(142a) Assessment of Minimum Energy and Operation Flexibility of Complex Dividing Wall Columns by Use of the Vmin-Diagram

The dividing wall distillation columns (DWC) find increased use in industrial practice. The benefits are the combined savings in both energy and capital cost. Most of the industrial applications are three-product columns, but also 4-product DWCs have been constructed. A simulation study has shown that a four-product DWC with multiple partition walls for an aromatics feed can save up to 50% energy compared to conventional sequences. The internal arrangement is more complex and it is important to predict the necessary internal flow rates both for making proper sizing of the internals and also for control structure design. Thus, there is still a need to focus on development of simple-to-use methods for making assessment on how complex columns like the DWC will perform for any given application. This paper will demonstrate the usability of the Vmin-diagram method for quick assessment of column configurations for multicomponent separation. 

The Vmin-diagram is obtained from feed data only, and gives a direct picture of the minimum vapor requirement for any specification of product splits. It is particularly well suited for fully thermally coupled distillation configurations like the generalized Petlyuk arrangements and dividing wall columns. The Vmin-diagram was first presented by Halvorsen and Skogestad (2003). The deduction was based on the simplifying assumptions of constant molar flows, constant pressure and constant relative volatility. It can also be constructed from simulations on a real mixture.

The minimum vapor flow rate in a DWC is given by the most difficult of the split between two adjacent product groups as if each split were to be carried out in a standard two-product column. For sharp split specifications, these are found as peaks in the Vmin-diagram. In addition, a base solution for the internal flow rates in each internal column section can be found directly.

In a DWC we fortunately have some tolerance in how to set the internal flows since there is normally more than a single optimal operating point. This flexibility arises when there are different minimum required flow rates in the upper and lower part of the arrangement. This tolerance can be explored in the Vmin-diagram and be used to specify operational margins on the internal flow rates. Knowledge of the operational tolerances for optimal operation is important in order to design and operate a column that obtains the theoretical energy savings in practice.