Continuous‐Flow Di‐N‐Alkylation of 1H‐Benzimidazole in a Fixed‐Bed Reactor

A new process for a continuous‐flow di‐N‐alkylation of 1H‐benzimidazole to 1H‐benzimidazole‐3‐ium iodide by methylene iodide in the presence of potassium carbonate in a fixed‐bed reactor is presented. The synthesis was transferred from batch to continuous operation with similar yields and conversion rates. Moreover, the influence of temperature and residence time in the continuous flow setup was characterized; optimized conditions led to a doubling of yield. In addition, the continuous flow allowed for a better control of the two‐step reaction by adding an additional tube reactor after the fixed bed that further enhanced the overall performance. With this, the continuous‐flow system presented itself as superior due to higher available temperatures and a better controllability.     

Modeling of Flow Burst,Flow Timing in Lab‐on‐a‐CD Systems and Its Application in Digital Chemical Analysis

A theoretical model is developed to predict the time‐dependent behaviors of “flow burst” and centrifugal capillary flow in Lab‐on‐a‐CD systems. Enabling the calculation of flow field, flow burst, and flow timing as a function of fluid properties and system specifications, the model is a useful design tool for understanding and controlling flow behaviors in Lab‐on‐a‐CD devices. More importantly, the model shows great promise in the development of a new method for digital chemical analysis.     

Continuous‐Flow Surface Aeration Systems

An aeration process in an activated sludge plant is a continuous‐flow system. In this system, there is a steady input flow (flow from the primary clarifier or settling tank with some part from the secondary clarifier or secondary settling tank) and output flow connection to the secondary clarifier or settling tank. The experimental and numerical results obtained through batch systems can not be relied on and applied for the designing of a continuous aeration tank. In order to scale up laboratory results for field application, it is imperative to know the geometric parameters of a continuous system. Geometric parameters have a greater influence on the mass transfer process of surface aeration systems. The present work establishes the optimal geometric configuration of a continuous‐flow surface aeration system. It is found that the maintenance of these optimal geometric parameters systems result in maximum aeration efficiency. By maintaining the obtained optimal geometric parameters, further experiments are conducted in continuous‐flow surface aerators with three different sizes in order to develop design curves correlating the oxygen transfer coefficient and power number with the rotor speed. The design methodology to implement the presently developed optimal geometric parameters and correlation equations for field application is discussed.     

Direct Numerical Simulations of Three‐Layer Viscosity‐Stratified Flow

Two‐dimensional simulations of flow instability at the interface of a three‐layer, density‐matched, viscosity‐stratified Poiseuille flow are performed using a front‐tracking/finite difference method. This is an extension of the study for the stability of two‐layer viscosity‐stratified flow of Cao et al., Int. J. Multiphase Flow, 30 , 1485‐1508 (2004). We present results for large‐amplitude non‐linear evolution of the interface for varying viscosity ratio m, Weber number We, and phase difference between the perturbations of the two interfaces. Strong non‐linear behaviour is observed for relatively large initial perturbation amplitude. The higher viscosity fluid is drawn out as a finger that penetrates into the lower viscosity layer. The finger originates at the crest of the perturbation at the interface. The simulated interface shape compares well with previously reported experiments. Increasing interfacial tension retards the growth rate of the interface as expected, whereas increasing the viscosity ratio enhances it. The sinuous instability appears to evolve faster than the varicose one. For certain flow parameters the high‐viscosity finger displays a bulbous tip, which is also seen in our previously conducted experiments and two‐layer results, although it is less pronounced. The low‐viscosity intruding finger does not display this curious bulbous tip. Drop formation is precluded by the two‐dimensional nature of the calculations.     

Two‐Phase Bubbly Flow Structure in Large‐Diameter Vertical Pipes

The two‐phase flow structure of an air‐water, bubbly, upward flow in a 20 cm diameter pipe is presented with particular emphasis on the local interfacial area concentration. The radial distribution of void fraction, bubble velocity, bubble size, bubble frequency, and interfacial area concentration were measured using a local dual‐optical probe. The experimental results showed that the saddle‐type distribution of void fraction and interfacial area concentration, which are common for bubbly flow in small diameter pipes, only appeared in the present experiments under conditions of very low area‐averaged void fraction (<?> < 0.04). The values for the interfacial area concentration were higher in large diameter pipes when compared with data obtained under the same flow conditions in small pipes. The area‐averaged void fraction data were correlated using the drift‐flux model.     

