Y y yeq

Equation (7.27) may be integrated to obtain, Z, in terms of KG*a, but for many situations the first term on the left-hand side is very close to unity. Since the number of transferred units Ntog is defined as yi (1 - y)m * dy

then

Ntog can, as shown, be related to the height of the packing and the height per transfer unit, termed Htog. The height per transfer unit is an experimental quantity, but it is more convenient to use it than KG*a and other mass-transfer coefficients in the design of towers. Htog has the dimension of length. The subscript, tog, is used, as seen in the terms Ntog and Htog to indicate that these terms are based on an overall driving force y - yeq within the gas phase. These terms, therefore, represent the vertical distance between the operating line and equilibrium curve at any liquid concentration on a graph plotted in mole fractions.

The quantity (1-y)m is the average concentration of non-diffusing gas at either end of the diffusion path. (1-y) is the concentration of the main body of the gas and (1-yeq) that at the liquid gas interface.

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