Slurry-Phase Hydrocracking

Slurry-phase hydrocracking converts residue in the presence of hydrogen under severe process conditions - more than 840°F (450°C) and 2000 to 3000 psig (13,891 to 20,786 kPa). To prevent excessive coking, finely powdered additives made from carbon or iron salts are added to the liquid feed. Inside the reactor, the liquid/powder mixture behaves as a single phase due to the small size of the additive particles. Residue conversion can exceed 90%, and the quality of converted products is fairly good. 

Unfortunately, the quality of the unconverted pitch is poor, so poor that it can t be used as a fuel unless it is blended with something else - coal or heavy fuel oil. Even then, its high metals and sulfur content can create problems. 

At the 5,000 b/d CANMET demonstration plant in Canada, the pitch is sent to a cement kiln for use as clinker. Other slurry-phase processes include COMBIcracking (developed by Veba Oel), Aurabon (UOP), and HDH Cracking (Intevep). Although several slurry-phase demonstration plants have been built, the pitch-disposal problem has kept it from gaining industry-wide acceptance. 

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Zhang, S., Liu, D., Deng, W., Que, G. 2007. A review of slurry-phase hydrocracking heavy oil technology. Energy Fuels 21(6) 3057-3062. 

Slurry-phase hydrocracking systems convert heavy vacuum residues however, these processes are not yet fully commercialized. The feed to this type of reactor is the petroleum residue plus a solid carrier (commonly known as additive). The purpose of the additive is to provide a surface for the deposition of converted asphaltenes and metals, as the residue is hydrocracked. Slurry reactors operate at high temperature and pressure, and residue conversions higher than 90% (Kressmann et al., 1998). Unfortunately, these units produce poor-quality, hydrogen-deficient distillate and vacuum products that cannot be used as fuel, unless blended with something else, for example, coal or heavy fuel oil, due to their high content of sulfur and metals (Ancheyta and Speight, 2007). 

At high temperatures Sasol tests have shown that the slurry bed has a lower conversion than the fixed fluidized-bed (FFB). At these high temperatures the wax is hydrocracked, ie, there is a negative wax production. To avoid this, the reactor temperature has to be lowered which, of course, means a further drop in conversion. It is not possible to load as much catalyst per unit reactor volume in a slurry phase reactor as in a normal FFB reactor. This gives the latter system an intrinsic advantage. (Increasing the catalyst content of a slurry increases its viscosity, hence the bubble size increases, resulting in a shorter gas residence time.)... 

Dr Dry from Sasol proposed in 1982 to use slurry phase reactors to ultimately produce mainly diesel with naphtha as a significant co-product by using a scheme in which the reactor wax is hydrocracked (19, 20). It was further proposed that this naphtha is a good feedstock for thermal cracking to produce ethylene. Gulf Oil in 1985 proposed the use of a slurry reactor with a modem precipitated cobalt catalyst to produce mainly diesel as a final product (21). The advent of still more active cobalt catalysts has now resulted in the ability to consider gas velocities for the LTFT reactors that are in line with those used for the HTFT fluid bed reactors. 

Exxon Mobil developed the AGC-21 process (Advanced Gas Conversion for 21st century) for converting natural gas to liquid fuels. The process involves methane reforming, slurry-phase FT synthesis with a cobalt-based catalyst, and hydroisomerization and hydrocracking of the waxes (12). A 6.3 Mt per year FT plant was designed to operate in Qatar, but the plans were cancelled in 2007 (10). 

The MRH process is a hydrocracking process designed to upgrade heavy feedstocks containing large amount of metals and asphaltene, such as vacuum residua and bitumen, and to produce mainly middle distillates (Sue, 1989). The reactor is designed to maintain a mixed three-phase slurry of feedstock, fine powder catalyst and hydrogen, and to promote effective contact. 

The DCL process contains two steps dissolution of coal in the preheater (accompanied by several fast reactions) and subsequent hydrogenation/hydrocracking reactions (slow reactions) in a three phase slurry reactor. 

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