Bubble Column Experiments Research Articles

Transport of biochar colloids under unsaturated flow condition: Roles of chemical aging and cation type

Biochar colloids released from biochar materials are ubiquitous in the environment and undergo environmental transformation processes that may alter their properties. Natural subsurface environments are usually under unsaturated conditions, which could affect the transport of biochar colloids. This study investigated the transport of pristine and aged biochar colloids under unsaturated conditions by aggregation test, bubble column experiment, and sand column experiment. After aging, the biochar showed a more negative, hydrophilic, and rougher surface. Compared with pristine biochar colloids, aged biochar colloids in NaCl solution were not retained at the air-water interface (AWI) due to their more hydrophilic and rougher surface. In CaCl2 solution, more pristine and aged biochar colloids were retained at the AWI because Ca2+ weakened the electrostatic repulsion between biochar colloids and the AWI. With the decrease in saturation, the transport of pristine and aged biochar colloids decreased by 17 %‑67 % through the retention at AWI and air-water-solid (AWS) interface. The transport of biochar colloids in NaCl solution was increased by 10 %‑20 % after aging as the aged biochar was not retained at the AWI. The difference of transport between pristine and aged biochar colloids in CaCl2 solution (<8 %) was lower than that in NaCl solution due to the enhanced retention of aggregated biochar colloids at the AWI and AWS interfaces. These results highlight the importance of the surface structure of biochar on its behavior in the environment, which is essential for assessing the potential of biochar application for carbon sequestration and environmental protection.

Preliminary development and validation of ACENA code for heavy liquid metal-gas two phase flow simulation

To simulate the heavy liquid metal-gas (HLM-gas) two phase flow phenomena that may occur during postulated accidents of Lead-based fast reactors (LFRs), a 2D multiphase fluid dynamics code, ACENA, is preliminarily developed and validated in this study. A brief description of theoretical models, including governing equations, auxiliary models and material properties are listed in this study. The capability of simulating flow and heat transfer characteristics between gas and liquid phases has been examined by typical air-water bubble column experiment, LBE-N2 two phase flow experiment and Lead-Ar heat transfer experiment, and the comparison results show that this code could attain acceptable accuracy in simulating flow and heat transfer characteristics of HLM-gas two phase flow with appropriate interfacial drag coefficient and heat transfer coefficient models.

Effects of ionic strength and cation type on the transport of perfluorooctanoic acid (PFOA) in unsaturated sand porous media

Current understanding of perfluorooctanoic acid (PFOA) transport in unsaturated porous media is still limited with significant variability in solution chemistry. Column experiments were conducted to systematically evaluate the impacts of ionic strength (1.5−30 mM) and cation type (Na+ and Ca2+) on PFOA transport in unsaturated quartz sand. The results showed that an increase in ionic strength (1.5−30 mM) led to greater PFOA retardation in unsaturated columns. Meanwhile, Ca2+ caused more PFOA retardation than Na+ at the same unsaturated conditions. These findings were supported by bubble column experiments, which indicated greater PFOA adsorption at the air-water interface with increasing ionic strength or in the presence of Ca2+ in comparison to Na+. Furthermore, the air-water interfacial (AWI) adsorption coefficients calculated from surface tension isotherms also increased with increasing ionic strength or in the presence of Ca2+ in comparison to Na+. These results clearly confirm that higher ionic strength or cation valence significantly promoted PFOA adsorption at the air-water interface, and thus caused greater PFOA retardation during transport in unsaturated porous media. This work points out the importance of considering solution ionic strength and cation type in assessing the transport behavior of PFOA in unsaturated porous media.

