Fahim M.A., Sahhaf T.A., Elkilani A.S. Fundamentals of Petroleum Refining
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equilibria calculations. Equations based on the Redlich–Kwong equation
have proven to be the most successful for petroleum systems. The most
widely used modifications of the Redlich–Kwong equations are the mod-
ifications of Soave and Peng–Robinson. In this section, we will concentrate
on the latter equation.
The Peng–Robinson (PR) equation is a cubic equation in volume with
the form:
Z ¼
V
V b
aðTÞ
V ðV þ bÞþbðV bÞ
ð3:42Þ
where Z is the compressibility factor defined as PV/RT, and V is the molar
volume. The equation is characterized by the energy parameter which is a
function of temperature and the size parameter which is constant for a
specific fluid. The equations for calculating these constants given the critical
temperature and pressure and the acentric factor can be found in many
standard chemical engineering thermodynamics books (Smith et al., 1999).
This equation can be applied to mixtures by applying the mixing rules:
a ¼
X
i
X
j
x
i
x
j
a
ij
and b ¼
X
i
x
i
b
i
ð3:43Þ
The combining rule for a
ij
is given by
a
ij
¼ða
i
a
j
Þ
1=2
ð1 k
ij
Þ
where k
ij
is the binary interaction parameters. Values of k
ij
between hydro-
carbons are usually equal to zero. For non-hydrocarbon–hydrocarbon
interactions, the values of these parameters are tabulated.
3.6.2. Vapour–liquid Equilibrium
The equilibrium K
eq
value for a component in a vapour–liquid equilibrium
system is given by
K
eq
¼
’
i
l
.
’
i
v
ð3:44Þ
’
l
i
and ’
v
i
are called the fugacity coefficients of component i in the liquid
and vapour phase, respectively. These coefficients are functions of temper-
ature, pressure and composition and are calculated directly from the equa-
tion of state. The expressions for these coefficients for the PR equation of
state can also be found in standard thermodynamics textbooks. These K
eq
values are utilized in distillation or flashing in the well-known bubble, dew
and flash calculation.
Thermophysical Properties of Petroleum Fractions and Crude Oils 59
Example E3.12
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 A PI method of Riazi and Daubert
and Daubert’s method. Plot the results and compare.
Solution:
The results for the API method of Riazi and Daubert are shown in Table
E3.12.1 and plotted in Figure E3.12.1.
Table E3.12.1 Method of Riazi and Daubert
Volume %
distilled a b
D86
T,
C
D86
T,
F
TBP,
F
TBP,
C
0 0.9167 1.0019 36.5 97.7 57.4 14.1
10 0.5277 1.0900 54.0 129.2 92.0 33.4
30 0.7429 1.0425 77.0 170.6 156.1 69.0
50 0.8920 1.0176 101.5 214.7 214.9 101.6
70 0.8705 1.0226 131.0 267.8 275.3 135.2
90 0.9490 1.0110 171.0 339.8 356.9 180.5
95 0.8008 1.0355 186.5 367.7 381.4 194.1
Volume%
Temperature, ⬚C
100100
0
50
100
150
200
250
TBP vs ASTM D86 Distillation
20 30 40 50 60 70 80 90
TBP Curve
ASTM D86
Figure E3.12.1 API method
60 Chapter 3
The results for the Daubert’s method are shown in Table E3.12.2 and plotted in
Figure E3.12.2.
Example E3.13
A petroleum crude has an API of 34 and a TBP of 320
C. Calculate all properties
such as critical, refractive index, viscosity and molecular type composition.
Solution:
The calculated properties are shown in Table E3.13. Listed in details in Table
E3.13.
