A model for the prediction of the air composition in pressure saturators

The paper deals with the air composition in pressure saturators used for dissolved air flotation. Two mathematical models are developed: a kinetic model for describing the change in saturator air composition with time from start-up to eventual equilibrium, and an abbreviated model for only predicting the saturator air composition once equilibrium is reached. Both derivations are based on mole balances for the two major components of atmospheric air, nitrogen and oxygen, and the equilibrium between saturator air and outflowing water as predicted by Henryʹs law. The models take factors into account which had hitherto been ignored, such as saturator efficiency, saturator pressure and the oxygen saturation level in the incoming water. The kinetic model gives a clear demonstration of the changes in air composition after saturator start-up and how different operating parameters affect the rate of change. It is demonstrated that laboratory saturators, operated at low hydraulic loading, could take many hours to attain equilibrium, which could explain anomalous measurements reported in the literature. Both mathematical models were tested with data collected from a laboratory saturator operated under conditions similar to those encountered at full-scale. The experimentally determined values agree well with the predicted results, with the exception that the actual saturator air composition converges towards an equilibrium concentration which is slightly less enriched in nitrogen than predicted by the theoretical models.