Intermediate Heat Exchange for Fixed Separation Requirements: Applications to a Binary Sieve Tray Distillation Column for Energy Savings

Abstract

The aim of this study is to investigate different ways of introducing intermediate heat exchange into a given sieve tray distillation column by using computer simulation. The number of stages and the product qualities were kept constant before and after the introduction of the intermediate heat exchange. The intermediate heat exchange was achieved by applying an open heat pump circuit where column streams were withdrawn from the column and after heat exchange, they were returned to the column immediately. Different return positions were evaluated as well as arrangements for adding/removing heat to/from different positions in the column were studied. The rate of the side streams was also varied. The column has been considered as a stand-alone system. The intermediate heat exchange was analysed from different points of view. It was found that the entropy production could be decreased in a tray column if the side streams were rein-troduced in an optimal way. A 7% reduction in entropy production was obtained for the column studied. A decrease in external energy of about 14% has been calculated with the return of the optimal side streams into the intermediate heat exchange. With the same modifications, a 27% decrease in the variance of the driving forces was achieved, which indicates that the system has become more reversible in its nature. The liquid and the vapour flows change along the column due to the intermediate heating and cooling. This will of course affect the mixing on the trays and the residence time, and consequently, the Murphree tray efficiencies will change. An average value of the Murphree and Chan-Fair efficiency for the optimal case was slightly higher compared to that for the conventional case.