Determination of the micromixing level in a CSTR: transient step response of reactive tracers

An experimental technique for the determination of the level of micromixing in a CSTR is proposed. The technique involves the generation and the analysis of the transient step response of reactive tracers with non-linear kinetics. Two existing micromixing models, a two-environment model based on the spectral interpretation of turbulent mixing and Toor's hypothesis, are used to develop the predictive equations for the determination of the limits and the level of micromixing. The effects of the initial reactants concentration ratio, the intensity of mixing and the Damkohler number on the level of micromixing, predicted by the two models, are then compared.     

NETmix®, a new type of static mixer: Modeling,simulation, macromixing,and micromixing characterization

NETmix® is a new technology for static mixing based on a network of chambers connected by channels. The NETmix® model is the basis of a flow simulator coupled with chemical reaction used to characterize macro and micromixing in structured porous media. The chambers are modeled as perfectly mixing zones and the channels as plug flow perfect segregation zones. A segregation parameter is introduced as the ratio between the channels volume and the whole network volume. Different kinetics and reactants injection schemes can be implemented. Results show that the number of rows in the flow direction and the segregation parameter control both macro and micromixing, but the degree of micromixing is also controlled by the reactants injection scheme. The NETmix® model enables the systematic study of micromixing and macromixing for different network structures and reaction schemes, enabling the design of network structures to ensure the desired yield and selectivity. © 2009 American Institute of Chemical Engineers AIChE J, 2009     

SECONDARY MIXING OF IMPINGING SHEETS

The increase in the overall micromixing rate of two impinging sheets that results from subsequent impingement of the mixed sheet into a barrier was studied using mixing-sensitive chemical reactions. This “secondary micromixing” of the reactants yielded overall micromixing times that were up to an order of magnitude lower than the micromixing times without subsequent impingement of the mixed sheet (primary micromixing). Employing the same diffusion model for secondary micromixing, used previously to explain primary micromixing, suggests that the liquid viscosity, the mixed sheet velocity, and the characteristic dimension of the barrier are important parameters in determining the secondary micromixing rate. In addition, the distance from the impingement zone, at which the barrier is placed, is a key parameter in determining the amount of enhancement in the overall micromixing rate. The greatest enhancement was achieved by placing the barrier at the end of the mixed sheet where it breaks up into droplets. At this point the energy release resulting from the impingement of the mixed sheet into the barrier yielded calculated striation thicknesses that were typically 3 to 4 times smaller than the calculated striation thicknesses resulting from the energy release due only to the impingement of the single sheets.     

Influence of hydrodynamics on liquid mixing during Taylor flow in a microchannel

The miscible liquid‐liquid two phases based on Taylor flow in microchannels was investigated by high‐speed imaging techniques and Villermaux/Dushman reaction. The mixing based on Taylor flow was much better compared with that without introducing gas in microchannels, even the ideal micromixing performance could be obtained under optimized superficial gas and liquid velocities. In the mixing process based on Taylor flow, the superficial gas and liquid velocities affected the lengths and the velocities of Taylor bubble and liquid slug, and finally the micromixing performance. The formation process of Taylor flow in the inlets, the initial uniform distribution of reactants and the internal circulations in the liquid slug, and the thin liquid films all improved the mixing performance. Furthermore, a modified Peclet number that represented the relative importance of diffusion and convection in the mixing process was proposed for explaining and anticipating micromixing efficiency. © 2011 American Institute of Chemical Engineers AIChE J, 58: 1660–1670, 2012     

撞击流混合器微观混合性能的研究

撞击流混合器是一种新型的微观混合器,今研究了T型直流对撞(IS)、锥形对撞(CIS)、直流旋撞(VS)和二次旋流旋撞(TVS)四种结构的撞击流混合器的微观混合性能。然后以Villermauxu/Dushman快速平行竞争反应测定混合器的离集指数Xs,并考察了流体流速、流体流速比和混合器结构对离集指数的影响。混合器混合效果用离集指数来衡量,离集指数越小混合效果越好。结果表明:其余工艺条件相同的情况下,流体流速越大,离集指数Xs愈小;两股流体流速比越小,离集指数Xs愈小;喷嘴进口管道直径越小,离集指数Xs愈小。锥形的比直线形的、旋流比直接撞击流混合效果要好,而且旋流使物料在混合器中的停留时间延长;根据实验数据模拟计算,T形撞击流微混合器的微观混合时间在1 ms数量级;用Fluent 6.2.1商业软件模拟计算了混合器内的流场分布情况,发现模拟计算结果和实验结果基本吻合。     

Use of relative reaction rates of CO and H2 as a measure of micro-mixing in combustion systems

A technique is proposed for the characterization of the intensity of micromixing in well-stirred, homogeneous chemical reactors used to carry out fast, gaseous-phase reactions. This involves allowing the competitive combustion of a lean feed mixture of carbon monoxide and hydrogen in air to take place in the reactor. The ratio of the concentrations of the two reactants, CO and H₂, in the system is to be measured and its value, which will be higher than that of the feed due to the difference in reaction rates, used to infer the degree of micromixing by comparison with computational predictions. A simple population-balance model and a detailed kinetic scheme are employed to explore the range of usefulness of the suggested procedure and demonstrate its practical applicability in producing meaningful assessments of mixing in real systems.     

