Baffled Vessel Research Articles

Effect of an Inclined Slots on the Power Consumption and Vortices Size in a Rushton Turbine Agitated Tank

Mechanical agitation in baffled vessels with turbines plays a vital role in achieving homogeneous fluid mixing and promoting various transfer operations. Therefore, designing vessels with optimal energy efficiency and flow dynamics is essential to enhance operational performance and eliminate flow perturbations. Hence, the present research focuses on a numerical investigation of the impact of inclined slots with different angles installed at the sidewall of a cylindrical vessel equipped with a Rushton turbine. This study explores power consumption and vortex size while considering various rotation directions of the impeller with different rotation speeds. The numerical simulations are conducted for Reynolds numbers ranging from 104 to 105, using the RANS k-ε turbulence model to govern the flow inside the stirred vessel, accounting for mass and momentum balances. The results have shown that the installation of slots reduces power consumption and vortex size compared to conventional vessel configurations. Moreover, increasing the slot angle from 0 to 32.5° further reduces energy consumption and vortex size, especially with negative rotation speeds. On the other hand, increasing the Reynolds numbers leads to a decrease in power consumption and an increase in vortex size. The present research therefore proposes a design for constructing Rushton-turbine stirred vessels offering optimal operation, characterized by reduced energy consumption and minimized vortex size.

Large blade impeller application for turbulent liquid–liquid and solid–liquid mixing

AbstractApplication of large blade impellers to turbulent single‐phase and two‐phase mixing is investigated in this work in order to provide a quantitative basis for estimating the possible advantages in industrial mixing operations with respect to fast impeller types. The analysis is based on the discussion of three‐dimensional velocity fields collected in a single‐phase vessel stirred by a Maxblend impeller by stereoscopic particle image velocimetry and of dispersed phase distribution and liquid mixing time obtained in solid–liquid and liquid–liquid systems by electrical resistance tomography. The results highlight that turbulent two‐phase mixing can be efficiently performed in baffled vessels stirred by large blade impellers both in shear‐rate controlled and bulk‐motion controlled processes.

Characterisation of solid hydrodynamics in a three-phase stirred tank reactor with positron emission particle tracking (PEPT)

It is challenging to measure the hydrodynamics of stirred tank reactors when they contain multiphase flows comprising liquid, gas bubbles and particles. Radioactive particle tracking techniques such as positron emission particle tracking (PEPT) are the only established techniques to determine internal flow behaviour due to the inherent opacity and density of fluid and the vessel walls. The profiles of solids flow are an important tool for robust reactor design and optimisation and offer insight into underlying transport processes and particle–fluid–bubble interactions for applications such as froth flotation. In this work, measurements with PEPT were performed with two tracer particles differing in surface hydrophobicity to characterise the solids hydrodynamics in a baffled vessel agitated with a Rushton turbine. The location data from PEPT were averaged with time to estimate the probability density function (PDF) of particle velocity in individual voxels. The peaks of these voxel distributions were used to produce profiles of solids flow in different azimuthal and horizontal slices. Bimodal vertical velocity distributions were observed in the impeller radial jet which suggest the particles experienced trajectory crossing effects due to inertia. Statistical tests were performed to compare the velocity distributions of the hydrophilic and hydrophobic tracer particles, which indicated similar average flow behaviour in the liquid or pulp phase of the vessel and differences near the air inlet, in the impeller discharge stream and pulp–froth interface. With tracers designed to represent gangue and valuable mineral species, the differences in velocity reveal interactions such as bubble–particle attachment and entrainment. • Positron emission particle tracking (PEPT) measurements of multiphase flow. • Solid hydrodynamics in a three-phase stirred tank reactor of gas, liquid and solids. • Comparison of hydrophilic and hydrophobic particle flow behaviour. • Observations of particle interaction with the gas bubble flow field.

