PetroleumThermal and catalytic processes in petroleum refining 2003

Thermal
and
Catalytic
Processes
in
Petroleum Refining
Serge
Raseev
Consultant
for
UNESCO, Paris, France
and
former
Professor,
Institute
of
Petroleum
and
Gases,
Bucharest-Ploiesti,
Romania
Technical
editor
for the
English-language
version
G.
Dan
Suciu
MARCEL
DEKKER,

INC.
NEW
YORK
•
BASEL
Copyright © 2003 by Taylor & Francis Group, LLC
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Originally published in Romanian as Conversia Hidrocarburilor in 3 volumes, 1996–1997.
ISBN: 0-8247-0952-7
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PRINTED IN THE UNITED STATES OF AMERICA
Copyright © 2003 by Taylor & Francis Group, LLC
To my dear wife Irena
Copyright © 2003 by Taylor & Francis Group, LLC
Preface
This book is considered to be a completely new version of the original book pub-
lished in 3 volumes in Romania, in 1996–1997 under the title Conversia
Hidrocarburilor (‘‘the conversion of hydrocarbons’’).
Recent developments in petroleum processing required the complete revision
of some of the chapters, the elimination of outdated material and bringing up to
date the processes in which the technology was significantly improved.
Furthermore, the presentation of theoretical aspects has been somewhat expanded
and deepened.
The processes discussed in this book involve the conversion of hydrocarbons
by methods that do not introduce other elements (heteroatoms) into hydrocarbon
molecules. The fir st part is devoted to thermal conversion processes (pyrolysis, vis-
breaking, coking). The second part studies catalytic processes on acidic catalysts
(catalytic cracking, alkylation of isoalkanes, oligomerization). The third and fourth
parts analyze catalytic processes on metal oxides (hyd rofining, hydrotreating) and on
bifunctional catalysts (hydroisomerization, hydrocracking, catalytic reforming),
respectively.
The importance of all these processes resides in the fact that, when required,
they allow large variations in the proportion of the finished products as well as
improvement of their quality, as required by increasingl y stringent market demands.
The products of primary distillation are further processed by means of secondary
operations, some fractions being subjected to several processing steps in series.
Consequently, the total capacity of the conversion process es is larger than that of
the primary distillation.
The development of petroleum refining processes has made it possible to pro-
duce pro ducts, especially gasoline, of improved quality and also to produce synthet ic

chemical feedstocks for the industry. The petrochemical branch of the refining indus-
try generates products of much higher value than does the original refining industry
from which the feedstocks were derived.
Copyright © 2003 by Taylor & Francis Group, LLC
One should not overlook the fact that the two branches are of quite different
volume. A few percentage points of the crude oil processed in the refineries are
sufficient to cover the needs for feeds of the whole petrochemical and synthetic
organic industry and of a large portion of the needs of the inorganic chemicals
industry. The continuous development of new products will result in a larger fraction
of the crude oil than the approximately 10% used presently being consumed as
feedstocks for the chemical industry.
Hydrocarbons conversion processes supply hydrocarbons to the petrochemical
industry, but mainly they produce fuels, especially motor fuels and quality lubricat-
ing oils. The same basic processes are used in all these different applications. The
specific properties of the feedstocks and the operating parameters are controlled in
order to regulate the properties of the product for each application. In this book, the
processes are grouped by these properties, in order to simplify the presentation and
to avoid repetitions.
The presentation of each group of processes begins with the fundamentals
common to all the processes: thermodynamics, reaction mechanisms (including cat-
alysis when applicable), and, finally, process kinetics. In this manner, operating
parameters practiced in commercial units result as a logical consequence of earlier
theoretical discussion. This gives the reader a well-founded understanding of each
type of process and supplies the basis on which improvements of the process may be
achieved.
The presentation of commercial implementation is followed by a discussion of
specific issues pertaining to the design of the reaction equipment, which results in the
unity of the theoretical bases with the design solutions adopted for commercial
equipment and the quantitative aspects of implementation.
My warmest thanks to Prof. Sarina Feyer-Ionescu, to my son Prof. George

