Global Gas Holdup Research Articles

Comprehensive investigation of gas hold-up in a double coaxial mixer with shear-thinning fluids exhibiting yield stress: Experimental, numerical, and artificial neural network approaches

This study addresses the challenge of uneven gas dispersion in yield-stress, non-Newtonian fluids, commonly encountered in industries such as biopharmaceuticals, cosmetics, and food processing. While previous research demonstrated the advantages of dual coaxial mixers for pseudoplastic fluids, limited attention has been given to aerating yield-pseudoplastic fluids with higher aspect ratios. This study bridges that gap by investigating both local and global gas hold-up under various conditions, utilizing electrical resistance tomography and computational fluid dynamics. Key findings showed that increasing the anchor speed from stationary to 30 rpm significantly enhanced aeration efficiency (gas hold-up per specific power consumption), with improvements of 78 % in UP-CO mode and 25 % in UP-COUNTER mode at Nc = 350 rpm and Qg = 20 L/min. These results underscore enhanced gas dispersion under specific operating conditions, driving overall process intensification. To ensure accurate prediction of gas hold-up, both dimensional and dimensionless empirical correlations, along with an artificial neural networks (ANNs) model, were developed. The ANNs model exhibited superior accuracy, achieving R² values of 0.99 for both rotation modes, outperforming empirical models, which achieved R² values of 0.90 and 0.89 for UP-CO and UP-COUNTER modes, respectively.

Numerical and experimental estimation of global gas holdup in a bubble column using computational fluid dynamics (CFD)

Numerical and experimental estimation of global gas holdup in a bubble column using computational fluid dynamics (CFD)

Effects of SDS surface-active agents on hydrodynamics and oxygen mass transfer in a square bubble column reactor: Experimental and CFD modeling study

This work examined experimentally and numerically the effects of sodium dodecyl sulfate (SDS) surfactant agent on mass transfer coefficients kLaL and kL in a high-aspect square bubble column reactor. To this end, SDS aqueous solutions at concentrations between 5 ppm and 30 ppm were prepared. Superficial gas velocity was varied from 0.74 cm.s−1 to 9.44 cm.s−1 covering all flow regimes to analyze the interaction between different phases and the effect of liquid surface tension on gas-liquid interfaces. Experimental data indicated that the presence of the SDS in the liquid phase augmented the overall gas holdup up to 25% at a constant gas velocity (Ug = 7.3 cm.s−1) in the transition regime, which is due to the role of SDS in bubble interactions. The impact of SDS addition on the mass transfer is twofold. Firstly, it reduced kL because the SDS molecules migrate towards the bubble-liquid interface thanks to the lower O2 diffusivity in water, thus hindering the O2 transfer from one phase to the other. Secondly, it increased aL owing to the inhibition of bubbles coalescence and, therefore, the lower mean bubble size. Among these two effects, the latter is prevailing, which led to an increase in kLaL in contaminated systems. In the second part of this work, a population balance model (PBM) was coupled with a computational fluid dynamics (CFD) model based on large-eddy simulation (LES) to analyze the effect of both tap water and SDS contaminated systems on the breakage phenomena, local flow pattern, and the resulting hydrodynamics and mass transfer features in the reactor. Ultimately, experiments and numerical simulations agreed well in terms of global gas holdup and kLaL coefficient. In particular, the transition points in the case of the SDS media (except for foam formation) provided the model with adequate PBM definitions for the dispersed phase.

Computational Fluid Dynamics Modelling of Two-Phase Bubble Columns: A Comprehensive Review

Bubble columns are used in many different industrial applications, and their design and characterisation have always been very complex. In recent years, the use of Computational Fluid Dynamics (CFD) has become very popular in the field of multiphase flows, with the final goal of developing a predictive tool that can track the complex dynamic phenomena occurring in these types of reactors. For this reason, we present a detailed literature review on the numerical simulation of two-phase bubble columns. First, after a brief introduction to bubble column technology and flow regimes, we discuss the state-of-the-art modelling approaches, presenting the models describing the momentum exchange between the phases (i.e., drag, lift, turbulent dispersion, wall lubrication, and virtual mass forces), Bubble-Induced Turbulence (BIT), and bubble coalescence and breakup, along with an overview of the Population Balance Model (PBM). Second, we present different numerical studies from the literature highlighting different model settings, performance levels, and limitations. In addition, we provide the errors between numerical predictions and experimental results concerning global (gas holdup) and local (void fraction and liquid velocity) flow properties. Finally, we outline the major issues to be solved in future studies.

