chemical engineering thermal and catalytic processes in petroleum refining s. raseev

Thermal andCatalytic Processes in

Petroleum Refining

Serge RaseevConsultant for UNESCO, Paris, France and

former Professor, Institute of Petroleum and Gases,Bucharest-Ploiesti, Romania

Technical editor for theEnglish-language version

G. Dan Suciu

MARCEL DEKKER, INC. NEW YORK BASEL

Copyright 2003 by Taylor & Francis Group, LLC

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, 19961997.

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

http://www.dekker.com

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, microlming, 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 19961997 under the title ConversiaHidrocarburilor (the conversion of hydrocarbons).

Recent developments in petroleum processing required the complete revisionof some of the chapters, the elimination of outdated material and bringing up todate the processes in which the technology was signicantly improved.Furthermore, the presentation of theoretical aspects has been somewhat expandedand deepened.

The processes discussed in this book involve the conversion of hydrocarbonsby methods that do not introduce other elements (heteroatoms) into hydrocarbonmolecules. The rst 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 (hydroning, 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 nished 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 rening processes has made it possible to pro-duce products, especially gasoline, of improved quality and also to produce syntheticchemical feedstocks for the industry. The petrochemical branch of the rening indus-try generates products of much higher value than does the original rening industryfrom which the feedstocks were derived.


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 reneries aresufcient 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 fractionof 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. Thespecic 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 andto 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, nally, 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 ofspecic 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 scientic back-ground and reects the concepts that I developed over the years, of the manner inwhich the existing knowledge on chemical process technologyand especially on theprocessing of hydrocarbons and petroleum fractionsshould 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 nd 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 designof 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: rst, 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 howquestions at the level of the current technical knowledge. In this manner, I hope tocontribute to the education of specialists who will advance continuing developmentsin 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


Contents

PrefacePreface to the Romanian Edition

PART I THERMAL CONVERSION PROCESSES

1 Thermodynamic Analysis of Technological Processes1.1 Calculation of the Overall Thermal Effect1.2 Equilibrium Calculations for a Wide Range of Process ConditionsReferences

2 Theoretical Background of Thermal Processes2.1 Thermodynamics of Thermal Processes2.2 Reaction Mechanisms2.3 Kinetics of Thermal Processes2.4 Inuence of Operating ConditionsReferences

3 Reaction Systems3.1 Selection of Reactor Type3.2 Reaction SystemsReferences

4 Industrial Implementation of Thermal Processes4.1 Thermal Cracking at High Pressures and Moderate Temperatures4.2 Coking4.3 PyrolysisReferences


5 Elements of Reactor Design5.1 Design of the Reaction Section of Tubular Furnaces5.2 Design of Soakers, Coke Drums, and Reaction Chambers5.3 Systems Using Solid Heat CarrierReferences

PART II PROCESSES ON ACID CATALYSTS

6 Theoretical Basis of Catalytic Cracking6.1 Process Thermodynamics6.2 Cracking Catalysts6.3 Reaction Mechanisms6.4 Kinetics of Catalytic Cracking6.5 Effect of Process Conditions6.6 Catalyst RegenerationReferences

7 Industrial Catalytic Cracking7.1 Feed Selection and Pretreatment7.2 Process History, Types of Units7.3 Characteristic Equipment7.4 Operation Aspects7.5 Catalyst Demetallation7.6 Yield Estimation7.7 Economic DataReferences

8 Design Elements for the ReactorRegenerator System8.1 Some Fluidization Problems8.2 Fluidization with Solids Circulation8.3 Reaction Systems8.4 Catalyst Regeneration8.5 Catalyst Entrainment8.6 Catalyst Circulation, Transport LinesReferences

9 Other Processes on Acid Catalysts9.1 Oligomerization9.2 Isoparafn-Olen AlkylationReferences

PART III PROCESSES ON METALLIC CATALYSTS

10 Hydroning and Hydrotreating10.1 Process Thermodynamics10.2 Catalysts10.3 Reaction Mechanisms10.4 Process Kinetics

Contents


10.5 Effect of Process Parameters10.6 Industrial Hydroning10.7 Industrial Hydrotreating10.8 Design Elements for the Reactor SystemReferences

PART IV PROCESSES USING BIFUNCTIONAL CATALYSTS

11 Hydroisomerization of Alkanes11.1 Thermodynamics of Hydroisomerization11.2 Hydroisomerization Catalysts11.3 Reaction Mechanism11.4 Kinetics of Isomerization11.5 Inuence of Operating Parameters11.6 Industrial Hydroisomerization of Lower Alkanes11.7 Hydroisomerization of Lube Oils and Medium FractionsReferences

12 Hydrocracking12.1 Thermodynamics of Hydrocracking12.2 Catalysts12.3 Reaction Mechanisms12.4 Kinetics of Hydrocracking12.5 Effect of Process Parameters12.6 Commercial Hydrocracking of Distillates12.7 Residue Hydrocracking12.8 Processes Using Slurry Phase Reactors12.9 Production of High Grade Oils by HydrocrackingReferences

13 Catalytic Reforming13.1 Thermodynamics13.2 Catalysts13.3 Reaction Mechanisms13.4 The Kinetics of Catalytic Reforming13.5 The Effect of Process Parameters13.6 Catalyst Regeneration13.7 Commercial Processes13.8 Elements of Design and Modeling13.9 Production of Aromatics13.10 Dehydropolymerization of Lower AlkanesReferences

14 Process Combinations and Complex Processing Schemes14.1 Denition of Objectives14.2 Evolution of the Range and Specications of Products14.3 Additional Resources

Contents


14.4 Initial Data for the Selection of Renery Conguration14.5 Approach for Establishing the Conguration of a Modern

ReneryReferences

Appendix Inuence of the n=i-Alkanes Ratio in the Pyrolysis Feedon the Ethene/Propene Ratio in the Products

Contents


1Thermodynamic Analysis ofTechnological 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 tempera-tures and pressures in order to deduce optimum working conditions andperformances

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 specic 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 transformationis 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 thedifculties resulting from taking into account all reactions many times in the calcu-lation, simplied approaches are taken. Thus, one may resort to the approximationof limiting the number of the reactions taken into consideration, or to take accountonly the main reaction. Such approximations may lead to signicant errors.

Actually, the exact value of the thermal effect can be calculated without havingto resort to such approximations. Since the thermal effect depends only on the initialand the nal state of the system (the independence of path, as stipulated by thesecond principle of thermodynamics), it may be calculated based on the initial andnal 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:

H0rT X

pH0fT

XrH

0fT 1:1

H0rT X

rH0cT

XpH

0cT 1:2

may be written under the form:

HrT X

neHfT X

niHfT 1:3

HrT X

niHcT X

neHcT 1:4The heats of formation Hf and of combustion Hc for hydrocarbons and

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).

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 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 thereactor at 6008C are collected in Table 1.2.

