Numerical Investigation Of The Effect Of Vapor Split Ratio On The Performance And The Operability For DWCs

The divided wall column (DWC) or fully thermally coupled (Petlyuk) distillationcolumn to separate three components is receiving increasing interests in industrialapplications due to their potential in energy and capital cost savings. Due to theirability to save energy, these distillation arrangements have become very attractive.However industries have been reluctant to use the DWC arguing a lack of knowledgeand understanding, and scepticism towards practical issues such as control andoperation. Recent research indicates that these problems may be resolved andmotivate to further study this distillation configuration.This work focused on the study of the operability of DWCs by investigating theeconomical behaviors of the columns with a shifted vapor split ratio Rv, which wasoptimized in column design but changed in operation due to either the resistancedifference between the column sections at both sides of the dividing wall or changesin feed composition.Four cases that were defined by three-component mixtures with differentcompositions of C5~C7were analyzed in the research.In a design stage, the vapor split ratio Rv, which could be known as a decisionvariable in the DWC design was optimized by minimizing the Total Annual cost ofthe Petlyuk/DWC for each of the cases. Then, the operating flexibility of the designedcolumn in terms of economical and operational feasibility was investigated by processsimulations. The operating flexibility of the DWC was first investigated by varyingRv for each of the cases, for the scenarios where the resistances of both sides of thedividing wall in operation were actually different and then resulted in a different Rv inactual operation from the one in the design. Then, investigations were made bychanging both Rv and the feed compositions for each of the cases, for the scenariosthat the feed composition change would need Rv change for maintaining an optimumoperation, yet actual Rv in operation could not be changed.The results of the investigations in this work demonstrated that a) an error inestimating the resistances of both sides of the dividing wall in the design could lead tosignificant loss in economical efficiency of a DWC; b) economical behaviors of aDWC could be very sensitive to feed composition change in terms of not only theoperability but also the separation feasibility, and consequently, c) a significant energy reduction could only be achieved with a right DWC operation.