(99f) High-Pressure Co2/Ch4 Separation Using Sapo-34 Membranes

SAPO-34 membranes were prepared by in-situ crystallization onto tubular stainless steel supports (0.8-µm pores). The SEM image of the surface of membrane M2 shows intergrown zeolite crystals that are rectangular with the approximate dimension of (50-100) × (100-200) nm. The SEM cross section of the membrane indicated that the continuous zeolite layer was approximately 5-µm thick. The n-C4H10 permeance at 473 K for membrane M1 was below the detection limit of 1 × 10-12 mol/(m2?s?Pa). Both CO2 and CH4 single-gas fluxes through membrane M1 increased with pressure drop at all temperatures used. The highest CO2 flux measured was 45 kg/(m2h), which is more than an order of magnitude higher than most pervaporation fluxes through zeolite membranes. Carbon dioxide permeates faster than CH4 because the smaller CO2 diffuses faster and because it has higher adsorption coverages than CH4.The CO2 fluxes decreased as the temperature increased except at high pressures for the temperature increase from 297 to 373 K. The CH4 fluxes were almost independent of temperature. The membranes were used for separating CO2 from CH4 at feed pressures up to 3.1 MPa. The highest CO2 permeance was 2.4 × 10-7 mol/(m2s?Pa) for a 50/50 feed mixture at a pressure drop of 0.14 MPa. For a pressure drop of 3 MPa, the CO2/CH4 separation selectivities at 253 K were 140 ? 150; at lower pressure drops, the highest selectivity was 270. The highest CO2 flux was 21 kg/(m2h) at 295 K and a pressure drop of 3 MPa. Both CO2 and CH4 fluxes in an equimolar CO2/CH4 mixture increased with feed pressure at 295 K when the permeate pressure was kept constant at 84 kPa. The separation selectivity increased slightly as feed pressure increased because the permeance decreased proportionally more for CH4 than CO2. The separation selectivity was as much as a factor of 3 higher than the ideal selectivity, with the largest difference seen at high pressure drops. The CH4 coverage is lower in the mixture, relative to the single gas, because of competitive adsorption; CO2 has a higher heat of adsorption (24 kJ/mol) than CH4 (16 kJ/mol) in the SAPO pores. The M-S model was used to predict how CO2 and CH4 fluxes changed with pressure. The adsorption equilibrium constants measured for SAPO crystals were used. The CO2 and CH4 single-gas fluxes at low pressure were used to estimate the CO2 and CH4 diffusivities. The model indicates that the CO2 flux at the same CO2 pressure is lower in the mixture than for the single gas because the slower-diffusing CH4 slows the faster diffusing CO2. The model also indicates that the CH4 flux is lower in the mixture than for the single gas because CO2 inhibits CH4 adsorption, and this more than compensates for the increased CH4 diffusivity in the presence of CO2. The CO2 mixture fluxes from the model are in reasonable agreement with the measurements. The CH4 mixture fluxes from the model, however, are not in agreement with the measurements, because a significant fraction of the CH4 flux is through non-zeolite pores. When the feed pressure was kept constant at 3.1 MPa for an equimolar CO2/CH4 mixture and the permeate pressure was increased, both CO2 and CH4 fluxes decreased as the driving force for permeation decreased. Because the permeate was enriched in CO2, the partial pressure difference (and thus the driving force) across the membrane decreased more for CO2 than CH4 as the permeate pressure increased. Thus, the CO2 flux decreased proportionally more than the CH4 flux. Therefore, the CO2 permeate concentration decreased as the permeate pressure increased. The measured CO2 flux dependence on permeate pressure was in close agreement with that predicted from the M-S model. When the CO2 feed concentration was increased at 253 K, for a pressure drop of 3.0 MPa across membrane M1, the CO2 flux increased and the CH4 flux decreased. Thus, the CO2 permeate concentration increased. Even for a 25% CO2 feed, the CO2 permeate concentration was 94.6%. As the CO2 feed concentration increased from 25 to 70%, the CO2 adsorption coverage on the feed side increased from 65% to 92% of saturation, based on the mixture isotherm. The CO2/CH4 selectivity exhibited a maximum of 180 at a CO2 feed concentration of 50%.