Abstract
Pyrolysis-catalysis of high density polyethylene (HPDE) was carried out in a fixed bed, two stage reactor for the production of upgraded aromatic pyrolysis oils. The catalysts investigated were Y-zeolite impregnated with transition metal promoters with 1 wt% and 5 wt% metal loading of Ni, Fe, Mo, Ga, Ru and Co to determine the influence on aromatic fuel composition. Pyrolysis of the HDPE took place at 600 °C in the first stage of the reactor system and the evolved pyrolysis gases were passed to the second stage catalytic reactor, which had been pre-heated to 600 °C. Loading of metals on the Y-zeolite catalyst led to a higher production of aromatic hydrocarbons in the product oil with greater concentration of single ring aromatic hydrocarbons produced. The single ring aromatic compounds consisted of mainly toluene, ethylbenzene and xylenes, while the 2-ring hydrocarbons were mainly naphthalene and their alkylated derivatives. There was a reduction in the production of multiple ring aromatic compounds such as, phenanthrene and pyrene. The addition of the promoter metals appeared to have only a small influence on aromatic oil content, but increased the hydrogen yield from the HDPE. However, there was significant carbon deposition on the catalysts in the range 14–22 wt% for the 1% metal-Y-zeolite catalysts and increased to 18–26 wt% for the 5 wt% metal-Y-zeolite catalysts.
Highlights
The plastics contained in municipal solid waste consist of about 50e70% of packaging materials comprising polyethylene (high and low density polyethylene (HDPE & LDPE)), polypropylene (PP), polyethylene terephthalate (PET), polystyrene (PS) and polyvinyl chloride (PVC)
The product yield include gases, liquid, and carbon deposited on the catalyst; no pyrolysis residue or char was produced in these experiments
Loading transition metals which included nickel, iron, molybdenum, gallium, ruthenium and cobalt on the Y-zeolite catalyst promoted the formation of aromatic hydrocarbons (80e95%) in the product oil and increased hydrogen gas production
Summary
The plastics contained in municipal solid waste consist of about 50e70% of packaging materials comprising polyethylene (high and low density polyethylene (HDPE & LDPE)), polypropylene (PP), polyethylene terephthalate (PET), polystyrene (PS) and polyvinyl chloride (PVC).Of the plastic waste generated, ~70% is ascribed to polyethylene [1]. Waste plastics recycling processes include mechanical/reprocessing of the waste plastic to form new products, this route tends to be based around the production of low quality goods. An alternative process, is thermal recycling via pyrolysis/gasification techniques which produce gases, liquid and solid products that can be used as fuels and chemicals [1e6]. Catalysts may be incorporated into the pyrolysis process to enhance the quality of the end-product liquid and gaseous fuels or the production of higher value chemicals [2,6]. Polyethylene, for the production of fuels and chemicals has been studied by several researchers [7e13]. Silica-alumina and zeolite catalysts have been used for catalytic degradation of polyolefins to produce lower molecular weight distribution fuels and chemicals such as benzene, toluene and xylenes [6e15]
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