Is the Liquid-Phase H2O2-Based Ethylene Oxide Process More Economical and Greener Than the Gas-Phase O2-Based Silver-Catalyzed Process?

An ethylene epoxidation process concept that employs methyltrioxorhenium (MTO) as catalyst and H2O2 as oxidant and selectively produces EO (with no CO2 as byproduct) has been demonstrated recently by researchers at the University of Kansas Center for Environmentally Beneficial Catalysis (CEBC). On the basis of a plant-scale simulation of the CEBC process using Aspen HYSYS, preliminary economic and environmental assessments of the process are performed, both of which are benchmarked against the conventional silver-catalyzed ethylene epoxidation process. The capital costs for both processes lie within prediction uncertainty. The EO production cost for the conventional process is estimated to be 58 (cent/lb EO. The CEBC process has the potential to be competitive with the conventional process if the MTO catalyst remains active, selective and stable for at least one year at a leaching rate of approximately 0.11 lb MTO/h (or 0.7 ppm Re in the reactor effluent). While CO2 emissions as byproduct are eliminated in the CEBC process, comparative cradle-to-gate life cycle assessments (LCA) reveal that the quantitative overall environmental impacts on air quality, water quality, and greenhouse gas emissions are similar for both processes and lie within the uncertainties of such predictions. The LCA results point to sources outside the EO production plants as the major contributors to potential environmental impacts: natural gas-based energy required for raw material production (ethylene in both processes and hydrogen peroxide in the CEBC process) and to the significant requirements of coal-based electrical power for compressing large volumes of recycled ethylene and diluent gases in the conventional process. H2O2 production via a highly selective direct synthesis route and effective H2O2 utilization/recycle (without decomposing it) will further reduce the environmental footprint of the CEBC-EO process.