Application of ultrasound for start‐up of evaporative batch crystallization of ammonium sulfate in a 75‐L crystallizer

A positive effect of ultrasound on crystallization has been shown for many applications especially on small scale. Predictable scale‐up of sonocrystallization is a challenge due to the inherent dependency of ultrasound on scale. The presented research discusses the experimental application of ultrasound to induce nucleation at low supersaturation for start‐up of evaporative batch‐wise crystallization of ammonium sulfate in a 75‐L draft tube (DT) crystallizer. A comparison is made with a conventional start‐up procedure using primary nucleation or seeding. Ultrasound is applied in two geometrically different vessels of 1.2‐L connected to a 75‐L DT crystallizer. Application of ultrasound for start‐up of a 75‐L DT crystallizer shows that an optimum amount of ground seeds is better capable to suppress nucleation. A challenge for future research is to improve the efficiency of ultrasound to produce a large number of nuclei for start‐up of batch crystallization at larger scale. © 2011 American Institute of Chemical Engineers AIChE J, 2011     

Modeling ultrasound-induced nucleation during cooling crystallization

A model to simulate cooling crystallization processes with ultrasound is proposed by introducing an additional kinetic expression for nucleation mechanism due to the ultrasound irradiation. Applying the model to predict the concentration profiles, nucleation rates, and crystal-size distribution during the sonocrystallization process shows that the key parameter for understanding induced nucleation seems to be the reduction of the nucleation work, which verifies the assumption of a heterogeneous-nucleation mechanism. Hence, the specific foreign surfaces of the cavitation bubbles as well as the adsorption properties of the forming crystals on the cavitation bubbles play important roles in qualitative prediction of the number of induced nuclei. Furthermore, enhanced diffusion due to ultrasonic waves seems to have a minor effect on the nucleation mechanism.     

On supersaturation during mass crystallization from solution

Supersaturation is the driving force for crystallization from solution. For design and operation of crystallization processes, it is necessary to apply a set of criteria for the determination of appropriate or even optimum supersaturation. In this paper we present and discuss the results of our experimental and theoretical investigations on supersaturation during crystallization from solution. While the determination of the metastable zone width of the solution is frequently applied as a fast check on solution behaviour, we demonstrate that the determination of the induction period of the supersaturated solution is a useful aid in ascertaining the maximum allowable supersaturation. Theoretical considerations and experiments confirm that the induction period dependssoley on growth kinetics if secondary nucleation is the predominant mechanism. In order to avoid uncontrollable spontaneous value of between 70 and 130 μm during crystallization. Since growth kinetics turns out to be the only important system property for the determination of maximum possible supersaturation, we conclude the paper with some arguments concerning a closed representation of growth kinetics.     

Ultrasound assisted crystallization of cephalexin monohydrate:Nucleation mechanism and crystal habit control

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.     

How to measure supersaturation?

The existing methods for the measurement of supersaturation can be generalized insofar that physical properties are used that show a dependence on concentration of the measurands for supersaturation. The influence of impurities, foreign particles, or ions on the metastable zone width and on the kinetics of nucleation and crystal growth cannot be detected by most of the described methods. Thus, it is necessary to develop a supersaturation sensor which considers the actual crystallization process itself in its measurement method. The idea of that is to induce crystallization on the surface of the sensor by generating an additional supersaturation by cooling and to observe the time-dependent development of the formation of solid matter on the surface which leads finally to incrustation. Assuming a constant cooling rate and constant properties of the sensor surface, the starting time of the incrustation on the sensor surface depends only on the prevailing supersaturation in the process solution. Experimental results obtained for inorganic (KNO₃) and organic (adipic acid) crystallizing solutes proved the working of the new sensor.     

Effects of ultrasonic radiation on induction period and nucleation kinetics of sodium sulfate

The effects of ultrasound on crystal nucleation and particle size distribution of sodium sulfate were investigated via determining the induction period and particle size. Crystal nucleation parameters and equations for primary nucleation were calculated. The experimental results show that the induction time decreases under the ultrasound irradiation, therefore, we can get a shorter induction period at a higher supersaturation level. Based on these observations, the growth mechanism of sodium sulfate is continuous growth because the value of the surface entropy factor f is smaller than 3. The induction period was observed shorter and particle size was smaller when the ultrasonic radiation time increased. Crystal growth improved with the longer crystallization time.     

