Spent caustic
Spent caustic is a waste industrial caustic solution that has become exhausted and is no longer useful. Spent caustics are made of sodium hydroxide or potassium hydroxide, water, and contaminants. The contaminants have consumed the majority of the sodium hydroxide and thus the caustic liquor is spent, for example, in one common application H2S (gas) is scrubbed by the NaOH (aqueous) to form NaHS and H2O, thus consuming the caustic.
Types
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 and, and those contaminants are removed by absorption in the caustic scrubbing tower to produce and. The sodium hydroxide is consumed and the resulting wastewater 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 are often mixed and called refinery spent caustic. This spent caustic is contaminated with sulfides, carbonates, and in many cases a high fraction of organic acids.