Taylor Flow Regime Research Articles

Packed‐Bed Microreactor Under Taylor Flow for MOFs Catalyst Testing Demonstrated on Phenylacetylene Hydrogenation

AbstractMetal‐Organic Frameworks (MOFs) represent a highly promising class of materials with diverse applications, particularly as catalytic materials. However, their synthesis typically yields powders available only at laboratory‐scale quantities, usually in the gram range or less. This study addresses the challenge of testing limited amounts of MOF catalysts for demanding applications, such as multiphase gas‐liquid‐solid reactions in flow, utilizing a packed‐bed microreactor. Specifically, we investigate the performance of a nanoparticle (NP) ‐immobilized Pd@UiO‐66 MOF catalyst in the selective semi‐hydrogenation of phenylacetylene to styrene, serving as a model reaction. Maintaining the Taylor flow regime upstream of the catalyst bed was crucial to ensure reproducible and reliable experimental results. We conducted 88 experiments at varying liquid flow rates, temperatures ranging from 15 to 45 °C, and relative pressures spanning 0.28 to 8 bar. Styrene selectivity within the range of 80–92.3 % was achieved at phenylacetylene conversions below 20 %. Notably, the optimal condition for styrene selectivity (70 %) was attained at 98.9 % phenylacetylene conversion under the lowest H2 pressure and highest temperature, demonstrating the significance of low H2 concentration for achieving optimal styrene selectivity. Remarkably, the catalyst exhibited stable activity and selectivity over a 20 h testing period, indicating its robust performance under prolonged reaction conditions. This study demonstrates the potential of the proposed catalyst testing system as a rapid and efficient approach for early‐stage exploration studies, particularly when limited quantities of catalyst, typically in the gram scale or less, are available.

Innovative fast-dynamic tool to characterize maldistribution in gas-liquid multi-channel reactors

In the field of structured reactors applied to multiphase reactions, the apparatus performance is related to the quality of the spatial distribution of fluids at the reactor inlet. This holds particularly true for monolith-type reactors since no flow redistribution is possible downstream in between the different parallel channels.In order to characterize the gas-liquid flows and their distribution in all the channels, a resistive sensor consisting in a printed circuit board with several annular electrodes has been developed, which can be scanned with high temporal resolution, of up to 20,000 Hz. This technique allows investigating flow features, such as gas holdup, bubble velocity, and bubble frequency, which are assessed in this work by shadowgraphy for a 12-channel monolith fed by air and water.Several treatment methods are evaluated, allowing the flow characteristics to be indirectly measured for Taylor flow regime conditions, with an accuracy of ±10% for bubble frequency and of ±20% for gas holdup and bubble velocity, with respect to the shadowgraphy. Thus, the innovative sensor can be used for industrial applications in order to distinguish the performance of gas-liquid distributors to provide uniform flow in multi-channel structured reactors.

Nopol Production Over Sn‐MCM‐41 Immobilized in a Wall‐Coated Microreactor using Formaldehyde Generated Ex‐Situ

AbstractThe kinetic study of the synthesis of nopol from β‐pinene and formaldehyde generated ex‐situ was carried out in a continuous flow process using Sn‐MCM‐41 wall‐coated microreactors of 1/8“ stainless steel tubes. The presence of Sn2+ and Sn4+ species was identified through XPS and TEM; the homogeneous distribution of tin on the support throughout the surface of the coating was verified by SEM analysis. Taylor flow regime and the kinetic control of the reaction were verified with hydrodynamic studies. A determination coefficient of 0.92 was obtained for the mathematical adjustment of the equation that described the reaction pathway obtained under the assumption of a dual‐site Langmuir‐Hinshelwood formalism with product desorption as the limiting step.

Mass transfer characteristics of ferrofluids flowing through a microchannel under AC magnetic field

The gas absorption process in a Y-shaped microchannel utilizing magnetic nanofluids stimulated within an alternating current magnetic field (AC mf) was studied. Absorption of CO2 under different operation conditions, ferrofluid concentration, and magnetic field strengths were investigated to analyze the liquid-side overall mass transfer coefficient (KLa), CO2 absorption efficiency (E), nanoparticle enhancement factor (α), magnetic field enhancement factor (γ), and pressure drop (Δp). The magnetic field strengths were 7, 12.6, and 21 mT accompanied by various ferrofluid concentrations within 0.001–0.004 (v/v). The ferrofluids were water-based colloids of Fe3O4 magnetic nanoparticles. The gas-liquid flow patterns were Taylor flow regime in all experiments. Furthermore, the position of the microchannel in the magnetic field was studied as an effective factor in CO2 absorption enhancement. The position in which the microchannel was held on the top inside the magnetic field reflected more gas absorption compared to the other positions. While exerting magnetic field strength of 21 mT, a maximum enhancement of 72.2% was observed for KLa compared to the case in which the magnetic field was absent.