Crystallization and Precipitation: The Optimal Supersaturation

In this article optimal supersaturation for cooling and evaporation crystallization are presented. This information is based on a simplified model which describes the metastable zone width of activated nucleation. The model assumes that the nucleation step is very fast and the occurrence of the shower of nuclei which is used to define the metastable zone width is caused by the growth of activated nuclei. In future new sensors for the measurement of supersaturation will help to operate batch crystallizers at the optimal supersaturation during the entire batch time.     

Crystallization and Precipitation: The Optimal Supersaturation

In this article the optimal supersaturation for cooling and evaporation crystallization are presented. This information is based on a simplified model which describes the metastable zone width of activated nucleation. The model assumes that the nucleation step is very fast and the occurrence of the shower of nuclei which is used to define the metastable zone width is caused by the growth of activated nuclei. In future new sensors for the measurement of supersaturation will help to operate batch crystallizers at the optimal supersaturation during the entire batch time.     

Feststoffbildung durch Kristallisation und Fällung

Formation of solids by crystallization and precipitation. Formation of solids from solutions can take place by crystallization or precipitation. The principal factor is the relationship between solubility and supersaturation. In crystallization, the solubility of the crystallising substance is so high that the formation of solids occurs largely in the metastable zone in the immediate vicinity of the solubility limits. Crystal growth and nucleation are functions of supersaturation. If a high supersaturation is required at low substance-specific growth rates for an adequate crystal growth, high nucleation rates and hence small crystals will result. On this basis, the authors present information for the specific design of crystallization processes. In contrast, the solubility of the precipitated product must be very low for precipitation. Direct crystal formation is possible only for substance systems of high solubility. As a rule, however, the precipitated substance is so insoluble that solids are formed via amorphous intermediates. The results of a large number of experiments show the influence of various parameters of the precipitation process on the filtrability of the precipitated product.     

A study on crystallization kinetics of pentaerythritol in a batch cooling crystallizer

The crystallization kinetics of pentaerythritol (PeE) in aqueous solution in the presence of impurity or not in a batch cooling crystallizer was explored. Also, the solubility and the nucleation and crystal growth kinetics of PeE in aqueous solution were investigated. A second-order dependence of PeE growth rate on supersaturation is observed in pure PeE-water system. The crystal growth rate of PeE-water system in the presence of impurity is proportional to the supersaturation to the 3.5 power. The nucleation and crystal growth behaviors for PeE-water system in a batch cooling crystallizer were grasped according to Mersmann's criteria. The nucleation in this crystallizer was found to act with heterogeneous nucleation. In this system, it suggests that the crystal growth is controlled by a complex mechanism behavior of surface integrated and diffusion limited. Simplified relation was derived for calculation of mean crystal size of product crystals from the batch cooling crystallizer. The obtained relation was verified by a set of experiments.     

Continuous crystallization of adipic acid with ultrasound

The application of a novel continuous cooling crystallization system coupled with ultrasound technology to crystallize adipic acid from an aqueous solution was investigated. Both continuous ultrasound irradiation and silent (control) regimes were used in order to elucidate the effects of unstable cavitation on the duration of the transient period, steady state operating conditions, crystal habit, and particle size distribution.The results reveal that under continuous ultrasonic irradiation the steady state particle size distribution is achieved after shorter times, with a consistent reduction of the steady state supersaturation resulting in increased product yields relative to silent continuous crystallization experiments. Continuous crystallization with ultrasonic irradiation results in significantly smaller crystal sizes, reduced agglomeration and an improved habit of crystals. Furthermore, the influence of mean residence time using continuous ultrasound produces a smaller change in particle size and size distribution. Finally, the experiments reveal a noticeable reduction of the fouling on non-cooling surfaces and underline the need for intermittent discharge to prevent classification on withdrawal product.     

