Microkinetic Modeling of Oxidative Dehydrogenation of Ethane to Ethylene at High Temperatures

The large availability of low cost shale gas reserves has generated interest for the production of high value-added chemicals. Although methane is the main component in natural gas (often more than 85%), ethane is also available due to the large amount of natural gas processed. Olefin production occurs via steam cracking, a high energy and capital intensity process. The on-purpose conversion technologies are widely exploited for more efficient utilization of energy and resources. In this regard, the short-contact-time reactor is a promising concept for autothermal conversion of light paraffins into their corresponding olefins in compact reactors at high temperatures. In this work, we use detailed modeling to understand the process. Our simulations show that gas phase reactions occur within less than 1 ms at temperatures above 1173 K. This suggests that gas phase reactions are unavoidable for the process of interest. It has been experimentally shown that ethylene yields, approximately 60%, comparable to steam cracking can be obtained by partial oxidation of ethane at 1223 K with a contact time of ca. 1 ms using platinum-tin catalyst.¹ Different mechanisms from purely heterogeneous to combination of both heterogeneous and homogeneous and purely homogeneous reaction mechanisms have been proposed in the literature. In this study, we model this process by a detailed gas-phase reaction mechanism including more than 100 species and over 2500 reactions. Microkinetic simulations with and without oxygen bracket the experimental ethylene selectivity-ethane conversion profiles. This provides an indication for the role of platinum catalyst as a heat generator driving the homogeneous reactions in the gas phase and relatively lower reforming capability of platinum to quench the gas-phase reactions. Overall, the present study exemplifies the importance of gas phase chemistry at high temperature/short contact time of alkane oxidative dehydrogenation.

1. Bodke, A. S.; Olschki, D. A.; Schmidt, L. D.; Ranzi, E. High selectivities to ethylene by partial oxidation of ethane. Science 1999, 285 (5428), 712.