Wesselingh and Krishna (1990, 2000) note that difference equations instead of differential equations are much easier to solve and, in most cases, result in answers that are of acceptable accuracy. For example, the difference equation equivalent of Eq. (15-56b) is
Rearranging the equation, this becomes
The bars over terms mean they should be evaluated at the average conditions. If this is applied to mass transfer across a film, Δz = δ where δ is the film thickness, we can define the average ...