Unbaffled Vessel Research Articles

Blending in unbaffled and baffled mechanically agitated vessels – Effect of Reynolds number and liquid height

The standard vessel for mixing in the process industry is a baffled tank. This design is very common especially for low viscosity fluids. Nonetheless, unbaffled vessels are found in industry and the laboratory. The reasons given for using unbaffled vessels are primarily concerns around cleaning the vessel between batches and the possibility of stagnation zones in high viscosity fluids particularly when properties change over the processing time. In this paper we will examine the effect of baffles on the blend time using one vessel: with and without baffles. The blend time was measured using the iodine color change method using a range of viscosities and Reynolds Numbers. The depth of the vortex was also measured. The same axial flow-down pumping impeller is used. In this work we observed that the flow patterns are dramatically different between unbaffled and baffled tanks. At high Reynolds Numbers with the baffled configuration, we see the familiar turbulent diffusion throughout the vessel. Without the baffles the fluid segregates into two zones: a swirling inner zone near the shaft and a more turbulent outer zone. The overall blend time is longer in the baffled tank at the same rotational speed compared to the baffled tank. This difference decreases with increasing Reynolds Number. We identified two blend times in using unbaffled tanks, a short blend time for the outer zone and a much longer time for the inner vortex centered zone which defines the overall blend time for the vessel. At low Reynolds Numbers the baffled blending is in the transitional flow regime between turbulent and laminar with a combination of eddies and striations. The last region of the tank to become completely mixed is located behind the baffles. Unbaffled blend times are shorter and as the Reynolds Number decreases the difference increases. The vortex is also reduced with decreasing Reynolds number. We consider this work just a beginning step in a long study and hope this work will be picked up by other researchers as happened so many times with Professor Nienow’s work.

Mixing characteristics of unbaffled bioreactor with levitating radial impeller

In the pharmaceutical and biotechnology industries, ensuring the cleanliness and sterility of the process environment is of utmost importance. To achieve this, some commercially available bioreactors are designed as unbaffled vessels with an eccentrically mounted, magnetically driven impeller. Contrary to the well-studied nature of turbulent mixing in standard baffled reactors, the mixing performance in such atypical vessels has not been thoroughly explored in the literature. This study aims to fill the knowledge gap on this subject by focusing on the mixing time in an atypical reactor using two experimental techniques, conductometry, and decolorization. The conductometric results demonstrate that the mixing times of the atypical reactor remain similar to the baffled configurations up to a height-to-diameter ratio of 1.2. Above this ratio, mixing times rapidly increase. Based on the experimental results, correlations for the design of standard and atypical vessels with small radial impellers have been proposed, also concerning the liquid height. Furthermore, the decolorization experiments in the atypical reactor revealed the formation of the segregated vortex region, in which the homogenization process takes up to four times longer period than is required for the remainder of the reactor volume. The impact of this vortex formation on the vessel mixing characteristics is discussed.

Reduced power consumption in stirred vessel with high solid loading by equipping punched baffles

Solid–liquid suspension in stirred tank is a common operation in the chemical industry. The power consumption, flow pattern and flow field instability of three systems named as unbaffled stirred tank, traditional baffled stirred tank and punched baffled stirred tank (Pun-BST) were studied by using the computational fluid dynamic analysis. Results showed that perforating holes in the baffles could reduce power consumption of mixing. Meanwhile, the punched baffle system could maintain the solids in suspension as traditional baffle system. The results also showed that the baffles could increase the “effective flow” of stirred tank even though the whole velocity of the vessel is lower than un-baffled vessel. In addition, both the solid–liquid suspension and “effective flow” were related to instability of the flow field. Perfect solid–liquid suspension results always along with obvious instability of the flow field. But, the strengthening effect of punched baffle on flow field instability mainly happened in the near-wall area. It’s because the collision and aggregation among sub-streams induced by holes intensified the unstable fluid flow. On the whole, the Pun-BST system provided much better mixing characteristics and recommended to apply in the industrial process.

