Consider Dividing Wall Columns

These distillation units often can provide both capital and energy savings.

By John G. Pendergast, David Vickery, Patrick Au-Yeung and Joe Anderson, The Dow Chemical Company

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The experimental unit DWC uses random packing. The vertical partition is located in the middle of the column, giving rise to a 1:1 ratio for the vapor split between the feed (pre-fractionation) and the product (side-draw) sides. The liquid split ratio is determined via a numerical simulation of the application to allow the DWC to meet quality specifications of all three products and optimize the distribution of the middle-boiling component between the top and the bottom of the prefractionator to minimize energy (reboiler duty) requirements. To facilitate distribution of liquid reflux to the two sides, liquid leaving the rectification section is collected using a total liquid collector and drawn off to an accumulator. The liquid then is pumped back to the two dividing-wall compartments through two Coriolis mass flow meters to achieve precise control of the liquid split ratio. Typically, this ratio is kept constant in the operation and is adjusted only slightly to ensure product quality requirements are met. The top reflux/distillate streams and middle reflux/side-draw streams are processed in a similar manner.

Effective Control
Simultaneously maintaining the product specifications on three products  may appear to be a daunting task considering the efforts undertaken for composition control of a conventional distillation column with two products. The fundamental tenets of providing sufficient energy to separate the key components and maintain the material balance split apply to the DWC.

Reflux drum level is controlled either by the reflux flow rate (L) or distillate flow rate (D), depending on the reflux ratio. With high reflux ratios (L/D>5), the reflux flow is used. Similarly, where the middle product is withdrawn from the column, the accumulator level is controlled by either the middle product or the middle reflux flow rate. At the bottom of the column, either the bottoms flow rate or reboiler duty are suitable for controlling the sump level.

Product compositions are maintained by treating the DWC, in effect, as the equivalent of three distillation columns — with the pre-fractionation (feed side of the wall) section separating A from C, the top half of the column separating A and B, and the bottom half separating B and C. The reboiler supplies the energy to separate the components. The bottoms product specification is maintained by adjusting the reboiler duty or the bottoms flow rate, depending on which stream is controlling reboiler level. Dow’s typical implementation utilizes the reboiler duty to control the separation of B and C.

To maintain the material balance split between B and C, we control the MRT, because it’s the temperature most sensitive to composition changes within the column. Typically this occurs when the vapor space is 50% B (or 50% C). Adjusting the middle reflux rate (when the side-draw flow rate controls the middle product accumulator level) or the middle product draw rate (when the middle reflux controls the middle product accumulator level) changes the temperature of the column by regulating the inventory of B in the column, facilitating control of the separation. We adjust the MRT setpoint to maintain the middle product composition. Analogously, the separation between A and B has a MRT. By adjusting the reflux rate or distillate rate, the inventory of A in the column can increase or decrease, changing the composition in the column. Monitoring the composition of the A product provides feedback on changing the upper MRT setpoint.

The last element of control is obtaining the split between A and C on the pre-fractionation section of the column (feed side of the wall). During DWC design, simulation determines the split of reflux to each side of the wall. During operation, this split is maintained by having one stream control the accumulator level. A ratio controller adjusts the other stream’s flow rate to maintain the liquid split ratio. Dow has successfully operated several pilot-scale DWC projects using this control philosophy.

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