Page 68 - Packed bed columns for absorption, desorption, rectification and direct heat transfer

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Consequently:

AY
K x.a e 'X*-X* (223)

or after integration

So in the general case the number of mass transfer unite for the gas phase is
defined by the equation

NTU = f' f^ . (225)
m
m
k 2X -X
Because of the simple relation between HTUOY and Ky.a m respectively between
&Y m and NTUOY, for practical purposes either of the models, with Kf,a e or with
HTUr, can be used. The same is valid also when the concentrations are given
for the liquid phase.
K
Instead of indices X" and "Y" at HTU and NTU often the respective
H n
indices, L - for liquid, "G"- for gas, and "V- for vapour, are used.
It is possible to calculate the number of mass transfer units for each of
the phases, gas and liquid, using the respective driving force as a difference of
concentration in the bulk and the equilibrium concentration at the interface. In
these cases the overall mass transfer coefficients have to be replaced by the
partial ones.
Thus for the number of mass transfer units of the gas phase NTUa and
of the liquid phase NTUi, it can be written:

£'_*?_ is£ (226)
NTU =

and