Abstract
Dual-phase molten salt-ceramic membranes show high permselectivity for CO2 when molten carbonate is supported in a porous oxygen-ion and/or electron conductor. In this arrangement, the support likely contributes to permeation. Thus, if one is to understand and ultimately design membranes, it is also important to perform experiments with an inert support where permeation relies upon the molten carbonate properties alone. Here, a nominally inert material (Al2O3) was used in order to restrict permeation to molten carbonate. Model Al2O3 dual-phase membranes were fabricated using laser drilling to provide an order of magnitude difference in molten salt-gas interfacial area between feed and permeate sides. Molten carbonate thickness in the model membranes was also varied, independent of the molten salt-gas interfacial area. For all thicknesses studied, CO2 permeation rates showed a significant temperature dependence from 500 to 750 °C, suggesting an activated process was rate-limiting, likely a permeate-side molten salt-gas interfacial process, i.e. desorption of CO2. We applied these findings in asymmetric hollow-fibre supports, a geometry with inherent modularity and scalability, by developing a new carbonate infiltration method to control molten carbonate distribution within the hollow fibre. Compared to a conventionally prepared dual-phase hollow-fibre membrane with an uncontrolled distribution of carbonates, permeation rates were increased by up to 4 times when the molten salt was confined to the packed-pore network, i.e. without infiltrating the hollow-fibre micro-channels. X-ray micro-CT investigations supported the idea that the resulting increase in interfacial area for desorption of CO2 was the key structural difference contributing to increased permeation rates. For CO2 separation, where large volumes of gas must be processed, such increases in permeation rates will reduce the demand for membrane materials, although one must note the higher permeation rates achievable with oxygen-ion and/or electron conducting supports.
Highlights
Permselective separation of CO2 has been realised by dual-phase molten salt-ceramic membranes, e.g. overcoming the permeability requirements for economically-competitive post-combustion CO2 capture [1]
Through model membrane experiments, a new molten carbonate infiltration method and robust mechanical and physical characterisation, we demonstrate that improved permeation rates can be achieved by carefully controlling molten carbonate distribution within dual-phase molten salt-ceramic membranes
A total of 745 pores were drilled, providing a model membrane with a feed-side molten salt-gas interfacial area of 3.8 cm2 and a permeate-side molten salt-gas interfacial area of 0.15 cm2 (Fig. 3b)
Summary
Permselective separation of CO2 has been realised by dual-phase molten salt-ceramic membranes, e.g. overcoming the permeability requirements for economically-competitive post-combustion CO2 capture [1]. The molten salt (an alkali metal carbonate mixture) is typically supported in porous oxygen-ion and/or electron conducting solids [2,3,4,5,6,7]. In such cases, it has been proposed that co- or counter-current transport of oxygen-ions and/or electrons in the solid phase occurs concurrently with the transport of carbonate-like ions through the molten carbonate [7,8]. 33 the triple-phase boundary length (molten salt-gas-solid support) likely impacts permeation rate [7,9]. An improved understanding of permeation in molten carbonate supported in a nominally inert material could eliminate the requirement for more expensive oxygen-ion and/or electronconducting materials as supports
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