Refinery Conversion Units: FCC, Hydrocracker and Reformer
Key takeaways
- Distillation alone is not enough: the market wants more gasoline and diesel than crude gives directly, so heavy molecules are cracked.
- The FCC cracks vacuum gas oil on hot catalyst in seconds, making high-octane gasoline and LPG.
- The hydrocracker cracks with hydrogen at very high pressure, making clean, low-sulfur diesel and jet fuel.
- The reformer does not crack; it rearranges naphtha molecules, raising octane and producing the hydrogen the refinery needs.
- Each unit has its own hazard: catalyst flow reversal in the FCC, temperature runaway and hydrogen in the hydrocracker.
Educational content only: procedures and operating values vary between companies and sites. Always follow your company’s approved procedures, your supervisor’s instructions and the permits in force at your site.
Why do we need conversion units?
In the previous lesson we saw that the distillation unit only sorts crude by boiling point. The problem is that what crude gives directly does not match what the market wants. A barrel contains a large share of heavy oils and residue, while most demand is for gasoline, diesel and jet fuel. And straight-run naphtha has a low octane number, so it is not good enough for modern car engines.
The answer is to change the molecules themselves, which is what conversion units do. They break long molecules into shorter ones, or rearrange molecules to improve their quality. With these units a modern refinery can turn most of the barrel into light, high-value products.
Fluid catalytic cracking (FCC)
The FCC is the heart of many gasoline-oriented refineries. Its main feed is vacuum gas oil from the vacuum column.
The catalyst is a very fine zeolite powder that behaves like a liquid when gas is blown through it, hence the word "fluid". The process runs as a continuous loop:
- Riser: the feed is sprayed into a long vertical pipe where it meets very hot catalyst. It vaporizes and cracks in a few seconds at about 500 to 540 °C.
- Separation: at the top of the riser, cyclones separate the hydrocarbon vapours from the catalyst. The vapours go to the main fractionator.
- Stripper: steam is injected onto the spent catalyst to remove trapped hydrocarbons.
- Regenerator: the catalyst is now covered with coke that deactivates it. It enters a large vessel where air is blown in and the coke burns at about 700 °C.
- Return: clean, hot catalyst flows back to the riser. Its heat vaporizes and cracks the feed, so the unit is almost heat-balanced by itself.
FCC products: dry gas, LPG rich in propylene and butylenes, high-octane gasoline, a light oil called light cycle oil (LCO), and a heavy bottoms oil called slurry or decant oil.
Conversion is the share of feed turned into products lighter than light cycle oil:
Conversion = (Feed − LCO − Slurry) ÷ Feed
A unit runs 40,000 bbl/d of feed and makes 8,000 bbl/d of LCO and 2,800 bbl/d of slurry:
(40,000 − 8,000 − 2,800) ÷ 40,000 = 0.73
So 73% of the feed became gas, LPG and gasoline. Raising the riser temperature a little usually raises conversion, but also makes more dry gas and coke.
Hydrocracking
The hydrocracker cracks much the same heavy feed, but differently: in the presence of hydrogen, at very high pressure, typically 100 to 200 bar, and about 350 to 430 °C. Its catalyst is bifunctional: one part cracks molecules, another adds hydrogen to them.
The result is saturated, very clean products: excellent diesel and jet fuel, plus naphtha, with sulfur and nitrogen almost completely removed. That is why refineries that want a lot of diesel choose a hydrocracker, and those that want a lot of gasoline choose an FCC.
The reactions release heat (exothermic). So the reactor is split into catalyst beds, with cold hydrogen (quench) injected between them to control temperature.
Catalytic reforming
Straight-run heavy naphtha has a low octane number. The reformer does not crack molecules; it rearranges them, converting naphthenes and paraffins into high-octane aromatics. The catalyst contains platinum and the temperature is about 500 °C.
The main reaction absorbs heat (endothermic), so the feed cools inside the reactor. The unit therefore has three or four reactors in series with reheating furnaces between them. The product is called reformate, and its octane can exceed 95.
An important extra benefit: the reactions release hydrogen, which the hydrotreaters and the hydrocracker use. Sulfur must be removed from the naphtha before the reformer, because it poisons the platinum catalyst.
The coker
Some refineries go all the way to the last drop. A delayed coker heats vacuum residue to about 480 to 500 °C and holds it in large coke drums, where it cracks thermally. Light vapours go to a fractionator, and solid petroleum coke remains in the drum, cut out with high-pressure water.
Hazards and safety
- Catalyst flow reversal in the FCC: the riser holds hydrocarbons and the regenerator holds air. Slide valves and pressure differentials keep them apart. If flow reverses, air can meet hydrocarbons and cause an explosion.
- High-pressure hydrogen: leaks ignite very easily, and the flame is almost invisible in daylight.
- Temperature runaway: in a hydrocracker, temperature can climb fast if cooling is lost, which is why emergency depressuring systems exist.
- Opening coke drums: requires strict procedures, as the drum is hot and may hold pockets of liquid or steam.
- H2S and ammonia: produced in most of these units and sent with sour gas and sour water.
Takeaway for the technician
FCC for gasoline, hydrocracker for clean diesel, reformer for octane and hydrogen, coker for the bottom of the barrel. Integrating these units is what makes a modern refinery profitable. In the next lesson we see how sulfur is removed and turned into solid sulfur.
Test your understanding
Related lessons
- Crude distillation unit (CDU/VDU)
- Hydrotreating and sulfur recovery (Claus)
- Enhanced Oil Recovery: More Oil from the Same Reservoir
Sources
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