For a 0-D homogeneous system, the molar production of a species per unit volume, , is given by

(18–1)

where and are the stoichiometric coefficients for the gas and surface reactions, respectively, and and are the rate of progress of the gas-phase reactions and the surface reactions. The contribution to the rate of production of species from gas-phase reaction is therefore

(18–2)

and for a surface reaction,

(18–3)

where is the surface area of the material with which the reaction is associated. Further details about how the reactions contribute to net species production rates are found in Gas-phase Chemical Rate Expressions and Surface Chemical Rate Expressions . The reactor model computes normalized values of the reaction contributions to the species production and destruction rates. The normalized production-contributions for gas-phase reactions are given by:

(18–4)

and the normalized destruction values for the gas-phase reactions is given by:

(18–5)

The normalized production values for the surface reactions are given by:

(18–6)

and the normalized destruction values for surface reactions are given by:

(18–7)

Thus, the normalized contributions to production and destruction sum to one, as follows:

(18–8)

and

(18–9)

In transient simulations, when rate-of-production analysis is requested, the above calculations are performed at every time step.