Thermal and catalytic degradation of high density polyethylene into useful fuels

Developing environmentally friendly methods for the recycling and disposal of waste plastic is important for the efficient use of resources and environmental sustainability. Of the different strategies for waste plastic recycling, chemical recycling has shown great potential as an environmentally friendly method of disposing of a wide variety of plastics. The advantage of chemical recycling is that useful chemicals and fuels can be produced from waste plastic streams without significant effort spent sorting and purifying the plastic waste stream. The aims of this thesis were to demonstrate the potential of reactive extrusion as a method of degrading plastic into useful fuels and to investigate the reaction kinetics of the degradation of high density polyethylene (HDPE) through the use of population balances. The thermal degradation of HDPE was first studied using thermogravimetric analysis to identify important kinetic mechanisms and reaction rates. A population balance model was proposed to describe how the molecular weight distribution changed throughout the thermal degradation. The proposed model described the data well for all nonisothermal experiments however the model was less accurate at the slow reaction rates of the isothermal experiments. The thermal degradation of HDPE was then conducted in a reactive extruder for a variety of conditions. The important operating variables and behaviour of the reactive extruder were characterised, with the molecular weight distributions of the reactive extruder reaction products determined using gel permeation chromatography. A new population balance model was developed to describe the thermal degradation in the reactive extruder by incorporating two more model parameters, a parameter for the rate of scissions dependence on molecular size and a breakage kernel parameter that allowed departure from random breakage. The model parameters were estimated from the molecular weight data of the reactor products. The new model predicted the reactive extruder data well for all conditions, particularly so for the long residence time (slow screw speed) experiments. The new population balance model also performed better than the previous model when applied to the data obtained from thermogravimetric analysis. Finally the catalytic degradation of HDPE was conducted in the reactive extruder using two weight percent silica-alumina catalyst. All the catalytic degradation experiments produced a high yield of liquid with a liquid weight fraction ranging from 74 to 84 weight percent. The liquid produced was found to have a composition that was comparable to gasoline. The influence of the screw speed on the total mass flowrate of the reactive extruder was significantly less in the catalytic experiments compared to the thermal experiments. This was attributed to the decrease in effectiveness of screw to convey gaseous reaction products. A population balance model for catalytic degradation described the product distribution obtained from reactive extruder well. In conclusion this thesis has demonstrated the potential for developing a process to recycle waste plastic into useful fuels utilising a reactive extruder. Additionally population balances were successfully used to describe the molecular size distribution of HDPE undergoing both thermal and catalytic degradation.