Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
1220757
Separation of a C4-hydrocarbon mixture by
extractive distillation
.
The present invention relates to a process for the
separation, by extractive distillation with the aid of a
selective solvent, of a c4-hYdrocarbon mixture which con-
tains hydrocarbons which are more readily soluble in the
selective solvent, and those which are more sparingly
soluble in the said solvent.
Extractive distillation is a conventionaL method
for separating mixtures which cannot readily be separated
by conventional fractional distillation, for example where
the components to be separated form an azeotrope or have
small differences in the relative volatilities. In extrac-
tive distillation, a relatively sparingly volatile solvent
is introduced into the distillation column in an amount
such that the differences in the relative volatilities
of the components to be separated are increased, and
- separation by distillation therefore becomes possible.
Typical examples of the use of extractive distillation can
be found in, for example, C.S. Robinson et al., Elements
of Fractional Distillation, 4th edition, McGraw-Hill Book
Company, Inc., New York (1959), page Z91.
The separation of a C4-hydrocarbon mixture by
extractive distillation with the use of a selective solvent
in order to recover buta-1,3-diene has been disclosed, for
example in German Published Application DAS 1,568,902
and German Patent 1,163,795.
In the conventional processes, a relatively high
concentration, based on the C4 hydrocarbon/selective
solvent mixture formed in the extractive distillation
zone, of the selective solvent is maintained in the
extractive distillation zone in order to effect selec-
tive separation of the C4-hydrocarbons. However, this
relatively high concentration of the selective solve~nt
necessitates the circulation of large amounts of sol-
vent and therefore results in the consumption of large
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amounts of valuable energy.
The present invention is intended to improve the pro-
cedure and cost-efficiency of the conventional processes.
It is an object of the present invention to pro-
vide a process for the separation of a c4-hYdrocarbon
mixture with the aid of a selective solvent, wherein
both the complexity of the apparatus and hence the capi-
tal costs, and the amount of selective solvent circu-
lated and hence the energy consumption, can be reduced.
We have found that these and other objects and
advantages are achieved, in accordance with the inven-
tion, by a process for the separation, with the aid of
a selective solvent, of a C4-hydrocarbon mixture which
contains hydrocarbons which are more readily soluble in
the selective solvent and those which are more sparingly
soluble in the said solvent, wherein the C4-hydrocarbon
mixture is separated, in an extractive distillation
zone, into a top product (distillate), which contains
the more sparingly soluble hydrocarbons, and an extract
which contains the more readily soluble hydrocarbons and
the selective solvent, the extract taken off as the
bottom product is fed to a solvent recovery zone, in
which the extract is separated into a product which con-
tains the hydrocarbons, and the selective solvent which
has been partially or completely freed from the hydro-
carbons, and the concentration, based on the C4-hydro-
carbon/selective solvent mixture formed in the extractive
distillation zone, of the selective solvent in the said
zone falls below 75% by weight on one or more trays or, in
the case of a packed extractive distillation column, at
one or more points within the packing.
In the novel process, the amount of selective
solvent circulated can be reduced while maintaining the
same separation efficiency as in the conventional pro-
cesses operating with higher solvent concentration.This is surprising since the selectivity which is essen-
tial for the separation decreases when the solvent
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concentration in the extractive distillation zone is
reduced. Reducing the amount of solvent circulated per-
mits smaller apparatuses to be used, so that the capital
costs are correspondingly reduced. Furthermore, in the
case of the separation of butane and butene, the heat
losses which result when the recycled selective solvent
is recooled to the temperature at which the solvent
enters the extractive distillation can be kept small if
the heat remaining in the recycled solvent after heat
exchange with laden solvent is used profitably for the
separation in a distillation zone.
The process of the present application is
generally applicable to the separation of C4-hydrocarbon
mixtures which contain various unsaturated compounds.
In such systems, the more highly saturated compound is
the comPonent which is the more sparingly so~uble in the
selective solvent, and the less saturated compound is
the component which is more readily soluble in the said
solvent. In the case of isomers, for example an acetylenic
- 20 compound and a diolefin, the acetylenic compound is more
readily soluble than the diolefin.
