Minimum Impeller Speed Research Articles

Solid Suspension in a Boiling Stirred Tank with Radial Flow Turbines

This paper considers conditions in reactors in which solids must be maintained in suspension in a boiling liquid. The situation is analogous to, but differs from, the more commonly studied conditions when the liquid is well-below its boiling point in a gas-liquid-solid reactor. Impeller performance in both cases is usually controlled by the development of gas or vapor cavities behind the impeller blades. Power consumption, critical off-bottom suspension speed, and vapor holdup measurements are reported for impellers working in a three-phase model system of 0.1 mm glass beads at concentrations of 3, 6, 9, 15, and 21 vol %, suspended in deionized boiling water. The vessel was electrically heated, dished based, and of 0.48 m diameter. It was agitated by single 0.24 m diameter impellers. The three radial-flow disc turbines studied were a flat-blade Rushton turbine, RT-6, a hollow half-pipe blade turbine, Chemineer CD-6, and a hollow semi-elliptical blade turbine, HEDT. The modem CD-6 and HEDT impellers are highly efficient in handling high gas-phase loading and in maintaining high levels of power input throughout a range of impeller tip speeds. For all three impellers, the relative power demand (RPD, i.e., the ratio between the gassed and ungassed power at a given impeller speed) was only slightly affected by solids, even up to 21 vol %. In contrast to the effect of varying the gas sparge rate in a cold aerated system, the boil-off rate had only a limited effect on the RPD, the minimum impeller speed needed to maintain suspension, or the retained void fraction. When boiling, the void fraction was, as might have been expected, much lower than in cold systems at similar power input and off-gas rates. Correlations of vapor holdup and RPD for the three radial flow turbines are suggested as a reference for future industrial applications in which knowledge of both the power requirements and the actual liquid fraction retained in the reactor may be of commercial importance. The HEDT impeller is recommended for both vapor dispersion and solid suspension in ungassed boiling three-phase systems.

The Effect of Bottom Roughness on the Minimum Agitator Speed Required to Just Fully Suspend Particles in a Stirred Vessel

In this work, the effect of the vessel bottom roughness on the suspension of solid particles in stirred tank reactors is investigated. The experiments were performed in a baffled vessel, which was mechanically stirred with a 45° pitched blade turbine. In order to evaluate the influence of the bottom roughness on particle suspension, four bottoms of different roughness and 8 different sets of spherical particles were used. The density of the solid particles, ρ s, ranged from 2500 kg m −3 to 8743 kg m −3 and they were characterized by narrow size distributions with a mean diameter, d p, from 128 μm up to 1850 μm. Measurement of the minimum impeller speed for ‘just complete suspension’, N js, showed that the roughness of the bottom had a significant influence. The precise effect depends on the particle size compared to the size of the roughness elements and to the Kolmogoroff microscale, λ K.

Impact of floating suspended solids on the homogenisation of the liquid phase in dual-impeller agitated vessel

The study relates to the liquid phase mixing in dual-impeller agitated contactors of internal diameter 0.32 m in presence of floating solid particles. High-density polyethylene particles and tap water were used as the solid and liquid phase in all experiments. Mechanically agitated tank was provided with two down-pumping pitched blade turbines (PTD). The mixing time has been measured by conductivity measurements using a pulse technique, whereas the minimum impeller speed for the complete drawdown of floating solid particles, N JS, was determined using Joosten visual method. The effects of a number of variables such as floating solids concentrations, the impeller diameter, the off-bottom impeller clearance as well as the spacing between impellers on the critical impeller speed, mixing time and power consumption values were studied in detail. Previous investigations on this subject have been limited to single-impeller configuration only. The dual-impeller reactor behaviour has been found to be very complex and their design must be considered with a great care. Obtained results show that the presence of floating solids in the liquid significantly affects the mixing time of the liquid phase. The analysed parameters are a strong function of the flow patterns structure created by dual-impeller configuration applied. Using experimental data dimensionless correlation is proposed to predict mixing time beyond complete suspension regime.

An Experimental Investigation into the Complete Drawdown of Floating Solids in Dual-Impeller Stirred Vessels

Experimental work is reported on the drawdown of floating solids by a mechanically agitated vessel with multiple impellers. The main aim of this work was to study the effect of different types of dual-impeller systems on the achievement of the complete suspension state of floating solids. Previous investigations on this topic have been limited to single-impeller systems only. In the present work three types of impellers were employed: a three-bladed 30° pitched turbine down-flow (3-PTD), a four-bladed 45° pitched turbine down-flow (4-PTD), and a straight blade turbine (SBT). Tap water and polyethylene particles (PEHD) were used as liquid and solid phases, respectively. The effects of physical properties of suspension as well as the impacts of a number of geometrical variables of the systems on the critical impeller speed and power consumption were studied in detail. The results indicate that the appropriate location of the second impeller on the same shaft can improve the performance of the agitation system for the process of floating solids suspension. This improvement is noted through the decrease of FrJS and (P/m)JS values. For all dual-impeller systems examined, the correlations for the estimation of the minimum impeller speed to attain the just dispersed state of floating solid particles has been developed.

