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Holdup
Kinds of Holdup Selected AbstractsA Dynamic Theory of HoldupECONOMETRICA, Issue 4 2004Yeon-Koo Che The holdup problem arises when parties negotiate to divide the surplus generated by their relationship specific investments. We study this problem in a dynamic model of bargaining and investment which, unlike the stylized static model, allows the parties to continue to invest until they agree on the terms of trade. The investment dynamics overturns the conventional wisdom dramatically. First, the holdup problem need not entail underinvestment when the parties are sufficiently patient. Second, inefficiencies can arise unambiguously in some cases, but they are not caused by the sharing of surplus per se but rather by a failure of an individual rationality constraint. [source] Holdup and Pressure Drop in Vertical and Near-Vertical Three-Phase Up-Flow: A Collection of Flow RegimesASIA-PACIFIC JOURNAL OF CHEMICAL ENGINEERING, Issue 1-2 2002P.L. Spedding Three-phase oil, water and air data are reported for vertical and near vertical +86 5° upflow in a 0 026 m i d pipe In general, the liquid holdup for near vertical flow was greater than for the corresponding vertical upflow, the exception being at low liquid and superficial velocities under 0 6 m/s and high superficial gas velocities over 20 m/s Here the liquid holdup varied being sometimes below and other times above the corresponding vertical value These variations of liquid holdup were shown to depend on the fine structure of the flow patterns present The total pressure drop and its component parts showed significantly different patterns of behaviour depending on whether the superficial gas velocity was above or below the rise velocity of a Taylor bubble The total pressure drop generally was greater for near vertical flow compared to the vertical upflow case but reflected changes in the fine structure of the flow patterns A comprehensive collection of flow regimes is included in this paper [source] An Analysis of Pressure Drop and Holdup for Liquid-Liquid Upflow through Vertical PipesCHEMICAL ENGINEERING & TECHNOLOGY (CET), Issue 7 2007A. K. Jana Abstract The present study has attempted to investigate pressure drop and holdup during simultaneous flow of two liquids through a vertical pipe. The liquids selected were kerosene and water. The measurements were made for phase velocities varying from 0.05,1.2,m/s for both liquids. The pressure drop was measured with a differential pressure transducer while the quick closing valve (QCV) technique was adopted for the measurement of liquid holdup. The measured holdup and pressure drop were analyzed with suitable theoretical models according to the existing flow patterns. The analysis reveals that the homogeneous model is suitable for dispersed bubbly flow whereas bubbly and churn-turbulent flow pattern is better predicted by the drift flux model. On the other hand, the two fluid flow model accurately predicts the pressure drop in core annular flow. [source] Effects of Liquid Holdup in Condensers on the Start-Up of Reactive Distillation ColumnsCHEMICAL ENGINEERING & TECHNOLOGY (CET), Issue 11 2006H.-X. Wu Abstract Compared with start-ups in conventional distillation columns, those in reactive distillation (RD) columns are much more time and energy consuming, and generate a large amount of by-products which are not easy to deal with together. For several years, researchers have been trying out different methods to shorten the time required to lower the cost of the start-up. In this work, a rigorous dynamic model in the ChemCAD simulator is applied to model the start-up process for the esterification of ethyl acetate in a reactive distillation column. In the model, two sets of equations are employed: one for the fill-up and heating stage and the other for the equilibration process which follows. In the fill-up and heating stage, fluctuation curves of the reboiler temperatures with respect to time which are similar to those for conventional distillation columns are observed, while in the equilibration process it is found that the increase of the liquid holdup volume in the condenser reduces the time required to reach steady state for the reactive column and decreases the liquid holdup volume in the reboiler at the equilibrium state. This shows that the liquid holdup volume in the condenser has an important effect on the start-up of reactive distillation columns. [source] Simple Efficient Contracts in Complex EnvironmentsECONOMETRICA, Issue 3 2008Robert Evans This paper studies a general model of holdup in a setting encompassing the models of Segal (1999) and Che and Hausch (1999) among others. It is shown that if renegotiation is modeled as an infinite-horizon noncooperative bargaining game, then, with a simple initial contract, an efficient equilibrium will generally exist. The contract is robust in the sense that it does not depend on fine details of the model. The contract gives authority to one party to set the terms of trade and gives the other party a nonexpiring option to trade at these terms. The difference from standard results arises because the initial contract ensures that the renegotiation game has multiple equilibria; the multiplicity of continuation equilibria can be used to enforce efficient investment. [source] Prediction of