9
macropore size distribution depending upon the application and usually possess
bimodal pore size distribution. Distillate HDS catalysts are designed with small pores
(about 7–10 nm pore size distribution), while residue HDS catalysts have mesopores
in the range of 10–20 nm. HDM catalysts are designed with large mesopores in the
10–50 nm range. Support macroporosity is also a desirable property with regard to
increasing the access of reactants to the active catalyst surface and minimizing diffusional limitations. The total porosity of HDT catalysts is typically in the range of
0.5–0.6 or less in order to maintain catalyst pellet strength. Porosity is related to pellet
strength, and catalyst pellet strength usually decreases with increasing porosity. There
is an optimum combination of surface area and pore diameter, which gives the highest
catalyst activity. For a given catalyst of similar chemical composition but differing
pore size, feed properties also play an important role in the catalyst activity [6]. High
HDS activity could be obtained with the small pore catalyst of high surface area for
light gas oil feed due to less diffusion limitations of small molecules to the interior of
the catalyst in comparison with heavy feeds such as VGO and residue.
Macrolevel properties such as size and shape of the catalyst particles also influence the hydroprocessing catalyst performance in terms of activity and pressure
drop. Hydroprocessing catalysts are manufactured with different shapes and sizes
depending on the type of application. The size of the fixed bed hydroprocessing
catalysts generally ranges from about 1.5 to 10 mm in diameter with length-todiameter ratios of about 1 for pellets and about 3 or 4 for extrudates [7]. Large catalyst particles offer diffusion resistance, decrease the rate of reaction, and thus reduce
the catalyst activity per unit mass. In order to overcome the diffusion limitations,
large catalyst particles are manufactured with holes or various shapes in order to
increase the surface-to-volume ratio [8]. Although small particles do not pose diffusion issues, they are prone to higher pressure drop in fixed bed reactors. Shaped
catalysts such as trilobes and quadralobes are widely employed in hydrotreating due
to their advantages in terms of high external surface area and better accessibility to
the interior of the catalyst compared to extrudates.
Bambrick [9] showed the influence of particle size on the relative weight activity
for the same catalyst with different shapes such as cylinder and trilobes. It was
observed in general that the surface area is the principal factor for catalyst activity
while the size and shape of the catalyst particle will have influence on the pressure
drop in fixed beds. For a given equivalent particle diameter, catalyst shapes were
ranked according to the relative pressure drops as follows:
Rings < Beads < Pellet < Extrudates < Crushed particles
5 Characterization and Testing of HDT Catalysts
Characterization and testing of HDT catalysts are crucial for both refiners and catalyst manufacturers to understand the interaction between the composition, physicochemical properties, and catalyst performance. Since industrial HDT catalysts are
Recent Advances in Hydrotreating/Hydrodesulfurization Catalysts: Part I: Nature…
macropore size distribution depending upon the application and usually possess
bimodal pore size distribution. Distillate HDS catalysts are designed with small pores
(about 7–10 nm pore size distribution), while residue HDS catalysts have mesopores
in the range of 10–20 nm. HDM catalysts are designed with large mesopores in the
10–50 nm range. Support macroporosity is also a desirable property with regard to
increasing the access of reactants to the active catalyst surface and minimizing diffusional limitations. The total porosity of HDT catalysts is typically in the range of
0.5–0.6 or less in order to maintain catalyst pellet strength. Porosity is related to pellet
strength, and catalyst pellet strength usually decreases with increasing porosity. There
is an optimum combination of surface area and pore diameter, which gives the highest
catalyst activity. For a given catalyst of similar chemical composition but differing
pore size, feed properties also play an important role in the catalyst activity [6]. High
HDS activity could be obtained with the small pore catalyst of high surface area for
light gas oil feed due to less diffusion limitations of small molecules to the interior of
the catalyst in comparison with heavy feeds such as VGO and residue.
Macrolevel properties such as size and shape of the catalyst particles also influence the hydroprocessing catalyst performance in terms of activity and pressure
drop. Hydroprocessing catalysts are manufactured with different shapes and sizes
depending on the type of application. The size of the fixed bed hydroprocessing
catalysts generally ranges from about 1.5 to 10 mm in diameter with length-todiameter ratios of about 1 for pellets and about 3 or 4 for extrudates [7]. Large catalyst particles offer diffusion resistance, decrease the rate of reaction, and thus reduce
the catalyst activity per unit mass. In order to overcome the diffusion limitations,
large catalyst particles are manufactured with holes or various shapes in order to
increase the surface-to-volume ratio [8]. Although small particles do not pose diffusion issues, they are prone to higher pressure drop in fixed bed reactors. Shaped
catalysts such as trilobes and quadralobes are widely employed in hydrotreating due
to their advantages in terms of high external surface area and better accessibility to
the interior of the catalyst compared to extrudates.
Bambrick [9] showed the influence of particle size on the relative weight activity
for the same catalyst with different shapes such as cylinder and trilobes. It was
observed in general that the surface area is the principal factor for catalyst activity
while the size and shape of the catalyst particle will have influence on the pressure
drop in fixed beds. For a given equivalent particle diameter, catalyst shapes were
ranked according to the relative pressure drops as follows:
Rings < Beads < Pellet < Extrudates < Crushed particles
5 Characterization and Testing of HDT Catalysts
Characterization and testing of HDT catalysts are crucial for both refiners and catalyst manufacturers to understand the interaction between the composition, physicochemical properties, and catalyst performance. Since industrial HDT catalysts are
Recent Advances in Hydrotreating/Hydrodesulfurization Catalysts: Part I: Nature…