Interaction of plastics in mixed-plastics pyrolysis

The pyrolysis of mixed-plastic waste has been proposed as a means of recycl ing to produce petrochemical feedstock. The interaction of the main plastic types in plastic mixtures is significant in predicting the likely yield an d composition of products from different plastic mixtures. The six main pla stics in municipal solid waste are high-density polyethylene (HDPE), low-de nsity polyethylene (LDPE), polypropylene (PP), polystyrene (PS), poly(vinyl chloride) (PVC), and poly(ethylene terephthalate) (PET). Each of the plast ics was pyrolyzed individually in a fixed-bed reactor heated at 25 degrees C min(-1) to a final temperature of 700 degrees C. Polystyrene was then mix ed with each of the other five plastics in a ratio of 1:1 and pyrolyzed in the fixed-bed reactor under the same pyrolysis conditions. The yield and co mposition of the derived oil/wax and gases was determined. The main gases p roduced from the individual plastics were hydrogen, methane, ethane, ethene , propane, propene, butane, and butane and for the PET plastic carbon dioxi de and carbon monoxide. Hydrogen chloride was also produced with PVC. Analy sis of the oil/wax showed that the polyalkene plastics, HDPE, LDPE, and PP, gave a mainly aliphatic composition consisting of a series of alkanes, alk enes, and alkadienes. PVC gave a mainly aromatic oil, and PS and PET, which have aromatic groups hi their structures, also showed a more aromatic comp osition. There was a higher gas yield from the mixtures of the plastics wit h PS than would be expected from the pyrolysis of the individual plastics, coupled with a reduction in the oil/wax phase. The average molecular weight of the oil/wax from the mixed plastics was less than expected from the ind ividual molecular weights. Compositional analysis of the oil/wax showed tha t changes in the concentration of aromatic hydrocarbons and polycyclic arom atic hydrocarbons were produced with the mixture compared to that expected from the individual plastics.