Temperature hydrocracking

The analysis presented above uses empirical relations for the catalyst activity functions for the desulfurization and demetalization reactions. These activity functions assume that the major cause for the decline of the catalyst activity is the metal sulfide deposition. In reality, the activity decline would be caused by coke as well as metal deposition. The extent of activity decline due to coke deposition will depend on the extent of hydrocracking reactions. At high temperatures, hydrocracking reactions would be important. The studies of Oxenreiter et al.35 and Beuther and Schmid1 show that the coke deposition on an HDS catalyst can be very rapid initially, due to uncontrolled hydrocracking. This initial rapid coke deposition would, however, reach an equilibrium level. 

The H-Oil process is a high pressure, high temperature hydrocracking process, which uses an ebullated bed of catalyst to convert lower value heavy oils into upgraded higher value products. Deposit formation in the equipment downstream of the H-Oil reactor and high sediment accumulation in heavy fuel oil product streams are confining factors in current attempts to maximize H-Oil unit conversion. 

More severe hydrotreating, such as mild hydrocracking at an H2 partial pressure of >6.9 MPa (1000 psig) and temperature of >400° C, can stiU further reduce sulfur content in the FCC feed. Mild hydrocracking of a 25° API VGO reduced the sulfur content of the FCC feed 98%, from 1.7 wt % to 0.03 wt % (40). This reduction can be expected to lower the SO content in the FCCU regenerator flue gas by about 90%. 

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. 

For refinery units such as hydrocrackers in which the hydrogen partial pressure is much higher, e.g., above l,350psi and the operating temperatures are above 800°F, the 2 A Cr-1 Mo steel is eommonly used. The higher alloy... 

All of the above reactions are reversible, with the exception of hydrocracking, so that thermodynamic equilibrium limitations are important considerations. To the extent possible, therefore, operating conditions are selected which will minimize equilibrium restrictions on conversion to aromatics. This conversion is favored at higher temperatures and lower operating pressures. 

Bond breaking can occur at any position along the hydrocarbon chain. Because the aromatization reactions mentioned earlier produce hydrogen and are favored at high temperatures, some hydrocracking occurs also under these conditions. However, hydrocracking long-chain molecules can produce Ce, C7, and Cg hydrocarbons that are suitable for hydrode-cyclization to aromatics. 

Hydrocracking reaction conditions vary widely, depending on the feed and the required products. Temperature and pressure range from 400 to 480°C and 35 to 170 atmospheres. Space velocities in the range of 0.5 to 2.0 hr" are applied. Figure 3-8 shows the Chevron two-stage hydrocracking process. 

Toluene is dealkylated to benzene over a hydrogenation-dehydrogenation catalyst such as nickel. The hydrodealkylation is essentially a hydrocracking reaction favored at higher temperatures and pressures. The reaction occurs at approximately 700°C and 40 atmospheres. A high benzene yield of about 96% or more can be achieved ... 

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. 

The present state of technology is reviewed (mainly from German literature of 1993 -4) in the Add of three principal thermal methods used for plastics wastes, namely pyrolysis (high-temperature carbonisation, coking), hydrocracking and gasification. 36 refs. Articles from this journal can be requested for translation by subscribers to the Rapra produced International Polymer Science and Technology. 

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