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344 Applied Process Design for Chemical and Petrochemical Plants

I
p, dm. A bso r p t ion 1 patme Desorption

c
I b./cu.ft. Ib./cu. ft.

y mol froction
7---"
Y!

'.U
AYmean strip.
V X
Ihkuft. mol fraction
Figure 9-69. Mass transfer diagrams. The number of transfer units can be determined by the difference in concentration or vapor pressure,
particularly over ranges where the equilibrium line is essentially straight. Used by permission of Czermann, J. J., Gyokheqyi. S. L., and Hay, J.
J., Petroieum Refiner, V. 37, No. 4 (1958) p. 165; all rights reserved.

of transfer units) or difficulty of the transfer under the uid film controlling, the gas is relatively insoluble in the
conditions of operation with respect to system equilibri- liquid and the resistance to transfer is in the liquid film.
um, the system is evaluated as to the number of transfer Many systems are a combination of the two in various pro-
units NOG or NOL required. These can be determined portions. Without good data on such systems it is next to
experimentally and the data used for similar systems. How- impossible to expect to accomplish an exact design of
ever, it is also important to be in a position to estimate the equipment, although satisfactory designs are possible. To
number of transfer units for some foreign system when have some guidelines, system information is presented in
data are not available. Table 9-39. Other data for different systems exist in the lit-
erature in a scattered fashion.
NOG = - Z (9 - 79) For (1) dilute solutions or (2) equal molar diffusion
Z
or NoL = -
HOG HOL between phases (e.g., distillation)
where NOG = number of transfer units, based on overall gas Y1 Y1 -Y2
film coefficients dy (Y -Y*h - (Y -Y*)2
NOL = number of transfer units, based on overall liquid NoG =J -= In (Y - Y *)1 (9 - 80)
(Y-Y*)
film coefficients y2 (Y - Y "12
Z = height of packing, ft
HOG = height of transfer units, based on overall gas fih, where (y - p) = driving force, expressed as mol fractions
coefficients, ft y = mol fraction of one component (solute) at
HOL 6 height of transfer unit, based on overall liquid any point in the gas phases of the contacting
film coefficients, ft system
y* = mol fraction gas phase composition in equilib-
The transfer process is termed gas film controlling if rium with a liquid composition, x
essentially all of the resistance to mass transfer is in the gas x = mol fraction in the liquid at the same corre-
film. This means that the gas is usually quite soluble in, or sponding point in the system as y
reactive with, the liquid of the system. If the system is liq- 1,2 = inlet and outlet conditions of system