On the formation of Taylor bubbles in small tubes

The formation of Taylor bubbles and resulting bubble lengths were studied in a ID vertical tube for air-water and air-octane systems. In the co-flow tube/nozzle arrangement two nozzle sizes were used as gas inlets. Superficial velocities varied between 0.001- for the liquid and 0.002- for the gas. Three different mechanisms of initial bubble formation were observed. Of the three mechanisms, mechanism 3 is periodic (with period consisting of a bubble and a liquid slug), reproducible and can be simply modelled. After initial bubble formation further modifications may occur in the formed bubble size by coalescence or pairing. Bubble pairing is encouraged by smaller nozzles and liquid flow rates, while coalescence is observed only for cases where non-Taylor bubbles form initially.Two simple models have been proposed, the first predicts the size of the Taylor bubbles formed by mechanism 3 while the second attempts to predict the condition for bubble pairing to occur. Reasonable agreement with experimental results validates the predictions of the first model for a strong dependence of the volume of Taylor bubbles formed on the gas and liquid flow rates, a moderate dependence on nozzle diameter and a weak dependence (if at all) on the surface tension of the liquid used. Mismatch with the experimental results is caused (at least in part) by the experimental setup where there was no perfect axial alignment of the gas inlet. The experiments also suffered from problems at the outlet at low flow rates where smooth bubble disengagement could not be ensured for long Taylor bubbles. The second model for pairing predicts its occurrence for concentric tube/nozzle arrangements as a function of flow rates and channel diameters. The model over-predicted the range of liquid flow rates at which pairing was observed experimentally, but it captured the form of the boundary between different bubble volume modification mechanisms when represented on superficial velocity graphs.     

Mixing in bubble columns: a new approach for characterizing dispersion coefficients

Use of bubble columns as photobioreactors requires a quantitative knowledge of radial mixing in these columns. A complete model of liquid-phase dispersion was used to simultaneously characterize axial and radial mixing in a relatively large (0.06 m³, 2.3 m tall, 0.193 m in diameter) bubble column photobioreactor. Axial and radial dispersion coefficients and mixing times were determined in tap water and sea water for superficial aeration velocities of up to . The measured axial dispersion coefficients (D_z) were generally consistent with the predictions of the well established correlations, thus validating the complete dispersion model used in the analysis. The D_z values ranged from ∼150 to and were highly reproducible. There was evidence that the existing literature data on D_z in bubble columns are slightly underestimated, as consistent underestimation was found to be a characteristic of the widely used dispersion model that disregards radial dispersion. The value of the radial dispersion coefficient was typically about 1% of the D_z value under any given condition. Except at incipient aeration, the radial dispersion coefficient was not as sensitive to the magnitude of the aeration rate as was the axial dispersion coefficient. The mixing time data were generally consistent with the existing correlations.     

Evolution of hydrodynamic and statistical parameters of gas-liquid slug flow along inclined pipes

The development of slug flow along two long inclined pipes (2-90° from the horizontal) with internal diameters of 0.024 and was measured by three optical fiber probes. The probes were located in a measurement module at axial distances of between the fiber tips. To measure the evolution of slug flow, the module was placed at different positions along the pipe. Instantaneous elongated bubble velocities and corresponding elongated bubble and liquid slug lengths were determined by processing the optical probe signals. The evolution of the liquid slug and elongated bubble length distributions along the pipes is characterized by a gradual growth of the mean and mode values. The growth rate decreases with decreasing inclination. Mean elongated bubble lengths have a minimum at about 60°, while mean liquid slug lengths decrease slowly with decreasing inclination angle. The coalescence rate, defined as the decrease in the ensemble size, becomes almost negligible at x/D>60, independent of pipe diameter, flow rates and inclination angle. The slug frequency has a maximum at about 60° inclination.     

