Pitched Blade Downflow Turbine Research Articles

Performance of an engineered combination of plunging jet with stirred tank part I: Single impeller system

The present work proposes a new combination of two conventional reactors with the objective of developing improved gas-liquid/gas-liquid-solid reactor. Thus performance of plunging jet/s coupled with conventional mechanically agitated contactor (MAC) has been investigated experimentally by standard parameters. Plunging jet/s was/were used as a means to introduce gas phase into conventional MAC instead of a sparger. The plunging jet/s was/were located in such a way that the gas plunged-in by the jet can be sucked-in and efficiently dispersed by the impeller. Experiments were performed in 1 m i.d. stirred tank and power consumption was varied in the range of 0.05 to 3 kW/m3. Volumetric mass transfer coefficient (kLa), specific power consumption (PG/V) and the gas hold-up (ЄG) was measured for disk turbine (DT), Pitched blade downflow turbine (PBDT) and hydrofoil impeller for two different jet diameter and length of the jet and two number of jets. The performance of proposed reactor has been compared with gas inducing (GIC) and conventional MAC. The performance of proposed reactor was found to be superior as compared to conventional MAC and GIC.

SIMULATION OF SOLID–LIQUID SUSPENSION AND SCALE-UP OF AGAROSE GEL ACTIVATION REACTOR

Agarose gel activation reaction, which is of great importance in preparing the carrier of the column packing material for blood purification, would be significantly influenced by the configuration and parameter of reactor. In order to optimize the structure design of the reactor and operating parameters of the process, the characteristics of suspension system composed of agarose gel, NaOH, water, 3-allyl bromide, and activated alumina were simulated numerically utilizing an Eulerian multiphase flow model and multi-reference frame (MRF) approach. The effect of impeller configuration was studied with three typical impellers, including Rushton disk turbine (DT), pitched blade downflow turbine (PBTD45), and pitched blade upflow turbine (PBTU45). The results showed that the optimum solid suspension was obtained using PBTD45 impeller with a diameter of 100[Formula: see text]mm and critical suspension speed of 570[Formula: see text]rpm in a 20[Formula: see text]L stirred reactor. In addition, the critical suspension speeds using the three impellers were calculated and the errors were all within the engineering allowance. Finally, the feasibility of scale-up design for agarose gel activation reactor was preliminarily discussed. The results would be useful to the optimization and scale-up of relevant reactor design.

Comments to “CFD simulation of stirred tanks: Comparison of turbulence models. Part I: Radial flow impellers” and “Part II: Axial flow impellers, multiple impellers and multiphase dispersions”

Iread with great interest, articles of Joshi et al. (2011a,b) “CFD Simulation of Stirred Tanks: Comparison of Turbulence Models. Part I: Radial Flow Impellers” and “Part II: Axial Flow Impellers, Multiple Impellers and Multiphase Dispersions.” The articles cover a valuable review and analysis of CFD simulation of turbulent flow of mechanically agitated liquid and two-(or more-) phase mixtures with two frequently used high-speed impellers: standard Rushton impeller (RT) and pitched blade down flow turbine (PBT). Analysis of individual CFD procedures is made on the basis of results of laser Doppler velocimetry (LDV) in agitated systems with both investigated impellers. The commented articles bring very useful pieces of knowledge for application of CFD simulation when projecting and designing equipment with high-speed impellers and give many useful advices both to researches and people in industrial praxis for successful research and development of devices with investigated agitators. For deeper understanding and following usage of published items it seems that some of them could be clarified and completed: Experimental LDV data of velocity profiles (axial, radial and tangential) are presented in the time – and ensemble – averaged form, i.e. the average-time velocity is obtained over 360◦ of impeller revolution (Schafer et al., 1998), in contradistinction to the angle-resolved form, where angular (tangential) coordinate between two adjacent impeller blades is taken into consideration. It is not quite clear how the results of CFD simulation were evaluated by the same way when compared with LDV ensembleaveraged data. Moreover each velocity profile in observed agitated system (cylindrical baffled vessel with axially located impeller) depends on tangential position of the radial ray where the experiments were carried out. It seems from Figure 4 (Joshi et al., 2011a) and Figure 1 (Joshi et al., 2011b) that location of experimental radial rays was between opposite baffles, but presented experimentally determined radial profiles of all three components of the mean ensemble-average velocities better correspond to the radial ray in the midplane between adjacent baffles. The authors of commented articles described design of both investigated impellers. For PBT 30, 45 and 60 they do not mention a number of their blades, nor relative width of the blade. For RT they mentioned “standard Rushton turbine” and it is hoped that this impeller has six blades and exhibits the standardized simplexes of geometric similarity. Nevertheless, the power input of RT mentioned in the article of Joshi et al. (2011a) depends significantly on the thickness of the RT disk t (Bujalski et al., 1987; Rutheford et al., 1996) according to the formula for the impeller power number:

Gas Induction Characteristics of Hollow Self-Inducing Impeller

The mechanism of self induction has been investigated for the case of hollow pitched blade downflow turbines. The pressure difference between the orifice and the reactor head space results into self induction of gas. The dependence of pressure difference as well as the rates of gas induction on various parameters like impeller speed, blade angle, orifice location and the number of open orifices was investigated. Gas side and liquid side resistances were experimentally measured for blade angles of 30°, 45° and 60°. An attempt has been made to propose a predictive procedure for the rate of gas induction on the basis of pressure driving force as well as the gas and the liquid side resistances. The gas induction rates for wide range of blade angle (viz. PBTD 30°, 45° and 60°), various hole combinations (viz. 1; 1,2 and 1,2,3,4,5,6) and for submergence of S = 2T/3 and for 1,2,3,4,5,6 holes at S = T/3 submergences have been predicted with this procedure. The predictions are in good agreement with the experimental results.

Studies in multiple impeller agitated gas–liquid contactors

Experiments have been performed to study the effect of the density and the volume of the tracer pulse on the mixing time for two impeller combinations in the presence of gas in a 0.3 m diameter and 1 m tall cylindrical acrylic vessel. The tall multi-impeller aerobic fermenters, which require periodic dosing of nutrients that are in the form of aqueous solution, is a classic case under consideration. Conductivity measuring method was used to measure the mixing time. Two triple impeller combinations; one containing two pitched blade downflow turbines as upper impellers and disc turbine as the lowermost impeller (2 PBTD–DT) and another containing all pitched blade downflow turbines (3 PBTD) have been used. Other variables covered during experiments were the density and the amount of the tracer pulse, the impeller rotational speed and the gas superficial velocity. Fractional gas hold-up, Power consumption and mass transfer coefficient have also been measured for both the impeller combinations. Influence of aeration and impeller speed on the mixing time has been explained by the interaction of air induced and impeller generated liquid flows. Three different flow regimes have been distinguished to explain the hydrodynamics of the overall vessel (i.e., multiple impeller system). A compartment model with the number of compartments varying with the flow regimes have been used to model liquid phase mixing in these flow regimes. A correlation for the prediction of the dimensionless mixing time in the loading regime has been proposed in order to account the effect of the density and the amount of the tracer pulse on the mixing time. Correlations have also been proposed to predict fractional gas hold-up and k L a .

Local Gas Holdup Measurement in Sparged and Aerated Tanks by γ-Ray Attenuation Technique

In the present work, a γ-ray attenuation technique was used to measure gas holdup distribution in stirred tanks. Measurements have been carried out in a 0.57-m-i.d. tank agitated by a pitched blade downflow turbine (PBTD45°) and a disk turbine under two different conditions, namely, surface aeration and sparger aeration beneath the impeller. The measurements have been made at various operating speeds. In both the cases, the average gas holdup calculated by integration of the local gas holdup was found to match well with those obtained by visual observations based on volume expansion. The gas holdup profiles also enable identification of speeds of regime transition. This technique appears to be promising for the characterization of industrial stirred reactors.

Prediction of Flow Pattern in Stirred Tanks: New Constitutive Equation for Eddy Viscosity

The CFD simulations were carried out for the flow produced by two axial flow impellers, namely a pitched-blade downflow turbine (PBTD, Np = 2.1) and a hydrofoil impeller (Np = 0.34) by the impeller boundary condition approach. The tank was fully baffled, and the flow regime was turbulent. An attempt has been made to develop a new constitutive equation for Cμ in the description of eddy viscosity given by a standard k−ε model. The resulting predictions with the new eddy viscosity relation are compared with the experimental data. Also the comparison is sought with the predictions of impeller boundary condition approach using k−ε model with the standard and modified turbulence parameters (Ranade et al. Chem. Eng. Commun. 1989, 81, 225), zonal model of Sahu et al. (Ind. Eng. Chem. Res. 1998, 37, 2116) and the sliding mesh simulation using a standard k−ε model. For all these cases, energy balance has been established and compared with the experimental data. This paper also presents a critical review of the computational fluid dynamic (CFD) studies pertaining to the prediction of the flow produced by the axial flow impellers.

