混合二氯苯显微镜热台定向结晶过程实验研究

引　言近年来国内外不少研究工作者开展了降膜结晶分离提纯过程的研究[1～ 6 ].北京化工大学自 1 994年开始先后进行了单管降膜结晶实验并在此基础上提出了不同的计算模型[3～ 6 ].模型中均假设结晶前锋为光滑生长界面 .其中前两个模型过于简化 ,算得的晶层厚度偏低而晶层纯度偏高 .在数值模拟计算[6 ]中同时考虑温度和浓度在轴向和径向的变化 ,数值模拟结果有所改进 ,但所得晶层厚度仍然偏低 ,晶体纯度也偏高 ,为此模型中不得不引入由实验决定的杂质包藏系数ｎ .理论和实际之间的这一偏差很可能是将实际结晶过程的非光滑结晶前锋生长形态过…     

对二甲苯降膜结晶动力学

降膜结晶是工业生产对二甲苯的重要方法。以多孔介质分形理论为基础开展对二甲苯降膜结晶动力学的研究。通过动力学模型优化实验条件,结晶条件为进料速度为60 ml·min^-1、结晶温度-15℃、原料预冷温度25℃,发汗条件为升温速率1℃·min^-1、发汗终温5℃。在此条件下测定降膜结晶过程中对二甲苯结晶量以及液相夹带量,建立了晶体生长速率方程和液相夹带速率方程,相关系数分别为0.967和0.977,模型可靠。结果表明随着过饱和度的增加,液相夹带速率增长更快,晶层中夹带液相体积分数越大,晶层孔隙率越大。晶体生长速率方程和液相夹带速率方程的建立对工业降膜结晶生产对二甲苯过程中,通过调节液膜过饱和度控制晶层生长具有重要参考意义。     

对二甲苯降膜结晶动力学

降膜结晶是工业生产对二甲苯的重要方法。以多孔介质分形理论为基础开展对二甲苯降膜结晶动力学的研究。通过动力学模型优化实验条件,结晶条件为进料速度为60 ml·min-1、结晶温度-15℃、原料预冷温度25℃,发汗条件为升温速率1℃·min-1、发汗终温5℃。在此条件下测定降膜结晶过程中对二甲苯结晶量以及液相夹带量,建立了晶体生长速率方程和液相夹带速率方程,相关系数分别为0.967和0.977,模型可靠。结果表明随着过饱和度的增加,液相夹带速率增长更快,晶层中夹带液相体积分数越大,晶层孔隙率越大。晶体生长速率方程和液相夹带速率方程的建立对工业降膜结晶生产对二甲苯过程中,通过调节液膜过饱和度控制晶层生长具有重要参考意义。     

层熔融结晶法制备高纯度磷酸

动态层熔融结晶技术净化磷酸,具有分离效率高、能耗低、无污染等优点,绿色环保。采用程序冷冻法研究了动态层熔融结晶净化磷酸过程中的传质、传热、结晶动力学和杂质分布。研究表明,磷酸晶层生长速率随磷酸熔体起始浓度和冷端降温速率的增加而增加,达结晶平衡态时,晶层平均厚度随磷酸熔体起始浓度的增加和降温终点温度的降低而增加,研究建立了晶层的增长速率和平均厚度与过程操作参数的关联模型;晶层传热能力和杂质的分离效率随着磷酸熔体起始浓度和降温终点温度的降低而增加;采用优化的工艺条件,通过一次结晶纯化,可将质量分数为86.79%的磷酸溶液纯度提高至90.8%,杂质氟离子的质量分数由3×10~(-5)下降到4.5×10~(-6),杂质去除率达85%。     

降膜结晶分离过程热、质及动量传递的数值模拟(Ⅰ)分区域统一模型

基于混合物理论和实验观察所获得的有关结晶形态及形貌特征 ,建立了描述降膜结晶过程液相区、枝晶状固液两相共存区及固相区的动量、热量和质量传递过程的分区域统一数学模型 .模型的一个重要特点是认为在降膜结晶过程中 ,于熔体液相区和固相区之间存在一薄厚不匀且不断变化的固液两相共存区 .随着结晶过程的进行 ,二相区内固相体积分数不断增加并不断向液相区推移直至完全固化 .二相区的存在为降膜结晶时含杂质低纯度液体提供了在晶层内栖身的场所 ,这可能是产生杂质包藏的实际原因。引入多孔介质传递理论并采用Brinkman扩展的非稳态Darcy方程来描述固液两相区多孔介质内的流体流动 .进一步 ,采用VanDriest修正的Prandtl混合长度模型来描述降膜结晶过程中液体区的湍流流动与传热传质     

