Global Gas Holdup Research Articles

The Influence of Impeller Combination on the Gas-Liquid Dispersion Performance of a Coaxial Mixer in Viscous Fluids

Abstract As a kind of mixer with wide adaptability, coaxial mixer has a wide-application in the process industry. With the help of experiment and numerical simulation, the gas-liquid dispersion performance of four impeller combinations in viscous system was studied in terms of the global gas holdup, local gas holdup, local bubble size and liquid phase flow field. Wall-scraping anchor is the common outer impeller of the four impeller combinations, and the four inner impellers are Rushton turbine, six-straight-blade turbine (SBT), 45° six-pitched-blade turbine pumping downwards (PBTD) and 45° six-pitched-blade turbine pumping upwards (PBTU). The experimental results indicate that the global gas holdup of the Rushton impeller combination is the highest among the four impeller combinations and that of PBTU is the lowest. The local gas holdup of Rushton and PBTU combinations are all high, but the gas distribution with the Rushton combinations is more uniform. Besides, the local bubble size of the four impeller combinations has the same tendency with the local gas holdup. Under the same conditions, the flow field and distribution of gas holdup of the four impeller combinations in stirred tank were simulated numerically. Compared with the experimental results, the simulation results show a good agreement on the value and distribution of local gas holdup. Among the four coaxial mixers, the Rushton impeller combination is more suitable for gas-liquid dispersion.

Evaluation of gas–liquid mass transfer in gas‐induced stirred tank reactor using electrical resistance tomography

AbstractBACKGROUNDA gas‐entrainment impeller is attractive for multiphase reactive systems such as Fischer–Tropsch reaction and deep liquid‐phase oxidation of organics with the possibility of low gas conversion per pass, enhancement of mixing performance and better inter‐phase mass transfer compared with conventional impellers.RESULTSTomograms obtained from ERT measurement reveal that the conductivity of a gas–water system changes as a function of impeller Reynolds number, ReI. The global gas holdup profile behaviour with ReI exhibited a sigmoid character which was adequately captured by ϵG = ϵG,0 + ϵG,max(1 − exp(−τglReI)). The parity plot showed strong linearity between model predicted and experimental data. The radial gas holdup profiles showed an upward parabolic trend with higher gas holdup values for gas‐entrainment impeller systems than the conventional 4‐blade impeller. To assess the efficiency of power consumption, the mass transfer coefficient, kLa, for a gas–liquid system in a stirred tank reactor was correlated as a function of dimensionless combination of power number and gas holdup; where α is the bubble collision frequency (s−1) and β is the gas‐inducing enhancement factor.CONCLUSIONThree different regimes of volumetric mass transfer coefficient were identified in the stirred tank reactor for each case. Overall, the combination of qualitative and quantitative analyses of ERT along with dissolved oxygen concentration permitted an insightful analysis of the self‐gas inducing impeller as a superior mixing technology for potential industrial applications involving gas–liquid operations. © 2017 Society of Chemical Industry

Bubble swarm characteristics in a bubble column under high gas holdup conditions

Local bubble characteristics were measured using a novel monofibre optical probe manufactured for elevated pressures and high gas holdups. The objective of this study is to test the performance of the optical probe, as well as to investigate the impact of operating conditions on bubble characteristics in a bubble column when operating under high gas holdup conditions. Experiments were conducted in a 101.6mm diameter column operating at pressures up to 6.5MPa. A 0.5wt% aqueous ethanol solution was used in this study to simulate high gas holdups observed in many industrial reactors containing liquid mixtures with surface-active compounds. The probe struggled to detect all the bubbles due to significant bubble size reduction and non-rectilinear bubble rise. However, successfully detected bubbles were deemed representative of the mean bubble rise velocity as indicated by the dynamic gas disengagement technique. Global gas holdup profiles in conjunction with the drift flux analysis showed that pressure does not have a significant impact at elevated liquid flow rates as bubbles tend to follow the convective current of the liquid. Relatively narrow chord length distributions were observed, where 90% of the bubbles were 1.0mm or less. The energy dissipated through the gas–liquid distributor plate was shown to have a significant impact on the initial bubble size generated and high gas holdups were also achieved at atmospheric pressure by varying the open-surface area of the distributor.

