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%.     

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%.     

Kinetics of butterfat crystallization

Fractionation of butterfat by melt crystallization is a commercial process in many countries for making butter fractions with varying melting, textural and flavor properties for use as food ingredients. However, the crystallization phenomena in this complex system are poorly understood and difficult to optimize and control. In this study, the crystallization kinetics of anhydrous butterfat were determined by cooling a melted sample to the final crystallization temperature in either a lab-scale (2 L) batch crystallizer or a pilot-scale (20 L) crystallization vessel. The butterfat was cooled sequentially from an initial temperature of 60°C to final temperatures of 30, 20 and 15°C at a constant cooling rate. Crystals formed at each temperature were separated by vacuum filtration, with the liquid cooled to the next crystallization temperature. Nucleation rates were determined by counting the number of crystals in a given volume of suspension during the course of crystallization. Crystal growth rates were obtained from image analysis of optical photomicrographs. Changes in viscosity, turbidity and mass of crystals also were determined. Effects of impeller velocity (75, 100 or 125 rpm) on the crystallization kinetics were determined. Nucleation and mass deposition rates increased while crystallization lag times decreased with increasing agitator velocities. Growth rates increased with agitator rpm at 20 and 15°C, but decreased with agitator rpm at 30°C, indicating different growth mechanisms. At 20 and 30°C, aggregation was the primary mechanism of crystal growth, whereas little aggregation was observed at 15°C. Crystallization at the larger scale, 20 L, showed only minor differences.     

Kinetic roughening transition and missing regime transition of melt crystallized polybutene-1 tetragonal phase: growth kinetics analysis

The morphology and lateral growth rate of isotactic polybutene-1 (it-PB1) have been investigated for crystallization from the melt over a wide range of crystallization temperatures from 50 to 110°C. The morphology of it-PB1 crystals is a rounded shape at crystallization temperatures lower than 85°C, while lamellar single crystals possess faceted morphology at higher crystallization temperatures. The kinetic roughening transition occurs around 85°C. The nucleation and growth mechanism for crystallization does not work below 85°C, since the growth face is rough. However, the growth rate shows the supercooling dependence derived from the nucleation and growth mechanism. The nucleation theory seems still to work even for rough surface growth. Possible mechanisms for the crystal growth of this polymer are discussed.     

Characterizing cocoa butter seed crystals by the oil-in-water emulsion crystallization method

Unambiguous quantitative evidence for the catalytic action of seed crystals in cocoa butter is presented. We used an ultrasound velocity technique to determine the isothermal growth of solid fat content in cocoa butter oil-in-water emulsions, in which the probability of finding a seed crystal in any one droplet was around 0.37 at 14.2°C. The upper limit for the size of seed crystals in West African cocoa butter was around 0.09 μm, the Gibbs free energy for nucleation was 0.11 mj m⁻², and the concentration of seed crystals was in the range of 10¹⁶ to 10¹⁷ m⁻³. X-ray diffraction measurements showed that emulsified cocoa butter crystallizes in the α polymorph and does not appear to transform to the β′ form within the first 25 min of crystallization. Primary nucleation events in cocoa butter emulsions are accounted for by seed crystals. Collision-mediated nucleation, a secondary nucleation mechanism, in which solid droplets (containing seed crystals) catalyze nucleation in liquid droplets, is shown to account for subsequent crystallization. This secondary nucleation mechanism is enhanced by stirring.     