Table E3.12.2 Method of Daubert
Volume %
distilled
Index
iA
i
B
i
DT
i
DT
i
0
,
F
TBP,
F
TBP,
C
0 1 7.4012 0.6024 31.5 59.14 22.4 5.3
10 2 4.9004 0.7164 41.4 70.57 81.6 27.5
30 3 3.0305 0.8008 44.1 62.86 152.1 66.7
50 4 0.8718 1.0258 0.00 215.0 101.7
70 5 2.5282 0.820 53. 1 65.67 280.7 138.1
90 6 3.0419 0.755 72 76.81 357.5 180.8
7 0.1180 1.6606
224.42
F
VABP 106.9
C
206.14
F
MeABP 96.75
C
Vol%
TBP, C
100100
0
50
−50
100
150
200
TBP Curve
250
300
20 30 40 50 60 70 80 90
TBP Curve
Poly. (TBP Curve)
ASTM D86
y = 0.0000001x
5
− 0.0000287x
4
+ 0.0027144x
3
−
0.1208156x
2
+ 4.2608778x − 5.3573689
R
2
= 0.9999966
Figure E3.12.2 Daubert’s method
Thermophysical Properties of Petroleum Fractions and Crude Oils 61
Table E3.13 Calculations of properties
The following calculations require the API gravity and the mean average boiling point and other variable.
Calculation of Watson Characterization factor and Molecular weight
API gravity = 34.00
Specific gravity = 0.8550
MeABP, C = 320.00 593.15 K 494.59 R
Watson K Factor = 11.95
Mol. Weight 257.12
Calculation of Kinematic viscosity
Kinematic viscosity at 100 F, cSt 5.823
Kinematic viscosity at 210 F, cSt 1.916
Calculation of Critical Properties and Acentric factor
Critical Temperature, T
c
Critical Pressure, P
c
T
br
o
K R bar
765.49 1377.87 15.26 0.774867 0.7770
Refractive Index
Light Fractions MW = 70–300
Huang Characterization parameter, I = 0.284
Refrective index n = 1.481 Constants for I
Density at 68 F, g/cm
3
d = 0.8514 a 0.02266 0.02341
Refractivity intercept RI = 1.055 b 0.000391 0.000646
c 2.468 5.144
d 0.00057 0.00033
62 Chapter 3
Heavy Fractions MW = 300–600 e 0.0572 0.407
I = 0.308 f 0.72 3.333
n = 1.528
Density at 68 F, g/cm
3
d = 0.8514
Refractivity intercept RI = 1.102
Molecular type composition
Light fractions MW = 70–200 a 13.359
Viscosity gravity constant b 14.4591
Paraffins, mol pct. VGF = 0.9591 c 1.41344
Naphthenes, mol pct. 53.83 d 23.9825
Aromatics, mol pct. 15.03 e 23.333
TOTAL 31.14 f 0.81517
100.00 g 9.6235
h 8.8739
I 0.59827
Heavy fractions MW = 200–600
Kinematic viscosity at 100 F in Sybolt seconds 45.01
Viscosity gravity constant VGC = 0.8346
Paraffins, mol pct. 70.80
Naphthenes, mol pct. 5.86
Aromatics, mol pct. 23.33
TOTAL 100.00
Thermophysical Properties of Petroleum Fractions and Crude Oils 63
3.7. Calculating Properties Utilizing UNISIM
Software
Process simulators are used to characterize crude oil and determine the
thermophysical properties of crude oil and fractions. UNISIM simulator
(UNISIM, 2007) can be utilized in defining pseudo-components of a crude
oil, given its crude assay. It provides the option of selecting the thermody-
namic model for vapour–liquid equilibrium and thermodynamic properties
calculations. It is recommended to use Peng–Robinson equation of state
to model hydrocarbon and petroleum mixtures in UNISIM. Detailed
information about UNISIM simulator is given in Appendix C.
Example E3.14
Consider the following crude assay which has API ¼ 29:
Volume % TBP (
C) Volume % TBP (
C)
0.00 9.44 40.00 273.33
4.50 32.22 50.00 326.67
9.00 73.89 60.00 393.33
14.50 115.56 70.00 473.89
20.00 154.44 76.00 520.56
30.00 223.89 80.00 546.11
40.00 273.33 85.00 565.56
Use UNISIM to divide the crude into 10 pseudo-components and calculate
all cut properties. Compare the results with those obtained from the corr elations
in this chapter.
