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.     

Numerical and experimental investigations of liquid mixing in two-stage micro-impinging stream reactors

A two-stage micro-impinging stream reactor(TS-MISR) that combined a first pre-mixing stage with a second micro-impinging stream reacting stage for continuous multi-component reacting systems has been built from commercial T-junctions and steel micro-capillaries. Both of operating parameters and reactor configurations,such as jet Reynolds number(Rej), volumetric flow ratio(R), the first-stage junction angle(φ), the connecting capillary length(Lc) and connecting capillary diameter(dc), had significant effects on the micromixing efficiency of the reactor. Such effects were investigated for both of the two stage structures, respectively, by experimental and CFD methods and were optimized for the best micromixing performance. Intensified micromixing among at least three reacting components can be achieved in a continuous mode by using TS-MISR; therefore, it is expected that the TS-MISR will produce products of higher quality with more uniform and stable element distribution.     

LASP and Villermaux/Dushman protocols for mixing performance in microchannels: Effect of geometry on micromixing characterization and size reduction

The paper reports an experimental investigation on the effect of geometrical design of double jet impingement microchannels on mixing efficiency. Three arrangements of microchannel reactors (MCRs) were designed with 800 μm in diameter by 30 mm in length in various confluence angles of 45°, 90°, and 135°. Mixing performance of the microchannels was first evaluated via competitive parallel reactions of Villermaux/Dushman. The mixing quality was then described under various total liquid flow rates and initial acid concentration using segregation index (X_S). In the second protocol, mixing performance was further investigated via complicated liquid anti-solvent precipitation (LASP) process for nanodrug production. Curcumin was utilized as a model of an insoluble drug in water and particle size and SEM imaging were then employed to characterize the produced nanosuspentions. The whole results show that despite considerable differences in nature of these two processes, the microchannel with confluence angle of 135° works more efficiently in both protocols, due to its higher mixing quality.     

Characterization and modeling of micromixing performance in micropore dispersion reactors

Micropore dispersion reactors have been considered as one of the most promising micro-structured devices. For reliable design of this kind of microreactors, the micromixing performance in those microreactors with different detailed geometric structures have been investigated in this work. The pore size, pore shape, pore number and pore distance were varied and the micromixing performance was characterized by the Villermaux/Dushman parallel competing reaction. The results showed that the mixing performance was greatly influenced by the geometric structures. The segregation indexes, X_S, were found in the range of 10⁻² to 10⁻³, indicating that the micropore dispersion reactors have high micromixing efficiency. To deeply understand the micromixing process, CFD simulation was carried out to describe the flow fields in the reactors. Based on the simulation results a mathematical model was developed and a new area parameter, S, was defined by considering both mixing region surface and mass transfer distance. A linear relationship between S and lg(X_S) was obtained at last, which is very helpful for optimizing the structure design of micropore dispersion reactors.     

Mesomixing scale controlling and its effect on micromixing performance

To direct highly efficient microdevice design, the mixing performance of different mixing methods was investigated. Three different microstructured mixers representing three kinds of mixing methods were utilized in this work. The mesomixing scale was adjusted through different ways in these mixers and the micromixing performance was characterized by a parallel competing reaction. A dimensionless parameter of mesomixing scale with different forms in these mixers was defined by considering contacting surface and mixing volume, and its relation with the segregation index which characterized the micromixing performance was investigated. All results indicate the enhancement of the micromixing performance with the decrease of the mesomixing scale. The mixing potential of different mixing method was discussed and it shows that the droplet cross-flow mixing method has the highest mixing potential. A linear relationship between the dimensionless parameter of mesomixing scale and the segregation index has been obtained. The results could provide much better understanding of how mesomixing scales affect the micromixing performance, which are very helpful for designing new micromixing devices and optimizing the geometric structures and operation conditions.     

CFD analysis of water gas shift membrane reactor

Fuel cell based modular power generation can be achieved by miniaturization and process intensification of equipments in the process. Fuel cells require hydrogen rich gas which can be generated through reforming and water gas shift reaction. The water gas shift reactor being kinetically limited occupies more volume to achieve the required CO conversion. A membrane reactor integrates the reaction and hydrogen separation stages and hence reduces the volume requirement. Computational Fluid Dynamics offers virtual prototyping of the reactor and thus helps in design, optimization and scale up of reactors. In this study customized User Defined Functions (UDFs) were developed to analyze the performance of low temperature water gas shift membrane reactor. The models were validated using literature data for the parameters – synthesis gas compositions, time factor, sweep flow rate and steam to CO ratio. The effect of all these parameters on the reactor was analyzed for CO conversion, H₂ recovery, DaPe, concentration polarization, concentration profiles and conversion index. The simulations have showed that the UDFs developed were capable of simulating the membrane reactor and this can be used for the design and optimization of the membrane reactor for any process conditions.     

