T2VOC is a TOUGH module for three-phase flow of water, air, and a volatile organic compound (VOC). T2VOC was designed to simulate processes such as the migration of hazardous non-aqueous phase liquids (NAPLs) in variably saturated media, forced vacuum extraction of organic chemical vapors from the unsaturated zone (soil vapor extraction), evaporation and diffusion of chemical vapors in the unsaturated zone, air injection into the saturated zone for removal of volatile organics (air sparging), direct pumping of contaminated water and free product, and steam injection for the removal of NAPLs from contaminated soils and aquifers.
Features & Capabilities
In the T2VOC formulation, the multiphase system is assumed to be composed of three mass components: air, water, and a volatile, water-soluble organic chemical. Although air consists of several components (nitrogen, oxygen, etc.), it is here treated as a single "pseudo-component” with averaged properties. The three fluid components in T2VOC may be present in different proportions in any of the three phases, gas, aqueous, and NAPL, except that the (usually) small solubility of water in the NAPL phase has been neglected. The aqueous-phase density model is applicable not only to slightly but also to strongly water-soluble organic compounds. Water exists in the aqueous and vapor phases; air and VOCs may partition into the aqueous, gas and NAPL phases. In addition, VOC may be adsorbed by the porous medium.
Each phase flows in response to pressure and gravitational forces according to the multiphase extension of Darcy's law, including the effects of relative permeability and capillary pressure between the phases. Transport of the three mass components also occurs by multicomponent diffusion in the gas phase. No allowance is made for molecular diffusion in the aqueous and NAPL phases, or for hydrodynamic dispersion. It is assumed that the three phases are in local chemical and thermal equilibrium, and that no chemical reactions are taking place other than (a) interphase mass transfer, (b) adsorption of the chemical component to the solid phase, and (c) decay of VOC by biodegradation. Mechanisms of interphase mass transfer for the organic chemical component include evaporation and boiling of the NAPL, dissolution of the NAPL into the aqueous phase, condensation of the organic chemical from the gas phase into the NAPL, and equilibrium phase partitioning of the organic chemical between the gas, aqueous, and solid phases. Interphase mass transfer of the water component includes the effects of evaporation and boiling of the aqueous phase, and condensation of water vapor from the gas phase. The interphase mass transfer of the air component consists of equilibrium phase partitioning of the air between the gas, aqueous, and NAPL phases.
Heat transfer occurs due to conduction, multiphase convection, and gaseous diffusion. The heat transfer effects of phase transitions between the NAPL, aqueous and gas phases are fully accounted for by considering the transport of both latent and sensible heat. The overall porous media thermal conductivity is calculated as a function of water and NAPL saturation, and depends on the chemical characteristics of the NAPL. Water properties in the liquid and vapor state are calculated, within experimental accuracy, from the steam table equations given by the International Formulation Committee. Thermophysical properties of the NAPL phase such as saturated vapor pressure and viscosity are calculated as functions of temperature, while specific enthalpy and density are computed as functions of both temperature and pressure. Vapor pressure lowering effects due to capillary forces are not presently included in the simulator. Gas phase thermophysical properties such as specific enthalpy, viscosity, density, and component molecular diffusivities are considered to be functions of temperature, pressure, and gas phase composition. The solubility of the organic chemical in water may be specified as a function of temperature, and Henry's constant for dissolution of organic chemical vapors in the aqueous phase is calculated as a function of temperature. The Henry's constants for air dissolution in aqueous and NAPL phases are small and for simplicity have been assumed to be constant.
The necessary NAPL/organic chemical thermophysical and transport properties are computed by means of a very general equation of state. This equation of state is largely based on semi-empirical corresponding states methods in which chemical parameters are calculated as functions of the critical properties of the chemical such as the critical temperature and pressure. Because these data are available for hundreds of organic compounds, the NAPL/organic chemical equation of state is quite flexible in its application. The porous medium porosity may be specified to be a function of pore pressure and temperature, but no stress calculations are made.
See T2VOC User's Guide.