Upgrading of Methane under Homogeneous Thermal Conditions: An

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Energy & Fuels 1997, 11, 1204-1218

Upgrading of Methane under Homogeneous Thermal Conditions: An Environmental and Economic Imperative S. H. Bauer,* S. Javanovic, C.-L. Yu, and H.-Z. Cheng Department of Chemistry, Cornell University, Ithaca, New York 14853 Received March 25, 1997X

After a brief outline of current estimates of the amount of methane annually injected into the atmosphere from fossil sources, we list various techniques that have been proposed and tested for converting methane to higher molecular weight hydrocarbons and alcohol to facilitate transport to locations where these chemicals could be utilized either directly to generate power or as feed stock for further processing. We propose a thermal conversion under homogeneous conditions that appears attractive, based on an extensive computer analysis of the kinetic mechanisms that control pyrolysis of methane when mixed with selected radical generators, at relatively modest temperatures (∼900 °C), that initiate C-H bond fission during the early stages of pyrolysis. Product distributions were computed to illustrate theoretically attainable temperature and time dependent levels of conversion. Therein, two key assumptions were made: “kinetic trapping” of radical-initiated products can be achieved; no solid deposits were generated. We utilize initiators available in commerce at low cost in bulk quantities. An extended series of tests were completed using a newly designed highly stirred “batch” reactor operating at pressures up to 25 atm that was later adapted for “flow-through” operation. It appears that conversion to a C/H solid deposit on the quartz-lined reactor walls does limit the amounts of hydrocarbons that can be recovered in the gas phase. Additional tests were run in a single-pulse shock tube, where the reactants are highly diluted in Ar and the residence time at the pyrolysis temperature is milliseconds. No C/H deposits were generated, and the analytical data confirmed the modeling calculations.

Introduction Methane is the most abundant organic constituent of the Earth’s atmosphere. Like carbon dioxide, it is an important contributor to the greenhouse effect. On a mass basis it is approximately 10 times as effective as CO2 with respect to global warming.1a Indeed, it has been proposed1b that about 55 million years ago a giant release (estimated duration of 10 000 years) of methane from the ocean generated a thermal pulse of greenhouse heating that altered the course of evolution on land. Most methane is destroyed in the troposphere by reaction with OH radicals, but 10-20% gets transported to the stratosphere.2 Whereas the mean methane content of the atmosphere is fairly well established, and its budget has been under study for several decades, the complete balance of sources and sinks has not yet been fully resolved.3 An extended description of a threedimensional model of the global methane cycle was presented by Fung et al.4 Since 1900 the methane content of the Earth’s atmosphere has risen sharply at a rate of increase of 1.0-1.5% per year,5,6 equivalent to 10-11 ppb/yr. However, since 1992 that has leveled off to 2 ppb/yr. The major sources are biogenic.6,7 CH4 of recent (or current) origin incorporates 14C. It has been Abstract published in Advance ACS Abstracts, September 15, 1997. (1) (a) Lashof, D. A.; Ahuja, D. R. Nature 1990, 334, 529. (b) Kerr, R. A. Science 1997, 275, 1267. (2) Crutzen, P. J. Nature 1991, 350, 380. (3) PETC Review, Spring 1995; p 19. (4) Fung, I.; et al. J. Geophys. Res. 1991, 13, 65. (5) Pearman, G. I.; Fraser, P. J. Nature 1988, 332, 489. (6) Cicerone, R. J.; Oremland, R. S. Global Biogeochem. Cycles 1988, 2, 299.

estimated that 5 × 107 tons/yr are produced by termites and a comparable amount by cattle. But concurrently, a huge biogenic depletion process operates; the bacterial enzyme methane mono-oxygenase converts about 1 × 109 tons/yr of methane to methanol.8a On the basis of measured “nonradio” methane, Lowe et al.8b estimated that 32% was derived from fossil sources, such as mine emissions, pipeline transmission losses, undersea venting, and oil-well blow-off. An inventory of natural gas resources indicated that recoverable gas reserves worldwide are 8 × 1015 ft3 (equivalent to 1.5 × 105 million metric tons (MMT)) of which 3.9 × 1015 ft3 were “established” from drilling data.9 This estimate does not include the enormous quantities locked in clathrate structures under the tundra and beneath the sea near the continental shelves.10 Rough estimates are cited at 3 × 1020 ft3. Reassuring as these figures are with respect to total energy reserves, the sobering aspect is their unavailability at centers of utility. Its extraction and transport are significant limiting factors. Of the antropogenic sources, Cicerone6 estimated that gas drilling, venting, and transmission line losses inject into the atmosphere 45 MMT annually out of a global natural gas production of 100 MMT/yr.11 Hence, even

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(7) Seiler, W. In Current Perspectives in Microbiological Ecology; Klug, M. J., Reddy, C. A., Eds.; American Association for Microbiology: Washington, DC, 1984; p 468. (8) (a) Shu, L.; et al. Science 1997, 275, 515. (b) Lowe, D. C.; et al. Nature 1988, 332, 522. (9) Dreyfus, D. A.; Ashby, A. B. Energy 1989, 14, 773. (10) (a) Holbrook, W. S.; et al. Science 1996, 273, 1840. (b) Stern, L. A.; et al. Science 1996, 273, 1843; 1995, 268, 647.

© 1997 American Chemical Society

Upgrading of Methane

if a fraction of the above could be captured, converted to a readily transportable state and channeled into power generation, or used as chemical feed stock, there would result not only a net loss of insult to the environment but also a significant gain in hydrocarbon utility. With respect to ozone-forming potential, were CH4 converted to LPG and utilized as an alternate fuel for vehicles, reduction by 40-50% (per mile) may be achieved.11b Conversion Processes. The indirect conversion of methane to liquid fuels that could be readily transported has been practiced for decades, based on the FischerTropsch process. This appears to be environmentally undesirable and relatively inefficient. The most favored approach is oxidative coupling of methane mediated by various metal oxide catalysts. A careful balance between homogeneous and heterogeneous reactions must be maintained. An extensive literature developed covering the multifold aspects of the overall process.12 Earlier reviews have been assembled by Amenomiya et al.,13 Mimoun et al.,14 and McCarty.15 It has been suggested that the useful metal oxide combinations have already been optimized.16 However, Tonkovich and coworkers17a reported that the oxidative coupling of methane to generate C2H4 and C2H6, using a countercurrent moving bed chromatographic reactor, gave 65% conversion with 80% selectivity for C2’s. Choudhary et al.17b described a nonoxidative activation of methane over H-galloaluminosilicate zeolite at relatively low temperatures (400-600 °C) to obtain conversions to higher hydrocarbons and aromatics in the range of 1045%. Relatively high yields of methanol were produced in a single-step partial oxidation of CH4 using a supported ferric molybdate catalyst.18 Even higher conversions were claimed by Jiang and co-workers.19 Numerous patents for practical processes have been awarded during the past 2 decades. Upgrading under homogeneous conditions has also been extensively explored, primarily directed at discovering controlled conditions for partially oxidizing natural gas to obtain high yields of methanol.20 Subjecting CH4 to microwave plasma (10-100 Torr) generated C2 hydrocarbons,21 and with added O2 produced methanol.22a (11) (a) Sheppard, J. C.; Westburg, H.; Hopper, J. F.; Ganeran, K. J. Geophys. Res. 1982, 87, 1305. (b) Chang, T.-Y.; et al. Environ. Sci. Technol. 1991, 25, 1190. (12) (a) Natural Gas Conversion; Holman, A., Jens, K.-J., Kolboe, S., Eds.; Symposium Proceedings, Oslo, August 1990: Elsevier: Amsterdam, 1991. (b) Natural Gas Upgrading, American Chemical Society Symposium, San Francisco, April, 1992; Huff, G. A., Scarpiello, D. A., Eds.; American Chemical Society: Washington, DC, 1992. (c) Fox, J. M., III. Catal. Rev. Sci. Eng. 1993, 35 (2), 169. (d) Nozaki, T.; et al. Energy Fuels 1993, 7, 432. (e) Bharadwaj, S. S.; Schmidt, L. D. J. Catal. 1994, 146, 11. (f) Hirschon, A. S.; Wu, H.-J.; Wilson, R. B.; Malhorta, R. J. Phys. Chem. 1995, 99, 17483. (g) Taniewski, M.; et al. Ind. Eng. Chem. Res. 1994, 33, 185. (13) Amenomiya, Y.; et al. Catal. Rev. Sci. Eng. 1990, 32, 163. (14) Nimoun, H.; Robine, A.; Bonnaudet, S.; Cameron, C. J. Appl. Catal. 1990, 58, 269. (15) McCarty, J. G. In Symposium on Methane Upgrading, Division of Petroleum Chemistry, American Chemical Society, Atlanta 1991. Prepr. Pap.sAm. Chem. Soc., Div. Pet. Chem. 1991, 36, 141. (16) Labinger, J. A. Prepr. Pap.sAm. Chem. Soc., Div. Pet. Chem. 1991, 36, 151. (17) (a) Tonkovich, A. L.; Carr, R. W.; Aris, R. Science 1993, 262, 221. (b) Choudhary, V. R.; Kinage, A. K.; Choudhary, T. V. Science 1997, 275, 1286. (18) Chellapa, A. S.; Viswanath, D. S. Ind. Eng. Chem. Res. 1995, 34, 1933. (19) Jiang, Y.; Yentekakis, V.; Vayenas, C. G. Science 1994, 264, 1564.

