Microkinetics and Mass Transfer Study of Treatment of Aqueous and Alcoholic Potassium Hydroxide on Poly(Butylene Terephthalate) Waste

Abstract

Depolymerization of poly(butylene terephthalate) (PBT) waste sample was undertaken with aqueous as well as alcoholic (methanolic and ethanolic) solutions of potassium hydroxide at various temperatures (35–98°C) for various time periods (0.5–60 min) at atmospheric pressure in a batch operation. Methanolic alkaline solution showed greater reaction effect as compared to that of aqueous as well as ethanolic solutions. 1,4-Butanediol (BD) and dipotassium terephthalate salt (K2-TPA) remain in liquid phase. The K2-TPA was converted into terephthalic acid (TPA) using HCl. The BD and TPA were analyzed and confirmed. The PBT waste sample was also analyzed. Acid value as well as carboxyl-group concentrations of TPA significantly increase with reaction time. Kinetic analysis was undertaken. A microkinetic-mass transport model was developed and simulated data was compared with experimental data. The model showed good agreement with experimental data. The model prediction was undertaken using our own laboratory data. The mass transport was described using a classical diffusion equation according to Fick's law and a modified Wilke-Chang diffusion model. Diffusion coefficient and mass transfer coefficient were calculated with change in temperature as a function of operation time to give explanation of process phenomena. Thermodynamics were undertaken. Activation energy for methanolic KOH hydrolysis of PBT calculated from an Arrhenius plot was 49.9 kJ/mole, and the Arrhenius constant was recorded as 175080 L/min/cm2.