Pd-based membrane reactors for one-stage process of water gas shift

Abstract

The water gas shift (WGS) reaction is the upgrading stage in the cycles of hydrogen production by, for example, steam reforming of light hydrocarbons from fossil or renewable sources. This is a thermodynamics limited reaction and CO conversion is furthermore depleted by the presence of products, such as hydrogen as often/always happens in industrial applications. WGS industrial processes consist of two reactors in series: the first one operates at high temperature (300–400 °C), exploiting the advantages offered by a fast kinetics; the second one works in the low temperature range (200–300 °C) to the benefit of the higher thermodynamic conversion. This work proposes the use of one Pd-based membrane reactor (MR) operating at the same temperature range as the high temperature WGS reactor as a suitable alternative to the whole traditional reactor (TR) process. The hydrogen permeation allows the increase of the equilibrium conversion close to the total value and thus to operate in the higher temperature range exploiting the greater kinetics offered by Fe–Cr based catalysts. The values of gas hourly space velocity (GHSV), temperature, H₂O/CO feed molar ratio, feed composition, etc. used in the simulations are those typical of an industrial application of a WGS upgrading stage. A reference value of 15 bar of feed pressure was assumed since this is the strength limit of the self supported Pd–Ag membrane considered in the simulations. However, a feed pressure of 30 bar was also considered as that used in industrial processes. The pressure proves to be one of the most interesting variables of MR processing. The proposed analysis demonstrates how only one stage based on a Pd-alloy MR can replace the two reactors of the traditional process, which also gives better performance for, e.g., CO conversion, pure hydrogen recovered on the permeate stream, etc. An intensified process with a smaller reaction volume, higher conversion and GHSV, etc. is the outcome.