This article presents an extensive study on the gas holdup, axial liquid dispersion, and mass transfer for packed, trayed, and empty bubble columns. Four types of structured packings (Super-Pak, Flexipac, Mellapak, and Gauze) and two types of perforated partition trays (with 25% and 40% tray open area) were used to characterize the packed and trayed bubble columns, respectively. It was observed that the gas holdup and mass transfer characteristics of the packed and trayed bubble columns are superior to those of an empty bubble column, while the axial dispersion coefficients are much lower. This article discusses in detail the effect of the liquid and gas flow rates, liquid-phase viscosity, and type of internals. Additionally, experimental data of the packed, trayed, and empty bubble column are correlated by dimensionless numbers. Novel empirical correlations are proposed for the gas holdup, Bodenstein number (for the axial dispersion coefficient), and Stanton number (for the volumetric mass transfer coefficient), as a function of the Froude and Galileo dimensionless numbers.

Gas holdup, axial dispersion, and mass transfer studies in bubble columns

Despite their drawbacks, chemical techniques are often the only means of determining volumetric gas—liquid mass-transfer coefficients kLa in gas—liquid and gas—liquid—solid reactors. Two techniques are considered here: the oxidation of sodium sulphite by oxygen in a viscous CMC solution and the oxidation of hydrazine catalysed by a copper ammonia complex. The latter technique is shown to be of use even when the liquid is not perfectly mixed. Furthermore, since hydrazine solutions exhibit coalescence, by adding sodium sulphate the coalescence behaviour of the liquid can be modified without affecting the kinetics of the reaction. In each case, the range of values of the mass-transfer coefficient kLa and the interfacial area allowing the use of this technique are given.

A review of recent chemical techniques for the determination of the volumetric mass-transfer coefficient KLa in gas-liquid reactors

Time-dependent gas holdup variation in an air-water bubble column

BACKGROUND: In order to improve the performance of a counter-current bubble column, radial variations of the gas hold-ups and mean hold-ups were investigated in a 0.160 m i.d. bubble column using electrical resistance tomography with two axial locations (Plane 1 and Plane 2). In all experiments the liquid phase was tap water and the gas phase air. The superficial gas velocity was varied from 0.02 to 0.25 m s−1, and the liquid velocity varied from 0 to 0.01 m s−1. The effect of liquid velocity on the distribution of mean hold-ups and radial gas hold-ups is discussed.



RESULTS: The gas hold-up profile in a gas–liquid counter-current bubble column was determined by electrical resistance tomography. The liquid velocity slightly influences the mean hold-up and radial hold-up distribution under the selected operating conditions and the liquid flow improves the transition gas velocity from a homogeneous regime to a heterogeneous regime. Meanwhile, the radial gas hold-up profiles are steeper at the central region of the column with increasing gas velocity. Moreover, the gas hold-up in the centre of the column becomes steeper with increasing liquid velocity.



CONCLUSIONS: The value of mean gas hold-ups slightly increases with increasing downward liquid velocity, and more than mean gas hold-ups in batch and co-current operation. According to the experimental results, an empirical correlation for the centreline gas hold-up is obtained based on the effects of gas velocity, liquid velocity, and ratio of axial height to column diameter. The values calculated in this way are in close agreement with experimental data, and compare with literature data on gas hold-ups at the centre of the column. Copyright © 2010 Society of Chemical Industry

The effect of gas-liquid counter-current operation on gas hold-up in bubble columns using electrical resistance tomography

The objective of this study was to develop a unified correlation for the volumetric mass-transfer coefficient (k L a) and effective interfacial area (a) in bubble columns for various gas-liquid systems using support vector regression (SVR-) based modeling technique. From the data published in the open literature, 1600 data points from 27 open sources spanning the years 1965-2007 for k L a and 1330 data points from 28 open sources spanning the years 1968-2007 for a were collected. Generalized SVR-based models were developed for the relationship between k L a (and a) and each design and operating parameters such as column and sparger geometry, gas-liquid physical properties, operating temperature, pressure, superficial gas velocity, and so on. Further, these models for k L a and a are available online at http://www.esnips.com/web/UICT-NCL. The proposed generalized SVR-based correlations for k L a and a have prediction accuracies of 99.08% and 98.6% and average absolute relative errors (AAREs) of 7.12% and 5.01%, respectively. Also, the SVR-based correlation provided much improved predictions compared to those obtained using empirical correlations from the literature.

Development of Unified Correlations for Volumetric Mass-Transfer Coefficient and Effective Interfacial Area in Bubble Column Reactors for Various Gas−Liquid Systems Using Support Vector Regression

The hydrodynamic and mass transfer characteristics in countercurrent, cocurrent and liquid batch operations with various Newtonian liquids were studied experimentally using the same bubble column. Taking the effect of gas sparger geometries, operating variables and liquid properties into account, empirical correlations were obtained for the gas hold-up and the volumetric liquid-phase mass transfer coefficient.

Shinji Tsuyutani

Papers

Mass transfer in countercurrent and cocurrent bubble columns