(UNISIM Design Tutorial and Applications, Honey Well, 2007)
Solution:
The crude assay (vol% versus TBP) is entered the oil environment and oil
manager data entry of UNISIM, and the number of pseudo-components (10
cuts) is entered in the Blend calculation. The properties calculated by UNISIM
are listed in Table E3.14.1.
64 Chapter 3
The corresponding properties calculated using the correlations described in
this chapter are listed in Table E3.14.2.
Comparing the results included in Tables E3.14.1 and E3.14.2 shows that
values of the properties calculated using UNISIM and the correlations are in
good agreement.
Table E3.14.1 Cut property results from UNISIM simulator
Cut
NBP
(
C) MW
Density
(kg/m
3
) o P
c
(kPa)
T
c
(
C)
Viscosity
(cP)
NBP_29 28.64 64.60 703.31 0.218 3544.42 202.21 0.16
NBP_81 81.13 86.71 743.36 0.299 3459.30 262.09 0.20
NBP_133 133.30 110.54 775. 15 0.388 2932.88 318.84 0.28
NBP_187 187.03 141.39 808. 52 0.468 2479.23 374.36 0.42
NBP_242 242.17 181.05 841. 61 0.615 2111.10 429.01 0.69
NBP_292 292.25 222.75 868. 27 0.716 1824.02 476.34 1.11
NBP_346 345.58 272.21 894. 35 0.853 1563.23 524.94 1.89
NBP_399 399.12 322.66 919. 05 0.990 1342.14 572.40 3.51
NBP_482 481.97 422.22 955. 54 1.131 1065.91 644.06 11.85
NBP_585 585.11 556.72 1000.19 1.383 809.38 731.80 124.42
Table E3.14.2 Cut proper ty results from correlations
Cut
NBP
(
C) MW
Density
(kg/m
3
) o P
c
(kPa)
T
c
(
C)
Viscosity
(cP)
NBP_29 28.64 68.15 738.41 0.257 3044.42 252. 27 0.128
NBP_81 81.13 89.67 778.95 0.312 2959.30 288. 45 0.160
NBP_133 133.30 114.41 815.45 0.425 2432.88 319.06 0.224
NBP_187 187.03 144.00 849.91 0.497 1979.23 346.49 0.336
NBP_242 242.17 179.53 882.58 0.655 1611.10 371.38 0.552
NBP_292 292.25 217.19 910.29 0.786 1324.02 441.74 0.888
NBP_346 345.58 264.03 938.06 0.871 1063.23 511.51 1.512
NBP_399 399.12 319.38 964.37 1.015 842.14 579.73 2.808
NBP_482 481.97 425.45 1002.46 1.220 565.91 655.31 9.480
NBP_585 585.11 603.15 1046.17 1.413 309.38 683.66 99.536
Thermophysical Properties of Petroleum Fractions and Crude Oils 65
Questions and Problems
3.1. A naphtha fraction has the following ASTM D86 distillation data:
Volume % Temperature (
C)
0.0 138.8
10.0 149.6
30.0 158.8
50.0 165.8
70.0 169.9
90.0 178.1
95.0 180.4
Obtain the TBP curve using the API method. Extrapolate the curve to
100% volume distilled.
3.2. Plot the volume percent versus specific gravity, for the naphtha fraction
given in problem 3.1, knowing that its specific gravity is 0.801.
3.3. Use the following crude assay data with crude API of 36 to estimate cut
vol%, critical properties and molecular weight for Light Naphtha (90-
190
F) and Kerosene (380-520
F ). In addition, calculate the fractions
of paraffins, naphthenes and aromatics in the two cuts.
ASTM D86 (
F) Volume % Cum vol% SG
86 0.0 0.0
122 0.5 0.5 0.6700
167 1.2 1.7 0.6750
212 1.6 3.3 0.7220
257 2.7 6.0 0.7480
302 3.1 9.1 0.7650
347 3.9 13.0 0.778 0
392 4.7 17.7 0.789 0
437 5.7 23.4 0.801 0
482 8.0 31.4 0.814 0
527 10.7 42.1 0.8250
584 5.0 47.1 0.845 0
636 10.0 57.1 0.8540
689 7.8 64.9 0.863 0
742 7.0 71.9 0.864 0
794 6.5 78.4 0.889 0
– 20.8 99.2 0.9310
66 Chapter 3
3.4. A gas oil has the following TBP distillation data
Volume % TBP (
C)
0 216
10 243
30 268
50 284
70 304
90 318
95 327
100 334
It also has an average boiling point of 280
C and an average density of
0.850 g/cm
3
.
