October 3, 2011
Oligomerize FCC ethylene to useful hydrocarbon fractions. Fluidized catalytic cracking (FCC) is a key process in a petroleum refinery. The object of FCC is to convert low-value fuel oil to fractions suitable for blending gasoline. The major fraction from an FCC unit is the C5–C12 cut, which is high in octanes; it is sometimes called FCC naphtha. The lighter C3 and C4 cuts are highly olefinic; the C4 olefins are commonly treated with isobutane to give a fraction called alkylate.
Alkylate has a high octane rating because of its highly branched structure, and it is a key blending stock for gasoline. Propylene from the C3 cut is also used to produce alkylate and sometimes petrochemicals.
The ethylene in the C2 cut is usually not used for making gasoline or petrochemicals for several reasons:
- Ethylene is present in too small a quantity;
- it does not oligomerize very readily; and
- impurities in the C2 cut include H2S, NH3, and CO, all of which can poison catalysts.
To overcome this problem, inventors C. P. Nicholas, A. Bhattacharyya, and D. E. Mackowiak developed catalysts that can oligomerize dilute ethylene streams to fractions that are suitable for blending into gasoline or diesel fuel. The catalysts are resistant to sulfur-, oxygen-, and nitrogen-containing impurities.
In one example, the inventors tested a catalyst consisting of 1.5 wt% Ni/SiO2–Al2O3 for ethylene oligomerization. The conditions were 280 °C, 6.9 MPa pressure, and an olefin gaseous hourly space velocity of 586 h–1 in a fixed bed over 10 mL catalyst. The feed stream consisted of 23 wt% ethylene, 14 wt% ethane, 35 wt% methane, 13 wt% hydrogen, 13 wt% nitrogen, 1 wt% CO, 1.5 wt% CO2, and 10 ppmw H2S. Ethylene conversion was 75%, and the selectivities to each fraction were C2–C4, 9%; C5–C10, 18%; and C10+, 73%.
A key finding was that between 27 and 44 h onstream, even with 1 ppm NH3 added, conversion and selectivity did not change. In contrast, zeolite catalysts gave good initial activity, but the activity quickly declined. (UOP LLC [Des Plaines, IL]. US Patent 8,021,620, Sept. 20, 2011; Jeffrey S. Plotkin)