The homogeneous conversion of methane to higher hydrocarbons in the presence of ethylene in the temperature range K

Transcription

1 The homogeneous conversion of methane to higher hydrocarbons in the presence of ethylene in the temperature range K ALAIN R. BOSSARD AND MARGARET H. BACK Department of Chemistry, University of Ottawa, Ottawa, Ont., Canada KIN 6N5 Received June 26, 1990 ALA~N R. BOSSARD and MARGARET H. BACK. Can. J. Chem. 69, 37 (1991). Mixtures of ethylene and methane have been pyrolyzed in the temperature range K for the purpose of converting methane to higher hydrocarbons. Addition of methane to thermally-reacting ethylene increases the rate of formation of propylene but decreases the rates of formation of the other major products, ethane, acetylene, and butadiene. Hydrogen abstraction from methane is a major propagation reaction and causes a shift in the radical distribution from ethyl and vinyl radicals, the main radicals in the pyrolysis reactions of ethylene alone, to methyl radicals, which lead to the formation of propylene. At 1023 K with a pressure of ethylene of 6.5 Torr and of methane of 356 Torr, 1.5 mol of methane is converted to higher molecular weight products for every mole of ethylene reacted. The rate of conversion of methane in the homogeneous system is lower than in catalytic reactions but the product is entirely hydrocarbon and no methane is lost to carbon monoxide or carbon dioxide. Key words: methane, ethylene, kinetics, pyrolysis, fuels. ALA~N R. BOSSARD et MARGARET H. BACK. Can. J. Chem. 69, 37 (1991). Dans le but de transformer du mtthane en hydrocarbures de poids moltculaires plus tlevts, on a pyrolyst des mtlanges d'tthylbne et de methane a des temperatures allant de 925 a 1023 K. L'addition de mtthane a de I'tthylbne qui rtagit d'une faqon thermique provoque une augmentation de la vitesse de formation du propylbne; elle provoque aussi une diminution des vitesses de formation des autres produits majeurs : tthane, acttylbne et butadibne. L'abstraction d'un atome d'hydrogbne du mtthane est une des principales rtactions de propagation et elle provoque un dtplacement important dans la distribution radicalaire qui passe d'une prtpondtrance des radicaux tthyles et vinyles, les radicaux principaux dans les reactions de pyrolyse de I'tthylkne seul, vers les radicaux mtthyles qui conduisent a la formation du propylbne. A 1023 K, a des pressions d'tthylbne de 6,5 Torr et de mtthane de 356 Torr, 1,5 mole de mtthane se transforme en hydrocarbures de poids moltculaires plus tlevts pour chaque moltcule d'tthylbne qui rtagit. La vitesse de conversion du mtthane dans le systbme homogbne est plus faible que dans les rtactions catalytiques; toutefois, les produits ne sont que des hydrocarbures et il n'y a pas de perte de methane en monoxyde et en dioxyde de carbone. Mots clis : mtthane, tthylbne, cinttique, pyrolyse, combustibles. [Traduit par la revue] Introduction Most of the efforts directed to the conversion of natural gas into higher molecular weight fuels have focussed on the use of catalysts as a means of overcoming the thermodynamic disadvantage of the overall process. For example, several metal oxides have been found effective in the oxidative coupling reaction of methane and their development and improvement has received considerable attention in recent years ( 1-4). Catalysts, however, while effective in many regards, have a number of limitations. In the methane conversion the presence of the oxide leads to the formation of carbon monoxide and carbon dioxide and represents a loss of carbon as fuel. Homogeneous conversion of methane would not involve this loss process and such systenis therefore warrant investigation along with catalytic processes. Direct thermal decomposition of methane is not practical because of its high stability; at the high temperatures necessary for reaction rapid conversion to carbon occurs (5, 6). To overcome this problem a sensitizer may be used to induce decomposition at lower temperatures, favouring the production of the less stable hydrocarbon products. One such system involves the pyrolysis of mixtures of methane and ethylene. A description of this system and some results from the pyrolysis of these mixtures have been reported (7) and only a brief summary of the reactions will be given here. The thermal reactions of ethylene at temperatures in the neighbourhood of 800 K involves both decomposition and polymerisation and occurs by a chain reaction mechanism (8). Initiation by the bimolecular reaction of ethylene, [I] 2CzH4+ CzH3 + C2H5 is followed by addition, abstraction, decomposition, and isomerisation of the ethyl, vinyl, and other radicals. In pure ethylene methane and ethane are the only saturated products and are formed by hydrogen-abstraction by the corresponding radical. Addition of methane will not add new initiation processes, since methane is stable at these temperatures, but will add a new propagation reaction. Thus in the ethylene system, where the ethyl radical is one of the major chain-canying radicals, methane is converted to ethane. When the relative amount of methane is small, the actual and relative concentrations of the various radicals in the system will not be altered; only the rate of formation of ethane will be increased. With larger amounts of methane, however, reaction [4] will shift the balance of radicals in favour of the methyl radical and the products of the chain reaction will be altered. As expected under these conditions, propylene becomes the product most affected by the presence of methane. The ratio CH4/C2H, is therefore an important parameter in the selectivity of the products. Equally important is the concentration of ethylene which controls the overall rate of the reaction. Thus optimum performance of the system with respect to conversion of methane requires a balance between these two factors.

