Product

The C₄ Cut is an olefin-rich C₄ hydrocarbon fraction recovered as the bottoms product of the LPG splitter from sweetened FCC LPG. It consists of a mixture of butenes (n-butenes and isobutylene) and butanes (isobutane and n-butane), typically 50–70 vol% olefinic, with residual trace sulfur below fuel-grade limits but above the requirements of sulfur-sensitive downstream catalysts. Its value lies in its high-octane olefins and isobutane content, and it is destined for conversion or gasoline blending rather than direct fuel LPG 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

• Total C₄ olefins (butenes): 50–70 vol% — comprising n-butenes (1-butene, cis/trans-2-butene), isobutylene (typically 10–25% of the C₄s), and traces of butadiene

• Isobutane: 20–35 vol% — valuable alkylate co-reactant

• n-Butane: 10–25 vol%

• C₃s: below ~1%, spec-controlled as bottoms contamination (propane/propylene slip reduces alkylation efficiency)

• C₅s: below ~1–2%, spec-controlled (light gasoline carryover)

• Butadiene: typically 0.5–2 vol% — a concern for some downstream routes (acid consumption in alkylation, catalyst fouling)

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

Contaminants (post-treating)

• Total sulfur: typically 5–50 ppmw, depending on FCC feed pretreatment severity; mainly residual mercaptans (ethyl/propyl) and trace sulfides

• COS: 1–20 ppmw

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

• Butadiene and trace dienes: catalyst fouling and acid consumption concerns downstream

• Trace nitrogen and oxygenates: guard-bed targets where applicable

Indicative Properties

• Molecular weight: ~55–57 g/mol

• Liquid density at 15 °C: ~0.56–0.60 g/cm³

• Vapor pressure at 37.8 °C: ~4–6 bar

• Boiling range: approximately −12 to −0.5 °C

• High olefin content gives high octane value — research octane of the olefinic fraction well above 90

Downstream Fates

• Alkylation unit feed (HF or H₂SO₄): the primary route — isobutylene and n-butenes react with isobutane to produce high-octane alkylate; sulfur, butadiene, and water are key feed contaminants requiring control

• MTBE / ETBE production: selective etherification of the isobutylene content; residual raffinate-1 C₄s then go to alkylation or blending

• Polymerization (polygas) unit: olefins converted to polymer gasoline

• Butene-1 recovery or isomerization units (where petrochemical integration exists)

• Direct gasoline blending (limited by RVP) or LPG sales (saturated portion only, low value for an olefinic stream)