Étude expérimentale sur le court-circuit de l'alimentation dans un réservoir imparfaitement agité en régime continu

The influence of various parameters on the degree of channeling of the feed is shown for a continuous, imperfectly mixed, stirred tank. Various forms of agitators have been used, more particularly the marine propeller type. In general, the amount of channeling decreases as the agitator speed increases. However, more channeling is often observed at low speeds than when the agitator is stopped. The nature of the maximum short-circuit observed at a given agitator speed is explained. Channeling usually decreases as the agitator diameter increases and as the distance between inlet and outlet increases. Channeling becomes more important as the flow rate increases. When baffles are present, the degree of channeling is hardly affected by their width, their position or whether they are slanted.     

Effet du diamètre de l'agitateur et de la largeur des chicanes sur le niveau d'agitation en régime continu et sur la puissance dissipée

The system studied is made of a vertical cylindrical tank with a coaxial turbine agitator and baffles. The feed is in the bottom of the tank, the outlet at the surface, and both are coaxial with the agitator. A model proposed by Cholette and Cloutier, in which the parameters m and n are involved, was used to evaluate the effect of different variables. At a given agitator speed, the level of mixing is decreased when baffles are present, proportionally to the width of the latter. A lowering of the short-circuit is also observed. The torque varies with the level of mixing; it increases when baffles are present, and more so for large agitator diameters. The authors propose a flow pattern to explain the experimental results.     

Study on the flow characteristics and mixing performance of multi-blade combined agitator

Developing an agitator suitable for wide viscosity range is of great significance to the energy saving and efficiency improvement by the intensification of fluid flow and mixing process. The power characteristics, flow field distribution, turbulence characteristics and mixing performance of multi-blade combined (MBC) agitator under laminar to turbulent flow state were studied experimentally and numerically at the level of large eddy simulation. The predicted power curve is consistent with the experimental results. Tangential flow is the main flow in laminar flow. With the increase of Reynolds number (Re), axial and radial flows in the vessel gradually increase. When Re reaches 486, the velocity field distribution is basically the same as that in the turbulent flow. At the same energy consumption level, MBC agitator is superior to the commercial Maxblend agitator in mixing high viscosity fluid. The intensification of axial and radial flows is due to the dispersed arrangement of the blades, enabling the MBC agitator to achieve larger axial and radial flows from the transitional flow to the turbulence state. Moreover, the turbulent kinetic energy is evenly distributed and the mixing process is significantly accelerated.     

多叶片组合式搅拌桨釜内流动特性和混合性能研究

开发可适用于较宽黏度范围的搅拌桨,强化釜内的流体流动和混合过程对于搅拌釜的节能增效具有重要的意义。实验与数值模拟相结合,在大涡模拟层面研究了多叶片组合式搅拌桨(MBC桨)从层流到湍流状态下,釜内的功率特性、流场分布、湍流特性和混合性能。结果表明:预测的功率曲线与实验结果一致;层流状态下釜内以切向流动为主,随着Reynolds数(Re)的增大,釜内轴向和径向流动逐渐增强,当Re达到486时,速度场分布与湍流状态下基本一致;在相同的能耗水平下,MBC桨对高黏度流体的混合性能优于商业Maxblend桨。桨叶的分散组合布置,强化了釜内的轴向和径向流动,使得MBC搅拌桨在从过渡流到湍流状态下均可实现较大的轴径向流动,湍动能分布较为均匀,混合过程显著加快。     

Flow of slightly settling slurries in a horizontal rotary drum

An experimental study was conducted to examine the flow of slightly settling slurries in a horizontal rotary drum. The effects of the slurry flow rate, feed solids concentration, particle settling velocity and drum rotational speed on the hold-up solids concentration were investigated. At low drum speeds, the hold-up solids concentration was much higher than that of the feed. However, at higher drum speeds, the hold-up solids concentration approached that in the feed. At a given drum speed, the hold-up solids concentration was found to vary linearly with the feed solids concentration. A semi-empirical correlation was developed to predict the hold-up solids concentration using the drum Froude number, the single particle drag coefficient and a dimensionless slurry feed rate.     

