Computational Fluid Dynamics modeling of micromixing performance in presence of microparticles in a tubular sonoreactor

This paper reports the results of CFD modeling for evaluating micromixing efficiency in presence of polymeric microparticles in a continuous tubular sonoreactor. The studied tubular sonoreactor was equipped with four 1.7 MHz ultrasound transducers and micromixing efficiency was analyzed using Villermaux/Dushman reaction. The main objective of this study is to illustrate the simultaneous effects of 1.7 MHz ultrasound waves and polymeric microparticles on micromixing performance from the fluid dynamics point of view. In order to model the presence of these microparticles, the Eulerian multiphase model was applied based on kinetic theory of granular flow. The dynamic mesh method was used to model the vibration of 1.7 MHz piezoelectric transducers. CFD modeling results indicate the positive effects of the presence of microparticles on micromixing efficiency and more efficient velocity distribution inside the sonoreactor. This was interpreted as the ability of high frequency ultrasound waves (1.7 MHz) to move and disperse the microparticles.     

CFD Modeling of Micromixing and Velocity Distribution in a 1.7‐MHz Tubular Sonoreactor

The effect of high‐frequency (1.7 MHz) ultrasound waves on the mixing rate in a new continuous tubular sonoreactor was investigated by CFD modeling. Modeling of piezoelectric transducer (PZT) vibrations was done based on the dynamic mesh model. Results indicate that the acoustic streams were in the direction of wave propagation and their maximum velocity near the PZT surface agreed well with experimental measurements. The micromixing efficiency of the sonoreactor was studied by adopting the Villermaux/Dushman reaction in the modeling. Comparison of the calculated relative segregation index from modeling results with experimental data revealed reasonable accordance.     

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.     

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.     

Macro- and micromixing in a novel sonochemical reactor using high frequency ultrasound

This paper deals with the influence of ultrasound on macro- and micromixing in a new developed sonochemical reactor. Unprecedented piezoelectric transducer arrangement with a high frequency of 1.7 MHz has been used in this novel reactor. Macromixing quality has been investigated visually and the Dushman reaction (iodide-iodate) coupled with a neutralization reaction have been examined in order to characterize micromixing quality. In addition, the effect of liquid viscosity on the segregation index has been studied. The results show that this new developed reactor can establish reasonable macro- and micromixing inside the reactor. Moreover, the performance of this reactor has been compared with a stirred tank reactor equipped with a Rushton turbine impeller. It is found that with the same input electrical power, the obtained segregation index for stirred tank reactor is approximately 10% more than proposed new ultrasound reactor, which means the sonoreactor works more efficiently.     

Experimental investigation on proper use of dual high-low frequency ultrasound waves—Advantage and disadvantage

An experimental study was conducted to investigate the proper use of dual frequency ultrasound irradiation in sonoreactors. For this purpose, two waves with frequencies of 1.7 MHz and 24 kHz were combined. A dual frequency rig was equipped with a high frequency 1.7 MHz ultrasound transducer and a low frequency one generating 24 kHz waves. Dushman competitive reactions were used to study micromixing, Weissler reaction for evaluating cavitation activity and ammonia degradation experiment to investigate the superposition effect of two coupling waves on decomposition strength. The results revealed that combining these waves has a positive effect on number of generating cavitation bubbles and micromixing performance. However, this combination reduced the strength of the low frequency wave and diminished the ammonia degradation process efficiency. The micromixing enhancement in terms of the segregation index in dual frequency irradiation mode is approximately 46.9% and 40.1% more than single low and high frequency irradiation, respectively. On the other hand, the ammonia removal efficiency is reduced by 10.3% in dual frequency irradiation compared to single low frequency layout.     

基于制备超细复合含能材料的对置受限式撞击流反应器微观混合性能

由于对置受限式撞击流反应器较高的传热及混合效率,能够制备出粒径小、均匀且分布范围窄的超细颗粒。本文采用碘化物-碘酸盐平行竞争反应体系研究入射速度、结构尺寸对对置受限式撞击流反应器微观混合效果的影响规律,并将混合腔尺寸同等比例放大一倍进行对比研究。结果表明,随着入射速度的增大,离集指数减小,微观混合效果提高。喷嘴间距与喷嘴直径比的增大使得离集指数增大,微观混合效果降低。离集指数随着混合腔高度、混合腔出口尺寸的增大呈现先增大后减小的趋势,且混合腔高度对混合效果的影响较混合腔出口尺寸显著。将对置受限式撞击流反应器混合腔尺寸扩大一倍,离集指数增加到原来的2.4倍,微观混合效果显著下降,但是在较大入射速度下,两种结构的混合效果差距减小。研究结果可为纳米复合含能材料的制备提供高效、安全的技术支持。     

