Effect of hydrodynamic heterogeneity on micromixing intensification in a Taylor–Couette flow reactor with variable configurations of inner cylinder

Effect of hydrodynamic heterogeneity on micromixing intensification in a Taylor–Couette flow (TC) reactor with variable configurations of inner cylinder has been investigated by adoption of a parallel competing iodide-iodate reaction system. Two types of inner cylinder, circular inner cylinder and lobed inner cylinder (CTC and LTC), were used to generate hydrodynamic heterogeneity, focusing on the effects of the Reynolds number, the acid concentration, and the feeding time on the micromixing performance. Segregation index (Xs) was employed to evaluate the micromixing efficiency. It is revealed that Xs decreases with the increase of Reynolds number and feeding time but increases with the increase of acid concentration for both the CTC and LTC. However, the LTC does present a better micromixing performance at various operating conditions than that of the CTC as affirmed by both the experimental and computational fluid dynamics simulation results.     

Facile synthesis of graphene oxide in a Couette–Taylor flow reactor

A practical approach to bulk-scale graphene-based materials is critically important for their use in the industrial applications. Here, we describe a facile method to prepare graphite oxide (GO) using a Couette–Taylor flow reactor for the oxidation of bulk graphite flakes. We found that the turbulent Couette–Taylor flow in the reactor could be engineered to result in the efficient mixing and mass transfer of graphite and oxidizing agents (KMnO₄ and H₂SO₄), thereby improving the efficiency of graphite into GO. As compared to the standard Hummers’ method, higher fraction of a single- and few-layer graphene oxide (G-O) can be yielded in a dramatically shortened reaction time, by optimizing the processing parameters, we have shown that ∼93% of G-O yield could be achieved within 60 min of reaction time. This method also allowed for the in-situ functionalization of G-O with metal oxide nanoparticles to give a nanoparticle-decorated G-O hybrid material. Our method for facile and large-scale production of graphite oxide may find utility in a range of applications including energy storage, composites and supporting frameworks of catalyst.     

Continuous emulsion polymerization of styrene in a single Couette–Taylor vortex flow reactor

The effect of the geometrical and operational parameters on the mixing characteristics of a Couette–Taylor vortex flow reactor (CTVFR) were investigated and were correlated with the same parameters by using the tank‐in‐series model. Continuous emulsion polymerization of styrene was conducted at 50°C in a CTVFR to clarify the effects on kinetic behavior and reactor performance of operational parameters such as rotational speed of inner cylinder (Taylor number), reactor mean residence time, and emulsifier and initiator concentrations in the feed streams. It was found that steady‐state monomer conversion and particle number could be freely varied only by varying the Taylor number. In order to explain the observed kinetic behavior of this polymerization system, a mathematical model was developed by combining the empirical correlation of the mixing characteristics of a CTVFR and a previously proposed kinetic model for the continuous emulsion polymerization of styrene in continuous stirred tank reactors connected in series (CSTRs). On the basis of these experimental results, it was concluded that a CTVFR is suitable for the first reactor (prereactor) of a continuous emulsion polymerization reactor system. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 80: 1931–1942, 2001     

The onset of instability in the flow induced by an impulsively started rotating cylinder

The onset of initial instability in a developing Couette flow following the impulsive starting of an inner rotating cylinder is analyzed using linear theory. It is well known that there is a critical Taylor number Ta_c at which Taylor vortices first appear between two concentric cylinders. For Ta>Ta_c Taylor-like vortices occur at a certain elapsed time. In the present study, the critical time t_c to represent the onset of this initiating instability, which then grows as toroidal vortices, is analyzed using propagation theory. For this purpose a self-similar transformation is forced through scaling analysis. The resulting stability criteria compare well with the available experimental data for vortices in water. The new measures represent the onset of the fastest growing instability and also suggest the detection time for the manifestation of secondary flow in the primary time-dependent Couette flow.     