Prediction of Layer‐Inversion Behavior of Down‐Flow Binary Solid‐Liquid Fluidized Beds

The layer‐inversion behavior of down‐flow binary solid‐liquid fluidized beds is predicted using the property‐averaging approach. The binary pair in this case consists of a larger solid species which is also heavier than its smaller counterpart, while both are lighter than the fluidizing medium. The model is based on using the generalized Richardson‐Zaki correlation for evaluation of the bed void fraction wherein mean values of particle properties are used. However, unlike the maximum bulk density condition for the conventional up‐flow binary solid fluidized bed, the model is based on a minimum bulk density condition for occurrence of layer inversion. This is due to the fact that the volume contraction phenomenon associated with the mixing of unequal solid species leads to a decrease in bulk density of the bed. Model predictions are also compared using the limited data available in the literature. Predictions are consistent with the observed mixing behavior.     

Homogeneous Tubular‐Flow Process for Monoolein Preparation

Esterification of fatty acids with glycerol is characterized by negligible solubility of the two liquid phases. The reactions to mono‐, di‐ and triglycerides taking place in the fatty acid phase, are limited by chemical equilibrium. The scope of this study is to investigate in a tubular reactor the conversion of a homogeneous mixture of oleic acid and glycerol in tert‐butanol. The liquid composition in this study was 1 mol of oleic acid, 6 mol of glycerol and 14 mol of tert‐butanol. Experiments were conducted in a tubular reactor at 35 atm over a temperature range of 200–240 °C and residence times of 0.7–17.6 h to determine the kinetics and the chemical equilibrium. The selectivity to monoolein was >95 mol %. A reversible second order reaction fits the data well.     

Short‐Circuit Flow in Hydrocyclones with Arc‐Shaped Vortex Finders

Several new arc‐shaped vortex finder structures were developed to restrain the circulation flow and reduce short‐circuit flow. These problems were investigated by conducting laboratory experiments and using a computational fluid dynamics numerical simulation method. To verify the accuracy of the numerical simulation, a type A vortex finder was employed to perform a particle image velocimetry test under identical working conditions. Then, these new structures were compared with a type O vortex finder to reveal the differences in pressure field, velocity field, and separation performance. The arc‐shaped vortex finder was able to restrain the circulation flow and reduce the maximum pressure drop at the outer wall of the vortex finder.     

Simulation and Experimental Validation of Two‐Phase Flow in an Aerosol Counter‐Flow Reactor using Computational Fluid Dynamics

A simulation of the hydrodynamic behavior of an aerosol‐counter flow reactor was conducted using an Euler‐Lagrange method. The simulation results were then verified with experiments. The process simulated was a separation process required during the production of biodiesel (fatty acid methyl ester). In this process, the liquid ester/glycerol phases are continuously injected through a hollow cone nozzle with an overpressure of 10⁶ Pa into the reactor, operated at 15000 Pa. The liquid is atomized because of the pressure drop and a liquid particle spray is generated with an inlet velocity of 44.72 m/s. Water vapor of temperature 333 K is injected tangentially through two side, gas inlets with an inlet velocity of 1.2 m/s. Excess methanol is subjected to a mass transfer from the liquid phase into the gas phase, which is withdrawn through the head of the reactor and condensed in an external condenser unit. The stripping of the methanol off the liquid leads to a sharp interface between the glycerol and the ester phase, which can then be easily separated by gravity or pumping. The gas velocity field, pressure field and the liquid particle trajectories were calculated successfully. Simulated dwell time distribution curves were derived and analyzed with the open‐open vessel dispersion model. Experimental dwell time distribution curves were measured, analyzed with the open‐open vessel dispersion model, and compared with the simulated curves. A good consistency between simulated and measured Bodenstein numbers was achieved, but 25 % of the simulated particles exited at the reactor's head, contrary to experimental observations. The difference between simulated and measured dwell times was within one order of magnitude.     