Dispersion of Floating Solid Particles in Aerated Stirred Tank Reactors: Minimum Impeller Speeds for Off-Surface and Ultimately Homogeneous Solid Suspension and Solids Concentration Profiles

Effects of impeller design, baffle, and gas flow rate on distributions of floating solid particles were examined in a stirred tank of 0.2 m i.d. for solid concentrations up to 50 vol %. Dual small cross-section impeller systems, i.e., dual four-flat blade disk turbines and dual four-pitched blade downflow disk turbines, and large cross-section impellers, i.e., Maxblend impeller and Fullzone impeller, were used. The minimum impeller speeds for off-surface floating-particle suspension decreased with aeration because bubbles rising near the tank wall enhanced the breakup of the floating-particle stagnant layer formed on the liquid surface and then the dispersion of floating particles into the liquid. The minimum impeller speeds for ultimately homogeneous floating-particle suspension also decreased with aeration. These results are contrary to those for the settling particles. The local solid particle concentrations at different heights in the stirred tank were measured. The axial solid particles concentration profiles were examined using the proposed one-dimensional floating-particle dispersion model. The Peclet numbers for floating-particle dispersion in the model were reasonably correlated in terms of impeller speed, power consumption, and forces exerted on floating particles. It was found that the large cross-section impellers could disperse completely floating particles into the liquid with less agitated speed and power consumption as compared with the dual small cross-section impeller systems used in this work.

Power consumption and solid suspension performance of large-scale impellers in gas–liquid–solid three-phase stirred tank reactors

An experimental investigation into power consumption and solid suspension performance of large-scale impellers was carried out under turbulent conditions. Two types of large-scale impellers, i.e. Maxblend and Fullzone impellers, were employed. For reference, a triple-impeller system, i.e. two four-pitched blade downflow disk turbines (DTs) at middle and upper positions and one Pfaudler type impeller at lower position, was also used. The power consumption and the minimum impeller speeds for off-bottom solid suspension and minimum impeller speeds for ultimately homogeneous solid suspension were measured in unaerated and aerated systems. At a given rotational speed, the power consumption of the Maxblend impeller was roughly half of that of the Fullzone impeller. The decrease in power consumption due to aeration for large-scale impellers was smaller as compared with that for the triple-impeller system. The proposed correlation for power consumption of large-scale impellers in three-phase systems fit the experimental data reasonably well. Interesting and unexpected solid movements caused by the large-scale impellers in the vessels having oval bottom were observed. Since the large-scale impellers create strong axial liquid recirculation flowing downward near the impeller shaft and upward near the wall, usually particles are expected to move outward on the tank bottom. On the contrary, however, solid particles near the bottom moved to the center of the base from the side along the oval tank bottom. The large-scale impellers were found to be more efficient for solid suspension than the triple-impeller system. The Maxblend impeller provided the best solid suspension ability among the three impellers used in this work. We proposed a correlation for power consumption of large-scale impellers in gas–liquid–solid three-phase systems. Empirical correlations were also proposed for the minimum impeller speeds for off-bottom solid suspension, minimum impeller speeds for ultimately homogeneous solid suspension and power consumption at the minimum impeller speeds for ultimately homogeneous solid suspension.

Three phase mixing studies for nickel precipitation

Hydrodynamics, temperature, pH and various other physico-chemical factors influence the morphology of nickel produced via hydrogen reduction. The focus of the current work is the effect on hydrodynamics of changing the impeller and reactor configurations in a 75 l stirred vessel with draft tube and baffles. The aim was to determine which configuration resulted in maximum particle suspension and local gas hold-up while using the minimum impeller speed and power consumption. A response surface methodology of experimental design was employed. This ensured that the number of candidate variables to be tested was reduced to a minimum and that interactive effects between variables were taken into account. The impeller configurations tested were a single Rushton turbine, a single axial flow impeller, and a double impeller system consisting of a combination of the two. The reactor configurations, tested at different gas flow rates, were varied to test the effects with and without baffles. It was found that optimum mixing could be achieved using a baffled vessel with an upper axial flow impeller and a lower Rushton turbine, and by keeping a minimum impeller clearance from the vessel bottom. This is in agreement with [Mineral Processing, 1–2 August 2002].

