(256e) Critical Assessment of Energy-Saving Potential of Extractive Divided-Wall Column

One of a common way to separate azeotrope in industry is to add a heavy entrainer so that the relative volatility of the original two components can be greatly enhanced. Thus, one original component can go overhead in an extractive distillation column and the other component will go with the heavy entrainer to the column bottoms. A second entrainer recovery column is designed to separate this stream so that the entrainer can be recycled back to the extractive distillation column. By observing the composition profile of stripping section of the extractive distillation column, a prominent remixing effect is exhibited with one composition going through a maximum and then decreasing toward the column bottoms. A way to eliminate this remixing effect to save energy is to design a thermally coupled two-column system so that the vapor traffic of the extractive distillation column is provided by a sidedraw from the entrainer recovery column. A special internal wall can be designed so that the original two-column system becomes one extractive divided-wall column (EDWC).

However, one potential drawback on the energy-saving of the EDWC design is that two reboilers in the original design should be combined into one reboiler. Since the entrainer is the heaviest component in the system, there are cases with reducing total reboiler duty but  adversely increasing of the total steam cost of the separation system. Three industrial separation systems of acetone-methanol-water, isopropanol-water-DMSO and dimethyl carbonate-methanol-aniline will be used as demonstrating examples for critical assessment of  the energy-saving potential of EDWC design. Dynamic controllability and operability of the EDWC system are also investigated to properly reject common feed rate and feed composition disturbances with a suitable overall control strategy.