Computational Investigation of Hydrodynamics, Mixing and Crystallization in a Batch Stirred Vessel

Abstract

In this work, the hydrodynamics, mixing and suspension quality of solids are numerically investigated using computational fluid dynamics (CFD). The transient CFD simulations are performed to obtain the flow field. Here multiple reference frame and sliding mesh approach are used to predict the flow field along with the standard k-ε turbulence model. The velocity field is analyzed spatially and temporally, and liquid circulation is calculated at various impeller speeds to find an optimum impeller speed. To improve the flow field in a batch stirred vessel, various draft tube baffle configurations are introduced. The optimum baffle system (DTB-IV) is identified that supports liquid circulation, mixing and suspension of solids in the batch stirred vessel. It is found that suspension quality is strongly dependent on the prevailing hydrodynamics in the stirred vessel. Further, the optimised baffled stirred vessel (DTB-IV) is used to carry out the cooling crystallisation process. The primary difficulty in the design and scale-up of the crystallization process is the lack of understanding of the flow field, growth and nucleation at different scales. Here, the performance of an unbaffled stirred vessel is compared with a baffled stirred vessel system. To predict crystal size distribution (CSD) in batch stirred vessel system, the population balance equation (PBE) is used. The PBE is solved using the quadrature method of moments. The PBE accounts for both the size and the number of the particles, and it is coupled with the CFD model. This coupled algorithm integrates solubility data, nucleation and growth kinetics. To examine the crystallization process in a batch stirred vessel, potassium dihydrogen phosphate – water system is chosen. This is analyzed for unbaffled and baffled batch stirred vessel to quantify the growth and nucleation rates of the crystals. Further, the effect of seed mass, its size and temperature on the crystallization process is investigated. The results showed that baffled stirred vessel is more advantageous and supports the crystallization process.