Page 45 - Gas Purification 5E

Introduction 35

approach this goal. Normally contactors of this type are used when the equilibrium vapor
pressure of the absorbate over the product liquid is extremely low and can be neglected.
Under these conditions, the equation for the number of transfer units reduces to

NN = In (Y(1 -E)) ( 1-24)

where E = absorption efficiency, expressed as a fraction.

In accordance with equation 1-24, removal efficiency and NoG are related as follows:

Removal Efficiency, % NOG
90 2.3
95 3.0
99 4.6

The NOG values can be used to extrapolate test or operating data on a spray contactor to
other systems or conditions. For example, if one spray unit provides 90% removal (2.3 trans-
fer units), it can be expected that two identical units in series will provide about 99%
removal (4.6 transfer units). Since commercial spray systems are widely variable, the devel-
opment of a generalized design approach based on fundamentals is quite difficult. As a
result, spray contactors are usually designed on the basis of previous experience with similar
systems.

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