A design model for dividing wall distillation columns
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Traditionally, the production of three high purity products from a three component mixture requires the use of two distillation columns. A Dividing Wall Column (DWC) offers an alternative to this approach. The DWC contains a vertical partition, dividing the column into two sides. The feed side separates the lowest and highest boiling products, while the product side separates the intermediate component. This configuration reduces capital costs by utilizing only one column, reboiler, and condenser and reduces the thermodynamic losses by partitioning the feed and side product. Previous theoretical studies have found that a DWC can also produce as high as 30-50 percent energy savings over a traditional multi-column scheme. Despite these advantages, validated predictive models, which will facilitate widespread adoption of the technology, are lacking. In this research, experimental results were obtained over a wide range of operating conditions using a constructed six inch diameter pilot plant data is used to develop a fully validated DWC. The pilot DWC ran several experimental tests with both an alcohol and hydrocarbon feed. Both systems were tested with an equimolar and a 10/80/10 feed composition. Internal flow rate and composition data were available that has not been published in research, allowing for a complete model validation, including heat loss and heat transfer across the dividing wall. Both Model Predictive Control and traditional PID control was tested and resulted in high quality steady state data obtained. An extensive hydraulic study was conducted for both structured and random packing. Mass transfer studies and air-water experiments were conducted to fundamentally characterize the column hydraulics. These studies included confirming the vapor split is determined by the wall location and not impacted by the pressure drop. Pressure drop and wetted area studies were performed on circular and dividing wall column structures. The dry pressure drop, irrigated pressure drop, and hold-up were compared with existing correlations. Additionally, the dividing wall wetted area was determined. A DWC model was validated with the experimental pilot data obtained as well as comparing with open literature data. With the results from the validated model, optimizations were conducted to aid in DWC design. It was shown that feed temperature is an important variable to consider in design. The vapor split did not have as much of an impact on energy savings as the liquid split. An accurate energy calculation was performed on a pilot scale column and a scale-up to industrial column diameters. The scale-up shows that the impacts from heat loss and heat transfer are not as significant in a large scale column for product purities and column liquid and vapor loads. DWC columns averaged approximately 30% energy savings. The comprehensive validated model lays the groundwork for DWC industrial expansion.