2 38 CAN. 1. CHEM. VOL. 69, 1991 In the earlier study covering the temperature range K (7) the conversion of methane and the selectivity of the products was measured as a function of the relative and absolute concentrations of methane and ethylene. Although the selectivity for formation of ethane and propylene was high, the rate of the reaction was too slow to be practical; the limitation was the rate of the initiation of the chain reaction by ethylene. These studies have now been extended to higher temperatures, K, for the purpose of increasing the overall rate of the conversion of methane. Significant changes in the mechanism of the reaction which caused considerable shift in the product distribution were observed in the higher temperature region and this paper describes the results of these studies. Experimental The experiments were performed in a static pyrolysis system. A quartz reaction vessel of 0.21 L was placed in the center of a three-zone Lindberg Furnace, Model Mixtures of reactants were admitted to the reactor by expansion from calibrated volumes. Samples were removed for analysis after the required time of reaction. Details of the procedures for preparation of the mixtures were given previously (7, 9). Products were analyzed using a Hewlett-Packard Gas Chromatograph Model 5710A with flame detector. Two columns were used to obtain analysis for a wide range of products. A silica gel column (40160 mesh, 1.5 m x 6.5 rnrn 0.d. copper tubing) at 30 C separated small quantities of the product ethane from the large quantities of the reactants. After elution of the ethylene the column was programmed at 16 C min-' to 250 C and analysis was obtained for benzene and toluene. Other C6 or C7 compounds of similar structure, such as cyclohexene or cyclohexadiene, may have been included in these peaks but no attempt at further separation was made. A Durapak column (phenylisocyanate/porasil C, 80/100 mesh, 6 m x 6.5 mm 0.d. copper tubing) maintained at 25OC gave good separation of acetylene, allene, propylene, and butenes. A single peak corresponding to a C5 compound was also detected and analyzed in certain experiments. Research Grade methane (99.99%) and Research Grade ethylene (99.99%) were obtained from Matheson Gas Products, Whitby, Ont., and, except for rigorous degassing, were used without further purification. Impurities in the reactants, mainly ethane and acetylene, were too small to affect the kinetics of the reaction but correction to the yields of these products were made where necessary. Results and discussion Twenty series of experiments, in which yields of products were measured as a function of the reaction time, were done at the following temperatures: 925, 974, and 1023 K. The conditions of initial pressures of ethylene and methane are summarized in Table 1. The pyrolysis of ethylene alone was also studied at each temperature. A. Initial stages of the reaction The main products from the pyrolysis of ethylene alone at 925 K are, in order of importance, butadiene, propylene, ethane, methane, acetylene, and butene-1. This distribution remained much the same at the two higher temperatures except that by 1023 K acetylene replaced propylene as the second most abundant product, next to butadiene. At this temperature butadiene accounted for 80% of the products and acetylene 10%. Examples of the yields of the major products as a function of time at 1023 K in the presence and absence of methane are shown in Figs The reaction was too fast in this temperature range to allow accurate measurement of initial rates under all conditions. Relative initial rates were, however, TABLE 1. Initial pressure of ethylene and ratios of methanelethylene for each temperature Pressure of Temperature ethylene 3 (K) (Torn) C2H4 Time /s FIG. 1. Yield of ethane as a function of time. Initial pressure of ethylene 12.5 Torr. T = 974 K. Pressure of methane, 0: 50 Tom; A : 200 Torr. reliably measured by extrapolation of the ratios of the yields of products to zero time. These ratios are summarized in Table 2. The mechanism of the thermal reactions of ethylene in the temperature region of 800 K was discussed in an earlier report (7). Following initiation by reaction [I], addition and abstraction reactions of the ethyl and vinyl radicals and addition, isomerization, and decomposition reactions of higher radicals led to the formation of the major products, methane, ethane, propylene, and butene-1. Carbon-carbon bond