Study And Multi-Phase CFD Modelling Of Ammoium Sulphate Crystallization

Ammonium sulphate is an important nitrogenous fertilizer for agriculture. It is usually precipitated from coke oven gas (COG) of steelworks and the byproduct of caprolactam production in industry. There are many problems in precipitating ammonium sulphate from a saturator, such as small crystal size, non-uniform crystal size distribution (CSD), nonstandard chroma and aggregation of crystals. A systematic study on the crystallization process of ammonium sulphate has been performed in this thesis. Computational Fluid Dynamics (CFD) simulations and optimization of the crystallizers have been carried out in conjunction with multi-phase model.The solubility, supersolubility and metastable zone of ammonium sulfate in pure water, sulfuric acid solution and ferric sulfate solution have been experimentally determined using a laser monitoring observation technique. The experimental data can be correlated with temperature by a double logarithmic empirical equation, Apelblat.An unsteady state method was employed to measure the nucleation and growth kinetics. The CSD of ammonium sulphate has been measured, consequnently, the paratmeters of equations of nucleation and growth rate in pure water and raw mather liquid has been estimated by regression. The effects of acidity of solution and impurity in the raw material on nucleation and growth rate were investigated.The effects of parameters, such as seeds added, agitation speed, evaporation temperature, pH of solution etc., on the product quality have been studied. And an optimal operating condition of crystallization of ammonium sulphate has been determined.CFD modellings, based on the physical properties and optimal operation conditions from experiments, have been carried out to simulate (NH4)2SO4 crystallizations in a pilot scale crystallizer (2.5L) and an industrial crystallizer drafted tube baffled (DTB) (456.78m3). The velocity magnitude, vector and shear rate of fluid inside the different scale crystallizers have been observed, form which the theoretical inputting power of impellers has been calculated. The effects of impeller types (a PBT impeller and a marine propeller) on the processes have also been studied.Multi-phase CFD simulations of the crystallization process of ammonium sulphate in an industrial batch DTB crystallizer have been performed using Euler-Euler method. A multi-group method has been developed to solve the population balance equation of crystallization, in which an interphase mass transfer model has been established and deduced to involve the nuclear rate and growth rate equations into CFD calculation.An industrial continuous DTB crystallizer of ammonium sulphate has been optimized through the results of Euler multi-phase simulation of the batch DTB. The flow fields and solid concentrations at different agitation speeds have been studied. Lagrange Discrete phase model has been employed to trace the trajectories of crystal particles at different diameters of 10μm, 800μm and 2000μm in order to understand crystals'bahaviour inside the continuous DTB.