On the measurement of bubble size distribution in gas–liquid contactors via light sheet and image analysis

Particle image velocimetry techniques coupled with advanced image processing tools are receiving an increasing interest for measuring flow quantities and local bubble-size distributions in gas–liquid mechanically agitated vessels.When trying to analyze image information the problem arises that bubble sizes are generally underestimated, due to the fact that the laser sheet used for lighting the system randomly cuts bubbles over non-diametrical planes, leading to an apparent bubble size distribution even in the ideal case of single sized bubbles. Clearly in the case of bubbles with a size distribution the experimental information obtained is affected by the superposition of effects.Aim of this work is that of providing a numerical procedure able to reconstruct actual bubble size distributions from relevant apparent size distributions obtained by laser sheet illumination and image analysis. The procedure proposed is robust and viable and can account for laser sheet thickness. The procedure is shown to provide fully satisfactory results even with quite extreme distributions. BSD resolution dependence on the numerousness of raw data processed is discussed. Use of the proposed procedure for extracting BSD from bubble chord raw data obtained by other devices, such as point probes, is straightforward.     

Effective interfacial area for mass transfer in the liquid–liquid slug flow capillary microreactors

Liquid–liquid biphasic reactions play an important role in the chemical and pharmaceutical industries. The liquid–liquid slug flow capillary microreactor offers considerable potential benefits over the conventional liquid–liquid contactors. Though the hydrodynamics and mass transfer have been investigated for this reactor concept, so far the effective interfacial area available for mass transfer has not been experimentally quantified. Despite the well-defined flow patterns arising in the capillary microreactor, the wetting behaviour of the liquids at the capillary wall is inadequately integrated into the models and thus, the true interfacial area being used for mass transfer is uncertain.     

On the prediction of gas hold-up in two-phase flow systems using an Euler–Euler model

We quantify the ability of the two-fluid Euler–Euler model to predict the overall gas hold-up during two-phase flow in vertical columns using a combination of experiments and simulations. Gas hold-up in a bubble column and gas hold-up in the less-frequently studied co-current flow are investigated. For homogeneous flow characterized by nearly uniform bubble size, Euler–Euler model predictions are within 10% of the experimental values for both modes of operation, if the bubble diameter supplied as input to the model is the average bubble diameter in the physical system. This also holds true for heterogeneous flow in bubble columns despite the presence of a broad distribution of bubble sizes, if turbulence and bubble swarm effects on momentum exchange between phases are properly accounted for. Swarm corrections adequate for bubble columns, are less successful for co-current heterogeneous flow, for which gas hold-up predictions are least accurate (average error of 22%).     

Micro-computed tomography for the investigation of stationary liquid–liquid and liquid–gas interfaces in capillaries

For better understanding and optimization of multiphase flow in miniaturized devices, micro-computed tomography (μCT) is a promising visualization tool, as it is nondestructive, three-dimensional, and offers a high spatial resolution. Today, computed tomography (CT) is a standard imaging technique. However, using CT in microfluidics is still challenging, since X-ray related artifacts, low phase contrast, and limited spatial resolution complicate the exact localization of interfaces. We apply μCT for the characterization of stationary interfaces in thin capillaries. The entire workflow for imaging stationary interfaces in capillaries, from image acquisition to the analysis of interfaces, is presented. Special emphasis is given to an in-house developed segmentation routine. For demonstration purposes, contact angles of water, liquid polydimethylsiloxane, and air in FEP, glass, and PMMA are determined and the influence of gravity on interface formation is discussed. This work comprises the first steps for a systematic 3D investigation of multiphase flows in capillaries using μCT.     

