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

Abstract

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