MECHANISM OF CRYSTALLIZATION IN AGITATED SOLUTIONS

A new concept of molecular cluster dynamics in turbulent flows is used to develop a physical model and mechanism of homogeneous and heterogeneous nucleation in solutions. The proposed physical model considers the motion of molecular clusters in centrifugal fields of free eddies induced in turbulent flows. The clusters are moved to an eddy boundary region where they are concentrated and aggregated to form critical nuclei.

It is shown theoretically that a critical fluid velocity exists, below which nuclei are not formed, and that its value depends upon the degree of solution supersaturation. The nucleation theory takes into account the hydrodynamic parameters of the process and permits calculation of the number of nuclei formed. Satisfactory agreement was obtained between calculated values and published experimental data.     

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.     

Molecular dynamics simulation of orientation and crystallization of polyethylene during uniaxial extension

Molecular dynamics simulations of realistic, united atom models of polyethylene undergoing uniaxial extension are described. Systems composed of chains ranging from 25 to 400 carbons have been studied, under a variety of processing histories, including isothermal deformation at constant applied stress below the melt temperature T_m, isothermal deformation below T_m followed by annealing, isothermal deformation above T_m followed by crystallization at a quench temperature below T_m, and non-isothermal crystallization during simultaneous deformation and cooling through T_m. Extension and orientation of large segments of flexible chains by uniaxial deformation accelerates the primary nucleation rate to a time scale accessible by molecular dynamics simulation. Entanglements operative during active deformation promote extension and orientation without nucleation of a crystal phase, while relaxation of stress at constant strain is sufficient to allow slippage of chains past pinning points and rapid nucleation and growth of crystallites as neighboring oriented chains come into registry. Isothermal crystallization of pre-oriented systems shows an apparent increase in nucleation density at lower temperatures; the resulting ordered regions are smaller and more closely aligned in the direction of orientation. During non-isothermal deformation, where stretching and cooling occur simultaneously, a first order transition is observed, with discontinuities in the volume and global order parameter, when the system crystallizes.     

BTS极性微晶玻璃恒温场晶粒定向生长机理

Halliyal和Ruessel认为，极性微晶玻璃的晶粒定向生长是由于高温梯度场驱动的。但这难以解释BTS极性微晶玻璃的特殊恒温场晶粒定向晶化工艺。基于实验和理论分析，提出BTS极性微晶体的偶极子模型，证明在特殊恒温场下，极性微晶诱导产生的静电作用，是晶粒定向生长的一个重要因素。模拟计算表明，极性微晶体诱导电场能Ec=0.35eV,约为晶体形成活化能ΔGa(2.39eV)的15％，改变了晶粒在不同方向的生长速率，显著影响微晶玻璃的晶粒定向生长过程，有利于极性微晶体的择优取向生长。     

螺旋桨搅拌槽中液体的流动结构

在直径１６００ｍｍ的平底柱形有机玻璃槽中，利用热膜风速仪测量了２个搅拌浆－槽体系中流体的时均速度，脉动速度的均方根值，湍流强度，纵向积分尺度，纵向微分尺度和能谱函数。研究了时均速度及湍流微结构在搅拌槽中的分布规律及搅抖转速的影响。     

超临界流体结晶技术研究进展

综述了超临界流体结晶技术的理论和应用进展，重点介绍了三种超临界流体结晶技术：超临界溶液快速膨胀结晶法，超临界流体抗溶剂结晶法和超临界流体梯度结晶分离法，并指出了它们的研究方向。     

CRYSTAL GROWTH LIMITATION IN AQUEOUS FOAM CRYSTALLIZATION

We present results concerning the limitation of maximum crystal size by pre-foaming an aqueous supersaturated solution, followed by its crystallization. Foams are generated by adding specific surface active agents to the solution and by air injection under continuous high shear agitation, leading to a gas-liquid stable dispersion having polyhedral internal structures with thin separating liquid films. This spatial dispersion structuration, by locally reducing the amount of solute available for each crystal growth, permits the limitation of the resulting crystal sizes. It is shown that, under these conditions, the initial film thickness and mean bubble radius, obtained for a given volume fraction of gas in the foam, can control the final crystal size distribution. The foam expansion ratio thus appears to be a new operating parameter for crystal growth limitation when using a foam crystallization technique.     

SECONDARY NUCLEATION OF SUCROSE BY FLUID SHEAR IN AQUEOUS SOLUTIONS

The motivation for this work was based on two hypotheses: subcritical nuclei of sucrose may be generated by secondary nucleation using a fluid shear field acting on the surface of a growing seed crystal in aqueous solutions, and a near-dynamic equilibrium distribution of embryos may be attained. These were supported experimentally. The value of a σ-based model parameter, a, was obtained by describing the system by a Boltzman- like distribution model. The number concentration of subcritical nuclei was well described by the model. Despite differences in experimental method and in the basic nature of the solute, sucrose, the behavior of the system and results were similar to those obtained in the earlier work using aqueous salt solutions [Sung et al. (1973), Jagannathan et al. (1980)].     

