Note: Descriptions are shown in the official language in which they were submitted.
O. Z .noso/367~0
Recovery of a conjugated diolefin and/or an olefin
from a C4- or C5-hydrocarbon mixture
The present invention relates to a process for
recovering a conjugated diolefin andtor an olefin or ole-
f;n mixture from a C4- or C5-hydrocarbon m;xture containing
the conjugated diolefin andtor the olefin or olefins, by
extractive distilla~ion using a selective solvent.
Extractive distillation is a well-known method for
separating mixtures which cannot readily be separated by
conventional fraction3l distillation, for example where
the components to be separated form an azeotrope or differ
only slightly in their relative volatilities. In extrac-
tive distillation, a relatively sparingly volatile
solvent is introduced into the distillation column in an
amount such that the differences between the relative
volatilities of the components to be separated are
;ncreased, and separation by distillat;on is therefore
made possible. Ty?;cal examples of the appl;cation of
extractive distillat;on are described in, for example,
C.S. Robinson et al, Elements of Fractional D;st;llation,
4th Edition, McGraw-Hill Book Company, Incn, New York
(1959), page 291.
It has been disclosed, for example ;n German
Published Application DAS 1,568,902, German Patent
- 25 1,163,795 or The Soviet Chemical Industry, No.11,
November 1971, pages 719-7~3, that conjugated d;olefins
can be recovered from a C4- or C~-hydrocarbon mixture by
extractive distillation, using a selective solvent or
solvent mixture. The selective solvent can be used in a
substant;ally anhydrous form. Furthermore, in order to
increase the selectivity and, if required, to lower the
boiling po;nt, water may be mixed with the selective sol-
vent. The use of two extractive distillat;on stages in
succession has proven particularly useful for the recovery
of a conjugated diolefin -from a C4- or C5-hydrocarbon
,~
- 2 - O.Z.OOS0/36780
mixture.
For example, ;n the first stage of such a two-stage
extractive distillation~ a distillate (raffinate) contain-
ing the less solub~e hydrocarbons~ and an extract con-
taining the conjugated diolefin, the more soluble hydro-
carbons and the selective solvent, are separated off.
This extract is freed from the selective solvent, a mix-
ture of the conjuga~ed d;olefin and the more soluble
hydrocarbons being obtained. This mixture is subjected
to a second extractive distillation with the selective
solvent~ and the conjugated diolefin is obtained as a dis-
tillate, and an extract containing the more soluble
hydrocarbons and the selective solvent is also obtained.
The result;ng extract is then freed from the seLective
solvent to give a h-ydrocarbon stream containing the more
soluble hydrccarbons. Howeverr this process is not satis-
factory in every respect.
The pr sent invention is intended to improve the
procedure and cost-efficiency of the conventional pro-
cesses.
It is an object of the present invention to pro-
vide a process for recovering a conjugated diolefin and/or
an olefin or olefin mixture from a C4- or C5-hydrocarbon
mixture containing the conjugated diolefin and/or the ole-
fin or olefins, in which both the complexity of the appara-
tus and hence the capital costs and the amount of selec-
tive so~vent circulated and hence the energy consumption
can be reduced.