two-phase pressure drop and liquid holdup in co-current gas,liquid downflow of air,Newtonian systems through packed bedsJOURNAL OF CHEMICAL TECHNOLOGY & BIOTECHNOLOGY, Issue 1 2006Ponnan Ettiyappan Jagadeesh Babu Abstract The dependency of pressure drop and liquid holdup on phase velocities, geometry of the column and packing materials as well as on the physical properties have been analyzed. Our experimental data (825 data points obtained using four liquid systems and three different particles) along with those of the available literature (776 data point from five different sources) were used for the analysis. The applicability and the limitations of the literature correlations were evaluated using the available data. Based on the analysis, new correlations for the estimation of pressure drop and liquid holdup, valid for low and high interaction regimes have been developed using the available data, with a wide range of variables. Copyright © 2005 Society of Chemical Industry [source] Minimum fluidization velocity and gas holdup in gas,liquid,solid fluidized bed reactorsJOURNAL OF CHEMICAL TECHNOLOGY & BIOTECHNOLOGY, Issue 2 2002K Ramesh Abstract Experiments were performed to study the hydrodynamics of a cocurrent three-phase fluidized bed with liquid as continuous phase. Based on the 209 experimental data (with four liquid systems and five different particles) along with 115 literature data from six different sources on minimum fluidization velocity, a unique correlation for the estimation of minimum fluidization velocity in two-phase (ug,=,0) as well as in three-phase systems is developed. A data bank consisting of 1420 experimental measurements for the fractional gas phase holdup data with a wide range of variables is used for developing empirical correlations. Separate correlations are developed for two flow regimes observed in this present work. The proposed correlations are more accurate and simpler to use. © 2002 Society of Chemical Industry [source] Effect of a gas,liquid separator on the hydrodynamics and circulation flow regimes in internal-loop airlift reactorsJOURNAL OF CHEMICAL TECHNOLOGY & BIOTECHNOLOGY, Issue 5 2001Jaroslav Klein Abstract The role of the gas,liquid separator on hydrodynamic characteristics in an internal-loop airlift reactor (ALR) was investigated. Both gas holdup and liquid velocity were measured in a 30,dm3 airlift reactor with two different head configurations: with and without an enlarged separator. A magnetic tracer method using a neutrally buoyant magnetic particle as flowfollower was used to measure the liquid velocity in all sections of the internal-loop airlift reactor. Average liquid circulation velocities in the main parts of the ALR were compared for both reactor configurations. At low air flow rates the separator had no influence on gas holdup, circulation velocity and intensity of turbulence in the downcomer and separator. At higher superficial air velocities, however, the separator design had a decisive effect on the hydrodynamic parameters in the downcomer and the separator. On the other hand, the gas holdup in the riser was only slightly influenced by the separator configuration in the whole range of air flow. Circulation flow regimes, characterising the behaviour of bubbles in the downcomer, were identified and the effect of the separator on these regimes was assessed. © 2001 Society of Chemical Industry [source] Experimental validation of CFD simulations of a lab-scale fluidized-bed reactor with and without side-gas injectionAICHE JOURNAL, Issue 6 2010Jian Min Abstract Fluidized-bed reactors are widely used in the biofuel industry for combustion, pyrolysis, and gasification processes. In this work, a lab-scale fluidized-bed reactor without and with side-gas injection and filled with 500,600 ,m glass beads is simulated using the computational fluid dynamics (CFD) code Fluent 6.3, and the results are compared to experimental data obtained using pressure measurements and 3D X-ray computed tomography. An initial grid-dependence CFD study is carried out using 2D simulations, and it is shown that a 4-mm grid resolution is sufficient to capture the time- and spatial-averaged local gas holdup in the lab-scale reactor. Full 3D simulations are then compared with the experimental data on 2D vertical slices through the fluidized bed. Both the experiments and CFD simulations without side-gas injection show that in the cross section of the fluidized bed there are two large off-center symmetric regions in which the gas holdup is larger than in the center of the fluidized bed. The 3D simulations using the Syamlal-O'Brien and Gidaspow drag models predict well the local gas holdup variation throughout the entire fluidized bed when compared to the experimental data. In comparison, simulations with the Wen-Yu drag model generally over predict the local gas holdup. The agreement between experiments and simulations with side-gas injection is generally good, where the side-gas injection simulates the immediate volatilization of biomass. However, the effect of the side-gas injection extends further into the fluidized bed in the experiments as compared to the simulations. Overall the simulations under predict the gas dispersion rate above the side-gas injector. © 2009 American Institute of Chemical Engineers AIChE J, 2010 [source] A mechanistic model for roll