Question 5 of 5: Major refining conversion processes

Question 5 — Major refining conversion processes (10 marks)

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

Each process is summarised below by purpose, feed, and its desirable / undesirable reactions; a consolidated comparison table follows.

(a) Catalytic reforming

Purpose. To raise the octane number of heavy straight-run naphtha (and produce aromatics / BTX and by-product hydrogen) by rearranging low-octane paraffins and naphthenes into high-octane aromatics and branched paraffins. Feed. Hydrotreated heavy naphtha (C₇–C₁₀, boiling ≈90–180 °C) over a Pt–Re/chlorided-alumina bifunctional catalyst at ~500 °C and 5–25 bar of hydrogen. Desirable reactions: dehydrogenation of naphthenes to aromatics (the main octane and hydrogen source), dehydrocyclisation of paraffins to aromatics, and isomerisation of normal to branched paraffins. Undesirable reactions: hydrocracking of paraffins to light gas (C₁–C₄, losing liquid yield and hydrogen) and coke laydown (dealkylation/condensation) that deactivates the catalyst.

(b) Catalytic cracking (FCC)

Purpose. To convert heavy, low-value gas oils into lighter, high-value products — chiefly high-octane gasoline plus C₃/C₄ olefins (LPG) — by acid-catalysed carbon–carbon bond scission. Feed. Vacuum gas oil (VGO) and heavy atmospheric gas oil / residue over an acidic zeolite (Y-zeolite) catalyst at ~500–540 °C, near atmospheric pressure (no added hydrogen). Desirable reactions: cracking of long paraffins and side chains to gasoline-range olefins and paraffins, isomerisation and hydrogen transfer that build octane. Undesirable reactions: over-cracking to dry gas (C₁–C₂), and coke formation on the catalyst from condensation/dehydrogenation of aromatics and olefins (coke is burned off in the regenerator, but excess coke lowers yield).

(c) Hydrocracking

Purpose. To crack heavy gas oils and residua into lighter, saturated middle distillates (diesel, kerosene, naphtha) at high hydrogen pressure, giving high-quality, low-sulphur products with great feed flexibility. Feed. VGO, FCC cycle oils and residua over a bifunctional catalyst (Ni–Mo or Ni–W metal on an acidic zeolite/silica-alumina) at 100–200 bar and 350–430 °C. Desirable reactions: hydrocracking of large molecules to distillates, ring opening, and simultaneous hydrogenation (saturation) and heteroatom (S, N) removal. Undesirable reactions: over-cracking to light gas / excessive hydrogen consumption, and, if temperature runs away, the strongly exothermic reactions can cause a temperature excursion; coking is suppressed by the high H₂ pressure.

(d) Hydrotreating (hydrofining)

Purpose. To purify streams — remove sulphur, nitrogen, oxygen and metals and saturate olefins/aromatics — with little change in molecular weight, protecting downstream catalysts and meeting product specifications (e.g. ultra-low-sulphur diesel). Feed. Almost any fraction (naphtha, kerosene, diesel, VGO, FCC/reformer feed) over Co–Mo or Ni–Mo/alumina at 30–70 bar and 300–400 °C under hydrogen. Desirable reactions: hydrodesulfurization ($\text{{R-S}}+\text{{H}}_2\!\to\text{{RH}}+\text{{H}}_2\text{{S}}$), hydrodenitrogenation (→ NH₃), and olefin/aromatic saturation. Undesirable reactions: unwanted hydrocracking to lighter product (yield and hydrogen loss) and coke/gum formation that deactivates the catalyst; excessive aromatic saturation also wastes hydrogen.

(e) Alkylation

Purpose. To combine light C₃–C₅ olefins with isobutane into a high-octane, low-vapour-pressure, clean-burning branched paraffin (“alkylate”), a premium gasoline blendstock. Feed. FCC C₃/C₄ olefins (propylene, butylenes) plus isobutane, with a strong-acid catalyst (concentrated H₂SO₄ or HF) at low temperature (~5–40 °C). Desirable reactions: acid-catalysed addition of isobutane to the olefin to form branched C₇–C₈ iso-paraffins (e.g. isobutylene + isobutane → iso-octane). Undesirable reactions: olefin polymerisation to heavy “acid-soluble oils”/tar that consume and dilute the acid, and, with HF/H₂SO₄, side reactions that increase acid consumption; hence a large isobutane excess is maintained to suppress polymerisation.