Micromixing Time Research Articles

Continuous synthesis of doped layered double hydroxides in a meso-scale flow reactor

Layered double hydroxides are a class of low-cost structured nanomaterials with many potential applications in environmental catalysis and sustainable technologies. Their large-scale use is hindered by the challenge of reproducible synthesis at scale. Here we report a general, readily scalable process for the reproducible synthesis of transition metal doped hydrotalcites using a two-step process: co-precipitation in a mm-scale (meso-scale) continuous flow reactor, followed by aging. We have shown that co-precipitation in flow at a residence time close to the micromixing time affords good control of particle formation. Reproducible synthesis allowed us, for the first time, to investigate the formation of the pore morphology of hydrotalcites and their thermal stability as a function of metal doping. The obtained samples exhibited surface areas (80–150 m2 g−1) higher than those typically attained in batch syntheses, with very low standard deviation between the samples, a high degree of crystallinity and small crystallite sizes, in the range of 9.5–11.9 nm, depending on composition. A systematic characterization allowed us to elucidate the mechanism of the pore morphology formation: the crystallites were found to agglomerate into disk-like platelets, whereas the pore structure of the hydrotalcites is formed by agglomeration of the platelets.

Liquid Distribution and Mixing in Rotating Packed Beds

In this study, two different approaches for investigating micromixing in a rotating packed bed (RPB) with a Villermaux-Dushman protocol are presented and detailed information on the operation, design, and power consumption of the RPB is given. First, a novel nozzle liquid distribution design is investigated for micromixing. A structured approach for experimental investigations is introduced, and the influence of different liquid distribution parameters on the micromixing results is presented. It is shown that the outlet velocity from the nozzles and the liquid hold-up at the inner rim of the packing play a crucial role in the mixing process. Second, micromixing in RPBs with equal flow rates is investigated. A comparison with other continuous mixing devices is conducted based on flow rates, pressure drop, and the micromixing time parameter tm. It is demonstrated that RPBs show a large potential for fast mixing at high flow rates and low pressure drop.

Micromixing Study of a Clustered Countercurrent-Flow Micro-Channel Reactor and Its Application in the Precipitation of Ultrafine Manganese Dioxide.

A clustered countercurrent-flow micro-channel reactor (C-CFMCR) has been assembled by the numbering-up of its single counterpart (S-CFMCR). Its micromixing performance was then studied experimentally using a competitive parallel reaction system, and the micromixing time was calculated as the micromixing performance index. It was found that the micromixing time of C-CFMCR was ranged from 0.34 to 10 ms according to its numbering-up times and the operating conditions of the reactor, and it was close to that of S-CFMCR under the same operating conditions, demonstrating a weak scaling-up effect from S-CFMCR to C-CFMCR. The C-CFMCR was then applied to prepare ultrafine manganese dioxide in a continuous manner at varying micromixing time. It showed that the micromixing time had a major effect on the particle structure. More uniform and smaller MnO2 particles were obtained with intensified micromixing. By building a typical three electrode system to characterize their performance as a supercapacitor material, the MnO2 particles prepared by both S-CFMCR and C-CFMCR under optimal conditions displayed a specific capacitance of ~175 F·g−1 at the current density of 1 A·g−1, with a decline of ~10% after 500 charge-discharge cycles. This work showed that C-CFMCR will have a great potential for the continuous and large-scale preparation of ultrafine particles.

Heat transfer and turbulent mixing characterization in screen-type static mixers

A numerical investigation of one phase flow through tubular pipes equipped with screen mixers was undertaken to study their effect on turbulent mixing as well as their heat transfer performance. The current work investigates 2D simulations in order to get insight on transport phenomena parameters in these mixers. Micromixing time was found to be enhanced by the use of screens with smaller fraction open areas because of their consequent effect on the local variation of the turbulence energy dissipation rate. Moreover, heat transfer experiments also showed that the Nusselt number, which increases with Re0.68 independently of the screen geometry, is also inversely proportional to the fraction open area of the screen. When compared with published data, screen mixers showed that their heat transfer performance compare favorably with the performance of various commercially available static mixers.