Experimental investigation of the bubble behavior in an aerated coaxial mixing vessel through electrical resistance tomography (ERT)

To enhance the efficiency of the aerated mixing vessel, it is critical to investigate the influence of the flow hydrodynamics generated by the impeller on the bubble size and gas holdup. In the current work, the global and local gas holdup values and the Sauter mean bubble diameter for an aerated coaxial mixing vessel equipped with a central pitched blade turbine and an anchor impeller were estimated for the viscous corn syrup solution. The main characteristics of the bubbles including the number of bubble size classes and the contribution of each class of bubble in the overall gas holdup were measured by using the dynamic gas disengagement theory coupled with the electrical resistance tomography (ERT) data. The impacts of the speed ratio and the gas flow rate on the bubble behavior (size and dispersion) were also explored. Bubbles generated at the central impeller plane were smaller than those generated beneath and above the impeller. It was shown that the speed ratio had no effect on the number of the bubble classes existed in the predefined liquid levels of the aerated coaxial mixing vessel. However, the gas holdup related to each class of bubble was affected by changing the speed ratio. The minimum overall gas holdup was obtained at the speed ratio of 10 for all studied gas velocities. In addition, the effect of the speed ratio on the bubble breakup through anchor blades was demonstrated. Gas holdup was correlated to the speed ratio, aerated power per liquid volume, and gas velocity.

Global performance parameters for different pneumatic bioreactors operating with water and glycerol solution: experimental data and CFD simulation.

Global variables play a key role in evaluation of the performance of pneumatic bioreactors and provide criteria to assist in system selection and design. The purpose of this work was to use experimental data and computational fluid dynamics (CFD) simulations to determine the global performance parameters gas holdup ([Formula: see text]) and volumetric oxygen transfer coefficient (k L a), and conduct an analysis of liquid circulation velocity, for three different geometries of pneumatic bioreactors: bubble column, concentric-tube airlift, and split tube airlift. All the systems had 5 L working volumes and two Newtonian fluids of different viscosities were used in the experiments: distilled water and 10 cP glycerol solution. Considering the high oxygen demand in certain types of aerobic fermentations, the assays were carried out at high flow rates. In the present study, the performances of three pneumatic bioreactors with different geometries and operating with two different Newtonian fluids were compared. A new CFD modeling procedure was implemented, and the simulation results were compared with the experimental data. The findings indicated that the concentric-tube airlift design was the best choice in terms of both gas holdup and volumetric oxygen transfer coefficient. The CFD results for gas holdup were consistent with the experimental data, and indicated that k L a was strongly influenced by bubble diameter and shape.

Oxygen transfer in different pneumatic bioreactors containing viscous Newtonian fluids

Pneumatic reactors have been widely employed in various sectors of the chemical industry, especially where it required high heat and mass transfer rates. This study aimed to obtain correlations that allow the prediction of global gas hold-up (ɛG) and volumetric oxygen transfer coefficient (kLa), and compare these values, for three models of pneumatic reactors on two scales utilizing viscous Newtonian fluids. Values of kLa were obtained using the dynamic pressure-step method, while for ɛG was used a new proposed of measure. Comparing the three models of reactors studied, it was observed that the mass transfer was superior to draft-tube airlift of 10L working volume. All correlations showed good fit to the experimental data, and comparisons with correlations from the literature demonstrate the need for further similar studies due to shortage of data available, mainly for airlift reactors operated with viscous Newtonian fluids. The proposed correlations in the present study allow prediction of the main mass transfer parameters and thus contribute significantly for project, operation, and scale-up of bioprocesses involving high density cell cultures of Newtonian characteristic.

