Characteristics of multistage column crystallizer for gas-liquid reactive crystallization of calcium carbonate

Crystallization of calcium carbonate using a gas (CO₂)-liquid (Ca(OH)₂) reaction was conducted using a three-stage column crystallizer. In this study, the staging effect of the multistage column crystallizer and the effect of the operating parameters on the particle size distribution were experimentally and theoretically investigated. From an economic viewpoint, the multistage column crystallizer has been developed to replace a cascade of MSMPR crystallizers, which requires a large installation area and high operating cost because some crystallizers are connected in a series. In the multistage column crystallizer, a narrower particle size distribution was obtained compared to that obtained in the standard column crystallizer. The theoretical approach to predict the particle size distribution is based on the population balance equation which takes account of the back flow into the crystallizer. The calculation results of particle size distribution fairly coincide with the experimental results, which indicates the particle size distribution in the multistage column crystallizer can be roughly predicted by this theoretical model.     

Investigation of different crystal habits without chemical additives in a three-phase reactor

A lot of powders with fine particle sizes and specific crystal habits are produced by precipitation processes, in which growth and morphology of the particles are controlled by a wide range of chemical agents. This study investigates alternative ways to limit crystal growth and influence crystal morphology. Experiments are carried out in an ultrasound levitator, where a single droplet can be suspended against gravity by an acoustic levitation force. The ultrasound leviator is a three-phase reactor, which allows the investigation of precipitation with applying specific shear forces at growing crystals within the droplet. The investigated system is calcium carbonate. By variation of physical reaction parameters as temperature and level of applied shear forces it is possible to obtain different crystal habits and morphologies. The morphology of calcium carbonate produced in the leviator can vary between prismatic, shell-like and spherical shape. Also the particle size distribution of the precipitated product is influenced. Increasing mechanical stress leads to a shift of the particle size distribution to smaller sizes.     

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.     

Nucleation and Supersaturation in Drowning‐Out Crystallization using a T‐Mixer

In order to assess the performance of drowning‐out crystallization using a T‐mixer, nucleation and supersaturation were studied. The particle size was changed considerably with the solvent fraction and the feed rate, which were the main parameters controlling the supersaturation. At S = 1.7, the boundary point between homogeneous and heterogeneous nucleations was found from a log‐log plot of supersaturation rate versus maximum supersaturation. Drowning‐out crystallization using a T‐mixer could easily generate high supersaturations of up to 50, which were adjusted by the feed rate and the ratio of solvent to antisolvent. Nano‐ and micron‐sized particles can be prepared by drowning‐out crystallization using a T‐mixer.     

不相容聚合物共混物受限结晶行为的研究进展

不相容聚合物共混是一种获得综合性能优异的高分子材料的有效途径。当结晶聚合物作为分散相均匀地分布在不相容聚合物基体相中,并且其分散相数目大于活跃的异相成核物质数目时,在冷却过程中会发生受限结晶(分级结晶)现象。分散相中缺少活跃的异相成核物质是不相容共混物发生受限结晶的关键因素。从受限结晶的产生总结了近年来不相容共混物中受限结晶的研究进展,并概述了增容剂、填料、退火、分散相尺寸及其分布和结晶温度等因素对受限结晶行为的研究。     

有机金属磷酸盐/聚乳酸复合材料的非等温结晶动力学研究

采用DSC对有机金属磷酸盐/左旋聚乳酸(NA/PLLA)复合材料的非等温结晶动力学进行了研究,并对所得数据用Jeziony方程进行了处理。结果表明有机金属磷酸盐(NA)能有效地提高PLLA的结晶度。NA对PLLA结晶过程的影响是成核效应和阻碍分子链的运动的协同作用,而且冷却速率的提升提高了添加剂对分子链的阻碍作用。Jeziony理论分析表明PLA及其复合材料的Avrami指数n均在2～3之间,前期PLA及PLA/NA复合材料可能以二维生长方式生长,而且NA的加入并未改变PLA的成核机理和生长方式。     

The effect of seed preparation on the chirality of the secondary nuclei

Previous secondary nucleation experiments were conducted with chiral NaClO₃ crystal seeds prepared by “regular” cooling (1.6°C/h). In these experiments seeds were used prepared by “slow” cooling (0.1°C/day) and by evaporation. In all cases, at low supercoolings almost all the nucleated crystals have the same chirality as the seed. Above a certain supercooling, however, crystals of both chiralities are obtained. This transition is consistent with the embryos coagulation secondary nucleation mechanism, and it was found to be a strong function of the way the crystals are prepared. The transition supercoolings vary from 2.5-4°C for seeds prepared by evaporation, to 6.8-9°C by “regular” cooling and to 10.5-11°C by “slow” cooling. ESEM observations of the seed surface link the above variations to variations in the surface roughness and the bulk crystal strain. These findings have implication for the separation of chiral compounds by crystallization.     

