Simulation Aided Design and Development of an Adaptive Scheme with Optimally Tuned PID Controller for a Large Multistage Flash Seawater Desalination Plant - Part II: Control Structure, Optimal PID Tuning Based on Optimal First Order Plus Dead Time (FODT) Approximation

Abstract

In this paper the linearized model of an 18-stage MSF desalination plant is considered for six inputs and six outputs. The transfer function matrix of the resulting model is subjected to interaction analysis by the well known relative gain array (RGA) method and an appropriate control structure has been established. The design of an optimal PID controller for one of the most important loops, namely the top brine temperature (TBT) loop is considered in detail. The difficulties due to the high dimensionality of the model are discussed. The model is then optimally approximated to the standard first order plus dead time (FODT) form which happens to be the basis of all existing design methods of PID control. The results are discussed in the light of the approximation and its possible implications on the overall performance of the controller acting on the actual plant.