Bubble/droplet formation and mass transfer during gas–liquid–liquid segmented flow with soluble gas in a microchannel

Microchannels have great potential in intensification of gas–liquid–liquid reactions involving reacting gases, such as hydrogenation. This work uses CO₂–octane–water system to model the hydrodynamics and mass transfer of such systems in a microchannel with double T‐junctions. Segmented flows are generated with three inlet sequences and the size laws of dispersed phases are obtained. Three generation mechanisms of dispersed gas bubbles/water droplets are identified: squeezing by the oil phase, cutting by the droplet/bubble, cutting by the water–oil/gas–oil interface. Based on the gas dissolution rate, the mass transfer coefficients are calculated. It is found that water droplet can significantly enhance the transfer of CO₂ into the oil phase initially. When bubble‐droplet cluster are formed downstream the microchannel, droplet will retard the mass transfer. Other characteristics such as phase hold‐up, bubble velocity and bubble dissolution rate are also discussed. The information is beneficial for microreactor design when applying three‐phase reactions. © 2016 American Institute of Chemical Engineers AIChE J, 63: 1727–1739, 2017     

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.     

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.     

直接接触式沸腾换热过程气泡混合均匀度评价

针对直接接触式沸腾换热过程中气泡群扰动的多相流体混合性能评价问题,提出一种基于图像处理技术与像素偏差理论量化混合复杂流体流型均匀性的精确检测与度量新方法。通过气泡群RGB图像的灰度矩阵进行偏差计算,精确地定量描绘了直接接触式换热过程中的气-液混合状态,其值大小反映了气泡群的分布状况,而不同工况下混合均匀时间的差异反映了不同多相体系的均匀性差异;对偏差序列进行正态性检验,发现其近似符合正态分布;运用混沌检测方法对序列进行混沌特征分析,发现整个混合过程是混沌的。通过一个简单的指标实现了复杂条件下气泡群的量化和表征,为多相流流型识别提供了新的分析思路并指导换热过程的混沌强化。     

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.     

On the drag force of bubbles in bubble swarms at intermediate and high Reynolds numbers

An accurate and fast simulation of large-scale gas/liquid contact apparatusses, such as bubble columns, is essential for the optimization and further development of many (bio)chemical and metallurgical processes. Since it is not feasible to simulate an entire industrial-scale bubble column in full detail from first principles (direct numerical simulations), higher-level models rely on algebraic closure relations to account for the most important physical phenomena prevailing at the smallest length and time scales, while keeping computational demands low. The most important closure for describing rising bubbles in a liquid is the closure for the drag force, since it dominates the terminal rise velocity of the bubbles.Due to the very high gas loadings used in many industrial processes, bubble–bubble (or ‘swarm’) interactions need to be accounted for in the drag closure. An advanced front-tracking model was employed, which can simulate bubble swarms up to 50% gas hold-up without the problem of (numerical) coalescence. The influence of the gas hold-up for mono-disperse bubble swarms with different bubble diameters (i.e. Eötvös numbers) was quantified in a single drag correlation valid for the intermediate to high Reynolds numbers regime . Also the physical properties of the liquid phase were varied, but the simulation results revealed that the drag force coefficient was independent of the Morton number. The newly developed correlation has been implemented in a larger-scale model, and the effect of the new drag closure on the hydrodynamics in a bubble column is investigated in a separate paper (Lau et al., this issue).     

The Impact of Local Phenomena on Mass Transfer in Gas‐Liquid Systems

The impact of local phenomena on mass transfer from single bubbles and bubbles rising under swarm conditions with the bubble diameter and the viscosity as variation parameters is investigated by using laser‐induced fluorescence. It is reported that the wake shedding frequency of ellipsoid bubbles is decisively responsible for the convective mass transfer. A novel mass transfer correlation is developed considering the Strouhal number and swarm conditions.     

