Hydrodynamics and residence time distribution of liquid flow in tubular reactors equipped with screen-type static mixers

Hweij, Khaled Abou; Azizi, Fouad

doi:10.1016/j.cej.2015.05.100

Cited by 38 publications

(14 citation statements)

References 42 publications

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“…As presented in Figures and , the axial dispersion coefficient increases with the Reynolds number increase (or if the Peclet number would be analyzed it would become evident that the Reynolds number increases when the Peclet number decreases) due to the existence of much more intense turbulence (swirls and vortexes), which supports the mixing in the axial direction. This fact is consistent with the literature report . It also means that for the higher values of velocity, the contribution of axial dispersion phenomenon increases and the system moves away from the plug‐flow.…”

Section: Resultssupporting

confidence: 93%

“…According to the performed RTD analysis, it was proven that static mixers and even a pipe for Re > 100 cannot be described by use of any of the ideal states’ models because their mixing operation characteristics are different. However, due to the literature reports, it was adopted that the considered devices may be successfully modelled as non‐ideal reactors by use of a simple dispersion model.…”

Section: Resultsmentioning

confidence: 99%

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Study of fluid dynamic conditions in the selected static mixers part III—research of mixture homogeneity

Stec¹,

Synowiec

2018

Can J Chem Eng

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This paper contains the analysis of mixture homogeneity in two types of static mixers, Koflo and Kenics, based on both experimental and CFD study. The research was also expanded to the recognition of the pipe that was used as a background. The scope of work includes the analysis of the CoV coefficients (as a measure of mixing degree) obtained for the mentioned devices and their comparison. The research showing the impact of such parameters, i.e., pressure drop or L/d ratio on the mentioned mixture homogeneity was also presented. What is more, the axial dispersion model was introduced to describe the devices as non-ideal reactors. According to this, new relations for the CoV predictions (taking into consideration the turbulence intensity, the deviations from well-known ideal states, and the mixing elements' shape) were developed. As a result, it was proven that static mixers are highly efficient devices because the obtained CoVs were at around 2 % in the laminar flow and even less than that in the turbulent regime. When the pipe was used, the satisfactory value of the CoV was obtained when the Reynolds number was increased to 2600.

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Section: Resultssupporting

confidence: 93%

Section: Resultsmentioning

confidence: 99%

Study of fluid dynamic conditions in the selected static mixers part III—research of mixture homogeneity

Stec¹,

Synowiec

2018

Can J Chem Eng

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“…Two general types of devices are used to generate (approximately) the ideal case of homogeneous and isotropic turbulence: the flow behind a grid in a channel [32] or the oscillation of one or more grids in a tank [33]. The first experimental system conducted in wind and water tunnels had a mean flow component across the grid.…”

Section: Active Grid Apparatusmentioning

confidence: 99%

Micro-mixing measurement by chemical probe in homogeneous and isotropic turbulence

Lemenand

Valle

Habchi

et al. 2017

Chemical Engineering Journal

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h i g h l i g h t sValidation of a micro-mixing model under homogeneous and isotropic turbulence (HIT). HIT is obtained by a system of oscillating grids placed in a vessel. Chemical probe measurements are carried out by the iodide/iodate reaction system. IEM and EDD models are tested. EDD model provides the more accurate quantitative characterization of micro-mixing. Keywords: Active grid turbulence Chemical probe IEM (Interaction by Exchange with the Mean) model EDD (Engulfment, Deformation and Diffusion) model Homogeneous and isotropic turbulence Micro-mixing LDV a b s t r a c tThe chemical probe is commonly used to evaluate the performance of chemical reactors. By a localized injection of chemical reagents, it is possible to measure the local micro-mixing, which is readily related to the selectivity of chemical reactions, since mixing at the molecular scale is the limiting factor for a wide range of chemical systems. The raw result of the chemical-probe method is a segregation index that allows comparison of different situations (various locations in the reactor, various Reynolds numbers, various geometries, etc.). Beyond the qualitative assessment provided by this segregation index, it is possible to obtain the intrinsic micro-mixing time by means of a micro-mixing model, describing the temporal evolution of a chemical reaction whose rate is governed by the micro-mixing. Here a key step is the choice of micro-mixing model. Several micro-mixing models available in the literature have been used in some specific cases without evaluating their appropriateness for the problem in hand. The main difficulty in this evaluation is that the real flows often do not fully satisfy the basic model assumptions, in particular the condition of homogeneous and isotropic turbulence (HIT).The present work aims at assessing the validity of a micro-mixing model under ''ideal" experimental conditions, i.e. HIT with no mean flow, to avoid the bias due to the flow gradients. The HIT is obtained here by a system of oscillating grids placed in a vessel. The chemical probe measurements carried out by the iodide/iodate reaction system are applied to the two most commonly used phenomenological models in the literature: the IEM (Interaction by Exchange with the Mean) and the EDD (Engulfment, Deformation and Diffusion) models. The benchmark for the micro-mixing models is based on comparison of the local turbulent kinetic energy (TKE) dissipation rate both drawn from the micro-mixing time by the theoretical model of Bałdyga and the reference direct experimental determination by laser Doppler velocimetry measurements. It is shown that the engulfment model EDD seems the more appropriate to analyze the chemical data and provide a quantitative characterization of micro-mixing.Ó 2016 Elsevier B.V. All rights reserved. IntroductionUnderstanding of the mechanisms that underlie turbulent mixing and convective transfer is fundamental in many technological applications -chemical, pharmaceutical, food, and cosmetic, for http://dx.

