Most current Ansys Chemkin Reactor Models do not accommodate situations where the surface site
density is not constant for a specific surface-site phase. However, It is possible for a user
to write their own programs that include equations for performing site-density balances where
a non-zero net rate of production of surface sites is allowed. In such cases, it is possible
that a given surface reaction (or reactions) will not conserve the number of surface sites.
When sites are not conserved, the density of sites 
is not necessarily a constant. One must then be sure to use the correct
value of 
in relationships such as Equation 2–3
, which relating a site fraction
and a surface molar concentration. It may be necessary to add equations to calculate the
current value of the total site concentration of each surface phase. Because surface site
non-conservation is an issue that can alter the basic governing equations of the system, we
require that one acknowledge its use by adding a keyword on the REACTIONS
line (REACTIONS Line Options
of
the Chemkin Input Manual
Input Manual). It
is up to the user's program to ensure that the current site concentrations are correct.
Subroutines SKRAT, SKRATI, which return an array of
species production rates, also return an array of surface phase production rates, which would
all be zero if sites are conserved in every reaction.
In the case where the user overrides the default of site conservation, the
production rate
(in moles/cm2 /sec) for each
surface phase is
given by:
 | (4–48) |
where
 | (4–49) |
Here, the term 
is the net change in the number of surface sites of type 
for surface
reaction 
.