Process description of Spent Caustic Treatment

Process description of Spent Caustic Treatment

PSC Srl Engineering and Contracting’s staff have an extensive experience in all types of refinery wastewater.

The treatment of spent caustic effluent deserves a special mention.

The spent caustic effluents are usually two types:

  • Ethylene spent caustic
    ​It 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), some traces of oil, olefins, polymers 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 sulphides and carbonates and a small fraction of organic compounds.​

 

  • Refinery spent caustic
    It 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 sulphides, mercaptans, phenols and are removed from the product streams into the caustic phase. The sodium hydroxide is consumed and the resulting wastewaters (BTEX, oil, phenolic, cresylic for gasoline; naphthenic for kerosene/jet fuel; sulfidic for LPG spent caustics) are often mixed and called refinery spent caustic.
    All spent caustic are highly contaminated with: sulfides, carbonates, residual caustic, pH, salinity and in many cases heavy organic substances.

They cannot be discharged directly to surface water nor to conventional biological process or to common chemical physical treatment, due to their chemical characteristics and composition (not easily biodegradable and not easily treatable by common chemical oxidants, very toxic to biological bacteria).

It may require a specific pre-treatment, as an example, one of the possible process steps:

  • Equalisation tank to smooth the peak flow and pollutants concentration, in order also to minimise the next process sizing.
  • Special oxidation system to reduce almost as possible the above pollutants down to limits value suitable to be managed by the next polishing processes.
  • Post biological treatment designed to re-treat the oxidised effluent, usually with combined incoming wastewater streams, to discharge the final treated water to seawater or river.
  • Post advance oxidation treatment upstream or alternative to biological treatment to remove the residual contaminants, before final discharge.

The treated effluent can be discharged to the surface water.