We detail here the controller of the acid strength that we implemented in the Elf-Antar France refinery in Feyzin (France). The control technique used is new. It relies on the flatness property of the system to solve a constrained minimum time objective as successive linear-programming problems. This controller is in full service since January 1997. We detail the control technique, including the estimation and numerical problems and then give industrial results over 6 months.