CFD Prediction of Gas-Liquid Flow in an Aerated Stirred Vessel Using the Population Balance Model

Abstract The main topic of this study is the experimental measurement and mathematical modelling of global gas hold-up and bubble size distribution in an aerated stirred vessel using the population balance method. The air-water system consisted of a mixing tank of diameter T = 0.29 m, which was equipped with a six-bladed Rushton turbine. Calculations were performed with CFD software CFX 14.5. Turbulent quantities were predicted using the standard k-ε turbulence model. Coalescence and breakup of bubbles were modelled using the homogeneous MUSIG method with 24 bubble size groups. To achieve a better prediction of the turbulent quantities, simulations were performed with much finer meshes than those that have been adopted so far for bubble size distribution modelling. Several different drag coefficient correlations were implemented in the solver, and their influence on the results was studied. Turbulent drag correction to reduce the bubble slip velocity proved to be essential to achieve agreement of the simulated gas distribution with experiments. To model the disintegration of bubbles, the widely adopted breakup model by Luo & Svendsen was used. However, its applicability was questioned.

A novel airlift reactor enhanced by funnel internals and hydrodynamics prediction by the CFD method

Airlift reactors have been used widely in many industrial processes, but little work has been conducted on such reactors integrated with internals. In this study, a novel airlift reactor with a funnel internal was developed to achieve better flow conditions and advantages in biological processes. The CFD (computational fluid dynamics) simulation method was employed to investigate the effect of the funnel internals on hydrodynamic properties in the reactor. A CFD model was developed for gas–liquid two-phase flow simulation in a bench-scale reactor. Grid-independent simulation results were verified with global-scale experimental data. The results showed that the local or global gas holdup could be enhanced by 15% and that turbulent kinetic energy could be reduced by a maximum of 7.8% when the superficial gas velocity was 1cm/s. These features are beneficial for applications in stress-sensitive biological processes.

Unified study of flow regimes and gas holdup in the presence of positive and negative surfactants in a non-uniformly aerated bubble column

In this paper, a study on the global gas holdup and hydrodynamic flow regimes developed in a partially aerated bubble column at variable air superficial velocities ( U G ) in the presence of positive and negative surfactants is presented. According to the results obtained, despite the different liquid phase properties variation caused by the presence of positive (alcohols) and negative (electrolytes) surfactants, both reduce coalescence and the effect in the gas holdup ( ε G ) is equivalent: it increases with the surfactant concentration ( C) but only when the ( C/ C t ) ratio is clearly above 1, being C t the transition concentration. Contrary to the results obtained for totally aerated bubble columns, for lower values of the ( C/ C t ) ratio, the holdup remains practically invariable. Considering the crucial role that C and C t play in the resulting ε G , a new prediction equation for ε G accounting for the ratio ( C/ C t ) and U G is presented and its performance for both types of surfactants validated. Additionally, visual and wall pressure fluctuations studies reveal that the vortical flow (VF), characterized by an oscillating bubble plume, prevails in ultrapure water (UPW) but results destabilized in the presence of surfactants. This destabilization results in an evolution to a pseudo-steady flow regime, the double cell turbulent flow regime (DCTF), characterized by a quasi-static bubble jet, located at the column centerline that determines the appearance of two static symmetrical vortices

Gas Hold-Up and Mass Transfer in Three Geometrically Similar Internal Loop Airlift Reactors Using Newtonian Fluids

Global gas hold-up and volumetric oxygen transfer coefficient (kLa) were evaluated in three internal-loop airlift reactors (ALR) with different working volumes (2, 5, and 10 dm3) and similar geometric configuration utilizing eight Newtonian fluids. Superficial gas velocity in the riser (UGR) was ranged from 0 to 0.112 ms-1. The effects of the superficial gas velocity (UGR) and liquid viscosity had contrary effects on kLa. However, they presented the same order of magnitude showing that the viscosity effect on the oxygen mass transfer cannot be neglected. Two correlations based on dimensional analysis relating global gas hold-up and kLa to the geometric parameters, physical properties of the fluids, and operational conditions were proposed. Very good fittings between calculated and experimental data of and kLa were obtained. The influence of the internal diameter of the reactor on kLa was considered positive showing that in large-scale reactors an appropriate oxygen transfer can be reached under smaller aeration conditions, due to higher internal diameter.