SECONDARY NUCLEATION OF SUCROSE BY FLUID SHEAR IN AQUEOUS SOLUTIONS

The motivation for this work was based on two hypotheses: subcritical nuclei of sucrose may be generated by secondary nucleation using a fluid shear field acting on the surface of a growing seed crystal in aqueous solutions, and a near-dynamic equilibrium distribution of embryos may be attained. These were supported experimentally. The value of a σ-based model parameter, a, was obtained by describing the system by a Boltzman- like distribution model. The number concentration of subcritical nuclei was well described by the model. Despite differences in experimental method and in the basic nature of the solute, sucrose, the behavior of the system and results were similar to those obtained in the earlier work using aqueous salt solutions [Sung et al. (1973), Jagannathan et al. (1980)].     

CRYSTAL GROWTH LIMITATION IN AQUEOUS FOAM CRYSTALLIZATION

We present results concerning the limitation of maximum crystal size by pre-foaming an aqueous supersaturated solution, followed by its crystallization. Foams are generated by adding specific surface active agents to the solution and by air injection under continuous high shear agitation, leading to a gas-liquid stable dispersion having polyhedral internal structures with thin separating liquid films. This spatial dispersion structuration, by locally reducing the amount of solute available for each crystal growth, permits the limitation of the resulting crystal sizes. It is shown that, under these conditions, the initial film thickness and mean bubble radius, obtained for a given volume fraction of gas in the foam, can control the final crystal size distribution. The foam expansion ratio thus appears to be a new operating parameter for crystal growth limitation when using a foam crystallization technique.     

溶胶-凝胶法制备CaAl_2SiO_8粉末及其结晶行为研究

以正硅酸乙酯、硝酸钙及异丙醇铝为原料，用溶胶-凝胶法制备了钙长石粉末．为调节铝酸盐的水解动力学，采用乙酰乙酸乙酯作为化学螯合剂，研究结果表明，不同的螯合剂和水的加入量对溶胶的聚合过程和所得凝胶特性有重要影响．在选定的实验条件下，制备的凝胶均匀透明，易于形成Al－O－Si网络，使结晶过程明显加快     

MICROMIXING EFFECTS ON BARIUM SULFATE PRECIPITATION IN AN MSMPR REACTOR

The Effects of non-ideal and nonhomogeneous mixing on barium sulfate precipitation in an MSMPR reactor were observed experimentally and analyzed theoretically. To generate nonhomogeneous mixing the unmixed feed streams were fed to the reactor at the same location (joint feeding mode) or a plug flow reactor was connected to the MSMPR reactor. These nonhomogeneous mixing conditions resulted in significant reductions in particle size and increases in particle numbers. These non ideal mixing effects were dependent on the impeller speed, feed stream velocity and residence time in the connected plug flow reactor and are believed to result from elevated supersaturation levels in a premixing zone which are controlled by turbulent micromixing

To model the effect of nonhomogeneous mixing (premixing) in the MSM PR reactor a plug flow-stirred lank reactor series model was developed. The plug flow reactor represents the premixing region of the MSMPR reactor in which turbulent micromixing is important, and the stirred tank reactor describes the homogeneous mixing region of the MSMPR reactor where particle growth is important. The model predicts that the premixing effect is strongly dependent on micromixing of the feeds in the premixing region, and thus, as the turbulent mixing intensity in this region is increased, the particle size in the product suspension is reduced and the particle population is increased. These predictions of the model arc in good agreement with the experimental data. An interesting prediction of the model is that as the impeller speed increases, the precipitation of barium sulfate in an MSMPR reactor deviates increasingly from the precipitation in a perfectly mixed (ideal) reactor.     

CRYSTALLIZATION KINETICS OF 2-CHLORO-4,6-DINITRORESORCINOL BY BATCH COOLING CRYSTALLIZATION METHOD

Crystallization kinetics of 2-chloro-4,6-dinitroresorcinol in ethanol was studied by the method of intermittent dynamic analysis. The nucleation rate and crystal growth rate of 2-chloro-4,6-dinitroresorcinol under different crystallization temperatures and stirring rates were estimated. The results show that with an increase of crystallization temperature, both the nucleation rate and crystal growth rate increase. It is further found that when the stirring rate increases, the nucleation rate increases and the crystal growth rate decreases. The technological conditions of cooling crystallization of 2-chloro-4,6-dinitroresorcinol were studied. It is found that when the stirring rate is 180 rpm and the solution is cooled slowly to ? 8°C, the particle size of the products is even and the highest yield of 2-chloro-4,6-dinitroresorcinol is 40.6%.     