Table 1.1

Component

Reactor inlet feed + recycle

(wt %)

Reactor Outlet

(wt %)

H2 - 1.0

CH4 - 0.6

C2H6 - 0.7

C2H4 - 0.7

C3H8 - 0.7

C3H6 - 1.4

C4H10 0.3 1.2

C4H8 - 2.2

C4H6 - 0.2

i-C5H12 79.3 55.8

i-C5H10 16.6 17.1

C5H8 0.8 12.1

n-C5H12 1.8 0.8

n-C5H10 1.7 1.7

1,3-C5H8 - 2.0

coke - 1.8


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

Hr;873 X

neHf 873 X

niHf 873 1599 2243:1 644 kJ/kgSince 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 Table 1.1, 793 558 235g, isopentane reacts

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

Hr 644

235 72:15 197:7 kJ/mole

and per mole of produced isoprene:

Hr 644

113 68:11 388:2 kJ/mole

If only the main reaction:

i C5H12 i C5H8 2H2

Table 1.2

Component

Heat of formation

H0f (kcal/mol) [2] Inlet Outlet

800

(K)

900

(K)

873=600

(K) (8C)ni

(mol/kg)

niH0f 873

(kcal/kg)

ne(mol/kg)

neH0f 873

(kcal/kg)

H2 0 0 0 - - 9.92 0

CH4 20.82 21.15 21.05 - - 0.37 7.79C2H6 24.54 24.97 24.85 - - 0.23 5.72C2H4 9.77 9.45 9.54 - - 0.25 2.39

C3H8 30.11 30.58 30.45 - - 0.16 4.87C3H6 0.77 0.35 0.46 - - 0.33 0.15

C4H10 36.41 36.93 36.79 0.05 1.84 0.21 7.73C4H8 6.32 6.84 6.70 - - 0.39 2.61C4H6 23.25 22.95 23.03 - - 0.04 0.92

i-C5H12 44.13 44.65 44.61 10.99 489.16 7.73 344.06i-C5H10 13.45 13.93 13.80 2.37 32.71 2.44 33.67C5H8 14.16 13.82 13.91 0.12 1.67 1.78 24.76

n-C5H12 42.28 42.85 42.70 0.25 10.68 0.11 4.70n-C5H10 12.23 12.78 12.63 0.24 3.03 0.24 3.031,3-C5H8 14.17 13.73 13.85 - - 0.29 4.02

C 0 0 0 - - - -

Totalkcal/kgkJ/kg

-

-

535.752243.1

-

-

381.941599.1


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

Hr Hf C5H8 Hf C5H12 13:91 44:51 58:42 kcal=mol 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 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-nient to perform the calculation in weight units, by modifying the terms n and Haccordingly.

For liquid petroleum fractions, the heats of combustion may be determined byusing the graph of Figure 1.1 [3], from the known values of the specic gravity andthe characterization factor.

The characterization factor of residues may be determined graphically from theviscosity, by means of Figure 1.2 [3].

The heat of combustion of coke is determined experimentally or less preciselyon 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:

HaCnHm nHf CO2 m

2Hf H2O Hf CnHm 1:5

This heat of combustion of gases must be brought to the same reference state asthat of liquid fractions, i.e. 158C and liquid water. For these conditions, Eq. (1.5)becomes:

HaCnHm 393:77n 143:02m Hf CnHm 1:6It 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 technicalnotation, 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 residueby visbreaking. The composition of the produced gases is given in Table 1.3. Theyields and the characterization factors, KUOP for the feed and the fuel oil wereobtained from Table 1.4.

The characterization factor and the specic 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:

Qr 43,645 0:0244 51,319 0:1166 46,827 0:859 43,233 204 kJ/kg

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


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 combustionof the liquid fractions by direct calorimetric methods is recommended.

Figure 1.1 Heat of combustion of petroleum fractions. Final state: gaseous CO2 and liquidwater at 158C.


Graphs and empirical relations are given [47] for the calculation of the ther-mal effect in the petroleum rening 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.

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

H 117,230Ma MpMa Mp

1:7

Figure 1.2 KUOP as 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.

Using the data from example 1.2 (see the Table 1.4), this equation gives:

H 117,230 440 253440 253 197 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 isuseful, since the thermal effect thus becomes actually independent of conversion [5].

1.2 EQUILIBRIUM CALCULATIONS FOR A WIDE RANGE OFPROCESS 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 thespecics 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 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.

The use of classical methods for computing equilibrium compositions for thelarge number of temperaturepressure values needed for thermodynamic analysis ofa broad range of process conditions necessitates a large number of calculations. A

Table 1.3

Component

Composition

(wt %)

H0288C(kJ/kg)

(H0283C fraction(kJ/kg)

CH4 22.32 55,540 12,396C2H6 18.84 51,910 9,780C3H8 4.57 50,330 2,300C3H6 20.56 50,380 10,358i-C4H12 7.97 48,950 3,901n-C4H12 2.20 49,390 1,093i-C4H10 9.20 49,540 4,558n-C4H10 1.85 48,170 8911-C4H8 3.50 48,470 1,696cis-2-C4H8 0.55 48,340 266trans-2-C4H8 2.37 48,270 1,144C4H6 2.01 47,020 945C5+ 4.06 49,050 1,991

Total (H0288C fraction 51,319 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 rela-tion:

G0T H0T TS0T 1:8

Figure 1.3 Thermodynamic equilibrium of propene dimerization. Parameter: conversion xas %.


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

At equilibrium, GT 0 and G0T RT lnKa (1.9)

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:

Ka Kp ixpn

1:10

Here, ix 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.

Equating Eqs. (1.8) and (1.9), and replacing Ka with the expression (1.10),dividing the right and left sides by TH0T , and effecting some elementary transfor-mations, one obtains:

1

T Rn

H0Tln pS

0T R ln ix

H0T1:11

For a given chemical reaction and a temperature range of 2003008C, which issufcient for a process analysis, H0T and S0T can be considered constants. Inthese 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:

2:3Rn band

R lnix dEquation (1.11) becomes:

1

T b

H0Tlog pS

0T d

H0T1:12

The parameter b depends only on the stoechiometric form of the chemicalreaction. Parameter d depends both on the stoechiometry and on x, the conversionat equilibrium. Both b and d are independent of the nature of the chemical sub-

Table 1.4

Yields

(wt %) Density

Viscosity

(cSt)

Characterization

factor

(KUOP

Thermal

eect

(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


stances that take part in the reaction and have been calculated [9] for chemicalreactions of various stoechiometric forms (Table 1.5).

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 examplex 0:95 from the table will have the meaning x 0:05 for the reverse reaction).

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:

p 1bar 1T S

0T d

H0T

p 10 bar 1T S

0T d bH0T

p 0:1 bar 1T S

0T d bH0T

1:13

The calculation is illustrated by the Example 1.3.

Example 1.3. For the reaction:

2C3H6 C6H12determine the equilibrium graph for pressures comprised between 1 and 100 bar andtemperatures between 4508008C.

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:

H0900 20020 2 350 20,720 cal/molS0900 143:65 2 89:75 35:85 cal/mol K

Using the Eq. (1.13) corresponding to the pressure of 1 bar and the values of theconstant 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

1=T 103 1.22 1.38 1.46 1.54 1.60 1.65 1.70 1.76 1.82 1.90 2.04 2.17 2.50For the pressure of 10 bar and x 0:5 and using the constant b from the Table

1.5, one obtains, according to the Eq. (1.13):

1=T 1:48 103

By using the obtained values, the equilibrium is represented in Figure 1.3.Note that for temperature ranges of not more than 2003008C that intervene in

the analysis of industrial processes, the variations with the temperature of H0 andS0 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 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$B 0 9.13 5.85 4.37 2.75 1.68 0.81 0 0.81 1.70 2.75 4.37 5.85 9.132A$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


thermal and catalytic processes in petroleum rening. 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 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 rafnage 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 Renery 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.


2Theoretical Background of ThermalProcesses

Thermal processes are chemical transformations of pure hydrocarbons or petroleumfractions under the inuence 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 s

propene for the chemical industry. The pyrolysis of liquid feed stocks, leads also tobutadiene, isoprene, and C6-C8 aromatics.

Characteristic for the pyrolysis process are temperatures of about 9009508Cand 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 1520 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 (4955108C) and high pressure (2040 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 515 bar in coke drums, and uid coking realized at about 5708C and23 bar, in a uidized bed.

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

2.1 THERMODYNAMICS OF THERMAL PROCESSES

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 parallelsuccessive reactions that may take place results inthe nal 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 pressureof 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 C4H10 C2H6 + C2H4.


