A review on single bubble gas–liquid mass transfer

It is common to empirically correlate volumetric mass transfer coefficient k_La for predicting gas–liquid mass transfer in industrial applications, and the investigation of single bubble mass transfer is crucial for a detailed understanding of mass transfer mechanism. In this work, experiments, models and simulations based on the experimental results were highlighted to elucidate the mass transfer between single bubbles and ambient liquid. The experimental setups, measurement methods, the mass transfer of single bubbles in the Newtonian and the non-Newtonian liquid, models derived from the concept of eddy diffusion, the extension of Whitman's, Higbie's and Danckwerts' models, or dimensionless numbers, and simulation methods on turbulence, gas–liquid partition methods and mass transfer source term determination are introduced and commented on. Although people have a great knowledge on mass transfer between single bubbles and ambient liquid in single conditions, it is still insufficient when facing complex liquid conditions or some phenomena such as turbulence, contamination or non-Newtonian behavior. Additional studies on single bubbles are required for experiments and models in various liquid conditions in future.     

Chemisorption of CO2 in a gas–liquid vortex reactor: An interphase mass transfer efficiency assessment

To develop cost-effective CO₂ capture technology process intensification will play a vital role. In this work, the capabilities of a gas–liquid vortex reactor (GLVR) as novel process intensification equipment are evaluated by studying its interphase mass transfer parameters to build up the fundamentals for its future application to for example, CO₂ capture. The NaOH-CO₂ chemisorption system and Danckwerts' model are applied to obtain the effective interfacial area and liquid-side mass transfer coefficient. Results show that the gas–liquid contact in the GLVR is capable of both generating a large interfacial area in a small reactor volume and creating a region with high-energy dissipation to improve mass transfer. A comparison of the volumetric mass transfer coefficients with data reported in literature for conventional and intensified reactor types confirms a superior mass transfer efficiency and, most importantly, a favorable energetic efficiency of the GLVR.     

Effect of surfactant type on interfacial area and liquid mass transfer for CO2 absorption in a bubble column☆

The effect of different surfactants (n-octyltrimethylammonium bromide (OTABr), sodium dodecyl benzene sulfonate (SDBS) and Tween 80) with different critical micelle concentrations (CMC) on the CO2 abso...     

Dynamic modelling of a bubble column for particle formation via a gas–liquid reaction

This paper presents a dynamic model of a bubble column reactor with particle formation, accomplished by adopting a hybrid CFD-reaction engineering approach. CFD is employed for estimating the hydrodynamics and is based on the two-phase Eulerian–Eulerian viewpoint. The reaction engineering model links the penetration theory to a population balance that includes particle formation and growth with the aim of predicting the average particle size. The model is then applied to the precipitation of CaCO₃ via CO₂ absorption into Ca(OH)₂aq in a draft tube bubble column and draws insight into the phenomena underlying the crystal size evolution.     

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.     

Effect of ultrasonic waveforms on gas–liquid mass transfer in microreactors

This study aims to investigate the effect of ultrasonic waveforms on the gas–liquid mass transfer process. For a given load power (P), continuous rectangular wave yielded stronger bubble oscillation and higher mass transfer coefficient (k_La) than continuous triangular and sinusoidal wave. For pulsed ultrasound, the k_La decreased monotonically with decreasing duty ratio (D), resulting in weak enhancement at low D (≤33%). For a given average load power (P_A), concentrating the P for a shorter period resulted in a higher k_La due to stronger cavitation behavior. For a given P_A and D, decreasing the pulse period (T) led to an increase in k_La, which reached a constant high level when the T fell below a critical value. By optimizing the D and T, a k_La equivalent to 92% of that under continuous ultrasound was obtained under pulsed ultrasound at a D of 67%, saving 33% in power consumption.     

