Thermal and Catalytic Processes in Petroleum Refining
Library of Congress Cataloging-in-Publication Data A catalog record for this book is available from the Library of Congress.
Originally published in Romanian as Conversia Hidrocarburilor in 3 volumes, 1996–1997.
ISBN: 0-8247-0952-7 This book is printed on acid-free paper. Headquarters Marcel Dekker, Inc. 270 Madison Avenue, New York, NY 10016 tel: 212-696-9000; fax: 212-685-4540 Eastern Hemisphere Distribution Marcel Dekker AG Hutgasse 4, Postfach 812, CH-4001 Basel, Switzerland tel: 41-61-260-6300; fax: 41-61-260-6333 World Wide Web The publisher offers discounts on this book when ordered in bulk quantities. For more information, write to Special Sales/Professional Marketing at the headquarters address above. Copyright # 2003 by Marcel Dekker, Inc. All Rights Reserved. Neither this book nor any part may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, microfilming, and recording, or by any information storage and retrieval system, without permission in writing from the publisher. Current printing (last digit): 10 9 8 7 6 5 4 3 2 1
PRINTED IN THE UNITED STATES OF AMERICA
To my dear wife Irena 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Recent developments in petroleum processing required the complete revision of some of the chapters, the elimination of outdated material and bringing up to date the processes in which the technology was significantly improved.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 first 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 (hydrofining, 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 increasingly 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 processes is larger than that of the primary distillation.
The development of petroleum refining processes has made it possible to pro- duce products, especially gasoline, of improved quality and also to produce synthetic 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.
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 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- 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 attempting 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
Contents
Contents
Contents
Contents
Appendix Influence of the n/i-Alkanes Ratio in the Pyrolysis Feed on the Ethene/ Propene Ratio in the Products
In order to evaluate the influence of the n/i-alkanes ratio in the feed on the ethene/ propene ratio in the pyrolysis effluent, three representative hydrocarbons were selected: n-octane, 4-methyl-heptane and 2,5-dimethyl-hexane.
For these hydrocarbons, the product’s composition was calculated using the F.O. Rice method at a temperature of 1100 K, which is typical for those used in pyrolysis.
For n-octane, the complete decomposition of the radicals formed by different substitution reactions gives: ðaÞ
ðbÞ ðcÞ
ðdÞ ðeÞ
ðfÞ The dissociation energies admitted by Rice for the bonds between hydrogen and the primary, secondary, and tertiary carbons atoms (92.0, 90.8, and 88.0 kcal/ mol), give for 1100 K, according to Eq. (2.4): r r
sec tert : :
¼ 1 7 ¼ 5
7 r r
prim prim
Each of the two last reactions have two possible paths for the decomposition of the initial radicals (c,d and e,f). The relative rates of these may be calculated using the dissociation energies given in It results:
138 ; 000 113 ; 000
r
d 1100 R
¼ e ¼ 15 :
6 r
c 130 ; 000 121 ; 000
r
f 1100 R :
¼ e ¼ 2
5 r
e
The relative rates of the reactions (a)-(f) will be:
Reactions Relative rate Conversion per mol n-octane (a) 0.22 1 6 ¼ 6
(b)
1 :7 4 ¼ 6:8
0.26
1 (c)
0.02
1 :7 4 ¼ 0:436
15 :6
14 :6 (d)
0.24
1 :7 4 ¼ 6:364 15 :6
1 (e)
0.10
1 :7 4 ¼ 2:72
2 :5
1 :5 (f)
0.16
1 :7 4 ¼ 4:08 2 :5
Taking into account the number of molecules of ethene and propene formed in the reactions (a)-(f), it results that for 1 mol of reacted n-octane the following amounts of ethene and propene will be obtained:
— 2.14 mol ethene and 0.22 4 + 0.26 2 + 0.24 2 + 0.10 + 0.16 — 0.26 mol propene The molar ratio ethene/propene = 8.23 For 4-methyl-heptane the different substitution reactions give:
ðaÞ ðbÞ
ðcÞ ðdÞ ðeÞ ðfÞ ðgÞ