Raseev, and especially to my technical editor Dr. G. Dan Suciu, for their support in
preparing the English-language version of this book.
Serge Raseev
Copyright © 2003 by Taylor & Francis Group, LLC
Preface to the Romanian Edition
This book is the fruit of many years of work in the petrochemical industry, and in
research, and of university teaching. It sums up my technical and scientific back-
ground and reflects the concepts that I developed over the years, of the manner in
which the existing knowledge on chemical process technology—and especially on the
processing of hydrocarbons and petroleum fractions—should be treated and con-
veyed to others.
While initially the discipline of process technology was taught mainly by
describing the empirical information, it soon changed to a quantitative discipline
that considers the totality of phenomena that occur in the processes of chemical
conversion of industrial interest.
The objective of process technology as a discipline is to find methods for the
continual improvement of commercial processes. To this purpose it uses the latest
advances in chemistry, including catalysis, and applies the tools of thermodynamics
and kinetics toward the quantitative description of the processes. In this manner it
became possible to progress from the quantitative description provided by the reac-
tion mechanisms to the mathematic formulation for the evolution in time of the
processes.
In order to implement the chemical process on a commercial scale, a series of
additional issues need to be addressed: the effect of the operating parameters and the
selection of the optimal operating conditions, selection of the reactor type, the design
of the reaction equipment and of the other processing steps, the limitations due to
the heat and mass transfer, and the limitations imposed by the materials of construc-
tion.
Process technology thus becomes the convergence point of several theoretical
and applicative disciplines called upon to solve in an optimal manner the complex

interrelations among quite different sciences and phenomena (chemistry, hydraulics,
heat transfer, etc.). This situation requires a multifaceted competence and the full
understanding and control of the entire complex phenomenon that is the implemen-
Copyright © 2003 by Taylor & Francis Group, LLC
tation of chemical conversions in the conditions of the commercial units. Without it,
one cannot address the two basic questions about process technology: first, why the
commercial processes have been developed in the manner they are presently imple-
mented and second, how they can be continually improved.
In this manner, by mastering the complex phenomena involved, the process
engineer is fully equipped to answer the ‘‘why’’ and ‘‘how’’ questions, and will be
able to become one of the important driving forces of technical progress. This is the
concept that has guided me during my entire professional activity.
This book treats the conversion of hydrocarbons and petroleum fractions by
thermal and catalytic methods, while atte mpting to answer the ‘‘why’’ and ‘‘how’’
questions at the level of the current technical knowledge. In this manner, I hope to
contribute to the education of specialists who will advance continuing developments
in processing methods.
I am thankful to Mr. Gavril Musca and Dr. Grigore Pop for their help in
creating this book. My special gratitude goes to Prof. Sarina Feyer-Ionescu, for her
special contributions.
Serge Raseev
Copyright © 2003 by Taylor & Francis Group, LLC
Contents
Preface
Preface to the Romanian Edition
PARTITHERMALCONVERSIONPROCESSES
1 Thermodynamic Analysis of Technological Processes
1.1 Calculation of the Overall Thermal Effect
1.2 Equilibrium Calculations for a Wide Range of Process Condi tions
References

2 Theoretical Background of Thermal Processes
2.1 Thermodynamics of Thermal Processes
2.2 Reaction Mechanisms
2.3 Kinetics of Thermal Processes
2.4 Influence of Operating Conditions
References
3 Reaction Systems
3.1 Selection of Reactor Type
3.2 Reaction Systems
References
4 Industrial Implementation of Thermal Processes
4.1 Thermal Cracking at High Pressures and Moderate Temperatures
4.2 Coking
4.3 Pyrolysis
References
Copyright © 2003 by Taylor & Francis Group, LLC
5 Elements of Reactor Design
5.1 Design of the Reaction Section of Tubular Furnaces
5.2 Design of Soakers, Coke Drums, and Reaction Chambers
5.3 Systems Using Solid Heat Carrier
References
PARTIIPROCESSESONACIDCATALYSTS
6 Theoretical Basis of Catalytic Cracking
6.1 Process Thermodynamics
6.2 Cracking Catalysts
6.3 Reaction Mechanisms
6.4 Kinetics of Catalytic Cracking
6.5 Effect of Process Conditions
6.6 Catalyst Regeneration
References

7 Industrial Catalytic Cracking
7.1 Feed Selection and Pretreatment
7.2 Process History, Types of Units
7.3 Characteristic Equipment
7.4 Operation Aspects
7.5 Catalyst Demetallation
7.6 Yield Estimation
7.7 Economic Data
References
8 Design Elements for the Reactor–Regenerator System
8.1 Some Fluidization Problems
8.2 Fluidization with Solids Circulation
8.3 Reaction Systems
8.4 Catalyst Regeneration
8.5 Catalyst Entrainment
8.6 Catalyst Circulation, Transport Lines
References
9 Other Processes on Acid Catalysts
9.1 Oligomerization
9.2 Isoparaffin-Olefin Alkylation
References
PARTIIIPROCESSESONMETALLICCATALYSTS
10 Hydrofining and Hydrotreating
10.1 Process Thermodynamics
10.2 Catalysts
10.3 Reaction Mechanisms
10.4 Process Kinetics
Contents
Copyright © 2003 by Taylor & Francis Group, LLC
10.5 Effect of Process Parameters

