Published papers

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.

In regions with a low oscillatory Reynolds number, an oscillatory baffled reactor (OBR) exhibits the folding and stretching of fluid, resulting in the mixing observed within the baffle sections. In this study, an analysis of the mixing mechanism and an evaluation of mixing performance were conducted using experimental methods and numerical simulations to visualize the boundary between the liquid phases and track its shape changes. The ratio of oscillation stroke to baffle interval length, the open ratio of the baffle's cross-sectional area, and the length of the baffle intervals were varied. A glycerin-water solution was used, and the boundary line was colored with rhodamine, forming a film-like layer at the orifice, which was then visualized using sheet laser fluorescence. Oscillatory flow was applied, and changes in the length of the boundary line and the boundary area were measured for one oscillation cycle. For the numerical calculations, the velocity field was first computed, followed by the arrangement of virtual particles to represent the boundary line. The shape of the boundary line obtained from the experiments was consistent with the simulation results, and the trends in the length of the boundary lines for the first cycle were generally consistent as well. Therefore, the experimental results confirmed that the boundary area increases exponentially, consistent with the simulation results.

This study aimed to investigate the effect of self-excited sloshing, induced by a bubble plume, on mass transfer. In a rectangular tank, experiments were conducted by intentionally varying the bubble diameter using different gas spargers and by adding 1-butanol to the liquid phase. The characteristics of the liquid surface oscillation and the mass transfer coefficient kL were measured. The results showed that decreasing the bubble diameter promoted sloshing and increased its amplitude. However, a further reduction in bubble size through the addition of 1-butanol suppressed the oscillation. This was attributed to the formation of a foam layer, which dissipated energy through friction with the tank walls. Regarding mass transfer, the mass transfer coefficient kL tended to increase with the gas flow rate, suggesting that sloshing could potentially enhance mass transfer. Nevertheless, no simple correlation was found between the sloshing amplitude and the value of kL. The study concludes that bubble diameter and liquid properties significantly alter sloshing characteristics, but further detailed investigation, including the improvement of CFD predictions, is required to fully elucidate the relationship between the fluid dynamics and mass transfer.

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.

Although the mechanism of ultrafine bubble generation has not yet been elucidated, we believe that the state of gas-saturated water before the generation of ultrafine bubbles is the most important factor in the mechanism of ultrafine bubble generation. Therefore, we proposed the hypothesis that the spatial distribution of dissolved gas molecules among the water molecules before the ultrafine bubble formation is stared has a significant effect on the formation of ultrafine bubble suspending water. Based on this hypothesis, we succeeded in generating ultrafine bubbles of sufficient number concentration in the condensate by rapidly condensing a well mixture of non-condensable gas and vapor by cooling. The number concentration of the generated ultrafine bubbles increased almost in proportion to the condensation rate of the vapor. Therefore, it was found that promoting the formation of bubble nuclei from dissolved gas molecules existing between water molecules in the condensate and suppressing bubble growth led to an increase in the number concentration of ultrafine bubbles. The principle of ultrafine bubble generation in this invention not only simplifies the generation of ultrafine bubble water but also enables the control of the number concentration of ultrafine bubbles in water.

液相への溶解効率の高い直径1 mm以下の微細気泡,すなわちサブミリバブルを分散させるためのガス分散器を開発した.著者らにより過去に提案されたスリット型オリフィスを採用し,多量の気泡を分散させるため複数のスリット型オリフィスを有するマルチスリット型のサブミリバブル分散器を開発した.この分散器について,ガスホールドアップおよび物質移動容量係数を測定し,従来型のガス分散器よりも高いガス溶解効率が得られることを確かめた.

To reduce the release of greenhouse gas from massive emission sources into atmosphere, a slurry bubble column suspending lithium silicate is proposed to remove hot CO2 from flue gas or pre-combustion effluent gas. As the liquid phase, the molten salt consisting of binary carbonates was used. The slurry suspending solid particles of lithium silicate in the molten salt promoted to mix gas and slurry in the column so that the absorption rate of CO2 was improved. In this study, the effects of some important operating conditions such as superflcial flue gas velocity, the concentration of lithium silicate powder in the slurry and the height of the slurry on CO2 absorption were investigated to optimize the system. The CO2 absorption increased with increasing superflcial gas velocity and decreasing slurry concentration. Fractional CO2 recovery increased with increasing the height of the slurry. The apparent rate constant and the apparent activation energy were determined in the reaction system. Moreover, an endurance test was conducted to confirm stable recovery capacity, in which the proposed CO2 recovering system performance was repeatedly maintained. © 2012 The Society of Chemical Engineers, Japan.

In this study, the new temperature-dependent phosphor, Y2O 3:Tb3+=Tm3+, was investigated for high-temperature thermometry. The photoluminescence intensity at 456nm emitted from Tm3+ was strong at temperatures higher than 1100K, whereas the peak intensities emitted from Tb3+ decreased due to the thermal quenching effect. Thus, the intensity ratio between those emissions showed an appropriate variation for thermometry over a wide temperature range. In addition, the phosphors showed a distinct change of visible emission colors from green to blue with increasing temperature. These findings suggest the applicability of these phosphors in visual thermo-sensors. © 2011 Optical Society of America.

