管式反应制备氧化锆纳米晶微粉的研究

本文采用管式ＳＫ－１０／２０型表态混合反应器对ＺｒＯＣｌ２·８Ｈ２Ｏ·ＹＣｌ３与ＮＨ·Ｈ２Ｏ进行液相共沉淀反应以制备ＺｒＯ２（Ｙ２Ｏ３）纳米晶微粉。研究了表态混合器反应过程中的流速、混合特性和反应浓度对粉体粒径与粒径分布的关系,并进行了分析探讨。     

Numerical characterisation of folding flow microchannel mixers

Micromixers have been considered in numerous recent studies with the aim of mixing different liquid streams for the common circumstance of non-inertial flow, i.e., in the Stokes flow regime. Under such conditions, the diffusion of momentum is dominant but the diffusion of species remains weak because the Schmidt number of liquids is large. Most mixers that have potential for application in the Stokes regime make use of a folding flow pattern that approximates the baker's transformation. In the work presented here, the general scaling of mixers of this type is developed from the exact equation for species transport and computations are made for a specimen mixer geometry to test the effectiveness of the resulting scaling. The scaling relation developed is found to give an excellent representation of the actual mixing characteristics of the specimen mixer over the entire range of Péclet number of practical interest. Finite volume computations are employed to solve the governing equations up to around Pe=10³. At higher Péclet numbers, where finite volume numerical solution becomes inaccurate with affordable mesh sizes, the species equation is solved using a Monte Carlo method instead. Finally, the scaling relation is used to develop the design relations needed to determine the number of mixing elements, the pressure drop incurred and the Péclet number of operation to achieve a given mixture uniformity within a specified mixing time.     

Residence Time and Concentration Distribution in a Kenics Static Mixer

Static mixers, often referred to as motionless mixers, are in-line mixing devices that consist of mixing elements inserted into a length of pipe. Most of the experimental works in this field have concentrated on establishing design guidelines and pressure drop correlations. Due to experimental difficulties, few articles have been published on the investigation of the flow and mixing mechanisms. In this work, a Kenics KMX static mixer was utilized to study concentration and residence time distribution (RTD) and effect of Reynolds number on mixing. The static mixer had six mixing elements arranged in-line along the length of the tube, and the angle between two neighboring elements was 90°. The length of the mixer was 0.98 m with internal and external diameters of 5.0 cm and 6.0 cm, respectively. The main continuous fluid was water, and NaCl solution was used as a tracer. All experiments were conducted with three replications at three Reynolds numbers, Re = 1188.71, 1584.95, and 1981.19. A dispersion model was used to model the RTD data. The experimental results were compared with the model results and reasonable agreement was achieved.     

双层撞击流混合器浓度时间序列的多重分形分析

利用平面激光诱导荧光技术(PLIF)对双层撞击流混合器的浓度场进行了实验测量,获得了不同径向位置处的浓度时间序列,通过多重分形去趋势波动方法(MF-DFA)研究了浓度时间序列中的多重分形特性,得到不同喷嘴间距和射流雷诺数下的多重分形谱,实现了流场特性的定量表征。结果表明,随着喷嘴间距增大,浓度时间序列的奇异性减弱,随着射流雷诺数增大,浓度时间序列的奇异性增强,喷嘴间距对浓度时间序列奇异性的影响比射流雷诺数更为显著。浓度信号的奇异性越强,流体粒子脉动越剧烈,混合效果越强,喷嘴间距对混合的促进作用大于射流雷诺数。通过分析多重分形谱参数(Δα, αmin, α0)随径向位置的变化规律得出了流型的转变区域,并将双层撞击流混合器由撞击中心处向下分为二次撞击区、涡旋区、一次撞击区。浓度信号的奇异性二次撞击区>一次撞击区>涡旋区,对混合的促进作用二次撞击区>一次撞击区>涡旋区。研究结果为更深入地揭示撞击流混合器内部流动模式和撞击流复杂无序的流动机理提供了理论依据。     

Powder mixing in a horizontal batch mixer

The mixing process in a horizontal batch mixer with a twin spiral rotor has been investigated.A mixture of crushed wheat and sodium chloride was mixed in a model mixer. The concentration of sodium chloride in mixture samples was checked as a function of the mixing time and sampling place for various degrees of mixer filling. The sample composition was determined by conductometric determination of sodium chloride in water extracts of the sample. From the values obtained, the mixing degree was calculated as defined by the variation coefficient along the whole mixer, as a function of the mixing time.In analysing the data and empirical correlation between the concentration of component under study and the place and time of mixing, C/C? = f(x, t) was obtained and compared with industrial experiments using radionuclides as tracers.     

