(532b) Analysis of a Cyclic Redox Process for Oxidative Dehydrogenation of Ethane

Authors: 
Haribal, V. P., North Carolina State University
Neal, L., 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 a high temperature (>1100 K) and a large steam generation load, making it highly energy intensive (17-21 GJ/tonne ethylene). The process also emits significant amount of NOx and CO2 and requires periodic shutdowns for coke burnout. Moreover, single-pass ethylene yield is limited by thermodynamic equilibrium. Oxidative dehydrogenation (ODH) of ethane, where ethane is partially oxidized to ethylene and water, represents an alternative route. The partial burning of the product hydrogen provides the required heat while shifting the equilibrium towards the product side. As a result, higher single-pass ethylene yield can potentially be achieved with reduced energy consumptions. The requirements for highly-selective catalyst, co-feed of combustible oxygen and ethane mixture, as well as cost associated with oxygen separation represents key challenges for such a process. A chemical looping based ODH (CL-ODH) scheme addresses the abovementioned challenges. In this process, the required oxygen is supplied by lattice oxygen from a redox catalyst composed of mixed metal oxides. The reduced catalyst is oxidized with air in a separate reactor, where it is regenerated and then recirculated to the ODH reactor. This allows built-in air separation with minimal parasitic energy loss and better temperature control. Overall, CL-ODH transforms the upstream of a traditional steam cracking process and has the potential to enhance ethylene production with significant energy savings.

Mn-based oxides have been shown to be effective as redox catalysts for the ODH of ethane. Selective combustion of hydrogen to water, over the redox catalyst, shifts the equilibrium towards ethylene and also provides the heat required for the endothermic dehydrogenation reaction. In this work, the ODH of ethane is modeled using ASPEN Plus® and is compared with a conventional stream cracking process. Results show that CL-ODH with 85% ethane conversion provides over 70% reduction in the overall energy demand with >75% reduction in the overall CO2 emissions. The exothermic nature of the regenerator and elimination of the steam requirement lead to major reductions in the upstream energy consumption. Combustion of hydrogen further reduces the downstream separation loads by >20%, with >25% drop in the compression work. For every tonne of ethylene produced, steam cracking requires an external fuel input equivalent to 1.5 GJTh, whereas the CL-ODH results in surplus fuel energy in excess of 6 GJTh. Increase in single-pass ethane conversion further increases the energy savings. Preliminary results from CHEMKIN-PRO® are in close agreement with literature values in terms of product compositions for ethane cracking. Addition of surface kinetics of the redox catalyst towards hydrogen combustion provides further understanding of the dependence of product output on the process conditions.