rupture was the only decomposition process of the radicals which was sufficiently fast to compete with addition to ethylene. In the present experiments the appearance of butadiene and acetylene as major products indicates that at higher temperatures dissociation of radicals by carbon-hydrogen bond splitting is an important loss process. As these dissociation reactions begin to compete effectively with additions to ethylene the product distribution shifts from monoolefins to more unsaturated products, of which butadiene, acetylene, and benzene were the main examples identified in the present experiments. The trend to unsaturation leads also to the formation of polyaromatic

3 BOSSARD AND BACK Time/s FIG. 2. Yield of propylene as a function of time. Initial pressure of ethylene 12.5 Ton. T = 974 K. Pressure of methane, 0: 0 Ton; 0: 50 Ton; A : 200 Torr. Time / s i, FIG. 3. Yleld of acetylene as a function of time. Initial pressure of I ethylene 12.5 Torr. T = 974 K. Pressure of methane, 0: 0 Torr; 0. 50Ton;~:200Ton. j I hydrocarbons and eventually to carbon. Thus the yields of the light products approached constant or maximum values with time even at conversions less than 1%. This contrasts with the auto-acceleration of the rates of formation of these products observed in the lower temperature region. The following simplified mechanism shows these modifications to the mechanism outlined for the low temperature region, and accounts for the main features of the reaction in the initial 1 stages i Time/s FIG. 4. Yield of butadiene as a function of time. Initial pressure of ethylene 12.5 Ton. T = 974 K. Pressure of methane, 0: 0 Ton; 0: 50 Ton; A : 200 Ton. [I] 2C2H4 + C2H5 + CzH3 121 CzH5 + C2H4 + CzH6 + CzH3 131 CzH5 + CzH4 % n-c4h9 [4] CzH5 % C2H4 + H [5] n-c4hg + S-C4H9 [6] S-C~H~ + C3H6 + CH3 171 CH3 + C2H4 4 CH4 + C2H3 [8] CH3 + CzH4 % C3H7 [9] C3H7 4 C3H6 + H 1101 C2H3 + C2H4S C4H7 [I]] C4H7+C4H6+H [12] H + CzH4 + H2 + CZH3 [13] CzH3 4 C2H2 + H [14] CzH5 + CH4 + C2H6 + CH CzH3 + CH4 + C2H4 + CH3 [I61 H+CH4+H2+CH3 No decomposition of methane itself was observed under the conditions of the present experiments; its effect is solely as a source of methyl radicals in the propagation reactions, as indicated by reactions [14]-[16]. The occurrence of these reactions completely changed the product distribution. At the highest temperature in the absence of methane, butadiene accounted for -80% of the products, whereas with the highest pressure of methane the products contained -85% propylene. Addition of methane increased the rate of formation of propylene, but decreased the rate of formation of all other products. This trend towards increasing importance of propylene in the presence of methane was observed in the previous experiments and was more noticeable at the higher temperatures. At the

4 40 CAN. 1. CHEM. vol. 69, 1991 where where and TABLE 2. Ratios of rates of formation of products CZH6 C2H2 C4H6 C2H1 C2H4 CH T(K) (TOE) (TOE) C3H6 C3H6 C3H6 C4H6 d= kll k-10 + kll lowest temperature, 774 K, methane did not significantly affect the rate of formation of propylene but at 853 K the rate of formation of propylene was increased more than that of ethane. The rise in importance of propylene is a direct consequence of the participation of methane in the chain propagation reactions. Radical abstraction from methane shifts the radical distribution in favour of methyl radicals at the expense of the ethyl and vinyl radicals and methyl radicals lead directly to propylene. Propylene will therefore become a major product as, with increasing temperature, abstraction is increasingly favoured over addition. The more surprising observation was that the rates of formation of the other main products, ethane, acetylene, and butadiene, all decreased in the presence of methane. This suggests that abstraction from methane by the radicals depicted in reactions [I41 and [15] is not fast enough to compete with their decomposition. Methane therefore reacts mainly with the hydrogen atoms produced in the decomposition rather than with the parent radical. In the absence of methane the hydrogen atoms are converted back to ethyl and vinyl radicals by addition and abstraction from ethylene; in the presence of methane they are converted to methyl radicals. Thus propylene will be formed through reactions [7]-[9] at the expense of the products formed by reactions of the ethyl and vinyl radicals. On the basis of the simplified mechanism