A novel sensing system of bubble shape, surface area, volume and rising velocity using three coupled laser sensors was developed. To confirm the reliability of the system, the bubble shape was simultaneously measured using both a high-speed video camera and this system for an identical rising single bubble. The bubble’s outline analyzed using the laser scanning system corresponded well with that analyzed by the video image analysis. The measured rising velocities of bubbles agreed well over a wide range of liquid viscosity with the data of Grace (1973), and Bhaga and Weber (1981). Moreover, the measured surface area of a bubble agreed well enough with the estimation of Tadaki and Maeda (1961) under their experimental condition.
With this system, not only rising velocity of a single bubble but also its three-dimensional shape and surface area could be more precisely and quickly measured in real-time.

View full abstract

The gas holdup and the volumetric mass transfer coefficient are very important parameters for mass transfer operation in bubble columns. These parameters have been major subjects for many researches for a long time. In this study, a novel technique of ultrasonics and neural network has been proposed to determine the local gas holdup and the volumetric mass transfer coefficient in a two-dimensional bubble column simultaneously. The measurement parameters of the energy attenuation and the time shift difference of ultrasound are used to obtain the local gas holdup and the volumetric mass transfer coefficient in dispersed systems of air–water and air–glycerol solution using the neural network reconstruction. In the range studied, the representative results of the proposed technique show a good agreement with the experimental data.

View full abstract

In this contribution the gas induced injection and blending of an inhibitor solution in a stationary low viscosity liquid media are handled. For the description of the transient turbulent gas/liquid-flow with dispersed gas phase in a vessel or tank a two-fluid Eulerian approach is employed.
Time averaged continuity and momentum equations are solved using a modified k–ε-turbulence model and the finite-volume method. The experimental investigations of the flow field are done with laser Doppler anemometry. The calculated results are compared with a number of experimental data and satisfactory agreements are obtained.

View full abstract

The effect of partitioning plates in the riser of external-loop airlift reactors (ELAs) was studied to increase the gas holdup by the restraint of bubble coalescence and the control of the liquid circulation velocity, and to improve the mass transfer in highly viscous liquids. The liquid circulation velocity, the gas holdup in the riser and the volumetric mass transfer coefficient were measured by changing the following parameters, that is, the number of stages, opening ratio of partitioning plates, diameter of a perforated hole and physical properties of liquids. The following conclusions were obtained; the liquid circulation velocity in the riser decreases with the increased number of stages. The gas holdup in the riser is affected by the liquid circulation velocity and bubble coalescence caused by partitioning plates. The gas holdup in the riser and the volumetric mass transfer coefficient increase with the increased number of partitioning plates. In highly viscous pseudoplastic solutions, the mass transfer rate was improved by the installation of partitioning plates into the riser of ELAs.

View full abstract

Experiments were conducted to study the hydrodynamics of a three-phase jet-loop-reactor with liquid as a continuous phase. By using a perspex flow channel with a special geometry and different optical measuring systems the simultaneous measurement of the flow field as well as of the movement of bubbles and solids in three-phase flows was possible. In order to measure the distribution and movement of fine, suspended solids (ρ_solid = 2400 kg/m³, d_P =150 μm; 500 μm) within the bubble wakes, a specific fraction of particles was coated by a fluorescent dye. This made it possible to track single particles even at solid concentrations up to 10% by the use of an optical filter. The results indicates that the rise velocity of bubbles in three-phase flows depends on the accumulation of solid particles in bubble wakes due to the inertia of the solids. Particles which are able to follow the wake structure will increase the mass force acting on a bubble due to a higher suspension density of the wake with the consequence of a lower relative velocity. The decrease in relative bubble velocity influences the pressure drop and circulation velocity on loop reactors. Based on the measured results and the analyses of the forces acting on the bubble a model was developed which predicts the relative bubble velocity reasonably well.

View full abstract

A non-biological floc system consisting of clay minerals and an organic flocculating agent was used for the determination of mechanical stresses in turbulent multiphase flows. The floc disintegration kinetics i.e. the time and stress dependent size distribution of the flocs were observed with an optical in-line particle-system analyser and the raw data transformed. The concomitant measurement of floc disintegration in a reference system, a couette system, enabled the assignment of the effective mechanical stresses. The volumetric power input resulting of isothermal expansion of gas has been derived to be the governing factor for the effective shear stresses in bubble columns and airlift loop-reactors. Furthermore the effects of gas phase distribution (originated from the sparger design) and solids loading on shear stresses were determined for fluid systems with low viscosities. Especially the interactions between finely dispersed bubbles and particles seemed to cause a drastic change of turbulence.

