Carbon dioxide stripping in aquaculture – Part II: Development of gas transfer models

The basic mass transfer equation for gases such as oxygen and carbon dioxide can be derived from integration of the driving force equation. Because of the physical characteristics of the gas transfer processes, slightly different models are used for aerators tested under the non steady-state procedures, than for packed columns, or weirs. It is suggested that the standard condition for carbon dioxide should be 20 °C, 1 atm, CCO2=20 mg/kgCCO2=20 mg/kg, and XCO2=0.000285XCO2=0.000285. The selection of the standard condition for carbon dioxide based on a fixed mole fraction ensures that standardized carbon dioxide transfer rates will be comparable even though the value of CCO2* in the atmosphere is increasing with time. The computation of mass transfer for carbon dioxide is complicated by the impact of water depth and gas phase enrichment on the saturation concentration within the unit, although the importance of either factor depends strongly on the specific type of aerator. For some types of aerators, the most accurate gas phase model remains to be determined for carbon dioxide. The assumption that carbon dioxide can be treated as a non-reactive gas in packed columns may apply for cold acidic waters but not for warm alkaline waters.

► It is proposed that the standard condition for carbon dioxide stripping should be 20 °C, 1 atm, CCO2=20 mg/kgCCO2=20 mg/kg, and XCO2=0.000285XCO2=0.000285.
► This standard condition will ensure transfer rates will be comparable even though the value of CCO2* in the atmosphere is increasing with time.
► The computation of mass transfer for carbon dioxide is complicated by the impact of gas phase enrichment and the hydration reaction.
► Carbon dioxide can be treated as a non-reactive gas in packed columns for cold acidic waters but not for warm alkaline waters.