Page 308 - Materials Chemistry, Second Edition

VOC-Laden Air Treatment 291

a catalytic incinerator with a recuperative heat exchanger. Determine the
required dilution air flow rate, if needed.

Solution:
The heating value of the off-gas has been determined as 11.6 Btu/scf or
160 Btu/lb in Example 7.8, which exceeds the 10-Btu/scf or 135-Btu/
lb limit. Thus, air dilution is required, and the dilution air flow rate
can be determined by using Equation (7.18) as:

 H   160 
Q dilution =  w − 1 Q w =  − 1(200) = 37scfm


 H i  135 
Discussion:
For the same off-gas, 800 ppmV of xylenes, air dilution is required for
catalytic incineration, but not required for direct incineration.

7.4.2 Supplementary Heat Requirements

For catalytic incineration of off-gases from soil/groundwater remediation,
supplementary heat is often provided by electrical heaters. If natural gas is
used, use Equation (7.19) to determine the supplementary fuel flow rate. Before
calculating the supplementary heat requirement, the following two equa-
tions should be used to estimate the temperature of the flue gas, T , which
out
can achieve the desired destruction efficiency without damaging the catalyst.
The T can be estimated with the temperature of the waste gas after the heat
out
exchanger and before the catalyst bed, T , and the heat content of the gas:
in
T out = T in + 50 H w (7.24)
On the other hand, the equation can be modified to determine the required
influent temperature to achieve a desired temperature in the catalyst bed:

T in = T out − 50 H w (7.25)
where H is the heat content of the waste air stream in Btu/scf only. These
w
two equations assume a 50°F temperature increase for every 1 Btu/scf of heat
content in the influent air to the catalyst bed.

Example 7.14: Estimate the Temperature of the Catalyst Bed
Referring to the remediation project described in Example 7.13, an off-gas
stream (Q = 200 scfm) containing 800 ppmV of xylenes is to be treated by
a catalytic incinerator with a recuperative heat exchanger. After the heat
exchanger, the temperature of the diluted waste gas is 550°F. Estimate the
temperature of the catalyst bed.