Micromixing efficiency in a rotating packed bed with non-Newtonian fluid

Micromixing performance of reactors has great influence on fast reactions such as polymerization, pharmaceutical and crystallization applications. In this work, based on Villermaux–Dushman iodide–iodate reaction system, an experimental investigation of micromixing with non-Newtonian fluid (CMC solution) in a rotation packed bed (RPB) was carried out. And a two-dimensional RPB Computational Fluid Dynamics (CFD) model coupled with shear-thinning rheological property was developed. Both experimental and numerical results indicated that increasing rotational speed could dramatically improve the micromixing performance due to the shear-thinning behavior in the RPB. Especially, the CMC concentration was an important factor that should be carefully considered, which showed quite different impacts on micromixing efficiency with liquid flow rate. In addition, based on incorporation model and experimental results, the micromixing time of the RPB with non-Newtonian fluid was determined as 4.1 × 10−4 s–1.9 × 10−3 s. This fundamental research had guiding significance for practical operations of RPBs when processing non-Newtonian fluids.

Vitiated High Karlovitz n-decane/air Turbulent Flames: Scaling Laws and Micro-mixing Modeling Analysis

Turbulent flames with high Karlovitz numbers have deserved further attention in the most recent literature. For a fixed value of the Damkohler number (ratio between an integral mechanical time and a chemical time), the increase of the Karlovitz number (ratio between a chemical time and a micro-mixing time) by an order of magnitude implies the increase of the turbulent Reynolds number by two orders of magnitude (Bray, Symp. (Int.) Combust. 26, 1–26 1996). In the practice of real burners featuring a limited range of variation of their turbulent Reynolds number, high Karlovitz combustion actually goes with a drastic reduction of the Damkohler number. Within this context, the relation between the dilution by burnt gases and the apparition of high Karlovitz flames is discussed. Basic scaling laws are reported which suggest that the overall decrease of the burning rate due to very fast mixing can indeed be compensated by the energy brought to the reaction zone by burnt gases. To estimate the validity of these scaling laws, in particular the response of the quenching Karlovitz versus the dilution level with a vitiated stream, the micro-mixing rate is varied in a multiple-inlet canonical turbulent and reactive micro-mixing problem. A reduced n-decane/air chemical kinetics is used, which has been derived from a more detailed scheme using a combination of a directed relation graphs analysis with a Genetic Algorithm. The multiple-inlet canonical micro-mixing problem includes liquid fuel injection and dilution by burnt gases, both calibrated from conditions representative of an aeronautical combustion chamber. The results confirm the possibility of reaching, with the help of a vitiated mixture, very high Karlovitz combustion before quenching occurs.

A study on the micromixing performance in microreactors for polymer solutions

Microreactors are widely applied for various polymerization processes to produce polymers with controlled molecular weight and structures. In this work, the mixing of polymer solutions in capillary microreactors was investigated with the aid of the biazo‐coupling reaction system and high‐speed camera. A smaller inner diameter, a higher Dean number and a lower Torsion number could promote the micromixing performance in microreactors. Unlike the mixing of Newtonian fluids in microreactors or micromixers, the mixing of polymer solutions follows different mechanisms and it is more difficult to reach homogeneous condition. However, the good micromixing performance in capillary microreactor systems for polymer solutions still could be obtained with high shear rates and long enough capillary length at relatively high correlated Reynolds numbers (NRe > 29). Furthermore, the micromixing time in the capillary microreactors was evaluated based on a modified stretching efficiency model and its value was in the range of 0.1–8.0 ms. © 2018 American Institute of Chemical Engineers AIChE J, 64: 3479–3490, 2018

Micromixing performance and the modeling of a confined impinging jet reactor/high speed disperser

In order to get a better micromixing performance, an intensification reactor that couples a confined impinging jet reactor and high speed disperser (CIJR-HSD) was designed. The influences of operating and structural parameters on the micromixing performance were investigated by using the iodide-iodate reaction system. The results showed that the segregation index decreased with the increase of both the rotational speed and the number of rotors; whilst there was little influence due to both inlet flow rate and the presence of packing. The coupling distance between the centre of the CIJR and the first rotor of the HSD also had a non-negligible effect on segregation index. The micromixing time was determined by the Engulfment model, and could also be evaluated by the energy dissipation model based on fluid energy transformation. It was found that the micromixing time for the CIJR-HSD device was in the range of 0.1–0.3 ms, while that for a single CIJR was in the range of 0.35–0.5 ms.

