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

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 CaMn_0.9Fe_0.1O₃, 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, CaMn_0.9Fe_0.1O₃ 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 CaMn_0.9Fe_0.1O_3. The reduction of Na₂WO₄-promoted CaMn_0.9Fe_0.1O₃ 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 CaMn_0.9Fe_0.1O₃ 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.