ðhÞ Estimating the relative rates of the reactions (a)/(b), (d)/(e), and (f)/(e) as in the pyrolysis of n-octane, it results:
Conversion per mol Reactions Relative rate 4-methyl-heptane
1 (a)
0.07 1 6 ¼ 2
3
2 (b)
0.14 1 6 ¼ 4
3
1 (c) :7 4 ¼ 6:8
0.24
1 (d)
0.02
1 :7 4 ¼ 0:436 15 :6
13 (e) :6
0.21
1 :7 4 ¼ 5:928
15 :6
1 (f)
0.02
1 :7 4 ¼ 0:436 15 :6 (g)
5 :7 1 ¼ 5:7
0.20 (h) 0.10 1 3 ¼ 3
The number of moles of ethene and propene formed from one mole of 4- methyl-heptane decomposed will be: 0.07 + 0.14 2 + 0.24 + 0.21 + 0.20 + 0.10 = 1.10 moles ethene and 0.14 + 2 0.24 = 0.62 moles of propene The molar ratio ethene/propene = 1.77 For 2,5-dimethyl-hexane, a similar reasoning gives:
ðaÞ ðbÞ
ðcÞ ðdÞ
ðeÞ to propene and atomic hydrogen, since in this case the dissociation energy is only 167 kJ/mol. Analogously, in this example it was considered that the ethyl radical is decomposed completely to ethene and atomic hydrogen. Both of these considera- tions correspond to the temperature and pressure conditions that are specific to pyrolysis. The relative rates of the reactions (a)-(e) will be:
Conversion per mol Reactions Relative rate 2,5-dimethyl-hexane 1 (a)
0.04 1 12 ¼ 0:77 15 :6
14 :6 (b) 0.52 1 12 ¼ 11:23
15 :6 (c) 1 5.7 = 5.7
0.26
1 (d) 0.01 1 4 ¼ 0:26
15 :6
14 :6 (e) 0.17 1 4 ¼ 3:74
15 :6
The number of moles of ethene and propene formed from a mol of 2,5- dimethyl-hexane will be: 0.52 moles ethene and 0.52 + 2 + 0.26 + 0.17 = 1.47 moles propene The molar ratio ethene/propene = 0.35 Finally, it results:
Feed C H /C H ratio in the effluent
2
4
3
8 n
- C H
8.23
8
18
2-C H CH
1.77
7
15
3 2,5-C H (CH )
0.35
6
12
3
2
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 expressed 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. 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.
Accordingly, the classic equations, which give the thermal effect of a chemical reaction:
X X ¼
ð1:1Þ H rT p H fT r H fT
X X ¼
ð1:2Þ
rT r cT p cT
H H H may be written under the form:
X X ¼ n n ð1:3Þ
H rT e H fT i H fT
X X ¼ n n ð1:4Þ
rT i cT e cT
H H H for hydrocarbons and The heats of formation H f and of combustion H c 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 dehydrogena- 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 1.1
Reactor inlet feed + recycle Reactor Outlet Component (wt %) (wt %)
- H
1.0
2 CH -
0.6
4 C 0.7 - H
2
6
- C H
0.7
2
4
- C H
0.7
3
8 C H
1.4 -
3
6 C H
0.3
1.2
4
10 C H
2.2 -
4
8
- C H
0.2
4
6 i -C H
79.3
55.8
5
12 i -C H
16.6
17.1
5
10 C H
0.8
12.1
5
8 n -C H
1.8
0.8
5
12 n -C H
1.7
1.7
5
10 1,3-C H
2.0 -
5
8
- coke
1.8 Table 1.2
Heat of formation (kcal/mol) [2] Inlet Outlet H f 800 900 873=600 n n n n i i e e
H f 873 H f 873 Component (K) (K) (mol/kg) (kcal/kg) (mol/kg) (kcal/kg) (K) (8C)
- 9.92 - H
2
20.82
21.15
21.05 - - CH
0.37
7.79
4 C H
24.54
24.97
24.85 0.23 - 5.72 -
2
6
9.77
9.45
9.54 - - C H
0.25
2.39
2
4
30.11
30.58
30.45 - C - H
0.16
4.87
3
8 C - - H
0.77
0.35
0.46
0.33
0.15
3
6 C H
36.41
36.93
36.79
0.05
1.84
0.21
7.73
4
10 C H
6.32
6.84
6.70 0.39 - -
2.61
4
8
23.25
22.95
23.03 - C H 0.04 0.92 -
4
6 i -C H
44.13
44.65
44.61 10.99 489.16 7.73 344.06
5
12 i -C H
13.45
13.93
13.80
2.37
32.71
2.44
33.67
5
10 C H
14.16
13.82
13.91
0.12
1.67
1.78
24.76
5
8 n -C H
42.28
42.85
42.70
0.25
10.68
0.11
4.70
5
12 n -C H
12.23
12.78
12.63
0.24
3.03
0.24
3.03
5
10 1,3-C H
14.17
13.73
13.85
0.29 4.02 - -
5
8
- C
kcal/kg 535.75 381.94 - -
Total - kJ/kg 2243.1 1599.1 -
According to Eq. (1.3), the overall thermal effect per unit mass (kg) of feed will be:
X X ¼ n n ¼ 1599 ð 2243:1Þ ¼ 644 kJ/kg
H r; 873 e H f 873 i H f 873 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, according to 793 558 ¼ 235g, isopentane reacts and 121 8 ¼ 113g, isoprene is formed. In these conditions, the thermal effect expressed per mole of reacted isopentane is:
644 ¼ 72:15 ¼ 197:7 kJ/mole
r
H 235 and per mole of produced isoprene:
644 H ¼ 68:11 ¼ 388:2 kJ/mole
r
113 If only the main reaction: i C H ¼ i C H þ 2H
5
12
5
8
2 is taken into account, then according to the Eq. (1.1) one obtains: Þ Þ ¼ 13:91 ð 44:51Þ ¼ 58:42 kcal=mol
H r f H f C H ¼ ð H 5 12 C5H8
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 H accordingly.