Importance of surface roughness on perfluorooctanoic acid (PFOA) transport in unsaturated porous media

Understanding the subsurface transport of perfluorooctanoic acid (PFOA) is of considerable interest for evaluating its potential risks to humans and ecosystems. In this study, packed-column experiments were conducted to examine the influence of surface roughness on PFOA transport in unsaturated glass beads, quartz sand and limestone porous media. Results showed decreasing moisture content significantly increased the air-water interfacial adsorption of PFOA and led to greater retardation in all three types of porous media. Particularly, rougher surface (limestone > quartz sand > glass beads) and smaller grain size (i.e. a larger solid specific surface area, SSSA) significantly enhanced PFOA retardation under unsaturated conditions. These results were further supported by bubble column experiments and SSSA analysis of porous media, which demonstrate that except for the factors affecting PFOA transport in solid-water interface (e.g. surface charge and chemical heterogeneity), the greater retardation of PFOA during transport is attributed to the larger air-water interfacial areas associated with rougher surface and smaller grain size and hence greater interfacial adsorption of PFOA. Our results indicated the importance of surface roughness on the retention and transport of PFOA in the unsaturated zone.

Review of drag coefficients on gas – liquid tower: the drag coefficient independent and dependent on bubble diameter in bubble column experiment

Drag coefficient independent on bubble diameter is required to ease design sieve trays or bubble column, through simulation of computational fluid dynamic. In this paper, the drag coefficients, independent or dependent on the diameter, are reviewed for gas-liquid system. A number of drag coefficients are used for Computational Fluid Dynamics (CFD). Different forces are entered to the liquid-bubble separation surface in diverse directions. Forces are investigated with mathematical proving for Newtonian fluids and Eulerian coordinate. Finally, the external force as a new force, enter to the drag coefficient equations. Drag coefficient is included force coefficient. Drag force is entered in momentum equations. Drag coefficient is used in two-phase systems which bubbles and liquid are activated as dispersed and continuous phase, respectively. Bubbles and liquid are in contact with each other in separation surface on bubble. Drag force is created slip on separation surface. The drag coefficients are investigated depended on the size and configuration of bubbles. The drag coefficient of Krishna et al is used dependent on bubble diameter. Schiller - Nauman model drag coefficient is estimated with 9% error and dependence on bubble diameter. In this article, the modern drag coefficients are studied independent on the diameter and shape of the bubble. The Drag coefficients are resulted theoretical, mathematical and experimental independent and dependent of diameter bubble. The new Drag coefficient is presented dependent on surface tension and diameter of the tower hole with 6.3 of error approximately.

The critical bubble diameter of the lift force in technical and environmental, buoyancy-driven bubbly flows

The lift force acting on particles, drops, and bubbles in a shear field is a well know effect that was extensively investigated since the 1980s. Experiments with single bubbles in a linear shear field reveal that the lift force coefficient has at a specific bubble size a zero where the coefficient switches its sign from positive to negative. Solving the lateral force balance for a polydisperse bubbly flow with the liquid flow field usually found in a bubble column, the lift force causes a spatial separation of small, which tend to the wall, and big bubbles, which tend to the center. Bubbles with a zero lift force coefficient on the other hand are equally distributed over the cross section. In order to investigate the influence of the flow field on the force balance, simulations with the Euler-Euler two-fluid model were conducted. The simulations are in good agreement with experiments and showed that the liquid flow field found in the used bubble column has a minor influence on the steady state distribution of the bubbles. From evaluating six different bubble column experiments conducted in air/water, the critical diameter at which the lift force coefficient is zero is determined between 5.1 and 5.2 mm. This result is very close to the critical diameter that we were able to determine in previous, single-bubble experiments. Therefore, the conclusion is possible that the lift force coefficients that are determined with single bubble experiments are applicable to the complex flow field found in polydisperse bubbly flows at low void fractions and low turbulence levels. Moreover, a very simple experimental setup is described to determine the critical diameter in complex flow situations, which is the most important point when it comes to modelling the lift force. Such a setup is therefore beneficial when complex substance mixtures used in a specific industrial or environmental application needs to be investigated since lift force correlations are only available for clean, ideal substances.