View full abstract

Longitudinal concentration distributions have been measured under various conditions for oxygen absorption in standard bubble columns. The disadvantages of standard bubble columns are considerable because of backmixing in the continuous liquid phase and the decrease of interfacial area due to bubble coalescence in the viscous liquids. To overcome these disadvantages, modified bubble columns, such as multistage bubble columns partitioned with perforated plates, have been used. When the catalytic particles suspended in the slurry bubble column are brittle and easily break down, the packing of the brittle particles into a partitioning plate is considered, which is called the multistage bubble column with a partitioning plate packed with particles (PMBC). The aims of this study are to make experimentally and theoretically clear the effect of operating parameters on the mass transfer characteristics of the PMBC, and to elucidate the concentration distributions of dissolved oxygen in the PMBC by a revised back flow model.

View full abstract

Three-dimensional direct numerical simulations of the motion of a gas bubble rising in viscous liquids were carried out by a coupled level-set/volume-of-fluid (CLSVOF) method which is comprised of a best mix of the advantages of both thevolume-of-fluid (VOF) and level-set (LS) methods. In our study, physical properties of real gas–liquid systems were introduced in the computations: the density ratio between both phases can be up to about 800 and the viscosity ratio reaches –50000. As a result, it will be presented that we can numerically predict three-dimensional bubble motion including nonlinear dynamics for the full range of bubble shapes and physical parameters (e.g. high Reynolds number) with good agreement with experiments. We shall describe the effectiveness and validity of our numerical (CLSVOF) method for two-phase flow.

View full abstract

Turbulent flow simulations in bubble columns are performed in the Euler–Lagrange approach. In the simulation, a large eddy simulation (LES) model is used as a turbulence model. In addition, bubble contact is considered by using a distinct element model (DEM), and bubble coalescence is taken into consideration as well as bubble break-up by solving a coalescence model based on the contact time and coalescence time. Furthermore, a new bubble-fluid drag model depending on distances among bubbles is introduced in DEM simulations, and it is shown that bubble clusters with high terminal velocities may play an important role for high superficial gas velocities. The present models are next applied to several simulations for bubble columns, and the comparison of the computational results with experimental data shows their effectiveness.

View full abstract

Characteristics of bubbling behavior and bubble properties were investigated and diagnosed in a gas–liquid countercurrent bubble column bioreactor which is 0.152 m in inside diameter and 3.5 m in height. The effects of gas and liquid velocities and bubble distribution mode (even, wall-side, central or asymmetric distribution) on the bubble properties such as chord length, frequency, rising velocity, holdup and bubbling behavior were examined. For the analysis of resultant bubbling behavior, pressure fluctuation signals were measured and analyzed by adopting the concept of the chaos theory; the signals were interpreted by means of an attractor in the phase space portraits and correlation-dimension. It was found that the resultant bubbling behavior could be detected effectively and quantitatively in terms of the phase space portraits and correlation dimension of pressure fluctuations in the bubble column bioreactor. The bubble size, frequency and holdup increased with increasing gas (U_G) or liquid velocity (U_L). The rising velocity of bubbles increased with increasing U_G, whereas decreased with increasing U_L. The uniformity of bubble size distribution and bubble holdup decreased when the distribution mode of bubbles at the gas distributor was changed from even to wall-side, central or asymmetric. The central distribution of bubbles was better than the asymmetric mode but worse than wall-side distribution, on considering the bubble holdup and uniformity of distribution. The bubble holdup and size were well correlated in terms of correlation dimension of pressure fluctuations elucidating the bubbling phenomena in the gas–liquid countercurrent bubble column bioreactor.

View full abstract

During cultivation of a microalga Chlorella in a bubble column and a split-cylinder airlift, turbulence characteristics were measured using a constant temperature anemometer equipped with a hot film sensor. From the digitized output signal of the anemometer, such parameters as the mean liquid phase velocity, turbulence intensity, integral scale, and turbulent diffusivity were calculated. The non-dimensional power spectrum distributions of liquid velocity fluctuations fit well with the equation which assumes that the correlation function of turbulent velocity fluctuations is exponential.

View full abstract

The rectangular airlift bubble column where anaerobic and aerobic regions coexisted was used, and the treatment of wastewater including nitrogen was performed. The effects of equipment and operation conditions on the gas holdup, liquid-phase volumetric mass transfer coefficient and liquid circulation flow rate were investigated in two different-size columns. On the basis of oxygen balance, the distribution of dissolved oxygen concentration in the column was calculated by using the experimental equations of flow characteristics. The results of wastewater treatment proved that the nitrogen removal was controlled by the volume fraction of anaerobic region.

