Comparative analysis of Extractive Distillation and Pressure Swing Distillation for different azeotropic mixtures

Abstract

One of the most used and important unit operation to separate mixtures in the chemical industry is distillation. Actually approximately 95% of all the separations are carried out by distillation processes, and indeed, a heuristic indicate that whenever possible distillation must be the first option to separate mixtures. There is situations where is not possible the separation by this operation as a result of closely boiling points (below 50ºC), low relative volatilities or the presence of azeotropes. In the first two cases, a high number of trays would be required for the column. In the case of azeotropes, the separation is impossible due to the composition of liquid and vapor is the same. For that reason, enhanced distillation techniques are necessary. In this work, a comparative analysis about these available techniques to break binary azeotropic mixtures is conducted to conclude which one is more convenient. In particular, it focus on the extractive distillation (ED) and the distillation by variation of the pressure known as pressure swing distillation (PSD). ED is one of the most used and known techniques characteristically by the addition of an extractive agent (EA), with high boiling point and non-volatile, which modifies the relative volatilities thus allowing the recovery of one pure compound at extractive distillation column (EDC). To obtain the other compound and recovery the extractive agent, a second column is required: recovery distillation column (RDC). The disadvantages of this technique are the cost associated with the recovery of the EA and that always there will be traces of the added compound impurifying the products obtained. In the case of PSD, no additional components are added but is exploited the sensitive of the mixture by changes of the pressure. There are two columns one of which works at high pressure (HPC) and a second at low pressure (LPC), thus varying the azeotropic composition and obtaining the pure compounds. This second is the least used because it is necessary for the mixture to be sensitive to changes in the pressure. Actually, there is a general tendency to wonder that by working with pressure the process is more expensive or less feasible.Therefore, one of the first questions in the early stages of conception of a process is the selection of which available techniques explained is the more feasible to break an azeotrope. Unfortunately, in my knowledge there is no works that focus on the comparison of both alternatives. The projects often focus on one of the techniques and there are some available articles which compares both for a specific mixture in basis economical terms. To answer that question, an intense bibliographic research is carried out to select the different cases of binary mixtures which are sensitive to pressure changes. A list of 26 mixtures is selected and going to be studied out more thoroughly. The information available and necessary it is searched at one simulator, databases and one book thus reducing the list at 23 mixtures. The simulator selected is Aspen Plus V10. Then, based on the corresponding material balances and applying a simplified mathematical model (infinite/infinite analysis), the different energy efficiencies are obtained. For this calculus it is necessary to know the compositions and boiling-points. The variation of the azeotropic composition with the pressure and the sensitive is also being studied. Consecutively, the efficiencies achieved in the two alternatives are compared distinguishing azeotropes of minimum and maximum boiling point. Finally, to find the reason of the results obtained a critical comparison is conducted to establish useful general indications on the selection during the first stages of the design process