The Effects of Caustic Tower Operations and Spent Caustic Handling on the Zimpro® Wet Air Oxidation (WAO) of Ethylene Spent Caustic
by Clayton Maugans, Ph.D. and Mike Howdeshell, Ph.D. - Siemens Water Technologies; Steve De Haan - Lummus Technology

Ethylene plant spent caustic is frequently treated using a Spent Caustic Oxidation
Unit (SCOU). Commonly, this is a medium pressure (~400 psig) Wet Air Oxidation (WAO) system. There is spent caustic storage for feeding the system. Events that cause excessive downtime in the WAO unit are not acceptable because the spent caustic storage capacity is limited. The hazardous and malodorous nature of the spent caustic makes large inventory unappealing; and ad-hoc storage difficult. WAO system operations are affected by the composition of the spent caustic. Off-spec feed can cause disruptions or even damage to the WAO system. The design and operations of the caustic tower, gasoline wash, and spent caustic storage affect the spent caustic composition, which ultimately affects WAO operations. While such disruptions are rare, when they do occur the most common cause is excessive organic contamination (red oil) in the spent caustic. This paper describes the WAO system and the spent caustic parameters that affect WAO operations. Red oil in particular affects WAO operations and the upstream handling, and
techniques to best manage this material are discussed.

Disposal of Spent Caustic at the Repsol YPF Refinery in La Pampilla, Peru
by Clay Maugans - Siemens Water Technologies; Felix Huaman - Repsol YPF

Repsol YPF’s La Pampilla refinery began using a Zimpro® wet air oxidation (WAO) system for the treatment of refinery spent caustic in 2005. The spent caustic comes from a variety of sources including caustic treatment of gasoline, kerosene, and LPG. Prior to installing a WAO system, the refinery was dependent on less satisfactory disposal options. The WAO system allows for on-site treatment without the production of odorous off-gas. System performance includes high chemical oxygen demand (COD) destruction and complete destruction of sulfides and mercaptans to below analytical detection limits. WAO transforms the material from an odorous and toxic feed to a readily biodegradable effluent that is free of noxious odors. The effluent is discharged to the sea.

Wet Air Oxidation Treatment of Spent Caustic in Petroleum Refineries
by Bryan Kumfer, Chad Felch, and Clay Maugans - Siemens Water Technologies

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 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.

Wet Air Oxidation of Ethylene Plant Spent Caustic
by Claude E. Ellis, Robert J. Lawson, Bruce L. Brandenburg - Siemens Water Technologies

Spent caustic scrubbing liquor is commonly the most problematic waste stream generated by an olefin plant. The most effective means for on-site treatment of spent caustic is wet air oxidation which can achieve the oxidation of reactive sulfide to soluble thiosulfate, sulfite and sulfate. This treated stream is then suitable for biotreatment in the plant's wastewater treatment system.

This paper provides insight into the design of a wet air oxidation system as it is applied on a full scale basis for spent caustic treatment.

Wet Air Oxidation of Refinery Spent Caustic: A Refinery Case Study
by Tania Mara S. Carlos - Refinaria de Petroleos de Manguinhos; and Clayton B. Maugans - Siemens Water Technologies

In 1995, a wet air oxidation system was put into operation for treatment of refinery spent caustic from gasoline sweetening, gasoline and LPG prewashing and from gasoline and LPG mercaptans extraction at the Refinaria de Petroleos de Manguinhos, S.A. (RPDM) in Rio de Janeiro. RPDM is a producer of liquid fuels and is located near residential property within Rio de Janeiro.

This paper discusses the technology selection process as well as performance of the wet air oxidation system.

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