661
Product Improvement and Treating
Although subsequent olefin reactions occur in thermal reforming, the product contains appreciable amounts of unstable unsaturated compounds. In the presence of catalysts and of hydrogen
(available from dehydrogenation reactions), hydrocracking of paraffins to yield two lower paraffins
occurs. Olefins that do not undergo dehydrocyclization are also produced. The olefins are hydrogenated with or without isomerization, so that the end product contains only traces of olefins.
The addition of a hydrogenation-dehydrogenation catalyst to the system yields a dual-function
catalyst complex. Hydrogen reactions—hydrogenation, dehydrogenation, dehydrocyclization, and
hydrocracking—take place on one catalyst; and cracking, isomerization, and olefin polymerization
take place on the acid catalyst sites.
Under the high-hydrogen partial pressure conditions used in catalytic reforming, sulfur compounds are readily converted into hydrogen sulfide, which, unless removed, builds up to a high
concentration in the recycle gas. Hydrogen sulfide is a reversible poison for platinum and causes
a decrease in the catalyst dehydrogenation and dehydrocyclization activities. In the first catalytic
reformers the hydrogen sulfide was removed from the gas cycle stream by absorption in, for example, diethanolamine. Sulfur is generally removed from the feedstock by use of a conventional desulfurization over cobalt-molybdenum catalyst. An additional benefit of desulfurization of the feed
to a level of <5 ppm sulfur is the elimination of hydrogen sulfide (H 2 S) corrosion problems in the
heaters and reactors.
Organic nitrogen compounds are converted into ammonia under reforming conditions, and this
neutralizes acid sites on the catalyst and thus represses the activity for isomerization, hydrocracking, and dehydrocyclization reactions. Straight-run materials do not usually present serious problems with regard to nitrogen, but feeds such as coker naphtha may contain around 50 ppm nitrogen
and removal of this quantity may require high-pressure hydrogenation (800–1000 psi) over nickel–
cobalt–molybdenum on an alumina catalyst.
The yield of gasoline of a given octane number and at given operating conditions depends on the
hydrocarbon types in the feed. For example, high-naphthene stocks, which readily give aromatic
gasoline, are the easiest to reform and give the highest gasoline yields. Paraffinic stocks, however,
which depend on the more difficult isomerization, dehydrocyclization, and hydrocracking reactions,
require more severe conditions and give lower gasoline yields than the naphthenic stocks. The end
point of the feed is usually limited to about 190°C (375°F), partially because of increased coke
deposition on the catalyst as the end point during processing at about 15°C (27°F). Limiting the feed
end point avoids redistillation of the product to meet the gasoline end-point specification of 205°C
(400°F), maximum.
Dehydrogenation is a main chemical reaction in catalytic reforming, and hydrogen gas is consequently produced in large quantities. 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.
24.2.2.1 Fixed-Bed Processes
Fixed-bed, continuous catalytic reforming can be classified by catalyst type: (1) cyclical regenerative with non-precious metal oxide catalysts and (2) cyclic regenerative with platinum-alumina
catalysts. Both types use swing reactors to regenerate a portion of the catalyst while the remainder
stays on-stream.
The cyclic regenerative fixed-bed operation using a platinum catalyst is basically a low-pressure
process (250–350 psi), which gives higher gasoline yields because of fewer hydrocracking reactions,
as well as higher octane products from a given naphtha charge and better hydrogen yields because
of more dehydrogenation and fewer hydrocracking reactions. The coke yield, with attendant catalyst
deactivation, increases rapidly at low pressures.
Product Improvement and Treating
Although subsequent olefin reactions occur in thermal reforming, the product contains appreciable amounts of unstable unsaturated compounds. In the presence of catalysts and of hydrogen
(available from dehydrogenation reactions), hydrocracking of paraffins to yield two lower paraffins
occurs. Olefins that do not undergo dehydrocyclization are also produced. The olefins are hydrogenated with or without isomerization, so that the end product contains only traces of olefins.
The addition of a hydrogenation-dehydrogenation catalyst to the system yields a dual-function
catalyst complex. Hydrogen reactions—hydrogenation, dehydrogenation, dehydrocyclization, and
hydrocracking—take place on one catalyst; and cracking, isomerization, and olefin polymerization
take place on the acid catalyst sites.
Under the high-hydrogen partial pressure conditions used in catalytic reforming, sulfur compounds are readily converted into hydrogen sulfide, which, unless removed, builds up to a high
concentration in the recycle gas. Hydrogen sulfide is a reversible poison for platinum and causes
a decrease in the catalyst dehydrogenation and dehydrocyclization activities. In the first catalytic
reformers the hydrogen sulfide was removed from the gas cycle stream by absorption in, for example, diethanolamine. Sulfur is generally removed from the feedstock by use of a conventional desulfurization over cobalt-molybdenum catalyst. An additional benefit of desulfurization of the feed
to a level of <5 ppm sulfur is the elimination of hydrogen sulfide (H 2 S) corrosion problems in the
heaters and reactors.
Organic nitrogen compounds are converted into ammonia under reforming conditions, and this
neutralizes acid sites on the catalyst and thus represses the activity for isomerization, hydrocracking, and dehydrocyclization reactions. Straight-run materials do not usually present serious problems with regard to nitrogen, but feeds such as coker naphtha may contain around 50 ppm nitrogen
and removal of this quantity may require high-pressure hydrogenation (800–1000 psi) over nickel–
cobalt–molybdenum on an alumina catalyst.
The yield of gasoline of a given octane number and at given operating conditions depends on the
hydrocarbon types in the feed. For example, high-naphthene stocks, which readily give aromatic
gasoline, are the easiest to reform and give the highest gasoline yields. Paraffinic stocks, however,
which depend on the more difficult isomerization, dehydrocyclization, and hydrocracking reactions,
require more severe conditions and give lower gasoline yields than the naphthenic stocks. The end
point of the feed is usually limited to about 190°C (375°F), partially because of increased coke
deposition on the catalyst as the end point during processing at about 15°C (27°F). Limiting the feed
end point avoids redistillation of the product to meet the gasoline end-point specification of 205°C
(400°F), maximum.
Dehydrogenation is a main chemical reaction in catalytic reforming, and hydrogen gas is consequently produced in large quantities. 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.
24.2.2.1 Fixed-Bed Processes
Fixed-bed, continuous catalytic reforming can be classified by catalyst type: (1) cyclical regenerative with non-precious metal oxide catalysts and (2) cyclic regenerative with platinum-alumina
catalysts. Both types use swing reactors to regenerate a portion of the catalyst while the remainder
stays on-stream.
The cyclic regenerative fixed-bed operation using a platinum catalyst is basically a low-pressure
process (250–350 psi), which gives higher gasoline yields because of fewer hydrocracking reactions,
as well as higher octane products from a given naphtha charge and better hydrogen yields because
of more dehydrogenation and fewer hydrocracking reactions. The coke yield, with attendant catalyst
deactivation, increases rapidly at low pressures.