Ind. E n g . C h e m . R e s . 1988,27, 755-759
755
S, 7704-34-9; Clz, 7782-50-5; K, 7440-09-7; Ca, 7440-70-2; poly-
McGill, 1987). Other promising materials include silicized steels (Brown et al., 1982) and steels coated with manganese-rich surfaces (Suzuki et al., 1986). Many nonmetals in the coke are probably due to impurities in the water used to make the steam that is a diluent for the hydrocarbon feedstock. The presence of sulfur could be either because of sulfur-containing compounds in the water or sulfur compounds often deliberately added as deactivators of the stainless steel surfaces. More data are needed to determine if or how nonmetals affect coking and decoking. Information obtained on cokes produced from atactic polypropylenes clarify some aspects of coke formation in ethylene furnaces and especially the coke produced from tarry liquids. Escape of the gases through coke deposits and surface-catalyzed reactions are apparently two important factors when high molecular weight liquids are p yr olyzed .
propylene, 9003-07-0.
Literature Cited Albright, L. F.; McGill, W. A. Oil Gas J. 1987, Aug 31, 46-50. Albright, L. F.; Tsai, T. C. In Pyrolysis: Theory and Industrial Practice; Albright, L. F., Crynes, B. L., Corcoran, W. H., Eds.; Academic: New York, 1983; Chapter 10, pp 233-254. Baker, R. T. K.; Harris, P. S. In Chemistry and Physics of Carbon; Walker, P. L., Thrower, P. A,, Eds.; Marcel Dekker: New York, 1978; Vol. 1, Chapter 1. Brown, D. E.; Clark, J. T. K.; Foster, A. I.; McCarroll, J. J.; Sims, M. L. In Coke Formation on Metal Surfaces; Albright, L. F., Baker, R. T. K., Eds.; ACS Symposium Series 202; American Chemical Society: Washington, D.C., 1982; Chapter 5, pp 23-44. Dunkleman, J. J.; Albright, L. F. In Industrial and Laboratory Pyrolyses; Albright, L. F., Crynes, B. L., Eds.; ACS Symposium Series 32, American Chemical Society: Washington, D.C., 1976; Chapter 14, pp 241-260. Graff, M. J.; Albright, L. F. Carbon 1982,20, 319. Marek, J. C.; Albright, L. F. In Coke Formation on Metal Surfaces; Albright, L. F., Baker, R. T. K., Eds.; ACS Symposium 202, American Chemical Society: Washington, D.C., 1982a; Chapter 7, pp 123-150. Marek, J. C.; Albright, L. F. In Coke Formation on Metal Surfaces; Albright, L. F., Baker, R. T. K., Eds.; ACS Symposium Series 202; American Chemical Society: Washington, D.C., 1982b; Chapter 8, pp 151-176. Suzuki, G.; Uchida, M.; Ohsaki, K.; Onodera, T.; Unemura, T.; Sundaram, K. M. "Development of Coke Deposition Retarding Bimetallic Tubes for Ethylene Cracking Furnaces". Paper presented at the National Meeting of American Institute of Chemical Engineers, New Orleans, April 6-10, 1986. Tsai, T. C.; Albright, L. F. In Industrial and Laboratory Pyrolyses; Albright, L. F., Crynes, B. L., Eds.; ACS Symposium Series 32; American Chemical Society: Washington, D.C., 1976; Chapter 16, pp 274-195.
Summary Materials of construction are needed for the coil, transfer line, and TLX plus possibly the preheater coils that will minimize surface-catalyzed coke formation and will also minimize spalling of particles containing nickel, iron, and chromium. Operating modifications that will likely result in less coke formation have been suggested. Acknowledgment Financial support for this investigation was provided by Gulf Research Foundation, Alon Processing, Inc., and Purdue Research Foundation.