Investigation of power consumption, torque fluctuation, and local gas hold-up in coaxial mixers containing a shear-thinning fluid: Experimental and numerical approaches

The performance of a coaxial bioreactor in the intensification of gas dispersion in shear-thinning fluids in terms of the power consumption of the central and anchor impellers, torque fluctuation for the anchor impeller, global and local gas hold-up, and cavity size was assessed. Electrical resistance tomography and computational fluid dynamics methods were used to evaluate the effects of the central impeller type, impeller speed, pumping direction, and rotating mode on the efficacy of the aerated coaxial mixers. Unlike the coaxial mixer equipped with a radial-flow impeller, for the coaxial mixer furnished with an axial-flow impeller, the rotating mode had a pronounced effect on the relative power demand. Surprisingly, it was observed that increasing the central impeller speed increased the contribution of the anchor impeller to the total power consumption of the co-rotating coaxial mixer furnished with a down-pumping impeller. Furthermore, by increasing the central impeller speed the torque fluctuation of the coaxial mixer was diminished due to the uniform distribution of the energy dissipation rate. Unexpectedly, for a coaxial mixer furnished with a down-pumping impeller in both rotating modes and an up-pumping impeller in the counter-rotating mode, the gas hold-up decreased by a further increase in the central impeller speed.

Effects of flow velocity and bubble size distribution on oxygen mass transfer in bubble column reactors-A critical evaluation of the computational fluid dynamics-population balance model.

Computational fluid dynamics (CFD) is used to simulate a bubble column reactor operating in the bubbly (homogenous) regime. The Euler-Euler two-fluid model, integrated with the population balance model (PBM), is adopted to compute the flow and bubble size distribution (BSD). The CFD-PBM model is validated against published experimental data for BSD, global gas holdup, and oxygen mass transfer coefficient. The sensitivity of the model with respect to the specification of boundary conditions and the bubble coalescence/breakup models is assessed. The coalescence model of Prince and Blanch (1990) provides the best results, whereas the output is shown to be insensitive to the breakup model. The CFD-PBM study demonstrates the importance of considering the BSD in order to correctly model mass transfer. Results show that the constant bubble size assumption results in a large error in the oxygen mass transfer coefficient, while giving acceptable results for gas holdup. PRACTITIONER POINTS: Constant bubble size (CBS) and population balance model (PBM) are compared for a bubble column reactor. Both PBM and CBS can predict gas holdup; however, PBM can correctly predict gas-liquid mass transfer whereas CBS cannot. Best practices for selecting coalescence, breakup, and drag models are determined.

Computational Modeling of the Impact of Solid Particles on the Gas Hold-up in Slurry Bubble Columns

Three-phase bubble column reactors, despite their broad diffusion in chemical process, are often difficult to describe given the intricate mutual phase interactions. Moreover, validation of computational models is likewise complex, since it is considerably challenging to obtain exhaustive and precise experimental data from these systems. The aim of this work is to predict reliably the effect of disperse solid particles on the overall fluid dynamics of a bubble column by computational fluid dynamics.The small size of particles allowed us to approximate the solid-liquid mixture as a single pseudo-homogeneous phase in the Euler-Euler framework. The simulation settings were tuned on two-phase gas-liquid systems and then extended to the three-phase gas-solid-liquid columns. Experimental data from different setups and correlations for the global gas hold-up in slurry systems were used to validate the model. In particular, what emerges from experiments is that the presence of the solid reduces gas hold-up, as a consequence of the higher density of the slurry in comparison to the pure liquid and the increased coalescence of the bubbles.Results show that this model is able to predict the reduction of the hold-up with a solid volumetric loading up to 20%, achieving good agreement with experimental data. Moreover, what stands out from the simulations is that the addition of the solid also changes the shape of the flow map curve (global hold-up as a function of the gas superficial velocity), switching to a smoother transition between the behavior at low and high gas superficial velocity.

