Page 289 - Materials Chemistry, Second Edition

272 Practical Design Calculations for Groundwater and Soil Remediation

where M GAC is the mass, V GAC is the volume, and ρ is the bulk density of the
b
GAC, respectively.
The following procedure can be used to determine the adsorption capacity
of a GAC adsorber:

Step 1: Determine the theoretical adsorption capacity by using
Equation (7.1).
Step 2: Determine the actual adsorption capacity by using Equation (7.2).
Step 3: Determine the amount of activated carbon in the adsorption
unit (also called the adsorber).
Step 4: Determine the maximum amount of contaminants that can be
held by the adsorber by using Equation (7.3).

Information needed for this calculation:

• Adsorption isotherm
• COC concentration in the influent air stream, P VOC
• Volume of the GAC, V GAC
• Bulk density of the GAC, ρ b

Example 7.1: Determine the Capacity of a GAC Adsorber
The off-gas from a soil-venting project is to be treated by GAC adsorbers.
The m-xylene concentration in the off-gas is 800 ppmV. The flow rate out of
the vacuum pump is 200 cfm, and the temperature of the air is ambient. Two
1000-lb activated-carbon adsorbers are proposed. Determine the maximum
amount of m-xylene that can be held by each GAC adsorber before being
exhausted. Use the isotherm data in Table 7.1.

Solution:
(a) Convert the xylene concentration from ppmV to psi as:
P VOC = 800 ppmV = 800 × 10 atm = 8.0 × 10 atm
−4
−6
= (8.0 × 10 atm)(14.7 psi/atm) = 0.0118 psi
−4
Obtain the empirical constants for the adsorption isotherm
from Table 7.1 and then apply Equation (7.1) to determine the
equilibrium adsorption capacity as:
a P
q = ( VOC ) m = (0.527)(0.0118) 0.0703 = 0.386 lb/lb
(b) The actual adsorption capacity can be found by using Equation (7.2)
as:

=
q design = (50%) theoretical = (50%)(0.386) 0.193 lb/lb
q