Page 295 - Materials Chemistry, Second Edition
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278 Practical Design Calculations for Groundwater and Soil Remediation
Example 7.5: Determine the Amount of GAC Required
for On-Site Regeneration
Referring to the remediation project described in Example 7.3, an on-site
regeneration GAC is proposed to deal with the high COC loading. The system
consists of three adsorbers. Two of the three adsorbers are in the adsor ption
cycle, and the other one is in the regeneration cycle. The adsorption cycle
time is six hours. Determine the amount of GAC required for this system.
Solution:
The total amount of GAC required in all three adsorbers can be deter-
mined by using Equation (7.10) as:
R removalad N des (0.65 lb/min)(360 min) 1
T
M GAC = 1+ = 1+ = 1,818 lb
q N ad (0.193 lb/lb) 2
So, a total of 1,818 pounds of GAC is required (606 pounds in each bed).
7.3 Thermal Oxidation
Thermal processes are also commonly used to treat VOC-laden air. Thermal
oxidation, catalytic oxidation, and internal combustion (IC) engines are pop-
ular thermal processes for these applications. The key components of thermal
treatment system design are the three Ts, which are combustion temperature,
residence time (also called dwell time), and turbulence. They affect the size of
a reactor and its destruction efficiency. For example, to achieve good thermal
destruction, the VOC-laden air should be held inside a thermal oxidizer for
a sufficient residence time (normally 0.3–1.0 s) at a high temperature, at least
100°F above the auto-ignition temperatures of the COCs in the VOC-laden gas
stream. In addition, sufficient turbulence must be maintained in the oxidizer
to ensure good mixing and complete combustion of the COCs. Other impor-
tant parameters to be considered include heating value of the influent and the
requirements of auxiliary fuel and supplementary air.
Discussion on the combustion basics for thermal oxidation will be pre-
sented here, and it is essentially applicable to other thermal processes.
7.3.1 Air Flow Rate versus Temperature
The volumetric air flow rate is commonly expressed in ft /min in the US
3
customary system, i.e., cubic feet per minute (cfm). Since the volumetric flow
rate of an air stream is a function of temperature and the air stream under-
goes zones of different temperatures in a thermal process, the air flow rate is
