| Summary: | 碩士 === 國立臺灣大學 === 化學工程學研究所 === 85 === A fluidized-bed crystallizer was used to study the crystal growth and secondary nucleation of potassium alum in the presence of impurities. Two different types of impurity, including a dispersing agent (NaPO3)6, and an organic dye ( Bismarck Brown Y ), were tested.
The growth rate and secondary nucleation rate of potassium alum both increased in the presence of NaHMP, though opposite effects of impurity on the crystal growth and nucleation rates were predicted. The impurity Bismarck Brown Y decreased the growth and secondary rates of potassium alum because the blocking effect is more effective than the enhancement of interfacial supersaturation. Then a derived secondary nucleation rate expression was employed to analyze the impurity effect on supersaturation and interfacial supersaturation.
Finally, when the Bismarck Brown Y concentrations were increasing, the surface reaction became more and more important in the growth process as revealed by the two-step mode. The surface reaction step became the controlling step when the concentration of dye is above 150ppm. However, the surface reaction and diffusion are both significant when NaHMP was used as an additive.
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