Kinetics, Thermodynamics and Mechanism of Thermal and Thermo-catalytic Pyrolysis of Waste Polystyrene into Valuable Products

Abstract

Due to huge increase in polymer production, a tremendous increase in municipal solid waste is observed. Every year the existing landfills for disposal of waste polymers decrease and the effective recycling techniques for waste polymers getting more and more important. In the present study pyrolysis of waste polystyrene was performed in the presence and absence of catalyst using thermogravimetry and pyrolysis gas chromatography/mass spectrometry (GC/MS). Firstly, the pyrolysis of waste polystyrene (PS) was carried out without catalyst at heating rates of 5, 10, 15 and 20 °Cmin-1 and temperature range 40-600 °C under nitrogen atmosphere. The apparent activation energy (Ea) and pre-exponential factor (A) were determined using Kissinger-Akahira-Sunose (KAS), Ozawa Flynn Wall (OFW), Friedman (FM), Coats Redfern (CR) and Kissinger kinetic models. Activation energy and pre-exponential factor were found in the range of 105-202 kJmol-1 and 3.5×106 to 7.6×1014 min-1 respectively. The results demonstrated that the calculated values of Ea and A vary with fraction conversion, heating rates and the applied model. Moreover, pyrolysis of waste polystyrene was carried out in an indigenously manufactured furnace at temperatures ranging from 340 to 420 °C. Temperature and reaction time were optimized, and the results revealed that temperature of 410 °C and exposure time of 70 min are the best conditions for maximum oil yield. Methane and ethane were found as the main products in the gas phase constituting about 82% of the gaseous fraction. The liquid products composed of broad range of C2 – C15 hydrocarbons depending on the pyrolytic parameters. In the second step CuO catalyst was used for degradation of PS. The activation energy calculated applying CR, OFW, KAS and FM models were found in the range 105-148.48, 99.41-140.52, 103.67-149.15and 99.93- 141.25 kJmol-1 respectively. The pyrolysis products were collected and characterized using GC/MS and FTIR. The major components of the fuel oil were observed as styrene xvii monomers, dimers, trimers with some other compounds such as 𝛽-ketopropane, iso propenyl benzene and 1,3- diphenyl propane. From the results it has been concluded that CuO catalyst increased the liquid yield of pyrolysis process along with reduction in maximum degradation temperature. Thirdly, pyrolysis of waste polystyrene was investigated in the presence of mixed metal oxide (Nix=0.5Cu1-xO) under inert condition in pyrolysis chamber. Moreover, kinetic parameters were determined from thermogravimetric data and the activation energy and pre-exponential factor at maximum conversion were found to be in the range of 120 to134 kJmol-1 and 1.5x109 to 2.1x1010 min-1 respectively using KAS, OFW, FM and CR models. The results demonstrated that KAS method is the most suitable model for determination of kinetic parameters. Thermodynamic parameters such as enthalpy, entropy and Gibbs free energy were investigated for thermo-catalytic decomposition and found in the range of 88.79 to 113.98 kJ/mol, -137.12 - -36.96 J/mol and 125.29 - 137.96 kJ/mol respectively. The gaseous and liquid fraction obtained from waste PS were analyzed by GC/MS. Major components of liquid oil collected at optimum conditions were ethyl benzene, 𝛼 - methyl styrene and 1,1- diphenyl ethylene while the gaseous fraction was mainly composed of methane. By comparing the properties of liquid fraction with standard properties of gasoline, diesel and kerosene, it was found out that most of the properties are closely related with diesel oil. Use of Cox=0.5Cu1-xO catalyst not only decreased the activation energy, but it also increased the yield of oil from waste polystyrene. The study suggested that pyrolysis oil obtained from polystyrene waste holds great promise for replacing commercial oil. By comparing the pyrolysis products and kinetic parameters of catalytic degradation with those of non-catalytic degradation, a predictable increase in pyrolysis products and decrease in activation energy at maximum conversion were observed.