Micromixing Process Research Articles

Micromixing efficiency and enhancement methods for non-Newtonian fluids in millimeter channel reactors

Micromixing efficiency and enhancement methods for non-Newtonian fluids in millimeter channel reactors

Investigation of effects of impeller geometry on the spatial distribution of turbulent field and micro-mixing performance in a stirred reactor equipped with multiple impellers

There are few studies on the effects of spatial distribution of hydrodynamics on the micro-mixing in stirred reactors. In this work, a statistical approach is introduced to quantitatively assess the spatial distributions of hydrodynamic parameters, using the turbulent dissipation rate as an example. According to the results, the effects of impeller geometry and angular velocity on the turbulent dissipation rate can be well described in terms of the volume-averaged values and spatial distribution functions. In the present study, a more uniform distribution of the turbulent dissipation rate is achieved by the multiple impellers with an inclinational angle of 15° at the same rotational speed. However, increasing the inclinational angle can decrease the volume-averaged value. The cumulative percentage of area-averaged turbulent dissipation rate is well modeled by an exponential function. The effects of impeller types on the meso- and micro-mixing processes are also investigated. It is found that the micro-mixing process has more significant influences than the meso-mixing process in the bottom region and near the free surface. The effects of impeller types on the micro-mixing performance are further investigated using the Villermaux–Dushman reaction system, both experimentally and numerically. An empirical correlation that considers the spatial distribution of turbulent parameters for the reaction rate of I2 synthesis is proposed. The results show that the predictions of segregation index, which represents micro-mixing performance, are improved by a numerical model coupled with the new reaction rate model.

Preparation of microparticles and nanoparticles using membrane-assisted dispersion, micromixing, and evaporation processes

Preparation of microparticles and nanoparticles using membrane-assisted dispersion, micromixing, and evaporation processes

Reactive mixing performance for a nanoparticle precipitation in a swirling vortex flow reactor

Mixing performance for a consecutive competing reaction system has been investigated in a swirling vortex flow reactor (SVFR). The direct quadrature method of moments combined with the interaction by exchange with the mean (DQMOM-IEM) method was employed to model such reacting flows. This type of reactors is able to generate a strong swirling flow with a great shear gradient in the radial direction. Firstly, mixing at both macroscale and microscale was assessed by mean mixture fraction and its variance, respectively. It is found that macromixing can be rapidly achieved throughout the whole reactor chamber due to its swirling feature. However, micromixing estimated by Bachelor length scale is sensitive to turbulence. Moreover, the additional introduction of ultrasound irradiation can significantly improve the mixing uniformity, namely, free of any stagnant zone presented in the reactor chamber on a macroscale, and little variance deviating from the mean environment value can be observed on a microscale. Secondly, reaction progress variable and the reactant conversion serve as indicators for the occurrence of side reaction. It is found that strong turbulence and a relatively fast micromixing process compared to chemical reaction can greatly reduce the presence of by-product, which will then provide homogenous environment for particle precipitation. Moreover, due to the generation of cavitation bubbles and their subsequent collapse, ultrasound irradiation can further intensify turbulence, creating rather even environment for chemical reactions. Low conversion rate was observed and little by-products were generated consequently. Therefore, it is suggested that the SVFR especially intensified by ultrasound irradiation has the ability to provide efficient mixing performance for the fine-particle synthesis process.

Continuous process for preparation of 2,3-dimethyl-4-methylsulfonylbromobenzene via oxidation by in situ formed peracetic acid

Continuous process for preparation of 2,3-dimethyl-4-methylsulfonylbromobenzene via oxidation by in situ formed peracetic acid

On the Elucidation of Polymer Fouling Mechanisms and Ethylene Decomposition in High‐Pressure LDPE Tubular Reactors