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“…Para determinar las bondades de ajustes de los modelos tradicionales y propuesto, se determinó la desviación media absoluta (DMA%), desviación absoluta máxima que se indica en paréntesis, y la desviación media relativa (DMR%) como medidas del error entre los datos calculados y experimentales (Valderrama y Alvarez, 2005), así como también la sumatoria de los errores al cuadrado (SSE) y raíz del error cuadrático medio (RMSE) usada como bondad de ajuste en la DTR (Zeng et al, 2005;Abou y Azizi, 2015). En la Tabla 1 se presentan las bondades de ajustes de los modelos tradicionales para cada uno de los tiempos de retención y condiciones estudiadas.…”

Section: Ajuste De Los Datos Experimentales a Los Modelos De Dispersiunclassified

“…También se han obtenido buenos ajuste de los datos de DTR con el modelo de Gauss pero el número de dispersión se hace a través de la varianza (Abou y Azizi, 2015), por otro lado el modelo de TS ha presentado buenos ajustes de manera parcial (Chen et al, 2015), esto se debe a la cola alargada que describe la curva de DTR. Si no se consideran las condiciones de borde adecuadas podemos llegar a resultados diferentes tal como ocurrió en un reactor a escala de laboratorio cuando se tomaron condiciones de GDra dadas por Levenspiel, 1999(Lou et al, 2006, obteniendo mayores valores de dispersión al definir condiciones de borde adecuadas (Zeng et al, 2005), del mismo modo que se observó en un estudio a escala real (Avella, 2001;Peña et al, 2006), lo anterior indica que si no encontramos la dispersión del flujo como parámetro de ajuste del modelo podemos llegar a resultados y conclusiones diferentes, es por ello que esta investigación tiene un gran aporte al estudio del tipo de flujo presente en un reactor a través de un modelo de dispersión axial en un sistema de tratamiento de agua residual en reactores de flujo continuo.…”

Section: Ajuste De Los Datos Experimentales a Los Modelos De Dispersiunclassified

Modelo de Dispersión Axial para Sistemas de Flujo Continuo Ajustado a las Condiciones de Borde

2016

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ResumenSe desarrolló un modelo matemático a partir de la ecuación general de dispersión y se comparó con tres modelos tradicionales. Para obtener los datos experimentales se realizaron ensayos hidráulicos utilizando litio como trazador agregado de forma puntual en el influente de un Reactor Anaerobio de Flujo Ascendente de 518 litros, alimentado con agua residual de la ciudad de Maracaibo-Venezuela a tiempos de retención hidráulico teórico de 10, 8, 5 y 4 horas. Se tomó muestras en el efluente y al final del lecho y manto de lodo. Los mejores ajustes se lograron con el modelo propuesto con desviación media absoluta entre 5 y 33% y desviación media relativa entre -1,7 y -18%. En todos los casos, el número de dispersión fue mayor a 0.01 indicando que no existió flujo pistón en el reactor. Palabras clave: dispersión axial; modelo; trazador; inyección puntual; reactor de flujo ascendente Axial Dispersion Model for Continuous Flow Systems Adjusted Boundary Conditions AbstractIn this study a mathematical model was developed from a general dispersion equation, which was compared with three traditional models. Tracer tests were carried injected lithium as a pulse at the reactor influent. Several samples were collected in an Upflow Anaerobic Sludge Bed reactor o 518 liters to obtain the experimental data. The reactor was feeding from municipal wastewater of Maracaibo city, Venezuela. Several theoretical retention times were evaluated between 4, 5, 8 and 10 hours. Samples were taken at the final effluent and at the end of the sludge blanket and bed level. Best fits were obtained with the proposed model giving absolute mean deviation between 5.0 and 33% and relative mean deviation between -1.7 and -18%. In most of the studies the dispersion number was lower than 0.01 concluding that plug flow did not exist in the reactor.

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Hydrodynamics and residence time distribution of liquid flow in tubular reactors equipped with screen-type static mixers

Cited by 38 publications

References 42 publications

Study of fluid dynamic conditions in the selected static mixers part III—research of mixture homogeneity

Study of fluid dynamic conditions in the selected static mixers part III—research of mixture homogeneity

Micro-mixing measurement by chemical probe in homogeneous and isotropic turbulence

Modelo de Dispersión Axial para Sistemas de Flujo Continuo Ajustado a las Condiciones de Borde

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