(350a) Chemical Looping – Oxidative Dehydrogenation of Ethane Using Surface Modified Mixed Oxide Particles: Kinetic Studies and Process Demonstration

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. When compared to steam cracking, 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 to CaMn_0.9Fe_0.1O₃ based redox catalysts, a number of other mixed oxide redox catalyst particles with different alkali metal salt promoters were investigated, showing promising performance at different operating temperature ranges. Large packed bed studies were performed for over 1000 cycles to demonstrate the long term stability and performance of a few promising “low-temperature” redox catalysts. Autothermal operation and favorable heat of reactions in both reduction and oxidation steps were demonstrated. In addition, long-term fluidized bed studies were also performed on a “high-temperature” redox catalyst, showing near 70% single-pass ethylene yield and excellent long-term stability.

Both the kinetic studies and long-term experiments validate the feasibility and attractiveness of the CL-ODH approach.