An analytical relationship of concentration‐dependent interfacial solute distribution coefficient for aqueous layer freeze concentration

Freeze concentration (FC) is a subzero temperature solute concentration procedure, favoring the retention of high quality compounds such as food ingredients and biological materials. It is known that modeling solute inclusion in the ice layers or ice crystals formed in a convective environment requires the solute distribution coefficient function. The fluid flow velocity, ice‐growth rate and solute concentration are influential on this function. Some literature has reported certain expressions of the function, which are relatively complex. Here, an explicit format of this function has been derived for single solute system, and found to be satisfactory in correlating a wide range of experimental data on sucrose solutions for both the controlled flow layer crystallization process (flow in between two cooling plates) as well as the falling film crystallization process. This expression has captured the fundamental aspects of mass transfer, and it is relatively simple which should be very useful for correlating FC parameters and for simulating the layer FC processes. © 2015 American Institute of Chemical Engineers AIChE J, 61: 1334–1344, 2015     

连续结晶过程非稳态特性的研究

建立了连续溶液结晶过程非稳态系统模拟软件,并应用该软件对过程内在不稳定性进行了分析.在10L及200L复杂结晶器内进行动态行为实验研究,观察晶粒粒度分布(CSD)的动态规律及系统参数对稳定性的影响趋向,实验与模拟分析的结果一致.     

膜辅助添加晶种的过硫酸铵冷却结晶在线监测与过程调控

冷却结晶是经典的溶液结晶过程,常用于分离溶解度随温度变化较大的物质,制备高品质晶体产品。直接进行降温会导致成核速率不可控,得到的晶体产品质量差。在工业中通常选择在溶液结晶介稳区内投放适量晶种来诱导成核,但晶种制备过程复杂,而且成功的添加晶种过程取决于晶种的粒度分布、数量、投放时机和操作人员的经验等因素,降低了产品质量的批次重复性。本文利用聚四氟乙烯（PTFE）中空纤维膜组件为结晶溶液和冷却液提供换热界面,结晶溶液温度降低,在膜界面处形成较均匀的过冷度梯度,进而在低过饱和度下发生异相成核,实现膜辅助添加晶种的过硫酸铵冷却结晶过程调控。膜组件中产生的晶种进入结晶釜中继续生长,将成核和生长过程进行解耦。在线结晶检测系统捕捉到的照片证实了通过控制膜组件使用温度和时长两个操作参数便可得到具有较好的形貌、较窄的粒度分布的晶种。相比直接冷却结晶,在相近的降温速率下,膜辅助添加晶种过程制备的晶体产品具有更大的平均粒径,且粒度分布更集中,表面更加光滑。因此,膜辅助冷却结晶呈现了良好的成核控制能力,有望实现晶种自动制备和添加功能,为高附加值晶体产品的冷却结晶过程开发提供了新方向。     

Solid‐state polymerization of bisphenol A polycarbonate with a spray‐crystallizing method

A novel crystallization method for the production of high‐molecular‐weight bisphenol A polycarbonate by solid‐state polymerization is suggested. In this method, a low‐molecular‐weight polycarbonate prepolymer is dissolved in a solvent and then partially crystallized with a novel spray‐crystallizing method to prepare crystallized polycarbonate particles having a very uniform and porous structure with a narrow melting region. As a result, during solid‐state polymerization, the phenol byproduct can be easily removed from the polymerizing porous polycarbonate particles, and the polymerization rate is dramatically increased. In particular, the effects of the crystallization methods on secondary crystallization during solid‐state polymerization and the melting behavior have been investigated with differential scanning calorimetry studies. The final product, a high‐molecular‐weight polycarbonate, displays a very narrow molecular weight distribution and uniform physical properties. A simultaneous process and an adequate reactor design for spray crystallization and solid‐state polymerization are also suggested. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Thermodynamically-controlled crystal orientation in stressed polymers: 2. Effect of free energy of the amorphous chains surrounding an intramolecular crystal in a crosslinked system

Partial crystallization of a polymer chain in a crosslinked system affects inter alia the configurational entropy of such a chain. The change in free energy is orientation-dependent and can contribute to the distribution of crystal orientations. Using a simple tetrahedron model of the polymer network, the change in configurational free energy due to the partial crystallization of polymer chains has been derived and its effect on the distribution of crystal orientations analysed. Model numerical calculations have been performed for uniaxially stretched polyethylene, and compared with the effects of strain energy on crystals embedded in an uncrosslinked matrix and flow potential.     

