Page 189 - Materials Chemistry, Second Edition

172 Practical Design Calculations for Groundwater and Soil Remediation

2. This example assumed a perfect ideal flow, but it would not hap-
pen in reality. In other words, the time required to flush one pore
volume of the impacted zone will be much longer than 1,960 min.

5.2.5 COC Removal Rate

The COC removal rate (R removal ) can be estimated by multiplying the extracted
vapor flow rate (Q) with the vapor concentration (G):

R removal = (G)(Q) (5.8)
Care should be taken to have G and Q in consistent units, and G should be
in mass-concentration units. Equation (5.1) can be used to estimate the initial
vapor concentration if the free-product phase is present, while the procedure
as illustrated in Example 5.5 can be used to estimate the extracted vapor con-
centration in the absence of the free-product phase. It is worthwhile to note
again that the calculated vapor concentrations are the ideal and equilibrium
values. The actual values should only be fractions of these values, mainly
due to the fact that not the entire air stream passes through the impacted
zone and that limitations of mass transfer exist (the system will not reach
equilibrium in most, if not all, cases). Nevertheless, the calculated values pro-
vide useful information. One can compare them with the actual data from
the collected samples and establish the correlation between them. The cal-
culated data can then be calibrated, adjusted, and used for later predictions.
For example, if we know that only a fraction η of the air flows through the
impacted zone, Equation (5.8) should be modified as:

R removal = [(η)(G)](Q) (5.9)
The removal rate estimated from Equation (5.9) still represents the upper
limit of the vapor concentration because it does not consider mass-transfer
limitations. The factor η can be considered as an overall efficiency factor if it
takes into account the percentage of flow through the impacted zone and the
limitations of mass transfer.
The following procedure can be used to determine the COC removal rate:

Step 1: Determine the extraction vapor flow rate from field measure-
ments or from the procedure described in Section 5.2.4.
Step 2: Estimate the extracted vapor concentration using Equation
(5.1) if the free-product phase is present, while the procedure
illustrated in Example 5.5 can be used to estimate the extracted
vapor concentration in the absence of the free-product phase.
Step 3: Convert the vapor concentration into a mass concentration by
using Equation (2.1).
Step 4: Adjust the calculated concentration from Step 3 by an overall
efficiency factor, η.