Segmented Gas-liquid Flow Research Articles

Gas–liquid two-phase flow in meandering microchannels

Due to the laminar flow in microfluidic devices, geometrical adaptations are required to enhance the transport across the center line of a microreactor. In gas–liquid segmented flow, the liquid slug between two gas bubbles creates two counter rotating vortices. Using μ-PIV measurements for microchannels with hydraulic diameters of DH = 150 μm and 200 μm, we show that this recirculating movement is very sensitive to the channel geometry. Our results show that the vorticity and swirling strength have the same characteristics before and directly after channel curve. We present the influence of the superficial velocities, channel diameter and curve radius on the recirculating motion. We demonstrate that for segmented gas–liquid flow, an increase of mass transfer over the complete channel diameter is possible by using meandering channels. The flow pattern for straight channels, characterized by two counter rotating vortices symmetrical to the channel center line, changes to two different sized vortices providing strong mass transfer over the center line. Considering these cognitions, we finally propose an optimized mixing design for liquid mixing in a gas–liquid two-phase flow.

A computational study of axial dispersion in segmented gas-liquid flow

Axial dispersion of a tracer in a two-dimensional gas-liquid flow is studied computationally using a finite-volume/front-tracking method. The effects of Peclet number, capillary number, and segment size are examined. At low Peclet numbers, the axial dispersion is mainly controlled by the convection through the liquid films between the bubbles and channel walls. In this regime, the computational results are found to be in a very good agreement with the existing model due to Pedersen and Horvath [Ind. Eng. Chem. Fundam. 20, 181 (1981)]. At high Peclet numbers, the axial dispersion is mainly controlled by the molecular diffusion, with some convective enhancement. In this regime, a new model is proposed and found to agree well with the computational results. These Peclet number regimes are shown to persist for different slug lengths. The axial dispersion is found to depend weakly on the capillary number in the diffusion-controlled regime. Finally, computational simulations are performed for the cases of six bubbles to mimic bubble trains, and results are compared with the theoretical models.

A Microfabricated Gas–Liquid Segmented Flow Reactor for High‐Temperature Synthesis: The Case of CdSe Quantum Dots

A Microfabricated Gas–Liquid Segmented Flow Reactor for High‐Temperature Synthesis: The Case of CdSe Quantum Dots

A Microfabricated Gas–Liquid Segmented Flow Reactor for High‐Temperature Synthesis: The Case of CdSe Quantum Dots

A Microfabricated Gas–Liquid Segmented Flow Reactor for High‐Temperature Synthesis: The Case of CdSe Quantum Dots

Cover Picture: A Microfabricated Gas–Liquid Segmented Flow Reactor for High‐Temperature Synthesis: The Case of CdSe Quantum Dots (Angew. Chem. Int. Ed. 34/2005)

The two‐phase flow of the gas–liquid segmented flow reactor with multiple temperature zones shown in the cover picture makes it possible to realize both rapid mixing of reagents and narrow residence‐time distributions. The reactor design allows the preparation of colloidal CdSe quantum dots (bottom right) with excellent size distributions at high temperature. For more details, see the Communication by K. F. Jensen, M. G. Bawendi, and co‐workers on page 5447 ff. (The authors thank Jane Rempel for assistance with the cover picture.)

Micromixing of Miscible Liquids in Segmented Gas−Liquid Flow

We present an integrated microfluidic system that achieves efficient mixing between two miscible liquid streams by introducing a gas phase, forming a segmented gas-liquid (slug) flow, and completely separating the mixed liquid and gas streams in a planar capillary separator. The recirculation motion associated with segmented flow enhances advection in straight microchannels without requiring additional fabrication steps. Instantaneous velocity fields are quantified by microscopic particle image velocimetry (muPIV). Velocities in the direction normal to the channel amount to approximately 30% of the bulk liquid velocity inside a liquid segment. This value depends only weakly on the length of a liquid segment. Spatial concentration fields and the extent of mixing (EOM) are obtained from pulsed-laser fluorescence microscopy and confocal scanning microscopy measurements. The mixing length is reduced 2-3-fold in comparison with previously reported chaotic micromixers that use three-dimensional microchannel networks or patterned walls. Segmented gas-liquid microflows allow mixing times to be varied over several orders of magnitude between milliseconds and second time scales.

