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Christophe Gourdon - One of the best experts on this subject based on the ideXlab platform.
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A new numerical method for Axial Dispersion characterization in microreactors
Chemical Engineering Science, 2017Co-Authors: Maxime Moreau, Christophe Gourdon, Nathalie Di Miceli Raimondi, Nathalie Le Sauze, Michel CabassudAbstract:Axial Dispersion is a key phenomenon in reactor engineering that can affect yield and selectivity when reactions are carried out. Therefore its characterization is necessary for an adequate modelling of the reactor. The development of compact reactors to fit with process intensification expectations requires the use of characterization methods adapted to small-scale devices. An original method not-frequently used up to now for the estimation of Axial Dispersion Coefficients is presented and applied to millimetric wavy channels. It is based on CFD simulations to calculate velocity and concentration fields from which Axial Dispersion Coefficient can be estimated. This method is used to predict the impact of the wavy channel geometry and of the fluid velocity on Axial Dispersion in laminar flow regime. The investigated geometrical parameters are the hydraulic diameter (2–4 mm), the cross-sectional aspect ratio defined as the ratio between the channel width and its depth (0.25–1) and the internal curvature radius of the bends (2–3.4 mm). The range of Reynolds number considered is Re = 70–1 600. Axial Dispersion Coefficient increases with velocity, values range from 2.8 10-4 to 3.2 10-3 m2.s-1. It appears that Axial Dispersion varies slightly in function of the channel hydraulic diameter. Square wavy channels generate less Axial Dispersion than rectangular wavy ones. Finally, Axial Dispersion Coefficient increases with the internal curvature radius which shows the positive impact of sharp bends to reduce Axial Dispersion effect.
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effect of microchannel aspect ratio on residence time distributions and the Axial Dispersion Coefficient
Chemical Engineering and Processing, 2009Co-Authors: Joelle Aubin, Laurent E Prat, Catherine Xuereb, Christophe GourdonAbstract:Abstract The effect of microchannel aspect ratio (channel depth/channel width) on residence time distributions and the Axial Dispersion Coefficient have been investigated for Newtonian and shear-thinning non-Newtonian flow using computational fluid dynamics. The results reveal that for a fixed cross-sectional area and throughput, there is a narrowing of the residence time distribution as the aspect ratio decreases. This is quantified by an Axial Dispersion Coefficient that increases rapidly for aspect ratios less than 0.3 and then tends towards an asymptote as the aspect ratio goes to 1. The results also show that the Axial Dispersion Coefficient is related linearly to the Reynolds number when either the aspect ratio or the mean fluid velocity is varied. However, the fluid Peclet number is a linear function of the Reynolds number only when the aspect ratio (and therefore hydraulic diameter) is varied. Globally, the results indicate that microchannels should be designed with low aspect ratios (≤0.3) for reduced Axial Dispersion.
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residence time distribution of droplets within discs and doughnuts pulsed extraction columns via lagrangian experiments and simulations
Chemical Engineering Journal, 2003Co-Authors: Nathalie Bardinmonnier, Pascal Guiraud, Christophe GourdonAbstract:This paper is devoted to the study of the transport of single droplets in discs and doughnuts extraction pulsed columns. Video experiments are carried out on a pilot plant of industrial size (D=300 mm) in order to extract the values of the plug flow with Axial Dispersion transport model parameters (i.e. mean residence time and Axial Dispersion Coefficient). The same kind of results are established thanks to Lagrangian simulations carried out with the industrial computational fluid dynamics (CFD) code ESTET (EDF, SIMULOG). A detailed study of the influence of the simulation conditions (expressions of the forces, turbulent Dispersion and rebound modelling) is led in order to set the limits of such an approach. The agreement between experiments and simulations is around 20%. This study leads to the conclusion that the quality of the results seems to depend strongly on the prediction of the continuous phase flow turbulence.
