Fahim M.A., Sahhaf T.A., Elkilani A.S. Fundamentals of Petroleum Refining
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Metal contents such as nickel (Ni), iron (Fe) and vanadium (V) can be also
determined by elemental analysis. The methods used are in the category of
X-ray fluorescence, atomic absorption and argon plasma. ASTM D5708 can
be used for metal determination. Sulphur can be found by the X-ray method
by ASTM D2622.
2.5.2. Carbon Residue
The carbon residue of a crude oil, heavy oil or residue is the weight percent
of coke that remains after evaporation and cracking of the sample in the
absence of air. Depending on the testing procedure used, the result is
reported as Conradson or Ramsbottom carbon residue in weight percent.
The carbon residue indicates the asphalt content of the oil or the amount of
lubricating oils that can be produced from the processing of the oil. It is
measured by ASTM D189.
2.5.3. Detailed Hydrocarbon Analysis
Detailed component analyses can be performed on petroleum gases and
naphtha fractions by gas chromatography using different columns in a
prescribed sequence. An internal standard is used to provide concentrations
of the different compounds. A chromatogram can be used to calculate the
parafins, isoparrafins, olefins and aromatic (PIONA) content of a sample.
ASTM D5445 can be used in this case.
2.5.4. Hydrocarbon Family Analysis
Although gas chromatography can be used for the determination of indi-
vidual compounds in naphtha fractions, this cannot be said about heavier
cuts. Hydrocarbon family analyses by mass spectroscopy (MS) provide
information on the relative amounts of different types of saturated and
aromatic hydrocarbons. Examples of hydrocarbon families are paraffins,
monocycloparaffins, alkylbenzenes, diaromatics, and the sulphur containing
family of compounds like thiophenes. The results can be useful in yield
calculations, since the correlations for the different processes contain para-
meters related to hydrocarbon family type. The ASTM D5368 can be used
in this case.
2.5.5. Aromatic Carbon Content
The aromaticity of a petroleum cut is defined as the mole fraction of aromatic
carbon in the sample as determined by carbon-13 nuclear magnetic reso-
nance (NMR) spectroscopy. The ASTM D5292 can be used in this case.
28 Chapter 2
2.5.6. SARA Analysis
The SARA analysis determines the content of saturate aromatics, resins and
asphaltenes in crude oils, heavy cuts and residues. This information is useful
in refinery design and operation. The analysis is also preparative, meaning
that the test provides enough samples of the above four classes of com-
pounds for further testing and analysis by the chemical analysis testing
methods mentioned above (Speight, 1991).
The separation of these four classes of compounds is done in different
stages. First, asphaltenes is precipitated by n-heptane. The deasphalted oil
called maltenes is subjected to liquid chromatography. Liquid chromatog-
raphy is similar in principles to gas chromatography in that the compounds
are eluted from a packed column by a carrier, in this case a liquid instead of a
gas. The saturated hydrocarbons are eluted by n-heptane, the aromatics by a
2:1 volume mixture of n-heptane and toluene and the resins by an equal
volume mixture of dichloromethane, toluene and methanol. The ASTM
D4124 can be used in this case. In Figure 2.2 a schematic diagram for SARA
analysis is shown.
The testing grid for crude oil and different cuts obtained from atmo-
spheric and vacuum distillation is shown in Table 2.4 (Strum and Shay,
2000).
Crude Oil Sample
Heptane
n-C
7
Soluble
Fraction
n-C
7
Insoluble Fraction
(Asphaltene)
Liquid Column
Chromatographic Separation
Saturation Aromatics
Resins
Asphaltene
Figure 2.2 Schematic diagram of SARA analysis
Refinery Feedstocks and Products 29
Table 2.4 ASTM testing grid for crude oil and petroleum fractions (Strum and Shay, 2000)
TBP (
F)
ASTM distillation (D86/D1160)
True boiling point (D28 92))
Simulate distillation (D 5307)
API/SG (D1298)
Total sulphur (D4294)
Mercaptants (D3227)
H
2
S(D325)
Total nitrogen D4629
Viscosity (2 Temp) D445 (D446)
Pour point (D97)
Organic chlorides (D4929)
Acid number (D664)
Carbon residue (D189)
Metals (D5708)
Cetane index (D4737)
Aniline point (D611)
Smoke point (D1322)
Octane number (D2700)
Vapour pressure (D323)
Wax and asphaltene
Sediment and water (D473)
Salt content (D6470)
Refractive index (D12 18)
Crude
LPG
LSR 90–180
HSR 180–380
Kero 380–520
LGO 520–610
HGO 610–800
VGO 800–1050
VR 1050
þ
30 Chapter 2
REFERENCES
Chatila, S. (1995), ‘‘Evaluation of Crude Oils,’’ Chapter 8, In ‘‘Crude Oil Petroleum
Products Process Flowsheets’’ Petroleum Refining, Vol. 1, Wauquier, J. ed., TECH-
NIP, France.
