644
The Chemistry and Technology of Petroleum
The steam–methane reforming process described in the preceding would be an ideal hydrogen production process if it was not for the fact that large quantities of natural gas, a valuable resource, are
required as both feed gas and combustion fuel. For each mole of methane reformed, more than 1 mol
of carbon dioxide is coproduced and must be disposed. This can be a major issue as it results in the
same amount of greenhouse gas emission as would be expected from direct combustion of natural gas
or methane. In fact, the production of hydrogen as a clean burning fuel by way of steam reforming of
methane and other fossil-based hydrocarbon fuels is not in environmental balance if in the process, carbon dioxide and carbon monoxide are generated and released into the atmosphere, although alternate
scenarios are available (Gaudernack, 1996). Moreover, as the reforming process is not totally efficient,
some of the energy value of the hydrocarbon fuel is lost by conversion to hydrogen but with no tangible
environmental benefit, such as a reduction in emission of greenhouse gases. Despite these apparent
shortcomings, the process has the following advantages: (1) produces 4 mol of hydrogen for each mole
of methane consumed, (2) feedstocks for the process (methane and water) are readily available, (3) the
process is adaptable to a wide range of hydrocarbon feedstocks, (4) operates at low pressures (less than
450 psi), (5) requires a low steam/carbon ratio (2.5–3), (6) good utilization of input energy (reaching
93%), (7) can use catalysts that are stable and resist poisoning, and (8) good process kinetics.
Liquid feedstocks, either LPG or naphtha (q.v.), can also provide backup feed, if there is a risk
of natural gas curtailments. The feed handling system needs to include a surge drum, feed pump,
vaporizer (usually steam-heated) followed by further heating before desulfurization. The sulfur in
liquid feedstocks occurs as mercaptans, thiophene derivatives, or higher boiling compounds. These
compounds are stable and will not be removed by zinc oxide; therefore, a hydrogenation unit will
be required. In addition, as with refinery gas, olefins must also be hydrogenated if they are present.
The reformer will generally use a potash-promoted catalyst to avoid coke buildup from cracking
of the heavier feedstock. If LPG is to be used only occasionally, it is often possible to use a methanetype catalyst at a higher steam/carbon ratio to avoid coking. Naphtha will require a promoted catalyst unless a preformer is used.
23.5.8 steAm–nAPHtHA reFormIng
Steam–naphtha reforming is a continuous process for the production of hydrogen from liquid hydrocarbons and is, in fact, similar to steam–methane reforming that is one of several possible processes
for the production of hydrogen from low-boiling hydrocarbons other than ethane (Muradov, 1998,
2000; Brandmair et al., 2003; Find et al., 2003). A variety of naphtha types in the gasoline boiling
range may be employed, including feeds containing up to 35% aromatics. Thus, following pretreatment to remove sulfur compounds, the feedstock is mixed with steam and taken to the reforming
furnace (675°C–815°C [1250°F–1500°F], 300 psi), where hydrogen is produced.
23.5.9 syntHesIs gAs generAtIon
The synthesis gas generation process is a noncatalytic process for producing synthesis gas (principally hydrogen and carbon monoxide) for the ultimate production of high-purity hydrogen from
gaseous or liquid hydrocarbons.
In this process, a controlled mixture of preheated feedstock and oxygen is fed to the top of the
generator where carbon monoxide and hydrogen emerge as the products. Soot, produced in this part
of the operation, is removed in a water scrubber from the product gas stream and is then extracted
from the resulting carbon–water slurry with naphtha and transferred to a fuel oil fraction. The
oil–soot mixture is burned in a boiler or recycled to the generator to extinction to eliminate carbon
production as part of the process.
The soot-free synthesis gas is then charged to a shift converter where the carbon monoxide reacts
with steam to form additional hydrogen and carbon dioxide at the stoichiometric rate of 1 mol of
hydrogen for every mole of carbon monoxide charged to the converter.
