Control and operation of dividing-wall columns with vapor split manipulation

Abstract

Thermally coupled distillation arrangements with dividing-wall implemen- tation provide significant energy and capital savings compared to energy intensive conventional distillation arrangements. In this thesis, the focus is to study control and operation of such arrangements for three and four product separation. The study comprises of mainly simulation study for the Petlyuk arrangements while for the four-product Kaibel arrangement, simulation as well as experimental works are reported.

The first contribution deals with selecting control structures for a three- product Petlyuk (dividing-wall) column. Alternate control structures are considered, with and without the vapor split as a degree of freedom. This work also demonstrates the usefulness of the graphical Vmin diagram to visualize minimum boilup requirement and choose the appropriate control structure.

Next, for a four-product Kaibel column separating methanol, ethanol, propanol and n-butanol, a start-up procedure and steady state operation is demonstrated using experimental setup. A control structure with four temperature controllers is used for control and it can handle feed rate dis- turbances as well as setpoint changes. The experiment data also compares well with an equilibrium stage model.

Dividing-wall distillation columns offer large potential energy savings over conventional column sequences, typically up to 30 % for three-product (Petlyuk) columns and 40 % for four-product (Kaibel) columns. However, the energy required for a separation depends on using an optimal vapor split. Hence, the energy saving potential may be lost if the column is operated away from its optimal point, for example, due to feed composition changes. The following work demonstrates experimentally that the vapor split can be effectively used as a degree of freedom during operation for example, for temperature control in the prefractionator section. Together with an adjustable liquid split, the vapor split control allows for minimizing the energy requirements.

Finally, a control study on a four-product extended Petlyuk column op- erating close to minimum energy is reported. The study assumes an “ideal” case with all steady state degrees of freedom available for control, including the vapor split valves, which is required to achieve minimum energy under all conditions. Four decentralized control structures are proposed and tested against a wide range of disturbances. This work demonstrates again the use of the graphical Vmin tool which can be used to visualize the minimum boilup requirement for four-product Petlyuk arrangements.