基于CFD-PBE耦合的连续结晶过程数值模拟

通过CFD(计算流体力学)与PBE(粒数衡算方程)耦合对连续结晶过程进行数值模拟。采用QMOM(积分矩方法)求解PBE。在考虑成核、生长、聚并和破碎的情况下,通过网格独立性验证确定计算网格数量。在此基础上研究挡板设置、停留时间、搅拌转速以及搅拌桨类型等因素对CSD(晶体粒径分布)演变的影响,结果表明：挡板的设置增强了结晶器内湍流程度;高转速和低停留时间会获得平均粒径较小的颗粒。对比3种搅拌桨对CSD演变的影响,轴流式的螺旋桨功耗小,适用于大颗粒晶体结晶过程,而径流式的圆盘涡轮桨适用于微粒结晶过程。     

斯文森DTB型结晶器

一、前言近年来,化学工业对结晶器提出了全面而严格的要求,即:过程完全连续、结晶速度高、结晶颗粒大、产品结晶的粒径分布幅度窄(也就是粒度均匀)、结垢极少、能长时间正常地运转、操作稳定而简便。为满足这些要求,曾设计出许多新型的结晶器。其中,DTB型结晶器,由于除去了过剩的结晶核、混浊液在结晶罐内维持较高浓度、结晶在过饱和区循环长大、产品在同一罐     

搅拌反应器中氯化钠的结晶动力学研究

氯化钠广泛应用于多个领域,为更好地了解氯化钠的结晶原理,优化其结晶过程,对氯化钠在搅拌反应器中的结晶动力学进行了实验模拟,以研究氯化钠晶体在30℃条件下搅拌结晶过程中晶体的成核过程和生长过程,以及搅拌速度、过饱和度和悬浮密度对氯化钠晶体结晶过程的影响,并通过测得的多组数据采取多元非线性拟合的方法构建氯化钠晶体的成核和生长动力学模型。     

非牛顿煤灰配渣的结晶特性

气流床气化炉内熔渣的结晶会对渣层的黏度产生重要影响,使熔渣由牛顿流体转变为非牛顿流体。采用单热电偶在线观察系统(SHTT)、高温淬冷炉、X射线衍射(XRD)等对一种五元组分的模拟煤灰渣从结晶的角度进行研究,在线记录了晶体的生长形貌,获得了熔渣析晶的时间-温度-转变(TTT)曲线,研究熔渣在不同温度和冷却速率下的结晶特性以及晶体的类型。实验结果表明温度的降低使结晶驱动力增加,结晶孕育时间减少,但温度过低使黏度增加,晶体生长受限,因此生成的晶体尺寸较小。熔渣的结晶在不同温度区间析出的矿物不同,主要的晶体产物在高温区为透辉石,低温区有少量钙长石生成。冷却速率的增加会使晶体的生长尺寸减小,非晶态含量增加,但对晶体的种类影响不大。     

非牛顿煤灰配渣的结晶特性

气流床气化炉内熔渣的结晶会对渣层的黏度产生重要影响,使熔渣由牛顿流体转变为非牛顿流体。采用单热电偶在线观察系统(SHTT)、高温淬冷炉、X射线衍射(XRD)等对一种五元组分的模拟煤灰渣从结晶的角度进行研究,在线记录了晶体的生长形貌,获得了熔渣析晶的时间-温度-转变(TTT)曲线,研究熔渣在不同温度和冷却速率下的结晶特性以及晶体的类型。实验结果表明温度的降低使结晶驱动力增加,结晶孕育时间减少,但温度过低使黏度增加,晶体生长受限,因此生成的晶体尺寸较小。熔渣的结晶在不同温度区间析出的矿物不同,主要的晶体产物在高温区为透辉石,低温区有少量钙长石生成。冷却速率的增加会使晶体的生长尺寸减小,非晶态含量增加,但对晶体的种类影响不大。     