Transport and reaction in microscale segmented gas-liquid flow.

We use micro particle image velocimetry (microPIV) and fluorescence microscopy techniques to characterize microscale segmented gas-liquid flow at low superficial velocities relevant for chemical reactions with residence times of up to several minutes. Different gas-liquid microfluidic channel networks of rectangular cross section are fabricated in poly(dimethylsiloxane) (PDMS) using soft lithography techniques. The recirculation motion in the liquid segments associated with gas-liquid flows as well as the symmetry characteristics of the recirculations are quantified for straight and meandering channel networks. Even minor surface roughness effects and the compressibility of the gas phase induce loss of symmetry and enhance mixing across the centerline in straight channels. Mixing is further accelerated in meandering channels by the periodic switching of recirculation patterns across the channel center. We demonstrate a new, piezoelectrically activated flow injection technique for determining residence time distributions (RTDs) of fluid elements in multiphase microfluidic systems. The results confirm a narrowed liquid phase RTD in segmented flows in comparison to their single-phase counterparts. The enhanced mixing and narrow RTD characteristics of segmented gas-liquid flows are applied to liquid mixing and in sol-gel synthesis of colloidal nanoparticles.

Scaling up of a monolithic catalyst reactor with two-phase flow

Scaling up of the monolithic three-phase reactor has been studied in the hydrogenation step of anthraquinones for large-scale production of hydrogen peroxide. The influence of flow pattern (bubble flow and slug flow), flow distribution, pressure drop, and mass transfer has been investigated. It is illustrated that the segmented gas-liquid flow (slug flow) gives the highest production rate with very small scale-up effects.

Mass transfer and selectivity in liquid-phase hydrogenation of nitro compounds in a monolithic catalyst reactor with segmented gas-liquid flow

Mass transfer and selectivity in liquid-phase hydrogenation of nitro compounds in a monolithic catalyst reactor with segmented gas-liquid flow

ChemInform Abstract: THE SEGMENTED TWO‐PHASE FLOW MONOLITHIC CATALYST REACTOR. AN ALTERNATIVE FOR LIQUID‐PHASE HYDROGENATIONS

AbstractDie FIüssigphasen‐Hydrierung von Nitrobenzoesäure wird mit einem monolithischen Palladium‐Katalysator durchgeführt.

The segmented two-phase flow monolithic catalyst reactor. An alternative for liquid-phase hydrogenations

Liquid-phase hydrogenation of nitrobenzoic acid was performed in a monolithic palladium catalyst. The total effectiveness factor of the catalyst was enhanced due to the fact that the mass transfer in the channels of the monolith was increased by the segmented gas-liquid flow. The effectiveness factor was shown to be much higher than for a trickle bed under corresponding reaction conditions. A mathematical model for the mass transfer in the segmented gas-liquid flow derived and found to describe the phenomena well.

Solid-liquid mass transfer in segmented gas-liquid flow through a capillary

Solid-liquid mass transfer in segmented gas-liquid flow through a capillary

Axial dispersion in a segmented gas-liquid flow

Axial dispersion of a tracer in liquid flow segmented by gas bubbles is investigated by numerical calculations using a lumped parameter model. Slow mixing in the liquid segments is shown to be the major factor in determining the magnitude of axial dispersion, measured by the second central moment of the tracer concentration distribution. Experimental results obtained at a variety of tracer inputs substantiate the model predictions. Additionally, convolution and deconvolution of concentration profiles at the output are discussed. 22 refs.