Computational Fluid Dynamics Simulation of Hydrodynamics in a Two-Stage Internal Loop Airlift Reactor with Contraction-Expansion Guide Vane.

Global circulation and liquid back mixing adversely affect the continuous production of a multistage internal airlift loop reactor. A contraction-expansion guide vane (CEGV) is proposed and combined with a two-stage internal loop airlift reactor (TSILALR) to suppress the liquid back mixing between stages. A computational fluid dynamics (CFD) simulation is conducted to evaluate the performance of the CEGV in the TSILALR. The bubble size distribution and turbulent flow properties in the TSILALR are considered in the CFD simulation by using the population balance model and RNG k-ε turbulence model. The CFD model is validated against the experimental results. The deviations in the gas holdup and mean bubble diameter between the simulation and experimental results are less than 8% and 6%, respectively. The streamlines, flow pattern, bubble size distribution, and axial liquid velocity in the TSILALRs with and without the CEGV at superficial velocities of 0.04 and 0.08 m/s are obtained by CFD simulation. It has been shown that the CEGV generated local circulation flows at each stage instead of a global circulation flow in the TSILALR. The average global gas holdup in the TSILALR with a CEGV increased up to 1.98 times. The global gas holdup increased from 0.045 to 0.101 and the average axial velocity in the riser decreased from 0.314 to 0.241 m/s when the width of the CEGV increased from 50 to 75 mm at the superficial gas velocity of 0.08 m/s.

Bubble hydrodynamics and mass transfer in stirred tank with non-Newtonian fluids: Scale-up from laboratory to pilot-scale

Mass transfer is a crucial phenomenon in designing and scaling up chemical and biochemical stirred tanks. The literature lacks a pilot-scale study on investigating mass transfer in non-Newtonian fluids. A pilot-scale study is a prerequisite step before scaling up the process from laboratory to industrial-scale. Thus, a study using pilot-scale stirred tank was conducted to investigate bubble hydrodynamics and mass transfer in non-Newtonian fluids. This work is a scale-up study from laboratory to pilot-scale. Axial distributions of bubble–liquid mass transfer coefficient and interfacial area were obtained using dedicated in situ optical endoscope probes (oxygen and bubble size) simultaneously. Volumetric mass transfer coefficient was determined by recording local dissolved oxygen concentrations in liquid. Interfacial area was estimated by measuring local bubble size and global gas holdup. Bubble–liquid mass transfer coefficient was then deduced by combining the obtained values of volumetric mass transfer coefficient and interfacial area. Effects of operating conditions, fluid rheology, and probe axial positions (liquid height) on bubble–liquid mass transfer coefficient were considered. The operating conditions (power inputs and superficial gas velocities) were in the range of 30–250 W/m3 and 3.10–4.70 mm/s, respectively. Bubble–liquid mass transfer coefficient increased with an increase in operating conditions, whereas it decreased with an increase in fluid rheology and liquid height. Scale-up effects on mass transfer were higher for water than viscous fluids, as suggested by large deviation (9.6%) in values of bubble–liquid mass transfer coefficient.

