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.     

Determination of minimum speed required for solids suspension in stirred vessels using pressure measurements

The minimum speed required for complete suspension (N_js) is a major parameter for solids suspension in stirred tanks. Micale et al. 7th UK Conference on Mixing (2002) determined N_js by using a pressure gauge technique in a model vessel. In the present work N_js was measured by the same later technique in a more practical vessel with varying C/T ratio and the impeller direction of rotation. The results were compared to those obtained by Zwietering Chem. Eng. Sci. 8 , 244–253, (1958) correlation. Agreement was found between results from present work and predictions by Zwietering's correlation with maximum difference not exceeding 17%.     

The effect of flow reversal on solids suspension in agitated vessels

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.     

Critical impeller speed for solid suspension in gas inducing type mechanically agitated contactors

Critical impeller speed for solid suspension (N_CS), has been measured in gas inducing mechanically agitated contactors (GIMAC) fitted with two impellers. Five different impeller designs were employed. It was found that the multiple impeller system having both pitched blade downflow turbines was the most energy efficient for the suspension of solid particles. The interimpeller clearance (C₃), and the impeller bottom clearance (C₁) were found to be important parameters. Further, for this optimum combination, the design of the lower impeller was optimized in terms of impeller diameter (D), blade width (W), blade angle (A), and the number of blades (n_b). An empirical model has been proposed based on the mechanism of fluidization.     

Agitation requirements for complete solid suspension in an unbaffled agitated vessel with an unsteadily forward–reverse rotating impeller

Background: To develop a new type of solid–liquid apparatus, we have proposed the application of an agitation system with an impeller whose rotation alternates direction unsteadily, i.e., a forward–reverse rotating impeller. For an unbaffled agitated vessel fitted with this system, the suspension of solid particles in a liquid was studied using a disk turbine impeller with six flat blades. Results: The effects of the solid–liquid conditions and geometrical conditions of the apparatus on the minimum rotation rate and the corresponding impeller power consumption were evaluated experimentally for a completely suspended solid. The power consumption for a just suspended solid with this type of vessel was comparable with that for a baffled vessel with a unidirectionally rotating impeller, taking the liquid flow along the vessel bottom into consideration. Conclusion: Empirical relationships to predict the parameters of agitation requirements were found. A comparative investigation demonstrated the usefulness of the forward–reverse rotation mode of the impeller for off‐bottom suspension of solid particles. Copyright © 2007 Society of Chemical Industry     

Determination of the just-suspended speed,Njs, in stirred tanks using electrical resistance tomography (ERT)

N_js, 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 N_js 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 N_js. The N_js 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.     

The influence of baffle off-bottom clearance on the solids suspension performance of pitched-blade and high-efficiency impellers

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.     

The influence of solid properties on the just-suspended agitation requirements of pitched-blade and high-efficiency impellers

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.     

Energy efficient high solids loading agitation for the mineral industry

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.     

影响下沉颗粒三相体系临界搅拌转速的因素

探讨了搅拌桨型、挡板和气体分布器等结构因素及气体流量、颗粒质量分数等工艺因素对下沉颗粒三相体系临界搅拌转速的影响。结果表明,底层桨对临界搅拌转速的影响最大,最佳的底层桨为3叶后掠桨。指型挡板对颗粒的悬浮非常有利,此外,给出了几种较好桨型-挡板-分布环组合的临界搅拌转速关联式。     

改进型INTER-MIG搅拌槽内固液悬浮特性的数值模拟

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.     

精金矿生物氧化反应器固体悬浮及能耗研究

为了解决生物氧化反应器存在的高剪切及高能耗等问题,分别以石英砂和含砷难处理精金矿固体颗粒为研究对象,对临界离底悬浮转速Njs和功率进行研究。结果表明,Njs随着固含率的增大而增大,轴向桨Njs随通气量的增大而减小,而径向桨Njs随通气量的增大变化不大;功率随着搅拌转速的增大而增大,从釜底引气时功率会随着通气速率的增大而增大,从侧壁引气时功率则受通气速率的影响不大。     

声波测量在搅拌釜中固体颗粒临界悬浮转速测定的应用

基于声发射测量技术,结合声信号的频谱分析、小波分解和R/S分析,获得了代表颗粒运动的特征信号频段（d₁、d₂频段）。同时,根据声波特征信号频段能量随搅拌转速的规律性变化,以固体颗粒碰撞壁面产生信号高频区域的声能量分率值为特征参数,提出了搅拌釜临界悬浮转速的测量判据,即声能量分率快速减少并开始趋于稳定时所对应的搅拌转速为临界悬浮转速。以水-玻璃珠体系为例,与目测法相比,声波法测量值的平均相对误差为3.51%,具有较高的精度。利用经典的Zweitering临界悬浮转速计算公式对声波法测得的实验数据进行拟合,计算值与测量值之间的平均相对误差为3.17%,表明公式对于临界悬浮转速的计算具有较高的准确性。由此获得了一种快速、准确、安全的搅拌釜反应器临界悬浮转速测量技术,有利于工业生产流程的优化和控制。     

Design rules for suspending concentrated mixtures of solids in stirred tanks

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 N_js 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 N_js 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 N_js. 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 N_js than the PBT. Both impellers showed an effect of off-bottom clearance on N_js, but the effect of off-bottom clearance depended on both the solids concentration and on the solids used.     

Investigation of the accuracy of the extrapolation method for the lattice Boltzmann simulation of viscous fluid flow in a Maxblend impeller system

This paper offers a thorough assessment on the performance of the extrapolation method for the lattice Boltzmann simulation of viscous mixing flows. This method appears to be well-suited for the treatment of the complex boundary conditions found in various mixing systems. Here, the ability to simulate accurate power consumption and pumping capacity is evaluated on several configurations of the Maxblend mixing system, which has proven efficient in a wide range of applications. First, the impact of the boundary conditions on the spatial convergence of the lattice Boltzmann method (LBM) is determined on the 3D Couette flow, clearly showing that small modifications of the boundary conditions may reduce the accuracy of the predicted shear rate and power. Second, a parallel LBM scheme was used to simulate fluid flow within a Maxblend mixing system. For the unbaffled configuration, the simulated power consumption and the pumping capacity are observed to be in good agreement with experimental data and finite element simulation results. The effect of the bottom clearance is also successfully evaluated, suggesting that the standard bottom clearance is not optimum in the transitional regime. Lastly, results for the most geometrically complex case (baffled configuration) indicate that the power consumption is affected by numerical perturbations appearing around the moving impeller. Overall, these results show that, when combined with the extrapolation method for the treatment of boundary conditions, the LBM is an efficient tool for the investigation of viscous flow in mixers of industrial relevance.