View full abstract

A mini-scale external loop airlift bubble column (MELBC) of 20 ml in volume was utilized for evaluating the performance of a novel type biocatalyst consisting of glucose oxidase (GO), phospholipid vesicle, i.e., liposome and gel beads for immobilization. It was found that the glucose oxidase-containing liposomes (GOL) as well as the immobilized glucose oxidase-containing liposomes (IGOL) in the MELBC were extraordinarily stable and reusable at 40°C in the glucose oxidation under the initial glucose concentration of 10 mM and the superficial gas velocity U_G of 0.94 cm/s for the GOL or 1.4 cm/s for the IGOL. The simultaneous reaction and mass transfer model for the glucose oxidation catalyzed by the liposomal glucose oxidase above was proposed to determine the gas–liquid oxygen transfer coefficient, k_La, the permeability coefficient of glucose through liposome membrane, P_G, and the apparent effectiveness factor of the liposomal GO, α. It was shown that the values of k_La, P_G and α could be obtained based on the model with the observed steady-state oxygen concentration as well as the glucose consumption rate. The model proposed was applied to calculate the time courses of glucose concentrations in both inner and outer aqueous phases of the liposomes during the liposomal GO-catalyzed glucose oxidation. It was found that the model was useful to predict the earlier stage of the prolonged reaction since the deactivation of GO leaked from liposomes was considered to be negligible initially. The time course of the glucose concentration inside liposomes, which could not be measured, was reasonably predicted with the model proposed.

View full abstract

The growth and CO₂ concentrating mechanism of a filamentous cyanobacterium Anabaena cylindrica IAM-M1 in a bubble column have been studied by following the time courses of biomass concentration, pH, CO₂ accumulation rate, and transmittance of light. The biomass production increased with the increasing superficial gas velocity from 9.0 m·h^–1 to 12 m·h^–1 or with increasing mole fraction of CO₂ at the inlet from 0.03 to 0.06, while it decreased with the increasing superficial gas velocity from 12 m·h^–1 to 18.0 m·h^–1 or with the increasing fraction of CO₂ from 0.06 to 0.12. As biomass concentration increased the photosynthetic photon flux density of the transmitted light decreased. The CO₂ accumulation rate, which almost equaled in magnitude the CO₂ mass transfer rate (CTR), went up steeply in the early phase of cell growth. The photosynthetic CO₂ uptake rate (CUR) reached the maximum at the late logarithmic growth phase. The maximum value of the ratio CTR/CUR was 804.

View full abstract

The reaction kinetics for anaerobic biodegradation of volatile acid components, including acetic, propionic and butyric acids commonly contained in the wastewater from beer brewery factories, was investigated in a granular sludge bed. For the degradation of each single substrate component in the granular sludge bed reactor, the reaction scheme was specified and the Monod kinetic parameters were evaluated. In addition, the degradation sequences and their kinetics were also identified for multiple substrate systems with combinations of the three volatile acid components including acetic, propionic and butyric acids. The batch degradation sequences for the single and multiple substrates in the batch sludge bed reactor were explained well by the numerical solutions of multiple differential mass balance equations for the components involved on the basis of the Monod kinetics associated with or without competitive and non-competitive inhibition mechanisms for the specified combinations of the substrate components.

View full abstract

The integrated bioreaction-crystallization process proposed previously was experimentally studied for efficient continuous production of calcium gluconate crystals. The process consisted of two external loop airlift bubble columns in series with 2.5 l reaction solution in each column, one being bioreactor and the other crystallizer. The bioreactor was used to produce calcium gluconate solution through the air oxidation of glucose catalyzed by the immobilized glucose oxidase plus manganese dioxide gel beads at 30°C with simultaneous neutralization of the oxidation product, gluconic acid, by calcium hydroxide. The solution in the bioreactor was then fed into the crystallizer operated at 10°C to produce calcium gluconate crystals which was recovered through filtration from the calcium gluconate slurry in a recycling tube from the crystallizer to bioreactor with the successive addition of solid glucose to the bioreactor. In addition to the optimal operating conditions determined in our previous work, the more favorable pH, type of neutralizer and gel beads size were examined to keep the MnO₂ activity as high as possible in the gel beads by minimizing the dissolution of manganese dioxide into the liquid bulk throughout the reaction period. The 14-h and 60-h operations for the continuous production of calcium gluconate crystals were performed under the optimal operating conditions determined, which gave the crystal recovery yields of 77% and 72%, respectively at the filtration unit. Such low recovery yields were attributed mainly to the incomplete crystal recovery system. In conclusion, it has clearly been demonstrated that the feed of solid glucose and calcium hydroxide to the bioreactor with simultaneous removal of calcium gluconate crystals from the crystallizer realizes the continuous production of the crystals in the environment-friendly bioprocess.

