Experimental characterization and multi-scale modeling of mixing in static mixers. Part 2. Effect of viscosity and scale-up

Static micro-mixers are used in precipitation processes to avoid mixing limitations. The mixing performance of these mixers, which are used in this study to mix two streams of different viscosity, is characterized using competitive-parallel chemical reactions and computational fluid dynamics (CFD). This work is an extension of a previous paper where mixing of fluids with equal viscosity has been studied [Lindenberg, C., Schöll, J., Vicum, L., Brozio, J., Mazzotti, M., 2008. Experimental characterization and multi-scale modeling of mixing in static mixers. Chemical Engineering Science 63, 4135-4149]. It is found that the mixing performance in terms of reaction yield and mixing time decreases slightly with increasing viscosity ratio in a two jet vortex mixer (Roughton mixer). In the Y-mixer the trend is the same at low flow rates, but it is the opposite at large flow rates due to a symmetry breaking phenomenon. The Roughton mixer is scaled-up using the CFD model and a linear relationship between scale-up factor and mixing time is observed. Finally, it is shown that mixing times can be described satisfactorily as a function of velocity, jet diameter and viscosity.     

Experimental evaluation of gas mixing with a static microstructure mixer

Mixing plays an important role in chemical reaction engineering. In the last years several types of static microstructure mixers have been developed. The characterization of microstructure mixing is difficult to perform as the dimensions are too small for conventional methods. Therefore, we report a method to characterize the mixing of two gases directly by measuring the concentration of the gases at the outlet of the mixer. The experiments have been carried out up to gas flows of 5000 ml/min STP per passage. The mixing degree and mixing length were determined as well as the mixing time was calculated. These values depend on the properties of the gases and other parameters as temperature and gas velocity. Thus complete mixing is achieved after a mixing length, i.e., the distance to the microchannel outlet, of only 300-800 μm. Corresponding mixing times are just 100-600 μs. Furthermore, discontinuities in the mixing characteristic can be explained with the results obtained. Also design parameters for a further improvement of the mixer geometry individually for various applications could be set up.     

Combined static mixers for high viscous fluids mixing

A new static mixer Cross-over-Disc has been invented to strip off the boundary layer and to make strong radial mixing. The pressure drop of Cross-over-Disc is 12-26 times as large as that of empty pipe with equivalent diameter and length. The mixing performance of Cross-over-Disc with 14 elements has been investigated in the viscosity range of 190–250 Pa·s by decoloration method, and the gray analysis of images shows that mixing inhomogeneity is about 7.5% and 9.4% for the mixing ratio of 5:1 and 10:1, respectively. Furthermore, mixing inhomogeneity for a combination of static mixing elements (four from Cross-over-Disc and three pairs from Sulzer-type) can be decreased to 2.1%–3.1% within a reasonable range of pressure drop.     

CFD modeling of immiscible liquids turbulent dispersion in Kenics static mixers: Focusing on droplet behavior

The present study is concerned with the computational fluid dynamics(CFD) simulation of turbulent dispersion of immiscible liquids, namely, water–silicone oil and water–benzene through Kenics static mixers using the Eulerian–Eulerian and Eulerian–Lagrangian approaches of the ANSYS Fluent 16.0 software. To study the droplet size distribution(DSD), the Eulerian formulation incorporating a population balance model(PBM) was employed. For the Eulerian–Lagrangian approach, a discrete phase model(DPM) in conjunction with the Eulerian approach for continuous phase simulation was used to predict the residence time distribution(RTD) of droplets.In both approaches, a shear stress transport(SST) k-ω turbulence model was used. For validation purposes, the simulated results were compared with the experimental data and theoretical values for the Fanning friction factor, Sauter mean diameter and the mean residence time. The reliability of the computational model was further assessed by comparing the results with the available empirical correlations for Fanning friction factor and Sauter mean diameter. In addition, the influence of important geometrical and operational parameters, including the number of mixing elements and Weber number, was studied. It was found that the proposed models are capable of predicting the performance of the Kenics static mixer reasonably well.     