Void Fraction Distributions in Cold-gassed and Hot-parged Three Phase Stirred Tanks with Multi-impeller

Vertical distributions of void fraction in gas-liquid and gas-liquid-solid stirred tanks have been measured in a fully baffled dished base vessel of 0.48 m diameter, using a conductivity probe. The impeller configuration (a hollow half elliptical blade dispersing turbine below two up-pumping wide blade hydrofoils, identified as HEDT+2WH U) recommended in previous work has been used in this work. The operating temperatures were 24°C and 81°C, identified as cold and hot respectively. The effects of superficial gas velocity, agitator speed and the corresponding power input on the local void fraction in two-phase systems are investigated and discussed. Results show that the increasing of agitator speed or gas flow rate leads to an increase in local void fraction at the majority of measurement points in both cold and hot systems. However, the uniformity of gas dispersion does not always increase as the raising of agitator speed and power input. In either cold or hot sparged conditions, the two- and three-phase systems have similar vertical profiles for void fraction, with maxima in similar locations; however, the void fractions are significantly lower in hot sparging than with cold. In cold operation the presence of particles leads to a lower void fraction at most points, although the local void fractions increase a little with the addition of solid particles at high temperature, in good agreement with the global gas holdup results, and the possible reasons are discussed in this paper. This work can give a better understanding of the differences between cold-gassed and hot-sparged three phase stirred tanks.

Simultaneous Computational Fluid Dynamics (CFD) Simulation of the Hydrodynamics and Mass Transfer in a Partially Aerated Bubble Column

In this work, simultaneous hydrodynamics and mass-transfer computational fluid dynamics (CFD) studies in a rectangular partially aerated bubble column were conducted. The proposed transient Eulerian−Eulerian model was applied to a two-phase flow composed of polydispersed air and water. Four different kl correlations were implemented in the CFD code, and the results were compared. At superficial gas velocity values ranging from 2.4 mm/s to 21.3 mm/s, the developed flow regimes were experimentally characterized by means of visual observations, global gas holdup, plume oscillation period, and Sauter mean bubble diameter measurements. Simultaneously, the transfer of oxygen from the disperse phase to the initially deoxygenized water was characterized by the measurement of the evolution of the dissolved oxygen concentration with time. Comparison between the experimental and simulated parameters previously mentioned allows for the validation of the computational model. According to the results presented in this ...

Study on Mass Transfer of Isopropylbenzene and Oxygen in a Two-Phase Partitioning Bioreactor in the Presence of Silicone Oil

A two-phase partitioning bioreactor to treat gas effluents polluted by volatile organic compound has been developed. In this work, both the mass transfer of isopropylbenzene (IPB) and oxygen have been considered in relation to their influence on the hydrodynamics of the reactor and the type of silicone oils used as a second phase. The synergistic effect of silicone oil and stirrer speed on the global oxygen mass transfer coefficient (KLa) and gas holdup (up to 12%) have been investigated. The addition of 10% of low viscosity silicone oil (10 cSt) in the reactor does not significantly affect the oxygen transfer rate. The very high solubility of IPB in the silicone oil leads to an enhancement of driving force term, especially for high fraction of silicone oil. However, it does not seem useful to exceed a volume fraction of 10% since KLa(IPB) decreases sharply at higher proportions of silicone oil. KLa(IPB) and KLaO2 evolve in the same way with the proportion of silicone oil. These results confirm the potentialities of our bioreactor to improve both the oxygen and pollutant gas transfer in the field of the treatment of gaseous pollutants, even for highly concentrated effluents.

