Page 363 - gas transport in porous media

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Chapter 22: Environmental Remediation of Volatile Organic Compounds
characteristics of the condensation front, causing it to spread out over a larger distance.
This spreading of the condensation front by noncondensible gas injection can be used
to minimize the accumulation and mobilization of NAPL at the condensation front
(Farber, 1997).
If the NAPL boiling point is lower than the steam temperature, then any NAPL not
mobilized by the steam front will boil as the condensation front passes through it. If
however, the NAPL has a higher boiling point, some of the trapped NAPL can remain
behind the condensation front in the steam zone. The rate at which this trapped NAPL
evaporates is of interest because in some cases the trapped high boiling point NAPL
can still evaporate fast enough to keep up with the advancing steam condensation
front (Falta et al., 1992; Yuan and Udell, 1993).
The speed of the NAPL evaporation front in the steam zone can be estimated by
assuming a constant, immobile NAPLsaturation, local chemical equilibrium between
the gas and NAPL phases, and the ideal gas law. Using the chemical vapor pressure
at the steam temperature, p c vap , the saturated vapor concentration is calculated, and
multiplied by the steam darcy velocity to get the NAPL evaporation rate. This is
divided by the mass of NAPL per unit volume to get the NAPL evaporation front
velocity (Falta et al., 1992):
c
v s (p c vap M /RT s )
wt
v ef = (22.12)
φρ n S nr
c
where M wt is the molecular weight of the chemical, and S nr is the residual NAPL
◦
saturation. It appears that NAPLs with boiling points below about 175 C will not
tend to exist in the steam zone based on the analyses by Falta et al. (1992) and Yuan
and Udell (1993), and on the experiments of Stewart and Udell (1988), Basel (1991)
and Yuan and Udell (1993).

22.4.3 Stripping of Dissolved Chemicals during Boiling
One of the interesting phenomena that can occur during steam injection operations
is boiling of pore water in hot zones during depressurization (Udell, 1994). This
boiling can be stimulated in field operations by stopping the steam injection once
the treatment zone is hot, and starting active vapor extraction operations to create
a vacuum. As the pore water boils into vapor, the associated volume change of a
factor of about 1,600 leads to a very strong vapor stripping mechanism that can
remove dissolved contaminants from the liquid water. Following Udell (1994), this
steam stripping mechanism can be analyzed using a mass balance on the dissolved
contaminant. It is assumed that the porous media is already at the steam temperature,
and that depressurization is causing boiling of the pore water. The mass of chemical
in the pore water can be written as

M l C l
M c = (22.13)
ρ l