BHET Crystallization in Water-Free PET Glycolysis Systems

Research in the last years has predominantly focused on optimizing the depolymerization step of polyethylene terephthalate (PET) glycolysis, often overlooking the at least equally critical downstream steps in recovering the monomer bis(2-hydroxyethyl) terephthalate (BHET). This study focused on BHET crystallization, a pivotal unit operation in the downstream of PET glycolysis. We successfully implemented a water-free PET glycolysis process, eliminating complications associated with internal solvent and homogeneous catalyst recycling found in the most commonly applied PET glycolysis processes. Based on BHET solubility measurements in real reaction mixtures, crystallization experiments were performed in a 180 mL batch lab-scale setup. The simplest method (natural cooling, no stirring) yielded the most efficient result, whereas the first experiments employing established crystallization techniques (seeding, controlled cooling, stirring) did not succeed. The simple approach resulted in a crystallization time of 180 min, and a filtration time of about 4.5 min, giving an overall process yield of 71%. Further, our BHET monomer/dimer product was successfully repolymerized, demonstrating the potential of our more environmentally friendly PET glycolysis process compared to conventional literature methods. This study lays the groundwork for future experiments aimed at recycling solvent and homogeneous catalysts within the process to minimize resources and energy demand.