Investigation of micromixing and precipitation process in a rotating packed bed reactor with PTFE packing

A rotating packed bed (RPB) reactor plays an important role in process intensification of micromixing and mass transfer. However, the commonly used stainless steel wire mesh packing suffers from serious corrosion in various chemical processes. Polytetrafluoroethylene (PTFE) material, which is well-known for its excellent anticorrosion property, was employed as packing in the RPB reactor. Micromixing efficiency of the RPB reactor with PTFE packing was studied experimentally by adopting the iodide-iodate reaction system. Results showed that the micromixing efficiency of the RPB reactor with PTFE packing was better than that with the conventional stainless steel wire mesh packing. The micromixing time (tm) was calculated using an incorporation model based on the experimental data. Furthermore, the RPB reactor with PTFE packing was applied to a precipitation process for the preparation of magnesium hydroxide (Mg(OH)2) nanoparticles. The obtained Mg(OH)2 nanoparticles have small size and narrow particle size distribution, which indicates that the RPB reactor with PTFE packing is a promising reactor for preparing small size nanoparticles with its excellent micromixing efficiency.

Influence of micromixing time and shear rate in fast contacting mixers on the precipitation of boehmite and NH4-dawsonite

The textural properties of γ-alumina catalyst supports largely depend on the solid properties of their precursors. Precipitation in fast contacting mixers of two precursors, boehmite and NH4-dawsonite, were investigated in order to obtain different and enhanced solid features. The mixing devices, a sliding surface mixing device (SSMD) and a rotor-stator mixer (RSM), were operated continuously. Micromixing was characterized by using a competitive parallel reaction system (iodide-iodate). It was possible to achieve micromixing time from 1 to 200 ms according to the type of mixer and the operating conditions. The micromixing times assessed experimentally are in correct agreement with the theoretical ones. For boehmite precursor, the micromixing time had not a decisive influence on the crystallite size. Better control of the particle surface area was obtained considering the shear rate level, maybe due to a disordered aggregation. Conversely, reduction of the crystallite size with a decrease of the micromixing time was observed with NH4-dawsonite. A possible explanation lies in a higher local supersaturation, leading to a more intense primary nucleation. Moreover, it was possible to adjust the pore volume of the NH4-dawsonite’s aggregates with the operating conditions to quite a large extent (0.2 up to 0.8 cm3 g–1).

Geometrical improvement of inline high shear mixers to intensify micromixing performance

The micromixing performance of two inline high shear mixers (HSMs) was studied using the iodide-iodate reaction system, considering the effects of rotor speeds, flux ratios, viscosities and fluid concentrations. The segregation index (Xs) was adopted to assess the micromixing performance. It is indicated that the value of Xs decreases with the increase of rotor speed and the flux ratio of two feed solutions. The dual rows ultrafine-teethed HSM exhibits the poorer micromixing performance at low rotor speeds, comparing with the single-row blade-screen HSM. Then we modified the geometric configurations of the teethed HSM with the aid of computational fluid dynamics (CFD) analysis, including the modification of the clamp nut with blades and the addition of a liquid distributor, and reevaluated the micromixing performance in the modified HSM in order to improve mixing in the reactor. The results indicate that the fluid dispersion and turbulence intensity in the inlet region of the HSM play an important role in enhancing the micromixing level, the liquid distributor can greatly improve the micromixing performance of the dual rows ultrafine-teethed HSM and the estimated micromixing time can reach 10−5s. The results obtained here will provide much insight for the applications of the inline HSMs and other reactors in the field of intensifying fast chemical reaction.

Micro-mixing measurement by chemical probe in homogeneous and isotropic turbulence

The chemical probe is commonly used to evaluate the performance of chemical reactors. By a localized injection of chemical reagents, it is possible to measure the local micro-mixing, which is readily related to the selectivity of chemical reactions, since mixing at the molecular scale is the limiting factor for a wide range of chemical systems. The raw result of the chemical-probe method is a segregation index that allows comparison of different situations (various locations in the reactor, various Reynolds numbers, various geometries, etc.). Beyond the qualitative assessment provided by this segregation index, it is possible to obtain the intrinsic micro-mixing time by means of a micro-mixing model, describing the temporal evolution of a chemical reaction whose rate is governed by the micro-mixing. Here a key step is the choice of micro-mixing model. Several micro-mixing models available in the literature have been used in some specific cases without evaluating their appropriateness for the problem in hand. The main difficulty in this evaluation is that the real flows often do not fully satisfy the basic model assumptions, in particular the condition of homogeneous and isotropic turbulence (HIT).The present work aims at assessing the validity of a micro-mixing model under “ideal” experimental conditions, i.e. HIT with no mean flow, to avoid the bias due to the flow gradients. The HIT is obtained here by a system of oscillating grids placed in a vessel. The chemical probe measurements carried out by the iodide/iodate reaction system are applied to the two most commonly used phenomenological models in the literature: the IEM (Interaction by Exchange with the Mean) and the EDD (Engulfment, Deformation and Diffusion) models. The benchmark for the micro-mixing models is based on comparison of the local turbulent kinetic energy (TKE) dissipation rate both drawn from the micro-mixing time by the theoretical model of Bałdyga and the reference direct experimental determination by laser Doppler velocimetry measurements. It is shown that the engulfment model EDD seems the more appropriate to analyze the chemical data and provide a quantitative characterization of micro-mixing.

