Combined Balance Control Structure for Distillation Columns

A new control structure for distillation columns, called the “combined balance structure” is presented. The combined balance structure consists of a linear combination of material balance and energy balance control (conventional control) that can outperform traditional control structures such as LV, DV, ratio structures, etc. Mathematical derivation of this new structure is based on a rigorous transformation technique. The combined balance method unifies the analysis of distillation column control structures and leads to general relative gain array (RGA) relationships that can be used to compare alternative distillation control structures. The importance of inventory control dynamics and the ability of the new optimum combined balance control structure to outperform other conventional control structures are demonstrated by closed-loop comparisons using transfer function representations.