【作者】 向爱双；

【导师】 许松林；

【作者基本信息】 天津大学 ， 化学工程， 2004， 硕士

【摘要】 采用分子蒸馏技术在高真空条件下,实现非常规混合物系在液体状态下的分离,该项应用研究可以追溯到上世纪初期。伴随着高真空技术的进步,人们对分子蒸馏技术的应用及研究越来越深入。尤其是到了20世纪末期,随着人们对天然产物越来越青睐,分子蒸馏作为一种高效的液体分离手段,在天然产物分离中的应用越来越受重视。随着该项技术的广泛应用与发展,人们对该技术的理论研究也越来越深入。但是在众多的数学理论模型中,尚未有一种能真正准确描述分子蒸馏过程的。利用计算机对分子蒸馏过程进行流体力学(CFD)数值模拟,是一种较新的理论研究方法。课题主要利用计算流体力学软件对刮膜式分子蒸馏蒸发液膜的流体力学性能进行模拟,并通过一项分子蒸馏的应用实验研究,从实验现象和结论方面,对流体力学模拟结果进行评判。课题对液膜的研究,主要集中在液膜厚度、膜内速度分布及流动状态上。课题基于三个主要假设,适当简化计算流域为二维模型,在恒温恒压条件下,对刮膜式分子蒸馏蒸发液膜进行了纯流体力学模拟。在旋转坐标系下进行模拟计算,选用了RNG和Realizable两种k-ε湍流模型进行对比研究,并利用VOF多相流模型追踪气液界面,采用增强壁面函数法加强对近壁面流动的处理,并通过模拟结果比较了两种湍流模型的优越性。通过比较,发现两种湍流模型在研究中差别不大,但前者具有微弱优势,又鉴于后者已被采用过,因此选用RNG模型进行进一步研究。分别改变刮膜转速及进料流量,模拟得出液膜内的速度分布、气液界面的波形变化以及湍流分布等的变化情况。模拟结果表明,蒸发液膜呈现层流、湍流、波动流等复杂流动形态,湍流主要出现在刮膜器前的弓形波内。并且随着转子转速增加或进料量减小,弓形波波动程度增大,湍流强度也增大,湍流耗散也越快。在远离刮膜转子处,蒸发液膜呈现层流状态,受转子转速或进料量影响较小。通过分子蒸馏精制纯度为95.0%的3-羟基丙腈中间产品的实验研究,考察了不同转速和进料流量等条件,即不同液膜流动状态,对产品分离效果的影响,发现增加转速或减小进料速率,产品纯度增大,分离效果越好。对比模拟与实验结果,得出另外一个结论:增加转速或减小进料量,增大了蒸发液膜的表面积,强化了传质传热过程,从而能获得较好的分离效果。更多还原

【Abstract】 Application of molecular distillation can be traced back to the beginning of last century efficiently separating unusual mixture under high vacuum and liquid state. With the development of high vacuum technique, application and research on molecular distillation are carried out more and more deeply and widely. Especially up to late 20th century when people gradually become enjoying natural material, significance of molecular distillation has been revealed as an effective natural material purification method. At the same time, theoretical research on molecular distillation has also been carried out largely. However, among all kinds of existing mathematical models none has really and correctly described the molecular distilling process. Moreover, computational fluid dynamics (CFD) simulation belongs to an innovative method of studying molecular distilling process. CFD simulations with software were mainly accomplished to study flow field of evaporating film in a wiped molecular distillatory, and an experiment of applying wiped molecular distillation was also completed to observe phenomenon of the thin liquid film flow and obtain qualitative conclusions. According to these conclusions and phenomenon, simulating results were evaluated and verified. What’s more in this dissertation, study on liquid thin-film was concentrated on film thickness, velocity distribution and flow state in the film. Taking advantage of CFD simulating software, Fluent, and based on three hypotheses to simplify modeling and computation process, mathematic theoretic models were built in two dimensions and pure fluid dynamics simulations were executed of evaporating liquid thin-film in the wiped molecular distillation under atmospheric temperature and pressure. RNG and Realizable k-εturbulent models were compared with simulation results in rotating reference frame to estimate their effectiveness in this study. At the same time, volume of fluid (VOF) was also built to track interface between liquid and gas as well as enhanced-wall function method to further deal with near-wall flow. By comparison, it is found that little difference exists between the above two turbulent models. And the RNG turbulent model was then applied to further study the liquid film. With respectively changing the wiper speed and feed flow, velocity distribution in the thin-film was simulated as well as turbulence distribution and wave interface between liquid and gas. Simulating results show that laminar, turbulent and wavy flow states are simultaneously subsisting in the evaporating liquid thin-film, and turbulence appears in the arched wave before wipers. With increase of wiper speed or decrease of feeding flow, the undulation becomes more enhanced as well as its turbulent intensity and dissipation. Away from wipers, the liquid film flow belongs to laminar state and is less affected by wiper rotation and feeding speed. Furthermore, an experiment on purifying 3-hydroxypoinitrile from a nearly pure medium product by wiped molecular distillation was carried out to obtain the flow phenomenon and separation degree with change of wiper speed and feed flow, that is, under different thin-film flow state. Comparison of simulating and experiment results, another conclusion can be induced that with increase of wiper speed or decrease of feeding flow, better separation degree can be attained because surface area of the evaporating liquid film is enlarged and then its mass and heat transfer process are enhanced.更多还原