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

C2H6 C2H4 H2 aC3H8 C3H6 H2 bC2H4 C2H2 H2 c

Figure 2.2 The thermodynamic equilibrium for reaction C4H10 C4H8 + H2.


Many studies [1,2,109111] 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 efuent areclose to those corresponding to the equilibrium of the reactions (a)(c), Figures(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 illustrates the signicant increase of the conversion of ethane toethene as the temperature increases from 800 to 9508C. This justies the use oftubes of special heat-resisting alloys, which make possible the continuous increaseof 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, inuences



the nal product distribution. The nal 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 theirpurication 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%



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 proportionof 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-tions of decomposition of the C4 hydrocarbons take place with high velocity andreduced the amounts of the butadiene produced. An analogous situation occurs inthe dehydrogenation of isopentene to isoprene.



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 thegraph for the dimerization of propene (Figure 1.3). It results that polymerizationcannot 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 exemplied inFigure 2.7, by the conversion of metilcyclohexane to toluene. While these reactions

Figure 2.7 Reaction C5H11 CH3 C6H5 CH3 + 3H2.


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

2.2 REACTION MECHANISMS

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:

RR1 k1

k1RR1 a

The reaction energy, which represents in this case the dissociation energy, isgiven by the equation:

Hr Da E1 E1Since the activation energy for the reverse equation, the recombination of

radicals, is equal to zero, the equation becomes:

Hr Da E1The rate of reaction (a), may be expressed by the relation:

ra AaeDa=RT RR1 2:1The molecule RR1 may also crack to form other two free radicals:

RR1 k2

k2R2 R

3 b

For this reaction also, one may write:

rb AbeDb=RT RR1 2:2The ratio between the relations (2.1) and (2.2) is:

rarb AaAb

eDbDa=RT 2:3

For reactions for which data for the values of the pre-exponential factors A areunavailable, as their values are of the same order, it may be assumed that their valueis approximately equal to 1016 [46], and relation (2.3) becomes:

rarb eDbDa=RT 2:4

By means of relations (2.3) and (2.4) respectively it is possible to calculate therelative rates of breaking of different bonds within the same molecule and in this wayto determine which is the initial step of the thermal decomposition process.

The dissociation energies from Table 2.1 make possible such calculations forhydrocarbons, sulfur compounds, and nitrogen compounds.


Table 2.1 Dissociation Energies of Some Hydrocarbons andRelated Substances

Bond Dissociation energy (kJ/mole) Ref.

H H 435 7

CH2 H 356 8

CH3 H 431 8

C2H5 H 410 7

n-C3H7 H 398 9

i-C3H7 H 394 10

n-C4H9 H 394 11

i-C4H9 H 390 12

tert-C4H9 H 373 10

neo-C5H11 H 396 10

CH2 CHH 435 7

CH2 CHCH2 H 322 13

CH2 C(CH3)CH2 H 322 14

CH3CHCHCH2 H 335 14

C6H5 H 427 11

C6H5CH2 H 348 15

o-CH3C6H4CH2 H 314 10

m-CH3C6H4CH2 H 322 10

p-CH3C6H4CH2 H 318 10

(C6H5)2CHH 310 16

389 16

CH3 CH3 360 10

C2H5 CH3 348 17

C2H5 C2H5 335 17

n-C3H7 CH3 339 16

sec-C3H7 CH3 335 16

n-C4H9 CH3 335 16

n-C3H7 C2H5 327 16

sec-C3H7C2H5 322 16

n-C3H7 n-C3H7 318 16

sec-C3H7 sec-C3H7 320 16

n-C3H7 sec-C3H7 314 16

n-C4H9 C2H5 322 16

CH3CH2(CH3 n-C3H7 314 18CH3C(CH3)2 n-C3H7 301 18

CH3C(CH3)2 CH(CH3)CH3 295 18

n-C4H9 n-C3H7 314 16

n-C4H9 n-C4H9 310 16

tert-C4H9 tert-C4H9 264 18

CH2 CH2 502 11

CH2 CHCH3 394 11

CH2 CHCH2 CH3 260 10

CH2 CHCH2 CHCH2 310 11

CH2 CHCHCH2 435 11

CH2 CHCH2 CH2CHCH2 176 11

CH2 C(CH3)CH2 CH3 268 11

CH3CHCHCH2 CH3 275 11

C6H5 CH3 381 16

C6H5 C2H5 368 16

C6H5 n-C3H7 360 16


C6H5CH2 CH3 264 10

C6H5CH2 C2H5 240 10

C6H5CH2 C3H7 272 10

C6H5 C6H5 415 16

C6H5CH2 CH2C6H5 198 16

(C6H5)2CHCH2C6H5 159 16

(C6H5)2CHCH(C6H5)2 105 16

(C6H5)3C C(C6H5)3 46 16

"

310 16

"

293 16

423 16

318 16

364 16

406 16

HSH 377 10

CH3 SH 293 10

C2H5 SH 289 10

n-C3H7 SH 295 10

tert-C4H9 SH 272 10

C6H5CH2 SH 222 19

CH3 SCH3 301 10

C2H5 SCH3 297 10, 20

C2H5 SC2H5 289 10, 20

C6H5CH2 SCH3 214 21

CH3S SCH3 306 10

C2H5S SCH3 301 10

C2H5S SC2H5 293 10

CH3 NH2 335 22

CH3 NHCH3 364 22

C6H5CH2 NH2 394 22

429 16

417 16

Table 2.1 Continued


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:

C2H6 2CH3

C2H6 C2H5 H

may be calculated by using relation (2.4) with R 8:319 kJ/moldegree and the datafrom Table 2.1:

rarb e410;000360;000=8:3191;173 168

By using the data of Table 2.2, a more accurate result is obtained:

ra=rb 58Both calculations lead to the conclusion that the reaction of breaking a C C

bond, will be prevalent over breaking a C H bond.In the same way, the initial prevalent reaction may be determined for other

hydrocarbons or hetero-atomic compounds.Examination of the dissociation data of Table 2.1, shows that for alkanes in

general, the energy of the C C bonds is much lower than the energy of the C Hbonds. Therefore, in the pyrolysis not only for ethane but also of other alkanes, aC 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 forC C bonds.

Table 2.2 Kinetic Constants of Some Dissociation Reactions

Reaction A (s1) E (kJ/mol) Ref.

C2H6 ! 2CH3 5.248 1016

5.185 1016372.0

380.0

23

24

C2H6 ! C2H5 H

1.3 1015 360.1 25

C3H8 ! C2H5 C

H3 1.995 1016

2.074 1016353.8

366.9

26

24

n C4H10 ! 2C2H5 1.5 1016

5.0 1015343.7

339.1

27

26

n C4H10 ! 1 C3H7 C

H3 9.0 1016 357.6 27

i C4H10 ! 2 C3H7 C

H3 2.0 1016 343.3 27

C3H6 ! C2H3 C

H3 8.0 1017 397.7 27

C3H6 ! C3H5 H

3.5 1016 360.1 27

1 C4H8 ! C3H5 C

H3 8.0 1016 309.8 28

2 C4H8 ! C3H5 C

H3 2.0 1016 298.5 27


From the same table one may note that the energy of the C C bonds in n-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 lowerby 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 singleC C bond. The energy of C C bonds in the position to a double bond is higherthan that of bonds in the position, and signicantly lower than the energy of theC C bonds in alkanes.

The energy of the C6H5C6H5 bond 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 conrmedexperimentally. The presence of methyl and ethyl radicals in the pyrolysis of ethanein a tubular reactor was rst identied by mass spectroscopy [2931].

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 102 mole/liter, the concentrations of the radicals inthe efuent ranged between 106.6 and 108.3 in the pyrolysis of ethane and between107.0 and 109.3 in the pyrolysis of propane.

