Hydroprocessing of Petroleum - Spent Hydroprocessing Catalysts - page 22

naphtha) derived from heavy feeds by carbon-rejecting processes may contain olefins, which
have to be removed to ensure stability of the final products. Again, the mechanism of reactions
occurring during the hydroprocessing of distillate feeds is well documented [49-53] compared
with that for heavy feeds, particularly those containing resins, asphaltenes, and metals.
The reactions occurring during hydroprocessing of the feeds boiling below 350 ◦ C are
common with those for the feeds boiling above 350 ◦ C, such as VGO and HGO. However, for
the latter, HCR and HIS reactions may be an important part of the overall mechanism,
particularly if the production of middle distillates for transportation fuels and lube base stock
are the objectives of hydroprocessing. In this case, a high level of dewaxing and HDAr may be
necessary to meet specifications of the final products.
For asphaltenes and metals containing feeds, HCR, HDAs, and HDM are the most important
reactions, as it is documented later in the topic. In multistage systems, hydroprocessing will be
dominated by different reactions in different stages. The HDM and HDAs are always the main
reactions occurring in the first stage. While these reactions may be still important, the
conversion of resins may become important in the second stage and stages following after until
the overall hydroprocessing is governed by HYD, HDS, HDN, and HDO reactions in the final
stage. For atmospheric and VRs, the reactions occurring during the final stage resemble those
occurring during hydroprocessing of DAO and VGO. However, the extent of these reactions in
different stages depends also on the origin of heavy feed and the type of catalyst. Therefore,
the selection of catalysts for every stage requires attention.
It is generally known that the structural changes of hydrocarbons increase with the increasing
acidity of catalysts. This supports the involvement of the HIS and HCR reactions. To a certain
extent, such reactions proceed via a carbocation mechanism. Because the thermal scission of
the C C bond to form free radicals begins above 600 K, the latter may be formed under typical
hydroprocessing conditions. Therefore, both carbocations and free radicals may be part of the
overall mechanism of hydroprocessing.
3.3 Hydroprocessing Catalysts
These are extensive information on all aspects of hydroprocessing catalysts. This information
has been periodically reviewed by several authors [15,27,44-54] . For the purpose of this topic,
a brief and general account of the chemical composition and physical properties of
hydroprocessing catalysts will only be given. The Mo(W)-containing supported catalysts,
promoted either by Co or Ni, have been used for hydroprocessing for decades. The -Al 2 O 3
has been the predominant support. In recent years, other supports, e.g., silica-alumina,
zeolites, TiO 2 , etc., have been gradually introduced with the aim of improving catalyst
performance. The enhancement in the rate of HCR and HIS reactions was the reason for using
more acidic supports. The operating (sulfided) form of the catalysts contains the slabs of the