QEEFoam: A Quasi-Eulerian-Eulerian model for polydisperse turbulent gas-liquid flows. Implementation in OpenFOAM, verification and validation

In this paper we present a new multiphase computational model for polydisperse turbulent gas-liquid flows. In this model the gas phase is transported by a single convection equation and the effect of turbulent dispersion is addressed by including a diffusion term. In order to close the system of equations, the gas phase velocity is calculated by employing the slip velocity concept or by solving an ODE. This procedure shares similarity with the Euler-Lagrange (E-L) method, in which the gas velocity is updated by bubble Lagrangian tracking, and with the Euler-Euler (E-E) method, and for this reason it is called the Quasi-Eulerian-Eulerian (Q-E-E) method. In order to account for polydispersity one single transport equation is added to describe the effects of bubble breakage and coalescence on the mean bubble size. The novel Q-E-E method was implemented in the open-source code OpenFOAM-7 and was used to simulate turbulent gas-liquid flows with three different geometries operating under different conditions. The predictions for the dynamical vortex structures, local phase fraction, global gas holdup, mean bubble size and vertical/horizontal liquid velocities were verified against the solution provided by the E-L solver or against published experimental data. Good agreement was found and with extremely small computational costs.

Prediction of gas holdup in partially aerated bubble columns using an EE-LES coupled model

Transient gas-liquid flow dynamics in partially aerated square cross-sectioned bubble columns were studied using an Eulerian–Eulerian based large eddy simulation approach. The numerical model was validated with the interfacial closures evaluated for drag, lift and virtual mass forces. The simulation data showed good agreement with both numerical predictions and experimental measurements in the literature. The current model was also shown to be able to predict the global gas holdup in the bubble column up to a very high value. Further simulations were performed to study the influences of initial liquid phase height to width ratio and inlet area to column cross-sectional area ratio on the global gas holdup in the bubble column. The numerical results indicated that the global gas holdup increases linearly with the superficial gas velocity. Thus, an empirical multivariate regression model was proposed to predict the global gas holdup with an adjusted R-square value of 0.99.

Bubble size and liquid-side mass transfer coefficient measurements in aerated stirred tank reactors with non-Newtonian liquids

Oxygen transfer is a key element in aerobic fermentations, especially if the culture broth’s rheology is non-Newtonian, as in the case of cultures of filamentous fungi. Viscosity negatively affects the volumetric mass transfer coefficient (kLa), but mechanisms involved in terms of change of interfacial area (a) and liquid-side mass transfer coefficient (kL) have still not been clearly identified. This lack of knowledge is in part due to the difficulty in measuring bubble size in viscous fluids. In this study, an innovative technique was used to measure bubble Sauter diameter (d32) in water and in xanthan gum solutions. Additional experiments were carried out to obtain the volumetric mass transfer coefficient and the global gas holdup (αG). It was then possible to obtain the liquid-side mass transfer coefficient and to compare it with existing models. Finally, empirical correlations were proposed to estimate d32,αG,kLa and kL in a mechanically stirred tank.

Hydrodynamics and mass transfer in a slurry external airlift loop reactor integrating mixing and separation

A pilot external airlift loop reactor (EALR) integrating mixing and separation was first proposed by combining directional flow in the EALR and liquid-solid separation in two parallel hydrocyclones. In this developed reactor, mixing, mass transfer, and liquid-solid separation can be simultaneously realized without extra energy input. Technical feasibility was firstly verified, and then hydrodynamics and mass transfer were measured. It was found that the minimum diameter of solid particles retained for circulation was 6.82 μm, and the treatment capacity of clean liquid was up to 3.0 m3/h. Additionally, global gas holdup, liquid circulating velocity, and volumetric mass transfer coefficient increased monotonously with increasing the superficial gas velocity while declined with the increase of solid concentration. Axial inhomogeneity of solid particles was decayed with increasing the superficial gas velocity and solid concentration. This new type of slurry EALR is promising to be applied in the gas-liquid-solid catalytic industries.