二乙胺导向合成中空纤维负载型SAPO-34分子筛膜

采用价格低廉的二乙胺为模板剂,通过球磨晶种诱导二次生长法制备中空纤维负载型SAPO-34分子筛膜用于CO_2/CH_4气体分离。系统考察了诱导晶种大小、膜合成液中二乙胺含量、铝源含量与晶化时间对膜结构形貌以及分离性能的影响。结果表明:相比于原始晶种,球磨晶种诱导制备SAPO-34分子筛膜层更加致密。随着膜合成液中二乙胺含量增加,膜表面分子筛晶体逐渐由SAPO-11向SAPO-34转变,当二乙胺含量过高时,载体表面未形成SAPO-34膜。当合成液中铝源含量较低时,分子筛膜晶化不够完全,当铝源含量过高时,膜表面晶体粒径逐渐减小甚至难以成核,膜层厚度减薄,不易生成连续的膜层。随着晶化时间的增加,膜层厚度逐渐增加,膜表面趋于致密。当膜合成液摩尔组成为1.0Al_2O_3∶0.9P_2O_5∶0.6SiO_2∶2.0DEA∶100H_2O,晶化时间为36 h时,球磨晶种诱导制得的SAPO-34分子筛膜分离性能最佳,膜的CO_2渗透性为1.11×10~(-6)mol·m~(-2)·s~(-1)·Pa~(-1),CO_2/CH_4分离选择性达80。     

NUMERICAL MODELING OF COUPLED MOMENTUM,HEAT AND MASS TRANSPORT IN FALLING FILM CRYSTALLIZATION PROCESS (Ⅱ)SIMULATION

采用液固二相分区域统一模型 ,以二元有机熔体 (萘 -硫茚 )为例 ,对实验管式降膜结晶器的动量、热量和质量传递过程进行了数值模拟 ,获得了液相区和固相区随时间的变化和枝晶状液固两相区内固相含量的变化及全场温度分布以及主组分萘分布及流场速度分布随时间的变化 .由积分得到的结晶层厚度及萘纯度与相应的实验数据进行了比较 ,所得结果在未引入任何经验常数的条件下与实验结果符合较好 .所建立的液固二相分区域统一模型可以较好地处理实际降膜结晶过程中枝晶状固液两相共存区晶体生长现象及不规则液固相变界面     

降膜结晶分离过程热、质及动量传递的数值模拟(Ⅱ)模拟计算

采用液固二相分区域统一模型 ,以二元有机熔体 (萘 -硫茚 )为例 ,对实验管式降膜结晶器的动量、热量和质量传递过程进行了数值模拟 ,获得了液相区和固相区随时间的变化和枝晶状液固两相区内固相含量的变化及全场温度分布以及主组分萘分布及流场速度分布随时间的变化 .由积分得到的结晶层厚度及萘纯度与相应的实验数据进行了比较 ,所得结果在未引入任何经验常数的条件下与实验结果符合较好 .所建立的液固二相分区域统一模型可以较好地处理实际降膜结晶过程中枝晶状固液两相共存区晶体生长现象及不规则液固相变界面     

硅溶胶辅助制备Beta分子筛膜

采用二次生长法，通过在晶种涂覆液中添加硅溶胶，在不锈钢丝网载体上制备牢固的Beta分子筛膜，系统考察晶种层中硅溶胶含量、晶种涂覆量、结晶温度和结晶时间对分子筛膜生长的影响。实验中发现，硅溶胶的存在可以增加涂覆层的牢固度，促进Beta分子筛膜的生长；硅溶胶既具有黏合剂的作用又可以补充硅源。当涂覆量较低时，无法得到一个完整的分子筛膜，分子筛膜负载量随着涂覆量的增加而增加；当结晶温度升至一定程度，分子筛膜组成不再单一。晶化过程中伴随着晶种的脱落和溶解。制备的Beta分子筛膜对N₂O催化分解具有良好的催化活性。     

多组分降膜蒸发传热计算方法研究

在降膜蒸发器设计过程中,现有的多组分降膜蒸发传热计算方法未考虑传质的作用,其计算结果与实际工程设计相比存在较大的偏差。文中探讨了多组分降膜蒸发传热机制,充分考虑降膜蒸发的传热传质耦合作用,提出了适用于多组分降膜蒸发传热计算的关联式。针对多组分物料物性随蒸发浓缩发生变化的特点,建立了降膜蒸发器蒸发传热的分段设计方法。完成了丙二醇水溶液降膜蒸发不同单元数的分段传热计算,计算结果表明:分段计算与不分段计算的管内传热系数与总传热系数的偏差率分别为23%—30%及12%—16%。随着计算单元数的增加,降膜传热系数偏差率的变化会逐渐减小,设计计算过程中可以根据精度要求确定最佳计算单元数。此研究工作为降膜蒸发器的工艺设计提供了参考。     

