Stirred Vessel Research Articles

Determination of the Rate of Salt-Induced Rapid Coagulation of Polystyrene Latex Particles in Turbulent Flow Using Small Stirred Vessel

In our study, we revisited a previously reported method for evaluating the mixing intensity of uniform colloidal spheres in terms of their collision frequency, with the aim of evaluating the validity of this method in the case of a small stirred vessel equipped with an impeller with four paddles. The rates of the salt-induced rapid coagulation of polystyrene latex (PSL) particles with five different diameters were measured as functions of the rotation rate. The ad-hoc assumption of the linear additivity of the perikinetics and the orthokinetics of the coagulation process was used for the analysis. Our previously proposed equation for the rate of turbulent coagulation as a function of the particle diameter, determined for an end-over-end rotation mixing device, was confirmed to be valid. However, it was found that, for small particles and low-mixing rates, i.e., for low Peclet numbers, the rate of coagulation becomes larger than that predicted on the basis of linear additivity because of the coupling effect of Brownian motion and the fluid flow during turbulent mixing. This increase occurred even though the rate was lowered by the wall effect, which resulted in an inhomogeneous distribution of the mixing intensity.

Determination of Volumetric Mass Transfer Coefficient in Gas-Solid-Liquid Stirred Vessels Handling High Solids Concentrations: Experiment and Modeling

Rigorous analysis of the determinants of volumetric mass transfer coefficient (kLa) and its accurate forecasting are of vital importance for effectively designing and operating stirred reactors. Majority of the available literature is limited to systems with low solids concentration, while there has always been a need to investigate the gas-liquid hydrodynamics in tanks handling high solid loadings. Several models have been proposed for predicting kLa values, but the application of neuro-fuzzy logic for modeling kLa based on combined operational and geometrical conditions is still unexplored. In this paper, an ANFIS (adaptive neuro-fuzzy inference system) model was designed to map three operational parameters (agitation speed (RPS), solid concentration, superficial gas velocity (cm/s)) and one geometrical parameter (number of curved blades) as input data, to kLa as output data. Excellent performance of ANFIS's model in predicting kLa values was demonstrated by various performance indicators with a correlation coefficient of 0.9941.

Generation Rate Measurement Method for Attrition Crystal Fragments and Abraded Volume of Potassium Alum in a Stirred Vessel Filled with Anti-Solvent

Generation Rate Measurement Method for Attrition Crystal Fragments and Abraded Volume of Potassium Alum in a Stirred Vessel Filled with Anti-Solvent

A Study of the Mixing Performance of Different Impeller Designs in Stirred Vessels Using Computational Fluid Dynamics

Design and operation of mixing systems using agitated vessels is a difficult task due to the challenge of obtaining accurate information on impeller-induced turbulence. The use of Computational Fluid Dynamics (CFD) can provide detailed understanding of such systems. In this study, experimental tests and computational fluid dynamics simulations were performed to examine the flow characteristics of four impeller designs (anchor, saw-tooth, counter-flow and Rushton turbine), in achieving solution homogeneity. The impellers were used to mix potassium sulfate granules, from which values of electrical conductivity of the solution were measured and used to estimate the distribution pattern of dissolved solid concentrations within the vessel. CFD models were developed for similar mixing arrangement using commercial software, ANSYS Fluent 18.1 solver and the standard k-epsilon (ε) turbulence model. The Multiple Reference Frame (MRF) approach was used to simulate the impeller rotation. Velocity profiles generated from the simulations were in good agreement with the experimental predictions, as well as with results from previous studies. It was concluded that, through CFD analysis, detailed information can be obtained for optimal design of mixing apparatus. These findings are relevant in choosing the best mixing equipment and provides a basis for scaling up mixing operations in larger systems.

Progressive stirred freeze-concentration of ethanol-water solutions

Progressive freeze-concentration is a technology to separate water from solutions by freezing. In the present investigation, ethanol-water solutions were freeze-concentrated by the progressive stirred technique. The freezing stage was carried out in a stirring vessel. Solute recovery by the fractionated thawing of ice was also studied. The effects of stirring speed (500, 1000, and 2000 rpm), initial concentration of the solution (3%, 5%, and 8% ethanol), and temperature of the thawing stage (0, 10, and 20 °C) on the solute yield and average distribution coefficient were determined using response surface analysis. The ethanol concentration was found to have increased by 1.3 and 2.1 times at the end of the freeze concentration process. It was found that the initial concentration had a significant effect on the distribution coefficient. In addition, the average yield was increased by 28% by fractionated thawing. Subsequently, a non-dimensional analysis of the distribution coefficient was developed to yield a model to predict the distribution coefficient as a function of the Reynolds number, the relationship between the average ice growth rate and the stirring speed, the agitator diameter, and the liquid fraction. This technique proved to be valid with respect to the concentration of ethanol-water solutions, with better yields being obtained at low initial concentrations. This model is the first of its kind to describe the ethanol-water interaction in agitated freeze-concentration systems.

