Bubble Residence Time Research Articles

On the Importance of Modelling Bubble Dynamics for Point-mass Numerical Calculations of Bubble Columns

For decades, CFD (computational fluid dynamics) has been applied for numerical calculations of the rather complex flow in bubble columns using the two-fluid approach (Euler/Euler method) as well as the Euler/Lagrange approach, both based on the point-mass approximation. Numerous different models and closures have been proposed and used for describing the fluid dynamic forces acting on bubbles, modelling bubble induced turbulence (BIT) and transport of bubbles by turbulence structures (Sommerfeld, 2004). However, the dynamics of bubbles, i.e., oscillations and tumbling motion, is mostly neglected in such calculations since a physically based model in the frame of a point-particle approximation is not readily available. Such a model was developed, implemented in the frame of LES-Euler/Lagrange calculations (LES: Large Eddy Simulations) and validated based on detailed experiments (Sommerfeld and Bröder, 2009). The temporal evolution of bubble eccentricity was randomly generated based on measured correlations for the mean and rms (root mean square) values of eccentricity in connection with a theoretical oscillation time scale. Naturally, the state-of-the-art fluid forces acting on the bubbles were considered and the transport of bubbles by sub-grid-scale-turbulence (SGS) was modelled by a Langevin approach, as well as turbulence modification by the bubbles being considered in the frame of the LES context. Similar to experimental observations, such a thorough model yielded a bubble tumbling motion even in the point-particle approximation. The correct anisotropic bubble fluctuating velocities were thereby reproduced and good agreement with experiments was obtained for all other velocities. Note that bubble velocity fluctuations were so far never considered for the validation of calculations, but they are of immense importance when also considering mass transfer. Hence, the bubbly dynamics model provided the correct bubble residence time and, as a result, a gas hold-up identical to measured values.

Improving the CO2 fixation rate by increasing flow rate of the flue gas from microalgae in a raceway pond

Residence time of flue gas bubbles with different solution velocities and the influence of NO X and SO2 from flue gas on pH values of culture solutions were analyzed based on large-scale raceway reactors. Microalgal growth and CO2 fixation rates were also investigated with different gas flow rates. Bubble residence time was ∼1.1 s when the solution velocity was 20 cm/s. The NO X and SO2 effects on microalgal growth were negligible, although 66% NO X and 95% SO2 were captured by the microalgal solution. Microalgal biomass productivity increased from 10.3 to 14.1 g/m2/d when flue gas flow rate increased from 20 to 50 m3/h. CO2 fixation and microalgae biomass productivity increased further from 26.3 to 31.9 g/m2/d and from 14.1 to 17.1 g/m2/d, respectively, upon increase of flue gas flow rate from 50 to 150 m3/h.

Experimental investigation on bubble dynamic behavior in a confined space bioreactor with bacterial suspension

AbstractIn this work, a confined space bioreactor with bacterial suspension was developed to experimentally investigate the adsorption of microorganisms at the gas–liquid interface and the effects of inner diameter, inclined angle, and gas flow rate on bubble dynamic behavior. The results indicated that the adsorption of microorganisms at the gas–liquid interface was affected by the bacterial chemotaxis and the gas–liquid interface expansion, and the confined space was conducive to accelerate the detachment of smaller bubble and exacerbate the fluctuation of gas–liquid interface. Moreover, the larger inner diameter induced that the bubble was squeezed by the 2 side walls of the confined space bioreactor, restricting the bubble shocking and slowing down the bubble rising velocity for increasing the bubble residence time. Furthermore, the smallest bubble departure size and the shortest inrush time were observed at horizontal capillary orifice, but the largest inrush bubble number was obtained at down‐45° capillary orifice. In addition, the inrush bubble number increased with increasing gas flow rate, but the bubble departure sizes were not much different. The results can be used to optimize the design of aeration system for improving the performance of the bioreactor.

