Impeller Spacing Research Articles

Gassed Power Consumption and Gas Hold-up for a Dual Hollow Blade Impeller System in a Stirred Vessel

Gas dispersion characteristics, i.e. ungassed and gassed power consumption, flow patterns and gas hold-up of the dual hollow blade disk turbines (6HB), which had been recently developed by a manufacturer to obtain a better gas dispersion performance than the traditional 6-bladed Rushton disc turbine (6DT), were investigated experimentally. The effect of impeller spacing in the dual 6HB impeller system on the unaerated and aerated power consumption was examined thoroughly.The gassed power consumption and gas hold up for the dual 6HB were compared with those obtained for the dual 6DT (radial flow impellers) and the dual Lightnin A315 (axial flow characteristics), of which performance has been also advanced recently. An enhancement of the gas handling capacity of the 6HB was achieved.

Dispersion Characteristics of Gas-Liquid Contactors Agitated by Single and Double-Stage Rushton Turbines

The study of the loading/complete dispersion transition is of great importance especially in processes with enhanced mixing requirements. In the present work, new data and correlations concerning the dispersion characteristics in gas-liquid contactors agitated by single and dual Rushton turbine systems are reported. The maximum amount of gas which can be completely dispersed, in the presence of gross, well defined recirculation patterns of gas at a given stirrer speed is predicted. Under these conditions, an increase of flow number with increasing Froude number could always be estimated. With decreasing impeller diameter, d, the same gas amount could be dispersed at higher stirrer speeds. At impeller spacing ΔH=2 d, for d equal to 0.06 and 0.08 m, and ΔH=1.54 d, for d=0.10 m, the complete dispersion conditions of the dual impeller systems were slightly better than the corresponding conditions of the single impeller systems.

Power Consumption in Gas-Liquid Contactors Agitated by Double-Stage Rushton Turbines

The dependence of power consumption on impeller spacing in unaerated and aerated gas-liquid contactors agitated by dual Rushton turbine systems was studied, and the gas flow rate and viscosity effects were measured in relation to these parameters. The experiments were carried out in a 0.19 m i.d. vessel stirred by two Rushton turbines with a diameter d = 0.10 m; with blade length and blade height 0.25 d and 0.2 d, respectively. In tap water the impellers acted independently for spacings greater than 1.65 d, while in glycerol solutions the two impellers already acted independently at an impeller spacing equal to 1.2 d. In aerated systems, a notable increase in the power consumption with increasing impeller spacing could be detected for small gas flow rates and low viscosities, while a decrease in the Newton number with increasing Froude number could be observed at constant impeller spacing. The Newton number was not affected by flow number at high viscosity values.

The effect of impeller pumping and fluid rheology on solids suspension in a stirred vessel

AbstractPumping velocities of different impellers were measured using laser Doppler velocimetry. It was found that there is a link between impeller pumping capacity and the 5 parameter in the Zwietering correlation. Measurements showed that Njs increases with impeller spacing for dual impeller configurations. A dual‐impeller configuration shows less sensitivity to solids loading change than a single‐impeller configuration, and the change in Njs is larger for small impeller spacing than for large impeller spacing. An increase in slurry cloud height and a reduction in Njs resulted at a small increase of non‐Newtonian viscosity.

Power Consumption in Agitated Vessels Provided with Multiple-Disk Turbines

The power dissipated by one-, two-, or three-disk turbines (Rushton type) in stirred vessels was experimentally determined under turbulent conditions. The power dissipated by each individual impeller (in single- or multiple-impeller configurations) and the total power consumption were measured by means of strain gauges mounted on the shaft and were reported as individual or total power numbers. Results were obtained for different combinations of number of impellers, off-bottom clearance of the lowest impeller, and spacing among impellers. These variables were found to play a very important role in the power drawn by individual impellers and, hence, the total power consumption. The impeller closest to the vessel bottom generally consumed significantly less power than the other impeller(s). This was especially true for combinations of low impeller clearances and small impeller spacing. Proximity of two impellers also lowered their power consumption. Only when the impellers were significantly spaced apart an...

Experimental and computational study of multiple impeller flows

A computational and experimental study is conducted of viscous flow in a stirred reactor with multiple impellers. The vessel is cylindrical in shape with a stack of four 45 pitched blade impellers, four rectangular side-wall baffles and an ellipsoidal shaped bottom. The flow is computed with an incompressible Navier-Stokes solver which uses the pseudocompressibility technique of coupling the velocity and pressure fields. The laminar viscous flow field is solved using an approximate steady-state technique which neglects relative motion between the impellers and baffles and solves the flow at a single impeller position in a rotating frame of reference. The resulting velocity field is spatially averaged and compared with time-averaged experimental results. Computed results for the velocity field are shown to agree very well with experimental laser Doppler velocimetry (LDV) data for two different impeller configurations. This work illustrates the utility of the numerical method for studying complex multiple impeller flows at low Reynolds number. A variety of different impeller configurations are studied numerically and the effect of relative impeller sizing, impeller spacing and baffling on flow distributions within the stirred vessel is investigated. It is shown that global circulation patterns within the tank are strongly dependent on relative impeller size and spacing. It is concluded that obtaining good global circulation and mixing performance is sensitive to relative impeller sizing and spacing. Improper impeller spacing or sizing can result in compartmentalization of the flow inside the vessel and hence poor global circulation.