Discharge liquid flow produced by closed impeller with flat blades for agitation

For a baffled vessel agitated by turbine type impellers with six flat blades, the liquid flows discharged by the impellers were measured using the particle tracking velocimetry (PTV). The impellers were used in the two configurations for the blades to be open and closed. For the the two configurations of impellers, the time-averaged flow with induction of a secondary flow and the turbulence in the flow were evaluated and the discharge flow characteristics were compared. The external flow of the closed impeller has an increased inflow from the surrounding of discharge region and a decreased turbulence downstream near the baffles.

CFD simulations of early- to fully-turbulent conditions in unbaffled and baffled vessels stirred by a Rushton turbine

Laboratory scale unbaffled tanks provided with a top cover and a baffled tank both stirred by a Rushton turbine were simulated by carrying out RANS simulations. Three different turbulence models were adopted (k-ωSST, k-ε and the SSG Reynolds stress model) to predict the flow field and the relevant performance parameters (power and pumping numbers) of the tank operated from early to fully turbulent conditions. CFD results were compared with literature experimental data and DNS simulation results to validate and properly compare the models. In the range of Reynolds numbers investigated, results showed that, for the unbaffled tank, the SSG model based on Reynolds stresses is a better choice at larger Re, while the k-ω SST model better reproduces the experiments at lower values. Conversely, no significant differences between the predictions of the three models were found in the baffled vessel.

The Liquid–Liquid Dispersion Homogeneity in a Vessel Agitated by a High-Shear Sawtooth Impeller

The agitation of immiscible liquids or solid suspensions is a frequent operation in chemical and metallurgical industries. The product quality yield and economy of the processes are significantly affected by mixing conditions. Prediction of mean drop size distribution (DSD) during agitation is fundamental for processes in many branches of industry where the mass transfer is crucial. This contribution aims to analyze the homogeneity of a dispersed system in a vessel agitated by a high-shear sawtooth impeller. The homogeneity of liquid–liquid dispersion is determined by comparison of Sauter mean diameters and drop size distribution (DSD) from different measured regions and for various dispersion times. The experiments were carried out in a baffled vessel for various impeller speeds. The sizes of droplets were obtained by the in-situ measurement technique and by the image analysis (IA) method.

Experimental Analysis of Conditions of Gas-liquid-floating Particles System Production in an Agitated Vessel Equipped with Two Impellers

The characteristic flow patterns of the three-phase gas-floating particles-liquid system produced in a baffled agitated vessel with two high-speed impellers on the common shaft were experimentally determined. The minimum impeller speeds adequate to obtain loading, complete dispersion and just drawdown (of floating particles) flow patterns for such multiphase system were identified. The results confirmed that there is influence of gas flow rate Vg and particles concentration X on the value of minimum impeller speeds for each flow model (nJD, nL, nCD, nJDG) and gas hold-up ( in analyzed systems.

Dispersion kinetics in mechanically agitated vessel

Agitation of two immiscible liquids or solid-liquid suspension is a frequent operation in chemical and metallurgical industries. Prediction of mean drop/particle size and drop/particle size distribution (DSD) is vital for emulsification, suspension polymerization, solid particle dispersion or crystallization. Simulation of particulate systems requires the knowledge of DSD and its time evolution. The time evolution of drop size distribution was investigated in baffled vessel mechanically agitated by a Rushton turbine and a high-shear tooth impeller. The system water –silicone oil was used as a model liquid. The volume fraction of the dispersed phase was 0.047 %. The drop sizes were determined by image analysis. The time evolution of the drops size dp32 was studied for both impellers tested. The model used involves the first order kinetics. Finally, the following correlations predicted by the Kolmogorov-Hinze theory were evaluated at steady state: dp32/D = C1.We-0.6 and dpmax/D = C2.We-0.6, where We is the impeller Weber number.

