632
The Chemistry and Technology of Petroleum
capabilities and requirements of a hydrogen plant becomes ever more important to overall refinery
operations as a means of making the best use of hydrogen supplies in the refinery.
The chemical nature of the crude oil used as the refinery feedstock has always played the major
role in determining the hydrogen requirements of that refinery. For example, the lighter, more paraffinic crude oils will require somewhat less hydrogen for upgrading to, say, a gasoline product than
a heavier more asphaltic crude oil (Speight, 2000). It follows that the hydrodesulfurization of heavy
oils and residua (which, by definition, is a hydrogen-dependent process) needs substantial amounts
of hydrogen as part of the processing requirements.
In general, considerable variation exists from one refinery to another in the balance between
hydrogen produced and hydrogen consumed in the refining operations. However, what is more pertinent to the present text is the excessive amounts of hydrogen that are required for hydroprocessing
operations, whether these be hydrocracking or the somewhat milder hydrotreating processes. For
effective hydroprocessing, a substantial hydrogen partial pressure must be maintained in the reactor
and, in order to meet this requirement, an excess of hydrogen above that actually consumed by the
process must be fed to the reactor. Part of the hydrogen requirement is met by recycling a stream
of hydrogen-rich gas. However, the need still remains to generate hydrogen as makeup material to
accommodate the process consumption of 500–3000 scf/bbl depending upon whether the heavy
feedstock is being subjected to a predominantly hydrotreating (hydrodesulfurization) or to a predominantly hydrocracking process.
In some refineries, the hydrogen needs can be satisfied by hydrogen recovery from catalytic
reformer product gases, but other external sources are required. However, for the most part, many
refineries now require on-site hydrogen production facilities to supply the gas for their own processes. Most of this non-reformer hydrogen is manufactured either by steam–methane reforming or
by oxidation processes. However, other processes such as steam–methanol interaction or ammonia
dissociation may also be used as sources of hydrogen. Electrolysis of water produces high-purity
hydrogen, but the power costs may be prohibitive.
An early use of hydrogen in refineries was in naphtha hydrotreating, as feed pretreatment for
catalytic reforming (which in turn was producing hydrogen as a by-product). As environmental
regulations tightened, the technology matured and heavier streams were hydrotreated. Thus, in the
early refineries, the hydrogen for hydroprocesses was provided as a result of catalytic-reforming
processes in which dehydrogenation (Chapters 15 and 24) is a major chemical reaction and, as a
consequence, hydrogen gas is produced. The light ends from the catalytic reformer contain a high
ratio of hydrogen to methane so the stream is freed from ethane and/or propane to get a high concentration of hydrogen in the stream.
The hydrogen is recycled through the reactors where the reforming takes place to provide the
atmosphere necessary for the chemical reactions and also prevents the carbon from being deposited
on the catalyst, thus extending its operating life. An excess of hydrogen above whatever is consumed
in the process is produced, and, as a result, catalytic-reforming processes are unique in that they are
the only petroleum refinery processes to produce hydrogen as a by-product. However, as refineries
and refinery feedstocks evolved during the last four decades, the demand for hydrogen has increased
and reforming processes are no longer capable of providing the quantities of hydrogen necessary
for feedstock hydrogenation. Within the refinery, other processes are used as sources of hydrogen.
Thus, the recovery of hydrogen from the by-products of the coking units, visbreaker units and catalytic cracking units is also practiced in some refineries.
In coking units and visbreaker units, heavy feedstocks are converted to petroleum coke, oil,
light hydrocarbons (benzene, naphtha, and liquefied petroleum gas [LPG]) and gas (Chapter 18).
Depending on the process, hydrogen is present in a wide range of concentrations. Since coking processes need gas for heating purposes, adsorption processes are best suited to recover the hydrogen,
because they feature a very clean hydrogen product and an off-gas suitable as fuel.
