Improving mixing performance by curved-blade static mixer

This article addresses design modification to a flat-blade static mixer to enhance mixing performance. The static mixer elements used in this work consist of four blades with curvature made to intensify turbulent-like flow, while reducing the pressure drop. The blades were mounted on a cylindrical housing with 45° rotation relative to the axial direction. The mixer assembly was used in three different arrangements of 8, 10, and 14 elements for a range of Reynolds number between 600 and 7,000. The coefficient of variance (COV) of samples was used to measure the mixing quality. The curved-blade mixer provides considerable improvement in mixing quality compared with the flat-blade mixer and comparable to the SMX mixer. Compared with the flat-blade static mixer, the new design reduces the COV by up to about 50%. This effect is more pronounced when the number of mixing elements increases. Furthermore, the friction factors for the modified mixer, obtained at a wide range of Reynolds number, were apparently smaller than those for the flat-blade, SMX, and SMV mixers. The correlation presented for the friction factor, when all mixer arrangements and aspect ratios were considered, supports the experimental data with ±15% deviation.     

SV型静态混合器湍流阻力的初步研究

为了获得流体在SV型静态混合器中湍流流动时的流动阻力规律,提出一种新的含有SV型静态混合器重要几何结构参数的流体阻力计算模型。对于不可压缩流体,将其在SV型静态混合器中的运动分解成沿管壁与轴线方向平行和沿混合元件凹槽方向的直线运动。运用流体力学理论,分别求解出流体作2种运动时所产生的湍流流体阻力的计算式,并计入相邻混合元件交接部分的局部阻力,然后进行叠加得到流体阻力理论计算式。以水为实验介质,对SV型静态混合器流体湍流阻力进行了实验测量,与理论结论进行比较分析,得出摩擦因子λ与Re-0.2呈线性关系的结论。     

Residence Time and Concentration Distribution in a Kenics Static Mixer

Static mixers, often referred to as motionless mixers, are in-line mixing devices that consist of mixing elements inserted into a length of pipe. Most of the experimental works in this field have concentrated on establishing design guidelines and pressure drop correlations. Due to experimental difficulties, few articles have been published on the investigation of the flow and mixing mechanisms. In this work, a Kenics KMX static mixer was utilized to study concentration and residence time distribution (RTD) and effect of Reynolds number on mixing. The static mixer had six mixing elements arranged in-line along the length of the tube, and the angle between two neighboring elements was 90°. The length of the mixer was 0.98 m with internal and external diameters of 5.0 cm and 6.0 cm, respectively. The main continuous fluid was water, and NaCl solution was used as a tracer. All experiments were conducted with three replications at three Reynolds numbers, Re = 1188.71, 1584.95, and 1981.19. A dispersion model was used to model the RTD data. The experimental results were compared with the model results and reasonable agreement was achieved.     

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。     

The design and performance of a vane mixer based on extensional flow for polymer blends

A new laboratory‐scale mixing device called the “Vane Mixer” was designed, built, and tested. The vane mixer consists of three vane plasticizing and conveying unit. In comparison with the existing laboratory mixers, material flow in this vane mixer is characterized by a high contribution from extensional flow. As the mixer has mixing chamber of very simple geometry, the cleaning is very easy and the material lost is very small. The influences of mixing time and rotor speed on dispersed phase size were characterized and discussed. Morphology data on model immiscible polystyrene/high density polyethylene (PS/HDPE) blend have proved the high distributive and dispersive mixing efficiency. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 41551.     

Three‐dimensional flow measurements in a static mixer

Laminar heat and mass transfer are central to a wide range of industrial processes, encompassing (thermal) processing of viscous fluids, compact equipment for process intensification, and emerging microfluidic devices. Many of these applications incorporate the “static‐mixing principle” (stirring of a throughflow by internal elements) for mixing and heat‐transfer enhancement. Investigations on static mixers primarily concern numerical simulations. Experimental studies, on the other hand, are relatively rare and to date restricted to visualization of mixing patterns or integral quantities as for example, pressure drop and heat‐transfer coefficients. The present study expands on this by quantitative experimental analysis of three‐dimensional (3‐D) flow fields and streamline patterns in a representative static mixer using 3‐D particle‐tracking velocimetry. This necessitates tackling of (internal) refractions and reflections caused by the complex mixer geometry. Comparison of experimental results with numerical predictions reveals a good agreement. © 2012 American Institute of Chemical Engineers AIChE J, 59: 1746–1761, 2013     

