advantageous to utilize a moderate concentration of Ni, as in 10–15 wt%, so that the
trapping catalyst will still have sufficient surface area to trap silica.
Alkali metals can originate from various crudes, from mis-operation of the crude
desalter, and from various operations which utilize caustic soda. Alkali metals
adsorb to the acid sites of the catalyst support, reducing activity. Their presence
may also be indicated by a rapid increase in pressure drop, especially in the layers
above the primary hydrotreating catalyst(s). During catalyst regeneration, the
presence of alkali metals tends to sinter the catalyst surface, resulting in surface
area loss beyond what is typically observed.
Mercury contaminants can generally be classified as hydrocarbon soluble, water
soluble, and asphaltenic. These can be found, in various proportions, within those
crudes extracted near tectonic plate boundaries. Elemental mercury and alkylmercury compounds will exit a crude column with the product streams according
to their relative volatility, from the LPG through to the heavy diesel. Water-soluble
mercury would be found in the heels of crude storage vessels and in the aqueous
drawoff from desalters. Asphaltenic mercury can be found in the atmospheric resid
and vacuum resid refinery streams. Asphaltenic mercury can be converted to the
lighter, hydrocarbon-soluble forms of mercury through hydroprocessing, which can
be problematic for an equipment that processes finished product streams, since
elemental mercury will form amalgams with Cu-containing alloys and with equipment constructed at least in part of aluminum, leading to embrittlement failures. In
general, mercury is not substantially trapped by typical hydrotreating catalysts, so
installation of a posttreat bed of mercury-trapping adsorbent may be needed to
prolong downstream equipment life and to otherwise satisfy market requirements
(Hadden et al. 2010).
Silica guard catalysts, loaded above the primary HDS catalysts in naphtha
hydrotreaters, are designed with high surface area. These catalysts are often
designed with a modest hydrogenation function via the NiMoS active phase. The
heat release of hydrogenation in this reactor drives the silica capacity higher than an
equivalent silica guard support without a NiMoS active phase. The adsorption
mechanism is directed by the available surface area and the operating temperature,
such that higher temperatures and surface areas result in greater silica capacities.
Silica adsorption can be tracked by proxy via the regular monitoring of the
hydrodenitrogenation performance, rather than the hydrodesulfurization performance. Several products of polydimethylsiloxane decomposition have been identified. It has been proposed that the decomposition products’ silanol groups react
with the alumina support hydroxyl groups (Chainet 2012).
Halide Removal
Organic halides, such as chlorides or bromides, can be present in petroleum
fractions at trace levels. Under hydrotreating conditions, organic halides are largely
Hydrotreating in Petroleum Processing
379
trapping catalyst will still have sufficient surface area to trap silica.
Alkali metals can originate from various crudes, from mis-operation of the crude
desalter, and from various operations which utilize caustic soda. Alkali metals
adsorb to the acid sites of the catalyst support, reducing activity. Their presence
may also be indicated by a rapid increase in pressure drop, especially in the layers
above the primary hydrotreating catalyst(s). During catalyst regeneration, the
presence of alkali metals tends to sinter the catalyst surface, resulting in surface
area loss beyond what is typically observed.
Mercury contaminants can generally be classified as hydrocarbon soluble, water
soluble, and asphaltenic. These can be found, in various proportions, within those
crudes extracted near tectonic plate boundaries. Elemental mercury and alkylmercury compounds will exit a crude column with the product streams according
to their relative volatility, from the LPG through to the heavy diesel. Water-soluble
mercury would be found in the heels of crude storage vessels and in the aqueous
drawoff from desalters. Asphaltenic mercury can be found in the atmospheric resid
and vacuum resid refinery streams. Asphaltenic mercury can be converted to the
lighter, hydrocarbon-soluble forms of mercury through hydroprocessing, which can
be problematic for an equipment that processes finished product streams, since
elemental mercury will form amalgams with Cu-containing alloys and with equipment constructed at least in part of aluminum, leading to embrittlement failures. In
general, mercury is not substantially trapped by typical hydrotreating catalysts, so
installation of a posttreat bed of mercury-trapping adsorbent may be needed to
prolong downstream equipment life and to otherwise satisfy market requirements
(Hadden et al. 2010).
Silica guard catalysts, loaded above the primary HDS catalysts in naphtha
hydrotreaters, are designed with high surface area. These catalysts are often
designed with a modest hydrogenation function via the NiMoS active phase. The
heat release of hydrogenation in this reactor drives the silica capacity higher than an
equivalent silica guard support without a NiMoS active phase. The adsorption
mechanism is directed by the available surface area and the operating temperature,
such that higher temperatures and surface areas result in greater silica capacities.
Silica adsorption can be tracked by proxy via the regular monitoring of the
hydrodenitrogenation performance, rather than the hydrodesulfurization performance. Several products of polydimethylsiloxane decomposition have been identified. It has been proposed that the decomposition products’ silanol groups react
with the alumina support hydroxyl groups (Chainet 2012).
Halide Removal
Organic halides, such as chlorides or bromides, can be present in petroleum
fractions at trace levels. Under hydrotreating conditions, organic halides are largely
Hydrotreating in Petroleum Processing
379