The governing equations for diffusion mass transfer are [35]

(385) |

and

(386) |

where

(387) |

and

(388) |

In these equations is the mass flux of species A, is the mass diffusivity, is the mass fraction of species A and is the density of species A. Furthermore, is the rate of increase of the mass of species A per unit volume of the mixture. Another way of formulating this is:

(389) |

and

(390) |

where

(391) |

and

(392) |

Here, is the molar flux of species A, is the mass diffusivity, is the mole fraction of species A and is the molar concentration of species A. Furthermore, is the rate of increase of the molar concentration of species A.

The resulting equation now reads

(393) |

or

(394) |

If and are constant, these equations reduce to:

(395) |

or

(396) |

Accordingly, by comparison with the heat equation, the correspondence in Table (16) arises.