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Clear liquid height on sieve plates in the froth, mixed and spray regimes

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Abstract

The clear liquid height on sieve plates has been measured in a hydraulic simulation column. Experimental work and investigation of published data show that three different foaming regimes have to be distinguished, as Chen recognised, and the hole diameter has an influence on the clear liquid height as Dhulesia proposed. The Hofhuis-Zuiderweg correlation has been modified to describe the clear liquid height in the froth, mixed and spray regimes, respectively. Different formulae have been developed for each of the foaming regimes. Two new parameters have been introduced: the hole diameter and the free cross section.
... To access the horizontal liquid velocity U L , the knowledge of clear liquid height h Lc is required. Thus for empirical correlations, different authors have used the flow ratio Ψ instead of the flow parameter FP (Dhulesia, 1983 andMustafa, 1991 and1997 …): ...
Article
Experimental measurements of hydrodynamic and interfacial area parameters are carried out over two rectangular pilot scale valve tray columns. The effect of tray path length on extrapolation between the two columns is studied and phenomenological correlations for hydrodynamic and interfacial area are proposed. Correlations are compared both to literature and to industrial results showing good agreement and a significant improvement for the prediction of industrial conditions. Discrepancies preventing an accurate description of industrial trends are highlighted through comparison between typical emulsion height profiles on both columns.
... The flow regime, which characterizes the dispersion, is central to fundamental hydraulic and mass transfer models on trays. To address the flow regimes in modeling, researchers chose one of the following options: (a) a semi-empirical model that extrapolates into adjoining regimes (Chen & Chuang, 1993) taking advantage of the gradual change in properties or (b) a separate model for each regime (Bekassymolnar and Mustafa, 1991;Zuiderweg, 1982) coupled with correlations to detect the regime boundaries. ...
Conference Paper
The bi-phases on distillation trays can function in several different regimes depending on the physical properties of the system and the liquid-to-vapor ratio. Predicting the fraction of the vapor transported as jets, or fraction jetting, on a tray operating in the mixed-froth regime can bridge froth and spray regime models, explain the gradual changes in tray efficiency during the froth-spray transition, and eliminate the need to predict the froth-spray transition point when separate froth and spray regime models are used. Fraction jetting models will also facilitate multi-regime models, such as the Syeda, Afacan, and Chuang (2007) sieve tray efficiency model, that are valid for both froth and spray regimes. Current fraction jetting models are empirical and developed from limited data. In this paper, the factors affecting fraction jetting on a tray are discussed and a new semi-empirical model to predict fraction jetting as an alternative to existing models is presented.
... A change of the flow regime in the normal operating region implies a change in the dispersion structure on the tray at the point of phase inversion warranting separate models for each flow regime. To address the flow regimes in modeling, researchers chose one of the following options: (a) a semi-empirical model that extrapolates into adjoining regimes (Chen and Chuang, 1993), or (b) a separate model for each regime (Bekassymolnar and Mustafa, 1991;Zuiderweg, 1982) coupled with correlations described in Section 2.3. ...
... These regimes have been studied and empirical functions created relating various hydrodynamic parameters for each regime [2]. The distillation literature contains many searches for theory and empirical data that accounts for the formation of foam and spray, the flow patterns that form upon and effect the hydraulics of a distillation tray, and the prediction of densities within the different regimes [34]. Yet, such a comprehensive model of the stage plate would require more information about the components than is readily available and contains more information than needed. ...
Article
The present work proposes a new approach for measuring the effective froth height on column trays. This approach is applied on the two-phase dispersion data gathered by a novel multi-probe sensor installed inside a large-scale tray column mockup. A physical explanation of the proposed approach describes how to distinguish between the liquid-continuous and gas-continuous regions in the froth. Accordingly, the effective froth height distributions are reported for selected tray loadings.
Thesis
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En este trabajo se estudian modelos de etapa para la simulación estacionaria de columnas de destilación de bandejas, los que, además del equilibrio de fases, incorporan explícitamente las ecuaciones constitutivas de transferencia de calor y materia multicomponente. Este enfoque, conocido como rate-based (RB) o modelos de etapa de no-equilibrio, se extiende a situaciones de flujo cruzado y se compara con el enfoque de eficiencia de etapa. Para tal efecto, se propone un modelo basado en un enfoque de velocidades globales de transferencia de materia, que evita las correcciones empíricas de mezclado y la suposición de propiedades interfaciales constantes, propias del modelo clásico de etapa de no-equilibrio y sus extensiones. Simulaciones de columnas y platos de destilación para diferentes casos de estudio muestran que los efectos de mezclado y flujo sobre la separación son tanto o más importantes que los efectos de transferencia de materia multicomponente. Las separaciones calculadas a nivel de plato para un amplio rango de condiciones de operación, indican que el modelo propuesto provee una mejor aproximación de los perfiles de concentración, comparado con los modelos de eficiencia multicomponente (MEF) y de etapa de no-equilibrio. Aunque el desempeño de modelos RB y MEF es similar, se muestra que el modelo MEF puede predecir la dirección equivocada de la transferencia de materia para componentes en rangos de dilución, debido a un tratamiento inconsistente del flujo molar interfacial neto. Las simulaciones que comparan modelos RB y modelos de eficiencia tradicionales, corroboran lo reportado en la literatura, en cuanto a que las eficiencias de Murphree no son iguales para cada componente y varían a lo largo de una columna, presentando a menudo variaciones abruptas. Considerando esta situación se recomienda utilizar modelos RB, dado que los resultados de simulación indican que el modelo de etapa de equilibrio con eficiencias