Global characterization of hydrodynamics and gas-liquid mass transfer in a thin-gap bubble column intended for microalgae cultivation

Global characterization of hydrodynamics and gas-liquid transfer in a 2D column with a 4mm thickness is carried out with water and aqueous glycerol solutions. This new design can be used as a photobioreactor of low thickness to intensify the volumetric productivity by increasing microalgae concentration. To mimic the presence of algae cultures, the liquid viscosity is adjusted with glycerol. Focuses are made on flow regimes and their transitions, mixing, and gas-liquid transfer at the scale of the reactor. In comparison with conventional bubble columns lower terminal velocities are obtained. So, in homogeneous regime, the residence time of bubbles is greater than in conventional bubble columns and the gas holdup is slightly higher. However, regime transitions appear at relatively low superficial gas velocities in the confined bubble column. In addition, two transition regimes are visible and have macroscopic flow structures very similar to the ones observed for conventional columns. For the mixing time, a dependence on the viscosity is observed and whatever the solution tested, this parameter decreases very quickly with the increase of the gas superficial velocity. Finally, global gas-liquid mass transfer coefficients measurements show lower values than in classical bubble columns at the same superficial gas velocity.

Alternatively permutated conic baffles generate vortex flow field to improve microalgal productivity in a raceway pond

Alternatively permutated conic (APC) baffles were proposed to generate vertical and horizontal vortex flow to intensify mixing and mass transfer in a raceway pond. Both clockwise vortexes were generated before and after conic baffles in the main stream to increase perpendicular velocity by 40.3% and vorticity magnitude by 1.7 times on vertical cross section. Self-rotary flow around conic baffles and vortex flow among conic baffles were generated to increase perpendicular velocity by 80.4% and vorticity magnitude by 4.2 times on horizontal cross section. The bubble generation time and diameter decreased by 25.5% and 38.7%, respectively, while bubble residence time increased by 84.3%. The solution mixing time decreased by 48.1% and mass transfer coefficient increased by 34.0% with optimized relative spacing (ε) and height (ω) of conic baffles. The biomass productivity of Spirulina increased by 39.6% under pure CO2 with APC baffles in a raceway pond.

Hydrogen Bubble Templated Electrodeposited Cu As CO2 Reduction Catalyst: Substrate Effect

Selective conversion of CO2 into high quality chemical feedstock and fuels represents an emerging energy storage technology1. The electrochemical reduction of carbon dioxide using renewable sources of energy is a promising way of utilizing the only intermittently available renewable energies to produce chemical feedstocks and fuels for later use. Presently, copper is the one metal in focus because of its ability to catalyze CO2 reduction towards energy-dense hydrocarbon products. However, the electrochemical reduction of CO2 on copper results in many different products at a wide range of overpotentials. The selectivity and efficiency of the Cu-based catalysts towards specific products can be improved by fine tuning the structure and composition2-4. In this work, high surface area porous Cu films were electrodeposited either on Cu or Pt substrates at various deposition currents and durations using the dynamic hydrogen bubble template method5. During electrodeposition at high current values, the size and residence time of H2 bubbles vary based on the substrate and on the in-situ deposited Cu, leading to significantly different catalyst morphologies. Consequently, well-defined dendritic structures with large pores are observed (inset of Fig. 1) for Cu deposited on Cu substrate (CuCat/Cu) as compared to Cu deposited on Pt substrate (CuCat/Pt). Interestingly, product quantification by gas chromatography during electrochemical measurements showed that the product selectivity observed for all variations of the catalysts deposited on a particular substrate does not depend significantly on the differences in pore size and film thickness of the material. However there is a difference in product selectivity for the respective catalysts deposited on different substrates as represented in Fig. 1, where ethylene and carbon-monoxide selectivities are shown at two different potentials. The formation of methane is significantly suppressed (<1%) on both substrates in the potential range where the catalysts show high selectivity for ethylene. From X-ray absorption measurements it is observed that the catalysts are composed of metallic Cu and CuI 2O species with no evidence of CuIIO species. The morphological changes, atomic state and composition of the catalyst may have a dialectic role in deciding the selectivity. In the follow-up work, the selectivity of various Cu films electrodeposited on both the substrates will be correlated with respect to surface area and CuI 2O content after comprehensive quantification of the gaseous and liquid reaction products by gas chromatography, liquid gas chromatography and HPLC. These studies are expected to reveal strategies by which catalysts can be designed for higher selectivity towards a specific hydrocarbon product. References M. Gattrell, N. Gupta and A. Co, J. Electroanal. Chem., 594, 1 (2006). 2. A. Dutta, M. Rahaman, N. C. Luedi, M. Mohos and P. Broekmann, ACS Catal. 6, 3804 (2016).H. Mistry, A. S. Varela, C. S. Bonifacio, I. Zegkinoglou, I. Sinev, et. al, Nat. Commun., 7, 12123 (2016).A. S. Varela, C. Schlaup, Z. P. Jovanov, P. Malacrida, S. Horch, et. al, J. Phys. Chem. C, 117, 20500 (2013).H. -C. Shin, J. Dong and M. Liu, Adv. Mater ., 15, 1610 (2003). Figure 1

