Page 29 - Gas Purification 5E

Introduction 19

The HTU for this case (based on an overall gas-phase driving force) is then defined as

(1-14)

Since NOG is dimensionless, will have the same units as h. Similarly, for the overall
liquid case:

(1-15)

As in the calculation of column height from &a or KLa data, it is theoretically correct to
use a logarithmic mean driving force when both the equilibrium and operating lines are
straight. For this case, the number of transfer units (overall gas) may be calculated from the
simple expression:

Nm = Y1 -Y2 (1-16)
(Y -Y~)LM

This equation may be combined with the equilibrium relation:

Ye = mx

and the material-balance expression:

to eliminate the need for values of ye. The resulting equation which was proposed by Col-
burn (1939) is given below:

(1-17)

Where: NOG = number of overall transfer units
m = slope of equilibrium curve dy&dx
x, = mole fraction solute in liquid fed to top of column
y1 = mole fraction solute in gas fed to bottom of column
y2 = mole fraction in gas leaving top of column
GM = superficial molar mass velocity of gas stream, lb moles/(hr)(sq ft)
LM = superficial molar mass velocity of liquid stream, lb moles/(hr)(sq ft)
It will be noted that the parameter mGM/LM appears several times in equation 1-17.
This parameter is called the stripping factor, S, and its reciprocal, LM/m&, is called the
absorption factor, A. The absorption factor is used in a number of popular techniques for the
design of both packed and tray absorbers. It can be considered to be the ratio of LM/GM. the
slope of the operating line, to m, the slope of the equilibrium line. Plots of equation 1-17,