Vapor transport, solution-diffusion model

The basic assumption for a mass transport limited model is that diffusion of water vapor thorugh air provides the major resistance to moisture sorption on hygroscopic materials. The boundary conditions for the mass transport limited sorption model are that at the surface of the condensed film the partial pressure of water is given by the vapor pressure above a saturated solution of the salt (Ps) and at the edge of the diffusion boundary layer the vapor pressure is experimentally fixed to be Pc. The problem involves setting up a mass balance and solving the differential equation according to the boundary conditions (see Fig. 10). 

Reasonably accurate (agreement with literature values better than 5%) heats of evaporation of some solvents (water, ether, dioxane) and NH4N03 solutions were obtained by TG methods (99). This technique also was used (100) to study the volatilization rates of some organic compounds that are of interest as environmental contaminants (naphthalene, hexachlorobenzene, 4-chlorobiphenyl, n-decane). Evaporation rates were influenced by the rates of heating (5, 10, and 25 K min-1)-A good representation of behavior was provided by the evaporation model, described in detail, provided that the surface area of the substance was known. It was assumed that equilibrium was established between condensed phase and vapor and that there was convective transport and diffusion to the container outlet. It was concluded that TG methods provide a useful method for studies of the evaporation of organic compounds. 

A numerical model was developed to simulate MeBr movement in soil and volatilization into the atmosphere. The model simultaneously solves partial differential equations for nonlinear transport of water, heat, and solute in a variably saturated porous medium. Henry s Law is used to express partitioning between the liquid and gas phases and both liquid and vapor diffusion are included in the simulation. Soil degradation is simulated using a first-order decay reaction and the rate coefficients may differ in each of the three phases (i.e., liquid, solid, or gaseous). 

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