Page 299 - Materials Chemistry, Second Edition

282 Practical Design Calculations for Groundwater and Soil Remediation

(b) To determine the heat content of the air containing 800 ppmV
xylene, we have to determine the mass concentration of xylene
in the air first (which has been previously determined in
Example 7.3):
800 ppmV of xylene = (800)(0.27 × 10 )
–6
= 2.16 × 10 lb xylene/ft air
3
–4
Use Equation (7.15) to determine the heating value of the off-gas:
Heatingvalue (inBtu/scf)
= 19,015 Btu/lb × (2.16 × 10 −4 lb/scf)= 4.11 Btu/scf

(c) Use Equation (7.16) to convert the heating value into Btu/lb:

Heatingvalue of an airstreamcontainingxylenes (inBtu/lb)
= 4.11 Btu/scf ÷ 0.0739 lb/scf = 55.6 Btu/lb

Discussion:
1. The heating value of xylene calculated from the Dulong’s for-
mula, 19,015 Btu/lb, is essentially the same as that found in the
literature, 18,650 Btu/lb.
2. The weight percentage of C is 90.57%, and a value of 90.57, not
0.9057, should be used in the Dulong’s formula.

7.3.3 Dilution Air
Some waste air streams contain enough organic compounds to sustain
burning (i.e., no auxiliary fuel is required, which means cost savings).
That is why direct incineration is favorable for treating air with high
organic concentrations. However, for hazardous air pollutant streams, the
concentration of flammable vapors to a thermal incinerator is generally
limited to 25% of the lower explosive limit (LEL), imposed by insurance
companies for safety concerns. Vapor concentrations up to 40% to 50% of
the LEL may be permissible if on-line monitoring of VOC concentrations
and automatic process control and shutdown are employed. Table 7.2 lists
the LELs and upper explosive limits (UELs) of some combustible com-
pounds in air.
When the off-gas has VOC content larger than 25% percent of the LEL (i.e.,
in most of the initial stages of the SVE-based cleanup projects), dilution air
must be used to lower the COC concentration to below 25% of its LEL prior
to incineration [3]. The 25% LEL corresponds to a heat content of 176 Btu/lb
or 13 Btu/scf in most cases.