The European Union 10-ppm sulfur fuel regulations present various challenges for operating and controlling refinery desulfurization units. Repsol has been using simulation to mitigate some of these challenges. In a joint project with AspenTech, the two companies developed a rigorous steady-state and dynamic Aspen HYSYS® model of Repsol’s Puertollano refinery’s hydrodesulfurization (HDS) unit.