AbstractIn high‐pressure free‐radical ethylene polymerization tubular reactors, fouling often occurs due to polymer deposition onto the reactor's wall surface. This results in a decrease in the overall heat transfer coefficient. Note that a high‐pressure tubular reactor must be capable of removing about half of the generated polymerization heat to the cooling water flowing into the reactor's jackets. Therefore, reactor fouling can have serious implications including decrease of heat removal rate and monomer conversion, increase in the overall initiator(s) consumption, change of polymer quality, decline of reactor's safe operation, and significant economic losses due to lower plant productivity and longer reactor maintenance periods. In rare cases, the loss of reactor's heat removal capacity may result in the appearance of local hot spots that eventually can trigger ethylene decomposition. In the present review paper, the fundamental physical and chemical phenomena regarding polymer fouling and ethylene decomposition in high‐pressure low density polyethylene (LDPE) reactors are critically discussed. The effects of shear rate, polymer molecular weight, and wall surface energy on polymer adsorption and desorption are analyzed. Moreover, the effect of characteristic initiator decomposition and micromixing times and process conditions on initiator dispersion, polymer fouling, and ethylene decomposition are assessed.

Residence time distribution and micromixing efficiency of a dynamic inline rotor–stator mixer

• Micromixing in a rotor–stator mixer was investigated by the iodide–iodate reaction. • Backmixing ( Bo = 1.450 – 5.051) was observed. • Dependence of micromixing efficiency on different variables was found. • New method for estimating backmixing from micromixing experiments was developed. An industrially available dynamic inline–mixer which operates on the rotor–stator principle was investigated for aqueous solutions with a total volume flow rate of up to 40 L/h to gain valid insight into the mixing behavior. In residence time distribution (RTD) experiments, mean residence times ranging between 29 – 305 s and Bodenstein numbers ranging between 1.4 – 5.1 were measured. Micromixing efficiency was investigated with the Villermaux–Dushman reaction at varying rotational speeds, total volume flow rates and injection inlets. The results suggest that micromixing processes are especially complex in a continuous rotor–stator mixer. As expected, the injection inlet had a decisive influence on micromixing efficiency. Surprisingly, even at higher volume flow rates backmixing was identified as a factor which significantly extends the micromixing time. A new method is proposed for estimating the degree of backmixing based on micromixing experiments. Micromixing times range between 3 · 10 –4 and 4 · 10 –3 s depending on the operating point.

A comparative analysis of micro-mixing process in a confined impinging jet reactor with/without applying ultrasound

• A multiphase CFD model coupling the cavitation model and chemical reactions is developed for the first time. • Influences of acoustic cavitation on “local” micro-mixing process in the CIJR are investigated. • An increase in the power of ultrasound can improve the micro-mixing efficiency. • Ultrasound has larger effects on the meso-mixing process than micro-mixing process at the tip of transducer head. • Meso-mixing process controls the overall process with or without applying ultrasound. Studies on the influences of ultrasound-induced cavitation on the micro-mixing process in a confined impinging jet reactor (CIJR) are still lacking. In the present study, experimental and numerical approaches are employed to investigate the effects of ultrasound intensification on the mixing processes in a CIJR. The numerical results are experimentally validated. The model is also coupled with chemical reactions for the first time. It is found that local eddies generated by the collapse of acoustic microbubbles and “turbulence”-like acoustic streaming can significantly reduce the micro-mixing time. In all cases, the micro-mixing efficiency is found to increase with an increase in the power of ultrasound. A reduction of 15–24% in micro-mixing time can be achieved at the inlet velocity of 0.2m/s when the power of ultrasound increases from 122.5W to 175W. It is also found that the effect of ultrasound intensification on the meso-mixing process is larger than that on the micro-mixing process in the vicinity of transducer head. The ratio of micro-mixing time to meso-mixing time in the case of P ultra =175W can be 88% larger than that in the case of P ultra =0W in the front of the transducer head at Re in =5000.

Controllable and Scale-Up Synthesis of Nickel-Cobalt Boride@Borate/RGO Nanoflakes via Reactive Impingement Mixing: A High-Performance Supercapacitor Electrode and Electrocatalyst.

