Correlations For Gas Holdup Research Articles

The effect of vessel diameter on time dependent gas hold-up variations in highly viscous impeller agitated liquids

Time dependent gas hold-up generated in the 0.3 and 0.6 m diameter vessels using high viscosity castor oil and carboxy methyl cellulose (CMC) solution was compared on the basis of impeller speed (N) and gas velocity (V G). Two types of hold-up were distinguished—the hold-up due to tiny bubbles (ε ft) and total hold-up (ε f), which included large and tiny bubbles. It was noted that vessel diameter (i.e. the scale of operation) significantly influences (i) the trends and the values of ε f and ε ft, and (ii) the values of τ (a constant reflecting the time dependency of hold-up). The results showed that a scale independent correlation for gas hold-up of the form ε f or ε ft=A(N or P G/V) a(V G) b, where “a” and “b” are positive constants is not appropriate for viscous liquids. This warrants further investigations into the effect of vessel diameter on gas hold-up in impeller agitated high viscosity liquids (μ or μ a>0.4 Pa s).

Development of an artificial neural network correlation for prediction of overall gas holdup in bubble column reactors

In the literature, several correlations have been proposed for gas holdup prediction in bubble columns. However, these correlations fail to predict gas holdup over a wide range of conditions. Based on a databank of around 3500 measurements collected from the open literature, a correlation for gas holdup was derived using a combination of Dimensional Analysis and artificial neural network (ANN) modeling. The overall gas holdup was found to be a function of four dimensionless groups: Re g, Fr g, Eo/ Mo, and ρ g/ ρ L. Statistical analysis showed that the proposed correlation has an average absolute relative error (AARE) of 15% and a standard deviation of 14%. A comparison with selected correlations in the literature showed that the developed ANN correlation noticeably improved prediction of overall gas holdup. The developed correlation also shows better prediction over a wide range of operating conditions, physical properties, and column diameters, and it predicts properly the trend of the effect of the operating and design parameters on overall gas holdup.

Mass transfer and hydrodynamic characteristics in a co-current downflow contacting column

Hydrodynamic and mass transfer characteristics of water–air system in a co-current downflow contacting column (CDCC) were studied for various nozzle diameters at different superficial gas velocities and liquid re-circulation rates. Gas hold-up and liquid-side mass transfer coefficient increased with increasing superficial gas velocity and liquid flow rate but decreased with increasing nozzle diameter. It is shown that correlations developed, which are based on liquid kinetic power per liquid volume present in the column, and superficial gas velocity explains gas hold-up and the mass transfer coefficient within an error 20% for all gas and liquid flow rates and nozzle diameters used. The constants of correlations for gas hold-up and mass transfer coefficient were found to be considerably different from other gas–liquid contacting systems. © 2003 Society of Chemical Industry

Influence of Hydrodynamic Flow Regimes on the Prediction of Gas Hold-up and Liquid Circulation in Airlift Reactors

The influence of the recently identified hydrodynamic flow regimes on the prediction of gas hold-up and liquid circulation velocity in airlift reactors has been investigated. Some hydrodynamic models based on momentum balance are considered. Experiments were conducted on a pilot plant, external loop airlift reactor using air-water system. The generalised equation of Joshi and Lali for the homogeneous regime predicts the overall gas hold-up in this region with satisfactory accuracy and the difference between the experimental and predicted values is lower than ± 8%. Hsu and Dudukovic model predicts the liquid circulation velocity based on the gas hold-up correlations proposed for the homogenous, transition and heterogeneous flow regimes and two-phase friction factor with satisfactory accuracy (within 4.9 %). The results also show that the friction factor has pronounced effect on the model predictions. It would appear, also that the characterisation of various flow regimes has profound effect on the predictions of these important design parameters. Superposition approach is implemented to develop a correlation for the prediction of the axial gas hold-up in the riser as a function of the height and superficial gas velocity. The proposed correlation gives an average error of 4.7%.