Photocatalytic degradation of phenol in a rotating annular reactor

Batch photocatalytic degradation studies of phenol were conducted in an annular slurry reactor, to evaluate its performance under different operating and design conditions. The reactor had two concentric cylinders with the inner one rotating at specified revolutions per minute. The reactor also had provisions for aerating the slurry present in the annular gap. The inner cylinder housed the UV-lamps. The effects of catalyst loading (0–8 g/L), inner cylinder rotation speed (0–50 rpm), annular gap-width (7.5, 17.5 and 32.5 mm), initial pollutant concentration (10–50 mg/L) and mode of illumination (continuous or periodic) were studied. Light intensity received by the slurry was measured using Actinometry. Depending on the catalyst loading, annular gap-width and number of illuminated lamps the intensity values ranged from 0.58×10^?4 to 6.4×10^?4 Einsteins/L min. Under well mixed conditions, the reactor performance was found to increase with increase in catalyst loading. At low/medium annular gap width configurations, agitation induced by continuous aeration was found to provide sufficient mixing even when the inner cylinder was stationary. Rotation of the inner cylinder was required only in the high gap width configuration at high catalyst loadings. Scale-up of the reactor was investigated by increasing the gap-width of the annulus and hence increasing the quantity of feed processed. Controlled periodic illumination created by Taylor vortices did not show any improved performance over the regular continuous illumination. Modeling of reaction kinetics was investigated with different approaches and their efficacy in fitting the concentration–time trends of both the primary pollutant and the intermediates are discussed.     

Regimes of Multiple Emulsions of W1/O/W2 and O1/W/O2 Type in the Continuous Couette‐Taylor Flow Contactor

The flow regimes of multiple emulsions in the continuous Couette‐Taylor flow (CTF) contactor and characterization of the dispersion state are reported. The proposed method of multiple emulsion preparation is a one‐step procedure on the contrary to the classical two‐step procedure. The effect of operating parameters in the CTF contactor on multiple emulsion appearance, structure (drop size and packing), and rheological behavior is discussed. The key factors affecting multiple emulsion preparation in the CTF apparatus were the phases ratio, the rotational flow, and an annular gap width. The influence of an axial flow was more significant in the range of small rotational rates. The operating conditions were optimized to find the best characteristic multiple emulsions (largest interfacial area). The paper presents the same exemplary data of using W₁/O/W₂ emulsions as emulsion liquid membranes (ELMs) in the extraction process and O₁/W/O₂ for control active agent (drug) release.     

Influence of Pressure on the Hydrodynamics and Mass Transfer Parameters of an Agitated Bubble Reactor

The present study deals with the pressure effects on the hydrodynamic flow and mass transfer within an agitated bubble reactor operated at pressures between 10⁵ and 100 × 10⁵ Pa. In order to clarify the flow behavior within the reactor, liquid phase residence time distributions (RTD) for different operating pressures and gas velocities ranging between 0.005 m/s and 0.03 m/s are determined experimentally by the tracer method for which a KCl solution is used as a tracer. The result of the analysis of the liquid‐phase RTD curves justifies the tank‐in‐series model flow for the operating pressure range. Good agreement is obtained between theoretical and experimental results assuming the reactor is operating as perfectly mixed. Two parameters characterizing the mass transfer are identified and investigated in respect to pressure: the gas‐liquid interfacial area and volumetric liquid‐side mass transfer coefficient. The chemical absorption method is used. For a given gas mass flow rate, the interfacial area as well as the volumetric liquid mass transfer coefficient decrease with increasing operating pressure. However, for a given pressure, a and k_La increase with increasing gas mass flow rates. The mass transfer coefficient k_L is independent of pressure.     

CFD simulation of Taylor–Couette flow in scraped surface heat exchanger

The flow between two concentric cylinders which is termed as Taylor–Couette flow has been studied in scraped surface heat exchanger with and without blades. Shear rate in annular flow with and without blades was measured by Dumont et al. (2000a) using electrochemical method and determined the onset of Taylor vortices at specific Taylor number in both cases for Newtonian flow. CFD simulations have been carried out to determine the transition zone from laminar Couette flow to Taylor vortex flow using the same geometry for which Dumont et al. (2000a) had carried out the experiments. The Reynolds stress model (RSM) and k–? model are used for Taylor vortex flow (Ta > 300) to characterize the flow pattern in annular flow and SSHE respectively. The aim of the present work is to analyze the effect of rotating scraper on the existing flow patterns in simple annular flow using CFD simulations.     