Measuring gas hold-up in gas–liquid/gas–solid–liquid stirred tanks with an electrical resistance tomography linear probe

An electrical resistance tomography (ERT) linear probe was used to measure gas hold-up in a two-phase (gas–liquid) and three phase (gas–solid–liquid) stirred-tank system equipped with a Rushton turbine. The ERT linear probe was chosen rather than the more commonly used ring cage geometry to achieve higher resolution in the axial direction as well as its potential for use on manufacturing plant. Gas-phase distribution was measured as a function of flow regime by varying both impeller speed and gas flow rate. Global and local gas hold-up values were calculated using ERT data by applying Maxwell's equation for conduction through heterogeneous media. The results were compared with correlations, hard-field tomography data, and computational fluid dynamic simulations available in the literature, showing good agreement. This study thus demonstrates the capability of ERT using a linear probe to offer, besides qualitative tomographic images, reliable quantitative data regarding phase distribution in gas–liquid systems.     

Characteristics of flow fields in the gas–liquid mini-bubble columns with particle image velocimetry measurements

As a new type of gas–liquid microreactors, the gas–liquid mini-bubble column has potential applications. However, few studies on the flow fields in the mini-bubble column can be found at present. In this work, particle image velocimetry (PIV) was used to visually study the velocity fields, vorticity fields and bubble dynamics in the gas–liquid mini-bubble columns with column inner diameters of 1–3 mm and mini-bubble diameters ranged from 0.7 to 1.3 mm. It is found that with the increase of superficial liquid velocity, bubbles rose from almost straight line to Z-shaped or S-shaped trajectory, and the bubble trajectory changed from one-dimension to three-dimension; when the bubble velocity changed, the bubble size and gas holdup decreased; bubble terminal velocity was controlled by bubble buoyancy and flow resistance, and increased slightly with bubble coalescence. These findings may provide basic reference for the design and scale-up of such a mini-bubble column reactor.     

Local bubble size distribution,gas–liquid interfacial areas and gas holdups in an up-flow ejector

Particle image velocimetry (PIV) was used to measure local bubble size distributions (BSD), gas–liquid interfacial areas and gas holdups in an up-flow ejector, based on the water–air system with different liquid and gas flow rates under the presence/absence of the swirl body. The results show that the bubble flow patterns are different whether to add the swirl body into the nozzle, especially at low gas flow rate because the bubbles formed “bubble chain” in the ejector with swirl. The mean bubble sizes D₃₂ of the two are both related to the pressure drop between import and export, gas ratio and liquid flow. The interfacial area and D₃₂ are both mainly dependent on the local gas holdups. The mean bubble sizes in the absence of swirl body are smaller than that in the presence of swirl under different operating conditions. The gas holdups and interfacial area are larger with swirl than those without swirl. With the increase of the gas fraction, the differences of D₃₂, a_t and e_G become smaller.     

Analysis of the flow pattern and periodicity of gas–liquid–liquid three-phase flow in a countercurrent mixer-settler

In contrast to the concurrent mixer-settler, the interaction between the mixing and settling chambers have to be taken into account in the simulation of the countercurrent mixer-settler, and no work has been reported for this equipment. In this work, a three-phase flow model based on the Eulerian multiphase model, coupled with a sliding mesh model is proposed for a countercurrent mixer-settler. Based on this, the dispersed phase distribution, flow pattern, and pressure distribution are investigated, which can help to fill the gap in the operation mechanism. In addition, the velocity vector distribution at the phase port shows an intriguing phenomenon that two types of vectors with opposite directions are distributed on the left and right sides of the same plane, which indicates that the material exchange in the mixing and settling chambers is simultaneous. Analysis of this variation at this location by a fast Fourier transform (FFT) method reveals that it is mainly influenced by the mixing chamber and is consistent with the main period of the outlet flow fluctuations. Therefore, by monitoring the fluctuation of the outlet flow and then analyzing it by the FFT method, the state of the whole tank can be determined, which makes it promising for the design of control systems for countercurrent mixer-settlers.     

