Page 147 - Materials Chemistry, Second Edition

130 Practical Design Calculations for Groundwater and Soil Remediation

Strategy:
It is a first-order reaction, and the reaction-rate constant is equal to 0.25/
min.
Solution:
Insert the known values into Equation (4.20) to find out the value of τ:

50 1
C out
= =
C in 1200 1 0.25+ τ
τ = 92 min

Example 4.12: A Low-Temperature Thermal Desorption Reactor
with Second-Order Kinetics (CFSTR)

A low-temperature thermal desorption reactor is used to treat soil that con-
tains 2,500 mg/kg of TPH. The required final soil TPH concentration is 100
mg/kg. From a bench-scale study, the rate equation was found to be

γ= − 0.12 C 2 in mg/kg/h

The reactor is rotated to achieve a good mixing. Assume that the reactor
behaves as a CFSTR. Determine the required residence time.

Strategy:
It is a second-order reaction, and the reaction-rate constant is equal to
0.12/(mg/kg/h).

Solution:
Insert the known values into Equation (4.22) (see Table 4.2) to find out
the value of τ:
100 1
= =
C out
τ
+
C in 1200 1 0.12 (100)
τ = 0.92 h = 55 min

4.4.3 PFRs

Let us now consider a steady-state PFR with a first-order reaction. As men-
tioned previously, by definition, there is no longitudinal mixing within the