Concentration and residence time effects in packed bed membrane reactors

A fixed bed reactor (FBR) and a packed bed membrane reactor (PBMR) were compared with respect to their performance in the oxidative dehydrogenation of ethane over VO_x/γ-Al₂O₃ catalyst. The experiments were carried out at high space velocities and under oxygen excess conditions. In the PBMR, the oxidant air was distributed from the shell side of the membrane.

At similar overall feed configurations, the conversion of ethane was found to be higher in the PBMR. This effect was most pronounced at the highest space velocity. Mostly ethylene yield was higher in the PBMR than in the FBR. However, the yield of carbon oxides increased more. Thus, an improvement of olefin selectivity was not observed. There were even sets of experimental conditions, where the ethylene yield in the PBMR fell below the corresponding value for the FBR. In the PBMR under oxygen excess conditions, the consecutive oxidation of ethylene is more favoured than in the FBR.

Two essential reasons for the observed differences in the reactor performances are discussed. At first, there are different local reactant concentrations. Secondly, there are essential differences in the residence time behaviour of the reactants in the FBR and PBMR. In order to exemplify the latter aspect additional experiments have been carried out using a cascade of three identical PBMRs. Varying the specific oxygen flow rates over the individual membrane segment walls different dosing profiles were implemented. The results obtained in this study emphasise the general potential, but also the limits of membrane reactors compared to the FBR.