Hydrodynamics and local mass transfer characterization under gas–liquid–liquid slug flow in a rectangular microchannel

Gas–liquid–liquid three-phase slug flow was generated in a glass microreactor with rectangular microchannel, where aqueous slugs were distinguished by relative positions to air bubbles and organic droplets. Oxygen from bubbles reacted with resazurin in slugs, leading to prominent color changes, which was used to quantify mass transfer performance. The development of slug length indicated a film flow through the corner between bubbles and the channel wall, where the aqueous phase was saturated with oxygen transferred from bubble body. This film flow results in the highest equivalent oxygen concentration within the slug led by a bubble and followed by a droplet. The three-phase slug flow subregime with alternate bubble and droplet was found to benefit the overall mass transfer performance most. These results provide insights into a precise manipulation of gas–liquid–liquid slug flow in microreactors and the relevant mass transfer behavior thereof.     

Mass transfer intensification and mechanism analysis of gas–liquid two-phase flow in the microchannel embedding triangular obstacles

An effective mass transfer intensification method was proposed by embedding different triangular obstacles to improve the gas–liquid mass transfer efficiency in microchannel. The influences of triangle obstacles configuration, obstacle interval and flow rate on the volumetric mass transfer coefficient, pressure drop and energy consumption were investigated experimentally. The enhancement factor was used to quantify the mass transfer enhancement effect of triangle obstacles. It was found that the isosceles or equilateral triangle obstacles are superior to the rectangular obstacles. The maximum enhancement factor of equilateral triangle obstacles was 2.35. Considering comprehensively mass transfer enhancement and energy consumption, the isosceles triangle obstacle showed the best performance, its maximum enhancement factor was 2.1, while the maximum pressure drop increased only 0.41 kPa (22%) compared to the microchannel without obstacles. Furthermore, a micro-particle image velocimetry (micro-PIV) was utilized to observe the flow field distribution and evolution, in order to understand and analyze the enhancement mechanism. The micro-PIV measurement indicated that the obstacle structure could induce the formation of vortex, which promotes convective mass transfer and thins the flow boundary layer, accordingly, the gas–liquid mass transfer efficiency is remarkably improved. This study can provide theoretical guidance and support for the design and optimization of microchannel with triangular obstacles.     

Numerical simulation of micro-mixing in gas–liquid and solid–liquid stirred tanks with the coupled CFD-E-model

The coupled CFD-E-model for multiphase micro-mixing was developed, and used to predict the micro-mixing effects on the parallel competing chemical reactions in semi-batch gas–liquid and solid–liquid stirred tanks. Based on the multiphase macro-flow field, the key parameters of the micro-mixing E-model were obtained with solving the Reynolds-averaged transport equations of mixture fraction and its variance at low computational costs. Compared with experimental data, the multiphase numerical method shows the satisfactory predicting ability. For the gas–liquid system, the segregated reaction zone is mainly near the feed point, and shrinks to the exit of feed-pipe when the feed position is closer to the impeller. Besides, surface feed requires more time to completely exhaust the added H⁺ solution than that of impeller region feed at the same operating condition. For the solid–liquid system, when the solid suspension cloud is formed at high solid holdups, the flow velocity in the clear liquid layer above the cloud is notably reduced and the reactions proceed slowly in this almost stagnant zone. Therefore, the segregation index in this case is larger than that in the dilute solid–liquid system.     

Modeling of Rayleigh convection in gas–liquid interfacial mass transfer using lattice Boltzmann method

