Bildgebende UV/VIS-Spektroskopie zur Untersuchung des Einflusses der Fluiddynamik auf die Entstehung von Haupt- und Nebenprodukt in schnellen konkurrierenden konsekutiven Gas-Flüssig-Reaktionen

To form many bulk chemicals, gaseous substances must be mixed and reacted with a continuous liquid phase. In this research work, we systematically investigate to what extent the formation of a side product in a fast competing consecutive reaction can be influenced by the time scales of mixing. For this purpose, a Taylor bubble experiment is used, in which the time scales of mixing can be varied systematically and reproducibly. It is shown, that the mixing in the wake of a bubble is of great importance for the formation of by-products.     

Influence of Bubble Bouncing on Mass Transfer and Chemical Reaction

The hydrodynamics of bubbly flows is dominated by bubble‐induced turbulence and bubble‐bubble interactions. Both phenomena influence the gas‐liquid mass transfer as well as the mixing of reactants. If the time scales of mass transfer and mixing are in the same order as the time scales of a parallel‐consecutive reaction, the yield and selectivity will be affected by the local hydrodynamics. An experimental setup is presented that enables the investigation of mass transfer during well‐defined and adjustable bubble collisions. The influence of CO₂ bubble collisions on mass transfer is measured and modeled with a modified Sherwood number correlation. Further visualization of the concentration field in the vicinity of O₂ bubbles by means of laser‐induced fluorescence demonstrates the dependency of mass transfer from a chemical reaction and permits the development of a first model approach.     

Influence of hydrodynamics on liquid mixing during Taylor flow in a microchannel

The miscible liquid‐liquid two phases based on Taylor flow in microchannels was investigated by high‐speed imaging techniques and Villermaux/Dushman reaction. The mixing based on Taylor flow was much better compared with that without introducing gas in microchannels, even the ideal micromixing performance could be obtained under optimized superficial gas and liquid velocities. In the mixing process based on Taylor flow, the superficial gas and liquid velocities affected the lengths and the velocities of Taylor bubble and liquid slug, and finally the micromixing performance. The formation process of Taylor flow in the inlets, the initial uniform distribution of reactants and the internal circulations in the liquid slug, and the thin liquid films all improved the mixing performance. Furthermore, a modified Peclet number that represented the relative importance of diffusion and convection in the mixing process was proposed for explaining and anticipating micromixing efficiency. © 2011 American Institute of Chemical Engineers AIChE J, 58: 1660–1670, 2012     

Reactive mass transfer at gas-liquid interfaces: impact of micro-scale fluid dynamics on yield and selectivity of liquid-phase cyclohexane oxidation

The impact of single-bubble wake dynamics on the reaction-enhanced mass transfer and on the yield and selectivity of the cyclohexane oxidation reaction was studied using a two-dimensional CFD-reaction model that was developed by our group. Temperature and the concentrations of the (desired) intermediate and (undesired) final products of this autocatalytic reaction were the parameters of this study. Two bubble types were studied: (a) a circular bubble with closed wake, and (b) an elliptical bubble with an unsteady, vortex-shedding wake. The main results of our work are: (1) Film theory over-predicts reaction-enhanced mass transfer since the assumption of an average film thickness is not justified. In order to study fast reaction systems on a reactor scale using coarse-grid CFD codes, a full bubble model, or correlations based on it, should be incorporated as a sub-grid micro model. (2) The bubble wake does not contribute to mass transfer in systems where reaction rates are low. For fast reactions, the local mass transfer rate in the wake can increase by several thousand percent. (3) Vortex shedding causes qualitatively different mixing since patches rich in the dissolved gas are quickly convected away from the bubble. Bubbles that cause vortex shedding will lead to a significantly higher conversion per volume than spherical bubbles. (4) Parallel-consecutive reactions with a high liquid-phase reactant concentration and with reaction rates that depend in an identical way on the dissolved gas concentration, are not micro-mixing sensitive in terms of selectivity. Since bubble shapes and sizes can be controlled by changing operating and design parameters, the yield of this reaction can be controlled.     