For liquid petroleum fractions, the heats of combustion may be determined by using the graph of [3], from the known values of the specific gravity and the characterization factor.
The characterization factor of residues may be determined graphically from the viscosity, by means of [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: m
Þ Þ þ Þ Þ ð1:5Þ
a f f f
ð H C H ¼ nð H CO ð H H O ð H C H n m 2 2 n m
2 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:
Þ ¼ 393:77n 143:02m Þ ð1:6Þ
a C H f C H
ð H ð H n m n m 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 effect of the processing of a vacuum residue by visbreaking. The composition of the produced gases is given The yields and the characterization factors, K for the feed and the fuel oil were
UOP obtained from
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 ¼ 43,645 ð0:0244 51,319 þ 0:1166 46,827 þ 0:859 43,233Þ
r
¼ 204 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 Heat of combustion of petroleum fractions. Final state: gaseous CO and liquid
Figure 1.1
2 water at 158C.
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 calorimetric methods is recommended.
K as function of the kinematic viscosity and density.
Figure 1.2
UOP
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: M M
a p
ð1:7Þ H ¼ 117,230
M M
a p Table 1.3
Composition Þ ( Þ fraction ð H 288 283 C H C
Component (wt %) (kJ/kg) (kJ/kg) CH 22.32 55,540 12,396
4 C H 18.84 51,910 9,780
2
6 C H 4.57 50,330 2,300
3
8 C H 20.56 50,380 10,358
3
6 i -C H 7.97 48,950 3,901
4
12 n -C H 2.20 49,390 1,093
4
12 i -C H 9.20 49,540 4,558
4
10 n -C H 1.85 48,170 891
4
10 1-C H 3.50 48,470 1,696
4
8 cis -2-C H 0.55 48,340 266
4
8 trans -2-C H 2.37 48,270 1,144
4
8 C H 2.01 47,020 945
4
6 4.06 49,050 1,991 + C
5 Total ( fraction 51,319 kJ/kg
Þ H 288 C
The calculated H is expressed in kJ/kg of feed. The sign is that used in the thermodynamic notation. Using the data from example 1.2 this equation gives: 440 253
¼ 197 kJ/kg H ¼ 117,230 440 253 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 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
Thermodynamic equilibrium of propene dimerization. Parameter: conversion x
Figure 1.3 as %.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: ð1:8Þ
G T ¼ H T T S T Table 1.4
Characterization Thermal
Yields Viscosity factor effect
(wt %) Density (cSt) (K (kJ/Kg)
ÞUOP Feed 1.0000 0.989 1,000 11.38 43,645 Products gases
51,319 - - - 0.0244
gasoline 0.1166 - 0.760 11.9 46,827 fuel oil 0.8590 1.000 630 11.1 43,233and as function of the equilibrium constant, by the expression: ¼ RT ln K (1.9)
T T a
At equilibrium, G ¼ 0 and G 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:
’ ðxÞ
i
K ¼ K ¼ ð1:10Þ
a p n
p 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 with the expression (1.10),
a
, and effecting some elementary transfor-
T
dividing the right and left sides by T H mations, one obtains:
1 R ln ½’ ðxÞ S T i
R n ¼ ln p þ
ð1:11Þ T H H
T T
For a given chemical reaction and a temperature range of 200–3008C, which is T T can be considered constants. In sufficient for a process analysis, H and S 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 R n ¼ b and R ln½’iðxÞ ¼ d
Equation (1.11) becomes: 1 b d
T
S ¼ log p þ
ð1:12Þ T H T H T 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- stances that take part in the reaction and have been calculated [9] for chemical reactions of various stoechiometric forms .
For reactions proceeding in the opposite direction, the sign of the constants 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 any other pressure. Thus, the whole family of lines may be obtained by selecting a pressure 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: 1 d S T p ¼ 1bar ¼
T H
T
1 d þ b
T
S ð1:13Þ p ¼ 10 bar ¼
T H T 1 d b
T
S p ¼ 0:1 bar ¼ T H T The calculation is illustrated by the Example 1.3.