Interfacial Rayleigh convection can be generated by concentration gradient near the interface in mass transfer processes. In the present study, a 2D time-dependent lattice Boltzmann method (LBM) with a double distribution model was established for simulating the liquid-phase Rayleigh convection in the mass transfer process of CO₂ absorption into various solvents. Two random parameters P and C_D denoting respectively the possibility and the magnitude of concentration perturbation at interface were introduced to model the interfacial disturbance, which is known as one of the necessary conditions of onset of Rayleigh convection. The values of the parameters were identified (0.05 ≤ P < 0.3 and 0 < C_D ≤ 10⁻⁹ kg m⁻³) by comparing simulated critical onset times of the Rayleigh convection with the experimental result from Blair and Quinn (1969) and theoretical predictions proposed by Kim et al. (2006) and 0245 and 0250. The maximum penetration depths, maximum transient Rayleigh numbers, and critical times for the onset of Rayleigh convection were obtained by the proposed model. The simulations captured the detailed information of the onset and the temporal–spatial evolution of Rayleigh convection, and gave the concentration contours of typical plume convection patterns which were well consistent with literatures. Enhancement of mass transfer by the Rayleigh convection was also demonstrated by comparing the simulated instantaneous mass flux across the interface with that predicted by penetration theory.     

Characteristics of liquid slugs in gas–liquid Taylor flow in microchannels

The hydrodynamics of liquid slugs in gas–liquid Taylor flow in straight and meandering microchannels have been studied using micro Particle Image Velocimetry. The results confirm a recirculation motion in the liquid slug, which is symmetrical about the center line of the channel for the straight geometry and more complex and three-dimensional in the meandering channel. An attempt has also been made to quantify and characterize this recirculation motion in these short liquid slugs (L_s/w<1.5) by evaluating the recirculation rate, velocity and time. The recirculation velocity was found to increase linearly with the two-phase superficial velocity U_TP. The product of the liquid slug residence time and the recirculation rate is independent of U_TP under the studied flow conditions. These results suggest that the amount of heat or mass transferred between a given liquid slug and its surroundings is independent of the total flow rate and determined principally by the characteristics of the liquid slug.     

Simultaneous measurement of velocity profile and liquid film thickness in horizontal gas–liquid slug flow by using ultrasonic Doppler method

Horizontal gas–liquid two-phase flows widely exist in chemical engineering, oil/gas production and other important industrial processes. Slug flow pattern is the main form of horizontal gas–liquid flows and characterized by intermittent motion of film region and slug region. This work aims to develop the ultrasonic Doppler method to realize the simultaneous measurement of the velocity profile and liquid film thickness of slug flow. A single-frequency single-channel transducer is adopted in the d...     

Entransy dissipation analysis of interfacial convection enhancing gas–liquid mass transfer process based on field synergy principle

An exploration of the gas CO2 absorbed into liquid ethanol accompanied with Rayleigh convection is performed by analyzing the mass entransy dissipation; this new statistical quantity is introduced to describe the irreversibility of mass transfer potential capacity. Based on the general advection–diffusion differential equation for an unsteady mass transfer process, the variation of the included angle between the velocity vector and concentration gradient fields is investigated to reveal the underlying mechanism of interfacial convection enhancing mass transfer. Results show some identical characteristics with the qualitative analyses of the synergy effects generated by the concentration and velocity fields after interfacial convection occurring for a boundary condition of fixed surface concentration. And the equivalent mass resistance for convective mass transfer process presents the similar variation with the reciprocal of instantaneous mass transfer coefficient. Accordingly, it is reasonable to be seen that mass transfer dissipation rate could be provided to assess the convection strength and explain fundamentally how Rayleigh convection improves mass transfer performance through establishing a close relationship between the mass transfer capacity and field synergy principle from the view of mass transfer theory.     

CFD and experimental investigation of the gas–liquid flow in the distributor of a compact heat exchanger

High performance of compact heat exchangers is conditioned by correct fluid distribution. This is especially true for gas–liquid heat exchangers where a uniform distribution is particularly delicate to obtain and where maldistribution entails significant performance deterioration. Several phenomena can lead to phase distribution problems: the fins may be subject to manufacturing defects or fouling, leading to shortcuts or dead zones. But the first source of maldistribution may be a poor distribution at the outlet of the entrance distributor. This distributor aims at mixing the phases and distributing them across the channels.     