Scaling inline static mixers for flocculation of oil sand mature fine tailings

Operations to reclaim mature fine tailings (MFT) ponds involve flocculation using high‐molecular‐weight polymers, for which inline static mixers are suited. Three different commercial static mixers were utilized to determine mixing parameters corresponding to optimal dewatering performance of flocculated MFT. MFT was treated with polymer solution under different mixing conditions. The dewatering rates passed through a peak with increasing mean velocity, V and Reynolds number, Re of the fluid. The greater the number of mixer elements, the lower the V and Re at which the peak dewatering rate occurred. Mixing parameters such as G‐value, residence time, and mixing energy dissipation rate of the most rapidly dewatering flocculated MFT were dependent on mixer type and setup. In contrast, peak dewatering rates converged when scaled with respect to specific mixing energy, E, demonstrating that E is a suitable scale‐up parameter for inline static mixing to produce optimally dewatering MFT. © 2015 American Institute of Chemical Engineers AIChE J, 61: 4402–4411, 2015     

静态混合器的化工应用研究

南京化学工业有限公司苯化工部将静态混合器应用于硝基苯生产的中和、水洗工序。文章介绍了静态混合器的混合原理，以及硝基苯生产的中和、水洗工序中使用釜式搅拌器和静态混合器存在的差别，并对硝基苯粗品中副产物去除效果进行比较。     

Computational study of convective–diffusive mixing in a microchannel mixer

Scalar mixing due to convection and diffusion in a microchannel mixer is studied using CFD. A method is developed to quantitatively measure the effect of false diffusion on scalar decay rate. This method computes an average false diffusivity from a given numerical solution and it is not limited to any particular numerical scheme. It is found that a range of molecular diffusivity exist in which average false diffusion is smaller than molecular diffusion and scalar decay rates can be computed accurately with CFD in the mixer. This range of molecular diffusivity covers most of the liquid solutions encountered in chemical and biochemical engineering. When effective diffusivity is used, this range can be further expanded. The predicted mixing structures agree well with experimental results in literature. The classical lamellar structures of the baker's transformation are strongly affected by diffusion. The striation doubling process is destroyed by diffusion broadening at very early stage in the mixer. The optimal mixing is achieved at low Re when the mixing mechanism in the mixer is the baker's transformation. At higher Re, secondary flow is generated and the mixing mechanism is the competition of the kinematics of the baker's transformation and the dynamics of the cross sectional flow. Results show that the secondary flow hinders mixing and the scalar decays at lower exponential rates than when the mixing is due to the baker's transformation alone.     

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.     

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.     

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.     

Hierarchical and Fractal Structuring in Polymer Processing

The potential of structuring thermoplastic polymers by convection only, using a combination of static mixer elements, which easily produce stratified structures with thousands of layers, and the black box concept that serves to elegantly combine materials in standard co‐extrusion technology is investigated. The aim is to obtain an alternative for routes that try to structure organic matter such as polymers down to submicrometer levels, usually via self‐organization based on phase separation. Structure is characterized by its complexity, here defined by the level of hierarchy. Horizontal stratification, parallel to the surface, is level 0. Vertical stratification connected to horizontal surface layers, is level 1. A series of horizontal stratifications in distinct places vertically connected to the surface layers is level 2. Higher levels of hierarchy finally result in dendritic structures that are fractal. Applications of complex structures with a huge interface and guaranteed cocontinuity throughout the whole cross section of the products are found in, for example, membranes for fuel cells and gas separators, and in miniaturizing electronic and optical devices such as photovoltaic cells.

     

Design and characterization of a Low-pressure-drop static mixer

Four-blade static mixer was designed for inline mixing of Newtonian fluids at Reynolds numbers from 700 to 6800. The mixer consists of four equally spaced blades mounted on cylindrical housing with 45° rotation relative to the circumference. It was tested in three different compartments of 6, 8, and 10 mixing elements; each element rotated 45° relative to the adjacent one. Multipoint sampling was used to measure concentration downstream the mixer. The mixing quality was measured by the coefficient of variance (CoV). The CoV decreases as the energy input per unit mass increases. This effect is more pronounced when the number of mixing elements increases. For the case of 10 mixing elements, a good mixing performance (typically more than 95% mixedness or CoV < 0.05) achieved, although a marginally good mixing performance could also be achieved by eight mixing elements. The friction factors were correlated as f = C₁/Re + C₂/Reⁿ with an average deviation of ±10% from experimental data. Furthermore, experimental friction factors were compared with existing models. For a wide range of Reynolds numbers, the friction factors are apparently smaller than those from SMV, KMX, and baffle-type static mixers. © 2018 American Institute of Chemical Engineers AIChE J, 65: 1126–1133, 2019     

Pyrolysis of hydrocarbons in a heat-carrier flow with fast mixing of the components

