RESIDENCE TIME DISTRIBUTION OF RHEOLOGICALLY COMPLEX FLUIDS PASSING THROUGH A SULZER SMX STATIC MIXER

Static mixers are widely used in continuous mixing, heat and mass transfer processes and chemical reactions. However, a proper understanding of the flow pattern is still missing when rheologically complex fluids are involved. This paper presents a study of the Residence Time Distribution (RTD) determination in a Sulzer SMX static mixer. A two-parameter RTD model, based on each mixing element, was proposed to describe the flow pattern of rheologically complex fluids in a such mixer. It was shown that in this model the flowrate fraction across the plug flow component increases with the generalized Reynolds number. However, the volumetric fraction of the plug flow component depends not only on the generalized Reynolds number, but also on the number of mixing elements, apparently due to memory effects of viscoelastic fluids.     

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     

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.     

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.     

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     

Liquid phase residence time distribution for gas–liquid flow in Kenics static mixer

The residence time distribution (RTD) of the liquid phase for co-current gas–liquid upflow in a Kenics static mixer (KSM) with air/water and air/non-Newtonian fluid systems was investigated. The effect of liquid and gas superficial velocities on liquid holdup and Peclet number was studied. Experiments were conducted in three KSMs of diameter 2.54 cm with 16 elements and 5.08 cm diameter with 8 and 16 elements, respectively, of constant L_e/D_e = 1.5 for different liquid and gas velocities. A correlation was developed for Peclet number, in terms of generalized liquid Reynolds number, gas Froude number and liquid Galileo number, where as for liquid holdup, a correlation was developed as a function of gas Reynolds number. The axial dispersion model was found to be in good agreement with the experimental data.     

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.     

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.     

Effects of geometry and flow rate on secondary flow and the mixing process in static mixers—a numerical study

A method based on computational fluid dynamics (CFD) for the characterization of static mixers using the Z factor, helicity and the rate of striation thinning is presented. These measures were found to be well-suited for the characterization of static mixers as they reflect the pressure drop, the formation of secondary flow, i.e. vortices, and their effect on the mixing process. Two commercial static mixers, the Kenics KM and Lightnin Series 45, have been characterized. In the mixers investigated, secondary flow is formed in the flow at the element intersections and due to the curvature of the mixer elements. The intensity of the vortices is higher in the Lightnin than the Kenics mixer due to edges in the middle of the Lightnin mixer elements. The formation of vortices affects the Z factor by an increase in the power requirement, and the rate of striation thinning by an increase in the stretching of the striations. The formation of vortices was observed at a Reynolds number of 10 in both mixers with aspect ratios of 1.5. However, the intensity of the vortices was greater in the Lightnin than the Kenics mixer, which was observed in not only the magnitude of the helicity, but also the Z factor, rate of striation thinning and the distribution of striation thickness.The distribution in striation thickness is shifted towards thin striations as the flow rate is increased from below to above the Reynolds numbers of which vortices were first observed, but some striations still pass the mixer elements almost unaffected, which can be seen in the skewness of the distribution of the striation thickness, which shifts from being negative to positive.     

Design and assessment of a novel static mixer

This paper examines the performance of a novel static mixer comprising a circular tube fitted with eight alternating equi‐spaced semicircular rigid insert (baffles) as the mixer elements. Experiments were carried out to obtain the coefficient of variance (CoV) for the mixing of two streams of water and brine for Reynolds number between 60 and 700. Decreasing the baffles clearance ratio significantly reduces the CoV but at a cost of an increase in the pressure drop across the static mixer. The presence of the mixing elements (baffles) promotes a non‐laminar, turbulent‐like flow which considerably enhances the mixing. The static mixer described here represents a cost effective, easy to manufacture, low maintenance, and flexible alternative to the more sophisticated static mixers currently in use.     

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.     

Experimental study on energy consumption characteristics of rotational–perforated static mixers

An ideal static mixer can achieve efficient mixing at low pressure drops. Owing to the excellent performance of the tridimensional rotational flow sieve tray (TRST) in a gas–liquid two-phase system, the TRST structure was modified into a rotational–perforated static mixer (RPSM) to enhance mixing in multicomponent liquid systems. The energy consumption characteristics of the RPSM were experimentally studied based on Reynolds numbers in the range of 986–7892, gap L = 0–80 mm, and relative angle γ = 0–45°. The effects of the element installation method, number, gap, relative angle, fluid Reynolds number, fluid properties, and other parameters on the RPSM pressure drop were also investigated. An interaction analysis of each factor was performed using the factorial design method and an empirical model of the RPSM Z-factor was established. Additionally, pressure drop in the RPSM was compared with those of other commonly used static mixers. Results show that, when the element is backward-installed, the pressure drop is higher than that in the forward direction because the fluid is constantly twisted. Moreover, the pressure drop increases with increasing element gap, and the average increase is 43.64% and 19.28% for the forward and backward installations, respectively. The influence of the relative angle on the pressure drop is mainly reflected when the gap L = 0. Subsequently, the degree of influence of each factor was determined, and the Z-factor was calculated and found to be consistent with the experimental values (relative error of less than 15%).     

Effect of Reynolds Number on Laminar Mixing in an Annular Tube Combining Two Crossed Secondary Flows

To improve static and continuous mixers, several methods have been proposed in the literature. Herein, two well‐known passive intensification configurations were applied to an annular laminar flow and numerically studied. Residence time distributions (RTDs) and Poincaré sections were examined to investigate the dispersion of the particles. These two approaches show that geometric perturbations of the walls improve the mixing level for all Reynolds numbers investigated. Above Re = 300, the increase in mixing can be attributed to chaotic advection within the flow. Finally, a general reactor model that allows the RTD in the annular tube to be predicted with high confidence is proposed.     

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     

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     

Experimental characterization and multi-scale modeling of mixing in static mixers

Mixing in static mixers is studied using a set of competitive-parallel chemical reactions and computational fluid dynamics (CFD) in a wide range of operating conditions. Two kinds of mixers, a wide angle Y-mixer and a two jet vortex mixer, referred to as Roughton mixer, are compared in terms of reaction yields and mixing times. It is found that the Roughton mixer achieves a better mixing performance compared to the Y-mixer. The effect of flow rate ratio on mixing in the Roughton mixer has been studied as well and it is shown that the mixing efficiency is not affected by the flow rate ratio. Moreover, experimental results and model predictions are in good agreement for all mixer geometries and operating conditions. CFD is used to calculate absolute mixing times based on the residence time in the segregated zone and it is shown that mixing times of less than 1 ms can be achieved in the Roughton mixer. In addition, CFD provides insight in local concentrations and reaction rates and serves as a valuable tool to improve or to scale-up mixers.     

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.     

不同结构静态混合器内熔体流动及混合效果的数值模拟

构建长度、直径相同,但具有不同扭曲率、分割次数的3种静态混合器,然后利用Polyflow软件模拟低密度聚乙烯(HDPE)熔体在静态混合器内的流动情况,得到熔体在静态混合器内的速度场、压力场、停留时间、分离尺度等参数,并通过分析示踪粒子在静态混合器内的分布情况,表征其混合效果。结果表明,扭曲率大的静态混合器,压降较大、横向速度分量较大、分离尺度较小、混合效果更佳;而分割次数多的静态混合器,压降增大,但是分割次数对横向速度、混合效果的影响较小。     

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.