Micromixing performance of the teethed high shear mixer under semi-batch operation

Semi-batch operated reaction processes are necessary for some competitive reaction systems to achieve a desirable process selectivity and productivity of fine chemical products. Herein the structural and operating parameters of the teethed high shear mixers were adjusted to study the micromixing performance in the semi-batch operated system, using the Villermaux/Dushman reaction system. The results indicate that the rising of the rotor speed and the number of rotor teeth, the decrease of the width of the shear gap and the radial distance between the feed position and the inner wall of stator can enhance the micromixing level and lead to the decrease of the segregation index. Additionally, computational fluid dynamics calculations were carried out to disclose the evolution of the flow pattern and turbulent energy dissipation rate of the semi-batch operated high shear mixer. Furthermore, the correlation was established with a mean relative error of 8.05% and R² of 0.955 to fit the segregation index and the parameters studied in this work, which can provide valuable guidance on the design and optimization of the semi-batch operated high shear mixers in practical applications.     

微通道反应器氯磷酸单丁酯与正丁醇酯化反应数值模拟

基于微通道反应器内氯磷酸单丁酯（MCP）与正丁醇（n-BuOH）酯化反应的实验结果,确定了有限速率（finite-rate）模型所需的反应动力学参数（活化能和指前因子）,进而通过此模型,采用数值模拟方法讨论了微通道几何尺寸及反应物进口流速对整个反应体系的影响。研究结果表明：在相同停留时间的前提下,管径减小使反应器出口位置的转化率升高,最高可达73%,并且反应器内物质分布的均匀性和一致性得到提升;在物料进口流速不变的条件下,随着反应器管长增加,出口位置反应物转化率有所提高,最高可达64%,但在实验条件的停留时间下,出口位置反应物转化率并没有达到峰值,峰值所在位置与出口位置相距约0.89m,存在一定的滞后现象;减小反应物进口流速会提高反应物的轴向转化率,但同时也会降低产品的质量流率。     

Estimation of the micromixing time in the torus reactor by the application of the incorporation model

On the basis of previous experimental results in a torus reactor, micromixing time is determined using the incorporation model. Obtained results allowed the characterisation of the performances of this new configuration of reactor in comparison to other reactors, such as the stirred tank reactor. In addition, a correlation is proposed for each incorporation law, in order to determine the micromixing time from the experimental micromixedness ratio (α). Finally, in terms of Kolmogorov's turbulence theory, a relationship between micromixing time and the local energy dissipation rate is obtained and compared to those previously published.     

微通道结构数值模拟及实验性能评价

Fluent软件的数值模拟结果显示伞形微通道结构比U形直线形微通道结构具有更加高效的混合效率,但该结构也存在流动死区,结合优化方向提出一种压降更小、混合效率较优更适用于液-液非均相反应的菱形微通道结构。同时,本文利用前述3种不同微通道结构反应器及常规反应器开展丙烯酸十四酯的合成评价对比实验,红外光谱及核磁谱图对反应产物的分析表征均证明了丙烯酸十四酯的生成。常规反应器和微反应器在相似工艺条件下,丙烯酸十四酯的收率仅为82%,停留时间更是长达300min,而微反应器中收率均达到90%以上,停留时间不超过3.5min,且菱形微通道结构反应器比U形直线形和伞形微通道结构反应器所得丙烯酸十四酯的收率都大,达到97%,说明菱形微通道结构更加有利于物料间的充分混合反应,这与菱形微通道结构数值模拟显示的结果是一致的,从而验证了数值模拟结果的可靠性。     

微通道反应器的一维放大及气液传质特性

微通道反应器能有效增强气液间传质,但处理能力受限。为了提高微通道的处理量,对微通道反应器的一维放大及气-液传质特性进行了研究。以乙醇胺（MEA）和甲基二乙醇胺（MDEA）混合水溶液吸收CO₂为研究物系,在通道深度恒定时,考察了微通道宽度、气液流速对传质特性的影响。结果表明,传质系数和体积传质系数均随通道宽度先增大后缓慢减小,在通道宽度为1000 μm时达到最大值。比表面积随通道宽度的增大而降低。因此,合理增大微通道宽度,可在提高处理能力的同时,仍然保持良好的传质特性。     

