Date of Award

Spring 5-3-2024

Level of Access Assigned by Author

Open-Access Thesis

Degree Name

Master of Science (MS)

Department

Chemical Engineering

Advisor

Sampath Gunukula

Second Committee Member

M. Clayton Wheeler

Third Committee Member

Peter Van-Walsum

Abstract

Apart from being a global issue, Improper disposal of plastics has increased the plastic contamination of environmental and industrial waste and has greatly impacted the efficiency of converting waste to energy. The impact on efficiency is due to water contamination of the plastics and inadequate separation before conversion. More so, depolymerization of plastic-based waste is very challenging because disassociation of the stable carbon-carbon bonds is only possible at a very high reaction temperature Lignin is the second most abundant biopolymer in nature, which is currently underutilized. Therefore, it is desirable to find a sustainable solution for increasing the use of underutilized lignin and plastic beyond incineration for energy. Processes like pyrolysis and combustion are associated with energy disadvantages as there is a need to dry the feedstock before processing. Also, mechanical recycling cannot handle mixed plastics. However, these shortcomings can be overcome with Hydrothermal liquefaction technology (HTL) due to its ability to upgrade wet materials to value-added products like high-quality fuel. In this research, the effects of HTL process conditions (like temperature and reaction time) and molecular weight on the quality and quantity of liquefaction and co-liquefaction products of plastics and lignin are delineated using the Elemental, GC-MS, and the TGA analysis. This research also focuses on the synergistic effect of co-liquefaction of plastics and lignin. We found that for HTL of Polyethylene (PE), an increase in temperature (from 300 ̊ C to 400 ̊ C) led to an increase in oil yield. However, for the HTL of Lignin (L) and the HTL of Polypropylene (PE), an increase in temperature beyond certain temperatures led to a decline in oil yield. Approximately 22% maximum oil yield was obtained from Lignin at 300 ̊ C – 30 minute reaction condition. For LMW PE and LMW PP, maximum yields were approximately 93% at 400°C for 30 minutes and around 88% at 375°C for 1 hour, respectively. For HMW PE and HMW PP, the maximum oil yield of 86% (375 ̊ C – 1 hour) and 83% (400 ̊ C – 1 hour) respectively were obtained. For HTL of both LMW and HMW PE, we observed, from the GC-MS, the presence oxygenated compounds whose yield decreased as the temperature increased. Meanwhile, these oxygenated compounds were only found in the HTL of HMW PP. The Elemental analysis of oil from HTL of PE confirmed that the increase in temperature indeed reduced the yield of the oxygenated aromatics. While the HHV values for the HTL of plastics were seen – at some temperatures and reaction times – to be close to those of conventional diesel and crude oil, the HHV of lignin HTL oil – at all observed temperatures and reaction times – much lower than those of conventional diesel and crude oil. We have also determined for co-liquefaction of lignin and plastics at some reaction conditions; some qualities of the co-liquefaction were improved when compared to those of HTL of Lignin. For HTL of lignin, the HHVs of the oil were between 20 – 35 MJ/Kg while those of L + LMW PE, L + LMW PP, L + LMW PP + LMW PE were between 15 – 47 MJ/Kg, 20 – 48 MJ/Kg, and 31 – 47 MJ/Kg respectively. For the combination of lignin and HMW plastics, the HHVs of oils from L + HMW PP and L + HMW PE + HMW PP, were 30 – 45 MJ/Kg and 32 – 47 MJ/Kg respectively. However, for L + HMW PE, none of the HHVs were close to those of conventional Diesel or crude oil. Also, from the GC-MS, we noticed more yield of the saturated hydrocarbons in the liquefaction oil than in the HTL oil of the individual plastics. Studying the synergistic effect of co-liquefaction, we hypothesized that the interactions of lignin and plastic intermediates at certain reaction conditions can increase the oil yield (synergistic effect). We provided possible explanations to explain these observations as well as other trends throughout this research.

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