Energy & Fuels, Vol. 11, No. 6, 1997 1205 Table 1. Equilibrium Constants (Keq) for Methane Conversion Reactions reactions

Keq at 1100 K

2CH4 ) C2H6 + H2 2CH4 ) C2H4 + 2H2 4CH4 ) C4H10 + 3H2 C2H4 + C2H6 ) C4H10

2.78 × 10-04 6.98 × 10-04 (atm) 8.12 × 10-10 4.19 × 10-03 (atm-1)

% converted at 20 atm 3.22 4.00 0.93

There are both thermochemical and kinetic constraints on the conversion of methane, illustrated by the equilibrium constants for the three initiating reactions as given in Table 1. Thus, neither ethane nor butane is favored. However, detailed kinetic analyses of the multitude of gas-phase reactions at elevated temperatures indicate that attack of methane by a sufficient level of free radicals points to the possibility of kinetic trapping of the desired products generated under homogeneous conditions, as proposed and calculated in Patent No. 5,214,226 (1993).22b In 1979 we described an experimental kinetic investigation of the early stages of methane pyrolysis23 and demonstrated that the key initial step is C-H bond fission. Even in rich CH4-O2-Ar mixtures the ratelimiting step is still C-H bond breaking; the oxidizer participates only in subsequent reactions with the hydrocarbon radicals. It has been known for decades that heating fuel-rich mixtures of methane and oxygen is not an efficient route for converting CH4 to alcohols and aldehydes because at the high temperatures required to initiate reaction further oxidation of the products cannot be avoided. A direct partial oxidation of methane, under homogeneous conditions, was tested by Danen et al.24a They provided a rapid quench by fast flow through a divergent nozzle; large-scale implementation of such a device appears problematic. A more complex device, designated “shock wave reactor”, was described by Knowlen and Mattick24b,c for a continuous flow process wherein ethane is rapidly heated by a shock wave and quickly quenched. In the currently operating unit, heat losses to the reactor walls led to considerably lower yields than were predicted. No attempts were made to pyrolyze methane. Another approach is described by Chun and Anthony,25 and a detailed kinetic model for homogeneous oxidation of CH4 was published by Hunter et al.26 Criteria for an Optimum Process. To approach the economic level for production of C2, C3, and C4 hydrocarbons currently derived from cracking of heavy petroleum stock, the proposed upgrading process should be feasible for operation in simple plants, constructed and operated at low cost, situated close to natural gas wells. More importantly, the compounds that readily generate free radicals should be available in large (20) (a) Casey, P. S.; et al. Ind. Eng. Chem. Res. 1994, 33, 1120. (b) Fouls, G. A.; et al. Ind. Eng. Chem. Res. 1993, 32, 780. (c) Zanthoff, H.; Baerns, M. Ind. Eng. Chem. Res. 1990, 29, 2. (d) Cheng, Q.; et al. AIChE J. 1994, 40, 521. (e) Feng, W.; Knopf, F. C.; Dooley, K. M. Energy Fuels 1994, 8, 815. (f) Foral, M. J. in ref 12b, p 34. (21) Huang, J.; Suib, S. L. J. Phys. Chem. 1993, 97, 9403. (22) (a) Huang, J.; et al. J. Phys. Chem. 1994, 98, 206. (b) Bauer, S. H.; Cheng, H.-Z. U.S. Patent No. 5,214,226, 1993. (23) Tabayashi, K.; Bauer, S. H. Combust. Flame 1979, 34, 63. (24) (a) Danen, W. C.; et al. In ref 15, p 141. (b) Knowlen, C.; Mattick, A. T. National Shock-Tube Symposium, Tokyo, Japan, 1996. (c) Hertzberg, A.; Mattick, A. T.; Russell, D. A. Patent No. 5,219,630, 1993. (25) Chun, J.-W.; Anthony, R. G. Ind. Eng. Chem. Res. 1993, 32, 796. (26) Hunter, T. B.; Wang, H.; Litzinger, T. A.; Frenklach, M. Combust. Flame 1994, 97, 201.

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quantities at low cost. The overall process should be no more environmentally objectionable than is now acceptable for the petroleum industry. We note that the use of free radical initiators is not a novel concept but stress that those proposed do not meet all the above criteria. For example, although Cl227 and N2O do generate copious levels of radicals that attack methane, they are not environmentally benign and are relatively expensive. On the basis of the above considerations, we suggested22b using isooctane as a free radical initiator. Here, we report the results of testing and validating this proposal by employing two experimental techniques: a “batch” reactor and a shock tube. We first describe the underlying kinetic model and note practical constraints based on computer simulations. Isooctane as a Free Radical Initiator. The activation energy for the methane C-H bond breaking is 103.8 kcal/mol. An ideal free radical initiator, therefore, should have a considerably lower bond-breaking activation energy. One potential candidate is 2,2,3,3-tetramethylbutane; fission of the central C-C bond in 2,2,3,3-tetramethyl butane, (CH3)3C-C(CH3)3, occurs with an activation energy (E) of 68.3 kcal/mol.28 The two resulting (CH3)3C• radicals then rapidly eject H• atoms to generate H2CdC(CH3)2 at E ) 38.2 kcal/mol. Unfortunately, 2,2,3,3-tetramethyl butane is a very expensive isomer. On the other hand, for the much more plentiful and low-cost isooctane, the central C-C bonds break with almost comparable ease at the carbon 2-3 and 3-4 positions, shown schematically with E’s, given in {}, in units of kcal/mol:

To obtain quantitative estimates, we performed kinetic simulations for a gas-phase methane-isooctane-argon mixture (details of the kinetic simulations, including reaction mechanism and thermal properties, will be cited below in the Kinetic Modeling section). Figure 1 is representative; it shows the calculated concentrations versus reaction time for various chemical species. Figure 1 illustrates two essential features. (i) The equilibrium distribution is attained at 102 s. Furthermore, comparisons of the calculated concentrations for [methane-isooctane-Ar] and [isooctane-Ar] clearly indicate methane conversion to higher hydrocarbons such as C2H4 and C2H6. (ii) High transient excursions in concentrations (note the log scale) is exhibited by 1-C3H6, C2H6, and i-C4H10 (and lesser species) over the time span 4 × 10-3 to 10-1 (27) Benson, S. W., U.S. Patent No. 4,199,533. (28) Chemical Kinetics Standard Reference Database No. 17, Version 6.8; NIST: Gaithersburg, MD, 1993.

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s. On a linear scale (Figure 1e) 19% of CH4 is converted within 0.8 s. Thus, to maximize the advantage of kinetic trapping of the higher hydrocarbons, residence times in the reactor should be about 0.01 s. In addition to isooctane-methane mixtures, we proposed22b the possible augmentation of free radical production (i.e., O• and HO•) by admixing O2 at levels comparable to that of isooctane (i.e., isooctane-oxygenmethane mixtures). This suggestion is based on free radical generation via oxygen-related chain branching and propagation reactions: {17.1}

H• + O2 98 HO• + O• {30.0}

CH3• + O2 98 H3CO• + O• {7.6}

CH3• + O2 98 H2CO + HO• In the present study, this concept was also experimentally examined. Experimental Apparatus Two types of reactors were utilized for the study of the proposed conversion process. (i) A quartz-lined, self-stirred “batch” reactor [flow-through conditions were also tested] designed to operate over the pressure range 1-25 atm and temperature range 900-1300 K with controlled residence times of 0.30-10 s [reduced to 0.08-0.10 s for flow-through runs]. (ii) A single-pulse shock tube for highly diluted reaction mixtures (in Ar) with a residence time of about 2 ms. Batch Reactor. Testing the free-radical-initiated conversion initially appeared more direct in a batch reactor than in a flow-through system. The micropilot plant as constructed and tested is illustrated in Figure 2; the final design evolved through a sequence of stages. Figure 3 is a diagram of the stainless steel reactor. Attention is directed to a few specific features. (i) To minimize the possibility that high-pressure O2 could be accidentally injected into the main line, a small intermediate cylinder was inserted, with a dedicated low-pressure gauge, so that low pressures of oxygen used for some mixtures could be controlled by V3 with V4 closed. (ii) There are two ports for sample extraction (for GC analysis): (a) from the mixing tank and preheater via SP1 and (b) from the product tank at SP2. (iii) All the heated lines are 0.25 in. stainless steel. The ambient temperature lines are 0.25 in. Cu. The line from V2 to V8 and the mixing tank is maintained at 80-100 °C to prevent condensation of the initiator. From V8 to PV13 the line is kept close to the temperature of the preheating tank; its temperature was determined to be just below that which would induce some conversion in the test mixture. The line between (and including) PV13 and PV17 is maintained about 100 K below that of the reactor. (iv) All pressures and temperatures are read electronically and are automatically recorded. Thermocouple TC1, which is situated inside the reactor, has a response time of 0.4 s (limited by the requirement that it operates at the indicated high pressures and temperatures). The others respond in about 4 s. Pressure transducer PT1 is used for the range 0-2 atm, while PT2 is used for pressures up to 30 atm. Both have response times of 1 ms. (v) PV13 and PV17 are solenoid-controlled, pneumatically actuated fast action, bellows-sealed valves rated for temperatures up to 900 K and pressures up to 40 atm. Their opening/ closing times are 0.1 s.


