The governing equations for diffusion mass transfer are [34]
![]() |
(227) |
and
![]() |
(228) |
where
![]() |
(229) |
and
![]() |
(230) |
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:
![]() |
(231) |
and
![]() |
(232) |
where
![]() |
(233) |
and
![]() |
(234) |
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
![]() |
(235) |
or
![]() |
(236) |
If and
are constant, these equations reduce to:
![]() |
(237) |
or
![]() |
(238) |
Accordingly, by comparison with the heat equation, the correspondence in Table (16) arises.