Cracking of low density polyethylene dissolved in benzene to liquid fuels using zeolite-based catalysts

Abstract

Researchers proposed numerous solutions to plastic pollution, with the hope to tackle the intractable problems brought by plastic especially to mankind and environment. One of the proposed methods of solving the problem is the conversion of plastic waste to chemicals and fuels through pyrolysis and cracking. However, previous studies focused on polymer cracking in a batch process, which resulted in the wide distribution of the products. Thus, there is a need to develop polymer cracking process in continuous mode and improve the product quality by using a suitable catalyst. The aim of this research is to investigate on catalytic cracking of low density polyethylene (LDPE) in a fixed bed reactor into liquid fuel. LDPE was dissolved in different solvents with similar solubility parameter and the most suitable solvent was selected. The catalytic cracking was then carried out on the LDPE solution using a fixed bed reactor at atmospheric pressure. Parent zeolites and nickel-impregnated zeolites were screened as catalysts for the cracking of LDPE. The change in product composition at different reaction conditions was also studied, and a plausible reaction mechanism was proposed. This was followed by parametric study of the process involving five factors, namely temperature (A), catalyst mass (B), feed flow rate (C), N2 flow rate (D), as well as concentration of LDPE solution (E), and the two responses were LDPE conversion (Y1) and liquid yield (Y2). Two level full factorial design was used to evaluate the factors. It was found that benzene is the most suitable solvent for LDPE dissolution. Catalytic cracking of the LDPE solution produced C1-C8 hydrocarbons in all runs. During the catalyst screening, zeolite Z2 (ZSM-5 zeolite, Si/Al: 1000) was found to be the most promising catalyst, as it was able to obtain high LDPE conversion (99.93%), high liquid yield (92.28%) and low coke formation (0.02%). The parametric analysis showed that four out of five factors (A, B, C and D) produced significant effects on Y1 and Y2. On the other hand, factor E was statistically insignificant on the responses. Analysis on products composition showed that cracking of LDPE over zeolite Z2 produced a high amount of aliphatic branched-chain compounds, together with the moderate amount of cyclic compounds (C7-C12). The reaction conditions also led to alkylation of benzene by the cracking products from LDPE. It is suggested that the catalytic cracking of LDPE is dominated by free radical mechanism, while the influence of carbenium ion mechanism is less pronounced due to low acidity of the catalyst. Hence, it is concluded that catalytic cracking of dissolved LDPE in fixed bed reactor with zeolite Z2 is able to convert LDPE into liquid fuel in gasoline range and has the potential to tackle the plastic pollution.