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lagrangian simulations contribution to the knowledge of discs and doughnuts pulsed solvent extraction columns hydrodynamics
Chemical Engineering and Processing, 2003Co-Authors: Nathalie Bardinmonnier, Pascal Guiraud, Christophe GourdonAbstract:Abstract This paper intends to evaluate in which way the Computational Fluid Dynamics (CFD), and more precisely Lagrangian simulations, can be used as a tool for getting information about the hydrodynamic behavior of the dispersed phase in extraction columns. As an example, the study is carried out on a particular contactor: the discs and doughnuts pulsed column. In a first part, details are given on the modeling and on the simulation hypothesis and principles. The second part is dedicated to the study of the dispersed phase transport via the plug flow with Axial Dispersion transport model parameters (mean residence time and Axial Dispersion Coefficient) for several operating conditions (pulsation amplitude and frequency, droplet diameter). The comparison of simulation results with those for ‘single drop’ experiment shows that the simulations perfectly reproduce the trends concerning the influence of the operating parameters, although a gap still remains. The last part of this paper is devoted to the presentation of information that can be extracted from the simulations in order to contribute to a better understanding of the contactor operation: drop spreading in the compartments, slip velocity related to the mass transfer, and the turbulent kinetic energy seen by the drops related to the breakage.
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numerical experimentation a tool to calculate the Axial Dispersion Coefficient in discs and doughnuts pulsed solvent extraction columns
Chemical Engineering Science, 1997Co-Authors: Aoun M Nabli, Pascal Guiraud, Christophe GourdonAbstract:In this paper, a correlation between Axial mixing in a discs and doughnuts pulsed extraction column, the geometrical characteristics of the column, and the pulsation conditions is established by numerical experimentation. The calculation method, based on the computational fluid dynamics approach and detailed on an example, consists in evaluating the Axial Dispersion Coefficient from simulations of tracer mixing, transported by the turbulent pulsed flow in the column. Hydrodynamic characteristics of flow and tracer transport are calculated by the ESTET code. The results concerning the mean velocity fields of the oscillatory turbulent flow show a transition from an unstable to a stable recirculation regime. The stability of the recirculations and their size depend only on one geometrical parameter which is the distance between successive discs and doughnuts. A quantitative analysis of the Dispersion process is obtained from the time variation's study of the tracer cloud's spatial moments. The results reveal that the distance between successive discs and doughnuts is the key parameter governing Axial mixing in such columns. This work also confirms the proportionality observed by other authors between the Axial Dispersion Coefficient and the pulsation intensity. Numerical results are compared with the experimental ones available.
J B Joshi - One of the best experts on this subject based on the ideXlab platform.
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hydrodynamic and mass transfer characteristics of asymmetric rotating disc extractors
Chemical Engineering Research & Design, 2009Co-Authors: B D Kadam, J B Joshi, R. N. PatilAbstract:Abstract Asymmetric rotating disc contactor (ARDC) offers potential advantages in having comparable mass transfer Coefficient ( k C a _ ) and lower back mixing over the conventional rotating disc contactor (RDC). In the present work, measurements of dispersed phase hold-up (∈D), effective interfacial area ( a _ ), continuous phase mass transfer Coefficient ( k C a _ ) and Axial Dispersion Coefficient for the dispersed as well as continuous phase have been performed in 80 mm and 150 mm i.d. ARDC over a wide range of operating variables and physical properties. From these measurements, the values of slip velocity and sauter mean drop diameter (d3,2) have been estimated. Suitable correlations have been proposed for ∈D, VS, k C a _ , a _ , kC, d3,2, PeC and PeD. The results on true mass transfer Coefficient (kC) have been compared with those published in the literature.
