Thermal (TC) and catalytic (FCC) cracking — process comparison: 23-Chem-B6 Petroleum Refining and Petrochemicals, Undated paper, Q4

Question 4: Thermal (TC) and catalytic (FCC) cracking — process comparison (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.

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The two printed sketches are redrawn below exactly as numbered on the paper: the TC box (equipment tag 3) has feed 8 entering from the left and three product streams leaving — 9 at the top, 10 at the right and 11 at the bottom; the FCC box (equipment tag 9) has feed 19 entering from the left and six product streams — 24 at the top, 25, 26, 27 and 28 on the right, and 29 at the bottom. The exam gives the stream numbers only, with no printed names, so part (e) is answered by ordering the products from lightest (top) to heaviest (bottom), which is the standard convention for a fractionated product slate and which makes part (g)'s pair — the top stream 24 and the bottom stream 29 — the lightest and heaviest FCC products. That assumption is stated here per the paper's own Note 1.

The printed process sketches

Figure 4a. The printed Thermal Cracking (TC) black box (equipment tag 3): feed stream 8 in; product streams 9 (top), 10 (side) and 11 (bottom) out. No catalyst enters or leaves — cracking is purely thermal.

Figure 4b. The printed Fluid Catalytic Cracking (FCC) black box (equipment tag 9): feed stream 19 in; six product streams out — 24 (top), 25, 26, 27 and 28 (side draws) and 29 (bottom), i.e. a full fractionated product slate from lightest to heaviest.

(a) Main functional role

TC (thermal cracking): uses heat alone, with no catalyst, to break large molecules by free-radical scission. Its main role in a modern refinery is residue upgrading — reducing the viscosity of heavy residua (visbreaking) and/or converting residue to distillates and solid coke (coking). FCC (fluid catalytic cracking): uses a solid acid catalyst to crack heavy gas oil selectively into lighter products; its main role is to maximize high-octane gasoline (and valuable C3/C4 olefins for alkylation and petrochemicals). In short, TC is a carbon-rejection residue process; FCC is the refinery's principal gasoline-making conversion unit.

(b) Typical temperature and pressure

Process	Temperature	Pressure
Thermal cracking (visbreaking / coking)	≈ 450–540 °C	low: visbreaking ≈ 0.5–2 MPa; delayed coking near atmospheric–few bar
FCC reactor (riser outlet)	≈ 500–540 °C	near atmospheric, ≈ 1–3 barg
FCC regenerator	≈ 680–730 °C	≈ 1–3 barg

(c) FCC catalyst

A fluidizable solid-acid zeolite catalyst: micron-scale crystals of a faujasite Y zeolite (usually ultrastable USY, often rare-earth exchanged) embedded in an amorphous silica-alumina/clay matrix and spray-dried into 60–70 µm microspheres that fluidize easily. Octane/olefin additives such as ZSM-5 are commonly co-circulated. The strong Brønsted acidity of the zeolite provides the carbenium-ion cracking activity.

(d) Effect of contact time, reactor temperature and catalyst activity

All three variables increase conversion, but the useful window is limited by overcracking and coke:

Contact (residence) time: longer time raises conversion, but past an optimum it promotes secondary reactions that overcrack gasoline to dry gas and coke, lowering gasoline selectivity. Modern units use short-contact-time riser cracking (a few seconds) for exactly this reason.
Temperature: higher riser temperature increases conversion and light-olefin yield, but too high shifts selectivity toward dry gas (C1–C2) and thermal (non-selective) cracking, and increases coke.
Catalyst activity: a more active (fresh, high-zeolite) catalyst gives higher conversion at a given severity, but also more coke and more chance of overcracking; activity is balanced against selectivity and regenerator coke burn.

The design philosophy is therefore a short contact time with a high-activity zeolite at controlled temperature, which maximizes gasoline while limiting overcracking and coke.

(e) Naming the feed and product streams

Each sketch shows one feed on the left and its products leaving top, side and bottom, so the product streams are named in order of decreasing volatility from the top of each box downwards — the standard reading of a fractionated product slate.

Stream	Unit	Identity
8 (feed)	TC	Heavy feed to the thermal cracker — atmospheric/vacuum residue or heavy gas oil
9 (product)	TC	Cracked gas: fuel gas and LPG (C1–C4, with olefins), the lightest product
10 (product)	TC	Cracked naphtha/gasoline — the middle, distillate-range product
11 (product)	TC	Cracked (visbroken) residue: heavy fuel-oil / tar bottoms
19 (feed)	FCC	Fresh FCC feed — heavy vacuum gas oil (VGO), often with coker gas oil and recycle
24 (product)	FCC	Wet gas / fuel gas — the lightest overhead (C1–C2 dry gas plus H₂S-bearing off-gas)
25 (product)	FCC	LPG: C3/C4 olefins and paraffins (propylene, butylenes, isobutane)
26 (product)	FCC	FCC gasoline (cracked naphtha), the main product
27 (product)	FCC	Light cycle oil (LCO), a middle-distillate side draw
28 (product)	FCC	Heavy cycle oil (HCO), the heavy side draw
29 (product)	FCC	Slurry / decant oil, the main-column bottoms, carrying catalyst fines

(f) Destination for streams 10 and 11 in the TC process

Stream 10 (cracked naphtha/gasoline): it is olefinic and sulphur-bearing and therefore unstable, so it is stabilized and hydrotreated and then routed to the gasoline blending pool — or, if its octane is too low, sent on to the catalytic reformer. Stream 11 (cracked residue / tar): its whole purpose is a lower viscosity, so it goes to heavy fuel-oil blending, where it needs far less light cutter stock than the original residue did; alternatively it is charged to a coker or to asphalt manufacture. Neither stream leaves the refinery as it stands: one goes to product upgrading, the other to fuel-oil blending.

(g) Destination of streams 24 and 29 in the FCC process

These are the lightest and the heaviest FCC products, and they go to opposite ends of the refinery. Stream 24 (wet gas / fuel gas): compressed and sent to the gas-concentration (vapour recovery) plant, then amine-treated to strip H₂S (which goes to the Claus sulphur plant) before the sweet dry gas joins the refinery fuel-gas system. Stream 29 (slurry / decant oil): routed to a slurry settler to drop out the entrained catalyst fines, after which the clarified decant oil goes to heavy fuel-oil blending or is sold as carbon-black feedstock or cutter stock; a portion may be recycled to the riser for additional conversion.

Concept & Theory

Topic: Thermal versus catalytic cracking — roles, conditions, catalyst and FCC severity variables.

Key relations: free-radical (thermal) vs carbenium-ion (catalytic) cracking mechanisms; conversion increases with temperature, contact time and catalyst activity but selectivity to gasoline peaks then falls as overcracking to gas + coke takes over.

Why this works: a solid acid zeolite steers cracking through carbenium-ion chemistry, which favours branched, aromatic, high-octane gasoline — something purely thermal (radical) cracking cannot do selectively. The severity variables all push conversion the same way, so the art of FCC operation is to gain conversion while staying short of the overcracking regime that destroys gasoline and makes coke, which is why short-contact-time riser cracking with a high-activity catalyst became standard.

Common pitfall: assuming “more severe is always better” — past the optimum, higher temperature/time/activity trade gasoline for dry gas and coke; and mixing up the reactor (≈500 °C) with the much hotter regenerator (≈700 °C).

Source: Gary, Handwerk & Kaiser, Petroleum Refining, Chs. 5–6 (thermal processes and FCC); Speight, Chemistry and Technology of Petroleum, Ch. on cracking.

Question 4 of 5: Thermal (TC) and catalytic (FCC) cracking — process comparison