AIChE 2014: CATALYSIS AND REACTION ENGINEERING DIVISION_20003 Reaction

Engineering for Combustion and Pyrolysis

Experimental and Modeling Study of 2-Butene and Isobutene

Pyrolysis

Kun Wang, Stephanie M. Villano, and Anthony M. Dean*

Chemical and Biological Engineering Dept., Colorado School of Mines, Golden, CO 80401

*corresponding author: amdean@mines.edu

Abstract:

Butene is the smallest alkene with isomeric structures (1-butene, 2-butene, and isobutene). The study of 1-C4H8 and 2-C4H8 pyrolysis kinetics allows analysis of the impact of the double bond location, while isobutene serves as a model compound for larger branched alkenes. Recent investigations on the combustion of butanol isomersi, and larger alkane fuelsii such as heptane and isooctane, have demonstrated the importance of the reactions of butene isomers.
Building on earlier work with 1-buteneiii, iv, v, the present study focuses on 2-C4H8 and i-C4H8 pyrolysis. The conversions of 2-C4H8 and i-C4H8 start at higher temperatures than 1-C4H8. This is consistent with the fact that cleavage of the bond C=C-C---C in 1-butene only requires 76 kcal/mol, while breaking the CC=CC---H in 2-C4H8 and C=C(C)C---H in iC4H8 both require 88 kcal/mol. However, 2-C4H8 is more active than the branched i-C4H8 due to the different secondary reactions, with the fuel conversion ~89% comparing with ~42% at 725ºC (pressure
0.8 atm and resident time ~5 sec). At similar fuel conversions, 2-C4H8 produces more propene,
1-butene, and 1,3-butadiene, while i-C4H8 forms more H2 and C3H4 isomers (allene and propyne). Methane and ethene are produced in comparable amounts for both fuels. Pyrolysis of i-C4H8 leads to production of more molecular weight products, with the distinction that it forms more C7 species while 2-C4H8 forms more C5 species.
A detailed kinetic mechanism has been developed that predicts the butane isomer conversions, production of both major and minor species, as well as the formation of molecular weight
growth species up to C10. The resonantly-stabilized free radicals (RSFR), 1-methyl-allyl (CC=CC. or C=CC.C) and 2-methyl-allyl (C=C(C)C.) in 2-C4H8 and i-C4H8 system, respectively, were found to play a critical role in the kinetics. High-level electronic structure calculations were used to generate high-pressure rate constants for these reactions, and the temperature- and pressure- dependent rate constants were obtained using a QRRK/MSC analysis. This approach has allowed significant new insights into the factors contributing to molecular weight growth during
pyrolysis of hydrocarbon fuels.

i S.M. Sarathy, S. Vranckx, K. Yasunaga, M. Mehl, P. OÃ?wald, W.K. Metcalfe, C.K. Westbrook, W.J. Pitz, K.

Kohse-Höinghaus, R.X. Fernandes, H.J. Curran, Combust. Flame 159 (2012) 2028-2055

ii Marco Mehl, William J. Pitz, Charles K. Westbrook, Henry J. Curran, Proceedings of the Combustion

Institute 33 (2011) 193â??200

iii Wang K., Al Shoaibi A., Villano M. S., and Dean M. A., â??Experimental & Modeling Study of Molecular Weight Growth Species Formation during Olefin Pyrolysisâ?, ACS 2013 National Spring Meeting, New Orleans, Louisiana

iv Wang K., Villano M. S. and Dean M. A., â??Experimental & Modeling Analysis of 1-Butene Pyrolysis: Characterization of Molecular Weight Growth Kineticsâ?, Western States Section of the Combustion Institute 2013 Fall Technical Meeting, Fort Collins, Colorado

v Wang K., Al Shoaibi A., Villano M. S., and Dean M. A., â??Experiment and Kinetic Modeling of Molecular

Weight Growth Kinetics during Olefins Decomposition: 1-Butene Pyrolysis� (to be submitted)