TMVOC is a numerical simulator for three-phase non-isothermal flow of water, soil gas, and a multicomponent mixture of volatile organic chemicals (VOCs) in multidimensional heterogeneous porous media. TMVOC is designed for applications to contamination problems that involve hydrocarbon fuel or organic solvent spills in saturated and unsaturated zones. It can model contaminant behavior under "natural" environmental conditions, as well as for engineered systems, such as soil vapor extraction, groundwater pumping, or steam-assisted source remediation. TMVOC is backwards compatible with T2VOC and can be initialized from T2VOC-style initial conditions.
The main enhancements in TMVOC relative to T2VOC are as follows:
- a multicomponent mixture of volatile organic chemicals can be modeled;
- any and all combinations of the three phases water - oil - gas are treated;
- several non-condensible gases may be present;
- diffusion is treated in all phases in a manner that is fully coupled with phase partitioning.
Features & Capabilities
In the TMVOC formulation, the multiphase system is assumed to be composed of water, non-condensible gases (NCGs), and water-soluble volatile organic chemicals (VOCs). The number and nature of NCGs and VOCs can be specified by the user. There are no intrinsic limitations to the number of NCGs or VOCs in the TMVOC formulation. NCGs may be selected by the user from a data bank provided in TMVOC; currently available choices include O2, N2, CO2, CH4, ethane, ethylene, acetylene, and air (a pseudo-component treated with properties averaged from N2 and O2). In most TMVOC applications, just a single NCG, air, will be present. Thermophysical property data for VOCs must be provided by the user. The fluid components may partition (volatilize and/or dissolve) among gas, aqueous, and NAPL phases. Any combination of the three phases may be present, and phases may appear and disappear in the course of a simulation. In addition, VOCs may be adsorbed by the porous medium, and may biodegrade according to a simple half-life model. Each phase flows in response to pressure and gravitational forces according to a multiphase extension of Darcy’s law, which includes effects of relative permeability and capillary pressure between the phases. Transport of the mass components may also occur by molecular diffusion in all phases. Multiphase diffusion is treated in a fully-coupled manner that can cope with diffusion of phase-partitioning components under conditions of variable phase saturations. In heterogeneous media dispersion is often caused by mass exchanges between pore regions with different fluid mobilities; such effects can be modeled with TMVOC using the method of “multiple interacting continua”.
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 chemical components to the solid phase, and (c) decay of VOCs by biodegradation. Mechanisms of interphase mass transfer for the organic chemicals include evaporation and boiling, dissolution into the aqueous phase, condensation of organic chemicals from the gas phase into a NAPL, and equilibrium phase partitioning of organic chemicals 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, condensation of water vapor from the gas phase, and water dissolution into the NAPL phase. The interphase mass transfer of the non-condensible gas components consists of equilibrium phase partitioning between the gas, aqueous, and NAPL phases.
Heat transfer occurs due to conduction and multiphase convection; heat flow associated with diffusive fluxes is neglected. 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. However, heat of dissolution effects for NCG dissolution in NAPL and aqueous phases and for water in the NAPL are neglected, as are heat effects associated with adsorption/desorption of VOCs. The overall porous medium thermal conductivity is calculated as a function of total liquid saturation (water and NAPL).
The saturation pressure, density and internal energy of water are computed, within experimental accuracy, using the International Formulation Committee correlations implemented in the TOUGH2 code. Dynamic viscosity of liquid water and steam is calculated using the correlation proposed by the International Association for the Properties of Steam. Thermophysical properties of the NAPL phase such as saturated vapor pressure and viscosity are calculated as functions of temperature and composition, while specific enthalpy and density are computed as function of temperature, composition, 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 coefficient for dissolution of organic chemical vapors in the aqueous phase is calculated as a function of temperature. The Henry’s coefficients for NCG dissolution in the aqueous phase are calculated as functions of temperature, whereas Henry’s coefficients for NCG dissolution in NAPL have for simplicity been assumed to be constant. Water solubility in a generic NAPL is computed as a function of temperature.
The necessary NAPL/organic chemical and transport properties are computed by means of a very general thermodynamic formulation, which uses 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 gas phase is treated as a mixture of “real” (not ideal) gases. Porosity may change as a function of fluid pressure and temperature, using simple concepts of pore compressibility and expansivity.
See TMVOC User's Guide.