658
The Chemistry and Technology of Petroleum
as iso-butene and iso-pentene, under the influence of acid catalysts. In the mid-1990s methyl-t-butyl
ether (MTBE) (Table 24.3), made by etherification of iso-butene with methanol, became the predominant oxygenate used to meet reformulation requirements for adding oxygen to mitigate emissions from gasoline-powered vehicles.
Environmental issues with MTBE have made it more desirable to dimerize iso-butene from the
catalytic cracking unit rather than etherify it. Fortunately, iso-butene dimerization may be achieved
with minimal modifications to existing MTBE plants and process conditions, using the same acidic
catalysts. Where olefin levels are not restricted, the extra blending octane boost of the di-isobutylene
can be retained. Where olefin levels are restricted, the di-isobutylene can be hydrotreated to produce
a relatively pure iso-octane stream that can supplement alkylate for reducing olefins and aromatics
in reformulated gasoline.
24.2.1 tHermAl reFormIng
Thermal reforming was a natural development from thermal cracking, since reforming is also
thermal decomposition reaction. Cracking converts heavier oils into gasoline; reforming converts
(reforms) gasoline into higher-octane gasoline. The equipment for thermal reforming is essentially
the same as for thermal cracking, but higher temperatures are used (Nelson, 1958).
In carrying out thermal reforming, a feedstock, such as a 205°C (400°F) end-point naphtha or a
straight-run gasoline, is heated to 510°C–595°C (950°F–1100°F) in a furnace much the same as a
cracking furnace, with pressures from 400 to 1000 psi. As the heated naphtha leaves the furnace,
it is cooled or quenched by the addition of cold naphtha. The quenched, reformed material then
enters a fractional distillation tower where any heavy products are separated. The remainder of the
reformed material leaves the top of the tower to be separated into gases and reformate. The higher
octane number of the product (reformate) is due primarily to the cracking of longer chain paraffins
into higher-octane olefins.
Thermal reforming is in general less effective than catalytic processes and has been largely
supplanted. As it was practiced, a single-pass operation was employed at temperatures in the
range of 540°C–760°C (1000°F–1140°F) and pressures in the range 500–1000 psi. Octane number
improvement depended on the extent of conversion but was not directly proportional to the extent
of cracking-per-pass.
The amount and quality of reformate is dependent on the temperature. A general rule is the
higher the reforming temperature, the higher the octane number of the product but the yield of reformate is relatively low. For example, a gasoline with an octane number of 35 when reformed at 515°C
(960°F) yields 92.4% of 56 octane reformate; when reformed at 555°C (1030°F) the yield is 68.7%
of 83 octane reformate. However, high conversion is not always effective since coke production and
gas production usually increase.
Modifications of the thermal reforming process due to the inclusion of hydrocarbon gases with
the feedstock are known as gas reversion and polyforming. Thus, olefinic gases produced by cracking and reforming can be converted into liquids boiling in the gasoline range by heating them under
high pressure. Since the resulting liquids (polymers) have high octane numbers, they increase the
overall quantity and quality of gasoline produced in a refinery.
The gases most susceptible to conversion to liquid products are olefins with three and
four carbon atoms. These are propylene (CH 3 .CH=CH 2 ), which is associated with propane in
the C 3 fraction, and butylene (CH 3 .CH 2 .CH=CH 2 and/or CH 3 .CH=CH.CH 3 ) and iso- butylene
[(CH 3 ) 2 C=CH 2 ], which are associated with butane (CH 3 .CH 2 .CH 2 .CH 3 ) and iso-butane
[(CH 3 ) 2 CH.CH 3 ] in the C 4 fraction. When the C 3 and C 4 fractions are subjected to the temperature and pressure conditions used in thermal reforming, they undergo chemical reactions that
result in a small yield of gasoline. When the C 3 and C 4 fractions are passed through a thermal
reformer in admixture with naphtha, the process is called naphtha-gas reversion or naphtha
polyforming.