the rates of formation of the main products in the presence of methane are given as follows: and the concentration of hydrogen atoms is controlled through equilibrium of reaction [4]. Table 2 shows that the ratios of the rates of formation of all products to the rate of formation of propylene decrease sharply as methane is added to the system until the ratio of methane/ ethylene reaches about four. Further additions of methane have much less effect on the ratios and in some cases a limiting value is obtained. This initial decrease shows that the second term in the expression for the rate of formation of propylene becomes the major contribution to its rate at relatively low ratios of methanelethylene, before the rates of formation of the other products are affected. With sufficiently large amounts of methane, however, the rates of formation of the other products become dominated by the terms involving methane and the ratios therefore eventually become independent of methane. This behaviour is illustrated by the ratio RC2H6/RC3H6 shown in Fig. 5 for the three lower temperatures of the previous study and for 925 K. The initial sharp decrease in the ratio observed at 925, 974, and 1023 K was progressively less pronounced with decreasing temperature and in fact at the lowest temperature the ratio increased with additions of methane, but still approached, a constant value. These ratios would not be expected to change much with further reaction and the system thus has the advantage of allowing a fixed product distribution by adjustment of the reactant ratio and the temperature. The limitations of the simplified mechanism are shown by the

5 BOSSARD AND BACK FIG. 5. Ratio of the initial rate of formation of ethane to the initial rate of formation of propylene. Initial pressure of ethylene 25 Tom. Temperature 0: 774 K; *: 824 K; A: 853 K; A: 925 K. ratios of acetylenelbutadiene. These are practically independent of methane concentration, as predicted, except for some increase at the highest ratio of methane to ethylene. They should, however, be inversely proportional to the concentration of ethylene whereas little change in the ratio was observed with a change in the initial concentration of ethylene. A possible loss process for butadiene is the Diels-Alder addition to ethylene. This reaction has been shown to be a major source of cyclic olefins in thermal reactions of ethylene and other light olefins (10) and its occurrence in the present system would act to increase the ratio C2H2/C4H6 at high pressures of ethylene. An additional source of acetylene could be the alternative decomposition of butenyl radicals, and the order of the rate of this reaction with respect to ethylene is the same as that for butadiene. Such additional reactions nevertheless represent relatively minor adjustments to the mechanism and it may be concluded that the simple scheme provides a reasonable description of the system. B. Later stages of the reaction An important question for this system is the efficiency of the conversion of methane into higher molecular weight products. What fraction of the products arise through reaction with methane and what fraction through reaction with ethylene? A series of experiments was performed at each of the temperatures 974 and 1023 K, using a low pressure of ethylene, 6.5 Torr, and a high pressure of methane, 356Torr, and allowing the reaction to proceed until the conversion was sufficient to allow quantitative measurement of the amount of ethylene reacted (up to about 15% conversion). At the same time the yields of all products up to C7 were measured. The relation between the yields of products and the conversion of the reactants is given as follows: At both temperatures propylene was the major product in the initial stages but with increasing conversion passed through a maximum yield and at 1023 K was surpassed by the secondary product, benzene. The results are shown in Figs. 6 and 7. Time /s FIG. 6. Yield of products as a function of time at 974 K. Initial pressure of ethylene, 6.5 Tom; initial pressure of methane 356 Torr. 0: C2H6; *: C2H2; A: C6H.5; A: C3H6. Time / s FIG. 7. Yield of products as a function of time at 1023 K. Initial pressure of ethylene 6.5 Tom; initial pressure of methane 356 Tom. 0: C2H6; e: C2H2; A: C6H6; A: C3H6. Butadiene is not shown on these figures because at 1023 K its yield closely matches that of acetylene; at 974 K its yield is slightly less than twice the yield of acetylene. At 1023 K the total quantity of product, expressed as CC/2, is compared to the loss of ethylene over the course of the reaction in Fig. 8. At low conversions the yield of products exceeds the ethylene reacted and the difference represents the product derived from methane