Production of commercial dyes via impingement-sheet mixing: Part I. Testing of a device suitable for industrial application

Impingement-sheet mixing is a proven technique for the rapid mixing of liquids on the laboratory scale. In this paper a practical mixer design for use on the industrial scale is presented. The industrial impingement-sheet mixer was tested at flow rate ratios typical of commercial applications and, compared with earlier laboratory results, only a slight loss in mixing speed was noted. At flow rates of the order of liters/minute and pressure drops up to 1.5 bar, the micromixing times of the industrial impingement-sheet mixer are of the order of tens of milliseconds for reactant stoichiometric ratios near 1.00. If one of the reactants is present in at least a 10% excess, then the micromixing time of the limiting reagent is reduced to several milliseconds.     

Simulating the effects of mixing on the performance of unpremixed flow chemical reactors

A parallel three environment model is developed to describe the effects of incomplete mixing on the performance of continuous flow chemical reactors fed separately by feedstreams which may have arbitrary flowrate and arbitrary residence time distribution. Unpremixed reactants are assumed to first enter partially segregated entering environments and subsequently transfer to partially mixed leaving environments at a rate and in amounts defined by an environment transfer function. Mixing in the environments is stimulated stochastically by Monte Carlo techniques. Calculations based on a kinetic model of the Michaelis-Menten type for arbitrarily selected pairs of residence time distributions suggest that performance is strongly influenced by the degree of overlap of the distributions and the method of feeding the reactants. The model is shown to compare directly with published experimental data obtained from a “jet-stirred” reactor and an equivalence found between the models' parameters and those of previously developed micromixing models.     

MODELLING AND SCALEUP OF DISPERSED PHASE LIQUID-LIQUID REACTORS

The problem of modelling dispersed phase liquid-liquid reactors is discussed from a global view. The two major areas of microscopic and macroscopic problems are addressed. The microscopic problem is concerned with the determination of the local rate of transfer of reactants and/or products between the two phases. Attention is focussed on approaches to obtain kinetic data; and results on recent important chemical systems such as nitrations, metal chelation reactions, and phase transfer catalytic reactions are discussed. The liquid jet recycle reactor is pointed out as a useful tool for obtaining laboratory data. Recent works employing the classical film and penetration theories to obtain flux expressions for complex reactions are described. The macroscopic problem deals with the reactor design question. Various models proposed to account for macromixing and/or micromixing effects are categorized into noninteraction and coalescence- dispersion or interaction models. The former approach includes the axial dispersion and CSTR models and can predict conversions at the extremes of micromixing. (See the previous paper in this series by Nauman for complete discussion of these models.) The basic formulations of these models and results are discussed in this paper, The latter approach discussed here includes population balance equations and Monte Carlo simulation methods. The ability of Monte Carlo simulation techniques to predict the effect of intermediate degrees of micromixing on conversion is demonstrated. The potential of the Monte Carlo simulation technique to account for local variations in dispersion properties, model droplet rate processes, and model complex reaction systems is also shown. (See the paper by Patterson in this series for more discussion of the application of Monte Carlo stimulation to complex reactions.)     

Modeling the effects of micromixing and start-up procedures on bulk copolymerization and copolymer in a tubular reactor

The combined effects of micromixing and start-up procedures on free radical, bulk copolymerization of styrene and acrylonitrile in an isothermal, premixed-feed tubular reactor have been theoretically analyzed. An axial dispersion model, which takes into account the entire range of backmixing, froms the basis of this analysis. Model predictions show that the overall conversion decreases with the increase of initial styrene content in the reactor, and is not affected by the degree of micromixing. However, for the nonazeotropic feed, the copolymer composition distribution becomes wider with the increase of initial acrylonitrile content in the reactor. For the azeotropic feed, broadening occurs with the decrease of initial acrylonitrile content in the reactor. Average copolymer composition is not affected either by micromixing or start-up procedure.     