A novel ANN-CFD model for simulating flow in a vortex mixer

The study presents a novel ANN-CFD model for simulating flow in a vortex mixer (an unbaffled vessel stirred by a magnetic stirrer). Large eddy simulations (LES) are performed to simulate flow and turbulence considering presence of both air and liquid phases. The flow fields in air and liquid phases are coupled by applying suitable boundary conditions at the air–liquid interface represented by the vortex. The shape of the vortex is built-in in the computational domain and obtained by using an artificial neural network (ANN) model trained and validated with the data obtained from experiments carried out to capture the vortex shape under different parametric conditions. ANN-CFD model is validated with PIV data reported in literature. The ANN-CFD model reduces the computational time significantly by obviating the need of performing computationally very intensive interface tracking simulations. The model is used to understand hydrodynamics and turbulence characteristics of flow in the vortex mixer.

Digital Twinning Process for Stirred Tank Reactors/Separation Unit Operations through Tandem Experimental/Computational Fluid Dynamics (CFD) Simulations

Computational fluid dynamics simulations (CFD) were used to evaluate mixing in baffled and unbaffled vessels. The Reynolds-averaged Navier−Stokes k–ε model was implemented in OpenFOAM for obtaining the fluid flow field. The 95% homogenization times were determined by tracer tests. Experimental tests were conducted by injecting sodium chloride into the vessel and measuring the conductivity with two conductivity probes, while the simulations replicated the experimental conditions with the calculation of the transport of species. It was found that the geometry of the system had a great effect on the mixing time, since the irregular flow distribution, which can be obtained with baffles, can lead to local stagnation zones, which will increase the time needed to achieve the homogenization of the solute. It was also found that measuring local, pointwise concentrations can lead to a high underestimation of the global mixing time required for the homogenization of the entire vessel. Dissolution of sucrose was also studied experimentally and by mathematical modeling. The dissolution of sucrose was found to be kinetically limited and a very good agreement was found between the experiments and the modeling approach. The extent of the applicability of CFD simulations was evaluated for enabling rapid process design via simulations.

Newly modified curved-bladed impellers for process intensification: Energy saving in the agitation of Hershel-Bulkley fluids

The power consumption in stirred tanks is a key parameter to determine the efficiency of a rotating impeller. This parameter is highly related to the viscous forces of the working fluid, especially when a complex non-Newtonian behavior exhibits. From this basis, an attempt is made in this paper to reduce the power requirements of Scaba 6SRGT impellers for stirring viscoplastic fluids in cylindrical unbaffled vessels. For this purpose, perforations are created in each blade of the impeller. Effects of the perforation diameter (dp/h = 0, 0.04, 0.07, 0.10 and 0.13), perforation shape (circular, lozenge, rectangular and triangular shapes) and number of perforation (nb = 0, 2, 4, 6, and 9) on the hydrodynamics induced and power consumption are investigated. The obtained results revealed an interesting decrease in power numbers by using the perforation technique. When changing the perforation diameter from 0 to 0.13 and the number of perforations from 0 to 9, Np is reduced by about 3.7 %–21.6 % and 2.6–6.7 %, respectively. In a comparison between the different shapes of perforation, the circular shape has allowed a reduction by about 3.8 % less than that for the other cases, which had almost the same power requirements.

Turbulent blend times with off‐centre v‐mounted agitators

AbstractExperimental measurements in a flat‐bottom tank with narrow‐blade hydrofoil and pitched‐blade impellers are used to develop guidelines for off‐centre, or eccentric, placement of vertical agitators in unbaffled tanks. The guidelines are based on providing a turbulent blend time that is no more than 20% longer than that of the same impeller operating at the same rotational speed on the centreline of a baffled tank. In addition to investigating the effect of impeller type, impeller diameter and off‐bottom clearance are also considered. The results support the commonly noted rule of thumb that as off‐centre distance is increased, performance in an unbaffled vessel approaches that in a baffled tank. A notable exception to this axiom occurs when a large impeller is located close to the tank base (specifically, D/T = 0.40, C/T = 0.10, and O/T = 0.25). In this case, a stable impeller tip vortex forms with both impeller types, with slow exchange of material between the vortex and bulk liquid leading to long blend times. Besides blend time decreasing with increasing off‐centre distance, the uncertainty or run‐to‐run variation in blend time also decreases dramatically. In most cases, the pitched‐blade turbine requires a smaller off‐centre distance than the hydrofoil impeller to approximate the blending performance provided during baffled operation.

