Segmented Gas-liquid Flow Research Articles

Modularization of Immobilized Multienzyme Cascades for Continuous-Flow Enantioselective C-H Amination.

Multienzyme cascades (MECs) have gained much attention in synthetic chemistry but remain far from being a reliable synthetic tool. Here we report a four-enzyme cascade comprising a cofactor-independent and a cofactor self-sustaining bienzymatic modules for the enantioselective benzylic C-H amination of arylalkanes, a challenging transformation from bulk chemicals to high value-added chiral amines. The two modules were subsequently optimized by enzyme co-immobilization with microenvironmental tuning, and finally integrated in a gas-liquid segmented flow system, resulting in simultaneous improvements in enzyme performance, mass transfer, system compatibility, and productivity. The flow system enabled continuous C-H amination of arylalkanes (up to 100 mM) utilizing the sole cofactor NADH (0.5 mM) in >90 % conversion, achieving a high space-time yield (STY) of 3.6 g ⋅ L-1 ⋅ h-1, which is a 90-fold increase over the highest value previously reported.

Investigation of mass transfer in valve-controlled gas–liquid segmented flow

Segment flow is widely employed in gas–liquid microreactor systems owing to its enhanced mass transfer through the biphasic interface. Previous studies evaluating an increased fluid flow rate (F) have reported an increased volumetric mass transfer coefficient (kLa) and reduced segment length (L). In a conventional microsystem, manipulating L without varying the residence time of the fluids is challenging. Hence, the relationship between kLa and L has not been systemically investigated under the same F. In this study, L control without changing F is implemented by introducing a high-speed valve into a gas–liquid microchannel system, and the influence of L on kLa is investigated by altering the valve frequency (f). The results indicate that kLa in the segmented flow is negatively correlated with L at constant F and reaches 0.149 s–1 (332.6 times larger than that in the bubble), whereas the specific area (a) of the segmented flow increases with increasing f before the liquid phase is saturated. Moreover, a and instinct kLa can be predicted using f, F, and the microchannel diameter. The outcomes of this research will improve the understanding of the mass transfer phenomenon in gas–liquid segmented flows and provide support for the design of microreactor systems.

Hydrophobic air pollutants removal at one second gas contact in a multi-channel capillary bioreactor

Biological processes are increasingly applied for gas purification as a sustainable and economical alternative to conventional physical-chemical processes (chemical absorption, incineration, adsorption). Although biological gas treatment is accepted as an economical, safe, and reliable air pollution control technology, it faces important limitations when applied for the treatment of poorly water-soluble compounds due to mass transfer limitations. A twenty-five capillary channels bioreactor was studied to characterize mass transfer coefficients and the removal of hydrophobic air pollutants under segmented gas-liquid flow pattern. The removal efficiency of hexane, toluene and α-pinene vapors reached values up to about 75%, 99% and 75%, respectively, at a gas contact time of less than 1 s, which is at least one, but closer to two orders of magnitude shorter than conventional biological gas purification systems. The bioreactor displayed stable operation for 100 days and was robust against common upsets, which opens the new opportunities for expanding the application field of biological processes for air pollution control and the mitigation of greenhouse gases in dilute air streams. SynopsisThis study breaks through existing barriers of gas treating biotechnologies which allows expansion of the application field of more sustainable processes for air pollution control.

Aerobic oxidation of 2‐tert‐butyl phenol within gas–liquid segmented flow: Mass transfer characteristics and scale‐up

AbstractQuantifying the mass transfer of gas–liquid segmented flow in practical reactions is of significant for the scale‐up design. However, there is a lack of theoretical guidance to predict operational conditions to meet required the mass transfer in a selective dimension‐enlarged microreactor. Herein, the oxidation of 2‐tert‐butyl phenol (2‐TBP) was performed in a selective dimension‐enlarged capillary microreactor of 4.35 mm i.d. and optimal operation conditions were obtained. A high 2‐tert‐butyl‐1,4‐benzoquinone (2‐TBQ) yield of 73% was achieved within 6 min. Moreover, a quantitative method to assess the mass transfer of gas–liquid segmented flow in practical reaction was developed by introducing a circulation frequency, fcir, which could provide theoretical guidance to predict operating conditions for the scale‐up production. The 2‐TBQ productivity is significantly increased from 0.0061 to 0.5068 kg/h, that is, an increase of 83 times, by selective dimension enlarging from 2 mm to 4.35 mm with predicted operating conditions.

