Chapter 7: Catalytic Hydrocracking. The interest in the use of hydrocracking has been caused by several factors, including The demand for petroleum products has shifted to high ratios of gasoline and jet fuel compared with the usages of diesel fuel and home heating oils,
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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
These streams are very refractory and resist catalytic cracking, while the higher pressures and hydrogen atmosphere make them relatively easy to hydrocrack.
Sometimes diesel boiling range material is included in 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.
Cracking and hydrogenation are complementary, for cracking provides oleﬁns for hydrogenation, while hydrogenation in turn provides heat for cracking.
Another reaction that occurs and illustrates the complementary operation of the hydrogenation and cracking reactions is the initial hydrogenation of a condensed aromatic compound to a cycloparafﬁn
Hydrocracking reactions are normally carried out at average 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).
For some types of hydrocracking catalysts, the presence of hydrogen sulﬁde in low concentrations acts as a catalyst to inhibit the saturation of aromatic rings.
In addition to the removal of nitrogen and sulfur compounds 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.
The decision to use a single- or two-stage system depends upon the size of the unit and the product desired.
The fresh feed is mixed with makeup hydrogen and recycle gas (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 reactor efﬂuent goes through heat exchangers to a 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 bottoms stream from the fractionator is mixed with recycle hydrogen from the second stage and sent through a furnace to the second-stage reactor.
With typical feedstocks it will take from two to four years for catalyst activity to decrease from the accumulation of coke and other deposits to a level which will require regeneration.