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

The conversion of methanol into olefins has been carried out in a gradientl ess quartz jet-loop reactor, thus ensuring the absence of mass and heat tra nsfer effects and the absence of wall reactions. The catalysts used were th ree samples of H-ZSM-5, each with a Si:Al ratio of about 100 and with a dif ferent crystal size. A thermodynamic analysis of the methanol (MeOH)-dimeth ylether (DME)-[CH2](n) system showed that in the jet loop reactor the MeOH- DME reaction was far removed from equilibrium, and this increased with incr easing temperature. No fundamental difference was observed in the selectivi ties of the hydrocarbon fraction at the same conversion of oxygenates betwe en the jet loop reactor and published data using fluidized bed and fixed be d reactors. Go-fed water reduced the conversion, probably by reducing the n umber of available sites due to preferential adsorption. In the jet loop re actor the alkylation of olefins with oxygenates appeared to occur to a less er extent than that which is usually observed in a fixed bed reactor. Moreo ver, significantly different behaviour was observed in the case of each cry stal size, with the least amount of DME forming when the largest crystals w ere used. It was possible to explain these differences in terms of the diff usional resistance experienced by DME inside the crystals. Pseudo rate cons tants were derived using a mechanistic model typical of the MTO reaction. M ethanol and DME were both involved to a similar extent in the alkylation of the [CH2](n) species. The model predicted that the first C-C bond formatio n was the slow step in the reaction sequence. Alkylation was faster than th e reversible MeOH to DME reaction. (C) 1999 Elsevier Science B.V. All right s reserved.