10.6 Industrial Hydrofining
10.7 Industrial Hydrotreating
10.8 Design Elements for the Reactor System
References
PARTIVPROCESSESUSINGBIFUNCTIONALCATALYSTS
11 Hydroisomerization of Alkanes
11.1 Thermodynamics of Hydroisomer ization
11.2 Hydroisomerization Catalysts
11.3 Reaction Mechanism
11.4 Kinetics of Isomerization
11.5 Influence of Operating Parameters
11.6 Industrial Hydroisomerization of Lower Alkanes
11.7 Hydroisomerization of Lube Oils and Medium Fractions
References
12 Hydrocracking
12.1 Thermodynamics of Hydrocracking
12.2 Catalysts
12.3 Reaction Mechanisms
12.4 Kinetics of Hydrocracking
12.5 Effect of Process Parameters
12.6 Commercial Hydrocracking of Distillates
12.7 Residue Hydrocracking
12.8 Processes Using Slurry Phase Reactors
12.9 Production of High Grade Oils by Hydrocracking
References
13 Catalytic Reformin g
13.1 Thermodynamics
13.2 Catalysts
13.3 Reaction Mechanisms
13.4 The Kinetics of Catalytic Reforming

13.5 The Effect of Process Parameters
13.6 Catalyst Regeneration
13.7 Commercial Processes
13.8 Elements of Design and Modeling
13.9 Production of Aromatics
13.10 Dehydropolymerization of Lower Alkanes
References
14 Process Combinations and Complex Pr ocessing Schemes
14.1 Definition of Objectives
14.2 Evolution of the Range and Specifications of Products
14.3 Additional Resources
Contents
Copyright © 2003 by Taylor & Francis Group, LLC
14.4 Initial Data for the Selection of Refinery Configuration
14.5 Approach for Establishing the Configuration of a Modern
Refinery
References
Appendix Influence of the n=i-Alkanes Ratio in the Pyrolysis Feed
on the Ethene/Propene Ratio in the Products
Contents
Copyright © 2003 by Taylor & Francis Group, LLC
1
Thermodynamic Analysis of
Technological Processes
The thermodynamic study of technological processes has two objectives:
Determination of the overall thermal effect of chemical transformations that
take place in the industrial process
Determination of the equilibrium composition for a broad range of tempera-
tures and pressures in order to deduce optimum working conditions and
performances

The manner in which the two objectives are approached within the conditions of
chemical technology is different from the classical approach and requires the use of
the specific methodology outlined in this chapter.
1.1 CALCULATION OF THE OVERALL THERMAL EFFECT
In practical conditions under which technological processes operate , the main reac-
tion may be accompanied by secondary reactions. In many cases the transformation
is of such complexity that it cannot be exp ressed by a reasonable number of chemical
reactions.
When calculating the heat of reaction in such situations, in order to avoid the
difficulties resulting from taking into account all reactions many times in the calcu-
lation, simplified approaches are taken. Thus, one may resort to the approximation
of limiting the number of the reactions taken into consideration, or to take account
only the main reaction. Such approximations may lead to significant errors.
Actually, the exact value of the thermal effect can be calculated without having
to resort to such approximations. Since the thermal effect depends only on the initial
and the final state of the system (the independence of path, as stipulated by the
second principle of thermodynamics), it may be calculated based on the initial and
final compositions of the system, without having to take in account the reactions that
take place.
Copyright © 2003 by Taylor & Francis Group, LLC
Accordingly, the classic equations, which give the thermal effect of a chemical
reaction:
ÁH
0
rT
¼
X

p
ÁH

0
fT
À
X

r
ÁH
0
fT
ð1:1Þ
ÁH
0
rT
¼
X

r
ÁH
0
cT
À
X

p
ÁH
0
cT
ð1:2Þ
may be written under the form:
ÁH

rT
¼
X
n
e
ÁH
fT
À
X
n
i
ÁH
fT
ð1:3Þ
ÁH
rT
¼
X
n
i
ÁH
cT
À
X
n
e
ÁH
cT
ð1:4Þ
The heats of formation ÁH

f
and of combustion ÁH
c
for hydrocarbons and
organic compounds, which are of interest in studying petrochemical processes, are
given in thermodynamic data books [1,2]. The values are usually given for tempera-
ture intervals of 100 K, within which linear interpolation is accurate. Thus, the
calculations that use the heat capacities may be avoided.
Example 1.1 shows how to perform the calculations by means of relations (1.3)
and (1.4).
Example 1.1. Compute the overall thermal effect of an industrial deh ydrogena-
tion process of isopentane to isoprene at 6008C.
The composition of the streams at the inlet and outlet of the reactor is given in
Table 1.1. The coke composition by weight, is 95% carbon and 5% hydrogen.
The calculations of the heat of formation at the inlet and the outlet of the
reactor at 6008C are collected in Table 1.2.
Table 1.1
Component
Reactor inlet feed + recycle
(wt %)
Reactor Outlet
(wt %)
H
2
-1.0
CH
4
-0.6
C
2