CFD simulation of convective heat transfer in vessel with mechanical agitation for milk.

Computational fluid dynamics (CFD) analysis using ANSYS Fluent software has been carried out to investigate velocity profiles and thermal characteristics of milk during heating under mechanically agitated condition. In earlier article experimental data on forced convection heat transfer coefficient and correlations of the form for cow milk, standardised milk and full cream milk in Baffled vessel and Unbaffled vessel with scraping, using Propeller, Flat Six Blade Turbine (FBT), Inclined Six Blade Turbine (IBT) and Paddle impellers were reported. It was noted milk in Baffled vessel with Paddle impeller provided highest even at lower rotational speeds followed by Propeller, FBT and IBT impellers. In Unbaffled vessel with scraping, Propeller provided the highest followed by FBT and IBT impellers. Hence, the present investigation has been carried out to validate and understand how different velocity of flow currents and their magnitude influence the heat transfer coefficient values in CFD simulation. It also justifies the relative performance of the impellers delineated in the earlier paper. In addition, theoretical values of heat transfer coefficients computed using CFD shows close agreement with experimental values.

CFD simulation of radially stirred baffled and unbaffled tanks

Stirred tanks typically employed in process industries are provided with baffles. Although the presence of baffles is known to guarantee good mixing rates, unbaffled vessels may be compulsory in some applications as crystallization, bioremediation, biotechnology and ore industry. A better understanding of unbaffled stirred vessels flow dynamics may allow (i) a proper design to be performed and (i) conditions/processes where baffle presence can be avoided to be recognized.In the present study, the k-? SST was used to simulate an unbaffled tank from early to fully turbulent regime (Re(600-33,000). The unbaffled tank simulated has a diameter T=0.19m and is stirred by a standard six-bladed Rushton turbine with diameter D=T/2 and clearance C=T/3. A corresponding baffled tank was also simulated in order to compare the the two systems. A time dependent Sliding Grid approach was employed for the baffled tank to account for the impeller-to-baffle relative rotation. Conversely, for the case of the unbaffled vessel, a reference frame rotating with the impeller was adopted. Experimental literature data concerning the power and pumping numbers were employed for the simulation validation. RANS results were in good agreement with the experimental data for the baffled case at the largest Re, whereas predictions for the unbaffled vessel exhibited a less satisfactory agreement with experimental data. The latter finding may be due to the poor capability of the two-equations model to manage the anisotropic turbulence typical of high swirling flows.

Surface vortex formation and free surface deformation in an unbaffled vessel stirred by on-axis and eccentric impellers

The present study investigated the mechanism of surface vortex formation and free surface deformation in an unbaffled stirred vessel with on-axis and eccentric impellers having four paddle blades. To shed light on the mechanism of surface vortex formation and free surface deformation, we conducted experiments and numerical simulation in an unbaffled cylindrical stirred vessel. The surface vortex was found to be small in the eccentrically stirred vessel due to a vertical flow caused by the interaction between the discharged flow and sidewall. In the case of on-axis impeller, low-pressure zones at the rear faces of blades pulled the free surface, that causes the air entrainment. These phenomena could be seen in both the experimental observation and numerical simulation. The three types of transient free surface deformation were observed: local vortex formation due to the low-pressure zone mentioned above, entire rotation of free-surface elevation due to the macro-instabilities, and local elevation and deflection due to trailing vortices.

Mechanism of small bubble breakup in an unbaffled stirred vessel

The present study investigated the mechanism of small bubble breakup in an unbaffled vessel stirred by a 4-blade paddle impeller. Bubbles were injected underneath the rotating blades. Numerical simulation and experimental observations were performed to investigate the related phenomena. The Volume of Fluid (VOF) method was utilized to simulate a gas-liquid multiphase flow. The injected bubbles were broken behind the impeller blade, where they were transported due to a decreasing pressure gradient caused by the impeller rotation. At the behind of the blade, bubbles were elongated and fragmented due to the trailing vortices developing from the rear surface of blade. The fragmented bubbles were entrapped at the cores of trailing vortices, and then floated up in the space where the trailing vortices became weak. The size of the broken bubbles was dependent on the impeller rotation rate.

