Development of An Economically Viable H2O2-based, Liquid-Phase Ethylene Oxide Technology: Reactor Engineering and Catalyst Development Studies (2012)

Ethylene Oxide (C2H4O, abbreviated as EO), a high volume chemical intermediate is used as a raw material for a variety of consumer products, such as plastic bottles, anti-freeze, sports gear, detergents and paints. In 2009, approximately 19 million metric tons of EO were produced and its demand is projected to grow at an average rate of 3-4% per year over the next decade. Currently, EO is manufactured by the silver catalyzed ethylene epoxidation process which is highly energy intensive and wasteful because much of the ethylene (feedstock) and EO (product) burns to form carbon dioxide, a greenhouse gas. Worldwide, commercial production of EO releases 3.4 million metric tonnes of CO2 each year making it the second largest emitter of CO2 among all chemical processes. Furthermore, loss of ethylene feedstock to burning represents a loss of $1.1 billion per year worldwide. In this dissertation, an alternative liquid phase ethylene epoxidation technology (henceforth referred to as CEBC EO process) has been demonstrated with both homogeneous Re-based and heterogeneous Ce- and W-based catalysts. In this process, the ethylene gas is compressed under pressure (50 bars) and dissolved in a liquid reaction medium containing the oxidant 50 wt% H2O2/H2O, promoter pyridine N-oxide and catalyst (methyl trioxorhenium or W-KIT-6 or W-KIT-5). The ensuing catalytic reaction produces EO with near complete selectivity with no CO2 detected in either the liquid or gas phases. Methanol is employed as a co-solvent to enhance the ethylene solubility in the liquid phase. At the operating conditions (P = 50 bars, T = 20-40 °C), the volumetric expansion studies reveal that the liquid reaction phase (methanol+H2O2/H2O) is expanded by up to 12% by compressed ethylene. The corresponding ethylene solubility is 22 mole %, converting ethylene from being the limiting reactant in the liquid phase at ambient pressure to an excess reactant at the higher pressures. Fundamental engineering studies (volumetric expansion, mass transfer and conversion studies) essential for achieving pressure-intensification established the optimum agitation speed for Re-catalyzed ethylene epoxidation to be 1200 rpm. Operating at conditions that enhanced the ethylene solubility and eliminated interphase mass transfer limitations maximized the EO productivity (1.61-4.97 g EO/h/g metal) on MTO catalyst, rendering it comparable to the conventional silver-catalyzed process. Further, intrinsic kinetic parameters, estimated from fixed time semi-batch reactor studies, disclosed the moderate activation energy (57±2 kJ/mol). Based on a plant-scale simulation of the CEBC EO 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 71.6 ¢/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 six months at a leaching rate of approximately 0.11 lb MTO/h (or 5 ppm Re in the reactor effluent). Comparative cradle-to-gate life cycle assessments (LCA) reveal that the overall environmental impacts on air quality, water quality and greenhouse gas emissions are similar for both processes given the uncertainties involved in such predictions. The LCA results implicate sources outside the EO production plants as the major contributors to potential environmental impacts: fossil fuel-based energy required for natural gas processing (used for producing ethylene, hydrogen and methanol) in both processes and to the significant requirements of coal-based electrical power for compressing large volumes of recycled ethylene and other gases in the conventional process. These results of the economic analysis prompted the evaluation of alternative catalysts that are inexpensive and exhibit the best performance metrics (high activity, near complete selectivity towards desired product and high stability). These evaluation studies identified tungsten and cerium based catalysts as possible alternatives. W-based catalysts formed EO with near complete selectivity and recycle studies established catalyst durability. Further, the EO productivity with these catalysts (0.3-3.2 g EO/h/g W) is of the same order of magnitude as the Re-based and Ag-based catalysts.