复分解法氯化铵生产中冷却结晶过程研究

研究了硝酸铵和氯化钾复分解法制取氯化铵的冷却结晶过程,考察了降温速率、搅拌速率、晶种添加量等因素对氯化铵产品的纯度及晶体粒度分布的影响。研究结果表明,采取适宜的降温速率、搅拌速率及适宜的晶种添加能有效地改善氯化铵晶体粒度,提高分离效果;氯化铵冷却结晶适宜操作条件为：降温速率为0.3 K/min、搅拌速率为350 r/min、添加晶种粒度为150~180 μm、添加晶种量为1.88%,在此条件下制得的氯化铵产品晶体粒度均一性好,平均粒径更大。     

Effect of cooling mode on product crystal size in seeded batch crystallization of potassium alum

Potassium alum was crystallized by seeding in a batch crystallizer under controlled and natural cooling modes. Regardless of the cooling mode, the product crystal size distribution (CSD) became bi-modal at low seed concentrations because of enormous secondary nucleation. The mean mass size of the product was smaller for the natural cooling mode compared to that for the controlled cooling mode with more intensive secondary nucleation. On the other hand, at high seed concentrations, the product CSD became uni-modal with the same mean mass size for both cooling modes, where the crystallization was dominated by seed growth. The low supersaturation caused by the growth of enough seeds plays a key role to produce uni-modal size distribution with suppressed nucleation. Adhering of small crystals (secondary nuclei) to growing seed crystals is also considered to be another mechanism for generating uni-modal CSD.     

SEEDING EFFECT ON CRYSTAL GROWTH IN HYDROTHERMAL SYNTHESIS OF LAYERED OCTOSILICATE

In the hydrothermal synthesis process of layered octosilicate ilerite, seed crystals were added to the starting solution (water glass) for control of the crystallization rate. Octosilicate platelets of 4.4 μm and their ground products with a size less than 1 μm were used as the seed. The crystallization rate increased with an increase in the additional amount of platelet seed. The crystalline products were obtained even if the seed had a relatively low crystallinity. The size of products depended strongly on both the additional amount and size of seed. When the seed-loading mass ratio of seed to the starting solution was 0.01, the crystal growth was a maximum and the size of products was about twice of that of seed. However, in the use of the ground seed, a finer product was obtained at the optimum seed-loading mass ratio due to increase in the number of seed crystals. To control crystal growth for reducing synthesis time and for obtaining crystals of a specific size, both number and size of seed crystals must be optimized because the silanol groups on the edge of seed crystals added become new reaction sites of crystallization.     

A model-based nucleation study of the combined effect of seed properties and cooling rate in cooling crystallization

Crystallization is a widely used unit operation for the production of pharmaceuticals, fertilizers, and fine chemicals. A commonly used crystallization operational objective is to produce large crystals under minimum nucleation rates. For cooling crystallization there are two key methods for minimizing nucleation, programmed cooling and seeding. In this paper, we evaluate the cooling and seeding methods through the detailed modeling of nucleation phenomena coupled with a population balance and dynamic optimization of this mathematical formulation. Extensive simulation results showed that initial seeding parameters, as proclaimed by others, do have a significant effect on the final product crystal size distribution. It also showed the significance of a combined seeding-cooling approach where a joint cooling and seeding optimization gives superior performance to just optimizing the seed. Importantly, the developed model was highly instrumental in rapidly determining optimal combined seeding-cooling profiles via dynamic model-based optimizations.     

膜辅助添加晶种的过硫酸铵冷却结晶在线监测与过程调控

冷却结晶是经典的溶液结晶过程,常用于分离溶解度随温度变化较大的物质,制备高品质晶体产品。直接进行降温会导致成核速率不可控,得到的晶体产品质量差。在工业中通常选择在溶液结晶介稳区内投放适量晶种来诱导成核,但晶种制备过程复杂,而且成功的添加晶种过程取决于晶种的粒度分布、数量、投放时机和操作人员的经验等因素,降低了产品质量的批次重复性。本文利用聚四氟乙烯（PTFE）中空纤维膜组件为结晶溶液和冷却液提供换热界面,结晶溶液温度降低,在膜界面处形成较均匀的过冷度梯度,进而在低过饱和度下发生异相成核,实现膜辅助添加晶种的过硫酸铵冷却结晶过程调控。膜组件中产生的晶种进入结晶釜中继续生长,将成核和生长过程进行解耦。在线结晶检测系统捕捉到的照片证实了通过控制膜组件使用温度和时长两个操作参数便可得到具有较好的形貌、较窄的粒度分布的晶种。相比直接冷却结晶,在相近的降温速率下,膜辅助添加晶种过程制备的晶体产品具有更大的平均粒径,且粒度分布更集中,表面更加光滑。因此,膜辅助冷却结晶呈现了良好的成核控制能力,有望实现晶种自动制备和添加功能,为高附加值晶体产品的冷却结晶过程开发提供了新方向。     

磷酸二氢钾结晶过程研究

研究了搅拌速度、降温方式、pH值及晶种对磷酸二氢钾结晶影响规律.搅拌速度为400 r/min时,料浆悬浮均匀,所得粒径最大;自然降温方式所得晶体平均粒度较大;pH值等于3.0时,介稳区宽度最小,结晶量最大;随着晶种加入量的增大,产品粒度分布趋于集中,加入晶种且过饱和度为3%时可得到较好的粒度分布.     

