Spent caustic is a waste industrial

caustic Caustic most commonly refers to: * Causticity, a property of various corrosive substances ** Sodium hydroxide, sometimes called ''caustic soda'' ** Potassium hydroxide, sometimes called ''caustic potash'' ** Calcium oxide, sometimes called ''caust ...

solution that has become exhausted and is no longer useful (or spent). Spent caustics are made of

sodium hydroxide Sodium hydroxide, also known as lye and caustic soda, is an inorganic compound with the formula NaOH. It is a white solid ionic compound consisting of sodium cations and hydroxide anions . Sodium hydroxide is a highly caustic base and alkal ...

potassium hydroxide Potassium hydroxide is an inorganic compound with the formula K OH, and is commonly called caustic potash. Along with sodium hydroxide (NaOH), KOH is a prototypical strong base. It has many industrial and niche applications, most of which expl ...

water Water (chemical formula ) is an inorganic, transparent, tasteless, odorless, and nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the fluids of all known living organisms (in which it acts as ...

, and contaminants. The contaminants have consumed the majority of the sodium (or potassium) 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 (''aq'') and H₂O (''l''), thus consuming the caustic.

Types

* Ethylene spent caustic comes from the caustic scrubbing of cracked gas from an

ethylene Ethylene ( IUPAC name: ethene) is a hydrocarbon which has the formula or . It is a colourless, flammable gas with a faint "sweet and musky" odour when pure. It is the simplest alkene (a hydrocarbon with carbon-carbon double bonds). Ethylene ...

cracker Cracker, crackers or The Crackers may refer to: Animals * ''Hamadryas'' (butterfly), or crackers, a genus of brush-footed butterflies * ''Sparodon'', a monotypic genus whose species is sometimes known as "Cracker" Arts and entertainment Films ...

. This liquor is produced by a caustic scrubbing tower. Ethylene product gas is contaminated with (g) and (g), and those contaminants are removed by absorption in the caustic scrubbing tower to produce (aq) and {{chem, Na, 2, CO, 3(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 Merox is an acronym for mercaptan oxidation. It is a proprietary catalytic chemical process developed by UOP used in oil refineries and natural gas processing plants to remove mercaptans from LPG, propane, butanes, light naphthas, kerosene and j ...

processing of

gasoline Gasoline (; ) or petrol (; ) (see ) is a transparent, petroleum-derived flammable liquid that is used primarily as a fuel in most spark-ignited internal combustion engines (also known as petrol engines). It consists mostly of organic ...

; the Merox processing of

kerosene Kerosene, paraffin, or lamp oil is a combustible hydrocarbon liquid which is derived from petroleum. It is widely used as a fuel in aviation as well as households. Its name derives from el, κηρός (''keros'') meaning " wax", and was reg ...

jet fuel Jet fuel or aviation turbine fuel (ATF, also abbreviated avtur) is a type of aviation fuel designed for use in aircraft powered by gas-turbine engines. It is colorless to straw-colored in appearance. The most commonly used fuels for commercial a ...

; 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

organic acids An organic acid is an organic compound with acidic properties. The most common organic acids are the carboxylic acids, whose acidity is associated with their carboxyl group –COOH. Sulfonic acids, containing the group –SO2OH, are ...

Treatment technologies

Spent caustics are malodorous

wastewater Wastewater is water generated after the use of freshwater, raw water, drinking water or saline water in a variety of deliberate applications or processes. Another definition of wastewater is "Used water from any combination of domestic, industri ...

s that are difficult to treat in conventional

processes. Typically the material is disposed of by high

dilution Dilution may refer to: * Reducing the concentration of a chemical * Serial dilution, a common way of going about this reduction of concentration * Homeopathic dilutions, Homeopathic dilution * Dilution (equation), an equation to calculate the rate ...

with

biotreatment Bioremediation broadly refers to any process wherein a biological system (typically bacteria, microalgae, fungi, and plants), living or dead, is employed for removing environmental pollutants from air, water, soil, flue gasses, industrial effluent ...

, deep well injection,

incineration Incineration is a waste treatment process that involves the combustion of substances contained in waste materials. Industrial plants for waste incineration are commonly referred to as waste-to-energy facilities. Incineration and other hig ...

wet air oxidation Wet oxidation is a form of hydrothermal treatment. It is the oxidation of dissolved or suspended components in water using oxygen as the oxidizer. It is referred to as "Wet Air Oxidation" (WAO) when air is used. The oxidation reactions occur in ...

Humid Peroxide Oxidation Advanced oxidation processes (AOPs), in a broad sense, are a set of chemical treatment procedures designed to remove organic (and sometimes inorganic) materials in water and wastewater by oxidation through reactions with hydroxyl radicals (·OH) ...

or other speciality processes. Most ethylene spent caustics are disposed of through wet air oxidation.

References

*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. *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
*Kumfer, B.; Felch, C.; Maugans, C. "Wet Air Oxidation of Spent Caustic in Petroleum Refineries", NPRA, March 2010, San Antonio, TX
WAO for Spent Caustic in Petroleum Refineries
Water pollution Waste