CFD Simulation of Solids Suspension in Mixing Vessels

AbstractA CFD approach for predicting minimum impeller speed (Njs) to achieve complete off‐bottom solids suspension in mechanically‐agitated vessels is presented. The method is based on observing the simulated transient profiles of solids volume fraction for the layer of cells adjacent to the vessel floor and has been shown to be reliable and robust. Predicted Njs values for typical impeller systems were compared with well‐known empirical correlations and good agreement was obtained.

Gas hold‐up in stirred tank reactors

AbstractBased on a study of the gas hold‐up data for stirred tank reactor generated in the present work and the data available in the literature for large stirred tank reactors (T = 0.57 m to 2.7 m) equipped with disc turbines and pitched blade downflow turbines a correlation is presented which reliably predicts gas hold‐up data over wide range of system configurations and operating parameters. The parameter used, N/Ncd, relates gas hold‐up at impeller speed N with respect to the gas hold‐up at minimum impeller speed for complete dispersion of the gas, Ncd. It is shown that the gas hold‐up data of different workers when compared on the basis of N/Ncd, shows unanimity.

An experimental investigation into vapor dispersion and solid suspension in boiling stirred tank reactors

Power consumption, gas hold-up, critical impeller speed for just complete off-bottom solid suspension and minimum impeller speed for ultimately homogeneous solid suspension were measured in boiling water using a 0.2 m i.d. stirred tank reactor with three four-pitched blade downflow turbines. Water and three different size glass beads were used as the liquid and solid phases, respectively. At higher vapor generation rates nucleation occurred at the heater, whereas at low vapor generation rates vapor was mainly generated from the impellers instead of the heater. At higher vapor generation rates, the boiling-to-nonboiling mechanical power ratio and the gas hold-up decreased and increased uniformly with increasing impeller rotational speed, respectively. At lower vapor generation rates, however, boiling-to-nonboiling mechanical power ratio exhibited a minimum and gas hold-up went through a maximum with varying impeller rotational speed. The changes in the nucleation site and solid suspension with impeller speed were responsible for these hydrodynamic behaviors. An empirical correlation for the Reynolds number corresponding to the minimum power consumption ratio or corresponding to the maximum gas hold-up in boiling liquids was developed using the present experimental data. The critical impeller speed for just complete off-bottom solid suspension and minimum impeller speed for ultimately homogeneous solid suspension in boiling systems were higher than those in gas-sparging systems. On the whole, the solid suspension in the boiling systems was poor as compared with that in the gas-sparging systems. Empirical correlations for critical impeller speed for just complete off-bottom solid suspension and minimum impeller speed for ultimately homogeneous solid suspension in boiling slurry stirred tank reactors were proposed.

Minimum agitation speed for liquid–liquid–gas dispersion in mechanically agitated vessels

The present investigation deals with the determination of the minimum agitation speed, N cdg, for complete liquid–liquid dispersion in mechanically agitated liquid–liquid mixtures sparged with a gas phase. The minimum impeller speed needed to completely disperse a liquid–liquid system was accurately predicted by the Skelland and Ramsay correlation [Skelland and Ramsey (Ind. Eng, Chem. Res., 1987; 26(1): 77–81)]. A new correlation is proposed for the prediction of minimum agitation speed, for three phase (liquid–liquid–gas) system as: Fr min =C 2 T D 2αφ 0.106 Ga · Bo −0.084 Re G −0.013 A plot of ( Fr) min predicted by the above equation and those obtained experimentally showed excellent agreement. This expression correlates experimental data of the present investigation with in a maximum error of ±15%.

Critical Impeller Speed for Solid Suspension in Mechanically Agitated Contactors.

Experiments were performed in mechanically agitated contactors with 6 (flat) blade turbine impellers, to study the effect of various operating parameters on minimum/critical speed required for complete suspension in two-phase (liquid-solid) as well as three-phase (liquid-solid-gas) systems. Based on 1212 experimental measurements, a unified empirical correlation is proposed, for the estimation of minimum impeller speed for solid suspension in both two-phase and three-phase systems in terms of fundamental, operational and geometrical variables.

Minimum Impeller Speeds for Complete Liquid-Liquid Dispersion in a Baffled Vessel.

The minimum impeller speed needed to obtain complete liquid-liquid dispersion in baffled vessels is examined experimentally for six-blade disk turbines, flat paddles, pitched paddles and four-blade propellers of three different sizes. The minimum impeller speed for complete liquid-liquid dispersion is detected by the method developed by using a multimeter. From the data obtained in this work, the impeller position above the vessel base to get the minimum power input for complete liquid-liquid dispersion is determined. The correlation for the minimum impeller speed for complete liquid-liquid dispersion in baffled vessels is also proposed.