Analysis of dynamic acoustic resonance effects in a sonicated gas–liquid flow microreactor

In this work, we characterize acoustic resonance phenomena occurring between gas bubbles in a segmented gas-liquid flow in a microchannel irradiated with a frequency around 500kHz. A large acoustic amplitude can be reached, leading to gas-liquid interface deformation, atomization of micrometer sized droplets, and cavitation. A numerical approach combining an acoustic frequency-domain solver and a Lagrangian Surface-Evolver solver is introduced to predict the acoustic deformation of gas-liquid interfaces and the dynamic acoustic magnitude. The numerical approach and its assumptions were validated with experiments, for which a good agreement was observed. Therefore, this numerical approach allows to provide a description and an understanding of the acoustic nature of these phenomena. The acoustic pressure magnitude can reach hundreds of kPa to tens of MPa, and these values are consistent with the observation of atomization and cavitation in the experiments. Furthermore, volume of fluid simulations were performed to predict the atomization threshold, which was then related to acoustic resonance. It is found that dynamic acoustic resonance gives rise to atomization bursts at the gas bubble surface. The presented approach can be applied to more complex acoustic fields involving more complex channel geometries, vibration patterns, or two-phase flow patterns.

3D-printed capillary force trap reactors (CFTRs) for multiphase catalytic flow chemistry

Figure of 3D illustration of a capillary trap force reactor (CFTR) with transiently trapped liquid nanoparticle catalysts in dimple-shaped capillary traps in the presence of a gas–liquid segmented flow, for the hydrogenation of 1-hexene to n-hexane.

Gas–liquid slug flow in microfluidic heat exchanger: Effect of gas hold-up and bubble size on pressure drop and heat transfer

This work is an extension of our recent article, which was inspired by discussions with the reviewers. The paper presents an experimental study of the gas-liquid segmented flow as a method of intensifying microfluidic heat exchangers (MFHE) with a focus on studying the influence of the gas volume fraction. It was found that the highest heat transfer coefficient is achieved when the bubble size coincides with the channel diameter, which corresponds to a gas hold-up of 12%. Experimental studies have also shown that segmented flow makes it possible to increase the Nu up to 1.54 and reduce thermal resistance up to 1.67 times compared to single-phase flow.

Continuous synthesis of 2-tert-butyl phenol oxidation in gas-liquid segmented flow and its kinetic investigation

The process to produce tert-butylhydroquinone from 2-tert-butyl phenol (2-TBP) is of great importance in the application of phenolic derivatives. However, the oxidation of 2-TBP to 2-tert-butyl-1,4-benzoquinone (2-TBQ) as the first step is usually performed in traditional batch reactors, suffering the disadvantages of mass transfer limitation and potential risk of explosion. Herein, a safer process to enhance the catalytic oxidation was developed by introducing gas–liquid segmented flow in a microchannel reactor. The 2-TBQ yield was approximately 70% within 29.02 min while the time was 8 h to obtained almost the same yield (65%) in batch systems. CFD simulation was conducted, confirming that secondary flow was formed in liquid slugs. The corresponding intensive convection ensured the continuous and stable operation without particle sedimentation. On the basis of the enhanced mass transfer, the kinetics of 2-TBP oxidation was also investigated. The obtained reaction orders of 2-TBP and oxygen were 2.0948 and 0.4073, respectively.