Large-scale synthesis of graphene-based nanomaterials in stirred tank reactor (STR) often results in serious agglomeration because of the poor control during micromixing process. In this work, reactive impingement mixing is conducted in a two-stage impinging jet microreactor (TS-IJMR) for the controllable and scale-up synthesis of nickel-cobalt boride@borate core-shell nanostructures on RGO flakes (NCBO/RGO). Benefiting from the good process control and improved micromixing efficiency of TS-IJMR, NCBO/RGO nanosheet provides a large BET surface area, abundant of suitable mesopores (2–5 nm), fast ion diffusion, and facile electron transfer within the whole electrode. Therefore, NCBO/RGO electrode exhibits a high specific capacitance of 2383 F g−1 at 1 A g−1, and still retains 1650 F g−1 when the current density is increased to 20 A g−1, much higher than those of nickel boride@borate/RGO (NBO/RGO) and cobalt boride@borate/RGO (CBO/RGO) synthesized in TS-IJMR, as well as NCBO/RGO-S synthesized in STR. In addition, an asymmetric supercapacitor (NCBO/RGO//AC) is constructed with NCBO/RGO and activated carbon (AC), which displays a high energy density of 53.3 W h kg−1 and long cyclic lifespan with 91.8% capacitance retention after 5000 charge-discharge cycles. Finally, NCBO/RGO is used as OER electrocatalyst to possess a low overpotential of 309 mV at a current density of 10 mA cm−2 and delivers a good long-term durability for 10 h. This study opens up the potential of controllable and scale-up synthesis of NCBO/RGO nanosheets for high-performance supercapacitor electrode materials and OER catalysts.

Microreactor synthesis of nanosized particles: The role of micromixing, aggregation, and separation processes in heterogeneous nucleation

The aim of the paper was to formulate the concept of controlled solution synthesis (a version of co-precipitation method) and to demonstrate its realization on several types of reactors, with a special focus on microreactors with free impinging jets (MRFIJ). The objects of this work are (i) co-precipitation method for synthesis of nanoparticles and (ii) various types of microreactors as a structured system consisting from dedicated elements. The procedure consisted on the decomposition of the total process into unit operations with further analysis of their spatial and temporal characteristics. It was shown that the elements for following unit operations should be presented in the microreactor: premixing; converging clusters (intense mixing in a thin layer/channel); ripening of nuclei; particle separation. Microreactors were shown as most appropriate devices for nanoparticles synthesis by co-precipitation method. Particularly, three case studies of co-precipitation synthesis in MRFIJ demonstrated possibility to produce nanoparticles having narrow particle size distribution (dmean: BiFeO3 – 20 nm, CoFe2O4 – 12 nm, GdFeO3 – 30 nm), without impurities of other phases and improved characteristics (e.g. low coercivity for GdFeO3 which makes it possible to use them as MRI-contrast agents) with high performance (4.3 m3/day). Besides, the temperature increase due to the energy dissipation in the liquid sheet in MRFIJ was analysed: it does not exceed 0.001 K and 0.2 K for characteristic jet velocities of 3.35 m/s and 40 m/s, correspondingly.

Parametric study on dual-fuel ignition characteristics under marine engine-relevant conditions