Power demand and mixing performance of coaxial mixers in a stirred tank with CMC solution

Experimental investigation was carried out in an el iptical based stirred tank with a diameter of 0.48 m to explore the power demand and mixing performance of coaxial mixers. Syrup and CMC solution (sodium carboxy methyl cellulose) were used as the Newtonian and non-Newtonian fluids, respectively. Four different coaxial mixers were combined with either CBY or Pfaudler impeller as the inner one, and anchor or helical ribbon (HR) as the outer one. Results show that Pfaudler-HR is the optimized combination among four coaxial mixers in this work, which provides the shortest mixing time given the same power consumption. Compared with the syrup solution, the increase of power input can make the mixing time decreasing more obviously in the CMC solution. The quantitative correlations for both syrup and CMC solutions were established to calculate the power draw and the mixing time of four coaxial mixers.     

Mechanisms of mixing of granular materials in drum mixers under rolling regime

Experimental investigations on mixing of non-ideal powders (granular tetraacetylendiamine (TAED)) are described. The evolution of mixing in rotating batch cylinders, in rolling regime has been addressed. Characterization and quantification of the local mixture composition have been obtained through an efficient solidification technique, coupled with computerized image analysis.Starting from a completely segregated configuration, the formation of a temporary, poorly mixed core at low rotation speed has been observed. Investigation of intermediate configurations during the mixing process allows to identify some unexpected granular mixing mechanism. The observed core has been explained in terms of transient axial convective fluxes superimposed on diffusive motion. Small differences of dynamic angle of repose between the two granular materials have been suggested to drive the axial convection, similarly to the mechanism reported in the literature to explain axial segregation phenomena. The differences in repose angle result from surface and shape irregularities typical of actual (i.e. non-ideal) granules.Convective fluxes due to the friction of powder with the end plates are also identified at the extremities of the mixer. Short-circuiting zones are created that hinder both axial diffusion and convection from the center of the vessel. Eventually, we suggest a mixing mechanism of non-ideal granular material where convection plays a major role.     

Drop breakage model in static mixers at low and intermediate Reynolds number

Drop break-up process for the flow of liquid-liquid dispersion in a static mixer has been investigated. Two new theoretical models for the drop break-up at low and intermediate Reynolds number for variant viscosity ratio of the dispersed phase to the continuous phase have been developed assuming that the flow through the static mixer elements is analogous to the flow through porous media. This concept has recently been established by Morançais et al. (Chem. Eng. Commun. 179 (1999) 77) and Legrand et al. (Chem. Eng. Res. Des. 79 (2001) 949). The boundary-layer shear force concept has been applied to predict the drop break-up at low Reynolds number and at intermediate Reynolds number, the effect of inertia on the drop break-up has been considered. The predicted drop sizes are in reasonable agreement with experimental results.     

Design and operation of an enhanced pervaporation device with static mixers

Pervaporation has a high potential for separating miscible solutions, particularly azeotropic mixtures. However, mass transfer limitations have long been a common concern in pervaporation device design. Therefore, in this work, we design a static mixer-based pervaporation device using water–ethanol separation as a model system and further develop computational fluid dynamics tools to investigate systematically all the influencing parameters. In the experiments, we use three-dimensional printed helical static mixers in the feed liquid channel to enhance mass transfer and implement a Sulzer pervaporation membrane for fast removal of water from ethanol. Using flow and mass-transfer simulations, we fit the membrane mass transfer coefficient and provide predictive models for optimal process design. Our pervaporation assembly exhibits promising performance and potential toward pervaporation processes for the removal of water from organics and is preferably scaled out by using stackable designs.     