waves for two-phase pipe flowAICHE JOURNAL, Issue 11 2009George W. Johnson Abstract A new two-phase roll wave model is compared with data from high pressure two-phase stratified pipe flow experiments. Results from 754 experiments, including mean wave speed, wave height, pressure gradient, holdup and wave length, are compared with theoretical results. The model was able to predict these physical quantities with good accuracy without introducing any new empirically determined quantities to the two-fluid model equations. This was possible by finding the unique theoretical limit for nonlinear roll amplitude and applying a new approach for determining the friction factor at the gas-liquid interface. © 2009 American Institute of Chemical Engineers AIChE J, 2009 [source] Volume-of-fluid-based model for multiphase flow in high-pressure trickle-bed reactor: Optimization of numerical parametersAICHE JOURNAL, Issue 11 2009Rodrigo J. G. Lopes Abstract Aiming to understand the effect of various parameters such as liquid velocity, surface tension, and wetting phenomena, a Volume-of-Fluid (VOF) model was developed to simulate the multiphase flow in high-pressure trickle-bed reactor (TBR). As the accuracy of the simulation is largely dependent on mesh density, different mesh sizes were compared for the hydrodynamic validation of the multiphase flow model. Several model solution parameters comprising different time steps, convergence criteria and discretization schemes were examined to establish model parametric independency results. High-order differencing schemes were found to agree better with the experimental data from the literature given that its formulation includes inherently the minimization of artificial numerical dissipation. The optimum values for the numerical solution parameters were then used to evaluate the hydrodynamic predictions at high-pressure demonstrating the significant influence of the gas flow rate mainly on liquid holdup rather than on two-phase pressure drop and exhibiting hysteresis in both hydrodynamic parameters. Afterwards, the VOF model was applied to evaluate successive radial planes of liquid volume fraction at different packed bed cross-sections. © 2009 American Institute of Chemical Engineers AIChE J, 2009 [source] Magnetic emulation of microgravity for earth-bound multiphase catalytic reactor studies,Potentialities and limitationsAICHE JOURNAL, Issue 5 2009Faïçal Larachi Abstract A method is proposed to generate Earth-bound artificial microgravity in a controlled facility capable of emulating lunar/Martian gravity or microgravity for experiments on passive/reactive catalytic multiphase flows. Its applicability was illustrated for trickle beds where flowing gas and liquid experience artificial microgravity inside the bore of a superconducting magnet generating large gradient magnetic fields to compensate for gravity. Artificial gravity is realized by commuting into apparent gravity acceleration the magnetization force at work on common "chemical engineering" non-magnetic fluids. The scaling property to be matched and maintained invariant in multiphase systems to achieve magnetic mimicry is phasic mass magnetic susceptibility. Hydrodynamic (liquid holdup, wetting efficiency, pressure drop) as well as catalytic reaction (conversion and selectivity) measurements were obtained. The main finding is a proof that magnetic fields affect reactor outcomes exclusively via hydrodynamic phenomena making them appealing proxies for emulating non-terrene reactor applications. © 2009 American Institute of Chemical Engineers AIChE J, 2009 [source] Multiresolution analysis on identification and dynamics of clusters in a circulating fluidized bedAICHE JOURNAL, Issue 3 2009Tung-Yu Yang Abstract A new wavelet-threshold criterion was developed to distinguish the cluster and the void phases from the transient solids holdup/concentration fluctuation signals when measured in a 108 mm-i.d. × 5.75 m-high circulating fluidized bed with FCC particles (dp = 78 ,m, ,p = 1,880 kg/m3). An appropriate level of approximation subsignal was systematically specified as a threshold for cluster identification, based on multiresolution analysis (MRA) of wavelet transformation. By the established threshold, the dynamic properties of clusters including the appearance time fraction of clusters Fcl, average cluster duration time ,cl, cluster frequency fcl, and local average solids holdup in clusters ,sc, at different radial and axial positions were determined under the turbulent, transition and fast fluidization flow regimes. The results also describe the dynamic properties of clusters and flow patterns in the splash zone along with the dense bottom region of the circulating fluidized beds. © 2009 American Institute of Chemical Engineers AIChE J, 2009 [source] Hydrodynamics of upflow anaerobic sludge blanket reactorsAICHE JOURNAL, Issue 2 2009Ting-Ting Ren Abstract The hydrodynamic characteristics of upflow anaerobic sludge blanket (UASB) reactors were investigated in this study. A UASB reactor was visualized as being set-up of a number of continuously stirred tank reactors (CSTRs) in series. An increasing-sized CSTRs (ISC) model was developed to describe the hydrodynamics of such a bioreactor. The gradually increasing tank size in the ISC model implies that the dispersion coefficient decreased along the axial