Process intensification of continuous starch hydrolysis with a Couette–Taylor flow reactor

Starch gelatinization and enzymatic hydrolysis was carried out in a continuous Couette–Taylor flow reactor with a water jacket. The degree of gelatinization and the concentration of reducing sugars produced via enzymatic saccharification were evaluated by varying operational variables: rotation speed of an inner cylinder, initial concentration of starch and reaction temperature. At the initial concentration of the starch suspension, 50 kg m⁻³, starch saccharification proceeded sufficiently even at low rotation speed of the inner cylinder and saccharification temperature. At the higher initial concentration, 100 and 150 kg m⁻³, a higher rotation speed of the inner cylinder and temperature of the saccharification section were required to obtain sufficient starch saccharification. Even in the case of C₀ = 100 and 150 kg m⁻³, the more reducing sugar was obtained by choosing an adequate rotation speed of the inner cylinder and a reaction temperature.     

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.     

An integrated system of a microreactor with a Taylor–Couette reactor for 2,2′-dibenzothiazole disulfide synthesis

An integrated microreaction system of a microreactor with a Taylor–Couette reactor (TCR) for continuous synthesis of 2,2′-dibenzothiazole disulfide has been developed, so as to improve the process efficiency and the stability with solid product generation compared with the current batch process. The homogeneous oxidation with hydrogen peroxide catalyzed by phosphotungstic acid was applied. In the microreaction system, two feedstocks can be rapidly mixed through the microreactor; stable particle flow ability and high conversion can be achieved through the TCR, due to the flow characteristics of high shear stress and limited back-mixing. Under the conditions of stable operation, the conversion can reach over 90%. The purity of the product is over 99%. The space–time yield can reach 160 g L⁻¹ h⁻¹, which is much higher than that in the batch reactor. The microreaction system is stable for long-term running, which provides an effective design strategy for continuous flow processes with solid generation.     

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.     

Hydrodynamics and droplet size distribution of liquid–liquid flow in a packed bed reactor with orifice plates

A packed bed reactor with orifice plates (PBR@OP) was designed by adding orifice plates periodically in packed beds. Hydrodynamics and droplet size distribution in PBR@OP were experimentally investigated using fatty acid methyl esters (FAME)/water as the model liquid–liquid system. In PBR@OP, the flow pattern was close to plug flow. Droplets with Sauter mean diameter (d₃₂) of 150–550 μm were generated. The pressure drop of orifice, flow velocity and plate spacing were key parameters to control the droplet size. The reactor performance was evaluated by analyzing a FAME epoxidation process. At the same d₃₂ and residence time, the length and total pressure drop of PBR@OP were about 1/3 and 1/4 of those of PBR without orifice plates, respectively. Furthermore, a semi-empirical correlation describing the d₃₂ change in PBR@OP was developed, revealing a relative mean deviation of 8.64%. PBR@OP presents a cost-effective option for the intensification of liquid–liquid medium rate reactions.     

Nature and characteristics of gas–liquid flow regimes in a micro-packed bed reactor

Micro-packed bed reactors (μPBRs) have the advantages of high heat and mass transfer efficiency and excellent safety, and they have been successfully applied to hydrogenation and oxidation reactions. However, the study of gas–liquid flow regimes in the μPBR, which is essential for the mass transfer modeling and reactor scale-up, is still insufficient due to the limitation of micro-scale and complexity of capillary force. In this work, the flow regimes in the two-dimensional μPBR were systematically studied by visual method utilizing a high-performance camera. Four typical flow regimes and characteristics were captured, and flow regime transition was revealed. Effects of gas and liquid superficial velocities, liquid physical properties, and particle sizes on liquid spreading areal fraction and pressure drop were investigated. Flow regime transition correlation of churn flow and pseudo-static flow in the μPBR was provided for the first time based on the summary of the current and previous published results.     

Hydrodynamics and mass transfer in a Couette–Taylor bioreactor for the culture of animal cells

A Couette–Taylor bioreactor device, commercialized by Synthecon for the culture of tissues, can exhibit interesting hydrodynamics and mass-transfer characteristics that could be well suited for the culture of animal cells in suspension. This is investigated through hydrodynamics and mass-transfer study on a plexiglass copy of the existing commercial equipment. Results of these studies show the potential of the Synthecon bioreactor for the culture of animal cells in suspension. The Synthecon bioreactor is then bought and actually used for the culture of CHO cells in suspension. First results confirm the approach developed on the plexiglass copy.     

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.     