H
6
-0.7
C
2
H
4
-0.7
C
3
H
8
-0.7
C
3
H
6
-1.4
C
4
H
10
0.3 1.2
C
4
H
8
-2.2
C
4

H
6
-0.2
i-C
5
H
12
79.3 55.8
i-C
5
H
10
16.6 17.1
C
5
H
8
0.8 12.1
n-C
5
H
12
1.8 0.8
n-C
5
H
10
1.7 1.7
1,3-C
5

H
8
-2.0
coke - 1.8
Copyright © 2003 by Taylor & Francis Group, LLC
According to Eq. (1.3), the overall thermal effect per unit mass (kg) of feed will
be:
ÁH
r;873
¼
X
n
e
ÁH
f 873
À
X
n
i
ÁH
f 873
¼À1599 ÀðÀ2243:1Þ¼644 kJ/kg
Since the process is performed at a temperature much above the critical point
and at low pressure, no deviations from the ideal state have to be considered.
In many cases it is convenient to express the thermal effect on the basis of the
reacted isopentane or of the formed isoprene.
For this example, accordi ng to Table 1.1, 793 À 558 ¼ 235g, isopentane reacts
and 121 À8 ¼ 113g, isoprene is formed. In these conditions, the thermal effect
expressed per mole of reacted isopentane is:
ÁH

r
¼
644
235
 72:15 ¼ 197:7 kJ/mole
and per mole of produced isoprene:
H
r
¼
644
113
 68:11 ¼ 388:2 kJ/mole
If only the main reaction:
i À C
5
H
12
¼ i ÀC
5
H
8
þ 2H
2
Table 1.2
Component
Heat of formation
ÁH
0
f
(kcal/mol) [2] Inlet Outlet

800
(K)
900
(K)
873=600
(K) (8C)
n
i
(mol/kg)
n
i
ÁH
0
f 873
(kcal/kg)
n
e
(mol/kg)
n
e
ÁH
0
f 873
(kcal/kg)
H
2
00 0 - -9.92 0
CH
4
À20.82 À21.15 À21.05 - - 0.37 À7.79

C
2
H
6
À24.54 À24.97 À24.85 - - 0.23 À5.72
C
2
H
4
9.77 9.45 9.54 - - 0.25 2.39
C
3
H
8
À30.11 À30.58 À30.45 - - 0.16 À4.87
C
3
H
6
0.77 0.35 0.46 - - 0.33 0.15
C
4
H
10
À36.41 À36.93 À36.79 0.05 À1.84 0.21 À7.73
C
4
H
8
À6.32 À6.84 À6.70 - - 0.39 À2.61

C
4
H
6
23.25 22.95 23.03 - - 0.04 0.92
i-C
5
H
12
À44.13 À44.65 À44.61 10.99 À489.16 7.73 À344.06
i-C
5
H
10
À13.45 À13.93 À13.80 2.37 À32.71 2.44 À33.67
C
5
H
8
14.16 13.82 13.91 0.12 1.67 1.78 24.76
n-C
5
H
12
À42.28 À42.85 À42.70 0.25 À10.68 0.11 À4.70
n-C
5
H
10
À12.23 À12.78 À12.63 0.24 À3.03 0.24 À3.03

1,3-C
5
H
8
14.17 13.73 13.85 - - 0.29 4.02
C000
Total
kcal/kg
kJ/kg
&
-
-
À535.75
À2243.1
-
-
À381.94
À1599.1
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is taken into account, then according to the Eq. (1.1) one obtains:
ÁH
r
¼ðÁH
f
Þ
C
5
H
8
À ÁH

