What Does Hydrocracking Mean?
Hydrocracking is a process by which the hydrocarbon molecules of petroleum are broken into simpler molecules, as of gasoline or kerosene, by the addition of hydrogen under high pressure and in the presence of a catalyst. This process employs hydrogen gas to improve the hydrogen-carbon ratio in the cracked molecules and to arrive at a broader range of end products.
The major products from hydrocracking are jet fuel and diesel, but low-sulfur naphtha fractions and LPG are also produced. All these products have a very low content of sulfur and other contaminants.
Corrosionpedia Explains Hydrocracking
Hydrocracking is a catalytic chemical process used in petroleum refineries for converting the high-boiling hydrocarbons in petroleum to low-boiling products such as gasoline, kerosene, jet fuel and diesel oil. The process takes place in a hydrogen-rich atmosphere at elevated temperatures (500–800 °F, 260–425 °C) and pressures (35–200 bars).
Any sulfur and nitrogen present in the hydrocracking feedstock are, to a large extent, also hydrogenated and form gaseous hydrogen sulfide (H2S) and ammonia (NH3), which are subsequently removed. The result is that the hydrocracking products are essentially free of sulfur and nitrogen impurities and consist mostly of paraffinic hydrocarbons.
Hydrocracking catalysts consist of active metals on solid, acidic supports and have a dual function, specifically a cracking function and a hydrogenation function. The cracking function is provided by the acid catalyst support and the hydrogenation function is provided by the metals.
The hydrocracking process depends on the nature of the feedstock and the relative rates of the two competing reactions. Heavy aromatic feedstock is converted into lighter products under a wide range of very high pressures (1,000–2,000 psi) and fairly high temperatures (750–1,500 °F, 400–815 °C), in the presence of hydrogen and special catalysts.
Catalytic hydrocracking involves three primary chemical processes:
- Cracking of high-boiling hydrocarbons found in crude oil into lower-boiling hydrocarbons
- Hydrogenating unsaturated hydrocarbons to obtain saturated hydrocarbons, usually referred to as paraffins or alkanes
- Hydrogenating any sulfur, nitrogen or oxygen compounds in the original feedstock into gaseous hydrogen sulfide, ammonia and water
Hydrocracking units are normally made of low-alloy steels with Type 347 cladding or weld overlay to avoid intergranular stress-corrosion cracking as well as avoid hydrogen attack.