Side-Tee混合器内微观混合行为研究

选用碘化物-碘酸盐体系,利用化学探针法对Side-Tee混合器在较高雷诺数下微观混合效率进行了研究。考察了不同影响参数(如雷诺数、体积流率比、反应物浓度、粘度、管径)对Side-Tee混合器微观混合性能的影响,并用离集指数来表征其微观混合特性。结果表明,雷诺数大于12 000后碘化物-碘酸盐反应体系对混合器内微观混合状况不敏感。基于团聚模型,估算出Side-Tee混合器在实验条件下的微观混合时间为3×10-5~3×10-4s。表明这一型式的反应器对于快速反应尤其是纳米材料制备过程具有替代传统搅拌槽反应器的优势。     

微通道反应器内黏性流体的微观混合

Micromixing efficiency of viscous media in Y-type micro-channel reactor was studied by using iodide-iodate test reaction as working system. Experiments were carried out in water-glycerin mixtures with 7 different viscosities. The experimental results showed that segregation index of the micro-channel reactor increases with the decrease of volumetric flow rate and the increase of solution viscosity. Based on the incorporation model, the micromixing time tm of the micro-channel reactor was estimated in the range of 10－4-10－3 s at different viscosities, which indicated that the micro-channel reactor possesses a much better micromixing performance compared to the stirred tank (tm 0.02-0.2 s).     

装填不同填料的管式反应器的微观混合性能

采用碘化物-碘酸盐平行竞争反应为工作体系,以离集指数（X_S）表征微观混合性能,实验考察了Reynolds数Re、物料体积流量比对装填扁环填料和网架填料的管式反应器的微观混合性能的影响及离集指数在管式反应器轴向、径向的分布。结果表明,X_S随着Re的增大而下降,而物料体积流量比的增加会导致X_S增加,相同条件下网架填料的X_S小于扁环填料,离集指数在轴向上呈现先明显降低后基本不变的趋势。在实验研究的基础上,利用团聚模型计算装填网架填料的管式反应器的微观混合时间（t_m）为4.82~2.27 ms。与扁环填料对比,网架填料最小值（2.27 ms）低于扁环填料的最小值（2.73 ms）,同时二者均小于空管的最小值（3.79 ms）,表明装填填料可以改善管式反应器的微观混合性能,网架填料的微观混合性能略优于扁环填料。     

CFD and experimental investigations on the micromixing performance of single countercurrent-flow microchannel reactor

Microchannel reactors are widely used in different fields due to their intensive micromixing and, thus, high masstransfer efficiency. In this work, a single countercurrent-flow microchannel reactor(S-CFMCR) at the size of ～1 mm was developed by steel micro-capillary and laser drilling technology. Utilizing the Villermaux/Dushman parallel competing reaction, numerical and experimental studies were carried out to investigate the micromixing performance(expressed as the segregation index XS) of liquids inside S-CFMCR at the low flow velocity regime.The effects of various operating conditions and design parameters of S-CFMCR, e.g., inlet Reynolds number(Re),volumetric flow ratio(R), inlet diameter(d) and outlet length(L), on the quality of micromixing were studied qualitatively. It was found that the micromixing efficiency was enhanced with increasing Re, but weakened with the increase of R. Moreover, d and L also have a significant influence on micromixing. CFD results were in good agreement with experimental data. In addition, the visualization of velocity magnitude, turbulent kinetic energy and concentration distributions of various ions inside S-CFMCR was illustrated as well. Based on the incorporation model, the estimated minimum micromixing time tmof S-CFMCR is ～2 × 10~(-4)s.     

CFD modeling of mixing intensification assisted with ultrasound wave in a T-type microreactor

This paper aims to demonstrate the effect of ultrasound wave on mixing in a T-type microreactor. In order to create vibration in this microreactor, a low frequency (42 kHz) piezoelectric transducer was used. A well-known parallel-competitive reaction (Villermaux–Dushman reaction) was employed to study the mixing in the microreactor and the segregation index values were found for layouts with and without sonication. Results show that the ultrasound waves have a significant favorable influence on product distribution and the segregation index at various total flow rates. In all cases, the segregation index decreased with increase in total flow rate. The results reveal that the segregation index improved up to 10–20% by consuming a low energy (2.45 W Kg⁻¹) by the piezoelectric transducer. Finally, the computational fluid dynamics (CFD) modeling was carried out to explain the observed experimental results.     

