Hydrocracking Flow Schemes

As described earlier, the most common form of hydrocracking process is a two-stage operation. " " This flow scheme has been very popular in the many refineries where it is necessary to maximize the yield of transportation fuels and has the flexibility to produce gasoline, naphtha, jet fuel, or diesel fuel to meet seasonal swings in product demand. 

The mild hydrocracking process uses operating conditions similar to those of a VGO desulfurizer to convert VGO to yield significant lighter products. Consequently, the flow scheme for a mild hydrocracking unit is virtually identical to that of a VGO desulfurizer. 

A severe hydrotreating plant will have a flow scheme similar to the hydrofinishing unit shown in Fig. 1.11. Hydrocracking is a highly exothermic reaction, so cold hydrogen must be injected at several points in the catalyst bed to moderate the temperature rise. Operating conditions are severe, with pressures of 100-180 bar and reaction temperatures of 350 20°C. 

A modem petroleum refinery may have four or more hydrotreating units. Strictly speaking, hydrotreaters are not conversion imits because the breaking of carbon-to-carbon bonds is minimal. However, it is convenient to discuss hydrotreating together with hydrocracking and mild hydrocracking because they employ similar catalysts and process flow schemes. 

Early fixed-bed hydrocrackers used a separate hydrotreat flow scheme, which resembles a 2-stage design with nothing but hydrotreating catalyst in the r stage. This flow scheme is discussed in further detail in Chapter 8 by Adrian Gruia. 

Figure 15-15. Optimized flow scheme for recovery of hydrogen from hydrocracker purge streams BoWngerotal., 1984. Beprodiwed with perniisskm from Chemical Engineering Progress, copyright 1984, American Instttute ofOimical Engineers...

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