Steam cracking and catalytic dehydrogenation are the primary industrial routes to ethylene and propylene, but the high energy consumption and CO2 emissions intensity of these processes have created a need for sustainable alternatives in light olefin production. Chemical looping represents a novel process approach for generating light olefins in which thermal cracking or catalytic dehydrogenation is coupled with selective hydrogen combustion (SHC) by a metal oxide redox catalyst, which enables autothermal operation, increased per-pass conversion, and greater-than-equilibrium yields. Recent studies indicate that Na2WO4-promoted perovskite oxides are effective redox catalysts with high olefin selectivity; however, the underlying redox reaction kinetics have yet to be investigated. Here we report kinetic parameters and reaction models for the reduction of unpromoted and Na2WO4-promoted (Ca,Sr)MnO3 redox catalysts by H2, ethylene, and ethane. We show that reduction of perovskite oxides by H2 is not significantly impeded by a Na2WO4 surface layer, with both types of materials exhibiting similar apparent activation energy of reduction in H2. Reduction rates of perovskite oxides under ethylene and ethane, conversely, were suppressed by Na2WO4. Model fitting of reduction kinetics (solid conversion X vs. t and dX/dt vs. X) showed good agreement with geometric contraction and reaction order-controlled models. Kinetic modeling results were consistent with time-on-stream fixed-bed experiments in H2/C2H4/C2H6, which showed enhanced hydrogen combustion selectivity (97.9% vs. 95.1%) and similar oxygen capacity (3.2 wt.% vs. 3.3 wt.%) for Na2WO4/SrMnO3 compared to SrMnO3. The kinetic models developed for H2 and hydrocarbon combustion can be used for reactor design and optimization in the context of olefin production via selective hydrogen combustion under a cyclic redox scheme.

Kinetic modeling of Na2WO4-promoted perovskite oxides for selective hydrogen combustion in the context of olefin production

2019 Spring National Meeting

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