Permeation and Surface Adsorption of Hydrogen On Pd-Ag Membranes In the Presence of Water Vapor

Permeation and surface adsorption of hydrogen on Pd-Ag membranes in the presence of water vapor

Jacopo Catalano, Marco Giacinti Baschetti, Giulio C. Sarti

Dipartimento di Ingegneria Chimica, Mineraria e delle Tecnologie Ambientali (DICMA)

 Alma Mater Studiorum - Università di Bologna, via Terracini 28, 40131, Bologna, Italy.

Abstract

In this work the influence of water vapour on the hydrogen flux through thin silver-palladium membranes was experimentally investigated and mathematically modelled.

The experimental tests were performed on 2.5 µm thick Pd-Ag (80-20% by weight) membranes considering pure hydrogen as well as binary mixtures containing also nitrogen or water vapour in the temperature range from 573 to 723K and at a transmembrane pressure differences up to about 3 bar. The membranes, supplied by NGK Insulator Ltd., Japan showed a very high hydrogen permeance and lifetime, as well as virtually infinite selectivity (exceeding 10000 for H₂-N₂ mixtures).

The experiments in hydrogen-nitrogen mixtures were carried out at different temperatures, hydrogen concentrations and feed flow rates in order to test the gas phase resistance to mass transport inside the experimental module usually present when highly selective and permeable membranes are considered. The typical behaviour of hydrogen flux with Sieverts' driving force is shown in Fig. 1 for the case of 623 K; Sieverts' law is not holding in case of mixtures and the flux presents a downward curvature suggesting the presence of non negligible concentration polarization phenomena. The experimental data were thus modelled considering also the gas phase mass transport coefficient that characterizes mass transport in the external gas phase. The description of the experimental behaviour was satisfactory (see solid line in Fig. 1) and a linear correlation between Sherwood and Péclet numbers was found to hold for the experimental module considered and in the range of operative conditions inspected.

Interestingly the hydrogen permeate fluxes measured with feeds containing H₂-H₂O mixtures resulted always lower than those obtained for the nitrogen-hydrogen mixtures performed at the same hydrogen mole fraction and operative conditions, as shown in Fig. 1. In particular, the hydrogen flux reduction increased with decreasing temperature and/or increasing the concentration of water vapour. All the experimental evidences suggest the presence of an interaction between water vapour and the metallic layer, leading to a lower hydrogen adsorption capacity of the membrane surface. That phenomenon is reversible, since the original permeance of the membrane was restored once the water vapour was removed from the feed, and is apparently due to a competitive H₂-H₂O adsorption on the Pd-Ag surface.

The experimental results were then analysed by using a specifically developed model that consider the competitive adsorption of different species on the metal interface validated in the case of mixtures containing carbon monoxide, for which abundant and reliable literature data are present. The model is a modification of the well-known approach proposed by Ward and Dao and takes into account for the different transport processes that the hydrogen undergoes into the gas phase as well as in the palladium based layer. All parameters entering the model equations were separately estimated using independent literature information, apart from the adsorption energy of water. In Fig. 2 a parity plot is presented showing that a rather good agreement between experimental data and calculated results was obtained with the use of a single adjustable parameter that is the adsorption enthalpy of water on the palladium membrane surface.

 

Keywords: Hydrogen permeation; palladium-silver membranes; water vapour competitive adsorption; mixed gas permeation

 

Figure 1. Experimental hydrogen permeate flux versus the average Sieverts' driving force in H₂-N₂ and H₂-H₂O mixture containing 96% and 88% vol of H₂ at 623 K. Dotted line represents pure hydrogen results while solid lines are calculated taking into account the gas phase resistance.

 

 

Figure 2. Parity plot between experimental and calculated data for H₂-H₂O mixtures considering the concentration polarization phenomenon and a competitive adsorption on the metal interface.