CFD simulation of effects of the configuration of gas distributors on gas-liquid flow and mixing in a bubble column

Numerical simulations of gas-liquid flow in a cylindrical bubble column of 400 mm in diameter at the superficial gas velocity were conducted to investigate effects of the configuration of gas distributors on hydrodynamic behaviour, gas hold-up and mixing characteristics. Eight different gas distributors were adopted in the simulation. The simulation results clearly show that the configuration of gas distributor have an important impact on liquid velocity and local gas hold-up in the vicinity of the gas distributor. Comparisons of the overall gas holdup and mixing time among different gas distributors have demonstrated that none of the adopted gas distributors was able to produce the highest interfacial area and also yield the shortest mixing time. The CFD modelling results reveal that an increase in the number of gas sparging pipes used in gas distributors is beneficial in improving the gas hold-up but is disadvantageous in reducing bubble size due to a decrease in turbulent kinetic dissipation. It has been demonstrated from the simulations that the appearance of asymmetrical flow patterns in the bubble column and the adoption of smaller gas sparging pipes for gas distributors are effective in improving the mixing characteristics.     

Numerical and experimental investigations of mixing in T-shaped and cross-shaped micromixers

Mass transfer within the T-shaped and cross-shaped micromixers has been studied using CFD and confocal laser scanning microscopy methods. The concentration profiles, based on flow regimes, were used to compare the T- and cross-geometries. The cross-shaped micromixer tends to intensify the mixing and this is occurring for lower flow rates in comparison to the T shape. The improvement made by the cross geometry is attributed to the stronger vortex stretching and high shear rate, which reduces the liquid transfer length. The presence of a single outlet in the T-shaped micromixer induces a smaller degree of freedom for the fluid. A higher pressure drop is calculated in T-shaped micromixer than in cross-shaped micromixer.     

CHARACTERIZATION OF THE DOWNFLOW SECTION OF AN AIRLIFT COLUMN USING BUBBLE SIZE DISTRIBUTION MEASUREMENTS

Bubble size distributions in an airlift column were investigated with an emphasis on the downflow section. Measurements have been made using direct photographic techniques in conjunction with image analysis in a split cylinder airlift column. Information extracted from these measurements includes local gas hold-up, variation of Sauter mean bubble diameter with column length, and liquid circulation velocity. An air-tap water system was studied for purposes of comparison, while effects of electrolyte concentration and viscosity were studied using salt water, and two carboxymethyl cellulose (CMC) concentrations in water, respectively. The effect of energy input was studied by varying air flow rate to produce superficial velocities ranging from 2·59 cm/s to 10·36 cm/s.     

Bubble size and mass transfer characteristics of sparged downwards two-phase flow

When gas is continuously fed through a sparger into a downflowing liquid in a pipe a ventilated cavity is often formed. The cavity remains attached to the sparger even in the presence of high liquid flow rates that would wash away a free slug bubble. Small bubbles are shed from the base of this cavity by the falling liquid film at the wall of the pipe and these bubbles are swept downwards forming a bubbly flow that is highly effective for mass transfer. The ventilated cavity is undesirable since it reduces the driving force for liquid circulation when the pipe is the downcomer of an external air loop fermenter or analogous gas/liquid reactors. The cavity also reduces the available interfacial area for mass transfer. It has been shown [Thorpe et al., 1997. Proceedings of the Fourth International Conference on Bioreactor and Bioprocess Fluid Dynamics; Lee, 1998. Ph.D Thesis, University of Cambridge, UK], that the length of the cavity can be reduced by replacing the common industrial design of a horizontal sparger (HS) with two novel spargers; a peripheral sparger (PS) and a plunging jet sparger (PJS) (Fig. 3). In this paper we investigate the effect of PS and PJS on mass transfer and the resulting bubble size.Experiments were carried out with air and water in a large circulating rig with a 0.105 m diameter test section. The local average bubble size in the bulk two-phase flow region below the ventilated cavity was determined using photography for three combinations of liquid and gas volumetric flow rates. The average bubble size was essentially the same (differences within 10%) for the PS, central spranger (CS) and HS. The PS created the largest bubble in all cases examined. The PJS created smaller bubbles than all the other spargers and did not allow the formation of cavities, which suggests that it has the superior performance. The estimated increase in k_La due to the smaller bubble size for the PJS was by a factor of 1.3.In order to check this result, the effects of sparger type on the volumetric mass transfer coefficient (k_La) were also measured. The k_La was determined with a dynamic method, by using unsteady state absorption of oxygen. The results confirmed the apparent superiority of PJS over the other spargers. An average increase of 19% in the k_La was observed when the PJS was used instead of the industrial design (HS). The CS and PS showed similar k_La values again within 10% of the HS.However the power consumption is larger when the PJS is used instead of the industrial design HS. Hence an attempt was made to adjust the bubble size and mass transfer coefficients of the PJS to account for the differences in energy consumption. When this is done the PJS and HS produce roughly the same bubble size and have the same mass transfer performance. Still the PJS had the important operational advantages of producing shorter cavities and having the greater resistance to stall at low liquid flow rates.     

