Numerical Simulation of Gas Holdup Distribution in a Standard Rushton Stirred Tank Using Discrete Particle Method

The discrete particle method was used to simulate the distribution of gas holdup in a gas-liquid standard Rushton stirred tank. The gas phase was treated as a large number of bubbles and their trajectories were tracked with the results of motion equations. The two-way approach was performed to couple the interphase momentum exchange. The turbulent dispersion of bubbles with a size distribution was modeled using a stochastic tracking model, and the added mass force was involved to account for the effect of bubble acceleration on the surrounding fluid. The predicted gas holdup distribution showed that this method could give reasonable prediction comparable to the reported experimental data when the effect of turbulence was took into account in modification for drag coefficient.

Simple Model for Gas Holdup and Liquid Velocity of Annular Photocatalytic External-Loop Airlift Reactor Under both Bubble and Developing Slug Flow

Based on the momentum conservation approach, a theoretical model was developed to predict the superficial liquid velocity, and a correlation equation was established to calculate the gas holdup of an annular external-loop airlift reactor(AELAR)in the bubble flow and developing slug flow pattern. Experiments were performed by using tap-water and silicone oil with the viscosity of 2.0 mm^2/s(2cs-SiO)and 5.0 mm^2/s(5cs-SiO)as liquid phases. The effects of liquid viscosity and flow pattern on the AELAR performance were investigated. The predictions of the proposed model were in good agreement with the experimental results of the AELAR. In addition, the comparison of the experimental results shows that the proposed model has good accuracy and could be used to predict the gas holdup and liquid velocity of an AELAR operating in bubble and developing flow pattern.

Experimental determination of gas holdup and volumetric mass transfer coefficient in a jet bubbling reactor

The hydrodynamics and mass transfer characteristics of a lab-scale jet bubbling reactor(JBR)including the gas holdup,volumetric mass transfer coefficient and specific interfacial area were assessed experimentally investigating the influence of temperature,pH and superficial gas velocity.The reactor diameter and height were 11 and 30 cm,respectively.It was equipped with a single sparger,operating at atmospheric pressure,20 and 40℃,and two pH values of 3 and 6.The height of the liquid was 23 cm,while the superficial gas velocity changed within 0.010-0.040 m·s^(-1)range.Experiments were conducted with pure oxygen as the gas phase and saturated lime solution as the liquid phase.The liquid-side volumetric mass transfer coefficient was determined under unsteady-state oxygen absorption in a saturated lime solution.The gas holdup was calculated based on the liquid height change,while the specific interfacial area was obtained by a physical method based on the bubble size distribution(BSD)in different superficial gas velocities.The results indicated that at the same temperature but different pH,the gas holdup variation was negligible,while the liquid-side volumetric mass transfer coefficient at the pH value of 6 was higher than that at the pH=3.At a constant pH but different temperatures,the gas holdup and the liquid-side volumetric mass transfer coefficients at 40℃were higher than that of the same at 20℃.A reasonable and appropriate estimation of the liquid-side volumetric mass transfer coefficient(kla)in a pilot-scale JBR was provided which can be applied to the design and scale-up of JBRs.

Prediction of Gas Holdup in Bubble Columns Using Artificial Neural Network

A new correlation for the prediction of gas hold up in bubble columns was proposed based on an extensive experimental database set up from the literature published over last 30 years. The updated estimation method relying on artificial neural network, dimensional analysis and phenomenological approaches was used and the model prediction agreed with the experimental data with average relative error less than 10%.

Reliability of gas holdup measurements using the differential pressure method in a cyclone-static micro-bubble flotation column

Gas holdup is one of the key parameters in flotation process. Gas holdup as measured by a differential pressure method was investigated and the relative errors compared to the average gas holdup from the volume expansion method. The errors were used to establish optimum measurement positions. The results show that the measurement position should be in the middle of the column and in the region half way from the center to the wall (the half-radius). The gas holdup along the axial direction is lower at the bottom and higher at the top of the floatation column. The gas holdup along the radial direction is lower near the wall and higher near the center of the flotation column. The average gas holdup measure- ment can be replaced by regional gas holdup values.

Effect of Temperature on Gas Hold-up in Aerated Stirred Tanks

Gas holdups in ambient gassed and hot sparged systems with multiple modern impellers and the effect of temperature on gas holdup are reported. The operating temperature has a great impact on gas holdup though the gas dispersion regime in the hot sparged system is similar to the ambient gassed condition. The gas holdup under the elevated temperature and the ambient gassed operation is successfully correlated. With the same total gas flow rate and power input, the gas holdup in the hot sparged system (say near the boiling point) is only about half of that in the ambient system. The results imply that almost all existing hot sparged reactors have been designed on the basis of incorrect estimates of the gas holdup during operation.