Mass transfer analysis for CO2 bubble absorption in methanol/Al2O3 nanoabsorbents

In this paper computational fluid dynamics (CFD) analysis is carried out to investigate CO2 bubble absorption characteristics in methanol/Al₂O3 nanoabsorbents. Bubble size, rising velocity and mass transfer rate are compared to the previous experimental results for validation. It is found that the distance traveled for each CO2 bubble increases as the concentration of Al2O3 increases, which, in consequence, increases the residence time between liquid and gas phases resulting in higher interfacial mass transfer rates. For the case of a bubble rising in the gap between walls, the wall shear stress has a major effect on the bubble diameter and rising velocity which in consequence affects the mass transfer coefficient. It is concluded that the mass transfer coefficient enhances by about 40% by adding Al₂O3 nanoparticles (0.01vol%) compared with pure methanol absorbent from the experimental and simulation results. It is also concluded that the use of nanoparticles has a higher impact on mass transfer rate than it does on mass transfer amount, which depends on the residence time and travel distance of CO2 bubbles.

A novel fuzzy-logic based method for determination of individual bubble velocity and size from dual-plane ultrafast X-ray tomography data of two-phase flow

Ultrafast X-ray tomography enables non-invasive imaging of gas-liquid flows with high spatial and temporal resolution. While it is relatively straightforward to extract e.g. gas fraction profiles from cross-sectional tomographic images, the extraction of bubble and gas-liquid interface information requires advanced image processing techniques. Thereby it is an important necessity to transform the temporal scale in the scanned sequences into a corresponding length scale for obtaining correct volumetric information. For bubbly flows this means that the velocity of the dispersed phase, e.g. the gas bubbles, has to be determined from dual-plane scans. A common and widely applied method to obtain gas phase velocities is cross-correlating the image sequences of the two scanning planes. This gives an averaged velocity for each position in the cross-section. In the present work, a new method is introduced, which determines the velocity of individual gas bubbles. This new method is termed as “bubble twinning method”, because it tries to identify twin-bubbles in both scanning planes. The developed algorithm compares essential bubble parameters, that is, volume, position and residence time in the slice, by applying a fuzzy-logic based membership function approach. The algorithm was tested for bubbly flow as well as slug flow conditions. Results are compared with established theoretical predictions as well as the cross-correlation method.

Using Lorentz forces to control the distribution of bubbles in a vertical tube filled with liquid metal

In this work, a method to increase the residence time of bubbles in tubes or pipes filled with liquid metal is investigated. Imposing a horizontal electric current and a perpendicular horizontal magnetic field generates an upward-directed Lorentz force. This force can counteract gravity and cause floating of bubbles. Even with homogeneous electric fields these float in the mean but fluctuate randomly within the swarm due to mutual interactions. In the present case the cylindrical shape of the container furthermore creates inhomogeneous electric currents and an inhomogeneous force distribution resulting in a macroscopic convection pattern stirring the bubbles and further homogenising the spatial distribution of the bubbles.

