Small bubble formation via a coalescence dependent break-up mechanism

A mechanism has been elucidated for the coalescence-mediated break-up of bubbles in gas-liquid systems. Images taken from dynamic systems (a coalescence cell and laboratory-scale bubble columns) show that in some instances the coalescence of two bubbles is accompanied by the formation of a much smaller daughter bubble. Following the coalescence process an annular wave is formed due to the very rapid expansion of the hole following the instant of film rupture. As the wave moves along the length of the bubble, away from the point of rupture it causes a rippling effect which distorts the newly coalesced bubble and may result in the formation of an unstable extension. Instabilities due to the annular wave pinch off a portion of this extension, resulting in the generation of a small daughter bubble. In coalescence dominated systems the process results in the generation of significant numbers of bubbles much smaller (100- diameter) than the Sauter mean diameter (3-).     

Chaos in bubbling—nonlinear analysis and modelling

In the present paper, nonlinear features and analytical results for the chaotic bubbling from a submerged orifice are described. A chain of air bubbles was produced from the single orifice of in diameter and micro-convection induced by the bubble generation was recorded using hot-probe anemometer located close to the orifice. The air flow rate was varied widely from q=100 to and the aspects of bubbling were observed by high-speed video. The nonlinear analysis is performed for the time series data of hot-probe anemometer especially in the range of q=435-. The calculated largest Lyapunov exponent shows that with increase of air volume flow rate, the time period for the process of liquid flow to lose stability becomes shorter and at high air flow rate such as , it is shorter than the time period between subsequent bubbles. To explain such chaotic behaviors of bubbling, a simple model has been proposed. The model simulates the process of interaction between the elastic bubble wall and liquid. Simulation results compared well with the analytical results of experimental data. Summarizing, it is concluded that one of the reasons for chaos appearance is the nonlinear character of interaction between an elastic bubble wall and the liquid stream.     

Spargers for controlled bubble size by means of the multiple slot disperser (MSD)

Sparging technology is crucial in the dispersion of gases in liquids. In this work, it was demonstrated that an effective, controllable sparger can be made by assembling an array of flat parallel slot-nozzles which may offer new options for sparger design and operation. To illustrate the technique, a compact, ‘multiple slot disperser’ (MSD) having a slot width of and a total slot length of 1.26 m was assembled from a series of 5-mm-thick graphite plates. In water containing a low concentration of frother, a dense three-dimensional bubble plume was produced. The MSD generated consistently narrow bubble size distributions with well-defined median bubble diameters in the range 2.6-3.1 mm, equivalent to a range of gas flow rate from 11 to 26 std l/min. The bubble sizes were readily predictable from established single slot bubble size correlations. The method of construction also allows for simple maintenance, repair and replacement of individual components as needed.     

Bubble sizes in agitated solvent/reactant mixtures used in heterogeneous catalytic hydrogenation of 2-butyne-1,4-diol

Catalytic hydrogenations reactions are frequently conducted in “dead-end” multiphase stirred reactors with the reactant dissolved either in an alcohol, or in water or a mixture of the two. In such systems, the rate of gas-liquid mass transfer, which depends on bubble size, may well be the overall rate-limiting step. However, a study of bubble sizes across the whole range of solvent compositions from entirely water to entirely organic has not been reported. Here, for the first time, a systematic investigation has been made in a 3 L, closed vessel simulating a “dead-end” reactor containing 1% by volume of air which is dispersed by a Rushton turbine in water, isopropanol (IPA) and mixtures of the two, with and without 2-butyne-1,4-diol simulating a reactant. Mean specific energy dissipation rates, , up to have been used and bubbles size distributions and mean size were measured using a video-microscope-computer technique. In the single component solvents (water, ; IPA, though the interfacial tensions are very different, irregular, relatively large bubbles of similar sizes were observed ( in IPA, and in water) with a wide size distribution. In the mixed aqueous/organic solvents, and especially at the lower concentrations of IPA (1%, 5%, 10%), the bubbles were spherical, much smaller (d₃₂ from 50 to ) with a narrow size distribution. The addition of the reactant (0.2 M 2-butyne-1,4-diol) to the mixed solvents had little effect on the mean size, shape or distribution. However, addition to water (thus producing a mixed aqueous/organic liquid phase) led to small spherical bubbles of narrow size distribution. Neither Weber number nor surface tension was suitable for correlating bubble sizes since σ decreased steadily from pure water to IPA whilst bubble size passed through a minimum at around 5% IPA. For any particular fluid composition, the functionality between d₃₂ and was similar, i.e. . The above observations are explained in terms of the polarisation of bubble surfaces in miscible mixed aqueous/organic liquids caused by preferential directional adsorption at low concentrations of the organic component with its hydrophilic part directed into the aqueous phase and its hydrophobic part into the gas phase. As a result, coalescence is heavily suppressed in the low-concentration miscible alcohol (or diol)/aqueous systems whilst strong coalescence dominates bubble sizes in water and the alcohol and at high concentrations of the latter.     