In Situ Observation and Physical–Chemical Characteristics of Rambutan (Nephelium lappaceum L.) Kernel Olein Crystals Obtained from Acetone Fractionation

Hard fat stocks containing highly saturated fats are a necessary ingredient for fabrication of trans-free plastic fats. Crystal fractions obtained from the fractionation of fats naturally containing saturated fatty acids (SFA) may be a promising approach to produce the desired hard fat stocks. Influences of cooling rate (0.4, 2.0 and 10.0 °C·min⁻¹) and fractionation temperature (15 and 20 °C) on the formation of solid fat crystals of rambutan (Nephelium lappaceum L.) kernel olein (RKOle) during acetone fractionation were examined using in situ observation with polarized light microscopy (PLM). The resulting stable crystals were then separated and characterized by their iodine values, fatty acid compositions, crystal polymorphism, solid fat index, and melting behavior. PLM results showed that cooling rate affected crystal formation. Entrained oil on the surface and number of small crystals increased at higher cooling rates of RKOle. Stable crystals were obtained at a cooling rate of 2.0 °C·min⁻¹ and 6 hours, which had lower iodine value and contained more SFA with a higher amount of solid fat than incipient RKOle. Crystals fractionated at 20 °C were larger in size, fewer in number, and had less entrained oil compared to those fractionated at 15 °C. Their main polymorph was the β' form with a melting range comparable to common fully hydrogenated oils. Results suggested that RKOle crystals have potential for use as hard fat stocks for various purposes.     

Seeding effects on solidification behavior of cocoa butter and dark chocolate. I. Kinetics of solidification

Effects of seeding of fat crystals on the crystallization kinetics of cocoa butter and dark chocolate were examined with a rotational viscometer. The seed crystals employed were cocoa butter, 1,3-distearoyl-2-oleoylglycerol (SOS), 1,3-dibehenoyl-2-oleoylglycerol (BOB) and 1,2,3-tristearoylglycerol (SSS). The seed powders were prepared by pulverization below —50°C, the dimensions being in a range from 20–70 μm. Particular attention was paid to the influence of polymorphism of the seed crystal. We found that all of the above seed materials accelerated the crystallization, the degree of acceleration being in a following order; SOS (β ₁) > cocoa butter (Form V) > SOS (a mixture ofβ’ andβ ₂) > BOB (β ₂) > BOB (pseudo-β’) > SSS (β). Precise measurements of the crystallization kinetics showed that the most influential factors in the seeding effects are the physical properties of the seed materials—above all, thermodynamic stability, and similarity in the crystal structure to cocoa butter are the most determinative.     

Internally generated seeding policy in anti-solvent crystallization of ceftriaxone sodium

A promising seeding strategy of internally generated seeds in producing ceftriaxone sodium with a uni-modal crystal size distribution (CSD) was experimentally investigated in batch anti-solvent crystallization. Effects of different seeding policies on product CSDs, both external seeding and internally generated seeding, were experimentally determined. A uni-modal product CSD was obtained without obvious nucleation at high external-seed loadings, while a bi-modal CSD was obtained with plenty of secondary nuclei at low external-seed loadings. The internally generated seeds, prepared by adding pure acetone into the non-seeded saturated solution of ceftriaxone sodium, were fine crystals with a uni-modal, narrow CSD, irrespective of the amount of the added diluent. These fine crystals were grown continuously as seeds by slowly adding a relatively low concentration diluent. The product CSD was uni-modal with relatively large mass-weighted mean size, and the product quality was as good as the one obtained by external seeding. Therefore, such an internally generated seeding policy is expected to be a favorable seeding technology to hopefully substitute for the external seeding policy in the pharmaceutical industry, where external seeding is not welcome because the external seeding policy would destroy the axenic conditions in crystallizer and lead to unstable quality of different batches.     

Application of controlled cooling and seeding in batch crystallization

Some aspects of batch cooling crystallization in the industrial practice are analysed by computer simulations. The results indicate that without appropriate kinetics and very accurate process control, even the qualitative effects of applying controlled cooling and seeding are highly unpredictable. An increase in product size by applying controlled cooling is likely to be successful only rather randomly and the size distribution becomes broader. A linear or weakly non-linear cooling curve usually produces larger crystals than a natural cooling curve, and a better reproducibility than a controlled cooling curve. Seeding increases both size and reproducibility, but is also likely to increase significantly the coefficient of variation of the product distribution. The product weight mean size and coefficient of variation may increase or decrease at increasing amount of seeds, depending on governing kinetics.     