Developing a water-circulating column photobioreactor for microalgal growth with low energy consumption

A water-circulating column photobioreactor (WCC–PBR) was developed to decrease bubble generation time and mixing time for growing microalgal biomass at low energy consumption. Bubble generation time was decreased by 60.4% and mixing time was decreased by 41.5% owing to an enhanced solution velocity with a water pump. Bubble residence time was decreased by 31.1% and mass transfer coefficient was decreased by 0.4% owing to a reduced distance between air aerator and solution surface. Microalgal growth rate was decreased by 12.7% from 128.9mg/Lday in an air-lifting column photobioreactor (ALC-PBR) to 112.6mg/Lday in a WCC–PBR because of the decrease in residence time of bubbles and an additional shear of cells in a water pump. However, total energy consumption of a WCC–PBR with an air compressor and a water pump was lower by 21.1% than that of an ALC–PBR with only an air compressor.

Turbulent flow field and air entrainment in laboratory plunging breaking waves

AbstractThis paper presents laboratory measurements of turbulent flow fields and void fraction in deep‐water plunging breaking waves using imaging and optical fiber techniques. Bubble‐size distributions are also determined based on combined measurements of velocity and bubble residence time. The most excited mode of the local intermittency measure of the turbulent flow and its corresponding length scale are obtained using a wavelet‐based method and found to correlate with the swirling strength and vorticity. Concentrated vortical structures with high intermittency are observed near the lower boundaries of the aerated rollers where the velocity shear is high; the length scale of the deduced eddies ranges from 0.05 to 0.15 times the wave height. The number of bubbles with a chord length less than 2 mm demonstrates good correlation with the swirling strength. The power‐law scaling and the Hinze scale of the bubbles determined from the bubble chord length distribution compare favorably with existing measurements. The turbulent dissipation rate, accounting for void fraction, is estimated using mixture theory. When void fraction is not considered, the turbulent dissipation rate is underestimated by more than 70% in the initial impinging and the first splash‐up roller. A significant discrepancy of approximately 67% between the total energy dissipation rate and the turbulence dissipation rate is found. Of this uncounted dissipation, 23% is caused by bubble‐induced dissipation.

Modeling of Liquid Level and Bubble Behavior in Vacuum Chamber of RH Process

In the Ruhrstahl-Heraeus (RH) refining process, liquid steel flow pattern in a ladle is controlled by the fluid flow behavior in the vacuum chamber. Potassium chloride solution and NaOH solution saturated with CO2 were respectively used as a tracer to investigate the liquid and gas flow behaviors in the vacuum chamber. Principal component and comparative analysis were made to show the factors controlling mixing and circulation flow rate. The liquid level and bubble behavior in the vacuum chamber greatly affect fluid flow in RH process. Experiments were performed to investigate the effects of liquid steel level, gas flow rate, bubble residence time, and gas injection mode on mixing, decarburization, and void fraction. The results indicate that the mixing process can be divided into three regions: the flow rate-affected zone, the concentration gradient-affected zone, and their combination. The liquid steel level in the vacuum chamber of 300 mm is a critical point in the decarburization transition. For liquid level lower than 300 mm, liquid steel circulation controls decarburization, while for liquid level higher than 300 mm, bubble behavior is the main controlling factor. During the RH process, it is recommended to use the concentrated bubble injection mode for low gas flow rates and the uniform bubble injection mode for high gas flow rates.