Influence of impeller diameter on overall gas dispersion properties in a sparged multi-impeller stirred tank

The impeller configuration with a six parabolic blade disk turbine below two down-pumping hydrofoil propellers, identified as PDT + 2CBY, was used in this study. The effect of the impeller diameter D, ranging from0.30 T to 0.40T(T as the tank diameter), on gas dispersion in a stirred tank of 0.48 m diameter was investigated by experimental and CFD simulation methods. Power consumption and total gas holdup were measured for the same impeller configuration PDT + 2CBY with four different D/T. Results show that with D/T increases from 0.30 to 0.40, the relative power demand(RPD) in a gas–liquid system decreases slightly. At low superficial gas velocity VSof 0.0078 m·s-1, the gas holdup increases evidently with the increase of D/T. However, at high superficial gas velocity, the system with D/T = 0.33 gets a good balance between the gas recirculation and liquid shearing rate, which resulted in the highest gas holdup among four different D/T. CFD simulation based on the two-fluid model along with the Population Balance Model(PBM) was used to investigate the effect of impeller diameter on the gas dispersion. The power consumption and total gas holdup predicted by CFD simulation were in reasonable agreement with the experimental data.

Study on gas–liquid flow characteristics in stirred tank with dual-impeller based on CFD-PBM coupled model

Study on gas–liquid flow in stirred tank with two combinations of dual-impeller(six-bent-bladed turbine(6BT)+six-inclined-blade down-pumping turbine(6 ITD),the six-bent-bladed turbine(6BT)+six-inclinedblade up-pumping turbine(6ITU))was conducted using computational fluid dynamics(CFD)and population balance model(PBM)(CFD-PBM)coupled model.The local bubble size was captured by particle image velocimetry(PIV)measurement.The gas holdup,bubble size distribution and gas–liquid interfacial area were explored at different conditions through numerical simulation.The results showed that the 4 mm bubbles accounted for the largest proportion of 33%at the gas flow rates Q=0.76 m^(3)·h^(-1) and 22%at Q=1.52 m^(3)·h^(-1) for combined impeller of 6BT+6ITU,while the bubbles of 4.7 mm and 5.5 mm were the largest proportion for 6BT+6ITD combination,i.e.25%at Q=0.76 m^(3)·h^(-1) and 22%at Q=1.52 m^(3)·h^(-1),respectively,which indicated that 6BT+6ITU could reduce bubble size effectively and promote gas dispersion.In addition,the gas holdup around impellers was increased obviously with the speed compared with gas flow rate.So it was concluded that 6ITU impeller could be more conductive to the bubble dispersion with more uniform bubble size,which embodied the advantages of 6BT+6ITU combination in gas–liquid mixing.

Local Gas Phase Flow Characteristics of a Gas-Liquid-Solid Three-Phase Reversed Flow Jet Loop Reactor

The local gas-phase flow characteristics such as local gas holdup(εg), local bubble velocity (V_b) and local bubble mean diameter(d_b) at a specified point in a gas-liquid-solid three-phase reversedflow jet loop reactor was experimentally investigated by a five-pointconductivity probe. The effects of gas jet flow rate, liquid jet flowrate, solid loading, nozzle diameter and axial position on the localεg, V_b and d_b profiles were discussed. The presence of solids atlow solid concentrations not only increased the local εg and V_b,but also decreased the local d_b. The optimum solid loading for themaximum local εg and V_b together with the minimum local d_b was0.16×10^-3 m^3, corresponding to a solid volume fraction ε_S＝2.5/100.

Experimental study on gas–liquid dispersion and mass transfer in shear-thinning system with coaxial mixer

The effects of impeller type, stirring power, gas flow rate, and liquid concentration on the gas–liquid mixing in a shear-thinning system with a coaxial mixer were investigated by experiment, and the overall gas holdup, relative power demand, and volumetric mass transfer coefficient under different conditions were compared. The results show that, the increasing stirring power or gas flow rate is beneficial in promoting the overall gas holdup and volumetric mass transfer coefficient, while the increasing system viscosity weakens the mass transfer in a shearing–thinning system. Among the three turbines, the six curved-blade disc turbine(BDT-6) exhibits the best gas pumping capacity; the six 45° pitched-blade disc turbine(PBDT-6) has the highest volumetric mass transfer coefficient at the same unit volume power.