Gas back-mixing studies in membrane assisted bubbling fluidized beds

Fluidized beds employing fine powders are finding increased application in the chemical and petrochemical industry because of their excellent mass and heat transfer characteristics. However, in fluidized bed chemical reactors axial gas back mixing can strongly decrease the conversion and selectivity. By insertion of membranes in fluidized beds large improvements in conversion and selectivity can be achieved, firstly by optimizing axial concentration profiles via distributive feeding of one of the reactants or selective withdrawal of one of the products, and secondly, by decreasing the effective axial dispersion via compartmentalization of the fluidized bed. Moreover, insertion of membrane bundles in a suitable configuration impedes bubble growth, thereby reducing reactant by-pass via rapidly rising large bubbles. In this work the influence of the presence and configuration of membrane bundles and the effect of gas addition via the membranes on the effective axial dispersion was studied experimentally.Steady state concentration profiles were measured where a CO₂-tracer was injected at different locations through a probe (point injection) or via the membranes (line injection) into a square fluidized bed containing glass particles (75-, 2550 kg/m³) fluidized with nitrogen distributed via a porous plate. Different bed configurations, viz. without internals, with vertical or horizontal membrane bundles were investigated and the effects of overall fluidization velocity and gas flow ratio of gas fed through the membrane bundles and the porous plate distributor were studied.Experimental results revealed that the insertion of vertical and horizontal membrane bundles decreases the effective axial dispersion considerably compared to a bed without internals. The point injection experiments indicated the importance of a non-uniform lateral emulsion phase velocity profile. The line injection experiments clearly pointed out the importance of bubble-to-emulsion phase mass transfer limitations. Gas addition through the membrane bundles decreases the effective axial gas dispersion enormously by almost annihilating the solids down flow along the walls and by decreasing the average bubble size and bubble fraction.     

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.     

Kinetic theory based computation of PSRI riser: Part I—Estimate of mass transfer coefficient

The PSRI benchmark challenge problem one is modeled using kinetic theory based CFD with the energy minimization multi-scale (EMMS) drag law. These computations give a better comparison than the previous models to measured solids mass flux, solids density and pressure drop.The computer model was also used to calculate axial and radial normal Reynolds stresses, energy spectra, power spectra, granular temperatures, the FCC viscosity and axial and radial dispersion coefficients. The computed cluster sizes agreed with the published empirical correlations. Then, the mass transfer coefficients and the Sherwood numbers are estimated based on particle cluster sizes. The conventional Sherwood number is scaled with the particle cluster diameter. The Sherwood number is the order of 10^-2 and the mass transfer coefficient is the order of . This Sherwood number is two orders of magnitude smaller than the diffusion controlled limit of two based on particle diameter, in agreement with the experimental data for fluidization of fine particles.     

Effect of contaminants on mass transfer coefficients in bubble column and airlift contactors

In this work, the effects of surface-active contaminants on mass transfer coefficients k_La and k_L were studied in two different bubble contactors. The oxygen transfer coefficient, k_L, was obtained from the volumetric oxygen transfer coefficient, k_La, since the specific interfacial area, a, could be determined from the fractional gas holdup, ε, and the average bubble diameter, d₃₂. Water at different heights and antifoam solutions of 0.5- were used as working media, under varying gas sparging conditions, in small-scale bubble column and rectangular airlift contactors of 6.7 and capacity, respectively. Both the antifoam concentration and the bubble residence time were shown to control k_La and k_L values over a span of almost 400%. A theoretical interpretation is proposed based on modelling the kinetics of single bubble contamination, followed by sudden surface transition from mobile to rigid condition, in accordance with the stagnant cap model. Model results match experimental k_L data within ±30%.     

On the influence of the liquid physical properties on bubble volumes and generation times

This work presents both theoretical and experimental studies about the specific influence of viscosity, surface tension and density in the formation of a gas bubble. The theoretical model includes both bubble formation and free rising and extends some previous work. As the previous bibliography provides rather scattered data regarding the effect of the liquid properties on bubble generation times, the results can be considered a step forward in the understanding of the subject because they separately describe the influence of viscosity, surface tension and density. The model yields satisfactory results of bubble shapes and formation times when compared to the experimental high-speed video observations obtained using non-Newtonian solutions of carboxymetyl cellulose in water at different concentrations (0.4-1.6% w/w). The complete experimental study also includes a range of different gas flow rates (1-) and orifice diameters (1.5 and 2 mm).     

Lateral mixing in trickle bed reactors

Knowledge of lateral mixing is essential to understand heat and momentum transfer parameters in both single-phase liquid and two-phase gas-liquid co-current down flow through packed bed columns. The reactors through which gas and liquid concurrently flow downwards through a bed of catalytic packing are called trickle bed reactors. Experimental data on lateral mixing coefficients from both the heat transfer and radial liquid distribution studies are obtained over a wide range of flow rates of gas and liquid using glass spheres (4.05 and 6.75 mm), ceramic spheres (2.59 mm), and ceramic raschig rings (4 and 6.75 mm) as packing materials covering trickle flow, pulse flow, and dispersed bubble flow regimes. In the present work, an expression for estimation of lateral mixing coefficient (αβ)_L is derived using the data on radial liquid distribution studies. The agreement between the values of (αβ)_L obtained from heat transfer studies and from radial liquid distribution studies using the experimental data shows that there exists an analogy between the heat transfer and radial liquid distribution in packed beds. Since (αβ)_L is an important variable for estimation of various heat and mass transfer parameters, a correlation for (αβ)_L based on present heat transfer study is proposed. The agreement between the (αβ)_L values estimated from the proposed correlation and experimental values is satisfactory with a standard deviation (s.d.) of 0.119.     

