Support mass transfer resistance of Pd/ceramic composite membranes in the presence of sweep gas

Abstract

The present paper aims at reducing the general lack of fundamental understanding about the effects of sweep gas on membranes supported on porous substrates. This is necessary to design better performing membranes for hydrogen purification and CO2 capture applications. Sweep gas is a widely considered option for enhancing the separation performance of hydrogen-selective Pd membranes in e.g. pre-combustion carbon capture and H2 production via low-temperature steam reforming. However, additional mass transfer resistance arises when using sweep gas with composite membranes due to its diffusion into the porous support. In order to assess this issue quantitatively, the influence of N2 sweep gas was systematically studied over a wide permeate pressure range using a 5µm thick Pd membrane supported on an asymmetric ceramic support tube. Experiments were carried out between 300 and 500°C at a trans-membrane pressure difference of 100kPa. These measurements were complemented with mathematical modelling to elucidate the mass transfer resistance associated with the sweep gas. In general, the mass transfer resistance of the support grew substantially with permeate pressure and increasing sweep rate, contributing up to 40% to the total mass transfer resistance at the highest investigated permeate pressure (1MPa). The negative impact of higher permeate pressures on the permeating fluxes could be effectively compensated by using relatively small sweep flow rates up to 100mLmin−1. Further increase of the sweep gas amount improved the permeation flux only by moderate margins albeit the sweep gas effect was a little stronger at higher permeate pressures. For example, the H2 permeation rate improved by ca. 29% (38%) when raising the sweep flow rate from 100mLmin−1 to 500mLmin−1 at 400°C and 100kPa (900kPa) permeate pressure. The relative permeation rate improvement with sweep rate became weaker with increasing temperature. The reduction of the support mass transfer resistance appears to be a promising strategy for enhancing hydrogen permeation through Pd composite membranes in situations that require the application of sweep gas.