水力空化混合强化超临界流体辅助雾化制备罗红霉素超细微粒

分析了超临界流体辅助雾化（SAA）过程,发现饱和器内超临界二氧化碳与溶液的混合是SAA成功的关键因素之一,由此引入了水力空化混合器以强化饱和器内两相间的传质。在自行组建的引入水力空化混合器的超临界流体辅助雾化（SAA-HCM）装置上,以罗红霉素为模型药物,考察了混合器压力、沉淀器温度、溶剂、进料中CO₂与液体溶液流量比（R）和溶液浓度对微粒形态和粒径的影响。结果表明,水力空化混合器能有效地强化两相间的传质,SAA-HCM工艺可制备出罗红霉素超细微粒,大部分微粒形态呈球形,通过改变操作参数可制得粒径在1～3 μm的适于吸入式给药的气溶胶药物微粒和粒径小于1 μm的超细微粒。     

An Analysis of Mixing in a Typical Experimental Set‐up to Measure Nnucleation Rates of Precipitation Processes

Mixing in a typical experimental setup to measure nucleation rates in precipitation processes was assessed. To determine these rates as a function of the driving force for concomitant polymorphs, it is necessary to perform these experiments at constant supersaturation. Therefore, the mixing time must be shorter than the time for the first nuclei to appear. For fast precipitation processes complete mixing has to be achieved within milliseconds. The mixing performance of a wide angle Y‐mixer was studied to see whether this is possible. An analysis of characteristic mixing times as a function of the average energy dissipation rate showed that turbulent dispersion of the feed streams determined the rate of the mixing process. The characteristic time for turbulent dispersion was of the same order as an arbitrarily set residence time in the Y‐mixer. However, CFD simulations of the flow showed large variation in the spatial distribution of the dissipation rate and revealed unsatisfying macromixing.     

风粉混合器内气固两相流动的模拟及试验研究

煤粉工业锅炉系统中风粉混合器是实现煤粉与一次风快速均匀混合的关键设备,测量及计算风粉混合器内煤粉、一次风气固两相流流场,对于优化风粉混合器结构,强化风粉混合效率及提高一次风粉的均匀稳定供给具有重要意义。笔者针对竖直结构及倾斜结构的2种风粉混合器,开展了数值计算及现场工程试验研究。基于几何拓扑学知识,采用ICEM软件针对2种风粉混合器划分了合适的三维网格;多相流理论模型中,多相连续介质模型中的双流体模型各相视为相互渗透、耦合但又保持各自运动特征的连续介质,相比于单流体模型,双流体模型考虑了固相的湍流输运以及气固两相间相互滑移引起的阻力,使得计算结果更接近实际情况;冷态双流体模型基本方程由守恒方程、相间耦合方程以及封闭方程构成,其中相间耦合方程用于表征气固相动量之间的耦合;为了探究不同停留时间下风粉混合器内气固两相的流场特征,采用非稳态数值计算方法,利用Fluent软件开展数值计算。基于两相流模型及Schilller-Naumann曵力系数模型研究了不同结构下风粉混合器内煤粉浓度分布随停留时间变化特征,采用德图testo425热敏风速仪测量了不同煤粉落料量下风粉混合器内负压变化规律。结果表明,竖直结构的风粉混合器内停留时间由0.25 s增至1 s时,混合器底部颗粒沉积的现象一直存在,即存在较长时间的颗粒流动死角区域;而对于倾斜结构的风粉混合器,当停留时间大于0.3 s,混合器内颗粒浓度基本降为0,较好避免了颗粒在混合器底部的沉积,该结构对于强化混合器内风粉混合及降低供料波动具有重要意义。不同落料量下的现场工程试验结果表明,高落料量下竖直结构的风粉混合器内平均负压偏小,几乎接近正压,且存在间断正压喷粉的现象,故该风粉混合器在高落料量下负压不足,易造成供料波动较大;高落料量下倾斜风粉混合器负压平均值仍大于-1 000 Pa,且无喷粉现象。相比于竖直结构,倾斜风粉混合器具有稳定且较宽的负压变化范围,能较好地克服供料波动大的现象。     

Experimental characterization and multi-scale modeling of mixing in static mixers