NUMERICAL SIMULATION OF TURBULENT FLOW OF AGITATED LIQUID WITH PITCHED BLADE IMPELLER

The turbulent flow field in an agitated system with baffles was solved numerically using the standard k-e model, an algebraic Reynolds stress model (ASM) and a differential Reynolds stress model (RSM). The commercial software FLOW3D (CFDS, Harwell Laboratories, 1991) was used for this purpose. The aim of the study was to investigate the influence of the impeller boundary conditions and turbulence models to the agreement with experimentally obtained laser-Doppler anemometry data. The boundary conditions for the impeller discharge used in the numerical calculations were obtained as whole-cycle-ensemble averages from experimental LDA-measurements (Fort et al., 1992). Since measurements of the rate of dissipation of turbulent kinetic energy ( ε) was not available the dissipation rate per unit mass in the impeller discharge was estimated from the expression:

where k is the turbulent kinetic energy per unit mass and L the macroscale of turbulence in the pitched blade impeller discharge. The macroscale of turbulence (L) in the impeller boundary condition for e was varied in order to optimize the fit of theoretically obtained profiles of turbulent kinetic energy with experimental data. The constant A was fixed to 0.85 according to Wu and Patterson (1989). The optimal values of L for the different turbulence models were compared with the projected height of the impeller blade (h). All three components of the mean velocity were compared with experimental data for the optimal ratio of L/h for six radial cross-sections in the tank.

The mean velocity field obtained from simulations showed good agreement with experimental data for all models, with somewhat better agreement for the k — e model. An optimal value of the ratio L/h was found to be equal to 2.0 for the k — ε model and 1.3 for the ASM. However, no such optimal value for the RSM could be determined in this study.     

ANALYSIS OF BATCH CRYSTALLIZATION PROCESSES

The analysis of batch crystallization processes normally requires the consideration of the time-dependent, batch conservation equations (e.g., population, mass, and energy balances), together with appropriate nucleation and growth kinetic equations. The solution of these integro-differential equations is relatively difficult, even by numerical techniques. This review outlines the advances that have been made in the experimental techniques and data analyses which can be used to study the crystallization kinetics and the crystal size distribution (CSD) in batch suspension crystallizers. Several simple and useful methods are discussed which include characterization of CSD maximum, cumulative CSD approach, SSBCR crystallizer, thermal response technique, maximum allowable growth rate, and desupersaturation curve technique.     

ANALYSIS OF BATCH CRYSTALLIZATION PROCESSES

The analysis of batch crystallization processes normally requires the consideration of the time-dependent, batch conservation equations (e.g., population, mass, and energy balances), together with appropriate nucleation and growth kinetic equations. The solution of these integro-differential equations is relatively difficult, even by numerical techniques. This review outlines the advances that have been made in the experimental techniques and data analyses which can be used to study the crystallization kinetics and the crystal size distribution (CSD) in batch suspension crystallizers. Several simple and useful methods are discussed which include characterization of CSD maximum, cumulative CSD approach, SSBCR crystallizer, thermal response technique, maximum allowable growth rate, and desupersaturation curve technique.     

CRYSTALLIZATION KINETICS OF 2-CHLORO-4,6-DINITRORESORCINOL BY BATCH COOLING CRYSTALLIZATION METHOD

Crystallization kinetics of 2-chloro-4,6-dinitroresorcinol in ethanol was studied by the method of intermittent dynamic analysis. The nucleation rate and crystal growth rate of 2-chloro-4,6-dinitroresorcinol under different crystallization temperatures and stirring rates were estimated. The results show that with an increase of crystallization temperature, both the nucleation rate and crystal growth rate increase. It is further found that when the stirring rate increases, the nucleation rate increases and the crystal growth rate decreases. The technological conditions of cooling crystallization of 2-chloro-4,6-dinitroresorcinol were studied. It is found that when the stirring rate is 180 rpm and the solution is cooled slowly to - 8°C, the particle size of the products is even and the highest yield of 2-chloro-4,6-dinitroresorcinol is 40.6%.     

SIMULATION OF CRYSTALLIZATION KINETICS AND MORPHOLOGICAL DEVELOPMENT DURING ISOTHERMAL CRYSTALLIZATION OF POLYMERS: EFFECT OF NUMBER OF NUCLEI AND GROWTH RATE

The effect of number of nuclei and growth rate on crystallization kinetics and detailed morphological development during isothermal crystallization of a polymer was investigated using a stochastic simulation. The results show that number of nuclei significantly affects both crystallization kinetics and polymer morphology. An increase in the number of nuclei hastens the crystallization process by speeding up the impingement phenomenon and increasing the levels of impingement. Growth rate has a stronger impact on crystallization kinetics, but it only helps speed up the impingement phenomenon without increasing the level of impingement. Although growth rate influences an average spherulite size and distribution of spherulite size during crystallization, it has no effect on final morphology. The quantitative understanding of morphological development obtained from this work will be a key element for constructing quantitative morphology-property relationships.