Adsorption Column Design for Solvent-Based Plastic Recycling

Multilayer plastic films used in food packaging are designed with several layers of polymers that improve the product shelf life, transportation, and aesthetic appeal. The presence of many polymers in multilayer plastics makes it impossible to mechanically recycle them, so they are ultimately either landfilled or burned. A potential solution to this recycling challenge is the solvent targeted recovery and precipitation (STRAP) process. In this process, each polymer layer is chemically dissolved within a selective solvent and recovered by precipitation (Fig. 1). The resins produced from STRAP can then be reused in the production of plastics.

The Chemical Upcycling of Waste Plastics center is currently developing STRAP technology to process municipal solid waste, producing plastic resins from the multilayer plastics and ethanol from the biogenic material. However, municipal solid waste contains impurities and additives like minerals and dyes that need to be separated from the solvent to improve the quality of the resin products. It may be possible to implement adsorption to purify the solvent for reuse. Adsorption is a purification process where solute particles adhere to an adsorbent through van der Waals forces (Fig. 2). An adsorption column, packed with activated carbon adsorbent, can thus remove impurities from the solvent.

The goal of this work was to investigate the potential economic and environmental impact of an adsorption process to purify the solvent. To this end, we developed an adsorption column model in BioSTEAM, an open-source software in Python for the design, simulation, and analysis of chemical production processes, and implemented the adsorption column in a STRAP model for processing municipal solid waste. We then conducted uncertainty and sensitivity analyses to determine the factors driving the economic and environmental impact (Fig. 3). The adsorption column was modeled based on first principles of mass transfer and adsorption equilibria and kinetics. Key parameters in the design of the adsorption column include the mass-transfer transfer coefficient, equilibrium constant, cycle time, solution flow rate, and solute concentration. Adsorption experiments were conducted to inform the adsorption equilibrium and mass transfer parameters.

A preliminary techno-economic analysis and life cycle assessment suggest a minimum selling price of $1.39/kg-resin and global warming potential of 38 g-eq-CO2/L-ethanol (Fig. 3). Additionally, our sensitivity analysis showed that the hydrolysis yield and processing capacity were the most impactful in the system while adsorption column parameters had less bearing. These results suggest that adsorption may be a cost-effective method to purify the solvent in the STRAP process.

References:

Azizian, S., & Setareh, E. (2021). Adsorption isotherms and kinetics. In M. Ghaedi (Ed.), Adsorption: Fundamental Processes and Applications (pp. 445-509). Elsevier. https://doi.org/10.1016/B978-0-12-818805-7.00011-4

Gabelman, Alan. (2017). Adsorption Basics: Part 1. CEP, July 2017, pp. 48-53. https://www.aiche.org/resources/publications/cep/2017/july/adsorption-ba...

Seader, J. D., Henley, E. J., & Roper, D. K. (2016). Separation process principles: With applications using process simulators (4th ed.). Wiley.