Bubble aspect ratio in dense bubbly flows: experimental studies in low Morton-number systems

Almost every modelling approach of bubbly flows includes assumptions concerning the bubble shape. Such assumptions are usually made based on single bubble experiments in quiescent flows, which is far away from the flow field observed in large-scale multiphase facilities. Considering low Morton-numbers and the highly deformable interface at medium and large Eötvös-numbers, the evaluation of the bubble shape in such systems under real flow conditions is highly desirable. In this study, we experimentally evaluate the bubble shape (in terms of aspect ratio), at low Morton-numbers, in different bubble column setups and a pipe flow setup under different operating conditions. The bubble shape in the bubble column experiments were obtained with cameras at Politecnico di Milano and Helmholtz-Zentrum Dresden Rossendorf (HZDR) whereas the shapes in the pipe flows were measured by the ultrafast electron beam X-ray tomography system (ROFEX) at HZDR. In the bubble column experiments almost the same shape is observed; conversely, the shape in the pipe flows distinctly depends on the flow conditions. In conclusion, in bubble columns the assumption of a constant shape regardless of the flow conditions is valid whereas in pipe flows the turbulence and shear rates can be strong enough to deform distinctly the bubbles.

Observations on bubble shapes in bubble columns under different flow conditions

The bubble shape is fundamental for every aspect of modeling bubbly flows. The interface is usually highly deformable so that the bubble shape is in general dependent on the surrounding turbulent flow field. Since recent work on this topic addressed almost entirely single-bubbles rising in quiescent flow, the extent of such flow field dependencies as well as swarm effects is rather unknown. This study examines the effect on the bubble shape in bubbly flows with a liquid background flow that is generated by natural convection in the bubbly flow regime when flow properties, i.e. the gas flow rate, sparger setup, and column geometry, are changed by evaluating six different bubble column experiments. The results of this integral approach reveal that the bubble shape of small bubbles is distinctly influenced whereas the shape of large bubbles is unchanged. Averaged over all flow rates, we find that the size-dependent bubble shapes are quite similar for all six experiments. Further studies focusing on single local effects like the shear rate or wake effects are highly desirable to obtain a deeper understanding of the underlying processes; for this purpose, the given results can help to assess the most important effect and in which extend it should be studied.

Modelling aerated flows with smoothed particle hydrodynamics

Modelling aerated flows is a complex application of computational fluid dynamics (CFD) since the interfaces between air and water change rapidly. In this work, the simulation of aerated flows with the smoothed particle hydrodynamics (SPH) method is investigated with a focus towards the application in engineering practice. To prove the accuracy of the method, the processes of air entrainment and rising air bubbles are studied. Through monitoring the evolution of the bubble contours it is shown that the novel approach of adding artificial repulsion forces at the interface does not alter the dynamics but stabilizes the flow. Building on these fundamental processes we extend the discussion to practical applications with a special focus on forced aeration. Since the employment of a detailed SPH model to practical problems remains out of bounds due to the high computational demand, we propose a combined experimental and numerical study where experimental bubble characteristics are imposed on the numerical simulation. Based on the data of the conducted bubble column experiment, the computational demand is significantly decreased such that the oxygen consumption due to biokinetic processes can be modelled. The future perspective is to apply SPH to urban water systems, e.g., for simulating detailed processes in wastewater treatment and sewer hydraulics.

Deposition and transport of graphene oxide in saturated and unsaturated porous media

In this work, sand and bubble column experiments were conducted to explore the deposition mechanisms of graphene oxide (GO) particles in porous media with various combinations of moisture content and ionic strength. Sand column experimental results indicated that retention and transport of GO in porous media were strongly dependent on solution ionic strength. Particularly, GO showed high mobility under low ionic strength conditions in both saturated and unsaturated porous media. Increasing ionic strength dramatically increased the retention of GO particles in porous media, mainly through secondary-minimum deposition as indicated in the XDLVO interaction energy profiles. Recovery rates of GO in unsaturated sand columns were lower than that in saturated columns under the same ionic strength conditions, suggesting moisture content also played an important role in the retention of GO in porous media. Findings from the bubble column experiments showed that the GO did not attach to the air–water interface, which is consistent with the XDLVO predictions. Additional retention mechanisms, such as film straining, thus could be responsible to the reduced mobility of GO in unsaturated porous media. The experimental data of GO transport through saturated and unsaturated porous media could be accurately simulated by an advection–dispersion-reaction model.