The isothermal selectivity of porous catalysts: competitive—consecutive reactions

The influence of mass transfer on the selectivity of three reaction types has been studied for an isothermal catalyst particle: Type 1 is a consecutive reaction, Types 2 and 3 are competitive—consecutive reactions. Type 1 and Type 2 are treated analytically for the case that the diffusivities are not equal. Equations and diagrams are presented that show the importance of different parameters to maximize the concentration of an intermediate in a plug-flow reactor. Type 3 has been dealt with on an analog computer. It can be proved that the selectivity loss for Type 3 is less than for Type 1.     

Modeling of bubble column slurry reactor for reductive alkylation of p-phenylenediamine

A bubble column slurry reactor (BCSR) model has been developed for the reductive alkylation of p-phenylenediamine (PPDA) with methyl ethyl ketone (MEK) to N,N^′-di-secondary-alkyl-p-phenylenediamine (Di-amine). This particular reaction system is commercially relevant and involves a combination of parallel and consecutive reactions comprising equilibrium non-catalytic (homogeneous) and catalytic (heterogeneous) steps. The proposed model is based on the ‘mixing cell approach’. In this work the mixing cell approach has been extended by including a liquid backflow stream from all but the bottommost mixing cell. The model incorporates the contributions of gas-liquid and liquid-solid mass transfer, heat effects, and complex multistep reaction kinetics. CFD model is used to estimate the extent of backflow among mixing cells and its dependence on operating parameters. The effect of gas and liquid velocities, catalyst loading, inlet PPDA concentration, and temperature on the conversion, selectivity, global rate of hydrogenation, and temperature rise is discussed. The comparison of the current approach with the traditional mixing cell model is discussed. The BCSR model presented here will be useful to provide guidelines for designing and improving overall performance of bubble column reactors.     

Micromixing in Reactive,Deformable Bubble and Droplet Swarms

Detailed, high‐resolution numerical simulations have been performed of the buoyancy‐driven motion of deformable, chemically reacting bubble and droplet swarms. Mass transfer rates and chemical reaction selectivities were determined and a comparison is presented between the results for bubble/droplet swarms and single bubbles. The mixing in the wake of bubbles was characterized as well. It was shown that for mixing‐sensitive reaction networks, the hydrodynamics of the bubble swarm may significantly impact the reaction selectivity. Four special cases are highlighted, i.e., highly exothermic reactions, heterogeneously catalyzed reactions, animal cell cultures and experiments. The most important outcome of this work is that bubble swarms and the impact of the swarm hydrodynamics on reacting systems can be studied in detail. This is the first study of this kind reported in the literature. Its importance is paramount as knowledge of the unique flow dynamics inside bubble swarms is crucial to understanding the mechanisms controlling mass transport and chemical reactions and is a prerequisite for effective process intensification.     

斜插穿流式挡板强化鼓泡塔内流体混合的研究

乙炔二聚反应制备乙烯基乙炔是氯丁橡胶生产等工艺中的重要操作单元,通常在全混式鼓泡塔反应器中进行。但塔内流体混合模式单一、流场对称性强、液相轴向返混较高,乙烯基乙炔容易滞留在催化剂中,并与乙炔进一步反应生成二乙烯基乙炔和高聚物。强化乙炔二聚鼓泡塔内的反应,提高乙炔单程转化率和选择性,有助于氯丁橡胶生产实现节能减排。应用FLUENT软件模拟,计算了鼓泡塔内流场结构及变化规律,发现斜插穿流式挡板可有效地调控乙炔二聚鼓泡塔内流场结构,减小漩涡尺寸,降低液相湍流流速,减小液相返混区域,强化鼓泡塔内乙炔二聚反应。     

Operating Characteristics and Performance of a Monolithic Downflow Bubble Column Reactor in Selective Hydrogenation of Butyne‐1,4‐diol

The effects of flow condition, bubble dispersion level, and liquid flow rate on the behavior of a novel monolithic downflow bubble column (M‐DBC) were investigated using a reaction model, the palladium‐catalyzed hydrogenation of butyne‐1,4‐diol. The stable and closely packed homogeneous bubble dispersion present in the bulk region of the M‐DBC allowed effective introduction of the gas‐liquid phase for formation of Taylor flow inside the monolith channels. The condition defined as the minimum level dispersion was required in order to obtain high selectivity towards the intermediate product, cis‐2‐butene‐1,4‐diol. Enhanced reaction rates were obtained at increasing the dispersion level and lowering the liquid flow rate. Comparison with the DBC employing 5 % Pd/C powder catalyst and 1 % Pd‐on‐Raschig‐ring revealed a better performance of the M‐DBC (1 % Pd loading) with the advantage of smaller reaction volume and intensified reaction rate. As an alternative to conventional three‐phase reactors, the M‐DBC was so simple due to its inherent characteristic operation and no specially designed device is required.     

