Optimization of microwave power and graphite susceptor quantity for waste polypropylene microwave pyrolysis

Abstract

Microwave power and the susceptor quantity are important operating parameters in the microwave-assisted pyrolysis. This work aims at finding the optimal power and susceptor quantity from a discrete set of microwave powers (300 W, 450 W, and 600 W) and graphite susceptor quantity (50 g, 200 g, and 350 g) to pyrolyze waste polypropylene. The effect of microwave power and the susceptor quantity on heating rate, conversion efficiency, and heat losses in pyrolysis were analyzed using a central composite design (CCD). It was observed that a higher microwave power and lower susceptor quantity yielded higher heating rates, and lower heating rates were obtained for a lower microwave power and higher susceptor quantity. For example, a very low heating rate, averaging to 8.5 °C/min was obtained for a low microwave power (300 W) and a high susceptor quantity (350 g), whereas high values of heating rate (71.7 °C/min) were obtained for a microwave power of 600 W with 50 g of susceptor quantity. For a set of microwave powers considered in this study, it was observed that the required bed temperatures were acquired faster at a low susceptor quantity compared to high susceptor quantity. The oil produced from the pyrolysis of WPP has a high heating value of 44 MJ/kg and the carbon number distribution in the range of C8 to C12. The char produced has rich carbon content (96 wt.%) and a high specific surface area (195 m2/g). Cyclo alkenes with a selectivity of 70% and monocyclic hydrocarbons with a selectivity of 49.6 % were produced at 450 W and 600 W powers respectively for a susceptor quantity of 50 g. From this study, it is found that the microwave power and susceptor quantity are the important parameters for minimizing the microwave energy requirement in the pyrolysis. The low susceptor quantity and high microwave power were found to be the best for achieving fast heating rates and low energy requirements. The microwave power significantly playing a vital role in the polypropylene pyrolysis product spectrum for value-added hydrocarbons.