Including levels in MPC to improve distillation control

This work examines the effects of including levels into MPC for distillation control via rigorous simulation of two different columns: a depropanizer and a propane/propylene splitter (C-3 splitter). Three different MPC implementations for distillation control were compared: regular implementation in which the levels are controlled by PI regulatory controllers, direct implementation in which the levels are controlled by MPC directly by manipulating flow rates, and cascade implementation in which MPC moves the set points to the regulatory level controllers. The results show that directly including levels in MPC improves composition control performance significantly for fast-responding distillation columns. The cascade implementation is recommended for columns that exhibit ill-conditioning for the [L, V] control configuration or for columns that require a long time to obtain steady state.