PetroleumThermal and catalytic processes in petroleum refining 2003

giáo trình mô tả về xúc tác trong công nghệ dầu khí và các xúc tac thường dung trong công nghệ hóa học. cho chúng ta cái nhìn tổng quan về xúc tác trong công nghệ dầu khí cũng như trong công nghệ hóa học

Thermal and Catalytic Processes in

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

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Marcel Dekker, Inc

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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 anymeans, electronic or mechanical, including photocopying, microfilming, and recording, or

by any information storage and retrieval system, without permission in writing from thepublisher

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

This book is considered to be a completely new version of the original book lished in 3 volumes in Romania, in 1996–1997 under the title Conversia

Recent developments in petroleum processing required the complete revision

of some of the chapters, the elimination of outdated material and bringing up to

Furthermore, the presentation of theoretical aspects has been somewhat expandedand deepened

The processes discussed in this book involve the conversion of hydrocarbons

by methods that do not introduce other elements (heteroatoms) into hydrocarbonmolecules 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 fourthparts analyze catalytic processes on metal oxides (hydrofining, hydrotreating) and onbifunctional 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 asimprovement of their quality, as required by increasingly stringent market demands.The products of primary distillation are further processed by means of secondaryoperations, some fractions being subjected to several processing steps in series.Consequently, the total capacity of the conversion processes is larger than that ofthe primary distillation

The development of petroleum refining processes has made it possible to duce products, especially gasoline, of improved quality and also to produce syntheticchemical feedstocks for the industry The petrochemical branch of the refining indus-try generates products of much higher value than does the original refining industryfrom which the feedstocks were derived

pro-One should not overlook the fact that the two branches are of quite differentvolume A few percentage points of the crude oil processed in the refineries aresufficient to cover the needs for feeds of the whole petrochemical and syntheticorganic industry and of a large portion of the needs of the inorganic chemicalsindustry The continuous development of new products will result in a larger fraction

of the crude oil than the approximately 10% used presently being consumed asfeedstocks for the chemical industry

Hydrocarbons conversion processes supply hydrocarbons to the petrochemicalindustry, but mainly they produce fuels, especially motor fuels and quality lubricat-ing oils The same basic processes are used in all these different applications Thespecific properties of the feedstocks and the operating parameters are controlled inorder to regulate the properties of the product for each application In this book, theprocesses 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 fundamentalscommon to all the processes: thermodynamics, reaction mechanisms (including cat-alysis when applicable), and, finally, process kinetics In this manner, operatingparameters practiced in commercial units result as a logical consequence of earliertheoretical discussion This gives the reader a well-founded understanding of eachtype of process and supplies the basis on which improvements of the process may beachieved

The presentation of commercial implementation is followed by a discussion ofspecific issues pertaining to the design of the reaction equipment, which results in theunity of the theoretical bases with the design solutions adopted for commercialequipment and the quantitative aspects of implementation

My warmest thanks to Prof Sarina Feyer-Ionescu, to my son Prof GeorgeRaseev, and especially to my technical editor Dr G Dan Suciu, for their support inpreparing 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 inresearch, 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 inwhich the existing knowledge on chemical process technology—and especially on theprocessing of hydrocarbons and petroleum fractions—should be treated and con-veyed to others

While initially the discipline of process technology was taught mainly bydescribing the empirical information, it soon changed to a quantitative disciplinethat considers the totality of phenomena that occur in the processes of chemicalconversion of industrial interest

The objective of process technology as a discipline is to find methods for thecontinual improvement of commercial processes To this purpose it uses the latestadvances in chemistry, including catalysis, and applies the tools of thermodynamicsand kinetics toward the quantitative description of the processes In this manner itbecame possible to progress from the quantitative description provided by the reac-tion mechanisms to the mathematic formulation for the evolution in time of theprocesses

In order to implement the chemical process on a commercial scale, a series ofadditional issues need to be addressed: the effect of the operating parameters and theselection 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 tothe heat and mass transfer, and the limitations imposed by the materials of construc-tion

Process technology thus becomes the convergence point of several theoreticaland applicative disciplines called upon to solve in an optimal manner the complexinterrelations among quite different sciences and phenomena (chemistry, hydraulics,heat transfer, etc.) This situation requires a multifaceted competence and the fullunderstanding 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 thecommercial 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 processengineer is fully equipped to answer the ‘‘why’’ and ‘‘how’’ questions, and will beable to become one of the important driving forces of technical progress This is theconcept that has guided me during my entire professional activity