氯化钾结晶器搅拌选型探研

反浮选冷结晶氯化钾生产的关键在于DTB结晶器中的控速分解和控速结晶,而控速的关键在于搅拌器的调速,因此本文通过对搅拌工作原理、类型及机理的粗浅分析,在保证不影响分解和结晶速率的情况下选用合适的搅拌型式,生产实践证明:推进式轴流型搅拌较之折叶式径漉型搅拌在提高氯化钾产品的粒度方面有极大的优越性.     

层叠流道中天然高分子挤压流动过程的数值模拟

为探究层叠流道中天然高分子材料挤压流动过程与混合特性，建立层叠流道的三维物理模型和有限元网格模型，利用POLYFLOW软件计算了食品熔体在不同的入口流量，挤压过程中的压力、速度、剪切速率分布。结果表明，压力沿着挤出方向呈梯度递减，随着入口流量增大中间位置压力差增大；剪切速率在流道中变化明显，剪切速率从流道中间位置向四周递减，从壁面到流道内部递减；速度在流道内沿着挤出方向先增大后减小，靠近壁面速度小；层叠扭转流道的结构和尺寸设计，有助于提高天然高分子物料的混合效果，并且第1、4层流道混合效果优于第2、3层流道，提高入口流量也有利于提高物料的混合效果。     

Mechanically Agitated Fluidized Bed Drying of Cohesive Particles at Low Air Velocity

The drying of micrometer-sized, cohesive particles belonging to the group C type materials according to Geldart's classification is always challenging. An agitated fluid bed dryer (AFBD) of pilot-scale capacity was designed to study the effect of the type of agitator and its speed, gas velocity, and inlet temperature and feed loading on the hydrodynamic performance of the AFBD. Key hydrodynamic parameters such as pressure drop have a profound influence on determining the fluidization characteristics. The pressure drop across the AFBD system was expressed in terms of flow coefficient, ξ, which can be conveniently used as a design parameter. The choice of agitator has an even greater influence on the drying kinetics and the results have been summarized, paving the way for a more efficient spiral agitator of the helical ribbon type.     

Parametric study on the production of phosphoric acid by the dihydrate process

In this work a computer program has been developed to simulate a three-CSTR pilot plant leaching process of phosphate rock with sulfuric acid for the production of phosphoric acid and precipitation of calcium sulfate dihydrate as a byproduct. The simulation model has been examined with real experimental data obtained from the Jordan Phosphate Mines Company (JPMC), a phosphoric acid pilot plant at Rusaifa, Jordan. The predicted results are in very good agreement with the experimental data with a relative absolute error of less than 3.5%.A parametric study has been made to find the optimum operating conditions of the pilot plant for a given phosphate rock feed flow rate, chemical composition, and particle size distribution. The effect of varying reactor(s) temperature, sulfuric acid feed rate, agitator–impeller speed, ratio of slurry recycle to feed rate, and ratio of return acid to feed rate have been investigated. A reactor temperature of 80°C, slurry recycle to feed ratio of 80, and return acid to feed ratio of 2.5 have been found to give best results. The optimum conditions for sulfuric acid feed rate and agitator speed are determined only from power limitations and economics of the plant itself.     

Mixing performance comparison of milliscale continuous high‐shear mixers

Mixing performance of two continuous flow millilitre‐scale reactors (volumes 9.5 mL and 2.5 mL) equipped with rotor‐stator mixers was studied. Cumulative residence time distributions (RTD) were determined experimentally using a step response method. Distributions were measured for both reactors by varying impeller speed and feed flow rate. The mixing effect was determined by measured RTDs. Computational fluid dynamics (CFD) were used to verify that the residence time distribution in the measurement outlet agreed with the outlet flow. The mixing power of both reactors was determined using a calorimetric method. The reactor inlet flow rate was found to affect mixing performance at 1–13 s residence times but the effect of impeller speed could not be noted. Both milliscale reactors are close to an ideal continuous stirred‐tank reactor (CSTR) at the studied impeller speed and flow rate ranges. The specific interfacial area was found to depend on the reactor inlet flow rate at constant impeller speed for the case of copper solvent extraction.

