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Gas Superficial Velocity (gas + superficial_velocity)

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Chemistry100%

Selected Abstracts

Particle scale study of heat transfer in packed and bubbling fluidized beds

AICHE JOURNAL, Issue 4 2009
Z. Y. Zhou
Abstract The approach of combined discrete particle simulation (DPS) and computational fluid dynamics (CFD), which has been increasingly applied to the modeling of particle-fluid flow, is extended to study particle-particle and particle-fluid heat transfer in packed and bubbling fluidized beds at an individual particle scale. The development of this model is described first, involving three heat transfer mechanisms: fluid-particle convection, particle-particle conduction and particle radiation. The model is then validated by comparing the predicted results with those measured in the literature in terms of bed effective thermal conductivity and individual particle heat transfer characteristics. The contribution of each of the three heat transfer mechanisms is quantified and analyzed. The results confirm that under certain conditions, individual particle heat transfer coefficient (HTC) can be constant in a fluidized bed, independent of gas superficial velocities. However, the relationship between HTC and gas superficial velocity varies with flow conditions and material properties such as thermal conductivities. The effectiveness and possible limitation of the hot sphere approach recently used in the experimental studies of heat transfer in fluidized beds are discussed. The results show that the proposed model offers an effective method to elucidate the mechanisms governing the heat transfer in packed and bubbling fluidized beds at a particle scale. The need for further development in this area is also discussed. © 2009 American Institute of Chemical Engineers AIChE J, 2009 [source]

Destabilisation of homogeneous bubbly flow in an annular gap bubble column

THE CANADIAN JOURNAL OF CHEMICAL ENGINEERING, Issue 4 2010
Fahd M. Al-Oufi
Abstract Experimental results are presented to show that there are very significant differences in the mean gas void fractions measured in an open tube and a annular gap bubble column, when operated at the same gas superficial velocity, using a porous sparger. Measurements were carried out in a vertical 0.102,m internal diameter column, with a range of concentric inner tubes to form an annular gap, giving diameter ratios from 0.25 to 0.69; gas superficial velocities in the range 0.014,0.200,m/s were investigated. The mean gas void fraction decreases with increasing ratio of the inner to outer diameter of the annular gap column and the transition to heterogeneous flow occurs at lower gas superficial velocities and lower void fractions. Two reasons are proposed and validated by experimental investigations: (1) the presence of the inner tube causes large bubbles to form near the sparger, which destabilise the homogeneous bubbly flow and reduce the mean void fraction; this was confirmed by deliberately injecting large bubbles into a homogeneous dispersion of smaller bubbles, and (2) the shape of the void fraction profiles changes with gap geometry and this affects the distribution parameter in the drift-flux model. Both of these effects serve to reduce the mean gas void fraction in an annular gap bubble column compared to an open tube at the same gas superficial velocity. Des résultats expérimentaux sont présentés pour montrer qu'il existe de très grandes différences dans les fractions de vide gazeux moyennes mesurées dans un tube ouvert et une colonne à bulles à espace annulaire, lorsqu'ils sont utilisés à la même vitesse superficielle de gaz, au moyen d'un aérateur poreux. On a effectué des mesures dans une colonne verticale avec un diamètre interne de 0.102,m, avec une portée de tubes internes concentriques pour former un espace annulaire, procurant des rapports de diamètre de 0.25 à 0.69; des vitesses superficielles de gaz de 0.014 à 0.200,m/s ont été étudiées. La fraction de vide gazeux moyenne diminue avec le rapport croissant du diamètre interne à externe de la colonne à espace annulaire et la transition à la circulation hétérogène se produit à des vitesses superficielles de gaz et fractions de vide plus basses. Deux raisons sont proposées et validés par les vérifications expérimentales: (1) la présence du tube interne provoque la formation de grandes bulles près de l'aérateur, ce qui déstabilize l'écoulement à bulles homogène et réduit la fraction de vide moyenne; cet état a été confirmé en injectant délibérément de grandes bulles dans une dispersion homogène de plus petites bulles et, (2) la forme des profils de fraction de vide change avec la géométrie de l'espace qui les sépare, ce qui a des conséquences sur le paramètre de distribution du modèle à flux de dérive. Ces deux effets servent à réduire la fraction de vide gazeux moyenne dans une colonne à bulles à espace annulaire, en comparaison avec un tube ouvert à la même vitesse superficielle de gaz. [source]

