Original Research PaperEvaluation of particle density effect for mixing behavior in a rotating drum mixer by DEM simulation
Graphical abstract
Introduction
The mixing of particulate materials is one of the crucial unit operations in many industries such as the manufacturer of chemicals, food and plastics. In most cases, particulate materials should be uniformly mixed to have the right quality of products. For many processes, a variety of particle mixing equipment have been used. It is important to understand that material characteristics such as frictional coefficient, restitution coefficient, particle size distribution, particle shape and particle density, have an effect on the mixing behavior for each equipment. Recently, high performance computers have enabled us to simulate precisely the mixing behavior. Especially, Distinct Element Method (DEM) has been widely used to analyze the mixing phenomena [1], [2], [3], [4], [5].
It is well known that particle size distribution leads to segregation [6], [7], [8], [9]. The large particle size difference leads to particles segregation easily. In addition, effects of frictional coefficient on mixing phenomena have been studied [10], [11]. The performance of the mixing equipment is correlated much better with frictional coefficient. Therefore, frictional coefficient is an important parameter to select the mixing equipment. Particle shape also has an effect on mixing behavior [12], [13], [14]. In order to demonstrate a non-spherical particle motion, a several kinds of model for DEM are suggested. In the DEM and the experimental results, no significant difference in the mixing behaviors of the non-spherical particles which are icosahedron, dodecahedron and hexahedron can be observed. On the other hand, the 2 or 3 dimensional DEM and experiment have been performed to analyze the influence of particle density for the mixing behavior [15], [16]. The experimental results show that the particles with larger density gather in the center of vessel, and the segregation is obviously observed. In the results of the 2 or 3 dimensional DEM, the segregation pattern is quite similar in comparison with the experimental results. However, it is not sufficiently investigated to make clear the mechanism of the segregation. Also, the technique to improve the mixing state of particles is not obviously reported.
In this study, the mixing behavior of particles in a rotating drum mixer (RDM) was analyzed by experiment and DEM to discuss the particle density effect. In order to analyze the mixing phenomena quantitatively, the new index related with the mobility of particles was developed. In addition, improvement of the particle mixing was performed by using DEM.
Section snippets
Experiment
Fig. 1 shows the RDM (our original equipment) used in the experiment. The container is made of stainless steel, and the lid is made of acrylic to observe mixing behavior in the drum with high speed camera. Maximum observation time of the high speed camera is 14.0 s. Particulate materials used in the experiment are alumina (NIKKATO Corporation: HD-3) and stainless steel (AS ONE Corporation: 1-9762-01) particles. Diameter of these particles is 3.0 mm, and the number of each particle is 377.
DEM simulation
Mixing behavior in the rotating drum was simulated by DEM. DEM is one of the most reliable and popular computer simulation methods for mixing behavior [15], [16], [17], [18], [19], [20]. The interaction forces in collision between two particles are represented by the Voigt model as shown in Fig. 3. This model is composed of a spring-dashpot and a slider for the friction in the tangential motion.
Determination of parameters
The DEM simulation parameters need to be determined so that mixing behavior simulated can correspond
Evaluation method of the mixture of particles
The mixing degree has been widely used to qualify the mixture of particles. In order to calculate the mixing degree, the video recordings of the experiment and the DEM simulation were converted to gray scale, and pixels belonging to each particle type (black and white) were detected based on their color value. The mixing degree was defined as:withwhere n is the number of samples, Ci is the average color value in sample i, C′ is the color value of a perfectly dispersed
Validity of the DEM simulation
The mixing behavior of alumina and stainless steel particles in the RDM is shown in Fig. 6. In the experimental results, segregation gradually starts (Fig. 6(a)). After 10 s (Fig. 6(b) and (c)), most of the alumina particles with lower density exist close to the container wall and in the vicinity of the surface of particle bed. The segregation of alumina and stainless steel particles is obviously observed. The mixing behavior of the DEM simulation is quite similar to that of the experimental
Conclusions
In order to analyze the particle density effect for mixing behavior in the rotating drum mixer, the experiment and the DEM simulation were performed. Validity of the DEM simulation was confirmed by comparing to the experimental results in terms of mixing behavior. The new index was used to make clear the segregation mechanism in a binary system. The results showed that the particles with lower density could move easily in comparison with higher density particles and the segregation could be
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