Page 61 - Materials Chemistry, Second Edition

44 Practical Design Calculations for Groundwater and Soil Remediation

Estimate the vapor pressure of benzene at 20°C and at 25°C using the
Antoine equation.
Solution:
(a) Use Equation (2.17), at 20°C
B 2788.51
ln P sat = A − = 15.9008 − = 4.322
+
TC (293 − 52.36)
So, P vap = 75.3 mm-Hg
(b) Use Equation (2.17), at 25°C

B 2788.51
ln P sat = A − = 15.9008 − = 4.557
+
TC (298 − 52.36)
So, P vap = 95.3 mm-Hg

Discussion:
1. The calculated benzene vapor pressure, 95.3 mm-Hg (at 25°C), is
essentially the same as that in Table 2.2, 95 mm-Hg.
2. The calculated benzene vapor pressure, 75.3 mm-Hg (at 20°C), is
essentially the same as that in Example 2.25, 75 mm-Hg, which
uses the Clausius-Clapeyron equation.
Example 2.27: Vapor Concentration in the Void with
Presence of Free Product
An industrial solvent, consisting of 50% (by weight) toluene and 50% ethyl
benzene, leaked from a UST at a site and entered the vadose zone. Estimate
the maximum toluene and ethyl benzene concentrations (in ppmV) in the
void of the subsurface. The temperature of the subsurface is 20°C.
Solution:
From Table 2.2, the vapor pressure of toluene (C H , MW = 92) is 22 mm-Hg
8
7
and that of ethyl benzene (C H , MW = 106) is 7 mm-Hg at 20°C.
10
8
For 50% by weight of toluene, the corresponding percentage by mole
(i.e., mole fraction) will be
= (moles of toluene) ÷ [(moles of toluene)
+ (moles of ethyl benzene)] × 100
= (50/92) ÷ [(50/92) + (50/106)] × 100
= 53.5%

The partial pressure of toluene in the void can be estimated from
Equation (2.18)
vap
= (P )(x )
A