(241g) 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.