Spent caustic


Spent caustic is an industrial caustic liquor that has become loaded and is no longer useful (spent). Spent caustics are made of sodium hydroxide, water, and contaminants. The contaminants have consumed the majority of the sodium hydroxide and thus the caustic liquor is spent (e.g. H2S(g) is scrubbed 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.

Treatment technologies


Spent caustics are malodorous wastewaters that are difficult to treat in conventional wastewater processes. Typically the material is disposed of by high dilution with biotreatment, deep well injection, incineration, wet air oxidation, or other speciality processes. Most ethylene spent caustics are disposed of through 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 superheated water at a temperature above the normal boiling point of water (100 °C), but below the critical point (374 °C).
The system must be maintained under pressure to avoid excessive evaporation of water. This is done to control energy consumption due to the latent heat of vaporization. It is also done because liquid water is necessary for most of the oxidation reactions to occur. Compounds oxidize under wet oxidation conditions that would not oxidize under dry conditions at the same temperature and pressure



Oxidation describes the loss of electrons / hydrogen or gain of oxygen / increase in oxidation state by a molecule, atom or ion



The enthalpy of vaporization, (symbol ΔvH), also known as the heat of vaporization or heat of evaporation, is the energy required to transform a given quantity of a substance into a gas.
It is often measured at the normal boiling point of a substance; although tabulated values are usually corrected to 298 K, the correction is often smaller than the uncertainty in the measured value.
The heat of vaporization is temperature-dependent, though a constant heat of vaporization can only be assumed for small temperature ranges and below Tr<<1.0. tr="1)" class="mw-redirect" title="Critical temperature" href="http://en.wikipedia.org/wiki/Critical_temperature">critical temperature the liquid and vapor phases don't coexist anymore.