Viscous flow on the outside of a horizontal rotating cylinder—II. Dip coating with a non-newtonian fluid

The flow of a viscous fluid on the outside of a partially submerged, rotating horizontal cylinder is one of the basic fundamental coating flows. The main features of this flow for a Newtonian fluid were described by Campanella and Cerro [Chem. Engng Sci.39, 1443 (1984)]. A rapid flow approximation [Cerro and Scriven Ind. Engng Chem. Fundam.19, 40 (1980)] is used to develop a film profile equation for a power-law type non-Newtonian fluid. The film profile equation allows to compute a unique critical flow regime for each kind of power law fluid characterized by two rheological constants, K and n. Theoretical results are shown to be in very good agreement with experimental results for a wide range of fluid properties.     

Effect of co-feeding carbon dioxide on Fischer–Tropsch synthesis over an iron–manganese catalyst in a spinning basket reactor

The effect of co-feeding CO₂ on the catalytic properties of an Fe–Mn catalyst during Fischer–Tropsch synthesis (FTS) was investigated in a spinning basket reactor by varying added CO₂ partial pressure in the feed gas. It was found that co-feeding CO₂ to syngas did not decrease the activity of the catalyst, on the contrary, a dramatic increase of the activity and an increase of methane selectivity were observed over the catalyst after removal of CO₂ from the feed gas. The addition of CO₂ led to an increase in olefin/paraffin ratios of low carbon hydrocarbons and a slight decrease in C₁₉⁺ selectivity. It also slightly decreased CO₂ formation rate on the catalyst by increasing the rate of reverse step of the water–gas shift (WGS) reaction and pushing the reaction towards equilibrium, and did not remarkably influence the hydrocarbon formation rate. However, the co-feeding CO₂ can significantly increase the water formation rate and the overall oxygenate formation rate under these reaction conditions.     

Effect of oil viscosity on the flow structure and pressure gradient in horizontal oil–water flow

The flow patterns and pressure gradient of immiscible liquids are still subject of immense research interest. This is partly because fluids with different properties exhibit different flow behaviours in different pipe's configurations under different operating conditions. In this study, a combination of oil–water properties (σ = 20.1 mN/m) not previously reported was used in a 25.4 mm acrylic pipe. Experimental data of flow patterns, pressure gradient and phase inversion in horizontal oil–water flow are presented and analyzed together with comprehensive comments. The effect of oil viscosity on flow structure was assessed by comparing the present work data with those of Angeli and Hewitt (2000) and Raj et al. (2005). The comparison revealed several important findings. For example, the water velocity required to initiate the transition to non-stratified flow at low oil velocities increased as the oil viscosity increased while it decreased at higher oil velocities. The formation of bubbly and annular flows and the extent of dual continuous region were found to increase as the oil–water viscosity ratio increased. Dispersed oil in water appeared earlier when oil viscosity decreased.     

Study of cell seeding on porous poly(d,l-lactic-co-glycolic acid) sponge and growth in a Couette–Taylor bioreactor

In this study, porous poly(d,l-lactic-co-glycolic acid) (PLGA) sponges were fabricated by using a solvent-free supercritical CO₂ gas-foaming technique. The sponges were then used as three dimensional scaffolds to culture rat bone marrow stroma (rBMS) cells. The rBMS cells were seeded on the scaffolds using a rocking shaker in three directions with different shaking speeds and times. The amount of cells attached onto the polymer sponges increased with the increase of initial cells amounts. A saturation level [Cell_{seeded max}] was achieved when the initial seeded cells were above 6×10⁴. The seeding curved followed closely to a mathematical model through which [Cell_{seeded max}] and seeding affinity (k) can be calculated. A Couette–Taylor bioreactor was subsequently used to culture the cell-sponge constructs. The Taylor vortex flow patterns generated in the bioreactor were characterized by particle imaging velocimetry (PIV). Computation fluid dynamics simulations were performed to reconfirm the vortex flow characteristics and determine the distribution of shear rate and shear stress quantitatively. The results indicated that moderate shear stresses (0.02–0.19 Pa) generated in the bioreactor improved the proliferation of rBMS cells to around 1.3 times of the static control with well maintained calcium deposition ability of the cells. Relative larger shear stresses (0.24 Pa) helped to improve the calcium deposition ability of the cells but inhibited their proliferations. Larger shear stress (>0.24 Pa) inhibited the proliferation of the cells as well as weakened the ability of cell membrane to preserve calcium ions.     

