Product

The C₃ Cut is an olefin-rich C₃ hydrocarbon fraction recovered as the overhead product of the LPG splitter from sweetened FCC LPG. It consists predominantly of propylene (65–75 vol%) and propane, carries residual trace sulfur (notably COS) below fuel-grade LPG limits but above polymer-grade requirements, and is destined for further separation and purification rather than direct fuel use.

Origin and Treating Status

• Recovered from the FCC gas plant via the main fractionator overhead system, gas concentration unit, and LPG stabilization

• Feed to the splitter has been amine-treated (H₂S removal) and Merox/caustic-treated (mercaptan extraction/sweetening)

• The stream is therefore sweetened, but not deeply desulfurized

Typical Composition

• Propylene: 65–75 vol% — the key value component, varying with FCC severity, ZSM-5 catalyst use, and feedstock

• Propane: 25–35 vol%

• C₂s (ethane/ethylene): below ~1%, spec-controlled at the splitter overhead

• C₄s: below ~1–2%, spec-controlled as overhead contamination

• Water: saturated at draw conditions, dried downstream if required

Contaminants (post-treating)

• Total sulfur: typically 5–50 ppmw, depending on FCC feed pretreatment severity

• COS: 1–20 ppmw — the key concern for propylene upgrading

• H₂S: below 1–5 ppmw residual after amine treating

• Trace nitrogen species, arsine, and dienes: guard-bed targets for downstream purification

Indicative Properties

• Molecular weight: ~42–43 g/mol

• Liquid density at 15 °C: ~0.50–0.52 g/cm³

• Vapor pressure at 37.8 °C: ~13–16 bar

• Boiling range: approximately −48 to −42 °C

Downstream Fates

• Feed to a PP splitter (C₃ splitter) producing polymer-grade (≥99.5%) or chemical-grade (~93–95%) propylene plus propane — requires guard beds (COS hydrolysis, molecular sieve) to reach ≤1 ppm S

• Direct feed to alkylation or polymerization (polygas) units

• Sales as chemical-grade propylene after treating upgrade