青霉素亚砜结晶生长与成核动力学

利用Mydlarz 和 Jones 模型（MJ2）,对乙酸丁酯中青霉素亚砜的成核与生长动力学进行研究。通过矩量法对MJ2模型进行处理后,利用晶体产品的粒度分布计算得到青霉素亚砜的生长速率与成核速率,然后利用最小二乘法拟合回归求解出成核与生长动力学方程参数。通过实验设计考察了过饱和度、温度与搅拌速度对青霉素亚砜晶体成核和生长过程的影响。研究表明青霉素亚砜晶体生长速率随过饱和度比的增加呈现指数型增长,确定青霉素亚砜晶体生长属于晶体表面生长控制过程。由于高速搅拌会增加青霉素亚砜晶体的破碎,促进了二次成核过程,随着搅拌速度的增加,晶体生长速率出现小幅下滑,而成核速率则明显升高。青霉素亚砜成核与生长动力学研究将有助于工业生产过程优化。     

Regulating the β′→β polymorphic transition in food fats

Hydrogenated cottonseed oil (HCSO) is commonly used as a β′-stable fat in margarines and shortenings. In the present study, the crystallization behavior of HCSO is altered via dilution, agitation, tempering regime, and the addition of an emulsifier [polyglycerol polyricinoleate (PgPr)]. Key properties assessed include crystal morphology (with polarized light microscopy), polymorphic behavior (with X-ray diffraction), and crystallization kinetics (with DSC). It is demonstrated that on considerable dilution with canola oil (4% w/w), HCSO can be crystallized in the β′ or β polymorph with associated changes in crystal morphology, depending on tempering regime. Crystallization from the melt to 25°C results in the β′-form, as there is insufficient supercooling to form the β polymorph but enough to form the metastable β′. With cooling from the melt to 5°C, there is adequate supercooling for the δ polymorph to form, with the presence of the canola oil facilitating the transformation toward this stable phase. Static vs. crystallization under agitation does not lead to visible changes in either polymorphic behavior or crystal morphology. However, there is extensive secondary nucleation and growth as a result of crystals breaking off accreting agglomerates. The presence of PgPr, added as a crystal modifier, does not affect the final crystal polymorph or morphology, except under one set of conditions—crystallization from the melt to 5°C with agitation, whereby it considerably alters crystallization behavior.     

Rheometry and polymorphism of cocoa butter during crystallization under static and stirring conditions

In this work the association between polymorphism and the crystal network structure developed by the TAG of cocoa butter (CB) was investigated under static and stirring crystallization conditions using a dynamic mechanical spectrometer. The results obtained showed that parameters obtained through oscillatory rheometry (i.e., phase shift angle, δ) followed the polymorphism of CB during static crystallization. Although standard DSC was not capable of differentiating the α to β′ phase transformation from the direct β′ crystallization from CB melt, δ rheograms measured these two processes separately. Additionally, through oscillatory rheometry, we followed the dimensionality of the crystal network during CB crystallization. Within this context, the pre-exponential term (In γ) from the weak-link regime equation for colloidal dispersions was much more sensitive than the fractal dimension (D) to differences in crystal size, spatial distribution of the crystal network, and melting temperature of the β′ phase of CB. On the other hand, torque measurements obtained during CB crystallization under stirring conditions showed a shear rate effect that favored TAG development in the β phase at temperatures of 19, 22, and 26.5°C, particularly at shear rates of 120 and 400 rpm. In contrast, under static conditions CB did not develop in the β phase at any of the crystallization temperatures investigated (i.e., 18 to 26.5°C).     

Dynamic crystallization of cocoa butter. II. Morphological,thermal, and chemical characteristics during crystal growth

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.     

Molecular weight dependence of melt crystallization behavior and crystal morphology of low molecular weight linear polyethylene fractions

Melt crystallization behavior and corresponding crystal morphology of five low molecular weight (3,900 ≤ MW ≤ 20,800) linear polyethylene (PE) fractions have been investigated. The overall crystallization data indicate that the lower molecular weight (MW) fraction possesses a higher crystallization rate at the same undercooling (ΔT). On the contrary, at the same crystallization temperature (T_c) the rate increases with MW. The Avrami exponent (n) varies from ca. 3 to 4 with decreasing ΔT for the fractions studied, which implies the nucleation process changes from athermal type to thermal type as T_c increases. For the low MW PE’s, the different crystal growth regimes (regime I and II) have been first time identified via linear crystal growth rate (G) measurements. The regime I/II transition temperatures are close to previously reported data, which were obtained through a different method. As reported for intermediate MW PE’s, the transitions occur at an almost constant ΔT of 17.5±1 °C for each fraction studied. Morphological study shows that single crystals could be formed isothermally at low ΔT’s. Typical banded spherulites and axialites, which are MW and ΔT dependent, are also observed. Orthorhombic structure is ascertained to be the dominant crystal structure that exists irrespective of MW and crystal growth regime.     

