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

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

Ein Rahmensystem zur Ableitung dynamischer Siebprozessmodelle

For a wide field of applications in mechanical process engineering and materials preparation technology, screening as technical simple but still not satisfactorily understood process step is well suited to separate bulk materials according to their particle sizes. Frequently, particles prevail in broad size distributions, highly non-spherical shape and under moist conditions, complicating the handling of screening processes. The complexity and significance of this process even increase when screening is operated within combined solids processes, in which defined narrow particle size distributions are needed for subsequent process steps. Therefore, it is inevitable to understand the subprocesses of screening (stratification, particle passage and particle transport) and their interaction. Furthermore, flexible, simple and predictive tools for a quantitative representation of screening on the background of a transient description are significant in industrial applications. For the design and improvement of screening processes, detailed particle-based simulation approaches like the discrete element method (DEM) as well as various computational efficient phenomenological process models are available. Usually, the material-, operating-, and apparatus-specific parameters of process models are empirically determined by experiments, whereas, in this work, the parameters for dynamic screening models are directly obtained from DEM simulations, which are validated against experimental investigations. Within this work, discontinuous and continuous screening as well as its subprocesses in laboratory scale and realistic polydisperse systems are investigated. Therein, different screen geometries and characteristics are considered along with various mechanical excitations applying model and real particle shapes under the influence of various liquid amounts. Besides the inherent transient nature of batch sieving, screening processes under induced altered operational conditions are considered to study their dynamic behavior. In order to perform reliable DEM screening simulations, the exact determination of particle properties like size, shape, material and contact parameters is essential, which is performed in advance of the simulations in this thesis by a self-developed general straightforward calibration procedure. Furthermore, existing phenomenological process models of differing complexity are benchmarked based on the passage rate or residual mass obtained by DEM simulations of stationary operated discontinuous and continuous dry screening processes. Additionally, suitable process models are extended to represent screening processes under altered operational conditions and under the influence of moisture. A further extension is accomplished in order to predict screening results for different operational parameters and liquid amounts, after a material- and apparatus-specific parameter adjustment. As a result, derived process models can be applied as prototypes in dynamic process simulation frameworks together with other solids processes like grinding, agglomeration or air classifying.Für ein breites Anwendungsgebiet in der mechanischen Verfahrens- und Aufbereitungstechnik ist das Sieben als technisch einfacher, aber noch nicht ausreichend verstandener Verfahrensschritt gut geeignet, um Schüttgüter nach ihren Korngrößen zu trennen. Häufig liegen Partikel dabei in breiten Größenverteilungen, stark nicht-sphärischer Gestalt und unter feuchten Bedingungen vor, was die Gestaltung und Durchführung von Siebprozessen erschwert. Die Komplexität und Signifikanz dieses Prozesses erhöht sich, wenn das Sieben als Teil vernetzter Feststoffprozesse erfolgt, bei denen definierte, enge Partikelgrößenverteilungen für nachfolgende Prozessschritte benötigt werden. Daher ist es unumgänglich, die Teilprozesse des Siebens (Stratifikation, Partikeldurchtritt und Partikeltransport) und deren Wechselwirkung zu verstehen. Darüber hinaus sind flexible, einfache und prädiktive Werkzeuge für eine quantitative Darstellung des Siebens vor dem Hintergrund einer transienten Beschreibung in industriellen Anwendungen von Bedeutung. Für die Auslegung und Optimierung von Siebprozessen stehen detaillierte partikelbasierte Simulationsansätze wie die Diskrete-Elemente-Methode (DEM) sowie verschiedene recheneffiziente phänomenologische Prozessmodelle zur Verfügung. Üblicherweise werden die material-, betriebs- und apparatespezifischen Parameter der Prozessmodelle empirisch durch Experimente bestimmt, wohingegen in dieser Arbeit die Parameter für dynamische Siebmodelle direkt aus experimentell validierten DEM-Simulationen gewonnen werden. Im Rahmen dieser Arbeit werden diskontinuierliche und kontinuierliche Siebungen sowie deren Teilprozesse im Labormaßstab und in wirklichkeitsnahen polydispersen Systemen untersucht. Dabei werden verschiedene Siebgeometrien, -eigenschaften und -anregungen betrachtet, wobei Modellkörper und reale Partikelformen unter dem Einfluss verschiedener Flüssigkeitsmengen herangezogen werden. Neben der instationären Natur der Chargensiebung werden induziert veränderte Betriebsbedingungen von Siebprozessen betrachtet, um deren dynamisches Verhalten zu untersuchen. Für die zuverlässige Durchführung von DEM-Siebsimulationen ist die genaue Bestimmung von Partikeleigenschaften wie Größe, Form, Material- und Kontaktparametern essentiell, was im Vorfeld der Simulationen dieser Dissertation durch einen selbstentwickelten, universell einsetzbaren und unkomplizierten Kalibrierungsansatz erfolgt. Darüber hinaus werden existierende phänomenologische Prozessmodelle unterschiedlicher Komplexität auf der Grundlage der Durchtrittsrate oder der Rückstandsmasse, die durch DEM-Simulationen stationär betriebener diskontinuierlicher und kontinuierlicher Trockensiebungen erhalten werden, beurteilt. Zusätzlich werden geeignete Prozessmodelle zur Darstellung von Siebprozessen unter veränderten Betriebsbedingungen und unter Feuchteeinfluss erweitert. Eine zusätzliche Neuerung besteht in der Vorhersagbarkeit von Siebergebnissen für verschiedene Betriebsparameter und Flüssigkeitsmengen nach einer material- und apparatespezifischen Parameteranpassung. Die hergeleiteten Prozessmodelle können schlussendlich als Prototypen in dynamischen Prozesssimulationsumgebungen zusammen mit anderen mechanischen Prozessen wie Mahlung, Agglomeration oder Sichtung eingesetzt werden.DFG, SPP 1679, Dynamic simulation of interconnected solids processes (DYNSIM-FP

