Hydrocracker Operation

Heavy fractions (e.g., vacuum gas oils) and residues HDP might involve both, hydrotreatment and hydrocracking operations. HDT, in this case, is a feed pretreatment, for preparation to another process unit, which might be a HCK unit. This process combination, HDT-HCK can be used on Cycle Oil (FCC, coker), VGO (SR and coker) and SR residues (atmospheric and vacuum). It can be carried out in a single reactor with more than one catalyst, or in more than one reactor. 

To produce the maximum yield of finished gasoline in a hydrocracker-reformer combination, the hydrocracker should be operated to give maximum liquid yields, followed by a modem low pressure catalytic reformer to give the desired octane improvement (6). More severe hydrocracker operation produces higher octane naphtha but leads to an increase in the production of butanes with reduced yields of finished... 

For all of the hydrocracker operations, the feed properties are those of the heavy naphtha from the base crude mix blended with a specified fraction of light cycle oil from the base FCC operation. For all of the motor reformer operations, the feed properties are those of the motor naphtha from the base crude mix blended with heavy hydrocrackate from the base hydrocracker operation. For all of the BTX reformer operations, the feed properties are those of the BTX naphtha from the base crude mix blended with light hydrocrackate from the base hydrocracker operation. Finally, for each process unit, the process simulator computes the change in plant performance associated with a fixed perturbation of each feed property about the base operation. 

Figure 1. Calculated hot-spot formation in modeling hydrocracker operation. Reprinted with permission from S. B. Jaife, Ind. Eng. Chem. Process Dev. 15,410 (1976). Copyright 1976 American Chemical Society.

Development of a mathematical model for the gas-liquid-solid reactor where significant evaporation of the liquid occurs. The reaction could occur either only in liquid phase or in both liquid and gas phases. Both isothermal as well as nonisothermal operations should be considered. A practical example of such a reactor is the reactor used in high-severity hydrocracking operations. 

As illustrated earlier, for various forms of more conventional hydrocracking, the type of catalyst used can influence the product slate obtained. For example, for a mild hydrocracking operation at constant temperature, the selectivity of the catalyst varies from about 65% to about 90% by volume. Indeed, several catalytic systems have now been developed with a group of catalysts, specifically, for mild hydrocracking operations. Depending on the type of catalyst, they may be run as a single catalyst or in conjunction with a hydrotreating catalyst. 

Further processing of the hydrocracker residue is needed and Fig. 1.12 indicates the extra steps. Extraction and hydrotreatment are desirable to remove traces of polycyclic aromatics and improve product quality. De-waxing is essential because the hydrocracker residue is invariably waxy and distillation is needed to adjust boiling range and viscosity of the base oil. The economics of making special base oils from fuel hydrocracker residue are determined both by the hydrocracker operation and the additional processing at a conventional base oil plant, which is often at a separate site. 

The residual hydrotreated shale oil was mixed with regular refinery cat cracker feed at a rate of 3%, with no detectable shifts in yields or other adverse consequences. Similarly, the gasoline range cut had no detectable effects on hydrocracker operations at 1.5% of feed. 

Table 6 gives typical data concerning a two-step hydrocracking operation. 

Another application of the hydrocracking process could become important during the next decade, when the transformation of natural gas into liquid fuels is considered to be economically feasible. In a route going via the manufacture of synthesis gas and the production of heavy paraffins by Fischer-Tropsch synthesis, a hydrocracking operation could be the final step in order to produce a high quality diesel engine fuel. 

Ammonia and HCN are found in gas streams from delayed cokers, visbreakers, and fluid catalytic cracking units (FCCUs). Ammonia, HCN, and H2S are also produced during hydrotreating and hydrocracking operations. When gases from these units are treated, the ammonia is readily absorbed by aqueous alkanolamine solutions. HCN, which is a weak add and very water-soluble, is chemically absorbed by alkanolamine solutions and is released, along with the absorbed ammonia, in the amine regenerator. 

The presence of hydrogen in, for example, hydrotreating and hydrocracking operations, increases the severity of high-temperature sulfidic corrosion. Hydrogen coverts organic sulfur compounds in feed stocks to hydrogen sulfide corrosion becomes a function of HjS concentration. 

These results highlight the importance of using simulation tools to determine real conversion and yields in hydrocracking operations. The proposed approach can be employed not only for hydrocracking but also for other processes, which due to specific requirements have the need of introducing nonconventional feeds to the reactor. 

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