of the UASB reactor and that its hydrodynamic behavior was basically dispersion-controlled. Experimental results from both laboratory-scale H2 -producing and full-scale CH4 -producing UASB reactors were used to validate this model. Simulation results demonstrate that the ISC model was better than the other models in describing the hydrodynamics of the UASB reactors. Moreover, a three-dimensional computational fluid dynamics (CFD) simulation was performed with an Eulerian-Eulerian three-phase-fluid approach to visualize the phase holdup and to explore the flow patterns in UASB reactors. The results from the CFD simulation were comparable with those of the ISC model predictions in terms of the flow patterns and dead zone fractions. The simulation results about the flow field further confirm the discontinuity in the mixing behaviors throughout a UASB reactor. © 2008 American Institute of Chemical Engineers AIChE J, 2009 [source] Effect of hydrodynamic multiplicity on trickle bed reactor performanceAICHE JOURNAL, Issue 1 2008Werner van der Merwe Abstract Multiple hydrodynamic states in trickle bed reactors have been the subject of numerous hydrodynamic investigations. The extent of variation in the hydrodynamic parameters (like holdup and pressure drop) is large and this variation can be expected to have a significant impact on the conversion in a reaction system. This study presents reaction data for ,-methyl styrene hydrogenation in a trickle bed reactor over a range of conditions that include gas and liquid limitations. It is seen that liquid flow rate variation induced hysteresis has a large impact on the conversion. For gas-limited reactions, the upper branch of the pressure drop hysteresis loop has a higher conversion than the lower branch at the same linear fluid velocities and catalyst weight, while for liquid-limited reactions the lower branch has a higher conversion than the upper branch (the difference in productivity being up to 20%). These trends cannot be explained by differences in wetting efficiency. Instead, it is proposed that for this system the gas,liquid mass transfer rate is the limiting step in gas-limited reactions, while the liquid,solid mass transfer rate is the limiting step in liquid-limited reactions. © 2007 American Institute of Chemical Engineers AIChE J, 2008 [source] A CFD,PBM coupled model for gas,liquid flowsAICHE JOURNAL, Issue 1 2006Tiefeng Wang Abstract A computational fluid dynamics,population balance model (CFD-PBM) coupled model was developed that combines the advantages of CFD to calculate the entire flow field and of the PBM to calculate the local bubble size distribution. Bubble coalescence and breakup were taken into account to determine the evolution of the bubble size. Different bubble breakup and coalescence models were compared. An algorithm was proposed for computing the parameters based on the bubble size distribution, including the drag force, transverse lift force, wall lubrication force, turbulent dispersion force, and bubble-induced turbulence. With the bubble breakup and coalescence models and the interphase force formulations in this work, the CFD-PBM coupled model can give a unified description for both the homogeneous and the heterogeneous regimes. Good agreement was obtained with the experimental results for the gas holdup, liquid velocity, and bubble size distribution. © 2005 American Institute of Chemical Engineers AIChE J, 2006 [source] Gas holdup in a three-phase fluidized bedAICHE JOURNAL, Issue 7 2002M. Safoniuk First page of article [source] Gas-solids flow behavior: CFB riser vs. downerAICHE JOURNAL, Issue 9 2001H. Zhang Comparisons are made in a circulating fluidized-bed riser/downer system between a 15.1 m high, 0.10 m ID riser and a 9.3 m high, 0.10 m ID downer, based on the measurements of the radial distributions of the local solids holdups and local particle velocities along the two columns. Although the core-annulus flow structures exist in both the riser and downer, the radial flow structure in the downer differs largely from that in the riser. The radial distributions of solids holdup and particle velocity in the downer are much more uniform than those in the riser, thus ensuring the low back mixing and the narrow particle residence time distribution in the downer. The axial flow structure in the downer is also more uniform than that in the riser. Due to the high particle acceleration and the high particle velocity in the downer, the overall solids holdup is significantly lower than that in the riser. The microflow structure in the downer, characterized by the low intermittency indices, is also more uniform than that in the riser. These key properties of the downer make it a very promising candidate for industrial applications where short reaction times and high product selectivity are required. [source] Hamiltonian view on process systemsAICHE JOURNAL, Issue 8 2001Katalin M. Hangos The thermodynamic approach of analyzing structural stability of process plants was extended to construct the simple Hamiltonian model of lumped process systems. This type of model enables us to design a nonlinear PD feedback controller for passivation and loop shaping. This approach is applicable for lumped process systems where Kirchhoff convective transport takes place together with the transfer and sources of various types, and