Water gas shift (WGS) is a thermodynamically limited reaction which has to operate at low temperatures reducing kinetics rate and increasing the amount of catalyst required to reach valuable CO conversions It has been widely demonstrated that the integration of hydrogen selective membranes is a promising way to enhance WGS reactors performance a Pd based MR operated successfully overcoming the thermodynamic constraints of a traditional reactor thanks to the removal of hydrogen from reaction environment In the first part of a MR the H(2) partial pressure starts from a minimum value since the reaction has not started As a consequence if the carner gas in the permeation zone is sent in counter current which is the most efficient configuration in the first reactor section the H(2) partial pressure in reaction zone is low while in the permeation zone is high potentially implying back permeation This means a bad utilization of the first part of the membrane area and thus a worsening of the MR performance with lower H(2) recovery and lower CO conversion with respect to the case in which the whole selective surface is properly used To avoid this problem different MR configurations were evaluated by a 1 D pseudo homogeneous model validated with WGS industrial data reported in scientific literature It was demonstrated that the permeated H(2) flow rate per membrane surface, ie the membrane flux strongly improves if selective membrane is placed only in the second part of the reactor in fact if the membrane is placed at L(m)/L(tot) = 0 5 the membrane flux is 0 2 kmol/(m(2)h) about if it is placed along all reactor tube (L(m)/L(tot) = 1) flux is 0 05 kmol/(m(2)h) The effect of the L/D reactor ratio and of the reactor wall temperature on the CO conversion were also assessed (C) 2010 Professor T Nejat Veziroglu Published by Elsevier Ltd All rights reserved