302543 Thin Carbon Molecular Sieve Membrane for Propylene/Propane Mixture Separation
Xiaoli Ma(1), Xiaotong Wei(2), Jay Kniep(2), Jerry Y.S. Lin(1*)
1. School for Engineering of Matter, Transport and Energy, Arizona State University, Tempe, Arizona 85287
2. Membrane Technology and Research, Inc, Newark, CA 94560
* To whom correspondence should be addressed. Email: jerry.lin@asu.edu
Abstract: Carbon molecular sieve (CMS) membrane is one promising membrane candidate for olefin/paraffin separation due to their superior molecular sieving capability and stable membrane structure. To improve mechanical strength for uses on the output of industrial crackers or on petrochemical purge streams, both of which are typically at elevated pressures, CMS membranes are preferred to be prepared in composite configurations using inorganic supports. This paper will report the synthesis and propylene/propane mixture separation properties of ultra-thin CMS membranes prepared on ceramic supports using a proprietary polymer as precursor. The ceramic supports used here 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.
In this work, the thickness of the polymer film (and CMS membrane) was reduced by using polymer precursor solution with lower concentration and dip-coating with shorter dip-coating time. No penetration into support pores was observed even for the solution with concentration as low as 0.25wt%. High-quality CMS membrane as thin as 450 nm could be prepared on mesoporous alumina supports by an economical single polymer-coating/pyrolysis step. The membrane is permeable to water and methanol, but not ethanol, indicating an average pore diameter of around 0.4nm. The membrane possesses excellent propylene/propane mixture separation performance with a propylene permeance of 8.2×10-9 mol/Pa.s.m2 and a C3H6/C3H8 mixture selectivity of 36 when separating 50/50 propylene/propane mixture gas at room temperature. The separation performance of our membrane is well above the upper-bound trade-off curve between permeance and mixture selectivity for the reported CMS membranes. Both propylene and propane permeances increase while the selectivity decreases with increasing feed pressure. This could be due to the change in solubility coefficients of propylene and propane or the flexible nature of the carbon membrane. The permeances for the two gases are strongly dependent on the operation temperature. The activation energy for the permeation of propylene and propane through the membrane is 15.0 and 23.2 kJ/mol, respectively.