Mixing dynamics in bubbling fluidized beds

Solids mixing affects thermal and concentration gradients in fluidized bed reactors and is, therefore, critical to their performance. Despite substantial effort over the past decades, understanding of solids mixing continues to be lacking because of technical limitations of diagnostics in large pilot and commercial‐scale reactors. This study is focused on investigating mixing dynamics and their dependence on operating conditions using computational fluid dynamics simulations. Toward this end, fine‐grid 3D simulations are conducted for the bubbling fluidization of three distinct Geldart B particles (1.15 mm LLDPE, 0.50 mm glass, and 0.29 mm alumina) at superficial gas velocities U/Umf = 2–4 in a pilot‐scale 50 cm diameter bed. The Two‐Fluid Model (TFM) is employed to describe the solids motion efficiently while bubbles are detected and tracked using MS3DATA. Detailed statistics of the flow‐field in and around bubbles are computed and used to describe bubble‐induced solids micromixing: solids upflow driven in the nose and wake regions while downflow along the bubble walls. Further, within these regions, the hydrodynamics are dependent only on particle and bubble characteristics, and relatively independent of the global operating conditions. Based on this finding, a predictive mechanistic, analytical model is developed which integrates bubble‐induced micromixing contributions over their size and spatial distributions to describe the gross solids circulation within the fluidized bed. Finally, it is shown that solids mixing is affected adversely in the presence of gas bypass, or throughflow, particularly in the fluidization of heavier particles. This is because of inefficient gas solids contacting as 30–50% of the superficial gas flow escapes with 2–3× shorter residence time through the bed. This is one of the first large‐scale studies where both the gas (bubble) and solids motion, and their interaction, are investigated in detail and the developed framework is useful for predicting solids mixing in large‐scale reactors as well as for analyzing mixing dynamics in complex reactive particulate systems. © 2017 American Institute of Chemical Engineers AIChE J, 63: 4316–4328, 2017     

Drag coefficient of relatively contaminated gas bubbles

Based on the velocity profiles around clean and relatively contaminated gas bubbles, the tangential and normal stress, pressure distribution at the surface around the gas bubbles, and finally the viscous, form and total drag coefficient of the gas bubbles were calculated and compared with experimental values for dilute aqueous solutions of alcohols. The theory verifies the experimental observation that the total drag coefficient of single gas bubbbles increases with the contamination factor. The effect of the void fraction of the gas bubble swarm on the total drag coefficient was examined by the use of a void function. Most of the void functions for the relatively contaminated gas bubble swarms were between the values of potential flow and that of solid spheres.     

A unified theoretical model for breakup of bubbles and droplets in turbulent flows

Pressure has a significant effect on bubble breakup, and bubbles and droplets have very different breakup behaviors. This work aimed to propose a unified breakup model for both bubbles and droplets including the effect of pressure. A mechanism analysis was made on the internal flow through the bubble/droplet neck in the breakup process, and a mathematical model was obtained based on the Young–Laplace and Bernoulli equations. The internal flow behavior strongly depended on the pressure or gas density, and based on this mechanism, a unified breakup model was proposed for both bubbles and droplets. For the first time, this unified breakup model gave good predictions of both the effect of pressure or gas density on the bubble breakup rate and the different daughter size distributions of bubbles and droplets. The effect of the mother bubble/droplet diameter, turbulent energy dissipation rate and surface tension on the breakup rate, and daughter bubble/droplet size distribution was discussed. This bubble breakup model can be further used in a population balance model (PBM) to study the effect of pressure on the bubble size distribution and in a computational fluid dynamics‐population balance model (CFD‐PBM) coupled model to study the hydrodynamic behaviors of a bubble column at elevated pressures. © 2014 American Institute of Chemical Engineers AIChE J, 61: 1391–1403, 2015     

Water blown free rise polyurethane foams

A model is developed in this work for predicting the bubble size distribution in polyurethane foams generated by using water as a chemical blowing agent. The model combines equations of energy balance, kinetics of the reactions of isocyanate with water and polyol, and nucleation and growth of CO₂ bubbles. It is found that as the water content of the reaction mixture is increased, the mean bubble size decreases and the bubble size distribution become narrower. Exactly the opposite occurs in polyurethanes foamed with a physical blowing agent, e.g., DuPont Freons. This suggests that a combination of physical and chemical blowing agents can be employed to control bubble size distribution.     

Hydrodynamics of slug flow inside capillaries

Understanding the motion of long gas bubbles (gas slugs) inside capillaries is a challenging problem that is relevant in many processes of chemical and biological interests. It has been proved by many workers that such long gas bubbles can be used successfully in enhancing mass and heat transfer in many chemical and biological processes. In order to quantify and understand this enhancement a light was shed on the hydrodynamics of such a flow pattern. The volume of fluid method implemented in the commercial CFD package, Fluent, is used for this numerical study. Velocity and bubble profile were obtained as functions of capillary number. Computed values of the bubble velocity and diameter were in excellent agreement with published experimental measurements. The detailed velocity field around the bubble was also computed and compared favourably with those experimental results reported in literature.     