Mixing in static mixers is studied using a set of competitive-parallel chemical reactions and computational fluid dynamics (CFD) in a wide range of operating conditions. Two kinds of mixers, a wide angle Y-mixer and a two jet vortex mixer, referred to as Roughton mixer, are compared in terms of reaction yields and mixing times. It is found that the Roughton mixer achieves a better mixing performance compared to the Y-mixer. The effect of flow rate ratio on mixing in the Roughton mixer has been studied as well and it is shown that the mixing efficiency is not affected by the flow rate ratio. Moreover, experimental results and model predictions are in good agreement for all mixer geometries and operating conditions. CFD is used to calculate absolute mixing times based on the residence time in the segregated zone and it is shown that mixing times of less than 1 ms can be achieved in the Roughton mixer. In addition, CFD provides insight in local concentrations and reaction rates and serves as a valuable tool to improve or to scale-up mixers.     

Effect of Impeller Spacing on the Flow Field of Yield-Pseudoplastic Fluids Generated by a Coaxial Mixing System Composed of Two Central Impellers and an Anchor

The three-dimensional flow field generated by a coaxial mixer composed of double Scaba impellers and an anchor in the mixing of the xanthan gum solution, a non-Newtonian yield-pseudoplastic fluid was investigated using the computational fluid dynamics (CFD) technique. The mixing time measurements were performed by a non-intrusive flow visualization technique called electrical resistance tomography (ERT). To evaluate the influence of the impeller spacing on the hydrodynamics of the double Scaba-anchor coaxial mixer, the upper impeller submergence was set to 0.140?m while the lower impeller clearance and the spacing between two central impellers were changed within a wide range. The experiments and simulations were conducted for both co-rotating and counter-rotating regimes at different impeller spacing. The analysis of the collected data with respect to the power number, flow number, mixing time, and pumping effectiveness proved that the co-rotating mode had superiority over the counter-rotating regime. Furthermore, the impact of the impeller spacing in the co-rotating mode was assessed with respect to the mixing time, power number, and mixing energy. The results demonstrated that a coaxial mixer with the impeller spacing of almost equal to the central impeller diameter (C₂?=?0.175?m) and the impeller clearance of C₃?=?0.185?m was the most efficient configuration compared to the other cases. Additionally, the influence of the impeller spacing on the flow pattern was assessed in terms of the radial velocity, tangential velocity, axial velocity, shear rate, and apparent viscosity profiles. When the impeller spacing (C₂) was varied, the merging flow and parallel flow patterns were observed.     

The influence of viscosity on micromixing in turbulent flows

Viscosity has a negligible effect, if any, on macromixing parameters (e.g. velocity distribution and blending time when turbulent flow is fully developed). It does, however, influence micromixing parameters (e.g. Batchelor concentration microscale (νD²/ε)^1/4 and time constant for decay of fine-scale concentration fluctuations). The product distribution of two rapid, competitive, consecutive reactions (diazo coupling between 1-naphthol and diazotized sulphanilic acid) is sensitive to reagent concentration gradients on the molecular scale. It is shown that if all the independent micromixing parameters are kept constant, except the viscosity, the product distribution changes. The viscosity was varied by dissolving less than 0.5wt.% carboxymethyl cellulose (Hercules, type 7MF) in the aqueous reagent solutions. The viscosity then depends upon many factors (CMC concentration, temperature, shear rate, pH, chemical composition of the solution and mixing sequence during make-up of the solution), and CMC is not an ideal additive. Nothing better seems to be available. At least the spectrophotometric analytical method and the rate constants are unaffected by low CMC concentrations (0.5 wt.%). Three reactors (rotor---stator high intensity mixer, flow in a pipe, stirred tank) were operated in the turbulent flow regime. Increasing the viscosity caused more secondary product to be formed. This effect was described quantitatively by our earlier micromixing model.     