Roussel, J., and Boulet, R. (1995a) ‘‘Characterization of Crude Oils and Petroleum Frac-
tions,’’ Chapter 3, In ‘‘Crude Oil Petroleum Products Process Flowsheets,’’ Petroleum
Refining, Vol. 1, Wauquier, J. ed., TECHNIP, France.
Roussel, J., and Boulet, R. (1995b) ‘‘Composition of Crude Oil and Petroleum Products,’’
Chapter 1, In ‘‘Crude Oil Petroleum Products Process Flowsheets,’’ Petroleum Refin-
ing, Vol. 1, Wauquier, J. ed., TECHNIP, France.
Roussel, J., and Boulet, R. (1995c) ‘‘Fractionation and Elemental Analysis of Crude Oil and
Petroleum Fractions,’’ Chapter 2, In ‘‘Crude Oil Petroleum Products Process Flow-
sheets,’’ Petroleum Refining, Vol. 1, Wauquier, J. ed., TECHNIP, France.
Speight, J. (1991). ‘‘The Chemistry and Technology of Petroleum,’’ Marcel Dekker.
Strum, G., and Shay, J. Y. (2000). Comprehensive Report of API Crude Oil Characteriza-
tion Measurements, Technical Report 997, American Petroleum Institute.
Refinery Feedstocks and Products 31
CHAPTER THREE
Thermophysical Properties of
Petroleum Fractions and Crude Oils
3.1. Introduction
A petroleum refinery is a collection of unit operations, such as frac-
tionation towers, pumps, and heat exchangers. Analysis and design of these
units require knowledge of the thermodynamic and physical properties of
the petroleum fluids. Designing a crude distillation tower requires knowl-
edge on how hydrocarbons in crude oil are distributed on each tray of the
tower, that is vapour–liquid distribution, and the densities of the mixture.
Heat exchanger design depends on enthalpies, thermal conductivity and
viscosity of the flowing streams. Table 3.1 lists the thermophysical proper-
ties required for the design and operation of almost all processing equipment
in a refinery.
Due to the complexity of the composition of petroleum fractions and
crude oils it is not possible to measure or calculate accurately all of these
properties. Furthermore, calculation methods developed for pure hydro-
carbons are not always applicable. Therefore, over the years chemical and
petroleum refining engineers have developed special methods or correla-
tions to estimate the properties of petroleum fraction from easily measured
properties like normal boiling point (NBP) and specific gravity. Such
methods and schemes characterize these petroleum fractions. Traditionally
these correlations are developed to be simple to use, requiring minimum
input data, and are usually presented in graphical form. At this stage, it is
instructive to point out that there is a difference between the characteriza-
tion of crude oils and narrow boiling petroleum fractions. For crude oils,
mixture bulk properties cannot directly be used to estimate the properties
of the mixture. While for narrow boiling fractions, mixture properties such
as the average boiling point, specific gravity, and molecular weight may
directly be used to predict many other properties as outlined in the API
Technical Data Book—Petroleum Refining (API-TDB) (API, 1993).
Fundamentals of Petroleum Refining
#
2010 Elsevier B.V.
DOI: 10.1016/ B978-0-444-52785-1.00003-6 All rights reserved.
33
3.2. Basic Input Data
The characterization of petroleum fractions requires several measur-
able laboratory properties:
Specific gravity (SG)
Boiling point curve (ASTM or true boiling point (TBP distillation)
Kinematic viscosity at 37.8
C(100
F) and 98.9
C (210
F) (n
100
, n
210
)
Refractive index (n)
Molecular weight (M)
With the exception of TBP distillation and molecular weight, these proper-
ties can be readily measured in any petroleum characterization laboratory.
