(22f) Corrosion Control in the Wet Air Oxidation System

Wet air oxidation (WAO) represents a highly effective and well-established technology for the on-site treatment of spent caustic within ethylene production facilities. This process facilitates the transformation of reactive sulfides in the spent caustic solution into soluble thiosulfates, sulfites, and sulfates, ultimately rendering the treated stream suitable for subsequent biological treatment within the facility's wastewater management system.

This paper aims to provide insights into corrosion phenomena observed in feed effluent exchangers constructed from high-grade metallurgy Inconel 600. A failure analysis conducted by an independent third party attributed the corrosion primarily to organic acids, notably formic and acetic acid. The corrosion was characterized as uniform, with the microstructure displaying no evidence of intergranular or trans-granular cracking. The root cause assessment indicates that corrosion is exacerbated by the presence of excessive hydrocarbons in conjunction with occasional low concentrations of caustic. Furthermore, this paper examines the impact of caustic tower performance—including fouling and chemical treatment—on the corrosivity of the system, which ultimately influences the selection of materials for the WAO exchanger and its associated components.

The information presented in this paper is based on the operational experiences of CPChem’s two wet air oxidation systems: one associated with an ethane cracker and the other with a flexi-cracker.