Chemical Engineering Research & Design, Vol.79, No.8, 1011-1016, 2001

Effect of particle size on simulation of three-dimensional solid dispersion in stirred tank

This work concerns the effect of particle size on three-dimensional solid dispersion in a baffled tank stirred with a disc turbine. The system studied consists of a cylindrical flat bottom four-baffled tank, 30 cm in diameter, with a six-blade disc turbine impeller, 10 cm in diameter, filled with water. Baffle width is 0.1-tank diameter and impeller clearance from tank bottom is 0.3-tank diameter. The height of liquid level in the tank is equal to tank diameter. The density of solid particle used is 2360 kg m(-3) and the diameters are 87 mum, 50 mum and 10 mum. Average solid concentration in the tank is 5% and 20% by volume. The impeller rotation speed is 13.3 rps. In this work the three-dimensional solid concentration distribution was predicted using the Algebraic Slip Mixture model (ASM model) under FLUENT1 5.1 facility. Standard k-epsilon turbulent model is used to model fluid flow field in the tank, and swirling radial jet model (SRJ model) approximates the velocity profile at the impeller tip. The simulation results are presented as concentration contour on r-z plane and r-theta plane in several positions in the tank for different average solid concentration and particle sizes. The solid concentration contour for 87,mum diameter solid and 5% average concentration shows that there is still a rather high solid concentration in the centre of the tank bottom, while in the liquid surface around the shaft the solid concentration is very low. This indicates the accumulation of solid particles in the centre of the tank bottom. For the other particle sizes the pattern of solid concentration distribution is similar. Below the impeller region the solid concentration near to the shaft is higher than the concentration far from the shaft. However, in the above impeller region the solid concentration near to the shaft is lower than the concentration far from the shaft. The distribution of the smaller particles is more uniform than the larger particles. Under the same rotation speed of 13.3 rps the distribution of 10 mum particles is almost uniform. The simulation result was verified using experimental data. The simulation result agreed very well with experimental data up to average solid concentration, 20% by volume.