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1.
The available literature on the crystal structure of the metastable alumina polymorphs and their associated transitions is critically reviewed and summarized. All the metastable alumina structures have been identified as ordered or partially ordered cation arrays on the interstitial sites of an approximately close-packed oxygen sublattice (either face-centered cubic or hexagonal close packed). The analysis of the symmetry relations between reported alumina polymorphs having an approximately face-centered cubic packing of the oxygen anions allows for an exact interpretation of all the complex domain structures that have been observed experimentally. Possible mechanisms for the phase transitions between the different alumina polymorphs also are discussed. 相似文献
2.
Condensation, involving nucleation, growth, and ripening from a metastable state, is an important but complex phase transition process. The effect of physical parameters, including temperature, on condensation dynamics, the competition between homogeneous and heterogeneous (seeding) nucleation, and the separation of polymorphs are among several issues of practical interest. We present a model based on population dynamics that describes the time evolution of the particle size distributions for condensation of the fluid phase and consequent decline in supersaturation. The crucial effect of interfacial curvature on energy, and hence on particle size (Gibbs-Thomson effect), causes larger particles to be less soluble, so that smaller particles dissolve and eventually vanish (denucleate). Numerical solutions of the governing equations show the transition from nucleation and growth to ripening occurs over a relatively long time period. The influence of temperature on these phenomena is primarily through its effect on interfacial energy, growth rate coefficients, and equilibrium solubility. Temperature programming is proposed as a potential method to control the size distribution during the phase transition. The model suggests conditions to suppress homogeneous nucleation by seeding. We also explore how a temperature program for cooling crystallization based on different properties of the crystal forms can separate two crystal polymorphs. 相似文献
3.
利用液滴-微流体法研究了温度、液滴大小及溶剂组成对L-谷氨酸晶型的影响,并利用显微镜和PXRD进行了表征。研究结果表明,相同过饱和度下,温度越高越有利于亚稳态晶型的生成;反之,有利于稳态晶型的生成。相同过饱和度和温度下,液滴大小对晶型没有影响。溶剂中乙醇含量的增加有利于亚稳态晶型的生成。 相似文献
4.
《Chemical engineering journal (Lausanne, Switzerland : 1996)》2001,81(1-3):1-7
Taltirelin, a central nervous system activating agent, has two crystal forms (α- and β-forms). Previously it has been reported that methanol (MeOH) as a co-solvent promoted the solvent-mediated phase transformation from α-form to β-form. In the present study, the mechanism of the promotion was investigated. Of the three processes in the solvent-mediated transformation, namely dissolution of the metastable form, nucleation of the stable form, crystal growth of the stable form, MeOH promoted the latter two processes. In particular, the nucleation was strongly promoted. The nOe (nuclear Overhauser effect) data suggested that MeOH promotes the conformational change from the α-form to the β-form. The solute–MeOH interaction resulted in the formation of pseudo-polymorphs of the β-form (2H2O·MeOH or H2O·2MeOH solvate), although these pseudo-polymorphs were exchangeable to the β-form (4H2O solvate) under a humid atmosphere. Based on these findings, a possible mechanism of the transformation of taltirelin polymorphs in the presence of MeOH was discussed. 相似文献
5.
