Fahim M.A., Sahhaf T.A., Elkilani A.S. Fundamentals of Petroleum Refining
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AGO Steam
Atm. Feed
Off Gas
Waste Water
Naphtha
Kerosene
Diesel
AGO
Residue
2
29
8
1
2
3
1
2
3
1
2
3
28
9
10
11
22
18
21
17
16
Diesel Steam
Main Steam
S.S
S.S
S.S
P. A
P. A
P. A
P.A: Pumparound
S.S: Side Stripper
Figure E4.6.2 Distillation column
0.00
0.05
0.10
0.15
0.20
0.25
0.30
0.35
0.40
0.45
0.50
0 100 200 300 400 500 600 700
Boing Point (
o
C)
Liquid Volume Fraction of Total Oil__
2
1
3
4
5
6
7
8
1 Off Gas
2 Lt St Run
3 Naphtha
4 Kerosene
5 Light Diesel
6 Heavy Diesel
7 Atm Gas
8 Residue
Figure E4.6.3 Crude distribution curve
90 Chapter 4
The distillation column specifications are listed in Table E4.6.1.
The solved material and energy balance is shown in Table E4.6.2.
Table E4.6.1 Column specifications
Item Specification
Kero-SS product flow 61.61 m
3
/h
Diesel-SS product flow 127.5 m
3
/h
AGO-SS product flow 29.81 m
3
/h
Pumparound 1 rate 331.2 m
3
/h
Pumparound 1 duty 5.8 10
7
kJ/h
Pumparound 2 rate 198.7 m
3
/h
Pumparound 2 duty 3.7 10
7
kJ/h
Pumparound 3 rate 198.7 m
3
/h
Pumparound 3 duty 3.7 10
7
kJ/h
Naphtha product rate 152.4 m
3
/h
Liquid flow 23.19 m
3
/h
Kero reboiler duty 7.913 10
6
kJ/h
Vapour product flow 0.0
Reflux ratio 1
Table E4.6.2 Material and energy balance
Name
Va p o u r
fraction
T
(
C)
P
(kPa)
Molar flow
(kg mol/h)
Mass flow
(kg/h)
Molar
enthalpy
(kJ/kg mol)
Hot crude 0.6447 343.3 448.2 2843 5.829 10
5
2.673 10
5
Main steam 1.0 190.6 1034 188.8 3402 2.359 10
5
Diesel steam 1.0 148.9 344.7 75.54 1361 2.370 10
5
AGO steam 1.0 148.9 344.7 62.95 1134 2.370 10
5
Off gas 1.0 41.92 135.8 0 0 –
Naphtha 0.0 41.92 135.8 1283 1.124 10
5
1.921 10
5
Waste water 0.0 41.92 135.8 316.8 5707 2.841 10
5
Kerosene 0.0 236.5 205.8 320.9 5.079 10
4
2.678 10
5
Diesel 0.0 253.0 213.6 509.8 1.106 10
5
3.583 10
5
AGO 0.0 300.4 218.6 91.7 2.708 10
4
4.493 10
5
Residue 0.0 355.1 225.5 647.8 2.822 10
5
5.953 10
5
Crude Distillation 91
Questions and Problems
4.1. With 100,000 BPD of the following crude (API ¼ 36), estimate the
products of the atmospheric distillation column. If the atmospheric
residue of the crude is taken at 650þ
F. It enters in a vacuum distilla-
tion tower to give three products: light vacuum gas oil (650–850
F),
heavy vacuum gas oil (850–1050
F) and vacuum residue (1050þ
F).
Calculate the mass flow rate of these products. Then calculate the
sulphur content (lb/hr) for each product.
ASTM D86 (
F) vol% 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 0.7780
392 4.7 17.7 0.7890
437 5.7 23.4 0.8010
482 8.0 31.4 0.8140
527 10.7 42.1 0.8250
584 5.0 47.1 0.8450
636 10.0 57.1 0.8540
689 7.8 64.9 0.8630
742 7.0 71.9 0.8640
794 6.5 78.4 0.8890
– 20.8 99.2 0.9310
4.2. What is the concentration of salt in the washed solution for a desalted
oil containing 1 lb of NaCl/1000 bbl of oil, if the inlet oil contains
20 lb of NaCl/1000 bbl of oil? Assume the amount of water in oil
before and after desalting is the same and the wash water used is 10 vol
% of oil.
4.3. Sketch a combined atmospheric and vacuum distillation for a crude oil
feed.
4.4. Sketch a side view for the desalter.
92 Chapter 4
REFERENCES
Abdel-Aal, H. K., Aggor, M., and Fahim, M. A. (2003). ‘‘Petroleum and Gas Field Proces-
sing.’’ Marcel Dekker, New York.