This book treats the conversion of hydrocarbons and petroleum fractions bythermal 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 tocontribute 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 increating this book My special gratitude goes to Prof Sarina Feyer-Ionescu, for herspecial contributions

Serge Raseev

Preface

Preface to the Romanian Edition

1 Thermodynamic Analysis of Technological Processes

1.1 Calculation of the Overall Thermal Effect

1.2 Equilibrium Calculations for a Wide Range of Process ConditionsReferences

2 Theoretical Background of Thermal Processes

2.1 Thermodynamics of Thermal Processes

2.3 Kinetics of Thermal Processes

2.4 Influence of Operating Conditions

4 Industrial Implementation of Thermal Processes

4.1 Thermal Cracking at High Pressures and Moderate Temperatures

4.3 Pyrolysis

References

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

6 Theoretical Basis of Catalytic Cracking

6.1 Process Thermodynamics

6.2 Cracking Catalysts

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

8 Design Elements for the Reactor–Regenerator System

8.1 Some Fluidization Problems

8.2 Fluidization with Solids Circulation

10 Hydrofining and Hydrotreating

10.5 Effect of Process Parameters

11.5 Influence of Operating Parameters

11.6 Industrial Hydroisomerization of Lower Alkanes

11.7 Hydroisomerization of Lube Oils and Medium FractionsReferences

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

13.4 The Kinetics of Catalytic Reforming

13.5 The Effect of Process Parameters

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

Thermodynamic Analysis of

Technological Processes

The thermodynamic study of technological processes has two objectives:

Determination of the overall thermal effect of chemical transformations thattake place in the industrial process

Determination of the equilibrium composition for a broad range of tures and pressures in order to deduce optimum working conditions andperformances

tempera-The manner in which the two objectives are approached within the conditions ofchemical technology is different from the classical approach and requires the use ofthe specific methodology outlined in this chapter

In practical conditions under which technological processes operate, the main tion may be accompanied by secondary reactions In many cases the transformation

reac-is of such complexity that it cannot be expressed by a reasonable number of chemicalreactions

When calculating the heat of reaction in such situations, in order to avoid thedifficulties 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 accountonly 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 initialand the final state of the system (the independence of path, as stipulated by thesecond principle of thermodynamics), it may be calculated based on the initial andfinal compositions of the system, without having to take in account the reactions thattake place

Accordingly, the classic equations, which give the thermal effect of a chemicalreaction:

organic compounds, which are of interest in studying petrochemical processes, aregiven 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, thecalculations 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)

dehydrogena-tion process of isopentane to isoprene at 6008C

The composition of the streams at the inlet and outlet of the reactor is given inTable 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

According to Eq (1.3), the overall thermal effect per unit mass (kg) of feed willbe:

and 121
8 ¼ 113g, isoprene is formed In these conditions, the thermal effectexpressed per mole of reacted isopentane is:

900(K)

873=600(K) (8C)

ni(mol/kg)

ni
H0

f 873(kcal/kg)

ne(mol/kg)

ne
H0 f873(kcal/kg)

-

-
535.75

2243.1

-

-
381.94

1599.1

is taken into account, then according to the Eq (1.1) one obtains:

C5H8

HfÞC5H12 ¼ 13:91
ð
44:51Þ ¼ 58:42 kcal=mol

¼ 244:59 kJ=molthe 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 theoverall thermal effect is not based on the real compositions of the inlet and outletstreams of the reactor

Eq (1.4) makes it possible to compute the thermal effects by using the heats ofcombustion 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-

accordingly

For liquid petroleum fractions, the heats of combustion may be determined by

the characterization factor

The characterization factor of residues may be determined graphically from theviscosity, by means ofFigure 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) Forhydrocarbons, this equation takes the form:

An illustration of these calculations is given in Example 1.2

The characterization factor and the specific gravities were used to determinethe heats of combustion for all the liquid fraction from Figure 1.1

SOLUTION By introducing the values of the heats of combustion from Tables1.3 and 1.4 into Eq (1.4), one obtains:

¼
204 kJ/kg

Calculation of the thermal effects for a specific reaction, usually a small ber obtained as the difference of heats of combustion, usually larger numbers, is

num-associated with large errors, unless the determination of the values of the heats ofcombustion was made with high accuracy This fact is especially valid for liquidfractions, for which the graphical determination of the combustion heats may giveerrors In order to obtain exact results, the determination of the heats of combustion

of the liquid fractions by direct calorimetric methods is recommended

Figure 1.1 Heat of combustion of petroleum fractions Final state: gaseous CO2and liquidwater at 158C

Graphs and empirical relations are given [4–7] for the calculation of the mal effect in the petroleum refining processes The values calculated by their meansand the numerical values given in the literature must be critically analyzed, takinginto 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 incomputations.

ther-For the process of thermal cracking, the use of equation [8] is recommended:

Figure 1.2 KUOPas function of the kinematic viscosity and density

The calculated
H is expressed in kJ/kg of feed The sign is that used in thethermodynamic notation.

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 isuseful, since the thermal effect thus becomes actually independent of conversion [5]

PROCESS CONDITIONS

The computation of the equilibrium compositions for a wide range of process ditions (temperatures and pressures) has the purpose of identifying practical operat-ing conditions that will optimize the performance of the process Depending on thespecifics 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

con-In all cases, the composition at equilibrium, calculated on basis of namic principles, represents the maximum conversion that is possible to achieve inthe given conditions There is however no certainty that such performance will beactually obtained Nonthermodynamic factors, such as the reaction rate and theresidence time within the reactor will determine how close the actual performancewill approach the theoretical one

thermody-The use of classical methods for computing equilibrium compositions for thelarge 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 %)

ð
H0

288ÞC(kJ/kg)

(
H0

283ÞCfraction(kJ/kg)

method elaborated by the author many years ago [9] provides a simple method forthe 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 tion:

and as function of the equilibrium constant, by the expression:

Assuming that the substances participating in the reaction do not deviate fromthe behaviour of ideal gas, the equilibrium constant may be expressed by the rela-tion:

Ka¼ Kp¼ ’iðxÞ

Here,’iðxÞ is a function of the conversion at equilibrium x The form of this functiondepends on the stoechiometry of the reaction but is independent on the nature of thesubstances that participate in the reaction

transfor-mations, one obtains:

these conditions, using as coordinates log p and 1=T, the Eq (1.11) corresponds to afamily of parallel straight lines with the equilibrium conversion x as parameter.Simple plots are obtained, by writing:

at equilibrium Both b and d are independent of the nature of the chemical

sub-Table 1.4

Yields(wt %) Density

Viscosity(cSt)

Characterizationfactor(KUOPÞ

Thermaleffect(kJ/Kg)

stances that take part in the reaction and have been calculated [9] for chemical

For reactions proceeding in the opposite direction, the sign of the constants band d must be changed, and the meaning of the conversion x reversed (for example

Since in plots of log p versus 1=T the straight lines of constant conversion areparallel, it is enough to calculate one point of each line and to determine the slope ofall the straight lines by calculating just one point for any other pressure Thus, thewhole family of lines may be obtained by selecting a pressure of 1 bar for thedetermining one point on each straight line and a pressure of either 10 bar or 0.1bar 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:

The calculation is illustrated by the Example 1.3

Note that for temperature ranges of not more than 200–3008C that intervene inthe analysis of industrial processes, the variations with the temperature of
H0

and

S0

may be neglected, without consequently introducing any practical errors.Deviations from ideal conditions are important near the critical state and donot affect the results at temperatures much higher than the critical, as used in the

TABLE 1.5 Values of the Constants b and d for Various Reaction Stoichiometric Types

Reaction form b

x= equilibrium conversion0.99 0.95 0.90 0.80 0.70 0.60 0.50 0.40 0.30 0.20 0.10 0.05 0.01