Mixing performance of T, Y, and oriented Y-micromixers with spatially arranged outlet channel: evaluation with Villermaux/Dushman test reaction

This study aims to investigate the micromixing performance of three basic types of spatial shaped micromixers. New configurations of T, Y, and oriented Y-spatial mixers were designed with change in the angles of the confluence and the outlet channel to achieve the efficient micromixing. These micromixers offer advantages that are not attainable with the typical types of these mixers. Experimental tests were carried out in the laminar flow regime and the mixing efficiency was evaluated using Villermaux/Dushman test reaction. The geometries of the channels were cylindrical with the length of 30 mm and the diameter of 800 μm. The experimental results show that the angle of outlet channel has a significant effect on the pressure drop and segregation index. Generally, the results reveal that at various feed flow rates the spatial shape of channels can lead to considerable improvement in micromixing performance. In all T, Y, and oriented Y-mixers, significant enhancement by increasing the confluence angle was also seen because the fluid elements were stretched and folded in the two inlet fluid interfaces. Furthermore, the micromixing time for the more efficient geometry of three shapes of microchannels was determined based on the incorporation model, which it was in the range of 0.001–0.1 s.

Study on micromixing and reaction process in a rotating packed bed

The exploration on the microscale transport phenomena is of great significance to understand the mechanism of microscale mixing and chemical reaction in a rotating packed bed (RPB). In this paper, numerical investigation is carried out and compared with the experimental results. Two different scale reaction models are applied to simulate and the numerical result is in reasonable agreement with the experimental results. The fluid flow and the species transfer with a competitive and consecutive reaction are studied to obtain the evolution of the segregation index and flow field. Each environment changes and is updated continuously with the liquid film flowing outward, and the micromixing and reaction are influenced by the fractions p1 and p2 of different environments. It is evidenced that different factors such as rotational speeds, volume ratios and initial mixing concentration impose considerable influences on the micromixing and reaction processes. The micromixing time ranges about 5.2×10−2–1×10−3ms showing that micromixing performance in RPB is much better. The main product and by-product appear different evolution pattern along the package wires, which provide guideline for the optimization of RPB structure. The optimization on the RPB structure is provided and the simulation results clarify a deeper understanding on the micromixing and reaction features in RPB.

Computational Fluid Dynamics Analysis of the Micromixing Efficiency in a Rotating-Packed-Bed Reactor

Micromixing in rotating-packed-bed (RPB) reactors is of great significance for their process-intensifying performance. On the basis of the iodide–iodate reaction system, a two-dimensional computational framework of RPB was developed to investigate the micromixing efficiency in a RPB. The volume-of-fluid multiphase model, laminar finite-rate model, and the Reynolds stress model were adopted to simulate the volumetric fraction of the liquid phase, the concentration distributions, and the effects of rotating speed and liquid flow velocity on the micromixing performance of a RPB. The computational fluid dynamics results showed that the micromixing and reaction processes occurred mainly in the inlet region of RPB packing, which further confirmed the end effect of the packing. An increase of the rotating speed and liquid flow velocity could remarkably enhance the micromixing efficiency in a RPB. On the basis of the incorporation model, the micromixing time in a RPB was estimated as 0.05−0.30 ms, indicating a re...