Validation of bubble breakage, coalescence and mass transfer models for gas-liquid dispersion in agitated vessel

Bubble breakage and coalescence phenomena and multicomponent gas-liquid mass transfer were studied in a Rushton turbine agitated vessel. Local bubble size distributions (BSD) were measured from air-tap water system at several agitation conditions with capillary suction probe (CSP) technique. The CSP was compared to the digital imaging (DI) and phase Doppler anemometry (PDA) techniques in a stirred vessel. The volumetric BSDs between the CSP and DI were in agreement, but number BSDs showed notable deviation. The limitations of measurement techniques seem to be the main reason.A multiblock stirred tank model with discretized population balances for bubbles and two-film Maxwell-Stefan multicomponent mass transfer between gas and liquid was created for the agitated vessel. The model considers local mass transfer conditions in the vessel and is simple enough for the mathematical optimization of unknown model parameters. Unknown parameters in the mechanistic bubble breakage and coalescence models were fitted against measured local BSDs. After this, a parameter in the liquid film mass transfer correlation was adjusted against absorption and desorption experiments of oxygen. Local gas-liquid mass transfer areas were calculated from the population balance model. The simulations with the validated models show good agreement against experiments. On the other hand, the fitted parameters deviate from the theoretical values, which emphasizes the need of model validation against accurate experiments. Due to their fundamental character and the validation process, the fitted models seem to be useful tools for the design and scale-up of agitated gas-liquid reactors.     

Electrochemical performances of gel polymer electrolytes using tetra(ethylene glycol) diacrylate

A gel polymer electrolyte (GPE) was prepared using tetra(ethylene glycol) diacrylate monomer, benzoyl peroxide, and (). The LiCoO₂/GPE/graphite cells were prepared and their electrochemical properties were evaluated at various current densities and temperatures.The viscosity of the precursor containing the tetra(ethylene glycol) diacrylate monomer was around . The ionic conductivity of the gel polymer electrolyte at 20°C was around . The gel polymer electrolyte had good electrochemical stability up to vs. Li/Li⁺. The capacity of the LiCoO₂/GPE/graphite cell at rate was 63% of the discharge capacity at rate. The capacity of the cell at −10°C was 81% of the discharge capacity at 20°C. Discharge capacity of the cell with gel polymer electrolyte was stable with charge-discharge cycling.     

Numerical simulations of bubble formation on submerged orifices: Period-1 and period-2 bubbling regimes

The process of bubble formation is involved in several gas-liquid reactors and process equipment. It is therefore important to understand the dynamics of bubble formation and to develop computational models for the accurate prediction of the bubble formation dynamics in different bubbling regimes. This work reports the numerical investigations of bubble formation on submerged orifices under constant inflow conditions. Numerical simulations of bubble formation at high gas flow rates, where the bubble formation is dominated by inertial forces, were carried out using the combined level set and volume-of-fluid (CLSVOF) method and the predictions were experimentally validated. Effects of gas flow rate and orifice diameter on the bubbling regimes and in particular, on the transition from period-1 to period-2 bubbling regime (with pairing or coalescence at the orifice) were investigated. Using the simulation data on the transition of bubble formation regimes, the bubble formation regime map constructed using Froude and Bond numbers is presented.     

Effect of fiber length distribution on gas holdup in a cocurrent air-water-fiber bubble column

An experimental investigation is reported on the effect of fiber length distribution on gas holdup in a cocurrent air-water-fiber bubble column. Different combinations of 1 and 3 mm Rayon fibers are used to simulate different fiber length distributions. At a constant total fiber mass fraction, gas holdup generally decreases with increasing mass fraction of the 3 mm Rayon fiber while other conditions remain constant. Crowding factors estimated using four different methods (N_c=N_c,A, , N_c,L, and N_c,M) and the parameters and are tested on their performance to quantify the overall effects of fiber mass fraction and fiber length and its distribution on gas holdup. and provide the best characterization of the fiber effects on gas holdup in the cocurrent air-water-fiber bubble column. The crowding factor estimated using the model-based average fiber length (N_c,M) also provides a good characterization and is better than the other crowding factor definitions.     

