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The Chemistry and Technology of Petroleum
hydrogen sulfide and ammonia. Limited hydrocracking also occurs. After the hydrocarbon leaves
the first stage, it is cooled and liquefied and run through a hydrocarbon separator. The hydrogen is
recycled to the feedstock. The liquid is charged to a fractionator. Depending on the products desired
(gasoline components, jet fuel, and gas oil), the fractionator is run to cut out some portion of the
first stage reactor out-turn. Kerosene-range material can be taken as a separate side-draw product or
included in the fractionator bottoms with the gas oil. The fractionator bottoms are again mixed with
a hydrogen stream and charged to the second stage. Since this material has already been subjected
to some hydrogenation, cracking, and reforming in the first stage, the operations of the second stage
are more severe (higher temperatures and pressures). Like the outturn of the first stage, the secondstage product is separated from the hydrogen and charged to the fractionator.
23.3 FEEDSTOCKS
The most common, and perhaps the best, feedstocks for steam reforming are low-boiling saturated
hydrocarbons that have a low sulfur content; including natural gas, refinery gas, LPG, and lowboiling naphtha.
Natural gas is the most common feedstock for hydrogen production since it meets all the requirements for reformer feedstock. Natural gas typically contains more than 90% methane and ethane with only a few percent of propane and higher boiling hydrocarbons (Mokhatab et al., 2006).
Natural gas may (or most likely will) contain traces of carbon dioxide with some nitrogen and
other impurities. Purification of natural gas, before reforming, is usually relatively straightforward.
Traces of sulfur must be removed to avoid poisoning the reformer catalyst; zinc oxide treatment in
combination with hydrogenation is usually adequate.
Light refinery gas, containing a substantial amount of hydrogen, can be an attractive steam
reformer feedstock since it is produced as a by-product. Processing of refinery gas will depend on
its composition, particularly the levels of olefins and of propane and heavier hydrocarbons. Olefins,
which can cause problems by forming coke in the reformer, are converted to saturated compounds
in the hydrogenation unit. Higher boiling hydrocarbons in refinery gas can also form coke, either on
the primary reformer catalyst or in the preheater. If there is more than a few percent of C 3 and higher
compounds, a promoted reformer catalyst should be considered, in order to avoid carbon deposits.
Refinery gas from different sources varies in suitability as hydrogen plant feed. Catalytic
reformer off-gas (Chapter 24), for example, is saturated, very low in sulfur, and often has high
hydrogen content. The process gases from a coking unit (Chapter 18) or from a fluid catalytic cracking unit (Chapter 19) are much less desirable because of the content of unsaturated constituents. In
addition to olefins, these gases contain substantial amounts of sulfur that must be removed before
the gas is used as feedstock. These gases are also generally unsuitable for direct hydrogen recovery,
since the hydrogen content is usually too low. Hydrotreater off-gas lies in the middle of the range. It
is saturated, so it is readily used as hydrogen plant feed. Content of hydrogen and heavier hydrocarbons depends to a large extent on the upstream pressure. Sulfur removal will generally be required.
23.4 PROCESS CHEMISTRY
Before the feedstock is introduced to a process, there is the need for application of a strict feedstock
purification protocol. Prolonging catalyst life in hydrogen production processes is attributable to
effective feedstock purification, particularly sulfur removal. A typical natural gas or other light
hydrocarbon feedstock contains traces of hydrogen sulfide and organic sulfur.
In order to remove sulfur compounds, it is necessary to hydrogenate the feedstock to convert
the organic sulfur to hydrogen that is then reacted with zinc oxide (ZnO) at approximately 370°C
(700°F), resulting in the optimal use of the zinc oxide as well as ensuring complete hydrogenation.
Thus, assuming assiduous feedstock purification and removal of all of the objectionable contaminants, the chemistry of hydrogen production can be defined.
The Chemistry and Technology of Petroleum
hydrogen sulfide and ammonia. Limited hydrocracking also occurs. After the hydrocarbon leaves
the first stage, it is cooled and liquefied and run through a hydrocarbon separator. The hydrogen is
recycled to the feedstock. The liquid is charged to a fractionator. Depending on the products desired
(gasoline components, jet fuel, and gas oil), the fractionator is run to cut out some portion of the
first stage reactor out-turn. Kerosene-range material can be taken as a separate side-draw product or
included in the fractionator bottoms with the gas oil. The fractionator bottoms are again mixed with
a hydrogen stream and charged to the second stage. Since this material has already been subjected
to some hydrogenation, cracking, and reforming in the first stage, the operations of the second stage
are more severe (higher temperatures and pressures). Like the outturn of the first stage, the secondstage product is separated from the hydrogen and charged to the fractionator.
23.3 FEEDSTOCKS
The most common, and perhaps the best, feedstocks for steam reforming are low-boiling saturated
hydrocarbons that have a low sulfur content; including natural gas, refinery gas, LPG, and lowboiling naphtha.
Natural gas is the most common feedstock for hydrogen production since it meets all the requirements for reformer feedstock. Natural gas typically contains more than 90% methane and ethane with only a few percent of propane and higher boiling hydrocarbons (Mokhatab et al., 2006).
Natural gas may (or most likely will) contain traces of carbon dioxide with some nitrogen and
other impurities. Purification of natural gas, before reforming, is usually relatively straightforward.
Traces of sulfur must be removed to avoid poisoning the reformer catalyst; zinc oxide treatment in
combination with hydrogenation is usually adequate.
Light refinery gas, containing a substantial amount of hydrogen, can be an attractive steam
reformer feedstock since it is produced as a by-product. Processing of refinery gas will depend on
its composition, particularly the levels of olefins and of propane and heavier hydrocarbons. Olefins,
which can cause problems by forming coke in the reformer, are converted to saturated compounds
in the hydrogenation unit. Higher boiling hydrocarbons in refinery gas can also form coke, either on
the primary reformer catalyst or in the preheater. If there is more than a few percent of C 3 and higher
compounds, a promoted reformer catalyst should be considered, in order to avoid carbon deposits.
Refinery gas from different sources varies in suitability as hydrogen plant feed. Catalytic
reformer off-gas (Chapter 24), for example, is saturated, very low in sulfur, and often has high
hydrogen content. The process gases from a coking unit (Chapter 18) or from a fluid catalytic cracking unit (Chapter 19) are much less desirable because of the content of unsaturated constituents. In
addition to olefins, these gases contain substantial amounts of sulfur that must be removed before
the gas is used as feedstock. These gases are also generally unsuitable for direct hydrogen recovery,
since the hydrogen content is usually too low. Hydrotreater off-gas lies in the middle of the range. It
is saturated, so it is readily used as hydrogen plant feed. Content of hydrogen and heavier hydrocarbons depends to a large extent on the upstream pressure. Sulfur removal will generally be required.
23.4 PROCESS CHEMISTRY
Before the feedstock is introduced to a process, there is the need for application of a strict feedstock
purification protocol. Prolonging catalyst life in hydrogen production processes is attributable to
effective feedstock purification, particularly sulfur removal. A typical natural gas or other light
hydrocarbon feedstock contains traces of hydrogen sulfide and organic sulfur.
In order to remove sulfur compounds, it is necessary to hydrogenate the feedstock to convert
the organic sulfur to hydrogen that is then reacted with zinc oxide (ZnO) at approximately 370°C
(700°F), resulting in the optimal use of the zinc oxide as well as ensuring complete hydrogenation.
Thus, assuming assiduous feedstock purification and removal of all of the objectionable contaminants, the chemistry of hydrogen production can be defined.