Numerical analysis of wet separation of particles by density differences

Wet particle separation is widely used in mineral processing and plastic recycling to separate mixtures of particulate materials into further usable fractions due to density differences. This work presents efforts aiming to numerically analyze the wet separation of particles with different densities. In the current study the discrete element method (DEM) is used for the solid phase while the smoothed particle hydrodynamics (SPH) is used for modeling of the liquid phase. The two phases are coupled by the use of a volume averaging technique. In the current study, simulations of spherical particle separation were performed. In these simulations, a set of generated particles with two different densities is dropped into a rectangular container filled with liquid. The results of simulations with two different mixtures of particles demonstrated how separation depends on the densities of particles.Comment: This manuscript was accepted for the publication in the conference proceedings of ICNAAM 2016 conferenc

Extension of process models to predict batch screening results under the influence of moisture based on DEM simulations

Screening is a technical simple but still not fully understood process step, which can be used in a wide field of applications to separate bulk materials according to their particle sizes. A severe issue in screening technologies is that particles frequently prevail in moist conditions, due to effects related to the environment, the material or the process. This is often not preventable, although it is not preferred due to attractive forces altering the screening efficiency. For the design of dry screening processes, phenomenological models and detailed particle-based simulation approaches like the discrete element method (DEM) are available. The latter method has recently been extended and validated against experiments to calculate forces caused by liquid bridges formed out between particles or walls close to each other to meet the requirements to tackle real particle systems under moist conditions. In the investigation here, batch screening under the influence of moisture involving different sized glass spheres is investigated numerically with DEM simulations and by using process models. Therein, the related subprocesses stratification and passage as well as the influence of the operating parameters and the liquid amount on the fraction retained per size class are examined. Existing phenomenological process models, which can be applied efficiently for industrial applications due to their short calculation time, are extended to represent batch screening processes under moist conditions for the first time. Therefore, a benchmark is realized in which the fraction retained per size class over time for discontinuous screening under the influence of various amounts of liquid and different mechanical agitations obtained by DEM simulations and process models is compared. In this context, the process models are first adjusted to fit related simulation results and later used in a novel method to predict the outcome of screening with different operating parameters and liquid amounts. Thereby, process models, which consider the subprocesses stratification and passage, predict screening results for process parameters requiring interpolation or extrapolation in the investigated range very well. As a consequence, newly derived process models can function as prototypes to be applied in dynamic process simulation frameworks.DFG, SPP 1679, Dynamische Simulation vernetzter Feststoffprozess

Thermal lattice boltzmann simulation of diffusion/ forced convection using a double mrt model

The Lattice-Boltzmann method (LBM) is an alternative and flexible approach for computational fluid dynamics (CFD). Unlike many other direct numerical simulation (DNS) techniques, LBM is not solving the Navier-Stokes equations but is based on the kinetic theory and the discrete Boltzmann equation. LBM utilizes a Cartesian mesh and hence does not require a complex mesh derivation or a re-meshing in case of moving boundaries. Thermal LBM (TLBM) which is capable of solving thermal convection/diffusion problems relies on a set of two distribution functions, the so called double distribution function (DDF) approach; one for the fluid density and one for the internal energy. For the carried out numerical investigations a 3D TLBM framework is derived involving a multiple-relaxation-time (MRT) collision operator for both, the fluid and the temperature field which is yet not applied widely. Hydrodynamic and thermal boundary conditions are represented by interpolated bounce back schemes. The derived TLBM framework is applied to diffusion and convection-diffusion problems (e.g. forced convection) for plane and curved boundaries and is validated against analytical solutions, when available or compared to established correlations. The thermal MRT operator is further compared against an existing LBM model based on a thermal Bhatnagar-Gross-Krook (BGK) operator regarding accuracy and numerical stability. Averaged and local heat transfer coefficients are presented. The findings indicate that the double MRT framework with interpolated boundary conditions offers a highly accurate and efficient approach for the analysis of heat transfer problems especially for particle/fluid systems under detailed resolved flow

