Heater element for use in an in situ thermal desorption soil...

Hydraulic and earth engineering – Soil remediation – With treatment

Reexamination Certificate

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C405S128550, C405S128150, C405S128400, C405S128200, C405S128300

Reexamination Certificate

active

06632047

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to soil remediation. An embodiment of the invention relates to a heater element for raising soil temperature during an in situ thermal desorption soil remediation process.
2. Description of Related Art
Contamination of subsurface soils has become a matter of 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. 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 a food supply. Contaminants in subsurface soil may migrate into the food supply through bio-accumulation in various species that are part of a food chain.
There are many methods to remediate contaminated soil. “Remediating contaminated soil” refers to treating the soil to remove soil contaminants or to reduce contaminants within the soil to acceptable levels. A method of remediating a contaminated site is to excavate the soil and to process the soil in a separate treatment facility to eliminate or reduce contaminant levels within the soil. Many problems associated with the method may limit the effectiveness and use of the method. For example, one problem associated with the method is that excavation may generate dust that exposes the surrounding environment and workers to the soil contamination. Also, many tons of soil may need to be excavated to effectively treat even a small contamination site. Equipment cost, labor cost, transport cost, and treatment cost may make the method prohibitively expensive as compared to other available soil remediation methods.
Biological treatment and in situ chemical treatment may also be used to remediate soil. Biological and/or chemical treatment may involve injecting material into the soil. A material injected during a chemical treatment may be a reactant configured to react with the soil contamination to produce non-contaminated reaction products or volatile products that may be easily removed from the soil. The material injected during a chemical treatment may be a flooding agent configured to drive the contamination toward a production well that removes the contaminant from the soil. The flooding agent may be steam, carbon dioxide or other fluid. Soil heterogeneity and other factors may inhibit reduction of contaminant levels in the soil using biological treatment and/or chemical treatment to levels required by governmental regulations.
A process that may be used to remove contaminants from subsurface soil is a soil vapor extraction (SVE) process. An SVE process applies a vacuum to the soil to draw air and vapor through subsurface soil. The vacuum may be applied at a soil/air interface, or the vacuum may be applied through vacuum wells placed within the soil. The air and vapor may entrain and carry 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 eliminate, or reduce contaminants within the off-gas to acceptable levels. An SVE process may allow contaminants to be removed from soil without the need to move or significantly disturb the soil. An SVE process may operate under roads, foundations, and other fixed structures.
The permeability of the subsurface soil may limit the effectiveness of an 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 an SVE process has treated the soil. Reduced air permeability due to water retention, stratified soil layers, and material heterogeneities within the soil may limit the effectiveness of an SVE soil remediation process.
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 through subsurface soil.
In situ thermal desorption (ISTD) may be used to increase the effectiveness of an SVE process. An ISTD soil remediation process involves in situ heating of the soil to raise the temperature of the soil while simultaneously removing off-gas from the soil. Heating the soil may result in removal of contaminants by a number of mechanisms in addition to entrainment of contaminants in an air stream. Such mechanisms may include, but are not limited to: vaporization and vapor transport of the contaminants from the soil; entrainment and removal of contaminants in water vapor; and thermal degradation or conversion of contaminants by pyrolysis, oxidation or other chemical reactions within the soil. In situ heating of the soil may greatly increase the effectiveness of an SVE process.
An ISTD soil remediation process may offer significant advantages over SVE processes and processes that inject drive fluids or chemical and/or biological reactants into the soil. Fluid flow conductivity of an average soil may vary by a factor of 10
8
throughout the soil due in part to soil heterogeneities and water within the soil. Uniform mass transport through the soil may be a limiting factor in the remediation of a treatment site using an SVE process or a chemical and/or biological treatment of the soil. Thermal conductivity of an average soil may vary by a factor of about two throughout the soil. Injecting heat throughout soil may be significantly more effective than injecting a fluid through the same soil. Heating soil may result in an increase in the permeability of the soil. Heat transferred into the soil may dry the soil. As the soil dries, microscopic and macroscopic permeability of the soil may increase. The increase in permeability of heated soil may allow an ISTD soil remediation process to efficiently remediate the soil throughout a treatment area. The increase in soi

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