The Petrochemical Refinery
Feedstocks for the production of petrochemicals originate from refineries with
processes similar to those described earlier to produce fuels. Indeed, there are
only a few refineries worldwide that cater only to petrochemical requirements.
Most petrochemical feedstocks are produced by changing operating parameters of
the normal fuel refinery processes. In catering for the petrochemical needs, much of
the refinery product streams are tailored as follows:
• Aromatic streams – high in benzene, toluene, and xylenes
• Olefin streams – high in ethylene, propylene, and C 4 s
Producing the Aromatic Feedstock
The production of aromatic feedstocks originates with the catalytic reforming of a
refinery stream of a heavy naphtha range (say 120–420
F) and rich in naphthenes.
A typical stream that meets these criteria would be a naphtha stream from a
hydrocracker. Thus, in order to meet petrochemical needs, a hydrocracker may
form part of a fuel refinery configuration. This unit would be operated to maximize
naphtha production. This would mean running the unit at a low space velocity with
a higher oil recycle rate (i.e., most recovered product heavier than the naphtha
would be recycled back to the reactors).
Another source of high naphthene feed to the cat reformer would be hydrotreated
cat cracker naphtha. Of course the hydrotreating of unsaturates has a high demand
on the refinery’s hydrogen system, but this is balanced to some extent by the
additional hydrogen produced in reforming the naphthenes. Should the refinery
configuration include a thermal cracker and/or a steam cracker, the hydrotreating of
the naphtha cut from these units also yields high naphthene catalytic reformer
feedstock.
Catalytic reforming of the high naphthene-content naphtha produces aromatics,
but there are also some unreacted paraffins and some naphthenes present. The
downstream petrochemical units that separate and purify the aromatic reformate
are expensive both in capital and operating costs. The specification for the BTX
(benzene, toluene, xylene) feed is very stringent and excludes nonaromatic components as much as possible. Another process may therefore be included in the
refinery configuration to “clean up” this aromatic feed stream before leaving the
refinery. This is an aromatic extraction plant. This is a licensed process using a
solvent to separate the paraffins and aromatics by countercurrent extraction. The
rich aromatic stream is then forwarded to the BTX plant where benzene, toluene,
ethyl benzene, and o-xylene are separated by fractionation while the para-xylene is
usually separated by crystallization or by solid adsorption. The meta-xylene may
also be recovered by super distillation, but more often than not, it is converted into
o-xylene or p-xylene in an isomerization unit.
Introduction to Crude Oil and Petroleum Processing
47
Feedstocks for the production of petrochemicals originate from refineries with
processes similar to those described earlier to produce fuels. Indeed, there are
only a few refineries worldwide that cater only to petrochemical requirements.
Most petrochemical feedstocks are produced by changing operating parameters of
the normal fuel refinery processes. In catering for the petrochemical needs, much of
the refinery product streams are tailored as follows:
• Aromatic streams – high in benzene, toluene, and xylenes
• Olefin streams – high in ethylene, propylene, and C 4 s
Producing the Aromatic Feedstock
The production of aromatic feedstocks originates with the catalytic reforming of a
refinery stream of a heavy naphtha range (say 120–420
F) and rich in naphthenes.
A typical stream that meets these criteria would be a naphtha stream from a
hydrocracker. Thus, in order to meet petrochemical needs, a hydrocracker may
form part of a fuel refinery configuration. This unit would be operated to maximize
naphtha production. This would mean running the unit at a low space velocity with
a higher oil recycle rate (i.e., most recovered product heavier than the naphtha
would be recycled back to the reactors).
Another source of high naphthene feed to the cat reformer would be hydrotreated
cat cracker naphtha. Of course the hydrotreating of unsaturates has a high demand
on the refinery’s hydrogen system, but this is balanced to some extent by the
additional hydrogen produced in reforming the naphthenes. Should the refinery
configuration include a thermal cracker and/or a steam cracker, the hydrotreating of
the naphtha cut from these units also yields high naphthene catalytic reformer
feedstock.
Catalytic reforming of the high naphthene-content naphtha produces aromatics,
but there are also some unreacted paraffins and some naphthenes present. The
downstream petrochemical units that separate and purify the aromatic reformate
are expensive both in capital and operating costs. The specification for the BTX
(benzene, toluene, xylene) feed is very stringent and excludes nonaromatic components as much as possible. Another process may therefore be included in the
refinery configuration to “clean up” this aromatic feed stream before leaving the
refinery. This is an aromatic extraction plant. This is a licensed process using a
solvent to separate the paraffins and aromatics by countercurrent extraction. The
rich aromatic stream is then forwarded to the BTX plant where benzene, toluene,
ethyl benzene, and o-xylene are separated by fractionation while the para-xylene is
usually separated by crystallization or by solid adsorption. The meta-xylene may
also be recovered by super distillation, but more often than not, it is converted into
o-xylene or p-xylene in an isomerization unit.
Introduction to Crude Oil and Petroleum Processing
47