Search

Abstract: A technique to measure spatially resolved permeances through tubular membranes with a moveable mass spectrometer probe at the membrane surface is described. This technique was used to determine spatial permeance distributions for SAPO-34 zeolite membranes, alumina membrane supports, and a Vycor glass membrane. The distribution of local permeances provides more detailed information about membrane properties than global permeances. The CO2 and CH4 permeances of SAPO-34 membranes that had high permeances and selectivities at 4.6MPa were not spatially uniform across the membrane area. Membranes prepared using Al(OH)3 as the aluminum precursor had large regions that were nearly impermeable, whereas membranes prepared with Al(i-C3H7O)3 were more uniform, and thus had higher overall permeances. Scanning electron micrographs indicated that these differences in permeance were not due to differences in membrane thickness, and spatially uniform permeance distributions for alumina supports suggest that they were not due to non-uniformities in the supports. Defect locations in the SAPO-34 layers were detected using i-butane permeation, because i-butane is too large to enter the SAPO-34 pores, and the defect concentrations were low. These spatially resolved measurements indicate the potential to increase both CO2/CH4 selectivity and CO2 permeance, and help understand the factors that affect membranes properties. [Copyright &y& Elsevier]

Abstract: Separation of CO2/CH4 mixtures through tubular SAPO-34 membranes with high permeances was strongly affected by concentration polarization at feed pressures above about 1MPa. The faster-permeating CO2 was depleted near the membrane surface because it could not diffuse fast enough from the bulk phase. Concentration polarization increased as pressure increased at constant feed rates because fluxes through the membranes increased, gas diffusivities decreased, and the velocity across the membrane decreased because gas densities increased. The CO2/CH4 selectivity, at 298K and a feed pressure of 5.8MPa, increased by about 160% and the CO2 flux increased by about 80% when Teflon spacers were inserted inside the membrane tube and the feed flow rate was increased by a factor of three. The spacers and the increased feed rate increased the gas velocity and decreased the diffusion distance from the bulk phase to the membrane surface, and thus they decreased the polarization boundary layer. The spacers did not affect separations at low pressures, where gas velocities and diffusivities were higher and fluxes were lower. [Copyright &y& Elsevier]

Abstract: SAPO-34 zeolite membrane synthesis was scaled up by preparing membranes on seven-channel monolith alumina supports. These monoliths have CO2 permeances and CO2/CH4 separations selectivities at 4.6MPa feed pressure that were similar to SAPO-34 membranes on single channel supports. They also exhibited the same pressure dependence. SAPO-34 membrane preparation was improved by adding SAPO-34 seed crystals to the synthesis gel instead of placing seeds on the support surface. Membranes prepared by seeded-gel synthesis had higher CO2 permeances, higher CO2/CH4 separation selectivities, and smaller standard deviations for these values. Using seeded-gel synthesis also decreased the number of synthesis steps, but increased the amount of seeds needed by two orders of magnitude. [Copyright &y& Elsevier]

Abstract: SAPO-34 membranes had 95–100% higher CO2 permeances and 9–17% lower CO2/CH4 separation selectivities when their structure-directing templates were removed at 673K in flowing N2 or under vacuum instead of in flowing air. The average CO2 permeance was 8.2±2×10−7 mol/(m2 sPa) during CO2/CH4 separations at 4.6MPa feed pressure for five membranes whose templates were removed under vacuum. The average CO2/CH4 separation selectivity was 55±6. Membranes heated in N2 or under vacuum were white in color, whereas those heated in air were darker. Membrane preparation was reproducible. Temperature-programmed desorption (TPD) and oxidation (TPO) of SAPO-34 crystals showed that more of the templates decomposed and were removed at lower temperature in inert gas than in air or oxygen. Over 90% of the templates were removed by 773K during TPD in helium, and ethylene was the main decomposition product. In contrast, only 70% of the templates were removed by 773K during TPO in air or oxygen, and CO2 and water were the major products When the SAPO-34 crystals were heated in air or oxygen, some of the template, before it decomposed to form ethylene, was oxidized to more stable compounds that were only removed at higher temperatures. [Copyright &y& Elsevier]

Abstract: The CO2/CH4 separation selectivity of SAPO-34 membranes increased by 20–150% after soaking them in 1–2% aqueous β-cyclodextrin (CD) solution and drying them at 473K. The percentage increase was higher at higher pressures, and the selectivities increased more for membranes with more defects. The CD decreased the CO2 fluxes at high pressures by about 10–20%, whereas CH4 fluxes decreased by 30–60%. The flux of CF4, which is too large to enter the SAPO-34 pores and thus only permeates through defects, decreased 80% for one membrane (5.7MPa feed pressure) after CD deposition; this confirmed that defects were effectively blocked. Even before CD exposure, the CF4 permeance at 2.8MPa was only 0.25% of the CO2 mixture permeance for that membrane. Permporometry also showed that CD blocked defects; the maximum defect size decreased from greater than 30nm to less than 20nm. About 91% of the overall defect area in the original membrane was from defects smaller than 8nm, whereas 99% was for defects smaller than 2nm after CD deposition. The membrane separation performance did not change during 3 days of continuous measurements, nor did it change after long term storage at 473K in air. [Copyright &y& Elsevier]