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The hydrodynamics of single-phase liquid flow with relatively high fluid viscosities in a microchannel was investigated experimentally. The results showed that the conventional theory could predict the single-phase flow with high fluid viscosities in microchannels. Furthermore, the effect of viscosity on the slug flow of two immiscible liquid phases in a microchannel was studied with high-speed imaging techniques. It was found that a higher dispersed-phase viscosity quickened the flow pattern transition from slug flow to parallel flow and resulted in smaller slugs. A modified capillary number representing the mutual effects of the viscosities of the continuous phase and the dispersed phase was proposed for predicting the slug sizes in microchannels. [ABSTRACT FROM AUTHOR]

The miscible liquid-liquid two phases based on Taylor flow in microchannels was investigated by high-speed imaging techniques and Villermaux/Dushman reaction. The mixing based on Taylor flow was much better compared with that without introducing gas in microchannels, even the ideal micromixing performance could be obtained under optimized superficial gas and liquid velocities. In the mixing process based on Taylor flow, the superficial gas and liquid velocities affected the lengths and the velocities of Taylor bubble and liquid slug, and finally the micromixing performance. The formation process of Taylor flow in the inlets, the initial uniform distribution of reactants and the internal circulations in the liquid slug, and the thin liquid films all improved the mixing performance. Furthermore, a modified Peclet number that represented the relative importance of diffusion and convection in the mixing process was proposed for explaining and anticipating micromixing efficiency. © 2011 American Institute of Chemical Engineers AIChE J, 58: 1660-1670, 2012 [ABSTRACT FROM AUTHOR]

Abstract: In this work, the hydrodynamics and the micromixing characteristics in the packed and non-packed microchannels were studied experimentally at low Reynolds numbers (8–300). The mixing performances in microchannels were observed with high-speed CCD camera, and were evaluated in terms of a segregation index by the Villermaux/Dushman method. The fluid elements were drastically stretched, folded, and sheared with the effects of micro-particles in packed microchannels, resulting in extremely shorter diffusion distance and larger effective interfacial area, and much higher micromixing efficiency compared with those of non-packed microchannels. Under enough packing length and appropriate packing position of micro-particles, the ideal micromixing performance could be obtained in packed microchannels. Furthermore, the micromixing time in packed microchannels was determined based on the incorporation model, and its value was in the range of 0–0.1ms. [Copyright &y& Elsevier]

Abstract: Researches on two-phase transfer and reaction processes in microchannnels are important to the design of multiphase microchemical systems. In the present work, hydrodynamics and mass transfer characteristics in cocurrent gas–liquid flow through a horizontal rectangular microchannel with a hydraulic diameter of have been investigated experimentally. Liquid side volumetric mass transfer coefficients were measured by absorbing pure into water and a 0.3M / 0.3M buffer solution. Interfacial areas were determined by absorbing pure into a 1M NaOH solution. Two-phase flow patterns and pressure drop data were also obtained and analyzed. This paper shows that two-phase frictional pressure drop in the microchannel can be well predicted by the Lockhart–Martinelli method if we use a new correlation of C value in the Chisholm''s equation. Liquid side volumetric mass transfer coefficient and interfacial area as high as about and , respectively, can be achieved in the microchannel. Generally, liquid side volumetric mass transfer coefficient increases with the increasing superficial liquid or gas velocity, which can be described satisfactorily by the developed empirical correlations. A comparison of mass transfer performance among different gas–liquid contactors reveals that the gas–liquid microchannel contactor of this study can provide at least one or two orders of magnitude higher liquid side volumetric mass transfer coefficients and interfacial areas than the others. [Copyright &y& Elsevier]

Abstract: Methanol oxidation reforming is an important reaction in the process of hydrogen generation for PEMFC. In this paper, we report the wall coated catalysts in a microchannel reactor for methanol oxidation reforming. The preparation method of the wall coating catalyst was studied in detail, i.e., the sol–gel and solution-coating techniques. To prepare the catalysts for methanol oxidation reforming, the washing-coating layer of CuZnAl was prepared by the sol–gel technique, and then the active layer was coated on it by solution-coating technique with emulsion colloid containing Pd–ZnO particles. Both the supporting layer and catalyst on the stainless steel foils were characterized by scanning electron microscopy (SEM) and X-ray diffraction (XRD). The formed wash-coating layer with ordered arrays of petaloid microcrystallites in layer surface and Pd–Zn alloy crystal phase can be clearly observed. The reaction experimental results indicated that the catalyst prepared have high activity and relatively high stability. [Copyright &y& Elsevier]

