thermal crackers. The following methods again in part or as a whole are used to
reduce this olefin content:
• Alkylation and etherification. The light olefins of C 5 can be fractionated out, and,
following a simple sulfur removal step, are fed to an alkylation process to
produce good high-octane C 9 alkylate. Alternatively, the light fraction from
the cracked naphtha can be processed to the oxygenate TAME (tertiary-amyl
methyl ether).
• Isomerization of the C 6 and C 7 fractions. These two olefin components of
cracked naphtha may be hydrogenated and isomerized to provide a good octane
rating. The two olefin components are only small in quantity in gasoline but they
are highly reactive, and their removal and conversion is necessary to meet the
present restrictions in gasoline manufacture.
Meeting the gasoline sulfur content. Almost all of the sulfur in gasoline comes
from the catalytic cracker and other thermal crackers. The RFG program is
expected to reduce sulfur in gasoline to around 10 ppm by weight; European
specifications are also lowered to 10 wppm. To meet these new criteria, refiners
are taking the following approaches:
• Route the heavy end (about 20 % vol) of the catalytic naphtha into the middle
distillate pool. This does not reduce the sulfur, but it moves a large portion of it
and the heavy aromatics into other parts of the product slate, reducing the sulfur
and aromatic content of the gasoline.
• Either hydrotreat and/or caustic treat the lighter portion of the cracked naphtha. This is rich in olefins and sulfur. Hydrotreating does reduce the octane
value of the naphtha in that it saturates the olefins to lower octane paraffin or
naphthene. Treating with sulfur extraction wash (e.g., UOP’s Merox process)
is the often preferred route; however, caustic treating is often inadequate to
reach 10 ppm.
• The back end of cracked naphtha is high in sulfur, aromatics, and olefins. This
may be hydrogenated using a catalyst selective in removing sulfur and leaving
the aromatics essentially as they were.
• Hydrotreating of the catalytic cracker gas oil feed is quite common. This
significantly reduces the sulfur content of all the cracked products from the
catalytic cracker unit and increases the product yields from the FCC.
Manufacturing Gasoline
The requirements of the Clean Air Act of 1990 and later additions have changed the
refining requirements to meet this product’s need quite significantly. Prior to this
date, much of the gasoline finished product recipe consisted of normal light
naphtha, reformate, usually some cracked naphtha, and possibly alkylate; some
butane may be included if required to meet volatility. The Clean Air requirement
and its subsequent additions force a reduction of both reformate and the cracked
stock (see Tables 10 and 11).
74
D.S.J. Jones and S.A. Treese
reduce this olefin content:
• Alkylation and etherification. The light olefins of C 5 can be fractionated out, and,
following a simple sulfur removal step, are fed to an alkylation process to
produce good high-octane C 9 alkylate. Alternatively, the light fraction from
the cracked naphtha can be processed to the oxygenate TAME (tertiary-amyl
methyl ether).
• Isomerization of the C 6 and C 7 fractions. These two olefin components of
cracked naphtha may be hydrogenated and isomerized to provide a good octane
rating. The two olefin components are only small in quantity in gasoline but they
are highly reactive, and their removal and conversion is necessary to meet the
present restrictions in gasoline manufacture.
Meeting the gasoline sulfur content. Almost all of the sulfur in gasoline comes
from the catalytic cracker and other thermal crackers. The RFG program is
expected to reduce sulfur in gasoline to around 10 ppm by weight; European
specifications are also lowered to 10 wppm. To meet these new criteria, refiners
are taking the following approaches:
• Route the heavy end (about 20 % vol) of the catalytic naphtha into the middle
distillate pool. This does not reduce the sulfur, but it moves a large portion of it
and the heavy aromatics into other parts of the product slate, reducing the sulfur
and aromatic content of the gasoline.
• Either hydrotreat and/or caustic treat the lighter portion of the cracked naphtha. This is rich in olefins and sulfur. Hydrotreating does reduce the octane
value of the naphtha in that it saturates the olefins to lower octane paraffin or
naphthene. Treating with sulfur extraction wash (e.g., UOP’s Merox process)
is the often preferred route; however, caustic treating is often inadequate to
reach 10 ppm.
• The back end of cracked naphtha is high in sulfur, aromatics, and olefins. This
may be hydrogenated using a catalyst selective in removing sulfur and leaving
the aromatics essentially as they were.
• Hydrotreating of the catalytic cracker gas oil feed is quite common. This
significantly reduces the sulfur content of all the cracked products from the
catalytic cracker unit and increases the product yields from the FCC.
Manufacturing Gasoline
The requirements of the Clean Air Act of 1990 and later additions have changed the
refining requirements to meet this product’s need quite significantly. Prior to this
date, much of the gasoline finished product recipe consisted of normal light
naphtha, reformate, usually some cracked naphtha, and possibly alkylate; some
butane may be included if required to meet volatility. The Clean Air requirement
and its subsequent additions force a reduction of both reformate and the cracked
stock (see Tables 10 and 11).
74
D.S.J. Jones and S.A. Treese