Zeren Shang Mingchen Li Baohong Hou Junli Zhang Kuo Wang Weiguo Hu Tong Deng Junbo Gong Songgu Wu 《中国化学工程学报》2022,41(1):430-440
With the high-quality requirements for cephalexin monohydrate,developing a robust and practical crys-tallization process to produce cephalexin monohydrate with good crystal habit,appropriate aspect ratio and high bulk density as well as suitable flowability is urgently needed.This research has explored the influence of ultrasound on crystallization of cephalexin monohydrate in terms of nucleation mechanism and crystal habit control.The results of metastable zone width and induction time measurement showed the presence of ultrasound irradiation can narrow the metastable zone and shorten induction time.Cavitation phenomena generated by ultrasound were used to qualitatively explain the mechanism of ultrasound promoting nucleation of cephalexin monohydrate.Furthermore,on the basis of classical nucleation theory and induction time data,a series of nucleation-related parameters (such as crystal-liquid interfacial tension,radius of the critical nucleus and etc.) were calculated and showed a decreasing trend under ultrasound irradiation.The diffusion coefficient of the studied system was also determined to increase by 72.73% under ultrasound.The changes in these parameters have quantitatively confirmed the mechanism of ultrasound influence on the nucleation process.In further,the calculated surface entropy factor has confirmed that the growth of cephalexin monohydrate follows continuous growth mechanism under the research conditions of this work.Through the exploration of crystallization conditions,it is found that suitable ultrasonic treatment,seeding,supersaturation control and removal of fine crystals are conducive to improving the quality of cephalexin monohydrate product.Optimizing the crystalliza-tion process coupled continuous ultrasound irradiation with fine-crystal dissolution policy has achieved the controllable production of monodisperse cephalexin monohydrate crystal with good performance. 相似文献
6.
Form selection of concomitant polymorphs: A case study informed by crystallization kinetics modeling
Weiwei Tang Yufeng Quan Junbo Gong Jingkang Wang Qiuxiang Yin Tonglei Li 《American Institute of Chemical Engineers》2021,67(4):e17129
Molecular mechanisms and process kinetics of crystallizing concomitant polymorphs remain poorly understood. Solvent-mediated phase transformation and concomitant crystallization are difficult to be distinguished in practice, as multiple forms can be detected at the same time. Herein, we developed a population balance model to simulate a concomitant crystallization process of two polymorphs of tolfenamic acid. Our kinetic modeling aims to understand concomitant crystallization and help guide form selection of such a molecular system. Crystallization kinetics of ethanolic solutions were uncovered from induction time measurements, as well as seeded and unseeded crystallization experiments. Experimental and simulation results demonstrate that the stable form I crystallizes concomitantly with the metastable form II. The faster growing form II results in an intermediate decline in the composition of form I in crystallized samples, a characteristic feature of the concomitantly crystallized system. A four-quadrant scheme of attainable polymorph outcome was simulated under various crystallization conditions. 相似文献
7.
氨基酸的不同晶型往往表现出不同的理化性质和药用效果。因此通过调控氨基酸多晶型溶液结晶过程得到目标晶型非常重要,其中添加剂对不同晶型的影响直接且有效,是控制晶型生产常采用的手段。本文综述了不同种类添加剂对氨基酸多晶型溶液结晶过程的影响。首先,以水溶液中不能直接得到的甘氨酸γ晶型为例,阐明了酸、碱和无机盐对其成核的促进作用,从电荷补偿角度分析了该类添加剂对不同晶型生长过程的影响机理。其次,阐述了tailor-made添加剂对氨基酸晶体形态和晶型的调控作用,重点分析了不同结构的tailor-made添加剂对晶型转变的影响规律。介绍了醇-水等混合溶剂对不同氨基酸多晶型结晶过程的选择性影响。最后,在以上分析的基础上,对通过添加剂分子设计制备不同目标晶型进行了展望。 相似文献
8.
以红辉沸石为原料,采用两步水热法将其转化为A型分子筛,系统地探讨A型分子筛的转化机理。对不同晶化时间的样品进行系统表征,深入解析天然红辉沸石转化A型分子筛的成核机制、生长控制、晶型稳定转化等结构基础问题。研究表明,A型分子筛的生长可分为3个阶段:在成核诱导期(晶化时间≤0.5 h),凝胶中的β笼通过双四元环构筑成α笼,生成较多晶核及少量1μm小晶体;此后为快速生长期(晶化时间1~4 h),大量晶核通过聚集生长形成2μm大晶体;在生长稳定期(晶化时间5~8 h),通过二次成核凝胶将会转化为更多的分子筛晶体。A型分子筛的形成遵循:铝硅酸盐凝胶→四元环、六元环→双四元环、β笼→α笼的晶化过程,因而其转化过程符合液相转化机理。 相似文献
9.
