7.3. Gilliland Correlation for Number of Stages at Finite Reflux Ratio

A general shortcut method for determining the number of stages required for a multicomponent distillation at finite reflux ratios would be extremely useful. Unfortunately, such a method has not been developed. However, Gilliland (1940) noted that he could empirically relate the number of stages N at finite reflux ratio L/D to the minimum number of stages N_min and the minimum reflux ratio (L/D)_min. Gilliland did a series of accurate stage-by-stage calculations and found that he could correlate the function (N−N_min)/(N + 1) with the function [L/D − (L/D)_min]/(L/D + 1). This correlation as modified by Liddle (1968) is shown in Figure 7-3. The data points are the results of Gilliland’s stage-by-stage calculations and show the scatter inherent in this correlation.