The divided wall column(DWC)or fully thermally coupled(Petlyuk)di stillation column to separate three components is receiving increasing interests in industrial applications due to their potential in energy and capital cost savings．Due to their ability to save energy，these distillation arrangements have become very attractive． However industries have been reluctant to use the DWC arguing a lack of knowledge and understanding，and scepticism towards practical issues such as control and operation．Recent research indicates that these problems may be resolved and motivate to further study this distillation configuration．　　This work focused on the study of the operability of DWCs by investigating the economical behaviors of the columns with a shifted vapor split ratio Rv，which was optimized in column design but changed in operation due to either the resistance difference between the column sections at both sides of the dividing wall or changes in feed composition．　　Four cases that were defined by three-component mixtures with different compositions of C5～C7 were analyzed in the research．　　In a design stage，the vapor split ratio Rv，which could be known as a decision variable in the DWC design was optimized by minimizing the Total Annual cost of the Petlyuk／DWC for each of the cases．Then，the operating flexibility of the designed column in terms of economical and operational feasibility was investigated by process simulations．The operating flexibility of the DWC was first investigated by varying Rv for each of the cases，for the scenarios where the resistances of both sides of the dividing wall in operation were actually different and then resulted in a different Rv in actual operation from the one in the design．Then，investigations were made by changing both Rv and the feed compositions for each of the cases，for the scenarios that the feed composition change would need Rv change for maintaining an optimum operation，yet actual Rv in operation could not be changed．　　The results of the investigations in this work demonstrated that a)an error in estimating the resistances of both sides of the dividing wall in the design could lead to significant loss in economical efficiency of a DWC；b)economical behaviors of a DWC could be very sensitive to feed composition change in terms of not only the operability but also the separation feasibility，and consequently，c)a significant energy reduction could only be achieved with a right DWC operation.