Local Mass-Transfer Coefficients General Case

3 Local Mass-Transfer Coefficients General Case 

The interfacial compositions yA ( and xA t must satisfy simultaneously equation (3-29) 

Example 3.5 Absorption of Ammonia by Water Use of F-Type Mass-Transfer Coefficients 

B is water. Ignoring the vaporization of water (the vapor pressure of water at 300 K is only 3.5 kPa), we may assume that NBL = 0 and VAL - 1.0. Then, equation (3-29) 

Substituting into equation (3-30) the numerical values given and rearranging yields 

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The general expression given by Eq. (14-8) is more complex than normally is required, but it must be used when the mass-transfer coefficient varies from point to point, as may be the case when the gas is not dilute or when the gas velocity varies as the gas dissolves. The values of yi to be used in Eq. (14-8) depend on the local hquid composition Xi and on the temperature. This dependency is best represented by using the operating and equilibrium lines as discussed later. 

In extraction columns, it is possible to find droplet swarms where the local velocities near the droplet surface are higher, this being due to the lower free area available for the countercurrent flowing continuous phase. Wake and Marangoni influences make the prediction of a physical mass transfer coefficients difficult. With reactive extraction the influence of interfacial kinetics on overall mass transfer is generally not negligible. In any case, a combination of reactive kinetics with any eddy mass transfer model is recommended, whereas the latter could rely on correlations derived for specific column geometries. 

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