Interfacial Areas and Mass-Transfer Coefficients

Measurement of Interfacial Areas and Mass-Transfer Coefficients 

Physical measurements can be made of gas holdup a, bubble size, and specific surface area a in gas-liquid dispersions, as usually encountered in bubble columns, plate columns, mechanically agitated tanks, and spray towers. Any two of these interfacial parameters are sufiicient to define all three, since they are interrelated  

The gas holdup a is determined directly by measuring the height of the aerated liquid Z.J and that of the clear liquid without aeration (Z). The average gas holdup is then 

This method, used for plate columns (G2), bubble columns (F2, M7, H13, Y4, A3, E3), and mechanically agitated tanks (R7, V3, Y3, M16), is rapid but only accurate to 15-20%, especially when waves or foam occur on top of the dispersion. 

A more accurate, manometric, technique has been used by Reith et al. (R10) and by Burgess and Calderbank (B32). The gas holdup in the dispersion is computed from measurements of the clear liquid height in the dispersion at successive manometer taps on the side of the froth-containing vessel. IfY( andjCf-i are the clear liquid heights recorded on the manometer at vertical positions Z, and Zf i above the column floor, then the gas holdup at the midlevel (/, i - 1) is given by 

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A. Schumpe, W.D. Deckwer, Gas holdups, specific interfacial areas, and mass transfer coefficients of aerated carboxymethyl cellulose solutions in a bubble column, I EC Process Des. Develop. 21 (1982) 706-711. 

M. Bouaifi, G. Hebrard, D. Bostoul, M. Roustan, A comparative study of gas hold-up, bubble size, interfacial area and mass transfer coefficients in gas—liquid reactors and bubble columns, Chem. Eng. Proc. 40 (2001) 97-111. 

C.W. Robinson, C.R. Wilke, Simultaneous measurement of interfacial area and mass transfer coefficients for a well-mixed gas dispersion in aqueous electrolyte solutions, AIChE J. 20 (1974) 285-294. 

For example, it is found that the mass transfer coefficient, Kca, for gas-liquid processes, is mostly a function of the linear superficial gas velocity and the power per unit volume with the constant D/T ratio for various size tanks. This is because the integrated volumetric mass transfer coefficient over the entire tank can be quite similar in large and small tanks even though the individual bubble size, interfacial area, and mass transfer coefficient can vary at specific points within the small and large tanks. 

III. Measurement of Interfacial Areas and Mass-Transfer Coefficients. 35... 

In this section we consider the rate of absorption of gases into liquids that are agitated so that dissolved gas is transported from the interfacial surface to the interior by convective motion. The next section, based on this one, treats chemical methods for determining interfacial areas and mass-transfer coefficients in agitated gas-liquid reactors. 

When necessary, results of surface-renewal theories will be presented simultaneously, as these models may be applied directly to determine interfacial area and mass-transfer coefficients in the laboratory apparatus considered in Sections III,B and V. 

Chemical methods for determining gas-liquid interfacial areas and mass-transfer coefficients have been intensively developed for the last 10 years. The principles of these methods are deduced from the results presented in Section III,B,2 A gas A is absorbed into a liquid where it undergoes a reaction with a dissolved reactant B ... 

INTERFACIAL AREA AND MASS TRANSFER COEFFICIENT IN GAS-LIQUID DISPERSIONS... 

The main reason for the deficiencies of the present state of the art is, however, the maldistribution of gas and liquid in packed beds. Many studies reveal that there exist large deviations from plug flow of gas and in particular of hquid within the bed. (Hoek et al. 1986 Kammermaier 2008). The degree of maldistribution as well as its effect on mass transfer are unknown and, in turn, not accounted for in existing mass transfer models. Thus the pubhshed data for interfacial area and mass transfer coefficients comprise the maldistribution in an undefined manner. The data are not true but pseudo values which are not predictable within the plug flow model. 

Gas dispersion in agitated tanks may be described in terms of bubble size, gas hold-up, interfacial area and mass transfer coefficient. While gas dispersion in low viscosity systems [Smith, 1985 Tatterson, 1991 Hamby et al., 1992] has been extensively studied, little is known about the analogous process in highly viscous Newtonian and non-Newtonian media, such as those encoim-tered in polymer processing, pulp and paper manufacturing and fermentation applications. 

Sharma.M.M. and P.V.Danckwerts. "Chemical methods of measuring interfacial area and mass transfer coefficients in two-phase systems." Brit.Chem.Eng. 15 (1970) 522-528. 

Sridharan,k. and M.M.Sharma. "New systems and methods for the measurement of effective interfacial area and mass transfer coefficients in gas-liquid contactors". Chem.Engng.Sci. 31 (1976) 767. 

Film and overall mass-transfer coefficients in packed towers. As discussed in Section 10.5 it is very difficult to measure experimentally the interfacial area A m between phases L and V. Also, it is difficult to measure the film coefficients k and ky and the overall coefficients K and Kf Usually, experimental measurements in a packed tower yield a volumetric mass-transfer coefficient that combines the interfacial area and mass-transfer coefficient. 

The rate and the manner in which energy is delivered influence such important fluid-fluid parameters as dispersed-phase holdup, interfacial area, and mass transfer coefficient (Calderbank, 1958 Nagel et al 1972, 1973 KastanSc, 1976 Zahradnik et al 1982 Oldshue, 1983). Thus, an energy-based classification would be both useful and appropriate. In this classification, the contactors are essentially divided into three broad groups ... 

Dhanuka, V.R. and J.B. Stepanek, "Simultaneous Measurement of Interfacial Area and Mass Transfer Coefficient in Three-Phase Fluidized Bed". AIChE J. (1980) 1029-1038. 

Robinson, C.W., and Wilke, C.R., "Simultaneous Measurement of Interfacial Area and Mass Transfer Coefficients for Well-Mixed Gas Dispersion in Aqueous Electrolyte Solutions",... 

Specify the arrangement to be used and estimate the average gas-bubble diameter, gas holdup, specific interfacial area, and mass-transfer coefficient to be expected. 

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