CFD simulation of liquid-phase mixing in solid-liquid stirred reactor

A comprehensive CFD model was developed to gain an insight into solid suspension and its implications on the liquid-phase mixing process in a solid-liquid stirred reactor. The turbulent solid-liquid flow in a stirred reactor was simulated using a two-fluid model with the standard k-ε turbulence model with mixture properties. The multiple reference frames (MRFs) approach was used to simulate impeller rotation in a fully baffled reactor. The computational model with necessary sub-models was mapped on to a commercial solver FLUENT 6.2 (of Fluent Inc., USA). The predicted solid concentration distribution was compared with the experimental data of Yamazaki et al. [1986. Concentration profiles of solids suspended in a stirred tank. Powder Technology 48, 205-216]. The computational model was then further extended to simulate and understand the implications of the suspension quality on liquid-phase mixing process. The computational model and the predicted results discussed here will be useful for understanding the liquid-phase mixing process in stirred slurry reactors in various stages of solid suspension.     

Parametric analysis of Fischer‐tropsch synthesis in a catalytic microchannel reactor

Fischer‐Tropsch synthesis (FTS) involves highly exothermic conversion of syngas to a wide range of hydrocarbons, but demands isothermal conditions due to the strong dependence of product distribution on temperature. Running FTS in microchannel reactors is promising, as the sub‐millimeter dimensions can lead to significant intensification that inherently favors robust temperature control. This study involves computer‐based FTS simulations in a heat‐exchange integrated microchannel network composed of horizontal groups of square‐shaped cooling and wall‐coated, catalytic reaction channels. Effects of material type and thickness of the wall separating the channels, side length of the cooling channel, coolant flow rate, and channel wall texture on reaction temperature are investigated. Use of thicker walls with high thermal conductivities and micro‐baffles on the catalytic reaction channel wall favor near‐isothermal conditions. Response of reaction temperature against coolant flow rate is significant. Using cooling channels with smaller side lengths, however, is shown to be insufficient for temperature control. © 2011 American Institute of Chemical Engineers AIChE J, 2012     

Three-dimensional CFD model of the temperature field for a pilot-plant tubular loop polymerization reactor

A three-dimensional (3D) computational fluid dynamics (CFD) model, using an Eulerian-Eulerian two-fluid model which incorporates the kinetic theory of granular flow, the energy balance and heat transfer equations, was developed to describe the steady-state liquid-solid two-phase flow in a loop propylene polymerization reactor composing of loop and axial flow pump. The entire temperature field in the reactor was calculated by the model. The predicted pressure gradient data were found to agree well with the classical calculated data. Furthermore, the model was used to investigate the influences of the circulation flow velocity, the slurry concentration, the solid particle size and the cool water temperature on the temperature field in the reactor. The simulation results showed that the whole loop can be divided into four sections. In addition, the simulation results also showed that the continuous stirred-tank reactor (CSTR) assumption is invalid for the entire field in the loop reactor.     

CFD and experimental studies of single phase axial dispersion coefficient in pulsed sieve plate column

This paper intends to study the single phase axial dispersion in pulsed sieve plate column using a combination of computational fluid dynamics (CFD) simulations and experimental measurements. Experiments and CFD simulations were conducted on 0.076 m diameter pilot scale column having standard geometry of 0.05 m plate spacing, 0.003 m hole diameter and 0.21 fractional free area. The effect of density of tracer solution and radial probe position on axial dispersion coefficient has been studied to ensure precision of the experimental measurement method. The effect of pulse velocity from 0.01 to 0.025 m/s and superficial velocity of water from 0.01 to 0.03 m/s has been studied. Simulations were carried out using commercial CFD software, FLUENT 6.2.16, with standard k–? model for turbulence. An unsteady state tracer injection technique was used for axial dispersion measurement. The range of velocity ratio (ψ = Re_o/Re_n) employed in this work was 1–4 which is very low. Therefore the effect of superficial velocity, V_c was found to be greater than pulse velocity. These results were critically compared with published data and it has been found that single phase axial dispersion coefficient is directly proportional to effective velocity (Af + 0.5 V_c). The presented CFD predictions and validation with experimental data will provide useful basis for further work on single phase axial dispersion with various geometrical parameters and understanding the two phase flow patterns in pulsed sieve plate column.     

