Catalytic pyrolysis of LDPE leads to valuable resource recovery and reduction of waste problems

Abstract

Recycling of waste polymers has become a necessity because huge piles of those polymers represent a threat to the environment. Used polymers are also a source of energy and valuable chemicals. Used low density polyethylenes (LDPE) were catalytically pyrolysed in a home assembled batch reactor under atmospheric pressure. For maximum conversion into chemicals which could be used for feedstock recovery optimum conditions like temperature, catalyst weight and reaction time were optimized. A wide range of acidic and basic catalysts like silica, calcium carbide, alumina, magnesium oxide, zinc oxide and homogeneous mixture of silica and alumina were tried for this purpose. Though CaC 2 was better on the basis of reaction time, however the efficiency of conversion into liquid for SiO 2 was found to be maximum at optimum conditions. These two catalysts could be picked up as suitable catalysts for catalytic pyrolysis of polyethylene. The results of the column separation using different solvents indicate that the oxide containing catalyst could be best suited for selective conversion into polar and aromatic products while CaC 2 catalyst could be adopted for selective conversion into aliphatic products. The liquid product obtained from catalytic pyrolysis was also characterized by physical and chemical tests. Among the physical tests density, specific gravity, API gravity, viscosity, kinematic viscosity, aniline point, flash point, Watson characterization constant, freezing point, diesel index, refractive index, gross calorific value, Net calorific value and ASTM Distillation were determined according to IP and ASTM standard methods for fuel values. From the physical tests it was observed that the results for the liquid fractions are comparable with the standard results of physical tests for gasoline, kerosene and diesel fuel oil. From the Bromine water and KMnO 4 tests it was observed that liquid obtained is a mixture of olefin and aromatic hydrocarbons. This was further confirmed by Bromine number tests. The values of which lie in the range of 0.1–12.8 g/ml, which fall in the range for olefin mixture. Phenol and carbonyl contents were quantified using UV/Visible spectroscopy and the values lie in the range of 1–8920 μg/ml and 5–169 μg/ml for both phenols and carbonyls respectively. The components of different hydrocarbons in the oil mixture were separated by using column chromatography and fractional distillation followed by characterization with FT-IR spectroscopy. The interpretation of FT-IR spectra shows that catalytic pyrolysis of LDPE leads to the formation of a complex mixture of alkanes, alkenes, carbonyl group containing compounds like aldehydes, ketones, aromatic compounds and substituted aromatic compounds like phenols. It could be concluded, that catalytic pyrolysis of LDPE leads to valuable resource recovery and reduction of waste problem.