Modeling the Extraction Rate Coefficient for the Extraction of Yttrium by DEHPA Using Organic-Phase Recycle

ABSTRACT

Extraction of yttrium (Y) from sulfuric acid was studied using di(2-ethylhexyl) phosphate (DEHPA). A portion of the organic phase was recycled back into the mixer after extraction for the mixer to operate at a moderate organic-to-aqueous volumetric phase ratio while processing at a low organic-to-aqueous flow rate ratio. The effective performance of the mixer was evaluated when operating at different organic-phase flow fractions. To model the extraction rate coefficient, a 2-factor designed experiment was performed by conducting both equilibrium and mixer-settler tests. The organic-phase flow fraction was varied over four discrete levels while the extractant concentration was varied over three discrete levels. Increasing the organic-phase flow fraction yielded a continual increase in the extraction rate coefficient. In contrast, increasing the extractant concentration yielded an initial increase followed by a subsequent decrease in the extraction rate coefficient. The decline in the extraction rate coefficient was attributed to a decrease in the yttrium-extractant complex’s diffusion coefficient. High metal loading caused an elevated organic-phase viscosity and thus the low diffusion coefficient. An extraction rate coefficient model is proposed to describe the effects of extractant concentration, viscosity and organic-phase flow fraction. Mass transfer resistance was largely in the organic phase.     

A novel microfluidic mixer using aperiodic perturbation flows

A novel technique is proposed for enhancing the mixing performance of a ‘crisscross’ microfluidic mixer by means of aperiodically-varying perturbation flows. The effects of the perturbation and geometry parameters on the fluid flow characteristics and mixing performance are analyzed numerically. In performing the simulations, the flow field and species concentration field are obtained by solving the two-dimensional time-dependent Navier–Stokes equations and the convection–diffusion equation, respectively. In addition, the oscillating source used to modulate the perturbation flows is modeled using the Sprott system. The results show that the irregularly-alternating flow perturbations cause a repeated stretching and folding of the species streams and enhance the mixing performance as a result. It is shown that an effective improvement in the mixing performance can be obtained through a suitable choice of the Sprott system scaling factor. Moreover, it is shown that having assigned an appropriate scaling factor, the mixing performance can be further improved by specifying suitable values of the geometry and perturbation parameters.     

螺旋管式二次流混合器的混合性能

考察了以聚合氯化铝（PACl）作为混凝剂时,混合器的曲率、螺距、管长、水流速率以及混合过程的GT值等对螺旋管式二次流混合器混合性能的影响。研究发现,混合器的管长、水流速率以及曲率对该混合器混合性能具有高度显著影响,而螺距具有显著影响,四者影响的显著性由高到低依次为管长、水流速率、曲率和螺距;实验范围内,二次流管的最佳长度为12 m,且在电中和及卷扫混凝时均出现两个最佳GT值。     

幂律流体在Kenics型静态混合器中流动特性分析

为了探究Kenics型静态混合器内扭旋叶片剪切作用对幂律流体流动的影响,利用旋转流变仪测量了浓度为0.5wt%, 0.7wt%, 0.9wt%的羧甲基纤维素(CMC)水溶液的流变参数,采用数值模拟与实验研究了扭旋叶片作用下幂律流体流动阻力和剪切稀化特性。对流场研究表明,扭旋叶片诱导产生了内流涡旋、绕流涡旋和近壁面涡旋,有效强化了静态混合器内流体流动的剪切作用。受多个纵向涡旋分布的影响,扭旋叶片局域流场中周向45°位置速度最高,周向30°位置涡量与剪切应力最高而黏度最低。径向0.4倍半径位置速度最高,0.7倍半径位置黏度最高。静态混合器有效提高了流体的二次流流动速度和剪切应力,降低了幂律流体的黏度和流动阻力系数。     

Pyrolysis of hydrocarbons in a heat-carrier flow with fast mixing of the components

A scheme of autothermal pyrolysis of hydrocarbons in a reactor with fast mixing of the feedstock with a high-temperature heat-carrier flow is considered. The possibility of providing fast mixing of the components is verified in a series of aerodynamic experiments in a model setup, where the optimal geometry of the mixer for operation conditions of the pyrolysis reactor is determined. An experimental reactor is designed and manufactured for verification of the concept proposed. A scheme of the propane pyrolysis reactor is given. Products of combustion of a hydrogen-air mixture are used as a heat carrier. Experimental distributions of the flow temperature and concentration of the injected substance at the exit of the reactor agree with results obtained in the model setup. The mixing time is demonstrated to be smaller than the residence time of the mixture in the reactor mixer by more than an order of magnitude, which allows the process to be arranged under controlled conditions. In addition to experimental research, several detailed kinetic schemes of hydrocarbon pyrolysis are tested on the basis of available experimental data. Results of kinetic calculations for the operation conditions of the experimental setup are presented. __________ Translated from Fizika Goreniya i Vzryva, Vol. 44, No. 5, pp. 38–44, September–October, 2008.     

