Experiments and modeling of transport in composite Pd and Pd/Ag coated alumina hollow fibers

The mass transport processes are analyzed for H2 permeation through Pd and Pd/Ag hollow fiber composite membranes. Experimental measurements of pure gas and two-component feeds quantify the extent of retentate-side transport limitations (concentration polarization) in reducing the hydrogen flux. The effects of membrane thickness, feed composition and flow rate, temperature, and total pressure on the extent of concentration polarization are measured. The data reveal that concentration polarization increases with increasing temperature and total pressure and decreasing hydrogen feed concentration. A hierarchy of transport models of varying complexity is presented. The complex 2D models account for gas-phase axial and retentate-side radial concentration gradients, and both selective transport of hydrogen through the Pd membrane and non-selective transport through membrane defects. Satisfactory agreement between model predictions and experimental data is obtained using experimentally measured parameters. An analysis of the characteristic times of the pertinent transport processes identifies the rate limiting regimes and helps to determine the conditions when gas-phase transport limits the overall productivity. The findings underscore the utility of small diameter fiber supports in providing a high surface to volume ratio and reduced concentration polarization.