We have found that this and other objec~s and
advantages are achieved, in accordance with the invention,
by a process for recovering a conjugated diolefin and/or
an olefin or olefin mixture from a C4- or C5-hydrocarbon
mixture which contains the conjugated diolefin and/or the
olefin or olefins, hydrocarbons which are more sparingly
soluble in the selective solvent than are the said diolefin
and/or olefin or olefins, and hydroGarbons which are more
readily soluble in the said solvent than these are, in
- 3 O~z.0050/36780
which the C4- or C5-hydrocarbon m;xture is separated, by
means o-f extractive distillation, into a d;stillate con
taining ti1e more sparingly soluble hydrocarbonsO a product
stream containing the conjugated diolefin and/or the ole~
fin or olefin mixture, and a stream containing the more
readily soluble hydrocarbons, wherein the extractive dis-
tillation is carried out using
- a distillate take-off zone tseparation zone C),
- a feed zone for the starting C4- or C5-hydrocarbon
mixture tseparation zone A),
- a product take-off zone for the product stream ~separa-
tion ~one B) and
- a stripping zone in which the selective solvent is
recovered and from which the stream containing the more
readily soluble hydrocarbons is taken off tseparation
zone D), and
a) the starting C4- or C5-hydrocarbon mixture is fed to
separation zone A,
b) the selective solvent to be fed to the extractive dis-
tillation 1s introduced into the upper part of separa~
t;on zone C,
c) some of the laden selective solvent obtained in the
lower part of separation zone C ;s fed to the upper
part of separation zone A and the rema;nder ;s fed to
the upper part of separation zone a,
d) the laden select;ve solvent obtained in the lower part
of separation zone A~ and that obta;nPd in the lower
part of separation zone B, are fed to separat;on zone
D, from ~hich the stream containing the more readily
soluble hydrocarbons, and a stream of the selective
solvent wh1ch has been partially or completely freed
from the hydrocarbons, are removed,
e) a product stream containing the conjugated diolefin
and/or the olefin or olef;n mixture is taken off from
separat;on zone a, and
f) the d;stillate is removed from separation zone CO
In the novel process, under otherwise identical
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- 4 - ~.z.ooso/36780
cond;tions, the amount of selective soLvent circulated
can be greatly reduced. The diameter of the separation
columns can therefore be decreased, so that the capital
costs are correspondingly lower. At the same time, it is
also possible for the steam and electricity requirements
to be substantially reduced, so that~ in addition to th~
reduction in the cap;tal costs, there is at the same time
a subs~antial reduction in the energy consumpt;on.
Examples of hydrocarbon mixtures ~h;ch contain con~
1û jugated diolefins and can be used as starting mixture for
the process of the present inven~ion are C4- or C5 ~ fractions
obtained by thermal crack;ng a petroleu0 fraction teg.
LPG, naphtha, etc.), butadiene-containing fract;ons
obtained by dehydrogenating n-butane and/or n-butene, and
isoprene-containing fractions obtained by dehydrogenating
isopentane and/or isoamylene. In general, the C4-hydro-
carbon mixture contains buta-1,3-diene as the conjugated
dioLefin, together with butanes~ n-butenes, isobutene~
vinylacetylene~ ethylacetylene and buta-1,Z diene, with or
without small amoun~s of C3- and/or C5-hydrocarbons. The C5-
hydrocarbon mixture contains, as a rule, isoprene, trans-
and cis-penta-1,3-diene, penta-1,4-diene and cyclopenta-
diene as conjugated diolefins, together with pentanes, n-
pen~enes, isoamylene, cyclopentane, cyclopentene and
higher acetylenes.
In the case of these hydrocarbon m;xtures conta;n-
;ng conjugated diolsfins~ it is possible to use the novel
process not only for the recovery of the conjugated diole-
fin but also for the simultaneous recovery of an olefin
or an olefin mixture. auta-1,3-diene and at the same time
n-butene or n-butene mixtures can be particularly advan
tageously recovered from the C4-hydrocarbon mixtures.
Examples of sui~able selective solvents for the
novel process are butyrolactone, nitriles, such as aceto-
nitrile, propionitrile or methoxypropionitrile, ketones,such as acetone, furfurol~ N-alkyl substituted lower ali-
phatic acid amides, such as dimethylformamide~
~2~
- 5 - o.z.oo50/367~0
diethylfozmamide, dimethylace-tamide, diethylacetamide and
N-formylmorpholine, and N-alkyl-substitu-ted cyclic acid
amides (lactams), such as N-alkylpyrrolidones, in particu-
lar N-methylpyrrolidoneO In general, N-alkyl-substituted
lower aliphatic acid amides or N-alkyl-substituted cyclic
acid amides are used. Particularly advantageously used
solvents are dimethylformamide and especially N-methyl-
pyrrolidone.
E~owever, it is also possible to use mixtures of
these solvents with one another, for example of N-methyl-
pyrrolidone with acetonitrile, mixtures of these solvents
with water and/or with tert.-butyl ethers, eg. methyl tert.-
butyl ether, ethyl tert.-butyl ether, propyl tert~-butyl
ether or n- or isobutyl tert.-butyl ether, or with tetra-
hydrofuran.
