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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, 2015

Current literature relies almost exclusively on the power number to compare and characterize impellers. Industrial mixing requirements may rely on conditions far away from the impeller. A protocol is proposed to compare impellers designed for turbulent mixing on the basis of impeller hydrodynamic performance and mixing process objectives. A hydrofoil impeller (KPC), and a mixed-flow impeller (45° down-pumping PBT), each at two diameters, were used to test the protocol. Fourteen measures were considered. Five are recommended for full characterization: power number, momentum number, and peak rate of dissipation of turbulent kinetic energy to characterize conditions at the impeller; power at just-suspended speed to compare the efficiency of solids suspension at the bottom of the tank; and point of air entrainment as a measure of turbulence penetration to the free surface. These five measures provide complete information about mixing performance and good differentiation between the impellers and geometries. © 2011 American Institute of Chemical Engineers AIChE J, 58: 2573–2588, 2012

In-line rotor stator mixers differ from in-tank versions because the flow is often controlled independently of the rotor speed. For in-tank devices the turbulent power can be adequately described by single impeller type power number. For an in-line rotor-stator mixer it is found that the power transmitted by the rotor consists of a power term and a flow term. The constants in this expression are normally obtained from a multilinear regression of a large matrix of experiments. Two simplified methods of obtaining these constants from limited data sets by (1) torque measurement and (2) by heat balance are described herein. These are used to determine the constants for a Silverson 150/250 MS inline mixer with different rotor stator arrangements from the laminar to turbulent regimes. The power and Metzner-Otto constants determined are shown to be in good agreement to data for a larger matrix of experimental data. © 2011 American Institute of Chemical Engineers AIChE J, 58: 1683–1692, 2012

The present work involves Computational Fluid Dynamics (CFD) modeling and validation of head and power characteristics of a Top Shrouded Turbine with Nonradial Rectangular Blades (TSTNRB) operated in backswept mode. Following the successful validation reported here and in two previous studies by the authors. CFD has been used to simulate the performance of different kinds of impellers viz. Straight Blade Paddle (SBP), Rushton Turbine (RT), Top Shrouded Turbine with Radial Rectangular Blades (TSTRRB), Top Shrouded Turbine with Radial Trapezoidal Blades (TSTRTB), Top Shrouded Turbine with Non-radial Rectangular Blades (TSTNRB), Top Shrouded Turbine with Curved Blades (TSTCB), and Both side Shrouded Turbine with Radial Trapezoidal Blades (BSTRTB), to compare them for pump-mix action. Baffle-impeller interaction has been modeled using the sliding mesh approach. Standard k-e model has been used for turbulence modeling. The power number, head number, and flow number have been computed for each impeller. The impellers have been rated on the basis of head generated by them for equal specific power input. An attempt has been made to explain why different impellers operating at the same speed and handling the same inlet flow rate should give different heads. How this insight can be used to conceptualize better designs of pump-mix impeller is illustrated. © 2007 American Institute of Chemical Engineers AIChE J, 2008

This paper presents experimental research carried out in a model mixing vessel on suspending solid particles at high concentrations. It was found that the maximum solid concentration at which slurry suspension can be maintained is approximately 0.90 when normalised by the bed packing coefficient. A higher exponent a-value was found for the dependency of just-off-bottom impeller speed on solids loading in the high solids loading range. At high solids loadings, radial impellers require relatively less power than axial flow impellers to suspend solids, the opposite to that observed in the low solids loading range. When operating in high solid concentration, there is an increase in power number for pitch-bladed turbines and a reduction in power number for Rushton turbines, in comparison with operating in water.

Theoretical formulations for impeller performance were evaluated based on a blade-element theory. These enable the calculation of the head and power vs. flow-rate curves of axial-flow impellers. The technique uses the lift and drag coefficients of the blade section of an impeller to calculate the spanwise swirl-velocity distribution. Using the angular-momentum equation, it is possible to calculate the corresponding spanwise distribution of the energy head of the impeller. Integration of these distributions of head and torque gives the impeller's performance. Parameters including the flow number, the power number, the thrust force number, and the swirl velocity can be found at the impeller operating point, determined using the head curve and an experimentally calibrated resistance curve. A laser Doppler velocimetry (LDV) system was used to measure the velocity distribution for different axial flow impellers in mixing tanks. Calculated flow and power numbers agreed well with the experimental results. Using the blade's spanwise head distribution and a set of calibrated flow-resistance data, it is also possible to estimate an impeller's outlet axial-velocity distribution. Predictions compared well with LDV experimental data. The effect of impeller-blade angle, number of blades, blade camber, and blade thickness on the performance of axial-flow impellers was investigated using the Agitator software.

