Question 3 of 5: Fluid catalytic cracking (FCCR) flow sheet

Question 3 (III) — Fluid catalytic cracking (FCCR) flow sheet (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.

[Figure not reproduced: Fluid catalytic cracking (FCCR) loop redrawn from the exam flow sheet: reactor–regenerator catalyst circulation, air/flue-gas, and the main-fractionator product streams (7 wet gas from the reflux drum, 3 heavy-cycle-oil side draw, 1 clarified oil from the bottoms slurry settler). See the official exam paper.]

1. FCCR = Fluid Catalytic Cracking (and) Regeneration — the fluidised catalytic cracking unit consisting of a riser/reactor coupled to a catalyst regenerator.
2. Main feed = heavy vacuum gas oil (VGO) from the vacuum distillation unit (sometimes supplemented with atmospheric gas oil or residue): high-boiling, low-value gas oils.
3. Desulfurization first because sulphur (and nitrogen/metals) in the feed deactivate the zeolite catalyst and promote coke; sulphur otherwise leaves as SO_x in the regenerator flue gas (emissions) and as sulphur in the gasoline and light cycle oil (off-spec, corrosive). Hydrotreating the feed protects the catalyst, cuts SO_x, and improves conversion and product quality.
4. Main purpose = crack heavy, low-value gas oils into lighter, higher-value products — chiefly high-octane gasoline, plus LPG olefins (propylene, butylenes) and light cycle oil.
5. Catalyst = a solid-acid zeolite (crystalline aluminosilicate, e.g. Y-zeolite / faujasite) dispersed in a silica–alumina matrix, as a fine powder (~60–70 µm) so it can be fluidised.
6. Conditions: riser reactor ≈ 500–550 °C, near-atmospheric pressure ≈ 1–3 bar (10–30 psig), very short contact time ≈ 2–10 s (riser residence). The regenerator burns coke at ≈ 650–730 °C.
7. Phase: the vaporised feed reacts in the gas/vapour phase over the fluidised solid catalyst — a gas–solid catalytic reaction in which the catalyst behaves as a fluidised bed.
8. Steam is used to (i) atomise and help vaporise the feed at the riser base (dispersion steam), and (ii) strip entrained/adsorbed hydrocarbon from the spent catalyst in the stripper before it enters the regenerator — recovering product and preventing valuable hydrocarbon (and extra heat) from being burned in the regenerator; steam also serves as lift/fluidising medium.
9. Cyclones (in the first unit, the reactor/disengager at the top of the riser, and also in the regenerator) separate entrained fine catalyst from the product vapour and from the flue gas and return it to the bed, retaining the catalyst inventory and preventing catalyst carry-over into the fractionator and up the stack.
10. Streams: 7 = the uncondensed vapour from the top of the overhead reflux drum (after the condenser) — wet gas / off-gas (H₂, C₁–C₄) sent to the gas-concentration / vapour-recovery unit (the drum liquid, stream 6, is the cracked gasoline); 3 = the lowest side draw of the main fractionator — heavy cycle oil (HCO), which joins the line at the bottom of the sheet, partly recycled to the riser and partly withdrawn as product; 1 = the overflow from the top of the small vessel below the column, which is a slurry settler (decanter) fed with the fractionator bottoms (stream 2) — stream 1 is the clarified (decant) slurry oil product, while the settler underflow, rich in catalyst fines, is recycled to the riser.