Research Of Kinetic Modelling Of Crystallization From Solution

Crystallization from solution is an important unit operation process in the field of chemical engineering, which could be used to separate and purify the higher purity solid products with lower costs, and has been applied in most traditional industries. Recently, crystallization operations are being employed to obtain high value-added products such as new materials, biochemical products and novel energy sources. It has been demonstrated by the theory of super molecule that the properties and/or functions of solid product are not only depended on its chemical composition, but also on its molecule structure, that is called as "constitutive relationship". Currently, the crystallization processes is being one of the most interesting research focuses.Crystallization is a sophisticated process with multiphase flows, which is related to its equilibrium thermodynamics and kinetics. With the thermodynamic theories and techniques, it is relatively easy to determine the phase equilibrium data with enough accuracy, however there still exist much more difficulties in crystallization kinetics study even for a simple binary system. That is the reason that nucleation and crystal growth rate are generally represented in form of the empirical expressions. The crystallization kinetic is important for crystallizer design, process control and optimization, and it is strongly depended upon the accurate characterization of process information concerning with multiphase flows and the further disclose of its mechanisms with suitable mathematical models.The progress and current status of the kinetic study on crystallization from solution was briefly reviewed in this thesis. Considering the mass balance law for solute, and combining with the populance balance model and the McCabe's L law, a linear crystal growth rate model was presented for potassium nitrate aqueous system. The test variables of both the liquid and solid for kinetic parameter estimations were given on this model, and the experimental apparatus were also set up.In order to determine the concentration or supersaturation in crystallization process, an online test apparatus was set up based on the relationship between the concentration and the electroconductivity at a temperature for binary electrolyte systems, which was also used in similar fields to measure the electrolyte concentration. The results showed that the accuracy and the repeatability for online concentrationacquisition could not be assured with this approach even though the obtained electroconductivity curves were essentially consistent with those of Hlozny's, because the electrode constant will be changed in the crystallization operation. Substituted by a densimeter based on the Archimedes Laws, the online concentration measurement was further modified, and the quality of the measured data was improved with satisfactory accuracy and repeatability. Nevertheless, information obtained from the continuous phase (concentration or supersaturation measurements) was still not enough to completely define the system. In order to significantly reduce the limits of uncertainty in kinetic parameter estimations, it was also necessary to use information on the solid phase. Therefore, in addition to the measurement of dissolved solid concentrations obtained through online densimeter, transmittance measurements should be performed to provide online data about the second moment of CSD (Crystal Size Distribution). In this thesis, a light sensitive resistor served as the light/electricity conversion component was used to measure the transmittance. In order to efficiently improve the stability of the apparatus, a dual-optical detection circuit was adopted. To eliminate the multiple scattering effects on transmittance measurements, the range of the transmittance was limited to 70-100% by adjusting the width of the light flow cell and changing the intensity of the incident light through regulating the load voltage of the laser light source. Moreover, the data acquisition software was programmed in Microsoft Visual Bas...