Mixing of miscible liquids with density differences: Effect of volume and density of the tracer fluid

AbstractThe effect on the mixing time of the density difference between the tracer fluid and the bulk, the type and geometry of the impeller, and the amount of the fluid added as a tracer, has been investigated in a tank with diameter of 0.31 m. The amount of tracer pulse has been varied in the range of 2 to 8% of the bulk volume and the density difference between two fluids has been varied in the range of 54 to 145 kg/m3. A generalized correlation has been developed for the prediction of dimensionless mixing time (Nθ) in terms of amount of the tracer fluid, density differences between the bulk and the tracer liquid and the pumping effectiveness of the impeller. A pitched blade downflow turbine with diameter as small as possible is the most energy efficient among the impellers studied in this work.

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

AbstractCritical impeller speed for solid suspension (NCS), 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 (nb). An empirical model has been proposed based on the mechanism of fluidization.

Power Consumption in Gas-Inducing-Type Mechanically Agitated Contactors

Power consumption was measured in 0.57, 1.0, and 1.5 m i.d. gas inducing type of mechanically agitated contactors (GIMAC) using single and multiple impellers. The ratio of impeller diameter to vessel diameter was varied in the range of 0.13 < D/T < 0.59. The effect of liquid submergence from the top and impeller clearance from the vessel bottom was investigated in detail. In the case of multiple impeller systems, six different designs were investigated. The designs included pitched blade downflow turbine (PBTD), pitched blade upflow turbine (PBTU), downflow propeller (PD), upflow propeller (PU), straight bladed turbine (SBT) and disc turbine (DT). The effect of interimpeller clearance was studied for the multiple impeller system. The effect of impeller speed was studied in the range of 0.13 < N < 13.5 rotations/s. A mathematical model has been developed for power consumption before and after the onset of gas induction.

Gas hold‐up behavior of mechanically agitated gas‐liquid reactors using pitched blade downflow turbines

AbstractFractional gas holdup was measured in 0.57, 1.0, and 1.5 m i.d. vessels. Pitched blade downflow turbines (PTD) were used as the impeller. Design details of the impeller, such as the impeller diameter (0.22 T to 0.5 T) and blade width (0.25 D to 0.4 D), were studied. The effect of sparger type, geometry and size on fractional gas hold‐up has been investigated in detail. Four different types of spargers (pipe, conical, ring and concentric ring spargers) were used. Sparger location was varied for all the types studied. Further, design details of the ring sparger, which gave the highest hold‐up were then studied in detail. These included ring diameter, number of holes and hole size. All the reported correlations for fractional gas hold‐up in mechanically agitated gas‐liquid reactors were tested and compared. A better correlation has been developed for pitched blade turbines.

Effect of addition of alcohol on the design parameters of mechanically agitated three-phase reactors

The effect of the presence of isopropyl alcohol on the design parameters of gas-liquid-solid systems has been studied in a vessel with an internal diameter of 0.57 m. A pitched blade downflow turbine was used as an impeller. Isopropyl alcohol concentration was varied from 0 to 2 vol.%. Air and quartz particles of 2000 μm in size were used as the gas and solid phase, respectively. The solid loading ( X) was varied from 0 to 15 wt.%. The impeller speed and superficial gas velocity were varied in the range of 2.4 to 10 rev s −1 and 0 to 9.4 mm s −1 respectively. The variations of power number and mixing time with respect to impeller speed were found to be similar. The effect of isopropanol concentration on the power consumption, liquid phase mixing time, critical impeller speed for gas dispersion, critical impeller speed for solid suspension and fractional gas hold up have been studied.

Role of sparger design in mechanically agitated gas‐liquid reactors. Part II: Liquid phase mixing

AbstractLiquid phase mixing time was measured in 0.57, 1.0 and 1.5 m i.d. mechanically agitated gas‐liquid reactors. Transient conductivity technique was used for the mixing time measurement. Pitched blade downflow turbine was employed. The design details of PTD impellers such as diameter (0.22 T to 0.5 T) and blade width (0.25 D to 0.35 D) were studied. The influence of sparger types and their design on mixing time has been investigated. For this purpose, pipe, ring, conical, and concentric ring spargers were employed. The design details of the ring sparger, i.e. ring diameter, number of holes and hole size were also studied in depth. Sparger location with respect to the impeller was found to be the most important variable and, therefore, it was varied for practically all the spargers studied in this work. It was found that the liquid phase mixing time depends on the impeller design, sparger design, sparger location, impeller speed and superficial gas velocity. Correlations have been developed for the dimensionless mixing time.

