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Ind. Eng. Chem. Res. 1988, 27, 1348-1356
Connick, R. E., private communication, 1987. Hofmeister, H. Z.; Van Wazer, J. R. Inorg. Chem. 1962, 1, 811. Hudson, J. L. "Sulfur Dioxide Oxidation in Scrubber Systems". EPA-600/7-80-083, 1980; EPA, Washington, D.C. Huss, A., Jr.; Eckert, C. A. J. Phys. Chem. 1977,81, 2268. Huss, A., Jr.; Lim, P. K.; Eckert, C. A. J. Phys. Chem. 1982,86,4224, 4229. Lim, P. K.; Hws, A., Jr.; Eckert, C. A. J. Phys. Chem. 1982,86,4233. Littlejohn, D.; Chang, S. G. Enuiron. Sci. Technol. 1984, 18, 305.
Millen, D. J. J. Chem. Soc. 1950, 2589. Nair, V. S. K.; Nancollas, G. H. J. Chem. Soc. 1958, 4144. Ryss, I. G.; Drabkina, A. Kh. Kinet. Katal. 1973, 14, 242. Simon, A.; Wagner, H. Z. Anorg. Allg. Chem. 1961, 311, 102. Thilo, E.; von Lampe, F. Zn. Anorg. Allg. Chem. 1963, 319, 387. Received for review November 30, 1987 Revised manuscript received March 9, 1988 Accepted March 21, 1988
Asphaltene Reaction Pathways. 4. Pyrolysis of Tridecylcyclohexane and 2-Et hyltetralin Phillip E. Savaget and Michael T. Klein* Department of Chemical Engineering and Center for Catalytic Science and Technology, University of Delaware, Newark, Delaware 19716
The thermal reactions of the alkylnaphthenic and alkylhydroaromatic moieties likely present in petroleum asphaltenes were investigated Via pyrolysis of the model compounds tridecylcyclohexane (TDC) and 2-ethyltetralin (2ET), respectively. TDC pyrolyzed to the major product pairs cyclohexane plus l-tridecene and methylenecyclohexane plus n-dodecane, both in initial selectivities of about 0.08. Cyclohexene, methylcyclohexane, and n-tridecane also formed but in lower selectivities of about 0.0388,0.035, and 0.030, respectively. Arrhenius parameters of [log A (s-'), E * (kcal/mol)] = [14.9, 59.41 were determined over the temperature range 400-450 "C. 2ET pyrolyzed to 2-ethylnaphthalene, 2-ethyldialins, naphthalene, dialin, and tetralin as major products in yields of 1.73%, 1.19%, 1.13%, 1.09%, and 0.17%, respectively, a t low (=5%) conversions and 3.79%,0, 30.2%, 2.19%, and 6.49%, respectively, at high (=65%) conversions. The temporal variation of these products permitted deduction of the reaction network. It includes pathways for primary reaction of 2ET to dialin and the 2-ethyldialins and secondary reactions accounting for the formation of the other major products. The Arrhenius parameters for the disappearance of 2ET were [log A @I), E * (kcal/mol)J = [12.7, 53.51. The pyrolyses of both compounds were consistent with free-radical reaction mechanisms. The implications of the results of this model compound study to the reactions of the analogous moieties in asphaltenes are suggested. Petroleum asphaltenes, operationally defined as the heptane-insoluble, benzene-soluble fraction of heavy oils and residua, are a physically and chemically complex mixture of polar and aromatic compounds. Their ubiquity in heavy oils and their propensity for deactivating catalysts have rendered asphaltenes, and their reactions under hydroprocessing conditions, to be of considerable practical significance. Unfortunately, however, the complexity of both asphaltenes and their reaction products frustrates the deduction of reaction engineering fundamentals from direct experiments with precipiated asphaltenes. Resolved information is typically limited to the apparent kinetics of asphaltene disappearance and the yields of solubilitybased product fractions (Schucker and Keweshan, 1980; Savage et al., 1985; Ternan, 1983). This complexity motivates complementary studies wherein model compounds mimicking key reactive moieties in asphaltenes are reacted to probe asphaltene reaction pathways. Experiments with model compounds can provide intrinsic kinetics, operative reaction pathways, and, in favorable instances, reaction mechanisms. The literature suggests (Yen et al., 1984a,b; Takegami et al., 1980; Speight and Moschopedis, 1981; Speight and Pancirov, 1983) that condensed aromatic and naphthenic ring systems with peripheral aliphatic substituents are important moieties in petroleum asphaltenes. Therefore,
* Corresponding author. +Present address: Department of Chemical Engineering, University of Michigan, Ann Arbor, MI 48109. 0888-5885/88/2627-1348$01.50/0