DEM simulations of screening processes under the influence of moisture

In a wide field of applications, screening is required to separate bulk materials according to their particle sizes. Due to environmental, material or process related effects, particles frequently prevail in moist conditions, which is not preferred due to attractive forces altering the screening efficiency, but often not preventable. As for the design of dry screening processes detailed particle-based simulation approaches like the discrete element method (DEM) and phenomenological models are available, a step towards meeting the requirements for real particle systems under moist conditions is made. Therefore, batch screening under the influence of moisture is investigated experimentally and by using DEM simulations involving different sized polyoxymethylene and glass spheres. For this purpose, a DEM code is extended to calculate forces caused by liquid bridges, forming out between particles or walls close to each other under moist conditions. Thereby, the bridge formation and rupture and the liquid distribution are considered. First, the DEM framework is validated against experiments by monitoring the capillary and viscous force acting on two liquid bridge contact partners. Further extensive validations are performed by comparing the fraction retained over time and the final liquid distribution for discontinuous screening under the influence of various amounts of liquid for different mechanical agitations in experiments and simulations. Finally, the detailed liquid distribution over time in the DEM simulations is examined and general conclusions are drawn. The overall aim is to use the framework and the respective data, to extend phenomenological process models for screening under moist conditions in subsequent studies.DFG, SPP 1679, Dynamische Simulation vernetzter Feststoffprozess

A strategy to determine DEM parameters for spherical and non-spherical particles

In Discrete element method (DEM) simulations the choice of appropriate contact parameters is significant to obtain reasonable results. Particularly, for the determination of DEM parameters for non-spherical particles a general straightforward procedure is not available. Therefore, in a first step of the investigation here, methods to obtain the friction and restitution coefficients experimentally for single particles [Polyoxymethylene (POM) spheres and quartz gravel] will be introduced. In the following, these predetermined DEM coefficients are used as initial values for the adjustment of bulk simulations to respective experiments. In the DEM simulations, the quartz gravel particles are represented by non-spherical particles approximated by clustered spheres. The best fit approximation of the non-spherical particles is performed automatically by a genetic algorithm. In order to optimize the sliding and rolling friction coefficients for DEM simulations, the static and dynamic angle of repose are determined from granular piles obtained by slump tests and rotating drum experiments, respectively. Additionally, a vibrating plate is used to obtain the dynamic bed height which is mainly influenced by the coefficient of restitution. The adjustment of the results of the bulk simulations to the experiments is conducted automatically by an optimization tool based on a genetic algorithm. The obtained contact parameters are later used to perform batch-screening DEM simulations and lead to accurate results. This underlines the applicability of the in parts automated strategy to obtain DEM parameters for particulate processes like screening.DFG, SPP 1679, Dynamische Simulation vernetzter Feststoffprozess

Influence of Processing Parameters on Fibre Properties during Twin-Screw Extrusion of Poplar Wood Chips

For sustainable agriculture, the contentious input of peat in growing media needs to be replaced by a substitute with the best possible water-holding capacity (WHC). Wood from fast growing poplar trees, cultivated in short rotation coppices (SRC), is a suitable alternative if it is processed correctly in a twin-screw extruder. The processing parameters, such as the aperture setting of the extruder, moisture content, and specific energy demand (SED), during twin-screw extrusion, as well as their influence on fibre properties such as WHC and particle size distribution, are investigated. SRC-poplar wood chips from clone Max3 are the raw material used for this research. As a result, the best volume-based WHC (75%) at −1 kPa suction tension was achieved for dry extruded wood chip fibre at an aperture setting of 15 mm and an SED of 340 kWh*t−1. The smallest SED of 140 kWh*t−1 was measured at apertures of 35 mm and 40 mm, which resulted in a volume-based WHC of approximately 30% and a dry matter mass flow during processing of 0.289 t*h−1 (40 mm). The particle size distribution of semi-dry wood chips has the highest fine fraction as well as the smallest coarse fraction. Conclusively, poplar wood can be processed fresh and dry into fibre at an acceptable SED, which results in an acceptable WHC

