Nonlinear model reduction and control of high-purity distillation columns

We address the derivation of reduced-order nonlinear models of high-purity distillation columns with large recycle and internal flowrates. We show through a singular perturbation analysis of the detailed tray-by-tray model that the presence of large internal flowrates leads to a time-scale separation in the column: (i) a fast dynamics corresponding to the individual stages, and (ii) a low-order slow dynamics of the column that governs the overall nonlinear input-output behavior. A description of the low-order slow dynamics of the column is derived and used for the synthesis of a well-conditioned nonlinear feedback controller.