Example 1.3. For the reaction:
2C H Ð C H
3
6
6
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: ¼ 20020 2 350 ¼ 20,720 cal/mol
H 900 ¼ 143:65 2 89:75 ¼ 35:85 cal/mol K
900
S 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
3 ð1=TÞ 10 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):
3
1=T ¼ 1:48 10 Note that for temperature ranges of not more than 200–3008C that intervene in and the analysis of industrial processes, the variations with the temperature of H may be neglected, without consequently introducing any practical errors. S
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
TABLE 1.5
9.88
11.60
8.16
5.54
3.32
1.22
1.74
3.55
6.80
16.53 A+2B $C+D 4.57 26.04 16.33 12.03
9.45
7.52
4.66 2.42 0.443
1.45
3.44
5.76
9.15
12.25
18.87 A+3B $C+D 9.14 32.81 20.01 14.47
16.10 A+3B $2C 4.57 31.01 22.71 17.21
6.38
5.40 2.80 0.572
5.51
4.05
7.44 A+B $ C+D 18.25 11.70
8.73
5.51
3.37
1.61
1.61
3.37
8.73
3.03
11.70
18.25 A+2B $2C 2.29 33.42 19.07 14.76
10.20
7.41
4.77
3.20
0.18
0.69
8.84
1.51
0.58
7.44
3.22
4.66
6.50
9.20
14.32
20.09
35.03
2C $A+5B 18.28 17.22 10.56
3.87
7.34 3.99 2.418 0.594 0.744
1.30
0.99
3.29
5.84
8.95
13.25
20.76
28.54
1.96
18.28
3.64
6.37
6.08
9.56
12.64
19.28 A+4B $C+D 13.72 38.92 23.10 16.40
9.78
5.83 2.99 0.581
1.63
3.85
9.90
19.98 A $ B+4C
13.00
19.66 A+5B $C+D 18.28 44.54 25.79 18.00
10.50
6.19 3.09 0.538
1.77
4.06
6.62
10.19
13.31
2.49
0.63
Values of the Constants b and d for Various Reaction Stoichiometric Types Reaction form b x = equilibrium conversion
0.54
4.37
5.85
9.13
2A $B 4.57 15.85
9.14
6.37
3.56
1.84
0.57
1.70
1.61
2.67
3.90
5.66
7.18
10.48
3A $B 9.14 16.38 11.41
7.69
2.75
0.81
1.79
0.30
0.99
0.95
0.90
0.80
0.70
0.60
0.50
0.40
0.20
0.81
0.10
0.05
0.01 A $B
9.13
5.85
4.37
2.75
1.68
3.94
0.23
1.83
1.60
2.75
5.98
9.13
15.54 A+2B $C 9.14 25.07 15.20 11.48
7.73
5.58
4.03
2.75
0.46
1.14
0.83
2.62
4.17
7.53 A+3B $C 13.72 30.65 18.00 13.10
8.61
6.14
4.42
3.04
0.61
2.75
1.04
4.60
2.19
3.34
4.62
6.40
7.91
11.29 A+B $C 4.57 18.30 11.90
9.13
6.31
3.29
4.36
2.18
1.14
0.08
1.15
2.89
4.42
7.74 A+B $2C 21.01 14.45 11.48
8.26
6.12
47.3 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.
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 temperatures 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, generally of 15–20 bar. Recently, processes at much lower pressures, sometimes atmospheric, were also developed (Section 4.2.1).
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 5708C and 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. aluminum).
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.
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 pressure 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%
. This makes possible a comparison of the two possible reaction path- ways.
Figure 2.1 The thermodynamic equilibrium for reaction C H Ð C H + C H .4
10
2
6
2
4
Ð C
Figure 2.2 The thermodynamic equilibrium for reaction C H H + H4
10
4
8 2.
The products obtained from the thermal decomposition of ethane, propane, and ethene, are those one would expect from dehydrogenation reactions: C H Ð C H þ H
ðaÞ
2
6
2
4
2 C H Ð C H þ H
ðbÞ
3
8
3
6
2 C H Ð C H þ H
ðcÞ
2
4
2
2
2 Many studies [1,2,109–111] reached the conclusion that in the pyrolysis process, irrespective of the feedstock used, the values of the concentration ratios ethene/ethane, propene/propane and acetylene/ethene in the reactor effluent are close to those corresponding to the equilibrium of the reactions (a)–(c), Figures (2.3),
Even if such assertions are only approximately accurate, the equilibrium of these reactions is of high interest for determining optimum operating conditions in pyrolysis.
Figure 2.3 The thermodynamic equilibrium for reaction C H Ð C H + H .2
6
2
4
2
Ð C
Figure 2.4 The thermodynamic equilibrium for reaction C H H + H3
8
3
6 2.