A novel, baffleless, mixing tank configuration presented provides greater mixing efficiency than conventional technologies under a wide variety of operating conditions. The concept uses a cylindrical tank with a contoured bottom and a shroud that surrounds an axial flow impeller and supports a set of stationary antiswirl fins positioned in close proximity to the impeller blades. The angle and pitch of the fins impart an angular component to the flow that is equal and opposite to the swirl induced by impeller rotation, thereby eliminating the need for wall baffles. The contoured bottom eliminates dead space beneath the impeller and n the corners of flat-bottom mixing tanks. This alternative mixing system, named the ASSET system, is compared to conventional mixing configurations through experiments of power consumption, gas-liquid mass transfer, and solids suspension. Measurements of Power number vs. Reynolds number reveal that it has power requirements similar to those of high-efficiency axial-flow turbines in a conventional tank geometry. Unsteady-state oxygen absorption and stripping experiments are used to determine the interphase mass-transfer coefficient as a function of power input for each configuration at superficial gas feed velocities of 0.057 to 0.17 cm/s. With this system, K{sub L}a values are on average 30 tomore » 50% greater than those obtained in a conventional mixing tank at the same power input. Measurements of the height of the well-mixed region of suspended solids in a 15 wt. % mixture of 0.1-cm-dia. glass beads in water indicate that the ASSET system requires {approximately} 30% less power to achieve equivalent extents of solids suspension than the conventional setup.« less

The influence of shear thinning and viscoelasticity on the power required for the mixing of viscous liquids using six different helical ribbon agitators has been investigated. Four Newtonian and 12 non-Newtonian fluids prepared using several polymers dissolved in varying concentrations in different solvents cover a wide range of rheological properties. By a careful choice of test media, the specific and combined effects of shear thinning and viscoelasticity on the power requirement have been examined. Simple models are proposed to predict the effective shear rate in the tank from the knowledge of the torque or power number. The effective shear rate predictions compared with the effective shear rate estimated using the scheme of Metzner and Otto (1957) show that they slightly depend on the shear thinning properties. Fluid's elasticity increases appreciably the power requirement, and departures from the generalized Newtonian power curve in the laminar regime are observed at smaller Reynolds numbers for viscoelastic fluids. Bottom wall resistance of the mixing vessel makes a negligible contribution to the power consumption.

On the basis of a back-flow model, the rates of interstage mixing of continuous phase for RDC and Mixco columns are measured experimentally under flow and nonflow conditions. These rates are correlated into dimensionless formulas for wide combinations of operational condition, column geometry, and dimensions, and are put into a single formula by introducing the power number as an additional parameter. The final correlation covers the impeller Reynolds number from 3.5 × 103 to 1.0 × 106 and the diameter of columns from 4.1 to 218 cm.

The dynamics of agitating single- and two-phase liquid mixtures in the absence of an air-liquid interface was compared with that of similar systems with an interface. Two geometrically similar cylindrical vessels, 12 and 18 in. in diameter respectively, were used, each fitted with a similar six-bladed, disk-turbine impeller. New general correlation curves of power number with Reynolds number are presented which show that elimination of the air-liquid interface makes it possible to attain dynamic similarity in scale-up in unbaffled vessels but makes little difference in baffled vessels.

This investigation was undertaken to show how the efficiency of solute transfer between two immiscible liquids is influenced by the type, size, and submergence and rotational speed of the impeller and the degree of baffling, residence time, and phase ratio. The system water-kerosene-n-butylamine solute was agitated in a continuous-flow 14 3/4-in. -diam. vessel using propellers, spiral turbines, and flat-blade turbines from 4 to 10 in. in diameter. The feed streams were introduced at the wall at the bottom of the vessel and left at the top of the vessel. On the basis of the power required for a given level of stage efficiency, the best agitator design is any impeller that has a diameter about 40% of the vessel diameter and is centered in the unbaffled vessel. Without baffles the impeller type and depth of submergence have little effect on performance. The power increase required by the addition of baffles is small at the highest stage efficiencies, especially with the larger radial-flow impellers, but may be severalfold at efficiency levels of 70 to 90%. This presumably results from the lowered mass transfer driving force caused by the increase in end-to-end mixing due to wall baffles. Whether baffles have such a large adverse effect if the feed is introduced into the impeller rather than at the vessel wall is not fully established, but there is some evidence that baffling is also undesirable in this case. For baffled operation, impeller type and submergence do affect performance. A correlation of stage efficiency is presented in terms of Reynolds number and power number. In addition, energy input per volume of total flow is correlated in terms of residence time and stage efficiency for one size of baffled propeller. Spot samples taken from the vessel showed large, random-concentration fluctuations out to 20 residence times and perhaps indefinitely. Reproducible results were obtained by taking time-averaged samples.