(354i) Plant-Wide Modelling of Microwave-Assisted Plastics Upcycling Process

Large amount of plastics are getting accumulated in the waste handling facilities¹. Among all types of plastics that are widely used, the low-density polyethylene is one of the difficult ones to convert to value added products or to recycle. Some of the leading methods that are being investigated for handling low-density polyethylene are thermal and catalytic pyrolysis, and gasification among others. However, these processes are energy-intensive and produces large number of products that are difficult to separate without incurring excessive cost. In additions, much of the products are low-value products with small yield of ethylene monomer. To address these issues, microwave (MW)-based technologies are being developed for conversion of low-density polyethylene to ethylene monomers and other monomers that can replace and supplement a large portion of the monomers being produced today from petroleum-derived feedstocks. Our in-house MW-assisted reactor yields high energy efficiency, achieved due to the low reaction severity, high conversion, achieved due to thermal and non-thermal effects of the MW reactors leading to high carbon recovery and utilization, and yield of value-added products, achieved by highly selective MW-enhanced catalytic depolymerization and aromatization process.

In this work, a kinetic model of the microwave-assisted, catalytic degradation of low-density polyethylene is developed². The kinetic model yields products that are observed in an inhouse batch reactor. The reactor is ramped to the desired temperature following a temperature profile. Time-varying product composition from the lab reactor is analyzed. The laboratory data can fail to satisfy the mass balances and in particular for this process, can have error for carbon and hydrogen balances. If the raw data from the reactor are sued without satisfying mass balances, it can lead to incorrect estimate of the rate parameters in the reaction kinetic model. Therefore, dynamic data reconciliation is undertaken for exactly satisfying overall mass and atom balances in the experimental data. A kinetic mechanism is proposed based on the observation of product formation profile from the laboratory reactor. The reconciled dynamic data are utilized for optimal estimation of rate parameters by solving a nonlinear programming problem. The reactor model is scaled up to the commercial scale. A plant-wide model is developed for separation producing monomer-grade ethylene. The separation process has considerable differences in the operating conditions and sequence of distillation towers compared to the conventional ethylene production process using ethane crackers. Furthermore, no quench is necessary like the conventional process due to low temperature of the MW-assisted reactor. A cascaded refrigeration system is modeled for the separations section, first cooling with propylene refrigerant followed by ethylene refrigerant is considered. Following a sequence of towers, 99.2% pure ethylene that satisfies the monomer grade purity is obtained from the top of C2 splitter. Overall mass yield of ethylene per unit mass of recycled plastic is found to be more than 46%. A number of sensitivity studies are done by considering the time-varying product yield from the MW reactor and the operating conditions and sequence of the distillation towers.

References

1. https://www.epa.gov/facts-and-figures-about-materials-waste-and-recyclin...
2. Chandrasekaran, B.Kunwar, B. Moser, N.Rajagopalan, and B.Sharma, “Catalytic Thermal Cracking of Postconsumer Waste Plastics to Fuels. 1. Kinetics and Optimization”, ACS Energy Fuels 2015,29,6068-6077, doi: 10.1021/acs.energyfuels.5b01083.