comparision of discrete element method simulation ...

COMPARISION OF DISCRETE ELEMENT METHOD SIMULATION WITH EXPERIMENTAL DATA OF RADIOACTIVE PARTICLE TRACKING IN A TUMBLING CYLINDER

Ebrahim Alizadeh, Robert Legros, François Bertrand, Jamal Chaouki

Chemical engineering department, École Polytechnique de Montreal, C.P. 6078 succ. Centre-Ville, Montreal, Quebec, Canada, H3C 3A7

Abstract: Proper design and operation of powder mixers is critical for the production of high quality pharmaceutical drugs. Successful design can only be achieved from a comprehensive knowledge of solid dynamic in such mixers. Discrete Element Methods (DEM) have been found useful to simulate mixing in various unit operations. However, the opaque nature of powder mixing systems strongly limits the application of any experimental technique for validation of DEM results. In this study the Radioactive Particle Tracking (RPT) method were used to achieve this goal. RPT is known as a strong method for visualizing the solid movement in single and multi-phase systems. In this study experimental data of RPT were compared with DEM simulation results for size distributed granular material in a tumbling cylinder, in order to investigate how segregation affects mixing. RPT tests have been carried out with different sizes of tracers. It is shown that experimental RPT data can be used to validate the selection of DEM parameters, one can obtain simulation results that are in close agreement with RPT data. Keywords: Discrete element method, Radioactive particle tracking, Tumbling cylinder, Solid-solid mixing, Numerical simulation, Pharmaceutical process

1. INTRODUCTION Nearly 70 % of pharmaceutical drugs are in powder form. The active material in a typical formulation of powdery drug is less than 0.5 percent, and its concentration must be strictly maintained within a very small interval. Therefore, design and operation of solid-solid mixing operations are critical for the pharmaceutical industry . Due to limited knowledge of solid-solid mixing dynamics, formulators must often compose with empirical methods during the development of new drugs, hence leading to unreliable product quality (Muzzio et al. 2002). Most common problems are nonhomogenous mixing and unstable powder flow, especially when the active ingredient particle size is very different from other excipients and fillers. This causes the mixing to be nonhomogenous and segregation can occur. Appropriate knowledge of particle dynamics helps to provide a better understanding of mixing behaviour and leads to improved design of existing installation (Bertrand et al., 2005). Recently, work has been done (numerically and experimentally) to study the particle dynamics. In particular, Discrete Element Method (DEM) and Radioactive Particle Tracking (RPT), two methods that have significant advantages, were used in the present study to compare simulation and experimental results. The aim of this work is to study the dynamics of size distributed granules in a tumbling cylinder. RPT experiments were carried out with different sizes of tracers, which were prepared with particles identical to those present in the tumbler. Experimental RPT results were then used to select adequate DEM parameters required to obtain simulation results that were in close agreement to RPT data.

2. METHODOLOGY & RESULTS Several non-invasive methods that map the flow field by tracking the motion of a single tracer have been developed to study the motion of particles in one phase. Recently, the use of radioactive particle tracking has increased. This technique was originated by Lin et al. (1985) and is now used by research groups working with Chaouki (Larachi et al., 1994) and Dudukovic (Degaleesan et al., 2002). In our setup, 8 sodium iodide (NaI-Tl) detectors were strategically located around a tumbling blender and recorded the gamma disintegration events coming from a tracer (Fig. 1). The RPT algorithm construct the position of the tracer by solving a minimization problem between the measured counts and a rigorous model proposed by Beam et al. (1978) and applied by Larachi et al. (1994). Using an appropriate model, the exact location and velocity of the tracer can be obtained.

