(92c) Greener Ethylene Production Via Chemical Looping Oxidative Dehydrogenation

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 (up to 1100 °C) and a large steam generation load, making it highly energy intensive (17-21 GJ/tonne ethylene). The process also emits significant amount of NO_x and CO₂ 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 the 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. 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. The redox catalyst, which acts as an oxygen carrier, facilitates air separation with minimal parasitic energy loss. It also avoids direct mixing between ethylene and oxygen, rendering a safer process than conventional ODH.

We have identified Mg₆MnO_8­, a mixed oxide with a cation deficient rocksalt structure, to be an excellent model redox catalyst for ODH. It is capable of supplying lattice oxygen at rates comparable to the rate of hydrogen formation via thermal cracking of ethane. 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 ethylene. The facile combustion of hydrogen favored by the promoted redox catalyst leads to high ethylene yield while providing the heat required for the endothermic dehydrogenation reactions. 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 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. Preliminary results from CHEMKIN-PRO^® are in close agreement with literature values in terms of product compositions for ethane cracking. Incorporation of surface kinetics of the redox catalyst towards hydrogen combustion provides further insights of the dependence of product output on the process conditions.