(84g) Optimizing Energy- and Composition Control of Dividing Wall Columns

Distillation is the most widely used separation method in the chemical process industry and is a large energy consumer among process units. This fact has provided a continuous incentive towards the search for more energy-efficiency distillation systems. The divided-wall distillation column is a fully thermally coupled column, capable of separating three products in a single column shell with a single reboiler. Many papers discuss the steady-state design issues and propose heuristic and rigorous design optimization methods, but the dynamic control of the divided-wall column has been explored in few works. This paper addresses the task of simultaneous optimization of the energy consumption and control of the product composition of a divided wall column (DWC). Only a few papers have addressed both control and optimization at the same time. In a series of papers by Halvorsen and Skogestad (1999,, 2003, 2004) the steady-state relationships for fully thermally coupled arrangements or Petlyuk arrangements (where DWC is a subclass) has been analysed in detail. These results can be used to design the basic control structure for a given case. Then ordinary controllability analysis can be applied to find a practical solution for the total control and optimization task.

Here we suggest a basic general control structure based on 5-point control. This solution will be feasible for all types of ternary mixtures, provided that a sufficient number of separation stages have been designed for the given specifications and feed. In a majority of practical cases this general control structure can be simplified. How, and when depends on the feed properties and the purity requirements and value of the products. Thus, even one or two-point control may be sufficient for some applications. The key to the solution is to know how and when it is required to do on-line adjustment of the liquid split at top of the dividing wall or the vapour split at the lower end, or even both, in order to maintain minimum energy operation. The control loops for product composition must also be in operation. Simulations of the column were performed. The effect of reflux rate, number of column stages and the effect of the vapour split were investigated. The impact of some variables such as the number of trays in various sections and non-optimum design values of the vapour split on controllability and rangeability of the divided-wall column were also studied.

References:

E.A. Wolff and S. Skogestad, "Operation of integrated three-product (Petlyuk) distillation columns", Ind.Eng.Chem.Res., 34, 6, 2094-2103 (1995).

I.J. Halvorsen and S. Skogestad, "Optimal operation of Petlyuk distillation: steady-state behavior", J. Proc. Control, 9, 407-424 (1999).

I.J. Halvorsen and S. Skogestad, "Minimum Energy Consumption in Multicomponent Distillation. Part 1. Vmin Diagram for a Two-Product Column", Ind. Eng. Chem. Res., 42 (3), 594-604 (2003). "Part 2. Three-Product Petlyuk Arrangements", Ind. Eng. Chem. Res., 42 (3), 610-615 (2003). "Part 3. More Than Three Products and Generalized Petlyuk Arrangements", Ind. Eng. Chem. Res., 42 (3), 616-629 (2003).

I.J. Halvorsen and S. Skogestad, "Shortcut analysis of optimal operation of Petlyuk distillation", Ind. Eng. Chem. Res., 43 (14), 3994-3999 (2004).