Note on the power consumption of a separating element

The large power consumption required for separating isotope mixtures by irreversible processes has been pointed out by Onsager (1939) and London (1961). General calculations, valid for both steady and unsteady flows of isotopic mixtures, are given here. Using non-equilibrium thermodynamics, an upper bound is derived for the entropy efficiency. Then the power consumption is expressed in term of the rate of change of a certain function and it is found that this latter is the product of the value function and the molar concentration. Finally numerical values are obtained for two simple cases.