Micromixing Time Research Articles

Enhanced mixing characteristics of unbaffled U-shaped microreactor coupled oscillatory flow

The Microscale Oscillatory Flow Reactor (MOFR) can achieve good plug flow and micromixing performance simultaneously at laminar net flow conditions. An unbaffled U-shaped microreactor coupled with oscillating flow technology was designed to study the macro and micromixing performance. Firstly, the influence of oscillations on the flow performance was studied to reveal the formation rule of vortex. The simulation results showed that the continuous formation and destruction of periodic vortexes occurred in the microreactor with oscillation. With the increase of oscillation intensity, the vortex size in the radial direction first gradually increased and then becomes stable, and gradually moved axially, resulting in axial diffusion. Secondly, the effect of oscillation on the macromixing and micromixing performance were investigated. The results showed that the coupling oscillation could greatly improve the macromixing and micromixing performance. The macromixing and micromixing performance were promoted simultaneously at lower oscillation intensity and then tended to be flat due to the axial diffusion at high oscillation intensity. When φ>6.05, the minimum micromixing time and the maximum number of tanks can be achieved at the same time. At a velocity ratio of about 23, FoM reached a maximum of about 3.5.

Micro-mixing enhancement in a Taylor-Couette reactor using the inner rotors with various surface configurations

This study investigates the effects of various inner cylinder configurations on micromixing and fluid dynamics within a Taylor-Couette (TC) reactor using the inner cylinders with different surface designs, including the traditional smooth-surfaced rotor cylinder. The four innovative inner cylinders were specifically designed with axial corrugations (N40 and N80) and three-dimensional rough surfaces (NZ40 and NZ80). Micromixing efficiency was assessed experimentally using the iodide-iodate reaction as a probe. To further understand the impact of the rotors' surface structures on micromixing, computational fluid dynamics (CFD) modelling was utilized to analyse the fluid dynamics within the TC reactor. An incorporation model was employed to calculate the micromixing time. The experimental findings reveal that the segregation index decreases with increasing rotation speed for all inner cylinders. Besides, NZ80′s micro-mixing efficiency surpasses that of its counterparts, NZ40, N40, and N80. The CFD modelling results underscore the significant influence of the inner cylinder's surface configuration on the turbulence dissipation rate and volume probability distribution, which are likely to contribute positively to the micromixing efficiency within the TC reactor. Furthermore, the empirical correlations obtained have been established to understand the micro-mixing time within TC reactors using different rotating cylinders.

Influence of Viscosity on Variously Scaled Batch Cooling Crystallization from Aqueous Erythritol, Glucose, Xylitol, and Xylose Solutions.

This study presents a comprehensive comparison of the batch cooling crystallization performance of aqueous solutions containing sugars and sugar alcohols, namely, erythritol, glucose, xylitol, and xylose. Erythritol and xylitol are commonly used alternative sweeteners to replace sucrose. They can be obtained by fermentation-based bioprocesses, where glucose and xylose are typical raw materials. These model compounds were selected based on their differing rheological nature: saturated erythritol solution has a viscosity lower than 3 mPa·s, whereas xylitol has the highest viscosity: greater than 90 mPa·s in the studied temperature range. Viscosities and densities of saturated solutions as well as apparent viscosities of crystal-mother liquor suspensions were measured. The purpose was to evaluate their crystallization behavior within a specific temperature range from 40 to 20 °C and batch time of 2 h, with the aim of understanding the influence of viscosity on the process more comprehensively. The comparison within the selected compound systems was carried out in terms of the physical properties of the mother liquor and the crystalline product. In addition to empirical laboratory-scale (0.1 and 1 L) studies, larger-scale simulations (1 and 100 m3) were performed with the experimental data obtained on average particle size, density, and viscosity for mother liquor and crystal-mother liquor suspensions. Mixing characteristics, such as the dissipation energy, mass transfer coefficient, energy of collisions, and micromixing time, were calculated with VisiMix software when using a single or dual impeller mixer. Furthermore, the scaling up of erythritol, xylitol, glucose, and xylose batch cooling crystallization from 40 to 20 °C based on the scaling-up rule of constant tip speed and energy of dissipation was done with VisiMix to obtain overall data on mixing conditions with crystallizers of 1 and 100 m3 in volume. Furthermore, ANSYS CFD software was used to determine the strain rates close to the impeller tip and velocity profiles on various crystallizer scales.

