Unbaffled Agitated Vessel Research Articles

Solid–liquid mass transfer in relation to diffusion controlled corrosion at the outer surface of helical coils immersed in agitated vessels

The diffusion controlled dissolution of copper in acidified dichromate solution technique was used to study solid–liquid mass transfer at the outer surface of helical coils immersed in unbaffled agitated vessel. Variables studied were impeller rotation speed, impeller geometry (four blade 90° turbine and 45° pitched blade turbine), distance between coil rings (coil pitch) and physical properties of the solution. The data were correlated in the form of dimensionless equations, for four blade 90° turbine impeller, the data fit the equation:Sh=0.023 Sc0.33 Re0.59dcS0.49while for four blade 45° pitched turbine impeller, the data were correlated by the equation:Sh=0.13 Sc0.33 Re0.51The above equations were obtained for unbaffled vertical cylindrical vessel of flat bottom in the range: 4705<Re<52,750, 1104<Sc<1506 and 8.75<(dc/S)<17.5. The possibility of using the present correlations to predict the rate of diffusion controlled processes that may occur at the outer surface of such coils was highlighted.

Transport phenomena in an agitated vessel with an eccentrically located impeller

AbstractThe paper presents results of an experimental analysis of the transport phenomena at the vicinity of the wall of an unbaffled agitated vessel with an eccentrically located impeller. Distributions of the transport coefficients were experimentally studied using an electrochemical method within the turbulent regime of the Newtonian liquid flow. Measurements were carried out in an agitated vessel with the inner diameter T = 0.3 m. Liquid height in the vessel was equal to the inner diameter, H = T. The agitated vessel was equipped with a Rushton or a Smith turbine or an A 315 impeller. Eccentricity of the impeller shaft was varied from 0 to 0.53. Local values of the dimensionless shear rate, shear stress, dynamic velocity and friction coefficient were integrated numerically for the whole surface area of the cylindrical wall of the vessel. Averaged values of these quantities were correlated with the impeller eccentricity and modified Reynolds number. The proposed Eqs. (5)–(8), with the coefficients given in Table 2, have no equivalent in open literature concerning this subject. Distributions of the shear rate, γ/n, and friction coefficient, f, at the vicinity of the cylindrical wall of the unbaffled vessel equipped with eccentric Rushton or Smith turbine or A 315 impeller are very uneven and they depend significantly on the impeller eccentricity, e/R. Maximum local values of these variables are located on the wall section closest to the impeller blades. From among the tested impellers, the greatest effects of the impeller eccentricity, e/R, and the liquid turbulence (described by the modified Reynolds number Re P,M) on the averaged dimensionless shear rate (γ/n)m and friction coefficient, f m, are found for the radial-flow Rushton turbine located eccentrically in an unbaffled agitated vessel.

Effect of impeller clearance on liquid flow within an unbaffled vessel agitated with a forward–reverse rotating impeller

AbstractFor an unbaffled agitated vessel with an unsteadily forward–reverse rotating impeller whose rotation proceeds with repeated acceleration, deceleration, and stop–reverse processes, liquid flow was studied through visualisation and measurement using particle tracking velocimetry (PTV). A disk turbine impeller with six flat blades was used with varied height settings. The impeller clearance and its forward–reverse rotation cycle characterised the impeller region flow: the radially outward flow in the deceleration process for the larger clearance relative to the vessel diameter of 1/3, and the axially downward flow in the acceleration process for the smaller clearance relative to the vessel diameter of 1/8. The flow patterns within the vessel resulting from the impeller's larger and smaller clearances were outlined, respectively, by double loops and a single loop of circulation, resembling the pattern produced by unidirectionally rotating turbine‐type impellers. The discharge flow was revealed to contain a comparable level of periodic circumferential velocity component, irrespective of the impeller clearance.

Turbulent and Laminar Mixings in an Unbaffled Agitated Vessel with an Unsteadily Angularly Oscillating Impeller

For an unbaffled agitated vessel with an angularly oscillating impeller whose rotation continues while periodically reversing its direction at a set angle, the progress of mixing of Newtonian liquid phase was studied experimentally in relation to the impeller power characteristics. Results obtained for the angular oscillation mode of a disk turbine impeller with six flat blades with different amplitudes (Strouhal number) were compared to those obtained for the unidirectional rotation mode of the impeller of identical design. Using an angularly oscillating impeller, turbulent and laminar mixings were effective, respectively, with larger and smaller amplitudes. The mixing characteristics for turbulent and laminar flow regions were discussed on the basis of the flow field features, which the impeller power number reflected.

