Y.T. Shah

Hydrodynamic modeling of three-phase flows through a vertical column

A computational fluid dynamics model used to examine the structure of three-phase (air-water-glass beads) flows through a vertical column is presented in this paper. The present study proposes new correlations to modify the drag between the liquid and the gas phase to account for the effect of solid particles on bubble motion. The study also attempts to propose new correlations for drag between the solid particles and liquid phase to incorporate the effect of the bubbles. Solid-solid interactions were also accounted for in the model by a modification to the drag force acting on the solid phase. A k−e turbulence model was used for simulating the effect of turbulence on the flow field. Predictions were compared with experimental data for the axial variation of the solids concentration for model validation. Results of the radial variation of the phase velocities are also presented.

EFFECT OF HIGH VOIDAGE ON MASS TRANSFER COEFFICIENT IN A FLUIDIZED BED

Abstract Experiments were carried out to determine the gas-solid mass transfer coefficient for the air-naphthalene system under fast-fluidized conditions. The data were obtained under conditions of high voidage (as high as 98%) and high gas velocities (0·5 m/s-2·2 m/s). The results indicate that the correlation for the mass-transfer coefficient in a gas-solid fluidized bed under low voidage conditions established by previous investigators can be extended to the fast fluidization regime. Significantly lower mass transfer coefficients were, however, obtained under high voidage compared to low voidage conditions.

Gas-liquid mass transfer in jet bubble column

Abstract The jet bubble column consists of a conical entrance section which expands to a cylindrical column. Gas and liquid are co-currently introduced at the bottom of the column by a small diameter inlet pipe which acts like an ejector. The kinetic energy of the gas and liquid jet together with the conical geometry at the lower section of the column cause the formation and dispersion of small bubbles. Gas-liquid mass transfer in the jet bubble column (61 cm diameter) was measured by a dynamic response technique, in which a step change was made in the gas phase oxygen concentration and the aqueous dissolved oxygen concentration response was measured at various axial and radial locations. It was found that a continuous stirred tank reactor model could be used to evaluate experimental results. The volumetric mass transfer coefficient in this type of system was found to increase with increasing gas flow rate and was about 1.5 times larger than the values obtained at similar conditions in conventional bubble...

ROLE OF SUPPORT STRUCTURE ON KINETIC PARAMETER ESTIMATIONS IN A SHELL CATALYST

Abstract The kinetic parameters for VOC oxidation by a shell catalyst are generally obtained from the experimental data of conversion-temperature in a laboratory scale isothermal fixed bed reactor. The kinetics of these reactions are often represented by a bi-molecular Langmuir-Hinshelwood expresion. With the help of experimental data for n-hexane, the present study showed that the values of the kinetic parameters estimated depend significantly upon the location and width of the active catalyst layer as well as value of the effective diffusivity of the reactant within the porous structure. The present analysis showed that accurate a priori assessment of these parameters is very important for the reliable estimations of the kinetic parameters.

TRANSPORT OF GAS-LIQUID FLOWS THROUGH VERTICAL COLUMNS

Abstract Applicability of a theoretical model, based on the fundamental governing equations of fluid motion is investigated to predict two-phase bubbly air-water flow structure through vertical columns. The model predictions are compared to experimental data for a wide range of flow parameters. The relative importance of various modes of interfacial momentum transfer, under different flow conditions, is also examined. It is found that correct estimation of the interfacial momentum transfer is necessary for agreement between the predictions and experimental data. The present study shows that the flow structure for air-water flows through vertical columns depends on the inlet flow conditions. This model predicts the experimentally observed trends well. In most cases the experimental data of local liquid velocity and gas volume fraction agree well with the calculated values.

HYDRODYNAMICS, MIXING AND MASS TRANSFER IN A LARGE DIAMETER JET BUBBLE COLUMN

Experimental measurements for the axial and radial variations in gas holdup, axial and radial dispersion coefficients, volumetric gas-liquid mass transfer coefficient and liquid phase circulation velocity in a cone of a large diameter (122 cm) jet bubble column are presented. Two diameters of the inlet nozzle, namely 10.16 cm and 15.24 cm, three superficial gas velocities (based on cylinder diameter), 3 cm/sec, 6 cm/sec and 8 cm/sec and two superficial liquid velocities, 0.3 cm/sec and 0.6 cm/sec, are examined. The experimental data are obtained for two different bed heights. The experimental data showed significant axial and radial variations in the gas holdup. The volumetric average gas holdup was higher at higher gas velocity and larger nozzle diameter and somewhat higher at lower liquid velocity. The axial dispersion was high while the radial dispersion was low. The volumetric gas-liquid mass transfer coefficient was larger at higher gas velocity and larger nozzle diameter. The liquid recirculation be...

PHASE DISTRIBUTION AND MIXING IN A JET BUBBLE COLUMN

This paper describes the results of an experimental study to evaluate phase holdups and RTD for a jet bubble column. The experimental data were obtained in a 61 cm diameter jet bubble column with a conical inlet. Air and water were used as a two-phase system. The ranges of gas and liquid velocities examined were 0 to 9 cm/sec and 0 to 0·6 cm/sec respectively, both based on the cylinder diameter. The experimental data indicate that in the conical section of the column, the gas holdup first decreases with an increase in distance away from the cone inlet, achieves a minimum and then increases until it reaches a somewhat constant value within the cylinder. Gas holdup varies radially with the maximum at the center and the minimum near the wall. Radially-averaged gas holdup increased with gas velocity and remained essentially unchanged with liquid velocity. The RTD measurements were correlated by a two-dimensional dispersion model. The axial dispersion coefficient increased linearly from the cone inlet to the c...