Ultra-Thin Carbon Molecular Sieve Membrane for Propylene/Propane Separation

Carbon molecular sieve (CMS) membrane is one of the most promising membrane candidates to separate propylene/propane gas because of their superior separation performance and good stability. To provide mechanical strength for uses under the conditions of large transmembrane pressure, and to attain high gas permeance, CMS membranes are preferred to be prepared in composite configurations using inorganic supports. This paper will report the synthesis and properties of ultra-thin carbon molecular sieve membranes prepared on ceramic supports using a proprietary polymer as the CMS precursor. The ceramic supports used in this work have a smooth layer of mesoporous gamma-alumina on the surface, allowing coating of high quality polymer film with no penetration into the support pores. Ultra-thin and defect-free polymer film was coated on the surface of gamma-alumina layer by dip-coating method. The polymer/ceramic composite membrane was pyrolyzed in an oxygen-free inert gas environment to get the final CMS/ceramic structure with strong molecular sieving effect.

To obtain CMS with high permeance and good selectivity, the thickness of the membrane needs to be as thin as possible while keeping the membrane free of any non-selective defects. In this work, the thickness of the CMS was first reduced by decreasing the dip-coating time to form polymer film on support. The optimal dip-coating time was found to be 15 seconds which will give a defect-free carbon membrane of 1.4 µm in thickness when using 2 wt% dip-coating polymer solution. To further reduce the membrane thickness, the concentration of the dip-coating polymer precursor solution was decreased from 2 wt% to 1 wt% and 0.5 wt%. As observed in the SEM and EDS, no penetration of polymer into support pores took place even when using the most diluted 0.5wt% polymer solution. With decreasing the concentration of polymer solution, the performances of the carbon membrane were improved in terms of both permeance and selectivity. The improved permeances were caused by the thinner membranes thickness when using more diluted polymer solution. The 0.5 wt% carbon membrane is as thin as 450 nm, which is about 3 times thinner than the 1.4 µm thick 2 wt% membrane. On the other hand, the polymer dispersed more uniformly in a more diluted solution, improving the quality of polymer film formed on the support by dip-coating. And the possibility of forming non-selective defects during drying and pyrolysis process was also reduced for a thin membrane, resulting in a membrane with higher selectivity. Overall, ultrathin and good-quality carbon membrane was achieved for the 450 nm thick carbon membrane, which possesses a high C(3)H(6) permeacne of 7.0×10^{^}(-9) mol/Pa.s.m(^2) and a good C(3)H(6)/C(3)H(8) perm-selectivity of 40 at room temperature. At the operation temperature of 80 degree C, the permeance of C(3)H(6) achieves as high as 1.0×10^{^}(-8) mol/Pa.s.m(^2).