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cfd simulation of flow and Axial Dispersion in external loop airlift reactor
Chemical Engineering Research & Design, 2006Co-Authors: Swarnendu Roy, M T Dhotre, J B JoshiAbstract:In the present work, a CFD simulation, using two-fluid model, has been performed to obtain the flow pattern and the gas hold-up in external loop air-lift reactors (EL-ALR). An agreement was observed between the predicted and the experimental data available in the published literature. The validated CFD model has been extended for the simulation of mixing and the Axial Dispersion Coefficient. CFD predictions of mixing time are well in agreement with the experimental values published in literature, and the results of Axial Dispersion Coefficient also show good agreement with the experimental values published in literature except in the case of Vial et al. (2005), where the CFD model underpredicts. A systematic numerical study was undertaken to understand the relative contribution of convection and eddy diffusion to the mixing and Axial Dispersion.
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cfd simulations of bubble column reactors 1d 2d and 3d approach
Chemical Engineering Science, 2005Co-Authors: K Ekambara, M T Dhotre, J B JoshiAbstract:Abstract CFD simulations have been carried out for the predictions of flow pattern in bubble column reactors using 1D, 2D and 3D k – e models. An attempt has been made to develop a complete correspondence between the operation of a real column and the simulation. Attention has been focused on the cylindrical bubble columns because of their widespread applications in the industry. All the models showed good agreement with the experimental data for Axial liquid velocity and the fractional gas hold-up profiles. However, as regards to eddy diffusivity, only the 3D model predictions agree closely with the experimental data. The CFD model has been extended for the estimation of an Axial Dispersion Coefficient ( D L ) using 1D, 2D and 3D models. Excellent agreement was found only between the experimental values and the 3D predictions. The 1D and 2D simulations, however, yielded D L values, which were lower by 25–50%. For this, a mechanistic explanation has been provided.
Georges Guiochon - One of the best experts on this subject based on the ideXlab platform.
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Theoretical and experimental impact of the bed aspect ratio on the Axial Dispersion Coefficient of columns packed with 2.5 μm particles
Journal of chromatography. A, 2012Co-Authors: Fabrice Gritti, Georges GuiochonAbstract:The impact of the ratio of the column diameter to the average particle size (or bed aspect ratio) on the column performance was investigated from theoretical and experimental viewpoints. The experiments were conducted for two series of 100mm long columns, 2.1, 3.0, and 4.6mm in diameter, packed with 2.5 μm fully porous particles of Bridge Ethylene Hybrid (BEH) for one series and Charged Surface Hybrid (CSH) for the other. The heights equivalent to a theoretical plate (HETP) of two low molecular weight compounds, uracil (non-retained, k=0) and naphthalene (retained, k=2.5), were determined from the true moments of the recorded peak profiles. The results showed a systematic decrease of the column performance for uracil at high flow velocities with decreasing column inner diameter, in agreement with the theoretical predictions of the variation of the trans-column eddy Dispersion HETP term with decreasing bed aspect ratio. This result is consistent with the increasing volume fraction of the wall region of the column, in which the average linear velocity of the mobile phase over a distance of 5 particle diameters from the column wall is about 10% larger than in the bulk center of the column (infinite diameter column). For the retained compound, the discrepancies are levelled out due to the longer average residence time and larger particle diffusivities of retained compounds, which allow a more efficient relaxation of the radial concentration gradients. Further improvements of the performance of the larger I.D. columns (3.0 and 4.6mm I.D.) may be achieved by decreasing the harmful effect of this trans-column velocity bias by injecting and/or collecting the sample molecules in a wide central zone of the column. For 2.1mm I.D. columns, this approach would prove useful only when HPLC instruments providing a lower extra-column band broadening contribution will become available. Finally, the further minimization of the trans-column eddy Dispersion HETP term and the design of new, better inlet/outlet column endfitting/frit assemblies requires newer and more accurate models of eddy Dispersion in packed columns than those previously provided by Gunn and Giddings and the numerical calculation of band profiles using original functions to account for the distribution and collection of the sample molecules at the inlet and outlet of the column.
