Pure gas permeability coefficients and separation

Typically, laboratory experiments for gas permeability are performed on one gas at a time to simplify the interpretation of results. The separating ability of the membrane for a particular gas pair is described by the ratio of the individual gases permeability coefficients and is called the separation factor (a), where a = PJ Pb- This differs from pervaporation experiments, where it is not possible to calculate the composition of the permeant from only a knowledge of the feed composition and the rates of permeation for pure components, because the membranes usually swell in the feed liquid mixture. Therefore, the membrane s condition when it is being permeated by individual compounds can differ from its condition during permeation by a mixture. For liquid separations, mixtures are generally measured to see which components selectively permeate the membrane. The relative composition of the permeant solution is a direct measure of the separating ability of the membrane. 

Another industrial application of gas-separation membranes is the removal of carbon dioxide from natural gas. The CO2/CH4, selectivity is about 20 to 30 for polycarbonate, polysulfone, and cellulose acetate membranes at 35°C and 40 atm. A selectivity of over 60 can be obtained with Kapton , but this polymer is much less permeable than the others. Increasing the temperature raises the permeability of most polymers but generally causes a. slight decrease in selectivity. The operating temperature is chosen to be somewhat above the dew point of the residue gas. There is considerable COj absorbed in the membranes at high CO2 partial pressures, and the plasticization effect of CO2 increases the effective diffusion coefficients for all gases and makes the selectivity less than that based on pure-gas data. Methods of allowing for such nonlinear effects have been presented. ... 

Figure 6.4 Pure gas transport data at 25 °C of membranes AF1600 (O), AFl 6 350 30 fD), AF16 80 15 (A), AF16 80 30 (U), AF16 80 40 ( 0), silicalite-1 (O) as derived from literature data (see text), and predictions of the Maxwell model fora AF16/MFI30% membrane ( ) (a) Pure gas steady state permeability vs kinetic diameter of the permeating molecules (b) gas/methane separation factor (c) gas diffusion coefficients from time-lag experiments vs kinetic diameter (d) gas solubility vs the e/k Lennard-Jones parameter...

Osmotic coefficient (([)) - Defined by ( > = In aJ(MJLm-, where is the molar mass of substance A (normally the solvent), is its activity, and the are molalities of the solutes. [1] Osmotic pressure (II) - The excess pressure necessary to maintain osmotic equilibrium between a solution and the pure solvent separated by a membrane permeable only to the solvent. In an ideal dilute solution n = c RT, where is the amount-of-substance concentration of the solute, R is the molar gas constant, and T the temperature. [1,2]... 

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