氯化钾间歇冷却结晶生长规律及粒度控制实验研究

采用聚焦光束反射测量技术（FBRM）考察了氯化钾（KCl）间歇冷却结晶过程中晶体成核和生长规律,重点研究了降温速率对KCl水溶液冷却时产生过饱和度的影响,以及添加晶种的相关条件（如晶种粒径和添加量等因素）与KCl晶体产品粒度的关系。同时,采用直接冷却刺激起晶产生“晶种”,并控制其生长达到控制晶体产品粒度的目的。结果表明,在添加晶种条件下,程序降温过程产生的低过饱和度不易引起爆发成核,且晶种的添加量决定着晶体产品的平均粒度与理想生长模型的偏差。另外,降温速率是冷却刺激起晶产生“晶种”粒径的关键因素。     

Modeling of crystal growth and nucleation rates for pentaerythritol batch crystallization

The kinetics of crystallization – nucleation and crystal growth – was determined for a seeded batch cooling process. Several experiments were done utilizing always the same condition: initial concentration, seed mass and size distribution, and cooling rate. From one experiment to other the agitation speed was varied. As the utilized reactor is able to measure torque of the impeller, the power dissipated in agitation was monitored during the crystallization, as well as reactor temperature and turbidity of the suspension. Turbidity monitoring and the measurement of particle size distribution from seeds and final product allowed obtaining the evolution of the second moment of the particles during the crystallization. The crystallization process was modeled utilizing the Method of Moments and the nucleation and crystal growth kinetics were obtained from least-square minimization of calculated second moments of the crystals. A crystal growth kinetic was determined and the secondary nucleation rate was described as a function of dissipated power and as functions of impeller tip speed. Additional experiments were done, in which cooling rate, seed mass and seed size were varied. The calculated kinetics could satisfactorily describe the results of the additional experiments, corroborating the quality of the modeling.     

Modeling batch crystallization processes: Assumption verification and improvement of the parameter estimation quality through empirical experiment design

In this work, experimental data of different batches was used for estimation of the kinetic parameters for the secondary nucleation framework of Gahn and Mersmann [Gahn, C. and Mersmann, A., 1999. Brittle fracture in crystallization processes. Chem. Eng. Sci. 54, 1273–1292].An empirical experiment design procedure was used to design an informative batch experiment through optimization of the seed quality, size and mass and process conditions at seeding. The parameters estimated using the data of the designed experiment showed smaller magnitudes of the confidence ellipsoids and standard deviations as compared to those obtained by using the data of conventional (un)seeded batch experiments. It was shown that the designed experiment allowed reducing uncertainty in the initial conditions, namely, the mass and crystal size distribution of the initial population of crystals and the initial supersaturation.It was also demonstrated that the main reason for the model/process mismatch was the origin of nuclei. Dynamic experimental data could be described better if the state of the crystals forming the crystallization system corresponded to the assumptions of the used kinetic model. Differences in the crystal surface properties, shape, and strain content could be responsible for a divergent nucleation and growth behavior in batches that were initiated either by primary nucleation, seeding with small ground seeds or seeding with coarse crystals from the product of the previous batch.     

Feedback control of crystal size distribution in a continuous cooling crystallizer

Control of crystal size distribution (CSD) in a 21.8 L continuous cooling KCl crystallizer was attempted. Feed saturated at 54°C with potash, nearly saturated with NaCl and containing 0.75 g MgSO₄/100 g of H₂ O was cooled to the crystallizer temperature at 40°C. The control scheme consisted of a proportional-integral controller with the rate of fines dissolution/removal as the input variable and the fines suspension density (crystals smaller than 150 μm) as the output variable. The measured/controlled variable was a temperature difference, ΔT, corresponding to the temperature of a slurry sample containing representative fines, before and after the fines were dissolved by heating. An increase in the product weight-mean crystal size and a decrease in the coefficient of variation of product were observed in the controlled runs.     

