Crystallisation of bis(2-hydroxyethylene) terephthalate as a part of a bottle-to-bottle recycling concept for poly(ethylene terephthalate)

The chemical recycling of poly(ethylene terephthalate (PET) is very attractive as PET bottle waste provides an abundant clean material with low levels of additives. One of the most promising processes is glycolysis, which depolymerises PET in the presence of ethylene glycol. For this process, it is necessary to think through the whole concept from the waste material to the newly polymerised virgin polymer. Most research ends with determining the yield of bis(2-hydroxyethyl)terephthalate (BHET) after glycolysis. Some research includes anti-solvent crystallisation with water to separate BHET from ethylene glycol. However, the subsequent separation of water and ethylene glycol is an energy intensive process. Therefore, this work aims at the direct crystallisation of BHET from ethylene glycol. For this reason, the crystallisation of BHET was investigated experimentally. Crystallisation was simulated using gPROMS Formulated Products with aim of estimating kinetic parameters and using these to optimise an industrial process. Kinetic parameters were determined by model validation including primary and secondary nucleation and crystal growth. The best fitting set of kinetic parameters was used to optimise BHET crystallisation in batch and continuous mode by minimising equipment costs. Impeller parameters were found to have a great influence on crystallisation performance. Ultimately, the continuous and batch processes gave comparable results in terms of equipment cost, with the batch process giving larger crystals and higher yields, but the continuous process requiring a smaller crystalliser.