Energy & Fuels, Vol. 11, No. 6, 1997 1207

Figure 1. (a-d) Computed time dependent concentrations (log scale) for the mixtures 3.53% CH4-0.068% isooctane-Ar (solid lines) and 0.068% isooctane-Ar (dash lines) at 1229 K, based on the mechanism compiled in Table 2. Note the large excursions at early times for all species except C2H2. (e) Decay of the CH4 concentration with time (linear scale). (vi) For flow-through operations, both PV17 and SV19 were left open so that the product gases were collected at the ambient temperature in the final tank. (vii) The entire reactor assembly is framed within a custom designed, fully automated data acquisition and control system (DACS) that consists of a data translation multichannel A/D and D/A board (Model 2811) plugged into an IBM AT PC and controlled by a program developed using LabTech Notebook software. In addition to reading and saving all the time dependent pressures and temperatures, the DACS controls the residence time of the mixture in the reactor by opening PV13, PV17, and SV19 valves in a preset sequence. (viii) The heating unit for the reactor consists of a stack of four cylindrical heaters, controlled individually by variacs, all fed by a large Sola voltage regulator. Although well lagged, a strictly uniform internal temperature was not achieved. A sliding thermocouple inserted in the central gas distributor

(in the PT3 inlet) showed a uniform trend with a spread of 20 °C from the bottom to the top of the reactor. (ix) To minimize the extent of heterogeneous reactions that inevitably take place on the reactor walls (a hard black deposit is produced; its FT-IR absorption spectrum will be given in the Results and Discussion section), the quartz liner was coated with a fine layer of KCl crystals to serve as an anticatalyst. The disturbing deposition of {CnHm} was not eliminated. Material balance closure is difficult to achieve in this type of reactor, as attested in many related literature reports, although we took particular pains to arrive at closure by estimating the amount of solid deposited. A crucial unknown proved to be the amount of hydrogen in the product samples. Our GC is equipped with an FID detector only. However, even with a TCD detector, H2 is not readily quantified owing to its highly nonlinear response factor. The amount of H2 was estimated indirectly by measuring the gas pressure


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Figure 2. Schematic of the micropilot plant assembly. in the product tank both before and after condensing the hydrocarbons upon immersing the tank in liquid nitrogen. Those data, combined with GC measurements of the hydrocarbon content, allowed estimates to be made of the extent of solid deposition. For the gas-phase analyses we used a Nicolet gas chromatograph (Model 9630). This is a modular designed, highperformance instrument controlled by an IBM processor via a separate terminal interfaced to the main unit. A newly developed, high-efficiency Alltech Carbograph 80/120, 10 ft × 1/8 in. column was used to separate saturated and unsaturated C1-C8 hydrocarbons in a temperature-programmed mode. The unit was run at 80 °C for 3 min followed by a ramp rate of 20 °C/min up to 300 °C, terminating after 7 min. All hydrocarbons of interest could be detected within 20 min. Ultrahigh purity helium (with a purity higher than 99.997%) was used as the carrier gas. The gas chromatograph and the column (GC) were calibrated with a Scott natural gas reference calorimetric standard. The output signals from the GC were recorded with a Hewlett-Packard integrator (Model 3396A). Finally, it is worth noting the difficulty of accurately assessing the degree of conversion of the methane for low levels of conversion (below 5%), since the amount reacted must be derived from the difference between two relatively large levels. A small experimental error of 1% at 80% methane detected in the product tank leads to a significant uncertainty in the amount of methane lost. Assuming that the entire gas sample was exposed to the same temperature (well-stirred reactor), calibrations must be made to determine that temperature and in particular its excursions with time. This is a complex problem, since estimates need to be made of heat-transfer rates from the walls, cooling due to gas expansions and compressions, and cooling due to endoergic reactions. The magnitudes of the temperature and pressure changes that should be considered (for conversions at constant enthalpy) are illustrated in Figure 4 computed for the system simulated in Figure 1. (In this respect, we question the validity of temperatures cited in the literature for conversions in similar batch or flow-through reactors for runs with short residence times. We also question the significance of claims that “closure-adjusted” was attained when the amounts and compositions of solid deposits on reactor walls remain undetermined.) One approach, extensively used in conventional shock tube kinetics investigations for a similar problem, is to measure in

the reactor a conversion for which temperature dependent rates are well established. For such an application one must select a relatively simple, preferably unimolecular process that has an activation energy so high that the net conversion (from 20 to 80%) in the vicinity of 1000 K could be measured reliably for residence times of 1-10 s. For optimum calibration, the reference reaction should match that being investigated in endoergicity and heat capacity under reaction conditions. We used the dissociation reaction system C4F8 f 2C2F4, which approaches but does not strictly satisfy all these criteria.29 Suitable corrections were made. Shock Tube. To circumvent the restrictions imposed by deposition of C/H solids on the batch reactor walls, we undertook a series of experiments in a single-pluse shock tube. This is a well-characterized and reliable technique for investigating reactions at elevated temperatures for short durations (1-2 ms) under homogeneous conditions with the reactants highly diluted in an Ar carrier.30 After exposure to the heating shock wave, the mixture of products is rapidly quenched with an expansion wave. In this study, a stainless steel and heatable tube with 1 in. i.d. was used. The tube has the lengths of the driver and driven sections of 120 and 170 cm, respectively. A damp-tank attached to the driven section, adjacent to the diaphragm holder, quenches multiple wave reflections in the tube. Shocks were generated by increasing the pressure of the He driver until the Mylar diaphragm broke. Two piezoelectric pressure sensors are stationed 10.00 cm apart at the end of the driven section. Their combined output was recorded and digitized in a Biomation 8100 plus Tracor Northern signal analyzer and stored in an IBM AT computer (a standard wave diagram is shown in Figure 5). The effective heating time was defined as the elapsed time from the beginning of the reflected shock wave to the point where the pressure signal decayed to 80% of the maximum level. The storage tank, gas-handling line, and the shock tube were maintained at ∼110 °C through(29) Details described in a manuscript accepted for publication in Int. J. Chem. Kinet. (30) (a) Greene, E. F.; Toennies, J. P. Chemical Reactions in Shock Waves; Academic Press: New York, 1964. (b) Tsang, W.; Lifshitz, A. Annu. Rev. Phys. Chem. 1990, 41, 559. (c) Michael, J. V.; Lim, K. P. Annu. Rev. Phys. Chem. 1993, 44, 429. (d) Chemical Reactions in Shock Waves, Special Edition. Isr. J. Chem. 1996, 36, 223ff. (e) Dean, A. M. J. Phys. Chem. 1990, 94, 143. Chen, C. J.; Back, M. H.; Back, R. A. Can. J. Chem. 1976, 54, 3175.



Kinetic Modeling The reaction mechanism used to compute the graphs shown in Figure 1 is listed in Tables 2. For every reaction cited the reverse was automatically included. The reverse rate coefficients were calculated based on detailed balancing. The reaction mechanism evolved through several stages wherein initially omitted species22b were successively added (final total number of 46) along with corresponding reactions (final total number of 192). A number of reaction mechanisms (e.g., in ref 30e) have been proposed for simulating the pyrolysis of methane, but none of them specifically addressed the effect of comixed isooctanes. Our model is comparatively modest, but we have demonstrated that additional reactions do not modify the final computed distributions of C/H species. Table 3 is a summary of the thermochemical parameters for the species considered. Attention is directed to parts of Table 2a. References are cited for all but 25 out of 192 steps. The values labeled “estimated” were derived according to the method of Ranzi et al.70 Slight adjustments were made in the published values of reactions 7, 68, 80, and 101

Figure 3. High-pressure, high-temperature reactor.

Figure 4. Computed temperature (T) and pressure (P) profiles in the reactor for constant enthalpy. out these experiments. Methane purchased from Matheson was of ultrahigh purity as were the He and Ar used for the driving and diluent gases. Immediately after each shock, a 16 mL sample of gas was collected through the sampling valve located at the end of the driven section for GC analysis. This instrument was recalibrated, and the response factors were found to be independent of the sample pressure.

