Troubleshooting a Depentanizer MPC Controller with Rigorous Process Models

A new GC analyzer was installed to the overhead stream of a depentanizer column and the overhead control objective was changed from controlling benzene to cyclopentane (boiling point is 55 F less than benzene). Shortly after installing the new GC, a new model predictive controller (MPC) was implemented. After several months of good control performance, the unit operators reported that the overhead benzene sometimes  got off-spec even though overhead cyclopentane was in good control. To resolve the issue, several tests were conducted to develop benzene related models. However, root causes of the issue were not well understood, and the control change did not work well. Further investigation with laboratory data and rigorous process models showed that the benzene breakthrough was due to feed composition changes. This presentation will discuss the modeling steps used to understand the root cause of the issue and the resulting control strategy changes.