Hydroprocessing of Petroleum - Spent Hydroprocessing Catalysts - page 21

3.2 Hydroprocessing Reactions
The presence of large molecules indicates a significant complexity of the reactions occurring
during hydroprocessing of heavy feeds compared with light feeds. Because of the increasing
involvement of asphaltenic molecules, the complexity increases from distillates, through
VGO/HGO towards VRs and topped heavy crude. In every case, the primary objective is the
conversion of large molecules to those present in distillates. This may be accomplished via
HCR of resins and asphaltenes simultaneously with the conversion of porphyrin structures.
Therefore, for heavy asphaltenic feeds, a high rate of the hydrodeasphaltization (HDAs) is
required to achieve a desirable rate of hydrodemetallization (HDM). Thus, the desirable level
of the removal of metals cannot be achieved before most of asphaltenes are depolymerized to
smaller entities.
In the case of VGO, HGO, and DAO feeds, a high level of hydrogenation (HYD) of aromatics,
i.e., a high rate of hydrodearomatization (HDAr), must be achieved, when the feed preparation
for FCC is the objective. Moreover, for such feeds, even traces of metals and asphaltenes as
well as nitrogen have to be removed to prevent poisoning of FCC catalyst, unless a more
advanced process, i.e., residue FCC (RFCC) process, is used. In the case of catalytic dewaxing
of these feeds, catalyst must posses an adequate HCR activity and selectivity to ensure a high
yield of middle distillates and lube base oil fractions. For dewaxing catalysts, the HIS of
n-paraffins to isoparaffins becomes an important catalytic functionality to ensure low freezing
point and pour point of the final products. In addition, to be suitable for preparation of
lubricants, lube base oil must exhibit good viscosity behavior. For this purpose, aromatic
structures must be converted to naphthenic compounds. For most of the VGO, HGO, and
DAO, desirable properties of the products (e.g., lube base oil and diesel oil) cannot be attained
in one stage.
To various degrees, hydrodesulfurization (HDS), hydrodenitrogenation (HDN), and HDO
reactions occur simultaneously with HYD, HCR, HDM, HDAs, and HIS. The mutual effects
of these reactions are rather complex. Kinetic measurements can be used to quantify the
progress of these reactions. The kinetic and mechanistic aspects indicate that the mutual
effects of hydroprocessing reactions are rather complex even for light feeds. To a certain
extent, these effects may be influenced and/or controlled by the properties of catalysts as well
as by the experimental conditions.
The extensive information on various aspects of the mechanism of hydroprocessing reactions
has been published in the literature. Several authoritative reviews were devoted to specific
reactions, i.e., HDS [1,2,27,48] , HDN [49,50] , HDO [51] , and HYD [52] . Focus has been on
both model compounds and real feeds. Usually, the objective of hydroprocessing of the
conventional feeds boiling below 350 ◦ C has been the removal of heteroatoms and HYD of
aromatics to meet specifications of the conventional fuels. The distillate fractions (e.g.,