Loop Interaction Analysis and Control Structure Selection: Application to a Fluid Catalytic Cracking Unit

A simulation approach for the assessment of variables interaction and consequent control structure selection of a fluid catalytic cracking unit (FCCU) is presented in this paper. The simulator which was implemented in Matlab draws from an earlier mathematical model of the FCCU, was used as a virtual FCCU for studying the dynamic response of the riser temperature (T rx ), the regenerator temperature (T rg ) and the regenerator flue gas oxygen concentration (O d ) to step changes in air flow rate (F a ), regenerated catalyst flow rate (F rc ), gas oil feed rate (F gr ). The results show strong interaction in FCCU variables, with F a affecting T rg and O d ; F rc affecting T rx, T rg and O d ; F gr affecting T rx , T rg and O d . A linearised state-space model based on the first-principle model was deduced and transformed to a 3x3 input-output model. Three channel interaction measures: Relative Gain Array (RGA), Effective Relative Gain Array (ERGA) and the Normalized Relative Gain Array (RNGA) were applied to the selection of FCCU control structure. All the measures point to a diagonal scheme with the following pairings: (T rx /F gr ), (T rg /F a ) and (O d /F rc ) ,for the decentralized control of the riser temperature, the regenerator temperature and the flue gas oxygen concentration respectively. The suggested control structure offers a high promise of stability, with a Niederlinski index (NI) of 101.79. DOI : 10.7176/CTI/8-03