A Systematically-Generated, Pressure-Dependent Mechanism for High-Conversion Ethane Pyrolysis. Part II: Radical Disproportionations, Missing Reaction Families, and the Consequences of Pressure-Dependence

Published

October 16, 2008

Author(s)

D M. Matheu, J M. Grenda

Abstract

Part I of this work systematically developed a pressure-dependent reaction mechanism for the high-conversion pyrolysis of ethane [1], as studied experimentally by Glasier and Pacey [2]. By combining conventional equilibrium, sensitivity, and reaction pathway analyses, that model identified a complex set of reaction pathways governing the formation and destruction of the important minor products acetylene, propylene, 1,3butadiene, and benzene. But many questions raised by that study were left unexplored. This work examines: 1. the potential importance of large sets of radical disproportionation reactions and the uncertainty of rates for these reactions; 2. the consequence of ignoring broad classes of reactions and specific reaction pathways during the generation step; 3. the effects of the plug-flow assumption used to model the experimental reactor, and 4. the importance of pressure-dependence to the minor products.We find that the predicted benzene concentration is mildly sensitive to the presence of a specific, large collection of radical disproportionation reactions. Reaction families for Diels-Alder, ene-reaction, and triplet ethylene formation are safely ignored during model construction for these conditions. Two specific reactions proposed in the literature, which rapidly convert fulvene to benzene, are found to potentially explain the underprediction of benzene by the generated model. However, the rates proposed for these reactions are either unrealistically fast, or unconfirmed, so that the question of what process explains the additional benzene production remains open. This work finds the plug-flow assumption reasonable in most respects, but the neglect of H-atom diffusion could cause the systematic underprediction of hydrogen concentration at longer residence times. Finally, accurate predictions of the minor product predictions appear to require the systematic treatment of pressure-dependence employed by our mechanism generator. Our efforts suggest that changing the pressure in the experimental reactor could substantially change the distribution of the minor products.

Citation

Journal of Physical Chemistry A

Pub Type

Journals

Keywords

automated mechanism generation, chemical kinetic modeling, ethan pyrolysis, pressure-dependence, pyrocarbon deposition, radical disproportionation

Citation

Matheu, D. and Grenda, J. (2008), A Systematically-Generated, Pressure-Dependent Mechanism for High-Conversion Ethane Pyrolysis. Part II: Radical Disproportionations, Missing Reaction Families, and the Consequences of Pressure-Dependence, Journal of Physical Chemistry A (Accessed December 13, 2024)