Received f o r review September 19, 1986 Revised manuscript received December 8, 1987 Accepted December 21, 1987
Registry No. C2H4,74-85-1; C2H6,74-84-0; C3HB,74-98-6; Fe, 7439-89-6 Cr, 7440-47-3; Ni, 7440-02-0; Si, 7440-21-3; P, 7723-14-0;
Mechanistic Model for Formation of Coke in Pyrolysis Units Producing Ethylene Lyle F. Albright* and James C. Marekt School of Chemical Engineering, Purdue University, W e s t Lafayette, Indiana 47907
Coke is formed by three mechanisms during pyrolysis processes used to produce ethylene. The mechanisms result in metal-catalyzed coke, noncatalytic coke formed from tars, and pyrocarbon coke that is produced when small precursors react with free radicals on the coke surface. The relative importance of each of the above mechanisms depends on the operating conditions, hydrocarbon feedstock, and type of reactor used. Although much has been reported in the past on coke formation and on various surface reactions that occur during the pyrolysis of various hydrocarbons, no known model has yet been proposed to explain several features of coke formation in both laboratory and industrial furnaces. Albright and Marek (1988a,b) have recently obtained extensive data that are particularly applicable to pyrolysis units producing ethylene and other unsaturates. Their results and those in the literature were employed to develop a mechanistic model in which coke is produced by three different methods. This model is likely also ap'Present address: E. I. du Pont de Nemours & Co., Inc., Aiken, SC 29801.
0S88-5885/88/2627-0755$01.50/0
plicable to other high-temperature processes including the dehydrochlorinationof 1,2-dichloroethaneto produce vinyl chloride. Three Coking Mechanisms The three mechanisms by which cokes are produced in the coil, transfer line, and transfer-line exchanger (TLX) of an ethylene furnace are reviewed next. Mechanism 1 involves metal-catalyzed reactions in which metal carbides are intermediate compounds and for which iron and nickel are catalysts (Baker and Harris, 1978; Trimm, 1983). The resulting fiiamenteous coke often contains 1-2 w t % metal; the metal is positioned primarily at the tips of the filaments. Other catalytic cokes have 0 1988 American Chemical Society
756 Ind. Eng. Chem. Res., Vol. 27, No. 5, 1988
been designated as arrowhead or needle cokes (Albright et al., 1979); such cokes may, however, be mixtures of filamenteous coke and noncatalytic coke produced by mechanisms 2 and 3, to be discussed later. Filamenteous coke is produced a t temperatures from about 400 OC up to at least 1050 "C (Albright and Marek, 1988a), and it is often the predominant coke formed on clean stainless steel surfaces. It does not form, however, on noncatalytic surfaces such as glass or aluminized steel surfaces, as prepared by Alon Processing, Inc. Filament thickening that causes an increase in the diameter of the filaments occurs by mechanism 3. Mechanism 2 has been described by Lahaye et al. (1977). Aromatic3 (Ar-H) are probably the most important intermediates; some aromatics are produced by trimerization and other reactions involving acetylene. Starting with simple aromatics, the following sequence of condensation and dehydrogenation reactions occurs in the gas phase to produce tar droplets and soot particles:
-H2
nAr-H polynuclear aromatics (tars) tar droplets
-Hz
nucleation
and condensation*
--H2
semitar droplets coke particles (soot)
Bennett (1981) using an electron microscope has seen the droplets hit solid surfaces. The droplets either rebound from the surface or adhere to it. The tar or semicoke adhering to the surface decomposes to form hydrogen and coke containing numerous surface radicals (Baker, 1985). Tars with low viscosities wet the solid surfaces and spread out on it; a relatively featureless coke results. Semitar droplets with high viscosities, however, retain their sphericity and often cluster. This coke is metal-free if quartz or Vycor glass surfaces and a metal-free feedstock are used. In industrial furnaces, however, metal particles or compounds spall from the tube walls; some of these particles collect in the tar droplets that eventually are converted to coke. Mechanism 2 is generally relatively unimportant at temperatures of 700 "C or less. The exact temperatures depend to some extent on the hydrocarbons being pyrolyzed. Spherical coke particles on the surface grow in size, however, by mechanism 3. Mechanism 3 involves as a first step the