Numerical Simulation of Taylor–Couette Flows with Rotating Outer Wall Using a Hybrid Spectral/Finite Element Method

Theoretical Foundations of Chemical Engineering - Taylor–Couette flow between independently rotating cylinders with a relatively small aspect ratio Γ = 2.4 has been investigated...     

Two-phase flow hydrodynamic study in micro-packed beds – Effect of bed geometry and particle size

Microscopic visualizations nearby the wall region of micro-fixed beds and hydrodynamic measurements during gas–liquid two-phase flows were carried out with an aim to investigate the effect of particle size and capillary tube shape on the bed pressure drop, flow regime transition, hysteresis and bed transient response to flow-rate step perturbations. Visualizations through inverted microscopy revealed that a decrease in particle size leads to early inception of a high interaction flow regime whereas changing capillary shape from circular to square had no effect on flow regime changeover. The effect of particle size on the wetting pattern hysteresis in square micro-packed beds was also investigated in both imbibition and drainage paths. It was found that wetting pattern hysteresis decreases with a decrease in particle size. Finally, the transient behavior of micro-fixed beds of circular and square geometries packed with particles of two different sizes were studied by monitoring the bed pressure drop variations upon step changes in liquid flow rate at iso-G conditions. Larger particle sizes and square geometry showed shorter transient times as compared to smaller particle sizes and circular geometry.     

Modeling of dynamic systems with a variable number of phases in liquid–liquid equilibria

Modeling of dynamic systems with a variable number of phases is still a challenge, especially for multiple liquid phases. A common approach from literature derives first-order Karush–Kuhn–Tucker (KKT) conditions of the Gibbs free energy minimization and relaxes these if a phase does not exist. It aims at enabling dynamic simulation in all phase regimes of systems in vapor–liquid equilibrium by following a nonphysical continuous solution. In this work, we demonstrate that this continuous solution is not always possible in liquid–liquid equilibrium problems. The demonstration is done both theoretically and for illustrative examples. To overcome the demonstrated issues, we review the use of negative flash approach that allows negative molar amounts of nonexisting phases and propose a hybrid continuous formulation that explicitly assigns phase variables in the single-phase regime and solves flash equations otherwise. Various dynamic case studies demonstrate the applicability and limitations of all three approaches. © 2018 American Institute of Chemical Engineers AIChE J, 65: 571–581, 2019     

Mixing and recirculation characteristics of gas–liquid Taylor flow in microreactors

The effects of operating parameters (capillary and Reynolds numbers) and microchannel aspect ratio (α=w/h=[1;2.5;4]$\alpha =w/h=[1;2.5;4]$) on the recirculation characteristics of the liquid slug in gas–liquid Taylor flow in microchannels have been investigated using 3-dimensional VOF simulations. The results show a decrease in the recirculation volume in the slug and an increase in recirculation time with increasing capillary number, which is in good agreement with previous results obtained in circular and square geometries (Thulasidas et al., 1997). In addition, increasing the aspect ratio of the channel leads to a slight decrease in recirculating volumes but also a significant increase in recirculation times.     

Coupled CFD–DEM of particle-laden flows in a turning flow with a moving wall

The nature of the particle–solid interactions and particle–fluid interactions in rectangular duct bend geometry with/without a moving wall is studied, taking into account particle collision, colloidal, and hydrodynamic forces, and four way coupling between the fluid flow and particles. The focus is on systems where particles and fluid phase have similar length scales, fluid Reynolds number (Re_f) ∼ 1, and particle's Stokes number (St) ∼ 1. Particles move toward the walls of the channel near the bend, and have long residence times in these regions. Buoyancy force has negligible effect on particle motion, where adhesion and drag forces lead to particle motion and agglomeration patterns. The effect of a free surface on agglomeration sites in the turning flow is elucidated.     

Effect of a NiO Additive on Interaction in a Ni–Al–W System in Self-Propagating High-Temperature Synthesis

An effect of a NiO additive on the combustion and structure formation in a Ni–Al–W system in self-propagating high-temperature synthesis (SHS) is under study. The stages of the combustion of compositions containing a NiO high-energy additive are shown. The interaction of W particles with Ni–Al melts during SHS results in the formation of globular decoration of particles on the basis of solid solutions of tungsten on the particle surface. This effect is observed only in compositions with an equimolar mixture of Ni–Al. With an NiO additive content in the initial sample more than 1 at.%, the globular decoration on the unreacted W particle surface does not occur. This effect can be associated with changes in the combustion temperature, deviation of the NiAl phase in the direction of a larger content of Ni, and the influence of oxide phases on diffusion processes.