On-Line control of supersaturation in a continuous cooling KCL crystallizer

Control of supersaturation in a 1-L continuous cooling KC1 crystallizer was investigated. The supersaturation was determined from on-line measurements of the density and temperature of clear liquor samples. A cascade control scheme was used to control the supersaturation through the manipulation of the co-saturated feed temperature set-point while maintaining the crystallizer temperature at 303.2 K. Experimental results showed that due to the suppression of spontaneous nuclearion, a decrease in the measured supersaturation resulted in a 23% increase in the mean crystal size and a 12% decrease in the amount of NaCl impurity in the KCI crystals.     

Kinetic analysis for crystal growth rate of NH4Cl in the NaCl‐MgCl2‐H2O system with a thermodynamic approach

A growth kinetic model has been developed from a rigorous thermodynamic perspective to describe the crystal growth rates of NH₄Cl on the basis of the difference of chemical potentials of NH₄Cl at solid–liquid interface in aqueous NH₄Cl, NH₄Cl‐NaCl, and NH₄Cl‐MgCl₂ solutions. The solid–liquid equilibrium and activity coefficient of NH₄Cl are calculated by the newly developed accurate Pitzer model with aid of Aspen Plus? platform. The predictions of the resulting model are in good agreement with the experimental data published in literature and determined in this work at 283.15–333.15 K within the supersaturation up to 0.1. The kinetic model was subsequently used to analyze the effect of several operation variables, including temperature (283.15–333.15 K), supersaturation (up to 0.1), and NaCl or MgCl₂ concentration (0～2.5 mol kg^?1), on the crystal growth rate of NH₄Cl. The crystal growth rate of NH₄Cl, with activation energy of 39 kJ mol^?1, is strongly temperature‐dependent and increases with increasing temperature in the three systems investigated. The advantage of MgCl₂ over NaCl on the recovery of NH₄Cl is theoretically and experimentally illustrated from the thermodynamic and kinetic perspectives with the aid of the established model. © 2011 American Institute of Chemical Engineers AIChE J, 2012     

An electrochemical method to study scaling by calcium sulphate of a heat transfer surface

The main purpose of this investigation was to devise an electrochemistry-based method, using the assessment of the oxygen-reduction cathodic reaction, in order to evaluate the intensity of calcium sulphate scale formation on a heat transfer surface. For this purpose, an experimental device, permitting the simultaneous measurement of the surface temperature and the current intensity relative to electrochemical oxygen-reduction on the heat transfer surface during the fouling process, was built. From the chronoamperometric curves, it is possible to deduce the induction time. The results were confirmed by the simultaneous direct measurement of the surface temperature. This coupling of thermal and electrochemical measurement gave additional information regarding the growth and the porosity of the formed scale layer. By this chronoamperometric method, it was shown that the supersaturation coefficient of a saturated calcium sulphate solution must exceed a critical value Ω_c of the order of 4 to observe the crystallization of the gypsum layer. For the same saturation level, the salt concentration has a stronger effect than the temperature on the precipitation process.     

A comparison of crystal growth kinetics by use of chemical potential difference and concentration difference

A Spherical Void Electrodynanic Levitator Trap (SVELT) was used to measure the water activity in the supersaturated aqueous solutions of six materials, NaCl, (NH₄)₂SO₄, KH₂PO₄, NH₄H₂PO₄, KA1(SO₄)₂ · 12H₂O and glycine. The relationship between chemical potential and concentration was obtained using a fifth order polynomial fit. A comparison of the order of the crystal growth rate obtained from the use of chemical potential difference and concentration difference was made. The order of crystal growth rate calculated from concentration difference was found to be close to that obtained from chemical potential difference at low supersaturation, while at higher supersaturation a deviation was noted.     