阿司匹林结晶过程的在线分析

晶体的颗粒尺寸分布和形状是结晶产品的两个关键质量指标,不仅影响结晶产品的性质,还影响下游的过滤、干燥及运输存储等过程。利用超声粒度分析仪、衰减全反射傅里叶红外光谱仪、浊度仪与二维成像系统等分析仪器在线测量了不同搅拌速率和不同降温速率下阿司匹林乙醇溶液结晶过程中温度、浓度、颗粒尺寸分布和形状的变化情况。实验结果表明：较低的降温速率或者较大的搅拌速率条件下得到含有大量细晶的阿司匹林结晶产品;较高的降温速率下得到长宽比较大的阿司匹林结晶产品。调节降温速率和搅拌速率是一种有效控制阿司匹林结晶产品尺寸分布与形状的方法。     

Influences of seed crystals on agglomeration phenomena and product purity of m-chloronitrobenzene crystals in batch crystallization

Changes in the fraction of agglomerates, agglomeration kinetics and product purity of m-chloronitrobenzene (CNB) crystals with the number and size of seed crystals were examined in batch crystallization experimentally, and influences of seed types (ground and well-defined) on agglomerate purities were discussed. From the fraction of agglomerates and agglomeration kinetics, it was found that agglomeration occurred more frequently when the number of seed crystals was larger and its size was smaller. The amount of purity decrease by agglomeration was smaller for the smaller number of seeds and for the larger ones. As the number of the elementary crystals constituting agglomerates was smaller, the purity of agglomerates was higher. With ground crystals, more frequent agglomeration occurred and the purity of agglomerates was lower than those for the well-defined crystals.     

Synthesis of zeolite L membranes with sub-micron to micron thicknesses

The growth of zeolite membranes with controlled thicknesses in the sub-micron to micron region is examined, using zeolite L as the example. It was demonstrated that by controlling the concentration of the zeolite solid load and suspension viscosity, dip-coating provided a method to prepare zeolite seed layers of controlled thicknesses. Disk-shaped zeolite L crystals, with size distribution of 0.5–2 μm in diameter, were used as seed crystals for the growth of 2–7 μm thick membranes. By choosing the composition of the secondary growth medium, as well as time and temperature, crystal growth primarily occurred along the xy-plane in these micron plus-thickness membranes, with eventual thickness similar to the originally deposited seed layer. For the synthesis of sub-micron-sized membranes, seed crystals from 20 to 60 nm were used and growth was observed along the z-axis. The optimum secondary growth conditions was found to occur at 60 h of growth time (40 h for sub-micron membranes) using a temperature of 110 °C with a solution composition of 10K₂O:1Al₂O₃:20SiO₂:2000H₂O. Membranes were characterized by electron microscopy and single gas permeation studies, providing confirmation of membrane densification.     

Preparation and mechanical properties of machinable alumina/mica composites

Using a mica-crystallizing glass powder in which a large amount of mica crystal was precipitated and a larger amount of MgF₂ component was contained as the raw materials of mica, machinable alumina/mica composites were obtained at 1400 °C. In the firing process, magnesia component in the mica crystals reacted with alumina to form spinel at 1150–1200 °C. The reaction made the mica crystals melt. However, the mica crystals were precipitated again during the cooling. Because a larger amount of MgF₂ component was contained in the mica-crystallizing glass powder, the nucleation of the mica crystals was caused during the cooling by the residual magnesium and fluorine in the liquid phase and succeedingly the mica crystals were precipitated. The precipitated mica crystals grew to anisotropicaly larger size than alumina grains, which lowered the bending strength and Vickers hardness and little heightened the fracture toughness.     

添加剂作用下阿司匹林结晶模拟和实验研究

晶体形貌作为晶体产品的重要质量指标,不仅会影响产品流动性、稳定性、溶出速率和生物可利用度等产品的质量指标,还会对过滤、干燥、压片等下游操作造成影响。通过分子模拟的方法指导阿司匹林冷却-反溶剂结晶过程的添加剂筛选,以添加剂作为晶体形貌的改性剂,降低阿司匹林晶体的长径比优化晶体形貌。通过单因素实验考查了添加剂浓度、晶种加入量、降温速率、搅拌速率和加水速率对阿司匹林晶体产品形貌、流动性和粒度分布等的影响,确定了较优的工艺条件。实验结果表明加入聚乙烯吡咯烷酮（PVP）作为添加剂可以降低阿司匹林晶体长径比,获得形貌为短棱柱状的晶体产品,能够显著改变晶体形貌优化产品的流动性。     

Effect of impeller type and position in a batch cooling crystallizer on the growth of borax decahydrate crystals

The effects of impeller type and position in a batch cooling crystallizer on the crystal growth kinetic as well as the shape, agglomeration and crystal size distribution of disodium tetraborate decahydrate (borax) were investigated in detail. Three types of impellers applied in this work generated completely different fluid flow patterns in the crystallizer allowing investigation of the influence of the liquid flow on the kinetic parameters as well. The ratio of impeller off-bottom clearance, C, to the liquid height, H, was varied from 0.1 to 0.5 for all impeller types used. Results indicate that the impeller type and off-bottom clearance influence the overall crystal growth rate and the crystal size distribution of the final product. It was also found that agglomeration of crystals produced was less pronounced as the radial fluid flow in the crystallizer became more dominant. In that system borax crystals tend to be smaller, but more regularly shaped.