Optimal Operation of Dividing Wall Columns

Abstract

This thesis discusses the control and operation of dividing wall columns. In particular the attention is on the Kaibel column, a four-product dividing wall column with two side-streams.

The Kaibel column has for a given feed potentially 6 degrees of freedom available for control once inventory control loops are in place. These degrees of freedom are the vapour rate, reflux rate, two side-stream rates and the split ratios for vapour and liquid. They can be used to operate the column in such a way as to meet predefined goals and specifications. The goals of operation may be formalized by an economic cost (or objective) function. Optimal operation is when the best possible operation in terms of the cost function is achieved. The normal (in literature) cost function to evaluate dividing wall columns is to minimize the energy input to the column while keeping the product purities at specified values. This is a good formulation to evaluate the potential energy savings of the dividing wall compared to conventional distillation sequences, and is particularly useful in a design phase. For an existing column within a processing plant, other operational objectives may also be important, however. In this work, optimal operation of a dividing wall column is investigated from a perspective of different operational modes (objectives).

It is common to separate a control system into different layers based on the time-scale that the different control loops operate. The supervisory control layer should help us achieve the operational objectives - optimal operation, while the faster regulatory control layer should deal with stabilization and fast control. In this thesis the emphasis is on selecting the correct controlled variables for both layers. Methods for selecting self-optimizing controlled variables are applied with an aim of optimal operation, while column dynamics are more important when selecting variables to control in the regulatory control layer.

For the regulatory control layer, the importance of utilizing the liquid split for closed loop control is emphasized.

A Kaibel column pilot plant has been constructed during the work of this thesis. Initial experiments have been reported and valuable lessons were learned about the design and operation of the column. A purposebuilt valve for adjusting the vapour split in the column has been included in the apparatus. The first experiment with closed loop control using the vapour split as a manipulative variable is reported.