Numerical Analysis of an Annular Centrifugal Contactor for Liquid-Liquid Separation

Abstract

Multiphase flows consist of two or more distinct phases flowing simultaneously in the mixture, including some degree of phase separation at the molecular level in the fluid mechanic's context. The mixing and separation phenomena of two phases occur not only due to the interaction of liquid interfacial force, but also the chemical properties of the liquids. Thus, the separating of two or more phases from the mixture became a significant challenge in the separation-related industry, especially for two liquids with slight density differences. Therefore, the separation processes are necessitated either by facilitating the mixing or mass transfer or by making multistage units’ operations. The separation process of two liquids from the mixture is a mechanical and chemical process. However, the mechanical process for separation is usually carried out in one of three types of devices such as pulse column, mixer settlers, and annular centrifugal contactors. In this study, the annular centrifugal contactor (ACC) has been considered an important piece of equipment for liquid-liquid separation processes. Although the equipment is studied previously, the geometrical guidelines are still empirical owing to the complexity of flow characteristics in the rotor of the ACC. The structure of the ACC is also complicated due to different internal parts; therefore, the proper design would enable further advancements to understand the complete flow characteristics. In addition, the flow and mass transfer details of the system must be understood to perform an appropriate analysis owing to the complex behaviour of the liquids, particularly inside the rotor. Since 1980, computational fluid dynamics (CFD) is increasingly being used for understanding the fluid mechanics in process equipment and many others have brought out the perspectives of CFD in terms of expectations, status, and the path forward. Therefore, CFD is useful for facilitating the design of future contactors and the critical evaluation of existing designs, as well as for supporting the deployment of the ACC. Moreover, it can be a substitute for experiments in certain situations to reveal the flow fields in the device, thereby contributing significantly to the investigation of flow and mass transfer mechanics. However, to perform the simulation in a meaningful and systematic manner, the separation efficiency relationship was first developed of the ACC with the effects of the geometric structure and the flow conditions considered using dimensional analysis. A newly designed ACC with a funnel–type weir zone is investigated numerically to analyze the flow fields and separation performance of two liquids with a slight density difference, such as palm oil and water. ANSYS CFX is used to simulate unsteady, turbulent, and multiphase flows in the ACC. The simulation is performed using the Eulerian–Eulerian method, where a homogeneous k–ɛ turbulence model with a scalable wall function is used. The sliding mesh method is adopted to solve the rotational effect between the stator and rotor. High-resolution discretization for both the advection and turbulence terms was used for the accurate calculation. Meanwhile, only the drag effect is considered in this work, because other effects are small in the liquid-liquid-dominated flow in the ACC. The drag effect of both drops and bubbles must be considered in the three-phase flow of the ACC, therefore, the Ishii–Zuber drag model was used to evaluate drag force because this model is applicable to general fluid particles (drops and bubbles). A detailed parametric analysis of the mixing and separation based on the Reynolds number, angular velocity, vane numbers, aspect ratio, different water outlet locations, and phase morphology variations is performed according to the dimensionless relationships. To analyze the performance of the ACC rotor, the oil quality and separation efficiency are calculated at the oil outlet. The visualization results show the separation process and dynamic of the liquids. The overall separation efficiency of the ACC is significantly affected by the Reynolds number, angular velocity, aspect ratio, different water outlet locations, and phase morphology. The separation performance of the ACC was relatively lower than expected for two liquids with slight density differences such as palm oil and water. However, a significant improvement in the separation performance was obtained gradually by the geometrical modification such as aspect ratio and water outlet variations. The better separation performance such as oil separation efficiency (91.26%) and oil quality (99%) of the ACC was obtained at the oil outlet for two liquids such as palm oil and water when the water outlet was placed at the bottom. Finally, it can be concluded that two liquids with a slight density difference can be separated adequately using the newly designed ACC. However, this equipment shows the feasibility of low-speed operations for liquid–liquid separation.