Baffled Tank Research Articles

Finite Element Based Viscous Numerical Wave Flume

A two-dimensional numerical wave flume (NWF) for viscous fluid flows with free surface is developed in this work. It is based on the upwind finite element solutions of Navier-Stokes equations, CLEAR-volume of fluid method for free surface capture, internal wave maker for wave generation, and sponge layer for wave absorbing. The wave generation and absorption by prescribing velocity boundary conditions along inlet and radiation boundary condition along outlet are also incorporated. The numerical model is validated against several benchmarks, including dam-breaking flow, liquid sloshing in baffled tank, linear water wave propagation and reflection from vertical wall, nonlinear solitary wave fission over sharp step, and wave-induced fluid resonance in narrow gap confined by floating structures. The comparisons with available experimental data, numerical results, and theoretical solutions confirm that the present numerical wave flume has good performance in dealing with complex interface flows and water wave interaction with structures.

A successive boundary element model for investigation of sloshing frequencies in axisymmetric multi baffled containers

This study presents a developed successive Boundary Element Method to determine the symmetric and antisymmetric sloshing natural frequencies and mode shapes for multi baffled axisymmetric containers with arbitrary geometries. The developed fluid model is based on the Laplace equation and Green's theorem. The governing equations of fluid dynamic and free surface boundary condition are also applied to proposed model. A zoning method is presented to model arbitrary arrangement of baffles in multi baffled axisymmetric tanks. The influence of each zone on neighboring zones is applied by introducing interface influence matrix which correlates the velocity potential of interfaces to their flux. By discretizing the flow boundaries, the integral equation governed on the boundary is formulated into a general matrix eigenvalue problem. The proposed method has a considerable effect on decreasing computational cost and a good accuracy in determining the sloshing natural frequencies. The obtained results for different types of container based on the application of the presented study are validated in comparison with the literature and very good agreement is achieved. Finally, the effect of baffle parameters on the sloshing natural frequencies was investigated and some conclusions are outlined.

Synthetic Batik Wastewater Pretreatment Progress by Using Physical Treatment

One of the most important manufacturing industries in Malaysia is textile and specifically batik making. Many local batik factories discharge their wastewater, which contains wax, resin, silicate and dyes, without treatment, directly to nature. A possible treatment system involving membrane filtration has been tested to remove the dyes. However the presence of wax the wastewater requires a pretreatment stage to remove the undesired wax. In this study the performance of baffle tank pretreatment (own designed and fabricated) was evaluated for 4 different types of synthetic batik wastewater, which contain wax, resin, sodium silicate and 4 different fibers reactive dyes namely, Remazol Turquoise Blue G133 (Blue 21), Remazol Red 194 and Remazol Yellow 14 and Reactive black 5. The removal efficiency (%), pH and COD of each sample were evaluated and recorded. Wax removal efficiency in all samples was more than 88%. The COD and pH of all samples decreased after 60 minutes pretreatment cycle.

How Bromate and Ozone Concentrations Can be Modeled at Full-Scale Based on Lab-Scale Experiments – A Case Study

This article presents a full-scale modeling study of an industrial ozonation unit for practical application. The modeling framework combines an integrated hydraulic model (systematic network) with a quasi-mechanistic chemical model. Dealing with natural water, the chemical model has to be parameterized, and the parameters calibrated. This was done based on lab-scale experiments. The calibration results showed that the chemical model is able to account for changes in contact time with ozone, pH, temperature, ozone dose, NOM concentration, bromide concentration. Comparison of residence time distributions showed that the hydraulic model accurately reproduces flow conditions. Six sampling points were installed along an industrial ozonation unit of 487 m3 consisting of two baffled tanks in series. Bromate and ozone concentrations were monitored under varying operational process conditions. After the selection of a value for the kLa, simulations were run. Using the lab-scale calibrated models, simulated and experimental data were found in close agreement: 84% of the simulated concentrations for ozone matched measurements (±experimental error), 60 % for bromate. A readjustment of the kinetics of a single reaction (out of 65) showed that seasonal changes in NOM activity may easily be taken into account based on regular concentration measurements (90% of the bromate concentrations were then modeled accurately).

