633
Hydrogen Production
cracking, the molecular mass of the main fraction of the feed is lowered, while another part is
converted to coke that is deposited on the hot catalyst. The catalyst is regenerated in one or two
stages by burning the coke off with air that also provides the energy for the endothermic cracking
process. In the process, paraffins and naphthenes are cracked to olefins and to alkanes with shorter
chain length, mono-aromatic compounds are dealkylated without ring cleavage, and di-aromatics
and poly-aromatics are dealkylated and converted to coke. Hydrogen is formed in the last type of
reaction, whereas the first two reactions produce light hydrocarbons and therefore require hydrogen.
Thus, a catalytic cracker can be operated in such a manner that enough hydrogen for subsequent
processes is formed.
In reforming processes, naphtha fractions are reformed to improve the quality of gasoline (Speight,
2000; Speight and Ozum, 2002). The most important reactions occurring during this process are
the dehydrogenation of naphthenes to aromatics. This reaction is endothermic and is favored by
low pressures and the reaction temperature lies in the range of 300°C–450°C (570°F–840°F). The
reaction is performed on platinum catalysts, with other metals, for example, rhenium, as promoters.
Hydrogen is generated in a refinery by the catalytic-reforming process, but there may not always
be the need to have a catalytic reformer as part of the refinery sequence. Nevertheless, assuming that
a catalytic reformer is part of the refinery sequence, the hydrogen production from the reformer usually falls well below the amount required for hydroprocessing purposes. For example, in a 100,000
bbl/day hydrocracking refinery, assuming intensive reforming of hydrocracked gasoline, the hydrogen requirements of the refinery may still fall some 500–900 scf/bbl of crude charge below that necessary for the hydrocracking sequences. Consequently, an external source of hydrogen is necessary
to meet the daily hydrogen requirements of any process where the heavier feedstocks are involved.
The trend to increase the number of hydrogenation (hydrocracking and/or hydrotreating) processes
in refineries (Dolbear, 1998) coupled with the need to process the heavier oils, which require substantial quantities of hydrogen for upgrading, has resulted in vastly increased demands for this gas.
Hydrogen has historically been produced during catalytic-reforming processes as a by-product
of the production of the aromatic compounds used in gasoline and in solvents. As reforming processes changed from fixed-bed to cyclic to continuous regeneration, process pressures have dropped
and hydrogen production per barrel of reformate has tended to increase. However, hydrogen production as a by-product is not always adequate to the needs of the refinery and other processes are
necessary. Thus, hydrogen production by steam reforming or by partial oxidation of residua has also
been used, particularly where heavy oil is available. Steam reforming is the dominant method for
hydrogen production and is usually combined with pressure swing adsorption (PSA) to purify the
hydrogen to greater than 99% by volume (Bandermann and Harder, 1982).
The gasification of residua and coke to produce hydrogen and/or power may become an attractive option for refiners (Campbell, 1997; Dickenson et al., 1997; Fleshman, 1997; Gross and Wolff,
2000). The premise that the gasification section of a refinery will be the garbage can for deasphalter
residues, high-sulfur coke, as well as other refinery wastes is worthy of consideration.
Several other processes are available for the production of the additional hydrogen that is necessary for the various heavy feedstock hydroprocessing sequences and it is the purpose of this chapter
to present a general description of these processes. In general, most of the external hydrogen is manufactured by steam–methane reforming or by oxidation processes. Other processes such as ammonia
dissociation, steam–methanol interaction, or electrolysis are also available for hydrogen production,
but economic factors and feedstock availability assist in the choice between processing alternatives.
The processes described in this chapter are those gasification processes that are often referred to
the garbage disposal units of the refinery. Hydrogen is produced for use in other parts of the refinery as well as for energy and it is often produced from process by-products that may not be of any
use elsewhere. Such by-products might be the highly aromatic, heteroatom, and metal containing
reject from a deasphalting unit or from a mild hydrocracking process. However attractive this may
seem, there will be the need to incorporate a gas cleaning operation to remove any environmentally
objectionable components from the hydrogen gas.
