Main-Product

The C₈ aromatics fraction is the mixed xylenes cut produced downstream of the aromatics extraction unit, containing four structural isomers with the molecular formula C₈H₁₀ — three xylene isomers and ethylbenzene. It is the second-most important aromatic product in world chemical consumption, ranking behind benzene.

Origin and Composition

The C₈ aromatics fraction originates primarily from catalytic reforming of naphtha, where the reformate stream is rich in C₆–C₈ aromatics. After solvent extraction (typically using Sulfolane or N-Formylmorpholine) separates the aromatic fraction from paraffins and naphthenes, the BTX block fractionates benzene and toluene overhead, leaving the C₈ cut as the heavier bottom product. A secondary source is pyrolysis gasoline (pygas) from steam cracking units.

The four components of the C₈ fraction are:

All four components share the same molecular weight (106.17 g/mol) and are colorless liquids at ambient conditions.

Separation Challenges

Because the four isomers have very similar boiling points (136–144°C), simple distillation is insufficient to separate them. The industry uses a combination of techniques:

• Fractional distillation: Separates OX (highest BP, 144.4°C) from the rest
• Simulated Moving Bed (SMB) adsorption: Selectively adsorbs PX from the mixed C₈ stream using molecular sieves
• Crystallization: Exploits PX's relatively high melting point (13.3°C vs. −47.9°C for MX) to freeze it out
• Isomerization: Converts the less valuable MX and OX back to an equilibrium mixture enriched in PX, feeding a recycle loop known as the "xylenes loop"

Downstream Value Chain

Para-xylene is by far the highest-value C₈ isomer, as it is the precursor to purified terephthalic acid (PTA), the monomer for PET bottles, polyester textiles, and films. Ortho-xylene feeds phthalic anhydride production (plasticizers, resins), while ethylbenzene is almost entirely converted to styrene for polystyrene and ABS plastics. The mixed C₈ stream as a whole is also used as a high-octane gasoline blendstock when further separation is not economically justified.