Effects of High Solute Concentrations on kG and kL

Effects of High Solute Concentrations on kc and kL As discussed previously, the stagnant-film model indicates that kG should be independent of yBM and kG should be inversely proportional to yBM. The data of Vivian and Behrman [Am. Inst. Chem. Eng. J., 11, 656 (1965)] for the absorption of ammonia from an inert gas strongly suggest that the film mocfel s predicted trend is correct. This is another indication that the most appropriate rate coefficient to use in concentrated systems is kG and the proper driving-force term is of the form (y - /, )/(/bm- 

The use of the rate coefficient kL and the driving force (xt - x)/xBM is believed to be appropriate. For many practical situations the liquid-phase solute concentrations are low, thus making this assumption unimportant. 

Influence of Chemical Reactions on kc and kL When a chemical reaction occurs, the transfer rate may be influenced by the chemical reaction as well as by the purely physical processes of diffusion and convection within the two phases. Since this situation is common in gas absorption, gas absorption will be the focus of this discussion. One must consider the impacts of chemical equilibrium and reaction kinetics on the absorption rate in addition to accounting for the effects of gas solubility, diffusivity, and system hydrodynamics. 

There is no sharp dividing line between pure physical absorption and absorption controlled by the rate of a chemical reaction. Most cases fall in an intermediate range in which the rate of absorption is limited both by the resistance to diffusion and by the finite velocity of the reaction. Even in these intermediate cases the equilibria between the various diffusing species involved in the reaction may affect the rate of absorption. 

TABLE 5-22 Mass-Transfer Correlations for Particles, Drops, and Bubbles in Agitated Systems 

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