Capillary Microreactor for Initial Screening of Three Amine-Based Solvents for CO2 Absorption, Desorption, and Foaming

Microreactor is a very attractive laboratory device for screening conditions and solvents in an efficient, safe and fast manner. Most reported work on microreactors for CO2 capturing deals with absorption and mass transfer performance with a limited number of studies on solvent regeneration. For the first time, foaming, which is a major operational challenge of CO2 capturing is being studied in combination with absorption and desorption in a capillary microreactor setup. To demonstrate the setup capabilities, three known amine-based solvents (MEA, MDEA, and AMP) were selected for the screening and evaluation studies. MEA had the highest CO2 absorption efficiency while MDEA had the lowest one. CO2 absorption efficiency increased with temperature, liquid flow rate, and amine concentration as per the literature. During the absorption work, the Taylor flow regime was maintained at the reactor inlet. CO2 desorption of loaded amine solutions was investigated at different concentrations and temperatures up to 85°C. MDEA solution had the highest desorption efficiency, followed by AMP and the least desorption efficiency was that of MEA. Foaming experimental results showed that MEA had a larger foaming region compared to AMP. However, more foaming happened with AMP at higher gas and liquid flow rates. A plug flow mathematical reactor model was developed to simulate the MEA-CO2 system. The model captured well the performance and trends of the studied system, however the absolute prediction deviated due to uncertainties in the used physical properties and mass transfer correlation. Selecting a solvent for chemical absorption depends on many more factors than these three studied parameters. Still, microreactor proves a valuable tool to generate experimental results under different conditions, with the least amount of consumables (less than 1 L solvents were used), in a fast manner, combined with a knowledge insight because of the uniqueness of the Taylor flow regime.

Investigation of two-phase flow regimes in square minichannels with different mixers created using additive technologies

In this paper, the effect of the junction angle on the Taylor flow regime characteristics was investigated. Using the Direct Light Processing method, minichannel mixers with different liquid and gas junction angles (from 30° to 150°) relative to the minichannel axis were manufactured. An experimental study of the two-phase flow formation was carried out, where Milli-Q® water was used as a working liquid, and high purity nitrogen was used as a working gas. A method of two-phase flow characteristic analysis using a high-speed shadow technique and automatic postprocessing was developed. Two-phase flow regimes were investigated in a wide range of gas and liquid flow rates. Size distribution of gas bubbles and liquid slugs and the distribution of bubble velocities were analyzed. The mechanisms of Taylor bubble formation and their characteristics, depending on the junction angle, were investigated.

Enhancement of gas-liquid mass transfer by nanofluids in a microchannel under Taylor flow regime

The gas-liquid two-phase flow and the enhancement of mass transfer by nanoparticles for the process of CO2 absorbed by SiO2 nanofluids were investigated in a microchannel under Taylor flow regime. The influences of the dispersed and continuous phase flow rates and particle concentration in slurry on liquid side volumetric mass transfer coefficient (KLa), mass transfer enhancement factor (E), CO2 absorption efficiency, and pressure drop were studied systematically. The results show that the presence of nanoparticles can effectively enhance the gas-liquid mass transfer. Both KLa and pressure drop increase, while the CO2 absorption efficiency decreases with increasing the gas-liquid flow rate ratio. Differently, KLa, CO2 absorption efficiency and pressure drop increase with the increase of particle concentration in slurry. Considering the energy consumption and mass transfer enhancement effect, the effective mass transfer enhancement efficiency is in the range of 1.2 ~ 2.4, which shows the excellent enhancement performance of nanoparticles on gas-liquid mass transfer.