Simulation of the Fluid Flow in Stirred Vessels Using MPS Method with the Integral Wall Model

Simulation of the Fluid Flow in Stirred Vessels Using MPS Method with the Integral Wall Model

Crystal Material and Stirring Condition Effects on Attrition Fragment Generation Rate and Parent Crystal Abrasion in a Stirred Vessel

Crystal Material and Stirring Condition Effects on Attrition Fragment Generation Rate and Parent Crystal Abrasion in a Stirred Vessel

Uniformity of Carbon Supported Pt Nanocatalysts Produced By Arc Plasma Deposition

We propose a new approach for the preparation of Pt/C catalysts by arc plasma deposition (APD), in which Pt nanoparticles are deposited on carbon in a vacuum; this method has been applied in a number of fields other than fuel cell catalysts. The solid cathode material is transformed into a completely ionized plasma that leaves the cathode with a velocity of 1-2 x 104 m/s(this number is the same for all cathode materials). This velocity corresponds to an ion kinetic energy of 20 eV (light ions such ascarbon) to 300eV (heavy ions such as platinum). Thus, the cathode material is eroded and forms anenergetic plasma jet. When carbon powder is irradiated with a platinum-based plasma, the Pt+ ions collide with the carbon and become atoms, which then migrate and interact with other Pt atoms to form nanoparticles (NPs) with dimensions of 2 to 3 nm on the carbon surface. Figure 1 shows a scanning electron microscopy (SEM) image of Pt NPs on Ketjen black. Although the surface of the Ketjen black is extremely rough, the NPs are seen to be uniformly distributed. We compared the electrochemical properties of Pt/C catalysts prepared by APD with those of the commercially available catalyst TEC10E50E. Although the electrochemical surface area (ECSA) was almost the same, the specific activity for Pt/C(APD) was about 2 times higher than that for TEC10E50E, which led to an approximately 2 times higher mass activity. However, one remaining problem with Pt/C(APD) is that NPs are not deposited in some regions. Figure 2 shows an SEM image of Ketjen black containing regions where no Pt NPs are present. To address this issue, we made improvements to the stirring vessel, and optimized deposition parameters such as the plasma pulse interval and the powder surface conditions. We will describe the effects of these optimizations on the electrochemical properties of Pt/C(APD). Figure 1

Effect of Various Curved-Blade Impeller Geometries on Drop Size in a Liquid–Liquid Stirred Vessel

An experimental study was performed to discuss the effects of curvature angles and central disk sizes of 6-curved-blade impellers on the mean drop size in an agitated vessel. A system with 1% oil in water in the presence of a surfactant solution was used. The effects of impeller speed on drop size were also investigated. The laser diffraction technique and RSM method were employed to measure and analyze data, respectively. Decreased curvature angles from 180° to 140° reduced the drop size up to 9%, 10%, and 10% at agitation speeds of 5, 6, and 7 rps, respectively. Moreover, a decrease in central disk size from 3/4D to 1/4D reduced the drop size up to 16%, 18%, and 22% at an agitation speed of 5, 6, and 7 rps, correspondingly. Two mathematical models were suggested and the most significant parameters of each experimental design were identified through the Analysis of Variance.

Measurement of Gas Hold-up Distribution in Stirred Vessels Equipped with Pitched Blade Turbines by Means of Electrical Resistance Tomography

Gas dispersion is a widespread operation in the process industry. The effectiveness of the dispersion affects fluid mixing, heat and mass transfer rates and, as a consequence, the chemical or biochemical reactions involved. In this work, the gas-liquid dispersion in stirred tanks equipped with pitched blade turbines (PBT) is investigated by means of Electrical Resistance Tomography (ERT). The main goal of this study is the assessment of gas-distribution in the vessel. Measurements are taken in case of different fluid dynamic regimes, which occurrence depends on the operating conditions. The effect of impeller rotational speed on gas-liquid dispersion is investigated in vessels having different sizes. The adoption of ERT allows the assessment of gas distribution through the vessel volume without any particular limitation about the maximum gas volume fraction, so that experimental conditions close to that adopted in industrial operation can be effectively managed. The transition between flooding and loading regime is investigated, in which the impeller starts to effectively distribute the gas throughout the liquid phase. The vessel size is found to play a role in determining the condition at which loading regime occurs. The data could be either used to set up simplified correlations for the transition between flooding/loading regimes or as a valuable benchmark for CFD simulation, given the detailed information available about the spatial distribution the dispersed gas.