reactions of microspecies with the free radicals on the coke surfaces. These microspecies with molecular weights of usually 100 or less are often acetylene (Harris and Weiner, 1984), probably ethylene or other olefins, butadiene, and free radicals such as methyl, ethyl, phenyl, or benzyl radicals. Acetylene reacts with surface radicals to form aromatictype rings; dehydrogenation of these rings results in more coke and more surface radicals that permit further reactions with microspecies. With ethylene, a free-radical polymerization sequence or a modification of it can be postulated. Dente et al. (1983) and Trimm (1983) suggested a polymerization technique but gave no details on the probable chemistry. Mechanism 3 explains the thickening of filaments and the growth of spherical coke particles on the surfaces. Figure 1 indicates a version of mechanism 3 in which methyl radicals react to form attached methyl groups. On the surface, C-H bonds are broken, re-forming surface radicals and producing hydrogen radicals or molecules. A chain reaction is in essence occurring, and the coke is formed one carbon atom at a time. Phenyl radicals (formed from benzene), benzyl radicals (formed from toluene), and ethyl radicals probably also react with surface radicals by a similar chain mechanism. With the larger
Microspecies such a s alkyl or phenyl radicals react with with s u r f a c e radicals (9) to start chain m e c h a n i s m . E x a m p l e with methyl radicals
Z
H
S-i-H
S-t.
+
H.
H
4 a n d 5 ) Reactions similar to 2 a n d 3 continue the chain
6
All C-H b o n d s a r e eventually b r o k e n
Figure 1. Postulated chemical steps when methyl radicals react with surface free radicals (one version of mechanism 3 coking reactions).
radicals, more than one carbon atom is added to the surface each time a radical reacts. Surface free radicals likely couple readily with gaseous free radicals; the coke which is quite graphitic is a highly delocalized aromatic system, and resonance stabilization aids the coupling step. Since the microspecies have low molecular weights, they diffuse rapidly in the high-temperature gases (Cleland and Wilhelm, 1956). Hence, the microspecies have fairly uniform concentrations at all radial positions in a tubular reactor and on all sides of spherical or filamenteous coke; such a conclusion is especially true in a laboratory unit operated with relatively low flow velocities. As a result, both filamenteous and spherical coke grow in size rather uniformly in all radial directions. Such a finding was reported by both Tibbetts (1984) and Albright and Marek (1988a). Explaxiation of Laboratory Coking Results Laboratory, as well as industrial coking, results can be explained in considerable detail by considering mechanisms 1, 2, and 3. When noncatalytic surfaces are used, as can easily be done in laboratory units, only mechanisms 2 and 3 need to be considered. The results of Albright and Marek (1988a) are used as an example for explaining laboratory results; their results are particularly suitable since they investigated a wide range of operating conditions and since some variables of importance were investigated for the first time. Their results indicate that coking rates and coke morphology varied significantly as the residence time of the reaction mixture at high temperatures increased. Plots of the rate of coke formation versus residence time indicated one or, even in one occasion, two maxima. In interpreting such results for acetylene runs, the coke precursors of importance for each of the three mechanisms must be considered. Acetylene is a highly effective precursor for both mechanisms 1and 3. The maximum rate of coke formation on stainless steel coupons and the coke morphology at low residence times (and a high concentrations of acetylene) were obviously caused by coke formed mainly by mechanisms 1 and 3. Acetylene, however, also reacts in the gas phase to produce various coke precursors, and Figure 2 is a qualitative prediction of the concentrations of such precursors as a function of time. Low molecular weight microspecies such as benzene are the initial precursors produced, and later reactions result in polycyclic aromatics, tar droplets,
Ind. Eng. Chem. Res., Vol. 27, No. 5, 1988 757
GROSPEC
H
I ES
1
2
3
4
,-, TAR DROPLETS
TPARTICLES
RESIDENCE T I M E
Figure 2. Pmtulated concentrations of coke precursors in gas phase for acetylene pyrolysis at -1000 OC and up to 10-15-8 residence time.
