CFD-DEM simulations of particle separation characteristic in centrifugal compounding force field
Graphical abstract
Introduction
Centrifugation is a technique which involves the application of centrifugal force to separate different particles from slurry according to their size, shape and density. This process is used to recover valuable minerals, but also to separate fine coal particles [1]. In centrifugal force field, the acceleration of particles can achieve tens to several hundred times the Earths gravitational acceleration, which produces a significantly increased sedimentation velocity differential between particles with different density and size [2,3]. More-dense particles of the mixture migrate away from the axis of the centrifuge, while less-dense particles of the mixture migrate towards the axis [4,5]. Therefore, a series of studies have been conducted to increase the inertia of fine particles by application of a centrifugal compounding force field. These efforts have led to the development of various centrifugal separators [6], such as Multi-Gravity Separator [7], Kelsey centrifugal jig [8], Falcon separator [9] and Knelson separator [10]. Falcon and Knelson separator is a vertical axis bowl-type centrifugal separator that performs its concentration duty using a fluidized bed mechanism. The centrifugal force created by its fast rotating bowl can reach 30–300 times the Earths gravitational acceleration, which enhances the relative settling velocity differential between particles of different size and density. Therefore, Falcon and Knelson separator have been proved to be a potential in processing large flow rates of fine particles slurry with a highly separation efficiency.
Although centrifugal separators have been widely used for fine coal separation and major advances in the understanding of centrifugal separator circuit operation made in the past, problems and challenges are frequently encountered in their operation in coal preparation plants. The common problems involve centrifugal settling and collisions of particles due to fluidization water pressure through the multiple fluidization holes in the inner shell [[11], [12], [13]], the structure parameters of centrifugal separator, and operating parameters. A number of academic papers have been published on the experimental investigation for improving separation efficiency of centrifugal separator [[14], [15], [16]]. Relatively little effort has been directed towards understanding and characterization of the physical analysis underlying particle-fluid, particle-particle and particle-wall interaction forces due to constraint of the research techniques and conditions. It is important to understand the reasons causing these problems and to develop effective methods for their control. Physical and mathematical modeling has been recognized as a useful approach to tackle these problems [17,18].
With the development of advanced computational techniques, the information of particle-fluid, particle-particle and particle-wall interaction forces can be generated by use of Computational Fluid Dynamics (CFD) and Discrete Element Method (DEM) [[19], [20], [21], [22], [23]]. In CFD model, the flow of coal particles can be modeled by use of the so-called Lagrangian Particle Tracking method [[24], [25], [26]]. However, the Lagrangian Particle Tracking method is just suitable for dilute flow since it only traces a single particle, and the effect of inter-particle interactions and the reaction of particles on the fluid are ignored. DEM is a numerical modeling approach used to predict the behavior of collision dominated particle flows. It tracks each particle in the flow and models all inter-particles collisions and collisions between particles and domain boundaries. Fatahi [27] use DEM simulation of laboratory Knelson concentrator to study the effects of feed properties and operating parameters. Its investigations demonstrated that DEM is an efficient tool to understand Knelson concentrator separation process at a particle scale and predicted the performance variability induced by operating parameters. Recently, CFD-DEM coupling method has been proven to be effective in modeling particle–particle and particle–fluid interactions [[28], [29], [30]]. However, it has not been used in the modeling of complex multiphase flow in systems like that found in centrifugal separator.
In this work, an innovative centrifugal separator is proposed base on Falcon and Knelson separator, which focus on the particle-fluid, particle-particle interaction forces, and particle separation characteristic in centrifugal compounding force field by use of CFD-DEM model. Some key features of the flow in centrifugal separator are described, and the effect of operating parameters on the separation performance is analyzed. The roles of particle–particle and particle–fluid forces are discussed based on the CFD-DEM results. The findings should be useful to better design and control centrifugal separator operations, particularly for various coal types with different particle density distributions.
Section snippets
Computational model
A CFD-DEM coupling model has been accepted as a highly effective approach to fundamentally investigate the mechanisms of particle–particle and particle-fluid flow [31,32]. Many researches paper has reviewed the CFD-DEM modeling [33,34], therefore, only a brief description of the CFE-DEM model is given in this work.
Simulation methods and conditions
An innovative centrifugal separator is assembled in this work. The separation principle in the rotation bowl of centrifugal separator is schematically shown in Fig. 1. The slurry (fine coal particle and water) is fed through a central pipe at the bottom of the bowl where it is promptly diffused towards the wall. Then it rises in a countercurrent fashion until it overflows the top of the rotating bowl. The centrifugal force is generated by high rotation rate that would cause deposition and
Model validation
Due to the complexity of flow in centrifugal separator, a proper validation of the established CFD-DEM model is absolutely necessary. Here, this is performed with literature data used in validation. However, only limited coal particle separation data were available, so the flow of coal particles was only validated qualitatively. In the modeling of fluid flow in an innovative centrifugal separator, the method is very similar to modeling the liquid-solid flow in Falcon separator. For this
Conclusion
In present work, A CFD-DEM model has been developed and used to systematically study the separation of particles in centrifugal compounding force field. Some key features of the flow in centrifugal separator are described by using CFD modeling. The DEM is used to model the motion of discrete particles by applying Newton's laws of motion. The following conclusions can be drawn:
There is a flowing film located around the wall of rotation bowl. The flow regime of flowing film can be defined as
Acknowledgement
The authors are grateful to the National Natural Science Foundation of China (NSFC) (No. 51574252) and National Natural Science Foundation for Young Scholars (No. 51604281) for the financial support of this work.
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