Modeling and simulation of forced circulation evaporation crystallizer

Abstract

Crystallization is an effective process of forming a solid phase from solution and crystallizer is the equipment used for industrial crystallization operations. There has been a
lot of experimental research work carried out for understanding and improving crystallization operations in the past years, several aspects relating to the crystal growth, nucleation, breakage, type of crystallization process, design of crystallizer and scale up of crystallizers. It is difficult to analyze complex hydrodynamic behaviour in crystallizer by experiment. In last decade a good number of works has been carried out using CFD simulation for different types of crystallizers to study the flow dynamics and crystal formation. In present work flow behavior, solid suspension distribution, population density and mean crystal growth studies have been carried out for the forced circulation evaporation crystallizer (FC crystallizer) by CFD simulations using software package; Ansys Fluent 13. The FC crystallizer is a continuously operating mixed suspension mixed product removal (MSMPR) type crystallizer used in salt, sugar and pharmaceutical industries for crystal production. In the present work simulation has been carried out in isothermal and non-isothermal conditions to characterize the hydrodynamics and population balance model to explain the population density. 3D modified computational geometries have been used to observe the vigorous 3D multiphase flow by Eulerian model. The process of crystallization of NaCl from its solution has been used for simulation. NaCl solution and water vapor has been considered as the two phase liquid and gas. Water and solid particles (NaCl) as two phase has been used to see the crystal size distribution. For population density study all three phases: NaCl solution, NaCl crystals, and vapor has been considered. RNG k-ϵ model has been used for turbulent fluctuations of velocities in multiphase phase flow. The present work is carried out under varying conditions of flow rates and inlet positions. From the results it is observed that phase distribution, velocity profiles and pressure is changing with change in flow rates for isothermal and non-isothermal both conditions Increase in inlet height found to affect the vapor distribution. Flow rate and inlet position are affecting the suspension distribution in crystallizer. Better distribution of crystals is seen for higher flow rate of solution and for
inlet at higher position. Population density of large crystals and mean crystal size is found to increase with time.