• Develop a novel parameter-sensitivity method to quantify IDT-dependence. • The addition of methanol has a larger impact on DF autoignition than methane. • Ambient-charge reactivity is significant to optimize the DF-engine performance. • Mixing degree and mixing-time are critical to control the ignition process. • High-T IDT-reduction of Methanol-addition due to CH 3 OH + HO 2 <=> CH 2 OH + H 2 O 2 . Dual-fuel (DF) combustion strategies with low-carbon fuels are attractive for the shipping industry because of their better economy and lesser emissions. However, DF ignition process is highly complex and difficult to control due to its strong sensitivities to mixture-states. As such, a comprehensive numerical study was conducted to investigate the influence of thermodynamic states, ambient-fuel reactivity and micro-mixing process on the autoignition characteristics of DF mixtures (methane/ n -heptane and methanol/ n -heptane) under marine engine-relevant conditions. A novel parameter-sensitivity evaluation method by log–log definition was firstly proposed to quantify the ignition-delay-time (IDT) dependence on thermodynamic trajectory and charge reactivity, based on a newly detailed kinetic model (NUIGMech1.1). It was found that the methanol-addition has a larger impact on IDTs than methane, and also exhibits ignition-promoting behaviors at high temperatures. Besides, the IDT displays a higher sensitivity to temperature under methanol/air atmosphere but a higher sensitivity to pressure under methane/air atmosphere, suggesting that methane/diesel DF engines are prone to occur cycle-to-cycle variations under diesel pilot-ignition (DPI) mode while methanol/diesel DF engines exist the larger cycle variations under reactivity-controlled compression-ignition (RCCI) mode. There are small IDT-changes for methane/ n -heptane mixtures with varying ϕ CH4 while large variations for methanol/ n -heptane as varying ϕ CH3OH . Moreover, methanol shows the more significant chemical-effect on ignition. The mixture fraction of methanol-based blends with the shortest IDT, defined as the most-reactive mixture fraction ( Z MR ), is larger than that of methane at a given temperature. This demonstrates that the mixing degree (the high-reactivity fuel to low-reactivity fuel ratio) plays an important role in controlling the ignition. The mixing-time is also critical for ignition behavior of fuels, and autoignition of methanol/ n -heptane blends exhibits higher sensitivity to mixing rates than methane/ n -heptane. Therefore, adjusting injection parameters could tailor mixing-trajectories to offset changes in fuel autoignition chemistry. Additional reaction sensitivity analyses demonstrate that the OH radicals play a key role in the whole system reactivity at low temperatures, while HO 2 becomes more important at intermediate-temperatures and CH 3 OH + OH <=> CH 2 OH + H 2 O displays the highest ignition-inhibition. Significantly, CH 3 OH + HO 2 <=> CH 2 OH + H 2 O 2 shows a high promoting effect on IDTs at high temperatures, resulting in the IDT-reduction.

Significance of Synthetic Cilia and Arrhenius Energy on Double Diffusive Stream of Radiated Hybrid Nanofluid in Microfluidic Pump under Ohmic Heating: An Entropic Analysis

This study investigates the thermal aspects of magnetohydrodynamic double diffusive flow of a radiated Cu-CuO/Casson hybrid nano-liquid through a microfluidic pump in the presence of electroosmosis effects. Shared effects of the Arrhenius activation energy and the Joule heating on the intended liquid transport are also incorporated. The inner wall of the pump is covered with electrically charged fabricated cilia mat that facilitates flow actuation and micro-mixing process. The governing equations for the proposed problem are simplified by utilizing the Debye-Hückel and lubrication approximations. The numerical solutions are calculated with the aid of shooting technique. The analysis reports that the substantial effects of electroosmosis contribute an important role in cooling process. Existence of electric double layer stimulates an escalation in liquid stream in the vicinity of the pump surface. The Arrhenius energy input strengthens the mass dispersion and regulates the thermal treatment. The proposed geometry delivers a deep perception that fabricated cilia in electroosmotic pumps are potential pharmaceutical micromixers for an effective flow and minimum entropy generation rate.

Effect of Thermal Energy and Ultrasonication on Mixing Efficiency in Passive Micromixers

Micromixing is a key process in microfluidics technology. However, rapid and efficient fluid mixing is difficult to achieve inside the microchannels due to unfavourable laminar flow. Active micromixers employing ultrasound and thermal energy are effective in enhancing the micromixing process; however, integration of these energy sources within the devices is a non-trivial task. In this study, ultrasound and thermal energy have been extraneously applied at the upstream of the micromixer to significantly reduce fabrication complexity. A novel Dean micromixer was laser-fabricated to passively increase mixing performance and compared with T- and Y-micromixers at Reynolds numbers between 5 to 100. The micromixers had a relatively higher mixing index at lower Reynolds number, attributed to higher residence time. Dean micromixer exhibits higher mixing performance (about 27% better) than T- and Y-micromixers for 40 ≤ Re ≤ 100. Influence of ultrasound and heat on mixing is more significant at 5 ≤ Re ≤ 20 due to the prolonged mechanical effects. It can be observed that mixing index increases by about 6% to 10% once the temperature of the sonicated fluids increases from 30 °C to 60 °C. The proposed method is potentially useful as direct contact of the inductive energy sources may cause unwanted substrate damage and structural deformation especially for applications in biological analysis and chemical synthesis.