活性汚泥を利用した廃水処理プロセスでは,微生物への効率的な酸素供給が必要不可欠である.本研究では旋回液流式のマイクロバブル発生器を利用したエアリフト型マイクロバブル曝気装置を開発した.本装置はコンパクトで可動式であり,酸素溶解性能が高いという利点を持つ.廃水処理性能の評価のため,モデル有機廃水を用いて曝気速度,溶存酸素濃度および装置形状が有機物分解速度に与える影響について検討した.

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.

The purpose of this study is to examine the possibility of establishing a novel CO 2 absorption process with molten alkali carbonate using a bubble column reactor. In our previous study, a hot CO 2 recovery process using Li 4 SiO 4 suspended in molten Li 2 CO 3 -K 2 CO 3 was developed. In the process, molten alkali carbonate itself showed great potential for CO 2 absorption at high temperature. If a hot CO 2 absorption process were established using only molten alkali carbonate, it could make the system simpler and the operating temperature range could be extended without the limitation of reaction temperature of solid absorbent. In the study, molten Li 2 CO 3 , Na 2 CO 3 , K 2 CO 3 and its eutectic mixture were selected as CO 2 absorbent. A bubble column was chosen as the device for gas absorption at high temperature. First, the CO 2 absorption performance of each single molten alkali carbonate was investigated. The result showed that the molten Li 2 CO 3 had a great ability to absorb CO 2 at high temperature. Li 2 O was thought to be produced by decomposition of Li 2 CO 3 during the melting and purging process and a reaction of CO 2 with Li 2 O occurred during the absorption process. Further, the CO 2 absorption performance of eutectic mixture increased exponentially with increasing the ratio of Li 2 CO 3 in composition. Second, the possibility of establishing a CO 2 absorption process using molten Li 2 CO 3 was examined. The overall CO 2 absorption process in the bubble column was investigated and the experimental results showed that the mass transfer of CO 2 into molten Li 2 CO 3 was the rate-controlling step. The operational conditions of the bubble column were optimized. The superficial gas velocity was an important operational parameter that affected both the CO 2 absorption rate and total amount of CO 2 finally absorbed in the bubble column. The operating temperature also greatly affected the amount of absorbed CO 2 .

For a variety of applications of microbubbles, it is necessary to clarify the dissolution dynamics and the lifetime of a microbubble. Although various models have been proposed, predicting the dissolution rate of a microbubble in a static fluid with a surfactant is still difficult. In this study, dissolution behavior was predicted with taking into account the mass transfer inhibition effect due to a surfactant at the bubble surface. The shrinking behavior of an oxygen microbubble was observed with several surfactants and the experimental results were in good agreement with model predictions.

Submicron-sized bubbles are now officially called ultrafine bubbles (UFBs) by the international standard. The concentration of UFBs is generally low (<109 particles/mL; <0.001 vol%) compared to other colloidal dispersions. To overcome this practical problem, we concentrated UFBs in ultrapure water prepared by a commercial UFB generator and quantified the effect of rotary evaporation of the dispersion media on the stability of UFBs. The UFB dispersions were characterized by a particle tracking analysis (PTA) instrument. The experimental results showed that the UFBs can be diluted and concentrated without changing the size distribution and there was little or no loss of UFBs. By using a rotary evaporator, UFB dispersions were about 30-fold concentrated and the resultant number concentration reached over 3 × 1010 particles/mL. Increasing the concentration of UFBs allowed for satisfactory dynamic light scattering (DLS) measurements. The differences among the three algorithms for analyzing the raw data, i.e., autocorrelation function, obtained by DLS are discussed, along with the characteristics of the particle size distribution derived from each algorithm.

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 microcapillary was combined with the liquid-liquid slug flow to build the separation field. The partition behavior of seventeen kinds of substrates was investigated by using six kinds of water/organic biphasic systems under hydrothermal conditions (25 - 190° C at 10 MPa). The scale of hydrophobicity (HF) of water/organic biphasic system was estimated. The HF value for the biphasic system used here was comparable to the conventional aqueous two-phase systems and depended on the (de)solvation of substrates by water and organic solvent. Besides, the vortex field in the slug contributed to the enhanced mass transfer of substrates. Those results are available for the selection of water/organic biphasic system as the reactive separation of target materials.

This study aims to improve the numerical stability of rheological parameter estimation for non-Newtonian food fluids by proposing a Sequential Herschel–Bulkley (SHB) model, which sequentially estimates yield stress (τ0), consistency index (K), and flow behavior index (n) using pressure drop and flow rate data obtained from tube flow. In contrast to the conventional Herschel–Bulkley (HB) model, which fits all three parameters simultaneously and is susceptible to numerical instability, the SHB model first determines τ0 using the Casson equation and then fits K and n with τ0 fixed. The method was validated using tamarind seed gum (TG) solution, xanthan gum (XG) solution, and ketchup. Flow curves derived from SHB-based tube-flow measurements showed improved agreement with rotational viscometer data, particularly for ketchup, where the mean absolute percentage error (MAPE) decreased from 4.49% for the HB model to 2.10% for the SHB model. In addition, repeated nonlinear fitting demonstrated that the SHB model consistently reduced the coefficients of variation and parameter variances relative to the HB model, indicating enhanced robustness. These findings demonstrate that the SHB model provides a concise, reliable, and numerically stable approach for estimating shear rheological parameters of yield-stress food fluids using practical in-line measurement signals.