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Hydrocracking is a chemical cracking process used to break down large hydrocarbon molecules found in petroleum feedstock (crude oil)into smaller molecules. The primary benefit of hydrocracking when compared to traditional hydrocarbon cracking process is the use of relatively high-pressure streams of hydrogen gas. Hydrogen gas is used efficiently remove toxic organic nitrogen and sulfur compounds from the resultant oil fractions. These compounds are primarily removed from the oil fractions in the form of ammonia and hydrogen sulfide gas. Hydrogen gas also prevents the creation of large aromatic compounds, alkenes, and alkynes by the process of hydrogenation.

History

Hydrocracking traces its roots back to 1927 in Leuna, Germany . The process was originally used to hydrogenate brown coal for use as liquid fuel. Despite its declining usage after World War II, research continued until Chevron developed a new process known as "isocracking" in the early 1960s. Further advancements including "zeolite-based" hydrocracking catalysts" increased the economic viability of hydrocracking. Likewise, the increase in the need for diesel and jet fuel for airplanes and newer diesel-powered trains caused a drastic surge in the importance of hydrocracking. Rapid growth of hydrocracking continued until the late 1970s, when high prices of hydrogen gas and lowered demand of diesel and jet fuel brought fluid catalytic cracking to the forefront once more.

Chemistry

The catalytic hydrocracking process is ionic and initiated by an acid catalyst. The first step is carbocation formation, either by hydrogen ion attack on centers of negative charge, such as aromatic pi electron systems, or hydride ion abstraction by a Lewis acid. This intermediate then lowers the free energy barrier on hemolytic carbon-carbon bond cleavage, which causes long-chain hydrocarbons to break down into smaller hydrocarbon species. Coke formation by this process is of some concern in industry, as it is produced by isomerization of the carbocation species on the surface of catalyst species and lowering its efficiency. Isocracking is a more efficient and cleaner method of hydrocracking, with the added benefit of removing a large fraction of SOx and NOx species, which are responsible for formation of acid rain, by hydrogenation. For feedstocks with high concentrations of sulfur, nickel, and/or aromatic species, nickel-molybdenum, nickel-tungsten, and cobalt-molybdenum catalysts are preferred, in addition to the standard alumina. Noble metal catalysts (platinum, palladium) are preferred when levels of these contaminants are low. Isocracking, by nature of its initiating step, also effectively degrades and removes most aromatic compounds, many of which are pollutants and found at high levels in heavy oil.

• Hydrocracking Efficiency
  • alumina catalyst: %15-30
  • amorphous silica/alumina: %25-40
  • zeolites: %40-60

Industrial Applications

Hydrocracking

The process of hydrocracking is most commonly used in the fuel refinery industry. An industrial application of hydrocracking is the production of valuable jet fuel and diesel from heavy, crude oil. In industry, the hydrocracking process is similar to the cracking process, but at a higher pressure of hydrogen gas and higher temperature. The major products of hydrocracking are jet fuel, diesel, and gasoline with a high octane rating. The products are also much lighter than the crude oil that is refined. Another benefit of hydrocracking in industry is that the crude oil is converted to a lighter fuel with low amounts of sulfur and nitrogen. The crude oil typically used contains high amounts of organic sulfur and nitrogen, which can be harmful to the environment.

Isocracking

Isocracking involves the same process as hydrocracking but at a lower temperature and pressure. This specific form of hydrocracking was developed in the late 1950s by Chevron in order to convert crude oil to high octane gas. This process is the most widely used form of hydrocracking in industry because it produces a higher yield of less contaminated oils and fuels. Also, the products of isocracking contain low amounts of aromatics, which are difficult to burn and can be carcinogenic. The products produced also have very low amounts of sulfur and nitrogen after the isocracking process. An application of the isocracking process in industries is converting heavy fuels that are typically only used in ships and power plants into a usable, lighter, and less contaminated fuel.

Summary of Hydrocracking versus Isocracking

Isocracking is the process most commonly used in fuel refinery industries. This is due to the higher yield produced by isocracking. The products of isocracking have less impact on the environment because the fuels produced are less contaminated. The products of hydrocracking typically contain more [sulfur] and [nitrogen] than the products of isocracking. The major difference between these two processes is that hydrocracking is performed at a much higher temperature and pressure than isocracking.

Technological Advances

A vast majority of hydrocracking technology lies in the advancement of catalyst materials. The main feature of a catalyst is the acid support; this component provides the hydrocarbon cracking function. The other component is the metal which facilitates the hydrogenation-dehydrogenation process. Prior to zeolite-based catalysts coming to prominence, the acidic supports mainly consisted of amorphous solids, such as silica-alumina; mixtures of amorphous solids and zeolite are used heavily today to provide middle distillates (mid-range boiling points). The main goal of catalyst research today is in the area of heavy-oil hydrocracking catalysts.