Characterization of dissimilar liquids mixing in T-junction and offset T-junction microchannels

Micromixing process in microfluidic devices has been broadly employed in bio-, nano-, and environmental technologies using either miscible or immiscible liquids. However, there are limited experimental studies investigating the mixing process of different densities and viscosities liquids in relation to microfluidics. Therefore, the mixing process of propan-2-ol and water, water and sodium chloride solution, propan-2-ol and sodium chloride solution were experimented and reported at 5 ≤ Re ≤ 50 in T-junction and offset T-junction microchannels. For miscible mixing experiments, i.e. propan-2-ol and water, water and sodium chloride solution, both microchannels show mixing index for each Reynolds number is directly proportional to the mixing time. At low Reynolds number, higher molecular diffusion takes place while at low flow rate, the residence time of fluid is high. The mixing performance is relatively good at high Reynolds number of 40 and 50 due to the significant convection which is caused by the effect of stretching and thinning of liquid lamellae. For immiscible propan-2-ol and sodium chloride solution mixing, offset T-junction microchannel offers better mixing performance than T-junction microchannel at both low and high Reynolds number. The chaotic mixing happened at the intersection of the T-junction microchannel due to the direct interaction of two liquids entering the junction at high momentum.

Micromixing in a gas–liquid vortex reactor

AbstractThe gas–liquid vortex reactor (GLVR) has substantial process intensification potential for multiphase processes. Essential in this respect is the micromixing efficiency, which is of great importance in fast reaction systems such as crystallization, polymerization, and synthesis of nanomaterials. By creating a vortex flow and taking advantage of the centrifugal force field, the liquid micromixing process can be intensified in the GLVR. Results show that introducing a liquid into a gas‐only vortex unit results in suppression of primary and secondary gas flow. The Villermaux–Dushman protocol is applied to study the effects of the gas flow rate, liquid flow rate, and liquid viscosity based on a segregation index. Based on the incorporation model and reaction kinetics, the micromixing time of the GLVR is determined to be in the range of 10−4 ~ 10−3 s, which is comparable to the highly efficient rotating packed bed and substantially better than a static mixer.

Experimental determination and computational prediction of the mixing efficiency of a simple, continuous, serpentine-channel microdevice

Micromixing devices often utilize complex architectures to mix miscible liquid streams and can be complex and expensive to fabricate. Here, we developed, built, experimentally tested, and computationally analyzed a serpentine micromixer that can be fabricated using simple tools and supplies available in non-microdevice dedicated laboratories. Fluorescence imaging was used to quantify its mixing effectiveness experimentally. A Computational Fluid Dynamics (CFD) software package (COMSOL) was used to model the micromixing process. The predictions were in excellent agreement with the experimental data. The serpentine micromixer can achieve significant levels of mixing efficiency. CFD predictions for a straight microfluidic channel of the same length as the serpentine favorably compared with previous theoretical predictions, indicating that the serpentine's mixing efficiency was vastly superior. Finally, CFD predictions were conducted for different and possibly improved designs of the basic serpentine. In all cases, the mixing efficiency was primarily associated with the number of 90o elbows in the device rather than the straight sections' length, with the first serpentine bend playing a significant role. Future design improvements should focus on incorporating as many elbows as possible in the device to maximize mixing efficiency and reduce the device size.

Characteristics analysis of the side-blown gas jet mixing process

The mixing performances of side-blown gas jets were studied by characterizing their manifold Betti numbers, namely 0-dimensional and 1-dimensional Betti numbers, through flow visualization and image processing techniques. The 0-dimensional Betti number reflects the mixing performance, and the 1-dimensional Betti number reflects the mixing unevenness degree. The mixing performances and flow fields were studied for different modified Froude numbers. As modified Froude numbers increase, the main flow was enhanced, promoting the macro-mixing of individual particles. The mixing performance of the stirring process was improved by a frequency conversion method, which destroyed the original flow pattern and promoted the micro-mixing process by chaotization.

