Hydrocracking is a catalytic chemical process used in petroleum refineries to break complex hydrocarbon molecules into simpler molecules of gasoline or kerosene, by addition of hydrogen under high pressure. This process uses hydrogen gas to improve the hydrogen-carbon ratio in the cracked molecules and arrive at a broader range of end products. The major products from hydrocracking are jet fuel and diesel, but low-sulfur naphtha fractions and LPG are also produced. All these products have very low sulfur content and few contaminants. Hydrocracking process takes place in a hydrogen-rich atmosphere at elevated temperatures of 260 to 425 degrees Celsius and pressures of 35 to 200 bars. Any sulfur and nitrogen present in the hydrocracking feedstock is, to a large extent, hydrogenated to form gaseous hydrogen sulfide (H2S) and ammonia (NH3), which are subsequently removed.
The importance of hydrocracking as an oil-conversion process is growing. With their well-defined pore structures, offered substantial advantages.3. The benefits of the zeolite catalyst, in terms of both initial activity and stability, are apparent. Some of the important development in hydrocracking has been mild hydrocracking and resid hydrocracking. Mild hydrcracking (MHC) is characterized by relatively low conversion (20-40%) as compared to convention hydrocracking which give 70-100% conversion of heavy distillate at high pressure.
The result is that hydrocracking products are essentially free of sulfur and nitrogen impurities and consist mostly of paraffinic hydrocarbons. Hydrocracking catalysts consist of active metals on solid, acidic supports and have a dual function, specifically a cracking function and a hydrogenation function.
The cracking function is provided by the acid catalyst support and the hydrogenation function is provided by the metals. The process depends on the nature of the feedstock and the relative rates of the two competing reactions.
Heavy aromatic feedstock is converted into lighter products under a wide range of very high pressure (1,000–2,000 psi) and fairly high temperatures (400 to 815 degree Celsius) in the presence of hydrogen and special catalysts. Catalytic hydrocracking involves three primary chemical processes:. Cracking of high-boiling hydrocarbons found in crude oil into lower-boiling hydrocarbons. Hydrogenating unsaturated hydrocarbons to obtain saturated hydrocarbons, usually referred to as alkanes. Hydrogenating any sulfur, nitrogen or oxygen compounds in the original feedstock into gaseous hydrogen sulfide, ammonia and water.
Hydrocracking units are normally made of low-alloy steels with type 347 cladding or weld overlay to avoid intergranular stress-corrosion cracking as well as hydrogen attack.
. Better balance of gasoline and distillate production. Greater gasoline yield. Improved gasoline pool octane quality and sensitivity. Production of relatively high amounts of isobutane in the butane fraction. Supplementing of fluid catalytic cracking to upgrade heavy cracking stocks, aromatics, cycle oils, and coker oils to gasoline, jet fuels, and light fuel oils In a modern refinery catalytic cracking and hydrocracking work as a team.
The catalytic cracker takes the more easily cracked paraffinic atmospheric and vacuum gas oils as charge stocks, while the hydrocracker uses more aromatic cycle oils and coker distillates as feed. Cracking, while the higher pressures and hydrogen atmosphere make them relatively easy to hydrocrack.
The new zeolite cracking catalysts help improve the gasoline yields and octanes from catalytic crackers as well as reduce the cycle stock and gas make. However, the cycle oil still represents a difficult fraction to crack catalytically to extinction. One alternative is to use the cycle stock as a component for fuel oil blending, but this is limited as it is a relatively poor burning stock and burns with a smoky flame. For this reason a limit is placed on the percentage that can be blended into distillate fuel oils. The cycle oils that result from cracking operations with zeolite catalysts tend to be highly aromatic and therefore make satisfactory feedstocks for hydrocracking. Vacuum and coker gas oils are also used as hydrocracker feed. Hydrocracker feed to make jet and motor gasoline products.
Both straight-run and FCC LCO can be used and in some cases 100% LCO is used. In cases where 100% LCO is the feed, there is a follow-up with a high-pressure hydrotreater to reduce the aromatic content and increase the smoke point to meet specifications. When the feed contains large amounts of LCO, the major effects are increased heat release and lower smoke point of the jet fuel product. In addition to middle distillates and cycle oils used as feed for hydrocrack- ing units, it is also possible to process residual fuel oils and reduced crude by hydrocracking. Provides olefins for hydrogenation, while hydrogenation in turn provides heat for cracking.
The cracking reaction is endothermic and the hydrogenation reaction is exothermic. The overall reaction provides an excess of heat because the amount of heat released by the exothermic hydrogenation reactions is much greater than the amount of heat consumed by the endothermic cracking reactions. This surplus of heat causes the reactor temperature to increase and accelerate the reaction rate. This is controlled by injecting cold hydrogen as quench into the reactors to absorb the excess heat of reaction.
Complementary operation of the hydrogenation and cracking reactions is the initial hydrogenation of a condensed aromatic compound to a cycloparaffin. This allows subsequent cracking to proceed to a greater extent and thus converts a low-value component of catalytic cycle oils to a useful product. Isomerization is another reaction type that occurs in hydrocracking and accompanies the cracking reaction. The olefinic products formed are rapidly hydrogenated, thus maintaining a high concentration of high octane isoparaffins and preventing the reverse reaction back to straight-chain molecules. An interesting point in connection with the hydrocracking of these compounds is the relatively small amounts of propane and lighter materials that are produced as compared with normal cracking processes. Catalyst temperatures between 550 and 750°F (290 to 400°C) and at reactor pressures between 1200 and 2000 psig (8275 and 13,800 kPa). The circulation of large quantities of hydrogen with the feedstock prevents excessive catalyst fouling and permits long runs without catalyst regeneration.
