Reactant distribution by inert membrane enhances packed-bed reactor stability

Operational stability of the packed-bed membrane reactor (PBMR) in methanol partial oxidation reaction over industrially used Fe-Mo oxide catalyst was studied at temperatures 200-250degreesC and excess oxygen feed conditions. The reaction network is as follows: CH3OH(1)+-->(HCHO)-H-1/2O2 + H2O(2)+-->(CO)-C-1/2O2 + H2O. Oscillations in carbon monoxide production were observed and their amplitude was taken as a measure of reactor operational instability. Three reactor configurations were investigated. The conventional fixed-bed reactor (FBR), with both reactants fed directly to the catalyst bed, exhibited the largest oscillation amplitude. The PBMR with oxygen permeating through the membrane (PBMR-O) and methanol sent directly to the catalyst bed, exhibited decreased oscillation amplitudes, while the PBMR with methanol permeating the membrane (PBMR-M) and oxygen fed directly, was found to be stable in most cases. It is hypothesized that the instability in reactor operation is generated by the spatial non-uniformity in reaction conditions along the catalyst bed coupled with strong methanol adsorption. The consumption of methanol along the reactor alters the propagation rate of deviations in methanol concentration from its steady-state value. As shown by model considerations, this may result in packed-bed operational instability. Stability enhancement, obtained when using an inert membrane for distributed addition of methanol in the PBMR-M, is due to a more uniform methanol concentration profile along the catalyst bed. It is shown that distributed addition of a reactant to catalyst bed is an effective remedy from spatial nonuniformity induced instabilities. (C) 2002 Elsevier Science Ltd. All rights reserved.