(a) Split this gas oil fraction into five pseudo-components. Calculate T
c
, P
c
and o for each pseudo-component.
(b) Calculate T
c
, P
c
and o for the whole gas oil fraction.
(c) Calculate the enthalpy of this gas oil fraction at 400
C using the Lee–
Kessler correlation with a reference state of ideal gas at 273.15 K.
REFERENCES
American Petroleum Institute. (1993). ‘‘Technical Data Book—Petroleum Refining.’’
American Petroleum Institute, Washington, DC.
Daubert, T. (1994). ‘‘Petroleum Fractions Distillation Interconversion’’ Hydrocarbon Pro-
cessing, 9, p. 75.
Kessler, M. G., and Lee, B. I. (1976). Improve prediction of enthalpy of fractions. Hydrocar-
bon Proc. 55, 153–158.
Pedersen, K. S., Fredenslund, A., and Rasmussen, P. (1989). ‘‘Properties of Oils and Natural
Gases,’’ Chapter 7. Gulf Publishing Company, Houston, TX.
Riazi, M. R., and Daubert, T. (1980). Simplify property prediction. Hydrocarbon Proc. 3,
115–116.
Riazi, M. R. (2005). ‘‘Characterization and Properties of Petroleum Fractions,’’ Chapter 4.
ASTM International, Conshohocken, PA.
Riazi, M. R., and Al-Sahhaf, T. A. (1996). Physical properties of heavy petroleum fractions
and crude oils. Fluid Phase Equilib. 117, 217–224.
Smith, J. M., Van Ness, H. C., and Abbott, M. M. (1999). ‘‘Introduction to Chemical
Engineering Thermodynamics,’’ 9th ed. McGraw-Hill, New York.
UNISIM Design Tutorial and Application, Design Suite R370 (2007). Honeywell Process
Solutions, Calgary, Alberta, Canada.
Thermophysical Properties of Petroleum Fractions and Crude Oils 67
CHAPTER FOUR
Crude Distillation
4.1. Introduction
Crude distillation unit (CDU) is at the front-end of the refinery, also
known as topping unit, or atmospheric distillation unit. It receives high flow
rates hence its size and operating cost are the largest in the refinery. Many
crude distillation units are designed to handle a variety of crude oil types.
The design of the unit is based on a light crude scenario and a heavy crude
scenario. The unit should run satisfactorily at about 60% of the design feed
rate. Seasonal temperature variation should be incorporated in the design
because changes in the cut point of gasoline can vary by 20
C (36
F)
between summer and winter.
The capacity of the CDU ranges from 10,000 barrels per stream d ay
(BPSD) or 1400 metric tons per day (tpd) to 400,000 BPSD (56,000
metric tpd). The economics of refining favours larger units. A good size
CDU can process about 200,000 BPSD. The unit produces raw products
which have to be processed in downstream unit to produce products of
certain speci fica tions. This involve s the removal of undesirabl e compo-
nents like sulphur, nitrogen and metal compounds, and limiting the
aromatic contents.
Typical products from the unit are:
Gases
Light straight run naphtha (also called light gasoline or light naphtha)
Heavy gasoline (also called military jet fuel)
Kerosene (also called light distillate or jet fuel)
Middle distillates called diesel or light gas oil (LGO)
Heavy distillates called atmospheric gas oil (AGO) or heavy gas oil
(HGO)
Crude column bottoms called atmospheric residue or topped crude.
Fundamentals of Petroleum Refining
#
2010 Elsevier B.V.
DOI: 10.1016/ B978-0-444-52785-1.00004-8 All rights reserved.
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