Fig. 1. Strategically located detectors around a tumbling cylinder with required accessories The RPT experiments were carried out with free flowing granules. Three tracers, of different sizes were prepared from particles used in the experiments, which one tracer represents middle size particles and other tracers represent particles from lower and upper tail of particle size distribution (PSD). A hole was drilled inside a selected particle and a small quantity of powder 46Sc (half life of 84 days) was poured inside. The tracers were activated in the nuclear reactor of École Polytechnique de Montreal to a radioactive level of 125 microcurie. The tracers were then safely stored for 5 days in order to get rid of emission associated with the activated sodium (half life of ~15 hr) and other elements present in the particle. For the calibration of the RPT measurement technique, a tracer was inserted at a known position inside a hollow bar placed within the tumbler. Gamma disintegration counts from each detector were recorded and processed using the accessories shown in figure 1. Sampling time and dwell time for each calibration measurement were 14.4 sec and 10 ms respectively. During actual RPT experiments, the sampling time was increased to minimize the effects of fluctuations within the recorded events. For RPT experiments, the tumbling cylinder was filled with a certain quantity of granules and the tracer of a particular size was added. Separate experiments were carried out with each of the three tracers. Counts were recorded and processed using a FORTRAN code running on an IBM cluster containing AMD Opteron 246 (2.0 GHz) processors. Typical results are presented in Fig.2 for a tumbler angular velocity of 20 rpm.

Fig. 2. Typical result of the mean velocity field obtained from processing of RPT experimental data (Doucet, 2008).

As seen in Fig. 2, the tracer simply moves around a specific trajectory, which is different from one tracer size to another. This clearly shows that particles with different sizes become confined to different regions of the tumbler, thus exposing the segregation behaviour that can lead to mixing inadequacy and nonhomogenous products. A simulation based on DEM model (as presented in Lemieux et al., 2007) is very sensitive to input parameters such as friction factors (particle/particle and particle/wall) and rolling friction factor. By varying the input data, acceptable results have been got which were in agreement with the experimental data of RPT. The simulations were performed by home-made codes as described in Lemieux (2007). More results will be shown in conference.

3. CONCLUSION The aim of this study is to compare the experimental data of RPT with the simulation results of DEM for size distributed granules. RPT experiments were carried out with tracers of three different sizes and it was seen that each size becomes confined within a specific zone of the tumbler, hence showing segregation behaviour. DEM was used to simulate the RPT experiments under the same conditions. It was shown that when adequate input parameters are selected for the DEM, simulation results are in close agreement with RPT data. REFERENCES Beam G.B., L. Wielopolski, R.P. Gardner, K. Verghese (1978). Mont Carlo calculation of efficiencies of rightcircular cylindrical NaI detectors for arbitrarly located point sources, Nuclear instruments and methods 154 (3) pp. 501-508 Bertrand, F., L.A. Leclaire, G. Levecque, (2005). DEM-based models for the mixing of granular materials. Chem. Eng. Sci. 60 (8-9), pp. 2517-2531. Degalseen S., M.P. Dudukovic, Y. Pan (2002). Application of wavelet filtering to the radioactive particle tracking technique, Flow measurement and instrumentation 13 (1-2) pp. 31-34 Doucet J. (2008). Mesure et characterisation dumelange dans les systems granulaires denses, PhD thesis, Chapter 5, Ecole Polytechnique de Montreal Larachi F., G. Kennedy, J. Chaouki (1994). Gamma-ray detection system for 3-D particle tracking in multiphase reactors, Nuclear instruments & methods in physics research, Section A, Accelerators, Spectrometers, Detectors and Associated Equipment A338 (2-3) (1994) 568-576 Lemieux M., F. Bertrand, J. Chaouki, P. Gosselin, (2007). Comparative study of the mixing of free-flowing particles in a V-blender and a bin-blender. Chem. Eng. Sci. 62(6) pp. 1783-1802. Lin J.S., M.M. Chen, B.T. Chao (1985). A novel radioactive particle tracking facility for measurement of solids motion in gas fluidized beds, AIChE Journal 31 (3) pp. 465-473 Muzzio F.J., T. Shinbrot and B.J. Glasser, (2002). Powder technology in the pharmaceutical industry: the need to catch up fast, Powder technology 124 pp. 1–7