化学偶合法研究IS-RPB反应器微观混合特性

以α-萘酚与对氨基苯磺酸重氮盐之间的串联竞争偶合生成单偶氮和双偶氮的反应为测试体系,研究了超重力因子、喷嘴初速、浓度、撞击间距、填料等参数对撞击流-旋转填料床(IS-RPB)反应器离集指数和微观混合时间的影响规律,得出适宜的操作参数。在相近的操作条件下,对比了IS,RPB,IS-RPB 3种反应器的微观混合性能。实验结果表明:IS-RPB反应器秉承了IS反应器和RPB反应器的优点,其微观混合性能明显优于IS和RPB等反应器,是一种具有应用潜力的新型设备。     

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。     

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.     

泡沫陶瓷填料旋转填充床微观混合性能

采用碘化物-碘酸盐平行竞争反应为工作体系,以离集指数（X_S）表征微观混合性能,实验考察了物料体积流量、H⁺浓度、旋转填充床转速、物料体积流量比等对两种不同孔径的新型整体泡沫陶瓷填料旋转填充床的微观混合性能的影响。结果表明,孔径较小的泡沫陶瓷填料更利于微观混合;H⁺浓度、进料体积流量比的增加会导致X_S增加;而旋转填充床转速、进料流量的增大都可使X_S下降。在实验研究的基础上,利用团聚模型计算泡沫陶瓷填料旋转填充床微观混合时间（t_m）,得到t_m范围为0.385～8.55 ms。与传统不锈钢丝网填料对比,泡沫陶瓷填料t_m最小值（0.385 ms）低于不锈钢丝网填料的t_m最小值（1.6 ms）,表明泡沫陶瓷填料的微观混合性能优于传统不锈钢丝网填料。     

Micromixing efficiency of rotating packed bed with premixed liquid distributor

Rotating packed bed (RPB), in which high gravity is simulated by a centrifugal force, plays an important role in process intensification of fluid mixing and mass transfer. However, uneven initial liquid distribution in RPB leads to poor micromixing efficiency in local areas of the packing. Therefore, a premixed liquid distributor is proposed in this work to improve the liquid distribution in RPB. Micromixing efficiency of RPB with the premixed liquid distributor was studied by adopting the iodide–iodate reaction as working system and show better micromixing efficiency compared to that of RPB with non-premixed liquid distributor. Also, the effects of operating conditions (e.g. rotational speed, acid concentration, volumetric flow rate) and geometries using the premixed liquid distributor on micromixing efficiency (characterized by segregation index X_S) were investigated. The results show that segregation index X_S decreases with the increase of rotational speed, and the decrease of acid concentration and volumetric flow rate.     

机械搅拌反应器中挡板的结构设计

研究了内径为0．786m的搅拌釜中挡板尺寸及结构对圆盘透平桨RT和翼型桨k5及其组合在气液两相中的气体分散与混合特性的影响。对不同形式的挡板的搅拌功率、气含率及气液混合特性进行了对比分析。研究结果表明：挡板尺寸结构应根据搅拌特性需要进行优化设计;挡板系数为0．12时,组合浆的功率输入已与同一转速下的全挡板系数时的功率输入相近,它同时可改善微观混合、提高混合效率：采用开槽挡板可提高复杂快反应的选择性,混合效率提高20％～25％。     

Micromixing effects on a parallel reaction in flow reactors

Using the micromixing concepts of Danckwerts and Zwietering, the Peclet number Pe has been correlated mathematically to the degree of segregation J for the axial dispersion model. The results were applied to compare the micromixing effects on a model, mixed-order parallel reaction system in continuous flow reactors. Axial dispersion model, and Ng and Rippin's two-environment model were used to find the micromixing effects in tubular and stirred tank reactors, respectively. The performance of these reactors, with varying geometries, has been evaluated in terms of overall conversion, selectivity, and yield under identical operating and reaction conditions. The overall conversion increases in a tubular reactor with the increase in J, irrespective of the kinetic orders. However, in a stirred tank reactor, the conversion is found to be micromixing-sensitive, depending on the order of reaction. For m = 1 and n = 2 (case 1), the conversion is fairly insensitive to micromixing effects while it decreases for m = 0.5 and n = 1 (case 2) with increasing J. For the same extent of micromixing, a tubular reactor gives, in both cases, a higher conversion than a stirred tank reactor. The selectivity, in either case, decreases in both reactors with increasing segregation effects. However, in each case, the selectivity of a tubular reactor was fairly close to that of a stirred tank reactor at the same value of J. As far as the yield is concerned, both reactors achieve nearly the same value, without significant micromixing effects.     

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

Micromixing 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.