2A$B 4.57 15.85 9.14 6.37 3.56 1.84 0.54
0.57
1.61
2.67
3.90
5.66
7.18
10.483A$B 9.14 16.38 11.41 7.69 3.94 1.79 0.23
1.04
2.19
3.34
4.62
6.40
7.91
11.29A+B$C 4.57 18.30 11.90 9.13 6.31 4.60 3.29 2.18 1.14 0.08
1.15
2.89
4.42
7.74A+B$2C 0 21.01 14.45 11.48 8.26 6.12 4.36 2.75 1.14
0.61
2.75
5.98
9.13
15.54A+2B$C 9.14 25.07 15.20 11.48 7.73 5.58 4.03 2.75 1.60 0.46
0.83
2.62
4.17
7.53A+3B$C 13.72 30.65 18.00 13.10 8.61 6.14 4.42 3.04 1.83 0.63
0.58
2.49
4.05
7.44A+B$ C+D 0 18.25 11.70 8.73 5.51 3.37 1.61 0
1.61
3.37
5.51
8.73
11.70
18.25A+2B$2C 2.29 33.42 19.07 14.76 10.20 7.41 4.77 3.20 0.18
0.69
3.03
6.38
9.45
16.10A+3B$2C 4.57 31.01 22.71 17.21 11.60 8.16 5.54 3.32 1.22
1.74
3.55
6.80
9.88
16.53A+2B$C+D 4.57 26.04 16.33 12.03 7.52 4.66 2.42 0.443
1.45
3.44
5.76
9.15
12.25
18.87A+3B$C+D 9.14 32.81 20.01 14.47 8.84 5.40 2.80 0.572
1.51
3.64
6.08
9.56
12.64
19.28A+4B$C+D 13.72 38.92 23.10 16.40 9.78 5.83 2.99 0.581
1.63
3.85
6.37
9.90
13.00
19.66A+5B$C+D 18.28 44.54 25.79 18.00 10.50 6.19 3.09 0.538
1.77
4.06
6.62
10.19
13.31
19.98A$ B+4C
18.28 7.34 3.99 2.418 0.594
0.744
1.96
3.22
4.66
6.50
9.20
14.32
20.09
35.032C$A+5B
18.28 17.22 10.56 7.44 3.87 1.30
0.99
3.29
5.84
8.95
13.25
20.76
28.54
47.3

Copyright © 2003 by Taylor & Francis Group, LLC

thermal and catalytic processes in petroleum refining If corrections as such arehowever needed, they can be accomplished by using the methods elaborated in theoriginal work [9].

This method of equilibrium representation will be widely used in the followingchapters for the analysis of practical process conditions

REFERENCES

1 FD Rossini, KS Pitzer, RL Arnett, RM Braun, GC Pimentel Selected Values of Physicaland Thermodynamical Properties of Hydrocarbons and Related Compounds, Pittsburgh:Carnegie Press, 1953

2 DR Stull, EF Westrum Jr., GC Sinke The Chemical Thermodynamics of OrganicCompounds, 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, NewYork: Reinhold Publishing Co., 1955

9 S Raseev, Stud Cercet Chim 5 (2): 267, 285, 1957

Theoretical Background of Thermal

Processes

fractions under the influence of high temperatures Most of the transformations arecracking by a radicalic mechanism

The thermal processes comprise the following types of industrial processes:

PYROLYSIS (STEAM CRACKING) Main purpose: the production of ethene and spropene for the chemical industry The pyrolysis of liquid feed stocks, leads also tobutadiene, isoprene, and C6-C8aromatics

Characteristic for the pyrolysis process are temperatures of about 900–9508Cand 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 wasrealized 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 about4908C, and a 5–15 bar in coke drums, and fluid coking realized at about 5708C and2–3 bar, in a fluidized bed

Of some importance is the production of needle coke, which is used for theproduction of electrodes especially for electrometallurgy processes (e.g aluminum)

Thermodynamic calculations show that the thermal decomposition of alkanes ofhigher molecular weight may take place with high conversions even at relativelylow temperatures Thus, n-decane may convert to over 90% to form pentene andpentane at 3508C and 1 atmospheric pressure

The great number of parallel–successive reactions that may take place results inthe final product distribution being controlled by the relative rates of the reactionsthat take place and not by the thermodynamic equilibrium.

