The history of the process goes back to the late 1920s when it was realized that there was a need for gasoline of a higher quality than that obtained by catalytic cracking; this led to the development of the hydrocracking process.
One of the 1st plants to use hydrocracking was commissioned for the commercial hydrogenation of brown coal at Leuna in Germany. Tungsten sulfide was used as a catalyst in this one-stage unit, in which high reaction pressures, 2,900–4,350 psi, were applied.
After the World war II, commercial hydrocracking was very expensive but by the end of the 1950s, the process had become economic. The development of improved catalyst made it possible to operate the process at considerably lower pressure, viz. 1,000–2,200 psi. This in turn resulted in a reduction in equipment wall thickness, whereas simultaneously advances were made in mechanical engineering, especially in the field of reactor design and heat transfer.
These factors, together with the availability of relatively low-cost hydrogen from the steam reforming process, brought hydrocracking back on the refinery scene.
The modern hydrocracking processes were initially developed for converting refractory feedstocks to gasoline and jet fuel; process and catalyst improvements and modifications have made it possible to yield products from gases and naphtha to furnace oils and catalytic cracking feedstocks.
The concept of hydrocracking allows the refiner to produce products having a lower molecular weight with higher hydrogen content and a lower yield of coke. In summary, hydrocracking facilities add flexibility to refinery processing and to the product slate. Hydrocracking is more severe than hydrotreating, there being the intent, in hydrocracking processes, to convert the feedstock to lower-boiling products rather than to treat the feedstock for heteroatom and metals removal only.
Hydrocracking is an extremely versatile process that can be utilized in many different ways, and one of the advantages of hydrocracking is its ability to break down high-boiling aromatic stocks produced by catalytic cracking or coking. To take full advantage of hydrocracking, the process must be integrated in the refinery with other process units.
In gasoline production, for example, the hydrocracker product must be further processed in a catalytic reformer as it has a high naphthene content and relatively low octane number. The high naphthene content makes the hydrocracker gasoline an excellent feed for catalytic reforming, and good yields of high-octane-number gasoline can be obtained.
The mechanism of hydrocracking is basically similar to that of catalytic cracking, but with concurrent hydrogenation. The catalyst assists in the production of carbonium ions via olefin intermediates and these intermediates are quickly hydrogenated under the high-hydrogen partial pressures employed in hydrocracking.