Influence of the lift force closures on the numerical simulation of bubble plumes in a rectangular bubble column

Numerical Eulerian–Eulerian simulations of the unsteady gas–liquid flow in a centrally aerated two-dimensional bubble column were carried out in order to understand the effect of different formulations of the lift force coefficient ( C L ) on the computational results. Three different values of the superficial gas velocity ( U G =2.4, 12.0 and 21.3 mm s −1) that ensure the existence of different flow regimes were experimentally and computationally studied. The validation of the simulated results was based on visual observations and measurements of the global gas hold-up ( ε G ) and the plume oscillation period (POP). The results presented reveal that, at U G =12.0 and 21.3 mm s −1, using C L <0 results in under- and over-estimation of the ε G and POP, respectively. On the other hand, taking C L >0 does not affect the POP while it leads to increasingly higher ε G values, which are different from those experimentally reported. At U G =2.4 mm s −1, the effect of the lift force is not so evident, although it slightly improves the prediction of experimental values. Particularly interesting is the case of C L >0.4 at U G =21.3 mm s −1, producing a non-symmetric bubble plume oscillation. Since using Tomiyama's lift coefficient correlation does not improve the results, including the lift force into the simulation of bubble plumes is not recommended.

Effect of solid characteristics on hydrodynamic and mass transfer in a fixed bed reactor operating in co-current gas–liquid up flow

In order to understand better which parameters control the mass transfer efficiencies in biofilters, hydrodynamic and mass transfer parameters have been investigated for seven different packings in a three phase fixed bed reactor operating in co-current gas–liquid up flow. For this purpose, the packing characteristics (bed porosity, particle size and particle shape) and hydrodynamics parameters have been determined. Thus, the pressure drops, global gas hold-up, static gas fraction, slip velocities and bubble size have been investigated. For each type of solid, volumetric oxygen mass transfer coefficients have been measured under different operating conditions of gas (air) and liquid (tap water). The experiments were performed in a cylindrical PVC column with co-current up-flow of gas and liquid at laboratory temperature and atmospheric pressure. Air was injected at the bottom of the column using a porous disc diffuser. The superficial gas velocity ranged from 2.3E−3 m/s to 2.5E−2 m/s, the superficial liquid velocity varied from 1.2E−3 m/s to 3.3E−3 m/s. The column of 0.15 m in diameter and 4 m height was filled with packing. The following values were determined: • Pressure drop through the bed height was measured using differential manometers. • The average values of the global gas hold-up and static gas fraction were determined in the experimental set up by a new technique which is based on a monitoring of liquid flow. • The bubble sizes and slip velocities were determined by image acquisition and data treatment systems on a particular glass cell. • The volumetric oxygen mass transfer coefficient was determined by gas and liquid mass balance techniques. These different techniques applied on the fixed bed bubble column enabled for each type of solid a complete characterisation of hydrodynamic and mass transfer to be described. These experimental tools were used to compare and evaluate the influence of solid characteristics on the reactor performances and to initiate a mass transfer model.

Gas hold‐up estimation in bubble columns using passive acoustic waveforms with neural networks

AbstractPassive acoustic waveforms produced experimentally from a bench‐scale two‐phase bubble column were recorded using a miniature hydrophone at three axial positions. The generated acoustic waveforms were processed and trained using artificial intelligence against global gas hold‐up measurements. Two neural network architectures, the radial basis function (RBF) neural network and the recurrent Elman neural network, were employed. Both neural network techniques achieved accurate gas hold‐up estimation, characterised by low mean square errors of 2.70 and 1.68% for the RBF and recurrent Elman networks respectively. The designed and trained neural networks were found to be a powerful tool for learning and replicating complex two‐phase patterns. Passive acoustic waveforms were found to be a useful measuring technique for gas hold‐up estimation in bubble columns under moderate operating conditions. Copyright © 2006 Society of Chemical Industry

