Preparation of highly selective polyamide membranes for gas separation applications.

Abstract

The use of reverse osmosis (RO) and ultrafiltration (UF) membranes for separation of gas mixtures is a relatively new development. Most of asymmetric RO and UF membranes which are useful in liquid separation fail to be effective in gas separation because some of the pores on the membrane surface are too large and have no separation capacity. The selectivity of these membranes can be improved by blocking the large pores with a second material. This process is known as coating and has proven to be highly effective in upgrading RO & UF membranes for gas separation. In this study membranes were prepared from an aromatic polyamide (PA) polymer and coated by silicone rubber. In a set of experiments, the permeation rates of six experimental gases through these membranes were measured at different pressures. The ideal separation factors higher than those reported in the literature were obtained. These results show that PA membranes are highly capable of separating the gases under investigation. The resistance model was employed to analyze the data and to explain the performance of this polymeric membrane. In another set of experiments, mixtures of CH$\sb4$/CO$\sb2$ with known compositions, and O$\sb2$/N$\sb2$ (air), were separated using coated PA membranes. The actual separation factors of 64 to 166 for CO$\sb2$/CH$\sb4$ and 8.7 for O$\sb2$/N$\sb2$ systems obtained in this set of experiments were higher than those reported in the literature. A "membrane-self-sealing" gas separation cell design allowed for a leak-free system with a leak rate of ${}1.2\times10\sp{-9}$ cm$\sp3$/s. This level of leak is few orders of magnitude lower than similar existing systems. This cell design facilitates experiments involving the separation of air in which a perfect sealing is necessary in order to prevent the oxygen enriched permeate to mix with the leaking air. The new separation cell was installed in a fully automated system which operates gas permeation experiments using a process control computer program. The system is designed such that measurement of permeation rates as low as $1\times10\sp{-8}$ cm$\sp3$/s becomes possible. The new system is ideal for experiments involving homogeneous membrane and/or low permeable gases such as nitrogen and methane.