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Thermal Remediation of CVOCs (i.e., halogenated VOCs)

TerraTherm has completed numerous projects worldwide involving remediation of Chlorinated Volatile Organic Compounds (CVOCs) and CVOC-Dense Non-Aqueous Phase Liquids (DNAPLs) in a wide variety of geological settings. Our technologies have been proven well suited to these contaminants, typically achieving MCLs in groundwater and/or >99.9% reductions in soil concentrations (e.g., <1 mg/kg in source zones). Contaminants commonly included within this category include:

  • Tetrachloroethene or Perchloroethene (PCE)
  • Trichloroethene (TCE)
  • cis -1,2-Dichloroethene (cis-DCE)
  • Vinyl Chloride (VC)
  • 1,1,2,2-Tetrachloroethane (1,1,2,2-PCA)
  • 1,1,1-Trichloroethane (1,1,1-TCA)
  • Carbon Tetrachloride (CT)
  • Chloroform (CF)
  • Methylene Chloride (MC)


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Non-halogenated VOCs, including Benzene, Toluene, Ethylbenzene and Xylenes (BTEX) can also be treated.

TerraTherm’s processes are highly cost competitive relative to other approaches, including excavation, with unit treatment costs diminishing with the scale and configuration of the treatment volume (the deeper the cheaper).

A number of our technologies are applicable to the treatment of CVOCs and CVOC-DNAPLs including In Situ Thermal Desorption (ISTD), Steam Enhanced Extraction (SEE), and Electro-Thermal Dynamic Stripping Process (ET-DSP™). It should also be noted that a combination of these technologies, such as ISTD plus SEE can be applied for optimal results in sites containing both low permeability layers and high velocity aquifer units.

Thermal remediation process for CVOCs and DNAPL

With most CVOCs, we recommend a target temperature of 100°C (i.e., boiling point of water at ambient elevation/pressure). As the site is heated, the vapor pressures and Henry’s Law Constants of the various CVOC constituents increase. Therefore, as the temperature is increased, CVOCs will readily enter the gas phase, enabling their removal by vapor extraction and/or multiphase extraction.

Of all the contaminant property changes that occur during heating, the increase in vapor pressure is the most dramatic. Vapor pressures of TCE and PCE, for example, increase exponentially during heating from ambient temperature to the boiling point of water.

The Heterogeneous Azeotrope (or “Eutectic Point”) is the temperature at which a drop of CVOC-DNAPL and liquid water in contact with it will begin to co-boil. As a result of Raoult’s Law, the vapor pressures of the two components are additive, such that they co-boil at a temperature below the boiling points of each of the pure substances.

The following table presents the heterogeneous azeotropes of several common chlorinated solvents (Gmehling and Onken 1977). Once the temperature has risen past the heterogeneous azeotrope, DNAPL can no longer exist as a separate phase. If thorough removal is required, however, it is still desirable to heat to the boiling point of water, so that steam distillation will also remove sorbed, gaseous, and dissolved constituents.

Chlorinated
Solvent

 

Pure Substance Boiling Point

 

(°C)

Heterogeneous Azeotrope with Water-Boiling Point

(°C)

PCE

121

88

TCE

87

73

1,1,2-TCA

114

86

CT

77

67

CF

61

56

MC

40

39

Source: Gmehling and Onken 1977

Treatment approaches and wellfield configurations for CVOCs vary depending on the nature of the site and which technology or combination of technologies is chosen. The main design objectives are to achieve the selected target temperature throughout the Target Treatment Zone (TTZ), and ensure recovery of the resulting vapors and steam. When properly designed, such a system prevents condensed vapors from being left behind in cold spots.

Achieving 100°C within the treatment zone is sufficient to boil off CVOC DNAPLs and to volatilize and steam-strip dissolved and sorbed CVOCs. Removal (i.e., boiling off) of 10 to 30% of the water within the treatment zone is sufficient to achieve MCLs in groundwater and/or >99.9% reductions in soil concentrations (e.g., <1 mg/kg in source zones).

 

 

 

 
   
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