A study with PUREX aqueous-organic pair in Taylor-Couette mixing field

The hydrodynamics and mass transfer in the Taylor-Couette mixing field are important activities for concurrent process intensification work for miniaturization of the chemical reactor volume and enhancing the energy input per unit volume. In the literature, many studies have been reported for mixing of conventional two-phase systems of industrial interest. However reported studies on mixing of PUREX aqueous organic pair in Taylor-Couette field are rare and few. Utilizing an indigenously developed Taylor-Couette column, hydrodynamic and mass transfer studies with PUREX aqueous-organic pair are reported in this study. Mass transfer and hydrodynamics runs were conducted with PUREX aqueous-organic pair. Aqueous dispersed conditions were employed for d→c mass transfer. Mass transfer performance was highly dependent on rotor speed till a critical value. Macro, meso and micromixing time values were evaluated for Taylor-Couette mixing. Prototyping of the system was completed with a special run with 0.1 M TODGA/n-dodecane nitric acid pair for mutual separation of Cs(I) and Sr(II).

Image‐based measurement of engulfment vortex to quantitatively assess micromixing efficiency in a gas–liquid stirred tank

AbstractMicromixing in reactors is typically characterized by chemical test reactions. A novel image‐based measurement system of engulfment vortex (IMEV system) is developed to assess the micromixing efficiency. Over 2000 vortices were recognized from images captured under various experimental cases, and key frames for their formation, engulfment and dissipation were identified. Inspired by the engulfment model (i.e., E model), this work quantified the micromixing time and local turbulent energy dissipation rate with the vortex lifetimes. The micromixing time obtained through the IMEV method is comparable to its value estimated by the micromixing model and segregation index of the test reaction, with a deviation of less than 20%. In turbulent gas–liquid systems, bubbles play a dual role on the local micromixing due to frequent disturbance and phase interface limitation. To summarize, this work offers an alternative approach for micromixing studies and provides valuable insights into dispersed phase effects on the micromixing efficiency.

Micromixing Performance in a Taylor–Couette Reactor with Ribbed Rotors

The Taylor–Couette reactor (TCR) is becoming an increasingly significant topic in chemical industry. This study investigates the micromixing performance of a ribbed TCR with axial flow in the Villermaux–Dushman reaction system. The local micromixing mechanism of the ribbed TCR was analyzed, and the volume-averaged energy dissipation rate was calculated using CFD. The effects of operating parameters and rib structural parameters on micromixing performance were investigated. The results show that the introduction of ribs eliminates the high shear region between the vortex pairs, resulting in the strong micromixing region being situated on the inner and outer cylinder wall surfaces and the ribbed surface region. Smaller rib spacing, larger rib width, and rib height can strengthen micromixing and result in a smaller segregation index. Micromixing times of ribbed TCRs were calculated using the incorporation model, tm, in the range of 2.0 × 10−5 to 8.0 × 10−3. The results show that ribbed TCRs require a lower energy consumption to achieve a lower tm than other rotating reactors. A correlation equation between tm and five parameters was developed, with a correlation coefficient of 0.951. The accuracy of the volume-averaged energy dissipation rate obtained via CFD was verified through experimental analysis. The correlation between the micromixing time and the volume-averaged energy dissipation rate was established in a form that satisfies Kolmogorov’s turbulence theory for tm. To convert the volume-averaged energy dissipation rate into a local energy dissipation rate, a factor ϕ was introduced and solved using the engulfing diffusion model. This study provides insights into the design and optimization of ribbed TCRs.

