Meeting sulfur content. Sulfur content of 0.05 % wt or less is now on its way out
as a common requirement for diesel fuel around the world. The current common
standard is between 10 and 20 wppm in many areas. In most cases, this level of
sulfur can only be met by extensive hydrotreating or hydrocracking to remove the
sulfur from the middle distillates that will be used to blend the diesel product.
Some refineries select their crude slate with low-sulfur crudes to reduce the
hydrotreating demand. The impact on refiners to meet this parameter extends
beyond the severity of hydrotreating that may exist. It impacts the availability of
hydrogen in the front end and the capacity for handling the sour gases (H 2 S in
particular) in the back end of the process. It is likely that the legislative sulfur
content in diesel may be reduced still further. Ultralow sulfur diesel (ULSD) is now
the norm in many areas of the world. This makes the selection between crude slate
demand for low-sulfur crude and a modified refinery configuration and operation a
critical one.
Reducing aromatic content. With the increasing demand for diesel fuel, particularly in North America and Europe, refiners will need to use streams normally
routed to heating oils as components for the diesel product blend. These streams are
of lower quality and, in particular, contain higher quantity of aromatics than will be
acceptable as diesel blend stocks without severe treating to improve their blending
characteristics. High-severity hydrotreating to remove sulfur and nitrogen helps in
reducing aromatics in acceptable diesel blend stocks, it will not be sufficient to
improve the quality of the heating oil and fuel oil blend stocks to meet the diesel
pool requirement. Hydrotreating using nickel molybdenum catalyst
(dearomatization process) is used for the purpose of upgrading these poorer blend
stocks. This dearomatization process usually consists of two stages. The first is
processing over a conventional Co–Mo hydrotreating catalyst to remove sulfur and
then to process over the Ni–Mo catalyst. The nickel catalyst is poisoned by sulfur,
thus the two stages. The economics of using the dearomatization process must be
carefully evaluated because the metal content of the nickel catalyst represents a
significant investment. The process however operates at a higher space velocity and
lower pressure than the conventional Co–Mo hydrotreating process.
Improvement of cold flow properties. Problems with meeting the cold flow
properties of diesel fuel are associated with the presence of straight chain paraffins.
Although these have higher cetane numbers, they pose cold flow limitations because
they tend to precipitate as a wax at low temperatures. The simplest and probably the
best solution to this cold flow property of these paraffins is the use of additives.
These additives are however costly, ranging from below $50 per thousand bbls for
pour point reduction to as high as $250 per thousand bbls for cloud point reduction
(these are year 2000 dollars). Another alternative is to reduce the diesel FBP and add
kerosene. This action removes the heavier paraffinic molecules, which tend to
precipitate at higher temperature. Reducing the cut point from 700
F to 640
F
reduces the yield of diesel by, perhaps, 6 % vol on crude. Solvent dewaxing is
another option. In this process, the wax-forming components are selectively
removed from the diesel product. This is only considered however for stocks from
Petroleum Products and a Refinery Configuration
81
as a common requirement for diesel fuel around the world. The current common
standard is between 10 and 20 wppm in many areas. In most cases, this level of
sulfur can only be met by extensive hydrotreating or hydrocracking to remove the
sulfur from the middle distillates that will be used to blend the diesel product.
Some refineries select their crude slate with low-sulfur crudes to reduce the
hydrotreating demand. The impact on refiners to meet this parameter extends
beyond the severity of hydrotreating that may exist. It impacts the availability of
hydrogen in the front end and the capacity for handling the sour gases (H 2 S in
particular) in the back end of the process. It is likely that the legislative sulfur
content in diesel may be reduced still further. Ultralow sulfur diesel (ULSD) is now
the norm in many areas of the world. This makes the selection between crude slate
demand for low-sulfur crude and a modified refinery configuration and operation a
critical one.
Reducing aromatic content. With the increasing demand for diesel fuel, particularly in North America and Europe, refiners will need to use streams normally
routed to heating oils as components for the diesel product blend. These streams are
of lower quality and, in particular, contain higher quantity of aromatics than will be
acceptable as diesel blend stocks without severe treating to improve their blending
characteristics. High-severity hydrotreating to remove sulfur and nitrogen helps in
reducing aromatics in acceptable diesel blend stocks, it will not be sufficient to
improve the quality of the heating oil and fuel oil blend stocks to meet the diesel
pool requirement. Hydrotreating using nickel molybdenum catalyst
(dearomatization process) is used for the purpose of upgrading these poorer blend
stocks. This dearomatization process usually consists of two stages. The first is
processing over a conventional Co–Mo hydrotreating catalyst to remove sulfur and
then to process over the Ni–Mo catalyst. The nickel catalyst is poisoned by sulfur,
thus the two stages. The economics of using the dearomatization process must be
carefully evaluated because the metal content of the nickel catalyst represents a
significant investment. The process however operates at a higher space velocity and
lower pressure than the conventional Co–Mo hydrotreating process.
Improvement of cold flow properties. Problems with meeting the cold flow
properties of diesel fuel are associated with the presence of straight chain paraffins.
Although these have higher cetane numbers, they pose cold flow limitations because
they tend to precipitate as a wax at low temperatures. The simplest and probably the
best solution to this cold flow property of these paraffins is the use of additives.
These additives are however costly, ranging from below $50 per thousand bbls for
pour point reduction to as high as $250 per thousand bbls for cloud point reduction
(these are year 2000 dollars). Another alternative is to reduce the diesel FBP and add
kerosene. This action removes the heavier paraffinic molecules, which tend to
precipitate at higher temperature. Reducing the cut point from 700
F to 640
F
reduces the yield of diesel by, perhaps, 6 % vol on crude. Solvent dewaxing is
another option. In this process, the wax-forming components are selectively
removed from the diesel product. This is only considered however for stocks from
Petroleum Products and a Refinery Configuration
81