CFD modelling of liquid–liquid multiphase microstructured reactor: Slug flow generation

Microreactor technology, an important method of process intensification, offers numerous potential benefits for the process industries. Fluid–fluid reactions with mass transfer limitations have already been advantageously carried out in small-scale geometries. In liquid–liquid microstructured reactors (MSR), alternating uniform slugs of the two-phase reaction mixture exhibit well-defined interfacial mass transfer areas and flow patterns. The improved control of highly exothermic and hazardous reactions is also of technical relevance for large-scale production reactors. Two basic mass transfer mechanisms arise: convection within the individual liquid slugs and diffusion between adjacent slugs. The slug size in liquid–liquid MSR defines the interfacial area available for mass transfer and thus the performance of the reactor. There are two possibilities in a slug flow MSR depending on the interaction of the liquids with the solid wall material: a dispersed phase flow in the form of an enclosed slug in the continuous phase (with film—complete wetting of the continuous phase) and an alternate flow of two liquids (without film—partial wetting of the continuous phase). In the present work, a computational fluid dynamics (CFD) methodology is developed to simulate the slug flow in the MSR for both types of flow systems. The results were validated with the experimental results of Tice et al. (J.D. Tice, A.D. Lyon and R.F. Ismagilov, Effects of viscosity on droplet formation and mixing in microfluidic channels, Analytica Chimica Acta507 (1) (2004), pp. 73–77.).     

Liquid–liquid slug flow: Hydrodynamics and pressure drop

In this paper, the hydrodynamics and the pressure drop of liquid–liquid slug flow in round microcapillaries are presented. Two liquid–liquid flow systems are considered, viz. water-toluene and ethylene glycol/water-toluene. The slug lengths of the alternating continuous and dispersed phases were measured as a function of the slug velocity (0.03–0.5 m/s), the organic-to-aqueous flow ratio (0.1–4.0), and the microcapillary internal diameter (248 and 498 μm). The pressure drop is modeled as the sum of two contributions: the frictional and the interface pressure drop. Two models are presented, viz. the stagnant film model and the moving film model. Both models account for the presence of a thin liquid film between the dispersed phase slug and the capillary wall. It is found that the film velocity is of negligible influence on the pressure drop. Therefore, the stagnant film model is adequate to accurately predict the liquid–liquid slug flow pressure drop. The influence of inertia and the consequent change of the slug cap curvature are accounted for by modifying Bretherton’s curvature parameter in the interface pressure drop equation. The stagnant film model is in good agreement with experimental data with a mean relative error of less than 7%.     

Effective interfacial area for mass transfer in the liquid–liquid slug flow capillary microreactors

Liquid–liquid biphasic reactions play an important role in the chemical and pharmaceutical industries. The liquid–liquid slug flow capillary microreactor offers considerable potential benefits over the conventional liquid–liquid contactors. Though the hydrodynamics and mass transfer have been investigated for this reactor concept, so far the effective interfacial area available for mass transfer has not been experimentally quantified. Despite the well-defined flow patterns arising in the capillary microreactor, the wetting behaviour of the liquids at the capillary wall is inadequately integrated into the models and thus, the true interfacial area being used for mass transfer is uncertain.     

Hydrodynamics and local mass transfer characterization under gas–liquid–liquid slug flow in a rectangular microchannel

Gas–liquid–liquid three-phase slug flow was generated in a glass microreactor with rectangular microchannel, where aqueous slugs were distinguished by relative positions to air bubbles and organic droplets. Oxygen from bubbles reacted with resazurin in slugs, leading to prominent color changes, which was used to quantify mass transfer performance. The development of slug length indicated a film flow through the corner between bubbles and the channel wall, where the aqueous phase was saturated with oxygen transferred from bubble body. This film flow results in the highest equivalent oxygen concentration within the slug led by a bubble and followed by a droplet. The three-phase slug flow subregime with alternate bubble and droplet was found to benefit the overall mass transfer performance most. These results provide insights into a precise manipulation of gas–liquid–liquid slug flow in microreactors and the relevant mass transfer behavior thereof.     

