Search

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]

• The effect of liquid viscosity (0.89–45.6 mPa·s) on Taylor flow is investigated. • Higher viscosity leads to more stable instantaneous flow rate during formation stage. • The film thickness at short planes are much larger than predicted in literature. • A distinct bubble shape with sharpened rear cap is observed. • A correlation representing the effect of recirculation on pressure drop is proposed. Characteristics of gas-liquid Taylor flow with different liquid viscosities are investigated in a rectangular microchannel, aiming at providing knowledge and aid in the design of processes involving viscous fluid, such as polymers and ionic liquids. The effect of liquid viscosity on the bubble formation dynamic, film thickness, bubble velocity, and pressure drop is investigated. The results reveal a specific viscous effect compared to those in square or circular channels. For the same capillary number, both the liquid film thickness at the corners and at the short planes are much larger than in square channels. New correlations are proposed for predicting the film thickness in the rectangular channel. The bubble shape sheared by the liquid phase is also distinct from literature observations that a smaller radius occurs at the rear cap. For the conditions studied (0.00065 < Ca < 0.0525), increasing the viscosity leads to an increase in the instantaneous flow rate, and also an later shift from filling stage to squeezing stage. As the bubble formation is driven by both squeezing pressure and shearing force, the bubble/slug length is affected by both capillary number and flow rate ratio. The recirculation inside liquid slugs is found to play an important role in the pressure drop, which can be well described by an empirical correlation including dimensionless liquid slug length and capillary number. [ABSTRACT FROM AUTHOR]

• Efficient glucose-to-FA conversion was achieved in a continuous flow microreactor. • Possible intermediates were extensively examined while DFT was used to calculate different C–C bond cleavage energies of glucose and fructose. • Plausible reaction pathways were proposed for the conversion of glucose to FA catalyzed by HPA-2. • Sources of CO 2 influencing the FA yield were discovered for the conversion of glucose catalyzed by HPA-2. The process of converting glucose into formic acid (FA) has been explored as a promising route for using biomass as a renewable feedstock for green chemical manufacturing. However, the most commonly used batch-tank reactor systems currently suffer from several drawbacks, including extended processing time, limited product selectivity, and high energy consumption, while the underlying reaction paths have been poorly understood. In this work, a continuous flow microchannel reactor was employed for the glucose conversion catalyzed by H 5 PV 2 Mo 10 O 40 with O 2 as an oxidant. The influence of various parameters on the conversion rate of glucose and the FA yield was characterized. Experimental results indicated that with a residence time of less than 3 min, the glucose was completely converted, and the highest FA yield reached 82.40 %. Extensive examination on the apparent by-products revealed that most of them acted as intermediates to produce FA through various pathways, including glyoxal, glyceraldehyde and glycolaldehyde as key intermediates for the oxidation of glucose to FA. Furthermore, the density functional theory (DFT) method was used to determine the bond energies of different C–C bond cleavage modes of substrate glucose and fructose produced by the isomerization of glucose. Experimentally, the conversion of three main intermediates and some other possible intermediates and their FA yield were measured with different residence time. It was also found that the CO 2 was produced through the decarboxylation of α-hydroxy and α-carbonyl carboxylic acid compounds, while the aldehyde groups in the compounds were more likely converted to FA by the α-carbon bond cleavage. Finally, plausible reaction pathways were proposed for the process of glucose-to-FA catalyzed by HPA-2, providing useful guidance for the identification of side reaction pathways and further improvement of FA yield. [ABSTRACT FROM AUTHOR]

• The flow distributions of 16-, 8-, 4-, 2-channels microreactors were investigated and compared. • The pressure drops of microreactors were investigated by experimental, empirical and numerical methods. • Increasing the channel length after the bifurcation point is beneficial to the flow distribution. • Incorporating a bypass pressure equalization chamber at the bifurcation point is in favor of the flow distribution. In this work, the flow distributions of 16-, 8-, 4-, 2-channels microreactors with 2-D constructal inlet distributors were measured. The results showed an increase in channel numbers caused a decreasing uniformity of the flow distribution. Both asymmetrical velocity profiles before a bifurcation point and asymmetrical flow resistance after a bifurcation point played an important role on the flow distribution in microchannels. The uniformity of flow distribution showed a trend of firstly increasing and then remaining unchanged with the increase of Re (4 ∼ 680). The pressure drops of microreactors were also investigated and the empirical method is in the acceptable range for estimating the energy consumption. Extending the channel length after bifurcation point from 124.46 mm to 160.95 mm and incorporating a bypass pressure equalization chamber at bifurcation point could improve the uniformity of flow distribution. [ABSTRACT FROM AUTHOR]

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]