Bubble size in a forced circulation loop reactor

BACKGROUND: The bubble size distribution in gas‐liquid reactors influences gas holdup, residence time distribution, and gas‐liquid interfacial area for mass transfer. This work reports on the effects of independently varied gas and liquid flow rates on steady‐state bubble size distributions in a new design of forced circulation loop reactor operated with an air–water system. The reactor consisted of a cylindrical vessel (～26 L nominal volume, gas‐free aspect ratio ≈ 6, downcomer‐to‐riser cross‐sectional area ratio of 0.493) with a concentric draft tube and an annular riser zone. Both gas and liquid were in forced flow through a sparger that had been designed for minimizing the bubble size. RESULTS: Photographically measured bubble size distributions in the riser zone could be approximated as normal distributions for the combinations of gas and liquid flow rates used. This contrasted with other kinds of size distributions (e.g. bimodal, Gaussian) that have been reported for other types of gas‐liquid reactors. Most of the bubbles were in the 3 to 5 mm diameter range. At any fixed low value of aeration rate (≤1.8 × 10^?4 m³s^?1), increase in the liquid flow rate caused earlier detachment of bubbles from the sparger holes to reduce the Sauter mean bubble size in the riser region. CONCLUSION: Unlike in conventional bubble columns where bimodal and Gaussian bubble size distributions have been reported, a normal bubble size distribution is attained in forced circulation loop reactors with an air–water system over the entire range of operation. Copyright © 2007 Society of Chemical Industry     

Impact of fluid path geometry and operating parameters on l/l-dispersion in interdigital micromixers

The dispersion of two immiscible fluids in interdigital micromixers was investigated using silicon oil/water and n-heptane/water as test systems. The experiments revealed the dependence of the average droplet size and size distribution on geometrical parameters of the micromixer and operating conditions. The mean droplet size was found to be correlated with total volume flow, volume flow ratio and corresponding pressure drop of the two liquids for a given micromixer geometry, which could be explained in light of the energy input. As a major focus, the effect of mixing chamber geometry and feeding system geometry was investigated with regard to droplet size distribution by systematically varying decisive dimensions in the mixer and by changing the feeding structure. It was shown that reducing the slit height and the number of feeding channels had a significant effect on droplet size distribution, leading to a smaller mean droplet size. Furthermore, the dispersion efficiency was also investigated by an extraction process.     

DISPERSION OF GAS AND LIQUID IN BUBBLE COLUMNS OF HOMOGENEOUS BUBBLE FLOW REGIME

Mean gas holdup, lateral distribution of gas holdup and axial mixing of gas and liquid were measured in bubble columns of 12 and 19cm i.d. The lateral distribution of gas holdup was strongly dependent on the flow regimes in the column. The axial mixing of liquid in the homogeneous bubble flow regime was much smaller than that in the heterogeneous bubble flow regime, and was not expressed by existing correlations. The axial mixing of liquid in the homogeneous bubble flow and the intermediate flow regime was simulated with a flow model based on the lateral distribution of buoyancy force and the effective viscosity. The axial mixing of gas was larger than that of liquid.     

RADIAL DISPERSION AND BUBBLE CHARACTERISTICS IN THREE-PHASE FLUIDIZED BEDS

The effects of gas and liquid velocities, liquid viscosity and particle size on the radial dispersion coefficient of liquid phase (D_r) and the bubble properties in three-phase fluidized beds have been determined. A new flow regime map based on the drift flux theory in three-phase fluidized beds has been proposed.

In three-phase fluidized beds, D, increases with increasing gas velocity in the bubble coalescing and in the slug flow regimes, but it decreases in the bubble disintegrating regime. The coefficient exhibits a maximum value in the bed of small particles with increasing liquid velocity at lower gas velocities. However, it increases with increasing liquid velocity at higher gas velocities. In two and three-phase fluidized beds of larger particles (6,8 mm), D_r exhibits a maximum value with an increase in liquid viscosity at lower gas velocities, but it increases at higher gas velocities. The mean bubble chord length and its rising velocity increase with increasing gas velocity and liquid viscosity. However, the bubble chord length decreases with an increase in liquid velocity and it exhibits a maximum value with increasing particle size in the bed. The radial dispersion coefficients in the bubble coalescing and disintegrating regimes of three-phase fluidized beds in terms of the Peclet number in the present and previous studies have been well represented by the correlations based on the concept of isotropic turbulence theory.     

