The Thermodynamic Bubble problem and its relevance for In-Situ thermal remediation

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ABSTRACT

This paper presents the thermodynamic bubble problem and its relevance for successful design of an in-situ thermal remedy of DNAPL in the water saturated zone. This investigation is relevant to the mass transfer of chemicals that have been volatilized as a result of an in-situ thermal remediation process. The volatilized chemicals exist as numerous entrained bubbles in the water phase within the porosity of the saturated soil.
A pressure gradient is created in the soil by the operation of extraction and injection wells. The pressure gradient is towards the extraction wells which operate at a much lower pressure than the initial pressure of the aquifer and the injection wells. This gradient forces the water and entrained bubbles to flow in a horizontal direction towards the extraction wells for recovery of the chemicals from the soil.

In addition to the pressure gradient, a vertical upward force acts on the entrained bubbles. This force is a direct result of the density difference between the water and gas phase and is referred to as the buoyancy force. Consequently, as the entrained bubbles are moving in a horizontal direction towards the extraction wells, they are also moving vertically. If the bubble is not extracted from the soil rapidly it could rise into cooler regions and condense into a liquid phase. This results in a vertical migration of the chemicals and is detrimental to the efficiency of the in-situ thermal remediation process. The thermodynamically aggravated vertical migration of contaminants has been observed in the field. Data from the Turtle Bayou Superfund site provides evidence of the vertical migration of DNAPL.

The design of the extraction system becomes critical to the success of an in-situ thermal remediation process. The distance between extraction and injection wells, the layout of wells, and the rate of extraction are critical design considerations for the prevention of vertical migration of the bubbles. We have derived a simple mathematical model that is based on the physics of a thermodynamically created bubble in porous media. The model tracks the flow path of the bubbles in the soil as a function of temperature and pressure, the distance between wells, and extraction rate. The model can be used by the engineer to optimize the design of an extraction system while accounting for geology and chemical composition of the DNAPL.

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