CFD simulations of dense solid–liquid suspensions in baffled stirred tanks: Prediction of the minimum impeller speed for complete suspension

In the literature on mechanically agitated solid–liquid systems, several methods are described to estimate the minimum impeller speed Njs at which all particles are suspended, but few studies have been devoted so far to their critical comparative assessment [67].In the present paper, several alternative Njs prediction methods are applied to CFD results obtained for selected test cases covering a broad range of suspension conditions and impeller speeds. Results are compared with one another and with classic empirical correlations [88]. The aim of the work is to assess the adequacy of different methods for predicting Njs and, more generally, to contribute to a viable CFD-based strategy for the design of solid–liquid mixing equipment.To this purpose, transient RANS simulations using the sliding grid algorithm were carried out. An Unsuspended Solids Criterion (USC) was introduced to judge whether the solids contained in a generic control volume should be regarded as suspended or unsuspended. Based on this criterion, the concept of impeller speed for sufficient suspension, Nss, was proposed. The results suggest that it may be convenient to base the design of solid–liquid contactors on the sufficient suspension speed Nss rather than on the traditional Njs concept.

Experimental study on gas–liquid–liquid macro-mixing in a stirred tank

In this paper, experimental data on the mixing time of the continuous phase and power consumption of gas–liquid–liquid dispersions in a mechanically agitated baffled tank are presented. The electrical conductivity method is taken for the measurement of mixing time and the shaft-torque method for power consumption measurement. Tap water is used as the continuous phase, and kerosene and air as the dispersed ones. The effects of probe/tracer injection position, agitation speed, type of impeller, clearance of impeller off tank bottom, oil volume fraction, gas holdup and physical properties of the dispersed liquids on the macro-mixing of the gas–liquid–liquid system have been investigated. The phenomenon of gas–liquid–liquid macro-mixing in a stirred tank is largely similar to that of liquid–liquid and gas–liquid stirred tanks. Our experiments indicate that the gas–liquid–liquid macro-mixing can be enhanced at higher gas holdups while damped at low gas holdups. Contrary to gas effect, the dispersed oil phase at low holdups increases the macro-mixing intensity but at higher holdups decreases the macro-mixing intensity of the continuous phase. The experimental results show that axial impellers are more energy efficient for gas–liquid–liquid macro-mixing than radial impellers. A simple correlation is developed for predicting the mixing time in gas–liquid–liquid three-phase systems and satisfactory agreement with experimental data is observed.

Direct numerical simulation of the turbulent flow in a baffled tank driven by a Rushton turbine

AbstractWe present a direct numerical simulation (DNS) of the turbulent flow in a baffled tank driven by by a Rushton turbine. The DNS is compared to a Large Eddy Simulation (LES), a Reynolds Averaged Navier‐Stokes (RANS) simulation, Laser Doppler Velocimetry data, and Particle Image Velocimetry data from the literature. By Reynolds averaging the DNS‐data, we validate the turbulent viscosity hypothesis by demonstrating strong alignment between the Reynolds stress and the mean strain rate. Although the turbulent viscosity νT in the DNS is larger than in the RANS simulation, the turbulent viscosity parameter Cμ = νTϵ/k2, is an order of magnitude smaller than the standard 0.09 value of the k‐ϵ model. By filtering the DNS‐data, we show that the Smagorinsky constant CS is uniformly distributed over the tank with CS ≈ 0.1. Consequently, the dynamic Smagorisnky model does not improve the accuracy of the LES. © 2012 American Institute of Chemical Engineers AIChE J, 2012

Energy efficient solids suspension in an agitated vessel–water slurry

Power consumption required to suspend water–solid slurries in a mechanically agitated tank has been studied over a wide range of design and solids conditions with the goal to improve the agitation energy efficiency. It is demonstrated that the power required for providing off-bottom solids suspension and solids dispersion could be reduced dramatically with baffles removed, as in comparison with conventional agitator designs where vertical baffles were used.Axial-flow impellers in baffled tanks were found more energy efficient to suspend solids off the bottom, consistent with the literature data. However, with baffles removed, radial-flow impellers were found more energy efficient than axial flow impellers. The effect of removal of baffles on the solids dispersion was also included. The impeller geometrical effects include pitch angle of blade, number of blades on energy efficiency were studied. The impacts of solid concentration and particle size on power consumption were also investigated.

A reduced order model for liquid sloshing in tanks with flexible baffles using boundary element method

SUMMARYIn order to study the interaction of sloshing and structural vibrations of baffled tanks, a reduced order model based on modal analysis of structure model and boundary element method for fluids motion is developed. For this purpose, the governing equations of elastic structure and incompressible flow are used to derive simple models to simulate both fields. Using the modal analysis technique, the structural motions are applied to the fluid model and on the other hand by using boundary element method, the fluid loads are applied to the structural model. Based on this formulation, a code is developed which is applicable to an arbitrary elastic tank with arbitrary arrangement of baffles. The obtained results are validated with literature data and then the effects of baffle flexibility on the sloshing frequencies and structural vibration frequencies are investigated. Copyright © 2011 John Wiley & Sons, Ltd.

