Gas phase:

(9–1)

In Equation 9–1, is the net molar production rate (in units of mol/s) of the -th gas-phase species due to interactions with bulk phases. For example, when liquid-bulk phases are present, there could be Vapor-Liquid-Equilibrium (VLE) and the corresponding species conversion rate (as discussed in Phase Transfer Models ).). The represents the net mass-inflow-rate of the gas-phase through all inlets while is the net outflow rate of the gas phase. These quantities will be non-zero (applicable) only for the open reactor model.

Bulk phase:

The total mass of a bulk phase , , is simply the sum of the masses of the components (species) in the bulk phase

(9–2)

where is the number of species in the bulk phase and is the mass of species in the bulk phase .. The represents the net mass-inflow-rate of the bulk phase through all inlets while is its net outflow rate. These quantities will be non-zero (applicable) only for the open reactor model.