Separation Curves of Screening and Air Classifying Processes at Low Material Loadings

Zur Abtrennung von Partikeln mit unterschiedlicher Größe sind in der Feststoffverfahrenstechnik trockene Klassierprozesse zur Herstellung von engen Fraktionen von großer Bedeutung. Um das Trennverhalten von Strömungs- und Siebklassierprozessen genauer vorhersagen zu können, wurden experimentelle Resultate mit DEM-Simulationen verglichen. Bei der Siebklassierung wurde der Einfluss der Partikelform auf den Trennprozess betrachtet, während das Partikelverhalten (Partikel-Partikel- und Partikel-Wand-Stöße) zwischen den Schaufeln eines Abweiseradsichters zum ersten Mal mit einer DEM-Simulation modelliert wurde. Die vorliegende Arbeit zeigt, dass die Kombination aus DEM und Experiment die Möglichkeit eröffnet Modellvorhersagen durch Ermittlung prozessspezifischer Parameter zu verbessern und Klassiermodelle abzuleiten, welche in dynamischen Feststofffließschemasimulationen Verwendung finden.For the separation of particles with different sizes, dry classification processes are of great importance for the production of narrow fractions in the solid processing technology. In order to be able to more accurately predict the separation behavior of flow and sieve classifying processes, experimental results were compared with DEM-simulations. In screening the influence of the particle shape on the separation process was considered while the particle behavior (particle-particle and particle-wall-collisions) between the paddles of a deflector classifier was modeled for the first time with a DEM simulation. The present work shows that the combination of DEM simulation and experiment makes it possible to improve model predictions by identifying process-specific parameters and to develop classification models, which are applicable in dynamic flowsheet simulations of solids processes.DFG, SPP 1679, Dynamische Simulation vernetzter Feststoffprozess