(31) Baulch, D. L.; Cobos, C. J.; Cox, R. A.; Frank, P.; Hayman, G.; Just, T.; Kerr, J. A.; Murrells, T.; Pilling, M. J.; Troe, J.; Walker, R. W.; Warnatz, J. J. Phys. Chem. Ref. Data 1994, 23, 847. (32) Davidson, D. F.; DiRosa, M. D.; Chang, E. J.; Hanson, R. K.; Bowman, C. T. Int. J. Chem. Kinet. 1995, 27, 1179. (33) Duran, R. P.; Amorebieta, V. T.; Colussi, A. J. J. Phys. Chem. 1988, 92, 636. (34) Tsang, W.; Hampson, R. F. J. Phys. Chem. Ref. Data 1986, 15, 1087. (35) Sillesen, A.; Ratajczak, E.; Pagsberg, P. Chem. Phys. Lett. 1993, 201, 171. (36) MacKenzie, A. L.; Pacey, P. D.; Wimalasena, J. H. Can. J. Chem. 1984, 62, 1325. (37) Brodsky, A. M.; Kalinenko, R. A.; Lavrovsky, K. P. J. Chem. Soc. 1960, 4443. (38) Tsang, W.; Walker, J. A. J. Phys. Chem. 1992, 96, 8378. (39) Tsang, W. J. Phys. Chem. Ref. Data 1991, 20, 221. (40) Tsang, W. Ind. Eng. Chem. Res. 1992, 31, 3. (41) Kinsman, A. C.; Roscoe, J. M. Int. J. Chem. Kinet. 1994, 26, 191. (42) Tsang, W. J. Phys. Chem. Ref. Data 1990, 19, 1. (43) Knyazev, V. D.; Dubinsky, I. A.; Slagle, I. R.; Gutman, D. J. Phys. Chem. 1994, 98, 11099. (44) Bencsura, A.; Knyazev, V. D.; Xing, S.; Slagle, I. R.; Gutman, D. Twenty-Fourth Symposium (International) on Combustion; Combustion Institute: Pittsburgh, 1992; p 629. (45) Dean, A. M. J. Phys. Chem. 1985, 89, 4600. (46) Tsang, W. Combust. Flame 1989, 78, 71. (47) Anastasi, C.; Arthur, N. L. J. Chem. Soc., Faraday Trans. 2 1987, 83, 277. (48) Tsang, W. J. Phys. Chem. Ref. Data 1988, 17, 887. (49) Tsang, W. J. Am. Chem. Soc. 1985, 107, 2872. (50) Knyazev, V. D.; Dubinsky, I. A.; Slagle, I. R.; Gutman, D. J. Phys. Chem. 1994, 98, 5279. (51) Bryce, W. A.; Kebarle, P. Trans. Faraday Soc. 1958, 54, 1660. (52) Pacey, P. D.; Wimalasena, J. H. J. Phys. Chem. 1980, 84, 2221. (53) Knyazev, V. D.; Slagle, I. R. J. Phys. Chem. 1996, 100, 5318. (54) Zhang, H.; Back, M. H. Int. J. Chem. Kinet. 1990, 22, 537. (55) Mitchell, T. J.; Benson, S. W. Int. J. Chem. Kinet. 1993, 25, 931. (56) Baldwin, R. R.; Walker, R. W.; Walker, R. W. J. Chem. Soc., Faraday Trans. 1 1979, 75, 1447. (57) Axelsson, E. I.; Brezinsky, K.; Dryer, F. L.; Pitz, W. J.; Westbrook, C. K. Twenty-First Symposium (International) on Combustion; Combustion Institute: Pittsburgh, 1986; p 783. (58) Westbrook, C. K.; Warnatz, J.; Pitz, W. J. Twenty-Second Symposium (International) on Combustion; Combustion Institute: Pittsburgh, 1988; p 893. (59) He, Y. Z.; Cui, J. P.; Wallard, W. G.; Tsang, W. J. Phys. Chem. 1988, 92, 1510. (60) Nicholas, J. E.; Vaghjiani, G. L. J. Chem. Phys. 1989, 91, 5121. (61) Tsang, W. Int. J. Chem. Kinet. 1973, 5, 929. (62) Daby, E. E.; Niki, H.; Weinstock, B. J. Phys. Chem. 1971, 75, 1601. (63) Tsang, W.; Walker, J. A. Twenty-Second Symposium (International) on Combustion; Combustion Institute: Pittsburgh, 1988; p 1015. (64) Seres, L.; Nacsa, A.; Arthur, N. L. Int. J. Chem. Kinet. 1994, 26, 227.


Bauer et al.

Table 2. Reaction Mechanism for Kinetic Calculations and Parameters for Central Broadening Factor Fc (a) Reaction Mechanism for Kinetic Calculations forward rate coefficients k ) ATn e-E/(RT) no.

reactions

1 2

H + H + M a H2 + M CH3 + H (+M) a CH4 (+M)

3 4 5 6

CH4 + H a CH3 + H2 CH3 + CH3 a C2H5 + H CH3 + CH3 a C2H4 + H2 CH3 + CH3 (+M) a C2H6 (+M)

7 8 9 10 11 12 13

CH3 + CH4 a C2H5 + H2 CH3 + CH4 a C2H6 + H C2H3 + H a C2H4 C2H4 + H a C2H3 + H2 C2H4 + CH3 a C2H3 + CH4 C2H3 + C2H6 a C2H5 + C2H4 C2H4 + H (+M) a C2H5 (+M)

14 15 16 17 18

C2H5 + H a C2H4 + H2 C2H5 + H a C2H6 C2H6 + H a C2H5 + H2 C2H5 + CH3 a C2H4 + CH4 C3H8 (+M) a C2H5 + CH3 (+M)

19 20 21 22 23 24 25 26 27 28 29 30

C2H6 + CH3 a C2H5 + CH4 C2H5 + C2H4 a 1-C3H6 + CH3 C2H5 + C2H4 a 1-C4H8 + H C2H5 + C2H5 a C2H6 + C2H4 C2H5 + C2H5 a C4H10 C2H6 a C2H4 + H2 3-C3H5 a C3H4 + H 3-C3H5 + C2H5 a C3H4 + C2H6 1-C3H6 + H a C2H4 + CH3 1-C3H6 + H a 3-C3H5 + H2 1-C3H6 + H a n-C3H7 i-C3H7 (+M) a 1-C3H6 + H (+M)

31 32 33

1-C3H6 + CH3 a 3-C3H5 + CH4 i-C4H9 a 1-C3H6 + CH3 s-C4H9 (+M) a 1-C3H6 + CH3 (+M)

34 35 36 37

1-C3H6 + C2H5 a 3-C3H5 + C2H6 1-C3H6 + i-C3H7 a C3H8 + 3-C3H5 1-C3H6 + n-C3H7 a C3H8 + 3-C3H5 n-C3H7 (+M) a C2H4 + CH3 (+M)

38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53

C3H8 a i-C3H7 + H i-C3H7 + CH3 a 1-C3H6 + CH4 i-C3H7 + CH3 a i-C4H10 i-C3H7 + C2H6 a C3H8 + C2H5 i-C3H7 + n-C3H7 a C3H8 + 1-C3H6 i-C3H7 + i-C3H7 a C3H8 + 1-C3H6 C3H8 + H a n-C3H7 + H2 C3H8 + H a i-C3H7 + H2 C3H8 + CH3 a n-C3H7 + CH4 C3H8 + CH3 a i-C3H7 + CH4 C3H8 + C2H3 a n-C3H7 + C2H4 C3H8 + C2H3 a i-C3H7 + C2H4 1-C4H7 a C3H4 + CH3 n-C4H9 a 1-C4H8 + H s-C4H9 a 1-C4H8 + H t-C4H9 (+M) a i-C4H8 + H (+M)

54 55 56 57 58 59 60 61 62 63 64 65 66

1-C4H8 + CH3 a 1-C4H7 + CH4 i-C4H8 + CH3 a p,i-1-C4H7 + CH4 n-C4H9 a C2H5 + C2H4 t-C4H9 + CH3 a i-C4H8 + CH4 t-C4H9 + t-C4H9 a ((CH3)3C)2 t-C4H9 + t-C4H9 a i-C4H10 + i-C4H8 i-C4H10 + H a i-C4H9 + H2 i-C4H10 + H a t-C4H9 + H2 i-C4H10 + CH3 a i-C4H9 + CH4 i-C4H10 + CH3 a t-C4H9 + CH4 neo-C5H11 a i-C4H8 + CH3 C8H17 a t-C4H9 + i-C4H8 C8H17 a C7H14 + CH3

A

(cm3

mol-1 s-1)

6.53 × 10+17 2.11 × 10+14 6.17 × 10+23 1.33 × 10+04 3.01 × 10+13 1.00 × 10+11 3.61 × 10+13 1.27 × 10+41 2.00 × 10+14 8.00 × 10+13 5.36 × 10+14 5.42 × 10+14 4.16 × 10+12 6.02 × 10+02 3.98 × 10+09 4.72 × 10+18 1.81 × 10+12 1.00 × 10+14 1.45 × 10+09 1.15 × 10+12 1.10 × 10+17 2.17 × 10+17 1.51 × 10-07 2.30 × 10+07 5.20 × 10+07 1.45 × 10+12 1.15 × 10+13 6.90 × 10+16 1.50 × 10+11 9.64 × 10+11 7.23 × 10+12 1.73 × 10+05 1.30 × 10+13 8.76 × 10+07 2.17 × 10+17 1.40 × 10+11 2.00 × 10+13 2.73 × 10+10 1.47 × 10+52 2.23 × 10+0 6.60 × 10-02 2.23 × 10+0 1.23 × 10+13 4.42 × 10+49 6.31 × 10+05 2.19 × 10+14 6.63 × 10+14 8.43 × 10-03 5.14 × 10+13 2.56 × 10+12 1.33 × 10+06 1.30 × 10+06 9.03 × 10-01 1.51 × 10+0 6.02 × 10+02 1.02 × 10+03 1.00 × 10+14 2.51 × 10+13 1.29 × 10+13 2.18 × 10+09 1.38 × 10+57 1.40 × 10+08 2.20 × 10+09 1.06 × 10+13 3.79 × 10+15 1.25 × 10+16 3.46 × 10+12 1.81 × 10+06 6.02 × 10+05 7.59 × 10+11 1.41 × 10+11 7.94 × 10+13 1.90 × 10+06 1.10 × 10+04

n

E (cal/mol)

ref

-1.00 0.00 -1.80 3.00 0.00 0.00 0.00 -7.00 0.00 0.00 0.00 0.00 0.00 3.30 1.28 0.00 0.00 0.00 1.50 0.00 0.00 0.00 6.00 0.00 0.00 0.00 0.00 0.00 0.84 0.00 0.00 2.50 0.00 1.76 0.00 0.00 0.00 1.11 -10.60 3.50 4.00 3.50 -0.10 -10.00 0.00 -0.68 -0.57 4.20 -0.35 0.00 2.54 2.40 3.65 3.46 3.30 3.10 0.00 0.00 0.00 1.48 -12.00 0.00 0.00 0.00 -1.00 -1.50 0.00 2.54 2.40 0.00 0.00 0.00 0.00 0.00