Intensification of liquid,liquid two-phase mass transfer by gas agitation in a microchannel

AICHE JOURNAL, Issue 8 2009
Yuanhai Su
Abstract In this experiment, the inert gas is used to agitate two immiscible fluids in microchannels. The mass transfer performances with or without gas agitation are investigated. 30% TBP (in kerosene)-acetic acid-water is chosen as testing system, and nitrogen as agitating gas. The superficial velocities of the immiscible liquid,liquid two phases and gas phase are varied in the range from 0.02 to 1.2 m/s, and 0 to 3.0 m/s, respectively. In microchannels, with enough gas agitating intensity, high dispersion between two immiscible liquid phases can be obtained. The overall volumetric mean mass transfer coefficients are two-folds higher than those without gas agitating, which are in the range of 3.8,30.6 s,1. Some parameters which impact on the mass transfer process, such as the mixture superficial velocity of the immiscible liquid,liquid two phases, the gas superficial velocity, the microchannel structure, the gas inlet locations and the sampling time are experimentally investigated. © 2009 American Institute of Chemical Engineers AIChE J, 2009 [source]

Particle scale study of heat transfer in packed and bubbling fluidized beds

AICHE JOURNAL, Issue 4 2009
Z. Y. Zhou
Abstract The approach of combined discrete particle simulation (DPS) and computational fluid dynamics (CFD), which has been increasingly applied to the modeling of particle-fluid flow, is extended to study particle-particle and particle-fluid heat transfer in packed and bubbling fluidized beds at an individual particle scale. The development of this model is described first, involving three heat transfer mechanisms: fluid-particle convection, particle-particle conduction and particle radiation. The model is then validated by comparing the predicted results with those measured in the literature in terms of bed effective thermal conductivity and individual particle heat transfer characteristics. The contribution of each of the three heat transfer mechanisms is quantified and analyzed. The results confirm that under certain conditions, individual particle heat transfer coefficient (HTC) can be constant in a fluidized bed, independent of gas superficial velocities. However, the relationship between HTC and gas superficial velocity varies with flow conditions and material properties such as thermal conductivities. The effectiveness and possible limitation of the hot sphere approach recently used in the experimental studies of heat transfer in fluidized beds are discussed. The results show that the proposed model offers an effective method to elucidate the mechanisms governing the heat transfer in packed and bubbling fluidized beds at a particle scale. The need for further development in this area is also discussed. © 2009 American Institute of Chemical Engineers AIChE J, 2009 [source]

Transient fluidization and segregation of binary mixtures of particles

AICHE JOURNAL, Issue 11 2000
A. Marzocchella
Fluidization of binary mixtures of particles belonging to group B of the Geldart classification of powders was studied. Beds tested were prepared by mixing in different proportions particles with almost equal density (,2,500 kg/m3) and dissimilar size (125 ,m silica sand and 500 ,m glass beads). Experiments were carried out using a segmented fluidization column equipped with multiple pressure transducers. Experimental procedures included continuous monitoring of pressure drop at different locations along the bed during quasi-steady or stepwise changes of gas superficial velocity, and characterization of particle-size distributions in each segment of the fluidization column after fluidization of the bed for given times. Three ranges of gas superficial velocity were recognized for each solids mixture. At low velocity the bed behaves as a fixed bed. At high velocity, it is fully and steadily fluidized. In an intermediate velocity range, transient fluidization takes place: an initially uniform fluidized bed eventually undergoes segregation, giving rise to a defluidized bottom layer rich in the coarser solids and to a "supernatant" fluidized layer where finer particles prevail. The thresholds between these velocity ranges are rather sharp and were characterized as functions of initial bed composition. Rates at which the defluidized solids layer builds up from initially uniform beds, and the ultimate compositions of the defluidized bottom and fluidized top layers are characterized for beds with different compositions at variable gas superficial velocity. [source]