Measuring precipitation kinetics of sparingly soluble salts using Shock-Freeze Cryo-TEM

The precipitation of barium sulphate in a rapid mixing device (Y-mixer) coupled with an instantaneous sample freezing device was studied in order to accurately measure the nucleation rate. In this Shock-Freeze Cryo-TEM (SFCT) approach, a small volume of solution was removed from the stream directly into liquid ethane by means of a gravitational guillotine. Using different pipe lengths to measure various residence times, it was possible to determine the particle size, morphology, numbers of primary particles and thus nucleation rate. The nucleation rates were found to be a strong function of Reynolds number, up to Re=3×104 (Re being inversely related to the micromixing time). The measured nucleation rates ranged between 2.79×1014 and 3.81×1017 # m−3s−1 at supersaturations between 4×106 and 1×108. The measured particle sizes in this work (ranging from 62 to 320nm for supersaturation values between 4×106 and 1×108) were smaller than those measured by previous researchers, possibly because previous work did not quench the reaction sufficiently fast and thus allowed the particles a longer time to increase in size. In summary, this work successfully achieved a fast, accurate and reproducible nucleation rate measurement that could also give information about particle morphology.

Theoretical analysis of fluid mixing time in liquid-continuous impinging streams reactor

The mixing time of impact zone in liquid-continuous impinging streams reactor (LISR) is theoretically calculated by empirical model and modern micromixing model of the fluid mixing process, and the variation laws of macromixing time and micromixing time are quantitatively discussed. The results show that under a continuous and stable operating condition, as the paddle speed increases, the macromixing time and micromixing time calculated by the two models both decrease, even in a linkage equilibrium state. Simultaneously, as the paddle speed increases, the results figured by the two models tend to be consistent. It indicates that two models both are more suitable for calculation of mixing time in high paddle speed. Compared with the existing experimental results of this type of reactor, the mixing time computed in the speed of 1500 r/min is closer to it. These conclusions can provide an important reference for systematically studying the strengthening mechanism of LISR under continuous mixing conditions.

Micromixing characterisation in rapid mixing devices by chemical methods and LES modelling

The work reported here describes a Computational Fluid Dynamics (CFD) modelling of micromixing using the Large Eddy Simulation (LES) approach. Two models are proposed in order to define the micromixing time constant from the LES parameters. The modelling is applied to study mixing efficiency in three high-intensity mixers of the same family and commonly used: T-shaped tube, Y-shaped tube with a 90° angle and Hartridge–Roughton mixing device. Micromixing is also experimentally investigated using two chemical methods, the “iodide–iodate” micromixing test reaction and an acid–base neutralization test. Micromixing time values determined by both methods lead to the same mixer classification, the Hartridge–Roughton configuration being much more efficient than the two other ones. Numerical results are consistent with experimental ones, which validates the LES micromixing models developed. The analysis of the flow field and the turbulent structures inside the three mixers shows that the Hartridge–Roughton tube is characterised by a high rotating central structure whereas, contrary to the Y-shaped tube which reveals a very weak presence of turbulent structures.

Cationic Polymerization Initiated By Electrochemically Generated Dendritic Cations

Dendritic diarylcarbenium ions, which were generated by the electrochemical oxidation of dendritic diarylmethylsilane, were found to initiate the cationic polymerization of vinyl ethers in flow microreactor systems. Extremely fast micromixing and high-resolution residence time control are responsible for narrow molecular weight distribution of the resulting linear-dendritic hybrid polymers. The end-functionalization was also successfully achieved.

Flow Characteristics in a Scaled-up Multi-inlet Vortex Nanoprecipitation Reactor

The microscale multi-inlet vortex reactor (MIVR) has been developed for use in flash nanoprecipitation, a technique to generate functional nanoparticles. A scaled-up MIVR is motivated by the desire for a higher output of nanoparticles than the microscale reactor can provide. As the first step of this scaling process, the flow characteristics in a macro-scale MIVR have been investigated by stereoscopic particle image velocimetry. The studied Reynolds numbers based on the inlet geometry range from 3290 to 8225, resulting in a turbulent swirling flow within the reactor. The flow in the mixing chamber is found to be unstable with a wandering vortex center. The vortex wandering is constrained to a small area near the center of the reactor and has little effect on the mean velocity field. However, the measured turbulence kinetic energy and Reynolds stresses are found to be sensitive to the vortex wandering. The flow characteristics of the macro-scale MIVR are compared with the microscale MIVR in terms of swirl ratio and micromixing time. It is found that the swirl ratio and micromixing time of the flow increases as the MIVR is scaled up, indicating a flow with stronger swirl yet less mixing effectiveness in the scaled-up reactor.