Medium/high temperature water gas shift reaction in a Pd–Ag membrane reactor: an experimental investigation

Abstract

In the hydrogen production cycle the syngas stream coming out from reformers or coal gasification plants contains a large percentage of H₂ (ca. 50%) and CO (ca 45%). In fact, it is systematically upgraded by a water gas shift reaction (WGS) in order to convert the CO present and, at the same time, to produce further hydrogen. Water gas shift is an exothermic thermodynamics limited reaction and the presence of products like H₂ further depletes the CO conversion achievable. In traditional applications this upgrading stage is usually performed in two reaction stages: firstly, the syngas stream is fed to a reactor operated in a high temperature range (300–400 °C) to exploit the advantages offered by a fast kinetics; then, the outlet stream of the first reactor is fed, after cooling, in a low temperature stage, for the benefit of the thermodynamics. This final stream is, thus, fed to a separation unit to recover the hydrogen from the rest of the gaseous stream. In this work the upgrading of a syngas stream is experimentally investigated in a Pd-based membrane reactor (MR) operated in the medium/high temperature range (340–375 °C). The advantages on the MR performance offered by fast kinetics and permeation rate have been analyzed as a function of the feed pressure (up to 1100 kPa), feed molar ratio, and gas hourly space velocity (GHSV). The values of these variables used in the experiments are closer to those used in the industrial applications. The MR performance, compared with the ones of a traditional reactor (TR) operated in the same conditions, was evaluated also in terms of volume index which evidences the lower catalyst volume required by an MR for achieving the same conversion of a TR. Owing to the fast kinetics and permeation rate, the limit due to the presence of product (i.e., H₂) in the feed stream added to that imposed by the thermodynamics were successfully overcome with the MR operated in the high temperature range. A CO conversion significantly higher than the thermodynamics upper limit of a TR was achieved, also at high values of GHSV and less than 30% of the reaction volume of a TR was required for achieving a conversion equal to 90% of the traditional reactor equilibrium conversion.