The hydrodynamics of liquid slugs in gas–liquid Taylor flow in straight and meandering microchannels have been studied using micro Particle Image Velocimetry. The results confirm a recirculation ...motion in the liquid slug, which is symmetrical about the center line of the channel for the straight geometry and more complex and three-dimensional in the meandering channel. An attempt has also been made to quantify and characterize this recirculation motion in these short liquid slugs (Ls/w<1.5) by evaluating the recirculation rate, velocity and time. The recirculation velocity was found to increase linearly with the two-phase superficial velocity UTP. The product of the liquid slug residence time and the recirculation rate is independent of UTP under the studied flow conditions. These results suggest that the amount of heat or mass transferred between a given liquid slug and its surroundings is independent of the total flow rate and determined principally by the characteristics of the liquid slug.
► Measurement of flow fields in gas–liquid Taylor flow in microchannels using microPIV. ► Determination of the extent of dead zones in the flow via the analysis of velocities. ► Quantification of liquid slug flow and determination of characteristic time scales.
Although a number of definitions of mixing have been proposed in the literature, no single definition accurately and clearly describes the full range of problems in the field of industrial mixing. An ...alternate approach is proposed which defines segregation as being composed of three separate dimensions. The first dimension is the intensity of segregation quantified by the normalized concentration variance (
CoV); the second dimension is the scale of segregation or clustering; and the last dimension is the exposure or the potential to reduce segregation. The first dimension focuses on the instantaneous concentration variance; the second on the instantaneous length scales in the mixing field; and the third on the driving force for change, i.e. the mixing time scale, or the instantaneous rate of reduction in segregation. With these three dimensions in hand, it is possible to speak more clearly about what is meant by the control of segregation in industrial mixing processes. In this paper, the three dimensions of segregation are presented and defined in the context of previous definitions of mixing, and then applied to a range of industrial mixing problems to test their accuracy and robustness.
•Intensified technologies are reviewed to transform waste cooking oil in fatty acid methyl esters.•Intensified technologies are compared on mass-transfer limited reactions.•The possible catalysts and ...the anticipation of the downstream separation steps are discussed.•A summary table is proposed to compare the different technologies.
This article reviews the intensification of fatty acid methyl esters (FAME) production from waste cooking oil (WCO) using innovative process equipment. In particular, it addresses the intensification of WCO feedstock transformation by transesterification, esterification and hydrolysis reactions. It also discusses catalyst choice and product separation. FAME production can be intensified via the use of a number of process equipment types, including as cavitational reactors, oscillatory baffled reactors, microwave reactors, reactive distillation, static mixers and microstructured reactors. Furthermore, continuous flow equipment that integrate both reaction and separation steps appear to be the best means for intensifying FAME production. Heterogeneous catalysts have also shown to provide attractive results in terms of reaction performance in certain equipment, such as microwave reactors and reactive distillation.
This article reviews existing methods for the characterisation of mixing and flow in microchannels, micromixers and microreactors. In particular, it analyses the current experimental techniques and ...methods available for characterising mixing and the associated phenomena in single and multiphase flow. The review shows that the majority of the experimental techniques used for characterising mixing and two-phase flow in microchannels employ optical methods, which require optical access to the flow, or off-line measurements. Indeed visual measurements are very important for the fundamental understanding of the physics of these flows and the rapid advances in optical measurement techniques, like confocal scanning laser microscopy and high resolution stereo micro particle image velocimetry, are now making full field data retrieval possible. However, integration of microchannel devices in industrial processes will require on-line measurements for process control that do not necessarily rely on optical techniques. Developments are being made in the areas of non-intrusive sensors, magnetic resonance techniques, ultrasonic spectroscopy and on-line flow through measurement cells. The advances made in these areas will certainly be of increasing interest in the future as microchannels are more frequently employed in continuous flow equipment for industrial applications.
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•A flow regime map for CO2-ethanol at 40 °C and for pressures up to 90 bar is presented.•CO2-ethanol flow regimes correlate with thermodynamic phase equilibrium.•Taylor bubble size ...increases linearly with UG/UL and follow models for ambient flows.•Bubble size decreases by gas dissolution until equilibrium is reached in the channel.•Relative bubble to unit cell length correlates with UG/UL, similar to ambient flows.
