(530e) The Chemistry and Kinetics of Polyvinyl Chloride (PVC) Pyrolysis

Pyrolysis is a technology to convert plastic wastes into an oil that can be used for further upgrading into fuels or chemicals. Polyvinyl chloride (PVC) is often found in waste plastics and can cause corrosion problems.¹ During the thermal decomposition of PVC, HCl is unavoidably formed and corrodes the stainless-steel components of the pyrolysis reactor system. One to three wt% of PVC in a waste plastic feed leads to 5,000-10,000ppm of Cl concentration in the crude pyrolysis oil product.¹ The objective of this research is to investigate the kinetics and chemistry of PVC decomposition using combination of experimental and computational modeling to develop a thermal treatment method to remove Cl efficiently.

PVC degradation occurs in two stages, dehydrochlorination and further decomposition of residual PVC. Dehydrochlorination is an autocatalytic reaction that begins at tertiary Cl structural defects. HCl is the main product from this reaction with small amounts of benzene (less than 2.77 wt.% of product formed). The formation rate of benzene has a similar trend as the formation of HCl because HCl can potentially catalyze a homolytic C-C bond cleavage, which gives rise to benzene and an aliphatic fragment. In this work, we studied the decomposition of PVC based on TGA results and used two models in series: a first order model to represent the dehydrochlorination process with the apparent activation energy of 116.7 kJ/mol, and a parallel first order model to represent the decomposition of PVC residue with the main apparent energy of 230.6 kJ/mol. The experimentally determined activation energy was in reasonable agreement with the predicted value for the HCl–catalyzed dehydrochlorination from density functional theory (DFT) calculations. We developed a pretreatment process for PVC and found out that the longer the isothermal treatment held at 320 ⁰C, the more Cl could be removed. The highest Cl removal efficiency reached 99.97%. FTIR and NMR were used to study the structure of treated PVC. The decomposed PVC had around 23% of quaternary carbon, 33% of C=C, 43% of aliphatic carbons, and 1% of primary carbon. The high concentration of the quaternary carbon indicates the treated PVC has a high crosslinked concentration. The most probable structure of the crosslinked center is cyclohexadiene according to our NMR analysis. Our results provide more insights into the PVC degradation chemistry with a proposed lumped PVC degradation mechanism.

1. J. Scheirs, W. K., Feedstock Recycling and Pyrolysis of Waste Plastics: Converting Waste Plastics into Diesel and Other Fuels. John Wiley & Sons, Ltd: 2006.