semitar droplets, and coke or soot particles. Maxima in the rates of coking a t intermediate residence times are explained by these heavier coke precursors and the major importance of mechanism 2. Toluene and all reaction products formed from it are however not highly effective coke precursors for mechanism 1;hence, mechanism 1is of relatively minor importance when toluene is used. Toluene, however, produces higher molecular weight precursors such as tar;mechanism 2 is hence of primary importance especially at intermediate or higher residence times. Based on experimental results, mechanism 3 apears also to be of major importance when toluene is used as a feedstock. This finding suggests that benzyl and/or phenyl radicals are important and effective precursors. Prior to coke formation on a solid surface, coke precursors must either react on or collect on the solid surfaces in the reactor. These precursors must therefore be transferred to the solid surfaces; both the geometry of the reaction system and gravity were found to have an important effect on the transfer steps of the higher molecular weight precursors (Albright and Marck, 1988a). By use of tar droplets as an example in a coil reactor, improved transfer to the surface causes coke formed by mechanism 2 to occur over a longer length of the coil and/or a t positions in the reactor that are further upstream. Although the flow velocity of the gas stream in the reactor has not been extensively investigated as an operating variable in coking experiments, the velocity likely has a major effect on the transfer steps of the heavier coke precursors. To obtain extremely high velocities in a laboratory unit that are similar to those in industrial units will not be easy. The degree of graphicity of a coke likely depends on several factors which include a t least the following: 1. Relative Importance of Mechanisms 1 , 2 , and 3 in the Coking Process. Cokes formed by mechanism 1 and by mechanism 3 (Tibbetts, 1983a,b) are quite graphitic, whereas those formed by mechanism 2 are presumably mostly amorphous. 2. Type of Precursor. In the case of mechanism 3, the nature of the microspecies is likely very important; phenyl, benzyl, or other aromatic radicals may produce a more graphitic coke than alkyl radicals. Acetylene likely also produces a graphitic coke. 3. Surface of Metal, Coke, etc., on Which Coke Is Deposited. Considering mechanism 3, a more graphitic coke
of f i l a m e n t o u s c o k e o n " c l e a n " steel rurface e a r l y i n r"".
I.
Forvation StalnleSS
2.
T h i c k e n i n g o f f i l a m e n t s a n d collection of t a r or seni-tdr droplets. Incomplete f i l l - i n near metal surface.
3.
Fill-in mechanism c r e a t e s e ~ s e n t i a l l y "on-porous c o k e away f r o m t h e m e t a l j u r f a c e , b u t some p o r o s i r y 11 retailed near the metal.
L .
dvnaaic c o k e grov:il, i h i c h moves LO i ! c 1 time, crea:i?q 6 1 4 *:indl ~ ~ _ c x ? ~ s s n o n - p o - 3 ~ ~i 7 k e . S p h e r e s g r ? ~' r ~ m a d 5 2 r 3 i l n n and ieac:.on of I I C T O S D ~ C ~ ~ ~ . R e a ~ o no f c h e cl:ht
of
Figure 3. Growth of coke deposits on stainless steel surfaces as a function of time.