Simulation of the hydrodynamics and mass transfer in a falling film wavy microchannel

The flow in a liquid falling film is predominantly laminar, and the liquid-side mass transfer is limited by molecular diffusion. The effective way to enhance the mass transfer is to improve the liquid film flow behavior. The falling film behaviors of water, ethanol and ethylene glycol in nine different wavy microchannels were simulated by Computational Fluid Dynamics. The simulation results show that the falling film thickness exhibits a waveform distribution resulting in a resonance phenomenon along the wavy microchannel. The fluctuation of liquid film surface increases the gas–liquid interface area, and the internal eddy flow inside the liquid film also improves the turbulence of liquid film, the gas–liquid mass transfer in falling film microchannels is intensified. Compared with flat microchannel, the CO_2 absorption efficiency in water in the wavy microchannel is improved over 41%. Prediction models of liquid film amplitude and average liquid film thickness were established respectively.     

CFD analysis of hydrodynamic, heat transfer and reaction of three phase riser reactor

Catalytic cracking reaction and vaporization of gas oil droplets have significant effects on the gas solid mixture hydrodynamic and heat transfer phenomena in a fluid catalytic cracking (FCC) riser reactor. A three-dimensional computational fluid dynamic (CFD) model of the reactor has been developed considering three phase hydrodynamics, cracking reactions, heat and mass transfer as well as evaporation of the feed droplets into a gas solid flow. A hybrid Eulerian-Lagrangian method was applied to numerically simulate the vaporization of gas oil droplets and catalytic reactions in the gas-solid fluidized bed. The distributions of volume fraction of each phase, gas and catalyst velocities, gas and particle temperatures as well as gas oil vapor species were computed assuming six lump kinetic reactions in the gas phase. The developed model is capable of predicting coke formation and its effect on catalyst activity reduction. In this research, the catalyst deactivation coefficient was modeled as a function of catalyst particle residence time, in order to investigate the effects of catalyst deactivation on gas oil and gasoline concentrations along the reactor length. The simulation results showed that droplet vaporization and catalytic cracking reactions drastically impact riser hydrodynamics and heat transfer.     

双催化层固定床甲烷化反应器CFD模拟

温度分布直接影响着固定床甲烷化反应器的甲烷产量和设备安全性。以年产12.75亿立方煤制天然气绝热甲烷化反应器为研究对象,在建立真实设备三维模型的基础上,利用ANSYS-CFX有限元数值模拟的方法,建立多孔介质内化学反应、热交换与质量传递的气-固两相反应器模型,获得了双段固定床甲烷化反应器内部温度、压力、速度场的分布规律及甲烷产率分布。对不同床层结构对应的特征场分布进行了探索,分析了床层结构对各特征场分布的影响,确定了床层结构优化方案,MCR催化剂床层出口处支撑延长的结构更有利于温度场沿反应器径向的均匀分布和甲烷质量分数的提高。对反应器入口温度、空速、压力对特征参数分布的影响进行了研究,提出了针对本工艺的允许入口参数波动范围。     

The NETmix reactor: Pressure drop measurements and 3D CFD modeling

NETmix is a new static mixing technology based on a network of mixing chambers interconnected by channels. Three NETmix reactors with different geometries were used to obtain experimental data for pressure drop and a generalized model for pressure drop in NETmix reactors has been developed. This model features a single adjustable parameter and it is only dependent on the geometric configuration of the NETmix design. The Z factor and the power number were also determined to compare the performance of different NETmix configurations with other existing mixers. The dynamic measurement of pressure drop was used to evaluate the mixing dynamics in the NETmix chambers and, above the critical Reynolds number, the natural oscillation frequency was quantified. Furthermore, a three-dimensional computational fluid dynamic transport model was also developed and validated. The energy performance of the three NETmix prototypes was quantified and shown to be very competitive with the compared existing static mixers. The developed 3D CFD transport model, validated by the reported experimental data, enables the computation of transport properties for any geometrical design and fluid properties, and avoids the need for experimental data each time a new NETmix configuration is designed.     

Characterisation and comparison of the micromixing efficiency in torus and batch stirred reactors

The mixing at a molecular scale (micromixing) plays an important role on selectivity, yield and quality of final products of a large range of competing fast chemical reactions. In this study, we have compared, by the use of iodide–iodate reaction tests, the micromixing in two reactors, the first one is the standard batch stirred reactor and the second is the torus reactor. Various conditions of agitation and feed locations were used for this study. A comparative analysis of the micromixedness ratio (α) in the two reactors was carried out on the basis of the local rate of specific energy dissipation.