Energy-saving process development for the purification of propylene glycol based on MVR heat pump distillation combined with thermally coupled technology

Abstract

In the process of recovering organic compounds from wastewater generated by propylene oxide (PO) production plant, the low concentration of propylene glycol (PG) in the wastewater makes it difficult and energy-intensive to achieve the high concentrations purify of PG using conventional distillation technology. Additionally, there is no publicly available commercial technology for this purpose. Therefore, the aim of this work is to develop an energy-efficient PG purification process based on mechanical vapor recompression (MVR) heat pump distillation combined with thermally coupled technology. Several energy-saving methods were implemented as follows. Firstly, MVR distillation technology was used to purify PG from 2.22% to 33.3% in the PG concentration tower, and the gas at the top of the tower was compressed and pressurized to serve as the heating medium for the reboiler, which significantly reduced the consumption of fresh steam. Secondly, in order to achieve heat matching, the dehydration tower was divided into two parts. The surplus steam compressed by the compressor was then served as the heat source for the reboiler of the first dehydration tower to reduce steam consumption, while achieving 98.7% water removal. Thirdly, the condenser of the de-heavy tower and the scraping film evaporator were thermally coupled to reduce the consumption of heating and cooling media. Based on the aforementioned energy-saving measures, the developed optimization process can achieve PG purification of over 90%, while significantly reducing costs. The total annual cost (TAC) is reduced by 34.89% compared to the traditional distillation process.