Vacuum pyrolysis of commingled plastics containing PVC II. Product analysis

Abstract

Vacuum pyrolysis of individual plastics including high density polyethylene (HDPE), low density polyethylene (LDPE), polypropylene (PP) and polystyrene (PS) has been performed at a final temperature of 500°C and under a total pressure of 2 kPa. A PVC sample and a commingled plastic sample composed of HDPE/LDPE/PP/PS/PVC have been pyrolyzed in two consecutive steps at 360 and 520°C in order to separately trap HCl. A pyrolysis set-up consisting of a bench scale batch reactor with a condensation train was used to collect the pyrolysis products. The main gaseous and liquid products, including the chlorinated hydrocarbons, were identified by chromatography. Global and chlorine mass balances were performed. The yields and pyrolysis product composition from each single polymer were compared with those from the commingled plastics. At 520°C, 99.55 wt.% of the total chlorine was trapped as HCl corresponding to 4.6 wt.% in yield, on an original feedstock basis, followed by 62.0 wt.% wax, 31.1 wt.% oil, 1.0 wt.% gas and 1.2 wt.% solid char residue (loss was 0.1 wt.%). The pyrolysis oil contained 12 ppm Cl on a pyrolysis oil basis. This value was less than what was expected (153 ppm) from the theoretical values which were calculated when the individual polymers were pyrolysed. The chlorinated hydrocarbons formed during the pyrolysis of commingled plastics were different from those obtained during the thermal decomposition of pure PVC. However, the hydrocarbons produced in the oil fraction were similar to those identified during the pyrolysis of the single plastics.