DEVELOPMENTS ON WETTING EFFECTS IN MICROFLUIDIC SLUG FLOW

Wetting effects form a dimension of fluid dynamics that becomes predominant, precisely controllable, and possibly useful at the micro-scale. Microfluidic multiphase flow patterns, including size, shape, and velocity of fluidic particles, and mass and heat transfer rates are affected by wetting properties of microchannel walls and surface tension forces between fluid phases. The novelty of this field, coupled to difficulties in experimental design and measurements, means that literature results are scarce and scientific understanding is incomplete. Numerical methods developed recently have enabled a shortcut in obtaining results that can be perceived as realistic and that offer insight otherwise not possible. In this work the effect of the contact angle on gas-liquid two-phase flow slug formation in a microchannel T-junction was studied by numerical simulation. The contact angle, varied from 0 to 140 degrees, influenced the interaction of the gas and liquid phases with the channel wall, affecting the shape, size, and velocity of the slugs. The visualisation of the cross-sectional area of gas slugs allowed insight into the existence of liquid flow along rectangular microchannel corners, which was affected by the contact angle and determined the occurrence of velocity slip. The velocity profile within the gas slugs was also found to change as a function of contact angle, with hydrophilic channels inducing greater internal circulation, compared to greater channel wall contact in the case of hydrophobic channels. These effects play a role in heat and mass transfer from channel walls and highlight the value of numeral simulation in microfluidic design. Supplementary materials are available for this article. Go to the publisher's online edition of Journal of Chemical Engineering Communications to view the supplemental file.     

Effect of Viscosity on the Hydrodynamics of Liquid Processes in Microchannels

The hydrodynamics of single‐phase liquid flow with relatively high fluid viscosities in a microchannel was investigated experimentally. The results showed that the conventional theory could predict the single‐phase flow with high fluid viscosities in microchannels. Furthermore, the effect of viscosity on the slug flow of two immiscible liquid phases in a microchannel was studied with high‐speed imaging techniques. It was found that a higher dispersed‐phase viscosity quickened the flow pattern transition from slug flow to parallel flow and resulted in smaller slugs. A modified capillary number representing the mutual effects of the viscosities of the continuous phase and the dispersed phase was proposed for predicting the slug sizes in microchannels.     

Design of mixing in microfluidic liquid slugs based on a new dimensionless number for precise reaction and mixing operations

This paper reports mixing characteristics inside a microfluidic liquid slug using the computational fluid dynamics (CFD) simulations. Each slug is modeled as a single-phase flow domain. Slug-based microfluidics offers rapid mixing by internal circulation and transport with narrow residence time distribution, making it suitable for precise reaction and mixing operations. Miniaturizing the slug size to microscale allows high interactions between the slug internal fluid and the channel wall, leading to a highly effective internal circulation. However, quantitative understanding of mixing characteristics and the influences of operating parameters on mixing rate is crucial for the design of a liquid slug that ensures desired mixing rates. The simulation results provide insights into the influences of operating parameters on slug-based mixing rates. Based on the simulation results, the modified Peclet number, , is proposed for designing mixing in liquid slugs. A novel method using Pe^* to estimate mixing rates and design liquid slugs to obtain desired mixing rates is discussed. Using this method, both short (ms) and long (min) mixing timescales can be accessed in the same microfluidic device by simply varying the slug velocity.     

Mass transfer behavior of liquid–liquid slug flow in circular cross-section microchannel

An alkaline hydrolysis reaction was used as the model reaction to investigate the performance of liquid–liquid slug flow microchannel. The specific interfacial area was determined through the photographic snapshot method physically by means of measuring the lengths of relevant slugs. The overall volumetric mass transfer coefficients were calculated through the Danckwerts’ model chemically. The influences of various operating conditions on the slug length, the overall volumetric extraction rate and the mass transfer coefficient were investigated quantitatively. A decreasing trend of volumetric mass transfer coefficients along the channel length was found. The linear dependence of the volumetric extraction rate on the volumetric mass transfer coefficient indicates that the overall rate of the process is determined by the mass transfer process. In addition, the volumetric mass transfer coefficients were correlated for different channel lengths.     

Hydrodynamics of an external-loop gas-lift system with restrictions located in the downcomer

Experiments were conducted to study the fluid dynamics in the case that slug flow occurs in the riser of an external-loop gas-lift system with a restriction section located in the downcomer. Complex fluctuation behaviors of the liquid circulation velocity and the wall shear stress in the riser were observed and discussed. Based on the slug flow hydrodynamic behaviors and the balance of momentum and pressure drop over the circulating loop, a model was developed to predict the main parameters of the system: the liquid circulation velocity, the void fraction, the length and velocities of Taylor bubbles and liquid slugs. The predicted results of these parameters were compared with the experimental data and a good agreement was obtained.     

Characteristics of liquid slugs in gas–liquid Taylor flow in microchannels

The hydrodynamics of liquid slugs in gas–liquid Taylor flow in straight and meandering microchannels have been studied using micro Particle Image Velocimetry. The results confirm a recirculation motion in the liquid slug, which is symmetrical about the center line of the channel for the straight geometry and more complex and three-dimensional in the meandering channel. An attempt has also been made to quantify and characterize this recirculation motion in these short liquid slugs (L_s/w<1.5) by evaluating the recirculation rate, velocity and time. The recirculation velocity was found to increase linearly with the two-phase superficial velocity U_TP. The product of the liquid slug residence time and the recirculation rate is independent of U_TP under the studied flow conditions. These results suggest that the amount of heat or mass transferred between a given liquid slug and its surroundings is independent of the total flow rate and determined principally by the characteristics of the liquid slug.     

