Hydraulic and earth engineering – Soil remediation – With treatment
Reexamination Certificate
2000-04-14
2002-11-26
Shackelford, Heather (Department: 3673)
Hydraulic and earth engineering
Soil remediation
With treatment
C405S128150, C405S128400, C405S128200, C405S128350
Reexamination Certificate
active
06485232
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to soil remediation, and more particularly to a heater for an in situ thermal desorption soil remediation process.
2. Description of Related Art
Contamination of subsurface soils has become a matter of great concern in many locations. Subsurface soil may become contaminated with chemical, biological, and/or radioactive contaminants. Contamination of subsurface soil may occur in a variety of ways. Hazardous material spills, leaking storage vessels, and landfill seepage of improperly disposed of materials are just a few examples of the many ways in which soil may become contaminated. Contaminants in subsurface soil can become public health hazards if the contaminants migrate into aquifers, into air, or into the food supply. Contaminants in subsurface soil may migrate into the food supply through bioaccumulation in various species that are part of the food chain.
There are many methods for removal of contaminants from subsurface soil. Some possible methods for treating contaminated subsurface soil include excavation followed by incineration, in situ vitrification, biological treatment, and in situ chemical treatment. Although these methods may be successfully applied in some applications, the methods can be very expensive. The methods may not be practical if many tons of soil must be treated.
One process that may be used to remove contaminants from subsurface soil is a soil vapor extraction (SVE) process. A SVE process applies a vacuum to a well to draw air through subsurface soil. The air carries volatile contaminants towards the source of the vacuum. Off-gas removed from the soil by the vacuum may include contaminants that were within the soil. The off-gas may be transported to a treatment facility. The off-gas removed from the soil may be processed in the treatment facility to reduce contaminants within the off-gas to acceptable levels.
The permeability of the subsurface soil may limit the effectiveness of a SVE process. Air and vapor may flow through subsurface soil primarily through high permeability regions of the soil. The air and vapor may bypass low permeability regions of the soil. Air and vapor bypassing of low permeability regions may allow large amounts of contaminants to remain in the soil after a SVE process has treated the soil. Reduced air permeability due to water retention, stratified soil layers, and heterogeneities within the soil may cause regions of high and low permeability within subsurface soil.
Reduced air permeability due to water retention may inhibit contact of the flowing air with the contaminants in the soil. A partial solution to the problem of water retention is to dewater the soil. The soil may be dewatered by lowering the water table and/or by using a vacuum dewatering technique. These methods may not be effective methods of opening the pores of the soil to admit airflow. Capillary forces may inhibit removal of water from the soil when the water table is lowered. Lowering the water table may result in moist soil. Air conductivity through moist soil is limited.
A vacuum dewatering technique may have practical limitations. The vacuum generated during a vacuum dewatering technique may diminish rapidly with distance from the dewatering wells. The use of a vacuum dewatering technique may not result in a significant improvement to the soil water retention problem. The use of a vacuum dewatering technique may result in the formation of preferential passageways for air conductivity located adjacent to the dewatering wells.
Many types of soil are characterized by horizontal layering with alternating layers of high and low permeability. A common example of a layered type of soil is lacustrine sediments. Thin beds of alternating silty and sandy layers characterize lacustrine sediments. If an SVE well intercepts several such layers, nearly all of the induced airflow occurs within the sandy layers and bypasses the silty layers.
Heterogeneities may be present in subsurface soil. Air and vapor may preferentially flow through certain regions of heterogeneous soil. Air and vapor may be impeded from flowing through other regions of heterogeneous soil. For example, air and vapor tend to flow preferentially through voids in poorly compacted fill material. Air and vapor may be impeded from flowing through overly compacted fill material. Buried debris within fill material may also impede the flow of air and vapor through subsurface soil.
In situ thermal desorption (ISTD) may be used to increase the effectiveness of a SVE process. An ISTD soil remediation process involves in situ heating of the contaminated soil to raise the temperature of the soil while simultaneously removing offgas by vacuum. In situ heating may be preferred over convective heating by the introducing of a hot fluid (such as steam) into the soil because thermal conduction through soil is very uniform as compared to mass transfer through soil. Thermal conductivity of an average soil may vary by a factor of about two throughout the soil. Fluid flow conductivity of an average soil may vary by a factor of 10
8
throughout the soil.
Soil may be heated by radiant heating in combination with thermal conduction, by radiant by radio frequency heating, or by electrical formation conduction heating. Conductive heating may be a preferred method of heating the soil because conductive heating is not limited by the amount of water present in the soil. For soil contamination within about 2 feet of the soil surface, thermal blankets may apply conductive heat to the soil. For deeper soil contamination, heaters placed in wells may apply conductive heat to the soil. Coincident or separate source vacuum may be applied to remove vapors from the soil. U.S. Pat. No. 4,984,594 issued to Vinegar et al, which is incorporated by reference as if fully set forth herein, describes an ISTD process for soil remediation of low depth soil contamination. U.S. Pat. No. 5,318,116 issued to Vinegar et al., which is corporated by reference as if fully set forth herein, describes an ISTD process for eating contaminated subsurface soil with conductive heating.
A conductive heat ISTD soil remediation process may have several advantages ver a simple soil vapor extraction system. The heat added to the contaminated soil may aise the temperature of the soil above the vaporization temperatures of the soil ontaminants. If the soil temperature exceeds the vaporization temperature of a soil ontaminant, the contaminant will become a vapor. The vacuum may be able to draw the vaporized contaminant out of the soil. Even heating the soil to a temperature below the vaporization temperature of the contaminants may have beneficial effects. Increasing the soil temperature will increase the vapor pressure of the contaminants in the soil and allow an air stream to remove a greater portion of the contaminants from the soil than is possible at lower soil temperatures.
Most soil formations include a large amount of liquid water as compared to contaminants. Raising the temperature of the soil to the vaporization temperature of the water will boil the water. The resulting water vapor may volatize contaminants within the soil by steam distillation. An applied vacuum may then remove the volatized contaminants and water vapor from the soil. Steam distillation within the soil may result in the removal of medium and high boiling point contaminants from the soil.
In addition to allowing greater removal of contaminants from the soil, the increased heat of the soil may result in the destruction of contaminants in situ. The presence of an oxidizer, such as air, may result in the oxidation of the contaminants that pass through soil that is heated to high temperatures. Contaminants within the soil may be altered by pyrolysis to form volatile compounds that are removed from the soil by the vacuum.
Heating the subsurface soil may result in an increase in the permeability of the soil. A visible indication of the increase in permeability of soil may be seen in the surface of dry lake b
Stegemeier George L.
Vinegar Harold J.
Board of Regents , The University of Texas System
Meyertons Eric B.
Mitchell Katherine
Shackelford Heather
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