586
The Chemistry and Technology of Petroleum
distillate fractions, coal-derived liquid shale, bitumen, and synthetic fuels. However, further characterization on the biochemical mechanisms and bioprocessing issues involved in petroleum upgrading are required in order to develop reliable biological processes.
For upgrading options, the use of microbes has to show a competitive advantage of enzyme over
the tried-and-true chemical methods prevalent in the industry. Currently, the range of reactions
using microbes is large but is usually related to production of bioactive compounds or precursors.
But the door is not closed and the issues of biodesulfurization and bioupgrading remain open for the
challenge of bulk petroleum processing. These drawbacks limit the applicability of this technology
to specialty chemicals and steer it away from bulk petroleum processing.
Biodesulfurization is, therefore, another technology to remove sulfur from the feedstock.
However, several factors may limit the application of this technology, however. Many ancillary processes novel to petroleum refining would be needed, including a biocatalyst fermentor to regenerate
the bacteria. The process is also sensitive to environmental conditions such as sterilization, temperature, and residence time of the biocatalyst. Finally, the process requires the existing hydrotreater
to continue in operation to provide a lower sulfur feedstock to the unit and is more costly than conventional hydrotreating. Nevertheless, the limiting factors should not stop the investigations of the
concept and work should be continued with success in mind.
Once the concept has been proven on the scale that a refiner would be required, the successful microbial technology will most probably involve a genetically modified bacterial strain for
(1) upgrading distillates and other petroleum fractions in refineries, (2) upgrading crude petroleum
upstream, and (3) dealing with environmental problems that ace industry, especially in areas related
to spillage of crude oil and products. These developments are part of a wider trend to use bioprocessing to make products and do many of the tasks that are accomplished currently by conventional
chemical processing. If commercialized for refineries, however, biologically based approaches will
be at scales and with economic impacts beyond anything previously seen in industry.
In addition, the successful biodesulfurization process will, most likely, be based on naturally
occurring aerobic bacteria that can remove organically bound sulfur in heterocyclic compounds
without degrading the fuel value of the hydrocarbon matrix. Because of the susceptibility of bacteria to heat, the process will need to operate at temperatures and pressures close to ambient and also
use air to promote sulfur removal from the feedstock.
21.7 GASOLINE AND DIESEL FUEL POLISHING
Briefly, the two desulfurization processes used for fuels purification (desulfurization) are (1) sweetening (Chapter 24) and (2) hydrotreating (Chapter 21). Sweetening is effective only against mercaptans, which are the predominant species in light gasoline. Hydrotreating is effective against all
sulfur species and is more widely used.
In the sweetening process, a light naphtha stream is washed with amine to remove hydrogen
sulfide and then reacted with caustic, which promotes the conversion of mercaptans to disulfides.
R SH RSSR
-
Æ
The disulfides can subsequently be extracted and removed in what is referred to as extractive sweetening.
In the hydrotreating process, the feed is reacted with hydrogen, in the presence of a solid catalyst. The hydrogen removes sulfur by conversion to hydrogen sulfide, which is subsequently separated and removed from the reacted stream. As the reaction is favored by both temperature and
pressure, hydrotreaters are typically designed and operated at approximately 370°C (700°F) and
1000–2000 psi hydrogen. The lower ends of the ranges typically apply to gasoline desulfurization,
while gas oil desulfurization requires a more severe operation.
Hydrogen is provided in the form of treating gas at a purity that is typically around the order
of 90% by volume although gas with as little as 60% by volume hydrogen is reputed to be used.
The Chemistry and Technology of Petroleum
distillate fractions, coal-derived liquid shale, bitumen, and synthetic fuels. However, further characterization on the biochemical mechanisms and bioprocessing issues involved in petroleum upgrading are required in order to develop reliable biological processes.
For upgrading options, the use of microbes has to show a competitive advantage of enzyme over
the tried-and-true chemical methods prevalent in the industry. Currently, the range of reactions
using microbes is large but is usually related to production of bioactive compounds or precursors.
But the door is not closed and the issues of biodesulfurization and bioupgrading remain open for the
challenge of bulk petroleum processing. These drawbacks limit the applicability of this technology
to specialty chemicals and steer it away from bulk petroleum processing.
Biodesulfurization is, therefore, another technology to remove sulfur from the feedstock.
However, several factors may limit the application of this technology, however. Many ancillary processes novel to petroleum refining would be needed, including a biocatalyst fermentor to regenerate
the bacteria. The process is also sensitive to environmental conditions such as sterilization, temperature, and residence time of the biocatalyst. Finally, the process requires the existing hydrotreater
to continue in operation to provide a lower sulfur feedstock to the unit and is more costly than conventional hydrotreating. Nevertheless, the limiting factors should not stop the investigations of the
concept and work should be continued with success in mind.
Once the concept has been proven on the scale that a refiner would be required, the successful microbial technology will most probably involve a genetically modified bacterial strain for
(1) upgrading distillates and other petroleum fractions in refineries, (2) upgrading crude petroleum
upstream, and (3) dealing with environmental problems that ace industry, especially in areas related
to spillage of crude oil and products. These developments are part of a wider trend to use bioprocessing to make products and do many of the tasks that are accomplished currently by conventional
chemical processing. If commercialized for refineries, however, biologically based approaches will
be at scales and with economic impacts beyond anything previously seen in industry.
In addition, the successful biodesulfurization process will, most likely, be based on naturally
occurring aerobic bacteria that can remove organically bound sulfur in heterocyclic compounds
without degrading the fuel value of the hydrocarbon matrix. Because of the susceptibility of bacteria to heat, the process will need to operate at temperatures and pressures close to ambient and also
use air to promote sulfur removal from the feedstock.
21.7 GASOLINE AND DIESEL FUEL POLISHING
Briefly, the two desulfurization processes used for fuels purification (desulfurization) are (1) sweetening (Chapter 24) and (2) hydrotreating (Chapter 21). Sweetening is effective only against mercaptans, which are the predominant species in light gasoline. Hydrotreating is effective against all
sulfur species and is more widely used.
In the sweetening process, a light naphtha stream is washed with amine to remove hydrogen
sulfide and then reacted with caustic, which promotes the conversion of mercaptans to disulfides.
R SH RSSR
-
Æ
The disulfides can subsequently be extracted and removed in what is referred to as extractive sweetening.
In the hydrotreating process, the feed is reacted with hydrogen, in the presence of a solid catalyst. The hydrogen removes sulfur by conversion to hydrogen sulfide, which is subsequently separated and removed from the reacted stream. As the reaction is favored by both temperature and
pressure, hydrotreaters are typically designed and operated at approximately 370°C (700°F) and
1000–2000 psi hydrogen. The lower ends of the ranges typically apply to gasoline desulfurization,
while gas oil desulfurization requires a more severe operation.
Hydrogen is provided in the form of treating gas at a purity that is typically around the order
of 90% by volume although gas with as little as 60% by volume hydrogen is reputed to be used.