Effect of Gas Injection on Fluid Flow in Mold

A mathematical and physical model was adopted to compute the fluid flow distribution in case of localgas holdup in mold. The photography was used to show the fluid field. The predicted flow patterns show. reasonableagreement with experiment observations using actual water model.

Gas dispersion and solid suspension in a three-phase stirred tank with triple impellers

The hydrodynamics is still not fully understood in the three-phase stirred tank equipped with multi-impeller due to the intensive interaction between phases.In this work,the solid critical suspension speed(NJSG),relative power demand(RPD)and overall gas holdup(ε_G)were measured in an air–water–glass beads stirred tank equipped with multi-impeller,which consists of a parabolic blade disk turbine below two down-pumping hydrofoils.Results show that either the NJSGor the specific power consumption increases when increasing the volumetric solid concentration or superficial gas velocity.RPD changes less than 10%when solid volumetric concentration ranges from 0 to 15%.ε_G decreases with the increase of solid concentration,and increases with the increase of both superficial gas velocity and the total specific power consumption.The quantitative correlations of NJSG,RPD andεGwere regressed as the function of superficial gas velocity,specific power consumption,Froude number and gas flow number,in order to provide the reference in the design of such three-phase stirred tank with similar multi-impellers.

ERT Imaging of Gas-Liquid Mixing in Agitated Vessel

An investigation using Electrical Resistance Tomography （ERT） was carried out in order to characterize gas-liquid mixing in an agitated vessel. The experimental work was carried out in a 400 mm diameter agitated vessel that was fitted with four planes, 16 stainless steel electrodes. Agitation was carried out using the Lightnin Labmaster and Rushton turbine while conductivity data acquisition was carried out using the ITS P2000 ERT system. A Mathlab code was developed to construct a surface plot for gas hold-up from the ERT data. Various gas dispersion conditions such as flooded, loaded and fully dispersed were successfully characterized using the ERT technique.

Effect of solids on pulp and froth properties in flotation

Froth flotation is a widely used process of particle separation exploiting differences in surface properties. It is important to point out that overall flotation performance(grade and recovery) is a consequence of the quality and quantity of the solid particles collected from the pulp phase, transported into the froth phase, and surviving as bubble-particle aggregates into the overflow. This work will focus on studying these phenomena and will incorporate the effects of particle hydrophobicities in the 3-phase system. Solids are classed as either hydrophilic non-sulphide gangue(e.g. silica, talc), hydrophilic sulphide(e.g. pyrite), or hydrophobic sulphide(e.g. sphalerite). Talc is a surface-active species of gangue that has been shown to behave differently from silica(frother adsorbs on the surface of talc particles). Both are common components of ores and will be studied in detail. The focus of this work is to investigate the role of solids on pulp hydrodynamics, froth bubble coalescence intensity, water overflow rate with solids present, and in particular, the interactions between solids, frother and gas on the gas dispersion parameters. The results show that in the pulp zone there is no effect of solids on bubble size and gas holdup; in the froth zone, although hydrophilic particles solely do not effect on the water overflow rate, hydrophobic particles produce higher intensity of rates on water overflow and bubble coalescence, and many be attributed to the water reattachment.

The Effects of Frothers and Particles on the Characteristics of Pulp and Froth Properties in Flotation—A Critical Review

The pulp and froth zones are the main components of froth flotation as it defines both quality of the end product and overall efficiency. The importance of the properties of the two zones, which include pulp hydrodynamics, froth bubble coalescence rate, water overflow rate, air recovery, etc., is being increasingly recognized. The properties are depending not only on the type and concentration of the frother but also on the nature and amount of the particles present in the flotation system, and as well as the frother-particle interactions and potentially of bubble-particle interactions. To date, there is no specific criterion to quantify pulp and froth properties through the interactions between frothers and particles because the various related mechanisms occurring in the pulp and froth are not fully understood. Linking the properties to the metallurgical performance is also challenged. In order to better understand the effect of these issues in flotation, in this review paper, the past and recently published articles relevant to characterizations of pulp and froth properties are widely reviewed;the findings and the gap of knowledge in this area are highlighted for further research.

