Showing papers on "Slug flow published in 2016"

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

Abstract: 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.

Chemical conversion and liquid–liquid extraction of 5‐hydroxymethylfurfural from fructose by slug flow microreactor

Abstract: A dehydration of fructose in the water/methyl isobuthyl ketone (MIBK) biphasic system can yield 5-hydroxymethylfurfural (HMF) to be successfully extracted into the organic MIBK phase. The HMF production and yield in MIBK phase was discussed by using a simplified model taking into consideration of the slug flow. The extraction resistance of HMF across the interface between water and MIBK depended on the line velocity and the flow rate ratio. It was likely that the velocity field generated in the slug flow contributed to an increase in the mass transfer of HMF. © 2016 American Institute of Chemical Engineers AIChE J, 62: 2135–2143, 2016

Comparison of liquid-liquid extraction in batch systems and micro-channels

Abstract: Liquid-liquid extraction is an important mass transfer operation in the chemical, food processing and pharmaceutical industries. Our work focuses on experimentally quantifying mass transfer performance in a well stirred batch vessel and in stratified flow and slug flow in micro-channels. Extraction of propionic acid from toluene to water is chosen as a test system. The distribution of propionic acid in toluene and water at equilibrium was found to be non-linear. The batch experiments were carried out both with a flat interface and as a well-mixed dispersed system. The continuous experiments were carried out in micro-channels with a square cross-section. A lumped parameter model was used to quantify the mass transfer coefficient in batch mode. A lumped (distributed) model was used to describe the extraction in a slug (stratified) flow in a micro-channel. The overall extraction performance is characterized as a function of residence time of the phases and the hydrodynamics. It was found that the micro-channel gives a superior extraction performance in the slug flow regime as compared to the well mixed batch system although both have comparable sizes of the dispersed phase. This is attributed to the strong internal circulations induced by shear in the slug flow regime.

Computational Study of Saturated Flow Boiling Within a Microchannel in the Slug Flow Regime

Abstract: This paper presents a fundamental study of the flow dynamics and heat transfer induced by a slug flow under saturated flow boiling in a circular microchannel. Numerical simulations are carried out by utilizing the commercial CFD solver ANSYS FLUENT v. 14.5, with its built-in volume of fluid (VOF) method to advect the interface, which was improved here by implementing self-developed functions to model the phase change and the surface tension force. A continuous stream of bubbles is generated (by additional user-defined functions) by patching vapor bubbles at the channel upstream with a constant generation frequency. This modeling framework can capture the essential features of heat transfer in slug flows for a continuous stream of bubbles which are here investigated in detail, e.g., the mutual influence among the growing bubbles, the fluid mechanics in the liquid slug trapped between two consecutive bubbles, the effect of bubble acceleration on the thickness of the thin liquid film trapped against the channel wall and on other bubbles, and the transient growth of the heat transfer coefficient and then its periodic variation at the terminal steady-periodic regime, which is reached after the transit of a few bubble-liquid slug pairs. Furthermore, the results for a continuous stream of bubbles are found to be quite different than that of a single bubble, emphasizing the importance of modeling multiple bubbles to study this process. Finally, the outcomes of this analysis are utilized to advance a theoretical model for heat transfer in microchannel slug flow that best reproduces the present simulation data.

Investigation of the Dynamic Contact Angle Using a Direct Numerical Simulation Method.

Abstract: A large amount of residual oil, which exists as isolated oil slugs, remains trapped in reservoirs after water flooding. Numerous numerical studies are performed to investigate the fundamental flow mechanism of oil slugs to improve flooding efficiency. Dynamic contact angle models are usually introduced to simulate an accurate contact angle and meniscus displacement of oil slugs under a high capillary number. Nevertheless, in the oil slug flow simulation process, it is unnecessary to introduce the dynamic contact angle model because of a negligible change in the meniscus displacement after using the dynamic contact angle model when the capillary number is small. Therefore, a critical capillary number should be introduced to judge whether the dynamic contact model should be incorporated into simulations. In this study, a direct numerical simulation method is employed to simulate the oil slug flow in a capillary tube at the pore scale. The position of the interface between water and the oil slug is determine...

A New Void Fraction Measurement Method for Gas-Liquid Two-Phase Flow in Small Channels.