may be produced when the outer layers of the coke are highly graphitic as compared to those that are amorphous or microscopically rough. 4. Non-Hydrocarbon Gases in Reaction Mixtures. Hydrogen and steam, for example, may indirectly affect the graphicity of the coke. Both gases react with coke at high temperatures to gasify it. Amorphous coke may react more rapidly than graphitic coke; in such a case, the level of graphicity of the residual coke would be increased. Explanation of Industrial Coking Results A much improved understanding is now possible of the mechanisms by which cokes form in industrial furnaces as a function of run time. The results obtained by Albright and Marek (1988b) are particularly useful. The formation of coke on the stainless steel surface of an ethylene coil is divided into at least two time periods, as depicted by Figure 3. In a plant run that is often 4-8 weeks in length, filamenteous and other metal-catalyzed cokes form in high concentrations during the initial stages of the run (when decoked or clean stainless steel surfaces such as HK-40 or Incoloy 800 are present); mechanism 1 is then especially important. The filaments are excellent collection sites for tar or semitar droplets. Industrial cokes near the stainless steel surfaces frequently contain 1-2 wt % metals. A few industrial cokes have, however, been found with lower concentrations of metal in this region. When the metal content is high, the numerous filaments that form prevent the tar droplets from completely filling in the voids around the base of the filaments. Simultaneously the filaments thicken via mechanism 3. As tar droplets collect on the tops of the filaments, solid coke layers begin to form. When that occurs, tar droplets and microspecies cannot penetrate and complete the filling-in operation, and a porous layer of coke results near the metal surface. Filling in the void spaces occurs by means of both mechanism 2 and 3, and a relatively solid coke matrix is produced when fewer filaments are present. With time, the solid coke increases in thickness. Most of the metal to catalyze the production of more filaments (by means of mechanism 1)is transferred from the inlet or intermediate section of the coil in the ethylene furnace (Albright
758 Ind. Eng. Chem. Res., Vol. 27, No. 5, 1988
MECHANISM 2
MECHANISM 3
Figure 4. Approximate relative importance of mechanisms 1,2, and 3 for industrial and laboratory cokes.
and Marek, 1988b). The amounts of metals entrained and transferred in the gas stream affect the number of filaments produced and hence the importance of mechanism 1 as the solid coke layer grows in thickness. The hydrocarbon feedstocks used in pyrolysis units and the operating conditions affect the types and the concentrations of coke precursors; hence they affect the relative importance of the three mechanisms and the rates of coking. The precursors formed from ethane and naphtha feedstocks are for example obviously quite different. Various naphthas also often produce quite different precursors. In a TLX, the sudden decrease in gas temperatures results in condensations and the production of considerably more tar droplets. In addition, radical destruction by recombination is favored over production of radicals. Hence, the relative importance of the three coking mechanisms shifts when the gases enter the TLX’s, and mechanism 2 becomes more important. In the coil, mechanism 3 is likely most responsible for the fill-in operations. Photomicrographs of cokes from coils and TLX’s support these conclusions. Figure 4 is a prediction of the relative importance of mechanisms 1,2, and 3 during the production of coke in the coils and TLX’s of a typical ethylene unit. As already indicated, mechanism 1is of more importance immediately after decoking when both the coils and TLX’s have just been cleaned and are essentially coke free. The predictions of course also vary with operating conditions and hydrocarbon feedstocks. As also shown, mechanism 1is of little or no importance when glass surfaces or aluminized steel surfaces are used; both glass-coated and aluminized surfaces have been used in recent pilot plant and/or industrial plant runs. Some iron or nickel may, however, be present in such runs due to imperfections in the coatings, diffusion through the coatings, or impurities in the feed. Mechanism 3 is favored by higher temperatures and with higher concentrations of acetylene in the gas stream. A t these conditions, considerable amounts of very thick filaments were produced (Albright and Marek, 1988a). The mechanism proposed here is undoubtedly also applicable to coke production in industrial dehydrochlorination units used to produce vinyl chloride. Many of the coke precursors mentioned above are also formed in these vinyl chloride units, e.g., acetylene, benzene, and butadiene.
Recommendations Although the proposed mechanism contributes significantly to an understanding of coke formation in ethylene units, additional information is needed to clarify the following factors: 1. Role of steam and factors affecting coke gasification (or formation of carbon oxides) during pyrolysis. Dunkleman and Albright (1976) and Siklos et al. (1986) reported that carbon oxides are formed almost exclusively because of metal-catalyzed reactions on the walls of a coil; few or no carbon oxides formed when coke was deposited on glass reactors. The relative rates of gasification of graphitic and amorphous cokes should be clarified. 2. Source of metal and predominant method of metal transfer in the coil, transfer line, and TLX. Vaporization of metal sulfides is sometimes important (Marek and Albright, 1982). 3. Roles of nonmetals such as sodium, potassium, calcium, phosphorus, chlorine, and sulfur in coking and decoking. Some nonmetals promote decoking steps (Trimm, 1977). 4. Importance of hydrogen chloride and chlorine in the gas phase of vinyl chloride units. As metal chlorides form, surface compositions and roughness are affected. Ferric chloride is, for example, quite volatile. The roles of hydrogen chloride and metal chlorides should be investigated with respect to coke formation, gasification, and the kinetics of dehydrochlorination. Bernstein and Albright (1972) showed, for example, that chlorine sometimes promotes free-radical reactions on the surfaces. Acknowledgment Financial support for this investigation was provided by Gulf Research Foundation, Alon Processing, Inc., and Purdue Research Foundation. Registry No. C2H4, 74-85-1.