CRYSTALLIZATION KINETICS AND PRODUCT PURITY OF α-GLUTAMIC ACID CRYSTAL

A continuous mixed-suspension mixed-product removal lpar;CMSMPR) crystallizer was used to study the crystallization kinetics and product purity of glutamic acid crystal in pure solution and impure solution with L-phenylanaline as the impurity. Under the assumption that the crystal growth rate was a function of crystal size, the population balance equation was solved to give the crystal growth and nucleation rates by use of the steady-state population density data. The crystal growth and nucleation rates were suppressed by the presence of impurity. The impurity contents in the crystal products, which were analyzed by a HPLC pre-column method, were found to be related to crystal size, supersaturation, and impurity concentration of solution     

D-核糖结晶工艺探讨

结晶过程的原则是争取最大的结晶速度，尽可能提高成品收率，保证晶粒的均匀整齐。而影响结晶的因素有：溶液浓度、晶种质量、晶体生长速率、过饱和度的大小、结晶温度、溶液pH值、溶液杂质等。本文探讨了D-核糖结晶过程中溶液浓度、溶媒（乙醇）含量和降温方式等工艺的优化。     

Salting out crystallization of sodium sulfate

The growth and nucleation rates of sodium sulfate crystals salted out from their solution in a 38% w/w sulfuric acid solvent by the addition of an 80:20 w/w methanol:water solution, were determined from measurements of the steady-state crystal size distribution (CSD) generated in a continuous mixed-suspension, mixed-product-removal (MSMPR) salting out crystallizer. The effects of the supersaturation, the crystal suspension density and the temperature on these rates were investigated. The effect of the Cr⁺⁺⁺ impurity was also briefly studied. The growth rate kinetic order g was determined to be essentially unity. The nucleation rate kinetic order b was determined as 2.11. The corresponding kinetic rate constants K_G and K_B were fitted to an Arrhenius type temperature dependence to give. K_G = 43960 exp(-24600/RT) and K_B = 0.064 exp(70900/RT) where R is in kJ/kmol · K. An increase in chromate impurity concentration caused an increase in the growth rate constant K_G but had no significant effect on the nucleation kinetics. The kinetiics of crystallization as determined for this system would indicate that a high temperature, high crystal suspension density and long residence time are conditions which are favorable for the production of large sodium sulfate crystals.     

浓盐溶液的膜蒸馏机理研究

考察了高浓度氯化钠溶液在直接接触式膜蒸馏操作中的热侧温度，冷侧温度，流速和料液浓度对膜渗透通量的影响。发现膜渗透通量随氯化钠溶液浓度的高而降低。当氯化钠溶液浓度约为25％时，膜渗透通量急剧下降。氯化钠溶液达到饱和时，膜渗透通量逐渐趋于稳定。研究认为此现象是由于膜面上有NaCl结晶形成而导致的。     

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.     

Optimization of a Continuous Precipitation Process to Produce Nanoscale BaSO4

The effect of supersaturation, reaction temperature, and mixing intensity on particle size was investigated. Sterical stabilization of barium sulfate suspensions was applied to prevent formation of agglomerates. This allowed a reactant ratio of 1:1, thus maximizing product yield. The local supersaturation is strongly affected by the mixing intensity that can be characterized by Reynolds numbers. The significant decrease in particle size was observed by increasing the Reynolds number from 600 to 8000. A higher reactant concentration leads to a higher degree of supersaturation, and finer particles are precipitated. The particle size can be reduced with increasing reactant concentration. The degree of supersaturation increases with temperature reduction, i.e., the particle size will be reduced at low temperature. In addition, nucleation and growth kinetics are changed in a way that reduces the particle size. The optimized lab‐scale process is capable of producing over 1 kg h^–1 of nanoscaled BaSO₄ with a median diameter of 75 nm.     

Electrochemical bleaching of kraft bagasse pulp using a new cell design

In situ single‐stage electrochemical bleaching of kraft bagasse pulp was carried out in a cylindrical agitated vessel fitted with four graphite rod anodes and a cylindrical stainless steel screen cathode, using NaCl as an electrolyte. The effect of current density, pH, NaCl concentration, impeller rotational speed, temperature, and pulp consistency on the rate of bleaching was studied. It was found that the rate of bleaching increased with increasing current density, NaCl concentration, and temperature and decreased with increasing pH and pulp consistency. The effect of temperature was found to fit Arrhenius equation with an activation energy of 0.515 kcal/mol, which denotes a diffusion‐controlled mechanism. Energy consumption (EC) calculation showed that EC ranged from 0.225 to 3.11 kWh/kg dry pulp depending on the current density. The strength of bleached pulp was little affected by bleaching lying within an acceptable range.     