Acoustic resonance and atomization for gas-liquid systems in microreactors

It is shown that a liquid slug in gas–liquid segmented flow in microchannels can act as an acoustic resonator to disperse large amounts of small liquid droplets, commonly referred to as atomization, into the gas phase. We investigate the principles of acoustic resonance within a liquid slug through experimental analysis and numerical simulation. A mechanism of atomization in the confined channels and a hypothesis based on high-speed image analysis that links acoustic resonance within a liquid slug with the observed atomization is proposed. The observed phenomenon provides a novel source of confined micro sprays and could be an avenue, amongst others, to overcome mass transfer limitations for gas–liquid processes in flow.

Gas-liquid flow characterization and mass transfer study in a microreactor for oligomerization catalyst testing

The acceleration of the development of ethylene oligomerization processes by homogeneous catalysis requires a complete characterization of the catalytic system. This work proposes to use segmented gas-liquid flow in microchannels to achieve high experimental throughput for catalyst testing.Gas-liquid flow was studied in order to validate the microreactor setup. Results showed that bubble and slug lengths vary linearly with the superficial velocities ratio following the Garstecki et al. model [21]. The impact of lateral feed in Taylor flow was explored to mimic the catalyst feed in an oligomerization test. The experiments showed that the volume of the liquid slugs increased proportionally to the additional flow rate, thereby suggesting that the majority of the liquid in the lateral feed is ultimately retained in the liquid slug.A study of mass transfer showed that the volumetric mass transfer coefficient is between 0.27 and 0.55 s−1. These values lead to a Hatta number < 0.5, thus confirming the possibility of conducting the ethylene oligomerization reaction in the chemical kinetic regime.Finally, an ethylene oligomerization test performed in the microreactor showed that the continuous microreactor system can be used for catalyst screening purpose.

Continuous synthesis of monodisperse silica microspheres over 1 μm size

This study presents a robust and scalable method for synthesizing continuously the silica microspheres via the Stober method. Owing to the superb mixing accessible with gas-liquid segmented flow in coiled microtube, satisfactory morphology and mono-dispersity of silica spheres was obtained. The continuous method was firstly optimized through investigating the effects of various operation parameters on the morphology, particle size, size distribution and chemical structure of silica spheres. Without the surfactant, the size of silica spheres was limited < 850 nm due to the partial blockage of microchannel when synthesizing larger silica spheres. The addition of surfactant migrated the clogging issue, enabling the reliable synthesis of microspheres with diameters over 1 μm within a four times shorter residence time (30 min). Moreover, the mono-dispersity of silica microspheres could be further improved by increasing the gas/liquid ratio in segmented flow due to the more intensified mixing in liquid segments, as verified in visual flow field investigation.

Automated, high frequency, on-line dimethyl sulfide measurements in natural waters using a novel “microslug” gas-liquid segmented flow method with chemiluminescence detection

Dimethyl sulfide (DMS) is the major biogenic volatile sulfur compound in surface seawater. Good quality DMS data with high temporal and spatial resolution are desirable for understanding reduced sulfur biogeochemistry. Here we present a fully automated and novel “microslug” gas-liquid segmented flow-chemiluminescence (MSSF-CL) based method for the continuous in-situ measurement of DMS in natural waters. Samples were collected into a flow tank and DMS transferred from the aqueous phase to the gas phase using a vario-directional coiled flow, in which microvolume liquid and gas slugs were interspersed. The separated DMS was reacted with ozone in a reaction cell for CL detection. The analytical process was automated, with a sample throughput of 6.6 h−1. Using MSSF for DMS separation was more effective and easily integrated with CL detection compared with the commonly used bubbling approach. Key parameters of the proposed method were investigated. The linear range for the method was 0.05–500 nM (R2 = 0.9984) and the limit of detection (3 x S/N) was 0.015 nM, which is comparable to the commonly used gas chromatography (GC) method and sensitive enough for direct DMS measurement in typical aquatic environments. Reproducibility and recovery were assessed by spiking natural water samples (river, lake, reservoir and pond) with different concentrations of DMS (10, 20 and 50 nM), giving relative standard deviations (RSDs) ≤1.75% (n = 5) and recoveries of 94.4–107.8%. This fully automated system is reagent free, easy to assemble, simple to use, portable (weight ~5.1 kg) and can be left in the field for several hours of unattended operation. The instrumentation can provide high quality DMS data for natural waters with an environmentally relevant temporal resolution of ~9 min.

