Consistency of property estimators in distillation column control

This paper addresses the problem of input selection in inferential control of multicomponent distillation columns by introducing the concept of consistency. An estimator is consistent when a feedback (multivariable) control system that uses the estimates of the controlled variables guarantees low closed-loop steady-state offset in the true unmeasured controlled variables when disturbances enter the system (and/or set-point changes are considered). A definition of this property is given, and the relations between the estimator consistency and the closed-loop steady-state offset are derived for both single-input-single-output and multi-input-multioutput systems. A multicomponent distillation column case study is presented to show that the selection of the most "precise" inputs does not necessarily guarantee the lowest closed-loop offset in the presence of disturbances, whereas the use of less precise but more "consistent" inputs leads to a well-designed estimator that guarantees a lower closed-loop steady-state offset.