The Chemistry and Technology of Petroleum
as iso-butene and iso-pentene, under the influence of acid catalysts. In the mid-1990s methyl-t-butyl
ether (MTBE) (Table 24.3), made by etherification of iso-butene with methanol, became the predominant oxygenate used to meet reformulation requirements for adding oxygen to mitigate emissions from gasoline-powered vehicles.
Environmental issues with MTBE have made it more desirable to dimerize iso-butene from the
catalytic cracking unit rather than etherify it. Fortunately, iso-butene dimerization may be achieved
with minimal modifications to existing MTBE plants and process conditions, using the same acidic
catalysts. Where olefin levels are not restricted, the extra blending octane boost of the di-isobutylene
can be retained. Where olefin levels are restricted, the di-isobutylene can be hydrotreated to produce
a relatively pure iso-octane stream that can supplement alkylate for reducing olefins and aromatics
in reformulated gasoline.
24.2.1 tHermAl reFormIng
Thermal reforming was a natural development from thermal cracking, since reforming is also
thermal decomposition reaction. Cracking converts heavier oils into gasoline; reforming converts
(reforms) gasoline into higher-octane gasoline. The equipment for thermal reforming is essentially
the same as for thermal cracking, but higher temperatures are used (Nelson, 1958).
In carrying out thermal reforming, a feedstock, such as a 205°C (400°F) end-point naphtha or a
straight-run gasoline, is heated to 510°C–595°C (950°F–1100°F) in a furnace much the same as a
cracking furnace, with pressures from 400 to 1000 psi. As the heated naphtha leaves the furnace,
it is cooled or quenched by the addition of cold naphtha. The quenched, reformed material then
enters a fractional distillation tower where any heavy products are separated. The remainder of the
reformed material leaves the top of the tower to be separated into gases and reformate. The higher
octane number of the product (reformate) is due primarily to the cracking of longer chain paraffins
into higher-octane olefins.
Thermal reforming is in general less effective than catalytic processes and has been largely
supplanted. As it was practiced, a single-pass operation was employed at temperatures in the
range of 540°C–760°C (1000°F–1140°F) and pressures in the range 500–1000 psi. Octane number
improvement depended on the extent of conversion but was not directly proportional to the extent
of cracking-per-pass.
The amount and quality of reformate is dependent on the temperature. A general rule is the
higher the reforming temperature, the higher the octane number of the product but the yield of reformate is relatively low. For example, a gasoline with an octane number of 35 when reformed at 515°C
(960°F) yields 92.4% of 56 octane reformate; when reformed at 555°C (1030°F) the yield is 68.7%
of 83 octane reformate. However, high conversion is not always effective since coke production and
gas production usually increase.
Modifications of the thermal reforming process due to the inclusion of hydrocarbon gases with
the feedstock are known as gas reversion and polyforming. Thus, olefinic gases produced by cracking and reforming can be converted into liquids boiling in the gasoline range by heating them under
high pressure. Since the resulting liquids (polymers) have high octane numbers, they increase the
overall quantity and quality of gasoline produced in a refinery.
The gases most susceptible to conversion to liquid products are olefins with three and
four carbon atoms. These are propylene (CH 3 .CH=CH 2 ), which is associated with propane in
the C 3 fraction, and butylene (CH 3 .CH 2 .CH=CH 2 and/or CH 3 .CH=CH.CH 3 ) and iso- butylene
[(CH 3 ) 2 C=CH 2 ], which are associated with butane (CH 3 .CH 2 .CH 2 .CH 3 ) and iso-butane
[(CH 3 ) 2 CH.CH 3 ] in the C 4 fraction. When the C 3 and C 4 fractions are subjected to the temperature and pressure conditions used in thermal reforming, they undergo chemical reactions that
result in a small yield of gasoline. When the C 3 and C 4 fractions are passed through a thermal
reformer in admixture with naphtha, the process is called naphtha-gas reversion or naphtha
polyforming.