CO2 reduction in a microchannel electrochemical reactor with gas-liquid segmented flow

Abstract Electrochemical CO2 reduction is suffered from mass transfer limitation at high current densities, owing to the low carbon dioxide concentration over the surface of the catalyst. To enhance mass transfer, a novel annular microchannel electrochemical reactor was established, which consists of a cylindrical cation exchange membrane at the outside and an oxide-derived Cu foam rod as the electrode in the middle. The results show that, due to the enhanced gas–liquid mass transfer of CO2 in the channel, the limiting current density can increase by 49.3% from 6.7 mA cm−2 to 10.0 mA cm−2 and CO2 reduction rate increase by 55.3% from 21.5 × 10−9mol s−1 cm−2 to 33.4 × 10−9mol s−1 cm−2, compared when Cu electrode is in the open space. It further shows the limiting current density is influenced by the annulus channel width and gas–liquid ratio, and width of 1 mm gives the best performance at a gas–liquid ratio of 2:1. This work demonstrates the great potential of utilizing microchannel to intensify the mass transfer for efficient electrochemical CO2 reduction.

Gas-Directed Production of Noble Metal-Magnetic Heteronanostructures in Continuous Fashion: Application in Catalysis.

Complex nanomaterials produced by scale-up batch processes lack suitable control of shape, size distribution, chemical composition, and quality, because heat and mass transfer are seriously affected as the reactor volume increases. Here we use a novel continuous synthesis procedure, the active gas-liquid segmented flow, to produce noble metal-magnetic heteronanostructures with enormous interest in the fields of catalysis, biomedicine, environmental sensors, food monitoring, and chemical analysis. The microreactor technology proposed scales down the reaction volume to gain advantage of the large surface area to volume ratio with respect to conventional batch-type reactors, improving heat and mass transport and, consequently, promoting a uniform heating and mixing. The gas phase was introduced in the chemical reactor as gas slugs of nanoliter scale with a dual role: (1) passive mixing and (2) chemical directing agent to tune the crystallization of nanostructures in a continuous fashion. The shape, size, and magnetic properties of the resulting heteronanostructures, as well as the density, size, and composition of noble metal nanoparticles were tuned to show the versatility of the proposed approach in a timeline of 4 min. We demonstrated that the produced nanostructures provide excellent catalytic properties in the catalyzed hydrogenation of nitrophenols to aminophenols. Electron microscopy, UV-vis spectroscopy, and cyclic voltammetry studies showed the remarkable catalytic performance of the produced heteronanostructures.

Bioprocess Intensification Using Flow Reactors: Stereoselective Oxidation of Achiral 1,3-diols with Immobilized Acetobacter Aceti

Enantiomerically enriched 2-hydroxymethylalkanoic acids were prepared by oxidative desymmetrisation of achiral 1,3-diols using immobilized cells of Acetobacter aceti in water at 28 °C. The biotransformations were first performed in batch mode with cells immobilized in dry alginate, furnishing the desired products with high molar conversion and reaction times ranging from 2 to 6 h. The biocatalytic process was intensified using a multiphasic flow reactor, where a segmented gas–liquid flow regime was applied for achieving an efficient O2-liquid transfer; the continuous flow systems allowed for high yields and high biocatalyst productivity.

Enhanced Mixing of Microvascular Self-Healing Reagents Using Segmented Gas-Liquid Flow.

