Performance Parameters in Membrane Separation of Gases

How well economically a gas separation membrane system performs is largely determined by three parameters. The first parameter is its permselectivity or selectivity toward the gases to be separated. Permselectivity affects the percentage recovery of the valuable gas in the feed. For the most part, it is a process economics issue. The second is the permeate flux or permeability which is related to productivity and determines the membrane area required. The third parameter is related to the membrane stability or service life which has a strong impact on the replacement and maintenance costs of the system. 

A frequently used indicator of how much two gases (say, gas m and gas n) in a multicomponent gaseous mixture are separated with respect to each other through a membrane is called separation factor. It is defined as 

It is a theoretical overall selectivity of a membrane and is often referenced in the literature. 

From Eqs. (7-1) and (7-2), it follows that the separation factor is purely based on the compositions of the entering and exit streams regardless of their flows. Another measure of the separation efficiency of a membrane process is the extent of separation proposed by Rony [1968]. In the context of applying this index of separation efficiency between two comfionents, it is assumed that there is no difficulty in separating the third component Thus the segregation fractions, fiy, are obtained from the molar flow rates of the permeate and retentate streams on the basis of only two components. The extent of separation is defined as the absolute value of a determinant of a binary separation matrix consisting of the segregation fractions as follows  

Besides some measures of separation efficiency such as the separation factor and extent of separation defined above, some quantity indicative of the throughput rate of a membrane system is needed to compliment the permselectivity of the membrane. It is quite common and practical in the membrane technology to use a phenomenological expression to relate the permeate flux (Ja in the unit of cm (STP)/s-cm7) of a given gas (A) through the membrane to the driving force, the transmembrane pressure difference (Ap) as follows  

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