FIG. 14-8 Graphical method for a three-theoretical-plate gas-absorption tower with inlet-liquor composition xj and inlet-gas composition y
(zero for a pure solvent), and A = absorption factor = LM/mGM. Note that the absorption factor is the reciprocal of the expression given in Eq. (14-4) for packed columns.
Note that for the limiting case of A = 1, the solution is given by
Although Eq. (14-31) is convenient for computing the composition of the exit gas as a function of the number of theoretical stages, an alternative equation derived by Colburn [Trans. Am. Inst. Chem. Eng., 35, 211 (1939)] is more useful when the number of theoretical plates is the unknown:
The numerical results obtained by using either Eq. (14-31) or Eq. (14-33) are identical. Thus, the two equations may be used interchangeably as the need arises.
Comparison of Eqs. (14-33) and (14-23) shows that
thus revealing the close relationship between theoretical stages in a plate tower and mass-transfer units in a packed tower. Equations (14-23) and (14-33) are related to each other by virtue of the relation hT = HogNog = (HETP)N (14-35)
Algebraic Method for Concentrated Gases When the feed gas is concentrated, the absorption factor, which is defined in general as A = Lm/KGm and where K = y0/x, can vary throughout the tower due to changes in temperature and composition. An approximate solution to this problem can be obtained by substituting the "effective" adsorption factors Ae and A' derived by Edmister [Ind. Eng. Chem. 35, 837 (1943)] into the equation
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