Bubble coalescence at sieve plates: II. Effect of coalescence on mass transfer. Superficial area versus bubble oscillations

Bubble columns are among the most used equipments for gas-liquid mass transfer processes. This equipment's aim is to generate gas dispersions into a liquid phase in order to improve the contact between phases. Bubble coalescence has always been one of their greatest problems, since it reduces the superficial gas-liquid contact area. However, bigger bubbles can oscillate, and these oscillations increase the mass transfer rate by means of modifying the contact time as well as the concentration profiles surrounding the bubble. In the present work, the coupled effect has been studied by means of two-holed sieve plates with diameters of 1.5, 2 and 2.5 mm each, close enough to allow the coalescence and separated enough to avoid it. The results show that although coalescence decreases mass transfer rate from bubbles the deformable bubble generated can, in certain cases, balance the decrease in mass transfer rate due to the reduction in superficial area. This fact can then be used to avoid the harmful effect of coalescence on the mass transfer rate. Empirical and theoretical equations have also been used to explain the phenomena.     

Experimental study of mass transfer in a dense bubble swarm

We consider the liquid-side mass transfer coefficient k_L in a dense bubble swarm for a wide range of gas volume fraction (0.45%≤α_G≤16.5%). The study is performed for an air–water system in a square column. Bubble size, shape and velocity have been measured for different gas flow rates by means of a high speed camera. Gas volume fraction and bubble velocity have also been measured by a dual-tip optical probe. Both of these measurements show that the bubble vertical velocity decreases when increasing α_G in agreement with previous investigations. The mass transfer is measured from the time evolution of the dissolved oxygen concentration, which is obtained by the gassing-out method. The mass transfer coefficient is found to be very close to that of a single bubble provided the bubble Reynolds number is based on the average equivalent diameter 〈d_eq〉 and the vertical slip velocity 〈V_z〉.     

Solid‐Liquid Mass Transfer at Gas Sparged Tube Bundles

The solid‐liquid mass transfer characteristics of an in‐line tube bank immersed in a gas‐liquid bubble column were investigated by measuring the rate of diffusion‐controlled dissolution of copper surface in acidified dichromate solution. Variables studied were the number of rows in the tube bank, physical properties of the solution, and nitrogen flow rate. The mass transfer coefficient was found to increase with increasing nitrogen flow rate. Increasing the number of rows in the tube bank was found to decrease the mass transfer coefficient. The data were correlated for the following conditions: 0.0021 < Fr.Re < 0.1603, 1 < N_r < 5 and 850 < Sc < 1370 by the equation J = 0.15 ( Fr.Re )^–0.15. Comparison was made between the present mass transfer data and previous heat and mass transfer studies conducted in packed and empty bubble columns.     

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.     

Analysis of local heat transfer and hydrodynamics in a bubble column using fast response probes

A fast response probe is used to measure local heat transfer in a bubble column. It captured the variations in local heat transfer coefficients due to changes in local hydrodynamic conditions in radial and axial directions. These measurements have been used to identify flow regime transitions, variations in flow patterns and local hydrodynamic structure as obtained with different gas distributors and varying gas velocity. Standard deviations of pressure measurements obtained with a fast response probe have been compared with heat transfer coefficient fluctuations for the first time and the similarities and differences have been pointed out. Variations in average heat transfer coefficients and standard deviations in radial and axial directions point to different hydrodynamic conditions and are compared with literature studies. Relationships between local heat transfer measurements and hydrodynamic conditions are shown.     

Rise velocities and gas-liquid mass transfer of bubbles in organic solutions

Bubble size, shape, rise velocity and liquid side mass transfer coefficient have been experimentally determined for bubbles rising in organic systems, consisting of single or mutually soluble components, namely: alkanes (n-dodecane, n-hexadecane), alcohols (ethanol, 1-butanol, 1-octanol) and mixtures thereof. For pure solvents (alkanes and alcohols alike), it was found that the bubbles are non-spherical, and that both the rise velocity and the mass transfer coefficient are close to those expected for bubbles with a mobile surface.For alkane-alcohol solutions, on the other hand, the bubbles become almost spherical, and their rise velocity and mass transfer coefficient decrease, taking values intermediate between those of rigid bubbles and bubbles with a mobile surface. Trace concentrations of either alkane in alcohol or alcohol in alkane are enough for this effect to be observed. The bubbles, however, never become completely rigid in the whole range of concentrations between pure alkane and pure alcohol.Use of Higbie's equation with experimental value of slip velocity to calculate the mass transfer coefficient, k_L, (system n-dodecane/1-octanol) yields somewhat high predictions of k_L, but follows the trend of experimental k_L with concentration for most of the concentration range. However, for very small concentrations of either component, Higbie's equation gives completely wrong results, both in magnitude and in trend. The reason for this behaviour is unknown.     