微通道内气-液弹状流动及传质特性研究进展

气-液弹状流,又称Taylor流,是一种以长气泡和液弹交替形式流动的流动形态。微通道内气-液弹状流因其气泡与液弹尺寸分布均一、停留时间分布窄、径向混合强等优点,是一种适于强化气-液反应的理想流型。本文首先介绍了微通道内气泡的生成机理、气泡和液弹长度,以及气泡生成阶段的传质特征。其次系统综述了主通道中弹状流动及传质过程的研究进展,包括气泡形状与液膜厚度、液弹内循环和泄漏流特征、气-液传质系数的测量与预测,以及物理与化学吸收过程中的传质特性等方面内容。最后阐述了当前研究的不足并展望了气-液弹状流的研究方向。     

Modeling of extraction behavior of docosahexaenoic acid ethyl ester by utilizing slug flow prepared by microreactor

The liquid–liquid extraction dynamics of an ethyl ester of docosahexaenoic acid (DHA‐Et) with silver ion was investigated. The kinetic model was derived according to the following stepwise processes: Diffusion of DHA‐Et across the organic film, complex‐formation between DHA‐Et and silver ion at the interface, and diffusion of extracted complex across the aqueous film. The kinetic parameters for the complex‐formation reaction were determined from the investigation with the stirred transfer cell. With the proposed model and determined parameters, we predicted the uptakes of DHA‐Et for the extraction system utilizing a slug flow prepared by a microchip. The calculated uptakes showed good correlation to the experimental data. The theoretical investigation suggested that the fast equilibration realized for the slug flow extraction system was due to the large specific interfacial area of the slug caused by the presence of wall film and the thin liquid film caused by the internal circulation. © 2009 American Institute of Chemical Engineers AIChE J, 2010     

CFD Simulation of hydrodynamics of air sparging in a vertical tubular membrane

Computational fluid dynamics (CFD) simulation of the hydrodynamics of slug flow which is generated by air sparging in a vertical tubular membrane has been investigated. The results of simulation have been reported in the form of parameters such as shape, velocity profile, surface shear stresses and gas slug (Taylor bubble) rising velocities, and evaluated with experimental data which were presented in previous articles. This study showed that CFD modeling is able to accurately simulate the shape and velocity field around the gas slugs. Also the shear stress induced by slug flow passage and rising velocity of gas slugs for high-velocity liquid and low-velocity gas fit appropriately to values in reference data. Simulation results for gas slug rising velocity showed about 0.35–9% error in the different conditions investigated in respect to experimental data.     

Immiscible liquid–liquid slug flow characteristics in the generation of aqueous drops within a rectangular microchannel for preparation of poly(2-hydroxyethylmethacrylate) cryogel beads

The microchannel liquid-flow focusing and cryo-polymerization is an efficient method for the preparation of cryogel beads with a narrow diameter distribution. In order to prepare cryogel beads with expected diameters, it is necessary to get insights in the liquid–liquid immiscible flow characteristics of the flow-focusing fluid and the monomer solution in microchannels. In this work, the slug flow behaviors of two immiscible liquids regarding the preparation of poly(2-hydroxyethylmethacrylate) (pHEMA) cryogel beads in a rectangular cross-junction microchannel were investigated experimentally by the high-speed imaging method. Correlations of the immiscible liquid–liquid slug flow parameters like the aqueous slug velocity and length, the aqueous slug nose and tail lengths, the water-immiscible slug length as well as the aqueous droplet size were obtained. The pHEMA cryogel beads were prepared under certain flow conditions and the bead sizes were measured by laser particle size analyzer. The obtained correlations were then employed to estimate the bead sizes and compared with those obtained experimentally. The results showed that the present correlations gave reasonable estimations of the mean bead diameters at various conditions and thus, could be useful and helpful in the preparation of cryogel beads with expected size distributions in rectangular microchannels.     