Integration of a multi-enzyme system with a liquid membrane in Taylor flow regime for the production and in situ recovery of gluconic acid from cellulose

Since the demand for gluconic acid has increased during the last years, alternative processes to minimize its production costs associated to raw material and separation are being developed. In this study, a hybrid process consisting of gluconic acid production by a multi-enzyme system with an in situ gluconic acid recovery by a liquid membrane in Taylor flow is analyzed. Gluconic acid production was carried out from microcrystalline cellulose, using the following enzymes: cellulase, β-glucosidase, glucose oxidase and catalase. The liquid membrane in Taylor flow system was constituted by a membrane phase of trioctylamine/1-dodecanol, a receiving phase of sodium carbonate and the donor phase was the multi-enzyme broth. Enzymes did not show inhibitory effects by the organic phase of the liquid membrane. pH control by the liquid membrane in Taylor flow, as well as low ionic strength in the multi-enzyme broth, enhances gluconic acid production. Gluconic acid productivity and yield in the hybrid system were 1.6- and 1.7-fold higher than the values obtained using the multi-enzyme system with sodium acetate buffer, respectively. In situ gluconic acid recovery by the liquid membrane in Taylor flow system is a promising approach to enhance gluconic acid production by the multi-enzyme system.

Gas bubbles have controversial effects on Taylor flow electrochemistry

Electrochemistry is currently resurging in popularity amongst synthetic chemists due to the unique opportunities it provides to activate organic molecules. Simultaneously, continuous-flow technology has been used to enable scalability and to increase the efficiency of the developed electrochemical processes. Many of these processes involve a gaseous reagent or byproduct generated during the electrochemical process. The presence of a gas phase in flow reactors may lead to the generation of a so-called Taylor flow regime, where gas bubbles and liquid segments alternate. While Taylor flow has almost exclusive positive effects in flow chemistry due to increased mass and heat transfer, we show herein that the ramifications of gas bubbles on flow electrochemistry are essentially negative. Computational fluid dynamics (CFD) was used to gain a detailed understanding of the effects induced by the gas phase on the electrochemical process, taking the reduction of furfural to furfuryl alcohol carried out in an in-house developed electrochemical reactor as benchmark. We show that the gas bubble presents a local situation with infinite electrical resistance leading to a temporary passivation of the electroactive surface, while its presence also intensifies the mixing in the liquid slug reducing mass transfer limitations. Essentially, the larger the bubble, the higher the energy losses become and the less efficient the reactor is used. This results in a higher overall energy consumption for the electrochemical process. Moreover, we investigated the residence time distribution in the liquid slug, and the effect of different operational conditions (bubble size, gas holdup, interelectrode distance, electrolyte velocity and species concentration) on the overpotential and current density, providing guidelines for reactor design and operation. Based on the results described herein, we also discuss potential solutions to increase the efficiency of the electrochemical flow reactor.

Integration of a liquid membrane in Taylor flow regime with a fermentation by Lactobacillus casei ATCC 393 for in-situ lactic acid removal

A new type of liquid membranes called liquid membrane in Taylor flow was integrated to a lactic acid fermentation, using Lactobacillus casei ATCC 393, for lactic acid removal during fermentation. The performance in terms of lactic acid production of the hybrid batch system is compared to a conventional batch fermentation. Lactic acid removal rate increases proportionally with the LA concentration within the fermenter. The lactic acid, the biomass production and the LA productivity in the hybrid system increased by 41.8, 12 and 26.6%, respectively, as compared to the conventional batch fermentation. However, toxicity effects reduce LA to glucose yield in 15.9% as compared to conventional fermentation. Liquid membranes in Taylor flow results promising for enhancing batch and continuous fermentation processes by a hybrid system.

Furfural Production by Continuous Reactive Extraction in a Millireactor under the Taylor Flow Regime

This study demonstrates the use of a millireactor as intensified technology for the continuous production of furfural via acid dehydration of xylose in a biphasic media. Very rapid extraction of fu...

A new concept of liquid membranes in Taylor flow: Performance for lactic acid removal

A liquid membrane in Taylor flow regime is a novel alternative kind of contact in three-phase flow for liquid membranes that preserves the advantages of conventional emulsion liquid membranes while overcomes the stability problems of emulsion systems. As a proof of concept, this work presents experimental results of a liquid membrane in Taylor flow for lactic acid removal. Several operating conditions, such as injection times, delay times and flow of the membrane phase were tested for a channel length and inner diameter of 348.8 cm and 2.5 mm, respectively. The lactic acid removal is mainly affected by the driving force of lactic acid concentrations between donor droplets and the membrane interface, and the space-time. Thus, the lactic acid removal process through the liquid membrane in Taylor flow is enhanced at low injection times and high droplet velocity considering that enough space-time is provided. This technology results promising as an alternative to conventional liquid membranes and the intensification of chemical and fermentative processes.