In the accompanying drawings:
Figure 1 is a general flow chart of the
extractive distillation process according to the invention;
Figures 2 to 5 are flow charts of possible
designs for reducing into practise the process according to
the invention;
Figure 6 is a flow chart of another possible
design allowing simultaneous recovery of a further product
such as, for example, ~utenes.
As aforesaid, figure 1 is a general diagram
illustrating the extractive distillation process according
to the inventionO In this Figure 1, Ll to L8 indicate the
path of the selective solvent or of this solvent laden with
hydrocarbons, and Gl to G6 indicate the flow of the
hydrocarbons between and within the separation zonesO F is
the feed for the starting hydrocarbon mixture, R is the
take-off for the distillate, P is the take-off for the
product stream and N is the take off for the stream
3L~ 7~a~3
- 5a -
containing the more readily soluble hydrocarbons.
In the process according to the invention,
the selective solvent is generally fed into the upper
half, preferably into the upper third, and is advantageously
fed in at a point below the take-off Eor the distillate R.
The solvent fed in can be pre-laden with hydrocarbons, but
preferably is free of hydrocarbons or contains only a
small residual amount. The selective solvent which is
obtained from stripping zone D and has been partially or
completely freed from hydrocarbons is preferably fed to
separation zone C. Some of the laden selective solvent
taken off in the lower part, in general in the lower half,
preferably in the lower third, for example at the bottom,
g 3
- 6 - O.Z.0050/367~
of separation zone C ;s fed (v;a L2) into the upper part,
-in general the upper half, preferably the upper third, in
part-icular the upper fourth, of separation zone A, and the
remainder is fed (via L3) into ~he upper part~ in general
the upper half~ preferably the upper third, in particular
the upper fourth, of separation zone ~, preferably above
the po;nt at which the startin3 C4- or C5-hydrocarbon
mixture is fed into separation zone A, or above the point
at which the product stream is taken off from separation
zone B. The ratio in which the Laden solvent taken off
from separation zone C is distributed between separation
zones A and B is in general from 1:50 to 50:1, preferably
from 1:10 to 10:1, in par~icular from 5:1 to 1:5.
The hydrocarbons which rise in separation zones A
and B~ are fed countercurrent to the selective solvent
and are taken off in the upper part of separation zones A
and ~ v;a G1 and G2 are advantageously fed to the lower
part~ in general the lower haLf~ preferably the lower
third, in particuLar the lower fourth, of separation zone
C. The number of separation stages in separation 7ones A
and B can be ;dentical or different. The starting C4- or
C5-hydrocarbon mixture being separated is fed to separation
zone A (via F), advantageously in a middle region, pre-
ferably in a region extending between the upper fifth and
the lower fif~h, in particular between the upper fourth
and the lower fourth, of separa~ion 70ne A. The m;xture
can be introduced in gaseous or liquid form, a gaseous
feed being preferred. It may be advantageous if there is
- one further hydrocarbon feed, or a plurality of further
hydrocarbon feeds, into separation zones A~ B and C
and/or D. In general~ the further hydrocarbon f~eds are
hydrocarbon m;xtures which are to be separated and which
differ in composition from the starting C4~ or C5-hydro-
carbon mixture, and for which, as a rule, the feed point
chosen differs from that for the starting C~- or C5-
hydrocarbon mixture. If the further hydrocarbon mixture
contains relativeLy large amounts of hydrocarbons which
12174~9
- 7 - o.z.ooso/36780
are more read;ly soluble than the product as well as those
which are more sparingly soluble than the latter, the fur~
ther hydrocarbon mixture ;s in general preferabLy fed into
separation zone A. For further hydrocarbon feeds which,
in addition to containing the product hydrocarbon~ essen-
tially contain only hydrocarbons which are more sparingly
soluble than the product hydrocarbon, the feed point used
;s, as a rule, in separat;on zone B above the product take
off~ in separat;on zone C or in separat;on zone A. For
further hydrocarbon feeds which, in addition to containing
the product hydrocarbon, essentially contain only hydro-
carbons whirh are more readily soluble than the product hydro-
carbon~ the feed point chosen is, as a rule, in separation
zone 8 below the product take-off, in separation zone A or
in separation zone D~ However, it may also be advantageous
if the further hydrocarbon feeds are ;ntroduced at po;nts
other than the preferred ones described above, for example
in order to improve the stripping process, or if a dis-
t;llative separation of the secondary components passing
from this feed into the product is poss;ble and is
intended, or ;f the secondary components wh;ch pass in-to
t-he produc~ do not present problems dur;ng processing.
