固-液搅拌槽内桨型对颗粒悬浮特性影响的实验和模拟研究

以水为液相,玻璃珠为固相,在固-液搅拌槽内比较了传统径向流Rushton桨、轴向流下推式45°六斜叶桨以及新型半折叶搅拌桨的功耗、泵送能力和对固体颗粒的悬浮效果。并应用CFD （Computational fluid dynamics）方法研究了不同搅拌桨操作下颗粒的轴向速度分布。结果表明：在相同转速下,Rushton桨的功耗最大,新型半折叶桨与下推式45°六斜叶桨的功耗接近;新型半折叶桨的流量准数最大,泵送能力最强;在固-液体系中,新型半折叶桨与下推式45°六斜叶桨的流型类似,但3种桨中新型半折叶桨对固体颗粒的悬浮效果最好。     

Suspension of solid particles in multi‐impeller three‐phase stirred tank reactors

Solids suspension characteristics in gas—liquid–solid three‐phase stirred tanks with multi‐impellers were experimentally examined. Minimum impeller speeds for ultimately homogeneous solid suspension have been measured stirred tank reactors. Three impellers were installed: two four‐pitched blade downflow disk turbines and one Pfaudler type impeller chosen to provide good gas dispersion and to accomplish off‐bottom suspension of solid particles, respectively. Gas dispersion causes an increase in particle sedimentation associated with a decrease in power consumption and as a result, minimum impeller speeds for ultimately homogeneous solid suspension increase with increasing gas flow rates. A correlation was developed to predict minimum impeller speeds for ultimately homogeneous solid suspension. The proposed correlation, which agrees satisfactorily with the experimental results, is expected to be useful in design and scale‐up.     

Effect of scale on the draw down of floating solids

In this study, the draw down of floating solids from the liquid surface has been investigated using vessels of 0.61 and 2.67 m diameter. The importance of impeller type (mixed flow pitched blade turbine (PBT) and the narrow blade hydrofoil LE-20), pumping mode and position and the effect of varying liquid height have also been studied. Impeller speed and power consumption at which no solids remain at the surface for more than 2-4 s are determined by visual observations.Results from different scales are discussed in relation to the way in which solids are drawn down from the liquid surface. It has been shown that over a wide range of conditions the power required for drawing down solids can be reduced by operating in the upward rather than downward pumping mode and using an axial flow narrow blade hydrofoil rather than a mixed flow pitched blade turbine. Different scale up criteria, power per unit volume, tip speed and Froude number, are discussed for these systems. For scale up, specific power input is shown to be the most appropriate criterion for upward pumping impellers.     

CRITICAL IMPELLER SPEED FOR SOLID SUSPENSION IN MULTI-IMPELLER AGITATED CONTACTORS: SOLID-LIQUID SYSTEM

The critical impeller speed for suspension of solid particles (N_js) has been measured in multi-impeller mechanically agitated contactors of 0.15 and 0.30m id and 1.0 m height. Three types of impellers, i.e. disk turbine (DT), pitched turbine downflow(PTD) and pitched turbine upflow(PTU) were used. The number of impellers used in the 0.3 m and 0.15 m id reactors were three and four, respectively. The distance maintained between two impellers was equivalent lo the tank diameter. The effect of impeller type and diameter, particle size and loading, and clearance of the bottom impeller from the reactor bottom was studied and results compared with those of single impeller agitated contactors. PTD impeller was found to be more efficient for solid suspension. The N_js values obtained in reactors with multiple impeller are essentially the same as those observed in single impeller reactor and the bottom impeller plays dominant role in determining the N_js, values. An empirical correlation has been proposed for estimation of N_js and an attempt has been made to explain the mechanism of suspension in multi-impeller agitated reactor.     

GAS DISPERSION AND SOLID SUSPENSION IN A THREE-PHASE STIRRED TANK WITH MULTIPLE IMPELLERS

Despite much research on gas-liquid-solid systems and their widespread application in industry, gas dispersion with solid suspension in multistage stirred reactors equipped with multiple impellers has received little attention. We report here the critical just-suspension impeller speed for different concentrations of solid particles, gas holdup, and shaft power in a vessel of 0.48 m diameter with four baffles and dished base. Five agitator configurations, each with three impellers mounted on a single shaft, have been used in the experiments. Two novel impeller designs were used, a deep hollow blade (semi-ellipse) disc turbine (HEDT) and four-wide-blade hydrofoil impellers. The hydrofoils were used in both up-pumping (WH_U) and down-pumping (WH_D) modes. Glass beads of 50 ∼ 150 μm diameter and density 2500 kg · m^-3 were suspended at solid volumetric concentrations of 1.5, 3, 6, 9, and 15%. Results show that these suspended solids have little effect on the relative power demand. Agitators using the HEDT radial dispersing impeller at the bottom have a higher relative power demand (RPD = P_G/P_U) than those with WH_D or WH_U as the lowest one. For all impeller combinations there is little or no effect on gas holdup with increasing solid concentrations. Of the five different impeller combinations, those with an axial flow bottom impeller have significantly higher just-suspension agitation speeds and power consumption, so mounting the hydrofoil impeller at the bottom is not the optimal configuration for particle suspension. Of these impeller combinations, at a given gas flow rate the arrangement of HEDT + 2WH_U has the highest relative power demand, gas holdup, and power input for both the suspension of settling particles and gas dispersion.     

