561
21
Hydrotreating and
Desulfurization
21.1 INTRODUCTION
The trend of processing more heavy and sour crudes, the shift in demand away from gasoline
toward more distillate, and more stringent fuel qualities change the fuel and hydrogen balances in
most refineries (Speight and Ozum, 2002; Ancheyta et al., 2005; Hsu and Robinson, 2006; Gary
et al., 2007; Liu et al., 2009; Speight, 2011a). Finding the right solution to managing fuel gas and
hydrogen requirements has become essential for refineries to remain profitable. The refinery gas
value is higher for hydrogen generation than for generating power and slightly lower than the value
for using it as chemical feedstock.
In addition, the hydrogen requirement for product improvement, which is the hydrotreatment of
petroleum products, to insure that they meet utility and performance specifications is also increasing. Product improvement can not only involve hydrotreatment but also changes in molecular
shape (reforming and isomerization) or change in molecular size (alkylation and polymerization)
(Chapter 24) and hydrotreating can play a major role in product improvement.
The use of hydrogen in thermal processes was perhaps the single most significant advance in refining technology during the twentieth century (Dolbear, 1998). In fact, hydrogenation processes are the
principal processes used in the manufacture of naphtha and gasoline (Figure 21.1) (Brunet et al.,
2005). Indeed, with the influx of heavier feedstocks into refineries, hydroprocessing will assume a
greater role in the refinery of the future. The process uses the principle that the presence of hydrogen
during a thermal reaction of a petroleum feedstock terminates many of the coke-forming reactions
and enhances the yields of the lower boiling components, such as gasoline, kerosene, and jet fuel.
Hydrogenation processes for the conversion of petroleum fractions and petroleum products
may be classified as destructive and nondestructive. Destructive hydrogenation (hydrogenolysis or
hydrocracking) is characterized by the cleavage of carbon–carbon linkages accompanied by hydrogen saturation of the fragments to produce lower boiling products. Such treatment requires severe
processing conditions and the use of high hydrogen pressures to minimize polymerization and condensation that lead to coke formation. Many other reactions, such as isomerization, dehydrogenation, and cyclization, occur under the drastic conditions employed.
Furthermore, a growing dependence on high-heteroatom heavy oils and residua has emerged
as a result of continuing decreasing availability of conventional crude oil through the depletion of
reserves in the various parts of the world. Thus, the ever growing tendency to convert as much as
possible of lower grade feedstocks to liquid products is causing an increase in the total sulfur content in refined products. Refiners must, therefore, continue to remove substantial portions of sulfur
from the lighter products, but residua and heavy crude oil poses a particularly difficult problem.
Indeed, it is now clear that there are other problems involved in the processing of the heavier feedstocks and that these heavier feedstocks, which are gradually emerging as the liquid fuel supply of
the future, need special attention.
There are several valid reasons for removing heteroatoms from petroleum fractions. These include
(1) reduction, or elimination, of corrosion during refining, handling, or use of the various products,
(2) production of products having an acceptable odor and specification, (3) increasing the performance
and stability of gasoline, (4) decreasing smoke formation in kerosene, and (5) reduction of heteroatom
content in fuel oil to a level that improves burning characteristics and is environmentally acceptable.
21
Hydrotreating and
Desulfurization
21.1 INTRODUCTION
The trend of processing more heavy and sour crudes, the shift in demand away from gasoline
toward more distillate, and more stringent fuel qualities change the fuel and hydrogen balances in
most refineries (Speight and Ozum, 2002; Ancheyta et al., 2005; Hsu and Robinson, 2006; Gary
et al., 2007; Liu et al., 2009; Speight, 2011a). Finding the right solution to managing fuel gas and
hydrogen requirements has become essential for refineries to remain profitable. The refinery gas
value is higher for hydrogen generation than for generating power and slightly lower than the value
for using it as chemical feedstock.
In addition, the hydrogen requirement for product improvement, which is the hydrotreatment of
petroleum products, to insure that they meet utility and performance specifications is also increasing. Product improvement can not only involve hydrotreatment but also changes in molecular
shape (reforming and isomerization) or change in molecular size (alkylation and polymerization)
(Chapter 24) and hydrotreating can play a major role in product improvement.
The use of hydrogen in thermal processes was perhaps the single most significant advance in refining technology during the twentieth century (Dolbear, 1998). In fact, hydrogenation processes are the
principal processes used in the manufacture of naphtha and gasoline (Figure 21.1) (Brunet et al.,
2005). Indeed, with the influx of heavier feedstocks into refineries, hydroprocessing will assume a
greater role in the refinery of the future. The process uses the principle that the presence of hydrogen
during a thermal reaction of a petroleum feedstock terminates many of the coke-forming reactions
and enhances the yields of the lower boiling components, such as gasoline, kerosene, and jet fuel.
Hydrogenation processes for the conversion of petroleum fractions and petroleum products
may be classified as destructive and nondestructive. Destructive hydrogenation (hydrogenolysis or
hydrocracking) is characterized by the cleavage of carbon–carbon linkages accompanied by hydrogen saturation of the fragments to produce lower boiling products. Such treatment requires severe
processing conditions and the use of high hydrogen pressures to minimize polymerization and condensation that lead to coke formation. Many other reactions, such as isomerization, dehydrogenation, and cyclization, occur under the drastic conditions employed.
Furthermore, a growing dependence on high-heteroatom heavy oils and residua has emerged
as a result of continuing decreasing availability of conventional crude oil through the depletion of
reserves in the various parts of the world. Thus, the ever growing tendency to convert as much as
possible of lower grade feedstocks to liquid products is causing an increase in the total sulfur content in refined products. Refiners must, therefore, continue to remove substantial portions of sulfur
from the lighter products, but residua and heavy crude oil poses a particularly difficult problem.
Indeed, it is now clear that there are other problems involved in the processing of the heavier feedstocks and that these heavier feedstocks, which are gradually emerging as the liquid fuel supply of
the future, need special attention.
There are several valid reasons for removing heteroatoms from petroleum fractions. These include
(1) reduction, or elimination, of corrosion during refining, handling, or use of the various products,
(2) production of products having an acceptable odor and specification, (3) increasing the performance
and stability of gasoline, (4) decreasing smoke formation in kerosene, and (5) reduction of heteroatom
content in fuel oil to a level that improves burning characteristics and is environmentally acceptable.