Effect of processing conditions on physical properties of a milk fat model system: Microstructure

The effect of processing conditions on the microstructure of three blends of 30, 40, and 50% high-melting fraction [Mettler dropping point (MDP)=47.5°C] in the lowmelting fraction (MDP=16.5°C) of milk fat was studied. The effect of cooling and agitation rates, crystallization temperature, chemical composition of the blends, and storage time on crystalline microstructure (number, size, distribution, etc.) was investigated by confocal laser scanning microscopy (CLSM). To improve resolution, a mix of Nile blue and Nile red dyes was dissolved in the melted samples in proportions that did not modify the nucleation kinetics. Samples were then crystallized by cooling (0.2 or 5.5°C/min) to crystallization temperature (25, 27.5, and 30°C). After 2 h at crystallization temperature, a slurry was placed on a microscope slide and samples were stored 24 h at 10°C. During this period, more material crystallized. Slowly crystallized samples (0.2°C/min) formed different structures from rapidly crystallized samples (5.3°C/min). Crystals were sometimes diffuse and hard to distinguish from the liquid. Samples were darker as a result of this solid-mass distribution. However, rapidly crystallized samples had well-defined crystals and seemed to be separated by a distinct liquid phase. These crystals were not in touch with each other as was the case for slowly crystallized samples. Higher agitation rates led to smaller crystal size due to enhanced nucleation. Larger crystals were formed when crystallization occurred at higher temperatures. Storage time resulted in an increase of crystal size. Larger crystal size and structures with more evident links had a more elastic behavior with higher elastic modulus E’.     

Kinetics of the cooling crystallization of sodium sesquisulfate from 4.5 mol/l sulfuric acid

Simulating effluent from chlorine dioxide generators, the crystallization kinetics of sodium sesquisulfate from 4.5 mol/L sulfuric acid were studied under cooling conditions. The crystal growth and nucleation rates were determined using the population balance concept and the crystal size distribution from a continuous mixed suspension mixed product removal (MSMPR) crystallizer. Crystallization temperatures studied were 45, 50, 55 and 60°C. The crystal growth and nucleation rate data were correlated to supersaturation and temperature. Nucleation rate was found to be a function of suspension density (secondary nucleation). The activation energies for nucleation and growth are reported.     

Study of the polymorphism and the crystallization kinetics of tripalmitin: A microscopic approach

The morphology and kinetics of crystallization of tripalmitin have been examined in detail by optical microscopy. The α-crystallization process is characterized by a fast heterogeneous nucleation and spherulitic growth, even at low undercooling, resulting in intense birefringence and smooth spherulitic entities. Four different β’-microstructures have been found—grainy, fibrous, feathery and lamellar. Around 47°C, a clear change from a grainy to a fibrous β’-microstructure is observed. This transition seems to take place without a drastic change in nucleation or in crystal growth. At 50°C, both nucleation and crystal growth exhibit a clear discontinuity, indicating interference from β-crystallization. Around 52°C, the β’-form changes again from a fibrous to a more feathery microstructure; the transition is accompanied by a distinct decrease in crystal growth rate. The lamellar β’-structure exhibits the highest stability and can be obtained onlyvia an accelerated nucleation at low temperature, followed by further growth at elevated temperature near the melting temperature of the β’-form. Determination of the β-form on the basis of its microstructure is not always precise, because the microstructure strongly depends on whether the β-crystals are obtained from a transformation of α or β’, or whether β-crystallization occurs directly from the melt. Clear confirmation of the polymorphic nature of the solid state can be obtained from melting point determination.     