The product stream containing the conjugated
diolef;n and/or the olef;n or ole~F;n m;xture is taken off
from separat;on zone 8 (via P~, advan~ageously from a
middle region, preferably from a region which extends
between the upper fifth and the Lower fifth, in particular
between the upper fourth and the lower fourth, of separa-
tion zone B. It can be taken off in gaseous or liquid
form, advantageously in solution in the solvent, but is
preferably removed in gaseous form. It may be advan-
tageous to remove one further product stream or a plurality
of further product streams from separation zones A, ~, C
and/or D. In general, the further product streams are
hydrocarbon mixtures which differ in composition from the
first produc~ stream and whose take-off points differ from
the take-off point of the first product stream. For
~ Z i ~
~ ~ - O.Z.OUS0/367~0
example~ in the recovery of buta-1,3-diene from a C4-
hydrocarbon m;xture, it ~ay be advantageous, in addition
to the buta 1,3-diene product, to remove cis-but-2-ene
wh;ch has a low content of butane, buta-1,3-diene and
acetylene or is free of these~ or a butene mixture, as a
further product from separation stage B. Such butene mix-
tures can advantageously be employed, for example, for the
recovery of pure butenes.
The laden selective solvent taken off in the
lower part, ;n general in the lower half, preferably in
the lower third, for example at the bottom, of separation
zone A, and that taken off in the lower part, ;n general
;n the lower half, preferably ;n the lower third9 for
example at the bottom, of separatîon zone B, are fed (via
L6 and L7) to separation zone D, in wh;ch the selective
solvent contain;ng the more read;ly soluble hydrocarbons
;s part;ally or completely freed from the hydrocarbons.
The streams of laden solvent taken off from separat;on
zones A and 9 can first be combined and then fed to
separat;on zone D at a s;ngle po;nt, or they can be fed
to the said separat;on zone separately and at d;fferent
po;nts~ The solvent streams are advantageously fed ;nto
the upper part of separat;on zone D, preferably ;n~o the
upper half, ;n part;cular into the upper th;rd~ for exam-
ple to the top, of separation zone D. The selective sol-
vent wh;ch has been part;ally or compLetely freed from the
hydrocarbons is advantageously taken off from the lower
part of separat;on zone D, preferably from the lower haLf~
in part;cular from the lower third~ for example at the
bottom, of separat;on zone D. The select;ve solvent
removed ;s advantageously recycled to separation zone C.
The str;pp;ng off or voLatil;zat;on of the hydrocarbons
from the laden solvent in separat;on zone D is carr;ed
out in general by heating, for example via a reboiler,
advantageously ;n the lo~er part, preferably in the lower
third, in particular at the bottom, of separat;on zone D.
Th;s procedure not only frees the solvent from the
- 9 - O.Z.0050/36780
hydrocarbons but produces some or all of th~ r;sing hydro-
carbon vapors which are required in the separation zones
to separate the hydrocarbons. At the same time, the
stream containing tha more readily soluble hydrocarbons
is removed from separation zone D~ This stream is taken
off in general at a point above that at which the par-
t;ally or completely hydrocarbon-free solvent is removed,
and below the point at which the laden solvent is fed to
separation zone D, in general in a middle region wh;ch
extends between the lower fifth and the upper fifth, pre-
ferably between the lower fourth and the upper fourth, of
separation zone D. One or more expansion and compression
steps may be carr;ed out ~;thin separat;on zone D.
The hydrocarbons obta;ned from the upper part of
separat;on zone D, ;n general from the upper half, pre-
ferably from the upper th;rd, in particular from the upper
fourth, for example at the top, are advantageously dis-
tributed between separation zones A and B~ in general ;n
a ratio of from 1:50 to 50:1, preferably from 1:10 to
10:1, ;n par.;cular from 1:5 to 5:1, and are fed to these
separation zones advantageously in the lo~er part, prefer-
ably ;n the lower half, in particular ;n the lowe~ third,
for example at the bottom. In dividing the hydrocarbons
obta;ned from the upper part of separation zone D between
ZS separation zones A and 8, i~ is unimportant in practice
~hether the hydrocarbons are removed together from separa-
tion zone D and then d;vided, or are taken off directly as
two sep3rate streams. The same appl;es to the distribu-
tion, between separation zones A and B, of the stream of
laden solvent taken off from separation zone C.