the manipulable input variables are the flow rates. Systems with constant mass holdup and uniform pressure in every balance volume satisfy these conditions. General results are shown by simple examples of practical importance: on a bilinear heat exchanger cell and on an isotherm CSTR with nonlinear reaction. [source] Effect of chemical kinetics on feasible splits for reactive distillationAICHE JOURNAL, Issue 3 2001Nitin Chadda Feasible direct and indirect sharp splits for multicomponent single-feed continuous reactive distillation are predicted with a model, in which each column section is represented by a series of cocurrent isobaric flashes. In the limits of no reaction and equilibrium chemical reaction, the model reduces to conventional models for distillation lines, and each column section can be represented by the same equations. At intermediate reaction rates, however, the models for the column sections differ, and new results for fixed points and feasible products are obtained. A bifurcation study shows the limits of feasibility, including the influence of flow rate, catalyst level and holdup. Unlike distillation without reaction, limited ranges of feasibility in all of these variables are found. The method has been applied to five examples, one of which is described in detail. Feasibility predictions are validated by column simulations. [source] Phase distributions in a gas,liquid,solid circulating fluidized bed riserTHE CANADIAN JOURNAL OF CHEMICAL ENGINEERING, Issue 4 2010S. A. Razzak Abstract The distributions of the three phases in gas,liquid,solid circulating fluidized beds (GLSCFB) were studied using a novel measurement technique that combines electrical resistance tomography (ERT) and optical fibre probe. The introduction of gas into a liquid,solid circulating fluidized bed (LSCFB), thus forming a GLSCFB, caused the increase of solids holdup due to the significantly decreased available buoyancy with the lower density of the gas, even with a somewhat increased liquid velocity due to the decreased liquid holdup giving space for the gas holdup. The gas passed through the riser in the form of bubbles, which tended to flow more through the central region of the riser, leading to more radial non-uniformity in radial holdup of the phases. The gas velocity has the most significant effect on the gas phase holdup. While the gas velocity also has an obvious effect to the solids holdups, the liquid flow rate had a much more considerable effect on the phase holdups. The solids circulation rate also had a significant effect on the phase holdups, with increasing solids circulation rate causing much more increased solids holdup in the central region than close to the wall. A correlation was developed for the relative radial distributions of solids holdup in GLSCFB, as such radial profiles were found similar over a wide range of operating conditions, like those in a typical gas,solid circulating fluidized beds (GSCFB). Finally, the axial solids profiles in a GLSCFB was found to be much closer to those in an LSCFB which are very uniform, than those found in a GSCFB which are less uniform and sometime having a S shape. Water was used as the continuous and conductive phase, air was the gas phase and glass bead and lava rock particles were used as the solid and non-conductive phase. Les distributions des trois phases des lits fluidisés circulants gaz-liquide-solide (LFCGLS) ont été étudiées à l'aide d'une technique de mesure originale qui combine la tomographie à résistance électrique (ERT) et la sonde à fibre optique. L'introduction de gaz dans un lit fluidisé circulant liquide-solide, formant par conséquent un lit fluidisé circulant gaz-liquide-solide, a provoqué une retenue de solides en raison de la diminution importante de la flottabilité disponible avec la densité moindre du gaz, même avec une vitesse de liquide quelque peu accrue causée par la retenue de liquide diminuée qui donne de l'espace pour la retenue de gaz. Le gaz passé dans la colonne de montée sous forme de bulles, qui avaient tendance à circuler davantage dans la région centrale de la colonne, donne une non-uniformité plus radiale dans la retenue radiale des phases. La vitesse du gaz a aussi un effet évident sur les retenues des solides; le débit du liquide a un effet beaucoup plus considérable sur les retenues des phases. Le taux de circulation des solides avait également un effet important sur les retenues des phases, le taux de circulation des solides croissant causant beaucoup plus de retenues de solides accrues dans la région centrale que près du mur. Une corrélation a été créée pour les distributions radiales relatives de la retenue des solides dans le LFCGLS, puisque de tels profils radiaux ont été trouvés semblables sur une vaste gamme de conditions d'utilisation, comme celles dans un LFCGS typique. Enfin, on a découvert que les profils des solides axiaux dans un LFCGLS sont beaucoup plus près que ceux d'un LFCLS qui sont très uniformes, que ceux découverts dans un LFCGS, qui sont moins uniformes et qui ont à peu près la forme d'un « S ». [source] Bubble shape, gas flow and gas,liquid mass transfer in pulp fibre suspensionsTHE CANADIAN JOURNAL OF CHEMICAL ENGINEERING, Issue 3 2010L. K. Ishkintana Abstract Gas,liquid mass transfer in pulp fibre suspensions in a batch-operated bubble column is explained by observations