Reactors hydrocracking

At the H2 pressures normally inside the FT reactors, hydrocracking of hydrocarbons is thermodynamically favored but in practice it hardly occurs, if at all. 

Isomerization and polymerization, for higher octane gasoline, and aromatization for petrochemicals, are typically done using packed bed reactors. Hydrocracking, to increase gasoline yield from crude-oil distillates, and desulfurization, essential to clean gasoline and diesel fuels of polluting sulfur oxides, is done both in packed or trickle bed reactors, or in a fluidized bed reactor for desulfurization. 

As an illustration. Figure 12 shows case-study results from a rigorous model for a two-reactor hydrocracker with partial recycle of unconverted oil. The unit runs at about 60% conversion-per-pass. A fixed flow of unconverted bottoms is recycled. The remainder is exported to the FCC unit or heavy diesel blending. At present, the export comprises about half of the total unconverted oil. 

C. R. Cutier and R. B. Hawkins, "AppHcation of a Large Model Predictive Controller to a Hydrocracker Second Stage Reactor," Proceedings of... 

Shell Gas B.V. has constructed a 1987 mVd (12,500 bbhd) Fischer-Tropsch plant in Malaysia, start-up occurring in 1994. The Shell Middle Distillate Synthesis (SMDS) process, as it is called, uses natural gas as the feedstock to fixed-bed reactors containing cobalt-based cat- yst. The heavy hydrocarbons from the Fischer-Tropsch reactors are converted to distillate fuels by hydrocracking and hydroisomerization. The quality of the products is very high, the diesel fuel having a cetane number in excess of 75. 

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. 

In the two-stage operation, the feed is hydrodesulfurized in the first reactor with partial hydrocracking. Reactor effluent goes to a high-pressure separator to separate the hydrogen-rich gas, which is recycled and mixed with the fresh feed. The liquid portion from the separator is fractionated, and the bottoms of the fractionator are sent to the second stage reactor. 

Van Driesen and Stewart (V4) have reported temperature measurements for various locations in commercial gas-liquid fluidized reactors for the large-scale catalytic desulfurization and hydrocracking of heavy petroleum fractions (2500 barrels per day capacity). The hydrogenation was carried out in two stages the maximum and minimum temperatures measured were 774° and 778°F for the first stage and 768° and 770°F for the second. These results indicate that gas-liquid fluidized reactors are characterized by a high effective thermal conductivity. 

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