The multiphase micro-structured devices such as microchannels have been recently taken into consideration to replace the conventional equipment in different applications. The mass transfer coefficients and the interfacial area in the multiphase microchannels are much larger than those in the conventional equipment. This paper reviews the experimental studies on mass transfer in gas-liquid and liquid-liquid microchannels. Proposed flow pattern maps are compared. The featured researches corresponding to the effects of the operating conditions and the geometry of the microchannels on mass transfer are discussed. Literature review shows that influences of geometrical parameters are not investigated as extensively as those of operating conditions and thus, vague aspects of geometrical parameters effects exist. There are inconsistencies between results of some researchers and most of these are due to using dissimilar microchannel materials, unalike dominant mechanisms at different investigated operating conditions, different approaches to change the independent variable, etc.

Mass transfer between phases in microchannels : A review

Continuous-flow microreactor technology has been embraced by researchers in academia and industry due to its excellent transport properties and the increased safety and control over challenging chemical transformations. Despite its popularity, scaling the benefits associated with the microenvironment has proven to be a daunting challenge. This review provides an up-to-date overview concerning the different scale-up approaches of micro/milli-reactors, including numbering up (both internal and external), sizing up (increasing length, geometry similarity and constant pressure drop strategies) and a combination of numbering up and sizing up. The strategies, design principles and applications of each approach have been discussed in detail. Each scale-up approach has its merits and limitations. However, scale-up factors, that are required in the fine chemical and pharmaceutical industry, are within reach when different scale-up approaches are combined.

Scale-up of micro- and milli-reactors : An overview of strategies, design principles and applications

The hydrodynamic properties of plug flow were investigated in small channels with 0.5-, 1-, and 2-mm internal diameter, for an ionic liquid/aqueous two-phase system with the aqueous phase forming the dispersed plugs. Bright field Particle Image Velocimetry combined with high-speed imaging were used to obtain plug length, velocity, and film thickness, and to acquire velocity profiles within the plugs. Plug length decreased with mixture velocity, while for constant mixture velocity it increased with channel size. Plug velocity increased with increasing mixture velocity and channel size. The film thickness was predicted reasonably well for Ca > 0.08 by Taylor's (Taylor, J Fluid Mech. 1961;10(2):161–165) model. A fully developed laminar profile was established in the central region of the plugs. Circulation times in the plugs decreased with increasing channel size. Pressure drop was predicted reasonably well by a modified literature model, using a new correlation for the film thickness derived from experimental values.

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Effect of channel size on liquid-liquid plug flow in small channels

Capillary microreactors operated under the slug flow regime were investigated for the separation of lactic acid from the aqueous phase using liquid–liquid reactive extraction. The experiments were performed at a 1:1 flow ratio of the aqueous to organic phases in a setup consisting of an inlet Y-type mixer connected with a poly(tetrafluoroethylene) capillary microreactor and subsequently an outlet Y-shape phase splitter. The extraction of lactic acid (intake: 0.11 and 0.055 M in water) using 15% (v/v) tri-n-octylamine in n-octanol under ambient conditions approached equilibrium after about 90 s in microreactors without noticeable emulsion formation. The measured reactive extraction performance in microreactors can be well described by a physical mass transfer model according to the penetration theory (developed from a model experimental study for the extraction of acetanilide from water to n-octanol) combined with an instantaneous irreversible reaction assumption.

/pdf/lactic-acid-extraction-and-mass-transfer-characteristics-in-1v17ti1m6t.pdf

Lactic Acid Extraction and Mass Transfer Characteristics in Slug Flow Capillary Microreactors

Intensified Eu(III) extraction using ionic liquids in small channels

An alkaline hydrolysis reaction was used as the model reaction to investigate the performance of liquid–liquid slug flow microchannel. The specific interfacial area was determined through the photographic snapshot method physically by means of measuring the lengths of relevant slugs. The overall volumetric mass transfer coefficients were calculated through the Danckwerts’ model chemically. The influences of various operating conditions on the slug length, the overall volumetric extraction rate and the mass transfer coefficient were investigated quantitatively. A decreasing trend of volumetric mass transfer coefficients along the channel length was found. The linear dependence of the volumetric extraction rate on the volumetric mass transfer coefficient indicates that the overall rate of the process is determined by the mass transfer process. In addition, the volumetric mass transfer coefficients were correlated for different channel lengths.

Mass transfer behavior of liquid–liquid slug flow in circular cross-section microchannel

Based on computational fluid dynamics (CFD), the liquid-liquid mass transfer in a slug flow through circular capillaries was studied. In order to save computational resources and time, the numerical simulations were carried out in a unit cell containing an aqueous slug and an organic one with a moving frame of reference. The flow field within individual slugs and interphase mass transfer with reactions between two consecutive slugs were discussed. Concentration fields were obtained to describe the mass transfer between the liquid phases. In addition, the effects of fluid diffusivity D on mass transfer coefficient kLa and the chemical species distribution were studied in detail. In general, the internal circulations inside individual slugs were displayed by the motion paths of mass transfer components. Furthermore, the mass transport process is enhanced by the alkaline hydrolysis reaction as confirmed by the so-called enhancement factor E. Finally, the simulation results showed close agreement with published experimental data.

Bujian Xu

Papers

Mass transfer behavior of liquid–liquid slug flow in circular cross-section microchannel

CFD simulation of mass transfer intensified by chemical reactions in slug flow microchannels