CFD modelling of conjugate heat transfer in a pilot‐scale unbaffled stirred tank reactor with a plain jacket

AbstractSemi‐conjugate and fully‐conjugate computational fluid dynamic investigations of the flow and heat transfer in a 25 L pilot‐scale unbaffled stirred tank reactor with a plain jacket are reported. A hot heat transfer fluid (DW‐Therm) flows through the plain jacket and heats water in the vessel, which is agitated by a pitched three‐blade impeller. The shape of the free‐surface of the vortex formed in the unbaffled vessel is captured by the use of a homogeneous multiphase and free‐surface flow model. The simulations of flow and heat transfer are carried out using the ANSYS CFX (V.15) code. The semi‐conjugate simulation considers the vessel wall and the vessel contents, and the fully‐conjugate simulation also includes the flow in the jacket. The upward flow in the jacket is uneven and this dominates the distributions of heat transfer coefficients and shear stresses on the inner and outer surfaces of the vessel wall. The downward motion created by the impeller generates very high heat transfer on the base surface of the vessel, but this is nullified by a stagnation zone in the base region of the jacket. Overall heat transfer coefficients evaluated from average film heat transfer coefficients values from the simulations are around 15 % higher than those evaluated from correlations in the literature. In this pilot plant facility, the combined thermal resistances for the convection in the jacket and across the glass wall dominate the overall heat transfer coefficient.

Investigation of impeller modification and eccentricity for non-Newtonian fluid mixing in stirred vessels

A shear thinning fluid (1% carboxymethyl cellulose) was used to investigate mixing under laminar flow conditions in an unbaffled vessel. The effects of impeller modification in addition to eccentricity were studied. Quantitative measurements such as percentage of uncovered area and coefficient of variance (CoV) of a tracer solution distributed inside the vessel were obtained using planar laser-induced fluorescence (PLIF) method. Increased eccentricity was found to be more effective than increasing rpm alone in reducing isolated mixing regions size (determined by the percentage of uncovered area). The dual-flow pitched blade turbine (DF-PBT), which was the modified version of a standard pitched blade turbine (PBT), was designed to provide both upward and downward flow at the same time to induce more chaotic flow. Though numerical analysis showed this type of flow generated, DF-PBT did not return lower values for the percentage of uncovered area and CoV than PBT did. Power consumption data were also compared between the two impeller types and eccentric locations. Further analyses focusing on the interactions between the impeller blades and fluid rheology is needed to improve laminar mixing in stirred vessels by impeller modification.

Empirical Model for Optimizing Shea Butter Extraction in an Unbaffled Vessel Equipped with an Impeller

The yield of Shea butter extracted in an unbaffled vessel equipped with an impeller was optimized by varying kneading time, kneading temperature and kneading speed using response surface method (RSM). Helical shaped impeller was mounted on a variable speed Tecmix TM 1100 kneader to knead the Shea paste. Minitab 16.1 software was used for the design and optimization of the process variables. The study indicated that the temperature and speed were highly significant on Shea butter oil yield with p-values of 0.001 and 0.002, respectively. The residual plots of the yield show that the adopted model was efficient because the experimental and the predicted yields for the extraction are very close. It was concluded that the model to be adequate.

Determination of forced convection heat transfer coefficients and development of empirical correlations for milk in vessel with mechanical agitators.

