Chemical recycling, kinetics, and thermodynamics of hydrolysis of poly(ethylene terephthalate) (PET) waste in sulfuric acid in presence of phosphoric acid

Poly(ethylene terephthalate) (PET) powder was depolymerized at atmospheric pressure in sulfuric acid in presence of 2.5 N phosphoric acid at 88-140degreesC. Particle sizes ranging from 50 to 512.5 mum and reaction time 30-200 min required for PET hydrolysis were optimized. Terephthalic acid (TPA) and ethylene glycol (EG) were produced by acid-catalyzed heterogeneous PET hydrolysis. To avoid the carbonization/oxidation of EG and to reduce corrosion, reaction was undertaken below 150degreesC using less concentration of sulfuric acid at a lower reaction time. To enhance reaction rate, 2.5 N phosphoric acid was introduced during reaction. Ethylene glycol was recovered during hydrolysis by salting-out technique. Terephthalic acid was separated out by using NaOH solution and then being acidified. Subsequently, TPA and EG were analyzed qualitatively and quantitatively. Yields of TPA and EG were almost equal to PET conversion. A modified shrinking-core model based on obtained acid value of TPA explained depolymerization in sulfuric acid (in presence of phosphoric acid). Effective surface area is proportional to the degree of hydrolyzed PET (X), affected by formation and growth of pore and crack on PET powder. Hydrolysis rates as well as apparent rate constant are inversely proportional to particle size. Thermodynamics are also undertaken to determine activation energy, Arrhenius constant, equilibrium constant, Gibbs free energy, enthalpy, and entropy. Dependence of rate constant on reaction temperature was correlated by the Arrhenius plot, which shows an activation energy of 56.8 kJ mol(-1) and a Arrhenius constant of 530.3 L min(-1) CM-2.