Solids suspension agitation in square tanks

The Zwietering correlation for the just‐suspended speed has been modified to include baffling and geometric effects (impeller‐to‐tank diameter ratio and impeller off‐bottom clearance) for two impeller types (high‐efficiency and pitched‐blade). Baffles are generally not required when using square tanks although they improve solids suspension performance of pitched‐blade turbines. Comparison of square tank solids suspension to that in a fully baffled cylindrical tank has also been made. Very little difference was observed in the power number, but the just‐suspended speed, torque, and power requirements were higher in the square tank.     

GAS DISPERSION AND SOLID SUSPENSION IN A THREE-PHASE STIRRED TANK WITH MULTIPLE IMPELLERS

Despite much research on gas-liquid-solid systems and their widespread application in industry, gas dispersion with solid suspension in multistage stirred reactors equipped with multiple impellers has received little attention. We report here the critical just-suspension impeller speed for different concentrations of solid particles, gas holdup, and shaft power in a vessel of 0.48 m diameter with four baffles and dished base. Five agitator configurations, each with three impellers mounted on a single shaft, have been used in the experiments. Two novel impeller designs were used, a deep hollow blade (semi-ellipse) disc turbine (HEDT) and four-wide-blade hydrofoil impellers. The hydrofoils were used in both up-pumping (WH_U) and down-pumping (WH_D) modes. Glass beads of 50 ～ 150 μm diameter and density 2500 kg · m^?3 were suspended at solid volumetric concentrations of 1.5, 3, 6, 9, and 15%. Results show that these suspended solids have little effect on the relative power demand. Agitators using the HEDT radial dispersing impeller at the bottom have a higher relative power demand (RPD = P_G/P_U) than those with WH_D or WH_U as the lowest one. For all impeller combinations there is little or no effect on gas holdup with increasing solid concentrations. Of the five different impeller combinations, those with an axial flow bottom impeller have significantly higher just-suspension agitation speeds and power consumption, so mounting the hydrofoil impeller at the bottom is not the optimal configuration for particle suspension. Of these impeller combinations, at a given gas flow rate the arrangement of HEDT + 2WH_U has the highest relative power demand, gas holdup, and power input for both the suspension of settling particles and gas dispersion.     

The effect of impeller-to-tank diameter ratio on draw down of solids

The effect of impeller-to-tank diameter ratio (D/T) on the draw down of solids was investigated using a mixed flow impeller (pitched blade turbine) and a narrow blade hydrofoil (LE-20) of D=T/2 and T/3. Operational conditions (impeller speed and power consumption) at which solids do not remain at the liquid surface for more than 2- were determined for a given solid type (polyethylene particles) at a given concentration (X=1%). Under selected conditions, liquid velocity values were obtained from LDA measurements and CFD simulations to better interpret the findings.Both upward and downward pumping modes were studied with the impeller mounted over a range of submergences, and different mechanisms of draw down were noted. For LDA measurements and CFD simulations, only the upward pumping PBT was considered. These were carried out under conditions where draw down did not occur through air ingesting vortices.When solids were drawn down without air being entrained from the surface, a smaller diameter impeller (D=T/3) was more energy efficient, although it required higher speeds to achieve draw down. Results from LDA measurements and CFD simulations showed that the discharge flow from a larger impeller has a stronger radial component of flow, and as a result the liquid is directed towards the vessel walls rather than the surface which supported this finding. The only exception to this was when solids were drawn down along with air through vortices. In this case, the power demand could be lower with a larger diameter impeller.     

