Nonlinear Model Predictive Control of High Purity Distillation Columns for Cryogenic Air Separation

High purity distillation columns are critical unit operations in cryogenic air separation plants that supply purified gases to a number of industries. We have developed a nonlinear model predictive control (NMPC) strategy based on the assumption of full-state feedback for a prototypical cryogenic distillation column to allow effective operation over a wide range of plant production rates. The controller design was based on a reduced-order compartmental model derived from detailed mass and energy balances by exploiting time-scale separations. Temporal discretization of the compartmental model produced a very large set of nonlinear differential and algebraic equations with advantageous sparsity properties, enabling online solution of the NMPC problem. The synergistic combination of several real-time implementation techniques were found to be essential for further reducing computation time and allowing reliable solution within the 2-min controller sampling interval. Closed-loop simulation studies demonstrated the performance advantages of NMPC compared to linear model predictive control technology currently used in the air separation industry.