6
ammonium bisulfide, which otherwise causes corrosion in the downstream equipment. The liquid from the separator and flash drum is sent to a stripper to strip off
hydrogen sulfide from the liquid. The stripper bottom product is routed to a fractionator in which naphtha, kerosene, and diesel streams are separated.
The process flow of most of the diesel hydrotreaters is similar to the one discussed above, with minor differences in the separation system and number of reactors. In the case of two-stage hydrotreaters, H 2 S is removed from the first-stage
product using mostly hydrogen as stripping medium, and the sulfur-free diesel is
processed in the second stage over sulfur-sensitive noble metal-based hydrogenation catalysts to carry out deep aromatic saturation for cetane improvement. The
process flow of other feed hydrotreaters such as naphtha, kerosene, vacuum gas oil,
lube oils, and residue are also almost similar to that of diesel hydrotreating.
4 Physicochemical Properties
4.1 Chemical Composition of Catalysts
Hydrotreating catalysts comprise mainly two components, namely, active metals
and support. The metal components that promote hydrogenolysis and hydrogenation functions are usually supported on suitable inert material such as alumina.
These are basically supported metallic catalysts using either single metal or combination of two or more metals, but common catalysts contain two metals with one
metal as the active component and another as a promoter. Most of the hydrotreating
catalysts are made of sulfides of Group VI A metals (Mo, W) promoted by sulfides
of Ni or Co and supported on high surface area carriers such as alumina, silicaalumina, and zeolites. γ-Alumina is the most commonly used support material in
Fig. 2 Typical flow diagram of diesel hydrotreater unit
G. Valavarasu and B. Ramachandrarao
ammonium bisulfide, which otherwise causes corrosion in the downstream equipment. The liquid from the separator and flash drum is sent to a stripper to strip off
hydrogen sulfide from the liquid. The stripper bottom product is routed to a fractionator in which naphtha, kerosene, and diesel streams are separated.
The process flow of most of the diesel hydrotreaters is similar to the one discussed above, with minor differences in the separation system and number of reactors. In the case of two-stage hydrotreaters, H 2 S is removed from the first-stage
product using mostly hydrogen as stripping medium, and the sulfur-free diesel is
processed in the second stage over sulfur-sensitive noble metal-based hydrogenation catalysts to carry out deep aromatic saturation for cetane improvement. The
process flow of other feed hydrotreaters such as naphtha, kerosene, vacuum gas oil,
lube oils, and residue are also almost similar to that of diesel hydrotreating.
4 Physicochemical Properties
4.1 Chemical Composition of Catalysts
Hydrotreating catalysts comprise mainly two components, namely, active metals
and support. The metal components that promote hydrogenolysis and hydrogenation functions are usually supported on suitable inert material such as alumina.
These are basically supported metallic catalysts using either single metal or combination of two or more metals, but common catalysts contain two metals with one
metal as the active component and another as a promoter. Most of the hydrotreating
catalysts are made of sulfides of Group VI A metals (Mo, W) promoted by sulfides
of Ni or Co and supported on high surface area carriers such as alumina, silicaalumina, and zeolites. γ-Alumina is the most commonly used support material in
Fig. 2 Typical flow diagram of diesel hydrotreater unit
G. Valavarasu and B. Ramachandrarao