Numerical study of heat transfer during nucleate pool boiling

In this paper three-dimensional numerical simulation of the atmospheric saturated pool boiling was performed. The applied modelling and numerical methods enable full representation of the two-phase mixture behaviour on the heating surface with the inclusion of the swell level prediction. The three-dimensional investigation presented here was performed in order to take into account a convective heat transfer on the heated surface, as well as spatial effects of the vapour generation and a two phase flow such as phase dispersion within the two-phase mixture. The results are presented for a short period of time after the initiation of the heat supply and vapor generation on the heating surface. The replenishment of the heating surface with water and partial surface wetting for lower heat fluxes is shown. The influence of the density of nucleation sites and the bubble residence time on the wall on the pool boiling dynamics is discussed. Also, the influence of the heat flux intensity on the pool boiling dynamics is investigated. The applied numerical and modelling method showed robustness by allowing stable calculations for wide ranges of applied modelling boiling parameters.

Improving CO2 fixation with microalgae by bubble breakage in raceway ponds with up–down chute baffles

The aeration gas was broken into smaller bubbles with enhanced local solution velocity to improve CO2 fixation with microalgae in raceway ponds with up–down chute baffles. A high-speed photography system and online precise pH probes were used to measure bubble generation and residence times, which were affected by paddlewheel speed, aerator orifice diameter, gas flow rate, and solution depth. Bubble generation time (from gas reaching aerator orifice surface to completely escaping from the aerator) decreased because of the enhanced local solution velocity, whereas bubble residence time increased because of the vortex flow field produced by up–down chute baffles. Bubble generation time decreased by 27% and bubble residence time increased by 27% when paddlewheel speed was 10r/min with an aeration gas rate of 0.03vvm. The decreased generation time and increased residence time of aeration bubbles promoted microalgae biomass yield by 29% in optimized flow fields in raceway ponds.

Solvent Effects on the Photothermal Regeneration of CO2 in Monoethanolamine Nanofluids.

A potential approach to reduce energy costs associated with carbon capture is to use external and renewable energy sources. The photothermal release of CO2 from monoethanolamine mediated by nanoparticles is a unique solution to this problem. When combined with light-absorbing nanoparticles, vapor bubbles form inside the capture solution and release the CO2 without heating the bulk solvent. The mechanism by which CO2 is released remained unclear, and understanding this process would improve the efficiency of photothermal CO2 release. Here we report the use of different cosolvents to improve or reduce the photothermal regeneration of CO2 captured by monoethanolamine. We found that properties that reduce the residence time of the gas bubbles (viscosity, boiling point, and convection direction) can enhance the regeneration efficiencies. The reduction of bubble residence times minimizes the reabsorption of CO2 back into the capture solvent where bulk temperatures remain lower than the localized area surrounding the nanoparticle. These properties shed light on the mechanism of release and indicated methods for improving the efficiency of the process. We used this knowledge to develop an improved photothermal CO2 regeneration system in a continuously flowing setup. Using techniques to reduce residence time in the continuously flowing setup, such as alternative cosolvents and smaller fluid volumes, resulted in regeneration efficiency enhancements of over 200%.

Effects of liquid height on gas holdup in air–water bubble column

Effects of the initial liquid height, H0, of air–water bubble columns on the total gas holdup, αG, were investigated in this study. Systematic databases of αG in a rectangular and a cylindrical column of the hydraulic diameter DH of 200mm were obtained. An image processing method was applied to high-speed video images of the liquid height to obtain accurate gas holdup data. Ranges of the superficial gas velocity, JG, and the dimensionless liquid height, H0∗ (= H0/DH), were 0.025⩽JG⩽0.40m/s and 1.5⩽H0∗⩽5.0, respectively. The bubbly flows observed in these ranges could be classified into either the heterogeneous bubbly flow consisting of bubbles much smaller than DH or that with huge bubbles of the column-width scale. The main parameter governing the flow regime transition was JG. The gradient, dαG/dJG, of αG with respect to JG was of use in flow regime identification. The increase in H0∗ decreased αG because a long bubble residence time at a high H0∗ leads to the increase in the mean bubble diameter due to bubble coalescence. The Froude number using H0 as a characteristic length well correlates αG at various H0. An empirical correlation of αG in terms of the Froude number was then proposed. Comparing the correlation with the αG data showed that the correlation can give good evaluations of αG in the rectangular and cylindrical columns by tuning model parameters.

