Each component of mass being transferred between the continuous and particle phases satisfies the equation:

(6–43)

In this equation, is the mass of the constituent in the particle, is the mass fraction of component in the particle, is the mass fraction of component in the surrounding fluid, is the equilibrium mass fraction ratio, is the dynamic diffusivity of the mass fraction in the continuum, and is the Sherwood number given by:

(6–44)

If no value is set for the equilibrium mass fraction , a value of 1 is used.

The simple model assumes that the mass transfer is driven only by concentration differences. While this may be appropriate in some situations, such as solids having a small moisture content, it does not adequately account for the vapor pressure dependence on particle temperature, which is imported for evaporating liquids. In these situations, the liquid evaporation model, presented below, is more appropriate.

The mass source to the continuous fluid is obtained from:

(6–45)