Ethylene is commercially produced from light alkanes, like ethane and naphtha, via high-temperature pyrolysis in the presence of diluting steam. The process requires a high temperature (>1100 K) and a large steam generation load, making it highly energy intensive (17-30 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, the feedstock is partially oxidized into ethylene and water. The oxidation of hydrogen makes the ODH reaction exothermic, thereby reducing fuel needs while overcoming the equilibrium limitations for cracking reactions. However, to allow economical product separation, conventional ODH schemes require the use of pure oxygen. 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, a family of mixed oxides which can function as excellent redox catalysts for ODH. They are capable of supplying lattice oxygen at rates comparable to the rate of hydrogen formation via thermal cracking of the alkane feed. The bulk and near-surface properties of the oxide system can be improved by modifying the redox catalyst to 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 providing the heat required for the endothermic dehydrogenation reaction. CL-ODH of these light alkanes 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 the overall energy demand and CO2 emissions, with over 50% savings, for the naphtha feed. The exothermic nature of the regenerator and elimination of the steam requirement lead to major reductions in the upstream energy consumption.