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.</p
