Design hydrocracker

In a well-designed hydrocracking system, the hydrogenation function adds hydrogen to the tarry deposits. This reduces the concentration of coke precursors on the surface. There is, however, a slow accumulation of coke that reduces activity over a 1-2 year period. Refiners respond to this slow reduction in activity by raising the average temperature of the catalyst bed to maintain conversions. Eventually, however, an upper limit to the allowable temperature is reached and the catalyst must be removed and regenerated. 

Comparing the overall concentrations of these different carbons designated generally as structural patterns , measured before and after a process such as FCC or hydrocracking (see Chapter 10), enables the conversion to be monitored the simple knowledge of the percentage of condensed aromatic carbon of a feedstock gives an indication of its tendency to form coke. 

Pressure Vessels. Refineries have many pressure vessels, e.g., hydrocracker reactors, cokers, and catalytic cracking regenerators, that operate within the creep range, i.e., above 650°F. However, the phenomenon of creep does not become an important factor until temperatures are over 800°F. Below this temperature, the design stresses are usually based on the short-time, elevated temperature, tensile test. 

Catalytic reformers are normally designed to have a series of catalyst beds (typically three beds). The first bed usually contains less catalyst than the other beds. This arrangement is important because the dehydrogenation of naphthenes to aromatics can reach equilibrium faster than the other reforming reactions. Dehydrocyclization is a slower reaction and may only reach equilibrium at the exit of the third reactor. Isomerization and hydrocracking reactions are slow. They have low equilibrium constants and may not reach equilibrium before exiting the reactor. 

Superficially the Oryx GTL refinery design has much in common with the SMDS process, but there are important differences. There is no separate hydrotreater, which limits production of chemicals, such as waxes. The hydrocracker employs the Chevron Isocracking technology, which is based on a sulfided supported base-metal catalyst that was designed for crude oil conversion. The operating conditions of the hydrocracker are also more severe (>350°C, 7 MPa) than those required by the SMDS process (300-350°C, 3-5 MPa). Only intermediate products are produced (Table 18.13),5 with the naphtha slated as cracker feed and the distillate as blending component for diesel fuel. 

Figure 16.4 Process flow diagrams for major hydrocracker designs.

All the hydrocracking/hydrogenation experiments were carried out in 500 ml capacity spinning type autoclaves. Two autoclaves of this design were used and the autoclaves were compared in experiments using the model compound phenanthrene, chosen because phenanthrene and its hydro-derivatives represent a large proportion of the solvent which is recycled in coal liquefaction processes. 

Figure 24.1 is a sketch of a centrifugal pump, driven by a three-phase motor, with a turbine helper. This particular pump was charging a light gas oil stream to a high-pressure hydrocracker. The pump was operating quite close to its design conditions of... 

Hydrocracking is probably the most versatile of modem petroleum processes. This versatility has been achieved by the development of specific families of catalysts, of processing schemes designed to allow these catalysts to function efficiently, and of optimum refining relationships between hydrocracking and other refining processes. 

While the definitions of the various hydroprocesses are (as has been noted above) quite arbitrary, it may be difficult, if not impossible, to limit the process to any one particular reaction in a commercial operation. The prevailing conditions may, to a certain extent, minimize, cracking reactions during a hydrotreating operation. However, with respect to the heavier feedstocks, the ultimate aim of the operation is to produce as much low-sulfur liquid products as possible from the feedstock. Any hydrodesulfurization process that has been designed for application to the heavier oils and residua may require that hydrocracking and hydrodesulfurization occur simultaneously. 

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