Published papers

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 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.

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.

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.

© 2020 The Society of Chemical Engineers, Japan Many chemical and biocatalytic reactions are consuming gaseous species like oxygen, provided by the mass transfer across interfaces of multiphase contact apparatuses. For biocatalytic reactions a macroscopic aeration can lead to reduced enzyme activity by foaming and shear forces and for fast chemical reactions in multiphase flows, the mass transfer limitation is often the bottleneck for a process optimization. The present study investigates the use of bubbles with diameters less than 100 µm for aeration of a 3 L lab scale stirred tank reactor. For demineralized water and a solution of glucose and bovine serum albumin (BSA) as biocatalytic model protein solution, two different membrane spargers with a mean pore size of 1 µm and 2 µm are investigated. Determining the influence of the energy input on the hydrodynamics of the system, endoscopic measurements of bubble size distributions are carried out. The mass transfer performance of the two spargers is analyzed by measurements of the oxygen kla value for varying gas flow rates. As a result microbubble aeration shows a significant higher mass transfer performance compared to an open tube aeration saving 60% of the gaseous phase by reaching the same kla values. Besides high specific interfacial areas and long residance times, the Laplace pressure inside the bubble is identified as an enhancing force for mass transfer at microscale.

© 2019 Elsevier B.V. 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, U B , and the mass transfer coefficient, k L , 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 U B 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.

© 2019, © 2019 Taylor & Francis Group, LLC. This study was performed to investigate the synthesis, kinetic and reaction mechanism of Li 4 SiO 4 with CO 2 in a slurry bubble column reactor. The Li 4 SiO 4 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 Li 2 CO 3 –K 2 CO 3 having a molar ratio of 38:62. The experimental results show that Li 4 SiO 4 in the slurry bubble column absorbed approximately a stoichiometric amount of CO 2 . The kinetic study shows that the CO 2 reaction behavior on the Li 4 SiO 4 surface was fitted to a double exponential model and the limiting step of the reaction was lithium diffusion. The mass transfer coefficient of CO 2 and rate constant of reaction with Li 4 SiO 4 were studied to understand the overall absorption mechanism in the reactor. The resistance for the direct reaction of CO 2 on the Li 4 SiO 4 was much smaller than the resistance for the mass transfer of CO 2 to the Li 4 SiO 4 . We can conclude that the direct contact of CO 2 with Li 4 SiO 4 was the main path for the reaction.

© 2019 The Authors. Published by WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Carbon capture technology can be set up in combination with biocatalysis to utilize the bound CO2 as substrate in the Kolbe-Schmitt like enzymatic reaction. The exemplary whole cell biotransformation of catechol to 2,3-dihydroxybenzoic acid in a triethanolamine-mediated multiphase system shows increased equilibrium conversion. Apart from the beneficial thermodynamics, the inherent fluid properties of triethanolamine is enabling easy application of CO2 fine bubbles as highly efficient gassing method to minimize the CO2 demand and CO2 emissions.

© 2019 The Society of Chemical Engineers, Japan 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 .