Development and Validation of a Computational Fluid Dynamics Model for the Optimization of a Sink-Float Separator for Plastics Recycling

Abstract

The Circular Economy Action Plan, in the context of the European Green deal, introduces new policies which promote a more sustainable use of plastics. These policies also prioritize the optimization of plastics recycling processes by increasing both the amount and quality of plastic recyclates produced, which is to a large extend defined by the purity and yield achieved by state-of-the-art plastic sorting processes. Therefore, the presented research focuses on developing a Computational Fluid Dynamics (CFD) based model to predict and obtain better insights in the separation performance of sink float separators, more specifically Dense Medium Drum (DMD) separators. The flow inside the drum is simulated by adopting an Euler-Lagrangian approach, a widely employed model for the numerical simulation of multi-phase flows including solid particles. To validate the simulations, a transparent lab-scale sink float separator, using the principle of a rotating drum, is constructed. In all experiments, water is used as medium. The separation objects represent a single polymer type which is fed to the drum. For the present work these are Polypropylene (PP), Polystyrene (PS), High Density Polyethylene (HDPE) Acrylonitrilic Butadiene Styrene (ABS), Polycarbonate Acrylonitrile Butadiene Styrene (PC/ABS) grains with a length scale of 3 mm and High Density Polyethylene grains of 4 mm. The separation number is easily expressed as the ratio of objects leaving the sink or float outlet depending on the type of the polymer density to the total amount of separation objects. It was found that the separation numbers computed by the numerical model for all separated objects agree with the measured ones within a maximum error of 7%. When comparing the hydrophilic ABS to the hydrophobic PS, which have similar densities, it is shown that air bubbles have a large influence on the separation efficiency. With the validated CFD model, essential insights are gained on the physics of the flow field inside the separator which help in further optimizing these devices to achieve higher separation efficiencies.