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One of the key elements in stirred tank reactor set-ups is the submersed aeration system. Aeration with microbubbles provides high gas utility coupled with prolonged enzyme stability by lowering the interfacial area renewing rate depending on the aeration mode.

This study was performed to investigate the synthesis, kinetic and reaction mechanism of Li4SiO4 with CO2 in a slurry bubble column reactor. The Li4SiO4 powder sample was prepared via a sol...

Highlights • A method to remove ultrafine bubbles (UFBs) generated by two methods was presented. • Indirect ultrasonication destabilized UFBs in pure water and surfactant solution. • UFBs in the mixture with polystyrene beads were also selectively destabilized. • Destabilization mechanism related to zeta potential was proposed., Ultrafine bubble (UFB) is a bubble with a diameter of less than 1 μm. Little attention has been paid to the defoaming and removal of UFBs. This study proposes a method to destabilize UFBs by using indirect ultrasonic irradiation. Besides, the destabilization mechanism of UFB was investigated. The ultrasonic frequency was 1.6 MHz and the dissipated power was 30 W. UFB dispersions were prepared using two different types of bubble generators: pressurized dissolution method and swirling liquid flow method. The effects of ultrasonic irradiation on the stability of UFBs were evaluated by particle tracking analysis (PTA) and electrophoretic zeta potential measurement. Results showed that the indirect ultrasonic irradiation for 30 min reduced the number concentration of UFBs by 90% regardless of the generation method. This destabilization was attributed to a decrease in the magnitude of zeta potential of UFBs due to the changes in pH and electrical conductivity. These changes in the electrochemical properties were caused by the formation of nitric acid. To study the destabilization mechanism, the pH of the UFB dispersions were modified by titration; the chemical and mechanical effects of ultrasound were separately examined. It was found that not only the chemical effect caused by the formation of nitric acid but also the mechanical effect contributed to the destabilization of UFB. Feasibility studies were also performed for UFBs in an aqueous surfactant solution and UFBs in a solid particle dispersion. The proposed method selectively destabilized UFBs in the solutions.

The present study focuses on the aeration of aqueous triethanolamine acting as reaction medium for biocatalytic carboxylations. For enhancing mass transfer in a bubble column reactor, microbubble aeration is applied and compared to conventional macrobubble aeration. Application of a 0.5 μm porous sparger enables microbubble CO2 aeration with bubble size distributions below 150 μm in Sauter mean diameter, correlating with the highest measured mass transfer rates. During CO2 saturation of the aqueous triethanolamine, bubble size distributions changed according to the level of CO2 saturation. For microbubbles, less foaming was observed compared to macrobubble aeration by a 10 μm porous sparger. This microbubble effect is attributed to their accelerated dissolution assisted by the Laplace pressure lowering the amount of bubbles reaching the surface of the liquid. The experiments reveal that the rate of interfacial area generation, which is calculated based on measured bubble size distributions, influences the biocatalyst activity.

The present study focuses on the aeration of aqueous triethanolamine acting as reaction medium for biocatalytic carboxylations. For enhancing mass transfer in a bubble column reactor, microbubble aeration is applied and compared to conventional macrobubble aeration. Application of a 0.5 μm porous sparger enables microbubble CO

Because of their useful characteristics, microbubbles attracted much interest in industry and medicine. In this study, we developed a novel method of generating microbubbles through the chemical absorption of ammonia (NH3). By using an ultrahigh-speed camera, consecutive images of bubble formation were captured, and the microbubble generation mechanism was investigated. The effects of the gas flow rate and ammonia concentration in the liquid phase on the size of the microbubbles were examined. The hydrogen (H2) gas flow rate was found not to demonstrate a significant effect on the bubble diameter. The diameter of the microbubbles increased as the superficial velocity of the gas mixture (NH3 – H2) decreased.

The electrochemical CO2 reduction on boron-doped diamond (BDD) electrodes in an aqueous solution with fine bubbles was studied. The overpotential decreased and the production of carbon monoxide was promoted in the CO2 reduction with fine bubbles compared to that without fine bubbles. It is suggested that fine bubbles in solution could facilitate the mass transport and also act as the catalyst for producing carbon monoxide. Potential-dependent in-situ attenuated total reflectance-infrared (ATR-IR) measurements confirmed the catalytic activity, showing that in solutions with CO2 fine bubbles the CO2•– intermediate or the CO2 molecules themselves become stabilized near the electrode and these can be adsorbed on the electrode surface more easily compared to using solutions without CO2 fine bubbles. The reported work is a first step in understanding the effects of fine bubbles in CO2 reduction reactions.