Crystal growth of high silica ZSM-5 at low temperature synthesis conditions

At the temperature of 90°C and under atmospheric pressure, growth kinetics of high silica ZSM-5 was investigated through a long induction, nucleation and crystal growth periods. It was found the entire crystallization mechanism of ZSM-5 seems to be the combined process of the nucleation via solid-solid transformation, intergrowth among seed crystals and the normal growth in the reaction mixture. Nuclei were initially formed on the Si-rich surface of the amorphous intermediates, indicating that the reaction of TPA with Si species was prior to that with Al species. As the reaction time proceeded, various types of intergrowth among the seed crystals were observed along with the crystals growing independently. The intergrowth seems to play a role for forming typical ZSM-5 crystal shapes. And then ZSM-5 crystals further grew in the reaction mixture, so that the bulk Si/Al₂ ratio of crystals approached that of the initial reaction mixture.     

Crystallization Kinetics of Cocoa Fat Systems: Experiments and Modeling

Isothermal crystallization kinetics of unseeded and seeded cocoa butter and milk chocolate is experimentally investigated under quiescent conditions at different temperatures in terms of the temporal increase in the solid fat content. The theoretical equations of Avrami based on one-, two- and three-dimensional crystal growth are tested with the experimental data. The equation for one-dimensional crystal growth represents well the kinetics of unseeded cocoa butter crystallization of form α and β′. This is also true for cocoa butter crystal seeded milk chocolate. The sterical hindrance due to high solids content in chocolate restricts crystallization to lineal growth. In contrast, the equation for two-dimensional crystal growth fits best the seeded cocoa butter crystallization kinetics. However, a transition from three- to one-dimensional growth kinetics seems to occur. Published data on crystallization of a single component involving spherulite crystals are represented well by Avrami’s three-dimensional theoretical equation. The theoretical equations enable the determination of the fundamental crystallization parameters such as the probability of nucleation and the number density of nuclei based on the measured crystal growth rate. This is not possible with Avrami’s approximate equation although it fits the experimental data well. The crystallization can be reasonably well defined for single component systems. However, there is no model which fits the multicomponent crystallization processes as observed in fat systems.     

Crystal Nucleation in Soda–Lime–Silica Glass at Temperatures below the Glass Transition Temperature

The effect of crystal nuclei formed during the production of glass on the kinetics of nucleation of crystals in it at temperatures below the glass transition temperature is studied. A glass composition of 22.4Na2O ⋅ 28.0CaO ⋅ 49.6SiO2 (mol %) is studied (glass transition temperature 540°C). Using the development method, the dependence of the number of nucleated crystals on the heat treatment time is determined at a temperature of 500°C. The nuclei of crystals formed during the production of glass lead to an unusual, in the form of a step, initial section of the curve N(t). The rate of stationary nucleation of crystals with a long time of nucleation heat treatment exceeding the induction period of crystal nucleation is determined. The reasons for the unusual initial nature of the dependence N(t) are discussed. The data obtained make it possible to expand the temperature dependences of the stationary rate and the induction period of crystal nucleation for the given glass toward low temperatures.     

Kinetics of isothermal and non‐isothermal crystallization of poly(butylene terephthalate)/ liquid crystalline polymer blends

The melting behavior and isothermal and non‐isothermal crystallization kinetics of poly(butylene terephthalate) (PBT)/thermotropic liquid crystalline polymer (LCP), Vectra A950 (VA) blends were studied by using differential scanning calorimetry. Isothermal crystallization experiments were performed at crystallization temperatures (T_c), of 190, 195, 200 and 205°C from the melt (300°C) and analyzed based on the Avrami equation. The values of the Avrami exponent indicate that the PBT crystallization process in PBT/VA blends is governed by three‐dimensional morphology growth preceded by heterogeneous nucleation. The overall crystallization rate of PBT in the melt blends is enhanced by the presence of VA. However, the degree of PBT crystallinily remains almost the same. The analysis of the melting behavior of these blends indicates that the stability and the reorganization process of PBT crystals in blends are dependent on the blend compositions and the thermal history. The fold surface interfacial energy of PBT in blends is more modified than in pure PBT. Analysis of the crystallization data shows that crystallization occurs in Regime II across the temperature range 190°C‐205°C. A kinetic treatment based on the combination of Avrami and Ozawa equations, known as Liu's approach, describes the non‐isothermal crystallization. It is observed that at a given cooling rate the VA blending increases the overall crystallization rate of PBT.     