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

isomerization:

CH3CH2CH2! CH3C

HCH3 a

decomposition:

RCH2CH2! R

C2H4 b1

the reverse of which is the addition to the double bond:

RC2H4! RCH2C

H2 b2


substitution:

R1 R2H! R1HR

2 c

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 (C5 ), 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 or4 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].

Reactions such as (b1) are endothermic, with thermal effects of about 80160kJ/mole. Accordingly, at the high temperatures characteristic for thermal processes,the favored transformation is decomposition. Reactions of type (b2) are exothermicand characteristic for polymerization processes at low temperatures.

The cracking reactions (b1) of the radicals occurs by the breaking of the C Cbond in position to carbon with the odd electron. This fact is explained by theexcess 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 thebonds with the carbon atoms in position. As result of the polarizing of the elec-trons of the carbon atoms in toward the carbon with the odd electron, the bonds in 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 rolein the cracking.

The thermal effects for the cracking of some radicals are listed in Table 2.4,while the kinetic constants of such reactions are presented in the Table 2.5.

The decomposition of alkyl radicals or of alkyl chains takes place by successivecracking in position, occurring at time intervals corresponding to the life of theradicals.

Table 2.3 Kinetic Constants for the Isomerization ofSelected Radicals

Reaction A (s1) E (kJ/mole) Ref.

1 C4H9 ! 2 C

4H9 5.2 1014 171.7 42

1 C6H13 ! 2 C

6H13 3.2 1010 49.0 35

1 C6H13 ! 3 C

6H13 2.0 1011 78.3 35

2 C6H13 ! 1 C

6H13 5.0 1010 67.0 35

3 C6H13 ! 1 C

6H13 3.2 1011 96.3 35


For the methyl and ethyl radicals, at temperatures of 6009008C, the life is ofthe order of 103 s. It decreases as the temperature and the molecular mass of theradicals increase.

Since the ethyl and sec-propyl radicals possess only C H bonds in position,these will be the ones cracked.

Indeed, according to the Table 2.4, the thermal effect for the reaction:

C2H5! C2H4 H

is 165 kJ/mole;

while for the reaction:

C2H5! C

H3 CH2 is 310 kJ/mole

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 therebyformed. It also could undergo cracking, and a new radical with a lower molecularmass is formed. Such reactions may continue and the cracking reactions acquire thecharacter of a chain reaction.*

The kinetic constants of some substitution reactions are given in Table 2.6.Besides the substitution reactions, additions of the free radicals to double

bonds also may take place, leading to the formation of radicals with higher mole-cular mass. The kinetic constants of such interactions are given in Table 2.7.

The data of Table 2.6 suggest that reactivity diminishes with increasing mole-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):

Hn-C4H10 ! H2 1 C

4H9 a

CH3 n-C4H10 ! H2 1 C

4H9 b

it follows that for a temperature of 1100 K, the ratio of the reaction rates is:

rarb 1:5 10

11

3:5 1010 e48;600 40;600=11008:319 H

CH3 10:27 H

CH3

For similar reactions which lead to the formation of the 2-C4H9 radicals it

follows:

rarb 42:5 H

CH3

In both cases, the rates of the reactions initiated by the atomic hydrogen aresignicantly higher than those initiated by the methyl radical. This may explain theincrease of the reaction rates in pyrolysis caused by the introduction of hydrogeninto the reactor that was measured experimentally [39].

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

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


Table 2.4 Thermal Effects for the Decomposition of Selected Radicals

Reaction H (kJ/mole)

C2H5 ! C2H4 H

165

C2H5 ! C

H3 CH2 310

CH3 CH2 CH2 ! C2H4 CH3&

C3H6 H

115

163

CH3 CH CH3 ! C3H6 H

167

CH3 CH2 CH2 CH2 ! C2H4 C

2H5 107

CH3 CH2 CH CH3 ! C3H6 C

H3 113

CH3C CH3 ! i C4H8 H

jCH3

180

n C5H11 ! C2H4 C

3H7 98

n C6H13 ! C2H4 C

4H9 96

n C8H17 ! C2H4 n C

6H13 94

C C C C C C C C! C3H6 n C

5H11 103

C C C C C C C C! C4H8 n C

4H9&

1 C7H14 CH3

113

138

C C C C C C C C! C5H10 C

3H7&

C6H12 C2H5

121

130

90

73

C C Cj j jCCC

C! CCCC CH3j j

C C

130

C6H5CH2CH2 ! C2H4 C

6H5&

C6H5CH CH2 H

77

96

151

75

88


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.These transformations consist of successive breaking of C C bonds in position tothe odd electron, of the radicals produced following the initial cleavage (reactions ofthe type b1). The radicals of lower molecular mass produced will react with 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 SelectedRadicals

Reaction A (s1) E (kJ/mole) Ref.

C2H5 ! C2H4 H

3.2 1013 167.5 33

1 C3H7 ! C2H4 C

H3 4.0 1013 136.5 33

1 C3H7 ! C3H6 H

2.0 1013 160.8 33

2 C3H7 ! C3H6 H

2.0 10132.5 1013

162.0

170.8

27

34

1 C4H9 ! C2H4 C

2H5 4.0 1013 121.4 35

1 C4H9 ! 1 C4H8 H

1.3 1014 163.3 35

2 C4H9 ! C3H6 C

H3 1.0 1014 138.1 35

2 C4H9 ! C4H8 H

1.6 1014 171.6 35

i C4H9 ! i C4H8 H

3.3 1014 150.7 27

i C4H9 ! C3H6 C

H3 2.0 1014 142.3 35

i C4H9 ! 2 C4H8 H

4.0 1013 153.2 27

C5H11 ! C5H10 H

5.0 1013 153.2 33

C5H11 ! 1 C4H8 C

H3 3.2 1013 131.9 27

C5H11 ! C2H4 1 C

3H7 4.0 1012 120.2 33

1 C6H13 ! C2H4 1 C

4H9 1.0 1014 125.6 35

2 C6H13 ! C3H6 1 C

3H7 1.0 1014 129.8 35

3 C6H13 ! 1 C4H8 C

2H5 1.0 1014 129.8 35

3 C6H13 ! 1 C5H10 C

H5 1.0 1014 136.1 35

C2H3 ! C2H2 H

2.0 109 131.9 27

C3H5 ! C2H2 C

H3 3.0 1010 151.6 27

C4H7 ! C4H6 H

1.2 1014 206.4 33

C4H7 ! C2H4 C

2H3 1.0 1011 154.9 27

Methyl allyl! C3H4 CH3 1.0 1013 136.5 27

Methyl allyl! C2H4 C2H3 1.0 1012 117.2 27


Table 2.6 Kinetic Constants for Selected Substitution Reactions

Reaction

A

(L mol1s1)E

(kJ/mole) Ref.