An inexpensive strategy for facilitated recovery of metals and fermentation products by foam fractionation process

Microbial biosurfactants produce foam during aerobic-fermentation processes. The degree of foaminess and foam stability of the lipopeptide-biosurfactant produced by a marine Bacillus megaterium strain were investigated using simulated biosurfactant solution (SBS), biosurfactant broth without cells (BBWOC) and biosurfactant broth with cells (BBWC) in bubble column experiments. The experimental data for foam collapse were fitted using a first-order foam decay model. The first-order rate constant (k), a measure of foam stability, was maximum (k=0.0003S−1) for BBWOC in the pH range 6–9. However, maximum foam stability (k=0.0006S−1) was restricted to pH 7 for BBWC. Foam-based metal removal studies revealed that the metal removal followed a saturation model. The relative binding capacity of each divalent metal was greatly affected by the presence of other divalent metals. The order of lipopeptide binding capacity of the metals was Fe2+>Ca2+>Mg2+, with Fe2+ significantly influencing the foam stability. In case of Fe2+, Ca2+ and Mg2+, maximum metal recovery of 64.7±4.3%, 52±3.1% and 41.4±3.6% respectively was observed at pH 7. The enrichment (E) of the other media components, including cells, was comparatively insignificant. The results of this study have implications in designing and optimizing biosurfactant or protein recovery in situ by foam fractionation as an inexpensive strategy, and also in facilitated metal recovery from industrial effluents and ores.

Effect of surface modification on single-walled carbon nanotube retention and transport in saturated and unsaturated porous media

This work investigated the effect of different surface modification methods, including oxidization, surfactant coating, and humic acid coating, on single-walled carbon nanotube (SWNT) stability and their mobility in granular porous media under various conditions. Characterization and stability studies demonstrated that the three surface modification methods were all effective in solubilizing and stabilizing the SWNTs in aqueous solutions. Packed sand column experiments showed that although the three surface medication methods showed different effect on the retention and transport of SWNTs in the columns, all the modified SWNTs were highly mobile. Compared with the other two surface modification methods, the humic acid coating method introduced the highest mobility to the SWNTs. While reductions in moisture content in the porous media could promote the retention of the surface modified SWNTs in some sand columns, results from bubble column experiment suggested that only oxidized SWNTs were retention in unsaturated porous media through attachment on air–water interfaces. Other mechanisms such as grain surface attachment and thin-water film straining could also be responsible for the retention of the SWNTs in unsaturated porous media. An advection–dispersion model was successfully applied to simulate the experimental data of surface modified SWNT retention and transport in porous media.

미세조류 Botryococcus braunii의 배양조건 최적화 및 지질축적 향상

Several tests and experimental work have been done for identifying the best growth conditions and accumulated amount of lipid moiety in B. braunii, a microalga(UTEX 572) in terms of media composition. The specific growth rate was found to be the highest at 0.15 g/L-day when the phosphorus concentration was doubled with the other ingredients at the normal level. Experiments for varied media compositions revealed that the accumulation of lipid was the highest at 48% (dry cell weight based) in the nitrogen deficient medium and its corresponding specific growth rate was comparative to that in the normal BG 11 medium. In the bubble column experiments, carbon dioxide containing air produced four times more cell mass than air only. Light and glucose addition also enhanced cell mass with maximum, 1.8 g/L and accordingly 42% of lipid composition, which turned out to be a better strategy for higher lipid-producing microalgal culture.

