Pyrolysis of common plastics and their mixtures to produce valuable petroleum-like products

Abstract

In the present study, polystyrene (PS), low-density polyethylene (LDPE), high-density polyethylene (HDPE), polypropylene (PP), and their mixture undergo intermediate isothermal pyrolysis at 450 °C, 500 °C, 550 °C, and 600 °C inside a tubular semi-continuous reactor under an inert nitrogen atmosphere. The liquid pyrolytic oil samples recovered in each case were analyzed through gas chromatography-mass spectrometry (GC–MS) and flame ionization detector (GC-FID) to identify and quantify the different products formed. It was found that polyethylenes (HDPE and LDPE) behave similarly in these pyrolysis conditions. For instance, GC analysis showed consecutive triplet series of linear aliphatic hydrocarbons at each carbon number of the alkane, 1-alkene, 1,(N-1)-alkadiene where N is the carbon number. On the other hand, PP, LDPE, and HDPE pyrolytic oil were rich in olefinic compounds. However, the case was quite different for PS. The pyrolytic oil was constituted totally of aromatic compounds ranging from mono to tri aromatic hydrocarbons. Styrene was the main product, along with other products such as styrene dimer, styrene trimer, benzene, toluene, bibenzyl, and α-methylstyrene. Results showed an increase in lighter hydrocarbons as the temperature increased, especially for styrene, where the weight fraction in the oil increased from 40 wt.% to 60 wt.%. Ultimately, a synergistic effect was observed with mixed plastic pyrolysis, yielding lighter compounds with a lower liquid quantity. In addition, the quality of the oil seems to be improved by the increase of around 10 wt.% of both aromatic and light compounds from the expected value at the expense of the heavier fraction. The computed lower heating value (LHV) for gaseous and liquid products ranged between 40 MJ/kg and 51 MJ/kg. A quick glance at the possible end use of each oil was investigated along with a principal component analysis (PCA) on the pyrolytic oil.