微观混和对均相快速反应选择性的影响 (Ⅰ)双球模型及其模拟计算

均相快速复杂反应实质上是一个非定态的扩散-反应过程.反应产品分布取决于两股反应物料的快速微观混和.本文提出了一种混和模型——双球模型,并采用合适的数值计算方法,以竟争串连二级反应为例,模拟计算了各种参数对反应结果的影响.     

Investigation of mixing in a rotor shape modified Taylor-vortex reactor by the means of a chemical test reaction

The macro- and micromixing properties of a continuous flow Taylor-vortex reactor can be optimised by changing the conventional cylindrical rotor geometry into a novel ribbed one. A chemical test reaction, the micromixing-sensitive alkaline saponification of ethylacetate with separately fed reactants, was used to probe the mixing performance down to the molecular level. Experiments were performed in a continuous flow Taylor-vortex reactor equipped either with a conventional cylindrical rotor or with a novel ribbed rotor in a wide hydrodynamic range of 150<Ta<8000 and 0.8<Re<2.0.Through increase in the reaction temperature and the feed concentrations, the relaxation times of this reaction were reduced from 680 to 19 s and compared to micromixing times by monitoring the reactor conversion. The results show that a TVR with conventional rotor achieves intense micromixing at high rotor speed, but behaves like a CSTR. In contrast, a device with ribbed rotor shows macromixing features close to those of a plug flow reactor (PFR) in a wide range of rotor speeds; however, segregation of the two feeds could only be dissipated at slow reaction rates, i.e. relaxation times larger than 64 s.     

Regulating the micromixing efficiency of a novel helical tube reactor by premixing behavior optimization

In this work, a novel helical tube reactor (HTR) was constructed, including a pre‐mixer for adjusting the premixing behavior of reactants and a helical tube as a further mixing unit. The pre‐mixer was modified to optimize the premixing behavior by using two methods, named as tangential‐feeding and insertion of a helical baffle. The premixing behaviors were investigated via computational fluid dynamics (CFD) simulation. Simulation results indicated that both methods can change the fluid flow, enhance the turbulence kinetic energy, and improve the premixing performance in the pre‐mixers. Based on the results of CFD simulation, it could be predicted that the micromixing efficiency of the HTR can be regulated by these methods accordingly. Then the predicated results were confirmed experimentally by a parallel competing reaction. Furthermore, the relationship between the premixing performance increasing and the corresponding micromixing efficiency increasing of the HTR was quantitatively analyzed. © 2017 American Institute of Chemical Engineers AIChE J, 63: 2876–2887, 2017     

管式填料床反应器内的湍流微观混合研究

采用碘化物-碘酸盐平行竞争反应体系研究了管式填料床反应器内的湍流微观混合.通过沿程取样分析来考察离集指数(Xs)沿反应器轴向位置的分布.实验结果表明,反应物在管式填料床反应器内的离集主要发生在反应物开始相遇的区域且随着流速的增加离集状况得到明显改善.在实验基础上,采用团聚(Incorporation)模型计算得到管式填料床反应器的微观混合时间tm在1×10-3s到4×10-3s之间,明显小于搅拌槽反应器内的微观混合时间.相对于搅拌槽反应器,管式填料床反应器更适合于快速反应.同时研究表明,丝网填料的加入能提高微观混合效率.     

Micromixing-assisted preparation of TiO2 films from ammonium hexafluorotitanate and urea by liquid phase deposition based on simulation of mixing process in T-shaped micromixer

Preparation of well-ordered TiO₂ films from alkaline solution by liquid phase deposition (LPD) method is still challenging owing to the fact that, in traditional mixing process, several disadvantages exist in controllable homogenous chemical reactions. In this contribution, mixing process in a porous dispersed double T-junction micromixer was designed and investigated using numerical simulation. It is obtained that high mixing intensity can be achieved by micromixer with inlet velocity of 0.6?m?s^?1 and microfiltration meshes 10?μm in size. A micromixing-assisted platform consisted of two syringe pumps and a micromixer was manufactured to efficiently fabricate TiO₂ films via mixing of (NH₄)₂TiF₆ and CO(NH₂)₂ as reactants and fluorine doped tin oxide, F: SnO₂ (FTO), conducting glass substrates by LPD in alkaline solutions. The results suggest that surface morphology of TiO₂ films fabricated by micromixing can be easily controlled in comparison with that of specimens prepared by traditional stirring mixing process. The dense and well-ordered TiO₂ films derived from micromixing have enhanced adhesion strength, high hardness, good hydrophilic and excellent photoelectrochemical properties. Particularly, homogeneous reaction could be inhibited in micromixer and an ideal supersaturation solution would be formed via micromixing treatment, which significantly facilitates the formation of high quality TiO₂ films by heterogeneous nucleation on substrate surface. These obtained achievements are expected to promote further application of TiO₂ films in a variety of domains including photocatalysis and corrosion protection of metals.     

