Total liquid mass, liquid temperature, and pressure are the primary variables used in the liquid phase simulations. The species mass rather than the mass fraction or the mole fraction is used to describe the composition of the liquid mixture because, unlike its gas-phase counterpart, liquid does not fill the volume, therefore the liquid volume is not necessarily equal to the reactor volume. Since the chemical reaction rates depend on the molar concentrations, the total liquid volume must be known and is calculated from the total liquid mass and the density of the liquid mixture.