Page 229 - Materials Chemistry, Second Edition

212 Practical Design Calculations for Groundwater and Soil Remediation

• Mixing conditions inside the reactor: Better mixing conditions will
enhance the heat transfer and improve venting of the desorbed COCs.
• Volatility of the COCs: The more volatile the COCs are, the shorter the
required retention time will be.
• Size of the soil particles: The smaller the soil particles, the easier the
desorption will be.
• Types of soil: Clay has a stronger affinity with COCs and, thus, the
COCs will be harder to desorb from clayey material.

The rate of desorption or the required detention time to remediate a spe-
cific type of soil to a permissible concentration can be best determined from
a pilot study. The results from the pilot study should then be used for the
preliminary design of the full-scale operation. The desorption process can
be conducted in a batch mode or in a continuous mode. For the continuous
mode, the reactor can be modeled as a continuous-flow stirred tank reactor
(CFSTR) if the soil is relatively well-mixed inside the reactor. For the desorp-
tion reaction, a first-order type of reaction is a reasonable assumption. For a
first-order reaction, the relationship among the influent and final concentra-
tions, reaction rate constant, and residence time are as follows (see Chapter 4
for more detailed discussions):

Batch reactor

k
C f −τ −τ
= e or C f = () (4.16)
k
Ce
i
C i
CFSTR
1 1
= = (4.20)
C out
k
C in 1+ k VQ( /) 1+τ

Example 5.35: Determine the Residence Time for Low-Temperature
Heating (Batch Mode of Operation)
A batch-type low-temperature thermal desorption reactor is proposed to
treat soil containing 2,500 mg/kg of total petroleum hydrocarbon (TPH). A
pilot study was conducted, and it took 25 min to reduce the concentration to
150 mg/kg. First-order kinetics applies. If the required final soil TPH con-
centration is 50 mg/kg, what should be the design residence time of the soil
in the reactor?