Closed-loop identification and control of multivariable chemical processes: a case study

Closed-loop identification methods based on relay techniques have been analyzed and compared with open loop methods for an application to a multivariable nonlinear chemical process (a distillation unit: C-3 - C-4 splitter). Classical relay techniques and a recently proposed method for identification and control design (ATV(+)) are applied in a sequential procedure which leads to the design of decentralised controllers. Several disadvantages of open-loop identification techniques have been put into evidence, mainly the inconsistency of the obtained results in the case of a non linear process (model parameters depend on the amplitude of the perturbations used during the identification). Also, longer times are necessary for the experimental tests and a more complex controller tuning technique is required. Closed loop relay techniques are faster, identify the real dynamics of the system and allow a more direct design of the control system. The ATV(+) technique can be successfully applied to the process under study, after some modifications made necessary by the nonlinear nature of the system.