     

齿形盘元件的局部停留时间分布

采用自制荧光检测装置在线测量含齿形盘元件（TME）的双螺杆挤出机的部分停留时间分布（PRTD），利用去卷积方法计算TME的局部停留时间分布（LRTD），并将PRTD转换成停留体积分布（RVD）和停留转数分布（RRD）。研究表明，直齿的LRTD曲线形状比斜齿更宽，其混合能力更强；喂料速率（Q）和螺杆转速（N）的提高均使LRTD曲线向短时间方向移动。等流量转速比（Q／N）的RRD和RVD曲线重合；提高Q／N，使RRD向低转数的方向移动，而RVD向高体积方向移动。对于特定螺杆构型，不同螺杆转速和喂料速率下的RVD曲线仅是体积坐标方向的平移，RVD曲线形状主要取决于螺杆构型。     

Optimization of Process Parameters for Spray Drying of Fermented Soy Milk

Generally used as an inexpensive source of high-quality protein with many other health-promoting properties, soy milk offers an interesting alternative when fermented. Fermentation with lactic acid bacteria brings about the value addition to soy milk, making a nutritious probiotic food product. Dehydration helps to achieve longer shelf life and easier transportation and storage, enabling wider distribution of the product. The present work pertains to optimization of process parameters for spray drying of fermented soy milk using response surface methodology (RSM). The process parameters studied include inlet air temperature, aspirator speed (air flow rate), feed flow rate, and atomization pressure. The experiments were designed using the central composite design tool. Residual moisture content was found to be low at higher inlet air temperature, lower feed flow rate, higher atomization pressure, and higher air flow rate. Porosity reduced with increased atomization pressure. Higher product yield was obtained at high air flow rate and low feed flow rate. The increase in the atomization pressure increases the cohesiveness between particles, resulting in reduced flowability. An increase in the inlet air temperature greatly reduces the viability and the isoflavone aglycone content in the product. Protein denaturation during the process was found to reduce the product solubility.     

一种连续流反应器内射流耦合搅拌流的混合特性

连续式搅拌反应器采用连续化工艺,可以保证生产过程的连续性,相较于传统反应器而言,连续式反应器内流场更复杂,进料口射流对于流场的影响较为明显。对传统的双层搅拌桨结构的微波反应釜进行连续化改造,基于计算流体力学CFD(Computational Fluid Dynamics)方法对不同射流方案下反应器内醇油混合液的流动及混合特性进行了数值模拟,得到反应器内混合液的流动特性和混合时间特征。结果表明：射流通过直接影响射流区的流场和改变反应器内的循环涡流位置间接影响全局流动;射流方向对于反应器内流型影响更大,逆时针射流时反应器内的速度梯度大,流体之间的剪切作用更强,混合性能最佳,相较于传统的间歇式搅拌反应器,混合时间缩短了33%。     

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.     

闭式涡轮搅拌器搅拌釜内流场的数值模拟

利用计算流体力学(CFD)软件Fluent,对某工程实例的闭式涡轮搅拌器的内腔流场进行了三维数值模拟,分析了其流场的流动情况,包括正常工况下的流型、速度场分布情况,研究了搅拌器对物料流动特性的影响。计算结果表明,闭式涡轮搅拌器使釜内流场同时产生明显轴向流动和径向流动,搅拌混合效果良好,能够实现釜内物料的充分混合。     

Théorie et application de la distribution normale de θ1/3 aux temps de séjour des fluides dans les réservoirs agités en régime continu

Statistical models of residence time distributions in ideal stirred tanks in series are well approximated by a normal distribution of θ^1/2, θ^1/2 or θ according as the number of tanks is > 15, > 50 and very large respectively. For one stirred tank, the normal distribution of θ^1/3 approximates well the residence time distribution for both ideal and nonideal mixing. For ideal mixing, the mean x_m and standard deviation σ of the θ^1/2 distribution assume values of 8/9 and 1/3 respectively. For nonideal mixing, x_m is < 8/9 while σ is > 1/3. At constant fluid viscosities, when the rotational speed of the agitator increases, x_m increases gradually to reach its upper limiting value of 8/9 whilst σ decreases to reach its lower limiting value of 1/3. At fixed rotational speeds, if the viscosity increases, x_m decreases whilst σ increases. A simple mixing efficiency criteria derived from the normal distribution of θ^1/3 is proposed and is compatible with Danckwerts' segregation concept.     