Effects of hydrodynamics and Lagrangian transport on chemically reacting bubble flows

The objective of this study is to investigate the dynamics of flows occurring in the wakes of rising bubbles of different shapes and sizes. Different wake dynamics can result in qualitatively different mixing characteristics. In the case of fast gas-liquid reaction networks, reactions occur almost exclusively in the bubble wake. Thus, wake mixing can have a strong impact on the reaction yield and selectivity. Dynamic numerical simulations were performed to study the flow of liquid around bubbles of different shapes. The obtained velocity and pressure fields were used to investigate the liquid-phase mixing in the flow for each case. As a strong connection between mixing and chaos is known to exist, Lagrangian tracking of passive tracer particles was used to identify chaotic fluid transport in the flows. Chaotic dynamics lead to folding and stretching of fluid elements, which results in very effective mixing. To quantify mixing, stretching fields were computed for each flow case. Finally, different liquid-phase chemical reaction networks were tested to illustrate the effects of mixing on chemical reaction yields and selectivities.     

微通道内气-液两相传质过程行为及其应用

微通道内气-液两相体系中Taylor流和泡状流具有气泡尺寸均一、停留时间分布窄、可调控性强和比表面积高等优点,具有广泛的应用前景。从Taylor气泡和泡状气泡的传质过程出发,系统综述了微尺度下气泡的溶解规律、传质过程机理和传质/溶解模型等方面的研究进展,并介绍上述流型在反应或过程强化、基础物性及动力学数据测量和微纳材料合成方面的应用。最后总结并展望了技术领域的研究难点与研究方向。     

Interaction between micro- and macro-mixing during reactions in agitated tanks

The present investigation focusses on the interactions between micro-and macro-mixing in a stirred tank reactor, operated in the semi-batch mode. With long feed times only micromixing controls reaction, but below a critical feed time macroscopic gradients also become important. Experiments generated a coherent data set which can be used as a design guide and also for model validation. A competitive, consecutive azocoupling reaction was carried out, whereby imperfect mixing produces finite values of X_S (yield of secondary product) which can be conveniently used as a mixing index. The influences of feed location, feed rate, stirrer speed, concentrations and vessel size on selectivity were studied. The micromixing model of Baldyga and Bourne (1989) was extended to include interactions between macro-and micro-mixing and should be valid for feed times below the critical value, although not so short as the macromixing time in the vessel. Model predictions were compared with the experimental results.     

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。     

DNS-based prediction of the selectivity of fast multiphase reactions: Hydrogenation of nitroarenes

The catalytic hydrogenation of nitroarenes is a reaction of significant importance for the pharmaceutical, agro-chemical and dye-manufacturing industries. It proceeds through a hydroxylamine intermediate, which is toxic, carcinogenic and thermally unstable. While many studies have been conducted, attempting to alter the catalyst to minimize the accumulation of hydroxylamine, the influence of the hydrodynamics on the product and intermediate formation has so far not been investigated.Due to the fast rate of reaction most of the hydrogen dissolved from the bubbles will be consumed in close proximity of the bubble surface. In such cases local mixing in the bubble wake impacts the reaction network far surpassing that of the reactor-scale transport.For the purposes of studying the local hydrodynamics in bubbly flows, direct numerical simulations (DNS) were performed of freely deformable gas bubbles rising in a liquid phase. Different cases were considered, including both single bubbles and clusters of multiple bubbles. The hydrodynamic model has been coupled with the equations describing species transport and chemical reaction of all reactants, which allows the direct investigation of the effects of the local hydrodynamics on the product distribution. The effect of different Hatta and Schmidt numbers has been investigated. The simulation results have been compared with a solution obtained from film theory and suggestions have been made how to incorporate the local mixing effect into a modified film theory.We found that the different bubble wake types observed at different conditions will result in different rates of hydroxylamine formation and selectivity. These differences will persist even in the case of clusters consisting of multiple bubbles. The results of this study can be useful for the future design and operation of bubble reactors, as it provided a straightforward method for decreasing the accumulation of harmful intermediates in the catalytic hydrogenation of nitroarenes.     