For e%ample, the process according to the inven-
tion is advantageously used for the separation of a buta-
1,3-diene-containing C4-hydrocarbon mixture.
Such C4-hydrocarbon mixtures are obtained as,
for example, C4-fractions in the preparation of ethy-
lene and/or propylene by thermal cleavage of a petroleum
fraction, for example liquefied petroleum gas ~LPG),
naphtha, gas oil or the like. C5-fractions of this type
are also obtained in the catalytic dehydrogenation of
n-butane and/or n-butene. The C4 fractions contain,
as a rule, butanes, n-butene, isobutene, buta-1,3-diene,
vinylacetylene, ethylacetylene and buta-1,2-diene, with
or without small amounts of C5-hydrocarbons, the buta-
1,3-diene content being in general from 10 to 80, pre-
ferably from 20 to 70, in particular from 30 to ~0, X by
weight, while the total content of vinylacetylenes,
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ethylacetylene and buta-1,2-diene in the C4 fractions
generally does not exceed 5% by weight. The novel
extractive distilLation of these C4 fractions generally
gives the saturated and monoolefinically unsaturated C4-
hydrocarbons, such as butanes, n-butene and isobutene,
as the toP product of the extractive distillation zone,
and buta-1,3-diene together with other hydrocarbons
which are more readily soluble in the selective solvent,
such as vinylacetylene, ethylacetylene and bu~a-1,2-
diene, as the product of the solvent recovery solvent,This butadiene obtained as a product of the solvent
recovery zone is as a rule subjected to further purifi-
cation operations in order to obtain very pure butadiene.
Other suitable buta-1,3-diene-containing C4-
hydrocarbon mixtures which are advantageously separated
using the process according to the invention are crude
butadienes which in generaL contain not less than 90,
preferably not less than 95, and in particular not less
than 98, X by weight of buta-1,3-diene together ~ith, as
impurities, higher acetyLenes, such as vinylacetyLene and
ethylacetylene, and higher allenes, such as buta-1,~-diene.
Such crude butadienes are obtained, for example~ by
extractive distillation, eg. with the novel process, of
C4 fractions resulting from the thermal cleavage of
petroleum fractions or from the catalytic dehydrogenation
of n-butane and/or n-butene. In the separation of such a
crude butadiene by the novel process, bu~a-1,3-diene,
being a hydrocarbon which is more sparingly soluble in the
selective solvent, is obtained as the top product of the
extractive distillation zone, and the higher acetylenes and
some or all of the higher allenes, being hydrocarbons ~hich
are more readily soluble in the selective solvent, are
obtained as a product of the solvent recovery zone.
Other C4-hydrocarbon mixtures which are suitable
as starting C4-hydrocarbon mixtures for the novel pro-
cess are, for example, mixtures containing butanes, n-
butenes and isobutene, as obtained, for example, as a
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disti llate (raffinate) from a butadiene extraction
plant, mixtures containing butanes and n-butenes, as
obtained, for example, after separation of isobutene
from the raffinate described above, and mixtures con-
taining butanes and but-2-ene, as obtained, for example,
from plants for the dimerization of n-butenes. The
separation of the C4-hydrocarbons into the distillate,
containing the more sparingly soluble hydrocarbons, and
a product containing the more readily soluble hydrocar-
bons can be used not only to separate buta-1,3-diene
from the butenes and from the acetylenes, as described
above, but also, for example, to separate butane from
the but-1-enes, from the but-Z-enes or frôm buta-1,3-
diene.
Examples of suitable selective solvents for the
novel process are butyrolactone, nitriles, such as
acetonitrile, propionitrile or methoxypropionitrile,
ketones, such as acetone, furfurol, N-alkyl-substituted
_ lower aliphatic acid amides, such as dimethylformamide,
diethylformamide, dimethylacetamide, diethylacetamide or
N-formyLmorpholine, and N-alkyl-substituted cyclic acid
amides (lactams), such as N-alkylpyrrolidones, in par-
ticular N-methylpyrrolidone. In general, N-alkyl-
substituted lower aliphatic acid amides or N-alkyl-
substituted cyclic acid amides are used. Particularlyadvanta~eously used solvents are dimethylformamide and,
in particular, N-methylpyrrolidone.
