In the present study, NaY zeolite membranes were synthesized by the secondary growth process and the single gas and mixed gas permeation properties were evaluated under humid conditions.
Lab-made NaY zeolite crystals having size of 80nm were uniformly seeded on the outer surface of tubular ?-alumina support by vacuum-assited filtration process. Seeded support was horizontally positioned in Al2O3-10SiO2-14Na2O-840H2O hydrothermal solution. The secondary growth process carried out at 90℃ for 24h. As-synthesized zeolite membrane was water washed and dried prior to gas permeation test.
Gas permeation tests were proceeded for single gases and equimolar binary mixtures in the feed pressurization and permeate evacuation modes. For single gases, the CO2 permeance was around 2 x 10-7 mol/m2secPa and the CO2/N2 permselectivity was about 5 in both modes. Compared with single gas permeation data, the CO2 permeance slightly decreased to be less than 1x 10-7 mol/m2secPa and the CO2/N2 separation factor drastically increased up to around 50 for equimolar binary mixtures. The single gas and binary mixture permeation data reconfirmed that CO2 permeation through NaY zeolite membrane should be governed by surface diffusion and the high CO2/N2 separation factor be from the nitrogen blocking of adsorbed CO2 molecules.
In the present study, gas permeation tests were also conducted for 14%CO2-6%O2-80%N2 ternary mixture. The CO2 permeance for the ternary mixture was around 1 x 10-7 mol/m2secPa and CO2/N2 separation factor was around 10. It was clear shown that NaY zeolite membrane showed a good CO2 separation behavior even for the ternary mixture containing 4% of CO2.
Single CO2 gas permeation test was also conducted at 40 to 90℃ under highly humid conditions. As water content increased, the CO2 permeance was initially unchanged but later decreased. The critical water contnet, at which CO2 permeance started to decrease, increased with elevating permeation temperatures. The critical water content was more than 6 and 8wt.% at 40 and 90℃, respectively. The wet single gas permeation results informed that CO2 molecules permeate rapidly through NaY zeolite membrane even in humid conditions.
Gas permeation tests were also carried out at 90 and 110℃ for 14%CO2-6%O2-80%N2 ternary mixture under humid conditions. As water content increased, CO2 permeance was 70% reduced at 90℃. On the other hand, the CO2/N2 separation factor initially increased, showed a maximum at water centent of 1.5wt.%, and then decreased. This complex dependency of the separation factor on water content could be explained by a combination of the variations of CO2 occupancy and the N2 blocking effect of adsorbed water. At lower water concentrations, a considerable fraction of CO2 adsorption sites will be occupied preferentially by water molecules, but the CO2 occupancy is still enough to show a considerable CO2 permeance. At these conditions, adsorbed water molecules can take part in the N2 blocking, so that the CO2/N2 separation factor increases with increasing the water content. But, further water adsorption on CO2 adsorption sites will decrease CO2 occupancy, which derives to lowering CO2 permeance. Also, the lowered CO2 permeance means that the membrane shows the lower separation factor. But, NaY zeolite membrane still showed a good CO2 separation behavior for 14%CO2-6%O2-80%N2 ternary mixture under high humid conditions.
From the above-mentioned results and discussion, it could be concluded that hydrophilic NaY zeolite membrane is still one of promising membranes for post-combustion carbon capture.