Hydrodynamic studies in both bi-dimensional and three-dimensional bubble columns with a single sparger

Gas holdup in highly viscous fluid in two-phase bubble column with a single sparger operated continuously with respect to the gas flow and batch wise with respect to the liquid was investigated using bed expansion technique. Two- and three-dimensional columns of the same effective cross-sectional area were investigated. Liquid viscosity varied between 0.063 and 0.320 Pa s, static bed heights between 0.12 and 0.60 m, surface tension between 0.0248 and 0.035 N m −1 and density between 906 and 928 kg m −3. Columns were injected with air at superficial gas velocities between 0.0018 and 0.290 m s −1. A single nozzle sparger of 0.01 m diameter was used. The behavior of bubbles in two-dimensional bubble column was found to fall into the bubble flow regime, whereas the behavior in three-dimensional column was found to fall into the churn turbulent regime. Two correlations for gas holdup in the two- and three-dimensional bubble columns were proposed and compared with correlations listed in the literature.

Prediction of gas hold‐up in a bubble column filled with pure and binary liquids

AbstractA dimensionless correlation for gas hold‐up in a bubble column filled with pure liquids and binary mixtures has been developed. The criteria of bubble coalescence in pure liquids and foamability of binary mixtures have been included successfully in the proposed correlation. For the first time, comprehensive data of gas hold‐up with respect to concentration in four binary systems are presented. The often‐reported enhancement of gas hold‐up in aqueous alcohol mixtures has been observed experimentally. The proposed correlation is able to predict successfully the trend of gas hold‐up enhancement with respect to concentration. The estimated values are found to be within 7% of the measured values.

Studies of Gas Holdup in Bubble Column Reactors

Gas holdup in bubble columns has been investigated over a wide range of operational and geometrical parameters. A criterion has been developed for the prediction of the transitional velocity from the homogeneous to heterogencous flow regime. Correlations for gas holdup in both regimes are developed and verified against experimental data.

Heat and mass transfer modeling during the formation and ascension of superheated bubbles

A previously developed model for the heat and mass transfer of a single superheated bubble during the ascension stage was extended to include the formation stage. It allows variable properties and bubble radius changes, solving the gas conservation equations coupled to a bubble dynamics model. Its results were used to predict the existing experimental data in a direct contact evaporator with good agreement. A correction factor for isothermal gas hold-up correlations can be fully calculated by the model enabling good prediction of gas hold-up. The constant property assumption overestimates the gas hold-up and should not be used. Experimental data for steam bubbling process could be reasonably simulated using the superheated bubble model with some additional assumptions.

Simultaneous heat and mass transfer during the ascension of superheated bubbles

A model has been developed for the simultaneous heat and mass transfer during the ascension of superheated bubbles in a liquid column, allowing variable gas physical properties and bubble radius. A correction factor for isothermal gas hold-up correlations, based on bubble model results, has been derived in order to apply them to nonisothermal systems. Results for the bubble volume and the evaporated mass per bubble are used in the prediction of experimental data on gas hold-up and vaporization rate obtained in a direct contact evaporator with fairly good agreement. It has been shown that the constant property and constant bubble radius assumptions lead to overestimation of the vaporization rate and gas hold-up.

Gas holdup in homogeneous and heterogeneous gas—liquid bubble column reactors

AbstractThe velocity‐holdup relationship is the most important design parameter for gas—liquid bubble column reactors, providing the basis for the prediction of heat and mass transfer coefficients and information on hydrodynamic conditions. A summary of the literature on gas holdup in bubble columns is supplemented by new experimental results which extend the data range. A criterion for the gas velocity leading to the transition between homogeneous and heterogeneous regimes for perforated plate gas distributors has been developed. Correlations for gas holdup in both regimes are developed and verified against both new and existing data.