The present work focuses on the hydrodynamics variation and mass transfer characteristics of Taylor flow along long serpentine microchannels with a square cross-section. The volumetric mass transfer coefficient ( k L a ) is regarded as the transient change value to characterize the gas-liquid mass transfer process of CO 2 in water. All experimental data of Taylor bubble are obtained from 1000 continuously captured images. An online high-speed imaging method and the unit cell model are adopted in this study. The effects of gas and liquid flow rates, together with microchannel geometry are investigated on Taylor bubble characteristics in terms of length, velocity and the mass transfer performance. Taylor bubble length shrinks and subsequently plateaus out along the flow direction from the T-junction, resulting in the decrease in Taylor bubble velocity. k L a in a unit cell gradually decreases along the serpentine microchannel, and increases as the channel cross-sectional area decreases. As the gas flow rate increases under a given liquid flow rate, a critical point is found for the evolution of k L a and k L (that is the liquid phase mass transfer coefficient). The results indicate that the contribution of the circulation in the liquid slug to k L is dominant before the critical point compared to the leakage flow in the liquid film. All these findings in this work give important information to understand the dynamic change in gas-liquid Taylor flow mass transfer within microchannels. They will serve as basis for designing and optimizing gas-liquid multiphase microreactors in the future. [ABSTRACT FROM AUTHOR]

• A gas–liquid-solid multiphase catalytic oxidation process was regulated in a microchannel reactor under Taylor flow. • A theoretical model was developed for quantifying the variation of a single droplet volume in the microreactor. • The reaction characteristics in O/W Pickering emulsions in microchannels were revealed. • The interfacial catalytic mechanism of Pickering emulsions in microchannels was elucidated through multiphase mass transfer analysis. There are still technical challenges associated with surface fouling and clogging in microchannels when conducting multiphase catalytic reactions involving solid particles. In this study, a Pickering emulsion was employed to enhance the oxidation of benzyl alcohol (BNOH) catalyzed by Pd/SiO 2 nanoparticles under Taylor flow in a microchannel reactor. The reaction characteristics of benzyl alcohol/water Pickering emulsion in a microchannel reactor were studied experimentally. The characteristic results showed that the O/W Picking emulsion system was superior to the W/O Picking emulsion system for the catalytic oxidation reaction. The influences of a wide range of operational variables were characterized on the conversion of reactants and the selectivity of the desired product. The optimized reaction conditions included: reaction temperature 403 K, CB 5 wt%, O 2 /BNOH (mol/mol) 0.76 and tube length 8 m. Under the optimal operating conditions, the conversion of BNOH and the selectivity of benzaldehyde were found to be 86.53% and 99.79%, respectively. In addition, the interfacial transport-reaction mechanism in Pickering emulsions under Taylor flow in microchannels was proposed from the perspective of multiphase mass transfer. This development provided significant insights and guiding for effectively performing multiphase catalytic reactions involving solid catalyst particles in microchannel reactors. [ABSTRACT FROM AUTHOR]

[Display omitted] • The synthesis of Acifluorfen by continuous flow nitration was performed within droplet-based microreactor. • The effects of operating parameters were characterized by hydrodynamics and thermodynamics. • The apparent and intrinsic kinetic model was established. Aromatic nitration still faces unmet technical challenges in relation to being heterogeneous and highly exothermic. In this work, a continuous flow aromatic nitration was developed with mixed acid within droplet-based microreactors. The effects of key operating parameters were characterized on the nitration of 3-[2-chloro-4-(trifluoromethyl)phenoxy] benzoic acid. The optimized reaction temperature, M-ratio and N/S were found to be 308 K, 1.6 and 0.57, respectively. Under these conditions, a conversion of 83.03% and selectivity of 79.52% were achieved. The addition of acetic anhydride improved both the solubility of etherate in the organic phase and the absorbability of water produced from the reaction, leading to increase in conversion, however increased by-products accordingly with reduced selectivity. Pseudo-homogeneous kinetics models were developed of apparent and intrinsic reaction rate constant, supporting the second-order reaction assumption. The calculated results based on the kinetics model were found to be in good agreement with the experimental results. [ABSTRACT FROM AUTHOR]

• The effects of both μ c and μ d on slug flow are investigated. • The slug flow occurrence is predicted based on composite dimensionless numbers. • For squeezing regime, the droplet scaling law is different from the classical theory. • The film thickness increased more rapidly with Ca D than in gas–liquid slug flow. • Correlations were proposed to reflect the effect of recirculations in both phases on the pressure drop. The hydrodynamics of liquid–liquid slug flow with various fluid systems were investigated, aiming at revealing the viscous effects of both phases. The operation range, droplet formation, film thickness and pressure drop of slug flow were studied and discussed. The results reveal the importance of inertia and viscous forces of both phases in the transition of flow patterns. Accordingly, a universal flow map based on composite terms of Ca c Re c 0.5 and Ca d Re d 0.5 was adopted, which worked well for a wide viscosity range. For droplet formation in the present system, both the squeezing and shearing regimes deviated from the classical ideal models. Under higher capillary number, the film was accumulated at the corners and lateral planes, leading to distinct droplet shapes from literature. For the two-phase pressure drop, new empirical correlations were proposed to describe the recirculation inside both the droplets and slugs. The findings can provide better guidance for optimized reactor design. [ABSTRACT FROM AUTHOR]