Technology

In a steam cracker plant, the bottom product from the debutanizer is the C₅⁺ stream also known as Pyrolysis Gasoline, that can be processed in various ways.

The attached image shows a typical Pygas hydrogenation setup comprising two hydrogenation steps and multiple seperator columns. 

• C₃/C₄ Separator Unit (Debutanizer, part of the PE Plant):
  • Bottom product (C5+ Pygas) from the Polyethylene Plant Debutanizer is fed to a first Gasoline Hydrogenation Unit.
• First Gasoline Hydrogenation Unit:
  • Product from the first Gasoline Hydrogenation Unit is sent to the C₆/C₇ Separation Unit.
• C₆/C₇ Separator Unit:
  • Top product from the C₆/C₇ Separation Unit (C₅ + C₆) is fed to a second Gasoline Hydrogenation Unit.
  • Bottom product from the C₆/C₇ Separation Unit (C₇⁺) is fed to the C₈/C₉ Separator Unit (Deoctanizer).
• Second Gasoline Hydrogenation Unit:
  • Product from the second Gasoline Hydrogenation Unit II is fed to the C₅/C₆ Separator Unit (Depentanizer).
• C₅/C₆ Separator Unit (Depentanizer):
  • Top C₅ stream from the C₅/C₆ Separator Unit is recycled to the furnaces.
  • Bottom C₆ Stream from the C₄/C₅ Separator Unit is fed to a third party Benzene Extraction Unit.
• C₈/C₉ Separator Unit (Deoctanizer):
  • Top fraction of deoctanizer is benzene-depleted pyrolysis gasoline (C₇-C₈) having Benzene content less than 1.5%.
    It is a naphtha range product with a high aromatic content used for gasoline blending or feedstock for BTX extraction unit.
  • Bottom product from the deoctanizer is a C₉⁺ rich mix of heavy hydrocarbons from C₉ to C₁₅ (C₉⁺ Pygas).

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.