Integration of phase distribution from gamma-ray tomography technique with monolith reactor scale modeling

In this work, a monolith reactor model was developed to study the effect of phase distribution on the performance of the monolith reactors by integrating phase distribution data from the experiments. To obtain the phase distribution at the reactions conditions, gamma ray computed tomography (CT) was used. Effect of gas density, surface tension of liquid and operating conditions on the phase distribution were studied using gamma-ray computed tomography. Experiments were conducted in Taylor flow regime. With the increase in gas density, uniformity increases. Surface tension has little effect on the distribution in the investigated conditions. Liquid with lower surface tension and gas with lower density has higher cross-sectional liquid saturation. Further, the monolith reactor model with uniform phase distribution and actual distribution were compared. It was found that at higher velocities both gives the same reactor performance irrespective of the maldistribution, however at low velocities, they differ significantly due to the maldistribution. If the maldistribution is present at both low and high velocities, catalyst utilization plays a major role, thus it is recommended to operate at higher velocities where catalyst utilization is high due to high mass transfer.

Study of overall mass transfer coefficients in a liquid membrane in Taylor flow regime: Calculation and correlation

The use of a liquid membrane in Taylor flow regime is a recent technology, which extends and generalizes the definition of a membrane. It has been developed and tested for lactic acid removal. A challenge in understanding the technology is that the values of the overall volumetric mass transfer coefficients are not known, and it is unclear how they are related with the operational conditions. In this work, the overall volumetric mass transfer for the liquid membrane in Taylor flow was calculated from experimental results and three empirical models, two of which are from literature and one was developed in this work based on dimensional analysis. From combination of experimental results and the developed models the main variables (operational conditions) of the liquid membrane in Taylor flow that have a strong influence on the overall volumetric mass transfer coefficients (in the donor and in the membrane phase) were defined. The relative velocity results as the variable that has the largest influence on the performance of this new liquid membrane technology.

Batch to continuous photocatalytic degradation of phenol using TiO2 and Au-Pd nanoparticles supported on TiO2

A series of Au-Pd/TiO2 catalysts were synthesized in different weight % using sol-immobilization method. Of the range studied 1%Pd/TiO2 catalyst achieved 86.4% conversion of phenol to CO2 in a standard batch-slurry system utilizing UV. However under recycle or continuous operation Pd leaching from catalyst surface led to gradual deactivation. Au-Pd nanoparticles supported on TiO2 P25 were stable and recyclable, here Au species were found to help to anchor Pd species on TiO2, and no observable Pd leaching occurred. Utilizing UV, 1%Pd/TiO2 showed faster rate of phenol degradation in comparison to Au-Pd/TiO2, while 1%Au/TiO2 and 0.5%Au-0.5%Pd/TiO2 showed faster phenol degradation rates under visible light. The TiO2 P25 support was also found to be active, stable and recyclable in phenol degradation utilizing UV; and was hence considered suitable for continuous operation. However poor oxygen mass transfer led to the formation and lay-down of polymeric species when using a Trickle bed approach. Operation in the Taylor flow regime was demonstrated to increase oxygen saturation and significantly reduced deactivation. Hence continuous photocatalytic degradation of phenol could be achieved using TiO2 under Taylor Flow conditions.

Formation mechanisms and lengths of the bubbles and liquid slugs in a coaxial-spherical micro mixer in Taylor flow regime

One of the most important problems of the efficiency of mini and micro devices (with diameter larger than ∼1 mm) for gas-liquid systems is the maintenance of regular Taylor (slug) flow regime. The paper describes a coaxial-spherical mixer (CS-mixer), which allows to create a slug flow regime in a wide range of phase flow rates. For CS-mixer a flow map was build up and superimposed to that for C-mixer.It was shown experimentally with 2 mm ID microchannel that the position of the nozzle for the injection of the gas relative to the spherical extension of CS-mixer significantly affects the lengths of the bubbles and the slugs generated in the micromixer. Two nozzle positions in the narrowing part of spherical extension were considered as optimal from the point of view of uniformity of slug flow.For nozzle position D (middle cross-section), which was studied more detailed in this research, equations for bubbles and slugs lengths were obtained. Values of bubbles and slugs lengths calculated by these equations were compared with data of other authors for different types of micro-mixers. It was shown that CS-mixer allows to control the length of the bubbles and to generate bubbles with smaller length. The size of the slugs produced by CS-mixer demonstrated weaker dependence on superficial liquid flow rate compared to the other types of micromixers. Unexpected (for other types of micromixers) behavior of two-phase flow in microchannel by use of CS-mixer was observed: with growing gas superficial velocity the length of the bubbles is decreasing. Such behavior was explained by particular effects of bubble detachment from the nozzle during its formation in the spherical extension of CS-mixer.

