Slippery Slopes: How Should We Calculate the Equilibrium Slope in Multicomponent Distillation?

The calculation of tray efficiencies the HETP of packing requires, among other things, the slope of the equilibrium line. For binary systems the slope is unambiguously defined (although not always rigorously calculated). However, there is no unique definition of the slope for multicomponent systems because the vapor liquid equilibria is properly represented by a surface (in hyperspace if there are more than 3 compounds).

Thus, for multicomponent systems any pseudo-binary method for estimating tray efficiencies and HETP must rely, in part, on an approximate method of estimating the slope. But which approximation is best? And what does “best” mean when no rigorous definition is even possible?

Thus, our first purpose here is to demonstrate that different approximations can lead to dramatically different estimates of the equilibrium slope.

The second goal of this presentation is to investigate the extent to which the choice of slope estimation method matters in the design of distillation processes.