A hydrocracking unit takes heavy gas oil from the atmospheric tower, the vacuum tower, the FCCU, and the coking units as a feedstock. These gas oils are heavier than distillate fuel oil, and they have a higher boiling range as well. The hydrocracker “cracks” the heavy long-chain molecules into shorter chain molecules in the presence of hydrogen and catalyst. The resulting product is a clean-burning diesel, jet fuel, and gasoline.
There are two main reactions that occur in a hydrocracker. The first is the catalytic cracking of the heavy long-chain hydrocarbons in the lighter shorter-chain unsaturated hydrocarbons. The catalytic cracking uses heat and the feed is cooled as it progresses through the reactor. These lighter hydrocarbons are then saturated by adding hydrogen to the chains. The saturation reaction releases heat and causes the feed and products to heat up as they progress through the reactor.
Hydrogen is fed at several points into the reactor to assist in maintaining temperature control of the reactor. Therefore, the reactor temperature will not cool to the point that the cracking reaction will not occur, and it will also not increase to a point as to create a hazardous operation with respect to safety.
There are several fundamentals that establish the value of gasoil hydrocrackers, including:
- The two reactions in a gasoil hydrocracker provides a significant amount of volume expansion
- The yields are more to the distillate product side rather than the gasoline product.
- Distillate production is fungible for making diesel or jet fuel
- Gasoil hydrocracker products are very low in impurities such as sulfur and heavy metals, and are ideal for blending into finished products
- The reaction yields can be shifted by approximately 10% between the gasoline or distillate production
- Further yield adjustments can be accomplished by modifying the fraction controls downstream of the hydrocracking reactor(s).
There are a good many different proprietary hydrocracker designs available under license. There are also a number of different hydrocracker process equipment configurations:
- Single stage, once through hydrocracker: This configuration uses only one reactor and any uncracked residual hydrocarbon oil from the bottom of the reaction product fractionation tower is not recycled for further cracking. For single stage hydrocracking, either the feedstock must first be hydrotreated to remove ammonia and hydrogen sulfide or the catalyst used in the single reactor must be capable of both hydrotreating and hydrocracking.
- Single stage hydrocracker with recycle: This is the most commonly used configuration. The uncracked residual hydrocarbon oil from the bottom of reaction product fractionation tower is recycled back into the single reactor for further cracking. Again, for single stage hydrocracking, either the feedstock must first be hydrotreated to remove ammonia and hydrogen sulfide or the catalyst used in the single reactor must be capable of both hydrotreating and hydrocracking.
- Two stage hydrocracker: This configuration uses two reactors and the residual hydrocarbon oil from the bottom of reaction product fractionation tower is recycled back into the second reactor for further cracking. Since the first stage reactor accomplishes both hydrotreating and hydrocracking, the second stage reactor feed is virtually free of ammonia and hydrogen sulfide. This permits the use of a high performancenoble metal, such as palladium or platinum, catalysts which are susceptible to poisoning by sulfur or nitrogen compounds
MEI Maverick Engineering, a wholly owned subsidiary of Triple 5 Worldwide, can be of assistance with your Hydrocracker projects, and can work closely with whichever Licensor you select. MEI can provide the complete range of services, including front-end engineering design, detail engineering, procurement, construction management, start-up and operator training, to insure a successful project. Our expertise yields shorter project schedules, industry superior cost control, and minimal client risk.
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