However, the selective solvent used can also be
a mixture of these solvents with one another, for example
N-methylpyrrolidone with acetonitrile, a mixture of
these solvents with cosolvents, such as water and/or a
tert.-butyl ether, eg. methyl tert.-butyl ether, ethyl
tert.-butyl ether, propyl tert.-butyl ether or n- or
isobutyl tert.-butyl ether.
An essential feature of the novel process is
that the concentration, based on the C4-hydrocarbon/
selective solvent mixture formed in the extractive dis-
tillation zone, of the selective solvent in the said
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zone, ie. in the zone between the solvent feed point and
the solvent recovery zone falls below 75% by weight on
one or more trays in the case of tray-containing extrac-
- tive distillation zones or, in the case of packed extrac-
tive distillation zones, at one or more points within
the packing. In general, the concentration of the
selective solvent is from 30 to 75, preferably from 35
to 65, in particular from 40 to 60, ~ by weight. The
çoncentration of the said solvent in the extractive dis-
tillation zone is established, for example, by varyingthe amount of hydrocarbon reflux at the top of the said
zone, by varying the pressure or the solvent feed tem-
perature or, where the solvent contains water, also by
changing the water content of the solvent.
The distillate obtained at the top of the extrac-
tive distillation zone is advantageously taken off at
the top or as a sidestream. To remove small amounts of
selective solvent present in the distillate, the latter
_ is advantageously washed with water, or fed counter-
current to a liquid hydrocarbon reflux in a distillation
zone, the solvent being retained as a result.
The absolute pressure in the extractive dis-
tillation zone and the water-wash zone or the distilla-
tion zone is in general from 1.5 to 9, preferably from
2 to 8, in particular from 3 to 7, bar.
The temperature in the extractive distillation
zone is dependent on the number of trays or the height of
the packing, and on the pressure. In general, it is from
2û to 80C, preferably from 40 to 70C, at the point
at which the solvent concentration is lowest.
The pressure in the solvent recovery zone can be
lower than that in the extractive distillation zone. In
this procedure, the pressure in the solvent recovery
zone is in general from 1.2 to 3 bar. However, it may
also be advantageous, particularly in the separation of
a C4-hydrocarbon mixture containing butanes and butenes,
if the process is carried out in such a way that the top
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of the extractive distillation zone is connected to a
distillation zone, and the pressure in the solvent
recovery zone is not less than that in the extractive
distillation zone and the distillation zone, the pressure
in these zones being no lower than that required for the
condensation temperature of the top product of the dis-
tillation zone to be 30C or h;gher. The separation of
the C4-hydrocarbon mixture containing butanes and butenes
is preferably carried out using the distillation zone
in addition to the extractive distillation zone. It may
furthermore be advantageous if only some of the selective
solvent freed from the hydrocarbons is removed from the
solvent recovery zone and then recycled to the extrac-
tive distillation zone.
The extractive distillation can be carried out
in a column. In the case of a large number of trays, eg.
more than 1~0 trays, it may be advantageous to carry out
the extractive distillation in more than one column, in
general in two columns. Where two columns are used, it
is advantageous if the absorption stage above the point
at which the C4-hydrocarbon mixture is fed into the
extractive distillation zone is located in the first
column, and the concentration stage below the feed point
of the hydrocarbon mixture is located in the second
column, ie. the feed point of the hydrocarbon mixture is
at the top of the second column or, preferably, at the
bottom of the first column. It is preferable if there
is no compression stage between the absorption stage and
the concentration stage, but instead pressure conditions
as arise automatically in the extractive distillation
zone in the absence of compression and/or pressure-
reduction stages in the said zone are maintained in this
zone, so that the pressure at the bottom of the extrac-
tive distillation zone corresponds to that at the top of
the said zone, taking into account the usual pressure
loss in the columns. As a rule, the pressure difference
between the top and the bottom of the extractive
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distillation zone is from 0.1 to 3, preferably from 0.2
to 2, bar.
