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

P.N. Josiah, J.U. Nwalor, T.O. Ajayi


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 (Trx), the regenerator temperature (Trg) and the regenerator flue gas oxygen concentration (Od) to step changes in air flow rate (Fa), regenerated catalyst flow rate (Frc), gas oil feed rate (Fgr). The results show strong interaction in FCCU variables, with Fa affecting Trg and Od; Frc affecting Trx, Trg and Od; Fgr affecting Trx, Trg and Od. 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: (Trx/Fgr), (Trg/Fa) and (Od/Frc) ,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

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ISSN (Paper)2224-5774 ISSN (Online)2225-0492

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