Page 159 - Materials Chemistry, Second Edition

142 Practical Design Calculations for Groundwater and Soil Remediation

Example 4.20: CFSTRs in Series
Low-temperature thermal desorption reactors (assuming they are ideal
CFSTRs) are used to treat soil that contains 1,050 mg/kg of TPH. The required
final soil TPH concentration is 10 mg/kg. A reactor with a 20-min residence
time can only reduce the concentration to 50 mg/kg. Assume that this is a
first-order reaction. Can two smaller reactors (10-min residence time each) in
series reduce the TPH concentration below 10 mg/kg?

Strategy:
The reaction-rate constant was not given, so we have to find its value
first.

Solution:
(a) Use Equation (4.20) to find the rate constant:

50 1
C out
= =
+
C in 1,050 1( k)(20)
k = 1/min
(b) For two small reactors in series:
Use Equation (4.28) to find out the final effluent concentration,

 C 2   C 2   C 1   1   1 
=   =     =    
C 2
+
+
C 0  1,050  C 1   C 0   1(1)(10)  1(1)(10)
C = 8.7 mg/kg (It is below the cleanup level.)
out
Discussion:
This example again demonstrates that two smaller CFSTRs can do
a better job than a larger CFSTR with an equivalent total volume.
However, two reactors may require a larger capital investment (two
sets of process control, for example) and higher operating and main-
tenance (O&M) costs.

Example 4.21: PFRs in Series

UV/ozone treatment is selected to remove trichloroethylene (TCE) from an
extracted groundwater stream (TCE concentration = 200 ppb). At a design