Influence of rheological properties of stirred liquids on the axial and tangential forces in a vessel with a PMT impeller

Based on literature information, the role of both tangential and axial forces in stirred liquids on the formation of circulation regions is discussed. Rheological measurements of seven applied liquids show that five liquids are pure Newtonian and two other liquids exhibit shear-thinning rheology. Those liquids are used in experiments in a baffled vessel stirred by a Prochem Maxflo T (PMT) type impeller in a broad range of its rotational speed. Both the tangential and axial forces, exerted by the impeller on the stirred liquids, are calculated on the basis of measured values of torque and apparent mass change of the stirred tank for the studied range of agitator speed. The total, resultant force is also calculated. A significant impact of the stirred liquid viscosity on the relationship of the three forces vs. the impeller power consumption is determined. Relationships of the three forces on the impeller power are also analysed and relevant correlation equations are proposed.

APPLICATION OF THE TEMPERATURE OSCILLATION METHOD IN HEAT TRANSFER MEASUREMENTS AT THE WALL OF AN AGITATED VESSEL

The Temperature Oscillation Infra-Red Thermography (TOIRT) was used to measure convective heat transfer coefficients at the inner vertical wall of an agitated and baffled vessel. Two impellers were used: an axial six-blade impeller with pitched blades and the Rushton turbine. The TOIRT method represents an indirect method based on measuring the phase shift between the oscillating heat flux applied to one side of the heat transfer surface and the wall temperature response monitored by a contactless infra-red camera at the same side. On the basis of this phase shift, the TOIRT method can indirectly evaluate the heat transfer coefficient on the other side of the heat transfer surface. Two forms of experimental results are presented in this paper. The first one describes graphical dependencies of the local Nusselt number on the dimensionless distance from the vessel bottom for various Reynolds numbers. The second form describes the mean Nusselt number along the wall as a function of the Reynolds number.

Effect of mixing conditions on the wet preparation of ceramic foams

Ceramic foams are a promising alternative to conventional catalyst supports due to their macro-porosity, which should enhance mass transport properties during reactions. Whilst direct foaming is a straightforward production method, the use of kitchen mixers commonly reported in the literature to initially froth the ceramic slurry limits understanding of scale-up. This study reports a systematic experimental investigation of the impact of mixing parameters on the properties of the foams produced in an agitated baffled vessel of diameter, T=175mm, equipped with an up-pumping pitch blade turbine with diameter of either D=0.23T or 0.51T and a bottom round sparger with a diameter of 45mm. The flow conditions in the present study were in the low to mid transitional regime (50<Re<1000). Design of Experiments (DoE) was employed to generate a series of screening experiments by variation of sparging time, air flow rate, impeller speed and impeller diameter. The mixing behaviour was described as a function of relevant dimensionless groups (Re, Fr, Flg, etc.) whilst the gas–liquid flow regime was estimated by examination of a ceramic particles free system. The properties of the foams obtained were correlated with key dimensionless numbers, though the exponents obtained deviated from values in the published literature. In addition, the rheology of the foam was correlated to the bubble size distribution showing that rheology measurements have potential for at-line measurement to control the structure of the produced material.

Mixing Performance of Counter-Axial Flow Impeller using Computational Fluid Dynamics

In this study, turbulent flow fields in a baffled vessel stirred by counter-axial flow impeller have been investigated in comparison to the Rushton turbine. The resultant turbulence was numerically predicted using computational fluid dynamics (CFD). Turbulence models were developed in ANSYS Fluent 18.1 solver using the Navier-Stokes equation with the standard k-ε turbulence model. The Multiple Reference Frame (MRF) approach was used to simulate the impeller action in the vertical and horizontal planes of the stirred fluid volume. Velocity profiles generated from the simulations were used to predict and compare the performance of the two designs. To validate the CFD model, the simulation results were compared with experimental results from existing work and a satisfactory agreement was established. It was concluded that the counter-axial flow impeller could provide better turbulence characteristics that would improve the quality of mixing systems.