HH2 ! H2 H

4.6 1010 28.8 6

HCH4 ! H2 C

H3 7.24 1011 63.1 36

HC2H6 ! H2 C

2H5 1.0 1011 40.6 27

HC3H8 ! H2 1 C

3H7 1.0 1011 40.6 27

HC3H8 ! H2 2 C

3H7 9.0 1010

4.0 101034.8

34.3

27

37

H n C4H10 ! H2 1 C

4H9 1.5 1011 40.6 27

H n C4H10 ! H2 2 C

4H9 9.0 1010 35.2 27

H i C4H10 ! H2 i C

4H9 1.0 1011 35.2 27

CH3 H2 ! CH4 H

1.55 1010 64.9 36

CH3 C2H6 ! CH4 C

2H5 3.8 1011 69.1 27

CH3 C3H8 ! CH4 1 C

3H7 3.4 1010 48.1 27

CH3 C3H8 ! CH4 2 C

3H7 4.0 109 42.3 27

CH3 n C4H10 ! CH4 1 C

4H9 3.5 1010 48.6 27

CH3 n C4H10 ! CH4 2 C

4H9 3.5 109 39.8 27

CH3 i C4H10 ! CH4 i C

4H9 9.5 109 37.7 27

CH3 C6H14 ! CH4 C

6H13 1.0 108 33.9 38

C2H5 H2 ! C2H6 H

3.0 108 45.2 6

C2H5 CH4 ! C2H6 C

H3 2.3 108 47.7 36

C2H5 C3H8 ! C2H6 1 C

3H7 1.2 109 52.8 27

C2H5 C3H8 ! C2H6 2 C

3H7 8.0 108 43.5 27

C2H5n C4H10 ! C2H6 1 C

4H9 2.0 109 52.8 27

C2H5 n C4H10 ! C2H6 2 C

4H9 4.5 108 43.5 27

C2H5 i C4H10 ! C2H6 i C

4H9 1.5 109 43.5 27

C2H5 n C7H16 ! C2H6 n C

7H15 9.8 109 44.4 6

1 C3H7 n C4H10 ! C3H8 2 C

4H9 2.0 108 43.5 27

2 C3H7 n C4H10 ! C3H8 2 C

4H9 2.0 108 52.8 27

2 C3H7 i C4H10 ! C3H8 i C

4H9 1.0 108 56.1 27

HC2H4 ! H2 C

2H3 8.0 108 16.7 27

HC3H6 ! H2 C

3H5 2.5 109 17.2 28

H1 C4H8 ! H2 C

4H7 5.0 1010 16.3 33

H2 C4H8 ! H2 C

4H7 5.0 1010 15.9 27

H i C4H8 ! H2 Methyl allyl00 3.0 1010 15.9 27

CH3 C2H4 ! CH4 C

2H3 1.0 1010 54.4 27

CH3 C3H6 ! CH4 C

3H5 2.0 109 51.1 27

CH3 1 C4H8 ! CH4 C

4H7 1.0 108 30.6 33

CH3 2 C4H8 ! CH4 C

4H7 1.0 108 34.3 27

CH3 i C4H8 ! CH4 Methyl allyl00 3.0 108 30.6 27

C2H5 C3H6 ! C2H6 C

3H5 1.0 108 38.5 33


C2H5 1 C4H8 ! C2H6 C

4H7 2.0 108 34.8 27

C2H5 i C4H8 ! C2H6 Methyl allyl00 6.0 107 34.8 27

C2H5 1 C7H14 ! C2H6 1 C

7H13 3.1 108 34.8 6

1 C3H7 C3H6 ! C3H8 C

3H5 1.0 108 38.5 33

2 C3H7 C3H6 ! C3H8 C

3H5 1.0 108 42.7 33

C2H3 C3H8 ! C2H4 1 C

3H7 3.0 109 78.7 27

C2H3 C3H8 ! C2H4 2 C

3H7 1.0 109 67.8 27

C2H3 n C4H10 ! C2H4 1 C

4H9 1.0 109 75.4 27

C2H3 n C4H10 ! C2H4 2 C

4H9 8.0 108 70.3 27

C2H3 i C4H10 ! C2H4 i C

4H9 1.0 109 70.3 27

C3H5 C3H8 ! C3H6 1 C

3H7 1.0 109 78.7 33

C3H5 C3H8 ! C3H6 2 C

3H7 8.0 108 67.8 33

C3H5 n C4H10 ! C3H6 1 C

4H9 4.0 108 78.7 33

C3H5 n C4H10 ! C3H6 2 C

4H9 8.0 108 70.3 33

C3H5 iC4H10 ! C3H6 i C

4H9 1.0 109 79.5 27

Table 2.6 Continued

Table 2.7 Kinetic Constants for Selected AdditionReactions

Reaction

A

(L mol1s1)E

(kJ/mol) Ref.

C2H2 H! C

2H3 4.0 1010 5.4 27

C2H4 H! C

2H5 1.0 1010 6.3 33

C3H6 H! 1 C

3H7 1.0 1010 12.1 33

C3H6 H! 2 C

3H7 1.0 1010 6.3 27

1 C4H8 H! 2 C

4H9 1.0 1010 5.0 33

2 C4H8 H! 2 C

4H9 6.3 109 5.0 33

i C4H8 H! i C

4H9 1.0 1010 5.0 27

C2H4 CH3 ! 1 C

3H7 2.0 108 33.0 27

C3H6 CH3 ! 2 C

4H9 3.2 108 31.0 27

C3H6 CH3 ! i C

4H9 3.2 108 38.1 33

i C4H8 CH3 ! C

5H11 1.0 108 30.1 27

C2H4 C2H5 ! 1 C

4H9 1.5 107 31.8 33

C3H6 C2H5 ! C

5H11 1.3 107 31.4 33

C2H4 1 C3H7 ! C

5H11 2.0 107 31.0 33

C2H4 2 C3H7 ! C

5H11 1.3 107 28.9 33


will be repeated and will lead to the decomposition of a great number of feedmolecules, thereby conferring to the reaction the characteristics of a nonramiedchain.

The length of the chain is dened as the number of molecules cracked byreactions of types b1 and c by one radical formed by the initial breaking of aC C bond. In thermal processes, the length of the chain is 100200 and above.

The successive interactions of types b1 and c will be interrupted by reactionsthat lead to the disappearance of the radical. This may take place by a heterogeneousmechanism (adsorption on the wall) or by a homogenous mechanism such as theinteraction between two radicals.

The existence of the heterogeneous chain-interruption reaction may beobserved by increasing the ratio: walls surface/volume of the reactor. The resultingdecrease of the reaction rate indicates the existence of heterogeneous chain-interrup-tion reactions.

The surface to volume ratio was shown to inuence the chain length, in labora-tory studies performed in glass reactors, at subatmospheric pressures and at mod-erate temperatures [4345]. However, no effect was detected in metallic reactors andat the pressures used in industrial processes [45].

The nature of the reactor walls and the pretreatment of the metallic ones (byoxidation, reduction, suldation, etc.) has a strong inuence on the formation ofcoke [46], but has very small effect upon the kinetics of the overall pyrolysis process[45].

For these reasons, in studies of the reaction mechanism and the kinetics ofthermal decomposition reactions, only the chain interruptions by homogeneousmechanisms are taken into account.

The interaction between two radicals may be of two types: recombination anddisproportionation.

The recombination is the reverse of the initiation reaction and may be illu-strated by the reaction:

CH3 C

2H5! C3H8

The disproportionation can be illustrated by:

CH3CH2 CH3 C

H2! C2H4 C2H6

The rates of the disproportionation reactions is signicantly lower than thoseof recombination reactions.

Among recombination reactions, the recombination of two hydrogen atomswill take place at a very low rate, since a triple collision of the type:

2HM! H2 M

is required, with molecule M accepting the energy resulted from the recombination.Since the frequency of triple collisions is very low, the reaction takes place with avery low rate. A similar situation exists in the recombination of a methyl radical withatomic hydrogen.

In the case of radicals with higher molecular mass, the energy developed by therecombination of the radicals is dissipated over a larger number of interatomicbonds. This will favor the recombination of radicals.


The activation energy for the recombination and disproportionation of theradicals is practically equal to zero. Thus, the rates of these reactions is independentof temperature, and the rate constant is equal to the pre-exponential factor.

Pre-exponential factors for the recombination and disproportioning of someradicals are listed in Table 2.8.

Since in thermal processes the length of the kinetic chain exceeds 100200, itfollows that the composition of the products will be determined mainly by thesubstitution reaction and the decomposition of the radicals, that is, by the reactionsof developing the kinetic chain and not by those of initiation or interruption of thechain. This understanding made it possible to calculate the composition of theproducts resulting from the thermal decomposition of hydrocarbons.