Effect of electrolytes in aqueous solutions on oxygen transfer in gas–liquid bubble columns

The effects of inorganic electrolytes (NaCl, MgCl₂, CaCl₂) in aqueous solutions on oxygen transfer in a bubble column were studied. Electrolyte concentrations (c) below and above the critical concentrations for bubble coalescence (c_tc), and six superficial gas velocities (v_sg), were evaluated. The volumetric mass transfer (k_La) and the mass transfer (k_L) coefficients were experimentally determined. It was found that the concentration of electrolytes reduced the k_L, but the interfacial area (a) increased enough to result in a net increase of k_La. Using as independent variable a normalizing variable (c_r = c/c_tc), and maintaining fixed v_sg, similar values of k_La were observed regardless the kind of electrolyte; the same happened for k_L. This suggests that c_r quantifies the structural effects that these solutes exert on mass transfer. Also, once c_r = 1 was reached, no significant variations were found in k_La and k_L for constant v_sg. It is concluded that the gradual inhibition of bubble coalescence (c_r < 1) governs the significant changes in hydrodynamics and mass transfer via the reduction of bubble size and the consequent increment of a and gas holdup (?_g). Finally, regarding the effects of v_sg on mass transfer, transition behaviors between those expected for isolated bubbles and bubble swarms were observed.     

Micromixing in a gas–liquid vortex reactor

The gas–liquid vortex reactor (GLVR) has substantial process intensification potential for multiphase processes. Essential in this respect is the micromixing efficiency, which is of great importance in fast reaction systems such as crystallization, polymerization, and synthesis of nanomaterials. By creating a vortex flow and taking advantage of the centrifugal force field, the liquid micromixing process can be intensified in the GLVR. Results show that introducing a liquid into a gas-only vortex unit results in suppression of primary and secondary gas flow. The Villermaux–Dushman protocol is applied to study the effects of the gas flow rate, liquid flow rate, and liquid viscosity based on a segregation index. Based on the incorporation model and reaction kinetics, the micromixing time of the GLVR is determined to be in the range of 10⁻⁴ ~ 10⁻³ s, which is comparable to the highly efficient rotating packed bed and substantially better than a static mixer.     

Local and global gas–liquid mass transfer in a micro-packed bed reactor utilizing a noninvasive colorimetric technique

Micro-packed bed reactor (μPBR) presents great potential in the field of multiphase reactions due to the features of safety and high efficiency. However, the deeper cognition of mass transfer needs to be taken into consideration that is the foundation of reactor design. In this work, local and global gas–liquid mass transfer in the μPBR were studied utilizing a noninvasive colorimetric technique. In reactor level, the qualitative and quantitative comparisons were conducted; in particle level, liquid flow and mass transfer textures were assessed for the first time. The diversities of local mass transfer characteristics from temporal and spatial dimensions were obtained, and the heterogeneity of local and global mass transfer was revealed. The predicted correlations of $$ in μPBR with churn flow and pseudo-static flow were established with deviations generally within ±18%. This study contributes to improve the understanding of mass transfer and points out the process intensification direction of μPBR.     

An engineering model of a gas bubble–liquid turbulent flow in coiled tubing wound on a reel

An engineering model of a gas–liquid turbulent bubbly flow in coiled tubing wound on a reel (vertical helical coils) is developed. A simplified monodispersed bubble system is considered. A largest size of a bubble was selected as a characteristic bubble size. The bubble concentration distributions across and along tubing are calculated. The concentration across tubing periodically changes along a coil. The concentration variation amplitude increases with increasing the tubing diameter and decreasing the flow rate. The model developed can be considered as a framework for a full liquid–gas turbulent flow model taking into account bubble system polydispersity.     

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.     

Gas–liquid mass transfer and bubble size distribution in a multi-Cyclone separator

This article discusses the use of a multi-cyclone separator, which is a simplified form of a degassing hydrocyclone, in the separation of sweeping nitrogen bubbles and dissolved oxygen from water. The motion of the nitrogen bubbles and mass transfer of dissolved oxygen is discussed. It was observed that the Sauter mean diameter and gas volume in the swirling flow region as well as the total gas holdup increased as the volumetric ratio of gas to liquid flow increased. Almost all bubbles were found to exit through the gas outlet, indicating optimum performance of the bubble-separation process. The multi-cyclone separator was found to achieve good performance for the mass transfer of oxygen from water to nitrogen. This work is important in predicting the destination of bubbles and dissolved gas in swirling flow. © 2018 American Institute of Chemical Engineers AIChE J, 65: 215–223, 2019     

Measurements of the effective mass transfer areas for the gas–liquid rotating packed bed

Rotating packed bed(RPB) is one of the most effective gas–liquid mass transfer enhancement reactors, its effective specific mass transfer area(ae) is critical to understand the mass transfer process. By using the NaOH–CO₂ chemical absorption method, the aevalues of three RPB reactors with different rotor sizes were measured under different operation conditions. The results showed that the high gravity factor and liquid flow rate were major affecting factors, while the gas flow rate ex...     

