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Improving wet plastic recycling through innovative lagrangian particle-fluid simulations

Periodic Reporting for period 1 - IMPRECSIM (Improving wet plastic recycling through innovative lagrangian particle-fluid simulations)

Reporting period: 2015-06-01 to 2016-11-30

With the beginning of the 20th century plastic has become a broadly used material and today modern life is unthinkable without it. Unfortunately its advantages such as its durability, light weight and low cost, also makes it problematic when it comes to its end of its life phase. Tens of millions of tons of plastic waste float in the world's oceans as broken debris. In addition, plastic is not inert, and chemical additives of which some are endocrine disrupting substances can migrate into body tissues or the food chain. The massive pollution of the oceans with plastic waste is therefore emerging as a global challenge that requires a global response. Currently a coherent strategy to optimize plastic waste policy is built up in the European Union. However, still nearly 50% of plastic waste in the EU is landfilled.

Mechanical plastic recycling is currently one of the weakest points in the recycling system, because only a low percentage of plastic is reused compared to the amount of recovered metal, glass and waste paper. Recycling of wastes generally requires the separation of different materials. As the raw mixture of plastic waste usually includes various kinds of plastics (e.g. Acrylonitrile-butadiene-styrene (ABS), Polyethylene terephthalate (PET), Polystyrene (PS), Polyethylene (PE), Polypropylene (PP), Polyvinyl chloride (PVC)), the separation process should classify waste into a number of reclaimable plastic fractions, so as to meet the requirements for the purity and cleanliness of a polymer type that are needed in a high-quality plastic recycling process. The separation of different polymers by type is necessary because contamination in recycling of one type of plastic by another type can severely lower the quality of plastic products and cause serious processing problems.

Wet particle separation is used widely in mineral processing as well as plastic recycling to separate mixtures of particulate materials into further usable fractions due to density differences. Despite its wide usage wet particle separation processes are often attributed to operational problems especially if density differences of the feed material are low. Objective of this project was to develop a numerical method with which the wet plastic particle separation process can be simulated and to apply the developed framework for modelling of a technical wet separation process.
The research consists of two parts. In the first part, the development of a novel, fully Lagrangian particle-fluid modelling framework applicable to systems with many particles in a wet environment is performed. For modelling the particles, the Discrete Element Method (DEM) is employed. For the fluid part, the Lagrangian Smooth Particle Hydrodynamics (SPH) is used which enables handling free surfaces and large movements of the fluid, inherently. While unresolved fluid flow around particles is already used in mesh-based methods, coupling the DEM and SPH in one computational framework is a challenging task addressed in this project. In the first part of the project, a framework was developed for representing technical scale systems with many particles in a wet environment.

A comparative study on mesh-based and mesh-less (SPH) coupled CFD-DEM (Computational Fluid Dynamics – Discrete Element Method) methods to model particle-laden flow was performed. In general, results obtained using both approaches agreed well with analytic reference results. Numerical differences were found mostly due to difference in computed fluid fractions that result in different drag forces. The proposed new model to account for boundary conditions in the SPH approach was demonstrated to produce accurate results in the presented verification tests. They proved to be convenient and stable in the context of DEM-SPH simulations.

In the second part of the project, the developed framework was applied to the modelling of a wet separation process involving a sink-float drum separator for plastic recycling. The numerical analysis of polyethylene terephthalate (PET) particle separation from polypropylene (PP) particles in the rotating drum was performed. The influence of different operational and design parameters, such as the rotational velocity, the number of lifters, the feed rate etc., was analysed. Numerical results show, that the use of the separating vertical walls, lower rotational velocity, higher number of the lifters and the higher feed velocity by water increases the purity of the separated particles.

Dissemination of the results:

1) Published paper in a journal:
D. Markauskas, H. Kruggel-Emden, R. Sivanesapillai, H. Steeb. Comparative study on mesh-based and mesh-less coupled CFD-DEM methods to model particle-laden flow. Powder Technology. 305 (2017) 78-88.

A version of the final peer-reviewed manuscript accepted for publication is available on arxiv:1603.06808

2) Presentation of the results in a conference:
Talk: Numerical analysis of wet separation of particles by density differences. Presenter: D. Markauskas, Co-author: H. Kruggel-Emden. 14th International Conference of Numerical Analysis and Applied Mathematics, ICNAAM 2016, 19-25 September 2016, Rhodes, Greece.

3) Accepted manuscript for publication in a conference proceeding:
D. Markauskas, H. Kruggel-Emden. Numerical analysis of wet separation of particles by density differences. Proceedings of 14th International Conference of Numerical Analysis and Applied Mathematics, ICNAAM 2016, 19-25 September 2016, Rhodes, Greece, 4 p.

A version of the final peer-reviewed manuscript accepted for publication is available on arxiv:1609.08421

4) Submitted manuscript for publication in a journal:
D. Markauskas, H. Kruggel-Emden, V. Scherer. Numerical study of wet plastic particle separation using coupled DEM-SPH method. Manuscript is submitted for the review in the journal Powder Technology.
A DEM-SPH framework was developed in the research addressing technical scale particle/fluid systems for the first time. Such a framework is applicable not only to sink-float separation used for plastic recycling, but also to many other areas such as pharmaceutical sciences, bulk solids handling, mining, minerals processing, solids fuel processing and many more, where liquid/particle systems are of relevance. In the listed areas a DEM-SPH framework may help to investigate the motion of particles and related fluid flow in great detail. Such information is useful to improve process efficiency and is difficult to obtain experimentally. The coupled DEM-SPH is especially applicable for particle/liquid systems forming fluid free surfaces and for complex moving geometries, where mesh generation and adaptation would be too time consuming to apply.

The numerical modelling has been applied for the wet plastic particle separation for the first time. The lack of earlier research in this area can be explained by the lack of reliable, predictive computational methods being available that would be applicable for such analysis. A coupled DEM-SPH framework, developed during this research, is a powerful tool enabling a detailed analysis of this process with regards to operational parameters and design specifications. By numerical modelling time consuming extensive experimental investigations could be avoided and especially separation of plastic mixtures of similar density at elevated throughput could be optimized.
Numerical simulation of wet drum separation