Catalytic cracking is the most important process step for the production of light products from
gas oil and increasingly from vacuum gas oil and heavy feedstocks (Chapter 19). In catalytic
The Chemistry and Technology of Petroleum
capabilities and requirements of a hydrogen plant becomes ever more important to overall refinery
operations as a means of making the best use of hydrogen supplies in the refinery.
The chemical nature of the crude oil used as the refinery feedstock has always played the major
role in determining the hydrogen requirements of that refinery. For example, the lighter, more paraffinic crude oils will require somewhat less hydrogen for upgrading to, say, a gasoline product than
a heavier more asphaltic crude oil (Speight, 2000). It follows that the hydrodesulfurization of heavy
oils and residua (which, by definition, is a hydrogen-dependent process) needs substantial amounts
of hydrogen as part of the processing requirements.
In general, considerable variation exists from one refinery to another in the balance between
hydrogen produced and hydrogen consumed in the refining operations. However, what is more pertinent to the present text is the excessive amounts of hydrogen that are required for hydroprocessing
operations, whether these be hydrocracking or the somewhat milder hydrotreating processes. For
effective hydroprocessing, a substantial hydrogen partial pressure must be maintained in the reactor
and, in order to meet this requirement, an excess of hydrogen above that actually consumed by the
process must be fed to the reactor. Part of the hydrogen requirement is met by recycling a stream
of hydrogen-rich gas. However, the need still remains to generate hydrogen as makeup material to
accommodate the process consumption of 500–3000 scf/bbl depending upon whether the heavy
feedstock is being subjected to a predominantly hydrotreating (hydrodesulfurization) or to a predominantly hydrocracking process.
In some refineries, the hydrogen needs can be satisfied by hydrogen recovery from catalytic
reformer product gases, but other external sources are required. However, for the most part, many
refineries now require on-site hydrogen production facilities to supply the gas for their own processes. Most of this non-reformer hydrogen is manufactured either by steam–methane reforming or
by oxidation processes. However, other processes such as steam–methanol interaction or ammonia
dissociation may also be used as sources of hydrogen. Electrolysis of water produces high-purity
hydrogen, but the power costs may be prohibitive.
An early use of hydrogen in refineries was in naphtha hydrotreating, as feed pretreatment for
catalytic reforming (which in turn was producing hydrogen as a by-product). As environmental
regulations tightened, the technology matured and heavier streams were hydrotreated. Thus, in the
early refineries, the hydrogen for hydroprocesses was provided as a result of catalytic-reforming
processes in which dehydrogenation (Chapters 15 and 24) is a major chemical reaction and, as a
consequence, hydrogen gas is produced. The light ends from the catalytic reformer contain a high
ratio of hydrogen to methane so the stream is freed from ethane and/or propane to get a high concentration of hydrogen in the stream.
The hydrogen is recycled through the reactors where the reforming takes place to provide the
atmosphere necessary for the chemical reactions and also prevents the carbon from being deposited
on the catalyst, thus extending its operating life. An excess of hydrogen above whatever is consumed
in the process is produced, and, as a result, catalytic-reforming processes are unique in that they are
the only petroleum refinery processes to produce hydrogen as a by-product. However, as refineries
and refinery feedstocks evolved during the last four decades, the demand for hydrogen has increased
and reforming processes are no longer capable of providing the quantities of hydrogen necessary
for feedstock hydrogenation. Within the refinery, other processes are used as sources of hydrogen.
Thus, the recovery of hydrogen from the by-products of the coking units, visbreaker units and catalytic cracking units is also practiced in some refineries.
In coking units and visbreaker units, heavy feedstocks are converted to petroleum coke, oil,
light hydrocarbons (benzene, naphtha, and liquefied petroleum gas [LPG]) and gas (Chapter 18).
Depending on the process, hydrogen is present in a wide range of concentrations. Since coking processes need gas for heating purposes, adsorption processes are best suited to recover the hydrogen,
because they feature a very clean hydrogen product and an off-gas suitable as fuel.
Catalytic cracking is the most important process step for the production of light products from
gas oil and increasingly from vacuum gas oil and heavy feedstocks (Chapter 19). In catalytic