Gas Emission Control - Mathematics and Statistics for the Environment - page 470

100

32°F

77°F

140°F

212°F

10

300°F

3.0
2.0

1.5
1.0

0.001 1.0

0.01
Partial Pressure, psia

0.1

FIGURE 16.13 Adsorption isotherm for carbon tetrachloride on activated carbon (Adapted from USEPA,
APTI Course 415: Control of Gaseous Emissions , EPA 450/2-81-005, U.S. Environmental Protection Agency
Air Pollution Training Institute, Washington, DC, 1981, p. 5-8.)

Solution: In the gas phase, mole fraction Y is equal to the percent by volume:

Y = % Volume = 680 ppm = 680/(10) 6 = 0.00068

Obtain the partial pressure:

p = YP = 0.00068 × 14.7 psia = 0.01 psia

From Figure 16.13, at a partial pressure of 0.01 psia and a temperature of 140°F, the carbon capacity
is read as 30%. This means that, at saturation, 30 pounds of vapor are removed per 100 pounds of
carbon in the adsorber (30 kg/100 kg).

16.4.3.2 Isostere
The isostere is a plot of the ln p vs. 1/ T at a constant amount of vapor adsorbed. Adsorption isostere
lines are usually straight for most adsorbate-absorbent systems. Figure 16.14 is an adsorption iso-
stere graph for the adsorption of H 2 S gas onto molecular sieves. The isostere is important in that the
slope of the isostere corresponds to the heat of adsorption.

16.4.3.3 Isobar
The isobar is a plot of the amount of vapor adsorbed vs. temperature at a constant pressure.
Figure 16.15 shows an isobar line for the adsorption of benzene vapors on activated carbon.
Note that—as is always the case for physical adsorption—the amount adsorbed decreases with
increasing temperature. Because these three relationships (i.e., isotherm, isostere, and isobar)
were developed at equilibrium conditions, they depend on each other. By determining one, such
as the isotherm, the other two relationships can be determined for a given system. In the design
of an air pollution control system, the adsorption isotherm is by far the most commonly used
equilibrium relationship.