0 0 0 8038 13512 0 0 2762 22998 40000 982 14903 11127 10501 1292 755 0 0 7412 0 84388 64975 6047 0 0 0 0 81992 59716 -131 1302 2492 3261 35508 28216 8723 29954 31220 35047 6637 8066 6637 30203 35766 94700 0 0 8715 0 0 6756 5067 7153 5480 10502 8828 37000 35612 36362 36004 42303 0 0 27828 0 0 0 6756 2583 11566 8077 31500 0 0

31 31, k∞ 31, k0 31 31 32 31, k∞ 31, k0 23, adjusted 23 33 31 31 34 31, k∞ 31, k0 34 35 31 31 31, k∞ 31, k0 31 36 36 31 31 37 38 39 39 39 40 31, k∞ 31, k0 41 42 31, k∞ 31, k0 39 39 39 44, k∞ 44, k0 45 39 46 39 39 47 48 48 48 48 48 48 estimated 36 49 50, k∞ 50, k0 51 52 53 42 42 47 42 42 54 54 55 56 56



Table 2 (Continued) forward rate coefficients k ) ATn e-E/(RT) no. 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 139 140 141

reactions p,i-octane a neo-C5H11 + 1-C3H6 i-octane a t-C4H9 + i-C4H9 i-octane + H a p,i-octane + H2 i-octane + H a t-i-octane + H2 ((CH3)3C)2 + H a C8H17 + H2 C4H10 a CH3 + n-C3H7 C4H10 a n-C4H9 + H C4H10 + H a n-C4H9 + H2 C4H10 + H a s-C4H9 + H2 CH3 + C3H4 a p,i-1-C4H7 t-C4H9 + i-C4H8 a i-C4H10 + p,i-1-C4H7 CH3 + p,i-1-C4H7 a 2-M-B-1-ene 2-M-B-1-ene + H a t-C5H11 H + i-C4H8 a H2 + p,i-1-C4H7 H + i-C4H8 a CH3 + 1-C3H6 CH3 + i-C4H8 a t-C5H11 t-C5H11 a 2-M-B-2-ene + H H + i-C5H12 a H2 + t-C5H11 CH3 + i-C4H9 a i-C5H12 neo-C5H12 a t-C4H9 + CH3 CH3 + neo-C5H12 a neo-C5H11 + CH4 H + neo-C5H12 a neo-C5H11 + H2 t-C4H9 + neo-C5H12 a neo-C5H11 + i-C4H10 1-C3H6 + C2H4 a 1-C5H10 1-C5H10 a C2H5 + 3-C3H5 C2H4 + M a C2H2 + H2 + M C2H3 (+M) a C2H2 + H (+M) C2H3 + C2H3 a C2H2 + C2H4 C2H3 + H a C2H2 + H2 CH3 + C2H3 a C2H2 + CH4 CH3 + C2H3 a 1-C3H6 C2H4 + C2H4 a C2H3 + C2H5 C3H4 a CH3CCH CH3CCH + H a CH3 + C2H2 i-octane a neo-C5H11 + i-C3H7 pai-octane a i-C4H8 + i-C4H9 t-i-octane a i-C4H8 + t-C4H9 i-octane + H a pai-octane + H2 i-octane + CH3 a pai-octane + CH4 i-octane + CH3 a p,i-octane + CH4 i-octane + CH3 a t-i-octane + CH4 pai-octane a p,i-octane pai-octane a t-i-octane i-octane + C2H5 a pai-octane + C2H6 i-octane + C2H5 a p,i-octane + C2H6 i-octane + C2H5 a t-i-octane + C2H6 i-octane + i-C4H9 a pai-octane + i-C4H10 i-octane + i-C4H9 a p,i-octane + i-C4H10 i-octane + i-C4H9 a t-i-octane + i-C4H10 i-octane + t-C4H9 a pai-octane + i-C4H10 i-octane + t-C4H9 a p,i-octane + i-C4H10 i-octane + t-C4H9 a t-i-octane + i-C4H10 i-octane + i-C3H7 a pai-octane + C3H8 i-octane + i-C3H7 a p,i-octane + C3H8 i-octane + i-C3H7 a t-i-octane + C3H8 i-octane + neo-C5H11 a pai-octane + neo-C5H12 i-octane + neo-C5H11 a p,i-octane + neo-C5H12 i-octane + neo-C5H11 a t-i-octane + neo-C5H12 i-octane + p,i-1-C4H7 a pai-octane + i-C4H8 i-octane + p,i-1-C4H7 a p,i-octane + i-C4H8 i-octane + p,i-1-C4H7 a t-i-octane + i-C4H8 H + t-C4H9 a i-C4H8 + H2 3-C3H5 + t-C4H9 a C3H4 + i-C4H10 3-C3H5 + t-C4H9 a 1-C3H6 + i-C4H8 1-C3H6 + t-C4H9 a 3-C3H5 + i-C4H10 i-C3H7 + t-C4H9 a C3H8 + i-C4H8 i-C3H7 + t-C4H9 a i-C4H10 + 1-C3H6 C2H5 + t-C4H9 a i-C4H10 + C2H4 C2H5 + t-C4H9 a i-C4H8 + C2H6 i-C4H9 + t-C4H9 a i-C4H10 + i-C4H8 C2H6 + t-C4H9 a i-C4H10 + C2H5 C3H8 + t-C4H9 a n-C3H7 + i-C4H10 C3H8 + t-C4H9 a i-C4H10 + i-C3H7 n-C3H7 + t-C4H9 a i-C4H10 + 1-C3H6 n-C3H7 + t-C4H9 a i-C4H8 + C3H8

A (cm3 mol-1 s-1)

n

E (cal/mol)

ref

1.30 × 10+13 4.00 × 10+25 5.64 × 10+07 1.26 × 10+14 2.52 × 10+12 1.00 × 10+17 1.58 × 10+16 3.80 × 10+13 2.10 × 10+13 1.59 × 10+11 2.00 × 10+11 2.00 × 10+13 9.10 × 10+11 8.62 × 10+13 1.72 × 10+13 2.51 × 10+11 1.00 × 10+16 5.11 × 10+13 1.93 × 10+14 4.47 × 10+16 8.32 × 10+11 6.46 × 10+07 8.32 × 10+11 3.19 × 10+03 1.00 × 10+16 3.50 × 10+16 2.00 × 10+14 4.16 × 10+41 8.49 × 10+13 1.21 × 10+13 2.15 × 10+13 6.88 × 10+13 4.82 × 10+14 2.10 × 10+12 1.30 × 10+05 2.50 × 10+25 1.30 × 10+13 5.00 × 10+12 8.46 × 10+07 5.85 × 10+12 3.90 × 10+12 1.00 × 10+11 6.00 × 10+11 1.00 × 10+11 9.00 × 10+11 6.00 × 10+11 5.00 × 10+10 9.00 × 10+11 6.00 × 10+11 5.00 × 10+10 9.00 × 10+11 6.00 × 10+11 5.00 × 10+10 9.00 × 10+11 6.00 × 10+11 5.00 × 10+10 9.00 × 10+11 6.00 × 10+11 5.00 × 10+10 9.00 × 10+11 6.00 × 10+11 5.00 × 10+10 5.42 × 10+12 2.89 × 10+13 4.33 × 10+14 3.01 × 10-05 2.86 × 10+15 2.86 × 10+15 2.20 × 10+14 3.59 × 10+14 2.16 × 10+14 3.38 × 10-06 1.15 × 10-05 1.08 × 10-06 2.16 × 10+14 3.46 × 10+14

0.00 -3.00 2.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 -0.32 0.00 0.00 2.00 0.00 2.20 0.00 0.00 0.00 -7.50 0.00 0.00 0.00 0.00 0.00 0.00 2.50 -3.00 0.00 0.00 2.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 -0.75 -0.75 4.90 -1.10 -1.10 -0.75 -0.75 -0.75 5.17 5.00 5.16 -0.75 -0.75

29500 78000 7700 7300 11616 84700 98000 12189 7887 4968 10000 0 0 7996 3593 6690 39600 6021 0 82000 11000 7030 17650 34080 71333 71532 39740 45502 0 0 0 0 71532 60000 1000 78000 29500 29000 7700 11600 11600 7900 14100 16100 13500 13500 9500 13500 13500 9500 15000 15000 10714 14500 14500 12063 13500 13500 9500 13500 13500 9500 0 131 131 8361 0 0 0 0 0 9067 9554 8129 0 0

57 57, adjusted 58 58 59 45 45 60 60 61 55 61 62 63, adjusted 63 64 55 65 42 55 55 55 55 39 66 31 31, k∞ 31, k0 67 30 68 68 34 69 69 57, adjusted 57 57 57 57 57 57 57 57 estimateda estimateda estimateda estimateda estimateda estimateda estimateda estimateda estimateda estimateda estimateda estimateda estimateda estimateda estimateda estimateda estimateda estimateda 42 39 39 39 42 42 42 42 39 42 42 42 42 42


Bauer et al.