MIXING CHARACTERISTICS IN SLURRY STIRRED TANK REACTORS WITH MULTIPLE IMPELLERS

Hydrodynamic parameters such as power consumption, gas holdup, critical impeller speed for solid suspension and mixing time were measured in slurry stirred tank reactors with multiple impellers. The experiments were mainly conducted in a stirred tank of 0.2mi.d. with baffles. It contained two four-pitched blade downflow turbines for gas dispersion and one Pfaudler type impeller for solid suspension. As a part of scaling studies, additional experiments were also carried out in a larger stirred tank reactor (0,8m i.d.) geometrically similar to the smaller one. Glass beads and polymeric particles were used as a solid phase. Solid concentration was in the range of 0-20% (K/K). Tap water and methanol were used as a liquid phase The power consumption decreased due to an introduction of gas and the presence of solids caused a decrease in the extent of reduction in power consumption. A correlation for power consumption in aerated slurry systems was proposed, It was found that the presence of solids is responsible for a decrease in gas holdup. A new correlation for gas holdup in gas-liquid-solid three-phase stirred tank reactors was developed. It fit the present experimental data reasonably. The critical impeller speed for solid suspensions increased with increasing gas flow rate. However, its increase was rather smaller as compared with the predictions of the correlations available in the literature. We proposed a correlation of the critical impeller speed for solid suspension in the presence of gas. The mixing time complicatedly increased or decreased depending on gas flow rate, impeller speed, solids type and concentration.

Hydrodynamic Flow Regimes, Gas Holdup, and Liquid Circulation in Airlift Reactors

This study reports an experimental investigation into the hydrodynamic behavior of an external-loop airlift reactor (ALR) for the air−water system. Three distinct flow regimes are identifiednamely homogeneous, transition, and heterogeneous regimes. The transition between homogeneous and heterogeneous flow is observed to occur over a wide range rather than being merely a single point as has been previously reported in the literature. A gas holdup correlation is developed for each flow regime. The correlations fit the experimental gas holdup data with very good accuracy (within ±5%). It would appear, therefore, that a deterministic equation to describe each flow regime is likely to exist in ALRs. This equation is a function of the reactor geometry and the system's physical properties. New data concerning the axial variation of gas holdup is reported in which a minimum value is observed. This phenomenon is discussed and an explanation offered. Discrimination between two sound theoretical modelsnamely model I (Chisti et al., 1988) and model II (Garcia Calvo, 1989)shows that model I predicts satisfactorily the liquid circulation velocity with an error of less than ±10%. The good predictive features of model I may be due to the fact that it allows for a significant energy dissipation by wakes behind bubbles. Model I is now further improved by the new gas holdup correlations which are derived for the three different flow regimes.

Gas Holdup and Volumetric Liquid-Phase Mass Transfer Coefficient in a Spout-Fluid Bed.

The effects of gas velocity, liquid velocity and nozzle diameter on the gas holdup and the volumetric liquid phase mass transfer coefficient, kLa, were studied a experimentally in a spout-fluid bed by absorption of oxygen in air into tap water. Gas holdup was found to increase with increasing gas velocity and to decrease with increasing liquid velocity and not to depend on nozzle diameter. kLa was found to increase with increasing gas velocity and liquid velocity and to decrease with increasing nozzle diameter. kLa was found to be strongly dependent on the liquid velocity. Two correlations for gas holdup and kLa in spout-fluid bed were presented with maximum deviation of 8% and 7% respectively.

Modeling Hydrodynamics And Mass Transfer Parameters In A Continuous Ozone Bubble Column

Abstract A computer model based on the establishment of mass balance equations and on the model of fluids flow “stirred tank in series” was developed in order to calculate the ozone transfer coefficient kLa and kinetic constant kc of ozone consumption by water. On the basis of experimental data, the correlation for gas holdup eg and bubble diameter dvs, were proposed and used to calculate the specific interfacial area a. The liquid-phase mass transfer coefficient kL for ozone was evaluated from a and the kLa data.