Influence of ultrasound-assisted nucleation on freeze-drying of carrots

Ultrasound has been often used to assist nucleation process in liquid samples, while solid samples were barely taken into account. In this study, direct-contact ultrasound was used to realize the sonocrystallization for solid sample and the influence of ultrasound-assisted nucleation on freeze-drying process was investigated. First, a microscopic observation was performed to exam the effectiveness of direct-contact ultrasound on inducing nucleation for carrot slice at different supercooling degrees. Second, an ultrasonic vibrating plate was used to integrate the contact ultrasound into freeze-drying process and its influence on ice crystals size and sublimation rate was evaluated. The results showed that direct-contact ultrasound is applicable to stimulate nucleation in carrot slice at lower supercooling degree and obtain bigger ice crystals. Compared to the control sample, the sublimation time of the samples under sonocrystallization was significantly reduced, by 21.80%. Therefore, direct-contact ultrasound maybe a promising method to be applied in solid-sample freeze-drying process.     

电磁场对碳酸钙成核诱导期影响的实验研究

基于电导率随滴定液容积变化特征与碳酸钙结晶过程的对比分析,研究了电磁场作用对碳酸钙结晶过程中成核诱导期的影响。在不同频率的电磁场作用下观察了碳酸钙溶液临界过饱和度及成核诱导期的变化,大量实验结果表明:电磁场作用能够降低碳酸钙溶液临界过饱和度,加速碳酸钙成核,缩短成核诱导期。通过机理分析,电磁场对碳酸钙成核过程的主要作用为:在洛伦兹力的影响下,溶液中的Ca2+及CO32-等带电离子运动方向要向相反方向偏离,增加了Ca2+和CO32-离子间的碰撞几率,加快新相晶核的生成。     

Bubble and Crystal Formation in Lipid Systems During High-Intensity Insonation

The objective of the present research was to monitor bubble and crystal formation in lipids during high-intensity insonation. High-intensity ultrasound was generated with a 20-kHz probe. Bubble and crystal formations were recorded using a low-intensity ultrasound spectrometer operating at 1 MHz central frequency. Bubble formation was monitored in soybean oil (SBO) during insonation for different time periods (5, 10 and 60 s) and at different temperatures (22, 24, 26, 28, and 30 °C). Ultrasound attenuation due to the presence of bubbles was observed in all conditions tested. Attenuation increased with temperature and at intermediate frequencies (1 MHz). In addition, the presence of bubbles was detected after insonation was stopped, particularly for SBO sonicated for 60 s at 30 °C. Low-intensity ultrasound spectroscopy was used to monitor the crystallization behavior of interesterified SBO. The acoustic velocity increased during crystallization, but significantly decreased during insonation.     

Determination of metastable zone and induction time of analgin for cooling crystallization

The solubility, metastable zone width, and induction time of analgin for unseeded batch cooling crystallization in ethanol–aqueous system were experimentally determined. The solubility data could be well described by the van't Hoff equation model. The metastable zone width at various cooling rates was measured, and some parameters of nucleation kinetic were calculated using the Ny'vlt theory. Furthermore, the induction period of various temperatures and supersaturation ratios was also measured. According to classical nucleation theory, some nucleation parameters and interfacial energy was calculated through the induction time(t_(ind)) data. Homogeneous nucleation tended to occur when the supersaturation is high, whereas heterogeneous nucleation was more likely to occur when the supersaturation is low.     

Induction Time in Crystallization Fouling on Heat Transfer Surfaces

This work deals with the induction period in crystallization fouling on heat transfer surfaces. While the crystal growth period can be calculated, the influencing effects in the initial stage of crystallization fouling are not fully understood and quantified so far. To identify the factors influencing the induction time, experiments with calcium sulfate were performed on modified surfaces. As surface parameters the free surface energy, roughness and topography were determined by drop shape analysis and/or AFM. The visual information by microscopic studies with a SEM shows different behavior in the initial stage of crystallization on modified surfaces at constant process conditions. These microscopic findings could be verified with induction times of fouling experiments, leading to a prediction of the induction time based on surface parameters and supersaturation of the salt solution.     