Gas-liquid mass transfer in a slurry bubble column operated at gas hydrate forming conditions

Gas-liquid interphase mass transfer was investigated in a slurry bubble column under CO₂ hydrate forming operating conditions. Modeling gas hydrate formation requires knowledge of mass transfer and the hydrodynamics of the system. The pressure was varied from 0.1 to 4 MPa and the temperature from ambient to 277 K while the superficial gas velocity reached 0.20 m/s. Wettable ion-exchange resin particles were used to simulate the CO₂ hydrate physical properties affecting the system hydrodynamics. The slurry concentration was varied up to 10%vol. The volumetric mass transfer coefficient (k_la_l) followed the trend in gas holdup which rises with increasing superficial gas velocity and pressure. However, k_la_l and gas holdup both decreased with decreasing temperature, with the former being more sensitive. The effect of solid concentration on k_la_l and gas holdup was insignificant in the experimental range studied. Both hydrodynamic and transport data were compared to best available correlations.     

Correction of the penetration theory based on mass-transfer data from bubble columns operated in the homogeneous regime under high pressure

A new correction term was developed which allows the classical penetration theory to be applied successfully to k_La data obtained from oblate ellipsoidal bubbles formed in bubble columns operated in the homogeneous regime at various pressures . The correction factor is a function of both the Eötvös number Eo and dimensionless gas density ratio. The new correlation was compared with literature k_La data in 18 pure organic liquids, 14 adjusted liquid mixtures and tap water. In some of the liquids (tetralin, xylene and ethanol) not only air but also other gases (nitrogen, helium and hydrogen) were used. In total, 263 experimental k_La points are fitted with an average relative error of 10.4%.In the theoretical approach for the k_La prediction, the gas-liquid contact time (used in the penetration theory) is defined as the ratio of bubble surface to the rate of surface formation. All further calculations are based on the geometrical characteristics (bubble length and height) of an oblate ellipsoidal bubble. It was found that the new correction factor f_c gradually reduces with the increase of both superficial gas velocity u_G and gas density ρ_G (operating pressure P).     

Effect of surfactants on liquid-side mass transfer coefficients in gas-liquid systems: A first step to modeling

This paper focuses on the effect of surfactants on the mass transfer parameters (volumetric mass transfer coefficient k_La and liquid-side mass transfer coefficient k_L). Tap water and aqueous solutions with surfactants (anionic, cationic and non-ionic at concentrations up to are used as liquid phases. The bubbles are generated into a small-scale bubble column having an elastic membrane with a single orifice as gas sparger. To understand the effects of the surfactants on the mass transfer, not only the static surface tension is used, but also the characteristic adsorption parameters like the surface coverage ratio at equilibrium Se. The liquid-side mass transfer coefficient is obtained from the ratio of the volumetric mass transfer coefficient (measured by a chemical method) and the specific interfacial area. These two parameters are obtained simultaneously. The methods used to obtain these parameters are described in Painmanakul et al. [2005. Effects of surfactants on liquid-side mass transfer coefficients. Chemical Engineering Science 60, 6480-6491].Whatever the liquid phase, three zones are found on the liquid-side mass transfer coefficient variation with the bubble diameter. For bubble diameters less than 1.5 mm, whatever the liquid phases, the k_L values are roughly constant at . For bubble diameters greater than 3.5 mm, the k_L values do not vary much with the bubble diameter, but depend on the surfactant concentration. For bubble diameters between 1.5 and 3.5 mm, the k_L values increase from to the value reached at 3.5 mm. This increase depends on the surfactants. Higbie's model does not represent the k_L values for bubble diameters greater than 3.5 mm, even though there is a small amount of surfactant in the liquid phase. Thus, a model is proposed for each zone described above. Explanations are also proposed for the effect of the surfactant on the k_L values for each of the above zones.     

The important role of local dispersed phase hold-ups for the calculation of three-phase bubble columns

A computational fluid dynamics model is used to calculate a three-phase (air-water-solid particles) flow in a bubble column. The calculation of multi-phase flows is significantly influenced by the formulation of the inter-phase drag and the modelling of the turbulence. Both are influenced by the dispersed phases. The k-ε turbulence model extended with terms accounting for the bubble-induced turbulence is used to calculate the eddy viscosity of the liquid phase. Bubble-bubble and particle-particle interactions are considered as well as a direct momentum transfer between the two dispersed phases bubbles and solid particles. The local volume fractions of the dispersed phases are considered for the calculation of the drag coefficients between the dispersed phases and the continuous phase. Measured local gas and solid hold-ups as well as measured liquid velocities are compared with the corresponding calculated results. The measured and the calculated results show good agreement.     

Oxygen transfer prediction in aeration tanks using CFD

In order to optimize aeration in the activated sludge processes, an experimentally validated numerical tool, based on computational fluid dynamics and able to predict flow and oxygen transfer characteristics in aeration tanks equipped with fine bubble diffusers and axial slow speed mixers, is proposed. For four different aeration tanks (1;1493;8191 and ), this tool allows to precisely reproduce experimental results in terms of axial liquid velocities, local gas hold-ups. Predicted oxygen transfer coefficients are within ±5% of experimental results for different operating conditions (varying pumping flow rates of the mixers and air flow rates). The actual bubble size must be known with precision in order to have a reliable estimation of the oxygen transfer coefficients.