EFFECT OF IMPELLER DESIGN ON LIQUID PHASE MIXING IN MECHANICALLY AGITATED REACTORS

Liquid phase mixing time (θmix) was measured in mechanically agitated contactors of internal diameter 0.57 m, 1.0 m and 1.5 m. Tap water was used as the liquid phase. The impeller speed was varied in the range of 0.4–9.0 r/s. Three types of impellers, namely disc turbine (DT), pitched blade downflow turbine (PTD) and pitched-blade upflow turbine (PTU) were employed. The ratio of impeller diameter to vessel diameter (D/T) and the ratio of impeller blade width to impeller diameter (W/D) were varied over a wide range. The effects of impeller clearance from the tank bottom (C), the blade angle (φ), the number of blades (n b ), the blade thickness (k) and the total liquid height (H/T) were studied in detail. Mixing time was measured using the conductivity method. Mixing time was found to have a strong dependance on the flow pattern generated by the impeller. Mixing time was found to decrease by decreasing the impeller clearance in the case of DT and PTU. However in the case of PTD it increases with a decrease ...

POWER CONSUMPTION IN MECHANICALLY AGITATED CONTACTORS USING PITCHED BLADED TURBINE IMPELLERS

Power consumption was measured in mechanically agitated contactors of internal diameter 0.3 m, 0.57 m, 1.0 m and 1.5 m. Tap water was used as a liquid in all the experiments. The impeller speed was varied in the range of 0.3-13.33 r/s. Three types of impellers, namely disc turbine (DT), pitched-blade downflow turbine (PTD) and pitched blade upflow turbine (PTU) were employed. The ratio of the impeller diameter to vessel diameter (D/T) and the ratio of impeller blade width to impeller diameter (W/D) were varied over a wide range. The effects of impeller clearance from the tank bottom (C), blade angle (φ), total liquid height (H/T), number of impeller blades (nb) and blade thickness (tb) were studied in detail. Power consumption was measured using a torque table Power number was found to have a strong dependence on the flow pattern generated by the impeller. Unlike, DT and PTU, the power number of PTD was found to increase with a decrease in clearance. The PTD (T/3) was found to have the lowest power number in all the vessels and the power number increased with either a decrease or an increase in the impeller diameter from T/3. The dependence of power number on impeller diameter was found to be more prominent when the D/T ratio was more than 0.3. In general, the power number was found to increase with an increase in blade angle and blade width. The effect of blade width was found to be more prominent in larger diameter vessels. A correlation has been developed for power number in the case of PTD impellers.

FLOW GENERATED BY PITCHED BLADE TURBINES II: SIMULATION USING κ-ε MODEL

Experimental data on average velocity and turbulence intensity generated by pitched blade downflow turbines (PTD) were presented in Part I of this paper. Part II presents the results of the simulation of flow generated by PTD The standard κ-e model along with the boundary conditions developed in the Part 1 have been employed to predict the flow generated by PTD in cylindrical baffled vessel. This part describes the new software FIAT (Flow In Agitated Tanks) for the prediction of three dimensional flow in stirred tanks. The basis of this software has been described adequately. The influence of grid size, impeller boundary conditions and values of model parameters on the predicted flow have been analysed. The model predictions successfully reproduce the three dimensionality and the other essential characteristics of the flow. The model can be used to improve the overall understanding about the relative distribution of turbulence by PTD in the agitated tank

FLOW GENERATED BY PITCHED BLADE TURBINES I: MEASUREMENTS USING LASER DOPPLER ANEMOMETER

Abstract Mean flow and turbulence intensities have been measured using laser Doppler anemometer for pitched blade downflow turbines (PTD). Fully baffled, flat bottomed cylindrical vessels of 300 and 500 mm internal diameter were employed. The effect of impeller clearance on flow characteristics have been investigated. The influence of geometry of PTD, that is blade angle (30-60°), blade width (0.2D-0.4D) and impeller diameter (0.25T-0.5T) on the flow have been studied in detail. The energy balances have been established around all the impellers and the hydraulic efficiency values have been reported. This part provides a right set of boundary conditions to the model as well as provides necessary and sufficient set of data to evaluate the model performance