氟硅酸钾(钠)结晶生长的动力学研究

采用静态法实验研究了溶液初始浓度、温度、搅拌强度及挂片材质对氟硅酸钾 (钠 )在湿法磷酸溶液中结晶生长的影响 ;结果表明 ,初始浓度增加能明显提高结晶速度、缩短诱导期 ,搅拌的影响不明显 ,氟硅酸钾 (钠 )晶体生长过程由晶体的表面反应控制 ,并根据实验结果得到其结晶生长的动力学方程。采用低表面能的聚氯乙烯和聚四氟乙烯塑料可以显著降低结垢形成速率     

基于PBM的L-谷氨酸粒度分布控制优化

针对β型L-谷氨酸冷却结晶过程,为获取期望粒度分布,采用特征曲线法（MOCH）来建立关于粒度相关生长率的种群平衡方程（PBE）,然后通过对种群平衡模型（PBM）参数辨识后确定最优过饱和度及控温曲线。由于辨识模型参数的目标函数具有非线性和非凸型性,因而采用少量经济性的批量冷却结晶实验,结合图像分析晶种和产品粒度分布得到的统计数据,拟合模型参数。根据实际要求的结晶过程时间,为达到目标粒度分布,通过优化结晶过程的过饱和度获得最优调温曲线,实现基于恒定过饱和度的晶体生长过程优化控制。实验结果表明通过优化的控温曲线,实现了基于最优过饱和度控制的期望目标粒度分布。     

Parameters controlling supersaturation by DELOS using carbon dioxide

The present work pertains to estimation of the maximum degree of supersaturation that can be attained in an organic solution by the DELOS process using carbon dioxide (CO₂) as a cosolvent. The paper analyzes the effects of initial mole fraction of carbon dioxide, temperature and pressure on the degree of supersaturation of cholesterol in a CO₂—acetone—cholesterol solution. It has been observed that owing to liberation of large amounts of CO₂ very large temperature drops may be attained by depressurization, resulting in attainment of very high supersaturation. Within the ranges of the parameters studied in this work, the degree of supersaturation is higher with higher values of initial temperature and initial CO₂ mole fraction of the solution due to inverse interdependence of the final temperature and the residual CO₂ mole fraction in the depressurized solution. Copyright © 2006 Society of Chemical Industry     

水溶液中硫酸钾晶体生长动力学

The single crystal growth rates of potassium sulfate in pure aqueous solution under different conditions were determined by photomicrography in a flow system for crystal growth. The effects of themain controlling factors, such as supersaturation, crystal size, solution velocity and crystal growth temperature, on crystal growth rates of potassium sulfate were discussed in detail by using non-linear regression from the experimental data, and several empirical relationships were given. The results showed that the growth rates of crystals increased with supersaturation, crystal size, solution velocity and temperature. Moreover supersaturation was the most important controlling factor influencing growth rates of crystals, crystal size and solution velocity were the secondary and temperature was the least.Furthermore, It was found that the growth rate of crystals along the [100] crystallographic axis was higher than that along the [001] in the same condition. The effect of every factor on crystal growth rates along the [100] crystallographic axis was stronger than that along the [001].     

Growth and morphology of ZnO nanorods prepared from Zn(NO3)2/NaOH solutions

ZnO nanorods on ZnO-coated seed surfaces were fabricated by a solution chemical method using supersaturated Zn(NO₃)₂/NaOH solution. The seed surfaces were coated on glass substrates by sol–gel processing and PEG addition. The mechanism of crystal growth and the factors affecting the rod growth were elucidated. The morphology and structure of both the seed surface and successive nanorods were analyzed by using SEM, XRD, TEM and SAED. Nucleation on the ZnO seed surface is crucial for rod growth since rods can only be observed on ZnO-coated substrates. Supersaturation is also required for rod growth and the Zn²⁺ ion and NaOH concentrations must be varied synchronously to maintain the high level of supersaturation. The average diameter and length of the ZnO nanorods increase to different degree with increasing precursor concentration. The dependence of rod growth on temperature shows that the maximum rod growth rate at any given concentration of Zn²⁺ occurs at a specific temperature, and the optimal temperature increases with Zn²⁺ ion concentration. Densely thick nanorods oriented perpendicularly to the substrate can be obtained by controlling the seed surface with PEG assistance.     