The hydrocarbons obtained from the upper parts of
separat;on zones A and B, ;n general from the upper halves,
preferably from the upper th;rds, in part;cular from the
upper fourths, for example from the top, are advantageously
partly or completely fed to the lower part of separation
zone C, ;n general the lo~er half, preferably the lower
third, ;n particular the lower fourth. The hydrocarbons
~2~L7~
- 10 - O.Z.0050/367~0
can be first comb;ned and then fed ;nto separation zone C
at a s;ngle point, or they can be fed to separation zone D
separately and at different points.
In addition to the heating at the bottom of separa~
5 tion zone D descr;bed above, ;t ;s also possible to supply
heat to separation zones A, P, C and/or D. This additional
heat is preferably supplied at points below the feed of
the hydrocarbons or below the product take-off. Such
heating can be effected by~ for example, heat exchange
with the devolat;l;zed solvent.
It is also poss;ble to remove heat from separation
zones A, ~, C and/or 9. Preferably~ heat ;s removed from
separation zone A above the feed for the start;ng hydro-
carbon mixture, and from separation zone B or C. However,
it may be necessary to remove heat from separation zone D
too, for example to maintain a maximum temperature of a
compressor.
Separation zones A to D can be designed as separ-
ate columns, but it ;s also possible for parts of separa
tion zones, or one or more separation zones, to be con-
structed as a combined unit.
Figure 2 illustrates, by way of example, one of
the poss;ble des;gns~ In this figure, as in F;gures ~ to
6 belo~, the names of the streams and separat;on zones
Z5 have been taken from Figure 1, for the sake of clarity.
Separat;on zone C and separation zone A above the
feed F have been combined to form a single column, as have
separat;on zone A below the feed F and a part of separa-
tion zone D. Separation zone a is designed as a single
30 column~ wh;le separation zone D is distributed over two
columns and a gas separator.
A separation zone in which distillation takes
place is located at the top of separat;on zone C, in order
to retain the solvent and any other high-bo;ling com-
ponents. S;milar separation zones are also poss;ble forthe purificat;on of the product or products P and of the
byproduct N ~h;ch conta;ns the more read;ly soluble
~ O.Z.0050~367~0
hydrocarbons. However, it ;s also possibLe to employ for
this purpose gas/Liquid or liquid/liquid water washes,
partial condensers or distillation procedures~
In Figure 2, separation ~one B possesses a further
take-off point tP2), from which, for example ;n the
recovery of buta-1,3-diene from a C4 hydrocarbon mixture,
c;s-but-2-ene or a butene mixture can be taken off as a
further product, in addition to the buta-1,3-diene product
removed via line P1.
-Figure 3 shows another embodiment of the process
according to the invention.
It serves to illustrate that the product P need
not be removed as a s;de stream from one column, but can
be taken off as a side stream from separation zone 9,
which in this example is divided between two columns.
Figure 4 shows another embodiment of the process
according to the invention. Instead of a water wash or a
hydrocarbon wash, which`serve only to retain the solvent,
a d;st;llat;ve separation zone E is employed for puri-
fy;ng the product to a high degree~ When the novel pro-
cess is used for recovering buta-1,3-d;ene from a C4-
- --hydroca-rbon mixture, this procedure has the following
advantages:
1) Components ~hich are more readily solub~e than buta-
1,3-diene, eg. buta-1,~-d;ene and/or but-1-yne, as well
as those which are more sparingly soluble than buta-1,3-
diene, eg. the but-2-enes, are retained by separation zone
E. The concentrations of these in separation zone B can
therefore be higher than in the absence of the said zone
E. Since these are key components in the separation, it
;s poss;ble to reduce the amount of solvent circulated and
to save valuable energy. The wash section of the extrac-
tive dist;llation acts as the strippiny section of the
distillation with regard to but-2-ene, uhile the stripping
section of the extractive d;st;llat;on acts as the strip-
p;ng sect;on of the d;st;llat;on with regard to buta-1,2-
diene. Some or all of the hydrocarbon vapor which rises
4~
- 12 - O.Z.0050/367~0
;n separation zone B can be removed and fed into separation
zone E. The hydrocarbon vapor requ;red in the upper part
of separation zone 9 can be obta;ned by recycling from
separation zone E and/or by boiLing the solvent. It may
also be advantageous to remove a bleedstream from the
lower part of separation zone E and to feed this stream
into separation zones D and/or A below the feed, in order
to reduce the build-up of concentration.
