Control of a Process with Recycle: Robustness of a Recycle Compensator

Several studies on the dynamics of processes with recycle are reported in the literature. A complete summary of the effects of the recycle, along with a detailed analysis of the disturbances behavior when matter or energy is recycled, is presented in the first part of the paper. The control of processes with recycle is then discussed, including the benefits of adding a recycle compensator to a controller. Representing the system equations with Bode plots shows that the performance of a PI (proportional and integral) controller with a recycle compensator is usually better than the use of a PI controller alone. A systematic analysis of the sensitivity of the model demonstrates that this is true, even for important model errors in the recycle path. This work suggests that the negative result of recycle addition on the process dynamics can be overcome when the recycle effects are anticipated even though the recycle path model is not error free.