Comparative Kinetic Investigation of the Prolysis of Pure Hydrocarbons and their Mixtures

No Thumbnail Available
Aribike, D. S.
Journal Title
Journal ISSN
Volume Title
University of Lagos
The kinetic and product distributions of the thermal decompositions of the n-butane, n-heptane, cyclohexane and methyl-cyclohexane as well as liquified petroleum gas (LPG), cyclohexane methylcyclohexane (CYH-MCH) and cyclohexane - heptane - benzene (CYH-HEP-BEN) synthetic mixtures were investigated in a stainless steel annular reactor at one atmosphere pressure and with excessive nitrogen dilution. Experimental data were obtained over a wide range of conversions at temperatures from 6400 to 8600C and residence times of 0.26 - 1.87 seconds. Ethylene, propylene, methane and hydrogen were formed as major products of n-butane pyrolysis. Besides, mole percent of propylene and methane did not change with temperature below 7600C, while ethylene increased. Mole percent of propylene was considerably higher than that of methane in contrast to the results of Blakemore et al (74) who observed equal molar amounts of these two products. The ratio ([C3H6] + [CH4]}/{[C2H4] + [C2H6]) decreased with increasing temperature (conversion). Methane, propylene and ethylene were the main products of LPG pyrolysis. Also methane selectivity generally increased with residence time, while those of ethylene and propylene passed through maxima. There is evidence of the accelerating effect of the H2S additive on the cracking reactions of LPG. Relatively substantial amount of methane and less ethylene and propylene were produced in the pyrolysis of LPG than pure n-butane. In n-heptane pyrolysis selectivities of ethylene and methane increased steadily with conversion, while propylene selectivity went through a broad maximum and those of the higher &-olefins (1-butene, 1-pentene and 1-hexene) decreased. R-k theory is inadequate in predicting the secondary reactions of &-olefins, though it predicts their formation. N-heptane pyrolysis was well represented by first order kinetic law; the estimated Arrhenius parameters are E = 206.1' kJ mo1-1 and A = 5.88 x 1010 sec-1. Selectivities of ethylene, propylene and methane increased with conversion in cyclohexane, pyrolysis while 1, 3-butadiene decreased. In addition, ethylene and propylene yields increased with residence time at 7000C - 8600C, while yield of 1, 3-butadiene increased at 7000C - 8600C, passed through a maximum at 6200C and decreased at 8400C. Similar observations were made by Levush et al (30) in cyclohexane pyrolysis at 9000 - 13000C. Relatively lower selectivities of ethylene and propylene and higher 1,3-butadiene selectivity were observed in cyclohexane pyrolysis than n-heptane. Ethylene, propylene, methane and 1,3-butadiene were the major products of CYH-MCH pyrolysis. Appreciable yields of benzene and isoprene were also formed. Selectivities of ethylene, methane and 1,3-butadiene showed little or no change with increasing conversion, while propylene decreased tremendously. Less methane, ethylene, 1,3-butadiene and hydrogen and more of propylene were formed in MCH pyrolysis than cyclohexane. The Arrhenius plot of MCH pyrolysis showed appreciable curvature in the region of temperature below 8000C; an evidence of strong surface effect and hence of heterogeneous mechanism on the kinetics of MCH pyrolysis. Ethylene, propylene and 1,3-butadiene were the major products of CYH-MCH pyrolysis. The order of the quantities of ethylene formed from the pyrolyses of CYH-MCH, pure cyclohexane and MCH was: Cyclohexane > CYM-MCH > MCH Conversely more propylene was formed in MCH pyrolysis than the other two reactants. The comparison of the variations of the major product yields and selectivities with temperature and residence time for the three hydrocarbon reactants showed that the decomposition reactions of the components play central role in overall mixture cracking. Comparison of the Arrthenius parameters of pure components with those estimated for mixture cracking showed that cyclohexane strongly inhibited in former slightly. Ethylene, propylene, 1,3-butadiene, methane and hydrogen were the main products of CYH-HEP-BEN pyrolysis. The order of the amounts of ethylene formed in the pyrolyses of CYH-HEP-BEN, pure cyclohexane and heptane was N-Heptane > CYH-HEP-BEN > Cyclohexane. Conversely, the order in the case of 1,3-butadiene was Cyclohexane > CYH-HEP-BEN > N-Heptane. Selectivity to products of pure component decomposition was maintained in the mixture cracking, confirming the results of Murata et al (36). There was strong effect of component interaction on the overall decomposition reactions of CYH-HEP-BEN. Futhermore, higher yields of C2+C3 olefins and lower yields of liquid products (C6+) were produced in CYH-HEP-BEn pyrolysis than CYH-MCH. Mechanistic models that fit the pyrolyses of n-butane and n-heptane fairly well were developed. Simulated radical concentrations showed appreciable changes with reaction time thus the assumption of pseudo steady stage for radical concentrations is not valid in reality. Molecular model developed for cyclohexane pyrolysis predicted the product distributions fairly well.
Temperature , Hydrocarbons , Cyclohexane , Methylcyclohexane
Aribike, D. S.(1985). Comparative Kinetic Investigation of the Pyrolysis of Pure Hydrocarbons and their Mixtures. A Thesis Submitted to the School of Post Graduate Studies of the University of Lagos. In Partial Fulfilment of the Requirements for the Degree of Philosophy in Chemical Engineering.