Liquid flow in impeller region of an unbaffled agitated vessel with an angularly oscillating impeller

AbstractThe characteristics of a liquid flow were studied in the impeller region of an unbaffled agitated vessel with an angularly oscillating impeller whose unsteady rotation proceeds while periodically reversing its direction at a set angle. The measurement of the velocity of the liquid flow was performed by particle tracking velocimetry (PTV), abreast of that of the torque of the shaft to which the impeller was attached. When a disk turbine impeller with six flat blades was used with variations in operating conditions, such as the frequency and amplitude of impeller angular oscillation, a series of images obtained during one oscillation cycle were analyzed to characterize the internal and discharge streams inside and outside the impeller rotational region. Energy data were inferred on the basis of the circumferential and radial velocities of an internal flow. Results showed that although the total head provided to the liquid by the impeller blades is almost similar, independent of the amplitude of impeller angular oscillation, namely, the acceleration of its movement, the transformation of energy from the pressure head to the velocity head is more efficient at a larger amplitude. In addition, the discharge flow was characterized in terms of volumetric flow rates calculated from the radial and axial velocities. The operation at a smaller amplitude was shown to transform the flow more successfully from the radial direction to the upward and downward axial directions near the vessel wall.

Solid–liquid mass transfer characteristics of an unbaffled agitated vessel with an unsteadily forward–reverse rotating impeller

AbstractTo develop an enhanced form of solid‐liquid apparatus, an unbaffled agitated vessel has been constructed, fitted with an agitation system using an impeller whose rotation alternates unsteadily in direction, i.e. a forward‐reverse rotating impeller. In this vessel, solid‐liquid mass transfer was studied using a disc turbine impeller with six flat blades. The effect of impeller rotation rate as an operating variable on the mass transfer coefficient was evaluated experimentally using various geometrical conditions of the apparatus, such as impeller diameter and height, in relation to the impeller power consumption. Mixing of gas above the free surface into the bulk liquid, i.e. surface aeration, which accompanied the solid‐liquid agitation, was also investigated. Comparison of the mass transfer characteristics between this type of vessel and a baffled vessel with a unidirectional rotating impeller underscored the sufficient solid‐liquid contact for prevention of gas mixing in the forward‐reverse rotation mode of the impeller. Copyright © 2008 Society of Chemical Industry

Mixing time of a non-Newtonian liquid in an unbaffled agitated vessel with an eccentric propeller

AbstractExperimental study of mixing time of non-Newtonian shear-thinning fluids within the transitional regime (3 × 102 &lt; Re &lt; 3 × 103) of liquid flow is presented. The purpose of this study was to analyze the effect of eccentricity and pumping mode of the impeller as well as of position of the tracer dosage point into the agitated liquid on mixing time. The measurements were conducted in an unbaffled agitated vessel with inner diameter D = 0.7 m equipped with an up-or down-pumping propeller located centrically (e/R = 0) or eccentrically (e/R ≠ 0) in the vessel. Experiments were carried out by means of computer-aided unsteady-state thermal method for three positions of the tracer dosage point. The experimental data show that eccentric position of the propeller in an unbaffled vessel causes a decrease of the mixing time compared to that obtained in a vessel with a centrically located propeller. Mixing time depends also on the pumping mode of the propeller as well as on the position of the tracer dosage point.

Movement of Solid Particles on and off Bottom of an Unbaffled Vessel Agitated by Unsteadily Forward-Reverse Rotating Impeller

An unbaffled agitated vessel having an unsteadily rotating impeller was employed as an apparatus mixing liquid and solid particles with the density larger than that of liquid. For this type of vessel, the movement of solid particles on and off the vessel bottom was studied in relation to the liquid flow produced by the impeller. When a disk turbine impeller with six flat blades was rotated in the forward-reverse mode, the liquid flow and the particle movement were visualized. Concurrently, the agitation requirement for complete solid suspension where no particle remains on the vessel bottom for more than a short period and all particles are in motion was determined as a minimum rotation rate of impeller. The liquid flow and the particle movement around a tiny heap of solid particles configured on the vessel bottom were characterized through measurement of their velocities by the particle tracking velocimetry (PTV). The relative velocity of rising with off-bottom suspension of solid particles was uniform in its distribution and wholly large in its magnitude, compared with that in a baffled vessel with a unidirectionally rotating impeller of the identical design, which revealed an effectiveness of this type of vessel as an apparatus for the solid-liquid mass transfer.