Numerical simulation of turbulent flow in a baffled stirred tank with an explicit algebraic stress model

An explicit algebraic stress model (EASM) was used to simulate anisotropic turbulent flows in baffled stirred tanks equipped with a standard Rushton turbine. The quantitative predictions of velocity components, turbulence kinetic energy, Reynolds stresses and turbulence energy dissipation rate in the context of anisotropic turbulence were conducted to assess the comprehensive performance of the EASM. A lot of efforts have been made to ensure numerical stability during the calculations such as using a good initial flow field, manipulating source terms and adjusting under-relaxation factors. The predicted results were also compared with experimental data and other simulation results obtained using the standard k– ε model, algebraic stress model (ASM), Reynolds stress model (RSM) and large eddy simulation (LES). All the simulations were run with in-house codes. The simulation results show that agreement between the EASM predictions and experimental values is satisfactory. The EASM is consistently superior to the standard k– ε model when predicting both peak values and trend of variation in velocities and turbulence quantities. In comparison to the RSM, the EASM has almost the same predictive accuracy. The EASM is inferior to the LES on the prediction of turbulence kinetic energy. Nevertheless, the computational cost of the EASM is significantly lower than that of the LES, which is an obvious advantage in practical applications.

CFD simulations of dense solid–liquid suspensions in baffled stirred tanks: Prediction of suspension curves

Mixing of solid particles into liquids within contactors mechanically agitated by stirrers is a topic of primary importance for several industrial applications. A great research effort has been devoted to the assessment of the minimum impeller speed ( N js ) able to guarantee the suspension of all particles. Conversely, only little attention has been paid so far to the evaluation of the amount of solid particles that are suspended at impeller speeds lower than N js . In some cases the loss in available interfacial area between particles and liquid could be reasonably counterbalanced by a decreased mechanical power, making it of interest to evaluate the percentage of suspended solids at different impeller speeds < N js in order to quantify the possible economical advantage of adopting an incomplete suspension (filleting) regime inside the stirred tank. The present work deals with computational fluid dynamics simulations of dense solid–liquid partial suspensions in baffled stirred tanks and particularly focuses on the prediction of the amount of suspended particles at agitation speeds encompassing both the filleting and the complete suspension regime. An Eulerian–Eulerian Multi Fluid Model coupled with a standard k– ɛ turbulence model for the continuous (liquid) phase only was adopted for CFD simulations. Both the Sliding Grid (SG) and the Multiple Reference Frame (MRF) approaches were employed to simulate the impeller-tank relative rotation. Different turbulence corrections to the fluid-particle drag correlation were considered. Experimental evidence was used for validation purposes: data collected by using the Pressure Gauge Technique (PGT) [1] and snapshots of the simulated tank. Comparison between CFD predictions and all experimental data showed a satisfactory agreement; for large particle diameters the drag coefficient correlation due to Pinelli et al. [2] gave the best results. The influence of the impeller motion treatment was found to be negligible.

An assessment of different turbulence models for predicting flow in a baffled tank stirred with a Rushton turbine

This work presents a comprehensive study of different turbulence models, including the k– ε, SST, SSG–RSM and the SAS–SST models, for simulating turbulent flow in a baffled tank stirred with a Rushton turbine. All the turbulence models tested predict the mean axial and tangential velocities reasonably well, but under-predict the decay of mean radial velocity away from the impeller. The k– ε model predicts poorly the generation and dissipation of turbulence in the vicinity of the impeller. This contrasts with the SST model, which properly predicts the appearance of maxima in the turbulence kinetic energy and turbulence energy dissipation rate just off the impeller blades. Curvature correction improves the SST model by allowing a more accurate prediction of the magnitude and location of these maxima. However, neither the k– ε nor the SST model is able to properly capture the chaotic and three-dimensional nature of the trailing vortices that form downstream of the blades of the impeller. In this sense, the SAS–SST model produces more physical predictions. However, this model has some drawbacks for modelling stirred tanks, such as the large number of modelled revolutions required to obtain good statistical averaging for calculating turbulence quantities. Taking into consideration both accuracy and solution time, the SSG–RSM model is the least satisfactory model tested for predicting turbulent flow in a baffled stirred tank with a Rushton turbine.

