Caustic solutions are used to remove acidic contaminants from refinery and petrochemical plant streams. Spent caustics are toxic to aquatic life and cannot be disposed to waterways while also are unsuitable for direct addition to a biological treatment facility.

Refinery spent caustics are resulted from scrubbing of LPG, gasoline, kerosene, jet fuel, diesel oil, etc., and often have high levels of COD. Another type of spent caustic having lower COD is produced in olefine plants. Depending on the source of scrubbed stream, spent caustic is commonly classified into sulfidic, naphthenic and cresylic. Refinery spent caustics are more complex and can be found in all three types while olefine spent caustics are usually sulfidic. In ethylene plants, a polymer named “redoil” is also formed in caustic wash tower.

Based on spent caustic contaminants, the design of spent caustic treatment package differs. In general, the package includes extraction (and regeneration in case of lack of required solvent), oxidation, neutralization, separation and filtration. Desing of each section will be optimized based on available energy and material, client requirements and spent caustic characteristics.