Ptak, C., Bonfils, P., and Marty, C. (2000). ‘‘Distillation Absorption and Stripping in the
Petroleum Industry,’’ Chapter 5, In ‘‘Separation Processes,’’ Petroleum Refining,
Vol. 2, Wauquier, P., ed., TECHNIP, France.
Riazi, M. R., Nasimi, N., and Roomi, Y. (1999). Estimating sulfur content of petroleum
products and crude oils. I EC Res. 38, 11.
UNISIM Design Suite R370, Honeywell Process Solutions, Calgary, Alberta, Canada.
Crude Distillation 93
CHAPTER FIVE
Catalytic Reforming and
Isomerization
5.1. Introduction
Catalytic reforming of heavy naphtha and isomerization of light
naphtha constitute a very important source of products having high octane
numbers which are key components in the production of gasoline. Envi-
ronmental regulations limit on the benzene content in gasoline. If benzene
is present in the final gasoline it produces carcinogenic material on combus-
tion. Elimination of benzene forming hydrocarbons, such as, hexane will
prevent the formation of benzene, and this can be achieved by increasing
the initial point of heavy naphtha. These light paraffinic hydrocarbons can
be used in an isomerization unit to produce high octane number isomers.
5.2. Catalytic Reforming
Catalytic reforming is the process of transforming C
7
–C
10
hydrocarbons
with low octane numbers to aromatics and iso-paraffins which have high
octane numbers. It is a highly endothermic process requiring large amounts
of energy. A schematic presentation of the feedstock, products and process
condition is shown in Figure 5.1. The process can be operated in two modes:
a high severity mode to produce mainly aromatics (80–90 vol%) and a middle
severity mode to produce high octane gasoline (70% aromatics content).
5.2.1. Reformer Feed Characterization
Feeds are characterized by the Watson characterization factor (K), naphthenes
(N) vol% and aromatics (A)vol%inwhich(N þ 2A) must be defined. In
addition, initial boiling points (IBP) and end points (EP) for feeds must be
characterized. Feeds can be also characterized by the hydrocarbon family and
their number of carbon atoms. Naphthenic feeds give a much higher yield
than paraffinic feeds. The main feed comes from hydrotreated heavy naphtha,
and some feed comes from hydrotreated coker naphtha.
Fundamentals of Petroleum Refining
#
2010 Elsevier B.V.
DOI: 10.1016/ B978-0-444-52785-1.00005-X All rights reserved.
95
5.2.2. Role of Reformer in the Refinery and Feed Preparation
The catalytic reformer is one of the major units for gasoline production in
refineries. It can produce 37 wt% of the total gasoline pool. Other units such
as the fluid catalytic cracker (FCC), the methyl ter-butyl ether (MTBE)
production unit, alkylation unit and isomerization unit, also contribute to
this pool. These units will be covered in other chapters of the book.
The straight run naphtha from the crude distillation unit is hydrotreated
to remove sulphur, nitrogen and oxygen which can all deactivate the reform-
ing catalyst. The hydrotreated naphtha (HTN) is fractionated into light
naphtha (LN), which is mainly C
5
–C
6
, and heavy naphtha (HN) which is
mainly C
7
–C
10
hydrocarbons. It is important to remove C
6
from the
reformer feed because it will form benzene which is considered carcinogenic
upon combustion. Light naphtha (LN) is isomerized in the isomerization unit
(I). Light naphtha can be cracked if introduced to the reformer. The role of
the heavy naphtha (HN) reformer in the refinery is shown in Figure 5.2.
Hydrogen, produced in the reformer can be recycled to the naphtha hydro-
treater, and the rest is sent to other units demanding hydrogen.
5.2.3. Research Octane Number
The research octane number (RON) is defined as the percentage by
volume of iso-octane in a mixture of iso-octane and n-heptane that knocks
with some intensity as the fuel is being tested. A list of the RON of pure
hydrocarbon is given in Appendix D. It is seen from this appendix that the
RON of paraffins, iso-paraffins and naphthenes decrease as the carbon
number of the molecule increases. Aromatics have the opposite trend.
This is shown in Figure 5.3.
