CFD simulation of silica dispersion/natural rubber latex mixing for high silica content rubber composite production

High silica contents rubber composites are favored in the green tire industry for their ability to reduce rolling resistance. However, achieving effective silica dispersion in natural rubber, particularly at high silica content, poses a challenge. In addition, the choice of impeller configuration significantly influences mixing performance, especially in commercial production, which requires large mixing tanks. Therefore, understanding the scaling-up process for this mixing system is essential. This research aims to investigate the mixing of silica dispersion in natural latex, specifically focusing on a high silica content regime. The flow characteristics of each liquid phase were simulated by employing the Computational Fluid Dynamics (CFD) approach, with a two-fluid model serving as the model based. Analyses were conducted on two variants of stirred tank reactors including four baffles and flat bottoms. Four configurations of Rushton turbine impellers were considered: four 90 degrees blades (RT4-90), four 45 degrees blades (RT4-45), six 90 degrees blades (RT6-90), and six 45 degrees blades (RT6-45). The simulations revealed that the 90 degrees blade promoted the radial flow, while the 45 degrees blades enhanced axial flow, through the process of diverting a significant proportion of the fluid above impeller, this regime effectively increases the liquid's velocity. Increasing the number of blades led to a more homogeneous velocity profile within the impeller region. Additionally, higher fluid velocity was observed in a larger mixing tank. In a smaller tank, the impact of impeller design (number and angle of the impeller) on mixing time was less pronounced. However, the mixing time decreased with the increasing blade number in a larger tank. In addition, the 45 degrees blade angle tends to decrease the mixing time. The optimum design is the Ruston turbine with six blades set at a 45 degrees angle. Furthermore, the upscaling criteria that were proposed by Norwood and Metzner were used into this inquiry. The suggested scaling criterion was consistently applied to the mixing of high silica natural latex, with no deviation exceeding 10%. The impeller configuration significantly influences on the hydrodynamics and mixing performance. The optimum design is the Ruston turbine with six blades set at 45 angle. The Norwood and Metzner scaling criteria is reliable to scale up of natural latex containing high silica content stirred tank.