Dimethyl ether to hydrocarbons over ZSM-5: Kinetic study in an external recycle reactor
DME provides advantages in the valorization of feed streams of low H2/CO ratio, which are formed from feedstocks such as biomass or coal. It is currently accepted that the conversion of oxygenates, such as DME or methanol, to hydrocarbons proceeds via the dual-cycle mechanism. After inspection of several kinetic models developed for the methanol-to-gasoline (MtG) reaction, we show in this investigation that they have limited applicability when dimethyl ether (DME) is used as feedstock. Therefore, the development of a kinetic model able to represent the DME-to-hydrocarbons (DtH) over a ZSM-5 catalyst was necessary. In this research, we have developed a new kinetic model that considers main steps of the dual cycle mechanism, including: (i) formation of aromatic intermediates (polymethylbenzenes); (ii) dealkylation of the intermediates to produce ethylene and propylene; (iii) methylation of small olefins to increase the chain size of the hydrocarbons; (iv) hydrogenation of ethylene, propylene and butenes to produce the corresponding saturated compounds; and (v) dimerization reactions between propylene and butenes to produce higher hydrocarbons. Moreover, the kinetic parameters of this new model were estimated from regression analysis using kinetic data measured under gradientless conditions in a fixed-bed external recycle reactor over a wide range of conditions, with temperatures from 325 to 375 °C, weight hourly space velocities (WHSV) from 25 to 125 h−1, a total pressure of 1 bar and undiluted DME as feed. The newly developed model allowed a good description of the experimental results, showed better performance than models available in literature for the MtG reaction, and rendered kinetic parameters that met physicochemical and statistical constraints, showing good agreement with results from DFT calculations reported by other authors.