Abstract
Carbon molecular sieve membranes (CMSMs) are emerging as promising solution to overcome the drawbacks of Pd-based membranes for H2 separation since (i) they are relatively easy to manufacture; (ii) they have low production and raw material costs; (iii) and they can work at conditions where polymeric and palladium membranes are not stable. In this work CMSMs have been investigated in pure gas and gas mixture tests for a proper understanding of the permeation mechanism, selectivity and purity towards hydrogen. No mass transfer limitations have been observed with these membranes, which represents an important advantage compared to Pd-Ag membranes, which suffer from concentration polarization especially at high pressure and low hydrogen concentrations. H2, CH4, CO2 and N2 permeation at high pressures and different temperatures in presence of dry and humidified stream (from ambient and water vapour) have been carried out to investigate the effect of the presence of water in the feed stream. Diffusion is the main mechanism observed for hydrogen, while methane, nitrogen and especially carbon dioxide permeate through adsorption-diffusion at low temperatures and high pressures. Finally, H2 permeation from H2-CH4 mixtures in presence of water has been compared at different temperatures and pressure, which demonstrates that water adsorption is an essential parameter to improve the performance of carbon molecular sieve membranes, especially when working at high temperature. Indeed, a hydrogen purity of 98.95% from 10% H2—90% CH4 was achieved. The main aim of this work is to understand the permeation mechanisms of CMSMs in different operating conditions and find the best conditions to optimize the separation of hydrogen.
Highlights
Pd-based membranes are effective for hydrogen purification, the high cost of these membranes has greatly limited their larger scale applications [1,2,3]
The sieving mechanism exhibited by these membranes on gas separation makes them known as “carbon molecular sieve membranes”
The permeation mechanism with configurational-diffusion can be partly described, it should be noted that the activation energy depends on the activation temperature or, in other words, at the temperature the membrane was exposed to under inert conditions to desorb the water confined in the membrane pores
Summary
Pd-based membranes are effective for hydrogen purification (thanks to their high permeance and selectivity), the high cost of these membranes has greatly limited their larger scale applications [1,2,3]. Carbon molecular sieve membranes have been proven to be very effective for various applications to replace other traditional processes for the purpose of cost and energy saving. The sieving mechanism exhibited by these membranes on gas separation makes them known as “carbon molecular sieve membranes”. Since carbon layers by themselves are fragile, they are generally supported on a ceramic or metallic porous support [7], which provides increased mechanical resistance and thermal stability without decreasing the hydrogen permeance [8]
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