LOCAL HYDRODYNAMICS IN AN EXTERNAL LOOP AIRLIFT SLURRY REACTOR WITH AND WITHOUT A RESISTANCE-REGULATING ELEMENT

The hydrodynamic behavior of an external loop airlift slurry reactor (ALSR) with and without a resistance-regulating element was studied with a fiber optic probe and ultrasound Doppler velocimetry. The influences of the superficial gas velocity and solid holdup on the global gas holdup and radial profiles of the suspension circulation velocity in the downer and of gas holdup, bubble size, and bubble rise velocity in the riser were studied. Local measurements allow a better understanding of the flow behavior in the reactor and can be used for CFD modeling and validation. Experimental results show that the resistance-regulating element increases the gas holdup and decreases the suspension circulation velocity, indicating that an optimum design of the flow resistance is needed to obtain the maximum gas-liquid volumetric mass transfer coefficient for a specific superficial gas velocity. A high superficial gas velocity and low solid holdup are favorable for increased uniformity of the radial profile of the gas holdup and bubble rise velocity. Hydrodynamic models that predict the gas holdup and suspension circulation velocity were developed for an ALSR with and without a resistance-regulating element. Good agreement was obtained between measured and predicted values.

PIV measurements of flow in an aerated tank stirred by a down- and an up-pumping axial flow impeller

Liquid phase hydrodynamics in an aerated tank stirred by a down- and an up-pumping pitched blade turbine have been investigated using particle image velocimetry. The effect of agitator configuration and the gas phase on the mean velocity fields and turbulent quantities in the vessel have been investigated. The global mean gas holdup has also been evaluated for the two pumping conditions. For the gas flow rate used, the presence of gas only slightly alters the liquid flow patterns produced by both the down- and up-pumping configurations and causes a general decrease in the mean liquid velocities. The turbulent kinetic energy in the impeller discharge region was not affected by the presence of gas, but in the bulk of the tank, aeration caused a decrease in this value. Global gas holdup was found to be ∼36% greater for the up-pumping impeller and a large amount of gas was found to be entrained by the primary circulation loop.

Some effects of rheology on the spatial distribution of gas hold-up in a mechanically agitated vessel

The effects of rheology on gas hold-up in non-Newtonian circulation flow has been rarely discussed in the literature, especially so far as local gas distribution in mechanically agitated vessels is concerned. This study analyses experimental gas dispersion behaviour for a wide range of complex-rheology conditions. Non-Newtonian aqueous solutions of xanthan gum with concentrations from 0.5 to 5 kg/m 3 and Newtonian glycerol solutions of 450 and 800 kg/m 3 containing also 5% electrolyte are studied. Hold-up is measured conductometrically at 36 spatial points within the stirred vessel mainly around and above the impeller for a conventional geometry (Rushton standard configuration with T=0.2 m and D/ T=0.33). In addition to increasing viscosity, three aspects of rheology impact on gas hold-up and different dominances of component effects upon the overall gas dispersion mechanism are revealed. The results are presented as local and global gas hold-up vs. consistency, pseudoplasticity, and position. By comparing data obtained for Newtonian and non-Newtonian flow conditions and data obtained for the same polymer solution at constant plasticity and variable consistency, an attempt to reveal the net effects of consistency and pseudoplasticity is made. The boundaries of regions with large differences of mixing intensity reflected in terms of local gas availability are quantified.

Measurement of gas hold-up in bubble columns from low frequency acoustic emissions

This paper demonstrates a novel method of global gas hold-up estimation in a laboratory scale bubble column using acoustic emissions from within the column, which are caused by bubble generation. Bubble formation excites the column to resonate at its natural frequencies; these frequencies are determined by the vessel dimensions. Peaks in the 0–1000 Hz range of the acoustic spectrum were observed and identified as the column resonance frequencies. Measured changes in resonance frequencies for a range of global gas hold-ups (up to 20%) correlated well with theory. This method of global gas hold-up monitoring should be particularly useful for systems where measurements of changes in liquid height are not possible.