基于EMMS模型的搅拌釜内气液两相流数值模拟

采用欧拉-欧拉模型对搅拌釜内气液两相流进行了三维CFD模拟,重点研究了采用不同曳力模型时CFD模拟对搅拌桨附近排出流区两相流动的预测能力。模拟结果表明CFD能准确地预测排出流区的液相速度分布,但采用传统的Schiller-Naumann曳力一定程度上低估了排出流区的气液相间曳力,导致在完全扩散区CFD预测的分布器和桨叶下方区域气含率偏小,而基于气液非均匀结构和能量最小多尺度(EMMS)方法得到的DBS-Global曳力模型能更准确地描述完全扩散区气液搅拌釜内流动情况。与传统曳力模型相比,采用DBS-Global曳力模型能显著提高对气含率的预测。     

基于计算流体力学方法的啶虫脒结晶釜搅拌器优化设计

采用计算流体力学(CFD)方法对啶虫脒结晶釜进行了模拟与优化,考察了不同搅拌转速下的速度分布均匀性和晶体体积含量分布均匀性,结果表明,高效轴流型桨SP306对晶体悬浮的效果较好,搅拌转速对晶体体积含量分布均匀性有显著影响,确定了90 r/min为优化后的搅拌转速。经工业应用,验证了优化设计的合理性,产品粒径分布有明显改善。     

氯化锶搅拌结晶过程的数值模拟及影响因素分析

传统的氯化锶结晶方法是建立在实验的基础上,通过半经验的方式对结晶器的结晶过程及其放大进行研究,此方法不仅耗时久,而且由于结晶过程的放大,会引起更加复杂的流体流动而产生较差的效果。本课题拟利用在传统欧拉(Euler)双流体模型的基础上加载群体平衡模型(PBM),以考虑实际存在的晶体颗粒破碎等动力学行为,与CFD耦合计算了在不同搅拌转速和不同叶数的浆叶下氯化锶晶体颗粒的成长变化过程及其浓度分布的固液两相流场。从而得出在同一温度场下,氯化锶结晶过程的最佳转速为50 rpm与最优搅拌桨叶数为2叶,为工业氯化锶的生产提供了一定的参考和理论依据。     

黑钛液中七水硫酸亚铁结晶动力学性质

采用平衡法研究了七水硫酸亚铁在黑钛液中的溶解度,通过拟合得到了其溶解度方程,并利用FBRM在线监测溶液结晶情况,测量了其结晶介稳区.通过间歇动态法研究了七水硫酸亚铁的结晶动力学,采用矩量变化法求解了粒数衡算方程,并利用最小二乘法对动力学实验数据进行多元线性回归后得到七水硫酸亚铁成核速率和生长速率方程,结果表明:搅拌速率和降温速率对七水硫酸亚铁结晶过程影响显著,因此,针对搅拌速率和降温速率对结晶工艺的影响进行了分析,并对优化前后七水硫酸亚铁产品进行了对比,发现优化后晶体呈规则的棱形,产品粒度明显增大,小颗粒晶体显著减小.     

维生素C精制粒度影响因素考察

考察了维生素C精制过程中,结晶罐预热温度、搅拌速率、晶种量、晶种类型及晶种加入温度等几个因素对VC晶体粒度的影响。VC精制粒度最好的条件为:结晶罐预热27℃,小试搅拌2.25 r/s,晶种类型适合选200目晶粉或200目晶浆,晶种加入温度45~50℃。     

通气条件下桨型对自浮颗粒悬浮的影响

考察了五十二种搅拌桨的组合对搅拌釜内自浮颗粒的气液固三相体系混合问题的功耗、气含率和釜底部的颗粒含量的影响。研究表明 ,对自浮颗粒三相体系的搅拌混合应采用多层桨 ,且上层最好用上推式桨 ;实验发现 ,当高径比为 1 .6时 ,三层桨的混合参数优于两层桨的 ;给出了优异的搅拌桨型。     