Heavy naphtha
C
7
– C
10
RON : 20 – 50
P: 45– 65 vol%
N: 20–40 vol%
A: 15-20 vol%
Reaction Conditions
Temp : 500°C
Pressure : 5 -25 bar
Platinum-based
catalyst
High heat demand
SR: Semi-
Regenerative
CCR: Continuous
Catalyst
Regeneration
C
5
+
Reformate
RON : 90 – 100
H
2
, C
1
, C
2
,
C
3
, C
4
Coke
P : 30 – 50 vol%
N : 5 – 10 vol%
A: 45 – 60 vol%
Feedstock
Products
Catalytic Reforming
Figure 5.1 Catalytic reforming process
96 Chapter 5
CDU
D
F
I
HN
HT
HTN
LN
N
R
Crude distillation unit
Distillation
Flash
Isomerization
Heavy Naphtha
Hydrotreater
Hydro Treated Naphtha
Light Naphtha
Naphtha
Reformer
D
N
H
2
C
5
+
HN
Reformate
LN
C
4
−
C
3
−
C
4
−
iC
5
/iC
6
HTN
C
4
Gases
Feed
To other hydrogen
consuming units
F
I
R
HT
C
D
U
Figure 5.2 Role of reformer in the refinery
n-P : normal Paraffins i-P : Isoparaffins
N : Naphthenes A : Aromatics
RON
Number of C Atom
A
N
i-P
n-P
4
6
8
50
100
150
Figure 5.3 Variation of research octane number (RON) (Antos et al., 1995)
Catalytic Reforming and Isomerization 97
5.2.4. Reforming Reactions
5.2.4.1. Naphthene Dehydrogenation of Cyclohexanes
CH
3
3H
2+
CH
3
Meth
y
l C
y
clohexane Toluene
ð5:1Þ
+
Dimeth
y
l-C
y
clo
p
entane Toluene
CH
3
CH
3
CH
3
3H
2
ð5:2Þ
5.2.4.2. Paraffin Dehydrogenation
n-C
7
H
16
n
-heptane
n-C
7
H
14
n
-heptene
þH
2
ð5:3Þ
5.2.4.3. Dehydrocyclization
n-C
7
H
16
+ 4H
2
n-he
p
tane Toluene
CH
3
ð5:4Þ
All the above reactions are highly endothermic.
5.2.4.4. Isomerization
n-C
7
H
16
n
-heptane
iC
7
H
16
isoheptane
ð5:5Þ
Isomerization is a mildly exothermic reaction and leads to the increase of
an octane number.
98 Chapter 5
5.2.4.5. Hydrocracking Reactions
Hydrocracking reactions are the main sources of C
4
hydrocarbons (C
1
,C
2
,
C
3
and C
4
). The reactions are highly exothermic and consume high
amounts of hydrogen. Cracking results in the loss of the reformate yield.
Paraffin hydrocracking:
C
10
H
22
+ H
2
C
2
H
5
– CH – C
2
H
5
+ C
4
H
10
CH
3
Decane
Isohexane
ð5:6Þ
Hydrocracking of aromatics
CH
3
H
2
+
CH
4
+
ð5:7Þ
Other paraffins can crack to give C
1
–C
4
products.
5.2.4.6. Coke Deposition
Coke can also deposit during hydrocracking resulting in the deactivation of
the catalyst. The catalyst in this case has to be re-activated by burning off the
deposited coke. The catalyst is selected to produce a slow hydrocracking
reaction. Coke formation is favoured at low partial pressures of hydrogen.
Hydrocracking is controlled by operating the reaction at low pressure
between 5–25 atm (74–368 psia), not too low for coke deposition and not
too high in order to avoid cracking and loss of reformate yield.
A summary of reformer reactions and interactions is shown by the
reaction network in Figure 5.4.
5.2.5. Thermodynamics of Reforming Reactions
The dehydrogena tion reactio ns are t he main sour ce of re for mate pro duct
and are considered to be the most important reactions in reforming. These
are highly endothermic reactions and require a great amount of heat
to keep t he reaction going. Fo r this reason three reactors are usually
used in the reforming process with heating the product from each reactor
before entering the other.
Catalytic Reforming and Isomerization 99
The dehydrogenation reactions are reversible and equilibrium is
established based on temperature and pressure. It is usually important to
calculate the equilibrium conversion for each reaction. In reforming, a high
temperature around 500
C (932
F) and a low hydrogen pressure are
required. The minimum partial pressure of hydrogen is determined by the
amount of the desired aromatics conversion.
Example E5.1
The Gibbs free energy of the following reaction at 500
C and 20 atm is
calculated to be 20.570 kcal/mol
3H
2
(CH) (B)
+
Calculate the reaction equilibrium conversion and barrels of benzene
formed per one barrel of cycloh exane.
The hydrogen feed rate to the reactor is 10,000 SCF/bbl of cyclohexane.
Solution:
The Gibbs free energy of a reaction is defined as:
DG ¼RT ln KðÞ
Using this equation, the equilibrium constant can be evaluated as
20; 570 ¼1:987ðÞ773ðÞln KðÞ; K ¼ 6:55 10
5
M - Metal catalyst A - Acid catalyst
I – Hydrocracking II - Isomerization
III – Dehydro-Cyclizing IV – Dehydrogenation
Aromatics
Normal Paraffins
Iso Paraffins
M/A
M/A
M/A
I
III
I
III
Cracked
Product
Naphthenes
IV
M
M/A
A
II
Figure 5.4 Networkof reforming reaction
100 Chapter 5