Experimental and numerical investigation on mixing and axial dispersion in Taylor–Couette flow patterns

Taylor–Couette flows between two concentric cylinders have great potential applications in chemical engineering. They are particularly convenient for two-phase small scale devices enabling solvent extraction operations. An experimental device was designed with this idea in mind. It consists of two concentric cylinders with the inner one rotating and the outer one fixed. Moreover, a pressure driven axial flow can be superimposed. Taylor–Couette flow is known to evolve towards turbulence through a sequence of successive hydrodynamic instabilities. Mixing characterized by an axial dispersion coefficient is extremely sensitive to these flow bifurcations, which may lead to flawed modelling of the coupling between flow and mass transfer. This particular point has been studied using experimental and numerical approaches. Direct numerical simulations (DNS) of the flow have been carried out. The effective diffusion coefficient was estimated using particles tracking in the different Taylor–Couette regimes. Simulation results have been compared with literature data and also with our own experimental results. The experimental study first consists in visualizing the vortices with a small amount of particles (Kalliroscope) added to the fluid. Tracer residence time distribution (RTD) is used to determine dispersion coefficients. Both numerical and experimental results show a significant effect of the flow structure on the axial dispersion.     

Residence time distribution analysis from a continuous couette flow device around critical taylor number

This paper presents an experimental study of residence time distribution (RTD) analysis by pulse response technique in a continuous Couette flow device with rotating inner cylinder and stationary outer cylinder. Two kinds of experimental tests using pulses of tracer dye solution and particles resulting from a fast precipitation were performed in the region near the critical Taylor number characterizing boundary between laminar and laminar vortex flow. For most experiments performed in laminar and laminar vortex flow regime around the critical Taylor number over the ranges 0 < T_a < 120 and 0 < R_e < 5.5 the normalized response can be described by a dispersion model. The results of the critical Taylor number as characterized by the minimum dispersion number appear consistent with both theoretical predictions and other empirical observations.     

Reappearance of azimuthal waves in turbulent Taylor-Couette flow at large aspect ratio

Velocity field data were acquired for Taylor-Couette flow in the annular gap between an inner rotating cylinder and a stationary concentric outer cylinder using particle image velocimetry (PIV) in a meridional plane of the annulus. Data were acquired for several rotational Reynolds numbers with the ratio of the rotational and critical Reynolds numbers ranging from 6 to 200, corresponding to flow states ranging from laminar wavy Taylor vortex flow to turbulent Taylor vortex flow. Spatial correlations of velocity fluctuations were found to exhibit a sharp decrease as R, the ratio of Reynolds number to the critical Reynolds number, increases from 16, indicating the disappearance of azimuthal waves and the onset of turbulence, reaching a minimum at R=18. However, correlation lengths subsequently increase with increasing R, displaying a secondary peak from 20?R?38, suggesting the reappearance of azimuthal waves. The reemergence of azimuthal waves was confirmed through other methods including analysis of the axial velocity. At still higher Reynolds numbers, correlation lengths decay once again. The magnitude and Reynolds number associated with the secondary peak in the fluctuation velocity correlations were found to be dependent on the location of the basis point used in the calculations. Specifically, correlation lengths were longest near the outer cylinder in the inflow boundary and near the inner cylinder in the outflow boundary. This was shown to be due to the spatial dependence of Reynolds stresses in turbulent Taylor-Couette flow.     