The situation is different for the lower alkanes Thus, in order to achieve aconversion 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 thedehydrogenation 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 C4H10Ð C2H6+ C2H4

The products obtained from the thermal decomposition of ethane, propane,and ethene, are those one would expect from dehydrogenation reactions:

Many studies [1,2,109–111] reached the conclusion that in the pyrolysisprocess, irrespective of the feedstock used, the values of the concentration ratiosethene/ethane, propene/propane and acetylene/ethene in the reactor effluent are

(2.3),(2.4),and(2.5)

Even if such assertions are only approximately accurate, the equilibrium ofthese reactions is of high interest for determining optimum operating conditions inpyrolysis

Figure 2.3 The thermodynamic equilibrium for reaction C2H6 Ð C2H4+ H2

Figure 2.3 illustrates the significant increase of the conversion of ethane toethene as the temperature increases from 800 to 9508C This justifies the use oftubes of special heat-resisting alloys, which make possible the continuous increase

of the temperatures towards the coil outlet, as practiced in modern high-conversionpyrolysis furnaces

The formation of propene (Figure 2.4) is favored by the equilibrium, but thecompetitive reaction by which propane is cracked to ethene and methane, influencesFigure 2.4 The thermodynamic equilibrium for reaction C3H8 Ð C3H6+ H2.

the final product distribution The final product composition is determined by therelative rate of the two reactions.

The equilibrium conversion to acetylene is much lower than that to alkenes(Figure 2.5) Still, the presence of acetylene in the reaction products makes theirpurification necessary The continuous increase in coil outlet temperatures has madethe acetylene removing sections a standard feature of the pyrolysis unit

Pressure has a strong effect on the conversions at equilibrium of these threereactions Increased conversions are obtained as the operating pressure decreases.Thus, a 50% lowering of pressure causes a supplementary amount of about 10%Figure 2.5 The thermodynamic equilibrium for reaction C2H4 Ð C2H2+ H2

ethane to be transformed to ethene (Figure 2.3) The same effect is obtained byreducing the partial pressures of the hydrocarbon, e.g by increasing the proportion

of dilution steam introduced in the reactor

The equilibrium graph for the dehydrogenation of butene to butadiene (Figure2.6) shows that in pyrolysis it is possible with high conversions The parallel reac-

reduced the amounts of the butadiene produced An analogous situation occurs inthe dehydrogenation of isopentene to isoprene

Figure 2.6 The thermodynamic equilibrium for reaction C4H8 Ð C4H6+ H2

In visbreaking and delayed coking, the relatively mild operating temperatureslead to a product containing no acetylene and only minor quantities of butadiene.The equilibrium for the polymerization of alkanes may be illustrated by the

cannot take place in the conditions of pyrolysis, but it may be intense in visbreaking,delayed coking, and the older high pressure cracking processes

The dehydrogenation of alkylcyclohexanes to aromatics is exemplified inFigure 2.7, by the conversion of metilcyclohexane to toluene While these reactions

Figure 2.7 Reaction C5H11 CH3 Ð C6H5 CH3+ 3H2.

are thermodynamically favored in all thermal processes, the low reaction rates at theoperating temperatures limit the conversions achieved.

It is well-known that chemical transformations, which are expressed usually byglobal chemical reactions, actually take place through a large number of paralleland successive elementary reactions Their knowledge is necessary both for under-standing of the chemical aspect of the process and for correctly formulating thekinetic equations

The initial chemical phenomenon, which take place at high temperatures, is thebreaking of the hydrocarbon molecule in two free radicals:

to determine which is the initial step of the thermal decomposition process

hydrocarbons, sulfur compounds, and nitrogen compounds

Table 2.1 Dissociation Energies of Some Hydrocarbons andRelated Substances

For the lower hydrocarbons, more complete data are available, including thevalues of the pre-exponential factors (Table 2.2).

For instance, the relative rate of breaking of the molecule of ethane at atemperature of 9008C, according to the reactions:

C – C bond will be preferentially broken The cracking of the C – H bonds may beneglected, since their rate is approximately two orders of magnitude lower than for

C – C bonds

5.185 1016

372.0380.0

2324

2624

n
C4H10! 2C2H5 1.5 1016

5.0 1015

343.7339.1

2726

From the same table one may note that the energy of the C – C bonds in alkanes decreases as one moves towards the center of the molecule Thus, for n-hexane, the energy is about 318 kJ/mole for cracking in two propyl radicals, andapproximately 322 kJ/mole for cracking in ethyl and butyl radicals In the same way,the energy needed for the cracking of n-pentane to ethyl and propyl radicals is lower

n-by 8 kJ/mole than for cracking to methyl and butyl radicals

According to relation (2.4), such small differences of energy lead to ratios ofthe respective reaction rates of about 1.5-2.5 Thus, one may not neglect the variousways in which the C – C bonds of a molecule may crack

