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.

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.

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.

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.

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.

Computational fluid dynamics simulation of hydrodynamics in the riser of an external loop airlift reactor

Local hydrodynamics in the riser of an external loop airlift reactor （EL-ALR） are identified and the performances of three drag models are evaluated in computational fluid dynamics simulation. The simulation results show that the Schiller-Naumann drag model underestimated the local gas holdup at lower superficial gas velocity whereas the Tomiyama drag model overestimated that at higher superficial gas velocity. By contrast, the dual-bubble-size （DBS）-local drag model gave more reasonable radial and axial distri-butions of gas holdup in all cases. The reason is that the DBS-local drag model gave correct values of the lumped parameter, i,e., the ratio of the drag coefficient to bubble diameter, for varying operating conditions and radial positions. This ratio is reasonably expected to decrease with increasing superficial gas velocity and be smaller in the center and larger near the wall. Only the DBS-local drag model correctly reproduced these trends. The radial profiles of the axial velocity of the liquid and gas predicted by the DBS-local model also agreed well with experimental data.

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.

On the scale-up criteria for bubble columns

It is generally admitted that experimental data obtained in“laboratory-scale”bubble columns are representative of“industrial-scale”reactors if the well-known three“Wilkinson et al.scale-up criteria”are satisfied:(a)the diameter of the bubble column is larger than 0.15 m,(b)the sparger openings are larger than 1e2mm and(c)the aspect ratio is larger than 5.The aim of this communication is to contribute to the existing discussion.To this end,this communication collects relevant experimental investigation and include new experimental data:in particular,we have experimentally studied the combined effect of the aspect ratio(within the range of 1e15)and the sparger design(considering both“coarse”and“fine”spargers)on the gas holdup in a large-diameter and large-scale gas-liquid bubble column.The bubble column has been operated both in the batch mode and in the counter-current mode.Filtered air has been used as the gaseous phase in all the experiments,while the liquid phase has included deionized water and different aqueous solutions of organic(i.e.,ethanol)and inorganic(i.e.,sodium chloride,NaCl)active agents.It is found that the“Wilkinson et al.scale-up criteria”are valid for the air-water case in the batch mode for“very-coarse”spargers.Conversely,they are no more valid when considering different liquid velocity,and/or aqueous solutions of active agents,and other sparger openings.

Hydrodynamic behavior of an airlift reactor with net draft tube with different configurations: Numerical evaluation using CFD technique

In terms of gas holdup,liquid velocity,and volumetric mass transfer coefficient for oxygen(KLa),the hydrodynamic behavior of four configurations of an airlift reactor(ALR)with a net draft tube(NDT)of different net mesh sizes(ALR-NDT-3,6,12,and ALR)have been numerically simulated for a range of inlet air flow rates.The effect of various levels of ratio of height(H)to inner tube diameter(D)of the net draft tube(H/D:9.3,10.7,17.5,and 20)and ratio of inner cross-sectional area of the riser(Ar)to the inner cross sectional area of the downcomer(Ad)(Ad/Ar:1.3 and 7)for different air flow rates is also evaluated for each reactor configuration operating with an air-water system.The two-fluid formulation coupled with the k-εturbulence model is used for computational fluid dynamics(CFD)analysis of flow with Eulerian descriptions for the gas and liquid phases.Interactions between air bubbles and liquid are taken into account using momentum exchange and drag coefficient based on two different correlations.Trends in the predicted dynamical behavior are similar to those found experimentally.A good agreement was achieved suggesting that geometric effects are properly accounted for by the CFD model.After a comparison with experimental data,numerical simulations show significant enhanced gas holdup,liquid velocity,and Kta for the ALR-NDTs compared with the conventional ALR.Higher gas holdup values are achieved for ALR-NDT-3 than that for the other ALRs because it acts like a bubble column reactor as the holes present in the NDT are large.Maximum liquid velocities are seen in ALR-NDT-12,which operates like a conventional ALR.Moreover,the interaction between the NDT and upward gas flow leads to cross flow through the net,small bubbles,and high interfacial area as well as good mass transfer.This was significant in ALR-NDT-6 with maximum Kta value of 0.031 s-1.The applied methodology provides an insightful understanding of the complex dynamic behavior of ALR-NDTs and may be helpful in optimizing the design and scale-up of reactors.

CFD simulation of the hydrodynamics in an internal air-lift reactor with two different configurations

Computationalfluid dynamics(CFD)was used to investigate the hydrodynamic parameters of two internal airlift bioreactors with different configurations.Both had a riser diameter of 0.1 m.The model was used to predict the effect of the reactor geometry on the reactor hydrody-namics.Water was utilized as the continuous phase and air in the form of bubbles was applied as the dispersed phase.A two-phaseflow model provided by the bubblyflow application mode was employed in this project.In the liquid phase,the turbulence can be described using the k-εmodel.Simulated gas holdup and liquid circulation velocity results were compared with experimental data.The predictions of the simulation are in good agreement with the experimental data.