In this paper, preliminary experimental results are presented on pressure drop characteristics of single and two-phase flows through two T-type rectangular microchannel mixers with hydraulic diameters of 528 and 333 μm, respectively. It is shown that both N2 and water single-phase laminar flows in microchannels, with consideration of experimental uncertainties, are consistent with classic theory, if additional effects, such as entrance effects that will interfere with the interpretation of experimental results, are eliminated by carefully designing the experiments. The obtained pressure drop data of N2–water two-phase flow in micromixers are analyzed and compared with existing flow pattern-independent models. It is found that the Lockhart–Martinelli method generally underpredicts the frictional pressure drop. Thereafter, a modified correlation of C value in the Chisholm’s equation based on linear regression of experimental data is proposed to provide a better prediction of the two-phase frictional pressure drop. Also among the homogeneous flow models investigated, the viscosity correlation of McAdams indicates the best performance in correlating the frictional pressure drop data (mean deviations within ±20% for two micromixers both). Finally it is suggested that systematic studies are still required to accurately predict two-phase frictional performance in microchannels. [Copyright &y& Elsevier]

The synergistic effects of gas agitation and ultrasound on mass transfer between immiscible liquids were investigated in an in-house made ultrasonic microreactor. With the introduction of inert gas (N 2 ), a three-phase slug flow with slug bubbles either dispersed in continuous aqueous phase or encapsulated in oil plugs was observed. Under ultrasound irradiation, slug bubbles underwent surface wave oscillation and induced agitation in microchannel. In addition, microbubbles were generated by acoustic cavitation, oscillating intensely and resulting in the formation of O/W emulsion. Bubble oscillation (i.e., slug bubbles and microbubbles) as well as emulsification promoted liquid-liquid mass transfer significantly. Extraction of vanillin from aqueous solution to toluene was employed to demonstrate the mass transfer enhancement. Compared with silent operation, both mass transfer coefficient and extraction efficiency were largely improved by the combined use of gas agitation and ultrasound. With gas flow velocity being 0.005–0.083 m/s at fixed ultrasound power of 30 W, the overall mass transfer coefficients ranged from 0.047 s −1 to 0.429 s −1 , which was 2.33–17.20 times larger than the corresponding liquid-liquid two-phase process without ultrasound irradiation. [ABSTRACT FROM AUTHOR]

Highlights: [•] Shear stress plays an important role in determining bubble length in squeezing regime. [•] A correlation was developed to address the effect of μL and jTP on bubble length. [•] Bubble frequency reaches a maximum with an increase of the gas–liquid flow ratio. [•] Cross-sectional shape of Taylor bubbles is close to square at low Ca. [Copyright &y& Elsevier]

Abstract: This paper concerns an experimental investigation into the flow pattern transition and break-up mechanism of liquid film in single-channel falling film microreactors. Three major flow patterns were observed to be ‘corner rivulet flow’, ‘falling film flow with dry patches’, and ‘complete falling film flow’. The critical flow rate associated with the transition between each flow pattern was determined. Hysteresis was found, as the critical flow rate was higher when the flow pattern shifted from ‘falling film flow with dry patches’ to ‘complete falling film flow’ than it was when the flow pattern shifted in the opposite direction. There existed a minimum wetting flow rate (MWF) in order to the complete falling film flow pattern to be present. MWF was observed to increase with the width or depth of microchannel and could not be well predicted by traditional falling film correlations. Based on the obtained data, an empirical correlation has been proposed for the prediction of MWF in falling film microreactors, where the influence of fluid physical properties and channel dimension is revealed. [Copyright &y& Elsevier]

Abstract: In this work, the flow hydrodynamic characteristics and the mass transfer performance of immiscible fluids in the packed microchannels are investigated experimentally. Water–kerosene system is used for visually identifying the flow hydrodynamic characteristics in PMMA microchannels, and water–succinic acid–n-butanol is chosen for investigating mass transfer performance in stainless steel microchannels. Quartz sand micro-particles are used as packing particles. In packed microchannels, high liquid–liquid dispersions can be obtained, and the diameter of droplets produced in the packed microchannel can be even less than 10μm. It ensures better mixing performance and larger effective interfacial area of two immiscible fluids, and improves the mass transfer performance obviously. Compared to the extraction efficiency (46–61%) in the non-packed microchannel, it can reach 81–96% in the packed microchannel. The effects of packing length, micro-particle size on liquid–liquid dispersions and extraction efficiency are investigated. The pressure drop and the specific energy dissipation in the packed microchannels are also discussed. [Copyright &y& Elsevier]

Abstract: In this work, the influence of surface properties on the flow characteristics and mass transfer performance of two immiscible liquids are investigated in the opposed and cross-flow configuration microchannels, as the volumetric flux ratio is far <1. For visually identifying flow patterns in transparent PMMA microchannels, dyed de-ionized water and kerosene are selected as test fluids. To investigate the mass transfer characteristics in stainless steel microchannels, water–succinic acid–n-butanol is chosen as a typical system. Reynolds number varied between 11 and 275. Only at low ReM numbers, the dispersed phase flow pattern can occur in the opposed T-shaped PMMA microchannel before surface modification. At higher ReM numbers, only the continuous phase flow pattern (parallel flow) is observed before and after surface modification. Moreover, the fluctuation amplitude after surface modification is larger than before surface modification at the opposed T-junction. To eliminate the effects of the sampling intervals and separation process of oil–water two phases on mass transfer performance, one new testing method is established by manufacturing the novel oil–water separator based on the principle of siphon. For the conditions applied during the study, the overall volumetric mean mass transfer coefficients range from 0.19 to 11.96s−1, which are one or three orders of magnitude higher than those in typical conventional large scaled contactors. [Copyright &y& Elsevier]

