Contribution to the Study of Heat Integration of Pressure Swing Distillation

Abstract

Separation processes of mixtures are one of the most important operations in the chemical industry due to the need of product purification or components recovery. Distillation is a widespread method of purification for liquid mixtures. The basis of this process is the selective boiling and condensation of the components. When it concerns an azeotropic mixture, the separation is limited to the azeotropic composition, so from this point it is impossible to continue with the purification and an alternative method must be used. Pressure swing distillation (PSD) process is an enhanced distillation method to break the azeotrope of azeotropic mixtures which are pressure sensitives. Heat integrated pressure swing distillation (HIPSD) is an improvement to overcome the high operating costs generated in the PSD process exchanging heat between hot and cold streams of the process in order to minimise the external energy requirements. In this project, the mixture to be separated is composed of methyl acetate (MetAc) and methanol (MeOH). This mixture is obtained in the synthesis of polyvinyl alcohol (PVA), and the purification of the components is useful as MeOH is a raw material for the PVA process, and to sell / use MetAc as a solvent in production processes such as glue or paint production. The PSD process is designed according to heuristic rules for distillation, and using Aspen Plus as simulation software. The process consists of two columns. MeOH is obtained in the first one (operating at 1 atm) and MetAc is obtained in the second one (operating at 10 atm), both by bottoms and with a 99.45 % of purity. Once the design is done, partial and full HIPSD are studied to achieve energy costs savings. This is done by means of Aspen Energy Analyzer complement, included in Aspen Plus. Since this is the first time using this tool in the department, the designs are also developed by a traditional method (Temperature-Interval) to compare the results and see how it works. For the partial HIPSD the feasible heat exchange between the first column reboiler and the second column condenser is performed, but the heat duties are not equal, hence an auxiliary reboiler is needed. The energy costs savings achieved are 37.47 %. For the full HIPSD, the PSD process is modified to equalise the heat duties, so that an auxiliary reboiler is not needed, and to minimise the second column reboiler heat requirements. The energy costs savings for this case are 46.27 %.