Effect of gas distributor on gas–liquid dispersion and mass transfer characteristics in stirred tank

Aiming at the fact that the function of gas distributor is often neglected in the case of high viscosity conditions, a novel spherical micro-orifice gas distributor was proposed and investigated experimentally in water system and non-Newtonian viscous system, respectively. The comparisons of gas–liquid dispersion and mass transfer performance between novel spherical micro-orifice gas distributor and traditional ring distributor were also done. Furthermore, the effects of gas distributors on the gas–liquid dispersion and mass transfer characteristics of three wide-viscosity-range impellers including LDB impeller, FZ impeller and MB impeller were also researched, and an impeller-distributor configuration with good gas–liquid dispersion and mass transfer performance was achieved. The results show that, compared with ring gas distributor, the bubbles dispersed by novel spherical micro-orifice gas distributor are smaller, and the gas holdup and volumetric mass transfer coefficient are relatively higher under the same power consumption per unit volume. Meanwhile, the significant effect of gas distributor geometry on gas dispersion was found for high-viscosity liquid, but not relevant for low-viscosity batches. In addition, compared with other impeller-distributor configurations, the configuration consisting of FZ impeller and novel spherical micro-orifice gas distributor can achieve greater global gas holdup and higher oxygen mass transfer rate, which is recommended especially in viscous system with high gas flow rate. The research is beneficial to promote the application of novel spherical micro-orifice gas distributor in stirred tanks.

Gas distribution characteristics for heterogeneous flows in the slender particle-containing scrubbing–cooling chamber of an entrained-flow gasifier

Dual-tip conductivity probes and a high-speed camera were used to study the gas distribution characteristics in a cold model apparatus of the slender particle-containing scrubbing–cooling chamber. The three-dimension column was 200mm in internal diameter and 1353mm in height. The effects of superficial gas velocities, fiber volume fractions and fiber aspect ratios on global gas holdups were determined. The results showed that under the influence of reverse buoyancy and negative pressure gradients, plume gas flow went up along the outside of the downcomer in downcomer spout region. Local radial gas holdup which was closely related with the internals and flow patterns presented obviously proximate core peak distribution in bubble-breaking plate region. The foam region consisted of bubble coalescence and breakup as well as droplets formation and splashing. At higher gas velocities, the increasing rate of global gas holdup gradually reduced. With the increase of fiber volume fraction, global gas holdup slightly increased at low gas velocities due to suppressed bed turbulence and bubble loading, but decreased at high velocities for increased liquid viscosity and vortex-shedding. Due to suppressed small-scale velocity fluctuation, global gas holdup increased with increased fiber aspect ratio. A modified drift-flux model containing crowding factor and fiber number density for global gas holdup correlations was proposed and all of the data could be calculated with a relative deviation less than 10%.

Hydrodynamic performance of a single-use aerated stirred bioreactor in animal cell culture: applications of tomography, dynamic gas disengagement (DGD), and CFD.

The hydrodynamics of gas-liquid two-phase flow in a single-use bioreactor were investigated in detail both experimentally and numerically. Electrical resistance tomography (ERT) and dynamic gas disengagement (DGD) combined with computational fluid dynamics (CFD) were employed to assess the effect of the volumetric gas flow rate and impeller speed on the gas-liquid flow field, local and global gas holdup values, and Sauter mean bubble diameter. From the results obtained from DGD coupled with ERT, the bubble sizes were determined. The experimental data indicated that the total gas holdup values increased with increasing both the rotational speed of impeller and volumetric gas flow rate. Moreover, the analysis of the flow field generated inside the aerated stirred bioreactor was conducted using CFD results. Overall, a more uniform distribution of the gas holdup was obtained at impeller speeds≥100rpm for volumetric gas flow rates≥1.6×10-5m3/s.

Bottom Pressure Method for the Determination of the Flooding/Loading Transition in an Aerated Vessel Stirred by a Rushton Impeller

A novel method to determine the flooding/loading transition point (Nf) was proposed based on the measurements of the bottom pressure. Experiments were carried out in an aerated vessel stirred with a Rushton impeller in single-phase and two-phase systems. The results showed that the bottom pressure (P) in the single-phase system had a parabolic decrease with the increasing impeller speed (N), which followed the Bernoulli Effect; the flooding/loading transition was defined by a sharp change in the P–N curve. The data were well consistent with the results obtained by the global gas holdup method, as well as the existing literature data on flooding/loading transition.

