Published papers

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 °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 CO2 mass transfer in eutectic mixtures of molten carbonate in bubble column at elevated temperature was investigated. CO2 was chosen as gas species, eutectic mixtures of Li2CO3-K2CO3 (38:62mol%) binary molten carbonate and Li2CO3-Na2CO3-K2CO3 (43.5:31.5:25mol%) ternary molten carbonate were used as liquid phase. In those eutectic mixtures, CO2 solubilities were determined at the temperature from 673K to 1173K. The values of solubilities increased with increasing temperature. It was suggested that CO2 dissolved into binary and ternary molten carbonate with chemical interaction, CO2+CO32-↔C2O52-. Increasing the temperature shifted this equation to the right and then more CO2 chemically dissolved into molten carbonates as C2O52-. From the CO2 gas absorption rate in molten carbonates, CO2 liquid phase volumetric mass transfer coefficients, kLa were determined and the influence of temperature and superficial gas velocity on kLa was also investigated. The kLa decreased with increasing temperature. And the kLa 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. © 2012 Elsevier Ltd.

When both a gas and liquid are simultaneously introduced from an injector into a vertical mini-channel, the hydrodynamic pattern in the channel depends on the construction of the injectors. To evaluate the effect of the injector on the two-phase flow pattern, direct injection has been compared to a co-flow pre-injection and a cross-flow pre-injection in recent study. For the three different injections, the flow pattern regime maps, which are described as the relation between the superficial liquid velocity and superficial gas velocity, were developed. The direct injector realized better dispersion of uniform bubble flow, various flow patterns and stable slug flow than the other injections. To clarify the two-phase flow dynamics in a vertical mini-channel attached to a direct injector, the frictional pressure drop has been measured and illustrated by a contour. The frictional pressure drop through a mini-channel was correlated using the equation of Chisholm. The two-phase friction multiplier was plotted versus the Lockhart-Martinelli parameter. The constant of the Chisholm equation has been evaluated for the two-phase flow such as bubbly flow and slug flow. By using the equation, the frictional pressure drop through a mini-channel can be well predicted. © 2012 The Society of Chemical Engineers, Japan.

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 1mm. 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. © 2011 The Society of Chemical Engineers, Japan.

乱流状態を用いずに高粘度液体の高効率な混合を行うため,内部に機械的駆動部やエレメントを持たず,複数の支流から周期的に流体を押し出すことにより流体の引き伸ばしと折り畳みを行わせるノンエレメントミキサーを開発した.その混合特性を明らかにするため,レーザ誘起蛍光法により主流管中心軸上の断面を可視化し,引き伸ばし・折り畳みの過程について検討を加え,支流本数および押し出し周期が混合促進に与える影響を明らかにした.

剪断流動場において発熱反応により発生する振動現象について,可視化実験をもとに形成される流動パターンについて検討した.上流側で観察される規則的な振動パターンが下流側では複雑に変化することを明らかにし,フラクタル解析を用いて粘度比,全体粘度,流量比及び全流量の影響について検討した.さらに温度変動データについて時系列のフラクタル解析を行い,全体粘度が流動及び温度両方の振動パターン制御に重要な因子であることを示した.

発熱反応を伴う剪断流動場で発生する不安定振動現象について,粘性が流動パターンに与える影響を検討した.上流での振動発生機構についてはペトリネットを用いたモデル化とシミュレーションにより粘性の増加が振動の周波数に与える影響を明らかにした.一方,流動パターンが複雑に変化する下流ではフラクタル解析を用いたパターンの複雑性の定量化を行い,粘性が重要な支配因子であることを明らかにした.

期待するプロセス強化(PI)とその実現のための方法論について独自の見解を述べることを目的とした.PIにとってプロセスシステムの組織的な統合化,即ちコンパクト化が重要であり,その結果精密要素が緻密に詰まった複雑なシステムにおける精密で効率的かつ柔軟なダイナミック複雑系のマルチスケールのモデリング・シミュレーションの必要性を明らかにした.

Recently, molten salt has attracted attention as an attractive liquid phase for gas-liquid processes at high temperature. Then a method of estimating the gas–liquid interfacial area in molten salt is required. However, the experimental and theoretical methods to estimate the gas bubble size in molten salt systems have not yet established. Especially, there is a lack of experimental techniques to directly observe the bubble formation in a high temperature system. The experimental observation of growth, stretch and detachment of bubbles is also necessary for modeling and numerical simulation. Thus, at first, this study tried to construct a reliable experimental method for observing the bubble formation in a high-temperature liquid. Clear images of the progress of bubble formation could be recorded by the experimental setups proposed in this study. The bubble formed in sodium nitrate was clear, spherically shaped and the shape was maintained longer than in aqueous system. Secondly, using this images, the bubble size generated in molten sodium nitrate was measured. The measured value was compared with the estimated values of the empirical formula established in the aqueous system. At dynamic conditions, where the difference of the physical properties has no effect on the bubble size, the same equation was applicable to those two quite different solutions. However, at static conditions, the measured bubble size was larger than the estimated value. It suggested that the empirical formula established for the aqueous system could not sufficiently estimate the effect of the surface tension in a molten salt solution.

