GPS-X Technical Reference

189 Suspended Growth Models
Figure 6-26 - Schematic of High Purity Oxygen (HPO) System
The flow of gas and flow of water are modelled separately. The exchange of O 2 gas, N 2 gas and
CO 2 gas at the air/water interface is determined using Henry’s Law, corrected for temperature and
headspace pressure.
The regular biological models have been supplemented in the HPO object with stoichiometry to
calculate CO 2 gas generation. As the gas travels downstream, O 2 is transferred into the liquid and
consumed by the biological activity. N 2 and CO 2 are generated, and equilibrated with the gas
concentrations in each headspace. Consequently, the composition of the gas (fraction of the gas
that is O 2, N 2 and CO 2) can be displayed for each reactor in the plug-flow system. Display
variables for the gas composition can be found in the HPO System Variables display variables
menu.
As the HPO system is capped, and CO 2 gas is contained in the headspace at concentrations often
much higher than atmospheric values, HPO systems may have atypical pH. The GPS-X HPO
object contains a pH calculator that determines the pH in the liquid for each reactor. For details
on the pH model, refer to the pH discussion in the “Tools” chapter (the model in the Toolbox is
the same one used here). It is important to properly specify the anion and cation concentrations in
the tank to achieve a calibrated pH calculation.
GPS-X Technical Reference