Impact of impeller type on methyl methacrylate emulsion polymerization in a batch reactor

The emulsion polymerization of methyl methacrylate (MMA) was carried out in a lab‐scale reactor, which was equipped with a top‐entry agitator, four wall baffles, a U‐shaped cooling coil, and a temperature controller. Potassium per sulfate and sodium dodecyl sulfate as were used as the initiator and the surfactant, respectively. The experimental investigation demonstrated the impact of the impeller type (45° six pitched‐blade turbine and Rushton impeller), number of impellers (single and double impellers), and impeller speed (100–350 rpm) on the monomer conversion, polymer particles size, molecular weight, and glass transition temperature. The results revealed that the effect of the impeller speed on the characteristics of the polymer attained using the pitched‐blade turbine was more prominent than that for the Rushton turbine. It was also found that the impact of the impeller speed on the polymer characteristics was much more pronounced for the double pitched‐blade turbines rather than for the double Rushton turbines. However, more uniform size distribution was achieved with the Rushton turbine. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 40496.     

Concentrated slurry formation via drawdown and incorporation of wettable solids in a mechanically agitated vessel

This article describes the effect of vessel configurations on the drawdown and incorporation of floating solids to prepare concentrated alumina slurries in stirred tanks. The impeller speed and power draw required to incorporate all dry powder within four seconds, N_JI and P_JI, are used to evaluate incorporation performance. The effect of impeller type is assessed, with pitched blade impellers proving to be the most effective across the full range of solid contents considered. At higher solids content the energy demand is shown to increase dramatically, with a 100‐fold increase in energy required to add 1% w/w more solid at 50% by weight compared to 1% by weight. Analysis of impeller power numbers show this coincides with a transition from constant power number to a region where power number increases linearly with decreasing Reynolds number. Contrary to studies at low solids content, the presence of baffles is shown to inhibit drawdown. © 2018 The Authors AIChE Journal published by Wiley Periodicals, Inc. on behalf of American Institute of Chemical Engineers AIChE J, 64: 1885–1895, 2018     

On the Manifestation and nature of macroinstabilities in stirred vessels

The flow variations or macroinstabilities (MIs) occurring in a vessel stirred by a pitched blade turbine (PBT) are studied through particle image velocimetry (PIV) experiments. Proper orthogonal decomposition and fast Fourier transform techniques are applied to the PIV velocity data at one vertical and nine horizontal planes below the impeller, to identify and characterize the flow structures present in the vessel. It is shown that the PBT MI is manifested as a precessional movement around the impeller axis and an oscillation in the direction of the axial mean stream around the shaft axis. The identified flow structures are similar to those previously observed in vessels stirred by Rushton impellers and are characterized by two dominant frequencies, equal to one‐tenth and one‐fifth of the impeller rotational speed. The nature and extent of these structures and their interaction with the trailing vortices emanating from the turbine blades are discussed. © 2011 American Institute of Chemical Engineers AIChE J, 2011     

Design factors affecting the dynamic performance of soil suspension in an agitated,baffled tank

The effects of particle size, impeller clearance and impel er speed are assessed to show how condition variations influence power consumption in the water-solid slurry suspension in an agitated tank. The energy efficiency of slurry height variation, impel er type and diameter, and solid movement speed has been investigated with six soil series stirred in a soil-water slurry. Coarser sand particles are observed to significantly increase power consumption, while finer particles, for instance clay, decrease the stirring power requirement. The 3-blade HR100 SUPERMIX? impeller manufactured by SATAKE general y performs more efficiently than a conventional 4-pitched blade turbine. The impeller's geometric design, including diameter and number of blades influences the impeller's energy efficiency, and HR100 impellers with greater diameters remarkably reduce power consumption. The tests demonstrated that the power required to provide off-bottom solid suspension and solid dispersion can be reduced dramatically by increasing the slurry height rather than by accelerating the impel er, if this option is possible.     

Gas recirculation rate and its influences on mass transfer in multiple impeller systems with various impeller combinations

Both the numerical and experimental approaches were used to study the effects of the gas recirculation and non‐uniform gas loading on the mass transfer rate for each impeller in a multiple impeller system. By combining the calculated gas velocity and local gas holdup, the gas recirculation rate around each impeller was estimated. The local mass transfer coefficients for systems equipped with various combinations of the Rushton turbine impeller (R) and pitched blade impeller (P) were determined by using the dynamic gassing out method. It is found that the Rushton turbine impeller has to be served as the lowest impeller in order to have a better gas dispersion and to give a higher overall K_La for a multiple impeller gas‐liquid contactor. The upper pitched blade impeller always enforces the circulating flow around the Rushton turbine impeller just beneath it and gives a higher overall average mass transfer rate. However, the system equipped with only the pitched blade impellers results in a much lower mass transfer rate than the other systems owing to the poor gas dispersion performance of the pitched blade impeller.     

Which impeller should be chosen for efficient solid–liquid mixing in the laminar and transitional regime?

