Comparative Assessment of Different Static Mixers Using CFD

Abstract

A numerical investigation was carried out to compare the hydrodynamics and the mixing performance of three variants of static mixers under turbulent flow conditions where the pipe Reynolds numbers varied between 5,000 and 30,000. The investigated mixers are the standard helical Kenics mixer, the corrugated Sulzer SMV mixer and a modified geometry of the screen-type static mixer. This new geometry is based on the use of specially located divergent inserts downstream of a woven mesh in an attempt to improve its distributive mixing. Comparing the flows through these mixers was based on the pressure drop, velocity fields in addition to quantifying both the dispersive and distributive mixing efficiencies. The latter was accomplished by computing the dispersive mixing efficiency coefficient (i.e., extensional efficiency) and the intensity of segregation (i.e., coefficient of variation) at the outlet of the mixing chambers. Pressure drop was found larger in the new mixer geometry which generated values that are 1.2 and 3 times larger than those in the Kenics and SMV mixers, respectively. In addition, the Kenics and SMV mixers exhibited better distributive mixing than the newly proposed design since they were able to achieve the desired, commonly acceptable, homogeneity level. However, dispersive mixing was found to be improved in the new mixer where an average extensional efficiency of 0.68 was obtained compared to values of 0.57 and 0.53 in the Kenics and SMV mixers, respectively. The results suggest that the new mixer geometry can be further optimized to meet the desired mixing and power consumption criteria by optimizing various geometric parameters such as the geometry of the woven mesh, as well as the length, width, and location of the downstream inserts.