Careful preparation of the feed is also necessary in order to remove catalyst poisons and to give long catalyst life. Frequently the feedstock is hydrotreated to remove sulfur and nitrogen compounds as well as metals before it is sent to the firsthydrocracking stage or, sometimes, the first reactor in the reactor train can be used for this purpose.
Hydrocracking catalyst is susceptible to poisoning by metallic salts, oxygen, organic nitrogen compounds, and sulfur in the feedstocks. The feedstock is hydrotreated to saturate the olefins and remove sulfur, nitrogen, and oxygen compounds. Molecules containing metals are cracked and the metals are retained on the catalyst. The nitrogen and sulfur compounds are removed by conversion to ammonia and hydrogen sulfide. Although organic nitrogen compounds are thought to act as permanent poisons to the catalyst, the ammonia produced by reaction of the organic nitrogen compounds with hydrogen does not affect the catalyst permanently. And metals, it is also necessary to reduce the water content of the feed streams to less than 25 ppm because, at the temperatures required for hydrocracking, steam causes the crystalline structure of the catalyst to collapse and the dispersed rare-earth atoms to agglomerate.
Water removal is accomplished by passing the feed stream through a silica gel or molecular sieve dryer. On the average, the hydrogen treating process requires approximately 150 to 300 ft3 of hydrogen per barrel of feed (27 to 54 m3 hydrogen per m3 feed). The hydrocracking process may require either one or two stages, depending upon the process and the feed stocks used.
The GOFining process is a fixed-bed regenerative process employing a molecular-sieve catalyst impregnated with a rare-earth metal. The process employs either single-stage or two-stage hydrocracking with typical operating conditions ranging from 660 to 785°F and from 1000 to 2000 psig (350–420°C and 6900–13,800 kPa). The temperature and pressure vary with the age of the catalyst, the product desired, and the properties of the feedstock.
(high in hydrogen content) and passed through a heater to the first reactor. If the feed has not been hydrotreated, there is a guard reactor before the firsthydrocracking reactor. The guard reactor usually has a modifiedhydrotreating catalyst such as cobalt-molybdenum on silica-alumina to convert organic sulfur and nitrogen compounds to hydrogen sulfide, ammonia, and hydrocarbons to protect the precious metals catalyst in the following reactors. The hydrocracking reactor(s) is operated at a sufficiently high temperature to convert 40 to 50 vol% of the reactor effluent to material boiling below 400°F (205°C).
High-pressure separator where the hydrogen-rich gases are separated and recycled to the first stage for mixing both makeup hydrogen and fresh feed. The liquid product from the separator is sent to a distillation column where the C4 and lighter gases are taken off overhead, and the light and heavy naphtha, jet fuel, and diesel fuel boiling range streams are removed as liquid sidestreams. The fractionator bottoms are used as feed to the second-stage reactor system.
The unit can be operated to produce all gasoline and lighter products or to maximize jet fuel or diesel fuel products. Recycle hydrogen from the second stage and sent through a furnace to the second-stage reactor. Here the temperature is maintained to bring the total conversion of the unconverted oil from the first-stage and second-stage recycle to 50 to 70 vol% per pass. The second-stage product is combined with the first-stage product prior to fractionation. Both the first- and second-stage reactors contain several beds of catalysts. The major reason for having separate beds is to provide locations for injecting old recycled hydrogen into the reactors for temperature control.
In addition, redistribution of the feed and hydrogen between the beds helps to maintain a more uniform utilization of the catalyst. There are a number of hydrocracking catalysts available and the actual composition is tailored to the process, feed material, and the products desired. Most of the hydrocracking catalysts consist of a crystalline mixture of silica-alumina with a small uniformly distributed amount of rare earths contained within the crystalline lattice. The silica-alumina portion of the catalyst provides cracking activity while the rare-earth metals promote hydrogenation. Catalyst activity decreases with use, and reactor temperatures are raised during a run to increase reaction rate and maintain conversion.
The catalyst selectivity also changes with age and more gas is made and less naphtha produced as the catalyst temperature is raised to maintain conversion. For catalyst activity to decrease from the accumulation of coke and other deposits to a level which will require regeneration. Regeneration is accomplished by burning off the catalyst deposits, and catalyst activity is restored to close to its original level. The catalyst can undergo several regenerations before it is necessary to replace it.
Almost all hydrocracking catalysts use silica-alumina as the cracking base but the rare-earth metals vary according to the manufacturer. Those in most common use are platinum, palladium, tungsten, and nickel. Reactor Temperature. Reactor temperature is the primary means of conversion control. At normal reactor conditions a 20°F (10°C) increase in temperature almost doubles the reaction rate, but does not affect the conversion level as much because a portion of the reaction involves material that has already been converted to materials boiling below the desired product end point.
As the run progresses it is necessary to raise the average temperature about 0.1 to 0.2°F per day to compensate for the loss in catalyst activity. At low concentrations the presence of hydrogen sulfide acts as a catalyst to inhibit the saturation of aromatic rings. This conserves hydrogen and produces a product with a higher octane number because the aromatic naphtha has a higher octane than does its naphthenic counterpart. However, hydrocracking in the presence of a small amount of hydrogen sulfide normally produces a very low-smoke-point jet fuel. At high hydrogen sulfide levels corrosion of the equipment becomes important and the cracking activity of the catalyst is also affected adversely.