1. Discussion
Julie Christiansen, Gary Healea, Michael Dodson,Robert MD'anjou
While ERH will boil a vadose zone that is
under the influence of a vapor recovery
system, it is not possible to achieve a
temperature of 100ºC due to two mechanisms
working together to lower the boiling point in
the vadose zone:
• Subsurface Vacuum
• Evaporative Cooling
While not large, boiling temperature
depressions from the subsurface vacuum
applied during ERH are measurable, whereas
the reduction in boiling temperatures due to
Evaporative Cooling are substantial.
Evaporative Cooling
CONSTRAINTS ON ACHIEVING VADOSE ZONE
TEMPERATURE PERFORMANCE GOALS
Introduction
Global Remediation Solutions, LLC
Raoult’s law
In thermodynamics, the
partial vapor pressure of
each component of an
ideal mixture of liquids is
equal to the vapor
pressure of the pure
component multiplied by
its mole fraction in the
mixture.
At a typical ERH site, the average vadose zone temperature will be 85
to 90ºC. A normal distribution of vadose zone temperatures during
ERH is shown on Figure 1 The heat generated by ERH gradually brings
soil moisture to boil and produces an in situ steam source that opens
up the soil matrix, strips away contaminants, and carries them to vapor
recovery wells that are co-located with the electrodes.
Conclusion
Dalton’s Law
states that in a mixture
of non-reacting gases,
the total pressure
exerted is equal to the
sum of the partial
pressures of the
individual gases.
The reduction in boiling temperatures in the vadose due
to the air/moisture ratio under applied vacuum actually
increases VOC removal efficiency despite the ability to
reach and maintain a 100ºC temperature throughout the
treatment area. Despite these temperature constraints,
99.99% VOC reduction is achievable.
Partial
Pressure of
Water
Partial Pressure
of Benzene
Total
Pressure
+ =
H₂O = 23.76
torr
C₆H₆ = 214.52
torr
Final H₂O
pressure = 22.83
Just 4 grams of Benzene
added to 200mL of water will
lower the vapor pressure of
water from 23.76 to 22.83
torr, effectively increasing
the evaporation rate of the
mixed liquids. STP  200 mL of H₂O =
200 grams,
respectively
Questions or
feedback?
Contact:
Julie Christiansen
juliec@pnecorp.com
100%
90%
80%
70%
60%
50%
40%
30%
20%
10%
100 ᵒC90 ᵒC85 ᵒC75 ᵒC 95 ᵒC80 ᵒC70 ᵒC
Percent of Vadose Zone Below 100°C
Figure 1