Abstract: Flow and mass transfer properties under air–water Taylor flow have been investigated in two square microchannels with hydraulic diameters of 400 and 200μm. Experimental data on Taylor bubble velocity, pressure drop and liquid side volumetric mass transfer coefficient have been presented. It was shown that the measured Taylor bubble velocity in square microchannels could be well interpreted based upon an approximate measurement of the liquid film profile therein. Then, the obtained two-phase frictional pressure drop values in both microchannels were found to be significantly higher than the predictions of the correlation proposed by Kreutzer et al. [2005b. Inertial and interfacial effects on pressure drop of Taylor flow in capillaries. A.I.Ch.E. Journal 51, 2428–2440] when the liquid slug was very short, which can be explained by the inadequacy of their correlation to describe the excess pressure drop caused by the strong inner circulation in such short liquid slugs. An appropriate modification has been made to this correlation in order to improve its applicability in microchannels. Finally, the experimental values in the microchannel with hydraulic diameter of 400μm were found to be in poor agreement with those predicted by the existing correlations proposed for capillaries with diameters of several millimeters. The observed deviation was mainly due to the fact that mass transfer experiments in this microchannel actually corresponded to the case of short film contact time and rather poor mixing between the liquid film and the liquid slug, which was not in accordance with mass transfer assumptions associated with these correlations. A new empirical correlation has been proposed to describe mass transfer data in this microchannel. [Copyright &y& Elsevier]

Abstract: This paper describes two-phase flow pattern and pressure drop characteristics during the absorption of into water in three horizontal microchannel contactors which consist of Y-type rectangular microchannels having hydraulic diameters of 667, 400 and , respectively. With the help of a high-speed photography system, flow patterns such as bubbly flow, slug flow (including two sub-regimes, Taylor flow and unstable slug flow), slug-annular flow, churn flow and annular flow were observed in these microchannels. The applicability of the currently available correlations for describing flow pattern transitions in microchannels has been examined. Generally, the predicting performance of these correlations deteriorates as the channel diameter further reduces. Toward solving this discrepancy, an empirical correlation based on the superficial Weber numbers was developed to interpret the transition from Taylor flow to unstable slug flow in three microchannels. Taylor bubble formation process in microchannels was found to be in the squeezing regime at lower superficial liquid velocities (Ca ranging from 0.0019 to 0.029) while the transition to the dripping regime was observed at the highest superficial liquid velocity of 1.0m/s. Lengths of Taylor bubbles formed in the squeezing regime can be well represented by the scaling relation proposed by Garstecki et al. [Formation of droplets and bubbles in a microfluidic T-junction—scaling and mechanism of break-up. Lab on a Chip, 6, 437–446]. For flow patterns including slug-annular flow, annular flow and churn flow, a simple analysis based on the separated flow model has been performed in order to reveal the observed effect of the superficial liquid velocity on two-phase frictional multiplier in the present microchannels. Then, reasonable correlations for the prediction of two-phase frictional pressure drop under these flow patterns were suggested. [Copyright &y& Elsevier]

Abstract: T-type microchannel reactors were used to prepare monodispersed nanocrystals of barium sulfate and boehmite. The size of barium sulfate is strongly dependent on the velocity of the reactants in the channel. High velocity is beneficial for producing small particles. The concentration of reactants also affects the size. Boehmite was prepared by neutralization of NaAlO2 and Al2(SO4)3. Changing the proportion of NaAlO2 and Al2(SO4)3 will lead to different pH values which determine the phase of alumina. High efficiency of micromixing is the key factor of the purity of the alumina morphology. Na2CO3 was used to adjust pH at given value during aging process. Aging is indispensable for the amorphous alumina to crystallize to boehmite. Raising the age temperature would reduce the time the crystallization need. The morphology and purity were characterized by powder X-ray diffraction (XRD) and transmission electron microscopy (TEM). [Copyright &y& Elsevier]

Abstract: Ethylene is essential material for the petrochemical industry. Ethylene production via catalytic dehydration of ethanol over TiO2/γ-Al2O3 catalysts in multi-microchannel reactors is reported in this paper. The physicochemical properties of these catalysts are characterized by X-ray diffraction (XRD) and FT-IR spectrum. The effects of operation parameters, such as ethanol concentration, reaction temperature and liquid hourly space velocity, have been investigated experimentally. The reaction results indicate that the catalysts doped with TiO2 have high ethanol conversion of 99.96% and ethylene selectivity of 99.4%. Ethylene yield of 26g/(gcat h) can be achieved, which provides very favorable foundation for the process intensification and miniaturization of the ethylene production process using bioethanol. [Copyright &y& Elsevier]