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Influence of the sweep gas on the permeation of palladium supported membranes

pubblicazioni - Poster

Influence of the sweep gas on the permeation of palladium supported membranes

In questo lavoro è stata valutata l’influenza del gas di sweep sulla permeazione dell’H2 attraverso membrane al Pd supportate, sulla base di misure sperimentali. I risultati indicano che il flusso di H2 attraverso la membrana è più basso di quello atteso e può essere spiegato considerando la resistenza al mass transfer in un mezzo poroso in cui è presente un gas stagnante.

Palladium and palladium alloy membranes are extensively studied for their application in membrane reactors. A lot of studies are focused on the development of Pd composite membranes, where a dense Pd layer is deposited on cheaper porous supports. Composites membranes up to 1m long are currently available at R&D institutes worldwide. Research is now focusing on aspects which are linked to process development by evaluating parameters and conditions which can affect membrane performances. In this frame the characteristics of the porous support can play an important role on hydrogen separation, in particular when a sweep gas is present. The scope of this work is to evaluate the resistance of the porous support on the hydrogen permeation of Pd-composite membrane in presence of sweep gas in the support side. The phenomenon has been studied experimentally on a 20m Pd-composite membrane, prepared by electroless plating on a stainless steel tubular macroporous support. H2 permeation tests has been performed at 400C in a laboratory pilot loop. A nitrogen sweep gas has been fed in the support side at different flow rates. Pressure on the membrane side has been progressively increased, while sweep gas pressure was kept atmospheric. In absence of sweep gas, H2 flux can be described by the Sievertslaw and the n-exponent at 400C is 0.67. Support resistance, estimated with the dusty-gas model, causes an increase of the pressure at the interface between Pd-layer and support, with a consequent H2 flux reduction of less than the 10%. When the sweep gas is fed at higher pressure, H2 flux is 30% lower than expected by considering the Sievertslaw. A correspondent increase of the n-value from 0.67 to 0.9 suggests an increase in the mass transfer resistance. Such a resistance can be attributed to the presence of the stagnant gas in porous support, which negatively affects binary diffusion reducing the H2 permeation. The support resistance in presence of sweep gas has been evaluated according to the theory of gas mass transport in porous media through a stagnant gas, which considers the Knudsen diffusion flow, the viscous flow and the binary diffusion. This approach permitted to evaluate the pressure gradient across the porous support and the H2 partial pressure between Pd-layer and support. The total pressure at the interface in presence of sweep gas is always higher than the total pressure evaluated in absence of sweep gas, and also the interface H2 partial pressure is higher than the H2 partial pressure measured in the permeate side. Such an increase is due to the binary diffusion term which determines an increase of the resistance of hydrogen transport through stagnant nitrogen. Finally it can be noted that the developed methodology can be applied to evaluate the influence of any type of support of asymmetric membranes separating different type of gases. The binary diffusion coefficient, in fact depends on the gas involved in the permeation and on the support characteristics (porosity, thickness and mean pore radius). A parametric study of these parameters will be also presented.

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