Challenges and Disadvantages of CMS Membranes

In support of these observations, the authors have suggested the following parallel resistance model to represent the permeability of a weakly adsorbing gas in a CMS membrane  

Jones and Koros (1994b) have studied the effect of organic exposure on air separation performance for hollow-fiber CMS membranes. Membranes were first tested using dry air feeds to determine a baseline permeance and selectivity. The membranes were then exposed to various hydrocarbons, and then the permeance and selectivity to air was measured over time. For and heavier hydrocarbons, concentrations as low as 0.1 ppm, decreased the permeability and selectivity by over 90% in less than one day. Though the use of these membranes would normally be limited to streams that do not have organics, exposure may occur in certain sitoations. It is evident from these results that exposure to organics could greatly affect the separafion performance. 

Therefore, Jones and Koros (1994b) have shown that CMS membranes are particularly susceptible to organic exposure, but propylene exposure may be a regenerative technique that would allow CMS membranes to continue to be used if they are inadvertently exposed to organic feeds. 

There are some issues that should be addressed with this study, though. The (O2/N2) selectivity of the membranes stored in laboratory air and dry air were 9 and 7, respectively. Therefore, these membranes presumably have a pore size very similar to that of oxygen and nitrogen. The (O2/N2) selectivity of the membrane stored in nitrogen was only 3.3, therefore, the intrinsic pore size of these membranes is fairly large in comparison to the samples 

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