Microwave‐Assisted Continuous‐Flow Reactor Based on a Ridged Waveguide

Microwave‐assisted continuous‐flow reactors (MCFRs) are a valuable alternative to conventional reactors for accelerating chemical reactions. However, despite several interesting applications, only little quantitative research has been conducted on the temperature uniformity of the heating load. With water as a common inorganic solvent, a novel MCFR type based on a special ridged waveguide for heating water is studied by optimizing some parameters of micropipes in order to achieve better temperature uniformity. Compared to the original reactor, the standard deviation of the electric field decreased significantly when using the optimized reactor under the same heating conditions while the average electric field density increased. The optimized results were verified by experiments.     

Generation of Two‐Phase Air‐Water Flow with Fine Microbubbles

Aiming at the development of a low‐cost technology for multipurpose water and surface treatment in the chemical industry and beyond, using microbubbles, a novel scheme of liquid‐gas interaction within a specially designed bubble generator was tested. Its efficiency for the production of microbubbles with a size distribution in the micron range is confirmed. The basic element of the device is a vortex chamber with water supply through tangential ducts, while the gas (air) is introduced in a highly turbulent swirling flow of water in radial direction through the orifice in the gas supply duct, located on the chamber axis. Bubble diameters, bubble velocities in the pipe flow and effect of the output pressure on the bubble size distribution were studied.     

Improved Void Fraction Correlation for Two‐Phase Flow in Large‐Diameter Annuli

A flow pattern‐independent void fraction correlation for gas‐liquid two‐phase flow in vertical large‐diameter annuli is established. Two equations are proposed for the parameters of a drift flux model‐based correlation: the distribution parameter and the drift flux velocity. These equations are expressed as a function of two‐phase flow variables including void fraction, fluid properties, pipe geometry, and phase flow rates. Experiments were performed to study the void fraction of vertical air‐water two‐phase flow in large‐diameter annuli. The obtained experimental data along with the literature data of Caetano are used to verify the performance of the proposed void fraction correlation. The accuracy of this correlation is compared with nineteen frequently used correlations in literature. The proposed correlation was found to predict the void fraction consistently with a better accuracy.     

Supported Catalysis in Continuous‐Flow Microreactors

This review surveys the recent developments to perform heterogeneous catalysis in continuous‐flow microreactors. Three different types, namely, (i) packed‐bed, (ii) monolithic, and (iii) wall‐coated approaches are discussed to implement various kinds of catalysts in a microreactor. In addition, the applications of these supported catalysts to perform a variety of organic reactions are described. Furthermore, advantages of catalytic microreactors over classical batch reactors on one or more aspects of the reaction, such as rate, conversion, selectivity, and enantioselectivity are presented.

     

Distribution of Two‐Phase Flow across 2.7‐mm Vertically Split U‐Type Junctions

A visual observation of the two‐phase flow across vertically split U‐type junctions and its flow redistribution inside two 2.7‐mm diameter smooth tubes with curvature ratios (2R/D) of 3 and 7, respectively, are reported. The range of mass flux is between 100 and 700 kg/m²s and quality (x) ranges from 0.001 to 0.5. The ratio of liquid distribution between the upper and lower outlet legs is related to the inlet flow pattern, but its influence is reduced at higher mass flux. The difference in liquid flow rates in the lower and upper legs is significantly affected by gravity at a small inlet mass flux, but this difference becomes less profound when the inlet mass flux is increased. The difference between the liquid flux in the upper and lower leg is reduced for the smaller curvature radius due to the reduced effect of gravity between the upper and lower legs. However, there is no consistent trend of gas flow distribution across the U‐type junction as compared to liquid flow distribution. The air mass flux in the upper and lower legs always increase with an increase in both gas quality and the total mass flux.     

Full‐Loop Simulation of Gas‐Solids Flow in a Pilot‐Scale Circulating Fluidized Bed

In order to study the system hydrodynamics in a circulating fluidized bed (CFB), a 3D full‐loop simulation was conducted for a pilot‐scale CFB. The Eulerian‐Eulerian two‐fluid model with the kinetic theory of granular theory helped to simulate the gas‐solids flow in the CFB. The system hydrodynamics including pressure balance, vectors of gas and solids, distribution of solids holdup, and instantaneous circulating rates were obtained to get a comprehensive understanding of the system. It was predicted that the main driving force was the pressure drop of the storage tank. The storage height and valve opening were critical operating factors to control the riser operation. The effects of operating conditions including solids circulating rates and superficial gas velocity on the hydrodynamics were investigated to provide guidance for the stable operation of the CFB system.     