Table 2.8 Pre-exponential Factors for Some Radicalic Recombinationsand Disproportionations

Reaction A (L mol1s1) Ref.

HC

H3 ! CH4 1.3 1011 6

HC

2H5 ! C2H6 8.5 1010 25

1 C3H7; 2 C

3H7;

H+ any of the radicals: 1 C

4H9; 2C

4H9;

i C4H9;C

5H11

1.0 1010 33

CH3 C

H3 ! C2H6 1.3 1010 25

CH3 C

2H5 ! C3H8 3.2 109

1.0 101127

44

CH3 C

3H7 ! C4H10 3.2 109 27

C2H5 C

2H5 ! C4H10 7.9 109 36

C2H5 C

3H71; 2 ! C5H12 8.0 108 33

1 C3H7 1 C

3H7 ! C6H14 4.0 108 6

i C4H9 i C

4H9 ! C8H18 3.2 109 48

C3H3 C6H5C

H2 ! C6H5CH2CH3 1.6 108 49

HC

2H5 ! H2 C2H4 2.1 109 6

H1 C

3H7 ! H2 C3H6 1.3 109 6

CH3 C

2H5 ! CH4 C2H4 2.3 108 6

CH3 1 C

3H7 ! CH4 C3H6 1.9 108 6

C2H5 C

2H5 ! C2H6 C2H4 1.2 108 6

C2H5 1 C

3H7 ! C2H6 C3H6 1.3 108 6

1 C3H7 1 C

3H7 ! C3H8 C3H6 1.1 108 6


Such a calculation is based on the rates of the substitution reactions, corre-sponding to the extraction of different hydrogen atoms from the initial feed. The ratedepends on the number of hydrogen atoms that have the same position in themolecule. It will also depend on the dissociation energy of the C H bonds of theH atoms in different positions (relation 2.4).

The data of Table 2.1 illustrate the signicant differences that might be foundamong these bond energies.

From the analysis of the data available at that time, F.C. Rice [32] acceptedaverage relative rates of extraction by free radicals, of the hydrogen atoms linked toprimary, secondary and tertiary carbon atoms.

The average activation energies accepted by F.C. Rice have the values:

Extraction of Hydrogen from kcal/mole

Primary C 92.0

Secondary C 90.8

Tertiary C 88.0

which, according to Eq. (2.4) lead to the following expressions for the relative rates:

rsec=rprim e604=Trtert=rprim e2013=T

which makes possible their calculation for different temperatures.According to F.O. Rice [31], the relative rates at 6008C, have the values:

1 for primary

2 for secondary

10 for tertiary carbon atoms

The calculation of the product composition after F.O. Rice is illustrated bymeans of Example 2.1.

Example 2.1. Calculate the composition of the products resulting from the ther-mal decomposition of i-pentane at 6008C. The assumptions corresponding to dif-ferent possible substitution reactions are:

ASSUMPTION A. Hydrogen abstraction takes place from the rst carbon atomor equivalently, from the carbon atom of the side chain.

CH3CHCH2CH3 R! RH C

H2CHCH2CH3

j jCH3 CH3

Assumption (a1) corresponds to the breaking of the C C bond in the sidechain and (a2) to the breaking of the C C bond from the main chain. The chancesof breaking these bonds are identical. The assumption (a02) corresponds to the


decomposition of the radicals ethyl to ethene and atomic hydrogen, while assump-tion hypothesis (a002) corresponds to the absence of this decomposition.

The probability of the decomposition (a02), which is not taken into account byF.C. Rice, was estimated by A.J. Dintes, to account for 20% of the ethyl radicalsformed at 6008C. Taking into account the radical which is reproduced, the overallreactions may be written as follows:

i-C5H12 ! CH4 1-C4H8 a1i-C5H12 ! H2 C3H6 C2H4 a01i-C5H12 ! C2H6 C3H6 a002ASSUMPTION B. Abstraction of hydrogen from the tertiary carbon:

CH3CHCH2CH3 R! RH CH3C

CH2CH3!

j jCH3 CH3

! RH i-C4H8 CH3

Taking into account the radical which is reproduced, the overall reaction willbe:

i-C5H12 ! CH4 i-C4H8 bASSUMPTION C. Abstraction of hydrogen from the third carbon atom:

CH3CHCH2CH3 R!RH CH3CHC

HCH3!

j jCH3 CH3

!RH 2-C4H8 CH3

The overall reaction will be :

i-C5H12 ! CH4 2-C4H8 cASSUMPTION D. Abstraction of hydrogen from the 4th carbon atom:

CH3CHCH2CH3 R!RH CH3CHCH2C

2!

j jCH3 CH3

!RH C2H4 CH3CHCH3!RH C2H4 C3H6 H

The overall reaction will be:

i-C5H12 ! H2 C2H4 C3H6 dBy using the relative rates at 6008C given by F.C. Rice and taking into account

the number of carbon atoms that have the same position in the molecule, the relativereaction rates given in Table 2.9 were obtained.

The overall stoechiometric equation for the thermal decomposition of i-pen-tane, at 6008C may be written:

115 i-C5H12 18 H2 85 CH4 18 C2H4 12 C2H6 30 C3H615 1 C4H8 50 i-C4H8 20 2 C4H8


More detailed studies [50] have shown that, owing to steric hindrances, theextraction rates of the hydrogen atoms linked to secondary and primary carbonsdepends also upon the nature of the radical that effectuates the extraction. At 5458C,the ratio of the extraction rates is 1.75 for extraction by C

2H5; 2.6 for C

H3 and 4 for

H, as compared with the value of 1, for radicals of higher molecular mass.

The comparison of the theoretical calculations with a great number of experi-mental data [35] leads to the conclusion that it is necessary to take into account notonly the nature of the radical that acts in the substitution reactions but also therelative rates of decomposition of the radicals formed in these reactions.

For radicals of higher molecular mass the isomerization reaction must also betaken into account [40].

Table 2.10 lists the kinetic parameters, recommended for calculations on thebasis of these studies [35].

In commercial pyrolysis, it is of great interest to predict the ratio of ethene topropene produced, as a function of the ratio n-/i-alkanes in the feed.

To this purpose, the ratio ethene/propene, resulting from pyrolysis at 1100 Kof n-octane, 4-methyl-heptane and 2,5-dimethyl-hexane, were calculated in Appendix1:

C2H4/C3H6n-octane 8.23

4-methyl-heptane 1.77

2,5-dimethyl-hexane 0.35

This calculation proves that the ethene/propene ratio diminishes strongly withthe degree of branching of the alkane feedstock.

The pyrolysis of heavy petroleum fractions (atmospheric and vacuum gas oils)has prompted studies [107,108], aiming at the understanding of the mechanism of thethermal decomposition of the C13 C14 n- and i-alkanes.

The decomposition reactions of other classes of hydrocarbons may be exam-ined analogously, by using data collected in Tables 2.12.8.

Alkenes. The data of Table 2.1 show, that alkenes may undergo breaking of theC C bonds, except those in position to the double bond.

Table 2.9

Reactions

Hydrogen atoms

occupying

identical position

Relative rate

of hydrogen

extraction

Overall relative

rate

a 6 1 6 1 = 6a1 0:5 a 0.5 6 = 3.0a02 0:2 a2 0.2 3 = 0.6a002 0:8 a2 0.8 3 = 2.4b 1 10 1 10 = 10c 2 2 2 2 = 4d 3 1 3 1 = 3


The rate of such initiation reactions is comparable with that for breaking theC C bonds in alkanes.

