Reaction of primary and secondary products in a membrane reactor: Studies of ethanol steam reforming with a silica-alumina composite membrane

Abstract

The steam reforming of ethanol over Na-Co/ZnO catalysts was investigated in a conventional packed-bed reactor (PBR) and in a membrane reactor (MR) fitted with silica-alumina composite membranes. Promotion with a moderate amount of Na (0.2-1.0 wt%) produced catalysts with stable ethanol conversion and product selectivity. At 523K, the main products were H-2 (51%) and CH3CHO (42%) with minor amounts of CH4 (0.7%) while above 573 K the main products were H-2 (70%) and CO2 (15%) with a small amount of CH4 (2.4%). Higher cobalt loading, higher W:E ratio, higher reaction temperature and lower space velocity enhanced the conversion of ethanol to H-2 and CO2 while reducing the formation of undesirable acetaldehyde. Contact time studies indicated that acetaldehyde was a primary product of ethanol reforming, while CO2 and CH4 were secondary products. The composite membranes were prepared by a chemical vapor deposition (CVD) method and had moderate H-2 permeances ((5.2-6.8) x 10(-8) mol m(-2) s(-1) Pa-1 at 623 K). The MR produced positive yield enhancements of the secondary products, including hydrogen, but yield reductions for the primary product, acetaldehyde, and this is explained from the effect of the membrane separation on the reaction network. Two membranes with similar permeances but different H-2 selectivity ratios (H-2/CH4 = 60 and 350) were compared and it was found that the membrane with a higher H-2/CH4 selectivity ratio gave a higher yield enhancement. A survey of membrane reactor studies indicated that the conversion enhancement in reforming reactions could be described by a parameter denoted the operability level coefficient (OLC) which is related to 1/DaPe.