The Chemistry and Technology of Petroleum
The steam–methane reforming process described in the preceding would be an ideal hydrogen production process if it was not for the fact that large quantities of natural gas, a valuable resource, are
required as both feed gas and combustion fuel. For each mole of methane reformed, more than 1 mol
of carbon dioxide is coproduced and must be disposed. This can be a major issue as it results in the
same amount of greenhouse gas emission as would be expected from direct combustion of natural gas
or methane. In fact, the production of hydrogen as a clean burning fuel by way of steam reforming of
methane and other fossil-based hydrocarbon fuels is not in environmental balance if in the process, carbon dioxide and carbon monoxide are generated and released into the atmosphere, although alternate
scenarios are available (Gaudernack, 1996). Moreover, as the reforming process is not totally efficient,
some of the energy value of the hydrocarbon fuel is lost by conversion to hydrogen but with no tangible
environmental benefit, such as a reduction in emission of greenhouse gases. Despite these apparent
shortcomings, the process has the following advantages: (1) produces 4 mol of hydrogen for each mole
of methane consumed, (2) feedstocks for the process (methane and water) are readily available, (3) the
process is adaptable to a wide range of hydrocarbon feedstocks, (4) operates at low pressures (less than
450 psi), (5) requires a low steam/carbon ratio (2.5–3), (6) good utilization of input energy (reaching
93%), (7) can use catalysts that are stable and resist poisoning, and (8) good process kinetics.
Liquid feedstocks, either LPG or naphtha (q.v.), can also provide backup feed, if there is a risk
of natural gas curtailments. The feed handling system needs to include a surge drum, feed pump,
vaporizer (usually steam-heated) followed by further heating before desulfurization. The sulfur in
liquid feedstocks occurs as mercaptans, thiophene derivatives, or higher boiling compounds. These
compounds are stable and will not be removed by zinc oxide; therefore, a hydrogenation unit will
be required. In addition, as with refinery gas, olefins must also be hydrogenated if they are present.
The reformer will generally use a potash-promoted catalyst to avoid coke buildup from cracking
of the heavier feedstock. If LPG is to be used only occasionally, it is often possible to use a methanetype catalyst at a higher steam/carbon ratio to avoid coking. Naphtha will require a promoted catalyst unless a preformer is used.
23.5.8 steAm–nAPHtHA reFormIng
Steam–naphtha reforming is a continuous process for the production of hydrogen from liquid hydrocarbons and is, in fact, similar to steam–methane reforming that is one of several possible processes
for the production of hydrogen from low-boiling hydrocarbons other than ethane (Muradov, 1998,
2000; Brandmair et al., 2003; Find et al., 2003). A variety of naphtha types in the gasoline boiling
range may be employed, including feeds containing up to 35% aromatics. Thus, following pretreatment to remove sulfur compounds, the feedstock is mixed with steam and taken to the reforming
furnace (675°C–815°C [1250°F–1500°F], 300 psi), where hydrogen is produced.
23.5.9 syntHesIs gAs generAtIon
The synthesis gas generation process is a noncatalytic process for producing synthesis gas (principally hydrogen and carbon monoxide) for the ultimate production of high-purity hydrogen from
gaseous or liquid hydrocarbons.
In this process, a controlled mixture of preheated feedstock and oxygen is fed to the top of the
generator where carbon monoxide and hydrogen emerge as the products. Soot, produced in this part
of the operation, is removed in a water scrubber from the product gas stream and is then extracted
from the resulting carbon–water slurry with naphtha and transferred to a fuel oil fraction. The
oil–soot mixture is burned in a boiler or recycled to the generator to extinction to eliminate carbon
production as part of the process.
The soot-free synthesis gas is then charged to a shift converter where the carbon monoxide reacts
with steam to form additional hydrogen and carbon dioxide at the stoichiometric rate of 1 mol of
hydrogen for every mole of carbon monoxide charged to the converter.