Suspension of solid particles in vessels agitated by Rushton turbine imperllers

The “just suspended” condition is an important criterion for the design of agitators in solid−liquid mixing processes. This criterion is met when all the particles are in motion on the base of the vessel but not necessarily uniformly distributed throughout the liquid. Grenville et al. (2015) have proposed a new correlation for predicting the just suspension speed of axial flow impellers in standard, baffled vessels with dished bases. It is based on the physical model proposed by Davies (1986) and correlates data taken at three scales over a wide range of physical properties.Many previous workers have measured the just suspension speed for Rushton turbines, which produce a radial primary flow, and forced their data to the correlation proposed by Zwietering (1958) but without testing the statistical fit. The applicability of the Davies (1986) model and Grenville et al. (2015) correlation to radial flow impellers was recognized as an outstanding question and it is addressed in this paper.Chapman (1981) measured the just suspension speed for Rushton turbines at five vessel scales in standard baffled vessels with flat bases under gassed and ungassed conditions. The ungassed data have been successfully fitted to the Grenville et al. (2015) correlation but with a change in geometrical dependencies. The just suspension speed is independent of impeller clearance above the vessel base but strongly dependent on the impeller to vessel diameter ratio. These geometric effects are very different to the ones observed for axial impellers in dished base vessels and this suggests that further work should be carried out to reconcile these differences.

Effect of number of branches on the performance of fractal impeller in a stirred tank: Mixing and hydrodynamics

Number of blades of an impeller, impeller design and its positioning in a stirred tank is known to affect the mixing in a stirred tank reactor. The extent of non-uniformity in mixing from conventional impellers can be reduced significantly using a space-filling impeller like a fractal impeller. In this work, we report the effect of number of branches (and hence the number of blades) of fractal impeller on power consumption, mixing and hydrodynamics. Velocity measurements were carried out using ultrasonic velocity profiler (UVP). Measurements showed that the performance of fractal impeller with different configuration is equivalent, however, better than standard impellers in terms of mixing achieved per unit power consumption. No significant difference was observed in radial and axial mean velocity profiles for three different configurations. However, the tangential velocity was found higher for four branches than two and three branches FI. Two distinct circulation loops were observed in upper as well as lower half of the vessel in r–z plane. Strong tangential flow throughout the baffled vessel helps to achieve good mixing even at low rotational speeds.

Mixing studies in unbaffled stirred tank reactor using electrical resistance tomography

Unbaffled stirred tanks are extensively used in chemical process industries for variety of applications including semi-batch reactions. In un-baffled stirred tanks, impeller rotation generates a vortex and reactants are added into this vortex. There is a growing interest towards understanding the mixing performance of such unbaffled stirred vessels. The present work is aimed at providing experimental results on mixing time and solid particle distribution inside an unbaffled vessel using electrical resistance tomography (ERT). A methodology for using ERT for characterizing vortex and mixing in unbaffled stirred vessel was established. The ERT was used to measure the mixing time with and without solid particles (glass beads, 250μm) in a stirred reactor. In this study, ERT technique was effectively applied for imaging solid–liquid flow and developed suitable data processing methodologies. It observed that estimated liquid phase mixing time for Un-baffled vessel was more as compared to baffled vessel. Radial solid concentration profiles showed Gaussian distribution inside the vessel. The presented methodology of using ERT and experimental results will be useful for designing and estimating mixing and solid distribution in unbaffled stirred tanks.

Study of fluid flow in baffled vessels stirred by a Rushton standard impeller

The paper deals with an investigation of velocity field in a tank stirred by a standard Rushton impeller located in the middle of the tank height. Time-resolved PIV method was used to measure the angle-resolved velocity field near the impeller blades in horizontal and vertical planes as well. Based on a simplified criterion λ2 < 0 the location, size and strength of trailing vortices behind the blades were determined. The measurements were carried out in two tanks of diameters T = 0.3 and 0.4 m and at mixing Reynolds numbers in a range 3.3–4.9 × 104. Simultaneously a detached eddy simulation (DES) was performed for the tank diameter T = 0.3 m and rotation speed 200 rpm (ReM = 3.3 × 104). The simulation results are compared with the experimental data.