It should be observed that the bonding energy of the hydrogen linked to carbonatoms in position to the double bond is relatively low. Thus, for example in thepyrolysis of propene and isobutene, the initiation takes place by breaking theseweaker C H bonds. In both cases, the activation energy is of 322 kJ/mole and a

Table 2.10 Kinetic Data Recommended by E. Ranzi, M.Dente, S. Pierucci, and G. Bardi

Reaction

A (s1) or(L mol1s1) E (kJ/mol)

Hydrogen Abstraction Reactions (Primary H-Atoms)

by a primary radical 2.0 108 58.6by a secondary radical 2.0 108 67.0by a tertiary radical 2.0 108 71.2

Ratio between secondary and primary H-abstraction 9.63

Ratio between tertiary and primary H-abstraction 18.84

Radicals Isomerization (primary to primary hydrogen

transfer)

1-5 transfer 2 1010 58.61-4 transfer 1 1011 87.9

Radicals Decomposition Reactions

C2 C4 Radicals

C2H5 ! C2H4 H

3.2 1013 170.4

1 C3H7 ! C2H4 C

H3 5.0 1013 134.0

1 C3H7 ! C3H6 H

7.9 1013 159.1

2 C3H7 ! C3H6 H

1.1 1015 175.8

1 C4H9 ! C2H4 C

2H5 4.0 1013 121.4

1 C4H9 ! 1 C4H8 H

1.3 1014 163.3

2 C4H9 ! C3H6 C

H3 1.0 1014 138.2

2 C4H9 ! C4H8 H

1.6 1014 171.7

1; i C4H9 ! C3H6 C

H3 2.0 1014 142.4

1; i C4H9 ! i C4H8 H 1.0 1014 159.1

2; i C4H9 ! i C4H8 H

2.0 1014 175.8

Decomposition of Higher Radicals to Primary Radicals

primary radical 1.0 1014 125.6secondary radical 1.0 1014 129.8tertiary radical 1.0 1014 134.0

Correction to the Activation Energy (Related to Primary

Radical)

secondary radical 8.4

tertiary radical 16.7

methyl radical 6.3

Source: Ref. 35.


pre-exponential factor of 1013 sec1, which is comparable with the values of breakingof a bond a C C at alkanes.

By the breaking of a C C bond in alkenes, an allyl radical is formed. Similarly,an allyl radical will be formed by the breaking of the C H bond at the carbon in the position to the double bond, as well as in the interaction between a radical and analkene.

Thus, alkenes will preferably give allyl radicals of the formula:

a

b

or, give dienes:

H2CCHCHCH2R!H2CCHCHCH2 R

c

H2CCHCCH2R!H2CCHCCH2 R

j jCH3 CH3

d

To a lesser extent, higher allyl radicals may be formed. They may have the oddelectron in a position different than , as for example:

H2CCHCH2CH2CH2! C2H4 C

3H5

H3CCHCHCH2CH2! H3CCH CHCH CH2 H

H2CCHCH2CHCH2CH3! H2CCHCH2CHCH2 C

H3

Such radicals may appear also as result of breaking of a C C bond of a higheralkene.

The main types of radicals and the specic reactions that may be produced inthe case of alkene are those indicated by the letters (a)(d). Further, the additionalreactions of the radicals to the double bond given in the Table 2.7 must be consid-ered.

Radicals (a) and (b) are stabilized by the conjugation of the odd electron withthe electrons of the double bond, producing three conjugated electrons. The dis-tances between the carbon atoms become equal, intermediary to that correspondingto the single C C bond and double bond. The available energy will be distributedequally between the two marginal and the central carbon atoms. These radicals areinactive, since they do not posses sufcient energy for the extraction of a hydrogenatom from a feed molecule and for continuation of the reaction chain. They will beadsorbed on the wall or combined with another radical. If the radicals (a) or (b) wereformed as a result of the reaction of the alkene with an alkyl radical, the interruptionof the reaction chain will occur, leading to a slowdown of the process of thermaldecomposition.


Reactions (c) and (d) explain why at the high conversions, which are specic tothe pyrolysis process, liquid feed stocks produce large amounts of butadiene andisoprene in comparison with much lower amounts of other dienes.

The experimental studies of the pyrolysis of alkenes and the interpretation ofthe obtained results [38] are in full agreement with the above considerations.

Cycloalkanes. The bond energies between the carbon atoms of the 5 and 6 atomscycles (Table 2.1) were estimated by indirect calculations. Their values are compar-able to those between the carbon atoms in alkanes.

The breaking of the cycles leads to the formation of biradicals, which can reactaccording to one of the following schemes:

Among these, the formation of alkenes with the same number of carbonatomsreactions (b)was not be conrmed with certainty.

It accepted that in case of alkylcycloalkanes, the break will occur more easilybetween the carbon atoms of the side chains, than in the cycle. The formed alkylradical will follow the specic reaction paths of these radicals. The radical containingthe cycle will decompose according by cleavages in position, until nally, a cyclicradical is produced. A cyclic radical will also result following the action of a freeradical upon a cycloalkane.

The decomposition of cyclic radicals takes place as follows:

The methyl radicals may continue the reaction chain.


These reactions show the competition between the isomerization reaction, bymigration of the odd electron, and the reaction of decomposition of the radical.There are no reliable methods for estimating their relative rates at this writing.However, the high proportion of butadiene, which is formed in the pyrolysis ofcyclohexane, may be an indication of a higher rate of the isomerization comparedto the decomposition reaction.

The reactions of the radicals formed following the initial decomposition ofcyclohexane deserve special attention. Two reaction paths are observed:

a

(b)

Both decompositions can be justied by the effect of the odd electron on thebonding energies of the carbon situated in position.

The dissociation energy of the bonds broken according to reactions (a) and (b)are not known. It is however possible to estimate them by a calculation based uponthe use of relation (1.1) for the dissociation reaction [11,16].

For reaction (a) one obtains:

DC4H7a H0fT C4H6 H0fT H H0fT

C4H7

and for equation (b):

DC4H7b H0fT C2 H3 H0fT C2H4 H0fT C4 H7

By substitution in Eq. (2.4) it results:

rarb e

1RT H0fT C 2H3

H0fT C2 H4H0fT C4 H6H

0fT

H

h i

Using for radicals the data from Table 2.11 recalculated for the temperature of1000 K, and for hydrocarbons the heats of formation published by Stull [52], itresults:

rarb e330;63738;56095;208232;749

8:3191;000 e4:954 142

This means that during pyrolysis, the reaction (a) of formation of butadienewill be predominant, while (b) is negligible.

This method of calculation could be applied at the analysis of the results ofother reactions of thermal decomposition, for which the data on the dissociationenergy are not available.

The data of Table 2.11 make possible to compare on thermodynamic groundsthe expressions for rate constants for reversible reactions in which radicals partici-pate, such as the reactions of dissociation of the molecules versus those of recombi-nation of the resulting free radicals.


In favorable thermodynamic conditions, the cyclohexane and its alkyl deriva-tives may undergo dehydrogenation reactions with the formation of aromatic hydro-carbons.

The graph of Figure 2.7, which indicates the equilibrium for the dehydrogena-tion of methyl-cyclohexane to toluene, proves that such reactions may take placeduring pyrolysis, as well as in other thermal processes.

Experimental studies on the pyrolysis of unsubstituted poly-cycloalkanes [53]conrmed the above mechanisms for cyclohexane and lead to the reaction schemesfor other hydrocarbons.

