Chemical Recycling of Plastics by Dissolution

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Senior Design Reports (CBE)
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Biochemical and Biomolecular Engineering
Chemical Engineering
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There is significant upside potential for the recycling of plastic wastes, especially for waste polyethylene plastic bags. Plastic containers and packaging have the most tonnage of waste plastics sent to landfills, and there are limited number of communities collecting single-use polyethylene plastics bags for recycling. This project explores the dissolution-precipitation technique as a viable plastics recycling method. Beginning with a feed assumed to be primarily low density polyethylene (LDPE), plastic bag waste is cleaned and 50 tons per day of this cleaned LDPE waste enters the main process. The proposed design includes a dissolution and precipitation vessel. The design is modeled to induce polymer dissolution with a toluene solvent and polymer precipitation in granule form with an isopropanol nonsolvent. The slurry from the precipitation is filtered and dried, and the recycled LDPE is recovered with a 99.99% purity. The solvent and nonsolvent are separated using distillation, and the toluene and isopropanol are recycled to minimize raw material operating costs. Pumps, heat exchangers, purge streams and surge tanks are additionally included in the design. Heat and energy are optimized to minimize utility costs. A $14.9MM total capital investment is required to yield an IRR of 21.97% and an NPV of $5,896,300 over a proposed twenty-year lifetime. Based on this base case, this proposal is a recommended investment, but investors should be cautious when revisiting the economics as the process depends on the assumed recycled LDPE selling prices, high utility requirements, and waste trash bag acquisition costs. Future investigation and experiments should look to solidify the ternary phase diagram between the three components and research should be dedicated to producing advanced filtration techniques to remove pigment and dye contamination.

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