6 42 CAN. 1. CHEM. VOL. 69, 1991 TABLE 3. Relation between methane converted and ethylene reacted, o = 6.5 Tom; P& = 356 TOIT Time of T reaction AC2H4 CC ACH4 ACHl - (K) (s) (mol L-') (mol L-I) (mol L-') AC2H4 TABLE 4. Maximum rate of conversion of methane at each temperature Time /s FIG. 8. Relation between ethylene consumed AC2H4, and total product, CC/2, at 1023 K. Initial pressure of ethylene 6.5 Torr; initial pressure of methane 356 Torr. 0: AC2H4; *: CC/2. according to eq. [23]. At higher conversions the yield of products analyzed (up to C7) falls below the ethylene consumed and higher molecular weight products, not analyzed, including carbon, are important in this region. From Fig. 8 the maximum conversion of methane into products occurs at 180 s. At 974 K the maximum occurred at 480 s. Table 3 summarizes the relation between the ethylene reacted and the methane converted for both temperatures. At the maximum, at 974 K, 0.8 mol of methane react for every mole of ethylene, while at 1023 K the ratio is 1.5. This appears to indicate an efficient use of methane, but in the experiments the ratio of methanelethylene is high, making the percentage conversion of methane low. An important characteristic of the reaction is the rate of conversion of methane. Because of the small percentage conversion of methane in all the exueriments a direct measurement of the rate of conversion was not possible. In the experiments in the lower temperature region the rate of conversion was estimated from the increase in the rate of formation of ethane in the presence of methane. In the higher temperature region the main product increased by the presence of methane is propylene. The rate of loss of methane in these experiments may therefore be estimated from the rate of formation of propylene in the presence, R 2, and in the absence, R O, of methane, as follows: The maximum rate of conversion of methane at each temperature, usually obtained at the highest pressure of ethylene, is summarized in Table 4. Although these rates are below those achieved in catalytic reactions (typically in the neighbourhood of 5% s-i), an important feature of the reaction is that the products are entirely hydrocarbon and no methane is lost to the formation of carbon monoxide or carbon dioxide. The percentage conversion of Pressure of Pressure of Rate of T ethylene methane conversion (K) (Tom) (Tom) (% S-I) methane in this system is of necessity low because high concentrations of methane are required to prevent formation of polyaromatic hydrocarbons and carbon. For recovery of the light hydrocarbon products a recycling system would be essential. Conclusions As a sensitizer for the conversion of methane, ethylene has certain advantages. The selectivity of the products for ethane or propylene may be controlled through the temperature and the ratio of methane to ethylene. If the conversion is kept low the formation of polyaromatic compounds and carbon can be avoided even at temperatures of 1000 K. The rate of conversion may be increased and controlled through use of additional sensitizers. Acknowledgments The authors thank the Department of Energy, Mines and Resources, Canada, for the award of a contract in support of this project. They also thank Dr. J. Z. Galuszka, ERL, CANMET, for his continued interest in the project. 1. S. AHMED and J. B. MOFFAT. J. Catal. 121,408 (1990). 2. J. G. MCCARTY, M. A. QUINLAN, and H. WISE. Proc. 9th Int. Congres. Catal. 4, 1818 (1988). 3. J. S. LEE and S. T. OYAMA. Catal. Rev. Sci. Eng. 30,249 (1988). 4. C.-H. LIU, J.-X. WANG, and J. H. LUNSFORD. J. Catal. 111,302 (1988). 5. C.-J. CHEN, M. H. BACK, and R. A. BACK. Can. J. Chem. 54, 3175 (1976). 6. M. H. BACK and R. A. BACK. Pyrolysis: theory and industrial practice. Edited by L. F. Albright, B. L. Crynes, and W. H. Corcoran. Academic Press p A. R. BossARDand M. H. BACK. Can. J. Chem. 68, 1401 (1990). 8. M. H. BACK and R. MARTIN. Int. J. Chem. Kinet. 11, 757 (1979). 9. G. AYRANCI and M. H. BACK. Int. J. Chem. Kinet. 13, 897 (1981). 10. T. SAKAI. Pyrolysis: theory and industrial practice. Edited bj) L. F. Albright, B. L. Crynes, and W. H. Corcoran. Academic Press p. 89.