Hydrogen Production
cracking, the molecular mass of the main fraction of the feed is lowered, while another part is
converted to coke that is deposited on the hot catalyst. The catalyst is regenerated in one or two
stages by burning the coke off with air that also provides the energy for the endothermic cracking
process. In the process, paraffins and naphthenes are cracked to olefins and to alkanes with shorter
chain length, mono-aromatic compounds are dealkylated without ring cleavage, and di-aromatics
and poly-aromatics are dealkylated and converted to coke. Hydrogen is formed in the last type of
reaction, whereas the first two reactions produce light hydrocarbons and therefore require hydrogen.
Thus, a catalytic cracker can be operated in such a manner that enough hydrogen for subsequent
processes is formed.
In reforming processes, naphtha fractions are reformed to improve the quality of gasoline (Speight,
2000; Speight and Ozum, 2002). The most important reactions occurring during this process are
the dehydrogenation of naphthenes to aromatics. This reaction is endothermic and is favored by
low pressures and the reaction temperature lies in the range of 300°C–450°C (570°F–840°F). The
reaction is performed on platinum catalysts, with other metals, for example, rhenium, as promoters.
Hydrogen is generated in a refinery by the catalytic-reforming process, but there may not always
be the need to have a catalytic reformer as part of the refinery sequence. Nevertheless, assuming that
a catalytic reformer is part of the refinery sequence, the hydrogen production from the reformer usually falls well below the amount required for hydroprocessing purposes. For example, in a 100,000
bbl/day hydrocracking refinery, assuming intensive reforming of hydrocracked gasoline, the hydrogen requirements of the refinery may still fall some 500–900 scf/bbl of crude charge below that necessary for the hydrocracking sequences. Consequently, an external source of hydrogen is necessary
to meet the daily hydrogen requirements of any process where the heavier feedstocks are involved.
The trend to increase the number of hydrogenation (hydrocracking and/or hydrotreating) processes
in refineries (Dolbear, 1998) coupled with the need to process the heavier oils, which require substantial quantities of hydrogen for upgrading, has resulted in vastly increased demands for this gas.
Hydrogen has historically been produced during catalytic-reforming processes as a by-product
of the production of the aromatic compounds used in gasoline and in solvents. As reforming processes changed from fixed-bed to cyclic to continuous regeneration, process pressures have dropped
and hydrogen production per barrel of reformate has tended to increase. However, hydrogen production as a by-product is not always adequate to the needs of the refinery and other processes are
necessary. Thus, hydrogen production by steam reforming or by partial oxidation of residua has also
been used, particularly where heavy oil is available. Steam reforming is the dominant method for
hydrogen production and is usually combined with pressure swing adsorption (PSA) to purify the
hydrogen to greater than 99% by volume (Bandermann and Harder, 1982).
The gasification of residua and coke to produce hydrogen and/or power may become an attractive option for refiners (Campbell, 1997; Dickenson et al., 1997; Fleshman, 1997; Gross and Wolff,
2000). The premise that the gasification section of a refinery will be the garbage can for deasphalter
residues, high-sulfur coke, as well as other refinery wastes is worthy of consideration.
Several other processes are available for the production of the additional hydrogen that is necessary for the various heavy feedstock hydroprocessing sequences and it is the purpose of this chapter
to present a general description of these processes. In general, most of the external hydrogen is manufactured by steam–methane reforming or by oxidation processes. Other processes such as ammonia
dissociation, steam–methanol interaction, or electrolysis are also available for hydrogen production,
but economic factors and feedstock availability assist in the choice between processing alternatives.
The processes described in this chapter are those gasification processes that are often referred to
the garbage disposal units of the refinery. Hydrogen is produced for use in other parts of the refinery as well as for energy and it is often produced from process by-products that may not be of any
use elsewhere. Such by-products might be the highly aromatic, heteroatom, and metal containing
reject from a deasphalting unit or from a mild hydrocracking process. However attractive this may
seem, there will be the need to incorporate a gas cleaning operation to remove any environmentally
objectionable components from the hydrogen gas.