How do you calculate the number of stages for a liquid-liquid extraction?

For immiscible solvents with constant distribution coefficient, the Kremser equation gives: N = ln[(xF-x*N)/(x1-x*1)] / ln(E), where E is the extraction factor (KS/F). Graphically, stages are stepped off between equilibrium curve and operating line on a triangular or rectangular diagram. For systems with varying K or partial miscibility, rigorous stage-by-stage calculations or process simulators are needed.