Technology Unit

The sour gases carbon dioxide (CO₂) and hydrogen sulphide (H₂S) are removed from the cracked gas by washing the cracked gas with caustic.

They are converted to their respective salts:

• 2 NaOH + CO₂ → Na₂CO₃ + H₂O
• 2 NaOH + H₂S    →     Na₂S + 2 H₂O

Less than 1 volume ppm of H₂S and CO₂  is achieved in the lower and the middle section of the caustic scrubber.
Caustic concentration of approximately 3 % in the upper cycle is adjusted by the water flow received from the top water wash section.
The backwash water trays on top of the column are charged with BFW to prevent any entrainment of caustic to the cracked gas compressor.

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.