The structure-effect relationship between inline high shear mixers and micromixing: Experiment and CFD simulation

High shear mixers (HSMs) can facilitate rapid reactions and control nanoparticle preparation due to their excellent micromixing, but little is known about the sophisticated relationship between HSM geometry and micromixing. Herein, experiments and numerical simulations along with a modified finite rate/eddy dissipation model are used to investigate how structure influences micromixing. Micromixing and chemical reaction results at different rotor–stator relative positions are presented, and structure-effect relationships between rotor–stator structure and micromixing are explored. A larger rotor–stator row can help reduce fluid backmixing and short-circuiting, and a smaller shear gap may increase energy dissipation, as well as a wider slot that will extend the residence time of H+ in the high energy dissipation region. In all these ways, HSM micromixing will be improved. Micromixing time tm (about 10-4 s) is obtained based on E model and incorporation model, and the HSM selection guide for fast chemical reactions is presented.

Characterization of macromixing and micromixing performance of unbaffled U-shaped mesoscale oscillatory flow reactor

BackgroundUnbaffled U-shaped mesoscale oscillatory flow reactors (meso‑OFRs) are able to achieve low pressure drop, good plug flow and micromixing performance simultaneously at laminar net flow conditions under a wide ‘‘operating windows”. MethodWe focus on the macromixing and micromixing behaviours of unbaffled meso‑OFR, where the net flow and oscillation conditions were in the wide range of Ren=17–51 and Reo=16–1350, respectively. Significant findingsThe good plug flow and micromixing performance could be achieved simultaneously at low oscillation intensity with the wide ‘‘operating windows” of ψ<47. It was believed that oscillatory motion led to periodic destruction and reformation of vortices, as the oscillation intensity increased, the large-scale vortices were decomposed into small-scale vortices, resulting in backmixing or axial dispersion. Meanwhile, the empirical formula of micromixing time and velocity ratio in accordance with the power-law relationship was established:tm/t¯=0.00277ψ0.51658. Finally, the oscillatory mechanism was qualitatively validated by Computational Fluid Dynamics (CFD).

Micromixing Performance and Residence Time Distribution in a Miniaturized Magnetic Reactor: Experimental Investigation and Machine Learning Modeling

Miniaturization of mixers is a hot topic in process intensification, but efficient mixing at operational conditions with relatively large viscosity μ and large flow rate ratio R between two mixed solutions is still a great challenge for micromixers. Herein, a miniaturized magnetic reactor (MMR) is developed, and its micro- and macromixing performances are characterized by the Villermaux/Dushman reaction and residence time distribution experiments, respectively. The results show that the miniaturization of the premixing unit and the stirred unit is vital. By arranging the stirred units in series, the MMR is effective in reducing axial dispersion, intensifying mixing at operational conditions with large μ and R and achieving efficient energy utilization without introducing extra energy input. The micromixing time tm in the optimized MMR ranges from 0.24 to 1.42 ms when 4.72 mPa·s < μ < 73.30 mPa·s and 8 < R < 20, indicating superior micromixing performance. Moreover, machine learning models are established and compared for predicting tm with excellent accuracy and guiding mixer design, and the SHAP method is adopted for model interpretation.

Investigation of mass transfer and hydrodynamics of liquid-liquid extraction in spinning disc reactor by computational fluid dynamics simulation

This research investigated the mass transfer performance of the extraction of benzoic acid in water/n-heptane system in a spinning disc reactor (SDR). The effects of rotational disc speed (R), volumetric flow rate (Q) and disc surface on the liquid flow pattern, extraction efficiency (E) and overall mass transfer coefficient (kLa) were studied. Three disc surface types were considered including 1) smooth disc, 2) roughness A disc (metal sandpaper P36), and 3) roughness B disc (metal sandpaper P24). The highest values of E and kLa were 69.7% and 9.59 s−1 when using the roughness B disc surface operated at R of 1000 rpm and Q of 5 mL/s. A 3D-multiphase CFD model was applied to investigate the hydrodynamic characteristics in the SDR for further understanding of the mass transfer performance in the SDR. The dissipation rate (ε) and micromixing time (tm) were proposed to compare the turbulent degree of each disc surface. The mass transfer performance was significantly enhanced by using the metal sandpaper as the disc surface. In addition, the metal sandpaper disc surface has a great ability to reduce energy consumption in comparison with the smooth disc surface.