Temperature variations in the oxygen carrier particles during their reduction and oxidation in a chemical-looping combustion system

A particle reaction model including mass and heat transfer has been developed to know the temperature variations produced inside the oxygen carrier particles during the cyclic reduction and oxidation reactions taking place in a chemical-looping combustion (CLC) system. The reactions of the different oxygen carriers based on Cu, Co, Fe, Mn, and Ni during the reduction with fuel gas (CH₄, CO, and H₂) and oxidation (O₂) have been considered. In these systems, the oxidation reaction is always exothermic with subsequent heat release; however, the reduction reaction can be exothermic or endothermic depending on the metal oxide and the fuel gas. The heat generated inside the oxygen carriers during the exothermic reactions increases the particle temperature, and could affect the particle structure if the temperature increase is near to the melting point of the active materials. Several variables that affect the reaction rate and the heat transport process have been analyzed to know their effect on the internal particle temperature. For a given oxygen carrier and reaction, the maximum temperature of the particles depended mainly on the particle size, the reaction rate, and the external heat transfer resistance, being lower than the effect of the oxygen carrier porosity, type of inert material, and metal oxide content. The highest temperature variations were reached for the oxidation reactions, with the maximum corresponding to the Ni and Co oxygen carriers with values of for particles. The highest temperature increase observed during the reduction reactions corresponded to the reaction of CuO with CO, with values of for particles. For the rest of the reactions and metals, the variations in the particle temperature were below for particle sizes below . Under the typical operating conditions that exist in a CLC system, with particle sizes lower than , % of metal oxide content, and overall conversion times lower than , the increases of temperature with respect to the bulk conditions were lower than for any reaction of any oxygen carrier. Moreover, the temperature profiles inside the particles were near flat in most of the practical conditions, and no local points with high temperatures were found. Thus, changes in the solid porous structure of the carrier due to sintering during oxidation in fluidized bed reactors are not expected working at typical temperatures of CLC systems (1000-).     

Role of hydrodynamic flow parameters in lipase deactivation in bubble column reactor

The dynamic environment within the bioreactor and in the purification equipment is known to affect the activity and yield of enzyme production. In the present work, the effect of hydrodynamic flow parameters and τ_N,max) and interfacial flow parameters ( and ) on the activity of lipase has been comprehensively investigated in bubble column reactors. Lipase solution was subjected to hydrodynamic flow parameters in 0.15 and 0.385 m i.d. bubble column reactors over a wide range of superficial gas velocity (0.01<V_G<0.4-). The flow parameters were estimated using an in-house CFD simulation code based on k-ε approach. The extent of lipase deactivation in both the columns was found to increase with an increase in hydrodynamic and interfacial flow parameters. However, at equal value of any of these parameters, the extent of deactivation was different in the two columns. The rate of deactivation was found to follow first order kinetics. An attempt has been made to develop rational correlations for the extent of deactivation as well as for the deactivation constant. The rate of deactivation was found to be depending on the average turbulent normal stress and interfacial flow parameters such as bubble diameter and bubble rise velocity.     

The effect of magnetic fields on the electrodeposition of cobalt

The effect of a magnetic field on the electrodeposition of Co has been investigated with respect to the strength and the orientation of the magnetic field (B). Two different effects of the magnetic field B on the electrodeposition of cobalt have been observed. The first is the magnetohydrodynamic (MHD) effect caused by the Lorentz force (). The second is an effect due to the paramagnetic force (), caused by the concentration gradient () and therefore the gradient of the molar susceptibility (). The limiting current density and the deposition rate of Co increases if the B-field is oriented parallel to the electrode surface. This is mainly due to the expected convection induced by . Both, the Co deposition and the reduction of hydrogen ions, are affected by this. At high cathodic potentials the contribution of the hydrogen reduction to the process changed, which led to homogeneous deposits. A decreasing deposition rate was measured for B-fields oriented parallel or antiparallel to the flow of ions. These results are attributed to the effect of on the electrochemical processes close to the surface.     

A generalized analysis on material invariant characteristics for microwave heating of slabs

A generalized dimensionless formulation has been developed to predict the spatial distribution of microwave power and temperature. The ‘dimensionless analysis’ is mainly based on three numbers: wave number, ; free space wave number, ; and penetration number, , where is the ratio of sample thickness to wavelength of microwaves within a material, is based on wavelength within free space and is the ratio of sample thickness to penetration depth. The material dielectric properties and sample thicknesses form the basis of these dimensionless numbers. The volumetric heat source due to microwaves can be expressed as a combination of dimensionless numbers and electric field distributions. The spatial distributions of microwave power for uniform plane waves can be obtained from the combination of transmitted and reflected waves within a material. Microwave heating characteristics are obtained by solving energy balance equations where the dimensionless temperature is scaled with respect to incident microwave intensity. The generalized trends of microwave power absorption are illustrated via average power plots as a function of , and . The average power contours exhibit oscillatory behavior with corresponding to smaller for smaller values of . The spatial distributions of dimensionless electric fields and power are obtained for various and . The spatial resonance or maxima on microwave power is represented by zero phase difference between transmitted and reflected waves. It is observed that the number of spatial resonances increases with for smaller regimes whereas the spatial power follows the exponential decay law for higher regimes irrespective of and . These trends are observed for samples incident with microwaves at one face and both the faces. The heating characteristics are shown for various materials and generalized heating patterns are shown as functions of , and . The generalized heating characteristics involve either spatial temperature distributions or uniform temperature profiles based on both thermal parameters and dimensionless numbers ().