Table 2 (Continued) forward rate coefficients k ) ATn e-E/(RT) no.

reactions

142 143 144 145 146 147 148 149 150 151 152 153 154 155 156 157 158 159 160 161 162 163 164 165 166 167 168 169 170 171 172 173 174 175 176 177 178 179 180 181

C2H3 + t-C4H9 a i-C4H10 + C2H2 C2H3 + t-C4H9 a i-C4H8 + C2H4 i-C4H10 + t-C4H9 a i-C4H10 + i-C4H9 H + i-C4H9 a H2 + i-C4H8 C2H3 + i-C4H9 a i-C4H10 + C2H2 C2H3 + i-C4H9 a i-C4H8 + C2H4 C2H5 + i-C4H9 a i-C4H8 + C2H6 C2H5 + i-C4H9 a i-C4H10 + C2H4 C2H6 + i-C4H9 a C2H5 + i-C4H10 3-C3H5 + i-C4H9 a 1-C3H6 + i-C4H8 3-C3H5 + i-C4H9 a C3H4 + i-C4H10 1-C3H6 + i-C4H9 a 3-C3H5 + i-C4H10 i-C3H7 + i-C4H9 a i-C4H10 + 1-C3H6 i-C3H7 + i-C4H9 a i-C4H8 + C3H8 n-C3H7 + i-C4H9 a i-C4H10 + 1-C3H6 n-C3H7 + i-C4H9 a i-C4H8 + C3H8 C3H8 + i-C4H9 a i-C4H10 + n-C3H7 C3H8 + i-C4H9 a i-C4H10 + i-C3H7 i-C4H9 + i-C4H9 a i-C4H8 + i-C4H10 H + i-C4H8 a i-C4H9 4-M-P-1-ene a i-C3H7 + 3-C3H5 1-C3H6 + 1-C3H6 a 4-M-P-1-ene C2H4 + i-C4H8 a 2-M-P-1-ene 1-C6H12 a 1-C3H6 + 1-C3H6 1-C6H12 a 3-C3H5 + n-C3H7 CH2 + CH4 a CH3 + CH3 CH2(S) + CH4 a CH3 + CH3 CH2(S) + H2 a H + CH3 CH + H2 a H + CH2 CH + CH2 a H + C2H2 CH + CH3 a H + C2H3 CH + CH4 a H + C2H4 CH2 + H2 a H + CH3 CH2 + CH2 a H2 + C2H2 CH2 + CH3 a H + C2H4 CH2(S) + M a CH2 + M CH2(S) + H a CH + H2 CH2(S) + CH3 a H + C2H4 CH2(S) + C2H6 a CH3 + C2H5 H + CH2 (+M) a CH3 (+M)

182 183 184 185 186 187 188 189 190 191 192

1-C3H6 a H + 3-C3H5 1-C3H6 + C2H4 a C2H5 + 3-C3H5 1-C3H6 + C2H4 a C2H3 + n-C3H7 1-C3H6 + C2H4 a C2H3 + i-C3H7 1-C3H6 + 1-C3H6 a n-C3H7 + 3-C3H5 1-C3H6 + 1-C3H6 a i-C3H7 + 3-C3H5 i-C4H8 + C2H5 a C2H6 + p,i-1-C4H7 i-C4H8 + 3-C3H5 a 1-C3H6 + p,i-1-C4H7 i-C4H8 + i-C3H7 a C3H8 + p,i-1-C4H7 i-C4H8 + neo-C5H11 a neo-C5H12 + p,i-1-C4H7 1-C3H6 + neo-C5H11 a neo-C5H12 + 3-C3H5

A (cm3 mol-1 s-1)

n

E (cal/mol)

ref

3.59 × 10+14 2.20 × 10+14 5.06 × 10-06 9.04 × 10+11 8.43 × 10+11 8.43 × 10+11 8.43 × 10+11 8.43 × 10+11 2.89 × 10-01 2.05 × 10+13 7.83 × 10+11 2.23 × 10+0 1.42 × 10+13 2.56 × 10+13 1.45 × 10+12 7.23 × 10+11 9.03 × 10-01 1.51 × 10+0 7.83 × 10+11 1.26 × 10+11 5.00 × 10+09 3.55 × 10+09 1.78 × 10+11 3.98 × 10+12 7.94 × 10+15 2.46 × 10+06 1.60 × 10+13 1.30 × 10+14 3.32 × 10+08 4.00 × 10+13 3.00 × 10+13 6.00 × 10+13 5.00 × 10+05 3.20 × 10+13 4.00 × 10+13 9.00 × 10+12 3.00 × 10+13 1.20 × 10+13 4.00 × 10+13 2.50 × 10+16 3.20 × 10+27 2.50 × 10+15 5.78 × 10+13 6.03 × 10+13 7.23 × 10+15 2.53 × 10+14 4.88 × 10+13 6.00 × 10+11 6.00 × 10+11 6.00 × 10+11 6.00 × 10+11 3.00 × 10+11

-0.75 -0.75 5.17 0.00 0.00 0.00 0.00 0.00 3.70 0.00 0.00 3.50 -0.40 -0.40 0.00 0.00 3.65 3.46 0.00 0.00 0.00 0.00 0.00 0.00 0.00 2.00 0.00 0.00 1.79 0.00 0.00 0.00 2.00 0.00 0.00 0.00 0.00 0.00 0.00 -0.80 -3.14 0.00 0.00 0.00 -0.65 0.00 0.00 0.00 0.00 0.00 0.00 0.00

0 0 9067 0 0 0 0 0 9784 -131 -131 6637 0 0 0 0 9140 7451 0 0 44200 37000 38000 57695 70795 8270 -570 0 1670 0 0 0 7230 0 0 600 0 -570 -550 0 1230 86679 51579 75438 73620 55173 52304 13500 13500 11200 13500 13500

42 42 42 42 42 42 42 42 42 42 42 39 42 42 42 42 42 42 42 71 72 73 73 74 74 32 32 32 32 32 32 32 32 32 32 32 32 32 32 32, k∞ 32, k0 39 39 39 39 39 39 estimateda estimateda estimateda estimateda estimateda

(b) Parameters for the Central Broadening Factor Fcb,c no.

reactions

a1

a2

a3

a4

2 6 13 18 30 33 37 53 93 181

CH3 + H (+M) a CH4 (+M) CH3 + CH3 (+M) a C2H6 (+M) C2H4 + H (+M) a C2H5 (+M) C3H8 (+M) a C2H5 + CH3 (+M) i-C3H7 (+M) a 1-C3H6 + H (+M) s-C4H9 (+M) a 1-C3H6 + CH3 (+M) n-C3H7 (+M) a C2H4 + CH3 (+M) t-C4H9 (+M) a i-C4H8 + H (+M) C2H3 (+M) a C2H2 + H (+M) H + CH2 (+M) a CH3 (+M)

0.370 0.620 0.760 0.760 0.355 0.526 0.110 0.612 0.355 0.680

3315 73 40 1946 12 607 326 874 12 78

61 1180 1025 38 52000 11 5000 11 52000 1995

7370 2870 1662 3360 7370 5590

a Rate coefficients were estimated by the method proposed by Ranzi et al. (ref 70). b F ) (1 - a ) exp(-T/a ) + a exp(-T/a ) + exp(c 1 2 1 3 a4/T). c Collision efficiency for various gases: H2, 3.0; CH4, 3.0; C2H6, 4.0; i-C4H8, 4.0; i-octane, 4.0.

that facilitated attaining closer agreements between the computed and measured species concentrations. Reactions 68 and 101 are the two most sensitive steps that are critical for the fragmentation of the isooctane initiator. In this category one should include abstractions from isooctane by radicals and isomerization of the

three iso-C8H17 species (pai-octane, p-i-octane, and t-ioctane). The least sensitive are reactions 92-100 and 162-181. Selection of the final rate constants is justified within the listed references. All calculations were performed using Sandia’s Chemkin II, versions 2.8 and 3.2. To establish that the



Figure 5. Typical x-t diagram showing gas particle trajectories as well as pressure (P) and temperature (T) excursions during the incident and reflected shock regimes.

calculations are internally consistent, the time scale for several kinetics calculations was extended to 1 × 1014 s. The final leveled concentrations so derived are essentially identical with concentrations at equilibrium derived by minimizing the total system free energy; refer to Table 4 for the major species. Results and Discussion Batch Reactor Experiments. A total of 84 runs were made with mixtures of methane-isooctaneoxygen-nitrogen-hydrogen in a variety of combinations. Typical compositions ranged from pure methane or 2% isooctane in nitrogen to mixtures of 96-98% methane with 2% isooctane (molar basis) and diluents. Nominal reactor temperatures (TC1b, Figure 2) were varied from 983 to 1373 K but generally were set at about 1200 K. Total pressures in the reactor ranged from 2 to 25 atm. For “batch” operation, residence times were set either at 1.0 or 10 s (a few at 100 s). Under “flow-through” conditions, the estimated residence times were in the range 0.08-0.10 s. The quartz liner of the reactor was examined periodically. After several runs it was found to be covered with a hard, black deposit. Samples (suspended in KBr pellets) showed the FT-IR absorptions listed in Table 5. (65) Baldwin, R. R.; Walker, R. W. J. Chem. Soc., Faraday Trans. 1 1979, 75, 140. (66) Tsang, W. Int. J. Chem. Kinet. 1978, 10, 599. (67) Thorn, R. P.; Payne, W. A.; Stief, L. J.; Tardy, D. C. J. Phys. Chem. 1996, 100, 13594. (68) Fahr, A.; Braum, W.; Laufer, A. H. J. Phys. Chem. 1993, 97, 1502. (69) Hidaka, Y.; Nakamura, T.; Miyauchi, A.; Shiraishi, T.; Kawano, H. Int. J. Chem. Kinet. 1989, 21, 643. (70) Ranzi, E.; Dente, M.; Faravelli, T.; Pennati, G. Combust. Flame 1994, 95, 1.

