Investigation On Continuous Crystallization Of Trisodium Citrate Hydrates

Trisodium citrate is a common food additive, and widely used in chemicalengineering and pharmaceutical fields. Trisodium citrate has two hydrates: trisodiumcitrate dihydrate and trisodium citrate pentahydrate. To fully understand the propertiesof the two hydrates and to develop more enery-efficent manufacturing method forthese hydrates, the crystallization processes of trisodium citrate hydrates and theirproperties related with crystallization processes were investigated in detail in thisthesis. Specifically, the main research content of this thesis can be summarized asfollows.Material solubility, which determines the choice of crystallization methods andthe yield of crystallization process, is the basic data of crystallization process. In thisthesis, the solubility of trisodium citrate dihydrate and trisodium citrate pentahydratewas experimentally measured using gravimetric method. Using these data, it wasfound that the two hydrates were enantiotropic and their transition temperature was315.4±1.0K. Above this point, trisodium citrate dihydrate is thermodynamically stable.Below the transition point, trisodium citrate pentahydrate is thermodynamicallystable.To fully understand the stability of the two hydrates, the dehydration processes oftrisodium citrate hydrates were investigated in detail by using PXRD, TG/DSC, FTIRet al. By analyzing the experimental data, it was found that the dehydration oftrisodium citrate pentahydrate can be divided into two steps. The crystal structure ofpentahydrate changed into the same as trisodium citrate dihydrate after its firstdehydration process. In addition, Ozawa equation was used to analyze the dehydrationmechanisms of the two hydrates with TG data. It can be concluded that thedehydration mechanisms of the two hydrates were different.Kinetics data of crystallization can provide theoretical basis for the optimizationand design of crystallization process. In this thesis, the nucleation and growthmechanisms of trisodium citrate pentahydrate was studied. The induction period oftirsodium citrate pentahydrate was measured using laser scattering technique atdifferent temperatures. And the interfacial energy of trisodium citrate pentahydrate inaqueous solution was calculated by using classical nucleation theory, mononuclear and polynuclear models. According to the data of interfacial energy, theheterogeneous nucleation might happen during the primary nucleation process. Usinginduction period data and various crystal growth models, it was found that the growthmechanism of trisodium citrate pentahydrate in aqueous solution might be2Dnucleation mediated growth.Finally, the continuous crystallization process of trisodium citrate pentahydratewas investigated. In this thesis, the particle size distribution of trisodium citratepentahydrate in continuous crystallization was measured using FBRM and Malvernsizer respectively. The concentration of the solution was also measured. Using thesedata, the nucleation and growth equations of trisodium citrate pentahydrate in aqueouswere determined using size-independent growth model. According to the equationsand experimental data, the influences of temperature and residence time oncontinuous crystallization and particle size distribution of product were preliminarilyunderstood.No similar research results have been published in literatures up to date.