Micromixing performance of a miniaturized annular rotating flow mixer (MARFM)

Miniaturization of mixers is becoming an increasingly significant topic in process intensification. In this work, we develop a novel miniaturized annular rotating flow mixer (MARFM) by integrating the rotating flow field and micro-scale effect. The micromixing performance of MARFM is evaluated by the Villermaux/Dushman reaction system. Effects of various structural parameters on the micromixing performance are investigated. Two flow regions named stratification region and entrainment region have been firstly distinguished. Increasing rotational speed can remarkably improve micromixing performance under the stratification region rather than the entrainment region. In addition, the mixing performance at extreme flow conditions including high fluid viscosity and high flow rate ratio can be substantially improved in the MARFM. Segregation index is at least an order of magnitude smaller than that without the rotating flow field. Moreover, the micromixing time could range from 10−5 to 10−3 s in the MARFM, which is 1-3 orders of magnitude smaller than that of conventional mixers. Relationship between micromixing time and mean energy dissipation rate is established, which shows that higher energy utilization efficiency has been reached.

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

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 and 305 s and Bodenstein numbers ranging between 1.4 and 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.

Development of an aqueous lignin mixture thermophysical model for hydrothermal liquefaction applications using uncertainty quantification tools

A thermophysical model is developed that can predict the properties of two lignin mixtures, black liquor and lignosulfonates, up to 50% mass fractions, at hydrothermal conditions. An uncertainty quantification framework linked with classic thermodynamical modelling was included to account for the extreme variability of the raw material. An idealised flow simulation verified the model, where hot compressed water mixes with a cold, aqueous lignin stream in a T-piece reactor configuration. The uncertainty quantification procedure determined that density and heat capacity uncertainty significantly influence residence time, and viscosity uncertainty mainly affects mixing. Micromixing time is fivefold and ten-fold higher for black liquor and lignosulfonates mixtures, respectively, compared to pure water mixing. The uncertainty in all simulated quantities of interest caused by the thermophysical model is reduced by increasing flow rates. This study predicted chemical reactor behaviour under varying thermophysical conditions and their final effect in terms of confidence intervals.

A consecutive‐competitive reaction system for assessing homogeneous and heterogeneous liquid–liquid mixing efficiency

AbstractHere, the consecutive‐competitive reaction system about aspirin and sodium hydroxide was proposed as a unified chemical probe to characterize the homogenous and heterogeneous liquid–liquid mixing. The kinetics of the reaction system was clearly studied. Then the mixing efficiencies were assessed in a standard stirred tank with this chemical probe and the obtained segregation index (XS) could well characterize the mixing performance. Based on the incorporation model, the estimation formula of micromixing time was established as . The chemical probe could show the influence of stirring intensity, interphase resistance, and two‐phase ratio on the mixing efficiency sensitively when it was extended to heterogeneous mixing. Using this chemical probe, we found the mixing efficiencies of homogeneous and heterogeneous process tended to be the same at sufficiently high stirring intensity. The reason for this phenomenon was unveiled by investigating the dispersed phase size and calculating mass transfer time.

Intensification of liquid mixing and local turbulence using a fractal injector with staggered conformation

Two self-similar, tree-like injectors of the same fractal dimension are compared, demonstrating that other geometric parameters besides dimension play a crucial role in determining mixing performance. In one injector, when viewed from the top, the conformation of branches is eclipsed; in the other one, it is staggered. The flow field and the fractal injector induced mixing performance are investigated through computational fluid dynamics (CFD) simulations. The finite rate/eddy dissipation model (FR/EDM) is modified for fast liquid-phase reactions involving local micromixing. Under the same operating conditions, flow field uniformity and micromixing are improved when a staggered fractal injector is used. This is because of enhanced jet entrainment and local turbulence around the spatially distributed nozzles. Compared with a traditional double-ring sparger, a larger reaction region volume and lower micromixing time are obtained with fractal injectors. Local turbulence around the spatially distributed nozzles in fractal injectors improves reaction efficiency.