A numerical study of the effect of gas distribution profile across the pulp–froth interface on flotation performance

A solution to the Laplace equation developed using a semi-analytical method termed the method of false transients was used to study the impact of the distribution of gas across the pulp–froth interface on froth performance. Simulation results indicated that profiling gas across the pulp–froth interface has a huge impact on bubble breakage across the surface of the froth and consequently results in different air recoveries. Further, gas flux distribution across the interface also resulted in changes in both bubble and particle residence time distribution. Comparison of predictions from residence time distribution and air recovery to experimental flotation performance was in partial agreement. The discrepancy between model predictions and experimental results was attributed to the fact that the current model does not take into account important froth-phase sub-processes such as bubble coalescence and froth drainage.

CFD modelling of gas–liquid flow in an industrial scale gas-stirred leaching tank

The performance of a gas-stirred tank for cyanide leaching relies on the good mixing between solid particles and reagents, as well as sufficient oxygen mass transfer through gas bubbles. In this study, a gasliquid two phase computational fluid dynamics (CFD) model has been applied to investigate the gas-slurry flow in an industrial scale leaching tank. Following model validation using measurement data from a laboratory gas-stirred reactor, the CFD model has been extended to study flow dynamics in an industrial scale Pachuca tank with different designs. The likely effects of the introduced gas bubble size and the draft tube diameter on tank performance are assessed in terms of the overall flow patterns, gas holdup, bubble residence time distribution, slurry mixing, as well as the solid sedimentation region. It was found that the slurry flow in the tank is dominated by strong global re-circulations with slurries being pumped up inside the riser, followed by a downward motion in the downcomer before re-joining the riser from the draft tube bottom. There are no local re-circulations observed in the riser and downcomer regions for the original design and the proposed new designs. The new design with either reducing the gas bubble size or increasing the draft tube diameter increases gas holdup, promotes solid mixing and reduces the solid sedimentation region. Simulation results obtained demonstrate the feasibility of the present modelling approach as a useful numerical tool to help potential improvement of industrial scale Pachuca tank design and/or operation.

Application of Minute-Bubble Technique to Reactive Crystallization of Hydroxyapatite

Minimizing bubble formation in gas-liquid system helps achieve the following: i) acceleration of mass transfer and reactive absorption with an increase in the gas-liquid interfacial area, ii) increase in the average residence time of the bubbles with a decrease in buoyancy, and iii) occurrence of interactions at the gas-liquid interface1,2). Thus, by increasing the residence time of bubbles in the liquid phase, a quasi-homogeneous gas-liquid system can be obtained. Furthermore, attention is focused around minute-bubble interface which is higher concentration field compared with bulk liquid, an increase in the yield of main product can be achieved by the variations in the local supersaturation, which acts as the driving force for crystallization.

Froth transport characterization in a two-dimensional flotation cell

A study of the froth bubble transport in a two-dimensional (2D) flotation cell was performed. Experiments were developed as a 2×2 factorial design, in which the effect of superficial air rate (1.2–1.8cm/s) and froth depth (2–4cm) on the froth transport for a two phase (air–water) system was characterized.Using image analysis techniques, bubble residence times, air recovery, bubble path and bubble size increase through the froth were obtained. This information was complemented by froth surface velocity measurements using the Visiofroth system.It was found that bubbles transported from the pulp–froth interface up to the overflow, showed a minimum residence time for bubbles entering the froth near the lip wall. Also, the air-recovery significantly changes in a range of 7–20% at different operating conditions.Higher residence times promoted bubble size increase by coalescence for bubbles transported from the interface. Conversely, for lower residence times, a smaller increase in bubble size was observed.