The selection of an appropriate control configuration (structure) is the most important decision when designing distillation control systems. The steady state RGA is commonly used in industry for selecting the best structure. One counterexample to the usefulness of this measure is the DB configuration, which has infinite steady state relative gain array (RGA) values, but still good control performance is possible. This is indicated by high-frequency RGA values close to 1. In this paper it is stressed that decisions regarding controller design should be based on the initial response (high-frequency behavior) rather than the steady state.
Based on a frequency-dependent RGA analysis and optimal Pl controller designs of four different configurations, the (L/D) (V/B) configuration is found to be the best choice for two-point composition control. The conventional LV configuration performs poorer than the above system, but is preferable if one-point control is used.