Thermochemical Conversion of Polystyrene

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In Europe, 20.7 Mt of Polystyrene (PS) was produced in the year 2021, mostly consisting of packaging, insulation, and food utensils. Like many other kinds of plastics, they take hundreds of years to naturally degrade in the environment. To prevent the buildup of plastics in our environment, recycling technology will have to be employed. Current recycling technologies can be divided into three categories: organic, mechanical and chemical recycling. Organic recycling involves the use of biological organisms or enzymes to breakdown plastic into useful materials. Mechanical recycling technologies include processes such as dissolution and conventional mechanical recycling. Among chemical recycling, baseline technology includes
pyrolysis that involves higher temperatures (>500°C) which produces char, gasses, and complex oils. Hydrothermal Liquefaction (HTL) of PS has shown promising results in tackling the problem plastic waste buildup in the environment. This process utilizes water as solvent and subjecting it to mild temperatures and pressures to produce less complex oils that contain building blocks such as monomeric compounds and high value chemicals (HVC). These compounds can then be used as platform chemicals or as source to manufacture new materials, as well as closing the loop on PS waste.
This thesis focuses on the use of catalysis to increase the selectivity for PS conversion through HTL to increase the yields of monomeric compounds and high-value chemicals (HVCs) at lower temperatures (<370°C) than current approaches. In this work, different catalysts were screened and the best performing catalyst was used to optimize the HTL process. The HTL process was optimized using a design-ofexperience (DOE) approach for the following process conditions: temperature (330-350°C), catalyst loading (0-15%) and reaction time (30-60 minutes). The analytical techniques used to characterize the quality of oil as well as determine the amount of chemicals present are: Bomb Calorimetry, Ultimate analysis (CHN-analysis), Gas Chromatography-Mass Spectrometry with Flame Ionization Detection (GCMS-FID)
and Two-Dimensional Gas Chromatography with Flame Ionization Detection (GCxGC-FID).
Results from a screening campaign indicated high yields of oil (90 wt%). Following this, a simple distillation was conducted to separate the lighter fraction from the heavier ones within 90 wt%. Ultimate analysis of the compounds showed high C,H,and N ratios similar to what was found in literature. Bomb calorimetry of the PS-Crude oil product showed of 40 MJ/kg, indicating that oil product is comparable to what is found in literature. GCMS-FID indicated that 17 wt% of styrene was produced from this process,
along with 5.5 wt% of alpha-methyl styrene and other HVCs. The rest of the oil consists of heavy fractions (C-20) which could be go through another upgrading process.
After the screening campaign MgO was chosen as a catalyst to increase the yield of styrene. The yield of oil remain high with 80-90 wt% yield of oil in most cases. Aqueous phase and gas yields remain low (<5 wt%), while char can vary but also remains quite low (<10 wt%). Analysis of Bomb calorimetry showed similar results to screening with an average of 38 MJ/kg. CHNO also showed similar ratios of C,H, and N ratios. When looking at the GCMS-FID and GCxGC-FID, the amount of styrene remained at a maximum of 16-17 wt% with no significant increase. However, results of the research also showed that catalyst does have effect in increasing yield of styrene. Optimization for oil was conducted and optimum point for oil production is at 340°C, 34 minutes and 8% catalyst loading. The research will contribute to the growing body of research into processes for plastic waste valorization and offers more insight into catalytic
hydrothermal liquefaction as well as experimental methodology to separate the oils into its components.