Estimation of Mass Transfer Coefficients for Packed Towers

The individual film mass-transfer coefficients k a and k a depend generally upon Schmidt number, Reynolds number, and the size and shape of the packing. The interactions among these factors are quite complex. Hence, the correlations for mass-transfer coefficients are highly empirical. The reliability of these correlations is not too satisfactory. Deviations of up to 25% are not uncommon. A main difficulty arises because an overall coefficient or resistance is measured experimentally that represents the two film resistances in series. To obtain the single-phase film coefficient, the experiment is so arranged that the other film resistance is negligible or can be approximately calculated. 

To measure the liquid film mass-transfer coeffident k a, a system for absorption or desorption of very insoluble gases such as Oj or CO2 in water is used. The experiment gives K a, which equals k a, since the gas-phase resistance is negligible. 

To measure the gas-phase film coefficient we desire to use a system such that the solute is very soluble in the liquid and the liquid-phase resistance is negligible. Most such systems as NHs-air-water have a liquid-phase resistance of about 10%. By subtracting this known liquid phase resistance (obtained by correcting k a data for absorption of CO2 or O2 to NH3 data for k a) from the overall resistance in Eq. (10.4-24), we obtain the coefficient k a. Details of these are discussed elsewhere (Gl, SI, S2). 

The experimental data for the gas film coefficient in dilute mixtures have been correlated in terms of Hq, where 

Equation (10.8-1) can be used to correct existing data for absorption of solute Aina. gas on a specific packing to absorption of solute E in the same system and the same mass-flow rates. This is done by Eq. (10.8-2). 

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ESTIMATION OF MASS-TRANSFER COEFFICIENTS FOR PACKED TOWERS... 

Sec. 10.8 Estimation of Mass-Transfer Coefficients For Packed Towers... 

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