钙硅比对钙铁硅铁磁体微晶玻璃核化与晶化的影响

为了研制用作温热治疗癌症的铁磁体微晶玻璃热种子材料,制备了一组化学组成为40Fe2O3xCaO(60-x)SiO23B2O33P2O5玻璃(x=20、25、30、35质量分数),用XRD、DTA、VSM对其核化与晶化过程进行了研究.研究发现,钙铁硅微晶玻璃在还原气氛下热处理后,除了磁铁矿和硅灰石及少量赤铁矿晶相外,还会出现钙铁辉石、方石英晶相.钙硅比较小的玻璃,热处理温度-时间相图中钙铁辉石相区范围较大;钙硅比较大的,钙铁辉石相区范围较小.另外,钙硅比不同,玻璃的成核机理、开始析出磁铁矿的温度也不同.     

Barium Sulfate Crystallization Kinetics in the Used Quenching Salts Treatment Process

The research results and kinetic studies on the barium ions precipitation process by means of crystalline ammonium sulfate addition are presented. The influence of: barium, sodium, potassium and calcium chlorides concentration in the feeding solution and also the process temperature on barium sulfate mass crystallization kinetics are investigated. Experiments were carried out in the MSMPR crystallizer with internal circulation of suspension. The barium sulfate nucleation and crystal growth rates were estimated from crystal size distribution taking into account a size dependent growth (SDG). Research results may be utilized in the used quenching salts processing technology.     

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.     

聚苯硫醚/羟基改性多壁碳纳米管复合材料等温结晶动力学的研究

用差示扫描量热法(DSC)研究了聚苯硫醚(PPS)/羟基多壁碳纳米管(MWCNTs-OH)复合材料的等温结晶过程,用Avrami方程考察了相关的结晶动力学参数.结果表明,各体系的Avrami指数均大于4,说明少量MWC-NTs-OH (1%～2%)不会改变PPS均相成核的成核方式,但MWCNTs-OH的加人使PPS结晶速率常数增大,体系结晶速率加快.MWCNTs-OH添加量相同时,结晶温度越低,MWCNTs-OH对结晶速率的影响越明显.FTIR分析结果表明,PPS中对位苯撑取代(S-Ar-S)振动峰发生明显位移,表明MWCNTs-OH与PPS间存在较强的分子间作用力.     

Combined Crystallization and Drying in a Pilot-Scale Spray Dryer

Combined crystallization and drying of lactose solutions was performed in a pilot-scale spray dryer over a wide range of operating conditions. The effect of different parameters, including temperature, moisture content, atomizing air flow rate, liquid feed rate, main drying air flow rate, and particle size, on the degree of crystallinity of the spray-dried powders was analyzed. Water-induced crystallization (WIC) and modulated differential scanning calorimetry (MDSC) were used to assess the effect of these parameters on the degree of crystallinity of the spray-dried powders. The particles were characterized in terms of the final moisture content using WIC and distinctive differences in the peak heights, which are indicative of the particle crystallinity, were found for spray-dried particles using different drying conditions, supporting the results from MDSC. MDSC showed that decreasing the inlet air temperature by 40°C increased the degree of crystallinity in the particles threefold from 22 to 72%. A decrease in the inlet air temperature may decrease the particle temperature, resulting in wetter particles, and a lower temperature meant a longer particle drying time and allowed the particles to rearrange themselves into a more crystalline form. Up to 72% crystallinity is achievable in a pilot-scale spray dryer by suitable adjustment of the operating conditions. The results suggest differences in the rate of crystallization and particle size between small and pilot-scale spray dryers.     

Crystallization of sulfamethizole using the supercritical and liquid antisolvent processes

Sulfamethizole was crystallized using both the supercritical and liquid antisolvent processes. Acetone and N,N-dimethyl formamide (DMF) were selected as solvents for the pharmaceutical compound, and carbon dioxide and distilled water were used as antisolvents. In the supercritical antisolvent process, the effects of experimental conditions such as carbon dioxide injection rate, type of solvent, and temperature were investigated. In the liquid antisolvent process, the effect of ultrasound on the properties of crystal was examined. The various crystal habits such as tabular, platy, acicular, and prismatic were observed depending on the process and experimental conditions. Differential scanning calorimetry (DSC) measurement revealed that the carbon dioxide injection rate affected the crystallinity of sulfamethizole particles. Larger crystals were obtained at higher temperatures in the two antisolvent processes. The particle size distribution was mostly affected by the antisolvent injection rate and the application of ultrasound.     

Crystallization kinetics of two polyesters with different main chain rigidity

The crystallization kinetics of two polyesters, poly(p-phenylene sebacate) (PPS) and poly(p-phenylene isophthalate) (PPI), have been investigated by DSC. The nonisothermal data indicate that the relative crystallization rate of PPI was slower than that of PPS due to the more rigid nature of the PPI the chains. The isothermal crystallization data were analyzed by the Avrami equation, 1 − X_(t) = exp(−ktⁿ). PPS exhibited Avrami exponent (n), values of about 4, indicating that its isothermal crystallization followed a process of homogeneous nucleation, spherical growth and a growth type of interface control. PPI exhibited Avrami exponent values of about 3 indicating its isothermal crystallization followed a process of homogeneous nucleation, a possible disc growth geometry and an interface control growth type. Possibly, the bending structure and rigid nature of PPI forced its growth to follow a three dimensional growth during crystallization.     