Numerical Investigation of the Effect of Impeller Design Parameters on the Performance of a Multiphase Baffle‐Stirred Reactor

AbstractThe turbulent gas‐liquid flow field in an industrial 100‐m3 stirred tank was calculated by using computational fluid dynamics based on the finite‐volume method. Turbulent effects were modeled with the shear stress transport model, and gas‐liquid bubbly flow was modeled with the Eulerian‐Eulerian approach using the Grace correlation for the drag force interphase momentum transfer. The relative motion between the rotating impeller and the stationary baffled tank was considered by using a multiple frames of reference algorithm. The effects of Rushton and pitched‐blade impeller design parameters such as blade geometry, location, and pumping direction on the mixing performance were investigated. It was found that a combination of Rushton turbines with up‐pumping pitched‐blade turbines provides the best mixing performance in terms of gas holdup and interfacial area density. The approach outlined in this work is useful for performance optimization of biotechnology reactors, as typically found in fermentation processes.

Boundary element analysis of liquid sloshing in baffled tanks

The paper concerns the natural frequencies and mode shapes of liquid sloshing in three dimensional baffled tanks with arbitrary geometries. The liquid is considered to be inviscid and incompressible and amplitudes of oscillations are assumed to be small. The tank bottom and the baffle are treated as rigid. The boundary element method is used to solve the considered problem. Triangular curvilinear 6-node boundary elements are applied. In the present formulation is not necessary to introduce the zoning method, because the baffles are treated as double layers immersed in liquid. After discretization the problem is formulated as the standard eigenvalue problem, which is limited to free surface degrees of freedom only. Several examples are given to verify the proposed method.

Dense Solid-liquid Suspensions in Top-covered Unbaffled Stirred Vessels

In the present work solids suspension is investigated in an unbaffled tank stirred by a Rushton turbine and provided with a top-cover in order to avoid the formation of the well known central vortex. The data obtained are compared with those pertaining baffled stirred tanks via comparison with the well known Zwietering’s correlation. The dependence of Njs on particle concentration is found to be similar to that well established for baffled vessels, while, as a difference from the latter, Njs is found to decrease when liquid viscosity increases. Results also show that Njs is substantially independent of particle size, a feature that may advice the adoption of unbaffled tanks when large heavy particles are to be dealt with. On the other hand, a smaller exponent for the scale-up rule is found with respect to baffled tanks, which implies the need for larger specific power consumptions the larger the vessel size and may limit useful applications to small to medium plant sizes. Finally, a correlation akin to Zwietering’s correlation is proposed for top-covered unbaffled stirred tanks.

Experimental Study on Liquid–Liquid Macromixing in a Stirred Tank

In this paper, the experimental data on the mixing time and power consumption of two immiscible liquids in a mechanically agitated baffled tank are presented. The electric conductivity method was taken for the measurement of the mixing time and the shaft-torque method for the power consumption measurement. Tap water was used as the continuous phase and kerosene the dispersed phase. The effects of the agitation speed, type of impeller, clearance of the impeller off the tank bottom, volume fraction of the dispersed phase, physical properties of the liquids, and probe position on the macromixing of the liquid–liquid system were studied. The phenomena of macromixing are largely similar to those of single-liquid and gas–liquid stirred tanks. The experiment indicates that the flow field and turbulence can be dampened at high volume fraction of the dispersed phase while enhanced at low percentage. The mixing time becomes longer with increasing viscosity of the dispersed phase. The results show that the pitched blade turbine downflow is more efficient for macromixing than the others tested in this work.

Investigations of Mixing Time Scales in a Baffled Circular Tank with a Surface Aerator