Complete Drawdown and Dispersion of Floating Solids in Agitated Vessel Equipped with Ordinary Impellers.

The minimum impeller speeds for complete drawdown and concentration distributions of floating solids in an agitated vessel are measured for impellers used commonly in process industries, namely disc turbine impellers and pitched paddle impellers. A correlation for the minimum impeller speed is developed for disc turbine impellers and pitched paddle impellers rotating to create downward flow. The relatively uniform concentration distributions are obtained in spite of different impeller geometries compared to those reported for solids heavier than the liquid media.

Influence of Tank Bottom Shear Stress on Complete Fluidization of Spherical Solid Particles in a Square Surface Agitation Tank.

The influence of tank bottom shear stress on complete fluidization of spherical solid particles in a square surface agitation tank is investigated. The shear stress on the tank bottom is measured by the electrochemical method, which utilizes the reduction of dissolved oxygen on point electrodes embedded in the tank bottom.The shear stress has a different value for each point electrode. It is found that complete fluidization of particles is attained when the shear stress all over the tank bottom exceeds the minimum shear stress required for the fluidization of particles, which is dependent on the properties of the particle and surrounding liquid in the tank. The empirical equation, which shows the relationship between the minimum shear stress required for the fluidization of particle and the properties of particle and liquid in the tank, is obtained. By use of this equation, the relationship between the minimum impeller speed required for complete fluidization and experimental conditions is further discussed.

Minimum impeller speeds for complete dispersion of non-Newtonian liquid-liquid systems in baffled vessels

The minimum impeller speed needed to completely disperse a Newtonian fluid in a pseudoplastic continuous phase was accurately predicted by the Skelland and Ramsay correlation. The overall absolute deviation was 8.9% for 120 runs when Vermeulen et al.'s viscosity expression was combined with Metzner and Otto's definition of apparent viscosity, μ A , in place of μ C . The variables included four pseudoplastic fluids with flow behavior indices between 0.52 and 0.83, three Φ values (0.5, 0.33, 0.167), and four common impeller types in a baffled vessel

Effects of surface active agents on minimum impeller speeds for liquid-liquid dispersion in baffled vessels

L'etude experimentale porte sur le melangeage des systemes benzaldehyde-eau distillee et chlorobenzene-eau distillee. Les parametres etudies sont le type d'agent de surface (non ionique, anionique, cationique), la concentration en agent de surface; le type d'agitateur (a helices, a pales rectilignes ou curvilignes et a turbine) et la vitesse d'agitation

Minimum Impeller Speeds for Liquid-Liquid Dispersion in Baffled Vessels

Minimum Impeller Speeds for Liquid-Liquid Dispersion in Baffled Vessels

Mass transfer and hydrodynamic characteristics of gas inducing type of agitated contactors

AbstractPower consumption, rate of gas induction (Q), fractional gas hold‐up and mass transfer characteristics of 0.41, 0.57 and 1.0 m i.d. gas inducing type of agitated contactors were studied. The power consumption was measured by a torque table. The values of effective interfacial area (a), and liquid side mass transfer coefficient (kLa) were obtained by using chemical methods. Two types of impellers (pipe and flattened cylindrical) were employed. The impeller speed was varied from 3 to 11.7 r/s.The effects of impeller diameter, baffle position, orifice area on the impeller, number of impeller blades, position of impeller from the bottom and liquid submergence on the values of Q and a were investigated. Further, the effect of liquid viscosity and surface tension on the values of Q was studied.A satisfactory agreement was found between the observed and the predicted value of the minimum impeller speed at which gas induction occurs. The performance of the gas inducing type of agitated contactor has been compared with the conventional mechanically agitated contactor.

On minimum power requirements for emulsification of two‐phase, liquid systems

AbstractPower requirements were measured for several, two‐phase, liquid systems with viscosities ranging from 0.391 to 215 centipoise in a baffled, cylindrical vessel conforming to a standard tank configuration. A correlation was developed for the minimum power needed in order to maintain a uniform emulsion. For the systems studied it was found that the product of minimum impeller speed and the weighted average density was only a function of the respective continuous phase properties. Measurements, made over a range of impeller speeds from the minimum required to maintain an emulsion up to the point where gas entrainment occurred, were accurately described by single phase power curve. For more than 250 points with Reynolds numbers ranging from 500 to greater than 100,000, the average absolute error in the experimentally measured power number for two phase systems was found to be 4.0% based on the corresponding single phase power number curve