翼片间距和尾距对HEV型静态混合器混合特性的影响

以变异系数作为评价混合器混合性能的指标,利用实验和数值模拟相结合的方法研究了翼片间距和尾距对HEV型静态混合器混合特性的影响。首先利用实验方法测量了静态混合器出口处示踪流体的体积分数,基于实验结果对数值模拟中的5种湍流模型(standard k-ε、RNG k-ε、realizable k-ε、SST k-ω、standard k-ω)进行对比分析,选出最优计算模型,在此基础上,模拟研究了不同翼片间距在同一基准断面处示踪流体变异系数的变化规律,以及翼片间距在0.8D~2.0D之间时,出口处示踪流体的变异系数随着尾距的变化规律。结果表明:RNG k-ε模型模拟的结果与实验结果最为接近(相对误差为3.37%),即该模型最能真实地反映此混合器的流场特性;HEV型静态混合器混合的均匀程度随着翼片间距的增大呈现先提高再降低的趋势,当翼片间距 ≤1.7D时,随着翼片间距的增大,变异系数迅速降低,当翼片间距 ≥1.7D时,随着翼片间距的增大,变异系数呈上升趋势,翼片间距为1.7D时,CV值最低,混合效果最好;在一定长度范围内,出口处示踪流体的变异系数随着尾距的增加迅速降低,但是尾距增加到一定距离以后,继续增加尾距对混合效果的改善不明显。     

Analysis and optimization of low-pressure drop static mixers

Various designs of the so called Low-Pressure Drop (LPD) static mixer are analyzed for their mixing performance using the mapping method. The two types of LPD designs, the RR and RL type, show essentially different mixing patterns. The RL design provides globally chaotic mixing, whereas the RR design always yields unmixed regions separated by KAM boundaries from mixed regions. The crossing angle between the elliptical plates of the LPD is the key design parameter to decide the performance of various designs. Four different crossing angles from 90° to 160° are used for both the RR and RL designs. Mixing performance is computed as a function of the energy to mix, reflected in overall pressure drop for all designs. Optimization using the flux-weighted intensity of segregation versus pressure drop proves the existence of the best mixer with an optimized crossing angle. The optimized angle proves to be indeed the LLPD design used in practice: the RL-120 with θ = 120°, although RL-140 θ = 140° performs as good. Shear thinning shows minor effects on the mixing profiles, and the main optimization conclusions remain unaltered. © 2009 American Institute of Chemical Engineers AIChE J, 2009     

Laminar mixing of shear thinning fluids in a SMX static mixer

Flow and mixing of power-law fluids in a standard SMX static mixer were simulated using computational fluid dynamics (CFD). Results showed that shear thinning reduces the ratio of pressure drop in the static mixer to pressure drop in empty tube as compared to Newtonian fluids. The correlations for pressure drop and friction factor were obtained at Re_MR?100. The friction factor is a function of both Reynolds number and power-law index. A proper apparent strain rate, area-weighted average strain rate on the solid surface in mixing section, was proposed to calculate pressure drop for a non-Newtonian fluid. Particle tracking showed that shear thinning fluids exhibit better mixing quality, lower pressure drop and higher mixing efficiency as compared to a Newtonian fluid in the SMX static mixer.     

Numerical investigation of blade shape in static mixing

The mixing performance of the KMX and SMX static mixers have been compared using 3D high-resolution computational fluid dynamics (CFD) simulations. Although these mixers have a similar design composed of layers of blades, their blade shape is different: curved for the KMX and flat for the SMX. The flow of a Newtonian fluid in steady laminar regime has been considered as the benchmark of the study. The simulation was first validated by assessing the pressure drop vs. the number of mixer elements and the results were found to be in good agreement with experimental data. To evaluate the mixing quality, cross-section stream function, extensional efficiency, mean shear rate, residence time, intensity of segregation, stretching, and Lyapunov exponent have been selected. Analysis of the flow pattern and mixing parameters shows differences between the mixers and it appears that the curved blade is more efficient than the flat blade design at the expense of a slightly higher pressure drop. In practice, the KMX mixer should provide a higher mixing rate at high viscosity ratio than the SMX mixer. © 2004 American Institute of Chemical Engineers AIChE J, 51: 44–58, 2005     

Comparative analysis of different static mixers performance by CFD technique: An innovative mixer

The flow and mixing behavior of two miscible liquids has been studied in an innovative static mixer by using CFD,with Reynolds numbers ranging from 20 to 160. The performance of the new mixer is compared with those of Kenics, SMX, and Komax static mixers. The pressure drop ratio(Z-factor), coefficient of variation(CoV), and extensional efficiency(α) features have been used to evaluate power consumption, distributive mixing, and dispersive mixing performances, respectively, in all mixers. The model is firstly validated based on experimental data measured for the pressure drop ratio and the coefficient of variation. CFD results are consistent with measured data and those obtained by available correlations in the literature. The new mixer shows a superior mixing performance compared to the other mixers.     

