PID tuning methodology for processes with recycle

The main strategy to improve the productivity and performance of process with separation, reactions and classification is through the implementation of recycles of mass and energy. However, setting up recycles affect the general dynamic behavior of plants, doing them more sensitive to disturbance of low frequency. Strategies to design controllers for this kind of processes were proposed in the first middle of past century, focused in a linear analysis. Since then, no more relevant works have been proposed. This work presents an analysis of processes with recycle from state space point of view. Measures to quantify recycle effect on processes are defined, comparing the operation of the plant with ideal processes without recycle and total recycle. Those indexes are used finally to tune a PID controller that minimize the affection by recycle. Performance of proposed PID is compared with traditional designing methodologies for this kind of processes, proving to have the best regulation behavior facing disturbances.