Abstract The oxygen transfer rate is a parameter that characterizes the gas-liquid mass transfer in surface aerators. Gas-liquid transfer mech-anisms in surface aeration tanks depend on two different extreme lengths of time; namely, macromixing and micromixing. Small scale mixing close to the molecular level is referred to as micromixing; whereas, macromixing refers to mixing on a large scale. Using experi-mental data and numerical simulations, macro- and micro-scale parameters describing the two extreme time scales were investigated. A scale up equation to simulate the oxygen transfer rate with micromixing times was developed in geometrically similar baffled surface aerators. Keywords: Energy dissipation, Macromixing, Micromixing, Oxygen transfer rate, Surface aerators, Theoretical power per unit volume 1. Introduction Surface aeration is an important operation in chemical and other processing industries. Surface aeration is defined as the aeration or oxygen transfer that takes place at the gas-liquid surface when the liquid is agitated. The objective of studying aeration process was to interpret the laboratory result for the development of a field installation. This requires a geometrical similarity condition; that is to say the field installation should be built on a definite geometric ratio to that of the laboratory setup. Under a geometric similarity condition, an effective scale-up mandates an awareness of the relative importance of various process parameters at different levels of scrutiny.Inside a surface aeration tank, different scales of mixing are present, with regions where one of the scales of mixing will pre-vail, i.e., either macromixing or micromixing. Macromixing is mixing driven by the largest scales of motion in the fluid. Mi-cromixing prevails near the impeller; where small eddies define the velocity gradients surrounding the bubbles. According to Kolmogorov’s theory, the power per unit volume determines the energy and size of the turbulent eddies, which has traditionally been used in the design and scale up of equipment [1-4].Macromixing prevails adjacent to the micromixing region, which determines the internal homogenization, movement of bubbles across the reactor and flow pattern when the turbulent eddies are tank-sized. The contribution of the surface aeration of a tank, as well as the effect of the geometry of the impeller and the tank to the mass transfer the depend on this scale of mixing [1, 4-8].Unlike macromixing, which is associated with large-scale fluid motions that can be monitored directly through physical property measurements, micromixing deals with diffusive mix-ing at the molecular level, for which only indirect methods are available. Despite the availability of much literature on the mix-ing length scale, mixing scale behavior of surface aerators is typ-ically missing. Recently, Rao and Kumar [9] studied the micro-mixing behaviors of unbaffled surface aerators. However, baffled type surface aeration systems are mostly preferred in the mixing and chemical industry. Baffles are flat vertical strips set radi-ally along the tank wall, which avoid vortex formation. A baffled tank provides a better concentration distribution throughout the tank and; therefore, improved mixing efficiency is achieved. Thus, this study aimed to understand the prevalent mixing scale in baffled circular surface aeration systems and obtain scale-up or simulation equations for different mixing scales.

Solids suspension with angle‐mounted agitators in unbaffled vessels

AbstractSolids suspension using an angle‐mounted axial‐flow impeller in an unbaffled tank relies on proper placement of the impeller in the vessel such that swirling flow does not occur. Even with proper placement, the just‐suspended speed of an angle‐mounted agitator is often 75% higher than with vertical mounting on the centreline in a baffled tank, leading to torque and power requirements that average more than twice and five times higher, respectively. Angle‐mounted turbulent power numbers of axial‐flow impellers are generally similar to, but lower than the corresponding power numbers with vertical mounting on the centreline of a baffled tank.

Comparison of Power Number for Paddle-Type Impellers by Three Methods

Power numbers in a baffled stirring tank with paddle-type impellers were measured over a range of Reynolds numbers, from laminar- to turbulent-flow regions. The impellers studied included a two-blade flat-paddle impeller, a 45° two-blade pitched-paddle impeller, a 45° four-blade pitched-blade turbine, and a two-stage pitched-blade turbine. The impeller-diameter-to-tank-diameter ratio was 0.5 to 0.6. The measured power numbers were compared with those derived from two other methods: prediction by using correlations and computational fluid dynamics (CFD). Power number correlations showed that the values estimated from the empirical correlations proposed by Nagata agreed closely with those measured in the laminar-flow region but deviated significantly in the transitional- and turbulent-flow regions. The power numbers derived from the correlation proposed by Kamei and Hiraoka agreed well with the experimental results in the laminar- and turbulent-flow regions. The power number in a stirring tank was also simulated by the CFD method. The numerical results showed satisfactory agreement with the experimental data over a wide range of Reynolds numbers.

Large-eddy simulation of turbulent flow in a stirred tank driven by a Rushton turbine

Large-eddy simulation (LES) of mixing process in a baffled tank was presented. The impeller rotation was modeled using the sliding mesh technique. In this study the CFD code was used for simulation of a standard vessel agitated by a 6-blade Rushton turbine and results were evaluated in terms of the predicted flow field, power number, mean velocity components, mixing time, turbulent kinetic energy and turbulent dissipation rate using published experimental data. Subsequently, the effects of varying injection position of the passive scalar have been investigated. The results show that LES is a reliable tool to investigate the unsteady behavior of the turbulent flow in stirred tank.