KINETICS OF MELT CRYSTALLIZATION IN FALLING FILMS

An analysis of the solidification rates of binary mixture melts flowing as a thin film on a cold surface, useful in the separation or purification of eutectic mixtures, is presented. The analysis which incorporates the hydrodynamics of the falling film and the convective heat transfer at the melt/crystal interface was used to determine the average crystallization velocity. The effect of parameters such as Stefan number (subcooling), initial superheat and the melt loading rate on the average crystallization velocity was examined. Experiments were performed using three different binary organic mixtures at subeutectic compositions. The results of the theoretical and experimental studies showed good agreement validating the model.     

降膜结晶分离技术实验研究

采用萘－硫茚物系对新型高效降膜结晶技术的分离提纯机制进行了实验研究．考察了管外降膜结晶过程中结晶时间、料液流量、冷却水初始进口温度、冷却水降温速率及原料浓度对分离提纯效果的影响．实验发现降膜结晶过程中晶层生长并不完全是光滑界面的生长,而在某些条件下表现为粗糙界面的生长,即生长过程在某些条件下表现为两相区界面的生长．进一步对辅助提纯－发汗过程的工艺参数—发汗时间、发汗初始浓度的影响进行了实验研究．实验结果可用于降膜结晶传热传质机理的数值模拟．     

DIRECT CONTACT CONDENSATION OF DILUTE STEAM/AIR MIXTURES ON WAVY FALLING FILMS

Condensation from air-steam mixtures on falling water layers is investigated experimentally and theoretically. The thin film flows on the inner surface of a 5cmi.d. vertical pipe. This film is wavy turbulent while the gas phase is kept saturated with steam. Experiments are conducted with the gas mixture effectively stagnant, compared with the fast moving liquid film. Measurements are also made under a mild vacuum applied on the gas phase. Heat transfer coefficients averaged over the entire length of the condensing surface, tend to increase by decreasing the liquid flowrate, by increasing the steam fraction, and by applying a mild vacuum on the gas phase. However, for the cases examined, there is a liquid flowrate above which the heat transfer coefficient becomes almost constant.

Numerical predictions are made for a fully developed turbulent film using an eddy diffusivity model. The results indicate that for a system with a large amount of noncondensable gases-as in this study-the temperature profile in the liquid film is nearly uniform and that the major resistance to condensation resides in the gas phase. The analysis also shows that the relative contribution of sensible heat transferred through the gas phase is small relative to the latent heat released upon condensation. Comparison of predictions with experimental data suggests that a significant parameter in these analyses is the gas diffusion boundary layer thickness which seems to be comparable in size with the liquid film thickness. Finally, the possibility is discussed of correlating condensation heat transfer coefficients with already available statistical characteristics of the falling wavy layer. Theoretical predictions based on this idea are in good agreement with data.     

Aspects of the Two-Layer Model for Direct Contact Condensation of Steam on Wavy Falling Films

Detailed physical modeling of the direct contact heat transfer process of steam condensing on a falling liquid film is a very difficult task due to the complex hydrodynamics of the film. The present state of the art is restricted to film Reynolds number of the order of 100. On the other hand, empirical relations cannot offer any insight into the mechanism and features of heat transfer in liquid film. Phenomenological models are needed to bridge the gap between empirical relations and direct physical simulations. One of these models is the so-called two-layer model, which divides the falling film into a laminar conduction-dominated (substrate) layer flowing over the solid wall and a completely mixed layer representing the waves. This model is further developed here by focusing specifically on two aspects. First, the influence of temperature-dependent physical properties of the liquid on the structure and heat transfer characteristics of the substrate layer is studied, and typical results are presented in the absence of waves. Second, the relation of the main parameter of the model (i.e., thickness of the substrate layer) to local film state is discussed in detail. Generalized constitutive laws and an approach based on utilization of the experimental film thickness time series are proposed and discussed. The proposed procedures can be integrated to a generalized two-layer model for direct contact condensation.     