High mobility of SDBS-dispersed single-walled carbon nanotubes in saturated and unsaturated porous media

Knowledge of the mobility of carbon nanotubes (CNTs) in porous media is very important to assess their impacts on the environment. In this study, a series of laboratory experiments were conducted to explore the transport mechanisms of sonication shortened, sodium dodecylbenzene sulfonate (SDBS) dispersed single-walled nanotubes (SWNTs) in both saturated and unsaturated sand columns. Laboratory columns packed with quartz sand with different combinations of moisture content and grain-size distribution were used to examine the breakthrough behavior of the SDBS-dispersed SWNTs. Bubble column experiments were also conducted to study the interactions between the SDBS-dispersed SWNTs and the air–water interface. Packed-column experimental results showed that the SDBS-dispersed SWNTs were highly mobile for most of the experimental conditions tested. The surface deposition of the SWNTs in the sand columns was low because all the interactive surfaces were negatively charged. Physical trapping was not observed for the SWNTs in the saturated porous media of different grain-size distributions because the SWNTs might orient parallel to the streamlines in flow to reduce their retention. Retention of the SWNTs in unsaturated porous media occurred only at a very low moisture content (<0.10). Otherwise, reduction in moisture content showed little impact on the retention and transport of the SWNTs in unsaturated porous media. Findings from the bubble-column experiments confirmed that the SDBS-dispersed SWNTs did not attach to the air–water interface. A mathematical model based on the advection–dispersion equation coupled with reaction-rate laws successfully described the retention and transport of the SDBS-dispersed SWNTs in both water-saturated and unsaturated columns.

Impact of surface charge density on colloid deposition in unsaturated porous media

This research investigated the impact of surface charge density on colloid deposition in water unsaturated porous media using column experiments. The same-sized carboxyl functionalized microspheres with variable surface charge densities (0.56–1.95 μC/cm 2) were used as the model colloids in this research. Column experiments were conducted under steady-state water flow conditions with effective water saturation ranging from 0.2 to 1.0. Colloid transport was described by the advection-dispersion equation under water saturated conditions and the mobile–immobile two-region transport model under water unsaturated conditions. Under water saturated conditions, no colloids were retained in the porous media. Under water unsaturated conditions, colloid retention was a function of both colloid surface charge density and water saturation, i.e., colloid retention increased with increasing surface charge density; colloid retention decreased with increasing water saturation. In a separate experiment, we conducted bubble column experiments to provide evidence that the colloids used in this research did not attach to the liquid–gas interface. Under steady-state water flow conditions, colloid retention in the porous media was believed to be retained at the liquid–solid–gas three-phase interface, or more precisely the liquid–gas meniscus and the solid interface, where the water film thickness approached the size of the colloidal particles. The repulsive electrostatic interactions between the colloids and the liquid–gas interface aided colloids to overcome the repulsive electrostatic interaction barrier with the sediments, leading the colloids to attachment.

Period-adding bifurcations and chaos in a bubble column.

We obtained period-adding bifurcations in a bubble formation experiment. Using the air flow rate as the control parameter in this experiment, the bubble emission from the nozzle in a viscous fluid undergoes from single bubbling to a sequence of periodic bifurcations of k to k+1 periods, occasionally interspersed with some chaotic regions. Our main assumption is that this period-adding bifurcation in bubble formation depends on flow rate variations in the chamber under the nozzle. This assumption was experimentally tested by placing a tube between the air reservoir and the chamber under the nozzle in the bubble column experiment. By increasing the tube length, more period-adding bifurcations were observed. We associated two main types of bubble growth to the flow rate fluctuations inside the chamber for different bubbling regimes. We also studied the properties of piecewise nonlinear maps obtained from the experimental reconstructed attractors, and we concluded that this experiment is a spatially extended system.

