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An Investigation On The Purification Process Of Organic Compounds In An Inclined Column Crystallizer

Posted on:2008-02-18Degree:DoctorType:Dissertation
Country:ChinaCandidate:L ChenFull Text:PDF
GTID:1101360218462483Subject:Chemical processes
Abstract/Summary:PDF Full Text Request
Melt crystallization is a green technology for separating and purifying organic compounds; it not only can produce high purity products but also can separate isomers and heat-sensitive organic compounds which can't be separated economically by other methods. Meanwhile, among crystallization devices, column crystallizer is widely used because it realizes washing, recrystallization and sweating mechanisms at the same time in one single piece of equipment. Therefore, the development of a new crystallization technology with independent intellectual property rights and the design of a continuous column crystallizer with high efficiency, have a very important significance. The object of this study is to study the separation characteristics of an inclined column crystallizer, which is design for the organic systems according to its relatively small density difference between solid and liquid phase, to develop a new type inclined column crystallizer with independent intellectual property rights, and to discuss the scaling up of the inclined column crystallizer through mathematical analysis and CFD modeling.In order to facilitate the experimental observations, a glass column crystallizer (1200mm×50mm i.d.) with an inclination angle of 45 degrees is used to separate p-dichlorobenzene-o-dichlorobenzene simple eutectic system. The experimental results show that the rates of heat and mass transfer in inclined column crystallizer are slow, time for reaching steady state is about 600 minutes, and feed concentration and stirring rate had no significant effect on it. Feed concentration has a significant impact on the height of crystal bed and the melt concentration profile in purification section, and both of them increase with the increasing of feed concentration, and the melt in crystal bed is supersaturated. However, feed concentration had no significant effect on product purity, because the system forming eutectic mixture. Purity of product can easily reach 99.9% with a feed concentration between 62% and 86%.The main effect of stirring is to provide enough disturbances to promote countercurrent contact between melt and crystal phase which is good for heat transfer and mass transfer. It also prevents the uneven flow of melt and the caking on column wall, but has no effect on the height of crystal bed. Height of crystal bed is a necessary condition for purification, and can be divided into two parts, the settlement section at the top and the packed section at the bottom. In packed zone, there is a need for adequate stirring rate to ensure the well mixing. Melt concentration increases sharply when the crystal bed appears, and in packed zone, concentration increases slowly because sweat mechanism plays the dominant role.Further inspection of the inclined column crystallizer is performed by increasing the length of cooling section of the previous glass column crystallizer. The column crystallizer is operated in a continuous manner with the acrylic acid-propionic acid partial solid solution system, the effects of heating power, reflux ratio and suspension density are investigated. The experimental results show that the feed concentration of 90% is suitable for present glass column crystallizer, and the stable time is about 1200 minutes due to the increased temperature difference. Reflux ratio is the most import factor influencing the product purity, the bigger the reflux ratio, the higher the product purity, and the more the necessary theoretical plates. But the crystal bed is the basis for purification, it is necessary to maintain a sufficient suspension density. With a smaller suspension density, it is also difficult to produce high purity products even with a greater reflux ratio. Theoretical plates can be a measure of the separation efficiency of column crystallizer, the more the theoretical plates number, the higher the separation efficiency of the column crystallizer. In a specific range of suspension density, reflux ratio is linear with the number of the theoretical plates. The effect of heating power on product purity and melt concentration profile is realized by the changing of suspension density and reflux ratio, in most cases suspension density increases with reflux ratio.Through the above preliminary experiments, the agglomeration of crystals between adjacent scrapers which easily causing the blocking of the column crystallizer is observed. Therefore, a new designed stainless column crystallizer is used to separate the p-dichlorobenzene-o-dichlorobenzene simple eutectic system. Effects of stirring rate and manner, heating and cooling power, feed concentration and inclination angle are studied. The experimental results show that the accumulation of crystals between two adjacent blades is well eliminated by redesign of the temperature measuring taps, and the improved column crystallizer can be operated more than 30 hours. Inclination angle has a very significant effect on purification performance, and the best separation efficiency is achieved with the inclination angle of 45 degrees. Heating and cooling power have influences on suspension density, and they need a match. An effective heating power of 10w can stabilize the column operation. The stirring pattern, which is represented by the adjustment of rotating direction, is mainly to prevent column blocking, thus to guarantee the continuous operation of the column. With the stainless steel column crystallizer, the product purity of p-dichlorobenzene can reach 99.99%.The continuous operation of the improved stainless steel column crystallizer is carried out under the following conditions, inclination angle: 45 degrees, stirring rate: 15r/min, feed concentration: 80~93%, circulating cooling water temperature: 25~30°C. The column crystallizer can be continuously operated for more than 48 hours and the product purity can reach 99.997%.The purification efficiency of column crystallizer starts to decline when the product rate is larger than 1.7g/min. The column crystallizer can be stably operated with the product rate of 0.4-0.9g/min.During the experiments, increasing the reflux ratio can significantly improve the efficiency of the column crystallizer and the product purity. The effects of heating and freezing power are realized by changing the reflux ratio and suspension density. In order to simulate plant operation, another new type of column crystallizer is designed. It abolishes the temperature measuring taps and sampling taps, and uses a double screw propeller to transport crystals, this column crystallize has been authorized by two patents.In order to better describe the mass transfer and heat transfer processes between two phases in column crystallizer, a general mathematical model is developed by considering the various purification mechanisms. All the reported models in literatures can be formulated by simplifying the common model.With the same boundary conditions, a comparison of various mathematical models is performed. The computation results show that sweating-heat transfer model can best reflect the purification process in gravity column crystallizer. A systematic analysis of the impacts of various factors on the purification process is carried out by using the sweating-heat transfer model. The calculated concentration profile by simplified sweating-heat transfer model consist with the experimental results, thus confirm the Validity of the model.The solid-liquid two-phase fluid flow in column crystallizer is simulated by Fluent software. Calculation results confirm that the distribution of suspension density undergone three different regions, which coincide with the experimental observation, and with the one-dimensional mathematical model.The scaling up of column crystallizer is also discussed from the aspects of product purity, recovery, production yield, operation stability and equipment investment. Through mathematical analysis, the effects of operating conditions on scaling up are studied. These factors are reflux ratio, heating and freezing power, axial dispersion coefficient of melt, sweating rate, crystal settlement rate, crystal size and shape, and inclination angle.
Keywords/Search Tags:Melt crystallization, Column crystallizer, Organic compounds, Separation and Purification, Mathematic model, CFD, Scaling up
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