Single and binary gas transport of H2, CO and CO2 through a NaA-centrifugally casted alumina composite membrane
Louw, Willem Frederik Breedt
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NaA (or Linde Type A: [Na+12 (H20)27]8 [Al12Si12O48]8 -LTA) membranes were prepared on centrifugally casted alumina supports and tested for its permeation and separation properties using single gasses and binary mixtures of H2, CO, CO2. 2 membranes were prepared: one with a double and one with a triple NaA layer. From single permeation experiments it was concluded that the triple coated NaA membrane gave better selectivities than the double coating one, but the selectivities were still in the range of Knudsen selectivities. Significant permeation of SF6 was also observed, which indicated that intracrystalline diffusion could not be neglected. From the single gas permeation experiments, using the triple coated membrane, the important transport mechanisms were determined and it was shown that the resistance in the support can be as high as 60%, depending on the applied pressure difference and temperature. The resistance in the support itself is predominant Knudsen diffusion, due to the relative small average pore radius of the support. The permeability of the NaA layer was higher than NaA membranes presented in the open literature, which were tested under similar conditions, which could also be attributed to the occurrence of intracrystalline diffusion. The selectivity of the membranes in binary mixtures of CO2/H2, CO/H2 and C02/CO were very close to unity, showing that the used triple coated NaA membrane was not suited for the separation of any of the studied gases. A reason for the bad performance of the membrane could be the relative short time used for the growth of the NaA crystals. Microscopic inspection showed that the crystal growth was not complete. and together with the hydrophilic character of the NaA zeolite this was probably the main reason for the occurrence of intracrystalline diffusion. A longer synthesis time was therefore also recommended.
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