Optimization of enzymatic transesterification of rapeseed oil ester using response surface and principal component methodology

Statistical experimental design combined with principal component analysis was used to evaluate interdependence of process variables in enzymatic transesterification. The reaction studied was alcoholysis of rapeseed oil methyl ester (biodiesel) and trimethylolpropane. The reaction products are trimethylolpropane esters which can be used as raw materials for biodegradable hydraulic fluid and other lubricants. The methods used revealed some hidden factors that affect the reaction. Water activity and elimination of the by-product methanol were found to be the principal factors affecting the product yield. Mono-, di-, and trisubstituted esters were produced in different optimum conditions. The model developed for monosubstituted ester was verified with a bubble column experiment.

The effect of surface active additives on bubble coalescence and gas holdup in viscous aerated batches

An experimental study has been aimed at examining the possibility of modifying the behaviour of viscous liquids in bubble column reactors by addition of surface active compounds (aliphatic alcohols). Results of coalescence measurements showed that bubble coalescence in viscous saccharose solutions could be fully suppressed by addition of alcohols. The transition concentrations of individual alcohols were within the whole set ethanol–octanol successfully correlated as a function of the number of carbon atoms in the alcohol molecule. The bubble column experiments proved that the properties of viscous aerated liquids could be significantly modified by the addition of relatively small amount of alcohol ( c A ≃1×10 -5–1×10 -2 kmol m -3). The results of gas holdup measurements indicated significant increase of bubble bed voidage with increasing alcohol concentration. Fitting of gas holdup data with the Zuber and Findlay drift-flux model also implied favourable effect of the surface active additives on the radial uniformity of voidage profiles, indicated by decreasing values of the distribution parameter, C o. The effect of alcohol addition increased with increasing viscosity of the original medium and was most pronounced in highly viscous saccharose solutions ( μ L ⩾100 mPa s). In full agreement with the results of the coalescence studies, the favourable effect of alcohols on bubble bed voidage increased with increasing length of the carbon chain. Comparison of gas holdup data obtained with different distributor plates indicated significant synergistic effect of alcohol concentration and the mode of primary gas dispersion on the properties of resulting bubble bed and on the values of its voidage. Addition of alcohols thus compensated even for the negative effect of liquid viscosity on the formation and stability of the homogeneous bubbling regime.

Application of LDA to bubbly flows

The fluctuating velocity field in an air–water bubble column (i.d. 15.2 cm) at a gas fraction of 25% is investigated using backscatter LDA. Since the interpretation of LDA signals in bubbly flows is not straight forward also experiments on a single bubble train are reported. It is discussed that in the latter case when using seeding the backscatter LDA measures predominantly the liquid velocity. No improvement from thresholding on the discrimination between gas and liquid was found. The bubble column experiments show that the radial averaged liquid velocity profile represents the well known gross scale circulation present in the column. More interesting, it is also seen that the fluctuating velocity field can be studied in great detail. The velocity probability density functions directly indicate high turbulence intensity. Low frequency fluctuations are observed in agreement with visual observations. The data rate is an exponential function of the distance from the column wall. This limits the possibilities of spectral analysis in the central part of the flow. However, close to the wall the mean data rate is sufficient to study the frequency contents of the signal. It is shown that the power spectral density function obeys a −5/3 power law and that the autocorrelation function is of similar shape as reported in literature on bubbly flows.

Kinetics and mechanism of ozonation of free cyanide species in water

The reaction rates of ozone with free cyanide species were determined by a stopped-flow spectrophotometer in the pH range 2.5-12.0. The direct reaction of molecular ozone and the free radical reactions which contribute to the overall reaction were identified. A reaction mechanism consistent with the observations was proposed. The rate constant for the direct reaction of molecular ozone with cyanide ion was calculated to be 2600 +/- 700 M/sup -1/ s/sup -1/. Batch and bubble-column experiments indicated stoichiometric conversion of cyanide to cyanate and an ozone consumption of 1.2 +/- 0.2 mol/mol of oxidized cyanide.