Limitations of Gas Separations Using Polymer Membranes

Furthermore, the CO2/H2 pure gas selectivity of XLPEO reached a value of 40 at - 20°C (i.e., CO2 permeability = 52 Barrers). This is due to the increase of CO2 solubihty and CO2/H2 solubility-selectivity. It indicates that solubility-selectivity can be strengthened by increasing the condensability of gas at lower temperatures, leading to high selectivity. However, considerations about economic process conditions (e.g., pressure and temperature) in practical applications must first be evaluated. 

An ideal gas separation membrane must have high permeability and high selectivity. The gas permeation properties of polymer membranes have been extensively smdied, and a wide variety of polymers have been synthesized to be more permeable and selective. Nevertheless, there are stiU strong trade-off relations of gas permeability and selectivity in polymer membranes that will not easily be broken in the fiimre. Moreover, this tradeoff behavior of polymer membranes is not yet fully comprehended theoretically. However, it is beheved that this trade-off behavior for specific gas pairs is unique and related to parameters of the gas molecules. 

Based on an enormous survey of approximately numerous references, including journals, books, and patents, Robeson (1991) proposed the trade-off relations of polymer membranes for various gas pairs (i.e., O2/N2, H2/CH4, CO2/CH4, H2/N2, He/CH4, He/N2, He/H2, He/02, and H2/O2). The transport parameters, selectivity Oab and 

The following empirical equation was proposed to explain the upper-bound lines for given gas pairs  

According to Robeson (1991), the transport parameters, Pab and X.ab. for various gas pairs correlate with the Lennard-Jones kinetic diameters of gases  

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