The extract taken off as a bottom product from
the extractive distillation zone is fed to a solvènt
recovery zone, in which the extract is separated into
a hydrocarbon-containing product and the selective sol-
vent which is partially or completely freed from the
hydrocarbons. The solvent recovery zone can be operated
as, for example, a devolatilizer or a solvent stripper,
or a combination of these.
In general, the solvent recovery zone is supplied
with heat, for example via a reboiler. The selective
solvent which has been partially or completely freed
from the hydrocarbons and is obtained as the bottom pro-
duct of this zone is ~dvantageously recycled to the
extractive distillation zone.
The Example which follows illustrates the inven-
tion.
EXAMPLE
- An extractive distillation (cf. Figure) was
operated with N-methylpyrrolidone as the selective sol-
vent, this being fed in an amount of 3 kg/h via line 10
to the extractive distillation zone 2 equipped with
trays. The C4-hydrocarbon mixture employed, which was
fed in an amount of 1 kg/h via line 1 to the extractive
distillation zone 2, was derived from a butene dimeriza-
tion plant and had the following composition:
C3-hydrocarbons 0.76% by weight
butane 47.68% by weight
3û isobutane 11.57% by weight
isobutene 0.83% by weight
but-1-ene 1.18X by weight
cis-but-2-ene 10.54% by weight and
trans-but-2-ene 27.44% by weight.
In the distillation zone 4, traces of solvent
which were present in the distillate obtained at the top
of the extractive distillation zone were retained. At
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the same time, the distillation zone served to effect
further distillative separation of the distillate
obtained at the top of the extractive distillation zone,
with the result that the concentration of cis but-2-ene
in the distillate was reduced and this compound was
retained. The concentration of the solvent on the trays
of the extractive distillation was adjusted by varying
the amount of hydrocarbon reflux. For this purpose,
some of the raffinate taken off via line 5 and condensed
was recycled via line 6. Butane raffinate was removed
via line 7, while a stream containing butenes was taken
off via line 8. The selective solvent removed at the
bottom of the solvent recovery zone 3 was subjected to
heat exchange to adjust the temperature and then re-
cycled via lines 9 and 10 to the top of the extractivedistillation zone.
At a hydrocarbon reflux of 1 kg/h, the solvent
concentration fell below 51X by weight on several trays.
The stream taken off via line 8 contained 75X by weight
of butenes and 25% by weight of butanes and therefore
had a composition corresponding to that of a starting
mixture for the n-butene dimerization plant. The top
product removed via line 7 contained 75% by weight of
butanes. If the hydrocarbon reflux was increased to
just 2 kg/h, the solvent concentration fell below 47% by
weight on several trays, and at the same total butene
content of the butene stream 8, ie. 75X by weight, the
butane content of the top product obtained via line 7
increased to as much as 82X by weight.
Surprisingly, it was not necessary to reduce
the pressure of the solvent recovery zone 3, at 4.75 bar,
with respect to the pressure of the extractive distilla-
tion zone 2 and the distillation zone 4 in order to
obtain sufficiently pure products. Hence, compression
of the hydrocarbons stripped off in the solvent recovery
zone was not necessary. It was 'possible to condense the
top product with cooling water, a coolant being
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unnecessary.
COMPARATIVE EXAMPLE
When the experiment was carried out as described
above, with the exception that a minimum solvent conc-en-
tration of 75.5% by weight was established in theextractive distillation zone by adjusting the hydro-
carbon reflux to 0.18 kg/h and increasing the tempera-
- ture of the selective solvent, the top product contained
only 65% by weight of butane, ie. in the top product,
about 3/4 of the butenes present in the C4-hydrocarbon
mixture used were lost, so that only about 1/4 of the
butenes were obtained ;n the butene product; conse-
quently, th;s procedure ;s of no further interest from
- an economic point of view.