Mechanical property enhancement of semicrystalline polymers—A review

Processing plastics below their melting points has been shown to be an effective way to significantly enhance the mechanical, chemical, and physical properties of semi-crystalline thermoplastics. A number of techniques have been developed that are capable of producing highly oriented fibers, films, and shapes. In general, these techniques can be classified as drawing, extrusion, arid rolling, with some very specific modifications having been made to each, that affects the performance of the process. Of particular interest is the rate of production, the range of thicknesses achievable, the uniformity of properties in the product, and the extent, if any, of biaxiality in the product. In recent years, the productivity of such forming processes has been increased by reducing the level of molecular entanglement among neighboring molecules. As an alternate to these orientation processes, techniques have been developed which rely on strain Induced crystallization to enhance the modulus of molded parts and extrudates from a melt. This paper constitutes a review of these techniques with emphasis on the process parameters, polymers investigated, and properties of the resulting materials.     

Steady state modeling and simulation of an industrial sugar continuous crystallizer

The profile of supersaturation along a continuous crystallizer of sugar factories, is the decisive factor that determines the performance of this apparatus. In order to control this profile, a mathematical model was developed taking into account the main physicochemical phenomena involved in crystallization process. The model is based on flow pattern, which was assumed and validated against plant measurements using a tracer test. The steady state mathematical model developed describes the most important aspects of the crystallizer behavior in each compartment: supersaturation, crystal size distribution and flow rate of the product crystals. The model can also describe the undesirable behavior such as dissolution and nucleation. Validation of the developed model was performed using industrial data. A parametric sensitivity study confirmed that the syrup supply distribution is the main variable that should be manipulated to achieve good performance for the crystallizer.     

Raman spectroscopy for spinline crystallinity measurements. II. Validation of fundamental fiber‐spinning models

The original Doufas–McHugh two‐phase microstructural/constitutive model for stress‐induced crystallization is expanded to polyolefin systems and validated for its predictive capability of online Raman crystallinity and spinline tension data for two Dow homopolymer polypropylene resins. The material parameters—inputs to the model—are obtained from laboratory‐scale material characterization data, that is, oscillatory dynamic shear, rheotens (melt extensional rheology), and differential scanning calorimetry data. The same set of two stress‐induced crystallization material/molecular parameters are capable of predicting the crystallinity profiles along the spinline and fiber tension very well overall for a variety of industrial fabrication conditions. The model is capable of predicting the freeze point, which is shown, for the first time, to correlate very well with the measured stick point (i.e., the point in the spinline at which the fiber bundle converts from a solid‐like state to a liquid‐like state and sticks to a solid object such as a glass rod). The model quantitatively captures the effects of the take‐up speed, throughput, and melt flow rate on the crystallization rate of polypropylene due to stress‐induced crystallization effects. This validated modeling approach has been used to guide fiber spinning for rapid product development. The original Doufas–McHugh stress‐induced crystallization model is shown to be numerically robust for the simulation of steady polypropylene melt spinning over a wide range of processing conditions without issues of discontinuities due to the onset of the two‐phase constitutive formulation downstream of the die face, at which crystallization more realistically begins. Because of the capturing of the physics of polypropylene fiber spinning and the very good model predictive power, the approximations of the original Doufas–McHugh model are asserted to be reasonable. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

熔融结晶技术分离纯化有机化合物的研究进展

熔融结晶技术是利用被分离物质各组分间凝固点的差异,通过控制热量的输入和移出,使被分离组分从熔融液中结晶析出,经洗涤、发汗等操作,实现目标组分分离纯化的一种结晶技术。熔融结晶分离纯化技术由于具有不需要使用溶剂、能耗低、设备体积小、能得到高纯产品等优点,在有机化合物的分离纯化领域得到了广泛应用。本文简述了熔融结晶的方式,介绍了熔融结晶器的类型,综述了熔融结晶技术分离纯化有机同分异构体、有机化工原料、日用品、食品和药品的研究进展,分析了熔融结晶技术分离纯化有机化合物过程中存在的问题,展望了熔融结晶技术分离纯化有机化合物的发展方向。文中指出：随着熔融结晶技术的发展,以提高产品质量,减小能耗和降低成本为目的的耦合熔融结晶技术已成为熔融结晶技术发展的方向。以包含熔融结晶设备、工艺、晶体成核和生长动力学、发汗机理以及传热传质模型的系统工程将会成为熔融结晶分离纯化有机化合物的研究热点。     

Modeling and Growth Kinetics of Antisolvent Crystallization Applied to the Pharmaceutical Industry

With the aim of simulating the product crystal size, which is one of the important physical properties for active pharmaceutical ingredients, an antisolvent crystallization model is proposed, including only six experimentally determined kinetic parameters to develop a concise model. As a first step, the methodology to assess the growth rate parameters, which are some of the six kinetic parameters, is discussed. An approach for appropriately treating the size distribution data obtained by means of the laser diffraction/scattering method is suggested. The determined growth rate parameters could be used to simulate the crystal size indicating that the simulation by crystallization modeling is a practical application for the pharmaceutical industry.     

