共查询到20条相似文献，搜索用时 15 毫秒
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An innovative video technique indicates that the solids suspension performance of pitched-blade impellers in flat-bottomed tanks is improved by using a baffle off-bottom clearance in the range of one-fourth to one-half of the baffle width (standard baffle width being one-twelfth of the tank diameter). The solids suspension performance of high-efficiency impellers is relatively unaffected by baffle off-bottom clearances in the range of zero to one-half of the baffle width. A baffle off-bottom clearance equal to the baffle width is detrimental to the solids suspension performance of both pitched-blade and high efficiency impellers. 相似文献
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应用CFD软件Fluent 12.0和并行计算机工作站对双层改进型INTER-MIG桨式搅拌槽内的固液悬浮特性、临界离底悬浮转速及功率消耗进行数值模拟,分析了在固体体积分数as=30%下,转速n、桨叶离底距离C1和桨间距C2等因素对搅拌槽内颗粒悬浮特性的影响. 结果表明,在一定的转速和桨径下,改变C1和C2会改变流场的局部结构,选取适合的C1和C2可使固液混合更均匀,有利于颗粒悬浮和整个搅拌槽传质传热的进行. 最佳桨叶离底高度与槽径比为0.36,最佳桨叶间距与槽径比为0.44;在该最佳工况下临界离底悬浮转速Njs=118.3 r/min;得到既能达到完全离底悬浮、又能使搅拌功耗最小的最佳转速为n=124 r/min. 相似文献
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Baran Teoman Chadakarn Sirasitthichoke Andrei Potanin Piero M. Armenante 《American Institute of Chemical Engineers》2021,67(10):e17354
Njs, the minimum agitation speed to just-suspend dispersed solids in liquids in stirred tanks, is a critical parameter to operate industrial processes. The focus of this work was to develop a novel observer-independent method to experimentally obtain Njs in tanks that cannot be visually inspected internally using electrical resistance tomography (ERT). The mean bulk resistivity was measured across electrodes mounted on an ERT linear sensing probe placed inside a stirred tank containing water and glass beads. As the agitation speed increased, more solids became suspended and the resistivity measured by the probe changed. Plots of resistivity variation vs. agitation speed resulted in an S-shaped curve that could be analyzed to determine Njs. The Njs values obtained with this novel approach compared very favorably with those obtained using other methods requiring transparent tanks. It is expected that the ERT method proposed here could find applications in many industrial solid–liquid mixing processes. 相似文献
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Experimental measurements with six impeller types in solid‐liquid suspensions indicate that impeller power draw in the turbulent regime is approximately proportional to the solid‐liquid suspension density when the solids are distributed throughout the liquid; however, the accuracy of this approach is limited and there are clear differences in the behaviours of the various impellers. In general, power draw increases are less than suspension density increases for impellers with large blade‐trailing vortices, while power draw increases are equal to or greater than suspension density increases for impellers with smaller blade‐trailing vortices. The power draw data is well‐described using linear relations between the impeller power number and the density difference correlating parameter proposed by Micheletti et al.,[9] with the slope of the relation being dependent on impeller type. More extensive testing with a pitched‐blade turbine, using a greater variety of solids, found that the relation between the impeller power number and the density difference correlating parameter is independent of particle size for particles as large as 1 mm (1000 microns). For particles larger than 1.7 mm (1700 microns), in addition to suspension density, the solid volume fraction affects the pitched‐blade turbine power number; however, it is difficult to determine if this effect exists at all scales or if it is a result of the large particle size relative to the impeller dimensions in the experimental system. For large particles, the power draw is increased by the addition of neutrally‐buoyant particles that do not change the suspension density, with the magnitude of the increase being dependent on impeller type. 相似文献
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Design of mechanical agitators for solids suspension under gassed conditions can be achieved using a constant ungassed torque criterion if the solids suspension task is more difficult than the gas dispersion task. When gas dispersion is the more difficult task, design for complete gas dispersion will ensure solids suspension. Evidence supporting this approach is very strong for radial-flow impellers, but less so for up-pumping and down-pumping axial-flow impellers. 相似文献
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Mechanically agitated reactors with single and multiple impeller systems are used in the industry for the various three‐phase mixing processes such as crystallization, fermentation, and hydrogenation, etc. The paper reviews the experimental work reported in the literature along with different techniques used for the measurement of the specific quantities such as minimum or critical impeller speed for solid suspension. The work critically surveys the literature and makes specific recommendations for the use of appropriate correlations and conditions to be used for the success of such equipment. This assessment will put all the relevant literature on a common footing and will help to validate work reported earlier. 相似文献
