Chapter 14: Polarization and Inhibition by Carbon Monoxide in Palladium-based Membranes

The topic of this chapter concerns the effect of inhibition by CO and concentration polarization in Pd-based membranes used for hydrogen purification processes. In particular, the decrease of membrane performances due to a reduced permeance and driving force with respect to the pure-hydrogen case is analyzed as a function of several operating conditions. This study is carried out by using a coefficient recently introduced in the literature – the overall Permeation Reduction Coefficient (PRC) – whose definition allows the coupled effects of concentration polarization and inhibition by CO to be analyzed by identifying and separating their respective contributions. According to this approach, it is clearly shown that the decrease of effective permeance due to polarization and that due to inhibition cannot be simply added to each other for the overall permeance to be evaluated. Differently, a non-linear (quadratic) dependence is obtained, showing the notable result that the concentration polarization is reduced in presence of inhibition. Such behaviour is described by some Permeation Reduction Maps, which are graphical representations of the mixture-membrane system complexity. From these maps, useful information on membrane performance can be directly read without the need of solving numerically the complex mathematical model behind them, allowing a more precise design of Pd-membrane systems for hydrogen purification.