The use of a jet loop reactor to study the effect of crystal size and the co-feeding of olefins and water on the conversion of methanol over HZSM-5

Abstract

The conversion of methanol into olefins has been carried out in a gradientless quartz jet-loop reactor, thus ensuring the absence of mass and heat transfer effects and the absence of wall reactions. The catalysts used were three samples of H-ZSM-5, each with a Si:Al ratio of about 100 and with a different crystal size. A thermodynamic analysis of the methanol (MeOH)–dimethylether (DME)–[CH 2] n system showed that in the jet loop reactor the MeOH–DME reaction was far removed from equilibrium, and this increased with increasing temperature. No fundamental difference was observed in the selectivities of the hydrocarbon fraction at the same conversion of oxygenates between the jet loop reactor and published data using fluidized bed and fixed bed reactors. Co-fed water reduced the conversion, probably by reducing the number of available sites due to preferential adsorption. In the jet loop reactor the alkylation of olefins with oxygenates appeared to occur to a lesser extent than that which is usually observed in a fixed bed reactor. Moreover, significantly different behaviour was observed in the case of each crystal size, with the least amount of DME forming when the largest crystals were used. It was possible to explain these differences in terms of the diffusional resistance experienced by DME inside the crystals. Pseudo rate constants were derived using a mechanistic model typical of the MTO reaction. Methanol and DME were both involved to a similar extent in the alkylation of the [CH 2] n species. The model predicted that the first C–C bond formation was the slow step in the reaction sequence. Alkylation was faster than the reversible MeOH to DME reaction.