Generalized Geometric Interpretation of Temperature Control for Ternary Distillation

Abstract

A previous paper introduced the concept of traveling distance to quantify how much the composition profile in a distillation column has to change when certain tray temperatures are held constant under single-end temperature control. The larger the traveling distance for a given temperature-control selection, the slower the composition dynamics and, hence, the poorer the control performance are expected to be. This work generalizes the concept of traveling distance in distillation-temperature control for different levels of complexity in a ternary distillation column separating components A, B, and C, in order of decreasing volatility. For the direct separation between A and B/C with a high purity of A at the top, the analysis shows that the conflict between process and control direction becomes more severe (larger traveling distance) with (1) increasing overhead purity of A, (2) decreasing relative volatility between light and heavy key components (A and B, respectively), (3) increasing relative volatility between the heavy and heavier-than-heavy key components (B and C, respectively), and (4) decreasing feed composition of the light key component (A). The results can be inverted to the indirect separation between A/B and C with a high purity of C at the bottom, while substituting heavy for light in the above conclusions.