Ideal gas selectivity

In this equation the permeabilities of gases are measured with the gas mixture. The selectivity calculated from the ratio of pure gas permeabilities is sometimes called the ideal gas selectivity and is often higher than the actual selectivity value. Permeability Pi can be expressed as the product of two terms. One, the diffusion coefficient Z), reflects the mobility of the individual molecules in the membrane material the other, the Henry s law sorption coefficient ki, reflects the number of molecules dissolved in the membrane material. Thus, equation (6) can also be written as... 

From single-gas experiments with permeabilities Pj (for preferentially permeating gas) and Pj, the ideal gas selectivity a - is calculated according to Equation 5, thereby, assuming that both gases i and j would not affect each other in their permeation behavior. 

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