Technology Unit

Spent caustic is stripped with methane at low pressure to remove light hydrocarbons and benzene.
Caustic is mixed with steam and air in a caustic/air/steam mixer.
Sulphide and hydrogen sulphide are oxidized to thiosulphate and then to sulphate:

• 2 Na₂S + 1.5 O₂ + H₂O → Na₂S₂O₃ + 2 NaOH (100% conversion)
• 2 NaHS + 2 O₂ → Na₂S₂O₃ + H₂O (100% conversion)
• Na₂S₂O₃ + 2 NaOH + 2 O₂ → → 2 Na₂SO₄ + H₂O (50-70% conversion depending on residence time).

Excess air separated in the neutralisation drum and vent gas from the caustic tank are sent to the sour gas flare.
Effluent suitable for Biological Treatment to be sent to OSBL (pH 7.0 by sulphuric acid).
Sulphide content in oxidized effluent is less than 10 mg/l.
COD is reduced to 4,000 – 6,000 mg/l.
Source: Y.A. Sastry & Uday Krishna Dy, DUAL FEED CRACKER UNIT & ASSOCIATED UNIT, ONGC Petro additions Limited (OPaL), Uploaded on 28^th Jan 2018 on SCRIBD.