de Murphree especificadas por columna, sección o etapa, no permite representar satisfactoriamente los perfiles de concentración de todos los componentes. El análisis de los fenómenos propios de la transferencia de materia multicomponente, para los casos de destilación estudiados, indica que los efectos de interacción multicomponente cobran mayor importancia cuando el flujo molar interfacial del componente tiende a cero y, por lo tanto, el efecto sobre la separación es inherentemente pequeño. Para los casos estudiados, los efectos de interacción difusionales son de menor importancia práctica comparados con la modelación del equilibrio de fases, los modelos de flujo y la predicción de las velocidades de transferencia de materia. El desempeño menos satisfactorio del método de difusividades iguales y de difusividades efectivas según la fórmula de Wilke, no necesariamente indica que un enfoque pseudobinario es inadecaudo, considerando el desempeño superior que entrega el método de difusividades equivalentes utilizado en este trabajo. Asimismo se comprueba que situaciones consideradas hasta ahora relevantes para mostrar la presencia de efectos de interacción difusionales, como eficiencias puntuales tipo Murphree fuera del intervalo [0,1] y situaciones de transporte reverso, no implican necesariamente la presencia de efectos de interacción difusionales. Los resultados sugieren que el enfoque de transferencia de materia multicomponente en destilación, siendo necesario, a menudo puede aproximarse satisfactoriamente. La simulación de una columna de destilación industrial, utilizando el modelo de etapa de no-equilibrio más simple y la confrontación de resultados de simulación con datos de operación, revela la dificultad de obtener información experimental que permita caracterizar el desempeño de una columna industrial de separación de alta pureza, así como el gran número de variables que afecta a la separación. En el caso particular de la refinación final de metanol, para el nivel de precisión requerido, tanto el equilibrio de fases, las correlaciones específicas de transferencia de materia, el arrastre de líquido y las condiciones de flujo y mezclado son de importancia comparables en la simulación de la separación. Los modelos de etapa RB constituyen un avance significativo, especialmente de enfoque, para la simulación de columnas de destilación. No obstante, para el nivel de detalle de modelación pretendido, es necesario considerar otros fenómenos adicionales, especialmente aspectos fluidodinámicos. En este trabajo se abordó uno de los más importantes, es decir, la modelación del flujo cruzado. Otros efectos, especialmente vinculados con la descripción de la dispersión, requieren de un estudio más fundamental con el apoyo de experimentación.
Conference Paper
Full-text available
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Thesis
Full-text available
The air/water flow in a sieve tray type contact device was modeled using the CFD software CFX 15®. The Eulerian approach was employed, being air and water assumed as interpenetrating phases where the momentum exchange happens, essentially, through drag forces. The expression presented in 1999 by Krishna et al for the drag coefficient (CD) was modified, enabling to simulate schemes of two or more associated trays and covering the effect of the shape of non-spherical bubbles. The simulations were planned with the help of the Response Surface method and covered a wide range of conditions and all operating regimes observed for these devices. The simulation results for the dry tray pressure drop (hd) and for the height of clear liquid on the wet tray (hl) were compared to the predictions of several empirical and semi-empirical correlations available in the literature, showing that the values are equivalent considering the tolerable margin of uncertainty for calculations of this type and that the main observed discrepancies can be explained through the analysis of the fluid dynamic behavior of the flow. As expected, the downcomer may have a significant influence on the height of the dispersion above the tray and needs to be modeled. The validity of the metodology presented was tested against the values of pressure drop observed in trays found in a vacuum distillation tower and a debutanizer tower of Duque de Caxias Refinery, owned by Petrobras, in Rio de Janeiro. The results showed that despite the differences between the physical properties of the fluids and the geometrical simplifications of the schemes in relation to the geometry of a valve tray, there is coherence between the values of pressure drop observed in the trays and the predictions of the simulator. This conclusion confirms the applicability of hydraulic simulators and pilot-plants using air and water to predict the fluid dynamic behavior of trays.
Article
The two phases on distillation trays can function in several different regimes depending on the physical properties of the system and the liquid-to-vapor ratio. Predicting the fraction of the vapor transported as jets, or fraction jetting, on a tray operating in the mixed-froth regime can bridge froth and spray regime models, explain the gradual changes in tray efficiency during the froth–spray transition, and eliminate the need to predict the froth–spray transition point when separate froth and spray regime models are used. Fraction jetting models will also facilitate multiregime models, such as the Syeda et al. (Chem. Eng. Res. Dev.2007, 85, 269–277) sieve tray efficiency model, that are valid for both froth and spray regimes. Current fraction jetting models are empirical and developed from limited data. In this paper, the concept of fraction jetting on a sieve tray is discussed and a new semiempirical model is presented. The fraction jetting model was developed using the existing data of Raper et al. (Chem. Eng. Sci.1982, 37, 501–506) and no new data were generated. Furthermore, the applicability of the fraction jetting model to tray efficiency models that incorporate jetting is demonstrated using the Syeda et al. sieve tray efficiency model.
Article
An experimental study on separation performance of a sieve tray column in ternary distillation is made under total reflux conditions with different tray specifications. The effects of operation conditions, tray specifications, and feed concentration on the clear liquid height and mass transfer rate are studied. The dimensionless volumetric vapor phase diffusion flux is correlated with the free area and Reynolds number. Simulation of separation performance of tray column was carried out using the proposed correlations for heat and mass transfer rates and clear liquid height. Predicted values of concentration agree with the experimental data of the present study and some previous works.
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