The meaning of these properties will be explained later.
3.2.1. Specific Gravity
Specific gravity for liquid oils is defined as:
SG
oil
¼
r
oil
r
water
ð3:1Þ
where r
oil
is the oil density and r
water
is the water density.
Both densities of oil and water are at the same standard temperature
and pressure conditions, which are 1 atm (14.7 psia) and 15.6
C (60
F).
Since under the same conditions, most petroleum fractions are lighter than
water, i.e. r
oil
1:
Table 3.1 Thermophysical property prediction
Thermodynamic properties
Enthalpy
Heat capacity
Compressibility factors
Equilibrium K-values
Flash curves
Transport properties
Viscosity
Thermal conductivity
Diffusivity
Physical properties
Densities
Volumes
34 Chapter 3
Another parameter for oil density is API gravity defined as:
API ¼
141:5
SG
131:5 ð3:2Þ
Heavy oils have low API and light oils have high API gravities.
The density measurement of petroleum fractions and crude oils are
carried out using either a pycnometer or a Mettler/Parr densitometer.
The latter method is based on density-dependent frequency changes of an
oscillating glass U-tube.
3.2.2. Boiling Point Curves
ASTM distillation or TBP distillations characterize the volatility of petro-
leum fractions and crude oils. Both are batch distillations which differ
mainly in the degree of fractionation obtained during distillation.
3.2.3. ASTM Distillation
ASTM distillation is carried out in a relatively simple apparatus consisting of
a flask holding the sample connected to an inclined condenser, which
condensed the rising vapours. The fractions distilled are collected in a
graduated cylinder. The temperature of the rising vapours is recorded at
specific interval of the collected distillates. This is essentially a batch distilla-
tion with one equilibrium stage and no reflux and minimum separation of
the components of the fractions. For gasoline, kerosene, gas oil and similar
light and middle distillates, the ASTM method D86, which is carried out at
atmospheric pressure, is used. Heavy petroleum products, which tend to
decompose in the ASTM D86 method but can be partially or completely
vaporized at a maximum temperature of 400
C (750
F) and pressures
down to 1 mm Hg, are distilled using the ASTM D1160 method. It is
carried out at pressures between 1 and 760 mm Hg.
The temperature at which the first drop of condensate is collected is
called the initial boiling point (IBP). The end point (EP) is the maximum
vapour temperature when almost the entire sample is distilled (above 95%).
Boiling temperatures at subatmospheric pressures (less than 760 mm Hg)
can be converted to NBPs (at 760 mm Hg) using procedure 5A1.13 of
API-TDB (API, 1993).
Minimum fractionation occurs in ASTM distillation, and components in
the mixture do not distil one by one in the order of their boiling points, but
rather as mixtures of successively higher boiling points. Nevertheless,
because ASTM distillations are quickly conducted and have been success-
fully automated, require only a small sample, and are quite reproducible,
they are widely used.
Thermophysical Properties of Petroleum Fractions and Crude Oils 35
3.2.4. True Boiling Point Distillation
Data from TBP distillation provides more detailed characterization of the
volatility of crude oil or petroleum fraction. It is performed in columns with
15 theoretical plates or equilibrium stages and a reflux ratio of 5. Rising
vapours are condensed and collected either at a constant rate of boiling points
or constant rate of the sample vaporized. Operation is at 760 mm Hg for
boiling points below 400
C (750
F). For higher boiling point fractions, the
distillation is conducted at reduced pressures as low as 0.5 mm Hg. Results
from vacuum operations are extrapolated to atmospheric pressure. The high
degree of fractionation in this test gives an accurate component distribution.
Because the degree of separation for a TBP distillation test is much higher
than that of the ASTM distillation test, its IBP is lower and its EP is higher than
those of the ASTM test. The TBP curve (a plot of the NBP versus the percent
volume of sample distilled) is usually used as a basis for the characterization of
crude oil or a petroleum product for the purpose of design and analysis.
