Figure 1 - Schematic of ISTD Process

      A major advantage of ISTD over other remediation processes is that it increases the permeability of the soil by 5 to 8 orders of magnitude as the soil desiccates.  Thus, contaminants in silt and clay can be readily removed.  Moreover, much of the heat is transported by conduction so that impermeable barriers do not hinder the process.

      ISTD remediation time is determined by heater size and spacing, soil-water saturation and target temperature.  The well spacing determines the heat that can be added, while the amount of water to be boiled governs the heat needed.  Although larger well spacing slows remediation, ISTD is so fast that wider well spacing is favored, because the cost of constructing the site is reduced with the square of the well spacing.

Figure 2 - Temperature after 80 Days

      Figure 2 shows the temperature that will be achieved in the project after 80 days of heating.  The temperature in the 1/6th of an element of symmetry model is approximately 1500^oF at heater wells but exceeds 1550^oF at the heater-vacuum wells where the  air and oxidizing reactions are concentrated.  Between heaters, the temperature  is nearly 1000^oF, but the lower corners of the model are just hot enough for the larger PAH compounds like benzo(a)pyrene [B(a)P] to be desorbed.
 
      There are four major classes of PAH destruction reactions.  These are oxidation, pyrolysis (coking), water-gas reactions, and reactions of coke with water or air.  The reactions in ISTD are analogous to those that take place during in-situ combustion or pyrolysis of petroleum.  Combustion does not actually occur in ISTD because the concentration of fuel is not high where the flux of air is significant, e.g., at the heater-vacuum well. Most decomposition takes place throughout the heated soil matrix and in the very hot vapors.  Other reactions that occur are similar to pyrolysis, coking and steam reforming reactions, which are used in chemical plants and refineries to form olefins, coke and carbon oxides.  The steam reforming reactions that form carbon oxides are the same reactions used to make town gas at the site.
 

Figure 3 - Mole Fractions in ISTD

     Figure 3 compares the mole fractions of steam, air and PAHs that are predicted to be produced.  Initially, there is little oxygen (air) available unless it is being injected at the heater-vacuum well because all of the original oxygen is consumed by the contaminants in the soil matrix.  Moreover, water is being removed from the site.  This steam bank prevents air infiltration into the site. Thus, reactions with water and coking will be the primary destruction mechanism for the first six weeks.  Then, oxidation of PAH and coke become more important.

Figure 4 - Production with ISTD

     Eighteen simultaneous reactions were used to predict on that 99.999% of the production of the carcinogenic PAH benzo(a)pyrene (Figure 4) will be consumed after 60 days of heating.  The simulations show that coking and oxidation with air are primarily responsible for the destruction.  PAH oxidation should provide approximately 25% of the energy needed to desorb and destroy the hydrocarbons.  A description of the reaction mechanisms used in these and other simulations was published in 2002 and is available upon request.
 
     Simulations like these are invaluable in engineering ISTD projects.  Not only do they assist in selection of the spacing and design of the wells, but they also show how the wells should be operated, how long the process can take, how much insulation and energy is required, and how best to destroy the chemicals.  For more information on ISTD please visit TerraTherm’s website TerraTherm.com or contact MK Tech Solutions.