It is important to clearly distinguish the terms membrane permeance and permeability. The permeance represents the mass transfer performance of a porous membrane, while on the other hand, the permeability is an intrinsic characteristic of the dense material that composes the membrane. Thus, two PC membranes with different pore densities will have the same permeability for component i, but different permeances. The selectivity of porous membranes is quantified by the Retention (R), a parameter related to the concentration of component i in the feed and permeate as given by eqn. ( ):