The Effect of Impurities on Permeance Through Chabazite Zeolite Membranes

Chisholm, Nicholas Odell

Graduate Thesis Or Dissertation

The Effect of Impurities on Permeance Through Chabazite Zeolite Membranes Public Deposited

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Citeable URL: https://scholar.colorado.edu/concern/graduate_thesis_or_dissertations/6m311p46t

Abstract

Zeolite membranes selectively permeate gases based on differences in size, diffusivity, and loading. The chabazite zeolites SAPO-34 and SSZ-13 have 0.38-nm pores, which make them suitable for separating CO₂ (0.33-nm kinetic diameter) from CH₄ (0.38 nm) for natural gas purification. Natural gas also contains ethane, propane, butane, and higher hydrocarbon impurities that can adsorb in the membranes and affect their performance.

We studied the effect of small alkanes on H₂/N₂ separations in SAPO-34 by adding ethane, propane, or n-butane to the feed. Propane reduced the permeance of both gases, but it reduced the H₂ permeance less than the N₂ permeance, so that the selectivity increased from about 4 to about 9. n-Butane had a similar effect, but ethane caused no change in selectivity. Reduced H₂ loading due to competitive adsorption would decrease H₂/N₂ selectivity if competitive adsorption were the dominant mechanism. Therefore, diffusion plays a significant role and the alkanes must have decreased N₂ diffusivity more than H₂ diffusivity.

We studied the effects of toluene, which is too large (0.59 nm) to diffuse into zeolite pores, on CO₂ and N₂ permeance through SAPO-34 membranes. When 0.65 mol% toluene was added to the feed, the CO₂ permeance decreased by 45% and the N₂ permeance decreased by 50% at 172-kPa pressure. At 10-kPa feed pressure, 8.5% toluene decreased CO₂ permeance by 20%. Binary isotherm models predict that competitive adsorption between toluene and CO₂ on the external surface significantly reduced the CO₂ coverage, and thus the driving force for CO₂ transport.

Finally, we measured the effect of C₁ through C₄ alkanes on CO₂ permeance through SSZ-13 membranes. Propane and n-butane have kinetic diameters (0.43 nm) larger than the SSZ-13 pores, but they adsorb to high loadings in SSZ-13 crystals because these molecules are flexible. Methane decreased CO₂ permeance by 5%, and ethane decreased CO₂ permeance by 45% at the same concentration. Propane, however, only decreased CO₂ permeance by 10%, and n-butane decreased the permeance by 15%. Apparently, the larger alkanes did not adsorb into the SSZ-13 pores, likely due to an altered external pore structure, and propane and n-butane only reduced CO₂ permeance by adsorbing on the external surface.

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