f
Þ
C
5
H
12
¼ 13:91 ÀðÀ44:51Þ¼58:42 kcal=mol
¼ 244:59 kJ=mol
the value being the same whether expressed per mole of isopentane or of isoprene.
This example shows that large errors may result if the computation of the
overall thermal effect is not based on the real compositions of the inlet and outlet
streams of the reactor.
Eq. (1.4) makes it possible to compute the thermal effects by using the heats of
combustion. This is useful for the conversion of petroleum fractions of other feed-
stocks consisting of unknown components. In such cases it is usually more conve-
nient to perform the calculation in weight units, by modifying the terms n and ÁH
accordingly.
For liquid petroleum fractions, the heats of combustion may be determined by
using the graph of Figure 1.1 [3], from the known values of the specific gravity and
the characterization factor.
The characteriz ation factor of residues may be determined graphically from the
viscosity, by means of Figure 1.2 [3].
The heat of combustion of coke is determined experimentally or less precisely
on the basis of the elementary composition.
The heats of combustion of gaseous components may be found in data books
[1,2], or may be calculated from the heats of formation [2], by applying Eq. (1.1). For
hydrocarbons, this equation takes the form:
ðÁH
a
Þ

C
n
H
m
¼ nðÁH
f
Þ
CO
2
þ
m
2
ðÁH
f
Þ
H
2
O
ÀðÁH
f
Þ
C
n
H
m
ð1:5Þ
This heat of combustion of gases must be brought to the same reference state as
that of liquid fractions, i.e. 158C and liquid water. For these conditions, Eq. (1.5)
becomes:
ðÁH

a
Þ
C
n
H
m
¼À393:77n À 143:02m ÀðÁH
f
Þ
C
n
H
m
ð1:6Þ
It must be noted that Eq. (1.6) gives the heat of combustion in thermodynamic
notation, expressed in kJ/mole. Figure 1.1 gives the heat of combustion in technical
notation, expressed in kJ/kg.
An illustration of these calculations is given in Example 1.2.
Example 1.2. Calculate the thermal e ffect of the processing of a vacuum residue
by visbreaking. The composition of the produced gases is given in Table 1.3. The
yields and the characterization factors, K
UOP
for the feed and the fuel oil were
obtained from Table 1.4.
The characterization factor and the specific gravities were used to determine
the heats of combustion for all the liquid fraction from Figure 1.1.
SOLUTION. By introducing the values of the heats of combustion from Tables
1.3 and 1.4 into Eq. (1.4), one obtains:
Q
r

¼ 43,645 Àð0:0244 Â 51,319 þ 0:1166 Â46,827 þ0:859 Â 43,233Þ
¼À204 kJ/kg
Calculation of the thermal effects for a specific reaction, usually a small num-
ber obtained as the difference of heats of combustion, usually larger numbers, is
Copyright © 2003 by Taylor & Francis Group, LLC
associated with large errors, unless the determination of the values of the heats of
combustion was made with high accuracy. This fact is especially valid for liquid
fractions, for which the graphical determination of the combustion heats may give
errors. In order to obtain exact results, the determination of the heats of combustion
of the liquid fractions by direct calori metric methods is recommended.
Figure 1.1 Heat of combustion of petroleum fractions. Final state: gaseous CO
2
and liquid
water at 158C.
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Graphs and empirical relations are given [4–7] for the calculation of the ther-
mal effect in the petroleum refining processes. The values calculated by their means
and the numerical values given in the literature must be critically analyzed, taking
into account the characteristics of the feed, the operating conditions, and the con-
version. Only values that refer to comparable feeds and conditions should be used in
computations.
For the process of thermal cracking, the use of equation [8] is recommended:
ÁH ¼ 117,230
M
a
À M
p
M
a
 M

p
ð1:7Þ
Figure 1.2 K
UOP
as function of the kinematic viscosity and density.
Copyright © 2003 by Taylor & Francis Group, LLC
The calculated ÁH is expressed in kJ/kg of feed. The sign is that used in the
thermodynamic notation.
Using the data from example 1.2 (see the Table 1.4), this equation gives:
ÁH ¼ 117,230
440 À 253
440 Â 253
¼ 197 kJ/kg
which gives the same result as the heats of combustion method.
In the literature, the thermal effect of reactions is often expressed per unit mass
of main product and not per unit mass of feed. In some cases, this way of expression is
useful, since the thermal effect thus becomes actually independent of conversion [5].
1.2 EQUILIBRIUM CALCULATIONS FOR A WIDE RANGE OF
PROCESS CONDITIONS
The computation of the equilibrium compositions for a wide range of process con-
ditions (temperatures and pressures) has the purpose of identifying practical operat-
ing conditions that will optimize the performance of the process. Depending on the
specifics of the process, the problem may be limited to the calculation of the equili-
brium of the main reaction, or may be extended also to the secondary reactions.
In all cases, the composition at equilibrium, calculated on basis of thermody-
namic principles, represents the maximum conversion that is possible to achieve in
the given conditions. There is however no certainty that such performance will be
actually obtained. Nonthermodynamic factors, such as the reaction rate and the
residence time within the reactor will determine how close the actual performance
will approach the theoretical one.