Mass transfer in different flow regimes of three-phase fluidized beds

The volumetric gas-liquid mass transfer coefficient ( K L a) and axial dispersion coefficient ( D z ) of liquid phase have been studied in air-oxygen saturated water fluidized beds of 0.52 to 0.755 mm glass spheres in a plexiglass column. The effects of liquid velocity, gas velocity, particle size and loadings on K L a and D z were examined. According to the different correlations of mass transfer, three flow regimes of three-phase fluidized beds were distinguished. It was found that K L a and D z change with different regularities in these different flow regimes. The results have shown that the flow regimes distinguished by different correlations of mass transfer were substantially identical with those distinguished by gas holdup correlations as well as pressure fluctuation analysis as had been found in previous investigations.

Bubble Characteristics in the Buoyancy Region of a Vertical Bubbling Jet.

The flow field in a vertical bubbling jet in a cylindrical vessel with bottom blowing can be divided into four regions. They are named the momentum, transition, buoyancy and surface regions in order of increasing distance from the nozzle exit. In a previous paper, bubble behavior characterized by gas holdup, bubble frequency, bubble rising velocity and so on was clarified for the momentum region in which the inertia force of injected gas played an important role. In the present study, bubble behavior in the buoyancy region in which the buoyancy force of bubbles governed the flow was investigated by means of two kinds of electro-resistivity porbes, a high-speed video camera, and a laser Doppler velocimeter. The bubble behavior was not affected by the nozzle diameter and the bath diameter. Therefore, correlations for gas holdup, bubble frequency and bubble rising velocity were proposed as functions of gas flow rate.

Hydrodynamics and mixing in down flow jet loop bioreactor with a non-Newtonian fluid

Gas holdup and liquid circulation time were measured in a down flow jet loop bioreactor with a non-Newtonian fluid. It was observed that the circulation time decreases with increase in nozzle diameter, draft tube to column diameter ratio and shear thinning of the media. The gas holdup increases with increase in gas and liquid velocities. The optimum draft tube to column diameter ratio was found to be 0.438. Correlations for gas holdup and circulation time involving operational and geometrical variables were presented.

Hydrodynamics in down flow jet loop reactor

AbstractGas holdup and liquid circulation time were measured in a down flow jet loop reactor for air–water system. It was observed that the circulation time decreases with an increase in nozzle diameter, draft tube to column diameter ratio and liquid velocity. Th gas holdup increases with an increase in gas and liquid velocities. The optimum draft tube to column diameter ratio was found to be 0.438. Correlations for gas holdup and circulation time involving operational and geometrical variables are presented.

Correlation of heat transfer and gas holdup data from slurry bubble columns

Our recently reported gas holdup and heat transfer data for the air-water-magnetite system extending over a range of air velocities, particle diameters, and slurry concentrations have been correlated. The gas holdup correlation is based on the drift-flux theory, while the heat transfer correlation is based on an empirical modification of the Higbie's theory of unsteady-state mass transfer as applied to heat transfer and Kolmogoroff's theory of isotropic turbulence as developed by Deckwer and Deckwer et al.

底吹き円筒容器内水-空気系気泡噴流の浮力領域における気泡特性

The flow field in vertical bubbling jet in a cylindrical vessel with bottom blowing can be divided into four regions. They are named the momentum, transition, buoyancy and surface region from the nozzle exit. In a previous paper, bubble behavior characterized by gas holdup, bubble frequency, bubble rise velocity and so on was clarified for the momentum region in which inertia force of injected gas played an important role. In the present study, bubble behavior in the buoyancy region in which buoyancy force of bubbles governed the flow was investigated by means of two kinds of electro-resistivity probes, a high speed video camera, and a laser Doppler velocimeter. The bubble behavior was not affected by nozzle diameter and bath diameter. Therefore, correlations for gas holdup, bubble frequency and bubble rise velocity were proposed as functions of gas flow rate.