How do you design a control strategy for a distillation column?

Distillation control strategy design: identify degrees of freedom (typically 5: reflux, distillate, bottoms, reboiler duty, condenser duty), select product quality control points (direct composition or inferential from temperature), choose inventory controls (level in accumulator and column base, pressure), and address constraint handling. Common configurations: LV (reflux flow, boilup), DV (distillate flow, boilup), L/D-V/B (ratio control). Use RGA for variable pairing. Implement composition/temperature cascade, constraint control for flooding/weeping limits, feed-forward from feed rate/composition, and consider dual composition control for high-purity columns. MPC effective for multivariable optimization.