Value-added products from waste materials
Date
2022-05
Authors
Chen, Xiaolin
Major Professor
Advisor
Bai, Xianglan
Tessonnier, Jean-Philippe
Mba-Wright, Mark
Wen, Zhiyou
Hu, Shan
Committee Member
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Abstract
Billion tons of municipal solid waste (MSW) are generated annually worldwide. MSW recycling is hindered by its complex composition, secondary pollution, and low-value recycled products. Mismanagement of MSW has caused serious environmental issues and tremendous energy losses. Therefore, an efficient and economic approach to recycling MSW is highly desired. In this Ph.D. study, novel recycling strategies were developed to maximize the products recovered from a variety of waste materials.
The first study explores the recycling strategy for post-consumer carton packages. Carton is a laminated composite with a multi-material composition, making recycling very challenging. This study developed a multi-step approach to upcycle low-cost carton waste into value-added products. Tetra Pak carton consisting of 80% paper, 17% polyethylene (PE), and 3% polyamide (PA) was collected from refrigerated milk products. In the first step, the carton was converted in tetrahydrofuran (THF) in a mild condition. The paper fraction was selectively decomposed to valuable bio-based chemicals; up to 19.2% of levoglucosenone and 8.6% of furfural were produced. The solids collected from the conversion consist of PE, PA, and paper-derived char. In the second step, the solid mixture was treated with a solvent-dissolution method using xylene and 1-propanol. PE was selectively recovered as the xylene-soluble, verified by its functional groups, the composition of the pyrolysis products, and the melt rheology results. In the last step, the xylene-insoluble fraction consisting of PA and paper-derived char was pyrolyzed; caprolactam was produced as the only major vapor product of selective PA depolymerization and thermally stable paper-derived char could be utilized as a high-quality solid fuel. Overall, the demonstrated recycling approach could potentially maximize the values of the products by taking advantage of each component of carton waste.
With the experience of recycling paper-polymer composite, the second study investigated the recycling of comingled waste materials with a more complex composition. Among different plastic wastes, polyvinyl chloride (PVC) is the most challenging to recycle due to the large portion of chlorine content. Hydrogen chloride (HCl) released by PVC decomposition at a relatively low temperature could easily react with other organics to produce hazardous organic chlorides detrimental to the natural environment and human health. Most recycling technologies treat PVC as a contamination source and try to eliminate it. This study developed a novel recycling approach to use PVC-released HCl as an asset. The mixture of cellulose, PVC, and another type of plastic (PE, PP, or PS) was converted in THF, where PVC-released HCl worked as an in-situ acid catalyst to promote cellulose conversion. Up to 38.2% levoglucosan (LGA) and its isomer, 1,6-anhydro-β-D-glucofuranose (AGF), were produced. Meanwhile, the hydrocarbons from dechlorinated PVC and another plastic and a minor amount of cellulose-derived char remained in the solid phase. The solid composition was confirmed by a comprehensive study of solid characterizations such as TGA, FTIR, SEM, Py-GC/MS, and Elemental Analysis. The optimum HHV of 46.34 MJ/kg was obtained in the PP case, higher than that of diesel (44.4 MJ/kg). The microstructure indicates the plastic-derived hydrocarbons and cellulose-derived char were chemically interweaved into a uniform solid with a three-dimensional porous structure, which could be potentially utilized as an oil-cleanup absorbent; the solid obtained in the PE case performed the best absorption capacity. PVC-released HCl significantly modified the microstructure of PE/PP/PS, and cellulose-derived char enhanced the surface reactivity of the solid. All chlorine of PVC was transferred to the liquid phase as chloride ion and 4-chloro-1-butanol (CB), which can be removed by adding NaOH. The chlorine balance analysis indicates adding PE/PP/PS could inhibit the secondary reaction of HCl and thus enhance the solution acidity for a more complete cellulose conversion; PS promoted the cellulose in the most significant way with the least amount of cellulose-derived char left in the solid.
In the third study, red oak and PVC were selected to simulate the composition of WPC since they are both common materials for industrial WPC production. Besides, due to the negative effect caused by PVC, there is a lack of efficient methods for wood-PVC composite recycling. In this study, an equivalent amount of red oak and PVC were co-converted in γ-valerolactone (GVL), a green biomass-derivable solvent that can dissolve lignin well. In the microstructure of wood fiber, lignin fills the space between cellulose and hemicellulose in the cell wall and adds compressive strength and stiffness to the structure. Therefore, the dissolution of lignin in GVL could help expose cellulose and hemicellulose to the conversion. In GVL, PVC-released HCl acted as an acid catalyst for promoting red oak conversion; up to 14.4% levoglucosenone and 14.3% furfural were produced by the selective decomposition of cellulose and hemicellulose. In comparison, lignin, as a highly branched phenolic polymer, has a much higher thermal stability and thus remained in the solid phase with dechlorinated PVC. Verified by FTIR, TGA, and pyrolysis results, dechlorinated PVC with unsaturated C=C bonds and free radicals were very active and reacted with lignin molecule or fragmented lignin, forming into a chlorine-free, condensed structure with high thermal stability. The optimum HHV of the solids was 36.2 MJ/kg, higher than that of anthracite coal. After the co-conversion in GVL, more than 80% of PVC-contained chlorine turned into chloride ions, and the rest formed 5-chlorovaleric acid (CA), a functional monomer with medical uses. Due to the insolubility in water, CA crystals were easily recovered from the post-reaction liquid with a purity of 91.2%.
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dissertation