Drying in the Context of the Overall Process

Abstract

Although drying has traditionally been considered as a unit operation, it is strongly affected by upstream operations such as crystallization and solid–liquid separation, and in turn can affect downstream processes such as gas cleaning and micronization. Process design needs to consider the complete flowsheet and the interactions between the different steps as early as possible. Key points are the particle formation method and the final product specification. The mean particle diameter and particle size distribution are vital parameters throughout the process, as smaller particles and fines make solids handling, dewatering, and washing more difficult. This in turn affects the inlet moisture content to the dryer, and hence the heat duty and performance. Intermediate size enlargement or reduction may be used to give a more easily dried particle or agglomerate. There are important links to the new subject of product engineering, for example in the choice of processing route to achieve a given product quality and specification. The interactions between the different process steps can affect process design, equipment selection and troubleshooting, and this is illustrated by industrial case studies. A holistic approach is proposed to allow the whole solids processing flowsheet to be optimized as an entity, rather than optimising each unit operation in isolation and then finding a conflict between them.     

In‐Line Monitoring of Crystallization Processes Using a Laser Reflection Sensor and a Neural Network Model

Laboratory‐scale experiments were carried out for measuring the chord length distribution of different particle systems using a laser reflection sensor. Samples consisted of monodisperse, polydisperse and bimodal FCC catalyst and PVC particles of different sizes, ranging from about 20 to 500 μm. The particles were dispersed in water, forming suspensions with solid‐phase mass fractions ranging from ca. 0.2 % until ca. 30 %. The experimental results, consisting of the particle number counting per chord length class, were used in fitting a neural network model for estimating the mass concentration of particles in the suspension and the volume‐based size distribution, eliminating the effects of suspension concentration and particle shape. The results indicate the feasibility of using such a model as a software sensor in crystallization processes monitoring.     

天然胶乳包覆聚苯乙烯刚性微粒对聚丙烯结晶的影响

采用DSC、XRD分析方法研究了聚苯乙烯刚性微粒(XPS)和天然胶乳包覆聚苯乙烯刚性微粒(N-XPS)对聚丙烯(PP)结晶行为的影响。结果表明:XPS和N-XPS均可加快PP在高温区的成核速率,导致PP在较高的温度下提前结晶;它的存在也阻碍了PP球晶的生长速率,使总结晶度降低,相比较而言,其中N-XPS有更好的效果。XRD分析表明:在PP中添加2%的XPS或N-XPS,均可诱导PP形成β结晶,增加用量,β-PP结晶峰消失;PP/N-XPS试样中2θ=14.02°的α晶(110)峰明显比纯PP的要小这一事实,佐证了N-XPS阻碍了PP(α-110)晶面的生长,使其结晶度降低的结果。     

On the prediction of crystal shape distributions in a steady-state continuous crystallizer

Zhang and Doherty [2004. Simultaneous prediction of crystal shape and size for solution crystallization. A.I.Ch.E. Journal 50, 2101-2112] have provided a one-dimensional analysis of crystallization based on the assumption that the relative face-specific growth rates of a (2-D) crystal are independent of supersaturation and hence invariant with time. Subsequent work by these authors [Zhang, Y., Sizemore, J.P., Doherty, M.F., 2006. Shape evolution of 3-dimensional faceted crystals. A.I.Ch.E. Journal 52, 1906-1915) consider shape evolution of single three-dimensional crystals with morphological changes. In this work, we present a multidimensional population balance approach accounting for dependence of the relative face-specific growth rates on supersaturation, a situation more commonly encountered. For example, Joshi and Paul [1974. Effect of supersaturation and fluid shear on habit and homogeneity of potassium dihydrogen phosphate crystals. Journal of Crystal Growth 22, 321-327] and Mullin and Whiting [1980. Succinic acid crystal-growth rates in aqueous solution. Industrial & Engineering Chemistry Fundamentals 19, 117-121] report face-specific growth rates with different dependence on the supersaturation. Thus it has been observed that there exists significantly different crystal shapes in a crystallizer [Yang, G., Kubota, N., Sha, Z., Louhi-Kultanen, M. Wang, J., 2006. Crystal shape control by manipulating supersaturation in batch cooling crystallization. Crystal Growth and Design 6, 2799-2803]. Consequently, the population of crystals at any instant will have widely varying crystal shapes and sizes depending upon the initial crystal shape and size distribution. Computations are presented for the shape distributions of the crystal population emerging from a steady-state continuous crystallizer for two cases: (1) feed without crystals including nucleation for the formation of new crystals, and (2) feed with seed crystals of known shape, with suppressed nucleation. In the range of mean residence times investigated, the calculated crystal volume distributions for the first case show geometrically dissimilar shapes without morphological variations. However, in the second case, because the feed crystals of the chosen shape were susceptible to morphological changes, the volume distributions display this feature with shape and size distributions for each of a number of different morphologies. By varying operating conditions such as the flow rate, the inlet supersaturation, and the shapes of feed crystals, the proposed model can clearly be used to manipulate the crystal shape and size distributions and their morphologies.