View full abstract

The enzymatic hydrolysis of shredded office paper as a model of waste paper was carried out in an external loop airlift bubble column with continuous ultrasonic irradiation. The paper used was 6 × 12 mm in size and suspended much less easily compared to a microcrystalline cellulose powder of 35 to 65 μm in diameter. Oxygen in the sparging air exerted no inhibitory effect on the enzymatic hydrolysis and the location of a horn for ultrasonic irradiation in the downcomer was slightly more efficient than that in the riser to enhance the hydrolysis. Even in the case of no ultrasonic irradiation, the enzymatic hydrolysis in the airlift reactor proceeded as a two-stage process as opposed to that in the previous stirred tank reactor. This was ascribed to a shear stress acting on the surface of office paper in the gas-sparging section in the riser in the same way as in the ultrasonic irradiation section in the stirred tank. The enzymatic hydrolysis in the ultrasonic airlift reactor was enhanced by the gas-sparging as well as ultrasonic irradiation. As a result, the external loop airlift bubble column was found to be more advantageous for higher sugar concentrations compared to the stirred tank reactor. The time courses were kinetically analyzed and simulated based on the kinetic model proposed previously. It was shown that the ultimate sugar concentration increased with an increasing initial substrate concentration and ultrasonic intensity, and that the apparent kinetic constant increased with the intensity.

View full abstract

Polysaccharide alginate, which is composed of mannuronic acid (M) and guluronic acid (G), is used as a gel-former and viscosifier in a wide range of applications. The properties of gel-forming, water-binding, and immunogenicity are influenced by the fraction of M- and G-monomers in the alginate. In this study, we engineered an alginate synthesis using culture of Azotobacter vinelandii IAM 9046. The effect of growth conditions on the cell concentration, alginate synthesis, fraction of monomers, etc., were investigated. As a result, alginate with a high content of serial G-monomers was produced in a bubble column reactor. It is our goal to control the monomer fraction of alginate that is required for medical and industrial applications.

View full abstract

Using a bubble column fermentor, submerged culture of Agaricus blazei mycelium was carried out to find the potential for large-scale culture by bubble column fermentors. Round pellets of A. blazei mycelium of 5 mm to 20 mm in diameter were found in the fermentor. The optimum conditions on pH, cultivation temperature, compositions of nitrogen and carbon sources, and aeration rate were obtained, where the cell concentration and the overall growth rate maximum.

View full abstract

For the enhancement of ozone oxidation performance, a gas–liquid simultaneous injection nozzle was set at the upper end of the downcomer of bubble column. The flow characteristics and the decomposition performance were examined. Gas and liquid from the nozzle flowed downward in the downcomer and upward in the annulus between the column and the downcomer. A part of liquid in the annulus was entrained into the downcomer. Both the liquid circulation flow rate and volumetric mass transfer coefficient increased with increasing downcomer diameter and with decreasing nozzle diameter. In the dye decomposition by means of air containing ozone, the decomposition conversion of dye was enhanced by the liquid entrained from the annulus to the downcomer at the upper end of the downcomer. On the basis of the balance equations of ozone and dye, the concentration distributions of ozone and dye, and the decomposition conversion of dye were calculated by using the flow characteristics.

View full abstract

The rectangular airlift sonoreactor combined with sparging ozone was developed. Tetraphenyl porphine tetrasulfonic acid was used as a sample to be decomposed and the decomposition performance was experimentally studied. First, by using only ultrasound, the superficial gas velocity and the riser width were changed, and the decomposition conversion and the flow characteristics were examined. It was found that the decomposition conversion became higher with increasing liquid velocity above the ultrasonic oscillator because the sample was more effectively supplied to the ultrasonic reaction field. Next, the decomposition experiment was conducted by using both the ultrasound and ozone. It was confirmed that the decomposition performance was enhanced by the combination with sparging ozone.

View full abstract

In wet limestone gypsum Flue Gas Desulfurizers, flue gas is washed with CaCO₃–water slurry on the wet walls of a vertical absorption tower, where SO₂ and O₂ are absorbed to form CaSO₃ and CaSO₄. Though it is required to capture sulfur into calcium sulfate, which is neutral to the environment as deposited, CaSO₃ is not always oxidized completely there to CaSO₄ due to insufficient residence time of slurry on the wet walls and the low absorption rate of oxygen. The rest CaSO₃ should be oxidized in a bubbling tank just under the tower. Therefore appropriate air bubble injectors are required to supply oxygen, just as much as necessary and enough, into the slurry in the tank. Here is proposed an Air Rotary Sparger (ARS) for the vertically and horizontally uniform distribution of small air bubbles in the tank and its performance is investigated with a small-sized conventional paddle type model with a central air injection hole on the bottom and two ARS models.

View full abstract