646
The Chemistry and Technology of Petroleum
23.5.11 reCovery From Fuel gAs
Recovering hydrogen from refinery fuel gas can help refineries to satisfy high hydrogen demand.
Cryogenic separation is typically viewed as being the most thermodynamically efficient separation
technology. The higher capital cost associated with pre-purification and the low flexibility to impurity upsets has limited its use in hydrogen recovery.
The basic configuration for hydrogen recovery from refinery gases involves a two-stage partial
condensation process, with post purification via PSA (Dragomir et al., 2010). The major steps in this
process involve first compressing and pretreating the crude refinery gas stream before chilling to
an intermediate temperature (−60°F to −120°F). This partially condensed stream is then separated
in a flash-drum after which the liquid stream from is expanded through a Joule–Thompson valve
to generate refrigeration and then is fed to the wash column. Optionally, the wash column can be
replaced by a simple flash drum.
A crude LPG stream is collected at the bottom of the column, and a methane-rich vapor is
obtained at the top. The methane-rich vapor is sent to compression and then to fuel. The vapor from
the flash drum is further cooled in a second heat exchanger before being fed to second flash drum
where it produces a hydrogen-rich stream and a methane-rich liquid. The liquid is expanded in a
Joule–Thomson valve to generate refrigeration, and then is sent for further cooling. The hydrogenrich gas is then sent to the PSA unit for further purification—the tail gas from this unit is compressed and returned to fuel together with the methane-rich gas.
23.6 CATALYSTS
Hydrogen plants are one of the most extensive users of catalysts in the refinery. Catalytic operations
include hydrogenation, steam reforming, shift conversion, and methanation.
23.6.1 reFormIng CAtAlysts
The reforming catalyst is usually supplied as nickel oxide that, during start-up, is heated in a stream
of inert gas and then steam. When the catalyst is near the normal operating temperature, hydrogen
or a light hydrocarbon is added to reduce the nickel oxide to metallic nickel.
The high temperatures (up to 870°C, 1600°F), and the nature of the reforming reaction require
that the reforming catalyst be used inside the radiant tubes of a reforming furnace. The active agent
in reforming catalyst is nickel, and normally, the reaction is controlled both by diffusion and by heat
transfer. Catalyst life is limited as much by physical breakdown as by deactivation.
Sulfur is the main catalyst poison and the catalyst poisoning is theoretically reversible with the
catalyst being restored to near full activity by steaming. However, in practice the deactivation may
cause the catalyst to overheat and coke, to the point that it must be replaced. Reforming catalysts
are also sensitive to poisoning by heavy metals, although these are rarely present in low-boiling
hydrocarbon feedstocks and in naphtha feedstocks.
Coking deposition on the reforming catalyst and ensuing gloss of catalyst activity is the most
characteristic issue that must be assessed and mitigated.
While methane-rich streams such as natural gas or light refinery gas are the most common feeds
to hydrogen plants, there is often a requirement for variety of reasons to process a variety of higher
boiling feedstocks, such as LPG and naphtha. Feedstock variations may also be inadvertent due, for
example, to changes in refinery off-gas composition from another unit or because of variations in
naphtha composition because of feedstock variance to the naphtha unit.
Thus, when using higher boiling feedstocks in a hydrogen plant, coke deposition on the reformer
catalyst becomes a major issue. Coking is most likely in the reformer unit at the point where both
temperature and hydrocarbon content are high enough. In this region, hydrocarbons crack and
form coke faster than the coke is removed by reaction with steam or hydrogen and when catalyst
The Chemistry and Technology of Petroleum
23.5.11 reCovery From Fuel gAs
Recovering hydrogen from refinery fuel gas can help refineries to satisfy high hydrogen demand.
Cryogenic separation is typically viewed as being the most thermodynamically efficient separation
technology. The higher capital cost associated with pre-purification and the low flexibility to impurity upsets has limited its use in hydrogen recovery.
The basic configuration for hydrogen recovery from refinery gases involves a two-stage partial
condensation process, with post purification via PSA (Dragomir et al., 2010). The major steps in this
process involve first compressing and pretreating the crude refinery gas stream before chilling to
an intermediate temperature (−60°F to −120°F). This partially condensed stream is then separated
in a flash-drum after which the liquid stream from is expanded through a Joule–Thompson valve
to generate refrigeration and then is fed to the wash column. Optionally, the wash column can be
replaced by a simple flash drum.
A crude LPG stream is collected at the bottom of the column, and a methane-rich vapor is
obtained at the top. The methane-rich vapor is sent to compression and then to fuel. The vapor from
the flash drum is further cooled in a second heat exchanger before being fed to second flash drum
where it produces a hydrogen-rich stream and a methane-rich liquid. The liquid is expanded in a
Joule–Thomson valve to generate refrigeration, and then is sent for further cooling. The hydrogenrich gas is then sent to the PSA unit for further purification—the tail gas from this unit is compressed and returned to fuel together with the methane-rich gas.
23.6 CATALYSTS
Hydrogen plants are one of the most extensive users of catalysts in the refinery. Catalytic operations
include hydrogenation, steam reforming, shift conversion, and methanation.
23.6.1 reFormIng CAtAlysts
The reforming catalyst is usually supplied as nickel oxide that, during start-up, is heated in a stream
of inert gas and then steam. When the catalyst is near the normal operating temperature, hydrogen
or a light hydrocarbon is added to reduce the nickel oxide to metallic nickel.
The high temperatures (up to 870°C, 1600°F), and the nature of the reforming reaction require
that the reforming catalyst be used inside the radiant tubes of a reforming furnace. The active agent
in reforming catalyst is nickel, and normally, the reaction is controlled both by diffusion and by heat
transfer. Catalyst life is limited as much by physical breakdown as by deactivation.
Sulfur is the main catalyst poison and the catalyst poisoning is theoretically reversible with the
catalyst being restored to near full activity by steaming. However, in practice the deactivation may
cause the catalyst to overheat and coke, to the point that it must be replaced. Reforming catalysts
are also sensitive to poisoning by heavy metals, although these are rarely present in low-boiling
hydrocarbon feedstocks and in naphtha feedstocks.
Coking deposition on the reforming catalyst and ensuing gloss of catalyst activity is the most
characteristic issue that must be assessed and mitigated.
While methane-rich streams such as natural gas or light refinery gas are the most common feeds
to hydrogen plants, there is often a requirement for variety of reasons to process a variety of higher
boiling feedstocks, such as LPG and naphtha. Feedstock variations may also be inadvertent due, for
example, to changes in refinery off-gas composition from another unit or because of variations in
naphtha composition because of feedstock variance to the naphtha unit.
Thus, when using higher boiling feedstocks in a hydrogen plant, coke deposition on the reformer
catalyst becomes a major issue. Coking is most likely in the reformer unit at the point where both
temperature and hydrocarbon content are high enough. In this region, hydrocarbons crack and
form coke faster than the coke is removed by reaction with steam or hydrogen and when catalyst