Literature Cited Albright, L. F.; Marck, J. C. Ind. Eng. Chem. Res. 1988a, first in a series of three in this issue. Albright, L. F.; Marek, J. C. Ind. Eng. Chem. Res. 1988b, second in a series of three in this issue. Albright, L. F.; McConnell, C. F.; Welther, K. In Thermal Hydrocarbon Chemistry; Oblad, A. G., Davis, H. G., Eddinger, R. T. Eds.; Advances in Chemistry Series 183; American Chemical Society: Washington, D.C., 1979; pp 175-191. Baker, R. T. K., personal communications, 1985. Baker, R. T. K.; Harris, P. S. In Chemistry and Physics of Carbon; Walker, P. L., Thrower, P. A,, eds.; Marcel Dekker: New York, 1978; Vol. 1, Chapter 1. Bennett, M. J., personal communication, March 1981. Bernstein, L. S.; Albright, L. F. AIChE J. 1972, 28, 141. Cleland, F. A.; Wilhelm, R. H. AIChE J . 1956, 2 , 489. Dente, M.; Ranzi, E.; Barendregt, S., Jr.; Tsai, F. W. “Ethylene Cracker Transferline Exchanger Fouling”. Presented a t the National Meeting American Institute of Chemical Engineers, Houston, TX, March 1983. Dunkleman, J. J.; Albright, L. F. In Industrial and Laboratory Pyrolyses; Albright, L. F., Crynes, B. L., Eds.; ACS Symposium Series 32; American Chemical Society: Washington, D.C., 1976; Chapter 14, pp 241-260. Harris, S. J.; Weiner, A. M. Combust. Sei. Technol. 1984, 38, 75. Lahaye, J.; Badie, P.; Ducret, J. Carbon 1977, 15, 87. Marek, J. C.; Albright, L. F. In Coke Formation on Metal Surfaces; Albright, L. F., Baker, R. T. K., Eds.; ACS Symposium Series 202; American Chemical Society: Washington, D.C., 1982; Chapter 7 , pp 123-150. Siklos, P.; Izsaki, Z.; Varga, T. A.; Kalman, J. Erdol Kohle-Erdgas Petrochem. Brenstoff-Chem. 1986, 39, 243.
Znd. Eng. Chem. Res. 1988,27, 759-764 Tibbetts, G. G. GMR-4518 Report, Oct 21, 1983a; General Motors Research Laboratories, Warren, MI. Tibbetts, G. G. Appl. Phys. Lett. 1983b, 42(8), 666. Tibbetts, G. G., personal communications, 1984. Trimm, D. L. Catal. Reu.-Sci. Eng. 1977,16, 155. Trimm, D. L. In Pyrolysis: Theory and Industrial Practice; Al-
759
bright, L. F., Crynes, B. L., Corcoran, W. H., Eds.; Academic: New York, 1983; Chapter 9, pp 203-232.
