Sensitivity of Distillation Column Design to Uncertainties in Vapor-Liquid Equilibrium Information

The sensitivity of distillation column design [number of stages (N) and reflux ratio (R)] to changes in the mean relative volatility (α) across the column is examined to demonstrate the effect that uncertainties in the required vapor-liquid equilibrium (VLE) information has on N and R. The results, presented in graphic form for different levels of α, provide the design engineer with a simple method of establishing the required degree of accuracy in the VLE information used for a given separation. The sensitivity increases, as expected, with decreasing values of α, becoming very pronounced for difficult separations, but it is significant even for easier ones. Thus, for α = 1.1, a 2% reduction in α leads to a 80% increase in N or an over 120% increase in R, while for α = 1.5 the corresponding increases are about 20 and 25%, respectively. The dependence of this sensitivity on design parameters, such as the ratio of the key components in the feed, feed thermal condition, etc., is also examined. The effect of the sources of uncertainty in the VLE information: the values of the interaction coefficient and, on occasion, of the critical properties and acentric factor, is then demonstrated with two examples.