Process for isolating cyclopentane and/or cyclopentene

Details Process for isolating cyclopentane and/or cyclopentene Process for isolating cyclopentane and/or cyclopentene

2001-02-16

2002-10-22

Manoharan, Virginia (Department: 1764)

Distillation: processes, separatory

Plural distillations performed on same material

C203S029000, C203S095000, C203S098000, C585S264000, C585S800000, C585S802000

Reexamination Certificate

active

06468399

ABSTRACT:

The present invention relate to a process for isolating cyclopentane and/or cyclopentene from a partially hydrogenated pre-benzene fraction or a partially hydrogenated C
5
fraction by distillative separation. In a variant of the process of the invention, isopentane (2-methylbutane) can be isolated in addition to cyclopentane.
In recent times, cyclopentane has become important as a replacement for chlorofluorocarbons (CFCs) as blowing agent for polyurethane systems since it may be regarded as harmless to the ozone layer of the earth's atmosphere. In addition, cyclopentane is of importance as a special solvent. Cyclopentene is a valuable comonomer for polymers and, in addition, can yield further cyclopentane by hydrogenation. Finally, isopentane is a valuable raw material for alkylates and other chemical reactions.
The cracking of naptha in steam crackers or in fluid catalytic crackers (FCCs) forms a tailgas (mainly H
2
and CH
4
) plus valuable monomers such as ethylene, propene, 1-butene, 2-butene and isobutene and also butadiene (C
2
, C
3
and C
4
fractions).
The remaining part of the cracker products, known as pyrolysis gasoline, is worked up for the recovery of aromatics. The pyrolysis gasoline is first partially hydrogenated in order to hydrogenate acetylene compounds and diolefins to form alkenes or monoolefins. Subsequently, a partially hydrogenated pre-benzene fraction is obtained from the partially hydrogenated pyrolysis gasoline by distillation. This partially hydrogenated pre-benzene fraction contains a large number of isomeric C
5
-alkanes and C
5
-alkenes, plus cyclopentane and cyclopentene, small amounts of incompletely removed C
4
-hydrocarbons and a certain proportion of hydrocarbons having 6 or more carbon atoms. As an alternative, a partially hydrogenated C
5
fraction which contains a large number of isomeric C
5
-alkanes and C
5
-alkenes, cyclopentane, cyclopentene, small amounts of incompletely removed C
4
-hydrocarbons and also small amounts of hydrocarbons having 6 or more carbon atoms can be obtained from the partially hydrogenated pyrolysis gasoline by distillation.
The work-up of the pyrolysis gasoline can also be carried out in such a way that a non-hydrogenated pre-benzene fraction is first obtained by distillative separation. This is subsequently partially hydrogenated, again in order to hydrogenate acetylene compounds and diolefins to form alkenes or monoolefins. This partially hydrogenated pre-benzene fraction then contains a large number of isomeric C
5
-alkanes and C
5
-alkenes, plus cyclopentane and cyclopentene, small amounts of incompletely removed C
4
-hydrocarbons and a certain proportion of hydrocarbons having 6 or more carbon atoms. As an alternative, the work-up of the pyrolysis gasoline can also be carried out such that firstly only C
5
-hydrocarbons are separated off by distillation, giving a non-hydrogenated C
5
fraction. This C
5
fraction is subsequently likewise hydrogenated and then contains a large number of isomeric C
5
-alkanes and C
5
-alkenes, cyclopentane, cyclopentene, small amounts of incompletely removed C
4
-hydrocarbons and relatively small amounts of hydrocarbons having 6 or more carbon atoms.
A typical composition of a partially hydrogenated pre-benzene fraction or a partially hydrogenated C
5
fraction obtained by one of the above-described process variants is the following, where the sum of the percentages by weight is 100.
Component
(Hydrocarbons = HCs)
Content (% by weight)
C
4
-HCs
 0-4
Low-boiling C
5
-HCs
30-60
2-Methyl-2-butene
 8-20
Cyclopentane
 6-24
Cyclopentene
 8-27
C
6
+
-HCs
 0-25
C
5
-dienes
 0-0.5
The preferred composition of a partially hydrogenated pre-benzene fraction is the following, where the sum of the percentages by weight is 100.
Component