Gas‐liquid mass transfer behavior in a surface‐aerated vessel stirred by a novel long‐short blades agitator

For a recently developed long‐short blades (LSB) agitator, its critical rotational speed for the onset of gas entrainment, power number, and gas‐liquid mass transfer behavior in the case of surface aeration is investigated. The effect of the LSB configurations and the liquid level on the agitator performance has been studied in details. The obtained results clearly show several advantages of the LSB agitator in gas‐liquid mass transfer with respect to the agitators in the literature. It is found that its gas‐liquid volumetric mass‐transfer coefficient at a given specific power can be several times larger than those shown in the literature. It can also avoid decrease in the gas‐liquid mass transfer rate as the liquid level increases. In addition, the bubble distribution in the system is more uniform with respect to conventional agitators, resulting from better distribution of the dissipated energy for the LSB agitator. © 2015 American Institute of Chemical Engineers AIChE J, 62: 1322–1330, 2016     

水泥浆混合能量对其性能影响的实验研究

在固井施工过程中,水泥浆是处于不断混配的过程,混合能量对水泥浆的综合性能有所影响,从而影响到固井质量。水泥浆在高速率混合时,类似于在恒速搅拌器内高剪切速率的流动状态;在管内流动低速率混合时,类似于在六速旋转粘度计内低剪切速率的流动状态。因此,基于室内恒速搅拌器和旋转粘度计的实验过程分析,推导出旋转粘度计测试液体过程混合能量的计算式,并通过对水泥浆进行室内实验认为:水泥浆混合过程在混合能量为10-15kJ/kg区间时,流性指数呈现最高值,稠度系数呈现最低值;在水泥浆混合阶段增加水泥浆的混合能量有助于提高水泥石的强度;并建议在水泥浆混配过程中,先进行几分钟高速搅拌,再实施注水泥施工,这有助于提高水泥浆性能的稳定性,有利于注水泥过程中的顶替和水泥石强度的提高。     

新型行星式搅拌器开发及性能研究

介绍了新型行星式搅拌器的结构和设计原理。该新型搅拌器有一个传动轴和数个行星叶轮组成,行星轮自转的驱动力来自液体阻力,行星叶轮自转速度的大小与公转速度、公转半径和自转半径有关,而与行星叶轮高度无关;自转方向与公转方向相反。测试采用IKA EUROSTAR power control搅拌装置和Labword软件,试验介质为水和甘油。通过对搅拌器的搅拌性能试验研究发现,流体质点的轴向流线为高速螺旋线,主要流型为径向流和轴向流,因此,在主轴转速较低的情况下被搅拌液体亦可获得满意的混合效果。通过对试验数据分析处理,得到测试用行星式搅拌器的功率准数、混合时间数、排液量和循环量等搅拌性能曲线及经验公式。     

Continuous blending of cohesive granular material

Results are presented from discrete element method (DEM) computer simulations of cohesive particles in a periodic slice of a continuous blender. The influence of inter-particle cohesion at various impeller speeds and fill levels is reported. Although increasing cohesion does not significantly change axial flow rates, mixing rates in the transverse plane and axial direction are affected. Mixing is generally enhanced for slightly cohesive materials, but decreases for larger cohesion, similar to trends observed in tumbling batch mixers. Changes in fill level are also shown to affect axial transport rates and mixing. These results suggest that the controllable operating parameters, such as feed rate and impeller speed, may be adjusted for cohesive powder formulations to obtain optimal mixing performance.