Liquid Holdup (liquid + holdup)

Distribution by Scientific Domains

Selected Abstracts

Effects of Liquid Holdup in Condensers on the Start-Up of Reactive Distillation Columns

H.-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]

Prediction of two-phase pressure drop and liquid holdup in co-current gas,liquid downflow of air,Newtonian systems through packed beds

Ponnan 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]

Volume-of-fluid-based model for multiphase flow in high-pressure trickle-bed reactor: Optimization of numerical parameters

AICHE JOURNAL, Issue 11 2009
Rodrigo 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 limitations

AICHE JOURNAL, Issue 5 2009
Faïçal 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]

Effect of Gas Density on the Hydrodynamics of Bubble Columns and Three-Phase Fluidized Beds

Arturo 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]

Radioisotope tracer study in trickle bed reactors

Krishna 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]

Momentum Balance for Two-Phase Horizontal Pipe Flow Part 1: Friction Factors

P. L. Spedding
Estimations of gas wall, liquid wall and interfacial friction factors for two-phase horizontal co-current pipe flow are discussed critically after being checked against reliable data obtained under a wide range of conditions. The use of equivalent diameters and the Blasius relation were shown to be valid for estimation of the gas wall friction. Prediction of liquid wall and interfacial friction factors proved to be more difficult but estimation improved if consideration was given to the effects of liquid holdup and interfacial liquid shape. [source]

Holdup and Pressure Drop in Vertical and Near-Vertical Three-Phase Up-Flow: A Collection of Flow Regimes

P.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]

Hairy Root Culture in a Liquid-Dispersed Bioreactor: Characterization of Spatial Heterogeneity

Gary R. C. Williams
A liquid-dispersed reactor equipped with a vertical mesh cylinder for inoculum support was developed for culture of Atropa belladonna hairy roots. The working volume of the culture vessel was 4.4 L with an aspect ratio of 1.7. Medium was dispersed as a spray onto the top of the root bed, and the roots grew radially outward from the central mesh cylinder to the vessel wall. Significant benefits in terms of liquid drainage and reduced interstitial liquid holdup were obtained using a vertical rather than horizontal support structure for the biomass and by operating the reactor with cocurrent air and liquid flow. With root growth, a pattern of spatial heterogeneity developed in the vessel. Higher local biomass densities, lower volumes of interstitial liquid, lower sugar concentrations, and higher root atropine contents were found in the upper sections of the root bed compared with the lower sections, suggesting a greater level of metabolic activity toward the top of the reactor. Although gas-liquid oxygen transfer to the spray droplets was very rapid, there was evidence of significant oxygen limitations in the reactor. Substantial volumes of non-free-draining interstitial liquid accumulated in the root bed. Roots near the bottom of the vessel trapped up to 3,4 times their own weight in liquid, thus eliminating the advantages of improved contact with the gas phase offered by liquid-dispersed culture systems. Local nutrient and product concentrations in the non-free-draining liquid were significantly different from those in the bulk medium, indicating poor liquid mixing within the root bed. Oxygen enrichment of the gas phase improved neither growth nor atropine production, highlighting the greater importance of liquid-solid compared with gas-liquid oxygen transfer resistance. The absence of mechanical or pneumatic agitation and the tendency of the root bed to accumulate liquid and impede drainage were identified as the major limitations to reactor performance. Improved reactor operating strategies and selection or development of root lines offering minimal resistance to liquid flow and low liquid retention characteristics are possible solutions to these problems. [source]

An Analysis of Pressure Drop and Holdup for Liquid-Liquid Upflow through Vertical Pipes

A. 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]

Liquid-Liquid Stratified Flow through Horizontal Conduits

T. Sunder Raj
Abstract The stratified configuration is one of the basic and most important distributions during two phase flow through horizontal pipes. A number of studies have been carried out to understand gas-liquid stratified flows. However, not much is known regarding the simultaneous flow of two immiscible liquids. There is no guarantee that the information available for gas-liquid cases can be extended to liquid-liquid flows. Therefore, the present work attempts a detailed investigation of liquid-liquid stratified flow through horizontal conduits. Gas-liquid flow exhibits either smooth or wavy stratified orientations, while liquid-liquid flow exhibits other distinct stratified patterns like three layer flow, oil dispersed in water, and water flow, etc. Due to this, regime maps and transition equations available for predicting the regimes in gas-liquid flow cannot be extended for liquid-liquid cases by merely substituting phase physical properties in the equations. Further efforts have been made to estimate the in-situ liquid holdup from experiments and theory. The analysis considers the pronounced effect of surface tension, and attempts to modify the Taitel-Dukler model to account for the curved interface observed in these cases. The curved interface model of Brauner has been validated with experimental data from the present work and those reported in literature. It gives a better prediction of liquid holdup in oil-water flows and reduces to the Taitel-Dukler model for air-water systems. [source]