A study on mathematical modeling of direct synthesis of dimethyl ether in structured membrane reactors

Abstract

Catalytic transformation of CO2 into dimethyl ether (DME) is modeled in a membrane microchannel reactor. The reactor geometry includes an α-Al2O3 supported SOD membrane layer, capable of in-situ H2O removal, residing between a reaction channel and a parallel permeate channel. Reaction channels are washcoated with a physical mixture of Cu–ZnO/Al2O3 (CZA) and HZSM–5 catalysts, which are responsible for the synthesis and dehydration of methanol respectively. CO2-H2 reactant mixture (H2/CO2=3) is dosed to the reaction channel at space velocity=6x103 ml gcat-1 h-1 whereas, pure H2 at identical temperature and pressure, is fed to the permeate channel to sweep permeated steam. A steady-state isothermal reactor model based on the mass and momentum conservation equations is solved numerically using ANSYS software. Computed performance metrics show minimal deviation from the reported experimental data at 483–543 K, 30–50 bar. Incorporation of energy conservation to the model results in near-constant temperature profile and almost identical reactor performance, which validates isothermal operation. Owing to steam effux from and H2 influx to the reactive stream, membrane integration increases CO2 conversion (~60%) and DME yield (~30%) by a factor of >3 compared to membraneless operation. The benefits become more significant at higher temperature and pressures. Sending sweep H2 counter-currently offers superior H2O removal. Relative inlet velocity of the permeate inlet (vrat) affects membrane mass transfer dramatically. Adopting higher CZA/HZSM-5 mass ratio improves both performance metrics and the reactor capacity. With a ~7 m3 reactor system, 2.76x102 tons of DME can be synthesized from 1x103 tons of CO2 annually, provided that 1MW electrolyzer provides the required H2.