Page 241 - Separation process principles 2

206 Chapter 6 Absorption and Stripping of Dilute Mixtures

Then, the application of an algebraic method may be solute when the number of theoretical stages, N, and the
preferred. absorption or stripping factor are known. I
The Kremser method for single-section cascades, as
developed in Section 5.4, is ideal for absorption and strip-
ping of dilute mixtures. For example, (5-48) and (5-50) can
be written in terms of the fraction of solute absorbed or As discussed by Okoniewski [3], volatile organic compounds
stripped as (VOCs) can be stripped from wastewater by air. Such compounds
AN+' - Ai are to be stripped at 70°F and 15 psia from 500 gpm of wastewater
Fraction of a solute, i, absorbed = (6- 13) with 3,400 scfm of air (standard conditions of 60°F and 1 atm) in an
AN+' - 1
existing tower containing 20 plates. A chemical analysis of the
and wastewater shows three organic chemicals in the amounts shown in
sY+' - si the following table. Included are necessary thermodynamic proper-
Fraction of a solute, i, stripped = (6- 14) ties from the 1966 Technical Data Book-Petroleum Rejining of the
sY+' - 1 American Petroleum Institute. For all three organic compounds, the
where the solute absorption and stripping factors are, wastewater concentrations can be shown to be below the solubility
values.
respectively,
Ai = L/(Ki V) (6- 15) Concentration Solubility Vapor
in the in Water Pressure
Si = KiV/L (6- 16)
Organic Wastewater, at 70°F, at 70°F,
Values of L and V in moles per unit time may be taken as Compound m& mole fraction psia
entering values. Values of Ki depend mainly on temperature,
Benzene 150 0.00040 1.53
pressure, and liquid-phase composition. Methods for esti-
Toluene 50 0.00012 0.449
mating K-values are discussed in detail in Chapter 2. At
Ethylbenzene 20 0.000035 0.149
near-ambient pressure, for dilute mixtures, some common
expressions are It is desirable that 99.9% of the total VOCs be stripped, but the
plate efficiency of the tower is uncertain, with an estimated range of
Ki = P,S/P (Raoult's law) (6- 17)
5% to 20%, corresponding to one to four theoretical stages for the
Ki = yiy Pis/ P (modified Raoult's law) (6-18) 20-plate tower. Calculate and plot the percent stripping of each of
the three organic compounds for one, two, three, and four theoreti-
Ki = Hi/P (Henry's law) (6- 19)
cal stages. Under what conditions can we expect to achieve the de-
Ki = P,S/xf P (solubility) (6-20)
sired degree of stripping? What should be done with the exiting air?
The first expression applies for ideal solutions involving
solutes at subcritical temperatures. The second expression
SOLUTION
is useful for moderately nonideal solutions when activity
coefficients are known at infinite dilution. For solutes at Because the wastewater is dilute in the VOCs, the Kremser equa-
supercritical temperatures, the use of Henry's law may be tion may be applied independently to each of the three organic
chemicals. We will ignore the absorption of air by the water and the
preferable. For sparingly soluble solutes at subcritical tem-
stripping of water by the air. The stripping factor for each com-
peratures, the fourth expression is preferred when solubility
pound is given by S, = K,V/L, where V and L will be taken at en-
data in mole fractions, xf, are available. This expression is
tering conditions. The K-value may be computed from a modified
derived by considering a three-phase system consisting of an
Raoult's law, K, = y,~ P:/P, where for a compound that is only
ideal-vapor-containing solute, carrier vapor, and solvent; a slightly soluble, take y,~ = l/x:, where x: is the solubility in mole
pure or near-pure solute as liquid (1); and the solvent liquid fraction. Thus, from (6-20), K, = P:/x: P
(2) with dissolved solute. In that case, for solute, i, at equi-
librium between the two liquid phases,

But, The corresponding K-values and stripping factors are

K at
Therefore, Component 70°F, 15 psia S
Benzene 255 9.89
Toluene 249 9.66
and from (6- IS), Ethylbenzene 284 11.02

From (6-14),
The advantage of (6-13) and (6-14) is that they can be sN+' - s
Fraction stripped = -
solved directly for the percent absorption or stripping of a ~Nfl - 1