Semi-continuous hydrothermal terephthalic acid recovery from poly(ethylene terephthalate)

This research delves into the hydrothermal decomposition of PET [poly(ethylene terephthalate)] between 523 and 573 K temperature at 10 MPa pressure. PET was placed in a packed column, and a constant water flow was maintained with an average residence time of water between 403 and 430 s as it travelled from the reactor to the end of the system. Terephthalic acid (TPA) recovery ranged from 87 % to 94 %, increasing with the temperature. The time of full conversion of the 0.5 g PET by 1 g/min water flow rate reduced from 2500 to 1200 sec, with increasing temperature. To prevent premature precipitation of TPA during cooling, an alkaline quench stream was employed. Terephthalic acid isolation involved acidification-driven precipitation and subsequent filtration at atmospheric pressure. The solid dry TPA product underwent NMR analysis, showing structural conformity and DSC/TGA measurement, which presented a purity of 99.5 w/w%. The fractional end-of-pipe sampling showed accelerating reaction rates at each temperature, which supports the autocatalytic nature of hydrothermal disintegration of the PET. Using the determined residence time distribution in the system, from the end-of-pipe data a crude Fourier transform-based backward calculation was carried out to map the variation of the TPA concentration in the high-temperature and −pressure reactor.