Suspension of solid particles in vessels agitated by axial flow impellers

In the design of agitators for solid–liquid mixing processes there are two important criteria. The first is “just suspended” where all the particles are in motion on the base of the vessel but not necessarily uniformly distributed. The second is “uniform distribution” where the particles are distributed throughout the liquid phase so that, at each axial location within the slurry, the solids concentration is very close to the calculated average.The correlation proposed by Zwietering (1958. Chem. Eng. Sci. 8, 244) has been widely used for estimating the “just suspension” speed, NJS, and is recommended for agitator design in the Handbook of Industrial Mixing (Atiemo-Obeng et al., 2004. Solid-liquid mixing. In: Paul, Atiemo-Obeng, Kresta (Eds.), The Handbook of Industrial Mixing. John Wiley & Sons, Hoboken, NJ, p. 558). However, there is significant evidence that the correlation does not properly account for the effect of key variables, including liquid viscosity, density difference, and scale. This is likely related to the correlation's lack of physical basis and development using simple dimensional analysis, along with the fact that it was based on limited data taken over a narrow range of physical properties and scales. Another problem with the correlation is that in order to use it the value of a geometric specific constant must be known.This paper reports data taken at three vessel scales and over a wide range of liquid phase viscosities using axial and mixed flow impellers in standard baffled vessels with torispherical or dished bases. The model used to correlate the data is based on the work of Davies (1986. Chem. Eng. Proc. 20, 175). Analysis shows that there are two regimes; one where the large and medium sized particles interact with turbulent eddies in the inertial sub-range and a second where smaller particles interact with eddies approaching the viscous sub-range. The correlation developed for the inertial sub-range mechanism can be rearranged to show that the appropriate scale-up rule for the just suspension speed is constant power input per unit mass.

PEPT Validation of a CFD-DEM Model of a Fine Quartz Particle (60 μm) Behaviour in Stirred Water

A computational fluid dynamics (CFD) simulation of a Rushton impeller mixing water inside a baffled vessel was used to input steady state fluid drag forces into a discrete element method (DEM) simulation of a fine quartz particle of 60μm freely moving inside the vessel. To validate the simulated behaviour of the particle, a modular positron emission particle tracking (PEPT) camera was used to track a real quartz particle of a diameter between 53 and 63μm. The particle used as the tracer was created from a 1180μm particle labelled with 18F radionuclide inside a cyclotron beam. A comparison of typical trajectory and velocities of the particle for both the model (CFD-DEM) and the measurement (PEPT) is presented. The results show that the simulation approach used is suitable to represent the behaviour of small particle of this type in water like fluid as similar trajectories and velocities are present.

G1700103 クローズド平羽根回転翼撹拌槽における液相混合

G1700103 クローズド平羽根回転翼撹拌槽における液相混合

Minimum rotation speed to prevent coning phenomena in compendium paddle dissolution apparatus

The purpose of the present study was to investigate the applicability of the Zwietering equation to coning phenomena which often occur during dissolution testing. The minimum rotation speed at which coning phenomena disappeared (no coning rpm, NCrpm) was experimentally determined for various particle and fluid properties in a compendium paddle apparatus with a round-bottom unbaffled vessel. The particle size, relative density and kinematic viscosity exponents in the Zwietering equation were optimized for NCrpm. The particle size and relative density exponents were found to be similar with those for the general tank configurations of cylindrical flat-bottom baffled vessels. However, the kinematic viscosity exponent was significantly different. The equation obtained in this study showed sufficient accuracy (r2=0.98, average error=12rpm) to estimate the occurrence of coning. The Zwietering equation was found to be applicable to the coning phenomena in the compendium paddle apparatus.