Experimental Determination and Modeling of Bubble Size Distributions in Bubble Columns

Using a five point conductivity technique local values of bubble size,bubble velocity and gas fractionhave been experimentally determined in a 288 mmID and 4.3 m high bubble column as a function of axial andradial position for the air/water and CO_2/N_2/aqueous MDEA systems.The experimental results are comparedwith predictions from a fundamental two-fluid model.The implementation of a non-steady lateral drag term inthe two-fluid model has been shown.In addition to improving the physical realism of the model,it is found togive slight improvements in the predictions of the distributions of local bubble size.Predictions of bubble size arefound in reasonable agreement with experimental values in the heterogeous flow regime,whereas they are stil1found to be unreliable at low gas velocities.Local void predictions are found in reasonable agreement with experi-mental values,but deviations occur in the homogeneous flow regime towards the wall.This is attributed to defi-ciencies in the simplified bubble size mode     

Bubble characteristics in shallow bubble column reactors

The bubble characteristics have been investigated in an air–water bubble column with shallow bed heights. The effect of bed height, location and the presence of solids on the bubble size, bubble rise velocity and overall and sectional gas holdup are studied over a range of superficial gas velocities. Optimal shallow bed operation relies on the combined entrance and exit effects at the distributor and the liquid bed surface. The gas holdup is found to decrease with an increase in H/D ratio but the effect is diminishing at high H/D ratios. A H/D ratio of 2–4 is found to be suitable for shallow bed operation. The presence of solids causes the formation of larger bubbles at the distributor and the effect is diminishing as the gas velocity is increased.     

Continuous alcohol addition in vaporized form and its effect on bubble behavior in a bubble column

This study investigated the effect of alcohol on gas hold-up in two methods to add alcohol into a column. In the first method, a weighed amount of ethanol was poured into the column before the gas hold-up measurement (batch mode). In the second method, we added ethanol continuously in the form of vapor dispersed in the gas phase (continuous mode). The continuous mode was more effective in improving the gas hold-up in a heterogeneous flow regime than the batch mode. On the other hand, it had a negative effect on gas hold-up in a homogeneous flow regime. To investigate these phenomena in more detail, we measured the detachment period, bubble size distribution, and bubble break-up frequency during bubble formation in the continuous mode. When the liquid vapor was highly soluble in the continuous water phase, the detachment period and average bubble size increased and the bubble break-up frequency decreased. On the other hand, when there was little interaction between the liquid vapor and continuous water phase, the effect was negligible. This could be explained by liquid vapor diffusion from the bubble inside into the continuous water phase.     

Coalescence during bubble formation at two neighbouring pores: An experimental study in microscopic scale

This work studies the effect of the liquid properties and the operating conditions on the interactions between under-formation bubbles in a cell equipped with two adjacent micro-tubes (i.d. ) for the gas injection, placed 210, 700 and apart. This set-up simulates, though in a simplified manner, the operation of the porous sparger in a bubble column, and it is used to study the bubble interactions observed on the sparger surface. Various liquids covering a wide range of surface tension and viscosity values are employed, while the gas phase is atmospheric air. A fast video recording technique is used both for the visual observations of the phenomena occurring onto the tubes and for the bubble size measurements. The experiments reveal that the interactions between under-formation bubbles as well as the coalescence time depend strongly on the liquid properties, the distance between the tubes and the gas flow rate. Two correlations, which can be found helpful for the bubble column design, have also been formulated and are in good agreement with the available experimental data.     

BUBBLE FORMATION AT A PUNCTURE IN A SUBMERGED RUBBER MEMBRANE

Very large enhancements in volumetric mass transfer coefficients have recently been reported using a new type of sparger which is comprised of a punctured rubber membrane. The punctured sheet has been reported to produce very uniform emulsions of small bubbles, which leads to large apparent increases in gas voidage and mass transfer area. Flooding(slugging) is presumably repressed owing to the “elastic hole” phenomenom whereby the rubber sheet balloons and expands as applied pressure increases. Under conditions of expansion, a puncture in the sheet also expands thereby mitigating the occurence of jetting. In the present effort, we study a single puncture in the center of circular rubber sheets of 2, 3, and 4 inch diameters. By measuring bubble frequency and flow rate, we compute average bubble size. These results for flow rates from 0.01 to 2.0 cc/sec suggest that bubble size is practically constant over a nearly two decades of flow rate, until a flow of around 0.5 cc/sec, thence bubbles tend to follow the empirical correlation of L. Davidson and Amick (1956) and the inviscid theory of J.F. Davidson and Schuler (1960) both of which predict bubble volume increases as the 6/5 power of flow.

