CFD SIMULATIONS FOR SCALE UP OF WET MILLING IN HIGH SHEAR MIXERS
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Rotor-stator mixers are widely used in the chemical and pharmaceutical process industries. Up to now, however, few papers discuss the mean flow and turbulence fields generated by them and their influence on final product quality. In this work, CFD results at different scales are used to aid in the scale up of crystal wet milling processes. CFD simulations were performed to simulate different scale mixers. In addition, wet milling studies were conducted at the bench scale to complement the CFD results and predict wet milling performance in larger scale mixers.
The flow properties in a batch Silverson L4R rotor-stator mixer at 4000 and 6000 rpm were investigated. A hybrid technique was developed. The new method is computationally efficient compared with the standard sliding mesh method. Macro scale properties are predicted. The turbulent flow field and deformation rate field are compared and analyzed. After obtaining fully converged flow fields, one way coupled particle tracking calculations were performed using an efficient fast particle tracking code. Particles trajectories were recorded, and analyzed. To validate the simulated flow field, particle image velocimetry (PIV) experiments were conducted.
CFD simulations of Silverson inline L4R (bench scale), 450LS (pilot scale) and 600LS (plant scale) mixers were conducted at constant tip speed to investigate the scale up effect. The macro scale properties werer predicted. The mean velocity, turbulent and deformation rate fields were investigated. The flow properties of the 450LS and 600LS mixers are quite similar, but they are significantly different from those of the L4R (bench scale) mixer. Therefore, it may be resonable to scale up from pilot scale to plant scale by the general accepted tip speed scale up criterion. However, considering tip speed alone may lead to a significant discrepancy between bench scale and larger scales.
Bench scale wet milling experiment were performed at 4000, 6000 and 8000 rpm using sucrose and mannitol in the Silverson L4R inline mixer. The crystal size decreases with rotation rate at both free pumping conditions and constant flow rate conditions. To investigate the effect of flow rate, wet milling of granulated sucrose in the Silverson L4R inline mixer with constant rotor tip speed were performed at different flow rates. It is found that the crystal size increases with the flow rate.