The use of classical methods for computing equilibrium compositions for the
large number of temperature–pressure values needed for thermodynamic analysis of
a broad range of process conditions necessitates a large number of calculations. A
Table 1.3
Component
Composition
(wt %)
ðÁH
0
288
Þ
C
(kJ/kg)
(ÁH
0
283
Þ
C
fraction
(kJ/kg)
CH
4
22.32 À55,540 À12,396
C
2
H
6
18.84 À51,910 À9,780
C
3

H
8
4.57 À50,330 À2,300
C
3
H
6
20.56 À50,380 À10,358
i-C
4
H
12
7.97 À48,950 À3,901
n-C
4
H
12
2.20 À49,390 À1,093
i-C
4
H
10
9.20 À49,540 À4,558
n-C
4
H
10
1.85 À48,170 À891
1-C
4

H
8
3.50 À48,470 À1,696
cis-2-C
4
H
8
0.55 À48,340 À266
trans-2-C
4
H
8
2.37 À48,270 À1,144
C
4
H
6
2.01 À47,020 À945
C
5
+ 4.06 À49,050 À1,991
Total (ÁH
0
288
Þ
C
fraction À51,319 kJ/kg
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method elaborated by the author many years ago [9] provides a simple method for
the calculation and graphical representation of the equilibrium. The method is out-

lined below.
For any chemical reaction, the standard free energy is expressed by the rela-
tion:
ÁG
0
T
¼ ÁH
0
T
À TÁS
0
T
ð1:8Þ
Figure 1.3 Thermodynamic equilibrium of propene dimerization. Parameter: conversion x
as %.
Copyright © 2003 by Taylor & Francis Group, LLC
and as function of the equilibrium constant, by the expression:
At equilibrium, ÁG
T
¼ 0 and ÁG
0
T
¼ RT ln K
a
(1.9)
Assuming that the substances participating in the reaction do not deviate from
the behaviour of ideal gas, the equilibrium constant may be expressed by the rela-
tion:
K
a

¼ K
p
¼
’
i
ðxÞ
p
Án
ð1:10Þ
Here, ’iðxÞ is a function of the conversion at equilibrium x. The form of this function
depends on the stoechiometry of the reaction but is independent on the nature of the
substances that participate in the reaction.
Equating Eqs. (1.8) and (1.9), and replacing K
a
with the expression (1.10),
dividing the right and left sides by TÁH
0
T
, and effecting some elementary transfor-
mations, one obtains:
1
T
¼
RÁn
ÁH
0
T
ln p þ
ÁS
0

T
À R ln ½’
i
ðxÞ
ÁH
0
T
ð1:11Þ
For a given chemical reaction and a temperature range of 200–3008C, which is
sufficient for a process analysis, ÁH
0
T and ÁS
0
T can be considered constants. In
these conditions, using as coordinates log p and 1=T, the Eq. (1.11) corresponds to a
family of parallel straight lines with the equilibrium conversion x as parameter.
Simple plots are obtained, by writing:
2:3 RÁn ¼ b
and
R ln½’iðxÞ ¼ d
Equation (1.11) becomes:
1
T
¼
b
ÁH
0
T
log p þ
ÁS

0
T
À d
ÁH
0
T
ð1:12Þ
The parameter b depends only on the stoechiometric form of the chemical
reaction. Parameter d depends both on the stoechiometry and on x, the conversion
at equilibrium. Both b and d are independent of the nature of the chemical sub-
Table 1.4
Yields
(wt %) Density
Viscosity
(cSt)
Characterization
factor
(K
UOP
Þ
Thermal
effect
(kJ/Kg)
Feed 1.0000 0.989 1,000 11.38 43,645
Products
gases 0.0244 - - - 51,319
gasoline 0.1166 0.760 - 11.9 46,827
fuel oil 0.8590 1.000 630 11.1 43,233
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stances that take part in the reaction and have been calculated [9] for chemical

reactions of various stoechiometric forms (Table 1.5).
For reactions proceeding in the opposite direction, the sign of the constant s b
and d must be changed, and the meaning of the conversion x reversed (for example
x ¼ 0:95 from the table will have the meaning x ¼ 0:05 for the reverse reaction).
Since in plots of log p versus 1=T the straight lines of constant conversion are
parallel, it is enough to calculate one point of each line and to determine the slope of
all the straight lines by calculating just one point for an y other pressure. Thus, the
whole family of lines may be obtained by selecting a pressur e of 1 bar for the
determining one point on each straight line and a pressure of either 10 bar or 0.1
bar for which one calculates the one point needed to determine the slope of all lines.
For these values of the pressure, the relation (1.12) becomes:
p ¼ 1bar
1
T
¼
ÁS
0
T
À d
ÁH
0
T
p ¼ 10 bar
1
T
¼
ÁS
0
T
À d þb

