Synthesis Of Pressure Swing Distillation For Separation Of Ternary Mixtures With Azeotropes

In pressure-swing distillation?PSD?systems to separate azeotropic ternary mixtures,the separation may become so difficult if there are more than one distillation boundary lines and a pinch between them is formed in the residual curve map?RCM?that a feasible PSD is imposible.In this work,a method for synthesising a three-column PSD process to separate effectively ternary mixtures with azeotropes.The separation of ethyl acetate?C₄H₈O₂-3?/ethanol?C₂H₅OH?/2-butanone?C₄H₈O-3?ternary mixture and the acetonitrile?CH₃CN?/ethanol?C₂H₅OH?/benzene?C₆H₆?ternary mixture were taken as two examples to illustrate the implementation of this method in detail.?1?In the example of separating C₄H₈O₂-3/C₂H₅OH/C₄H₈O-3 ternary mixture,according to its RCM at atmospheric pressure,each of the three pairs of components form a minimum-boiling azeotrope.In this system,two distillation boundary lines exist and they joint at the point of the C₂H₅OH/C₄H₈O₂-3 azeotrope.The three pure components are located in different distillation regions,and the coexistence of the three azeotropes results in a distillation boundary pinch near the C₂H₅OH/C₄H₈O₂-3azeotrope.By comparing azeotrope composition at different pressures,it was found that the composition of the C₂H₅OH/C₄H₈O₂-3 azeotrope changes significantly with pressure.An effective three-column PSD process was designed.This was accomplished by increasing the pressure of the column?column T1?for the separationg between the C₄H₈O-3 component and the C₂H₅OH/C₄H₈O₂-3 azeotrope with a shifted feed composition by a stream of C₂H₅OH that was recycled from the column of the separation between the C₂H₅OH/C₄H₈O₂-3 azeotrope and the C₂H₅OH product.The characteristics of the C₄H₈O₂-3/C₂H₅OH/C₄H₈O-3 ternary mixtures were analyzed and the temperature series method was used to summarize and classify the ternary mixtures applicable to separation using this three-column procedure.In addition,under six different pressures of the high-pressure column?column T1?,a sequential iterative approach and the maximum slope changing rate were adopted to optimize the stage numbers,feed stages,reflux ratios and other design variables.By comparing the economics of the alternatives with different parameters,the optimal pressure of the T1 column was obtained,and heat integration was applied resulting in extra reduction in the total annual cost?TAC?for the optimal process.?2?The CH₃CN/C₂H₅OH/C₆H₆ ternary mixture forms three binary azeotropes and one ternary azeotrope at atmospheric pressure,and the ternary azeotrope and the C₂H₅OH/C₆H₆ binary azeotrope are all sensitive to pressure changes.When the pressure is increased to 380 kPa,the ternary azeotrope disappeared;with further increase of the pressure,the distillation boundary pinch was mitigated.By analyzing the RCMs under different pressures and using the above three-column separation flowsheet,the separation specifications of this ternary mixture were achieved;based on this,four processes with different pressures of T1column was optimized and the optimal pressure at 900 kPa was obtained.Finally,heat integration was applied to further reduce the annual total cost.