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peak parking method for measurement of molecular diffusivity in liquid phase systems
Journal of Chromatography A, 2009Co-Authors: Kanji Miyabe, Nobuho Ando, Georges GuiochonAbstract:Abstract The combination of series of measurements of band broadening made with the peak parking (PP) method, using successively an open capillary tube and a HPLC column, gives a convenient procedure for the measurement of the molecular diffusivity (Dm) of compounds in solutions, of their Axial Dispersion Coefficient (Dax,m) in chromatographic columns, and of the tortuosity or obstructive factor of the column bed. The molecular diffusivity measured for benzene in methanol was in excellent agreement with literature data. The ratio of the Axial Dispersion Coefficient to this diffusivity gives the obstructive factor (γm) of the packed bed, which was 0.74 for the column used. The values of Dm in other solutions were obtained from the Dax,m values measured by the PP method, by correcting the Dax,m values with the γm value. The Dm values determined by this method were in good agreement with those previously reported or estimated using literature correlations. These results showed that the PP method is effective for the experimental measurement of Dm.
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influence of the concentration dependence of the mass transfer properties on the chromatographic band profiles i apparent Axial Dispersion Coefficient in frontal analysis
Journal of Chromatography A, 1996Co-Authors: Georges Guiochon, Peter Sajonz, Guoming ZhongAbstract:The influence of the concentration dependence of Axial Dispersion on the profiles of breakthrough curves was studied using the equilibrium-dispersive model. A simple linear relationship between the apparent Axial Dispersion Coefficient and the concentration was assumed and calculations of band profiles were carried out for both increasing and decreasing values of the Axial Dispersion Coefficient with increasing concentration. A comparison of the profiles with those obtained with a constant Coefficient shows that the effect of a concentration dependence is significant even for moderate variations.
N Midoux - One of the best experts on this subject based on the ideXlab platform.
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experimental and theoretical analysis of Axial Dispersion in the liquid phase in external loop airlift reactors
Chemical Engineering Science, 2005Co-Authors: Ch Vial, Souhila Poncin, Gabriel Wild, N MidouxAbstract:Abstract A model has been developed for the prediction of the Axial Dispersion Coefficient in the riser of an external-loop airlift reactor. It takes into account both radial velocity and gas hold-up profiles as a function of flow regime, but also the two-phase turbulence. It is based on the mixing length model developed by [Vial et al., 2002. Chem. Eng. Sci. 57, 4745–4762] for the flow field prediction and uses a turbulent diffusion Coefficient to estimate the influence of two-phase turbulence on mixing. The relations established using experimental data from an airlift reactor have been validated experimentally using a second reactor. A comparison with theoretical Dispersion Coefficients deduced from CFD calculations and correlations from the literature is also provided. The results show that in all the hydrodynamic regimes, Dispersion stems from bubble-induced turbulence, despite the presence of a nearly parabolic liquid velocity profile in the homogeneous regime and marked liquid hold-up profiles in churn-turbulent flow.
Geoffrey W Stevens - One of the best experts on this subject based on the ideXlab platform.
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Axial Dispersion in a pulsed and nonpulsed disc and doughnut solvent extraction column
Industrial & Engineering Chemistry Research, 2017Co-Authors: Yong Wang, Kathryn A Mumford, Kathryn H Smith, Geoffrey W StevensAbstract:In solvent extraction columns mechanical agitation is usually introduced to improve the extraction efficiency. However, some industrial columns have been found to have higher extraction efficiency while running with no pulsation. In this study, Axial Dispersion Coefficients in the continuous phase were measured under pulsing and nonpulsing conditions using a 72.5 mm diameter disc and doughnut solvent extraction column. The Axial Dispersion Coefficients were measured using the unsteady tracer injection method. Under nonpulsing conditions, the Axial Dispersion Coefficient increased with increasing continuous phase velocity, but it did not change significantly with the increase of the dispersed phase velocity. With increasing pulsation intensity, the Axial Dispersion Coefficient increased. A correlation is proposed to predict the continuous phase Axial Dispersion Coefficient in a pulsing and nonpulsing disc and doughnut solvent extraction column.
