(322g) A New Control Structure for Divided-Wall Columns

Industrial applications of the divided-wall column for the separation of ternary mixtures have increased in recent years with about 40 columns reported to be in service. The divided-wall column is a practical way to implement the topology of the Petlyuk column that features two columns (a prefractionator into which the feed is introduced and a main column from which a sidestream product is withdrawn) with interconnected vapor and liquid streams arising from a single reboiler and a single condenser. Many papers discuss the steady-state design issues and propose heuristic and rigorous design optimization methods.

The dynamic control of the divided-wall column has been explored in a relatively small number of papers. Control is more difficult than with a conventional two-column separation sequence because there is more interaction among controlled and manipulated variables since the four sections of the column are coupled. The vapor split is fixed at the design stage and cannot be changed during operation, but the liquid split can be manipulated to achieve some control objective.

A number of control structures and algorithms have been proposed, but the reported results present a somewhat confusing picture. Different authors draw conflicting and inconsistent conclusions. Most papers control the purities of the three product streams using reflux flowrate, sidestream flowrate and vapor boilup. This paper proposes a new control structure that controls these purities and also minimizes energy consumption. This is achieved implicitly by controlling a composition of the heaviest component in the prefractionator. Disturbances in feed flowrate and feed composition are used to demonstrate the effectiveness of the proposed control structure. A comparison of the dynamic controllability of the divided-wall column with a conventional configuration is also provided.