(555c) Molecular Weight Growth Kinetics of Olefin Pyrolysis Under Low to Intermediate Temperatures

In many conversion processes, such as solid oxide fuel cell operation or hot gas mixing upstream of steam reformers, hydrocarbon fuels might be exposed for sufficiently long times at temperatures between 500^oC and 1000^oC to allow gas-phase reactions to occur. This chemistry not only leads to a change in gas phase composition that enters the process, but it might also produce molecular weight growth species (MWG) large enough to precipitate from the gas phase on the surface to form deposits. Deposit formation can reduce the efficiency of the technical application. For example it may deactivate the reforming catalyst or fouling in the solid oxide fuel cell, or in severe cases lead to a catastrophic failure of the unit. Other examples include formation of gas-phase deposit precursors within steam crackers. These limit the time between decoking cycles. MWG species contained in exhaust gas emissions from IC engines also constitute an environmental and adverse health problem.

There have been amount of work focusing on understanding the reaction kinetics under high temperature environment, e.g., combustion. However, few studies focused on low to intermediate temperatures, in particularly the MWG kinetics for unsaturated hydrocarbons, such as alkene, is poor. In this work, propene and the three butene isomers are selected as the model compounds. Pyrolysis experiment were performed in a tabular flow reactor at pressure ~0.82atm in temperatures of ~500-800°C, with nominal residence time ~2.4s. These fuels showed substantially different reactivities, as well as product distributions. The pyrolysis data showed that the three linear alkenes (e.g., propene, 1- and 2-butene) generated similar amount of MWG species under the same fuel conversions, while the branched isobutene tended to form as much as twice amount. A detailed kinetic model was developed to understand the impact of fuel structures on the fuel conversion, product formation and especially MWG kinetics during olefin conversion. The knowledge obtained from this combined experimental and theoretical work could be applied to real-world practices including reduce deposit formation in steam cracker and understand ignition kinetics of motor engines.