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TOUGH2 simulations of the TEVES Project including the behavior of a single-component NAPL

01 May 1996-

AbstractThe TEVES (Thermal Enhanced Vapor Extraction System) Project is a demonstration of a process designed to extract solvents and chemicals contained in the Chemical Waste Landfill at Sandia National Laboratories. In this process, the ground is electrically heated, and borehole(s) within the heated zone are maintained at a vacuum to draw air and evaporated contaminants into the borehole and a subsequent treatment facility. TOUGH2 simulations have been performed to evaluate the fluid flow and heat transfer behavior of the system. The TOUGH2 version used in this study includes air, water, and a single-component non-aqueous phase liquid (NAPL). In the present simulations, an initial o-xylene inventory is assumed in the heated zone for illustration purposes. Variation in borehole (vapor extraction) vacuum, borehole location, and soil permeability were investigated. Simulations indicate that the temperatures in the soil are relatively insensitive to the magnitude of the borehole vacuum or the borehole locations. In contrast, however, the NAPL and liquid water saturation distributions are sensitive to these borehole parameters. As the borehole vacuum and air flow rate through the soil decrease, the possibility of contaminant (NAPL) migration from the heated zone into the surrounding unheated soil increases. The borehole location can also affect the likelihood of contaminant movement into the unheated soil.

Topics: Borehole (60%), Soil vapor extraction (55%)

Summary (3 min read)

2.1 TEVES Parameters

  • For simplicity, the soil properties are assumed to be uniform with a permeability of 50 darcies (Phelan, 1993) , a porosity of 0.333 and an initial liquid water saturation of 0.20, which is less than the liquid residual saturation.
  • These and other parameters including some o-xylene properties are summarized in.

2.2 TOUGH2 Model

  • The borehole consists of a series of elements connected to the appropriate soil elements.
  • Flow resistances from the surrounding soil elements to the borehole elements were modified to reflect the borehole geometry.
  • Upon examination of the results, the heat loss to the borehole was not realistic and affected the results.
  • Therefore, the simulations were redone with an insulated boundary condition at the borehole, thereby eliminating the heat loss from the soil to the borehole.

2.3 TOUGH2 Simulations

  • In addition to the extraction location and borehole vacuum variations, the permeability of the soil was varied for inside extraction.
  • Based on Darcy's law, the gas velocity is a function of the product of the intrinsic permeability and the imposed pressure difference.
  • These results will be compared to evaluate the potential for scaling and are shown in Section 5.0.

3.0 Simulation Results -Inside Extraction

  • The results in this section are for vapor extraction from the 2 inside wells, or inside extraction.
  • Lb and c give the side views for the model looking at the long and short side of the heated zone, respectively.
  • The results presented in this section have been scaled to reflect the entire heated zone.
  • Therefore, when masses or flow rates are given, they refer to the entire heated zone or to the 2 wells, not to the results from the quarter symmetry model.
  • The planes are labelled for reference to the plots.

3.1 2.5 kPa Borehole Vacuum

  • The time variation of the soil temperature in the heated zone (average, minimum, and maximum) out to 180 days is shown in Figure 3 -3a, while the maximum soil temperature in the unheated zone is given in Figure 3 -3b.
  • The temperature in the heated zone increases over the first 10 days and then begins to level out as the liquid water evaporates.
  • After 60 days, the average heated zone temperature is about 246°C; at this time, the minimum and maximum heated zone temperatures are 122 and 361 "C, respectively.
  • The wide variation in temperatures is due to heat conduction to the unheated soil and to the overlying atmosphere, cooling by the air flowing through the soil, and evaporation.
  • At 180 days, the maximum unheated zone soil temperature is 46°C.

45.7 m Calculational Domain

  • Figure 3 -14a shows the various mass flow rates into the borehole.
  • The vapor flow rate into the borehole is essentially the same as the heated zone evaporation rate in the early stages.
  • The heated zone evaporation rate is negative after heating is stopped since condensation is occurring.
  • Results for a borehole vacuum of 1.0 kPa (4 in. of water) for inside extraction are given in this section.
  • Unlike the 2.5 kPa case, only the heatup phase of 60 days was simulated.

Gas flow velocity vectors are given in

  • The time variation is very similar to that for a 2.5 kPa borehole vacuum.
  • The approximate time when half of the NAPL remains is about 7 days.
  • Liquid water saturation contours for later times are also essentially the same as for 2.5 kPa; the three views at 60 days are given in Figure 3 -22.
  • The total mass flow rate into the borehole increases in the early stages of heating due to the evaporation of the NAPL (VOC) component in the heated soil.