Using the “point source” model of J.F. Davidson for bubble growth, we include additional effects of surface tension to derive the required detachment-time. This leads to a theory which includes inertial, buoyancy and surface tension effects. The theory gives flow rate as a function of final bubble size and agrees quite well with the 180 experiments reported. The new theory approaches the inviscid models in the limit of large gas flow rates. Finally, we present results which clearly suggest that hole area increases linearly with increasing plenum chamber pressure.     

BUBBLE FORMATION AT A PUNCTURE IN A SUBMERGED RUBBER MEMBRANE

Very large enhancements in volumetric mass transfer coefficients have recently been reported using a new type of sparger which is comprised of a punctured rubber membrane. The punctured sheet has been reported to produce very uniform emulsions of small bubbles, which leads to large apparent increases in gas voidage and mass transfer area. Flooding(slugging) is presumably repressed owing to the “elastic hole” phenomenom whereby the rubber sheet balloons and expands as applied pressure increases. Under conditions of expansion, a puncture in the sheet also expands thereby mitigating the occurence of jetting. In the present effort, we study a single puncture in the center of circular rubber sheets of 2, 3, and 4 inch diameters. By measuring bubble frequency and flow rate, we compute average bubble size. These results for flow rates from 0.01 to 2.0 cc/sec suggest that bubble size is practically constant over a nearly two decades of flow rate, until a flow of around 0.5 cc/sec, thence bubbles tend to follow the empirical correlation of L. Davidson and Amick (1956) and the inviscid theory of J.F. Davidson and Schuler (1960) both of which predict bubble volume increases as the 6/5 power of flow.

Using the “point source” model of J.F. Davidson for bubble growth, we include additional effects of surface tension to derive the required detachment-time. This leads to a theory which includes inertial, buoyancy and surface tension effects. The theory gives flow rate as a function of final bubble size and agrees quite well with the 180 experiments reported. The new theory approaches the inviscid models in the limit of large gas flow rates. Finally, we present results which clearly suggest that hole area increases linearly with increasing plenum chamber pressure.     

Gas–liquid flows in medium and large vertical pipes

Gas–liquid bubbly flows with wide range of bubble sizes are commonly encountered in many industrial gas–liquid flow systems. To assess the performances of two population balance approaches – Average Bubble Number Density (ABND) and Inhomogeneous MUlti-SIze-Group (MUSIG) models – in tracking the changes of gas volume fraction and bubble size distribution under complex flow conditions, numerical studies have been performed to validate predictions from both models against experimental data of Lucas et al. (2005) and Prasser et al. (2007) measured in the Forschungszentrum Dresden-Rossendorf FZD facility. These experiments have been strategically chosen because of flow conditions yielding opposite trend of bubble size evolution, which provided the means of carrying out a thorough examination of existing bubble coalescence and break-up kernels. In general, predictions of both models were in good agreement with experimental data. The encouraging results demonstrated the capability of both models in capturing the dynamical changes of bubbles size due to bubble interactions and the transition from “wall peak” to “core peak” gas volume fraction profiles caused by the presence of small and large bubbles. Predictions of the inhomogeneous MUSIG model appeared marginally superior to those of ABND model. Nevertheless, through the comparison of axial gas volume fraction and Sauter mean bubble diameter profiles, ABND model may be considered an alternative approach for industrial applications of gas–liquid flow systems.     

Experimental and modelling study of gas dispersion in a double turbine stirred tank

Gas dispersion in a double turbine stirred tank is experimentally characterised by measuring local gas holdups and local bubble size distributions throughout the tank, for three liquid media: tap water, aqueous sulphate solution and aqueous sulphate solution with PEG. For all these media, bubble coalescence generally prevails over breakage. Where average bubble size decreases, this can be attributed to the difference in slip velocity between different sized bubbles. Most of the coalescence takes place in the turbine discharge stream.A compartment model that takes into account the combined effect of bubble coalescence and breakage is used to simulate gas dispersion. The model predicts spatial distribution of gas holdup and of average bubble size, with average bubble size at the turbines as an input. Reasonable agreement between experiment and simulation is achieved with optimisation of two parameters, one affecting mainly the slip velocity, the other related mainly to the bubble coalescence/breakage balance. Different sets of parameters are required for each of the three liquid systems under study, but are independent of stirring/aeration conditions. The model only fails to simulate the smaller average bubble diameters at the bottom of the tank.     

