Permeation of chemical species:

Permeation of chemical species through membranes is achieved by applying a driving force across the membrane. This could be in the form of hydrostatic temperature difference, pressure difference, concentration difference, partial pressure difference or electrical potential gradient. Application of various kinds of driving forces as mentioned above across several types of membranes gives a wide classification of membrane separation procedures. Accordingly, the mechanism through that chemical species permeate depends on the type of membrane process. A permeation of chemical species across the membrane is kinetically driven through the application of chemical, mechanical, or thermal work.

The performance of the membrane is elaborates in terms of two simple parameters, viz. transmembrane flux and solute retention or solute associatively. Transmembrane flux or permeation rate is the volumetric or mass or molar flow rate of a fluid passing by unit area of membrane surface per unit time. Transmembrane flux of any species, i, is straight proportional to the driving force applied across the membrane and is inversely proportional to the effectual thickness of the membrane. If the driving force is described in words of hydrostatic pressure gradient (?P) or partial pressure difference (?p_i) or concentration difference (?C_i ), across the membrane for any species , i, then

J_i   =  P_i   ( ?P or ?p_i  or ?C_i ) / δ

where, J_i  is the transmembrane flux for species  i, P_i  is the permeability of species i, by the membrane and is the effectual membrane thickness.

The ratio of permeability of species i, to effectual membrane thickness is sometimes called permeance.

The retention or selectivity is a measure of the relative permeation rates of variant components by the membrane. It is frequent expressed as selectivity coefficient, α.

For two species A and B permeating across a membrane, the associatively coefficient for species A along with respect to B(α^A_B ) is given through as follows:

α^A_B      =  P _A / P _B

The selectivity coefficient can also be expressed as the ratio of concentration of the two species in the permeate to the concentrate stream. Accordingly,

α^A_B  = (C_A  / C_B)_p / (C_A / C_B )_c

where, C_A  and C_B   represent the concentration of species A and B, respectively and the subscripts p and c represent the permeate and the concentrate, respectively.