Polymeric membranes for super critical carbon dioxide (scCO2) separations
Kosuri, Madhava Rao
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Providing an energy efficient recycle for the Teflon® synthesis process is of great interest due to environmental and economic reasons. This recycle step involves separating CO2 from a stream containing scCO2 and valuable monomer (C2F4). Membranes provide economical and environmental friendly separations compared to conventional methods (e.g. distillation, amine absorption). Therefore, I am investigating membrane materials that are well-suited for this important separation. Developing a robust membrane that can withstand the aggressive scCO2 environment (~1070 psi of CO2) is a key challenge. Supercritical CO2 swells traditional polymeric membrane materials, thereby increasing segmental mobility of the polymer chains which leads to a decrease in separation capacity. There have been no polymeric membrane materials identified in the literature which are suitable for this separation. In this work, I have identified an advanced polymer, Torlon® (a polyamide-imide), that solves this problem. After determining the appropriate material, it is important to choose a membrane morphology that is industrially desirable. The asymmetric hollow fiber membrane morphology provides the highest productivity. I have successfully produced defect-free asymmetric hollow fiber membranes using Torlon® that withstand high pressure feeds. These membranes have been shown to provide selective separations under scCO2 conditions without being plasticized. To further improve the separation performance of Torlon® membranes, mixed matrix concept was explored. Zeolite 4A, which is relatively more permeable and selective compared to Torlon®, was chosen as the sieve material. Mixed matrix membranes from Torlon® and zeolite 4A were made and their separation performance was measured. Based on these experimental measurements and Maxwell modeling, challenges in making successful mixed matrix membranes were identified and feasible solutions for these challenges are suggested.