反应与溶析结晶过程强化及数值模拟研究进展

反应与溶析结晶技术被广泛地用于无机与有机化学品的生产，如催化剂和药物活性组分。反应与溶析结晶过程通常是在高过饱和度条件下进行，具有快速的成核与生长速率，因此需要在结晶过程开始前实现不同反应物或溶析剂与溶液之间快速充分的混合，以避免不良混合造成过饱和度的空间不均匀分布，破坏晶体产品的性质。模型和数值模拟方法的分析和预测能力可以加深对过程现象的机理认识，促进结晶设备和操作条件的优化设计。本文综述了反应与溶析结晶的过程强化方法和数值模拟的研究进展。首先从结晶设备、外加能量场和膜技术辅助结晶三个方面对过程强化研究进行了阐述；然后介绍了数值模拟中常用的微观混合模型，包括涡流卷吸模型和基于联合组成概率密度函数的微观混合模型；最后对文献中的液液反应结晶、气液反应结晶和溶析结晶的数值模拟进行了总结与分析。针对已有研究的不足，提出了未来发展方向的一些展望。     

The reversed two-environment model of micromixing and growth processes

A reversed two-environment model of micromixing in a flow reactor in which the feed enters a maximum mixedness environment and then passes to a segregated environment is presented. The performance equations for an arbitrary residence time distribution are developed and employed to analyse the behaviour of growth processes in a continuous stirred tank reactor under intermediate states of micromixing. The results, which are bounded by the two extremes of micromixing, indicate that micromixing is important in growth processes and that segregation appears to reduce the rate of cell production.     

Scalable synthesis of ytterbium and erbium codoped calcium molybdate phosphors as upconversion luminescent thermometer

Luminescent thermometry is a noninvasive method of temperature detection with high sensitivity and response speed. The present study demonstrated the process-intensified synthesis of ytterbium and erbium codoped calcium molybdate phosphors (CaMoO₄:Yb³⁺/Er³⁺). The experiment involved the initial premixing of the precursors using a high-gravity rotating packed bed (RPB) reactor and subsequent calcination processing. The pronounced mass transfer and micromixing of the reactants in the RPB facilitated the scalable and controllable synthesis of CaMoO₄:Yb³⁺/Er³⁺ particles with submicron sizes and regular morphologies. The CaMoO₄:Yb³⁺/Er³⁺ particles exhibited a bright-green emission with temperature-dependent luminescence characteristics under 980 nm laser irradiation. Furthermore, the maximum absolute sensitivity was determined to be 0.02837 K⁻¹. These results indicated that the synthesized product was a suitable candidate for application in upconversion luminescent thermometers capable of temperature sensing at the microscale.     

Makro- und Mikromischen im Rührkessel

Macro- and micromixing in stirred tanks. Macromixing time, based on turbulence theory, is calculated theoretically as a function of the specific power input, the kinematic viscosity, the Schmidt number, and the degree of segregation. A comparison of the micromixing time with data for macromixing found in literature reveals that the micromixing time governs the reaction for very small vessel diameters. If the vessel diameter is greater than the critical size than the turbulent macromixing time controls the mixing process. The micromixing time θ_mikro is calculated by means of the experimental results of the turbulent velocity distributions in different stirred vessels for several geometrical arrangements of stirred tanks and different positions of feed input. The calculated data for turbulent mixing times are only valid for geometrically optimized stirred vessels. The necessary mixing time may increase or, if a chemical reaction of 2nd order takes place, the desired yield of the product may decrease considerably if the geometrical conditions are not taken into account.     

Effect of high frequency ultrasound on micromixing efficiency in microchannels

In the present work, an investigation on the effect of high frequency ultrasound wave on micromixing in the studied microchannels was carried out. Three types of microchannels with different shapes are examined. A 1.7 MHz piezoelectric transducer (PZT) was employed to induce the vibration in these microchannels through an indirect contact. A method based on the Villermaux–Dushman reaction was employed to study the micromixing in these microchannels. The segregation intensity was determined for layouts with and in the absence of ultrasound irradiation. Further, the effect of ultrasound waves, in various flow rates and initial concentrations of acid, on the segregation index (X_S) and micromixing time (t_m) was investigated. The experimental results showed that the ultrasound waves have a significant influence on product distribution and segregation index at various flow rate ratios. The data obtained in all cases showed that the segregation index was reduced when the flow rate ratios were increased. Also the results demonstrate that in spite of a low energy consumption of PZT, the relative segregation index improved up to 18–36% at various flow rate ratios.