(621b) Enhancement of Dimethylether Yield by Reverse Shift Reaction in the Direct Dimethylether Synthesis

DME has been attentioned as a clean fuel. DME is commercially produced by methanol dehydration. However, the direct DME synthesis from synthesis gas has been suggested to be economically attractive as compared with the conventional two step processes of methanol synthesis and methanol dehydration. The optimized ratio of hydrogen to carbon monoxide is about 1.0. The conventional steam reforming produces syntheis gas with H₂/CO above 3.0. Therefore, many refoming processes such as autothermal and CO₂ reforming of methane have been issues for the direct DME synthesis. On the other hand, 1 mol of CO₂ is simultaneous produced with 1 mol of DME from the direct DME synthesis, indicating that DME process can be malign in the point of CO₂ mitigation, as compared with the two step DME process. In this study, we tried to use the reverse shift reaction to mitigate CO₂ and to ajust H₂/CO. The key concept is to convert CO₂ to CO using excess hydrogen from a reformer and CO₂ from a DME reactor. Membrane process is chosen for separating hydrogen from the reformer stream, where H₂/CO ratio can be ajusted to be 1.0~2.0 for DME synthesis. The separated hydrogen is combined with CO₂ from the DME synthesis reactor, which is introduced into a reverse shift reactor to convert CO₂ into CO. The simulation result of this concept showed DME yield of above 97 % based on feed carbon and could mitigate CO₂. The reverse water shift reaction temperature should be above 600 ^oC to achive 60 % conversion of CO₂. The commercial shift reaction catalysts such as Cu/Zn/Al or Fe/Cr catalysts at the temperature above 600 ^o were rapidly deactivated at the high temperature in CO₂-H₂ stream. However ZnO based catalyst showed the high activity and stability. The ZnO based catalyst was optimized for the reverse shift reaction and pelletized with the size of 4-5 mm and tested in 3.5 L of reator. CO₂ flow rate was 3 Nm³/h and the ratio of H₂/CO₂ was about 3.0. The CO₂ conversion of 60 % was achived at 610 ^oC with the pilot scale reactor, indicating that the reverse shift reaction was ready to be commercilized. The DME bench plant (5 kg DME/day) with the reverse shift reactor was installed to prove the simulation result. The operation results of the bench plant will be discussed.