Destabilisation of homogeneous bubbly flow in an annular gap bubble column

THE CANADIAN JOURNAL OF CHEMICAL ENGINEERING, Issue 4 2010
Fahd M. Al-Oufi
Abstract Experimental results are presented to show that there are very significant differences in the mean gas void fractions measured in an open tube and a annular gap bubble column, when operated at the same gas superficial velocity, using a porous sparger. Measurements were carried out in a vertical 0.102,m internal diameter column, with a range of concentric inner tubes to form an annular gap, giving diameter ratios from 0.25 to 0.69; gas superficial velocities in the range 0.014,0.200,m/s were investigated. The mean gas void fraction decreases with increasing ratio of the inner to outer diameter of the annular gap column and the transition to heterogeneous flow occurs at lower gas superficial velocities and lower void fractions. Two reasons are proposed and validated by experimental investigations: (1) the presence of the inner tube causes large bubbles to form near the sparger, which destabilise the homogeneous bubbly flow and reduce the mean void fraction; this was confirmed by deliberately injecting large bubbles into a homogeneous dispersion of smaller bubbles, and (2) the shape of the void fraction profiles changes with gap geometry and this affects the distribution parameter in the drift-flux model. Both of these effects serve to reduce the mean gas void fraction in an annular gap bubble column compared to an open tube at the same gas superficial velocity. Des résultats expérimentaux sont présentés pour montrer qu'il existe de très grandes différences dans les fractions de vide gazeux moyennes mesurées dans un tube ouvert et une colonne à bulles à espace annulaire, lorsqu'ils sont utilisés à la même vitesse superficielle de gaz, au moyen d'un aérateur poreux. On a effectué des mesures dans une colonne verticale avec un diamètre interne de 0.102,m, avec une portée de tubes internes concentriques pour former un espace annulaire, procurant des rapports de diamètre de 0.25 à 0.69; des vitesses superficielles de gaz de 0.014 à 0.200,m/s ont été étudiées. La fraction de vide gazeux moyenne diminue avec le rapport croissant du diamètre interne à externe de la colonne à espace annulaire et la transition à la circulation hétérogène se produit à des vitesses superficielles de gaz et fractions de vide plus basses. Deux raisons sont proposées et validés par les vérifications expérimentales: (1) la présence du tube interne provoque la formation de grandes bulles près de l'aérateur, ce qui déstabilize l'écoulement à bulles homogène et réduit la fraction de vide moyenne; cet état a été confirmé en injectant délibérément de grandes bulles dans une dispersion homogène de plus petites bulles et, (2) la forme des profils de fraction de vide change avec la géométrie de l'espace qui les sépare, ce qui a des conséquences sur le paramètre de distribution du modèle à flux de dérive. Ces deux effets servent à réduire la fraction de vide gazeux moyenne dans une colonne à bulles à espace annulaire, en comparaison avec un tube ouvert à la même vitesse superficielle de gaz. [source]

A Method for the Detection of Defluidized Zones in Slurry Bubble Columns

THE CANADIAN JOURNAL OF CHEMICAL ENGINEERING, Issue 3-4 2003
Ahad Emami
Abstract The formation of defluidized zones was studied in a laboratory slurry bubble column equipped with heat transfer probes. The probes were small thermistors 2.4 mm in diameter. Dionized water and air were used as a liquid and gas phase, respectively. Solids were fine ceramic particles with mean size of 19.2 ,m and density of 2244 kg/m3. The effects of solids holdup (up to 30 wt% on gas free basis), gas superficial velocity (0.01-0.09 m/s), sparger height (0.01-0.09 m) on defluidized zones formation was studied. Cycle analysis of the local heat transfer fluctuations reliably detected the local formation of defluidized zones for each condition. La formation de zones défluidisées a été étudiée dans une colonne à bulles avec bouillie, équipée de sondes de transfert de chaleur. Les sondes étaient des petits thermistors de 2.4 mm de diamètre. De l'eau déionisée et de l'air constituaient les phases liquide et gazeuse. Les solides étaient des fines particules en céramique avec une taille moyenne de 19.2 µm et une densité de 2244 kg/m3. Les effets de la rétention solide (jusqu'à 30 % en masses sur base sans gaz), de la vitesse superficielle du gaz (0.01-0.09 m/s) et de la hauteur du distributeur de gaz ont été étudiés. L'analyse de cycle du transfert de local de chaleur a détecté de façon fiable la formation locale de zones défluidisées pour chaque condition. [source]