of bubble size and shape made in a 2D column. Two pulp fibre suspensions (hardwood and softwood kraft) were studied over a range of suspension mass concentrations and gas flow rates. For a given gas flow rate, bubble size was found to increase as suspension concentration increased, moving from smaller spherical/elliptical bubbles to larger spherical-capped/dimpled-elliptical bubbles. At relatively low mass concentrations (Cm,=,2,3% for the softwood and Cm,,,7% for the hardwood pulp) distinct bubbles were no longer observed in the suspension. Instead, a network of channels formed through which gas flowed. In the bubble column, the volumetric gas,liquid mass transfer rate, kLa, decreased with increasing suspension concentration. From the 2D studies, this occurred as bubble size and rise velocity increased, which would decrease overall bubble surface area and gas holdup in the column. A minimum in kLa occurred between Cm,=,2% and 4% which depended on pulp type and was reached near the mass concentration where the flow channels first formed. le transfert de masse gaz-liquide dans des suspensions de fibres de pulpe, dans une colonne à bulles de traitement en lot, est expliqué par des observations faites dans une colonne 2D de la taille et de la forme des bulles. Deux suspensions de fibres de pulpe (pulpes kraft de bois dur et de bois tendre) ont été étudiées sur un intervalle de concentrations en masse des suspensions et sur un intervalle de débits de gaz. Pour un débit de gaz donné, on a observé que la taille des bulles augmentait avec l'augmentation de la concentration de la suspension, passant de petites bulles sphériques/elliptiques à des bulles plus grosses de forme quasi-sphérique avec capuchon à elliptique avec dépression. À des concentrations en masse relativement basses (Cm,=,2,3% pour le bois tendre et Cm,=,,7% pour la pulpe de bois dur), des bulles distinctes n'étaient plus observées dans la suspension. Au lieu de cela, un réseau de canaux se formait, au travers duquel le gaz s'écoulait. Dans la colonne à bulles, le taux de transfert de masse volumétrique gaz-liquide, kLa, diminuait avec l'augmentation de la concentration de la suspension. À partir des études 2D, cela se produisait lorsque la taille des bulles et la vélocité ascendante augmentaient, ce qui devrait faire diminuer la surface d'ensemble des bulles et la retenue de gaz dans la colonne. Un minimum de kLa a été observé avec Cm,=,2% et 4% (selon le type de pulpe) et était atteint à proximité de la concentration en masse pour laquelle les canaux d'écoulement commençaient à se former. [source] Local Bubble Dynamics and Macroscopic Flow Structure in Bubble Columns with Different ScalesTHE CANADIAN JOURNAL OF CHEMICAL ENGINEERING, Issue 6 2003Wei Chen Abstract Local bubble behaviours were investigated in three bubble columns with different diameters of 200, 400 and 800 mm. By means of a novel single-tip optical fibre probe employing laser Doppler technique, the local gas holdup, bubble frequency, bubble size and velocity were measured simultaneously at different locations of the columns. Measurements were performed in air-water system at superficial gas velocities up to 90 mm/s. The averaged profiles and instantaneous measurements were analyzed and compared for different columns. The presence of a coherent gross circulation structure spanning the entire column diameter in the larger column rather than a pair of symmetrical circulation cells observed in the smaller columns has been confirmed. Les comportements locaux des bulles ont été étudiés dans trois colonnes à bulles de diamètres différents, soient 200, 400 et 800 mm. Au moyen d'une nouvelle sonde à fibre optique à embout unique employant la technique laser Doppler, des mesures locales de la rétention de gaz, de la fréquence des bulles, de la taille des bulles et de la vitesse ont été effectuées simultanément à différents endroits dans les colonnes. Ces mesures ont été effectuées pour un système air-eau à des vitesses de gaz superficielles atteignant 90 mm/s. Les profils moyennés et les mesures instantanées ont été analysés puis comparés pour différentes colonnes. La présence d'une structure de circulation unique cohérente occupant tout le diamètre de la colonne dans la colonne la plus large plutôt qu'une paire de cellules de circulation symétriques observées dans les colonnes plus petites est confirmée. [source] A Method for the Detection of Defluidized Zones in Slurry Bubble ColumnsTHE CANADIAN JOURNAL OF CHEMICAL ENGINEERING, Issue 3-4 2003Ahad 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] Intensification of Slurry Bubble Columns by Vibration ExcitementTHE CANADIAN JOURNAL OF CHEMICAL ENGINEERING, Issue 3-4 2003Jürg Ellenberger Abstract We show that application of low-frequency vibrations, in the 30 to 60 Hz range, to the liquid phase of an air , water , silica catalyst slurry bubble column causes significant enhancement of both gas holdup and volumetric mass transfer coefficient over a wide range of superficial gas velocities. The increase in the gas holdup is attributed mainly to a significant reduction in the rise velocity of the bubble swarm due to the generation of standing waves in the column. Furthermore, application of vibrations to the liquid phase serves to stabilize the homogenous bubbly flow regime and delay the onset of the churn-turbulent