Liquid mixing in gas-liquid two-phase flow by meandering microchannels

The influence of the channel radius on the mass transfer in rectangular meandering microchannels (width and height of ) has been investigated for gas-liquid flow. Laser induced velocimetry measurements have been compared with theoretical results. The symmetrical velocity profile, known from the straight channel, was found to change to an asymmetrical one for the meandering channel configuration. The changes in the secondary velocity profile lead to an enhanced radial mass transfer inside the liquid slug, resulting in a reduced mixing length. In the investigated experimental range (superficial gas velocity and superficial liquid velocity ) the mixing time was reduced eightfold solely due to changes in channel geometry. An experimental study on the liquid slug lengths, the pressure drop and their relation to the mass transfer have also been performed. Experimental results were validated by a simulation done in Comsol Multiphysics^®. To obtain information for higher velocity rates, simulations were performed up to . These velocity variations in the simulation indicate the occurrence of a different flow pattern for high velocities, leading to further mass transfer intensification.     

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.     

Modelling the effect of liquid motion on bubble nucleation during beer dispense

Beer dispense involves ejecting supersaturated beer under gas pressure, from a nozzle into a receiving vessel. Bubble nucleation therefore occurs in a flowing liquid. This situation is encountered in other processes, but is not accounted for in current nucleation models. An experimental system was developed to measure bubble production rates and sizes in laboratory scale beer dispense. Experimental results indicate that bubble nucleation is affected by both liquid flow rate and dissolved gas composition. Pre-existing gas nuclei models have been adapted using bubble and droplet detachment models to include the effect of liquid motion and gas composition. The adapted nucleation models were compared to the experimental results. Predicted bubble detachment radii and overall nucleation rates were affected by liquid flow rate, direction of liquid flow, dissolved gas composition, the contact radius and the level of contact angle hysteresis. Accurate predictions were achieved for different surface orientations and liquid flow directions. Accurate predictions occurred at hysteresis levels of 3.5°, 7.5° and 20° for liquid flow rates of 0.6, 2.2 and , respectively. It is clear that the predicted overall nucleation rate however, also depended on the number of nucleation sites and how many of these were active; although values for these parameters were not experimentally determined in this case. Further understanding of the exact number and size of nucleation sites available and the contact angle for the particular combination of liquid and solid used is required to improve the fit of the model to the experimental data.     

Bubble size effect on low liquid input drift-flux parameters

We investigated the effect of bubble size on the drift-flux parameters at low liquid flow conditions by measuring the radial profiles of void fraction and phase velocities in a vertical bubbly pipe flow of diameter and height . To study the effect of the bubble size we used two different types of bubble inlets. We measured the local bubble fraction and velocity U_g by using single and four-point-optical fibre probes, and we used Laser Doppler Anemometry to determine the liquid velocity U_l. The distribution parameter C₀ and the weighted mean drift velocity |U_drift| were directly computed from the local measurements at a height on our experimental set-up. Both parameters were influenced by the bubble size. Provided no liquid flow reversal occurred at the near wall region, the distribution parameter reached a below unity minimum plateau value of C₀=0.95 for wall peaking void fraction profiles. At low liquid input conditions both the liquid input and bubble size had an influence on the distribution parameter. Extreme values such as C₀>2 were measured. From these measurements we developed models for the drift-flux parameters to take into account the effect of bubble size and input-flow conditions for our intermediate pipe diameter value. These models were tested and validated with separately collected experimental data.     

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.     