Microvascular self-healing systems have previously been demonstrated to restore large-scale damage and achieve repeated healing of multiple damage events in polymers. However, the healing performance of these systems is often limited because thelaminar nature of flow in microchannels results in poor mixing of two-part self-healing reagents. In this paper, we introduce segmented gas-liquid flow (SGLF) to enhance the mixing of reagents in microvascular self-healing systems. In SGLF, discrete liquid slugs containing self-healing reagents are separated by gas bubbles while flowing through a single microchannel. Recirculating streamlines within the liquid slugs can enhance the mixing of miscible liquids such as healing reagents. We investigate the effect of SGLF on mixing and healing for a two-stage chemistry used to restore large-scale damage in thermoset polymers. Additionally, we employ SGLF to deliver an epoxy-thiol chemistry, enabling the repeated recovery of fracture toughness in glass fiber-reinforced composites. In both systems, the mixing of healing agents delivered by SGLF is enhanced compared to alternative microvascular delivery strategies. For the two-stage chemistry, SGLF increases the maximum damage size that can be healed by 25% compared to laminar single-phase flow. Furthermore, there are concomitant increases in the extent of polymerization and the mechanical properties of the restored material, including a fivefold increase in the peak load sustained during a push-out test. For the epoxy-thiol chemistry, SGLF enables multiple healing cycles with healing efficiency above 100%. On the basis of these results, we envision that SGLF could improve performance for a variety of microvascular self-healing systems with different host materials, damage modes, and healing chemistries.

How valid is Taylor dispersion formula in slugs?

How valid is Taylor dispersion formula in slugs?

Photooxygenation in an advanced led-driven flow reactor module: Experimental investigations and modelling

The photooxygenation of α-terpinene was investigated as a benchmark reaction in an advanced LED-driven flow reactor module, both from an experimental and modelling point of view. Ethanol was used as a green solvent and rose Bengal was chosen as a cheap sensitizer of industrial importance. Firstly, the kinetic law based on all mechanistic steps was established for the chosen photooxygenation. From this, the set of operating parameters potentially influencing the photoreaction rate were identified. Subsequently, experiments were carried out under continuous-flow conditions to screen these operating parameters, namely concentration of α-terpinene, concentration of photosensitizer, residence time, structure of the segmented gas-liquid flow and nature of the reagent gas phase (air versus pure oxygen). Finally, the conditions enabling minimization of sensitizer bleaching were established. It was also shown that the hydrodynamic characteristics of the gas-liquid flow can have an effect on the conversion levels. From this, a simplified model was proposed to predict the conversion at the reactor’s outlet when pure oxygen was used.

Principle-based design of distributed multiphase segmented flow

The design of systems incorporating multiphase flows remains a significant challenge for applications including lab-on-chip and microreactors. Segmented two-phase flow is used for the purpose of distributing biological samples from one site to many, enhancing heat and mass transport over laminar single phase flow and rapid chemical synthesis. This paper examines hierarchical designs, regularly utilised in such applications, that transport a segmented gas–liquid flow over an area with maximal thermodynamic performance. Fundamental design rules, predicted using the constructal method, minimize flow resistance of elemental bifurcations. The predicted geometric configuration follows an alternative to the established Murray’s law (or Hess–Murray law) when the global pressure difference is dominated by short liquid slugs and dispersed gas phase. Breakup of phases in the junction region was examined numerically with a volume-of-fluid approach to develop a general criteria where junction losses are non-negligible. Although this loss is periodic, the interfacial pressure during the necking stage of the dispersed phase dominates. Using the findings for an elemental bifurcation, multiple scale hierarchical configurations for combined fluid and heat transport were assessed. The multiple scale tree-shaped design can be advantageous for low thermal resistance and pumping power requirements compared to a single scale serpentine layout. The principle-based method and results can reduce the design space in high-fidelity investigations of microscale segmented flows.

Continuous synthesis of nanostructured silica based materials in a gas–liquid segmented flow tubular reactor

Synthesizing core–shell particles SiO2@mSiO2, mSiO2and Au@SiO2in a continuous tubular segmented reactor.