Integral Models of Turbulent Micromixing in Processes with Complex Chemical Reactions

Turbulent mixing during an irreversible isothermal competitive–consecutive reaction is considered. The sensitivity of the calculated results to the choice of micromixing model is analyzed. The upper- and lower-bound estimates of the average concentration of the desired product and selectivity are obtained for integral models of micromixing. The initial concentrations of the reactants affect the process characteristics. Simple analytical expressions are derived for the conditions for complete conversion of the initial reactants. Calculations for portionwise adding of the reactants are also performed.     

Visualisation research on characteristics of the cryogenic slug flow in vertical and inclined tubes

The statistical parameters of Taylor bubbles in cryogenic slug flow along glass tube were studied experimentally for various inclination angles θ (0°–60° from the vertical direction) and four tube inner diameters D (14, 18, 25 and 32 mm) by using high speed digital camera. The distributions and mean values of initial formation position, and the length and velocity of Taylor bubbles along the tube at various inclination angles were obtained. Initial position of Taylor bubbles increases as tube inner diameter increases and the effect of tube diameter on Taylor bubble initial position becomes more obvious at θ ≥ 30°. Taylor bubble length shows an allometric decreasing trend with the increasing of tube inner diameter. The influence of inclination angle and axial position on the length and velocity of Taylor bubble in cryogenic slug flow agrees qualitatively with conventional air–water system. The maximum mean velocity of Taylor bubble occurs at 30° ≤ θ ≤ 45° and the minimum mean length of Taylor bubble occurs at 0° ≤ θ ≤ 20°. The Mean length of Taylor bubble increases along the tube for x/D ≤ 60. © 2011 Canadian Society for Chemical Engineering     

气泡大小对反应器内氧传递系数的影响

在气液反应过程中,气泡的大小对结果往往起了决定性的作用。通过减小气泡的尺寸,可以促进气液传递,加快反应的进程。在气液搅拌式反应器上安装了一种特殊的气体分布器,通过搅拌产生离心场,从而诱导生成泰勒涡柱,使大量进入反应器的空气气泡保持在泰勒涡柱的内部。由于减少了气泡间的凝并作用,气泡尺寸减小,与对照组相比,反应器中最小的气泡尺寸减小了近50%,气泡的比表面积增加近80%。通过对不同通气流量和搅拌速度下气液反应器内氧传递系数的测量,与对照实验比较,使用特殊气体分布器的反应器中,氧的传递系数增加了10%~40%,证明这种气体分布器确实可以增加气液间氧的传递。     

用统计方法研究未充分发展弹状流动

在一内径19 mm、长2 m的垂直有机玻璃管内,采用自制的电导探针对未充分发展的气-液二相弹状流中的弹状气泡上升速度、液塞上升速度、弹状气泡长度和液塞长度进行了测量。得到了各自随表观气速或表观液速的变化规律。结果表明:在未充分发展的弹状流状态下,弹状气泡的上升速度略高于液塞的上升速度:弹状气泡长度随表观气速的增大而增大,随表观液速的增大而减小。文章对弹状气泡长度进行了统计分析。未充分发展弹状流中弹状气泡长度符合正态分布律。     

The mixing sensitivity of polysulphide generation

The effect of mixing and mass transfer on polysulphide generation by catalytic oxidation of sodium sulphide was studied using two batch‐operated reactors. One was a sparged reactor operated at atmospheric pressure and low mixing intensities (0.37 to 28 W/kg), the other was an unsparged pressurized reactor characterized by high mixing intensities (17 to 3100 W/kg). The reaction parameters examined included the impeller speed, sparged gas flowrate, oxygen partial pressure, and catalyst loading and type. In both reactors the maximum polysulphide yield, selectivity and rate of formation increased with increasing energy dissipation. Increased gas sparging increased the rate of reaction, but had little effect on either yield or selectivity. Increased oxygen partial pressure increased the rate of oxidation but decreased both the yield and selectivity. The type of catalyst dramatically affected the yield of polysulphide produced for a given set of reaction conditions with improved mixing increasing reaction rate, yield and selectivity.