Technology

Pyrolysis Gasoline or Pygas (C₅⁺ cut) is a Naphtha-range liquid product with high Aromatics content produced in a steam cracker plant (Ethylene plant) processing Naphtha, Butane, or Gasoil. The stream originates as the bottom product from the C₃/C₄ separator (debutanizer) section of the polyethylene plant.

In its raw form, pygas has limited commercial value due to its high reactivity and low stability. However, it contains valuable components such as isoprene, benzene, toluene, and xylenes. To stabilize the stream and enable downstream recovery, it is processed in a pygas hydrogenation unit (PHU), followed by fractionation and extraction steps. Products derived from pygas treatment are used either for gasoline blending or as feedstock for a BTX extraction unit.

Process Flow Diagram of Downstream Treatment of C₃⁺ Stream in a Polyethylene Plant

Front-End Separation and Hydrogenation

The C₅⁺ pygas stream from the debutanizer (C₃/C₄ Separator Unit) is routed to the first gasoline hydrogenation unit:

• First Gasoline Hydrogenation Unit:
  The stream is selectively hydrogenated to remove diolefins and acetylenes and improve stability.

• C₆/C₇ Separator Unit:
  The hydrogenated product is fractionated:

  • Top product (C₅ + C₆) is sent to a second gasoline hydrogenation unit.

  • Bottom product (C₇⁺) is routed to the C₈/C₉ separator unit (deoctanizer).

• Second Gasoline Hydrogenation Unit:
  Further hydrogenation removes remaining olefins and sulfur compounds, producing a stabilized stream suitable for downstream processing.

• C₅/C₆ Separator Unit (Depentanizer):

  • Top C₅ stream is recycled to the furnaces.

  • Bottom C₆ stream is routed to a third-party benzene Extraction Unit.

Heavy Fractionation Section

• C₈/C₉ Separator Unit (Deoctanizer):
  The C₇⁺ stream is separated into:

  • Top fraction (C₇–C₈): benzene-depleted pyrolysis gasoline (<1.5% Benzene), a naphtha-range product rich in aromatics used for gasoline blending or as feedstock to a BTX extraction unit.

  • Bottom product: C₉⁺ Pygas, a heavier aromatic-rich stream (C₉–C₁₅).

Aromatics Recovery Section

Depending on configuration, the stabilized and fractionated streams are further processed as follows:

• Depentanizer (alternative configuration):
  The pygas may first be separated into Raw-C₅ and C₆⁺ fractions before hydrogenation.

• Hydrogenation Unit:
  The C₆⁺ Pygas undergoes two-stage hydrogenation to saturate olefins and remove sulfur, producing hydrotreated pygas.

• Deoctanizer:
  The hydrotreated stream is split into:

  • HEART CUT (C6–C8), sent to extraction.

  • C9+ Raffinate, routed to the Hydrodealkylation Unit.

• Extraction Unit:
  Aromatics are selectively extracted (e.g., via sulfolane):

  • Aromatic extract (BTX) sent to fractionation.

  • Non-Aromatic Raffinate used as blending component.

• Hydrodealkylation Unit:
  Converts heavy C9+ aromatics into a Benzene-rich Aromatic Mixture.

• Fractionation Unit:
  Produces high-purity Benzene, Toluene, and Xylenes.

References

1. enggcyclopedia, Jun 2012, Typical Process Flow Diagram (PFD) – Pygas Processing
2. YEOCHUN NCC, Intro to Plants, Aromatic Plant 2
3. Y.A. Sastry & Uday Krishna Dy, DUAL FEED CRACKER UNIT & ASSOCIATED UNIT, ONGC Petro additions Limited (OPaL), uploaded on SCRIBD: Jan 2018