Fundamental understanding of molecular weight growth kinetics in olefin pyrolysis

Abstract

Hydrocarbons, whether derived from petroleum or alternative sources, will most likely remain the primary fuel source for the foreseeable future. A challenge in many industry processes for hydrocarbon conversion is the formation of molecular weight growth (MWG) species that lead to deposits or soot formation. Detailed kinetic models provide the most versatile and informative way to characterize the MWG chemistry in these processes. Such models can be used to identify process conditions to minimize production of undesirable MWG species while maximizing fuel conversions. While the thermal decomposition of saturated hydrocarbons is relatively well understood, the low-temperature gas-phase pyrolysis chemistry of olefins is much less well characterized. The current literature mechanisms cannot even describe the temperature dependence for propene pyrolysis. Olefins form a large fraction of initial products from the decomposition of alkane or alcohol fuels, and their subsequent chemistry substantially impacts the final product distribution. Compared to the pyrolysis of their alkane counterparts, olefin pyrolysis, at similar temperatures or with similar fuel conversions, generate much higher concentrations of MWG species. For these reasons, an improved and more complete understanding of olefin pyrolysis will significantly advance the understanding of MWG kinetics during the conversion of hydrocarbon fuels. In this work, both experimental and theoretical approaches are developed to describe olefin pyrolysis, with a particular emphasis on the kinetic characterization of the generation of MWG products. Propene, 1-butene, 2-butene, and isobutene were pyrolyzed in a tabular flow reactor operating at ambient pressure (~0.83 atm). Temperature, residence time, and initial fuel dilution were varied to cover a large extent of conversion. Fuel conversions and product formation, including MWG species, were quantitatively characterized. These data provided a comprehensive database for the validation of the kinetic model developed in this work. The reactions and rate constants used in the model where based on literature data where available. However, the complete model contains hundreds of species and thousands of reactions; providing internally-consistent and accurate thermodynamic and kinetic data for these reactions presents a major challenge. A key development in this work was the use of the high-level electronic structure calculations to provide generalized rate rules that could then be systematically applied to a particular reaction type to facilitate the construction of comprehensive kinetic models. Another important improvement involves the systematic investigation of the reactions of resonantly-stabilized free radicals (RSFRs), including recombination, addition, dissociation, and hydrogen abstraction, which play pivotal roles in MWG kinetics. The updated model, without any attempts to adjust rate coefficients, accurately describes the pyrolysis kinetics of the olefins studied. The model also provided improved predictions for pyrolysis of C2-C6 alkanes. Analysis of the reaction pathways revealed the importance of some reactions involving RSFRs that had not been considered in previous studies.