Gas holdup, liquid circulating velocity and mass transfer properties in a mini-scale external loop airlift bubble column

The external loop airlift bubble column has been regarded as a promising type of gas-liquid or gas-liquid-solid biooreactor because of the liquid circulating flow between the riser and downcomer. A mini-scale column is useful and efficient in the process research and development for highly specialized materials such as fine chemicals, advanced bioproducts and biocatalysts utilized in two or three phase system. In this work, a mini-scale glass column of in volume was designed and characterized. The gas holdup ε_G in the riser was obtained by measuring the volume expansion through photographs taken with a digital camera. The liquid circulating velocity U_L was measured by observing the time required for a tracer particle to travel a fixed distance in the downcomer through analysis of the images taken by a video camera. The gas-liquid volumetric oxygen transfer coefficient k_La and liquid-solid oxygen transfer coefficient k_S were determined by our previous method in which the air oxidation of glucose was catalysed by the immobilized glucose oxidase gel beads suspended in the column to obtain a pseudo steady state concentration of the dissolved oxygen and the corresponding constant rate of glucose consumption. It was shown that even such a mini-scale external loop bubble column could be characterized in terms of gas holdup, liquid circulating velocity and mass transfer properties according to our previous correlations proposed for the bench to pilot scale column.     

Bubble formation from a free-standing tube in microgravity

The bubble characteristics and the bubble detachment mechanisms during injection of air from a free-standing capillary tube submerged in water were studied in microgravity. The experiments were conducted in the 2.2-s drop tower at the NASA Glenn Research Center. A tube, 0.51 mm in diameter and 150 mm long, in a rectangular test section ( long) served as the injector. Images of the bubbles during the drops were acquired using a high-speed camera for various gas flow rates. Bubble detachment was observed for all the Weber numbers tested (0.28-31.12). This observation was different from previous studies using plate orifices in microgravity when bubble detachment was observed only for Weber numbers larger than 8. In order to resolve these differences, experiments were carried out using plate orifices. It was found that the bubbles detached from the orifice for all Weber numbers and that the bubbles formed were larger than those formed with the tube injector, particularly at low gas flow rates. The availability of a large area for the bubble to anchor itself and the presence of the chamber underneath the orifice could cause these differences. The effects of the chamber volume on the unsteadiness of bubble formation in plate-orifices at low gas flow rates are discussed.     

Heat transfer characterization of gas-liquid flows in a trickle-bed

Instantaneous local fluid-solid heat transfer coefficient (h_t) in a laboratory scale trickle-bed was measured using a constant-voltage anemometry technique. It was observed that convective heat transfer rate in the liquid-rich pulses was approximately 4 times that in gas-continuous bases for the air-water system. Time-averaged heat transfer rate was found to be positively influenced by both gas and liquid flow rates, with a stronger dependence on the latter. Heat removal efficiency, taking pressure drop penalty into account, suggested an optimum at intermediate liquid flow rate. Based on the measurements, a four-parameter heat transfer model featuring heat transfer coefficients in liquid-rich pulses (h_tp) and gas-continuous bases (h_tb), pulsing frequency and pulse fraction was developed to characterize transient h_t under various flow regimes. This model can be used in any trickle-bed reactor simulation that accounts for the dynamic interactions of catalytic reactions and heat transfer. It was found that while h_tp and h_tb correspond to liquid-solid and gas-solid heat transfer, respectively, and are determined mainly by the fluid properties, pulsing frequency and pulse fraction are the factors characterizing different flow regimes. Pulsing frequency, which can significantly impact reaction, may be tuned by selecting appropriate packing size, since smaller sizes generate higher frequency pulses. For example, a two-fold higher frequency was detected in packing as compared to that with packing. Flow regime evolution along the column axial location was identified visually, while the dispersed bubbling flow retreating to pulsing flow owing to gas bubble coalescence was evidenced by the heat transfer measurements.     

Liquid-liquid two-phase flow patterns and mass transfer characteristics in rectangular glass microreactors

The flow of two immiscible fluids was investigated in rectangular glass microchannels with equivalent diameters of 269 and . Deionised water, dyed toluene and hexane were selected as probe fluids. Flow patterns were obtained for Y- and T-junction of two micro-channels and monitored by a photo-camera. Volumetric velocities of water and organic phase varied between 1 and 6 ml/h. The formation mechanism of slug and parallel flow was studied and the mass transfer performances of two flow patterns were compared. The shape of the interface between the immiscible liquids was controlled by a competition between the viscous forces and the local interfacial tension. The flow patterns could be correlated with the mean Capillary and Reynolds numbers. The mass transfer coefficients for parallel and slug flow were determined using instantaneous neutralisation (acid-base) reaction. The two flow patterns showed the same global volumetric mass transfer coefficients in the range of , being affected mainly by the base concentration in water for parallel flow and by the linear velocity in the case of the slug flow.     

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.