Membrane Gas Separation Principles
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ere are some present uses for membranes for gas separation, but they are limited by small separation factors, lack of corrosion resistance, and restricted operating temperatures. Gaseous diffusion separation of uranium isotopes is the most widely recognized commercial use of membranes. This process has demonstrated that gas separations with membranes can be performed on a large industrial scale. For example, at maximum operating power, the interstage flow rate at the feed point could be as high as 100 million pounds per day. Thousands of stages are needed to separate uranium isotopes. Multistage operation is required when the separation factor is not large enough to achieve the desired enrichment in a single stage. The pumping cost can be high with multistage operation, so it is economically desirable to perform a separation with a single stage. This article comments primarily on single-stage operations. Research and development activities should be directed toward achieving large separation factors that will allow single-stage operation. Separation Factors and Separation Efficiency It is common practice to refer to a separation factor when describing the ability of a membrane to separate two gases. This article uses the symbol / to denote a separation factor. A separation factor is only meaningful for a mixture containing two gases. Usually the separation factor referred to is an ideal separation factor rather than an operational separation factor. The ideal separation factor is the ratio of the specific flow rates (flow rate per unit area per unit pressure difference) extrapolated to very low pressures where there is no interaction (momentum exchange or effect on the solubility) between the two gases.
To assume an ideal separation factor, we must assure that there is no interaction between the two gases. Any dependence of specific flow on pressure can imply that there is opportunity for gas interaction. It is preferable to measure the specific flows of the individual gases at several mean pressures and extrapolate the specific flow to zero mean pressure. If there is gas interaction, the separation factor calculated from the ratio of individual specific flow measurements, even if measured at the same mean pressure, may not represent a meaningful separation factor. In addition, the ideal separation factor applies only if the ratio of the lowside pressure to the highside pressure of the membrane approaches zero. This ratio of lowside to highside pressure will be denoted by W. Even if independent ideal gas transport occurs for both gases (no gas interaction), the actual separation factor that applies to given operating conditions is always strongly influenced by the pressure ratio, W, across the membrane. If there is gas interaction, the actual separation factor may also be a function of pressure and temperature.2 Enrichment and Recovery While the ideal separation factor is the most important consideration in comparing membranes, enrichment and yield or fractional recovery of product are extremely important for determ
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