The energy of the C – C bonds of tertiary and especially quaternary carbonatoms is lower than the others; thus they will be preferentially cracked

The double and triple bonds have bonding energies much higher that the single

C – C bonds in alkanes

The energy of the C6H5 –C6H5bond is 415 kJ/mole, but it decreases greatly ifthe rings are bound by means of an alkylic bridge, or if the bridge is bound to morearomatic rings Thus, for the molecule (C6H5)3C – C(C6H5)3, the dissociation energydecreases to as low as 46 kJ/mole

For sulphur compounds, the energy of the C – S bonds is of the same order asthat of the C – C bonds in n-alkanes

From the above data, it may be concluded that in all cases, the initial step inthe thermal decomposition of hydrocarbons is the breaking of a C – C bond Thecracking of various C – C bonds takes place with comparable rates, with the excep-tion of those in the position to double bonds, to triple bonds, or to aromatic rings,the rate of which can be neglected

The formation of free radicals by the cracking of C – C bonds was confirmedexperimentally The presence of methyl and ethyl radicals in the pyrolysis of ethane

in a tubular reactor was first identified by mass spectroscopy [29–31]

The concentration of the various species of radicals formed in the pyrolysis ofethane and propane at 8508C, using dilution with steam in conditions similar tothose of industrial processes, was determined by Sundaram and Froment in 1977

At a concentration of the feed of 10–2mole/liter, the concentrations of the radicals in

10–7.0and 10–9.3in the pyrolysis of propane

Semenov [18] distinguishes three types of transformations undergone by theformed radicals:

Reactions (a), isomerization by odd electron migration, are understood to takeplace via the formation of a cyclic activated complex, followed by the transfer of anatom of hydrogen [35,38]

For higher radicals (Cþ5), a cyclic-activated complex of 5 or 6 atoms is formed.For the lower radicals (C5– C3), the formation of an activated cyclic complex of 3 or

4 atoms is necessary, which would require a higher activation energy This energy ismuch higher than that for decomposition or substitution reactions For these rea-sons, the isomerization reaction with migration of an odd electron may be neglectedfor alkanes, which have chains shorter than that of 5 atoms of carbon (Table 2.3)

In conclusion, isomerization by odd electron migration must be taken intoaccount only for molecules which have longer chains [35,40,41]

kJ/mole Accordingly, at the high temperatures characteristic for thermal processes,

and characteristic for polymerization processes at low temperatures

The cracking reactions (b1) of the radicals occurs by the breaking of the C – C

excess of bonding energy possessed by the carbon at which the odd electron islocated The excess will be distributed over the three bonds of this carbon atom.The electron-attracting effect of the carbon with the odd electron strengthens the

 position will be weakened This explains the preferred cracking in  position.The electrons of the C – H bonds are less prone to polarization and play no role

in the cracking

The decomposition of alkyl radicals or of alkyl chains takes place by successive

For the methyl and ethyl radicals, at temperatures of 600–9008C, the life is of

radicals increase

these will be the ones cracked

which shows that the second reaction is unlikely

The radicals, before or after cracking, may give substitution reactions of type(c) with molecules of the feed A new radical, of higher molecular mass, is thereby

mass is formed Such reactions may continue and the cracking reactions acquire thecharacter of a chain reaction.*

Besides the substitution reactions, additions of the free radicals to doublebonds also may take place, leading to the formation of radicals with higher mole-

The data of Table 2.6 suggest that reactivity diminishes with increasing cular mass, which is easy to explain by steric hindrances Thus, if the rates of thereactions which follow are compared, by using relation (2.5):

* The formulation of this mechanism for the thermal decomposition of hydrocarbons was first developed

by F.C Rice and K.F Herzfeld [32].

Table 2.4 Thermal Effects for the Decomposition of Selected Radicals

C
C
C
C
C
C
C
C ! C5H10þ C3H7

&

C6H12þ C2H5

121130

7588

The above information on the reactions generated by the radicals formed in theinitial breaking of the C – C bonds, make it possible to analyze the succession oftransformations that take place during the thermal decomposition of hydrocarbons.

the odd electron, of the radicals produced following the initial cleavage (reactions of

hydro-carbon molecules by way of substitution reactions of the type (c) and produce newradicals with a higher molecular mass The decomposition and substitution reactions

Table 2.5 Kinetic Constants for the Decomposition of Selected

2734