A scheme of autothermal pyrolysis of hydrocarbons in a reactor with fast mixing of the feedstock with a high-temperature heat-carrier flow is considered. The possibility of providing fast mixing of the components is verified in a series of aerodynamic experiments in a model setup, where the optimal geometry of the mixer for operation conditions of the pyrolysis reactor is determined. An experimental reactor is designed and manufactured for verification of the concept proposed. A scheme of the propane pyrolysis reactor is given. Products of combustion of a hydrogen-air mixture are used as a heat carrier. Experimental distributions of the flow temperature and concentration of the injected substance at the exit of the reactor agree with results obtained in the model setup. The mixing time is demonstrated to be smaller than the residence time of the mixture in the reactor mixer by more than an order of magnitude, which allows the process to be arranged under controlled conditions. In addition to experimental research, several detailed kinetic schemes of hydrocarbon pyrolysis are tested on the basis of available experimental data. Results of kinetic calculations for the operation conditions of the experimental setup are presented. __________ Translated from Fizika Goreniya i Vzryva, Vol. 44, No. 5, pp. 38–44, September–October, 2008.     

数值模拟静态混合器结构对PS/CO_2熔体温度的影响

利用专用CFD软件Polyflow对SMX型和Kenics型静态混合器中PS/CO_2发泡溶液进行数值模拟计算,分析比较不同板厚在不同元件个数条件下两种静态混合器消耗的压力损失,以及不同CO_2浓度对静态混合器压力损失的影响;并引入"离散系数"分析比较两种静态混合器出口温度均匀性的变化.数值模拟的结果表明:SMX型静态混合器冷却效果优于Kenics型静态混合器,并且SMX型静态混合器出口温度均匀性高于Kenics型静态混合器.     

The numerical simulation of a new double swirl static mixer for gas reactants mixing

For the nitrogen oxide removal processes, high performance gas mixer is deeply needed for the injection of NH₃ or O₃. In this study, a new type of double swirl static mixer in gas mixing was investigated using computational fluid dynamics (CFD). The results obtained using Particle Image Velocimetry (PIV) correlated well with the results obtained from simulation. The comparisons in pressure loss between the experimental results and the simulation results showed that the model was suitable and accurate for the simulation of the static mixer. Optimal process conditions and design were investigated. When L/D equaled 4, coefficient of variation (COV) was < 5%. The inlet velocity did not affect the distributions of turbulent kinetic energy. In terms of both COV and pressure loss, the inner connector is important in the design of the static mixer. The nozzle length should be set at 4 cm. Taking both COV and pressure loss into consideration, the optimal oblique degree is 45°. The averaged kinetic energy changed according to process conditions and design. The new static mixer resulted in improved mixing performance in a more compact design. The new static mixer is more energy efficient compared with other SV static mixers. Therefore, the double swirl static mixer is promising in gas mixing.     

Chaotic mixing in a barrier‐embedded partitioned pipe mixer

Inspired by the partitioned pipe mixer (PPM), a barrier‐embedded partitioned pipe mixer (BPPM) is designed and analyzed using a numerical simulation scheme. The BPPM is a static mixer, composed of orthogonally connected rectangular plates with a pair of barriers, which divide, stretch, and fold fluid elements, leading to chaotic mixing via the baker's transformation. The aspect ratio of the plate (α) and the dimensionless height of the barrier (β) are chosen as design parameters to conduct a parameter study on the mixing performance. The flow characteristics and mixing performance are analyzed using the cross‐sectional velocity vectors, Poincaré section, interface tracking, and the intensity of segregation. The results indicate that several designs of the BPPM significantly enhance the PPM's mixing performance. The best BPPMs are identified with regard to compactness and energy consumption. © 2017 American Institute of Chemical Engineers AIChE J, 64: 717–729, 2018     

Performance of Kenics static mixer over a wide range of Reynolds number

The present study deals with the numerical simulation of flow patterns and mixing behaviour in Kenics static mixer over a wide range of Reynolds number. Three different sets of Kenics mixer (aspect ratio = 1.5) comprised of 3, 9 and 25 elements each have been characterized. The Reynolds number was varied in the range of 1 to 25,000 (i.e., from laminar to turbulent flow regime). The numerical approach takes into account the aspects of the fluid flow at higher Reynolds number values including circumferential velocity profiles at different cross-sections within the Kenics mixer, which were neglected in previous studies. It was observed that cross-sectional mixing in the turbulent flow regime takes place up to 30% of each element length at element-to-element transition; beyond that velocity profiles were uniform. The experiments were also carried out to measure the circumferential and axial velocity profiles and pressure drop in three different Kenics Mixers using air as fluid. The pressure drop per unit element (ΔP/η) was found to be independent of the number of Kenics mixing elements used in the system. The total pressure drop across Kenics mixer obtained by CFD simulations were compared with the experimental pressure drop values and correlations available in the literature. The numerical results were found in good agreement with the experimental as well as the results reported in the literature. A new pressure drop correlation in the Kenics static mixer has been developed.