Page 230 - Materials Chemistry, Second Edition

Vadose Zone Soil Remediation 213

Solution:
(a) Determine the rate constant by using Equation (4.16):

150
C f −τ − k(25)
k
= e = = e
C i 2,500
So, k = 0.113/min

(b) Now, we use this rate constant and Equation (4.16) to determine
the required detention time:

50
C f −τ − (0.113)τ
= e = = e
k
C i 2,500
So, τ = 35 min

Discussion:
The rate constant is often obtained from bench-scale experiments by
using the batch-type reactors.

Example 5.36: Determine the Residence Time for Low-Temperature
Thermal Desorption (Continuous Mode of Operation)
A low-temperature thermal-desorption soil reactor is proposed to treat soil
containing 2,500 mg/kg of total petroleum hydrocarbon (TPH) in a continu-
ous mode of operation. Assume that the reactor is a CFSTR and that first-
order kinetics applies. A pilot study was conducted and the reaction-rate
constant was determined to be 0.3/min. The required final soil TPH concen-
tration is 100 mg/kg.

1. What should be the design residence time of the soil in the reactor?
2. The soil content of the reactor is to be kept at less than 30% of the total
reactor volume to allow for efficient mixing. Estimate the required
size of the reactor vessel to treat the impacted soil at a rate of 500
kg/h.

Solution:
(a) Determine the required retention time by using Equation (4.20):
1 100 1
= = =
C out
k
C in 1+τ 3,000 1(0.3)+ τ