【作者】 陈亮；

【导师】 李军；

【作者基本信息】 四川大学 ， 化学工艺， 2007， 博士

【摘要】 熔融结晶过程是一种分离提纯有机物的有效方法，它不仅能制备高纯度的有机物，而且还能分离其它分离方法难以分离的特种有机物系，如同分异构体、同系物和热敏性有机物。同时，在熔融结晶装置中，塔式结晶器由于能在同一设备中同时实现洗涤、重结晶和发汗提纯机理而被广泛使用。因此，开发具有自主知识产权的新型结晶分离技术，设计一个高效连续的熔融结晶装置就具有非常重要的意义。本文的研究目标是采用针对有机物系固液相间密度差较小而设计的倾斜结晶塔，研究有机物的分离提纯规律，开发出具有自主知识产权的倾斜型结晶塔，并通过对结晶过程的数学建模以及流体模拟，以获得结晶塔的设计放大规律。为了便于实验现象的观察，首先制作一个长为1200mm，内径为50mm的玻璃制倾斜结晶塔，研究对二氯苯一邻二氯苯低共熔体系在倾斜结晶塔中的分离规律。正交实验结果表明：结晶塔中固液二相的传质和传热较慢，所需的稳定时间约为600min，进料浓度和搅拌速率对稳定时间的影响不明显；进料浓度对结晶塔提纯段的浓度分布和结晶床层高度有显著影响，进料浓度增加，结晶床层越高，提纯段各点的浓度也升高，且晶体床层区域的熔融液处于过饱和状态，但是，进料浓度对产品纯度的影响不大，因为实验体系为低共熔物系，进料浓度为62％～86％时，产品纯度都达到了99.9％以上；搅拌对结晶塔分离提纯效果的影响表现在它可以提供足够的扰动，推动熔融液与晶体之间的逆流接触，促进两相之间的传质和传热，避免出现晶体悬浮密度分布不均匀和熔融液出现沟流的现象，还可以有效地防止塔壁晶体结块现象的发生，搅拌速率对晶体床层高度并没有影响；晶体床层的存在是结晶塔实现分离提纯的必要条件，且可以分为二段，上端为晶体沉降区，晶体量较少，固液二相混合较好；下端为紧密晶体床层区，晶体堆积紧密，需要有足够的搅拌速率，来保证固液二相的充分混合，开始出现结晶床层的部分，熔融液浓度上升很快，提纯效果十分明显，之后熔融液浓度的上升趋势变缓，发汗提纯机理在发挥作用。为进一步考察倾斜结晶塔对有机物分离提纯的适应性，在增加结晶段长度后，使用丙烯酸-丙酸部分固体溶液体系，研究了加热功率、返流比和晶体悬浮密度对结晶塔分离提纯效果的影响。实验结果表明：对于现有的玻璃制结晶塔，进料浓度为90％是一个合适的进料浓度；由于降温温差的增大，结晶塔到达稳定的时间为1200min;回流比越大，产品纯度越高，所需理论塔板数就越多；但是晶体床层是分离提纯过程的基础，如果悬浮密度较小，则即使回流比较大，也难以制得较高纯度的产品；理论塔板数反应了结晶塔的分离提纯效率，理论塔板数越多，则结晶塔的分离效率越高，在一定的悬浮密度范围内，回流比与理论塔板数呈线性关系；加热功率对产品纯度和浓度分布的影响，是通过其对回流比和悬浮密度的改变而实现的，回流比越大，晶体悬浮密度通常也较大。通过前期的实验观察，发现搅拌桨相邻桨叶之间容易聚集晶体造成结晶塔的堵塞，因而对结晶塔进行了重新设计，使用不锈钢制倾斜结晶塔，考察了搅拌方式与转速、加热和冷却功率、进料浓度和倾斜角度对结晶塔分离提纯效果的影响。实验结果表明：通过对结晶塔测温口的重新设计，消除了两相邻桨叶之间的晶体聚集，确保了结晶塔的连续操作，不锈钢制倾斜结晶塔已经可以运转30个小时以上；结晶塔的倾斜角度对结晶塔的分离提纯效果有明显的影响，倾斜角度为45度时分离效果最好；加热功率和冷却功率需要匹配，有效加热功率为10w时结晶塔能稳定操作，结晶塔的分离提纯效果较好；搅拌方式的影响主要是通过调节搅拌轴的正转和反转来防止结晶塔内的堵塞，保证了分离过程的连续进行；使用不锈钢制倾斜结晶塔分离提纯对二氯苯—邻二氯苯低共熔体系，塔底的产品纯度可以达到99.99％以上，且能够长时间连续运转。改进后的不锈钢制倾斜结晶塔，在倾斜角度为45°，搅拌速率为15r/min，进料浓度为80～93％，冷却循环水冷却温度为25～30℃时，能连续运转48小时以上，塔底产品纯度可达到99.997％；出料速率大于1.7g/min时，结晶塔的提纯效率开始下降，出料速率在0.4～O.9g/min之间，结晶塔能稳定高效地运转；在连续实验中，增加返流比，能显著提高结晶塔的分离效果和产品的纯度；加热功率和冷冻功率对结晶塔分离提纯效果的影响，是通过改变返流比和晶体悬浮密度来实现的。为了模拟工业装置的运行情况，还重新设计了新型的结晶塔，此结晶塔使用双螺旋输送桨来输送晶体，取消了提纯段的取样口和测温口，所设计的结晶塔已经获得了专利授权。为了更好地描述结晶塔内固液两相的传质和传热过程，综合各种提纯机理的数学描述，对结晶塔提纯段建立通用的数学模型，此通用数学模型能够经化简得到文献中已报导的各种数学模型。使用相同的边界条件，比较了各种数学模型，数学模型的计算结果表明，发汗一传热模型最能反映重力沉降型结晶塔内的提纯过程。使用发汗一传热模型，系统地分析了各种因素对结晶塔分离提纯过程的影响。使用简化的发汗模型，计算了结晶塔提纯段的熔融液浓度分布，计算结果与实验值一致，证实了该模型的有效性。使用Fluent软件对结晶塔内固液两相的流动过程进行了流体模拟，计算所得结果证实了结晶塔提纯段内晶体悬浮密度的分布经历了三个不同的区域，即熔融液占据区、晶体沉降区和紧密晶体床层区，模拟结果与实验观察到的现象完全吻合，且与一维数学模型的计算结果也一致。通过对结晶塔的进一步数学分析，从产品纯度、回收率、产量、结晶塔稳定性和设备投资五个角度，探讨了结晶塔的放大规律，研究了各操作条件对结晶塔设计放大的影响，这些影响因素包括返流比，回流比，加热和冷冻功率，熔融液轴向扩散系数，晶体发汗提纯速率，晶体沉降速率，晶体尺寸及形状和倾斜角度。更多还原

【Abstract】 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.更多还原