Received for review September 19, 1986 Revised manuscript received September 21, 1987 Accepted December 11, 1987
Thermal Decomposition of Cyclohexane at Approximately 810 "C F. Billaud,*t P. Chaverot,t M. Berthelin,* and E. Freundg Departement de Chimie Physique des Rgactions, U A 328, CNRS, INPL-ENSIC, 1 rue Grandville, 54042 Nancy, France, Direction de Recherches, Recherche et Ddueloppement, Institut Franqais du Pdtrole, Centre d'Etudes et de Ddveloppement Industriels, Solaize, BP 3, 69390 Vernaison, France, and Direction de Recherche Physico-Chimie Applique6 et Analyses, Institut Franqais du Pdtrole, 1 et 4 Avenue de Bois PrQau, BP 311, 92506 Rueil-Malmaison, France
In order to determine the behavior of cycloparaffins in steam cracking reactions, a study of the thermal decomposition of cyclohexane in the presence of n-decane has been carried out at ca. 810 "C by using the technique of plug flow reactor. Cyclohexane chiefly decomposes into ethylene, hydrogen, 1,3butadiene, and small amounts of cyclohexene. We propose a primary mechanism of the decomposition of cyclohexane initiated by n-decane. This mechanism leads to three main primary stoichiometries which account for the whole range of reaction products. 1. Introduction: Aims and State-of-the-Art Mushrush and Hazlett (1984) have pyrolyzed model compounds representative of shale crude to confirm that molecules containing long unbranched alkyl groups may be the source of n-alkanes in jet fuels derived from this source. We have used a rather similar approach in the present paper to describe and explain olefin production during the steam cracking of petroleum cuts liable to contain large amounts of naphthenic compounds in relation to linear alkanes. We began by investigating the n-decane-cyclohexane mixture. This model can simulate the behavior of cycloparaffins in naphtha. A better understanding of the initial decomposition processes of cyclohexane is thus very important for improving our understanding of the cracking and oxidation of cycloparaffins. Decompositions of naphthenic compounds have not been analyzed with as much precision as those of paraffins. Although a number of studies of cyclohexane or substituted cyclohexane pyrolyses have been reported, these were mainly concerned with product distributions and yields at high conversion and with possible contributions of surface effects (Fabuss et al., 1964a,b; Romavacek et al., 1972; Levish et al., 1969; Zdonik et al., 1967; Frey, 1949; Garnett et al., 1976); Gordon (1962) and Stein and Rabinovitch (1975) detected the isomerization of the cyclohexyl radical into methylcyclopentyl radicals in low-temperature experiments. Above 400 "C, cyclic radicals begin to split up (Gordon, 1962; Stein and Rabinovitch, 1975; Arai et al., 1960). Mechanistic modeling has been useful in studying the kinetics of pyrolytic reactions at low conversion (Tanaka et al., 1975; Powers and Corcoran, 1974; Murata and Saito, 1975). Few attempts have been reported at high conver-
sion levels similar to those of industrial cracking (Sundaram and Froment, 1978;Aribike et al., 1981). This stems from the large number of molecular species and free radicals with their associated reactions. The number of these reactive species drastically increases with conversion and leads to excessive computation time. Most of these studies have a particular emphasis on kinetics within the Rice (1931, 1933), Rice and Herzfeld (1934), and Rice and Kossiakoff (1943) theory parameters. Tsang (1978a,b) investigated the decomposition mechanism and initial velocities of cyclohexane from singlepulse stock tube experiments and demonstrated that the main initial process is the isomerization of cyclohexane into 1-hexene followed by the decomposition of 1-hexene. The initiation of chains during the dissociation of pure cyclohexane is difficult. The breaking of the C-H bonds requires an energy expenditure of 95.2 kcal/mol (Kondratiev, 1974), whereas breaking along the C-C bonds occurs at much lower velocity. This breaking requires only 77 kcal/mol and does not lead to the formation of any monoradicals. The isomerization of the biradical that is formed
through the intermediary of an activated complex with six centers considerably facilitates the initiation of chains following the formation of a weakened C-C bond in 1hexene. The breaking of this bond leads to a resonant allyl form (C3H5.):
+
C3H~
CH2=CH(CH2)&H3
-
I *CH2CH2CH3
1
t CNRS.
Institut Frangais d u PBtrole, Vernaison.
* Institute Frangais du PBtrole, Rueil-Malmaison. f
0888-5885/88/2627-0759$01.50/0
Ho
CH ,,
+
0 1988 American Chemical Society
C H j
+
CH2=CHCH2*