The vast majority of solid–liquid mixing studies have focused on high Reynolds number applications with configurations and impeller geometries adapted to this type of regime. However, the mixing of particles in a viscous fluid is an essential element of many contemporary industries. We used the computational fluid dynamics-discrete element method model previously developed in our group to investigate solid–liquid mixing with close-clearance impellers in the laminar regime of operation. We compared different geometries, that is, the double helical ribbon, anchor, Paravisc™, and Maxblend™ impellers. We investigated the impact of fluid viscosity and compared the results with those obtained with the pitched blade turbine, a more commonly used impeller, based on power consumption for equivalent mixing states. This study highlights that the higher the viscosity of the fluid, the more interesting it is to use close-clearance impellers for their ability to generate a strong shear stress and a strong bulk flow in the entire vessel.     

Experiments and Simulations on Bidisperse Solids Suspension in a Mixing Tank

Experiments and simulations of a solids suspension process in a lab‐scale stirred tank under turbulent conditions have been performed. Two impellers have been tested. The liquid‐solid suspension consists of water and a mixture of glass and bronze particles. The simulations are Eulerian‐Lagrangian with the liquid flow as the Eulerian part, being solved by means of a lattice‐Boltzmann method combined with a large‐eddy approach to turbulence modeling. Comparison with experimental visualizations indicates that the simulations are able to represent the start‐up of the suspension process from a zero‐velocity initial condition. Differences between experiment and simulation are observed near the bottom of the tank. The simulation data are used to highlight the collisional interaction between the two different types of solids, the role of baffles, and the effect of impeller type on the suspension process.     

Power Consumption in the Turbulent Regime for a Coaxial Mixer

The power consumption of a new coaxial mixer composed of an anchor impeller and a pitched‐blade turbine impeller, and a series of rods operated in a contra‐rotating mode has been characterized experimentally in the turbulent regime. It is shown that both the power curve and the turbulent power number vary significantly with the speed ratio between the impellers. Likewise with single impeller mixers, the transition regime starts at a Reynolds number above 100 and the turbulent regime between 10³ and 10⁴ irrespective of the definition of the Reynolds number used.     

几种单层桨搅拌槽内宏观混合特性的比较

为了丰富对向心桨的混合特性的认识,比较了向心桨、Rushton桨、三斜叶桨和穿流桨的单层桨搅拌槽内的宏观混合特性,考察了搅拌转速、桨叶离底高度对搅拌槽混合时间和功率特性的影响。结果表明,四种桨的宏观混合时间均随着搅拌转速的增加而减少,搅拌功率均随转速的增加逐渐增大。当转速相同时,四种桨型中Rushton桨的功率消耗最大,三斜叶桨功率消耗最小,向心桨的功率消耗仅仅比三斜叶桨高。桨叶离底高度的变化对四种桨型的混合时间和功率的影响不尽相同。混合效率的影响因素大小顺序为：搅拌转速>桨型>桨叶离底高度。在考察的四种桨型中,向心桨的混合效率最高。研究成果可为向心桨等新型搅拌桨的工业应用积累实验数据,为其优化设计和放大提供理论依据。     

Transitional Mixing of Shear‐Thinning Fluids in Vessels with Multiple Impellers

The mixing efficiency of shear‐thinning fluids was evaluated using carboxymethylcellulose sodium salt (Na‐CMC) aqueous solutions of varying mass concentrations and three types of impellers (Rushton turbine (RT), six‐flat‐blade turbine (FBT), six‐pitched‐down‐blade turbine (PBT)) which were mounted on a common shaft in combinations of three, four, and five impellers. The mixing time proved to be dependent on the number of impellers as well as on the distance between. The Reynolds number has a significant influence on the mixing time for all studied systems. The results of power consumption allowed to choose the impeller system with the best efficiency.     

Critical impeller speed for solid suspension in multi-impeller three phase agitated contactors

The critical impeller speed for solid suspension in gas-liquid-solid systems has been measured in multi-impeller agitated contractors of 0.15 and 0.30 m and ID and 1.0 m height. Three types of impellers, i.e. disk turbine (DT), pitched turbine downflow (PTD) and pitched turbine upflow (PTU) were used. Air, deionised water and sand particles were used as the gas, liquid and solid phases, respectively. The superficial gas velocity and solid loading were varied in the ranges 0–15 mm/s and 0.5 to 10% w/w, respectively. The effects of impeller type and its diameter, particle size and loading and gas flow rate were studied. Some measurements of gas hold-up and mixing time were also made in order to get some insight of the hydrodynamic behaviour of the reactor. The critical impeller speed for solid suspension in the presence of gas (n_isg) was found to be more than that in the absence of the gas and the increase of critical speed correlated well with the gas flow rate. The influence of particle—liquid parameters on solid suspension speed in the gassed system was similar to but relatively weaker than that in the ungassed condition.