3.3 0.5 kPa Borehole Vacuum

  • Early-time liquid water and NAPL saturation contours are given in Figures 3-28 to 3-30.
  • The liquid water and NAPL saturation contours look generally similar to the 1.0.
  • Wa case except that a zone of NAPL is left behind on the edges of the heated zone as shown at 11.6 days; these "pocketst' are areas that lagged behind in the 1.0 kPa case.
  • For much of the simulation, the mass fraction is predominantly water vapor.
  • Figure 3 -34b shows the evaporation rate in the heated and unheated zones along with the water vapor flow rate into the borehole.

Liquid water saturation contours also indicate an insufficient air sweep as shown in

  • Gas flow velocity vectors are essentially constant with time; the various view at 60 days are shown in Figure 3 -35.
  • In fact, some vectors in Figures 3-35a and 3-35b in the unheated soil are directly away from the borehole.
  • As shown in Figure 3 -36c, this region corresponds to NAPL migration into the unheated soil.
  • Therefore, when masses or flow rates are given, they refer to the entire heated zone or to the 4 wells, not the results from the quarter symmetry model.
  • Plots are given for parameters at the center of each element, and the planes shown in the plots are labelled on Figure 4 -2 for reference.

Heated Zone

  • Distance inside extraction, all air flow passed through the heated zone before entering the borehole; in the outside extraction case, probably the majority of the flow comes from the unheated zone, so the air sweep is not as efficient as for inside extraction.
  • A region of flow from the heated zone into the unheated soil is present in the lower corner of the long-side view since the borehole extraction depth is greater than the heated zone depth.
  • Also, a low-flow region exists from the borehole towards the symmetry planes.
  • This region is also obvious in the saturation plots shown earlier.

4.2 1.0 kPa Borehole Vacuum

  • Figures 4-22 through 4-24 present the liquid water saturation contours at 30 and 60 days for the various views.
  • Liquid water saturations higher than 20% are seen at the same locations as the NAPL pockets described above, indicating water migration into the unheated zone.
  • At 60 days, most of the migrated water has evaporated, although there still are some higher saturation zones.

5.0 Simulation Results -Soil Permeability Variation

  • The mass flow rate composition entering the borehole changed dramatically as the vacuum was reduced, changing from air dominated to water vapor dominated.
  • The gas velocity vectors indicated less uniform air sweep as the borehole vacuum was reduced, including directions directly away fiom the borehole.

6.2 Outside Extraction

  • The flow rate into the borehole is predominantly air in all cases since, with the location on the edge of the heated zone, most of the air will not pass through the heated zone.
  • In addition, since the borehole length is longer than the heated zone, air can flow fiom the heated zone into the unheated soil, increasing the likelihood of liquid water and NAPL migration into the unheated soil.

6.3 Soil Permeability Variation

  • Scaling the present results with Darcy's law was investigated by decreasing the soil permeability by a factor of 5 while increasing the borehole vacuum by the same factor.
  • Essentially all the results (temperature, liquid saturation, NAPL, and flow rates) were the same for the two cases indicating that Darcy's law may be used to estimate the effect of other conditions not explicitly analyzed.

7.0 Summary and Conclusions

  • From these results, sufficient air flow through the heated zone must be provided to contain the contaminants within the heated zone.
  • If the air flow rate is too low, water and NAPL may migrate into the unheated region outside the heated zone.
  • For lower permeability soils, the borehole vacuum will have to be increased, or the heating rate decreased, to prevent contaminant migration.

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Journal ArticleDOI
Abstract: Soil heating has been proposed as a method to enhance the vapor extraction of NAPLs from contaminated soils Three-dimensional fluid flow and heat transfer simulations have been performed for soil-heated vapor extraction to determine the transient system performance for a hypothetical configuration Soil layering has been considered in evaluation of the initial non-aqueous phase liquid (NAPL) distribution and in evaporation and transport to the vapor extraction location Results from this layered model are compared with results for a homogeneous system with an initially uniform NAPL, indicating the influence of layering, the initial NAPL distribution, the type of NAPL, and the possibility of enhanced vapor diffusion Not only is the NAPL removal time reduced significantly with the addition of heat, but the uncertainty in the removal time owing to a number of difficult to characterize in situ factors, such as layering and the initial NAPL distribution, is much less than for standard soil vapor extraction without heating, owing to the rise in temperature and increase in NAPL vapor pressure with time

14 citations