PYROLYSIS AND PYROLYSIS-CATALYSIS OF WASTE PLASTICS AND WASTE TIRES TO PRODUCE HIGH VALUE FUELS AND CHEMICALS

Abstract

This research focuses on the pyrolysis of waste plastic, waste tire, and 1:1 mixtures of tire and plastics using a fixed-bed reactor with the aim of determining the influence of co-pyrolysis on the yield and composition of the product oils and gases. The liquid oil produced from the pyrolysis of tires and polystyrene (PS) was mostly composed of aromatic compounds, such as BTEX, refers to benzene, toluene, ethylbenzene and xylenes; and PAHs refers to for example, naphthalene, anthracene, phenanthrene, pyrene and fluorene and their derivatives. The liquid oil formed from the pyrolysis of high density polyethylene (HDPE) and low density polyethylene (LDPE) was mostly waxes of high molecular weight consisting of aliphatic compounds, composed of a series of alkanes, alkenes, and alkadienes. Polypropylene (PP) produced more alicyclic compounds, such as methyl-cyclohexane. Polyethylene terephthalate (PET) pyrolysis oil consisted of compounds such as xylene and styrene but was mainly composed of oxygenated compounds, such as benzoic acid. Co-pyrolysis of 1:1 mixtures of tires and individual plastics involved interaction, resulting in significantly higher yields of gas than expected for all the plastic mixtures. The oil produced from the co-pyrolysis of the tire with polyalkene plastics showed interaction between the polymer pyrolysis products, resulting in higher yields of aliphatic compounds and lower yields for BTEX, PAHs, alicyclic, and aromatic compounds.

The study also investigated the pyrolysis-catalysis of waste plastic, waste tire, and a 1:1 mixture of the two materials using a two-stage fixed-bed reactor. ZSM-5 is used as a catalyst to investigate the influence on product distribution and composition of gases and oils. The results showed that pyrolysis-catalysis of tire over ZSM-5 reduced oil yield from 54.83 wt.% in thermal pyrolysis to 37.30 wt.% in pyrolysis-catalysis, with an increase in gas yield from 8.69 wt.% in thermal pyrolysis to 24.65 wt.% in pyrolysis-catalysis. Pyrolysis-catalysis of HDPE, LDPE, PP, and PS, also showed increased gas yields, mainly consisting of hydrocarbons (C1-C4), while decreasing oil production. PS produced the lowest gas yield, while PET produced the highest, with CO and CO2 being the main gases generated. The pyrolysis of individual tire and plastic over ZSM-5 produced valuable chemicals such as benzene, toluene, ethyl benzene, xylenes, and styrene. The efficiency of ZSM-5 to minimize the heavy aliphatic fraction was high, on the other hand, the ZSM-5 with low deactivation by coke deposition and a high selectivity to produce light olefins.

The co-pyrolysis-catalysis of the tire and plastics with the ZSM-5 catalyst showed interaction between the tire and plastics which changed the composition of the product oils and gases over what would be expected from mere addition. Plastics co-pyrolysed with tire and the presence of ZSM-5 catalyst promoted the high production of the aromatic content, particularly BTEX compounds at the expense of aliphatic content. Tire/HDPE, Tire/LDPE, and Tire/PP mixtures showed a reduction in aliphatic and heavy molecular weight compounds when co-pyrolysed with tires, and the presence of a ZSM-5 catalyst increased the production of aromatic content, particularly BTEX compounds. Tire/PS mixture produced aromatic compounds, and the addition of the ZSM-5 catalyst increased aromatics at the expense of styrene. Tire/PET mixture produced BTEX and aromatic (refers to benzene derivatives, biphenyls, limonene, terphenyl, and indene compounds) due to the selectively of ZSM-5 that promotes the decarboxylation of the oxygenated compounds, and it gave high gas yields of mainly CO and hydrocarbons.

The pyrolysis-catalysis of waste plastic, waste tire, and 1:1 mixtures of tire and plastics was conducted over the MCM-41/ZSM-5 catalyst layer to determine the influence of two catalysts in series on oil and gas composition. The MCM-41 with a higher pore size was followed by ZSM-5 with a smaller pore size. The pyrolysiscatalysis process optimizes the conversion of plastic pyrolysis products to low molecular weight hydrocarbon fuels and chemicals. The production of gases during catalytic pyrolysis increases at the expense of oil yields, with oil yields of 39.50, 57.25, 53.50, 58.50, 77.00, and 36.75 wt.% for Tire, HDPE, LDPE, PP, PS, and PET, respectively. The highest hydrocarbon gases were produced from the tire and plastics while PET produced the highest gas yield with more CO and CO2. Applying the two stages MCM-41/ZSM-5 has the advantage in the cracking of HDPE, LDPE, and PP. The primary products can pass through the large pores of MCM-41, then followed by ZSM-5 with (strong acidic sites and small pores) that promote the formation of light aromatic hydrocarbons and increased the production of BTEX.

The co-pyrolysis of tire/plastic mixtures over MCM-41/ZSM-5 catalyst in series showed an increase in gas yields compared to the individual tire and plastics results. The major influence of the co-pyrolysis was the reduction of paraffin and olefin compounds. According to literature, the high cracking of long chain polyolefins and the interaction between hydrocarbon radicals produced from tire rubber and the radicals produced from the thermal decomposition of PE and PP caused the reduction of both aliphatic and alicyclic contents in the mixtures of Tire/HDPE, Tire/LDPE, and Tire/PP. Tire/PET showed negative values of interaction for the oxygenated compounds.