Prediction of bubble behaviour in fluidised beds based on solid motion and flow structure

It has been demonstrated that the non-intrusive positron emission particle tracking (PEPT) could be a potential technique for observing bubble flow pattern, measuring bubble size and rise velocity in bubbling fluidised beds according to the solid motion in bubble and its wake. The results indicate that the behaviour of air bubbles varies greatly with the bed materials and superficial gas velocity. Three types of bubbling patterns (namely A, B and C) have been reported in this study, in which the pattern C is observed when the polyethylene fluidised bed is operated at the superficial gas velocity (U − U_mf) of 0.25–0.5 m/s and the ratio of bed height to bed diameter is unity. After the comparison of the results measured by the PEPT technique with the values calculated by using a number of empirical correlations, two modified correlations are recommended to calculate the bubble size based on the PEPT data.     

Imposing dynamic structures on fluidised beds

Structuring fluidised beds can increase the conversion and selectivity, and facilitate control and scale-up. Two methods for introducing a dynamic structure into gas–solid fluidised beds are compared based on their overall hydrodynamics: electric field enhanced fluidisation and distributed secondary gas injection by a fractal injector. It is shown that, under various conditions, these systems lead to significant decreases in bubble size and bubble hold-up and to an increase in the number of bubbles. It was found that the electric field enhancement can lead to homogeneous fluidisation at lower flow rates, and the distributed secondary flow leads to smaller bubbles at higher flow rates.     

Characterization of the Mixing Quality in Micromixers

The laminar flow patterns and mixing performance of two different micromixers have been investigated and quantified using CFD. The micromixer geometries consist of a channel with either diagonal or asymmetric herringbone grooves on the channel floor. The numerical results show that a single helical flow is produced for the diagonal mixer, whereas the herringbone mixer creates a double helical flow, composed of an alternating large and small vortex. Particle tracking of a tracer shows that very little convective mixing occurs in the diagonal mixer. However, in the herringbone mixer, very good mixing occurs. Quantitative analysis methods that are traditionally used for characterizing macro‐scale static mixers have been employed. Calculation of the variance of tracer dispersion and the stretching has shown to be well adapted for quantifying the mixing in the micromixers. However, methods based on the deformation rate appear to be less suitable. The results are in excellent agreement with previous experimental findings.     

CFD modeling of slurry bubble column reactors for Fisher-Tropsch synthesis

Industrial bubble column reactors for Fischer-Tropsch (FT) synthesis include complex hydrodynamic, chemical and thermal interaction of three material phases: a population of gas bubbles of different sizes, a liquid phase and solid catalyst particles suspended in the liquid. In this paper, a CFD model of FT reactors has been developed, including variable gas bubble size, effects of the catalyst present in the liquid phase and chemical reactions, with the objective of predicting quantitative reactor performance information useful for design purposes. The model is based on a Eulerian multifluid formulation and includes two phases: liquid-catalyst slurry and syngas bubbles. The bubble size distribution is predicted using a Population Balance (PB) model. Experimentally observed strong influence of the catalyst particles concentration on the bubble size distribution is taken into account by including a catalyst particle induced modification of the turbulent dissipation rate in the liquid. A simple scaling modification to the dissipation rate is proposed to model this influence in the PB model. Additional mass conservation equations are introduced for chemical species associated with the gas and liquid phases. Heterogeneous and homogeneous reaction rates representing simplified FT synthesis are taken from the literature and incorporated in the model.Hydrodynamic effects have been validated against experimental results for laboratory scale bubble columns, including the influence of catalyst particles. Good agreement was observed on bubble size distribution and gas holdup for bubble columns operating in the bubble and churn turbulence regimes. Finally, the complete model including chemical species transport was applied to an industrial scale bubble column. Resulting hydrocarbon production rates were compared to predictions made by previously published one-dimensional semi-empirical models. As confirmed by the comparisons with available data, the modeling methodology proposed in this work represents the physics of FT reactors consistently, since the influence of chemical reactions, catalyst particles, bubble coalescence and breakup on the key bubble-fluid drag force and interfacial area effects are accounted for. However, heat transfer effects have not yet been considered. Inclusion of heat transfer should be the final step in the creation of a comprehensive FT CFD simulation methodology. A significant conclusion from the modeling results is that a highly localized FT reaction rate appears next to the gas injection region when the syngas flow rate is low. As the FT reaction is exothermal, it may lead to a highly concentrated heat release in the liquid. From the design perspective, the introduction of appropriate heat removal devices may be required.