440
The Chemistry and Technology of Petroleum
at favored positions, although generally all the carbon–carbon bond positions are attacked to
some extent. For example, methyl-cyclopentane is converted (over a platinum–carbon catalyst) to
2-methylpentane, 3-methylpentane, and n-hexane.
Aromatic hydrocarbons are resistant to hydrogenation under mild conditions, but under more
severe conditions, the main reactions are conversion of the aromatic to naphthenic rings and scissions within the alkyl side chains. The naphthenes may also be converted to paraffins. However,
polynuclear aromatics are more readily attacked than the single-ring compounds, the reaction proceeding by a stepwise process in which one ring at a time is saturated and then opened. For example,
naphthalene is hydrocracked over a molybdenum oxide-molecular catalyst to produce a variety of
low-weight paraffins (≤C 6 ).
16.3.2 HydrotreAtIng
It is generally recognized that the higher the hydrogen content of a petroleum product, especially
the fuel products, the better is the quality of the product. This knowledge has stimulated the use of
hydrogen-adding processes in the refinery.
Thus, hydrotreating (i.e., hydrogenation without simultaneous cracking) (Chapter 21) is used
for saturating olefins or for converting aromatics to naphthenes as well as for heteroatom removal.
Under atmospheric pressure, olefins can be hydrogenated up to about 500°C (930°F), but beyond
this temperature dehydrogenation commences. Application of pressure and the presence of catalysts
make it possible to effect complete hydrogenation at room or even cooler temperature; the same
influences are helpful in minimizing dehydrogenation at higher temperatures.
A wide variety of metals are active hydrogenation catalysts; those of most interest are nickel, palladium, platinum, cobalt, iron, nickel-promoted copper, and copper chromite. Special preparations
of the first three are active at room temperature and atmospheric pressure. The metallic catalysts are
easily poisoned by sulfur-containing and arsenic-containing compounds, and even by other metals.
To avoid such poisoning, less effective but more resistant metal oxides or sulfides are frequently
employed, generally those of tungsten, cobalt, chromium, or molybdenum.
Alternatively, catalysts poisoning can be minimized by mild hydrogenation to remove nitrogen,
oxygen, and sulfur from feedstocks in the presence of more resistant catalysts, such as cobalt–
molybdenum–alumina (Co–Mo–Al 2 O 3 ). The reactions involved in nitrogen removal are somewhat
analogous to those of the sulfur compounds and follow a stepwise mechanism to produce ammonia
and the relevant substituted aromatic compound.
16.4 ISOMERIZATION
The importance of isomerization in petroleum-refining operations is twofold. First, the process is
valuable in converting n-butane into isobutane, which can be alkylated to liquid hydrocarbons in the
gasoline boiling range. Second, the process can be used to increase the octane number of the paraffins
boiling in the gasoline boiling range by converting some of the n-paraffins present into isoparaffins.
The process involves contact of the hydrocarbon feedstock and a catalyst under conditions favorable to good product recovery (Chapter 24). The catalyst may be aluminum chloride promoted with
hydrochloric acid or a platinum-containing catalyst. Both are very reactive and can lead to undesirable side reactions along with isomerization. These side reactions include disproportionation and
cracking, which decrease the yield and produce olefinic fragments that may combine with the catalyst and shorten its life. These undesired reactions are controlled by such techniques as the addition
of inhibitors to the hydrocarbon feed or by carrying out the reaction in the presence of hydrogen.
Paraffins are readily isomerized at room temperature, and the reaction is believed to occur by
means of the formation and rearrangement of carbonium ions. The chain-initiating ion R + is formed
by the addition of a proton from the acid catalyst to an olefin molecule, which may be added, present
as an impurity, or formed by dehydrogenation of the paraffin.
The Chemistry and Technology of Petroleum
at favored positions, although generally all the carbon–carbon bond positions are attacked to
some extent. For example, methyl-cyclopentane is converted (over a platinum–carbon catalyst) to
2-methylpentane, 3-methylpentane, and n-hexane.
Aromatic hydrocarbons are resistant to hydrogenation under mild conditions, but under more
severe conditions, the main reactions are conversion of the aromatic to naphthenic rings and scissions within the alkyl side chains. The naphthenes may also be converted to paraffins. However,
polynuclear aromatics are more readily attacked than the single-ring compounds, the reaction proceeding by a stepwise process in which one ring at a time is saturated and then opened. For example,
naphthalene is hydrocracked over a molybdenum oxide-molecular catalyst to produce a variety of
low-weight paraffins (≤C 6 ).
16.3.2 HydrotreAtIng
It is generally recognized that the higher the hydrogen content of a petroleum product, especially
the fuel products, the better is the quality of the product. This knowledge has stimulated the use of
hydrogen-adding processes in the refinery.
Thus, hydrotreating (i.e., hydrogenation without simultaneous cracking) (Chapter 21) is used
for saturating olefins or for converting aromatics to naphthenes as well as for heteroatom removal.
Under atmospheric pressure, olefins can be hydrogenated up to about 500°C (930°F), but beyond
this temperature dehydrogenation commences. Application of pressure and the presence of catalysts
make it possible to effect complete hydrogenation at room or even cooler temperature; the same
influences are helpful in minimizing dehydrogenation at higher temperatures.
A wide variety of metals are active hydrogenation catalysts; those of most interest are nickel, palladium, platinum, cobalt, iron, nickel-promoted copper, and copper chromite. Special preparations
of the first three are active at room temperature and atmospheric pressure. The metallic catalysts are
easily poisoned by sulfur-containing and arsenic-containing compounds, and even by other metals.
To avoid such poisoning, less effective but more resistant metal oxides or sulfides are frequently
employed, generally those of tungsten, cobalt, chromium, or molybdenum.
Alternatively, catalysts poisoning can be minimized by mild hydrogenation to remove nitrogen,
oxygen, and sulfur from feedstocks in the presence of more resistant catalysts, such as cobalt–
molybdenum–alumina (Co–Mo–Al 2 O 3 ). The reactions involved in nitrogen removal are somewhat
analogous to those of the sulfur compounds and follow a stepwise mechanism to produce ammonia
and the relevant substituted aromatic compound.
16.4 ISOMERIZATION
The importance of isomerization in petroleum-refining operations is twofold. First, the process is
valuable in converting n-butane into isobutane, which can be alkylated to liquid hydrocarbons in the
gasoline boiling range. Second, the process can be used to increase the octane number of the paraffins
boiling in the gasoline boiling range by converting some of the n-paraffins present into isoparaffins.
The process involves contact of the hydrocarbon feedstock and a catalyst under conditions favorable to good product recovery (Chapter 24). The catalyst may be aluminum chloride promoted with
hydrochloric acid or a platinum-containing catalyst. Both are very reactive and can lead to undesirable side reactions along with isomerization. These side reactions include disproportionation and
cracking, which decrease the yield and produce olefinic fragments that may combine with the catalyst and shorten its life. These undesired reactions are controlled by such techniques as the addition
of inhibitors to the hydrocarbon feed or by carrying out the reaction in the presence of hydrogen.
Paraffins are readily isomerized at room temperature, and the reaction is believed to occur by
means of the formation and rearrangement of carbonium ions. The chain-initiating ion R + is formed
by the addition of a proton from the acid catalyst to an olefin molecule, which may be added, present
as an impurity, or formed by dehydrogenation of the paraffin.