For decahydronaphthalene the mechanism is [53,54]:1. Radical formation

Table 2.11 Thermodynamic Constants of Free Radicals

Radical

H0f ,298(kJ/mol)

Sof ,298(J/mol grd)

Cp(J/mol grd)

Source300 K 500 K 1000 K

H 218.14 114.72 20.81 20.81 20.81 6CH3 142.36 193.02 28.89 29.30 30.14 6C2H3 288.90 235.73 40.61 54.84 78.30 6C2H5 108.86 250.38 46.47 68.25 107.61 6C3H5 169.99 265.46 59.04 83.74 125.61 6n-C3H7 87.93 286.81 71.60 105.51 159.52 6

i-C3H7 73.69 279.27 71.18 104.26 158.69 6n-C4H9 66.32 336.64 97.05 143.15 213.20 6sec-C4H9 51.67 339.06 96.47 141.81 212.26 6tert-C4H9 28.05 312.35 79.97 127.70 205.16 6n-C5H11 45.60 376.29 120.08 177.70 264.87 6sec-C5H11 30.94 378.50 119.50 176.36 264.03 6C6H5 293.0 11C6H5CH2 180 12 51o-CH3C6H4CH2 113 4 11m-CH3C6H4CH2 121 8 11p-CH3C6H4CH2 117 6 11For other temperatures, the following equations may be used:

H0fT H0f 298 CpT 298 S0fT S0298 Cp lnT

298

where: Cp CpT Cp298

2


2. Decomposition2.1 Cleavage in position

2.2 Isomerization followed by cleavage in position

Similarly, for perhydrophenanthrene [54]:1. Radical formation

2. Decomposition2.1

32%


2.2a,b

12%

30%

2.3

26%

Aromatics The aromatic ring is remarkably stable, the average binding energybetween the carbon atoms within the ring being 407 kJ/mole [10]. The bindingenergy of the hydrogen atoms bound to the carbons of the ring is also higher thanin other hydrocarbons (see Table 2.1). For these reasons, the decomposition reac-tions affect especially the side chains attached to the rings.

The interaction of the free radicals with alkylbenzenes is illustrated by n-pro-pylbenzene, i-propylbenzene and tert-isobutylbenzene.

For n-propylbenzene the following reactions will take place:

C6H5CH2CH2CH3 R! RH C6H5C

HCH2CH3

! RH C6H5CHCH2 CH3 a

C6H5CH2CH2CH3 R! RH C6H5CH2C

HCH3

! RH C6H5 C3H6 b

C6H5CH2CH2CH3 R! RH C6H5CH2CH2C

H2

! RH C6H5CH2 C2H4 c


Overall reactions are:

For i-propyl-benzene:

a

b

Both reactions lead nally to the formation of styrene and methane, which is inaccordance with the experimental data:

For tert-isobutylbenzene:

The occurrence of reaction (b) is doubtful. If and when it however takes place,the formed hydrocarbon will isomerize to -methyl-styrene, similar to the dehydro-genation of isopropylbenzene.

Thus, the overall reaction may be written:


The radicals formed by breaking the alkyl chains off the benzene ring willdecompose further, following a similar pattern.

The use of atmospheric and vacuum gas oils as pyrolysis feed stocks hasinduced studies of the pyrolysis of model compounds with naphthenic-aromatic,aromatic and substituted-aromatic structures [54].

The thermal cracking of substituted aromatics having a long aliphatic chainwas examined using dodecylbenzene as a model compound. It was conrmed thatthe radicals formed by the initial breaking of the side chain extract one of thehydrogen atoms from the alkyl chain. The resulting alkyl-aromatic radical furtherdecomposes by successive cleavages. The nal product distribution is dependent onthe position of the carbon atom from which the hydrogen atom was extracted. Thenal result of the decomposition was formulated by the authors [54] by means of sixoverall reactions. They are:

33%

Odd electron at carbon atom in odd position (1,3,...) from cycle

22%

5%

Odd electron at carbon atom in even position (2,4,6...) from cycle

22%

5%

13%


In this study the following breaking energies of the side chain bonds were used:

Position of the bond relative to ring and followingDissociation energy, kJ/mole 407 287 340 342

The decomposition of the naphthenic-aromatic hydrocarbons was examinedtaking as a model tetrahydro-naphthalene and octahydro-phenanthrene [5356]. Theresults of the studies indicated that the presence of the aromatic rings favors thedehydrogenation of the naphthenic rings. This leads to the conclusion that for thesetypes of hydrocarbons the dehydrogenation reactions are in competition with thethermal cracking reactions. The latter lead to the formation of monocyclic aromatichydrocarbons with short chains, which are refractory to subsequent decompositions.

The nonsubstituted condensed aromatic hydrocarbons are completely refrac-tory against the decompositions; but it seems that they have also a stabilizing effect,of hindering the decomposition of other hydrocarbons which might be present. Thiseffect was observed at the pyrolysis of n-decane in the presence of naphthalene [54].

The condensed aromatics with methyl groups attached have a different beha-vior. Thus, the methyl-naphthalene gives methyl and naphthyl radicals, the latterleading also to condensation reactions.

Heteroatomic compounds The data contained in Table 2.1 show that in com-pounds with similar structures, the energy of C S bonds is lower than that of theC C bonds in hydrocarbons. It results that the initiation of thermal reactions insulphur compounds will probably take place by breaking of the bonds C S orS S, with formation of H2S and mercaptans respectively, which agrees with theexperimental ndings.

The lower energy of the C-S bonds leads to a greater energy of the C H bondsfor the carbon atoms neighboring the sulphur atom, as compared with the energiesof the corresponding C H bonds in hydrocarbons.

Also, taking into account that the energy of the H S in hydrogen sulde is of377 kJ/mole, and the energy of the C S bond in mercaptans is of about 293 kJ/mole,it results that the energy of the H S bond in mercaptans is of the order of 2 377293 461 kJ/mole. This energy is greater than that of the C H bond in hydrocar-bons. Thus, the hydrogen atom in the H S of mercaptans can not be extracted by aradical.

As in the case of hydrocarbons, the composition of the products of thermaldecomposition of sulfur compounds is determined by the interaction of the feedmolecules with the radicals. Thus, for some sulphur compounds, schemes concerningthe result of such interactions can be formulated as:

From this reaction one can anticipate that the products obtained from thedecomposition of propylmercaptan are methylmercaptan and ethene or hydrogensulde and propene, for the two possible pathways shown.

For similar reasons, the ethylmercaptan is expected to decompose preferen-tially to ethene and hydrogen sulde.


The decomposition of n-butyl-mercaptan can take place according to the fol-lowing reactions:

It results that the decomposition of n-butyl-mercaptan and of higher mercap-tans will produce besides hydrogen sulde, methyl-mercaptan and hydrocarbons,also mercaptans with a carbon-carbon double bond, according to pathway c1.

The decomposition of the suldes may be illustrated by:

CH3CH2SCH2CH3 R! C

H2CH2SCH2CH3

! C2H4 CH3CH2 S

with formation of ethene and mercaptan, as nal products.The experimental results conrm the decomposition of suldes to alkenes and

mercaptans and the decomposition of mercaptans to alkenes and hydrogen sulde.Also, the industrial practice conrms the important amounts of methyl-mercaptanand hydrogen sulde expected in the cracking gases.

For disuldes and cyclic sulfur compounds (thiophenes, thiophanes, etc.) theavailable data do not allow the formulation of even hypothetical cracking mechan-isms. It is known that the cyclic compounds are much more stable than the acyclicones.

The presence of elementary sulfur in the heavy products resulting from pyr-olysis may be explained as result of the decomposition of disuldes. Thus, it may beassumed that the interaction of the disuldes with the radicals, followed by thecleavage in position, may lead, among others, to the formation of two types ofradicals:

R S

and RS S

The rst one, may produce a mercaptan:

R SR1H! RSHR

1

while the second, by a decomposition in , may give elemental sulfur:

RS S! S2 R

The thermal decomposition, of oxygen compounds produces mainly phenoliccompounds. They (especially m-cresol) can be recovered from the gasoline obtainedin thermal cracking.

In a similar manner, the nitrogen compounds give pyridyne bases. These can berecovered from gasoline fractions, and may be sold as useful byproducts


chemical engineering thermal and catalytic processes in petroleum refining s. raseev

Documents