Abstract: Based on a laser diode, a 12 × 6 photodiode array sensor, and machine learning techniques, a new void fraction measurement method for gas-liquid two-phase flow in small channels is proposed. To overcome the influence of flow pattern on the void fraction measurement, the flow pattern of the two-phase flow is firstly identified by Fisher Discriminant Analysis (FDA). Then, according to the identification result, a relevant void fraction measurement model which is developed by Support Vector Machine (SVM) is selected to implement the void fraction measurement. A void fraction measurement system for the two-phase flow is developed and experiments are carried out in four different small channels. Four typical flow patterns (including bubble flow, slug flow, stratified flow and annular flow) are investigated. The experimental results show that the development of the measurement system is successful. The proposed void fraction measurement method is effective and the void fraction measurement accuracy is satisfactory. Compared with the conventional laser measurement systems using standard laser sources, the developed measurement system has the advantages of low cost and simple structure. Compared with the conventional void fraction measurement methods, the proposed method overcomes the influence of flow pattern on the void fraction measurement. This work also provides a good example of using low-cost laser diode as a competent replacement of the expensive standard laser source and hence implementing the parameter measurement of gas-liquid two-phase flow. The research results can be a useful reference for other researchers’ works.

Pressure drop of three-phase liquid–liquid–gas slug flow in round microchannels

Abstract: In this paper we present a model for the calculation of pressure drop of three-phase liquid–liquid–gas slug flow in microcapillaries of a circular cross section. Introduced models consist of terms attributing for frictional and interfacial pressure drop, incorporating the presence of a stagnant thin film at the wall of the channel. Different formulations of the interfacial pressure drop equation were employed, using expressions developed by Bretherton (J Fluid Mech 10:166–188, 1961), Warnier et al. (Microfluid Nanofluid 8:33–45, 2010) or Ratulowski and Chang (Phys Fluids A 1:1642–1655, 1989). Models were validated experimentally using oleic acid–water–nitrogen and heptane–water–nitrogen three-phase flows in round Teflon or Radel R microchannels of 254- and 508-µm nominal inner diameter, for capillary numbers Ca b between 10−4 and 4.9 × 10−1 and Reynolds numbers Re between 0.095 and 300. Best agreement between measured and calculated values of pressure drop, with relative error between −22 and 19 % or −20 and 16 %, is reached for Warnier’s or Ratulowski and Chang’s interfacial pressure drop equation, respectively. The results prove that three-phase slug flow pressure drop can be successfully predicted by extending existing two-phase slug flow correlations. Good agreement of Bretherton’s equation was reached only at lower Ca numbers, indicating that an extension of the interfacial pressure drop equation as performed by Warnier et al. (Microfluid Nanofluid 8:33–45, 2010) or Ratulowski and Chang (Phys Fluids A 1:1642–1655, 1989) for higher capillary numbers is necessary. Additionally it was demonstrated that pressure drop increases substantially if dry slug flow occurs or if microchannels with significant surface roughness are employed. Those influences were not accounted for in the models presented.

Microreactor Mixing-Unit Design for Fast Liquid-Liquid Reactions

Abstract: Based on previous work studying complex microreactors, it was desired to further improve the mixing efficiency by varying the mixing unit design for fast liquid-liquid reactions. Different flow regimes were studied, including slug flow, parallel flow, and drop flow. The two-phase hydrolysis of 4-nitrophenyl acetate in sodium hydroxide solution was used to evaluate the overall volumetric mass transfer coefficients (Korga) as a function of the average rate of energy dissipation (e) for each microreactor design and all flow regimes. The liquid-liquid systems investigated used n-butanol or toluene as the organic phase solvent and a 0.5-M NaOH aqueous solution. The use of surfactant was also investigated with the toluene- water system. All microreactor geometry designs were based on contraction–expansion repeating units with asymmetric obstacles to aid the breakup of slugs and desynchronize the recombination of split streams. The investigated designs were chosen to avoid the formation of the parallel flow regime, contrary to curvature-based mixing-unit designs. The microreactor design can then be optimized to reduce the e required to reach drop flow, since Korga has been found to be constant at equal e for a given solvent system in this flow regime, regardless of the reactor selection. Additionally, the “3/7th” scaleup rule was applied and confirmed with the LL-Triangle mixer. It was found that, for low interfacial-tension systems (i.e., n-butanol-water), the onset of drop flow occurred at a lower e for the LL-Triangle mixer when compared with the Sickle or LL-Rhombus mixers.