Kyuya Nakagawa Wataru Murakami Julien Andrieu Sverine Vessot 《Chemical Engineering Research and Design》2009,87(8):1017-1027
Freeze-dried samples were prepared from D-mannitol solution by selected freezing conditions. Crystalline structures of prepared samples were determined by XRD analysis, and distributions of the various crystal structures of mannitol were obtained for each sample. It was found that the amount of each polymorph was quite well correlated to the freezing conditions, namely the ice crystal nucleation temperature and the cooling rate. In case of samples prepared at fast cooling rates, the samples where the ice crystals nucleated at higher temperature contained much more stable form than the samples nucleated at lower temperature. Besides, the samples prepared at slow cooling rates predominantly contained stable crystalline forms despite of the variation of nucleation temperatures. Moreover, the experimental results also indicated that the various polymorphs were also distributed vertically through the sample along the direction of the heat flux during freezing. The tendency of the polymorph distribution through the freeze-dried cake was also elucidated by using the simulated temperature profiles during freezing. Thus, the profiles of mannitol polymorphs after the freezing derived from the temperature distributions could predict the global tendency of polymorphism behaviour, and, consequently, would be useful to achieve quality control of freeze-dried formulations. 相似文献
10.
一水盐酸环丙沙星初级成核机理的研究 总被引:1,自引:0,他引:1
A general expression for the ralationship between induction period and supersaturation was developed based on polynuclear approach.Different mechanism of primary nucleation in solution can be illustrated by the expression.The results of induction period determined by laser scatering method shows that the crystallization of ciprofloxacin hydrochloride monohydrate in water/ethanol or aqueous solution is by the mechanism of primary nucleation followed by one-dimensional diffusion growth,and then one-dimensional continuous or “birth and spread” growth on crystal face. The growth mechanism on the crystal face is affected by temperature and solvent. 相似文献
11.
Martin Wijaya Hermanto Richard D. Braatz Min‐Sen Chiu 《American Institute of Chemical Engineers》2011,57(4):1008-1019
Polymorphism, a phenomenon in which a substance can have more than one crystal form, is a frequently encountered phenomenon in pharmaceutical compounds. Different polymorphs can have very different physical properties such as crystal shape, solubility, hardness, color, melting point, and chemical reactivity, so that it is important to ensure consistent production of the desired polymorph. In this study, an integrated batch‐to‐batch and nonlinear model predictive control (B2B‐NMPC) strategy based on a hybrid model is developed for the polymorphic transformation of L ‐glutamic acid from the metastable α‐form to the stable β‐form crystals. The hybrid model comprising of a nominal first‐principles model and a correction factor based on an updated PLS model is used to predict the process variables and final product quality. At each sampling instance during a batch, extended predictive self‐adaptive control (EPSAC) is employed as a NMPC technique to calculate the control action by using the current hybrid model as a predictor. At the end of the batch, the PLS model is updated by utilizing the measurements from the batch and the above procedure is repeated to obtain new control actions for the next batch. In a simulation study using a previously reported model for a polymorphic crystallization with experimentally determined parameters, the proposed B2B‐NMPC control strategy produces better performance, where it satisfies all the state constraints and produces faster and smoother convergence, than the standard batch‐to‐batch strategy. © 2010 American Institute of Chemical Engineers AIChE J, 2011 相似文献
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Permanent magnets of different intensities were used to investigate the effect of a magnetic field on the crystal growth of calcite suspended in a fluidized bed. The magnets were fixed on the suspended bed where the crystal grew. The growth rates of calcite were measured at various levels of supersaturation (σ), pH, and ionic strength (I) using the constant-composition technique. The calcite growth rates in the presence of the magnetic field were lower than those in the absence of the magnetic field, and higher magnetic intensity yielded a lower growth rate. The effect of a magnetic field on CaCO3 polymorphism was also studied. The percentage of polymorphs was dependent on the magnetization time. Aragonite was the predominant polymorphic form in the precipitate mixture, which was induced spontaneously by changing the solution pH after the supersaturated solution had been magnetized for 48 h. 相似文献
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15.