Flow-induced crystallization of polypropylene in stretched ribbons

Ribbons of molten polypropylene have been extruded into a thermostatted chamber and subjected to elongations ranging as high as 4400-fold. Some of these ribbons have been shown to possess the unusual ability of recovering nearly all of the strain from large levels of stretch (≤100 percent). This property, as well as the magnitude of the tensile yield drop, has been found to be directly proportional to the uniaxial character of the flow present at the moment of crystallization. The structure in the final solid films has been characterized by wide angle X-ray diffraction and crystal orientation distribution. Both of these factors have been found to be proportional to the character of flow at the moment of crystallization. Finally, the temperature in the thermostatted chamber has been shown to affect the character of the flow.     

Isothermal crystallization kinetics of high‐flow nylon 6 by differential scanning calorimetry

The isothermal crystallization kinetics have been investigated with differential scanning calorimetry for high‐flow nylon 6, which was prepared with the mother salt of polyamidoamine dendrimers and p‐phthalic acid, an end‐capping agent, and ε‐caprolactam by in situ polymerization. The Avrami equation has been adopted to study the crystallization kinetics. In comparison with pure nylon 6, the high‐flow nylon 6 has a lower crystallization rate, which varies with the generation and content of polyamidoamine units in the nylon 6 matrix. The traditional analysis indicates that the values of the Avrami parameters calculated from the half‐time of crystallization might be more in agreement with the actual crystallization mechanism than the parameters determined from the Avrami plots. The Avrami exponents of the high‐flow nylon 6 range from 2.1 to 2.4, and this means that the crystallization of the high‐flow nylon 6 is a two‐dimensional growth process. The activation energies of the high‐flow nylon 6, which were determined by the Arrhenius method, range from ?293 to ?382 kJ/mol. The activation energies decrease with the increase in the generation of polyamidoamine units but increase with the increase in the content of polyamidoamine units in the nylon 6 matrix. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

制药结晶中的先进过程控制

结晶作为固-液分离的重要手段,是实现医药产品高端高值化的关键技术。药物结晶过程的精准控制决定了晶体产品的多晶型、晶习、粒度及粒度分布等诸多特性,对生产效率和产品质量的提升具有重要的意义。基于国内外对药物结晶过程控制的研究现状,结合相关案例,系统总结了制药结晶中先进过程控制的理论模型、监测手段和控制策略,重点分析讨论了过程控制在产品工程中的应用并对其发展趋势进行了展望。     

Cycling Zone Adsorption: Variable-Feed Mode of Operation

Abstract

Relatively large separations are possible using a single cycling zone adsorption column when operated with product recycle. Such a recycle may cause a large cyclic step-change in feed concentration, resulting in significant differences in alternating product stream concentrations. It is concluded that such a “variable-feed mode of operation” may allow the separation of compounds whose solid—fluid equilibrium distribution coefficients are only slightly affected by changes in a thermodynamic potential, or compounds which are difficult to separate by classical techniques such as distillation or extraction. It will also be apparent that the “variable-feed mode of operation” of a single zone (column) produces separations that could only be previously developed by a number of constant feed concentration zones operating in a series. Theoretical and experimental studies show that longitudinal diffusion is the primary cause of effluent concentration profile distortion, limiting the duration of peak product concentration flow.     

