crude oils. The top correlation line is
for nitrogen and the bottom one is for sulfur, which is more scattered (has a lower R2 value)

Table 2.4 Properties of a Mexican crude oil and its residua at different cut points [5]
Whole Crude Residua
650 °F 950 °F 1050 °F
Yield, vol.% 100.0 48.9 23.8 17.9
Sulfur. wt% 1.08 1.78 2.35 2.59
Nitrogen, wt% 0.33 0.52 0.60
API gravity 31.6 17.3 9.9 7.1
Conradson Carbon wt% 9.3 17.2 21.6
Vanadium, ppm 185 450
Nickel, ppm 25 64
Kinematic Viscosity at 100 °F 10.2 890
Kinematic Viscosity at 210 °F 35 1010 7959
Pour Point, °F −5 45 95 120
References 37

References

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demic Press, London, pp 2028–2034
2. Annual Book of ASTM Standards, section 5: Petroleum products, liquid fuels, and lubricants,
Vol. 05.01 and Vol. 05.02. In: American society for testing and materials (ASTM) International,
West Conshohocken, PA, 2016
3. Gray JH, Handwerk GE, Kaiser MJ (2007) Petroleum refining—technology and economics, 5th
Edition, CRC Press
4. Lopes MS, Savioli Lopes M, Maciel Filho R, Wolf Maciel MR, Median LC (2012) Extension
of the TBP curve of petroleum using the correlation DESTMOL. Procidea Eng 42:726–732
5. Speight JG (2006) The chemistry and technology of petroleum, 4th edn. Marcel Dekker, Revised
and Expanded
6. Leffler WL (2008) Petroleum refining in nontechnical language. Fourth Edition, PennWell
7. Assays available for download. https://corporate.exxonmobil.com/en/company/worldwide-
operations/crude-oils/assays. (Retrieved 20 Sept 2018)