Investigation of a Sink-Float plastic separator through Computational Fluid Dynamics and Particle Image Velocimetry

The Circular Economy Action Plan aims to improve the quality standards of recycled plastics to enhance their competitiveness with virgin plastics and to support an increase in the current 9.5% share of recyclates in overall plastic production. This study addresses this goal by investigating the most widely used sink-float separation process in the plastics recycling industry using Computational Fluid Dynamics (CFD), Particle Image Velocimetry (PIV), and empirical experiments with plastic granules on a lab-scale separator. The CFD model used in this study is validated with PIV measurements, demonstrating the model's ability to predict the flow field of the separator with a maximum error of 5.3%. Moreover, the error of the CFD model in predicting the yield and purity of the separation is below 1%. This study quantifies the main cause of misplacement for the sinking fraction, which is the attachment of air bubbles to hydrophobic plastics, and suggests ways to address this issue based on experimental findings. The CFD model is used to assess the injection location for achieving the highest yield and purity for plastics of different densities, showing that plastics with a relative density difference greater than 0.04, compared to water, can be separated with a yield and purity of 99%. Finally, the CFD model developed in this study, along with the simulation strategy, is considered to encompass great potential for recycling companies to optimize the working parameters of their sink-float separators.