The pressure drop experiment to determine slug lengths in multiphase monoliths

The length of the liquid slugs, that separate the elongated bubbles in Taylor flow, is an important parameter for mass transfer, flow stability and pressure drop in capillary microchannels. In this work, pressure drop measurements are used to determine the length of slug in Taylor flow in downflow monoliths. The method is sensitive if the slugs are relatively short, less than 10 times the channel diameter. The pressure drop measurements are a cheap and fast alternative to tomographic or electric methods. Experiments using different distributors indicate that the slug length varies significantly with changes in the hydrodynamics in the feed section of the monoliths. Slug length correlations that are based on parameters inside the channels can therefore not safely be used for a different setup. As a result, the slug length should be measured in each experimental setup, which makes a inexpensive and robust method to do so very welcome.     

外环状弹性壁布膜器及超薄降膜厚度

研发了竖直管外环状弹性壁降膜分布器,可以产生微米量级厚度均匀的超薄降膜流动。由弹性薄壁轴对称变形协调性决定,该布膜器具有均匀性和稳定性内在机理。理论分析导出了初始膜厚及布膜流量与布膜器内液柱高度的线性关系,并通过布膜器操作参数及降膜流量等实验数据进行了验证。实验结果表明,该布膜器对降膜管壁的润湿性能已经达到由固液界面性质决定的最小润湿流率。     

水平管段塞流持液率的波动特性

气液两相段塞流是液塞和长气泡在空间和时间上的交替,在流动过程中表现出间歇性和不稳定性.今对水平管中段塞流持液率的波动特性进行了分析.结果表明:在同一折算液速下,随着折算气速的增加,段塞单元的平均持液率和液膜持液率先快速下降再缓慢下降,而液塞持液率先缓慢下降再快速下降.段塞流持液率的概率密度分布为双峰分布,高持液率峰对应于液塞区,低持液率峰对应于液膜区;概率密度函数中较完好的峰所对应的持液率与光滑分层液膜区和液塞区的平均持液率相一致.     

Design and operation of gas–liquid slug flow in miniaturized channels for rapid mass transfer

The influences of operating parameters such as channel size, flow rate, and void fraction on the mass transfer rate in the gas–liquid slug flow are investigated to establish a design method to determine the parameters for rapid mass transfer. From the experimental results, the turnover index, including the slug linear velocity, its length, and the channel size that represents the turnover frequency of the internal circulation flow, is proposed. For PTFE tube in which no liquid film exists in slug flow, a master curve is derived from the relationship between the mass transfer coefficient and the turnover index. For each channel material, the Sherwood number is also roughly correlated with the Peclet number. These correlations make it possible to arbitrarily determine a set of operating parameters to achieve the desired mass transfer rate. However, the turnover index and the Peclet number include the slug length, which cannot be controlled directly. The relationship between the slug length and the operating parameters is also investigated. The slug volume mainly depends on the inner diameter (i.d.) of a union tee. At a fixed union tee i.d., the slug length is controlled through the exit i.d. of the channel connected to the union tee and the void fraction. Thus, the final slug length depends on the union tee and exit channel inner diameters. At low flow rates, the gas and liquid collision angle is significant in determining the slug length.     

水平管段塞流气弹区液膜特性研究

为了研究气液段塞流相界面的结构特征,采用了双平行电导探针技术对水平管内段塞流气弹区的液膜特性进行了实验测量,并基于一维双流体模型导出液膜厚度的控制方程,该方程对液膜厚度的预测结果与实验结果吻合良好。结合液膜厚度计算模型提出了计算段塞流的机理模型,该段塞流机理模型的计算结果表明对于液相表观速度较高而气相表观速度较低的段塞流,机理模型中忽略液膜非平衡性时得到的平均液膜厚度和气弹区长度明显偏低。     

Hydrogenation of 2‐ethylanthraquinone under Taylor flow in single square channel monolith reactors

The hydrogenation of 2‐ethylanthraquinone (EAQ) to 2‐ethylanthrahydroquinone (EAHQ) was carried out under Taylor flow in single square channel monolith reactors. The two opening ends of opaque reaction channel were connected with two circular transparent quartz‐glass capillaries, where Taylor flow hydrodynamics parameters were measured and further used to obtain practical flow state of reactants in square reaction channels. A carefully designed gas‐liquid inlet mixer was used to supply steady gas bubbles and liquid slugs with desired length. The effects of various operating parameters, involving superficial gas velocity, superficial liquid velocity, gas bubble length, liquid slug length, two‐phase velocity and temperature, on EAQ conversion were systematically researched. Based on EAQ conversion, experimental overall volumetric mass transfer coefficients were calculated, and also studied as functions of various parameters as mentioned earlier. The film model, penetration model, and existing semi‐empirical formula were used to predict gas‐solid, gas‐liquid, and liquid‐solid volumetric mass transfer coefficients in Taylor flow, respectively. The predicted overall volumetric mass transfer coefficients agreed well with the experimental ones. © 2009 American Institute of Chemical Engineers AIChE J, 2009