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Evaluation of the porous support influence on the permeation of asymmetric membranes

pubblicazioni - Presentazione

Evaluation of the porous support influence on the permeation of asymmetric membranes

In questo lavoro si descrive l’approccio adottato per la valutazione dell’influenza del supporto poroso sulla permeazione di membrane per separazione di gas. Tale metodologia, sviluppata a partire da risultati sperimentali ottenuti su membrane al Pd per separazione di idrogeno, fornisce le linee guida per minimizzare l’influenza del supporto e ottimizzare il processo di separazione.

Hydrogen and oxygen separation membranes are extensively studied for their application in the CO2 emission-free coal or natural gas power plants. Due to the lower cost and higher permeation, research is now focused on the development of asymmetric membranes, where a thin dense membrane layer is deposited on cheaper porous supports, which is needed to increase the membrane mechanical stability. Gas transport mechanism in the porous media can play an important role on the gas separation, depending on porous support characteristics, in particular when a multicomponent mixture, in the process side or in the permeation side, is present. Literature data show an increase of the permeation of supported membranes for oxygen separation, when pure oxygen is used instead of air as process gas on the support side [ ]. Same results have been obtained by testing a Pd membrane for hydrogen separation in absence of sweep gas on the support side [2]. Hydrogen permeation tests have been performed on a supported Pd membrane, at 400°C by feeding N2 as sweep gas on the support side, in the flow rate range of 0 – 100 Nl/h. The feed pressure has been increased up to 7 bar, while the permeate side has been kept at atmospheric pressure. By considering experimental data, the gas transport trough the support has been modeled. Results show that in absence of sweep gas, the support resistance, estimated by considering the combination of the viscous flux and the Knudsen diffusion, leads an increase of the hydrogen partial pressure at the interface Pd layer/support and a consequent flux reduction of less than the 10%. In presence of sweep gas the hydrogen permeation is lower than expected and the flux reduction is up to the 30% at the higher sweep gas flow rate. The support resistance, when the binary mixture is present in the support, has been evaluated according to the theory of gas mass transport in porous media through a stagnant gas [3], which combines the Knudsen diffusion flow, the viscous flow and the binary diffusion. The evaluated pressure gradient across the membrane is higher than the value estimated 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. It can be noted that the approach used in this work 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, as porosity, tortuosity, thickness and mean pore radius. Moreover the developed methodology, combined with a parametric study, provides the guide lines to minimize the support influence and to improve the membrane permeation.

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