Pressure-Swing Distillation for Minimum- and Maximum-Boiling Homogeneous Azeotropes

Pressure-swing azeotropic distillation uses two columns operating at two different pressures to separate azeotropic mixtures by taking high-purity product streams from one end of the columns and recycling the streams from the other end with compositions near the two azeotropes. This configuration can be economically used when changes in pressure significantly shift the composition of the azeotrope. The larger the shift, the smaller the required recycle flow rates, so the smaller the energy requirements in the two reboilers. Pressure-swing distillation can be applied to both minimum-boiling and maximum-boiling homogeneous azeotropic mixtures. With minimum-boiling systems, the distillate streams are recycled. With maximum-boiling systems, the bottoms streams are recycled. Intuition would lead us to expect that recycling distillate streams would be more energy intensive than recycling bottoms streams. A distillate recycle must be boiled up in the column. A bottoms recycle stream is not boiled up. Therefore, we would expect that less pressure sensitivity of the azeotropic mixture would be required to make the pressure-swing configuration in a maximum-boiling system economical than in a minimum-boiling system. The purpose of this article is to explore this question. The results show that there is very little difference between the two systems in terms of the effect of pressure sensitivity and energy consumption.