3.2.5. Conversion between ASTM and TBP Distillation
Since TBP distillation is both tedious and time-consuming in comparison
with the ASTM method, there has been an incentive to develop correlation
to convert ASTM to TBP distillation while at the same time achieving the
benefit of the detailed separation of TBP with the little effort of the ASTM
distillation. The following equation suggested by Riazi and Daubert (1980)
and published by the API (1993) is used for the interconversion, referred to
as API method:
TBP ¼ aðASTM D86Þ
b
ð3:3Þ
where a and b are constants varying with percent of liquid sample distilled as
given in Table 3.2. TBP is true boiling point temperatures at 0, 10, 30, 50,
70, 90, and 95 volume percent distilled, in degrees Rankin. ASTM D86 is
Table 3.2 Constants for equation (3.3)
Volume %
distilled ab
0 0.9167 1.0019
10 0.5277 1.0900
30 0.7429 1.0425
50 0.8920 1.0176
70 0.8705 1.0226
90 0.9490 1.0110
95 0.8008 1.0355
36 Chapter 3
the observed ASTM D86 temperatures at corresponding volume percent
distilled, in degrees Rankin. Average error between the calculated and
measured TBP is in the range of 5
C (41
F).
More recently, Daubert (1994) published a new method for distillation
curves interconversion using the following equations:
T
0
50
¼ A
4
ðT
50
Þ
B4
; T
0
30
¼ T
0
50
DT
0
3
; T
0
10
¼ T
0
30
DT
0
2
T
0
0
¼ T
0
10
DT
0
1
; T
0
70
¼ T
0
50
þ DT
0
5
; T
0
90
¼ T
0
70
þ DT
0
6
T
0
90
¼ T
0
95
þ DT
0
7
ð3:4Þ
where DT
0
i
¼A
i
ðDT
i
Þ
B
i
, DT
1
¼ T
10
T
0
, DT
2
¼ T
30
T
10
, DT
3
¼ T
50
T
30
, DT
5
¼ T
70
T
50
, DT
6
¼ T
90
T
70
and DT
7
¼ T
f
T
90
.
The symbols T and T
0
stands for ASTM D86 and TBP temperatures,
respectively, both in
F. The subscripts 0 and f stand for the initial and final
temperatures, respectively. A
i
and B
i
are constants given in Table 3.3
The reported average error for this method is about 3
C (37.4
F).
Example E3.1
A petroleum cut has the following ASTM D86 Distillation data:
Volume % distilled 0 10 30 50 70 90 95
Temperature (
C) 36.5 54 77 101.5 131 171 186.5
Convert these data to TBP data using the API method of Riazi and Daubert
and Daubert’s method. Plot the results and compare.
Solution:
Application of the API method is straightforward using equation (3.3) and the
constants in Table 3.2. Results are shown in Table E3.1. For the Dauber’s
Table 3.3 Constants for Daubert’s distillation curves
interconversion method
Index number iA
i
B
i
1 7.4012 0.6024
2 4.9004 0.7164
3 3.0305 0.8008
4 0.8718 1.0258
5 2.5282 0.8200
6 3.0419 0.7750
7 0.1180 1.6606
Thermophysical Properties of Petroleum Fractions and Crude Oils 37
method, the constants in Table 3.3 are used to calculate TBP at 50 vol% T
0
50
.
The △T
i
are calculated from the ASTM data, the △T
0
for the TBP are then
calculated. Then T
0
for TBP are calculated using the formulas in equation (3.4).
The results are shown in Table E3.1.
As can be seen from Figure E3.1, the TBP distillation curve is below the
ASTM curve at volume distilled below 50% and above it for volume distilled
above 50%. The API and Duabert’s methods give comparable results except for
the low range, less than 10% distilled.
Table E3.1 Converted ASTM temperatures
Volume %
distilled
D86
T (
C)
TBP (
C) API
method equation (3.3)
TBP (
C) Daubert’s
method equation (3.4)
0 36.5 14.1 5.3
10 54 33.4 27.5
30 77 69.0 66.7
50 101.5 101.6 101.7
70 131 135.2 138.1
90 171 180.5 184.6
95 186.5 194.1 201.1
−50
0
50
100
150
200
0
Volume % Distilled
Temperature C
ASTM D86
TBP (API method)
TBP (Daubert’s method)
20 40 60 80 100
Figure E3.1 Conversion of ASTM D86 intoTBP
38 Chapter 3