Mass transfer modeling and simulation at a rotating cylinder electrode (RCE) reactor under turbulent flow for copper recovery

This work presents the numerical simulation of a laboratory reactor with rotating cylinder electrode (RCE) and a six-plate counter electrode that is used in studies on heavy metal recovery. The rate of electrode rotation and the potential applied are of such magnitude that the electrochemical reactor works in conditions of mass transport control under turbulent flow to obtain high recovery rates and formation of dendritic metal deposits. For hydrodynamics, the Reynolds averaged Navier–Stokes (RANS) equations were solved using the standard k–ε turbulence model, as well as wall functions based on the universal velocity distribution in the near-wall region. Results of 3-D simulations of the velocity field show clearly the formation of the turbulence Taylor vortex flow. For mass transfer, convection–diffusion equation was solved using the Kays–Crawford model for turbulent Schmidt number and Launder–Spalding wall functions adapted for mass transfer. Kinetics of copper recovery from aqueous solutions containing 0.019 M CuSO₄ and 1 M H₂SO₄, in the range of rotation speed of 400–1100 rpm, was adequately fit (error <8%) during the electrolysis time to achieve a final recovery of 85% for potentiostatic and 60% for galvanostatic experiments. The fitting parameter of the concentration wall function used in all experiments was A=2.9.     

Numerical Simulation of Sterilization Processes for Shear‐Thinning Food in Taylor‐Couette Flow Systems

The performance of the Taylor‐Couette flow apparatus as a heat sterilizer is numerically investigated. The destruction of Clostridium botulinum and thiamine (vitamin B1) was selected as model reaction. When Taylor vortices were formed in the annular space, the heat transfer significantly enhanced as compared to the case without vortex flow. As a result, the equivalent lethality calculated from the temperature field increased, which is regarded as a quantum leap. Conversely, the improvement of heat transfer induced destruction of thiamine. These results suggest that there is a trade‐off relationship between the enhancement of heat transfer and the avoidance of thermal destruction of nutritional components. In conclusion, the Taylor‐Couette flow sterilizer has the potential for process intensification in heat sterilization processes.     

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.     

Macro‐ and Micromixing in a Taylor‐Couette Reactor with Axial Flow and their Influence on the Precipitation of Barium Sulfate

A new set‐up for precipitation experiments capable of independent adjustment of micromixing and macromixing conditions is presented. The setup consists of a Taylor‐Couette (TC) reactor serving as the reaction zone and an external loop where the slower stages of precipitation processes take place. Micromixing in the TC reactor has been investigated with a chemical reaction system and with PIV‐measurements. Micromixing times range between 6·10^–3 and 8·10^–2 s. Tracer experiments reveal the macromixing performance of the whole set‐up which has been compared with the behavior of ideal reactors. Precipitation experiments with barium sulfate show some influence of micromixing intensity on the particle size and of macromixing on particle morphology.     

Heat and mass transfer in non-newtonian fluid flow with power function velocity profiles

Heat and mass transfer in laminar and turbulent non-Newtonian fluids is investigated in this work using the power function velocity profiles. Analytical solutions are presented for cases of mass transfer in laminar non-Newtonian fluid flows, namely for a flat velocity profile (plug flow), for the case of a constant velocity gradient at the solid boundary (Couette flow), and for the velocity distribution within a laminar boundary layer on a flat plate, and these are illustrated by rotating disks and cylinders in laminar Ostwald-de Waele fluids. Further, turbulent mass transfer processes (tubular flow, rotating disk, and rotating cylinder) in non-Newtonian fluids (Ostwald-de Waele fluid and drag-reducing fluid) at low and large Schmidt numbers are also discussed using the solutions of mass transfer in flows with power function velocity profiles. Reasonable agreement is found between the predictions of this work and the available experimental data and correlations.     