Heating of milk is an important unit operation to produce many milk based products. This process is done either in heat exchangers or in agitated vessels, where the mode and type of heat transfer plays a significant role. Use of mechanical agitator with suitable impeller would result in uniform agitation of the liquid. Data for forced convection heat transfer coefficients for milk in agitated vessel have not been documented. In the present investigation, forced convection heat transfer coefficients for milk, using mechanical agitators in vessels have been determined. Experiments have been conducted in vessels (Baffled as well as Unbaffled vessel) that were designed and fabricated under standard procedure employing: (1) Flat Six Blade Turbine impeller, (2) Inclined Six Blade Turbine impeller, (3) Three Blade Propeller impeller and (4) Two Bladed Paddle. Heat transfer coefficients were determined for for cow milk (3.5% fat), standardised milk (4.5% fat) and full cream milk (6.0% fat). The Two Bladed Paddle was found to as yield the highest heat transfer coefficient for all three types of milk. In addition, empirical correlations for the forced convection heat transfer using have been developed for each case. These heat transfer correlations derived will find application in large scale design and the experimental setup would be useful for future investigation with different liquids and impellers.

Scale-up and viscosity effects on gas–liquid mass transfer rates in unbaffled stirred tanks

The interest in the process industry on unbaffled stirred tanks has greatly expanded in the last years because they may bring about significant advantages in a number of applications, including biochemical, food and pharmaceutical processes where the presence of baffles is undesirable for several reasons. Despite their application potential, unbaffled vessels still lack fundamental information, due to the fact that only recently their capabilities have started being dug out. The lack of information on scale up effects is possibly the main reason hindering practical applications.In this work the influence of vessel size and liquid viscosity on the mass transfer performance in unbaffled stirred vessels, is investigated. As regards the first issue (scale-up) quite surprisingly results show that comparable mass transfer coefficients may be obtained in larger vessels, for a given power input per unit volume, with typical kLa values up to 2×10−3s−1 in ungassed conditions and of 2×10−2s−1 in gassed conditions. This is opposite to expectations based on the fact that the exchange surface in subcritical conditions only grows with D2 while volume grows with D3. Notably, the same result is obtained at an increased liquid viscosity (a feature often exhibited by biological cultivation broths) and confirms that unbaffled stirred tanks should be regarded as a strong candidate for industrial applications.Finally, also at the larger scale, in the super-critical regime unbaffled tanks are found to provide a performance comparable with that of standard (baffled & sparged) stirred tanks, hence resulting in a viable alternative to baffled tanks for all gas–liquid processes and bio-processes.

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.

S1740103 往復回転翼邪魔板無し撹拌槽における液相混合

To develop an enhanced form of apparatus for operations in chemical processes, an unbaffled vessel was constructed, fitted with an agitator employing an impeller whose rotation alternates unsteadily the direction at set range and rate, i.e., a forward-reverse rotating impeller. For this type of agitation vessel when the disk turbine impeller with six flat blades was used, liquid flow in the impeller rotational region was examined for the flow path between the neighboring blades of the rotating impeller. Internal flow of the impeller was compared with those of the unidirectionally rotating impeller of identical design in the baffled and unbaffled vessels. The results suggested that forward-reverse rotation of the impeller could have a function of the baffles. Additionally, it was found that the forward-reverse rotating impeller produced a characteristic disturbed liquid flow. Energy consideration provided a perception helpful for improvement of the unbaffled vessel agitated by the forward-reverse rotating impeller.

Formation of microcapsules by complex coacervation

AbstractThe process of encapsulation by complex coacervation includes several steps that have been investigated in order to reach a better control over the whole process. This has been carried out for the encapsulation of linseed oil by the classical gelatin/acacia gum pair of coacervating polymers. Fabrication of an oil‐in‐water emulsion is the first step that has been studied so as to reach a definite picture of the emulsification process under intermediate flow regime in a stirred unbaffled vessel. The classical scheme of oil droplet break‐up by turbulent eddies in the turbulent inertial regime gave a fair agreement with the experiments and can be used as a possible framework for a process scale‐up. Polynuclear microcapsules containing several oil droplets have been obtained during the deposition of the coacervate onto the surface of oil droplets. According to the emulsification conditions and microparticles growth caused by multiple droplets encapsulation, microcapsules of ∼130 µm were produced. A video probe immersed in the stirred vessel allowed an in situ monitoring of the successive events taking place as acid was added into the medium. This technique allows dynamic observations of microcapsules formation. These observations confirmed the deposition of coacervate droplets around oil drops and clearly showed that the formation of coacervate particles and their deposition onto oil droplets were successive events.

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.