Isothermal crystallization of metallocene‐based polypropylenes with different isotacticity and regioregularity

Three polypropylenes with various isotacticities and regioregularities were prepared with metallocene catalysts at different polymerization temperatures. WAXD results showed that the relative content of γ crystals in polypropylenes increased as isotacticity decreased. It was found that the γ crystal overcame the α crystal and became predominant in polypropylene of low isotacticity and high regioerrors. More γ crystals were also formed at higher crystallization temperatures. The isothermal crystallization kinetics of α and γ crystals were compared and the metastability of these two crystal phases was interpreted in terms of the crystallization kinetics. It was observed that the α crystal has a faster crystallization rate than that of the γ crystal and, thus, higher stability at low temperature. By contrast, the γ crystal tends to have a faster crystallization rate and becomes more stable at high temperature. The metallocene‐based polypropylenes with different isotacticities have similar Avrami exponents. As the content of the γ crystal increases, double melting peaks become more evident. Equilibrium melting temperatures were derived from Hoffman–Weeks analysis and very close equilibrium melting temperatures were obtained, 185.5 and 184.0°C, for two metallocene‐based polypropylenes containing major α crystals and one of 182.5°C for the polypropylene with predominant γ crystals. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 90: 3215–3221, 2003     

Effect of Overcooling and Stoichiometry Deviations on Kinetics and Mechanism of Crystallization of As 2 Se 3 Glass

A comparative study of the kinetics of massive bulk crystallization of As2Se3, As1.9Se3, and As2.1Se3 bulk glasses at 240°C is performed using the Kolmogorov–Avrami model. A theoretical analysis of the experimental dependences of the linear growth rate of As2Se3 crystals on temperature and viscosity of the As2Se3 glass is performed. It is shown that the dislocation growth of crystals in the form of spherulites appears to be probable in the As2Se3 glass under overcooling at ∆T = 30–135°С. For individual lamellar crystals As2Se3 with dislocation-free faces inside the spherulites, a growth mechanism with surface two-dimensional nucleation (2Dsg model) appears to be probable at ∆T = 75–135°С.     

Kinetics of nucleation and growth of L-sorbose crystals in a continuous MSMPR crystallizer with draft tube: Size-independent growth model approach

The experimental data concerning kinetics of a continuous mass crystallization in L-sorbose - water system are presented and discussed. Influences of L-sorbose concentration in a feeding solution and mean residence time of suspension in a working volume of laboratory DT MSMPR crystallizer on the resulting crystal size distributions, thus on the nucleation and growth kinetics, were determined. The kinetic parameter values were evaluated on the basis of size-independent growth (SIG) kinetic model (McCabe’s ΔL law). It was observed that within the investigated range of crystallizer productivity (220–2,200 kg of L-sorbose crystals m⁻³ h⁻¹), a crystal product of mean size Lm from 0.22 to 0.28 mm and CV from 68.8 to 44.0% was withdrawn. The values of linear growth rate show increasing trend (from 6.6·10⁻⁸ to 7.6·10⁻⁸ m s⁻¹) with the productivity enlargement (assuming constant residence time τ=900 s). Occurrence of secondary nucleation phenomena within the circulated suspension, resulting from the crystals attrition and breakage was observed. The parameter values in a design equation, matching linear growth rate and suspension density with nucleation rate were determined.