非晶态晶化导向剂在洗涤剂用4A沸石合成中的应用

用工业原料 ,在常温下合成非晶态晶化导向剂 ,并应用于水热合成体系制备 4A沸石。考察了晶化导向剂加入量以及胶化、陈化和晶化时间等因素对成品的影响 ,得到了结晶粒度高、粒度均匀、钙离子交换度 (>310mgCaCO3/g沸石 )高的产品 ,并使总反应时间由原来的 5h～ 7h降为 80min。     

Chemical engineering aspects of precipitation from solution

Precipitation is a rapid solidification. As a rule, it applies to substances which are sparingly soluble and is coupled with a chemical reaction or with a salting out process. The chemical engineer has to ensure that the product obtained has a constant and preferably coarse size distribution. This contribution compares the literature data on crystallization of soluble and sparingly soluble substances; it can be shown that, during precipitation processes, nuclei are formed, most probably by a primary mechanism. Analysis of the processes occurring during precipitation, such as chemical reaction, mixing and crystallization, leads to recommendations for the operation of precipitation processes.     

结晶过程晶体粒度分布控制研究进展

结晶是一种重要的晶态化学品分离纯化技术,通过结晶过程控制实现晶体产品粒度分布（CSD）的可控定制,对生产高附加值的精细化学品具有重要意义。尤其对于制药工业,CSD不仅会影响药物产品堆密度与溶出速率,还会影响到药品的生物利用度。基于国内外对结晶过程中CSD控制的研究现状,结合相关案例,系统总结了结晶过程CSD控制的方法,简要介绍了美国FDA新倡导的药物质量保证体系QbC模式,重点分析讨论了过程强化和过程控制策略在工业结晶过程CSD控制方面的应用,并对其发展趋势进行了展望。     

Auslegung und Maßstabsvergrößerung von Kristallisatoren

Design and scale-up of crystallizers . Industrial scale crystallizers have to be designed in such a way that the crystalline product has the design particle size distribution, crystal form, and purity. Data on multiphase flow, solubility, balances of mass, energy, and number of crystals, kinetics (crystal growth rate and nucleation rate), and heat transfer are required. The present article discusses problems of kinetics in greater detail than other aspects. Multiphase flow is considered only briefly and heat transfer not at all. Familiarity with the terms homogeneous and heterogeneous (primary) and secondary nucleation [1, 2] is also assumed. This contribution is based on evaluation of available literature.     

Numerical investigation of the influence of the activity coefficient on barium sulphate crystallization

We investigate in this paper the influence of the value and computational method for the activity coefficient on the crystallization process of BaSO₄. We consider for this a mixing-controlled configuration based on a coaxial pipe mixer. The corresponding flow is turbulent and this problem is solved using numerical simulations until reaching steady-state conditions. The precipitation model has been coupled with the method of moments to describe the crystal population, taking the first four moments into account. All supplementary physical models and transport equations have been defined as User-Defined Functions and Scalars and compiled into a commercial Computational Fluid Dynamics code (here Fluent 6.1). The simulations have been performed for various inlet concentrations with different values and models for computing the activity coefficient in order to analyze its influence on the product quantity and on the crystal size distribution at the pipe outlet. The supersaturation ratio, containing the activity coefficient in its definition, is the driving force for the crystallization process. This is the reason why, as concluded from the present results, it is important to model correctly and accurately the activity coefficient in order to obtain reliable data, a point which has been often neglected in the literature. Even relatively small variations of this coefficient can considerably affect the results, in particular for fast reactions.     

Surface Properties – A Key for Nucleation in Melt Crystallization Processes

Numerous organic products which are commercially refined by crystallization exhibit wide metastable zones, for example, xylene, bisphenol‐A, isocyanates, or pyridine derivatives. The practical meaning for layer crystallization processes is that a high degree of subcooling on crystallization surfaces is necessary to start nucleation at the beginning of a crystallization stage. The subsequent crystallization runs then uncontrolled, at much higher rates than designed until the subcooling has been dissipated. As a consequence dendritic crystal growth sets in, which is disadvantageous in terms of the separation efficiency of the crystallization process. A practicable countermeasure is seeding which, however, requires more complex equipment and generates additional process steps, resulting in additional costs. In this work an alternative way of reducing the negative impact of subcooling on crystallization, which is based on the reduction of the metastable zone itself rather than on the bypassing it, has been investigated. The width of the metastable zone depends on the activation energy for nucleation which in turn depends on the interfacial surface tension between the melt and the surface of the crystallization element. It has been shown in this work that the activation energy for nucleation and so the supercooling in a xylene isomer mixture can be considerably reduced when replacing stainless steel by PTFE as a material for the crystallization surface. In follow‐up trials it was found that the crystallization surfaces do not need to be wholly covered by PTFE but that just small PTFE nucleation zones on steel surfaces have the same positive effect on the separation by crystallization. Applied in industrial equipment such nucleation zones might contribute to the cost optimization of commercial layer crystallization processes.