flow regime. On montre que l'application de vibrations de faibles fréquences (entre 30 et 60 Hz) à la phase liquide d'une colonne à bulles à suspensions de catalyseur air-eau-silice, permet une amélioration significative à la fois du coefficient de rétention de gaz et du coefficient du transfert de matière volumétrique pour une vaste gamme de vitesses de gaz superficielles. L'augmentation de la rétention de gaz est imputée principalement à une réduction importante de la vitesse de montée de l'essaim de bulles qui est due à la création de vagues stationnaires dans la colonne. En outre, l'application des vibrations à la phase liquide sert à stabiliser le régime d'écoulement à bulles homogène et retarde l'apparition du régime d'écoulement agité-pistonnant. [source] Effect of Gas Density on the Hydrodynamics of Bubble Columns and Three-Phase Fluidized BedsTHE CANADIAN JOURNAL OF CHEMICAL ENGINEERING, Issue 3-4 2003Arturo Macchi Abstract Experiments were performed at ambient temperature and pressure in a 127 mm inner diameter column with a 55% wt. aqueous glycerol solution, 6-mm spherical borosilicate beads and four gases , helium, air, carbon dioxide and sulphur hexafluoride , giving a 35-fold gas density range. The dispersed bubble flow regime was sustained to higher gas velocities and gas holdups for denser gases. This finding appears to be due to the reduction of the maximum stable bubble size (i.e. enhanced bubble break-up), rather than to formation of smaller bubbles at the distributor with increasing gas density. The effect of gas density was significant both with and without the particles present, with gas holdup increasing, bed voidage increasing and liquid holdup decreasing with increasing gas density. The holdup correlations of Han et al. (1990) have been modified to incorporate the effect of gas density. On a mené des expériences à la température et à la pression ambiante dans une colonne de 127 mm de diamètre intérieur avec une solution de glycérol aqueux de 55 % en poids, des billes de verre de borosilicate sphériques de 6 mm et quatre gaz , hélium, air, gaz carbonique et hexafluorure de soufre , donnant une gamme de densité gazeuse multipliée par 35. Le régime d'écoulement bouillonnant dispersé est maintenu jusqu'à des vitesses et des rétentions de gaz supérieures pour les gaz plus denses. Ce résultat semble être dû à la réduction de la taille de bulle stable maximum (c.-à-d. une rupture de bulles améliorée), plutôt qu'à la formation de bulles plus petites au distributeur avec l'augmentation de la densité gazeuse. L'effet de la densité gazeuse est significatif avec ou sans la présence des particules, avec l'augmentation de la rétention de gaz, l'augmentation du vide du lit et la diminution de la rétention de liquide avec l'augmentation de la densité gazeuse. Les corrélations de rétention de Han et al. (1990) sont modifiées de sorte à introduire l'effet de la densité gazeuse. [source] Studies on Liquid,Gas and Three-Phase Fluidized Beds with Pulsating Air FlowsTHE CANADIAN JOURNAL OF CHEMICAL ENGINEERING, Issue 1 2003Christopher G. J. Baker Abstract The effects of air-flow pulsation and water and air flowrates on the hydrodynamics of liquid,gas and three-phase fluidized beds containing 3-mm glass beads have been studied in a 90-mm i.d. column. Under steady-flow conditions, both types of bed contained a relatively large number of small bubbles. With a pulsing air flow, however, a smaller number of much larger bubbles or slugs were formed. This was attributed to different mechanisms of bubble formation at the distributor. Variations in phase holdup were explained in terms of the effects of the operating parameters on the bubble characteristics. On a étudié dans une colonne de 90 mm de diamètre intérieur les effets de la pulsation air-écoulement et des débits d'eau et d'air sur l' hydrodynamique de lits fluidisés gaz,liquide et triphasiques contenant des billes de verre de 3 mm. Dans des conditions d'écoulement en régime permanent, les deux types de lit contenaient un nombre relativement important de petites bulles. Toutefois, avec un écoulement à air pulsé, il s 'est formé un moins grand nombre de bulles de très grande dimension ou de bouchons. Ceci est imputable aux différents mécanismes de formation des bulles au niveau du distributeur. Les variations dans la rétention des phases sont expliquées par les effets des paramètres opératoires sur les caractéristiques des bulles. [source] Modified volume expansion method for measuring gas holdupTHE CANADIAN JOURNAL OF CHEMICAL ENGINEERING, Issue 2 2002Annie X. Meng Abstract A simple, modified volume expansion method, or inclined tube method, is compared to the pressure difference method for determining gas holdup in an airlift bioreactor. The modified volume expansion method could be used for all pneumatic bioreactors where fluid fluctuation is vigorous and visual observation of the continuous phase rise is difficult. The overall gas holdup data measured using the inclined tube method are shown to be very close to overall gas holdup determined using a gamma ray density monitor system. However, the overall gas holdup measured by the pressure difference method is found to be significantly different. This difference is due to energy dissipation in the External Loop Airlift Bioreactor (ELAB) used in this study, which causes the pressure