model compounds of the basic asphaltene hydrocarbon framework should contain alkylaromatic, alkylhydroaromatic, and alkylnaphthenic moieties. We previously (Savage and Klein, 1987a,b) reported on the pyrolysis of n-pentadecylbenzene and l-phenyldodecane, prototypical alkylaromatics, and herein report on the pyrolyses of a mode alkylnaphthene, n-tridecylcyclohexane (TDC), and a model alkylhydroaromatic, 2-ethyltetralin (2ET). Few reports of the pyrolysis of singly substituted alkylcyclohexaneshave appeared in the literature. Fabuss et al. (1964) pyrolyzed several saturated cyclic hydrocarbons including cyclohexane and methyl-, ethyl-, propyl-, and n-butylcyclohexane,and they developed an empirical correlation between the first-order rate constant for substrate disappearance at 427 "C and molecular structure. Mushrush and Hazlett (1984) pyrolyzed tridecylcylohexane at 450 O C and observed n-alkanes, l-alkenes, alkylcyclohexanes, toluene, benzene, and methylcyclohexane as reaction products. However, this latter investigation provides only limited kinetics results and little information regarding the operative reaction pathways. The literature concerning the pyrolysis of alkyltetralins is likewise sparse, and only methyl-substituted tetralins have been previously pyrolyzed. For instance, Trahanovsky and Swenson (1981) subjected l-methyl- and 2methyltetralin to flash vacuum pyrolysis conditions (700-900 "C, 0.1 Torr), and they concluded that cleavage of alkyl groups was facile because the major products were naphthalene and 1,2-dihydronaphthalene. No reports of 2-ethyltetralin pyrolysis were found. 0 1988 American Chemical Society
Experimental Section TDC and 2ET were pyrolyzed at isothermal temperatures between 375 and 450 "C for batch holding times between 10 and 180 min. Materials. All chemicals were obtained commercially and used as received. Tridecylcyclohexane (98%) was from Pfaltz and Bauer, and 2-ethyltetralin (99%) was from API Standard Reference Materials. Pyrolyses were accomplished in batch reactors fashioned from either Swagelok tube fittings, as described previously (Savage and Klein, 1987b), or from Kimax glass capillary tubes. The latter reactors were used for the 2ET pyrolyses because only 1 mL of this model compound was available, and the low volume of these reactors permited a large number of experiments to be performed. Procedure. The tubing bomb reactors were typically loaded with 50-80 mg of the model compound and about 3-5 mg of biphenyl, a demonstrably inert compound, which served as an internal standard in the chromatographic analyses. All quantities were carefully (*0.1 mg) weighed. The glass capillary tubes used in 2ET pyrolyses were typically charged with 10 pL of a previously prepared stock solution containing the model compound and the internal standard. The reactors were then sealed and immersed in a preheated, isothermal, fluidized sand bath for the desired reaction time. Upon removal from the sand bath, the reactors were rapidly cooled to room temperature and then opened. The reaction products were extracted in spectrophotometric grade acetone, and these liquids samples were then analyzed by gas chromatography and GCMS. Analytical Chemistry. A Hewlett-Packard Model 5880 gas chromatograph equipped with a 50-m SE-54 fused-silica capillary column and a flame ionization detector was used for routine analyses of the acetone-soluble reaction products. Product identities were determined by retention time comparison with compounds of known identity and by GC-MS using a Hewlett-Packard Model 5890 GC coupled with a Model 5970 mass-selective detector. Gaseous products were separated via a 6-ft stainless steel column packed with 80/100 silica gel, and sample constituents were observed by a thermal conductivity detector. These analyses allowed calculation of product yields, expressed on a mole percent basis, as the number of moles of product (X100) divided by the number of moles of reactant initially charged to the reactor. Product selectivities were determined as the molar yield divided by the reactant conversion. Additionally, the Identified Products Index (IPI) was calculated as the total weight of the identified products (X100) divided by the weight of reactant initially charged to the reactor, and the molar balance on six-membered rings was determined for TDC pyrolysis as the moles of ring-containing products (x100) divided by the initial number of moles of TDC in the reactor. Note that the IPI is not a total mass balance because its basis is only the reaction products identified from the low-temperature pyrolyses and not all of the reaction products detected. Therefore, if these low-temperature products thermally decompose at the more severe pyrolysis conditions, the IPI could decrease. This factor, along with the observed formation of acetone-insoluble char at the more severe pyrolytic conditions, contributes toward an IPI of less than 100%.
Results The results are presented in terms of the temporal variation of the yields of the individual reaction products and product groups. Only the yields of the major products
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than 2% conversion was attained after 120 min at 400 OC (Savage, 1986). Thus, the presently observed decreasing selectivity and implied decomposition of 2-ethylnaphthalene warrant the further scrutiny and discussion that follow below. The yield of naphthalene displayed in Figure 5c increased with conversion, and the data intimate a non-zero initial slope and hence primary formation of this product from 2ET. Note however that these data are also consistent with rapid secondary formation of naphthalene. Likely precursors are dialin and 2-ethylnaphthalene. The yield of tetralin was less than 0.4% for low (
