Date of Award

2019

Document Type

Open Access Dissertation

Degree Name

Doctor of Philosophy in Chemical Engineering (PhD)

Administrative Home Department

Department of Chemical Engineering

Advisor 1

David R. Shonnard

Advisor 2

Julio C. Sacramento Rivero

Committee Member 1

Gerard T. Caneba

Committee Member 2

Joshua M. Pearce

Abstract

The increasing amount of plastic waste (PW) generation has become an important concern due to the leveled-off recycling rates. Therefore, governmental agencies around the world, including state governments in the United States, have proposed initiatives to minimize the amount of PW that is landfilled and encourage recycling or energy recovery. Circular economy is a strategy that attempts on reusing PW to produce new polymers while avoiding its disposal and the use of virgin material. Chemical recycling raises an interesting technology prospect due to the potential reduction of pollutant emissions and the establishment of a circular economy through the production of monomers and fuels.

This dissertation initially presents a resource assessment for available MSW in Mexico and concludes that when the organic and polyolefin plastic components are converted to liquid hydrocarbon transportation biofuels through a pyrolysis-based pathway, up to 7% of Mexico’s transportation-fuel consumption could be met. A preliminary carbon footprint analysis (CFA) shows that liquid transportation biofuels from the organic portion of MSW (paper, packaging, wood, yard trimmings) sequesters 9.5 g CO2 eq. per MJ biofuel, with significant pathway credits due to avoiding landfill CH4 emissions. The greenhouse gas (GHG) emissions from the conversion of the polyolefin plastic in the MSW are positive (88 g CO2 eq. per MJ), though still lower than current fossil transportation fuels in Mexico (95.5 g CO2 eq. per MJ).

In this Ph.D. research, pyrolysis vapors from waste high density polyethylene (HDPE) were subjected to secondary degradation by varying the temperature and vapor residence time (VRT) in the reaction zone of a newly-designed two-stage micropyrolysis reactor (TSMR). Temperature and VRT variations showed a strong effect on the product distribution, with low temperature (625 ºC) and short VRT (1.4 s) producing a wide range of gases and liquid products and with high temperature (675 ºC) and long VRT (5.6 s) producing mostly hydrocarbon gases (monomers) and mono- and poly-aromatics.

The last two chapters of the dissertation present a novel multiproduct/multiprocessing pyrolysis-based refinery design for the conversion of 500 tonnes/day of high-density polyethylene (HDPE) waste. The products obtained from the refinery are chemical grade ethylene and propylene, an aromatics mixture, and low- and high-molecular weight hydrocarbon mixtures (MWHCs). The energy efficiency was 72 and 77% for the base case and the heat integrated (HI) refinery, respectively. The net present values (NPVs) were 367 and 383 million U.S. dollars (MM USD), for the base case and the heat integrated process, respectively. The CFA results show that the GHG emissions of all products; ethylene, propylene, aromatics mixture, low molecular weight (MW) hydrocarbons (HCs), and high MW HCs, are equal to or less than fossil products for the HI scenario assuming US average electricity grid. Finally, the evaluation of regional electricity grids on GHG emissions for all products was conducted for all 50 states in the US. These results suggest energetic, economic, and environmental sustainability of the design and its promising application on an industrial scale.

This dissertation ends with overall conclusions and recommendations for future research.

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