A method for estimation of the volumetric mass transfer coefficient of an entire aeration tank for optimization of tank design was investigated. The gas holdup, bubble diameter, and volumetric mass transfer coefficient of an aeration tank were measured by experiments and the experimental results were compared with two estimation method. One method used empirical equations, while the other used CFD simulation. The rate of oxygen transfer into the aeration tank was assumed to be the sum of the rates at the bubble-liquid interface and free surface. The empirical equation method showed good agreement with the measured gas holdup by assuming that the aeration tank was divided two zones. On the other hand, the CFD simulation method provided a good estimation of the gas holdup and bubble diameter. The sum of the estimated volumetric mass transfer at the bubble-liquid interface and free surface showed good agreement with the measured volumetric mass transfer coefficient of the entire aeration tank.

Conventionally, both aeration tanks and bubble columns are supplied oxygen from dispersed bubbles. However, the geometry of the aeration tanks is greatly different from that of bubble columns. Although almost all oxygen is absorbed from dispersed bubbles in the water for bubble columns, the aeration tank has such a large free surface area that the oxygen transfer from atmosphere cannot be neglected. Therefore, it is very important to investigate oxygen transfer from not only the free surface, but also bubbles. In this study, the volumetric oxygen transfer coefficient, gas holdup and its two dimensional profile were measured in a large scaled aeration tank of typical size for industrial sewage treatment. Moreover, the effect of the aeration method on gas holdup and water circulation such as spiral flow aeration and whole floor aeration were also investigated. By analyzing the experimental results, a simultaneous oxygen transfer model which consists of the sum of oxygen transfer coefficients not only between bubbles and water, but also between the atmosphere and the free surface, is proposed. The model was evaluated by comparing with two alternative models for the experimental overall oxygen transfer coefficients. The proposed model estimated the experimental results better than the other models.

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.

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 solid-state reaction. The sample was characterized via X-ray diffraction (XRD) analysis and verified as a single phase. The median diameter of the sample was measured using the laser diffraction and scattering method as about 20 μm. The synthesized sample was suspended in binary molten carbonate of Li2CO3–K2CO3 having a molar ratio of 38:62. The experimental results show that Li4SiO4 in the slurry bubble column absorbed approximately a stoichiometric amount of CO2. The kinetic study shows that the CO2 reaction behavior on the Li4SiO4 surface was fitted to a double exponential model and the limiting step of the reaction was lithium diffusion. The mass transfer coefficient of CO2 and rate constant of reaction with Li4SiO4 were studied to understand the overall absorption mechanism in the reactor. The resistance for the direct reaction of CO2 on the Li4SiO4 was much smaller than the resistance for the mass transfer of CO2 to the Li4SiO4. We can conclude that the direct contact of CO2 with Li4SiO4 was the main path for the reaction.

This study investigated submicron-sized bubbles, more precisely ultrafine bubbles (UFBs). The number concentration, size, and scattered light intensity of UFBs were measured by particle tracking analysis (PTA). The results showed that storage temperature had a significant effect, whereas the storage containers had little effect on the long-term stability of UFBs. It was found that the second-order aggregation rate equation could predict the stability of UFBs; however, there were some differences from the theory at high storage temperatures. The features of the PTA were also discussed in detail.

Self-induced sloshing is an oscillatory phenomenon of a free liquid surface due to the flow of a fluid. This phenomenon has been reported in some gas–liquid reactors, and it is important to predict and prevent its occurrence for the safe operation of the reactors. However, the fundamental knowledge on the self-induced sloshing by bubble flow, such as the frequency and amplitude, is insufficient, and the occurrence condition has not been clarified. The purpose of this study is to investigate the characteristics of self-induced sloshing by bubble flow experimentally. We attempt to reproduce self-induced sloshing by using computational fluid dynamics (CFD) and establish a CFD model for the prediction of the occurrence of self-induced sloshing. In the experiments, air bubbles were dispersed into a liquid from the bottom of a rectangular vessel. The effects of the air flow rate and static liquid height on the characteristics of self-induced sloshing were investigated experimentally by image analysis. The occurrence of self-induced sloshing was confirmed by increasing the airflow rate at a specific static liquid height. The amplitude reached a maximum at the static liquid height, where self-induced sloshing was most likely to occur, and the frequency decreased with increasing static liquid height. Next, in order to reproduce the self-induced sloshing through CFD, an appropriate drag model of the bubbles was selected. Although the amplitude was overestimated due to the absence of the foam layer, the predicted frequency agreed well with the experimental value. Finally, the movement of the circulation flow was analyzed, and its correlation with the self-induced sloshing was clarified.