The effect of operating and design parameter on bubble column performance:The LOPROX case study

It is known that the performances of multi-phase reactors depend on the operating parameters(the temperature and the pressure of the system),the phase properties,and the design parameters(the aspect ratio(AR),the bubble column diameter,and the gas sparger design).Hence,the precise design and the correct operation of multi-phase reactors depends on the understanding and prediction of the fluid dynamics parameters.This paper contributes to the existing discussion on the effect of operating and design parameter on multi-phase reactors and,in particular,it considers an industrial process(e.g.,the LOPROX(low pressure oxidation)case study,which is typical example of two-phase bubble columns).Based on a previously-validated set of correlations,the influence of operating and design parameter on system performances is studied and critically analyzed.First,we studied the effects of the design parameter on the liquid–gas interfacial area,by keeping constant the fluid physical–chemical properties as well as the operating conditions;subsequently,we discussed for a fixed system design,the influence of the liquid phase properties and the operating pressure.In conclusion,this paper is intended to provide guidelines for the design and scale-up of multi-phase reactors.

Hydrodynamic Behavior in a Tapered Bubble Column

The experiment was conducted to explore the hydrodynamics in a conical column with a height of 3.00 m, and a taper angle of 1.91°. Three regimes occur in succession with increasing superficial gas velocity. Overall gas holdup increases with an increase in gas velocity and a decrease in solid concentration or static slurry height. Axial solid holdup becomes more uniform with increasing gas velocity, while axial gas holdup decreases from the bottom to the top. Both dry and wet pressure drops across the gas distributor increase with an increase in superficial gas velocity.

Flow Characteristics in an External-Loop Airlift Slurry Reactor

The local flow characteristics in an external-loop airlift slurry reactor were investigated.The axial profiles of the local gas holdup,the Sauter mean diameter and the rise velocity were obtained.It was found that the bubble size and rise velocity were influenced by the solid holdup,and the bubble coalescence was enhanced by the increase of the solid holdup.A new correlation was used to predict the slip velocity between the gas phase and the slurry phase by taking into account the local gas holdup,the bubble size,and the physical properties of the experimental system.By using this correlation,the local drag coefficient can be calculated in the bubble swarm.

Hydrodynamic behavior of an internally circulating fluidized bed with tubular gas distributors

To better understand the hydrodynamic behavior of an internally circulating fluidized bed, solids holdup in the down-comer （Eso）, solids circulation rate （Gs） and gas bypassing fraction （from down-comer to riser y^R, and from riser to down-comer yRD） were experimentally studied. The effects of gas velocities in the riser and in the down-comer （UR and UD）, orifice diameter in the draft tube （dor）, and draft tube height （HR） were investigated. Experimental results showed that increase of gas velocities led to increase in Gs and yDR, and slight decrease in yeD. Larger orifice diameter on the draft tube led to higher 8sD, Gs and yDR, but had insignificant influence on YRD. with increasing draft tube height, both Gs and YDR first increased and then decreased, while yRD first decreased and then increased. Proposed correlations for predicting the hydrodynamic parameters agreed reasonably well with experimental values.

Effects of particle size and concentration on bubble coalescence and froth formation in a slurry bubble column

A new approach for simulating the formation of a froth layer in a slurry bubble column is proposed. Froth is considered a separate phase, comprised of a mixture of gas, liquid, and solid. The simulation was carried out using commercial flow simulation software （FIRE v2014） for particle sizes of 60-150 μm at solid concentrations of 0-40vol%, and superficial gas velocities of 0.02-0.034m/s in a slurry bubble column with a hydraulic diameter of 0.2 m and height of 1.2 m. Modelling calculations were conducted using a Eulerian-Eulerian multiphase approach with k-ε turbulence. The population balance equations for bubble breakup, bubble coalescence rate, and the interfacial exchange of mass and momentum were included in the computational fluid dynamics code by writing subroutines in Fortran to track the number density of different bubble sizes. Flow structure, radial gas holdup, and Sauter mean bubble diameter distributions at different column heights were predicted in the pulp zone, while froth volume fraction and density were predicted in the froth zone. The model was validated using available experimental data, and the predicted and experimental results showed reasonable agreement. To demonstrate the effect of increasing solid concentration on the coalescence rate, a solid-effect multiplier in the coalescence effi- ciency equation was used. The solid-effect multiplier decreased with increasing slurry concentration, causing an increase in bubble coalescence efficiency. A slight decrease in the coalescence efficiency was also observed owing to increasing particle size, which led to a decrease in Sauter mean bubble diam- eter. The froth volume fraction increased with solid concentration. These results provide an improved understanding of the dynamics of slurry bubble reactors in the presence of hydrophilic particles.