provides a more even distribution of liquid in a trickle-flow reactor, which is the
prevalent flow regime in the hydrotreating of diesel-range streams and heavier.
Dense loading of at least the primary hydrotreating catalyst bed(s) is usually
preferred in order to maximize the reaction rate per unit reactor volume throughout
the catalyst life cycle. Dense loading delivers catalysts into a reactor at a controlled
rate, varied according to the rising bed height, so that the catalysts are oriented
similarly throughout the dense bed. This uniform orientation helps facilitate a
uniform vapor/liquid flow distribution and catalyst utilization. Catalyst particle
orientation is especially critical with respect to shaped, extruded catalysts in
trickle-flow reactors. Of all the factors influencing catalyst utilization, catalyst
loading has generally proven to be the most important factor. Another advantage
of dense loading is that it allows loading more catalyst in the reactor because of the
reduced void fraction in the catalyst bed. Thus, the catalyst life can be extended, or
else the unit can be operated at more severe conditions (lower product sulfur level,
increased feed rate) than if the catalyst had been sock loaded. Except for the
hydrotreaters that have reactor pressure drop limitations mainly due to operation
at higher-than-design throughputs, most hydrotreaters are dense loaded. Ex situ
presulfurized catalysts (see catalyst activation) are self-heating materials. Thus,
they should be loaded in an inert atmosphere though some loading contractors do
load them under air atmosphere.
Catalyst Activation
Hydrotreating catalysts have to be activated in order to be catalytically useful. The
activation of the catalyst is performed by conversion of metal oxides to metal
sulfides and is commonly called sulfiding, though several other names are used to
describe the same thing. Other names that are used to describe catalyst activation
techniques are pre-sulfiding or pre-sulfurizing. The metals on the catalysts are in an
oxide form at the completion of the manufacturing process. More often than not,
this is typically accomplished in situ; however, more refiners have started to use
catalyst which had the sulfiding compound loaded onto the catalyst outside the unit
(ex situ pre-sulfidation). It is likely that more and more refiners will opt to receive
the catalyst at the refinery site in pre-sulfided state to accelerate the start-up of the
unit and because it is more environmentally friendly (eliminates the VOC and odors
evolved when the sulfiding compound is introduced into the unit).
In situ sulfiding can be accomplished either in vapor or liquid phase. In vapor
phase sulfiding, the activation of the catalyst is accomplished by injecting a chemical
which decomposes easily to H 2 S, such as dimethyldisulfide (DMDS) or
dimethylsulfide (DMS). The usage of H 2 S/H 2 was common up until the end of the
last century, but now it is only rarely used because of environmental and safety
concerns. Liquid phase sulfiding can be accomplished with or without organo-sulfur
compounds added to the feedstocks. In the latter case, the feedstock is generally a
straight-run gas oil that contains sulfur compounds in ranges from a few thousand to
20,000 parts per million. The H 2 S necessary for the activation of the catalyst is
generated by the decomposition of the sulfur compounds. This method is in very little
use today, but it was “state of the art” in the 1960s and early 1970s. The preferred
Hydrotreating in Petroleum Processing
421
prevalent flow regime in the hydrotreating of diesel-range streams and heavier.
Dense loading of at least the primary hydrotreating catalyst bed(s) is usually
preferred in order to maximize the reaction rate per unit reactor volume throughout
the catalyst life cycle. Dense loading delivers catalysts into a reactor at a controlled
rate, varied according to the rising bed height, so that the catalysts are oriented
similarly throughout the dense bed. This uniform orientation helps facilitate a
uniform vapor/liquid flow distribution and catalyst utilization. Catalyst particle
orientation is especially critical with respect to shaped, extruded catalysts in
trickle-flow reactors. Of all the factors influencing catalyst utilization, catalyst
loading has generally proven to be the most important factor. Another advantage
of dense loading is that it allows loading more catalyst in the reactor because of the
reduced void fraction in the catalyst bed. Thus, the catalyst life can be extended, or
else the unit can be operated at more severe conditions (lower product sulfur level,
increased feed rate) than if the catalyst had been sock loaded. Except for the
hydrotreaters that have reactor pressure drop limitations mainly due to operation
at higher-than-design throughputs, most hydrotreaters are dense loaded. Ex situ
presulfurized catalysts (see catalyst activation) are self-heating materials. Thus,
they should be loaded in an inert atmosphere though some loading contractors do
load them under air atmosphere.
Catalyst Activation
Hydrotreating catalysts have to be activated in order to be catalytically useful. The
activation of the catalyst is performed by conversion of metal oxides to metal
sulfides and is commonly called sulfiding, though several other names are used to
describe the same thing. Other names that are used to describe catalyst activation
techniques are pre-sulfiding or pre-sulfurizing. The metals on the catalysts are in an
oxide form at the completion of the manufacturing process. More often than not,
this is typically accomplished in situ; however, more refiners have started to use
catalyst which had the sulfiding compound loaded onto the catalyst outside the unit
(ex situ pre-sulfidation). It is likely that more and more refiners will opt to receive
the catalyst at the refinery site in pre-sulfided state to accelerate the start-up of the
unit and because it is more environmentally friendly (eliminates the VOC and odors
evolved when the sulfiding compound is introduced into the unit).
In situ sulfiding can be accomplished either in vapor or liquid phase. In vapor
phase sulfiding, the activation of the catalyst is accomplished by injecting a chemical
which decomposes easily to H 2 S, such as dimethyldisulfide (DMDS) or
dimethylsulfide (DMS). The usage of H 2 S/H 2 was common up until the end of the
last century, but now it is only rarely used because of environmental and safety
concerns. Liquid phase sulfiding can be accomplished with or without organo-sulfur
compounds added to the feedstocks. In the latter case, the feedstock is generally a
straight-run gas oil that contains sulfur compounds in ranges from a few thousand to
20,000 parts per million. The H 2 S necessary for the activation of the catalyst is
generated by the decomposition of the sulfur compounds. This method is in very little
use today, but it was “state of the art” in the 1960s and early 1970s. The preferred
Hydrotreating in Petroleum Processing
421