The governing equations for diffusion mass transfer are [32]
![]() |
(118) |
and
![]() |
(119) |
where
![]() |
(120) |
and
![]() |
(121) |
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:
![]() |
(122) |
and
![]() |
(123) |
where
![]() |
(124) |
and
![]() |
(125) |
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
![]() |
(126) |
or
![]() |
(127) |
If and
are constant, these equations reduce to:
![]() |
(128) |
or
![]() |
(129) |
Accordingly, by comparison with the heat equation, the correspondence in Table (16) arises.