Hydrodynamic shrinkage of liquid CO2 Taylor drops in a straight microchannel

Hydrodynamic shrinkage of liquid CO2 drops in water under a Taylor flow regime is studied using a straight microchannel (length/width ~100). A general form of a mathematical model of the solvent-side mass transfer coefficient (ks) is developed first. Based on formulations of the surface area (A) and the volume (V) of a general Taylor drop in a rectangular microchannel, a specific form of ks is derived. Drop length and speed are experimentally measured at three specified positions of the straight channel, namely, immediately after drop generation (position 1), the midpoint of the channel (position 2) and the end of the channel (position 3). The reductions of drop length (Lx, x = 1, 2, 3) from position 1 to 2 and down to 3 are used to quantify the drop shrinkage. Using the specific model, ks is calculated mainly based on Lx and drop flowing time (t). Results show that smaller CO2 drops produced by lower flow rate ratios () are generally characterized by higher (nearly three times) ks and Sherwood numbers than those produced by higher , which is essentially attributed to the larger effective portion of the smaller drop contributing in the mass transfer under same levels of the flowing time and the surface-to-volume ratio (~104 m−1) of all drops. Based on calculated pressure drops of the segmented flow in microchannel, the Peng–Robinson equation of state and initial pressures of drops at the T-junction in experiments, overall pressure drop (ΔPt) in the straight channel as well as the resulted drop volume change are quantified. ΔPt from position 1–3 is by average 3.175 kPa with a ~1.6% standard error, which only leads to relative drop volume changes of 0.3‰ to 0.52‰.

Comparison of a monolith and a confined Taylor flow (CTF) reactor for propene epoxidation

Heterogeneous catalytic epoxidation of propene to propene oxide with hydrogen peroxide was investigated in a monolith and a confined Taylor flow (CTF) reactor in which titanium silicalite (TS-1) catalyst was coated on the walls. The influence of gas and liquid superficial velocity on the hydrodynamic characteristics of the monolith and CTF reactor was also investigated under Taylor flow regime at atmospheric and high pressure. The reactors showed distinctly different hydrodynamic properties which in turn led to different performance for propene epoxidation. The production rate of propene oxide was higher in the monolith reactor due to its larger catalyst coating area, larger mass-transfer surface area and more frequent recycling of liquid flow. A variation of reactor column structures confirmed that the propene oxide production was highly dependent on the catalyst coating area and cross-sectional area of the reactor column. High operating pressure made a significant impact on the length of Taylor bubbles and the propene oxide production rate was found to increase in proportion to the operating pressure.

Epoxidation of propene in a confined Taylor flow (CTF) reactor at atmospheric pressure

Heterogeneous catalytic epoxidation of propene to propene oxide with hydrogen peroxide as oxidant was investigated in a confined Taylor flow (CTF) reactor, a continuous monolith reactor, containing a long alumina rod coated with titanium silicalite (TS-1) catalyst in the centre of the reactor column. The effect of gas and liquid superficial velocity on the hydrodynamics of CTF reactor was also investigated under Taylor flow regime at atmospheric pressure. The variation of hydrodynamics had a profound impact on the production of propene oxide. When liquid superficial velocity was constant, the concentration of propene oxide produced decreased with increasing gas superficial velocity as Taylor bubble length and bubble rise velocity increase. However, when gas superficial velocity was constant, the concentration of propene oxide produced had no linear dependency on liquid superficial velocity as Taylor bubble length decreases but bubble rise velocity increases.

Axial slit wall effect on the flow instability and heat transfer in rotating concentric cylinders

The slit wall effect on the flow instability and heat transfer characteristics in Taylor-Couette flow was numerically studied by changing the rotating Reynolds number and applying the negative temperature gradient. The concentric cylinders with slit wall are seen in many rotating machineries. Six different models with the slit number 0, 6, 9, 12, 15 and 18 were investigated in this study. The results show the axial slit wall enhances the Taylor vortex flow and suppresses the azimuthal variation of wavy Taylor vortex flow. When negative temperature gradient exists, the results show that the heat transfer augmentation appears from laminar Taylor vortex to turbulent Taylor flow regime. The heat transfer enhancement become stronger as increasing the Reynolds number and slit number. The larger slit number model also accelerates the flow transition regardless of the negative temperature gradient or isothermal condition.