Particle Distribution in Unbaffled Stirred Vessels

Particle Distribution in Unbaffled Stirred Vessels

Assessment of Global Void Fraction in a Gas-liquid Stirred Vessel by Digital Image Processing

The main goal of the study presented in this paper is to propose a non-invasive method based on digital image processing to assess the global void fraction for both the vortex and the air bubbles that form in a gas-liquid stirred vessel. Three impellers with different geometries (circular, rectangular, and trapezoidal) were tested in a laboratory scale water tank for two aspect ratios of the water column. The trapezoidal impeller has perforated blades, a secondary goal of the study being to investigate whether the perforations could decrease the power demand of the impeller while providing a high global void fraction. The results obtained show that the operation of the impellers is characterized by two critical rotational speeds, ncr,1 and ncr,2. The transition from the sub-critical (non-aerated) to the super-critical (aerated) regime takes place at ncr,1. The vortex volume fraction increases up to ncr,2 and gets stabilized beyond it. The evolution of the bubble void fraction can be described by the logistic function. When enough experimental data is available, this function allows the estimation of a third critical speed beyond which the increase in bubble void fraction becomes negligible. The usage of perforated blades did not bring a decrease but an increase in power demand. The method based on digital image processing proved to be adequate for choosing the appropriate type of impeller and for optimizing new impeller designs in applications involving gas-liquid stirred tanks.

Effect of Shear Rate Distribution on Particle Aggregation in a Stirred Vessel

AbstractThe modeling of particle aggregation under a simple shear flow and the extension of the model to a stirred vessel is described. The model quantitatively demonstrates the change of the number of aggregates with time for each shear rate. This number increased with higher shear rate and, conversely, the aggregate size became small when raising the shear rate. This was because aggregates were broken by the stronger shear force. The number of aggregates for different impellers was determined. The shear rate was back‐calculated from the experimentally obtained aggregate size and the model equation. This shear rate was different from that estimated from the Metzner‐Otto method, consequently, some revisions of the Metzner‐Otto equation might be necessary for its application to particle aggregation.

Products distribution and influence of nickel catalyst on glucose hydrothermal decomposition

Biomass hydrothermal decomposition is a clean and rapid way for hydrogen production. In order to reveal the conversion mechanism, glucose with a concentration of 10 wt. % was gasified from 473 K to 673 K in 25 K increments in a stirring vessel and both the gaseous and liquid products were analyzed. Nickel based catalysts with different supports were used to promote the production of hydrogen as well as reduce the char formation. The concentration of furfural, 5-hydroxylmethylfurfural (5-HMF), levulinic acid, cyclopentanone and phenol in the liquid products were detected by a Gas chromatography–Mass spectrometry detector (GC–MS). It came out that the contents of phenol increased as reaction temperature increased, while 5-HMF, furfural and levulinic acid was consumed successively soon after their contents reached the maximum. The concentration of levulinic acid decreased and that of cyclopentanone increased sharply when nickel catalysts were used especially for magnesia-supported catalyst. The concentration of furfural decreased with the adding of catalysts and nearly approached to zero other than alumina-supported catalyst.

Maximizing Impeller Power Efficiency in Gas–Solid–Liquid Stirred Vessels through Process Intensification

In this work, suspension of ultrahigh concentration solids in the presence of gas at different solids concentrations, gas flow rates, and impeller types was studied in a mechanically agitated vessel. A term defined as the power efficiency factor (ejsg–1 (kg/W)) serves as an indication of the quantity of particles that could be suspended per unit of power consumed by the impeller. Accordingly, it was found that the ejsg–1 values can be maximized by operating the mixing tank with an optimum range of solids concentration, which is given as Cv ≈ 0.2–0.3 (v/v) for the systems studied in this work. It was observed that large mixed flow impellers were more energy-efficient when the tank was operated under aerated conditions with an optimum concentration of solids. A mathematical model was developed to predict the impeller specific power ejsg (W/kg), which can be used to determine the optimum solids concentration in three-phase stirred vessels, and its predictions exhibited reasonable agreement with the experimen...