ÁH
0
T
p ¼ 0:1bar
1
T
¼
ÁS
0
T
À d Àb
ÁH
0
T
ð1:13Þ
The calculati on is illustrated by the Example 1.3.
Example 1.3. For the reaction:
2C
3
H
6
Ð C
6
H
12
determine the equilibrium graph for pressures comprised between 1 and 100 bar and
temperatures between 450–8008C.
SOLUTION. The reaction corresponds to the form 2A $ B, in Table 1.5.
Using the thermodynamic constants for 900 K [2] and taking mean values for i-
hexenes, it results:

ÁH
0
900
¼À20020 À 2  350 ¼À20,720 cal/mol
ÁS
0
900
¼ 143:65 À2  89:75 ¼À35:85 cal/mol K
Using the Eq. (1.13) corresponding to the pressure of 1 bar and the values of the
constant d from the Table 1.5, following pairs of values are obtained:
X 0.01 0.05 0.10 0.20 0.3 0.40 0.50 0.60 0.70 0.80 0.90 0.95 0.99
ð1=TÞÂ10
3
1.22 1.38 1.46 1.54 1.60 1.65 1.70 1.76 1.82 1.90 2.04 2.17 2.50
For the pressure of 10 bar and x ¼ 0:5 and using the constant b from the Table
1.5, one obtains, according to the Eq. (1.13):
1=T ¼ 1:48 Â10
À3
By using the obtained values, the equilibrium is represented in Figure 1.3.
Note that for temperature ranges of not more than 200–3008C that intervene in
the analysis of industrial processes, the variations with the temperature of ÁH
0
and
ÁS
0
may be neglected, without consequently introducing any practical errors.
Deviations from ideal conditions are important near the critical state and do
not affect the results at temperatures much higher than the critical, as used in the
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TABLE 1.5 Values of the Constants b and d for Various Reaction Stoichiometric Types

Reaction form b
x = equilibrium conversion
0.99 0.95 0.90 0.80 0.70 0.60 0.50 0.40 0.30 0.20 0.10 0.05 0.01
A$B 0 9.13 5.85 4.37 2.75 1.68 0.81 0 À0.81 À1.70 À2.75 À4.37 À5.85 À9.13
2A$B 4.57 15.85 9.14 6.37 3.56 1.84 0.54 À0.57 À1.61 À2.67 À3.90 À5.66 À7.18 À10.48
3A$B 9.14 16.38 11.41 7.69 3.94 1.79 0.23 À1.04 À2.19 À3.34 À4.62 À6.40 À7.91 À11.29
A+B$C 4.57 18.30 11.90 9.13 6.31 4.60 3.29 2.18 1.14 0.08 À1.15 À2.89 À4.42 À7.74
A+B$2C 0 21.01 14.45 11.48 8.26 6.12 4.36 2.75 1.14 À0.61 À2.75 À5.98 À9.13 À15.54
A+2B $C 9.14 25.07 15.20 11.48 7.73 5.58 4.03 2.75 1.60 0.46 À 0.83 À2.62 À4.17 À7.53
A+3B $C 13.72 30.65 18.00 13.10 8.61 6.14 4.42 3.04 1.83 0.63 À0.58 À2.49 À4.05 À7.44
A+B$ C+D 0 18.25 11.70 8.73 5.51 3.37 1.61 0 À1.61 À3.37 À5.51 À8.73 À11.70 À18.25
A+2B $2C 2.29 33.42 19.07 14.76 10.20 7.41 4.77 3.20 0.18 À0.69 À3.03 À6.38 À9.45 À16.10
A+3B $2C 4.57 31.01 22.71 17.21 11.60 8.16 5.54 3.32 1.22 À1.74 À3.55 À6.80 À9.88 À16.53
A+2B $C+D 4.57 26.04 16.33 12.03 7.52 4.66 2.42 0.443 À1.45 À3.44 À5.76 À9.15 À12.25 À18.87
A+3B $C+D 9.14 32.81 20.01 14.47 8.84 5.40 2.80 0.572 À1.51 À3.64 À6.08 À9.56 À12.64 À19.28
A+4B $C+D 13.72 38.92 23.10 16.40 9.78 5.83 2.99 0.581 À1.63 À3.85 À6.37 À9.90 À13.00 À19.66
A+5B $C+D 18.28 44.54 25.79 18.00 10.50 6.19 3.09 0.538 À1.77 À4.06 À6.62 À10.19 À13.31 À19.98
A$ B+4C À18.28 7.34 3.99 2.418 0.594 À0.744 À1.96 À3.22 À4.66 À6.50 À9.20 À14.32 À20.09 À35.03
2C $A+5B À18.28 17.22 10.56 7.44 3.87 1.30 À0.99 À3.29 À5.84 À8.95 À13.25 À20.76 À28.54 À47.3
Copyright © 2003 by Taylor & Francis Group, LLC
thermal and catalytic processes in petroleum refining. If corrections as such are
however needed, they can be accomplished by using the methods elaborated in the
original work [9].
This method of equilibrium representation will be widely used in the following
chapters for the analysis of practical process conditions.
REFERENCES
1. FD Rossini, KS Pitzer, RL Arnett, RM Braun, GC Pimentel. Selected Values of Physical
and Thermodynamical Properties of Hydrocarbons and Related Compounds, Pittsburgh:
Carnegie Press, 1953.
2. DR Stull, EF Westrum Jr., GC Sinke. The Chemical Thermodynamics of Organic

