Controlling trace impurities in a dividing wall distillation column

Donahue, Melissa Mary
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Dividing wall distillation columns (DWCs) separate a feed mixture into three pure product streams using one column shell. Though attractive due to capital and operational savings, DWCs have yet to gain widespread industrial acceptance. One notable concern is controllability. The research within this document examines a four component feed mixture to evaluate the operational flexibility of a fixed-design DWC through experimental and simulation-based studies. A pilot DWC was successfully controlled at multiple operating points, and a dynamic model was developed to reflect the pilot dividing wall column. As a form of process intensification, DWCs have a higher risk for controller interaction making conventional PID control potentially inadequate. This work successfully used two PID temperature controllers to maintain the column at steady state, transition the column between steady states, and reject feed disturbances without controller interaction. These controller pairings were determined using conventional controller design techniques. Therefore, for this chemical system and column design, traditional approaches to distillation control are sufficient to handle the intensified nature of DWCs. Because more components are present in DWCs in larger amounts, there is concern that temperature control will no longer imply composition control. Temperature control proved successful in this study. Controlling two temperatures maintained column operation against feed disturbances. In addition, prefractionator temperature correlated well with reboiler duty for multiple feed qualities therefore serving as a promising control variable though more disturbances such as feed composition should be examined. The minimum energy controller was not tested experimentally. A steady state model with heat transfer matching the pilot data was scaled to the size of an industrial tower and used to generate a minimum energy response surface for different vapor and liquid split values. In summary, this research investigated the operational flexibility of a fixed-design DWC using a four component mixture, tested the ability of conventional distillation control design techniques to determine control structures for a DWC, and created a minimum energy operating surface that could be used to examine control structures. A technique to determine the overall heat transfer coefficients was developed, and the model closely matched experimental steady state data.