An evaluation of gas-phase micro-mixing models with differential mixing timescales in transported PDF simulations of sooting flame DNS

Abstract The use of transported probability density function (TPDF) models to predict soot has the strong advantage that the effects of turbulent fluctuations on soot source terms can be rigorously accounted for. However, soot processes are closely coupled to gas-phase composition. Among the open issues for gas-phase micro-mixing is the species-dependence of mixing timescales. The objective is to carry out an evaluation on the effect of incorporating differential mixing timescales among gas-phase species in a TPDF simulation for soot prediction. A DNS having the configuration of a temporally evolving, non-premixed ethylene flame with a four-step, three-moment soot model is considered as the target for evaluation. The DNS dataset is applied to provide key inputs for TPDF simulations to limit the sources of error to micro-mixing. TPDF simulations with the interaction by exchange with the mean (IEM) and modified Curl (MC) models, which impose the same mixing timescale to all species, underpredict soot mass fraction and overpredict extinction levels regardless of the prescribed mixing frequency. By incorporating differential mixing timescales among gas-phase species, IEM-DD and MC-DD models yield notable improvement in predictions of the overall extinction and soot levels, highlighting the benefit of accounting for differential mixing timescales. A TPDF simulation with the Euclidean minimum spanning tree (EMST) model yields even better predictions, illustrating that the localness in composition space remains a critical issue. The indicated species mixing frequencies by the EMST model are shown to follow the DNS results qualitatively, illustrating that the micro-mixing process based on the Euclidean distance in composition space reproduces to a certain extent the differential mixing timescales due to reaction. Finally, it is shown that incorporating differential mixing timescales of soot moments is expected to have limited value as the mixing timescales of soot moments are sufficiently large to safely neglect soot mixing.

Numerical investigation of the secondary flow effect of lateral structure of micromixing channel on laminar flow

Abstract Micromixing remains challenge for microfluidic applications. The outward lateral structure on micromixing channel has been adopted as a mild way to promote micromixing at low Reynolds condition. Herein, this paper presents the numerical investigation of secondary flow effect generated by lateral structure on laminar flows. The velocity difference between the lateral structure and the main channel is the necessary condition for generation of Dean Vortex. Especially, when the velocity difference along the main channel is small and the velocity difference perpendicular to the main channel is large, the greatest secondary flow effect would be generated. The optimal secondary flow effect was obtained through adjusting the size of semicircular lateral structure. The mixing enhancement resulted from lateral structure only showed advantage at Re = 1−4. The semicircular lateral structure was respectively applied to T-shape, Zig-zag and Spit and Recombine micromixers. Remarkable mixing enhancement was observed for both T-shape and Zig-zag serpentine mixer but little promotion for Spit and Recombine micromixer. The square wave serpentine micromixer with lateral structure was successfully utilized for preparation of liposome nanoparticle. This paper reveals the impact factors of the lateral structure on the micromixing process, and indicates the suitable micromixer category for lateral structure application.

Implementation of a Single Emulsion Mask for Three-Dimensional (3D) Microstructure Fabrication of Micromixers Using the Grayscale Photolithography Technique.

Three-dimensional (3D) microstructures have been exploited in various applications of microfluidic devices. Multilevel structures in micromixers are among the essential structures in microfluidic devices that exploit 3D microstructures for different tasks. The efficiency of the micromixing process is thus crucial, as it affects the overall performance of a microfluidic device. Microstructures are currently fabricated by less effective techniques due to a slow point-to-point and layer-by-layer pattern exposure by using sophisticated and expensive equipment. In this work, a grayscale photolithography technique is proposed with the capability of simultaneous control on lateral and vertical dimensions of microstructures in a single mask implementation. Negative photoresist SU8 is used for mould realisation with structural height ranging from 163.8 to 1108.7 µm at grayscale concentration between 60% to 98%, depending on the UV exposure time. This technique is exploited in passive micromixers fabrication with multilevel structures to study the mixing performance. Based on optical absorbance analysis, it is observed that 3D serpentine structure gives the best mixing performance among other types of micromixers.