FricDiff (friction difference) is a recently introduced separation technology. The separation occurs because of differences in interspecies friction in a multicomponent mixture, We present a description of a FricDiff unit. Such a unit consists of two compartments, the feed-side and the sweep-side, with a porous screen in between. The gas mixtures at the feed-side and the sweep gas interdiffuse through the screen. The basic modeling assumption is that the binary interaction of each feed-component with a counterflowing sweep gas is dominant. The interaction between the components diffusing in the same direction is neglected. This assumption leads us to introduce the number of "binary transfer units". We show that more detailed models introduced earlier are approximated well by this approach. Also, experiments seem to exhibit the same scaling, although the constants needed to fit the experimental results deviate quite a lot from the theoretical predictions. The equations derived can be used straightforwardly for incorporating a FricDiff unit in a process design.