Sub-Product

Pyrolysis Gasoline or Pygas (C₅⁺ cut) is a Naphtha-range liquid product with high Aromatics content produced in an Ethylene plant that processes Naphtha, Butane or Gasoil. 
This C₅⁺ cut, if left in its raw form, has little commercial value owing to its high reactivity and low stability. The stream contains components such as Olefins, Isoprene, Aromatics (Benzene, Toluene and Xylenes), and Paraffins (Alcanes) ranging from C₅'s to C₁₂'s. 

Pygas is generally used as a Gasoline blending mixture due to its high octane number and it can be blended with other hydrocarbons as a Gasoline additive, or distilled (in a BTX extraction process) to separate it into its components, including Benzene. Extracting these components can be highly economical and typically requires hydrotreating in a Pygas Hydrogenation Unit (PHU). 

References:

1. enggcyclopedia, Jun 2012, Typical Process Flow Diagram (PFD) – Pygas Processing
2. YEOCHUN NCC, Intro to Plants, Aromatic Plant 2.