Hydrodynamic characteristics of an aerated coaxial mixing vessel equipped with a pitched blade turbine and an anchor

AbstractBackgroundIn this work, computational fluid dynamics (CFD) simulation of an aerated coaxial mixing vessel composed of a centered impeller and a wall scraping anchor was conducted to investigate the effects of speed ratio, rotation modes (co‐rotating and counter‐rotating), and fluid viscosity on the local and global gas holdup values, flow pattern within the vessel, and turbulent kinetic energy. To validate the developed model, simulated gas holdup and gassed power uptake were compared with the measured experimental values. To gather experimental gas holdup values, an electrical resistance tomography technique was utilized.ResultsThe results demonstrated that the co‐rotating coaxial mixer with a speed ratio higher than 10 provided a higher gas volume fraction within the vessel. It was also shown that the turbulent kinetic energy attained in the counter‐rotating mode was lower than those for the co‐rotating coaxial mixer in most regions within the mixing tank, especially near the vessel walls. The size and the number of circulation loops developed within the coaxial mixer were affected by the speed ratio.ConclusionIt was demonstrated that speed ratio and rotation mode of the impellers affected the hydrodynamics developed within the aerated coaxial mixer. © 2017 Society of Chemical Industry

CFD-PBM Approach with Different Inlet Locations for the Gas-Liquid Flow in a Laboratory-Scale Bubble Column with Activated Sludge/Water

A novel computational fluid dynamics-population balance model (CFD-PBM) for the simulation of gas mixing in activated sludge (i.e., an opaque non-Newtonian liquid) in a bubble column is developed and described to solve the problem of measuring the hydrodynamic behavior of opaque non-Newtonian liquid-gas two-phase flow. We study the effects of the inlet position and liquid-phase properties (water/activated sludge) on various characteristics, such as liquid flow field, gas hold-up, liquid dynamic viscosity, and volume-averaged bubble diameter. As the inlet position changed, two symmetric vortices gradually became a single main vortex in the flow field in the bubble column. In the simulations, when water was in the liquid phase, the global gas hold-up was higher than when activated sludge was in the liquid phase in the bubble column, and a flow field that was dynamic with time was observed in the bubble column. Additionally, when activated sludge was used as the liquid phase, no periodic velocity changes were found. When the inlet position was varied, the non-Newtonian liquid phase had different peak values and distributions of (dynamic) liquid viscosity in the bubble column, which were related to the gas hold-up. The high gas hold-up zone corresponded to the low dynamic viscosity zone. Finally, when activated sludge was in the liquid phase, the volume-averaged bubble diameter was much larger than when water was in the liquid phase.

Analysis of gas holdup and bubble behavior in a biopolymer solution inside a bioreactor using tomography and dynamic gas disengagement techniques

AbstractBACKGROUNDThe interfacial mass transfer rate in an aerated system vessel in which the liquid phase is in contact with the gas phase is closely related to the interfacial area between two phases. The gas–liquid interfacial area strongly depends on the bubble size, bubble size distribution, and the gas holdup. For the first time, in this study the performance of the ASI impeller was assessed by determining the bubble classes, bubble size distribution, local gas holdup, and global gas holdup in an aerated mixing system containing a highly viscous and non‐Newtonian biopolymer solution. The effects of the volumetric gas flow, impeller speed, and fluid rheology on the gas dispersion attained by the ASI impeller were investigated.RESULTSElectrical resistance tomography (ERT) was coupled with the dynamic gas disengagement (DGD) technique to determine the bubble classes, to measure the gas holdup, and to calculate the Sauter mean bubble diameter. The performance of the ASI impeller was compared with those of the pitched blade turbine and the Rushton impeller in terms of the bubble size distribution and gas holdup. The experimental data revealed that three classes of bubbles were formed within the aerated system. An empirical correlation for the gas holdup as a function of the gas flow number and apparent viscosity of fluid was developed for the aerated mixing tank equipped with the ASI impeller.CONCLUSIONThe ASI impeller effectively enhanced the breakage of the bubbles and gas holdup at the higher gas flow rates in a yield‐pseudoplastic fluid. © 2017 Society of Chemical Industry