Chemical recycling of PET by catalyzed glycolysis: Kinetics of the heterogeneous reaction

Polyethylene terephthalate post-consume (PET-pc) glycolysis was investigated by the use of ethylene glycol (EG) and zinc acetate, as catalyst. It was focused the kinetic aspects through use of mathematical model specially developed for application in this study. The grains-lot was sieved in different size ranges and a relation between surface area and granulometry, surface area and temperature on the conversion and depolymerization rate was proposed. At temperatures ranging 180–190 °C the depolymerization rate is quite elevated and almost 100% of conversion is obtained up to 3 or 4 h reaction time. For lower temperatures (170–180 °C), equilibrium is achieved and it becomes more important as the temperature is decreased. The conversion profile showed an initial activation stage where the mass transfer between the liquid and solid phases is minimal. The proposed mathematic model was based on these findings and on reaction mechanism that differentiates the reaction sites present in the PET surface. By that model the value of rate constant (k) for each temperature, and the dependence of k with 1/T was calculated. Four our best knowledge it is the first time that a mathematical model considers the activation stage in the earlier times of PET depolymerization reaction. The inputs yielding, time and temperature were included in the used mathematical model that fits very well the experimental data obtained at temperatures higher than 180 °C. This model helps to predict the necessary mass of PET for producing a desired amount of products.

► Geometric surface area determination of the PET-pc grains by geometric models with two and three parameters.
► Determination of conversion (X) as a function of the granulometry of PET-pc.
► Evaluation of time and temperature influence on the heterogeneous depolymerization reaction.
► Developed a mathematical model for kinetic of heterogeneous PET glycolysis by ethylene glycol and zinc acetate.
► Determination of operating conditions to produce any quantity of products, providing a production planning tool for recycling of PET-pc.