Mixing and recirculation characteristics of gas–liquid Taylor flow in microreactors

The effects of operating parameters (capillary and Reynolds numbers) and microchannel aspect ratio (α=w/h=[1;2.5;4]$\alpha =w/h=[1;2.5;4]$) on the recirculation characteristics of the liquid slug in gas–liquid Taylor flow in microchannels have been investigated using 3-dimensional VOF simulations. The results show a decrease in the recirculation volume in the slug and an increase in recirculation time with increasing capillary number, which is in good agreement with previous results obtained in circular and square geometries (Thulasidas et al., 1997). In addition, increasing the aspect ratio of the channel leads to a slight decrease in recirculating volumes but also a significant increase in recirculation times.     

Spray and mixing characteristics of liquid jet in a tubular gas–liquid atomization mixer

For the design and optimization of a tubular gas–liquid atomization mixer,the atomization and mixing characteristics of liquid jet breakup in the limited tube space is a key problem.In this study,the primary breakup process of liquid jet column was analyzed by high-speed camera,then the droplet size and velocity distribution of atomized droplets were measured by Phase-Doppler anemometry (PDA).The hydrodynamic characteristics of gas flow in tubular gas–liquid atomization mixer were analyzed by computational fluid dynamics (CFD) numerical simulation.The results indicate that the liquid flow rate has little effect on the atomization droplet size and atomization pressure drop,and the gas flow rate is the main influence parameter.Under all experimental gas flow conditions,the liquid jet column undergoes a primary breakup process,forming larger liquid blocks and droplets.When the gas flow rate (Q_g) is less than 127 m~3·h~(-1),the secondary breakup of large liquid blocks and droplets does not occur in venturi throat region.The Sauter mean diameter (SMD) of droplets measured at the outlet is more than 140μm,and the distribution is uneven.When Q_g127 m~3·h~(-1),the large liquid blocks and droplets have secondary breakup process at the throat region.The SMD of droplets measured at the outlet is less than 140μm,and the distribution is uniform.When 127Q_g162 m~3·h~(-1),the secondary breakup mode of droplets is bag breakup or pouch breakup.When 181Q_g216 m~3·h~(-1),the secondary breakup mode of droplets is shear breakup or catastrophic breakup.In order to ensure efficient atomization and mixing,the throat gas velocity of the tubular atomization mixer should be designed to be about 51 m·s~(-1)under the lowest operating flow rate.The pressure drop of the tubular atomization mixer increases linearly with the square of gas velocity,and the resistance coefficient is about 2.55 in single-phase flow condition and 2.73 in gas–liquid atomization condition.     

Enhancement of entrainment rates in liquid–gas ejectors

Ejectors are widely used as effective distributors in many chemical and bioprocess industries. Gas entrainment rate as a function of liquid flow rate in ejectors is investigated using nozzles of different geometries. The data are analyzed through macro-energy balance for each phase considering air and water inlet line discharge coefficients. Nozzles with smaller discharge coefficients are effective in producing higher vacuum and hence higher entrainment rates. It has been observed that the factor limiting the air entrainment rate is the low discharge coefficient in the air inlet line. Higher air inlet line discharge coefficients can increase the entrainment rate.     

Gas—liquid interfacial area in stirred vessels: The effect of an immiscible liquid phase

The presence of an inert immiscible organic phase in gas—liquid dispersions in stirred vessels influences the interfacial area in a more complex fashion than hitherto reported. As the organic phase fraction is increased, the interfacial area expressed on the basis of a unit volume of dispersion or aqueous phase, first increases, passes through a maximum and then decreases. This trend is observed irrespective of whether the area is determined by chemical means or by physical method.It is found that for low values of inert phase fraction, the average bubble size decreases whereas the gas holdup increases, resulting in increased interfacial area. The lower average bubble size is found to be due to partial prevention of coalescence as the bubbles size generated in the impeller region actually increases with the organic phase fraction. The actual values of interfacial areas depend on the nature of the organic phase.It is also found that the organic phase provides a parallel path for mass transfer to occur, when the solubility of gas in it is high.