AM-10-104 Wet Air Oxidation Treatment of Spent Caustic in Petroleum Refineries

Description:

Spent caustic streams from refineries typically have a high chemical oxygen demand (20 - 500 g/L) and contain chemicals that are hazardous, inhibitory, and/or bio-refractory. The chemicals of concern in the spent caustic wastewater include reduced sulfur compounds such as sulfides and mercaptans as well as organic species such as the sodium salts of naphthenic and cresylic acids. Due to the types of chemicals contained in the spent caustic, the spent caustic wastewaters can be environmentally hazardous and difficult to treat with conventional biological treatment. Typical pretreatment techniques used for spent caustic waste water include acid neutralization and wet air oxidation (WAO). Acid neutralization will reduce a majority of the COD and foaming tendency by removing the sulfides and naphthenic acids but will not remove phenols or produce a biodegradable effluent. Low temperature wet air oxidation (110-120 °C) will oxidize sulfides, reduce but not totally oxidize mercaptans and will reduce the COD. Low temperature WAO will not oxidize phenol (cresols), naphthenic acids or eliminate foaming if the naphthenic acids are present. Mid temperature wet air oxidation (200-220 °C) will oxidize reduced sulfur compounds, phenols (cresols) and a majority of the COD while producing a biodegradable effluent. Mid temperature WAO will not eliminate the foaming tendency of the spent caustic if naphthenic acids are present. High temperature wet air oxidation (240-260 °C) will oxidize all reduced sulfur compounds and phenols (cresols), eliminate the foaming tendency of the spent caustic, and reduce a majority of the COD producing a biodegradable effluent. Laboratory and field data will be presented to show the effectiveness of wet air oxidation for treating spent caustic wastewater.