A comparative analysis of micro-mixing process in a confined impinging jet reactor with/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 Pultra=175W can be 88% larger than that in the case of Pultra=0W in the front of the transducer head at Rein=5000.

Micromixing efficiency intensification of a millimeter channel reactor in the high gravity field

A millimeter channel reactor which can achieve rapid mixing has been applied in many chemical processes. To intensify the micromixing performance, previous studies focused on the structure development and the optimization of operating conditions. However, using high gravity field for micromixing performance enhancement in the millimeter channel reactor has scarcely been assessed. In this work, a novel rotating millimeter channel reactor (RMCR) was proposed, which used centrifugal and Coriolis forces to further intensify the micromixing performance. The segregation index (Xs) and micromixing time (tm) of the RMCR were experimentally studied to evaluate the micromixing efficiency. Results showed that the micromixing efficiency of the RMCR was better up to 35% than that of the conventional millimeter channel reactor without rotation by the comparsions of Xs. According to the incorporation model based on the experimental data, the value of tm in the RMCR was calculated to be 10−6–10−4 s. Furthermore, the intensification mechanism analysis illustrated that the high gravity field effectively enhanced the contact with high velocity to intensify the premixing performance. This work provides a new strategy to improve the micromixing performance in the millimeter channel reactor via high gravity field.

Hydrodynamics and Mixing in a Novel Design of Compact Microreactors: Arc Flow Inverters

A novel tubular microreactor to intensify mixing, namely arc flow inverter (AFI) was proposed and the mixing behavior was investigated. The design concept of AFI expanded a design space of mixing/reactor devices consisting of curved tubes and 90 degrees bends compared with conventional coiled flow inverter (CFI). The fabrication difficulties were overcome by using 3D printing technology. Numerical study on hydrodynamics NRe ≤ 400 revealed that the profile of Dean vortices was spatially periodic along the axis of AFI. The narrower residence time distribution in AFI compared to CFI implied an enhanced lateral mixing due to the Dean vortices. Micro-mixing time in AFI was experimentally accessed via Dushman-Villermaux reaction. The AFI outperformed other fluidic devices in the literature in terms of the coefficient of mixedness.

A comprehensive investigation of the incorporation model for micromixing time calculation

The incorporation model is a widely applied micromixing model for the interpretation of micromixing experiments conducted by the Villermaux–Dushman test reaction. In this work, a deeper look into the model is taken and a modified incorporation model is proposed which leads to more reasonable results regarding the time to equalize concentration gradients, which corresponds to a micromixed state. The original definition of the micromixing time, tm, is revised and two characteristic times are introduced which can be deduced from the modified incorporation model. First, the time in which the acid–base and Dushman reaction are completed, tR, and the time until a complete equalization of concentration gradients is reached, tE, which corresponds to the actual micromixing time. In addition, an instruction to easily revise historical micromixing time calculations from the original incorporation model is given. From the incorporation model, concentration ranges for the kinetic investigation of the Dushman reaction relevant to mixing studies were extracted and are proposed for future kinetic studies of the Dushman reaction. The MatLab codes for implementing the incorporation model are published in GitHub for further scientific exchange.

High energy dissipation rates from the impingement of free paper-thin sheets of liquids: A study of the coefficient of restitution of the collision

The micromixing time of impinging thin liquid sheets depends upon the energy dissipation rate, which is typically104 to 106 W/kg. The energy dissipation rate in the impingement zone is a function of the coefficient of restitution (COR) of the collision. In this study theCOR was investigated both theoretically and experimentally. An analytical expression was developed that consists of a term that accounts for an inelastic collision of impinging sheets and a term that quantifies the “elasticity” imparted to the collision once the critical velocity for backflow is reached. The theoretical results were compared with experiments that determined theCOR by measuring the velocity of single sheets and mixed sheets. Not only do the experimental results show a jump in theCOR at the critical velocity, as predicted by theory, but calculated values of theCOR from the analytical expression compare well with measured values.