difference method to be incorrect. On compare une méthode d'expansion de volume modifiée simple, ou méthode à tubes inclinés, à une méthode de différence de pression pour déterminer la rétention des gaz dans un bioréacteur à air ascendant. La méthode d'expansion de volume modifiée pourrait être utilisée pour tous les bioréacteurs pneumatiques oú la fluctuation du fluide est vigoureuse et l'observation visuelle de l'ascension de la phase continue difficile. Les données globales de rétention de gaz mesurées à l'aide de la méthode à tubes inclinés s'avèrent très proches de la rétention de gaz globale déterminéd à l'aide d'un moniteur de densité à rayons gamma. Toutefois, on a trouvé que la rétention de gaz globale mesurée par la méthode de différence de pression était significativement différente. Cette différence est due à la dissipation d'énergie dans le bioréacteur à air ascendant à boucle externe (ELAB) utilisé dans I'étude, qui rend la méthode de différence de pression incorrecte. [source] Radioisotope tracer study in trickle bed reactorsTHE CANADIAN JOURNAL OF CHEMICAL ENGINEERING, Issue 6 2001Krishna Deo Prasad Nigam Abstract The residence time distribution (RTD) of liquid phase in trickle bed reactors has been measured for air-water system using radioisotope tracer technique. Experiments were carried out in a glass column of internal diameter of 0.152 m packed with glass beads and actual catalyst particles of two different shapes. From the measured RTD curves, mean residence time of liquid was calculated and used to estimate liquid holdup. The axial dispersion model was used to simulate the experimental data and estimate mixing index, ie. Peclet number. The effect of liquid and gas flow rates on total liquid holdup and Peclet number has been investigated. Results of the study indicated that shape of the packing has significant effect on holdup and axial dispersion. Bodenstein number has been correlated to Reynolds number, Galileo number, shape and size of the packing. La distribution de temps de séjour (DTS) de la phase dans des réacteurs a lits ruisselants a été measurée pour le système air-eau à d'une technique par traceurs radio-isotopes. Des expériences ont été menées dans une colonne de verre de 0,152 m de diamètre intérieur garnie de billes de verre et de particules de catalyseur réelles de deux formes différentes. à partir des courbes de DTS mesurées, ie temps de séjour moyen du liquide a été calculé puls utilisé pour l'estimation de la rétention de liquide. On a utilisé. On a utilisé le modèl de dispersion axiale afin de simuler les données expérimentales et d'estimer l'indice de mélange, soit le nombre de Peclet. On a étudié l'effet des débits de liquide et de gaz sur la rétention de liquide totale et le nombre de Peclet. Les résultats de l'étude indiquent que la forme du garnissage a un effet significatif sur la rétention et la dispersion axiale. Le nombre de Bodenstein a été corrélé au nombre de Reynolds, au nombre de Galilée, ainsi qu'à la forme et à la taille du garnissage. [source] Effect of natural surfactants released from athabasca oil sands on air holdup in a water columnTHE CANADIAN JOURNAL OF CHEMICAL ENGINEERING, Issue 4 2000Zhiang Zhou Abstract Natural surfactants released from Athabasca oil sands are a crucial component in water-based bitumen extraction processes. The effect of natural surfactant released from estuarine and marine oil sands on air holdup was investigated using a water column. The air holdup in the resultant supernatant of the conditioned oil sands slurry was found to be much higher than that in de-ionized water. A further increase in air holdup was observed with the supernatant obtained from the oil sands slurry conditioned with caustic. Aging of the ore prior to bitumen extraction caused an increase in air holdup. The presence of small amount of fine solids in the supernatant resulted in a higher air holdup for all the cases studied. The present study shows that the higher air holdup and poorer processibility of marine ores, compared with estuarine ores, could be accounted for by the release of larger amount of surface active species and the presence of more fine clays in the ores. Les surfactants naturels issus de sables pétrolifères d'Athabasca sont une composante essentielle des precédés d'extraction du bitume bases sur l'eau. On a étudié à l'aide d'une colonne à eau l'effet sur la rétention d'air de surfactants naturels de sables pétrolifères de mer et d'estuaire. La rétention d'air des boues de sables pétrolifères conditionnées dans le sumageant qui en résulte s'avère nettement superieure à celle constatée dans l'eau déionisée. Une rétention d'air encore plus grande est observée avec le surnageant issu de boues de sables pétrolifères conditionnées avec de la soude caustique. Le vieillissement des minerals avant l'extraction du bitume entraîne un accroissement de la rétention d'air. La présence d'une petite quantité de fines solides dans le surnageant entraîne une plus grande rétention d'air dans tous les cas étudiés. Cette étude montre que la plus grande rétention d'air et l'aptitude moins favorable à l'exploitation des minerals marins, par rapport aux minerals d'estuaire, peut s'expliquer par la production de plus grandes quantités d'espèces surfactives et la présence d'argiles plus fines dans les minerals. [source] | |||||||||||||