Fractional gas hold‐up in gas inducing type of mechanically agitated contactors

AbstractFractional gas hold up was measured in gas inducing type of mechanically agitated contactors (GIMAC) with single and multiple impellers. Three vessels of 0.57, 1.0 and 1.5 m i.d. were used. For the multiple impeller system, six different impeller designs were employed. The impeller speed was varied in the range of 0.30 to 15.45 s−1. The ratio of impeller diameter to tank diameter (D/T), the submergence (S) of the upper impeller, the clearance of the lower impeller from the tank bottom (C1) and the impeller spacing (C3) were varied over a wide range. The design of the lower impeller was optimized in terms of diameter (D), blade width (W), blade angle (Bø), number of blades (nb) and the blade thickness (tb). An optimum design has been proposed for the multiple impeller system. Rational correlations have been proposed.

Flow visualization scaleup studies for the mixing of viscoelastic fluids

Flow visualization studies have been performed with viscoelastic fluids in three-liter and 200-liter stirred tanks. The objectives were to determine the optimum agitator type, impeller diameter, impeller spacing, agitator speed, and feed location to minimize blend time and prevent formation of dead zones in the vessel for the blending of low viscosity materials into a viscoelastic fluid. Helical ribbon impellers and double-helical impellers (three-liter only), and multiple pitched-blade turbines (both scales) were studied. Helical impellers resulted in shorter blend times and more uniform mixing. Scaleup studies performed on the pitched-blade turbines demonstrated similar flow patterns and blend times to the smaller scale. Agitator spacing was critical between sets of turbines, with spacing of less than 1 2 the impeller diameter necessary to prevent zoning. Subsurface feed was shown to be superior for mixing low viscosity materials into the vessel. The formation of unmixed tori near the impellers was also noted.

Xanthan gum production in a fermentor with twin impellers

The production of xanthan gum by Xanthomonas campestris ATCC 13951 in a fermentor using twin turbine-type impellers with 6 flat blades was studied to determine the effects of impeller spacing on the specific production rate, the volume of the high shear stress region around the impellers, and the agitation power requirements, and a comparison was made with the use of a single impeller. The following results were obtained. (i) The specific rate of xanthan gum production was dependant on the volume of the high shear region in the fermentor, even in the twin-impeller system. (ii) The greater was the space between the two impellers, the larger was the volume of the high shear region. (iii) The volume of the high shear region and the specific rate of xanthan gum production were greater with two impellers than with a single impeller, even if the agitation rate was adjusted to consume the same power. These results strongly indicate that the production of xanthan gum depends on the overall volume of the high shear region, which is enhanced by duplication of impellers, and that a twin-impeller system is also economically advantageous in terms of the power consumption required for agitation.

Gas–liquid dispersion with dual Rushton impellers

Aerated and unaerated power consumption and flow patterns in a 0.56 m diameter agitated vessel containing water with dual Rushton turbines have been studied. Under unaerated conditions with a liquid height-to-diameter ratio of 2, an impeller spacing of 2 to 3 times the impeller is required for each to draw an amount of power equal to a single impeller. For aerated conditions, if a similar spacing is used, equations for the flooding-loading transition and for power consumption for a single Rushton impeller can be extended relatively easily to dual systems. All results for this spacing are explained by reference to bulk flow patterns and gassed-filled cavity structures and the proportion of sparged gas flowing through the upper impeller is also estimated. Such a spacing is generally recommended since it maximizes the power draw and hence the potential for oxygen mass transfer. Data are presented for other spacings but the results do not fit in easily with single agitator studies because strong impeller-impeller flow pattern interactions occur.

Mass transfer characteristics of horizontal agitated contactors

AbstractThe values of effective interfacial area, a, and liquid side mass transfer coefficient, kLa, were obtained in 0.385, 0.57 and 1.0 m i.d. horizontal agitated contactors by using chemical methods. Propeller type of impellers were employed for agitation. Two modes of operation were studied: (i) surface aeration and (ii) sparging of gas. The impeller speed was varied from 50 to 700 rev/min, and the nozzle gas velocity was varied from 40 to 160 m/s. The effects of impeller spacing, liquid submergence and nozzle gas velocity on the values of a and kLa were investigated. A scale‐up criterion is discussed.

Mass transfer characteristics of horizontal agitated contactors

AbstractThe values of effective interfacial area, a, and liquid side mass transfer coefficient, kLa, were obtained in 0.385, 0.57 and 1.0 m i.d. horizontal agitated contactors by using chemical methods. Propeller type of impellers were employed for agitation. Two modes of operation were studied: (i) surface aeration and (ii) sparging of gas. The impeller speed was varied from 50 to 700 rev/min, and the nozzle gas velocity was varied from 40 to 160 m/s. The effects of impeller spacing, liquid submergence and nozzle gas velocity on the values of a and kLa were investigated. A scale‐up criterion is discussed.