Page 304 - Materials Chemistry, Second Edition

VOC-Laden Air Treatment 287

H = heat content of the waste air stream, Btu/lb
w
H = heating value of supplementary fuel, Btu/lb (21,600 Btu/lb for
sf
methane)
If the temperature of the waste air stream after the heat exchanger (T )
he
is not specified, use the following equation to calculate T (Note: the heat
he
exchanger is to recuperate the heat from the exhaust of the oxidizer to heat
up the influent waste air stream):

 HR   HR 
T he =   T c +  1−  T w (7.20)
 100   100 

where
HR = heat recovery in the heat exchanger, % (If no other information is
available, a value of 70% may be assumed.)
T = temperature of the waste air stream before entering the heat
w
exchanger, °F

In Equation (7.20), T is the temperature of waste air stream after the heat
he
exchanger. (If no heat exchangers are employed to recuperate the heat, then
T = T .) The C value can be obtained from Figure 7.1.
w
p
he
Example 7.11: Determine the Supplementary Fuel Requirements
Referring to the remediation project described in Example 7.7, an off-gas
stream (Q = 200 scfm) containing 800 ppmV of xylene is to be treated by a
thermal oxidizer with a recuperative heat exchanger. The combustion tem-
perature is set at 1,800 °F. Determine the flow rate of methane as the supple-
mentary fuel, if required.

Solution:
(a) Assuming that the heat recovery is 70% and the temperature of
the waste air from the venting well is 65°F, the temperature of
the waste air after the heat exchanger, T , can be found from
he
Equation (7.20) as:
 HR   HR 
T he =   T c + 1 − T w
 100    100  

 70   70 
=   (1,800) + 1 − (65) = 1,280 °F
 100    100  
(b) The average specific heat can be read from Figure 7.1 as 0.0266
Btu/lb-°F at 1,800°F.