Modeling layered crystal growth at increasing supersaturation by connecting growth regimes

Mechanistic modeling facilitates rational crystallization engineering and design space screening. For an accurate model, the dominant growth mechanism operating on each face must be determined, which is highly dependent on supersaturation. Considering the case of centrosymmetric growth units, we developed and connected existing mechanistic expressions for spiral and two‐dimensional‐nucleation growth regimes, through application of stationary nucleation rate theory. Our approach enables calculation of crossover supersaturations and forms a framework to model the specific mechanism operating on each face under given crystallization conditions. Increasing supersaturation can change the crystal morphology; as face families switch growth mechanisms, they may grow out of the steady‐state shape, or influence its aspect ratio. Application of the model to naphthalene, biphenyl, pentaerythritol and β‐HMX shows the ability to capture experimentally observed examples of such supersaturation‐dependent crystal habits. This methodology broadens the applicability of mechanistic crystal growth modeling to include higher‐supersaturation industrial processes. © 2017 American Institute of Chemical Engineers AIChE J, 63: 1338–1352, 2017     

L-酒石酸水溶液结晶介稳区和诱导期的测定

在20~60℃温度范围内,采用激光法测定L-酒石酸在水中的结晶介稳区宽度和诱导期,分别研究了饱和温度、冷却速率和搅拌速率对L-酒石酸结晶介稳区宽度的影响以及过饱和度对L-酒石酸诱导期的影响。结果表明:结晶介稳区宽度随着饱和温度、搅拌速率的增大和冷却速率的降低而变窄,并推算出表观成核级数m,给出了成核速率方程,此外实验测定结晶诱导期随过饱和度S增大而变短,基于经典成核理论和诱导期数据,计算出L-酒石酸在水中20℃和25℃下的固液界面张力。     

Unseeded Induction Period and Primary Nucleation of Lithium Carbonate

A set of laser apparatus was used to study induction period and primary nucleation of lithium carbonate in reactive crystallization. The results show that induction period increases with the decrease of supersaturation degree and temperature. Magnetic field has nearly no impact on it, while the use of ultrasound reduces it obviously. The impacts of some impurities and additives on primary nucleation were investigated. It is found that the presence of Na2SO4, NH4Cl, (NH4)2SO4 and EDTA disodium prolongs induction period, whereas KCl, NaCl, NaBr and CH4N2O can decrease it. Many important parameters of primary nucleation are also obtained, as temperature goes up from 283 to 313 K, interfacial tension decreases from 44.8 to 41.5 mJ/m2, contact angle varies from 82.9o to 88.7o, nucleation order is between 5.0 and 5.4, surface entropy factors are all above 5, suggesting that the growth mechanism is a spiral growth. Besides, both the homogeneous and heterogeneous primary nucleations were monitored with on-line focused beam reflectance measurement.     

Design criteria for the crystallization of an organic salt from an aqueous solutionAuslegungsunterlagen zur kristallisation eines organischen salzes aus wässriger Lösung

Results from laboratory experiments and a 500 tonnes per month pilot plant have been used to draw up criteria for designing the crystallization of an organic salt from its aqueous solution. The crystallization kinetics were determined in a continuous forced circulation crystallizer. The experiments in the forced circulation evaporative crystallizer were carried out and evaluated in accordance with the MSMPR (mixed suspension mixed product removal) concept.The residence time was varied between 0.8 and 10 hours, and the density of the suspension between 180 and 870 kg m^?3. It was found that, within the residence time and suspension density ranges studied, growth and nucleation rates have the same dependence on the supersaturation. On the other hand, the nucleation rate decreases with increasing suspension density. If the crystallization kinetics and other design criteria, such as vapour velocity, i.e. supersaturation ratio in the vicinity of the evaporative surface, or height of liquid above the heat exchangers, are taken into account, it is possible to obtain a space—time yield which is five times higher than in the pilot plant, so that the plant to be built can be designed to be correspondingly smaller.