A dehydration of fructose in the water/methyl isobuthyl ketone (MIBK) biphasic system can yield 5-hydroxymethylfurfural (HMF) to be successfully extracted into the organic MIBK phase. The HMF production and yield in MIBK phase was discussed by using a simplified model taking into consideration of the slug flow. The extraction resistance of HMF across the interface between water and MIBK depended on the line velocity and the flow rate ratio. It was likely that the velocity field generated in the slug flow contributed to an increase in the mass transfer of HMF. © 2016 American Institute of Chemical Engineers AIChE J, 62: 2135–2143, 2016

Microbubbles are used in many applications. One of the major advantages of using microbubbles is to improve the mass transfer of gases to the bulk phase. In this study, mass transfer from single microbubbles that rise in ultrapure water and aqueous solutions of surfactants (SDS, Pluronic F68) or salt (NaCl) was investigated. The rising and dissolution behaviors of single air microbubbles were observed using a high-speed imaging technique by which the rising velocity, UB, and the mass transfer coefficient, kL, were optically evaluated. We found that the rising velocity of microbubbles obeys Stokes’ law, indicating that the surface of the microbubbles used in this study is immobile, despite their rising in ultrapure water. The obtained mass transfer coefficients were in good agreement with those predicted by Ranz and Marshall’s correlation for the Sherwood number, Sh. Using the two correlations for UB and Sh, a theoretical model that describes the mass transfer from a microbubble was constructed. The proposed model can predict the dissolution of single microbubbles rising in ultrapure water and aqueous solutions with an accuracy of ±10% in terms of dissolution time.

It is difficult to control the bubble in a liquid by the external operation, because the behavior of the bubble is controlled in buoyancy and flow of liquid. On the other hand, microbubbles, whose diameter is several decades μm, stably disperse in static liquid because of their small buoyancy and electrical repulsion. When an ultrasound, whose frequency was 2.4 MHz, was irradiated, the milky white microbubbles suspended solution became rapidly clear. In this study, the effects of surfactant addition on the removal of microbubbles from a liquid in an ultrasonic field were investigated. The efficiency of removal of microbubbles decreased with surfactant addition. Surfactant type influenced the size of agglomerated microbubbles, and the efficiency of removal of microbubbles changed. The surface of microbubble was modified by surfactant adsorption, and the steric inhibition influenced the removal of microbubbles.

The clarification of the partition property of substrates in a water / organic biphasic system is useful in the development of a reaction / separation process for value-added materials, especially under high temperature and pressure conditions. In this study, the water / methyl isobuthyl ketone (MIBK) biphasic system was investigated under various temperatures (25 – 190 o C) at 10 MPa. The partitioning behavior of substrates such as furfural derivatives, amino acids, and saccharides, depended on the temperature and the flow rate ratio of MIBK to the water phase. The scale of hydrophobicity (HF) of the water / MIBK biphasic system to determine the partitioning behavior of substrates was estimated based on the partitioning behavior of amino acids. The HF value for the water / MIBK phase flow system of was greater than that for the batch system and comparable to that for conventional aqueous two-phase systems such as polyethylene glycol / dextran and liposome membrane systems. It was probably because an increase of the surface-to-volume ratio and the vortex field in the slug flow contributed to the mass transfer of substrates and their distribution.

Recently, the practical uses of bubble columns and slurry bubble columns at elevated temperature have attracted much attention. However, there is less knowledge of mass transfer in high temperature slurry bubble column, even though mass transfer data is essential to design bubble column and slurry bubble column. In this study, the CO 2 mass transfer in eutectic mixtures of molten carbonate in bubble column at elevated temperature was investigated. CO 2 was chosen as gas species, eutectic mixtures of Li 2 CO 3 –K 2 CO 3 (38:62 mol%) binary molten carbonate and Li 2 CO 3 –Na 2 CO 3 –K 2 CO 3 (43.5:31.5:25 mol%) ternary molten carbonate were used as liquid phase. In those eutectic mixtures, CO 2 solubilities were determined at the temperature from 673 K to 1173 K. The values of solubilities increased with increasing temperature. It was suggested that CO 2 dissolved into binary and ternary molten carbonate with chemical interaction, CO 2 +CO 3 2− ↔C 2 O 5 2− . Increasing the temperature shifted this equation to the right and then more CO 2 chemically dissolved into molten carbonates as C 2 O 5 2− . From the CO 2 gas absorption rate in molten carbonates, CO 2 liquid phase volumetric mass transfer coefficients, k L a were determined and the influence of temperature and superficial gas velocity on k L a was also investigated. The k L a decreased with increasing temperature. And the k L a increased linearly with increasing superficial gas velocity. Regardless of unusual conditions in molten salt in bubble column at high temperature, where viscosity, surface tension and gas diffusivity were relatively high, it was suggested that the superficial gas velocity was most important operation condition in common with aqueous bubble column at ambient temperature.