Neue Aspekte zum Einfluß des Blasenwakes auf die Hydrodynamik und den Stoffaustausch in einer Blasensäule

New aspects of the influence of the wake of bubbles on fluid dynamics and mass transfer in bubble columns. Wake-phenomena behind solid and fluid particles moving in a continuous liquid are an important fluid-dynamic effect which has a pronounced influence on heat- and mass transfer in two-phase dispersions. In this paper results reported in the literature are used to analyse the influence of wakes behind bubbles on the hydrodynamics of bubble columns. A new class of model is introduced to describe the flow and mixing of the liquid phase, taking into account major hydrodynamic effects.     

Population balance modelling of bubbling fluidized bed reactors—I. well-stirred dense phase

A detailed population balance model is presented for a fluidized bed reactor incorporating: the formation of bubbles at the grid plate, their rise with velocities governed by their sizes, random coalescence between bubbles, gas exchange between bubbles and the dense phase, and a first order chemical reaction in the dense phase under well-mixed conditions. Reaction conversion is calculated as a function of dimensionless parameters relating the rates of various competing processes such as coalescence, dense phase mixing, mass exchange between bubbles and dense phase and reaction rate. Comparison of conversions with those of Davidson et al. (1977) show significant variations indicating that the dynamics of bubble size distributions could have non-trivial effects on the extent of reaction. Fluctuations in bubble populations did not seem strong enough to translate to strong fluctuations in reaction conversion.     

CFD Simulations of a Bubble Column Operating in the Homogeneous and Heterogeneous Flow Regimes

Bubble columns are operated either in the homogeneous or heterogeneous flow regime. In the homogeneous flow regime, the bubbles are nearly uniform in size and shape. In the heterogeneous flow regime, a distribution of bubble sizes exists. In this paper, a CFD model is developed to describe the hydrodynamics of bubble columns operating in either of the two flow regimes. The heterogeneous flow regime is assumed to consist of two bubble classes: “small” and “large” bubbles. For the air‐water system, appropriate drag relations are suggested for these two bubble classes. Interactions between both bubble populations and the liquid are taken into account in terms of momentum exchange, or drag‐, coefficients, which differ for the “small” and “large” bubbles. Direct interactions between the large and small bubble phases are ignored. The turbulence in the liquid phase is described using the k‐ϵ model. For a 0.1 m diameter column operating with the air‐water system, CFD simulations have been carried out for superficial gas velocities, U, in the range 0.006–0.08 m/s, spanning both regimes. These simulations reveal some of the characteristic features of homogeneous and heterogeneous flow regimes, and of regime transition.     

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.     

Hydrodynamic characteristics of gas–solid fluidization at high temperature

Effect of temperature on the hydrodynamics of bubbling gas–solid fluidized beds was investigated in this work. Experiments were carried out at different temperatures ranged of 25–600°C and different superficial gas velocities in the range of 0.17–0.78 m/s with sand particles. The time‐position trajectory of particles was obtained by the radioactive particle tracking technique at elevated temperature. These data were used for determination of some hydrodynamic parameters (mean velocity of upward and downward‐moving particles, jump frequency, cycle frequency, and axial/radial diffusivities) which are representative to solids mixing through the bed. It was shown that solids mixing and diffusivity of particles increases by increasing temperature up to around 300°C. However, these parameters decrease by further increasing the temperature to higher than 300°C. This could be attributed to the properties of bubble and emulsion phases. Results of this study indicated that the bubbles grow up to a maximum diameter by increasing the temperature up to 300°C, after which the bubbles become smaller. The results showed that due to the wall effect, there is no significant change in the mean velocity of downward‐moving clusters. In order to explain these trends, surface tension of emulsion between the rising bubble and the emulsion phase was introduced and evaluated in the bubbling fluidized bed. The results showed that surface tension between bubble and emulsion is increased by increasing temperature up to 300°C, however, after that it acts in oppositely.     

Bubble characteristics of a scaled‐down three‐phase fluidized bed

Experiments were carried out in a 0.29 m diameter column to provide information on bubble characteristics relevant to an industrial three‐phase fluidized bed reactor. Using a specially built electrical conductivity probe, together with a series of computer programs to gather and filter the data, local bubble properties (rise velocities, pierced chord lengths, frequencies, and direction of travel) were measured. The data are compared to existing correlations. Since the operating conditions were within the dispersed bubble regime, uniform bubbling was anticipated. However, the actual system indicated considerable non‐uniformity, with individual bubbles, large swarms of bubbles and particles traveling at all angles. As a result, it is shown that bubble rise velocities are not necessarily a good measure of the bubble properties and must be vectorized to provide a good means of estimating gas hold‐up.