High-speed observation of the effects of ultrasound on liquid mixing and agglomerated crystal breakage processes

A Kodak Ektapro Hs motion analyzer is applied to observe the ultrasonic effect on the mixing and breakage processes of agglomerated crystals suspended in ethanol. Mixing processes with stirring and with ultrasound are investigated by the dispersion of ink in the ethanol and by the motion of small crystals. The vessel is divided into several sections to investigate the local mixing process. Velocity variance is used to indicate the difference of turbulence when ultrasound and impeller stirring are applied respectively. The cavitation distribution in different sections is also recorded, and the effects on particles of different sizes are investigated.Observation of the breakage processes in different sections shows that both collisions between crystals and the vibration and implosion of cavitation bubbles contribute to the breakage process of agglomerated crystals. Finally, the effect of ultrasonic treatment is also compared with impeller stirring and with mechanical grinding.     

新型大双叶片搅拌器功率与混合特性的数值模拟

针对过程工业体系特性复杂多变的特点,开发了新型大双叶片宽适应性搅拌器,并建立了描述其流动、混合过程的综合数学模型,模拟分析了搅拌釜内的功率特性、流场特性和混合特性。模拟结果表明:新型大双叶片搅拌器所形成流场中存在着上部叶片区域循环流、下部叶片区域循环流以及纵贯全釜的整体循环流;随体系黏度增加,新型大双叶片搅拌器所产生流场的轴向速度、径向速度和切向速度均波动趋缓,且仍保持良好的功率与混合特性,对流态和黏度具有宽适应性;对新型大双叶片搅拌器而言,近液面加料方式不利于整体混合,适宜的加料点应在上下叶片之间,以缩短混合时间;在消耗相同单位体积功率的前提下,新型大双叶片搅拌器较同尺寸规格的FZ搅拌器的混合效率更高,具有高效节能的竞争优势。模拟结果对改进和优化新型大双叶片搅拌器的结构与运行具有参考价值。     

橡胶沥青搅拌罐内混合过程的数值模拟

针对橡胶粉与基质沥青混合过程中出现的漂浮、沉底、粘壁及挂料现象,建立了橡胶沥青搅拌罐的几何模型,基于计算流体力学软件对罐内混合过程进行非定常固液两相流数值模拟,分析了影响混合均匀性的因素,如桨叶直径、桨叶位置、挡板及搅拌速度等. 结果表明,尺寸适宜的桨叶直径与合适的桨叶位置有利于形成循环的轴向流,并减少定常流现象,安装挡板有效减少了切向流,搅拌器转速不影响内部流场的基本形态,但适宜的搅拌转速提高了混合均匀性. 混合均匀度与模拟结果印证,且当搅拌器直径800 mm、桨叶距离罐底680 mm、桨叶宽100 mm、搅拌速度280 r/min时,优化后橡胶粉的分布较均匀,混合均匀度为0.24,处于完全离底悬浮状态,模拟结果与实验结果较吻合.     

Influence of Floating Suspended Solids on the Homogenization of the Liquid Phase in a Mixing Vessel

The impact of floating suspended solids on the homogenization of the liquid phase in a stirred vessel was studied. The experiments were performed in a tank with an internal diameter of 0.32 m, equipped with a 45° pitched four-blade turbine (PTD) placed at varying positions in the vessel. Tap water was used as the liquid phase and polyethylene particles (PEHD) were used as the solid phase. The impeller speed was varied from N = 200–900 rpm. The mixing time of the suspended system was measured by a conductivity technique using a sodium chloride solution as the tracer, whereas power consumption was measured by the torque table. The influence of mean concentration of the suspended floating solids, average particle size, surface tension at the liquid/air interface and impeller diameter and its position on the mixing time and power consumption were analyzed.     