Pressure drop of pseudo-plastic fluids in static mixers

1 INTRODUCTIONUse of static mixers to process non-Newtonian fluids is quite commn.Data on thepressure drop of non-Newtonian fluids in Kenics static mixers are very useful in thedesign and engineering application of such mixers.However,only a few studies con-cerned with the pressure drop of non-Newtonian fluid flow in static ndxers can befound in literature.Wilkinson and Cliff presented pressure drop data for aqueousglycerine solutions(Newtonian fluids)and aqueous 1% polyacrylamide solution showingviscoelastic behavior.They found no difference between the friction factors of Newtonian     

Pressure drop and mixing in single phase microreactors: Simplified designs of micromixers

Continuous flow microreactors can greatly improve the safety and product yields of processes in the pharmaceutical and fine chemical industry by overcoming many of the drawbacks of traditional batch and semi-batch stirred reactors. This study compares on a common scale the pressure drop and mixing performance of different size commercial microreactor plates composed of a tangential, SZ-shaped or caterpillar mixer followed by a rectangular serpentine main channel. The pressure drop was fitted to a friction factor model, which suggests that the mixing zone had significant chaotic secondary flow patterns, whereas the main channel did not. Moreover, the mixing zone was the main contributor to the overall pressure drop. Mixing performance was then characterized using competitive parallel reactions. Upon the formation of chaotic secondary flows, typically due to the interactions of artificially induced vortices, the mixer performance was found to be independent of geometry for a given energy dissipation rate. However, the mixer geometry will affect the critical Reynolds number that induces chaotic advection and changes the mixing time scale.     

拉伸流动静态混合器混合性能的实验研究

设计了1种能够增强聚合物混合效果的拉伸流动静态混合器（EFM）,以高密度聚乙烯/聚苯乙烯（PE⁃HD/PS）作为混合体系,根据共混体系扫描电子显微镜（SEM）照片及分散相的平均粒径,研究了不同盘形结构和不同盘棱间隙（δ）（0.125~2.0 mm）下EFM的混合性能。结果表明,EFM的盘形入口结构对其混合性能影响较小,混合能力随盘棱顶端圆角半径的增大而有所提升,随δ的增大出现先降低再升高又降低的变化趋势。     

Shortstopping and jet mixers in preventing runaway reactions

Runaway reactions are continuing to be a major problem in the chemical industry (26% of major accidents). One of the main reasons for runaways is power failure. Runaway reactions could be inhibited in two ways: by the addition of cold diluents and by the addition of an inhibitor (chemical reaction stopper). This technology is called shortstopping. After a power failure, the process of adding an inhibiting agent and mixing it with the reactor contents becomes a major problem in the shortstopping process. Jets or impellers, driven by a small generator, however, can be used for mixing the inhibitor with the reactor contents.Dakshinamoorthy et al. [2006. CFD simulations of shortstopping runaway reactions in vessels agitated with impellers and jets. Journal of Loss Prevention in the Process Industries 19, 570-581] compared the efficiency of using jet mixers versus impeller stirred vessels in shortstopping runaway reactions. On the basis of equal power consumption, this comparative study showed that jet mixers are ineffective when used for shortstopping. One needs to identify additional factors, to effectively shortstop when using jet mixers.Due to the hazardous nature of runaway reactions, these factors cannot be determined with lab scale or pilot plant scale experiments. Recent developments with CFD make it possible to carry out virtual experiments. The computational model is solved using FLUENT. Shortstopping studies via the addition of a reaction inhibitor and cold diluent are discussed in detail. The results reported in this study identify the major and minor factors, which contribute to effective shortstopping; i.e., power requirements, locations for adding the inhibitor, the quantity of inhibitor added, rate of the inhibition, the use of cold diluent and the use of multiple nozzles. These results especially demonstrate the value of using CFD simulations in situations that are experimentally prohibitive.