Definition - What does Hydrocracking mean?
Hydrocracking is a catalytic chemical process used in petroleum refineries to break complex hydrocarbon molecules into simpler molecules of gasoline or kerosene, by addition of hydrogen under high pressure. This process uses hydrogen gas to improve the hydrogen-carbon ratio in the cracked molecules and 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 very low sulfur content and few contaminants.
Petropedia explains Hydrocracking
Hydrocracking process takes place in a hydrogen-rich atmosphere at elevated temperatures of 260 to 425 degrees Celsius and pressures of 35 to 200 bars. Any sulfur and nitrogen present in the hydrocracking feedstock is, to a large extent, hydrogenated to form gaseous hydrogen sulfide (H2S) and ammonia (NH3), which are subsequently removed. The result is that 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 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 pressure (1,000–2,000 psi) and fairly high temperatures (400 to 815 degree Celsius) 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 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 hydrogen attack.