Hydrocracking: Converting Heavy Oil into Diesel and Jet Fuel

Hydrocracking is a refinery process that converts gas oils to lighter, more valuable products such as diesel and jet fuel. Catalyst and a large amount of hydrogen are required. The conversion happens through three main reactions: aromatic saturation, which adds hydrogen to ring-shaped molecules like benzene; cracking, which breaks apart longer hydrocarbon chains; and isomerization, a rearrangement that results in branched molecules.

Key Operating Parameters

“Conversion” is a key parameter monitored for hydrocracking. It is defined as the percentage of feed that is converted into lighter products (diesel, jet fuel, naphtha). As you push toward higher conversion, there are tradeoffs such as over-cracking, which produces very light, less valuable hydrocarbons. Whatever is not converted is appropriately called “unconverted oil,” often abbreviated as UCO. This stream can be fed to the FCC unit, if the refinery has one.

Hydrocracking Products

In addition to conversion, hydrocracking units also remove sulfur and nitrogen through hydrotreating reactions. This results in middle distillates (jet fuel and diesel) that meet finished product specifications and have excellent cold flow properties, including diesel pour point and kero/jet fuel freeze point. [1]

The hydrocarbon products lighter than jet fuel are naphtha. Due to lower octane value, this stream may not be ready for direct blending into gasoline. However, naphtha is an excellent candidate for reformer feed due to its high naphthene content. Naphthene is a term used in refining for cyclic saturated hydrocarbons like cyclohexane, which has six carbons in a ring with single bonds.

Reformers take naphthenes like cyclohexane, remove a few hydrogens, and convert them into aromatics, which have much higher octane. Reformate is a key component for gasoline blending.

The Hydrogen Paradox

This is a paradoxical process when you think about it. Hydrocracking adds hydrogen to saturate aromatics, then reformers remove hydrogen to make aromatics. One of the key concepts here is that aromatics are useful in gasoline for octane, but problematic in jet fuel and diesel. To make high-quality diesel, you need high hydrogen content. Aromatics have extremely low hydrogen content and have a very negative impact on cetane number. Diesel fuel has a minimum specification for cetane number.

How Does Hydrotreating Compare to Hydrocracking?

Hydrotreating is a similar refinery technology that also uses catalyst and hydrogen. However, it operates at less severe conditions, meaning lower pressure and temperature. In hydrotreating, the goal is to remove sulfur and nitrogen compounds. For example, a diesel hydrotreater takes a sour distillate stream and produces Ultra Low Sulfur Diesel (ULSD). A hydrotreater does not convert the feed to lighter products. Minimal cracking occurs.

Works Cited

[1] Handbook of Petroleum Refining Processes, Chapter 6.1, Page 288

I write about oil refinery operations and technology. My goal is to write in plain English and provide an entry point to the downstream oil industry.