From particulates toscience of discontinua: generalization of particle simulation methods

In this work, we demonstrate that the rapid developments of methods of discontinua, when coupled with virtual experimentation and complementary discontinua based experimental and theoretical methods, are resulting in a significant paradigm shift from continuum-based analyses to either discontinuum and/or combined continuum discontinumbased approaches. Applications of these new approaches are so diverse (covering topics from traditional mineral processing to applications such as medical research, nano-science, social sciences, astrophysics, etc.) that what started as research on particulate media is rapidly transforming into the science of discontinua. In this paper, this trend is clearly demonstrated through a comparative study of both the fundamental developments in the core simulation technologies (together with synergies between different simulation tools) and their diverse fields of applications

On the 3D distribution and size fractionation of microparticles in a serpentine microchannel

Suitable methods to realize a multi-dimensional fractionation of microparticles smaller than 10 μm diameter are still rare. In the present study, size and density fractionation is investigated for 3.55 μm and 9.87 μm particles in a sharp-corner serpentine microchannel of cross-sectional aspect ratio h∕w = 0.25 . Experimental results are obtained through Astigmatism Particle Tracking Velocimetry (APTV) measurements, from which three-dimensional particle distributions are reconstructed for Reynolds numbers between 100 and 150. The 3D reconstruction shows for the first time that equilibrium trajectories do not only develop over the channel width, i.e. in-plane equilibrium positions but also over the channel height at different out-of-plane positions. With increasing Reynolds number, 9.87 μm polystyrene (PS = 1.05 g cm−3) and melamine (MF = 1.51 g cm−3) particles focus on two trajectories near the channel bisector. In contrast to this, it is shown that 3.55 μm polystyrene particles develop four equilibrium trajectories at different in-plane and out-of-plane positions up to a critical Reynolds number. Beyond this critical Reynolds number, also these particles merge to two trajectories at different channel heights. While the rearrangement of 3.55 μm polystyrene particles just starts beyond Re > 140 , 9.87 μm polystyrene particles undergo this rearrangement already at Re = 100 . As the equilibrium trajectories of these two particle groups are located at similar out-of-plane positions, outlet geometries that aim to separate particles along the channel width turn out to be a good choice for size fractionation. Indeed, polystyrene particles of different size assume laterally well-separated equilibrium trajectories such that a size fractionation of nearly 100% at Re = 110 can be achieved

Modelling of the dust release from bulk solids in the event of particle impact through enhanced dust detachment functions

In terms of environmental, health and explosion protection, it is important to assess the extent of diffuse dust emissions. In addition to standardized dustiness tests and measurements under field conditions on real dust emissions, numerical methods such as the Discrete Element Method (DEM) coupled to Computational Fluid Dynamics (CFD) are promising approaches for the prediction of the latter. Thereby the DEM calculates the bulk solid particle motion and the CFD provides information about the flow parameters of the fluid phases in terms of location and time. Due to computing time restrictions when simulating larger processes, each dust particle is usually not modelled in detail, but rather described using so-called dust detachment functions. In this study, detachment functions are adapted and developed based on a benchmarking with dust-resolved DEM simulations. Therefore, the adhesive contacts of fine dust particles attached to coarse bulk solid particles are modelled with an adhesive contact model (JKR approach). Depending on the bulk particle velocity, impact angle, material properties and bulk particle rotation in the case of single particle-wall and single particle-particle contacts, a good match between dust-resolved DEM and DEM with integrated detachment functions can be provided. For the further derivation and verification of the method, it is planned to compare numerical results also to experimental investigations. Therefore, the fine dust particle amount is reproducibly applied to coarse bulk solid particles by a well-defined calcium carbonate powder. The adhesion of the powder phase is then analysed before and after an impact