Liquid fuel production from catalytic pyrolysis of municipal plastic waste using synthesized Zeolite from Kaolin

Abstract

Municipal Plastic wastes are potential sources of alternative energy owing to their longchain hydrocarbon with high heating values. Plastic waste (PW) is the major constituent of municipal solid waste (MSW) and it is becoming one of the largest MSWs in developing countries. The accumulation of plastic wastes over a length of time in conjunction with the improper and conventional waste management strategies has led to major health and environmental hazards such as greenhouse gas emissions, groundwater pollution, and several other human health and aquatic inhabitant problems. To address the environmental problem associated with municipal plastic waste, it is necessary to explore the catalytic pyrolysis recycling method of plastic waste which is a promising method of Municipal plastic waste management. In this research four major used Municipal Plastic Wastes (MPW) namely Polystyrene (PS), Polypropylene (PP), Polyethylene (PE), and polyethylene terephthalate (PET) were investigated for the liquid-oil production individually and at mixed ratios. Three different samples of kaolin (G1, G3, and G10) obtained from Grahamstown, South Africa were used as the raw materials in the synthesis of ZSM-5 zeolite used as the catalyst. In the preparation of the kaolin-based ZSM-5, the required amount of G&W metakaolin and sodium hydroxide were dissolved in deionized (DI) water, and tetrapropylammonium bromide (TPABr) were also mixed separately with the required amount of DI water. The solution of NaOH/Kaolin and sodium silicate solution were added simultaneously to the solution of the TPABr while stirring. Nitric acid was used to control the pH until the solution mixture is homogenous. The synthesized gel was transferred to stainless steel Teflon-lined autoclave cup and was hydrothermally treated at 180 ℃ for two days. The resulting product was washed with DI until the pH is less than 8. The sample was dried overnight at 80 ℃ and calcined for 5 hours at 550 ℃. The resulting synthesized zeolites (G1/ZSM-5, G3/ZSM-5, and G10/ZSM-5) were then characterized using Fourier transforms infrared (FT-IR), X-ray diffraction (XRD), and scanning electron microscopy (SEM). The catalysts produced were applied in the production of liquid fuel from Municipal waste plastics such as Polystyrene (PS), Polypropylene (PP), Polyethylene (PE), and polyethylene terephthalate (PET) under an optimized catalytic pyrolysis reaction process. The operating parameters considered were catalyst loading, reaction time, and the temperature was investigated and optimized using response surface methodology (RSM) to obtain the best operating condition for the maximum yields. The optimized conditions established from the liquid fuels produced were used as a standard for the catalytic pyrolysis process condition for the single and mixed ratios. The catalytic pyrolysis of mixed plastic wastes in different ratios was conducted with the synthesized G1/ZSM-5, G3/ZSM-5, and G10/ZSM-5 zeolite catalysts separately. All the mixtures of PP and PE produced higher liquid oil yields than the single PP or PE feedstock. Also, the highest liquid oil yield was obtained from PS/PE/PP sample with G10/ZSM-5 zeolite, and the lowest yield was from PP/PE sample with G1/ZSM-5 zeolite catalysts. The highest gases and char yields were from PP/PE and PS/PE with G1/ZSM5 zeolite catalysts. The quality, quantity, and chemical composition of the products were analyzed. The liquid oils, produced from the selected types of plastic wastes using synthesized and commercial catalysts, mainly consisted of aromatic hydrocarbons such as styrene, ethylbenzene, benzene, azulene, naphthalene, and toluene with a few aliphatic hydrocarbon compounds as confirmed by GC–MS and FT-IR analysis. The analysis showed that the liquid oils produced had high HHV (30.6–45 MJ/kg), similar to conventional diesel. The physicochemical properties of the oil produced were also compared with South African (SANS) and International standards (ASTM). The synthesis of ZSM-5 zeolite was successfully carried out from locally sourced kaolin. The characterization results revealed that the patterns of G3/ZSM-5 and G10/ZSM-5 exhibit sharp reflections (2θ 7.8, 8.8, 23.1, 23.3, 23.7, and 24.3o ) with high intensity, which shows that the synthesized zeolite are solid crystals owing to their high Si/Al ratio. These catalysts were found to be effective and active in the oil conversion of both single and mixed feedstock ratios. The process of mixing the plastic wastes was found to be a very effective approach in the catalytic pyrolysis production process as it eliminates the need for sorting these wastes. Optimizing the process also helps in establishing operating parameters that produce optimum yield. The hydrocarbon properties obtained were within the international and South African standard specifications.