Ethylene and Refinery Spent Caustic

Source: Siemens Energy, Inc. - Water Solutions

In the last 10 years, we have successfully installed over two dozen spent caustic wet oxidation systems, with capacities varying from 0.5 to 18.5 m3/h (2 to 81 gpm). Our systems are the industry standard and are included in many of the new world-scale ethylene plants.

In the petroleum and petrochemical industries, spent caustic liquors typically come from two main sources:

  • ethylene production - from the scrubbing of cracked gas with aqueous sodium hydroxide
  • oil refining - from the extraction or treatment of acidic impurities, such as hydrogen sulfide, mercaptans and organic acids in hydrocarbon streams.

These spent caustic streams are a significant odor source as well as disruptive to the operation of a biotreatment facility. In addition, the Chemical Oxygen Demand (COD) load in spent caustic is usually very high (10,000 - 100,000 mg/l), and the contaminants in spent caustic typically include sodium carbonate, sodium sulfides, mercaptans, phenols, and emulsified hydrocarbons.

Wet air oxidation is commonly used for treating spent caustics. Depending upon specific contaminants and treatment objectives, spent caustic wet air oxidation systems can be divided into three ranges of operation:

  • Low temperature (<150°C): Achieves partial oxidation of sulfides to both sulfate and thiosulfate. Applicable for dilute sulfidic caustics when thiosulfate is not a concern for downstream treatment.
  • Medium temperature (approx. 200°C): Achieves complete oxidation of sulfides to sulfate. Also destroys mercaptans. Commonly used for sulfidic ethylene spent caustic treatment.
  • High temperature (approx. 260°C): Achieves complete oxidation of sulfides and mercaptans, plus destruction of organic contaminants such as cresylic acids. Applied to refinery spent caustics.