Catalytic conversion of dimethyl ether to lower olefins: Process and catalyst deactivation studies.

摘要

Coal or natural-based synthesis gas can be converted to dimethyl ether in a single-stage, liquid phase process. The process described here converts dimethyl ether to hydrocarbons, especially lower olefins. Thus, a novel process of producing lower olefins from coal via dimethyl ether has been introduced. The superior process advantages as well as its competitive economics quite clearly identifies this process to be quite promising when conducted on an industrial scale.; The process feasibility of dimethyl ether conversion has been evaluated and the range of products of this process have been identified. The effect of key operating parameters like temperature, partial pressure of dimethyl ether, and space velocity of dimethyl ether have been addressed and these parameters have been optimized to maximize the yield of lower olefins. The effect of the acidity (number of acidic sites and their distribution) of the zeolite catalyst on the product distribution has also been evaluated. The effect of operating times on dimethyl ether conversion, as well as on the coking contents of the zeolite catalysts is evaluated. Lastly, the economic analysis of this process is conducted to determine if this process is feasible to be conducted on an industrial scale.; This process uses dimethyl ether as the raw material, ZSM-5 type zeolite as the catalyst and a laboratory scale, once-through fixed-bed reactor as the mode of operation. Lower olefin selectivities, as high as 75-80 wt.%, have been achieved by dimethyl ether conversion over zeolite catalysts in a fixed bed reactor. These high selectivities can be accomplished by operating at higher temperatures, and low partial pressures as well as high space velocities of dimethyl ether. The lower olefin selectivity is further enhanced by utilizing a ZSM-5 zeolite with high silica to alumina ratios of 150. The zeolite catalysts investigated show remarkable thermal stability at extended periods of time on stream. Dimethyl ether conversion was maintained at 100%, and no appreciable changes in the product distribution were noticed when operating for as much as 60 hours on stream. The coking loadings on partially deactivated catalysts was evaluated and the change in the structure of the deactivated catalyst was examined by infra-red and x-ray diffraction techniques. The economic analysis of this dimethyl ether process indicate that olefins like ethylene, propylene, and butenes can be produced at prices that are comparable or cheaper than those of existing processes.