Nucleation of Ethylene Glycol Vapor and Growth of Sub-10-nm Particles in Nanoparticle Size Magnifier

We investigated the rates of heterogeneous and homogeneous nucleation of ethylene glycol vapor onto sub-10-nm particles in a newly developed condensation device called nanoparticle size magnifier (NanoPSM). The saturation ratio in the NanoPSM is precisely controlled by vapor-feeding system and mixing section, which are designed based on an earlier particle size magnifier (PSM) developed by Okuyama et al. (1984). Size-classified NaCl nanoparticles smaller than 10 nm in mobility diameter are used as heterogeneous nuclei for the condensation of ethylene glycol vapor. The activation efficiency and growth rate of the activated nuclei are determined by a pulse height analysis using an optical particle counter (OPC). A computer fluid dynamics (CFD) simulation is employed to calculate the profiles of the gas velocity, temperature, vapor concentration, and resulting supersaturation in the NanoPSM. Annular high-supersaturation region is generated around the mixing boundary between cold aerosol and hot vapor. The experimental activation efficiency is 50% for 4.5-nm and 0.8% for 2 nm NaCl particles, through the subsequent growth of droplets to 2 μm in diameter. The experimental data are in fairly good agreement with the predicted activation efficiencies based on the classical Kelvin-Thomson theory when the local profiles of supersaturation are taken into account.     

Effect of pressure and non-isothermal injection on re-crystallization by CO2 antisolvent: Solubility measurements,simulation of mixing and experiments

This work investigated for the first time a CO₂ antisolvent crystallization (SAS) operating in non-isothermal conditions, i.e. injection of a solution warmer than that of the CO₂ – in order to impose an additional driving for crystallization when CO₂ was not a strong antisolvent. The approach focused on phase equilibria (with a distinctive feature of being modeled by artificial neural network) and 3D-simulation of the mixing (considering both heat and mass transfers) in order to detail the supersaturation profiles in the mixing zone and confronted them to the crystallization results. The effect of pressure was investigated as well. The solubility of a model compound, mefenamic acid (MEFE) was measured in CO₂–acetone at 35 °C/8.5, 10 and 15 MPa, and 10 MPa/25, 35 and 45 °C and further correlated by a neural network to provide an easy-to-handle equation of MEFE concentration. Simulations results showed that supersaturation levels were low (around 2) and that the expanding jet spread similarly whatever the pressure. The effect of differential temperature on the mixing behavior and supersaturation was investigated. Compared to isothermal cases, higher superaturations were obtained but only if a more concentrated solution allowed by the higher temperature was processed as well. The benefit for the crystal size was difficult to evidence because of the long sizes of the needles and the difficulties of processing almost saturated solutions. Investigation of a less CO₂–acetone compound would be more promising.     

Synthesis of single phase aragonite precipitated calcium carbonate in Ca(OH)2-Na2CO3-NaOH reaction system

The formation behavior of precipitated calcium carbonate polymorphs was investigated in three different supersaturation levels. Because the most easily adjustable and influential variable determining supersaturation is the ion concentration of the major reactants — Ca²⁺ and CO3 ₃ ²⁻ — the supersaturation can be adjusted by changing the ion concentration of these two ions. At high supersaturation, free energy is necessary for a decrease in nucleation, promoting the formation of a sphere-shaped vaterite, while aragonite and calcite were seen to co-exist at medium supersaturation. At low supersaturation, aragonite was mainly formed by mixing with some calcite. Hence, we considered that lower supersaturation was necessary to obtain a single phase aragonite. Furthermore, we found that the solubility of Ca(OH)₂ was decreased with the addition of NaOH by a common ion effect. Thus, it is possible to perform an experiment at a lower Ca²⁺ concentration. The aragonite was synthesized by adding the Na₂CO₃ solution to the Ca(OH)₂ slurry containing several concentrations of NaOH solution at 75°C and under the addition rate of Na₂CO₃ at 3 ml/min. The formation yield of calcite decreased when the NaOH concentration was increased. In conclusion, in the case of the reaction of the 2.5 M NaOH solution over 210 minutes, single-phase aragonite with an aspect ratio of 20 was obtained.