| Summary: | 碩士 === 國立臺灣大學 === 化學工程學研究所 === 93 === Recently a pellet reactor, which plays an important role in chemical engineering process, has been developed for water softening, fluoride and phosphate removal, and heavy-metal recovery. In the design of a pellet reactor for water softening, we need to know the kinetics of CaCO3 in a fluidized bed. Most of the reported growth-rate model of calcite is rather empirical, for example, an overall growth-rate model. Although the over-all growth model is convenient to use, the applicability of it was limited to a certain range of operating variables. Therefore, the two-step growth model was used to modify the growth kinetics of calcite in a pellet reactor.
In this research, the measurement of crystal growth of calcite in a fluidized-bed was designed by using a constant-composition technique, which can be used to study the effects of solution properties. The calcite growth-rate data was analyzed by the two-step growth model to obtain the kinetic parameters of surface-reaction coefficient and mass-transfer coefficient. The effects of operating variables, such as pH, supersaturation, ionic activity ratio, and ionic strength on the coefficients were studied. In addition, an experiment was conducted to study the influence of pH on aragonite growth rate.
In the calcite growth experiment, several operation variables were fixed, such as L = 774 μm, v = 0.047 m/s, and T=298 K, then the crystal growth rate data were measured at various supersaturation under specified pH, activity ratio and ionic strength. In the range of the pH value between 8.5 and 9.5, the ionic strength (I) between 0.005 and 0.034 M, and the activity ratio between 0.4 and 5.54, the calcite growth rate increase with an increase in supersaturation, pH value, and ionic strength, however, the growth rate data shows a maximum at an activity ratio around 1.
Finally the growth-rate data were analyzed by the two-step crystal growth model by assuming r = 2. The calculated mass-transfer coefficient is rather independent of pH and activity ratio, but varies with ionic strength, the surface-reaction coefficient increase with pH and activity ratio but rather independent of ionic strength.
In the experiments of aragonite crystallization, the growth rate decreases with an increase in the pH range between 8.3 and 10, however, the effect of pH is not significant for pH being between 9.5 and 10.
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