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Flow patterns in agitated vessels are influenced by geometry, particularly impeller diameter and impeller off-bottom clearance. Large impellers and/or high off-bottom clearances lead to reversed flow in which the flow at the base of the vessel is radially-inward as opposed to radially-outward as expected with axial-flow impellers. Reversed flow is detrimental in solids suspension agitation because inordinately high torque and power are required to achieve suspension. This work experimentally characterizes the effect of flow reversal on solids suspension performance, including guidelines for avoiding flow reversal with straight-blade turbines, pitched-blade turbines, and high-efficiency impellers. 相似文献
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Michael T. Hicks Kevin J. Myers Andre Bakker 《Chemical Engineering Communications》1997,160(1):137-155
The focus of this work is the experimental determination of cloud height (the height to which solids are suspendedrpar; in an agitated slurry as a function of the agitation intensity, solid physical properties, impeller type (pitched-blade or high efficiency), and system geometry. Cloud height is not strongly dependent on impeller type or solid physical properties, except for extremely rapidly-settling particles. However, it is dependent on the impeller diameter to tank diameter ratio (D/T), impeller off-botiom clearance to tank diameter ratio (C/T), and solids loading. The use of multiple impellers allows solids to be suspended to higher levels in tall batches. An intermediate impeller separation leads to optimal performance in terms of the power requirement to achieve a desired cloud height. Lower impeller separations do not yield significant performance improvements over a single impeller, while higher impeller separations lead to poor performance caused by “zoning” between the impellers. 相似文献
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This paper presents results of a study on minimising the specific power for off‐bottom solids suspension in mixing tanks at a high solids concentration. It was found that major savings in the specific power required to suspend solids at high concentrations can be achieved by removal of baffles. This is particularly relevant to applications where chemical reaction rate is not affected by the mixing rate, such as that often occurs in the mineral industry. The specific power at the just off‐bottom solids suspension condition was found to decrease with the tank diameter at a rate a lot faster than previously reported in the literature. It appears that a larger power number axial flow impeller is more energy efficient than a lower power one to suspend solids at a very high solids loading. A mineral processing full‐scale tank is used as a benchmark in the study. Emphasis was placed on applications to the mineral industry, although results are also relevant to other industrial applications. 相似文献
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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 (nisg) 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. 相似文献
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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。 相似文献
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This study examines complete off-bottom suspension conditions for slurries containing mixtures of solids at a wide range of concentrations. Binary combinations of five different particles; bronze, two sizes of glass beads, ion exchange resin, and nickel were tested with two impellers: the Lightnin A310 and the down pumping 45° PBT. The effect of particle size ratio and density ratio of the two solid phases on Njs was investigated. Solids loadings were varied from 3 to 56 wt% (weight percent) with water as the liquid phase. The results showed that the highest particle Njs in a mixture of solids is not a sufficient design specification to ensure suspension of the mixture. For one of the cases studied, the particle–particle interactions became significant at high loadings and resulted in a decrease in Njs. Different behaviours were observed for the other mixtures. The performance of the two impellers was also compared. The A310 impeller consumes significantly less power at Njs than the PBT. Both impellers showed an effect of off-bottom clearance on Njs, but the effect of off-bottom clearance depended on both the solids concentration and on the solids used. 相似文献
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Extensive experimental data indicates that the particle property dependence of the just-suspended agitation speed of pitched-blade turbines and high-efficiency impellers can be described with reasonable accuracy using a Zwietering power-law correlation. However, there are indications that the effect of particle size is divided into two regimes, one for particle sizes less than 1000 μm and one for larger particles. Comparison of the two impeller types indicates that the pitched-blade turbine requires somewhat higher power inputs and substantially higher torque levels to produce the same level of agitation as the high-efficiency impeller. 相似文献
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