is an industrial caustic
liquor that has become loaded and is no longer useful (spent). Spent caustics are made of sodium hydroxide
, and contaminants. The contaminants have consumed the majority of the sodium hydroxide and thus the caustic liquor is spent (e.g. H2S(g)
via the NaOH(aq)
to form NaHS(aq) and H2O(l), thus consuming the caustic).
Types of spent caustic
- Ethylene spent caustic comes from the caustic scrubbing of cracked gas from an ethylene cracker. This liquor is produced by a caustic scrubbing tower. Ethylene product gas is contaminated with H2S(g) and CO2(g), and those contaminants are removed by absorption in the caustic scrubbing tower to produce NaHS(aq) and Na2CO3(aq). The sodium hydroxide is consumed and the resulting wastewater (ethylene spent caustic) is contaminated with the sulfides and carbonates and a small fraction of organic compounds.
- Refinery spent caustic comes from multiple sources: the Merox processing of gasoline; the Merox processing of kerosene/jet fuel; and the caustic scrubbing/Merox processing of LPG. In these streams sulfides and organic acids are removed from the product streams into the caustic phase. The sodium hydroxide is consumed and the resulting wastewaters (cresylic for gasoline; naphthenic for kerosene/jet fuel; sulfidic for LPG -spent caustics) are often mixed and called refinery spent caustic. This spent caustic is contaminated with sulfides, carbonates, and in many cases a high fraction of heavy organic acids.
Spent caustics are malodorous wastewaters that are difficult to treat in conventional wastewater
processes. Typically the material is disposed of by high dilution
, deep well injection, incineration
, wet air oxidation
, or other speciality processes. Most ethylene spent caustics are disposed of through wet air oxidation.
- Suarez, F. "Pluses and Minuses of Caustic Treating", Hydrocarbon Processing, pp 117-123, Oct 1996.
- Maugans, C.; Ellis, C. "Age Old Solution for Today's SO2 and NOx", Pollution Engineering, April 2004. [WAO for SOx NOx avoidance]
- Carlos T.; Maugans, C. "Wet Air Oxidation of Refinery Spent Caustic: A Refinery Case Study", NPRA Conference, San Antonio, TX, September 2000. [WAO for Refinery spent Caustic]