Introduction of Water As Aided Solvent in Melt Crystallization for Polyester-Grade Coal-to-Ethylene Glycol Separation

The industrial purification of coal-to-ethylene glycol (CtEG) faces the challenge of high energy consumption because of the boiling point azeotrope of EG and 1,2-butanediol (1,2-BDO). Melting crystallization has great potential for energy conservation in CtEG refining, only if a suitable separation efficiency is achieved. Herein, a water-aided strategy was investigated in the melt crystallization of CtEG. The polyester-grade CtEG was efficiently purified by the common process of suspension crystallization coupled with dry sweating when small amounts of water were added. Mechanism studies showed that water played an enhancing role in both crystallization and sweating stages by decreasing the melting point of EG in the eutectic system, namely, optimizing crystal growth in the crystallization while intensifying the residual liquid melting in the sweating at a constant operating temperature. Then, based on the inherent water in raw CtEG, a water-aided melt crystallization refining process of CtEG with an output of 200 kt/a was designed and evaluated. Its comprehensive energy consumption and total production cost were only about one-ninth of those of the industrial distillation. Moreover, scenario analysis was used to further improve the product competitiveness from the perspective of the EG recovery ratio. This work provides valuable insights into the solvent-aided crystallization strategy and a low-carbon and energy-saving CtEG refining technology.