439
Refining Chemistry
16.2.4 deHydroCyClIzAtIon
Catalytic aromatization involving the loss of 1 mol of hydrogen followed by ring formation, and
further loss of hydrogen has been demonstrated for a variety of paraffins (typically n-hexane and
n-heptane). Thus, n-hexane can be converted to benzene, heptane is converted to toluene, and octane
is converted to ethyl benzene and o-xylene. Conversion takes place at low pressures, even atmospheric, and at temperatures above 300°C (570°F), although 450°C–550°C (840°F–1020°F) is the
preferred temperature range.
The catalysts are metals (or their oxides) of the titanium, vanadium, and tungsten groups and are
generally supported on alumina; the mechanism is believed to be dehydrogenation of the paraffin
to an olefin, which in turn is cyclized and dehydrogenated to the aromatic hydrocarbon. In support
of this, olefins can be converted to aromatics much more easily than the corresponding paraffins.
16.3 HYDROGENATION
The purpose of hydrogenating petroleum constituents is (1) to improve existing petroleum products
or develop new products or even new uses, (2) to convert inferior or low-grade materials into valuable products, and (3) to transform higher molecular weight constituents into liquid fuels.
The distinguishing feature of the hydrogenating processes is that, although the composition of the
feedstock is relatively unknown and a variety of reactions may occur simultaneously, the final product
may actually meet all the required specifications for its particular use (Furimsky, 1983; Speight, 2000).
Hydrogenation processes (Chapters 21 and 22) for the conversion of petroleum and petroleum
products may be classified as destructive and nondestructive. The former (hydrogenolysis or hydrocracking) is characterized by the rupture of carbon–carbon bonds and is accompanied by hydrogen
saturation of the fragments to produce lower boiling products. Such treatment requires rather high
temperatures and high hydrogen pressures, the latter to minimize coke formation. Many other reactions, such as isomerization, dehydrogenation, and cyclization, can occur under these conditions
(Dolbear et al., 1987).
On the other hand, nondestructive, or simple, hydrogenation is generally used for the purpose of
improving product (or even feedstock) quality without appreciable alteration of the boiling range.
Treatment under such mild conditions is often referred to as hydrotreating or hydrofining and is
essentially a means of eliminating nitrogen, oxygen, and sulfur as ammonia, water, and hydrogen
sulfide, respectively.
16.3.1 HydroCrACkIng
Hydrocracking (Chapter 22) is a thermal process (>350°C, >660°F) in which hydrogenation accompanies cracking. Relatively high pressure (100–2000 psi) is employed, and the overall result is usually a change in the character or quality of the products.
The wide range of products possible from hydrocracking is the result of combining catalytic
cracking reactions with hydrogenation (Dolbear, 1998; Hajji et al., 2010). The reactions are catalyzed by dual-function catalysts in which the cracking function is provided by silica–alumina
(or zeolite) catalysts, and platinum, tungsten oxide, or nickel provides the hydrogenation function.
Essentially, all the initial reactions of catalytic cracking occur, but some of the secondary reactions are inhibited or stopped by the presence of hydrogen. For example, the yields of olefins and
the secondary reactions that result from the presence of these materials are substantially diminished
and branched-chain paraffins undergo demethanation. The methyl groups attached to secondary
carbons are more easily removed than those attached to tertiary carbon atoms, whereas methyl
groups attached to quaternary carbons are the most resistant to hydrocracking.
The effect of hydrogen on naphthenic hydrocarbons is mainly that of ring scission followed
by immediate saturation of each end of the fragment produced. The ring is preferentially broken
Refining Chemistry
16.2.4 deHydroCyClIzAtIon
Catalytic aromatization involving the loss of 1 mol of hydrogen followed by ring formation, and
further loss of hydrogen has been demonstrated for a variety of paraffins (typically n-hexane and
n-heptane). Thus, n-hexane can be converted to benzene, heptane is converted to toluene, and octane
is converted to ethyl benzene and o-xylene. Conversion takes place at low pressures, even atmospheric, and at temperatures above 300°C (570°F), although 450°C–550°C (840°F–1020°F) is the
preferred temperature range.
The catalysts are metals (or their oxides) of the titanium, vanadium, and tungsten groups and are
generally supported on alumina; the mechanism is believed to be dehydrogenation of the paraffin
to an olefin, which in turn is cyclized and dehydrogenated to the aromatic hydrocarbon. In support
of this, olefins can be converted to aromatics much more easily than the corresponding paraffins.
16.3 HYDROGENATION
The purpose of hydrogenating petroleum constituents is (1) to improve existing petroleum products
or develop new products or even new uses, (2) to convert inferior or low-grade materials into valuable products, and (3) to transform higher molecular weight constituents into liquid fuels.
The distinguishing feature of the hydrogenating processes is that, although the composition of the
feedstock is relatively unknown and a variety of reactions may occur simultaneously, the final product
may actually meet all the required specifications for its particular use (Furimsky, 1983; Speight, 2000).
Hydrogenation processes (Chapters 21 and 22) for the conversion of petroleum and petroleum
products may be classified as destructive and nondestructive. The former (hydrogenolysis or hydrocracking) is characterized by the rupture of carbon–carbon bonds and is accompanied by hydrogen
saturation of the fragments to produce lower boiling products. Such treatment requires rather high
temperatures and high hydrogen pressures, the latter to minimize coke formation. Many other reactions, such as isomerization, dehydrogenation, and cyclization, can occur under these conditions
(Dolbear et al., 1987).
On the other hand, nondestructive, or simple, hydrogenation is generally used for the purpose of
improving product (or even feedstock) quality without appreciable alteration of the boiling range.
Treatment under such mild conditions is often referred to as hydrotreating or hydrofining and is
essentially a means of eliminating nitrogen, oxygen, and sulfur as ammonia, water, and hydrogen
sulfide, respectively.
16.3.1 HydroCrACkIng
Hydrocracking (Chapter 22) is a thermal process (>350°C, >660°F) in which hydrogenation accompanies cracking. Relatively high pressure (100–2000 psi) is employed, and the overall result is usually a change in the character or quality of the products.
The wide range of products possible from hydrocracking is the result of combining catalytic
cracking reactions with hydrogenation (Dolbear, 1998; Hajji et al., 2010). The reactions are catalyzed by dual-function catalysts in which the cracking function is provided by silica–alumina
(or zeolite) catalysts, and platinum, tungsten oxide, or nickel provides the hydrogenation function.
Essentially, all the initial reactions of catalytic cracking occur, but some of the secondary reactions are inhibited or stopped by the presence of hydrogen. For example, the yields of olefins and
the secondary reactions that result from the presence of these materials are substantially diminished
and branched-chain paraffins undergo demethanation. The methyl groups attached to secondary
carbons are more easily removed than those attached to tertiary carbon atoms, whereas methyl
groups attached to quaternary carbons are the most resistant to hydrocracking.
The effect of hydrogen on naphthenic hydrocarbons is mainly that of ring scission followed
by immediate saturation of each end of the fragment produced. The ring is preferentially broken