PIV Study of the Turbulent Flow in a Stirred Vessel Equipped by an Eight Concave Blades Turbine

The purpose of this study is to experimental investigating of the hydrodynamics structure in a cylindrical stirred vessel generated by an eight concave blades turbine. Particle image velocimetry (PIV) system was used to evaluate the effect of the blade turbine in the flow fields. The flow is illuminated by a Nd: YAG 532 nm green pulsed laser source generated in 2x30 mJ. The acquisition of the tow-dimensional images data was taken with a CCD camera with 1600 x 1200 pixels² of resolutions. In addition, a mini-synchronizer was used to control the different PIV components. For the results, we are interested to present the distribution of the velocity fields, the vorticity, the turbulent kinetic energy, the dissipation rate of the turbulent kinetic energy and the turbulent viscosity. Three azimuthally planes were carried out which present the turbine blade plane (θ=30°), the upstream turbine blade plane (θ=10°) and the downstream turbine blade plane (θ=45°).

Numerical Study of Fluid Behavior Induced by Anchor Impeller in a Stirred Vessel

In the present paper, we have described in-depth numerical study of the basic fluid mechanics problem of yield stress fluid flow with regularization model of Berovier and Engelman in a cylindrical vessel not chicaned equipped with an anchor stirrer by using computational fluid dynamics (CFD) based on the finite volumes method discretization of Navier - Stokes equations formulated in variables (U.V.P). We study the effect of inertia and the yield stress influence by the variation of Hedström number on the flow; we have analyzed also the influence of rheological parameters on the hydrodynamic flow behavior, such as the velocity components and the power consumption.

Numerical study of the performance of multistage Scaba 6SRGT impellers for the agitation of yield stress fluids in cylindrical tanks

The 3-D flow fields and power consumption in a vessel stirred by multistage Scaba 6SRGT impeller have been investigated. The Xanthan gum solutions in water were used, which have a shear thinning behavior with yield stress. This study was carried out with the help of a CFD package (CFX 13.0, Ansys Inc.) which is based on the finite volume method to solve the momentum equations. The effects of stirring rate, fluid rheology, impeller number, impeller location and vessel size on the performance of such stirred system are presented. To validate the CFD model, our predicted results have been compared with other literature data and a satisfactory agreement has been found.

Numerical Analysis on the Vortex Pattern and Flux Particle Dispersion in KR Method Using MPS Method

The mechanically-stirring vessel is widely used in many fields, such as chemical reactor, bioreactor, and metallurgy, etc. The type of vortex mode that formed during impeller stirring has great effect on stirring efficiency, chemical reacting rate and air entrapment. Many efforts have been made to numerically simulate the fluid flow in the stirring vessel with classical Eulerian method. However, it is difficult to directly investigate the vortex mode and flux particle dispersion. Therefore, moving particle semi-implicit (MPS) method, which is based on Lagrangian method, is applied to simulate the fluid flow in a KR method in this practice. Top height and bottom heights of vortex surface in a steady state under several rotation speed was taken as key parameters to compare the results of numerical and published results. Flux particle dispersion behaviour under a rotation speed range from 80 to 480 rpm was also compared with the past study. The result shows that the numerical calculation has high consistency with experimental results. It is confirmed that the calculation using MPS method well reflected the vortex mode and flux particle dispersion in a mechanically-stirring vessel.

Effect of Gravity on the Hydrodynamics in an Unbaffled Stirred Vessel

AbstractThe hydrodynamics in an unbaffled stirred vessel were simulated in order to highlight the effect of gravity on pressure and velocity distributions. Two fluids with different viscosity were studied for the laminar and turbulent flow regimes, respectively. The simulation results were compared with experimental data from the literature. Results indicate that for the higher‐viscosity fluid, gravity only affects static pressure and that the effects of gravity on velocity and dynamic pressure are negligible. For the lower‐viscosity fluid, however, gravity imposes a pronounced impact on static pressure, dynamic pressure, velocity, and turbulent kinetic energy. These findings indicate that careful consideration is necessary for the role that gravity plays in the study of free‐surface hydrodynamics in unbaffled stirred vessels.