Aromatics Production Process (ExxonMobil)

CATEGORY: AROMATICS
Aromatics Production Process (ExxonMobil)
United States Patent Application 20160046544
Molinier; Michel ;   et al.   February 18, 2016
Applicant: ExxonMobil Chemical Patents Inc.
Abstract
In a process for producing para-xylene, at least one feed comprising C.sub.6+ aromatic hydrocarbons is supplied to a dividing wall distillation column to separate the feed into a C.sub.7- aromatic hydrocarbon-containing stream, a C.sub.8 aromatic hydrocarbon-containing stream and a C.sub.9+ aromatic hydrocarbon-containing stream. At least part of the C.sub.8 aromatic hydrocarbon-containing stream is then supplied to a para-xylene recovery unit to recover para-xylene from the C.sub.8 aromatic hydrocarbon-containing stream and produce a para-xylene depleted stream. The para-xylene depleted stream is contacted with a xylene isomerization catalyst in a xylene isomerization zone under conditions effective to isomerize xylenes in the para-xylene depleted stream and produce an isomerized stream, which is then at least partially recycled to the para-xylene recovery unit.
FIELD OF THE INVENTION
[0002] This invention relates to a process for the production of aromatic hydrocarbons and particularly for the production of para-xylene.
BACKGROUND OF THE INVENTION
[0003] Benzene, toluene and xylenes (BTX) are important aromatic hydrocarbons, for which the worldwide demand is steadily increasing. The demand for xylenes, particularly para-xylene, has increased in proportion to the increase in demand for polyester fibers and film and typically grows at a rate of 5-7% per year. Benzene is a highly valuable product for use as a chemical raw material. Toluene is also a valuable petrochemical for use as a solvent and an intermediate in chemical manufacturing processes and as a high octane gasoline component. However, in many modern aromatic complexes, some or all of the benzene and/or toluene is converted to further xylenes by either transalkylation or methylation or a combination thereof.
[0004] A major source of benzene, toluene, and xylenes (BTX) is catalytic reformate, which is produced by contacting petroleum naphtha with a hydrogenation/dehydrogenation catalyst on a support. The resulting reformate is a complex mixture of paraffins and the desired C.sub.6 to C.sub.8 aromatics, in addition to a significant quantity of heavier aromatic hydrocarbons. After removing the light (C.sub.5-) paraffinic components, the remainder of reformate is normally separated into C.sub.7-, C.sub.8 and C.sub.9+-containing fractions using a plurality of distillation steps. Benzene can then be recovered from the C.sub.7--containing fraction to leave a toluene-rich fraction which is generally used to produce additional C.sub.8 aromatics by either methylation or transalkylation with part of the C.sub.9+-containing fraction. The C.sub.8-containing fraction is fed to a xylene production loop where para-xylene is recovered, generally by adsorption or crystallization, and the resultant para-xylene depleted stream is subjected to catalytic conversion to isomerize the xylenes back towards equilibrium distribution and to reduce the level of ethylbenzene that would otherwise build up in the xylene production loop.
[0005] While catalytic technologies are becoming more efficient in achieving the desired chemical reactions to maximize para-xylene production while reducing loss of valuable aromatic molecules, there is a continuing need to achieve savings in hardware cost and energy consumption so as to reduce the overall para-xylene production cost.
SUMMARY OF THE INVENTION
[0006] According to the present invention, it has now been found that dividing wall distillation columns provide effective and energy-efficient means for separating hydrocarbon streams, particularly the C.sub.7-, C.sub.8, and C.sub.9+-containing fractions, encountered in certain para-xylene production complexes.
[0007] In a first embodiment, at least one feed comprising C.sub.6+ aromatic hydrocarbons is supplied to a dividing wall distillation column to separate the feed into a C.sub.7- aromatic hydrocarbon-containing stream, a C.sub.8 aromatic hydrocarbon-containing stream and a C.sub.9+ aromatic hydrocarbon-containing stream. At least part of the C.sub.8 aromatic hydrocarbon-containing stream is then supplied to a para-xylene recovery unit to recover para-xylene from the C.sub.8 aromatic hydrocarbon-containing stream and produce a para-xylene depleted stream, which is contacted with a xylene isomerization catalyst in a xylene isomerization zone under conditions effective to isomerize xylenes in the para-xylene depleted stream and produce an isomerized stream. At least part of the isomerized stream is then recycled to the para-xylene recovery unit.
[0008] In another embodiment, the process further comprises removing at least part of the aliphatic hydrocarbons from the .sub.C7- hydrocarbon-containing stream to produce a .sub.C7- aromatic hydrocarbon-enriched stream, which is supplied to a separation unit to separate the .sub.C7- aromatic-enriched stream into a benzene-containing stream and a toluene-containing stream. At least part of the toluene-containing stream and at least part of the .sub.C9+ hydrocarbon-containing stream is contacted with a transalkylation catalyst under conditions effective to produce a transalkylation product containing xylenes, which are supplied to the para-xylene recovery unit.
[0009] Desirably, the process further comprises supplying at least part of the C.sub.8 aromatic hydrocarbon-containing stream from (a1 or a2 or a3) to an ethylbenzene removal unit which is located upstream of the para-xylene recovery unit and which is operated under conditions effective to remove at least part of the ethylbenzene in the C.sub.8 aromatic hydrocarbon-containing stream. Desirably, the conditions in the ethylbenzene removal unit are effective to maintain the C.sub.8 aromatic hydrocarbon-containing stream substantially in the gas phase and the conditions in the xylene isomerization zone are effective to maintain the para-xylene depleted stream substantially in the liquid phase.
[0010] In a third embodiment, a feed comprising a mixture of C.sub.6+ aliphatic and aromatic hydrocarbons is provided to a distillation column to separate the feed into a C.sub.7- hydrocarbon-containing stream and a C.sub.8+ hydrocarbon-containing stream. At least part of the aliphatic hydrocarbons from the C.sub.7- hydrocarbon-containing stream are removed to produce a C.sub.7- aromatic hydrocarbon-enriched stream, which is supplied to a separation unit to recover benzene therefrom and produce a toluene-containing stream. At least part of the C.sub.8+ hydrocarbon-containing stream is contacted with an ethylbenzene dealkylation catalyst under conditions effective to dealkylate ethylbenzene in the C.sub.8+ hydrocarbon-containing stream and produce a dealkylation effluent comprising benzene and C.sub.8+ hydrocarbons, which is separated in a dividing wall distillation column into a C.sub.7- aromatic hydrocarbon-containing stream, a C.sub.8 aromatic hydrocarbon-containing stream and a C.sub.9+ aromatic hydrocarbon-containing stream. The C.sub.8 aromatic hydrocarbon-containing stream is then sent to a para-xylene recovery unit to recover para-xylene from the C.sub.8 aromatic hydrocarbon-containing stream and produce a para-xylene depleted stream that is contacted with a xylene isomerization catalyst under conditions effective to isomerize xylenes in the para-xylene depleted stream and produce an isomerized stream, which is recycled to the dividing wall distillation column. At least part of the toluene-containing stream and at least part of the C.sub.9+ hydrocarbon-containing stream is contacted with a transalkylation catalyst under conditions effective to produce a transalkylation product containing xylenes that is to the separation unit.
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