Effects of Concentration Polarization on Hydrogen Flux through Thin Pd80-Ag20 Ceramic Supported Membranes

The behavior of a palladium-based membrane module for hydrogen separation has been studied experimentally and mathematically modeled. These metallic membranes may have rather high hydrogen permeation rates and extremely high selectivity, and both aspects can lead to an accumulation of the non permeable gases and a depletion of hydrogen in the boundary layer close to the membrane [1-3]. In this work several tests have been performed using Pd based membranes formed by a 2.5µm Pd-Ag layer (about 20% wt Ag) deposited onto a porous ceramic support. The concentration polarization phenomenon was studied by measuring hydrogen permeance at different feed compositions (using both binary and ternary mixtures containing hydrogen, nitrogen and methane), feed flow rates, pressures (with total trans-membrane pressure differences between 0.5 and 6 bar), and temperatures (between 673 K and 773 K). The results obtained with mixture feeds have then been compared with pure hydrogen data. The membranes used exhibit excellent behaviour, maintaining high fluxes and a virtually infinite selectivity throughout the relatively long testing period which lasted over 3000 hours overall. The pure gas permeation behaviour is accurately described by Sieverts' law confirming that diffusion in metallic coating is controlling the process kinetics. On the contrary, the use gas mixtures in the feed definitely reduces H2 permeance, and also leads to hydrogen fluxes with significant deviations from Sieverts’ law. With gas mixtures in the feed, the permeate flux definitely increases by increasing both feed flowrate and feed hydrogen concentration. Such experimental findings document the effects of concentration polarization and the role of the mass transport resistance in the gas phase. The latter has been accounted for by a suitable mathematical model; the use of the gas phase mass transport coefficient allowed to obtain a very good representation of the experimental results and led to Sherwood numbers which follow a boundary layer type relationship as a function of Reynolds and Schmidt numbers. A new rigorous procedure, based on the sensitivity of the hydrogen flux on membrane permeance and gas phase mass transport coefficient, is also presented to properly compare the two resistances endowed with different physical dimensions. The analysis leads to the introduction of a new dimensionless number (concentration polarization number for hydrogen), which enables us to establish the relative role of membrane and gas phase resistances; its effective value contains only process variables and membrane permeance to hydrogen, and offers relevant information without requiring model simulations. This analysis shows that the experimental data collected span the range of operating conditions from the case in which the leading resistance for hydrogen flux is in the membrane to the case in which the controlling resistance is in the gas phase.