(619e) Evaluation and Comparison of PET Depolymerization Routes

Poly(ethylene terephthalate) (PET) is the most widely produced thermoplastic and is commonly used in consumer products such as bottles, packaging and textiles. There is potential economic incentive to recycle commercial PET waste because of its relatively high value and the ability to collect waste with high contents of PET. Collection infrastructure and mechanical recycling of PET bottles is well developed, but mechanical recycling is limited in its ability to purify waste PET to a virgin quality. Chemical recycling of PET offers a way to expand the scope of PET waste materials that can be recycled and reduce cleaning and sorting requirements of reclaimers. Chemical recycling involves the conversion of PET into relevant monomers conventionally used to produce PET and provides the advantage of enabling separation and purification processes such as crystallization and distillation, unavailable to polymers. This allows the monomers to be purified such that they can be polymerized into virgin quality PET.

The reversible nature of the ester linkages of PET means that the polymer can be converted into many different monomers. Our work investigates process modeling of the three primary routes to depolymerize PET: methanolysis, glycolysis, and hydrolysis. All three depolymerization routes are being developed for application on a commercial scale, but there is a lack of understanding of the differences among the depolymerization routes. We first compare the physical and thermodynamic properties of the products and reactants of the different routes and demonstrate how these properties lead to fundamentally different operating characteristics in the reaction and purification sections of the process. We then cover in detail the purification methods described in patents by different industrial technologies and demonstrate similarities and differences in the purification strategies of different depolymerization routes. We use our patent survey of the industrial depolymerization technologies to present a general framework for a PET depolymerization process.

The final part of this work involves the modeling of the different PET depolymerization processes using Aspen Plus. We build detailed models assuming common impurities and byproducts and simulate major unit operations. Process simulation is developed by integrating industrial literature review, knowledge of process design and heat integration. We present process innovations using advanced heat integration and process intensification techniques for each technology. During process evaluation, we consider uncertainties regarding optimal solvent rates for the reaction and purification sections and demonstrate the relationship between these variables and process energy demand.

From this investigation, we draw general conclusions about the advantages and disadvantages offered by each depolymerization route. Overall, the glycolysis process is the simplest and least energy-intensive process; however, challenges with isolating its monomeric product from oligomers and other heavy impurities make it less attractive for processing low-purity waste PET. Methanolysis has the potential to be competitive with glycolysis processes in terms of energy demand and the monomer dimethyl terephthalate (DMT) has a high enough volatility to be more easily purified by evaporation or distillation. Methanolysis is disadvantaged because DMT is used as a feed in only a few PET existing polymerization units. An extra reaction step following the purification of dimethyl terephthalate (DMT) by reacting with water to form terephthalic acid (TPA) may be necessary. The hydrolysis process is the least attractive of the depolymerization routes because of the necessity to convert the product to a carboxylate salt, which consumes an acid and base, the large amount of water to dissolve the carboxylate salt, and the added complexity handling a salt byproduct and separating ethylene glycol. We expect that a continuous, properly heat- integrated PET depolymerization process to have a total energy demand between 6,000-10,000 kJ per kg of PET.