A large volume of analytical data was accumulated, and their reduction was extensive (i.e., corrections for dead-volume, temperature corrections for endoergicity, etc). Considerable scatter appeared inevitable, owing to the variable amounts of material that remained in polymeric form on the wall. Typical product distributions, derived from GC measurements, are illustrated in the figures. Overall methane losses were calculated from these data, using the approximate relation

∆[CH4]% )

{ } χr1 - χp1 χr1

× 100

where χr1 and χp1 denote mole fractions of methane in the reactant gas mixture (preheater) and in the product mixture, respectively. A summary of conclusions follows. (i) Under the preset experimental protocol, the pyrolysis of neat CH4 yielded at most negligible amounts of higher hydrocarbons. However, some deposition of hard polymer on the reactor wall did occur. (ii) When isooctane was admixed (at ∼2%), there was significant production of hydrocarbons in the gas phase, but the yield was low, indicating substantial loss of C/H via deposition onto the reactor wall. (iii) Adding O2 (1-2%) to the methane and isooctane mixture had no discernible effect on the yields of gaseous products. (iv) Dilution of reaction mixtures with either N2 or H2 did not appreciably alter the distribution of species or the yields of hydrocarbons. Also, changing the operating pressures from a few to 25 atm had no measurable effect. (v) Coating the quartz liner with a film of KCl crystals reduced but did not eliminate the deposition of solids.


Bauer et al.

Table 3. Thermochemical Dataa,

species H H2 CH CH2(S) CH2 CH3 CH4 HCtCH HC4 dCH2 H2CdCH2 H2C4 sCH3 H3CsCH3 H2CdCdCH2 H3CsCtCH H2C4 sCHdCH2 H2CdCHsCH3 H3CsC4 HsCH3 H3CsCH2sC4 H2 CH3sCH2sCH3 H2CdCHsCH2sC4 H2

H2CdCHsCH2sCH3 H3CsCH2sC˙ HsCH3

H3CsCH2sCH2sC˙ H2 CH3sCH2sCH2sCH3

H3CsCH2sCH2sCHdCH2

CH2dCHsCH2sCH2sCH2sCH3

notation H H2 CH CH2(S) CH2 CH3 CH4 C2H2 C2H3 C2H4 C2H5 C2H6 C3H4 CH3CCH 3-C3H5 1-C3H6 i-C3H7 n-C3H7 C3H8 1-C4H7 p-i-1-C4H7

0 ∆Hf,298 kcal mol-1

b

0 S298 cal mol-1 K-1

C0p(T) (cal mol-1 K-1) 298

600

1000

1400

52.10 0.00 142.77 101.51 93.31 35.10 -17.90 54.35 71.64 12.50 28.36 -20.04 45.63 44.32 39.10 4.88 22.30 24.02 -24.82 46.00 29.00

27.42 31.23 43.75 45.10 46.58 46.36 44.49 48.02 55.92 52.38 59.06 54.75 58.18 59.34 61.87 63.70 69.15 69.18 64.57 76.50 69.80

4.97 6.89 6.97 8.07 8.07 9.18 8.47 10.52 10.43 10.23 12.07 12.52 14.07 14.51 15.15 15.39 15.81 17.02 17.58 19.69 20.69

4.97 6.99 7.11 8.98 8.97 11.50 12.44 13.89 15.59 16.86 19.35 21.28 22.04 21.80 25.04 25.81 27.32 28.78 30.75 33.42 34.04

4.97 7.21 7.71 10.61 10.46 14.07 17.16 16.23 19.43 22.43 25.78 29.29 27.99 27.71 32.12 34.53 37.36 38.13 41.73 43.30 44.40

4.97 7.64 8.42 11.62 11.66 15.87 19.97 17.82 21.57 25.49 29.45 33.73 31.27 31.06 35.99 39.22 42.76 43.22 47.67 48.63 49.98

i-C4H8

-4.10

70.90

21.03

34.78

46.60

53.26

1-C4H8 s-C4H9 t-C4H9

-0.13 16.73 12.36

73.59 77.78 76.41

20.40 21.85 18.70

35.05 37.54 34.80

46.85 50.20 48.93

53.89 57.11 56.41

i-C4H9

13.70

76.16

22.83

38.74

50.94

57.65

15.90 -30.06 -32.18

78.52 74.03 70.59

23.34 23.57 23.10

38.57 40.46 40.70

50.71 54.35 54.60

57.47 61.75 61.92

2-M-B-1-ene

-8.68

81.15

26.28

44.71

59.44

67.33

2-M-B-2-ene

-10.17

80.92

25.10

43.41

58.56

66.93

-5.08 4.60

82.64 88.80

26.19 24.22

44.52 43.99

59.21 61.30

67.01 70.89

neo-C5H11

8.70

78.80

27.39

49.01

64.59

72.14

neo-C5H12

-39.91

72.85

29.02

51.62

71.20

82.97

i-C5H12

-36.75

82.13

28.41

50.12

68.40

78.39

4-M-P-1-ene

-12.00

87.91

31.12

54.07

71.76

81.48

2-M-P-1-ene

-14.19

91.34

32.41

54.39

71.81

81.25

-9.96

91.97

31.62

53.87

71.54

80.81

-20.40

90.90

37.67

65.10

84.96

95.74

pai-octane

-5.40

106.30

44.20

77.35

100.80

113.45

t-i-octane

-12.80

106.90

44.54

77.06

100.73

113.66

n-C4H9 C4H10 i-C4H10

1-C5H10 t-C5H11

1-C6H12 C7H14



Table 3 (Continued)

species

notation p-i-octane

i-octane

C8H17

((CH3)3C)2

0 ∆Hf,298 kcal mol-1

C0p(T) (cal mol-1 K-1)

0 S298 cal mol-1 K-1

298

600

1000

1400

-6.40

105.50

44.57

77.34

100.82

113.51

-53.54

101.07

45.06

80.19

108.70

124.72

-6.70

99.90

44.74

78.83

102.05

113.99

-53.90

93.00

45.54

80.75

105.55

118.55

a Burcat, A.; McBride, B. Ideal Gas Thermochemical Data (1995); Technion-Israel Institute of Technology: Haifa, Israel, 1995. b Stein, S. E.; Rukkers, J. M.; Brown, R. L. NIST Structures and Properties Database and Estimation Program; NIST Standard Reference Database; NIST: Gaithersburg, MD, 1994.

Table 4. Comparison of Equilibrium and Kinetic Calculationsa,b species

equilibrium calculations

kinetic calculationsc

H H2 CH3 CH4 C2H2 C2H3 C2H4 C2H5 C2H6 C3H4 CH3CCH 3-C3H5 1-C3H6 C3H8 i-C4H8 1-C4H8 2-M-B-1-ene 2-M-B-2-ene

1.944 × 10-13 4.967 × 10-07 8.736 × 10-12 1.172 × 10-06 1.251 × 10-07 1.422 × 10-13 1.354 × 10-07 2.231 × 10-13 1.659 × 10-09 2.374 × 10-09 7.575 × 10-09 1.567 × 10-11 4.055 × 10-09 7.456 × 10-12 2.906 × 10-11 1.985 × 10-11 1.663 × 10-13 1.935 × 10-13

1.122 × 10-13 4.077 × 10-07 5.992 × 10-12 1.157 × 10-06 9.919 × 10-08 8.807 × 10-14 1.270 × 10-07 1.646 × 10-13 1.682 × 10-09 2.056 × 10-09 6.645 × 10-09 1.291 × 10-11 4.093 × 10-09 7.916 × 10-12 3.191 × 10-11 2.104 × 10-11 1.905 × 10-13 2.265 × 10-13

a Reaction condition: 3.527% CH -0.068% isooctane-Ar at a 4 temperature of 1229 K and a pressure of 4.09 atm. b The concentration is in units of mol/cm3. c The ending reaction time for kinetic calculations is 1014 s.

FT-IR of the deposits indicated that the composition of the polymer was higher in C-H (stretching) bond content for coated relative to uncoated quartz. (vi) Although the overall yield of gaseous species was always low, several methane conversions reached 50% in “batch” operations but only 10% under flow-through conditions (due to the much shorter residence times). (vii) The flow-through tests, with residence times as low as 0.10 s (some with 0.08 s) and temperatures somewhat in excess of 1300 K, generated the highest relative yields. (viii) Runs with tetramethylbutane were not significantly different from those with isooctane. The partition of products collected from several “batch” and “flow-through” runs is illustrated in parts a and b of Figures 6, respectively. Percentage mole fractions were derived from calibrated GC analyses of collected (gas-phase) products; the major constituent (CH4) was not included. The experimental conditions for the mixtures in parts a and b of Figure 6 are listed in parts a and b of Table 6, respectively.

Table 5. FT-IR Spectra of Black Deposit on Inner Walls of the Reactora frequencies (cm-1)

band characteristics (intensity; width)

vibrational modes

3423 1629 1422 1385 1239 1144 1042 816

(1) Quartz-Lined Reactor Wall very strong; medium C-H stretch strong; medium width CdC or aromatic medium; broad H-C-H bend very weak weak; medium very weak; medium C-C-H bend strong; very broad C-C stretch medium; very broad C-C stretch

3302 1242 1134 1029 798 728 653

(2) Stainless Steel Wall extremely strong; medium medium; medium very weak medium; very broad medium; very broad very weak medium; very weak

C-H stretch C-C-H bend C-C stretch C-C stretch C-C-C bend

a

Two samples were examined (solid dispersed in KBr pellet). General broad background absorption with superposed broad bands was observed.