2) The distillative separation zone E can be operated
~ith economicaL waste heat, for example with hot conden
sate or low~temperature steam, since the vaporization
takes place at a lo~ temperature.
3) Separation zone E simultaneously serves to retain the
selective solvent.
Where ~he distillative separation zone E is used,
its loca~ion, ie. whether it is, for example, located on
top of separation 8 or next to this~ is also not critical.
Other examples of embodiments are shown in Figures
5a and 5b. The parallel separation zones A and B are
located in one and the same column iacket, a vertical
separating p~ate being incorporated to separate the two
zones A and B from one another. If desired, the column
jacket may also contain further separation zones, eg. dis-
tillations~ which are likewise separated from the other
systems by means of a separating plate. For example,
Figure Sb shows a distillative separation zone E which is
contained within the column jacket~ together with separa-
tion zones A and B for extractive distillation~ Separa
t;on zone E in turn contains two separation zones which
run oarallel over part of their length.
Figures 6a and 6b show examples of embodiments for
the simultaneous recovery of a further product from the start-
ing C4- or C5-hydrocarbon mixture, for example the recovery
of not only buta-1,3-diene, as product P1, but also
butenes as product Pz, from a C4-hydrocarbon mixture.
Separat;on zone 8 is divided into ~wo part separation zones
B1 and B2, from which the produc~s P1 (eg. buta-1,3-diene)
~Z~7~
~ O.Z.0050/36780
and P2 (eg. a butene mixture) are removedO In addition to
the separation zones A, B, C and D, Figure 6a contains a
further separation zone G, in which the concentration of
the distillate R increases in an upward direction, and the
S concentrations of products P1 and P2 increase ;n a down-
ward direction.
The Example which follows iLlus~rates the invention.
EXAMPLE
In an experimental plant for the recovery of buta~
1,3-d;ene by means of extractive distillaeion, a C4~
hydrocarbon mixture was separated into a buta-1,3-diene
product~ a butane/butene mixture and a mixture containing
the C4-acetylenes, the separat;on being carried out
according to the flow diagram of Figure 3, and the selec-
tive solvent used being aqueous N-methylpyrrolidone con- --
taining water in an amount of 8X by weight, based on the
N-methylpyrrolidonetwater mixture. The C4-hydrocarbon mix~
ture employed had the following composition:
Butanes 4.1X by weight,
20 Butenes 49.3% by weight~
Buta-1,3-diene45~0% by weight,
C4-Acetylenes 0.9X by weight and
Other hydrocarb~ns 0.7X by weight.
00139 kg/hour of C4-hydrocarbon mixture was fed to
the experimenta~ p~ant via line F. 99.1~ pure buta-1,3-
diene ~as taken off-via line P. The butane/butene mixture
~as obtained as a distillate via line R, and the mixture
containing the C4 acetylenes was taken off via line N.
1.46 kg/hour of sel~ective solvent was fed to the experi-
mental plant Vi3 line L1, the selective solvent fed in
being divided between separation zones A and B in a ratio
of Z.7:1.
When~ on the other hand, the separation of the C~~
hydrocarbon mixture into 99.1X pure buta-1,3-diene, a
butane/butene m;xture and the mixture containing the C4-
acetylenes was carried out in a conventionally arranged
two stage ex~ractive distillation, it was necessary to feed
~ 14 ~ OoZ~0050/3678U
a total of 1.66 kg~hour of the selective solvent to the
first and second stages of the extractive distillation in
order to achieve the same separation effect~ ie. the amount
of solvent required in the novel process to achieve the
S same purity was 12X lower than the total amount of solvent
required in the convent;onal arrangement. With the novel
process~ it was therefore possible to save a substantial
amount of energy compared ~ith the conventional processO
Dra~ing
