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Overall transfer coefficients and transfer units

The driving force over the gas film is taken as (y, — y) and over the liquid film as (x — x,). If ye is the concentration in the gas phase in equilibrium with concentration x in the liquid phase, then (ye — y) is taken as the overall driving force expressed in terms of y. Similarly (x — xe) is taken as the overall driving force in terms of x, where xe is the concentration in the liquid in equilibrium with a concentration y in the vapour. The overall driving forces (Ay)o and (Ax)o may then be written as:

(Ay)0 = ye — y = (ye — y,) + (y, — y) (Ax)o = x — xe = (x — x,) + (x, — xe)

By analogy with the derivation for film coefficients, a series of overall transfer coefficients and overall transfer units based on these overall driving forces may be defined. Thus, the number of overall gas transfer units is given by:

J ye— y and the number of overall liquid transfer units is given by:

The heights of the overall transfer units are:

K'a where Kga and are overall transfer coefficients, based on gas or liquid concentrations in moles/unit time-unit volume-unit mole fraction driving force.

Relation between overall and film transfer units

From these definitions, the following equation may be written:

Thus: and:

dZ

dy

dZ

dy

hog

ye -y

hg

yi - y

dZ

dx

dZ

dx

hol

~ x -xe

h l

x xi

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