The hydrodynamics of gaseous and supercritical CO2-ethanol flows in microchannels under elevated pressure (45–90 bar) at 40 °C have been visualized experimentally. The effect of pressure and CO2-ethanol mixture composition on the flow regimes developed has been investigated and related to the thermodynamic phase equilibrium diagram. The characteristics of gas-liquid Taylor flow formed at elevated pressure have been analyzed and compared with Taylor flows at ambient conditions. The results of this study show that depending on the pressure and mixture composition, different flow regimes are formed, namely two-phase Taylor flow, dissolving Taylor flow, single-phase jetting-dissolving flow and single-phase supercritical jetting flow. The formation of these flows is explained with respect to the phase equilibrium diagram of CO2-ethanol. In the two-phase flow region, Taylor bubble size has been shown to depend on both CO2-ethanol mixture composition and operating pressure due to changes in CO2 mass flow rate and density, respectively. The bubble size formed has shown to increase linearly with UG/UL closely following the well-known bubble formation models for ambient conditions. Due to the non-negligible solubility of CO2 in ethanol, the Taylor bubble size decreases in the channel until equilibrium is reached. Mass transfer is not easily quantified however due to the mutually soluble nature of CO2 and ethanol and the dependency of this solubility on pressure.
The transformation of waste cooking oils for fatty acid methyl ester production is investigated in two intensified technologies: microstructured Corning
and oscillatory baffled NiTech
reactors, ...compared to a reference batch reactor to quantify the process intensification provided by each technology. Both reactors achieve high conversions in shorter times. For transesterification, 96 wt.% of esters are obtained in 1.4 min at 97°C in the Corning
reactor and 92.1 wt.% of esters in 6 min at 44°C in the NiTech
reactor, compared with 94.8 wt.% of esters in 10 min at 60°C in the batch reactor. For esterification, 92% conversion is obtained in 2.5 min in the Corning
reactor at 75°C compared with 20–30 min in the batch reactor at 60°C, and at 40°C, 96.8% conversion is achieved in 13.3 min in the NiTech
reactor, compared with 30 min in the batch reactor. The advantage of the Corning
reactor is that it can operate at higher pressures (1–20 bar) and temperatures (100°C), thereby providing faster kinetics than the NiTech
reactor. However, oils with a high free fatty acid level (73%) cause the Corning
reactor channels to be blocked. A wider range of operating conditions could be obtained in NiTech
with a pressure-resistant material.
The effect of fluid properties and operating conditions on the generation of gas–liquid Taylor flow in microchannels has been investigated experimentally and numerically. Visualisation experiments ...and 2D numerical simulations have been performed to study bubble and slug lengths, liquid film hold-up and bubble velocities. The results show that the bubble and slug lengths increase as a function of the gas and liquid flow rate ratios. The bubble and slug lengths follow the model developed by Garstecki et al. (Lab chip 6:437–446,
2006
) and van Steijn et al. (Chem Eng Sci 62:7505–7514,
2007
), however, the model coefficients appear to be dependent on the liquid properties and flow conditions in some cases. The ratio of the bubble velocity to superficial two-phase velocity is close to unity, which confirms a thin liquid film under the assumption of a stagnant liquid film. Numerical simulations confirm the hypothesis of a stagnant liquid film and provide information on the thickness of the liquid film.
This paper deals with the comparison of Eulerian methods to take into account the capillary contribution in the vicinity of a fluid–fluid interface. Eulerian methods are well-known to produce ...additional vorticity close to the interface that leads to non-physical spurious currents. Numerical equilibrium between pressure gradient and capillary force for the static bubble test case within a VOF framework has been reached in 35 with the height-function technique 14,35. However, once the bubble is translated in a uniform flow, spurious currents are maintained by slight errors induced by translation schemes. In this work, two main points are investigated: the ability of Volume of Fluid and Level Set methods to accurately calculate the curvature, and the magnitude of spurious currents due to errors in the calculation of the curvature after advection in both translating and rotating flows. The spurious currents source term is expressed from the vorticity equation and used to discuss and compare the methods. Simulations of gas–liquid Taylor flow at low capillary number show that the flow structure and the bubble velocity can be significantly affected by spurious currents.
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