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(706h) Kinetic Studies on Chemical Looping – Oxidative Dehydrogenation of Ethane Using Surface Modified Mixed Oxides

Tian, Y. - Presenter, North Carolina State University
Dudek, R. B., North Carolina State University
Neal, L., North Carolina State University
Westmoreland, P. R., North Carolina State University
Li, F., North Carolina State University
Ethylene is commercially produced from ethane via high-temperature pyrolysis in the presence of diluting steam a.k.a. steam cracking. The process requires high temperature (>1100 K) and a large steam generation load, making it energy intensive with an energy consumption of 17-21 GJ/tonne ethylene. Alternatively, Chemical Looping – Oxidative Dehydrogenation (CL-ODH) has the potential to substantially reduce the energy demand and to increase the single-pass ethane conversion. The ability of a redox catalyst for selective hydrogen combustion (SHC), i.e. selectively oxidizing hydrogen co-product from ethane dehydrogenation, represents an effective strategy for CL-ODH because it can shift the reaction equilibrium and facilitate autothermal operation.

In this work, redox kinetics of unpromoted and Na2WO4-promoted CaMnO3 based redox catalysts were investigated in the context of CL-ODH. In the case of unpromoted CaMn0.9Fe0.1O3, the reduction kinetics follow reaction-order models. The activation energy for C2H4 reduction is approximately three times greater than that for H2 reduction. The greater dependence on active lattice oxygen concentration and the larger energy barrier under C2H4 reduction result in an order-of-magnitude decrease in the reduction rate. Despite of its favorable SHC properties, CaMn0.9Fe0.1O3 redox catalyst leads to significant CO2 formation in CL-ODH of ethane. To further improve the redox catalyst’s selectivity towards hydrogen combustion, a Na2WO4 promoter was impregnated on CaMn0.9Fe0.1O3. The reduction of Na2WO4-promoted CaMn0.9Fe0.1O3 follows an Avrami-Erofe’ev nucleation and nuclei growth model. The addition of Na2WO4 more significantly suppressed C2H4 combustion relative to H2 oxidation. As a result, the reduction rate of Na2WO4-promoted CaMn0.9Fe0.1O3 under H2 was three orders of magnitude greater than that under C2H4, demonstrating its excellent SHC properties.

In addition, a number of other mixed oxide redox catalyst particles with different alkali metal salt promoters were also investigated, showing promising performance in terms of long-term stability and single-pass ethylene yield at different operating temperature ranges. Both the kinetic studies and long-term experiments validate the feasibility and attractiveness of the CL-ODH approach.