Conversion of Polyolefin Waste into Useful Products Via Hydrothermal Processing (HTP) and Sequential Selective Extraction and Adsorption (SSEA)
Abstract
There has been an exponential increase of plastics being produced, used, and disposed of over the last 60 years. Most of plastic waste (76%) is consigned to landfills, 12% is incinerated, 3% ends up in the oceans while only 9% is recycled. If the current trend continues, more than 30 billion tons plastic waste will be generated on this planet and more plastic than fish will be in the oceans by 2050. Plastics take more than 100 years to degrade into plastic debris, microplastics, and toxic chemicals, which pose serious threats to the ecosystems, water and food supply, and eventually human health. Polyolefins (Type 2 HDPE, Type 4 LDPE, and Type 5 PP), which are the majority (63%) of global plastic waste, are targeted in this study. Two methods, Hydrothermal Processing (HTP) and Sequential Selective Extraction and Adsorption (SSEA) were developed and tested. HDPE was converted into wax and oils using supercritical water under HTP at reaction temperature 400- 450 °C with reaction time 0.5- 3 hr. PP was converted into oils under supercritical water at 425 °C with reaction time 1- 3 hr. The oil products from HDPE and PP via HTP were composed of paraffins, olefins, cyclics, and aromatics with carbon numbers from C6 to C31. Reaction intermediates were analyzed using two-dimensional gas chromatography with a flame ionization detector (GC × GC-FID). The results were used to establish potential reaction pathways of HDPE and PP, respectively. PP was found to react faster than HDPE under the same HTP conditions while generating more olefins and cyclics than HDPE. Clean wax was obtained from PE waste via HTP. Its Fourier-Transform Infrared Spectroscopy (FT-IR) spectrum was almost identical to the one of commercial paraffin wax. Oils converted from PE waste via HTP was distilled into three fractions. The diesel-like fraction (170 °C≤ b.p. < 300 °C) has similar properties as No.1 ultra-low-sulfur diesel. It also can be used as a blendstock for No2. Ultra-low-sulfur diesel. SSEA methods were developed to recover pristine polymers from polyolefin waste via extraction and adsorption. Mixed solvents with higher selectivity, reduced toxicity, and lower costs were found based on their Hansen Solubility Parameters. Extraction conditions were investigated using model polyolefins. Selective mixed solvents were found for the separation of LLDPE from LLDPE/PP blend. Pristine PE polymers were recovered from dark green PE waste pellets. Preliminary analyses indicated HTP and SSEA have a higher energy efficiency and lower greenhouse gas (GHG) emissions than incineration, mechanical recycling, and pyrolysis. A combination of these two methods has the potential to convert 63% of the plastic waste into useful and profitable products. It also helps transform current linear path from crude oil to plastics to landfills, to a more sustainable circular path.
Degree
M.Sc.
Advisors
Wang, Purdue University.
Subject Area
Atmospheric sciences|Polymer chemistry|Sustainability
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