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by
Marten Ternan, CANMAR Engineering Inc.,
Selected literature describing catalytic hydrocracking of reaction mixtures containing vacuum residue will be reviewed. Vacuum residue can be considered to be molecules having nominal boiling points greater than 525°C. Emphasis will be placed on hydrocracking catalysts that are used commercially, CoSu-MoSx-Al2O3 or NiSv-MoSy-Al2O3 and FeSw. Extensive reviews are available on aspects of hydrocracking vacuum residue [1], hydrocracking model compounds [2], and the commonly used promoted molybdenum sulphide catalysts [3].
There are two essential changes required for the conversion of the large molecules present in vacuum residue. The size of the molecules must decrease and the atomic H/C ratio must increase if the products are to become useable as conventional fuel products. Although heteroatom removal reactions (hydrodesulphurization, hydrodenitrogenation, hydrodeoxygenation, and hydrodemetallization) are also necessary, by themselves they are not sufficient.
The critical function of a hydrocracking catalyst is to make hydrogen available to the reaction mixture. Hydrogen has an effect on several important phenomena that occur in a hydrocracking reactor. First, hydrogen must be added to obtain large yields of distillate products having acceptable H/C ratios. Second hydrogen can inhibit the undesirable side reaction that forms coke as a by-product. Third, the hydrogen content of the reaction mixture can affect the solubility parameters, δ, of the various species in the reactor that determine compatibility (mutual solubility) of the various species in the liquid.
Catalysts can have a large impact on hydrocracking processes even though considerable conversion occurs concurrently via thermal non-catalytic reactions. Both catalytic and thermal processes are important. In one study the maximum conversion that could be attained without coke formation via thermal reactions in the absence of a catalyst was 74%, whereas in the presence of a catalyst the conversion exceeded 99%. At certain reaction conditions both the hydrogen addition reaction and some heteroatom removal reactions were correlated with the same catalyst property. All of the catalyst reaction sites are not directly in contact with gas phase hydrogen since the reaction mixture is composed of a large proportion of liquid. As a result phenomena such as hydrogen solubility in the liquid and hydrogen transfer among the various species are important. Catalytic reaction mechanisms have been suggested in which hydrogen is added to ring structures prior to ring opening. In addition several studies have discussed thermal cleavage of aliphatic chains from condensed ring structures.
If hydrogen molecules are dissociated at reaction sites at a catalyst surface that is located within a porous solid such as an alumina support, the carbonaceous molecules that have the first opportunity to receive that hydrogen are the ones that are closest to those reaction sites. As a result liquid phase diffusion of large carbonaceous molecules within the catalyst pore structure can also have an influence on conversion reactions.
In general the hydrogen rich parts of molecules in the reaction mixture are cracked to form smaller molecular weight products that enter the vapour phase. The larger molecular weight species that remain in the liquid phase tend to be comparatively hydrogen poor. These are the species that require additional hydrogen to ensure their conversion to distillates rather than having free radical oligomerization and dehydrogenation reactions converting them to coke. In the course of the conversion reactions, as a minimum the rate of cracking reactions should not be out of balance with the rate of the hydrogen addition reactions.
Catalyst deactivation is caused by both coke formation and metals deposition. A steady state amount of coke tends to be deposited rapidly on the catalyst surface. Subsequently this steady state amount of coke remains comparatively constant with increasing catalyst time on stream. In contrast the quantity of metals deposited on the catalyst tends to increase rather uniformly with time until the catalyst pores are blocked.
There are circumstances in which there can be at least two liquid phases in the reaction mixture. One liquid phase would be a comparatively non-polar/naphthenic hydrocarbon liquid, having a comparatively large H/C ratio. The other liquid phase would initially be composed of microscopic micelles that include large polar/polyaromatic molecules having smaller H/C ratios.
The micelles can agglomerate to form mesophase that is a precursor to coke. Hydrogen can be substantially more soluble in a non-polar liquid than in a polar liquid. In an extreme case hydrogenation reactions could occur in the non-polar liquid phase while dehydrogenation reactions were occurring simultaneously in the polar mesophase. Solids or semi-solids formed from mesophase can precipitate causing incompatibility.
Hydrogen addition can also cause incompatibility. A NiSv-MoSy-Al2O3 hydrocracking catalyst has been used with and without an oil-soluble Mo compound that was converted to dispersed MoSx particles in the reactor. The observation that coke formation on the NiSv-MoSy-Al2O3 catalyst decreased in the presence of MoSx particles, was consistent with the expectation that dispersed MoSx particles could add hydrogen to the reaction mixture. However, the presence of MoSx particles also caused the precipitation of bulk quantities of coke in the reactor.
This suggests the addition of hydrogen to some parts of the reaction mixture caused precipitation of other parts of the reaction mixture. In unrelated work hydrogen deficient aromatics such as Fluid Catalytic Cracker decant oil have been added to maintain an appropriate balance between polar/aromatic species and non-polar/naphthenic species, in order to maintain compatibility. Both examples illustrate the importance of hydrogen management by hydrocracking catalysts.
References:
1. R.J. Quann, R.A. Ware, C.W. Hung, and J. Wei, Catalytic hydrodemetallation of petroleum, Adv. Chem. Eng. 14, 95-259 (1988).
2. M.J. Girgis and B.C. Gates, Reactivities, reaction networks, and kinetics in high-pressure catalytic hydroprocessing, Ind. Eng. Chem. Res. 30, 2021-2058 (1991).
3. H. Topsøe, B.S. Clausen, and F.E. Massoth, Hydrotreating Catalysis, in Catalysis, Science, and Technology, eds. J.R. Anderson and M. Boudart, 11, 1-310 (1996).
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