On the role of the coupled effect of Inhibition by CO and Concentration Polarization on Hydrogen Permeation through Pd-based Membranes

Abstract

In this work, H 2 permeation through Pd-based membranes is analyzed and discussed in presence of inhibition by CO and concentration polarization. Here, a complex hydrogen permeation model is used to evaluate the combined effect of these two phenomena by means of the following coefficients: Concentration Polarization Coefficient (CPC), Inhibition Coefficient (IC) and a novel overall Permeation Reduction Coefficient (PRC). The latter (PRC) takes into account the permeation rate decrease due to mass transfer resistance (polarization) and competitive CO adsorption (inhibition) on the active sites on membrane surface. According to their definition, when polarization and/or inhibition are relevant, CPC and/or IC tend to the unity, pushing also PRC to the same behaviour. On the contrary, when both are negligible, PRC approaches to zero. An important aspect arising from this study is that the polarization and inhibition effects on the overall permeance reduction are not simply additive, because it is found that CPC is lower in presence of inhibition. The present approach is general and provides a useful method to distinguish and quantify the negative effects of polarization and inhibition. Therefore, it can be applied also to other membrane-aided separation systems different from the here-considered one.

Water Adsorption Effect on Carbon Molecular Sieve Membranes in H2-CH4 Mixture at High Pressure

“…From the results it can be observed that the hydrogen permeance increases with temperature and that for hydrogen mixtures with CH 4 and N 2 , at the same partial pressure difference, no relevant deviation in the trend between 10% and 50% H 2 is observed, indicating that there is no mass transfer limitation affecting the permeation through the membrane. This is an advantage when compared to Pd-based membranes, where mass transfer resistances, commonly referred to as concentration polarization, limit the separation performance of the membrane [24][25][26][27][28]. Only in the case of the mixture with CO 2 a small deviation is observed at different partial pressures, which can be associated with the adsorption of CO 2 in the pores, which in turn depends on the CO 2 partial pressure in the system.…”