Compounds, New York: John Wiley, 1969.
3 P Wuithier. Le petrole raffinage et genie chimique, Vol. 1, 2nd Edition, Technip, Paris,
1972.
4. OA Hougen, KM Watson. Chemical Process Principles, vol. 1. New York: John Wiley,
1947.
5. S Raseev. Procese distructive de prelucrate a titeiului, Editura tehnica, Bucuresti, 1964.
6. G Suciu, R Tunescu. Editors. Ingineria prelucrdrii hidrocarburilor, Editura tehnica,
Bucuresti, 1973.
7. WL Nelson. Petroleum Refinery Engineering, New York: McGraw-Hill Book Co., 1958.
8. IH Hirsch, EK Ficher. The Chemistry of Petroleum Hydrocarbons, Vol. 2, Chap. 23, New
York: Reinhold Publishing Co., 1955.
9. S Raseev, Stud Cercet Chim 5 (2): 267, 285, 1957.
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2
Theoretical Background of Thermal
Processes
Thermal processes are chemical transformations of pure hydrocarbons or petroleum
fractions under the influence of high temperatures. Most of the transformations are
cracking by a radicalic mechanism.
The thermal processes comprise the following types of industrial processes:
PYROLYSIS (STEAM CRACKING). Main purpose: the production of ethene and s
propene for the chemical industry. The pyrolysis of liquid feed stocks, leads also to
butadiene, isoprene, and C
6
-C
8
aromatics.
Characteristic for the pyrolysis process are tempe ratures of about 900–9508C
and low pressures (less than 5 bar).
At the present, pyrolysis is the most important thermal process.

VISBREAKING. Used for producing fuel oils from heavy residues.
The process is characterized by relatively mild temperatures (around 5008C)
and pressures, general ly of 15–20 bar. Recently, processes at much lower pressures,
sometimesatmospheric,werealsodeveloped(Section4.2.1).
Of similar type was the old-time cracking process for gasoline production. It was
realized at relatively low temperatures (495–5108C) and high pressure (20–40 bar).
COKING. Used for producing petroleum coke from heavy residues.
There are two types of coking processes: the delayed coking realized at about
4908C, and a 5–15 bar in coke drums, and fluid coking realized at about 5708Cand
2–3 bar, in a fluidized bed.
Of some importance is the production of needle coke, which is used for the
production of electrodes especially for electrometallurgy processes (e.g. a luminum).
2.1 THERMODYNAMICS OF THERMAL PROCESSES
Thermodynamic calculations show that the thermal decomposition of alkanes of
higher molecular weight may take place with high conversions even at relatively
low temperatures. Thus, n-decane may convert to over 90% to form pentene and
pentane at 3508C and 1 atmospheric pressure.
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The great number of parallel–successive reactions that may take place results in
the final product distribution being controlled by the relative rates of the reactions
that take place and not by the thermodynamic equilibrium.
The situation is different for the lower alkanes. Thus, in order to achieve a
conversion of 90% in the decomposition of butane to ethene and ethane at a pressur e
of 2 bar, a temperature of near 5008C is required (Figure 2.1). In these conditions the
dehydrogenation reaction reaches a conversion at equilibrium of only about 15%
(Figure 2.2). This makes possible a comparison of the two possible reaction path-
ways.
Figure 2.1 The thermodynamic equilibrium for reaction C
4
H

10
Ð C
2
H
6
+C
2
H
4
.
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