Enhancement of Gas-Liquid Mass Transfer with Unsteadily Forward-Reverse Rotating Multiple Delta Blade Impellers in an Unbaffled Agitated Vessel

Enhancement of Gas-Liquid Mass Transfer with Unsteadily Forward-Reverse Rotating Multiple Delta Blade Impellers in an Unbaffled Agitated Vessel

Liquid‐phase mixing in an unbaffled agitated vessel with an unsteady forward–reverse rotating impeller

AbstractFor an unbaffled agitated vessel with an unsteady forward–reverse rotating impeller, whose rotation reverses its direction with a constant angular displacement, the progress of liquid‐phase mixing was studied experimentally in relation to the characteristics of the liquid flow produced by the impeller. The forward–reverse rotating disk turbine impeller with six flat blades has about 1.5 times the shear level of a unidirectionally rotating impeller of identical design. The turbulence generated in the liquid was more intense within the vessel agitated by the forward–reverse rotating impeller. In the forward–reverse agitation mode, the agitation function of the impeller demonstrably enhanced the liquid‐phase mixing, which shows a pattern with a higher level of mixedness. Copyright © 2007 Curtin University of Technology and John Wiley &amp; Sons, Ltd.

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

AbstractBackground: 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

LIQUID FLOW IN IMPELLER REGION OF AN UNBAFFLED AGITATED VESSEL WITH UNSTEADILY FORWARD-REVERSE ROTATING IMPELLER

For an unbaffled agitated vessel with an unsteadily forward-reverse rotating impeller whose rotation proceeds with repeated acceleration, deceleration, and stop-reverse processes, the liquid flow in the impeller region was studied based on photographs showing path lines of tracer particles. An image series taken during one cycle of the forward-reverse rotation was analyzed to characterize the internal stream inside the impeller rotational region and the discharge stream outside its region when a disk turbine impeller with six flat blades was rotated unsteadily. Because of the unsteady flow generated inside the impeller rotational region, the velocity vector of outflow from its region fluctuated periodically with the change of the impeller rotation rate. The circumferential velocity was almost in phase with the impeller rotation rate, oscillating periodically. The radial velocity exhibited larger values in the process for the impeller from decelerating to stopping and reversal. The radial flow, whose velocity decreased downstream outside the impeller rotational region, was clarified to be transformed into upward and downward axial flows that are almost uniform in the circumferential direction throughout the region near the vessel wall.

Modeling Turbulent Flows with Free-Surface in Unbaffled Agitated Vessels

In the present study, numerical simulations of turbulent flow with free-surface vortex in unbaffled vessels agitated by a paddle impeller and a Rushton turbine, which were investigated experimentally by Nagata (John Wiley & Sons: New York, 1974) and Ciofalo et al. (Chem. Eng. Sci. 1996, 51, 3557−3573), respectively, have been carried out. A homogeneous multiphase flow model coupled with a volume-of-fluid (VOF) method for interface capturing has been applied to determine the shapes of the gas−liquid interface and to compute the turbulent flow fields in unbaffled vessels. Turbulence is modeled using the k−ε/k−ω based shear-stress transport model (Menter, F. R. AIAA J. 1994, 32, 1598−1605) and a second-moment differential Reynolds-stress transport model. Calculations are carried out using the ANSYS CFX-5.7 CFD code (ANSYS: Canonsburg, PA, 2004). Validation of the predictions is effected against the measured free-surface profiles (Nagata, 1974; Ciofalo et al., 1996), and the mean velocity distributions (Nagata, 1974). The predicted liquid surface profiles using the VOF method in conjunction with both turbulence models are generally in good agreement with measurements. As for the mean velocity components, the Reynolds-stress transport model predictions are superior than those obtained using the shear-stress transport model.

Momentum transfer in an agitated vessel with off-centred impellers

AbstractMomentum transfer was investigated in an unbaffled agitated vessel of inner diameter 0.3 m equipped with different off-centred impellers. The distribution of the shear rate on the tank wall as a function of the impeller type and Reynolds number was studied in the turbulent regime of the Newtonian liquid flow. The dependences of the averaged dimensionless shear rate, friction coefficient, and dissipated energy on the Reynolds number and eccentricity ratio were approximated using four-parameter equations.

Behaviour of gas–liquid mixtures in an unbaffled reactor with unsteadily forward–reverse rotating impellers

AbstractThe behaviour of gas–liquid mixtures in the vicinity of the blades of an unsteadily rotating impeller in an unbaffled agitated vessel was studied by observations made with a rotating camera. The impellers used were a disk turbine impeller with six flat blades (DT) and a novel cross‐type impeller with four delta blades (CD). The behaviour of gas–liquid mixtures near the blades of the forward–reverse rotating impeller was unsteady in terms of the formation of cavities behind the blades and their dispersion into gas bubbles, and differed from that near the blades of a unidirectionally, steadily rotating impeller. The differences in relative power consumption between the forward–reverse rotating impellers in the unbaffled vessel and the steadily rotating impellers in the baffled vessel are discussed in relation to the differences in the behaviour of gas–liquid mixtures near the blades of each rotating impeller.© 2002 Society of Chemical Industry