Our review of the published reports related to the direct, noncatalytic conversion of methane indicated that most of the papers dealt with various aspects of partial oxidation of CH4, primarily to methanol (H3COH). Comparatively fewer papers deal with conversion to higher hydrocarbons: C2, C3, and aromatics. There are considerable differences in reported yields of products, mostly owing to differing experimental conditions, leading to claimed methanol selectivities that range from 10% to 80%. However, most investigators agree that selectivity of the desired products (hydrocarbons) decreases as methane conversion increases, which is consistent with our observations. The wellknown problem associated with high-temperature oxidative coupling of methane is the production of undesired byproducts, primarily CO or CO2. Rokstad et al.12a showed that significant amounts of “coke”, exceeding 60% of the products, are formed at methane conversions above 15% in the temperature range 1300-1500 K at 1 atm pressure. Nozaki et al.12d reported yields of the desired products C2 and C3, at levels of 2%, concurrently with the undesired CO and CO2 of 2-3%. This corresponds to methane conversions of 3-5% (at a temper-


Bauer et al. Table 6. Composition of Mixtures Used for Figure 6a and Methane Content in the Product Samples (a) Composition of Mixtures Used for Figure 6a mixture no.

CH4 (%)

isooctane (%)

O2 (%)

N2 %)

temp (K)

residence time (s)

R1 R2 β1 β2 γ1 γ2 δ1 δ2

12.3 12.3 12.50 12.50 97.0 97.0 11.75 11.75

0.6 0.6 0.15 0.15 1.0 1.0 0.25a 0.25a

1.2 1.2 0.15 0.15 2.0 2.0 0.50 0.50

85.9 85.9 87.20 87.20 0.0 0.0 87.50 87.50

1270 1282 1272 1279 1273 1277 1256 1262

1.0 10.0 1.0 10.0 1.0 10.0 1.0 10.0

(b) Methane Content in the Product Samplesb run no.

temp (K)

residence time (s)

pressure (atm)

CH4 (%)

flowh 21 flowh 23 flowh 22 flowh 31 flowh 32 flowh 41 flowh 42

1286 1277 1281 1335 1339 1372 1373

0.08 1.00 0.10 0.08 0.07 1.00 0.08

2.10 3.80 1.70 1.90 1.60 4.10 1.40

78.46 67.25 80.57 82.19 78.71 39.85 75.22

a 2,2,3,3-Tetramethylbutane was used as the free radical initiator. b For a mixture of 1.0% isooctane-2.0%O2-10.0% N2-87.0% CH4.

Table 7. Compositions and Reaction Conditions for the Shock-Heated Mixtures mixture no.

Ar (%)

CH4 (%)

isooctane (%)

O2 (%)

temp (K)

pressure (atm)

1 2 3 4 5 6

97.50 97.24 97.51 99.67 96.40 96.25

2.50 2.44 2.17 0.00 3.53 3.56

0.000 0.320 0.316 0.333 0.068 0.064

0.00 0.00 0.00 0.00 0.00 0.13

880-1390 1070-1280 1110-1270 1120-1330 1100-1360 1110-1350

4.7-6.0 4.7-5.8 4.6-5.1 4.7-5.2 3.6-4.8 4.1-5.0

4-6. We define a product carbon distribution (%)

i[Ci]

∑i i[Ci]

Figure 6. Distribution of products developed during the highpressure pyrolysis of various mixtures under the conditions listed in Table 6.

ature of 750 °C, pressure of 10 atm, and residence time of 0.6 s), which is consistent with our low-temperature run (less than 1100 K). Shock Tube Experiments. The compositions and reaction conditions of the six mixtures that were investigated are listed in Table 7. Typical pressures on the driver and driven sides were 80 psig and 200-300 Torr, respectively. About 75 tests were made. Residence times were somewhat under 2 ms. After every shock the inside of the tube was swabbed with a clean tissue to check whether any solid product was deposited on the wall. None was found. Clearly, the short duration of reaction time precluded C/H polymer growth as observed in the batch experiments. The first 10 experiments, using mixture 1, showed essentially negligible CH4 loss. Our major effort was devoted to mixtures

× 100%

where i is the carbon number of a chemical species and [Ci] is its concentration. Comparisons of the product carbon distributions derived for these three mixtures are illustrated in Figure 7. Despite the relatively large scatter of the data, two conclusions are evident from these graphs. (i) Admixing a low concentration of a free radical initiator clearly facilitates methane conversion. The contribution from the isooctane to the products is well below that measured for the mixture. (ii) Under shock conditions the addition of O2 is deleterious with respect to hydrocarbon production. Perhaps the rapidly generated OH radicals divert the essential H atoms to H2O. How well the present extended reaction mechanism accounts for the product distributions measured for mixture 5 is best illustrated in Table 8. The computed loss of methane is checked against that observed. For the C2 species the model overpredicts and for C3 it somewhat underpredicts the measured concentrations. Regarding Impure Methane from Well Blow-Off. In the design of conversion plants, it is essential to consider that the gaseous effluents from wells incorporate other species besides CH4, in the range from 5 to



Figure 7. (a-c) Experimentally determined product distributions for the C2, C3, and C4 species, comparing mixture 5 (CH4isooctane in Ar) with mixture 4 (isooctane in Ar). Experimental conditions for both mixtures are listed in Table 7. (d-f) Similar comparsion for mixture 5 with mixture 6 (added oxygen), illustrating the deleterious effect of oxygen.

15%, depending on the locale. Ethane, water, carbon dioxide, and hydrogen sulfide are the most common. Some of these components are “scrubbed” prior to injection into the pipeline distribution system. To assess the effects of such impurities, early model calculations of anticipated pyrolysis product distributions were made for the following mixtures.22b (i) One mixture at 20 atm consisted of [90% CH4 + 10% C2H6] + 0.2 atm O2 + 0.22 atm isooctane. The computed distributions indicated a 3.5% enhancement of butane production for a reaction time less than 1 s. On this basis, we postulate that the inclusion of some higher hydrocarbons, at levels comparable to the above, would have a salutary effect on the conversion process. Bossard and Back75 found that substantial levels of

comixed ethylene augmented the homogeneous conversion of methane to propylene (925-1023 K). (ii) To 20 atm of CH4 and 0.22 atm of isooctane, we admixed in sequence the following: 1% H2S + 1% O2; 5% H2S + 1% O2; 5% H2S + 5% O2. In these mixtures the major new products are S2, SO2, and H2CS. With the higher H2S levels methane conversion seems to be somewhat elevated but the fraction of desired products is clearly reduced. However, with 1% H2S or lower there is little change. (71) Kerr, J. A.; Parsonage, M. J. Evaluated Kinetic Data on GasPhase Addition Reactions; Butterworth: London, 1972. (72) Taniewski, M. J. Chem. Soc. 1965, 7436. (73) Richard, C.; Back, M. H. Int. J. Chem. Kinet. 1978, 10, 389. (74) King, K. D. Int. J. Chem. Kinet. 1979, 11, 1071. (75) Bossard, A. R.; Back, M. H. Can. J. Chem. 1991, 69, 37.


Bauer et al.

Table 8. Comparison of Model Predictions and Experimental Observations for the Shock-Heated Mixture 5 temp (K)

pressure (atm)

1145

4.52

1158

3.64

1189

4.41

1200

4.02

1229

4.09

1238

4.09

1256

4.18

1321

4.55

1345

4.70

exptl calcd exptl calcd exptl calcd exptl calcd exptl calcd exptl calcd exptl calcd exptl calcd exptl calcd

C1 (%)

C2 (%)

C3 (%)

C4 (%)

83.2 85.9 79.5 85.8 81.3 85.1 81.1 84.8 78.5 83.9 80.7 83.5 79.0 82.7 81.0 80.3 77.1 79.6

0.6 0.9 1.0 1.1 1.1 2.1 2.4 2.4 2.6 3.8 2.8 4.3 3.4 5.3 2.5 8.6 3.8 9.4

1.3 0.6 1.9 0.8 2.2 1.4 3.8 1.7 4.6 2.5 4.8 2.8 5.8 3.4 5.2 4.6 8.7 4.8

2.3 1.7 4.1 2.1 4.5 3.4 6.1 3.8 9.3 5.0 7.3 5.3 6.4 5.8 7.8 6.2 8.5 6.1

C5 (%) 0.02 0.6 0.03 0.1 0.5 0.1 0.6 0.2 0.8 0.2 0.3 0.2 0.1 0.1

C8 (%) 12.6 10.9 13.0 10.3 10.9 8.0 6.2 7.1 4.4 4.6 3.8 3.9 5.0 2.5 3.5 0.14 1.9 0.02

(iii) We also considered several combinations of methane with N2O, with and without added O2. Additional species were inserted in the mechanism along with corresponding reaction steps. None of these combinations yielded the desired product distributions at levels comparable to that of the methane-isooctane reference system.

Conclusions Under a wide range of experimental conditions we demonstrated that comixing a few percent of a low-cost, abundant radical initiator (isooctane) with methane facilitates its conversion to higher hydrocarbons under homogeneous conditions at temperatures at which methane alone undergoes little pyrolysis. This effect was predicted on the basis of extensive computer modeling that clearly showed kinetic overshoots in desired product concentrations for reactor residence times of ∼0.01 s. The practical application of this mode of upgrading may be limited by depletion of products that condense as C/H solids onto the reactor walls. The “cost” of methane at the well-head will also control the financial incentive for upgrading methane via this process. Additionally, the cost of recycling the unreacted methane and the cost of separating the generated hydrogen, either by low-temperature distillation or membrane separation, mitigate against the technological exploitation under present market conditions. Acknowledgment. These investigations were supported by NSF under the Chemical Reaction Processes Program, Award CTS No. 9306905. We are deeply indebted to Professor P. Harriott (Cornell, Chemical Engineering) for many consultations and sage advice. EF9700487

Upgrading of Methane under Homogeneous Thermal Conditions: An

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