Different efficiency toward the biomimetic aerobic oxidation of benzyl alcohol in microchannel and bubble column reactors: Hydrodynamic characteristics and gas–liquid mass transfer

The selective aerobic oxidation of benzyl alcohol to benzaldehyde has attracted considerable attention because benzaldehyde is a high value-added product. The rate of this typical gas–liquid reaction is significantly affected by mass transfer. In this study, CoTPP-mediated(CoTPP: cobalt(II) mesotetraphenylporphyrin) selective benzyl alcohol oxidation with oxygen was conducted in a membrane microchannel(MMC) reactor and a bubble column(BC) reactor, respectively. We observed that 83% benzyl alcoho...     

CFD simulation of gas–liquid flow in a high-pressure bubble column with a modified population balance model

In this study, based on the Luo bubble coalescence model, a model correction factor C_e for pressures according to the literature experimental results was introduced in the bubble coalescence efficiency term. Then, a coupled modified population balance model (PBM) with computational fluid dynamics (CFD) was used to simulate a high-pressure bubble column. The simulation results with and without C_e were compared with the experimental data. The modified CFD-PBM coupled model was used to investigate its applicability to broader experimental conditions. These results showed that the modified CFD-PBM coupled model can predict the hydrodynamic behaviors under various operating conditions.     

Transient gas–liquid mass transfer model for thin liquid films on structured solid packings

This paper describes a model for gas–liquid mass transfer through thin liquid films present on structured packings for gas–liquid operations under dispersed gas flow regime. The model has been derived for two cases: the absorption (or desorption) of a gaseous component into the liquid film and the transfer of the gaseous component through the liquid film to the packing surface where an infinitely fast reaction takes place. These cases have been solved for three bubble geometries: rectangular, cylindrical, and spherical. For Fourier numbers below 0.3, the model corresponds to Higbie’s penetration theory for both cases. The Sherwood numbers for cylindrical and spherical bubbles are 20% and 35% higher, respectively, than for rectangular bubbles. In case of absorption and Fourier numbers exceeding 3, the effect of bubble geometry becomes more pronounced. The Sherwood numbers for cylindrical and spherical bubbles now are 55% and 100% higher, respectively, than for rectangular bubbles. In case of an infinitely fast reaction at the packing surface, the Sherwood number corresponds to Whitman’s film theory (Sh=1$Sh=1$) for all bubble geometries. In this paper also practical approximations to the derived Sherwood numbers are presented. The approximations for both cases and all bubble geometries describe all the model data within an error of 4%. The application of the model has been demonstrated for three examples: (1) gas–liquid mass transfer for a structured packing; (2) gas–liquid mass transfer in a microchannel operated with annular flow; (3) gas–liquid mass transfer in a microchannel with Taylor flow.     

Hydrodynamics and mass transfer enhancement of gas–liquid flow in micropacked bed reactors: Effect of contact angle

Pressure drop, residence time distribution, dispersive behavior, liquid holdup, and mass transfer performance of gas–liquid flow in micropacked bed reactors (μPBRs) with different contact angles (CA) of particles are studied. The value of pressure drop for three types of beads can be obtained: copper beads (CA = 88.1°) > stainless steel beads (CA = 70.2°) > glass beads (CA = 47.1°). The liquid axial dispersion coefficient is 1.58 × 10⁻⁶ to 1.07 × 10⁻⁵ m²/s for glass beads and copper beads, which is smaller than those of trickle bed reactors. The liquid holdup of 400 μm copper beads is larger than that of 400 μm glass beads. The ratio of effective interfacial area enhancement is evaluated up to 55% for big contact angle beads compared with the hydrophilic glass beads. In addition, correlations of pressure drop, liquid holdup, and effective interfacial area in μPBRs with different wettability beads are developed and predicted values are in agreement with the experimental data.