管形对水平管降膜圆周膜厚和Nusselt数的影响

针对海水淡化系统的水平管降膜蒸发,建立了二维数值模型,分析了光滑圆管和6种不同截面形状的蛋形管外降膜流动及传热特性。采用VOF方法考察了不同管型对管外液膜分布和传热特性的影响。数值结果表明:蛋形管外液体沿周向流动较圆管稍快,且可获得更均匀更薄的液膜;液膜厚度随半轴比ε增大而减小,随周向先逐渐减小后迅速增加,圆管和蛋形管的液膜最小值分别出现在周向相对坐标0.69和0.70~0.84附近。蛋形管的膜内量纲1温度较圆管的小,其热边界层厚度较薄,具有更好的传热性能。拟合数据得到,ε为2.4的蛋形管具有最好的传热性能,其Nusselt数可达0.32,较圆管的高出12.68%。最后,将数值模拟结果与文献中的数据进行了对比,验证了数值模型的可行性和合理性。     

THE STEFAN AND DEBORAH NUMBERS IN POLYMER CRYSTALLIZATION

A model of the phenomenon of gradual crystallization of a polymer melt subjected to an unsteady temperature distribution is presented. The model allows for gradual changes of the degree of crystallization. It is shown that the values of two dimensionless parameters, the Stefan and Deborah numbers, simultaneously influence the behavior of the system. When the ratio of these two parameters, which we propose to call the Janeschitz-Kriegl number, becomes very large as compared to unity, the model degenerates into the classical abrupt crystallization model known in the literature as the Stefan problem. This asymptotic behavior is shown to hold through a modified boundary layer analysis, where the boundary layer stays attached to a moving, rather than a fixed boundary.     

Transfert de chaleur dans un film tombant autour d'un cylindre horizontal

As the initial flow of a falling film above one or several horizontal tube is known, we propose a model which permits the determination of the local heat transfer coefficient. The model is valid for Reynolds numbers lower than 2000 and for either a constant flux or a constant temperature. The fraction of the film which is evaporated after its passage over the exchange surface is taken into account. The calculated exchange coefficients, the film thickness and the Nusselt numbers are compared with the numerical and experimental results of different authors. The agreement is good, particularly for the case of estimation of the average Nusselt number for Reynolds numbers greater than 1000.     

磁场作用下垂直管外氨水降膜吸收的模型研究

在氨气吸收过程中增加了宏观磁场力，考虑了吸收过程中降膜溶液膜厚的变化、膜厚方向的对流以及氨水溶液物性的变化，建立了磁场条件下垂直管外氨水降膜吸收数学模型。在磁感应强度0－3T范围内，对数学模型进行数值求解，得到温度、浓度、速度分布等参数。结果显示磁场对于氨水降膜吸收过程有一定的增强作用。     

A numerical study of flow boiling in a microchannel using the local front reconstruction method

The rapid advances in performance and miniaturization of electronic devices require a cooling technology that can remove the produced heat at a high rate with small temperature variations, as is obtained in flow boiling. To obtain insight in flow boiling, we performed numerical simulations in a 200 μm square microchannel using the local front reconstruction method. Besides validation with literature results, a parametric study shows an increasing heat removal rate and bubble growth rate with increasing wall temperature, liquid mass density, and liquid heat capacity and decreasing inlet velocity indicating the importance of phase change compared to convective transport. Finally, the heat transfer in the liquid film is studied using a Nusselt number defined with the film thickness, which is comparable to Nusselt number for falling films on hot surfaces. It is observed that convective effects are more pronounced at the bubble rear compared to the bubble front.     

微细铜丝在水中自然对流换热

采用实验与数值模拟相结合的方法分别研究了封闭空间内水平放置的直径为39.9、65.8、119.1 μm的微细铜丝（微丝）在水中的对流换热,分析了微丝表面自然对流换热特性及机理。实验通过焦耳加热的方法测量了不同直径微丝在水中自然对流的传热系数及Nusselt数。同时建立三维不可压数学模型对微丝在水中的自然对流进行数值模拟,并将计算结果与实验值进行了对比。研究表明,数值模拟结果与实验值基本吻合,微丝在水中自然对流的传热系数随直径减小而显著增大,Nu则明显减小,且Nu随热通量增加的变化率也随直径减小而明显降低;微丝表面边界层厚度随直径减小而变薄,但边界层厚度与微丝直径的比值则逐渐增大;另外,对比微丝与常规尺度圆管表面自然对流的流场、温度场以及边界层分布,发现相同温差下微丝表面自然对流换热的边界层与常规尺度下沿壁面由底部向上发展的形状不同,而是沿微细丝表面呈椭圆形包裹于其上,因此削弱了表面对流换热强度,导致温度场呈现出较明显的导热特征。