(Hydrocarbons = HCs)
Content (% by weight)
C
4
-HCs
 0-3
Low-boiling C
5
-HCs
30-45
2-Methyl-2-butene
 8-15
Cyclopentane
 6-18
Cyclopentene
 8-20
C
6
+
-HCs
15-25
C
5
-dienes
 0-0.5
The preferred composition of a partially hydrogenated C
5
fraction is the following, where the sum of the percentages by weight is 100.
Component
(Hydrocarbons = HCs)
Content (% by weight)
C
4
-HCs
 0-4
Low-boiling C
5
-HCs
35-60
2-Methyl-2-butene
 9-20
Cyclopentane
 7-24
Cyclopentene
 9-27
C
6
+
-HCs
 0-2
C
5
-dienes
 0-0.5
EP-A-0 799 881 discloses carrying out a fractional distillation of partially hydrogenated pyrolysis gasoline for isolating cyclopentane and/or cyclopentene, where, if desired, a mixture of n-pentane and isopentane can be isolated by taking a mixture enriched in cyclopentane and cyclopentene from a side offtake of a column, taking of the remaining C
5
fraction at the top and obtaining hydrocarbons having 6 or more carbon atoms as bottoms. In the context of the process described in EP-A-0 799 881, unsaturated hydrocarbons can be subjected to catalytic hydrogenation. The disadvantage of this process is that a stream taken off at a side offtake of a first column into which a many-component mixture is fed is still a mixture of many components from which pure components can be isolated only by means of additional effort or only in unsatisfactory purity. A further disadvantage of the process described in EP-A-0 799 881 is that the desired isopentane can only be obtained together with the less desired n-pentane. A further disadvantage of this process is that the position of the side offtake changes with fluctuating composition of the partially hydrogenated pyrolysis gasoline used as a result of the associated changes in the temperature profile of the column, so that optimal recovery of cyclopentane and/or cyclopentene is not possible.
It has now been found that it is better from the points of view of energy consumption and the quality of the desired materials which can be obtained, i.e. for economic reasons, to provide separate distillation steps and to provide a side offtake only in a final distillation column, although a person skilled in the art would have to regard such a procedure as too complicated.
The invention relates to a process for isolating cyclopentane and/or cyclopentene by distillative separation of a partially hydrogenated pre-benzene fraction or a partially hydrogenated C
5
fraction, which is characterized in that
a) in a first fractional distillation of the partially hydrogenated pre-benzene fraction or the partially hydrogenated C
5
fraction low boilers having boiling points below that of cyclopentene are taken off as first top product and high boilers comprising cyclopentene and hydrocarbons having boiling points higher than that of cyclopentene are taken off as first bottom product and, either,
b1) the first bottom product is fed to a second fractional distillation and there cyclopentene is taken off as second top product and cyclopentane together with hydrocarbons having boiling points higher than that of cyclopentane is taken off as second bottom product and this second bottom product is subjected to a third fractional distillation where cyclopentane is isolated as third top product and hydrocarbons having boiling points higher than that of cyclopentane are separated off as third bottom product, or
b2) the first bottom product is subjected to a catalytic hydrogenation and is subsequently separated in a fractional distillation into cyclopentane as top product and hydrocarbons having boiling points higher than that of cyclopentane as bottom product, or
b3) the first bottom product is subjected to a fractional distillation where the hydrocarbons having boiling points higher than that of cyclopentane are separated off as bottom product and the top product is subjected to a catalytic hydrogenation to give cyclopentane.
The typical compositions of the partially hydrogenated pre-benzene fraction or the partially hydrogenated C
5
fraction correspond to those given above.
In the process of the invention, low boilers having boiling points below that of cyclopentene are separated off as first top product in a first frac

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