Stagnation‐point Flow towards a Stretching Surface

An exact similarity solution of the Navier‐Stokes equations is obtained. The solution represents steady axisymmetric stagnation‐point flow towards a stretching surface. It is shown that the flow displays a boundary‐layer structure when the stretching velocity of the surface is less than the free stream velocity. On the other hand, an inverted boundary layer is formed when the surface stretching velocity exceeds the free stream velocity. Temperature distribution in the flow is found when the surface is held at a constant temperature. It turns out that when the surface temperature exceeds the ambient temperature, heat flows from the surface to the fluid near the stagnation point but further away from the stagnation point, heat flows from the fluid to the stretching surface.     

Analysis of Flow Patterns in High‐Gravity Equipment Using Gamma‐Ray Computed Tomography

The capacity of today's gas‐liquid contacting equipment such as tray or packed columns is limited by the gravitational‐driven liquid flow. Intensified equipment applying centrifugal force offers great potential for enhancing the mass transfer and for reducing equipment size. Yet, detailed knowledge about the liquid flow inside rotating packings is scarce due to limited accessibility with conventional measurement systems. In this study, a gamma‐ray computed tomography is employed to quantify the liquid hold‐up and its distribution in the moving packing.     

Liquid‐Liquid Stratified Flow through Horizontal Conduits

The stratified configuration is one of the basic and most important distributions during two phase flow through horizontal pipes. A number of studies have been carried out to understand gas‐liquid stratified flows. However, not much is known regarding the simultaneous flow of two immiscible liquids. There is no guarantee that the information available for gas‐liquid cases can be extended to liquid‐liquid flows. Therefore, the present work attempts a detailed investigation of liquid‐liquid stratified flow through horizontal conduits. Gas‐liquid flow exhibits either smooth or wavy stratified orientations, while liquid‐liquid flow exhibits other distinct stratified patterns like three layer flow, oil dispersed in water, and water flow, etc. Due to this, regime maps and transition equations available for predicting the regimes in gas‐liquid flow cannot be extended for liquid‐liquid cases by merely substituting phase physical properties in the equations. Further efforts have been made to estimate the in‐situ liquid holdup from experiments and theory. The analysis considers the pronounced effect of surface tension, and attempts to modify the Taitel‐Dukler model to account for the curved interface observed in these cases. The curved interface model of Brauner has been validated with experimental data from the present work and those reported in literature. It gives a better prediction of liquid holdup in oil‐water flows and reduces to the Taitel‐Dukler model for air‐water systems.     

Three‐Dimensional CFD Simulation of Stratified Two‐Fluid Taylor‐Couette Flow

Two‐fluid Taylor‐Couette flow, with either one or both of the co‐axial cylinders rotating, has potential advantages over the conventional process equipment in chemical and bio‐process industries. This flow has been investigated using three‐dimensional CFD simulations. The occurrence of radial stratification, the subsequent onset of centrifugal instability in each phase, the cell formation and the dependency on various parameters have been analyzed and discussed. The criteria for the stratification, Taylor cell formation in each phase have been established. It can be stated that the analysis of single‐phase flow acts as the base state for the understanding of radial stratification of the two‐fluid flows. The extent of interface deformation also has been discussed. A complete energy balance has been established and a very good agreement was found between dissipation rate by CFD predictions and the energy input rate through the cylinder/s rotation.     

Characterization of Air‐Water Two‐Phase Vertical Flow by Using Electrical Resistance Imaging

The characterization of air‐water two‐phase vertical flow in a 12 m flow loop with 1.5 m of vertical section is studied by using electrical resistance tomography (ERT). By applying a fast data collection to a dual‐plane ERT sensor and an iterative image reconstruction algorithm, relevant information is gathered for implementation of flow characteristics, particularly for flow regime recognition. A cross‐correlation method is also used to interpret the velocity distribution of the gas phase on the cross section. The paper demonstrates that ERT can now be deployed routinely for velocity measurements and this capability will increase as faster measurement systems evolve.