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 requires periodic shutdowns for coke burnout. Moreover, single-pass ethylene yield is limited by thermodynamic equilibrium. Oxidative dehydrogenation of ethane (ODH) has been investigated as a potentially more efficient approach. In ODH, ethane is partially oxidized into ethylene and water. The oxidation of hydrogen makes the ODH reaction exothermic, thereby reducing fuel consumption while overcoming the equilibrium limitations for cracking reactions. However, to allow economical product separation, conventional ODH schemes require the use of pure oxygen co-fed with ethane. The oxygen generation, which uses cryogenic air separation systems, is capital and energy intensive. We propose a chemical-looping ODH (CL-ODH) approach that can address these issues. In this scheme, a metal oxide based redox catalyst provides oxygen for the ODH reaction from its lattice. The oxygen depleted redox catalyst is subsequently oxidized in air, regenerating the catalyst and releasing the heat needed for the process. This allows built-in air separation with minimal parasitic energy loss and better temperature control.
We have identified, in our previous work, that Mg6MnO8, a mixed oxide with a cation deficient rocksalt structure, is an excellent model redox catalyst for ODH. When promoted with alkali salts, changes in the bulk and near surface properties of the Mg/MnO system help produce ethylene with exceptional selectivity by suppressing deep oxidation of ethane or ethylene. The facile combustion of hydrogen favored by the promoted redox catalyst leads to high ethylene yield and provides the heat required for the endothermic dehydrogenation reaction. In the current study, CL-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 80% reduction in both the overall energy demand and CO2 emissions. The exothermic nature of the regenerator and elimination of the steam requirement lead to major reductions in the upstream energy consumption. Kinetic modeling using CHEMKIN-PRO® is performed to determine the effect of gas phase and surface reactions on the CL-ODH product distributions. The proposed CL-ODH process in the context of circulating fluidized bed operations is also discussed.