The pH-Deaeration Paradox: The pH-Temperature Operating Window for CO₂ Removal in High-Purity Steam-Water Cycles

ABSTRACT

Boiler feedwater pH in high-purity steam-water cycles is not freely adjustable when ammonia is used as the alkalizing agent but becomes the thermodynamic consequence of the coupled ammonia-carbon dioxide (NH₃-CO₂) system under deaerator conditions. While dissolved oxygen is readily removed, CO₂ can be stripped only in its molecular form, CO_2(aq), whose fraction decreases sharply at elevated pH as total inorganic carbon shifts to non-volatile species. When the temperature required to sustain this molecular fraction exceeds the temperature fixed by the deaerator's maximum allowable working pressure, pressure safety valve setpoint, and practical operating pressure, CO₂ removal becomes thermodynamically inaccessible. This pH-deaeration paradox explains how elevated feedwater pH can coexist with residual CO₂, enabling condensate side reacidification and increased iron dissolution in colder regions of the cycle. Integrating these thermodynamic and mechanical constraints yields a pH-temperature feasibility domain that defines the operating window within which CO₂ removal and pH control can be simultaneously satisfied.

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