Microbubbles have some different characteristics from conventional bubbles. To apply the useful properties for gas–liquid contact operation in industry, however, a separate technology of microbubbles has to be realized. In this study, promotion of microbubble separation using ultrasound was proposed. By irradiating with ultrasound, milky white microbubbles suspended solution changed instantaneously to be clear. The interesting behavior of microbubbles observed in the ultrasonic field was investigated by microscopic and macroscopic visualizations. The rapid ascent of microbubbles was caused by their agglomeration, where the Bjerknes force of attraction and electrical repulsive force on microbubble surface acted. Ultrasonic irradiation into microbubble suspended solution was very useful for dynamic operation of microbubbles.

In large-scale waste water treatment plants, the aerobic biochemical reactor is the most important process, where the oxygen supply into the microorganisms often limits the overall waste water treatment rate. On the other hand, several kinds of microbubble distributors have been developed to enrich the oxygen dissolution in water. Therefore, the application of microbubbles for a waste water treatment system was investigated in this study. The oxygen absorption performance of typical microbubble generators was compared with typical bubble generators. To evaluate each bubble generator, the liquid-phase volumetric oxygen transfer coefficient, gas hold-up and power consumption per unit liquid volume were measured in a bubble column attached to each bubble generator. All the microbubble generators allowed the oxygen to dissolve faster than the typical aerators. The spiral liquid flow type microbubble generator had the highest oxygen transfer coefficient even at a low air flow rate although it used more energy than the typical distributors. To improve an industrial waste water treatment system, a novel aeration system utilizing a spiral liquid flow type microbubble generator was proposed in this study. The present system has some advantages such as compact size, portability and fast oxygen dissolution rate. To ensure the performance for organic waste water treatment, the effects of the aeration rate, dissolved oxygen concentration and device properties on the specific consumption rate of model organic waste were investigated. For the novel aeration system, the most suitable conditions to treat organic waste were found.

A biological method is usually used in many industries and water purification plants to treat organic waste water. Aerobic microorganisms in active sludge decompose organic compounds to lower molecular weight compounds with oxygen consumption in aeration tank. On many aeration systems, the oxygen supply into water limits the rate of waste water treatment. To improve the oxygen dissolution into water, the size of air bubbles may be reduced. If bubbles are crushed by some additional power devices such as pump or stirrer, however, the cost increases. Therefore, in this study, a novel aerator which consumes almost no additional power was developed to generate bubbles having a diameter smaller than 1 mm.When a gas is inputted from a round orifice into water, the bubbles grow very stably and their size is large. On the other hand, when the shape of the orifice is slit-like, instability is caused by bubble expansion. By using this natural phenomena, a uniform dispersion of submilli-bubbles was realized. Moreover, to design submilli-bubble dispersers, a semi-empirical bubble formation model is proposed.

本研究では，超音波場におけるマイクロバブルの動的挙動に液相条件がおよぼす影響を調べた．イオン交換水中に発生させたマイクロバブルに2.4 MHzの超音波を照射すると，Bjerknes力により腹から斥力を受けて節の位置で気泡同士が近づきあい，凝集体を形成して急速に液中から離脱した．界面活性剤を添加すると，気泡表面に吸着し立体的阻害が起こり，凝集体を形成しにくくなった．また，液相のpH調製により，マイクロバブルのζ電位が0 mVに近い条件下では凝集した気泡同士の一部が合一した．液相条件を適切に設計することにより，マイクロバブルの凝集・合一挙動だけでなく，凝集体の大きさを操作し液中での上昇速度を外的操作により動的に制御できる可能性を見出した．

The purpose of this study is to examine the possibility of establishing innovative chemical-wise CO2 recovery system with a lithium silicate slurry bubble column, which could collect hot CO2 from massive emission sources. This is realized by using the composite of lithium and silica, called lithium orthosilicate, Li4SiO4, which can reversibly absorb and strip CO2 gas at high temperatures, i.e. ca. 700°C and 850°C, respectively. Therefore, continuous recovery of CO2 is feasible simply by thermal swing. The fundamental chemical reaction is expressed as follows: Li4SiO4+CO2 ⇄ Li2SiO3+Li2CO3. The reaction under high temperature conditions is possible by preparing molten salt as a reaction field. The lithium silicate is suspended in molten salt, and 20% CO2 containing gas is distributed to achieve absorption, both physically and chemically. To confirm the feasibility of the proposed system, two experiments were conducted to demonstrate CO2 capture ability: 1) physical absorption to Li-K-CO3 molten salt, and 2) chemical absorption to a Li4SiO4 suspended slurry. CO2 solubility and its level of mass transfer to Li-K-CO3 are investigated based on the results. Moreover, the performance of the CO2 recovery is investigated from the effects of gas feed rate, quantity of absorbent, and suspension concentration. Overall, the proposed system enables CO2 recovery under hot conditions, resulting in high efficiency recovery. This study establishes a lithium silicate suspended slurry system as an effective instrument for recovering CO2.