hydrokraking

Hydrocracking is a refinery process that converts heavy, high-boiling hydrocarbon fractions into lighter, more valuable products through catalytic cracking in the presence of hydrogen at elevated pressure and temperature. It combines hydrogenation and cracking in a single step, producing saturated hydrocarbons with low sulfur, nitrogen, and aromatic content. Compared with fluid catalytic cracking, hydrocracking operates under a hydrogen atmosphere and generally yields higher-quality middle distillates, particularly diesel and jet fuel.

The process uses bifunctional catalysts that provide both hydrogenation sites and acid cracking activity. Typical catalysts rely on metal components such as nickel, palladium, platinum, or molybdenum on acidic supports like silica-alumina or zeolites. The reaction sequence involves dehydrogenation and hydrogenation steps coupled with acid cracking. Feedstocks include vacuum gas oil, atmospheric gas oil, and residual oils, which are exposed to hydrogen under pressure to form lighter products such as naphtha, kerosene, and diesel.

Operating conditions include high hydrogen pressure (often tens of bar) and temperatures around 300–450 C, with reactors arranged in fixed-bed or trickle-bed configurations and, in many plants, hydrogen recycle systems. Products comprise naphtha, kerosene, diesel, and LPG, with reduced sulfur and nitrogen contents and lower aromatics, in addition to improved cetane for diesel.

Hydrocracking is widely used to upgrade heavy feeds and to produce cleaner fuels. It is energy- and hydrogen-intensive and is typically integrated with other refinery units, such as hydrotreating and distillation, to optimize yields and meet product specifications.