Flow and mass transfer in aerated viscous Newtonian liquids in an unbaffled agitated vessel having alternating forward–reverse rotating impellers

Flow and mass transfer characteristics in aerated viscous Newtonian liquids were studied for an unbaffled aerated agitated vessel with alternating rotating impellers (AAVAI), ie with multiple forward–reverse rotating impellers having four delta blades. The effects of operating conditions such as gas sparging rate, agitation rate and the number of impeller stages, and the liquid physical properties (viscosity) on the gas hold-up, ϕgD, and volumetric oxygen transfer coefficient, kLaD were evaluated experimentally. The dependences of ϕgD and kLaD on the specific total power input and superficial gas velocity differed, depending on the ranges of liquid viscosity. Empirical relationships are presented for each viscosity range to predict ϕgD and kLaD as a function of the specific total power input, superficial gas velocity and viscosity of liquid. Based on a comparative investigation of the volumetric coefficient in terms of the specific total power input between the AAVAI and conventional aerated agitated vessels (CAAVs) having unidirectionally rotating impellers, the usefulness of AAVAI as a gas–liquid agitator treating viscous Newtonian liquids is also discussed. © 2001 Society of Chemical Industry

Flow and mass transfer in aerated viscous Newtonian liquids in an unbaffled agitated vessel having alternating forward–reverse rotating impellers

AbstractFlow and mass transfer characteristics in aerated viscous Newtonian liquids were studied for an unbaffled aerated agitated vessel with alternating rotating impellers (AAVAI), ie with multiple forward–reverse rotating impellers having four delta blades. The effects of operating conditions such as gas sparging rate, agitation rate and the number of impeller stages, and the liquid physical properties (viscosity) on the gas hold‐up, ϕgD, and volumetric oxygen transfer coefficient, kLaD were evaluated experimentally. The dependences of ϕgD and kLaD on the specific total power input and superficial gas velocity differed, depending on the ranges of liquid viscosity. Empirical relationships are presented for each viscosity range to predict ϕgD and kLaD as a function of the specific total power input, superficial gas velocity and viscosity of liquid. Based on a comparative investigation of the volumetric coefficient in terms of the specific total power input between the AAVAI and conventional aerated agitated vessels (CAAVs) having unidirectionally rotating impellers, the usefulness of AAVAI as a gas–liquid agitator treating viscous Newtonian liquids is also discussed.© 2001 Society of Chemical Industry

SOLID-LIQUID MASS TRANSFER AT THE WALLS OF A RECTANGULAR AGITATED VESSEL

Rates of mass transfer between the walls of a rectangular agitated vessel and solution were studied by measuring the limiting current of the cathodic reduction of ferricyanide ion at the vessel wall. Variables studied were rotation speed of 45° pitched blade turbine impeller, physical properties of the solution and the presence of drag reducing polymer in lhe solution (Polyox WSR-301). The data were correlated for polymer free solution by the equation: Sh= 1.925 Sc0.33 Re0.5 Comparison of the present data with previous heat and mass transfer studies conducted in cylindrical vessels has shown that the mass transfer behaviour of agitated rectangular vessels lie between baffled and unbaffled agitated cylindrical vessels. Polyox addition was found to reduce the mass transfer coefficient by an amount ranging from 5.6 to 37% depending on polymer concentration. Practical implications of using drag reducing polymers in agitated vessels were discussed.

Vergleich der Effektivität von bewehrten und unbewehrten Rührsystemen in der chemischen Verfahrenstechnik

AbstractIn spite of the fact that vortex formation, in unbaffled agitated vessels, is in most cases an undesirable phenomenon, many cases exist in chemical engineering where unbaffled systems can be used with greater effectivness than systems with baffles, at the same or even smaller energy consumption levels. Satisfactory results can also be obtained in unbaffled agitatated vessels without an air‐liquid interface. These facts, viewed in the light of energy problems in the chemical industry, show the current importance of comparative studies of baffled and unbaffled systems. In the present review article some characteristic cases from the fields of fluid dynamics, homogeneous distribution processes, suspensions, as well as mass‐transfer and heat‐transfer operations in baffled and unbaffled agitated vessels are presented and comparatively discussed in relation to their effectivness.

Vortex depth in unbaffled single and multiple impeller agitated vessels

AbstractAlthough vortex formation is usually an undesirable phenomenon in the process industry, satisfactory process conditions and results can also be obtained in unbaffled agitated vessels in the presence of a vortex. This fact and especially the low power requirements in these systems, with their immediate relevance to the energy problem in the process industry, show the true importance of vortex formation in agitated vessels. This article reviews the literature results and the correlations proposed for the prediction of vortex depth in unbaffled agitated vessels with various types of single and multiple impeller systems and presents a critical discussion on the basis of a theoretical analysis.