The selective polymorph control in sulfamerazine (SMZ) using ultrasonic irradiation was investigated. Firstly, the behavior of polymorphs in the absence of ultrasonic irradiation with various degrees of supersaturation was monitored. As a result, only metastable form-I crystallized at the primary nucleation stage and crystal growth of only form-I was recognized. The nucleation rate of stable form-II was very small, and the transformation from form-I to form-II hardly occurred. Therefore, it is easy to obtain form-I without ultrasonic irradiation, as it does not depend on the level of supersaturation. Secondly, the behavior of polymorphs with various degrees of ultrasonic energy and supersaturation was monitored. As a result, only form-I was obtained in the case of small ultrasonic energy. However, in addition to the nucleation of form-I, an unknown form-III and/or the recognized form-II are induced to nucleate under the influence of the ultrasonic energy in the case of large ultrasonic energy. Eventually, all forms changed to form-II within a short period of time. Therefore, polymorph control of SMZ can be achieved easily by supplying an appropriate amount of ultrasonic energy. 相似文献
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17.
After an induction period, crystallization of cocoa butter under dynamic conditions at 26.5°C occurs in two stages, primary
and secondary. The primary stage involves nucleation, crystal growth, aggregation, and sintering. Crystals formed during the
primary stage were slightly or non-birefringent, and had long, irregular-shaped filaments. The secondary stage was initiated
by the formation of spherulites. Total crystallization time may depend upon the crystal growth rate in the primary stage and
the time that coca butters take to form the spherulitic crystals in the secondary stage. After the spherulitic crystals formed,
the crystal growth rates were rapid. Cocoa butters crystallized into two fractions during the primary and secondary stages.
The low-melting fractions had onset melting temperatures similar to those of polymorphs IV and V of cocoa butter. The high-melting
fractions, which were observed at the latter stages of crystallization, had differential scanning calorimetry endotherms with
peak maxima at approximately 34–36°C (Form VI). The concentrations of 1,3-stearoyl-2-oleoylglycerol (SOS) in the crystals
during growth were higher than those in the original cocoa butter. As crystallization progressed, crystals increased in their
proportions of SOS in the triacylglycerol fraction. Concentrations of the C18 free fatty acids were lower during early crystallization as compared to the original cocoa butter. 相似文献
18.
The solubility, secondary nucleation threshold, and growth kinetics of (RS)‐ibuprofen have been studied in an aqueous ethanol solvent. The metastable zone for secondary nucleation is very narrow at lower temperatures in this range, but greatly enlarged at higher temperatures. The crystal growth kinetics not only display significant dispersion of growth rates, but also a dead zone that is dependent on the growth rates of the crystals. Faster growing crystals display almost no dead zone, whereas the smallest crystals have a large dead zone. The size of the dead zone is largely responsible for the dispersion of crystal growth rates, perhaps due to differences in the thermodynamic stability of the different crystals. The mechanism of growth rate dispersion relates to that of the dead zone. 相似文献
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20.
Martin Wijaya Hermanto Min‐Sen Chiu Richard D. Braatz 《American Institute of Chemical Engineers》2009,55(10):2631-2645
Polymorphism, a phenomenon where a substance can have more than one crystal forms, has recently become a major interest to the food, speciality chemical, and pharmaceutical industries. The different physical properties for polymorphs such as solubility, morphology, and dissolution rate may jeopardize operability or product quality, resulting in significant effort in controlling crystallization processes to ensure consistent production of the desired polymorph. Here, a nonlinear model predictive control (NMPC) strategy is developed for the polymorphic transformation of L ‐glutamic acid from the metastable α‐form to the stable β‐form crystals. The robustness of the proposed NMPC strategy to parameter perturbations is compared with temperature control (T‐control), concentration control (C‐control), and quadratic matrix control with successive linearization (SL‐QDMC). Simulation studies show that T‐control is the least robust, whereas C‐control performs very robustly but long batch times may be required. SL‐QDMC performs rather poorly even when there is no plant‐model mismatch due to the high process nonlinearity, rendering successive linearization inaccurate. The NMPC strategy shows good overall robustness for two different control objectives, which were both within 7% of their optimal values, while satisfying all constraints on manipulated and state variables within the specified batch time. © 2009 American Institute of Chemical Engineers AIChE J, 2009 相似文献