Potential pitfalls in ratio control schemes

Abstract

We study the behavior of various ratio control schemes implemented on distillation columns with many trays. The results show that controller structures using ratios plus direct material balance manipulation (e.g., RR-B, D-BR) can produce underdamped open-loop systems (level loops closed, composition loops open). This underdamped behavior is due to the hydraulic lags in these large columns. Because of this very unusual behavior, it is difficult to derive the transfer function matrix of such ratio control structures by using standard identification techniques. A method is presented for finding the transfer function matrix by appropriately transforming the open-loop transfer function matrices of other conventional control structures (e.g., D-V). Once the necessary transfer function matrix is found, controller tuning is feasible, and the performance of the closed-loop system is very good when the two product composition control scheme is in effect