Continuous polymerization of methyl methacrylate in a taylor‐couette reactor. I. Influence of fluid dynamics on monomer conversion

A Taylor‐Couette reactor offers certain advantages for continuous polymerization over other reactor types. These advantages are its rather narrow residence time distribution (series of vortices) and its good heat transfer characteristics. Hydrodynamics in this type of reactor can be controlled by its geometry (diameter, gap, width, length) and its operational parameters (rotational speed, mean residence time, viscosity). In this article, a model is presented which is suited to answer the question of how hydrodynamics influences the productivity of a continuously operated Taylor‐Couette polymerization reactor. To this end, productivity is quantified by the rate of monomer conversion. The model is specified and experimentally validated for the free radical polymerization of methyl methacrylate with the solvent xylene and 2,2‐azoisobutyronitrile as the initiator. The model considers the following four phenomena: (i) Macromixing between the vortex cells is accounted for by an axial dispersion model. (ii) The dependence of viscosity on monomer conversion along the reactor is described by a viscosity model. (iii) Polymerization kinetics and its dependence on hydrodynamics are correlated from experimental data. (iv) The dependence of segregation index I_s on the local energy dissipation is used to characterize micromixing within the vortices. POLYM. ENG. SCI., 2013. © 2012 Society of Plastics Engineers     

Numerical flow simulation of viscoplastic fluids in annuli

This paper describes numerical solutions for the laminar flow of non-Newtonian fluids in vertical annuli using the Herschel-Bulkley model to describe the rheological behaviour of such materials. Numerical solutions have been obtained when there is both axial and tangential flows in either a concentric or eccentric annulus. The tangential flow arises from the rotation of the inner cylinder of the annulus and the axial flow from a constant axial pressure gradient. The flow is analysed by solving the momentum and continuity equation numerically using the finite element method. The dimensionless velocity, deformation and stress profiles with other quantities such as the apparent viscosity and pressure distribution have been calculated for various eccentricities, radius ratios, fluid properties and flow parameters; the results give insights into the flow behaviour in the annuli. It is shown that the inclusion of rotational effects, for a fixed pressure gradient, is likely to increase the axial volumetric flowrate over non-rotating situations in concentric geometries. New results reveal that, in eccentric annuli, the situation is reversed and the flowrate gradually decreases as the rotation rate is increased.     

A review on process intensification in HiGee distillation

This review paper describes the state‐of‐the‐art in the field of HiGee contactors used for gas–liquid mass transfer processes, with a special focus on distillation, and for heterogeneously catalyzed reactions. Several types of rotating beds are discussed, including single‐block rotating packed‐bed, split‐packing rotating bed, rotating zigzag bed, two‐stage counter‐current rotating packed bed, blade packing rotating packed bed, rotating bed with blade packing and baffles, counter‐flow concentric‐ring rotating bed and crossflow concentric‐baffle rotating bed. The working principles of HiGee technology, as well as the modeling, design and control aspects, and practical applications are explained and discussed. In addition, this paper addresses the advantages and disadvantages with respect to mass‐transfer performance, pressure drop, rotor complexity and suitability to perform continuous distillation and to be filled with catalyst packing for heterogeneous reactions. © 2017 Society of Chemical Industry     

Coagulation of bentonite suspension by polyelectrolytes or ferric chloride: Floc breakage and reformation

Coagulation process usually involves different hydrodynamic conditions, in particular when it is followed by a filtration step. In this study, coagulation performance was investigated under a wide range of shear stress. Floc behaviour was followed in-line by laser granulometry to determine size distribution and structure. Synthetic suspension of bentonite in tap water was used as a reference for mineral solids in surface water. Three cationic polymers (polyamine based and polyDADMAC) and ferric chloride were tested using different coagulation reactor geometries. Jar-test indicated coagulation performance under mild hydrodynamic conditions and Taylor–Couette reactors were used to create shear stresses up to 8 Pa. Flocs formed with ferric chloride are not able to grow under middle shear stress like 1.5 Pa. On the contrary, polyelectrolytes lead to large flocs, dense (D_f = 2.6) and resistant to shear stress. A qualitative comparison of floc resistance to shear depending on hydrodynamic conditions and coagulant type is given through the calculation of the strength factor. Fractal dimension measurements indicate a mechanism of particle erosion when flocs are subjected to a higher shear stress in Taylor–Couette reactor. Floc re-growth is also investigated, and breakage appears to be non-reversible regardless of coagulant and conditions experimented.