Reconstruction of hydrodynamic flow profiles in a rectangular channel using electrochemical methods of analysis

We propose a theoretical method for reconstructing the shape of a hydrodynamic flow profile occurring locally within a rectangular microfluidic channel based on experimental currents measured at double microband electrodes embedded in one channel wall and operating in the generator-collector regime. The ranges of geometrical and flow parameters providing best conditions for the flow profile determination are indicated. The solution of convection-diffusion equation (direct problem) is achieved through the application of the specifically designed conformal mapping of spatial coordinates and an exponentially expanding time grid for obtaining accurate concentration and current distributions. The inverse problem (the problem of flow profile determination) is approached using a variational formulation whose solution is obtained by the Ritz's method. The method may be extended for any number of electrodes in the channel and/or different operating regimes of the system (e.g. generator-generator).     

Continuous supercritical hydrothermal synthesis of controlled size and highly crystalline anatase TiO2 nanoparticles

The production of size-controlled and highly crystalline anatase titanium dioxide (TiO₂) nanoparticles was carried out under supercritical hydrothermal conditions (400 °C and 30 MPa) in a continuous flow apparatus with a residence time of 1.7 s. An industrially useful titanium sulfate (Ti(SO₄)₂) solution was used as the starting solution. KOH was used to change TiO₂ solubility and pH and thereby control the particle size. The apparatus comprised two micromixers operating at high temperature. The first mixer was configured to prepare a supercritical aqueous KOH solution from supercritical water (SC-H₂O) and KOH. The second mixer combined this KOH solution with aqueous Ti(SO₄)₂. In situ pH control and homogeneous nucleation were achieved in the second mixer. This two-step high-temperature micromixing process produced reasonably small and homogeneous particles. The particles were characterized by transmission electron microscopy (TEM) on the basis of morphology, average size, and size distribution, together with the coefficient of variation (CV). Powder X-ray diffraction (XRD) was used to determine the crystal structure and crystalline size. The weight loss of material was found through thermogravimetric (TG) measurement. The crystal structure of the product was assigned to the anatase single phase. The average particle size could be adjusted in the range 13–30 nm while maintaining a CV of 0.5 by changing the KOH concentration. At low pH, the powder XRD results for crystallite size were in good agreement with the average particle size measured by TEM, confirming that the products were single crystals of TiO₂ nanoparticles. When the reactor temperature was increased from 400 to 500 °C, the weight loss decreased from 4.5 to 2.5%, keeping the average particle size and high crystallinity of the TiO₂ particles unchanged.     

Investigation of the effect of impeller rotation rate,powder flow rate,and cohesion on powder flow behavior in a continuous blender using PEPT

In this paper, we examine the movement of particles within a continuous powder mixer using PEPT (Positron Emission Particle Tracking). The benefit of the approach is that the particle movement along the vessel can be measured non-invasively. The effect of impeller rotation rate, powder flow rate, and powder cohesion on the particle trajectory, dispersive axial transport coefficient, and residence time is examined. Increase in the impeller rotation rate decreased the residence time, increased the axial dispersion coefficient, and resulted in longer total path length. Effect of flow rate was different at two different rotation rates. At lower rotation rate, increase in flow rate increased the residence time, decreased the axial dispersion, and resulted in longer total path length. At higher rotation rate, increase in flow rate decreased the residence time, increased the total path length and showed a complex dependence on the axial dispersion coefficient. Increasing cohesion (measured using the flow index, dilation, and the Hausner ratio) did not affect the axial dispersion coefficient significantly, but had significant effects on the total particle path length traveled and the residence time. These results, relevant to pharmaceutical powders, provide better physical understanding of the influence of operating parameters on the flow behavior in the continuous mixer. In addition, one of the main obstacles of modeling continuous mixing of particles is to know the appropriate values for the modeling parameters as well as validate modeling approaches. One example is the dispersion coefficient which leads to an analytical solution for the axial dispersion model of a continuous blending process.     

A new method for the investigation of fluid-fluid mass transfer—II. Mass transfer in liquid-liquid systems

A new dynamic electrochemical method for the determination of liquid-liquid mass-transfer coefficients in a stirred cell and in a mixer cell is described. Current transients are recorded for different flow conditions and evaluated applying a non-linear regression analysis. The resulting mass-transfer coefficients correlate linearly with the stirring rate but the mean values over a long time period are considerably low. The extent of the investigated initial time dependence of the mass-transfer coefficient decreases with increasing electrolyte concentration.