(79a) Operable Version of Dividing-Wall Columns: A Control Perspective

Distillation is one of the most energy-intensive unit in chemical process industries. Conventionally, ternary mixtures are separated in two distillation columns by separating the most volatile component first (direct-split sequence), separating the least volatile component first (indirect-split sequence), or distributing the components (mid-split sequence). Columns in these sequences can have separate reboilers and condensers, or they can be thermally coupled to share liquid and vapor and operate with a single reboiler and condenser. The latter type of distillation columns are called Petlyuk columns. The most widely implemented dividing-wall column (DWC) provides capital and energy savings up to 40% compared to the conventional columns. DWC, is a single shell column with top, middle and bottom sections, where the middle section is divided into prefractionator and main sections by inserting a vertical wall. This arrangement allows the operation of the Petlyuk column in the DWC.

Despite the thermodynamical and economical advantages, a major drawback of DWCs that prevent them from being widely implemented in industries is the controllability issue that arise due to vapor split at the bottom of the DWC. The vapor split is proportional to the cross-sectional area of the prefractionator and the main columns, and cannot be manipulated during the operation because of the fixed physical location of the wall. Not only this results in the loss of a degree of freedom for controlling the operation of the DWCs, disturbances in the vapor split due to the external disturbances may not be well understood during the simulations or rejected well by the control systems during the operation.

Ramapriya et al.¹ proposed a method to synthesize thermodynamically equivalent DWCs by extending the middle-wall to the top and/or bottom of the DWC, adding additional condenser and/or reboiler, and transferring mass between the two sides of the wall through liquid flow transfer. This method can replace the vapor split at the bottom of the DWC with a liquid only transfer which is available for manipulation for control purposes. In this work, we present the control advantages of DWCs with extended wall to the top and/or bottom by considering the ternary separation of benzene, toluene and ortho-xylene in a DWC. We demonstrate the results by developing a new PID control strategy and study the responses of the column to disturbances in flow and composition of the feed. The DWC with extended wall not only can be practically operated, it can appropriately reject the external disturbances and deliver on-spec products.

References:

1. Madenoor Ramapriya, G., Tawarmalani, M. and Agrawal, R. (2017), A systematic method to synthesize all dividing wall columns for n-component separation—Part I. AIChE J. doi:10.1002/aic.15964