搅拌釜内液-液混合溶析沉淀法制备纳米姜黄素颗粒

研究了在搅拌釜内利用溶析沉淀法制备姜黄素纳米颗粒的过程,分别探讨了搅拌釜尺寸、搅拌形式、初始浓度以及搅拌转速对制备结果的影响。通过平面激光诱导荧光技术定量测量釜内液液混合行为,揭示了流体混合环境的控制是决定溶析沉淀产品过程的关键因素。实验表明,搅拌速度过低无法保证流体混合效率,将导致颗粒黏结;搅拌速度过高带来颗粒与流场的强剪切作用,引起颗粒破碎;局部过饱和度过大,引起颗粒的生长和聚团。搅拌釜内纳米颗粒制备需提供适度强化的流体混合环境。     

Influence of Impeller Diameter on Crystal Growth Kinetics of Borax in a Mixed Dual‐Impeller Batch‐Cooling Crystallizer

The influence of impeller diameter on crystal growth kinetics of borax decahydrate in a batch‐cooling crystallizer of non‐standard aspect ratio was evaluated. The dual‐impeller configuration consisted of a pitched‐blade turbine which was mounted below a straight‐blade turbine on a single shaft. Three different impeller‐to‐tank diameter ratios were investigated. In all experiments, mixing was conducted at just‐suspended impeller speed. To examine hydrodynamic conditions, mixing times were measured. The fluid flow pattern and velocity distribution were determined by computational fluid dynamics. Results showed that the smallest but also more regularly shaped crystals were produced in a system with standard diameter impellers. Product yield and power consumption were highest in this case.     

An arrangement of ideal zones with shifting boundaries as a way to model mixing processes in unsteady stirring conditions in agitated vessels

This paper investigates the way modelling mixing phenomena occur in unsteady stirring conditions in agitated vessels. In particular, a new model of torus reactor including a well-mixed zone and a transport zone is proposed. The originality of the arrangement of ideal reactors developed here lies in the time-dependent location of the boundaries between the two zones. This concept is applied to model the positive influence of unsteady stirring conditions on homogenization process: the model avoids a mass balance discontinuity when the transition from steady to unsteady stirring conditions is performed.To ascertain the reliability of the model proposed, experimental runs with highly viscous fluids have been carried out in an agitated tank. The impeller used was a non-standard helical ribbon impeller, fitted with an anchor at the bottom. The degree of homogeneity in the tank was observed using a conductivity method after a tracer injection.It is shown that for a given agitated fluid and mixing system, model parameters are easy to estimate and that modelling results are in close agreement with experimental ones. Moreover, it would appear that this model allows the easy derivation of a control law, which is a great advantage when optimizing the dynamics of a mixing process.     

A TORUS MODEL CONTAINING A SLIDING WELL-MIXED ZONE AS A WAY TO REPRESENT MIXING PROCESS AT UNSTEADY STIRRING CONDITIONS IN AGITATED VESSELS

This article investigates the modeling of mixing phenomena occur at unsteady stirring conditions in that agitated vessels. In particular, a new model of a torus reactor including a well-mixed zone and a transport zone is proposed. The originality of the arrangement of ideal reactors developed here is due to the time-dependent location of the boundaries between the two zones. This concept is applied to a model of the positive influence of unsteady stirring condition on a homogenization process; the model avoids mass balance discontinuity during the transitions from steady to unsteady stirring conditions.

To ascertain the reliability of the model proposed, experimental runs with highly viscous fluids have been carried out in an agitated tank. The impeller used is a nonstandard helical ribbon impeller, fitted with an anchor at the bottom. The degree of homogeneity in the tank is recorded using a conductivity method after a tracer injection.

It is shown that, for a given agitated fluid and mixing system, the parameters of the model are easy to estimate and that modeling results are in close agreement with experimental ones. Moreover, it appears that this model allows the easy derivation of the control law, which is a great advantage when optimizing the dynamics of a mixing process.