Note: Descriptions are shown in the official language in which they were submitted.
~2'~ 7
PROCESS FOR SEPARATING AN
ETHYLENICALLY UNSATURATED HYDROCARBON
FROM A.HYDROCARBON MIXTURE
This invention relates to an improved process
for the separation of an ethylenically unsaturated
hydrocarbon from a hydrocarbon mixture containing same.
Various hydrocaxbon mixtures are obtained
from the thermal cracking of petroleum products such
as naphtha, gas oil, light oil, crude oil, etc. A
typical hydrocarbon mixture from cracking operations
is pyrolysis gasoline which contains generally aro-
matic and cycloparaffinic compounds having 5-10
carbon atoms. A typical hy~rocarbon mixture obtained
following the removal of the hydrocarbons containing
5 carbon items is given in Table A:
.TABLE A.
Compound % bY Weight
15 Non-aromatics 16-18
Benzene 33-37
Toluene 16-20
Ethylbenzene 1-2
p/m Xylenes . 5-7
20 o-Xylene 2-3
~..... .
29,291~
.
- 2
Styrene 6-8
Dimethylcyclopentadiene <1
Cg Aromatics ~1
~-Methylstyrene <1
5 Vinyltoluene 2.5-3
Indene 2.5-3
Methyl indene <1
Naphthalene <1
Ehenylacetylene 0.1 or less
It should be understood that the above
hydrocarbon mixture is used to illustrate a typlcal
hydrocarbon mixture and is not presented to define
such mixture since such hydrocarbon mixtures may
vary greatly.
One of the commercially more valuable
components found in the above mixture is styrene.
An economical means or separating out this pre-
ferred component has been long sought after. The
removal of styrene from these mixtures by fractional
distillation is rendered unfeasible owing to the
pre~ence of other components in the hydrocarbon
mixture, especially o-xylene, which have boiling
points very close to that of styrene. Table ~
presents the boiling points of styrene and other
hydrocarbons.
TABLE B
Compound Boilin~ Point C
~-Methylstyrene 163.4
n-Propylbenzene 159.2
Cumene 152.4
Cyclooctane . 148.5
29,291-F -2-
3 ~Z~ 7
STYRENE 145.2
0-Xylene 1~4.4
3-Methyloctane 143-144
Cyclooctene 138-143
5 Phenylacetylene 142.4
4-Methyloctane 141-142
m-Xylene 139.1
p-Xylene 138.4
- Ethylbenzene 136.2
- 10 Toluene 110.6
- one of the more widely used processes for
utilizing this styrene involves hydrogenating the
styrene to ethylbenzene and thereafter separating
it from the xylenes by precision fractional dls-
tillation. Following this distillation, the ethyl-
benzene is then dehydrogenated to styrene and again
purified by another distillation. This method is
very complicated and expensive. The disadvantages
of the aforementioned method have promoted research
concerning the direct separation of styrene from
the hydrocarbon mixture without first converting
the styrene to ethylbenzene.
British Patent No. 1,038,606 proposes a
proc~ss utilizing an aqueous solution of a silver
salt such as AgNO3 to extract styrene from a hydro-
carbon mixture following treatment of the mixture
with fuller`'s earth to prevent slime formation.
This method has the disadvantage of being expensive
due to the use of silver salts.
29,291-F -3-
2~t7
Also, USP 3,328,267 proposes a process
consisting of extractive distillation for the
separation of styrene from o-xylene using a
di-lower-alkyl formamide such as dimethyl formamide
as the extractive distillation solvent. This pro-
cess also makes use of a polymerization inhibitor
such as quinone or hydroquinone or preferably
p-tert-butylpyrocatechol. However, the styrene
produced by this process has the undesirable char-
acteristic of being light yellow in color. Also,the polymerization inhibitor requires low tempera-
tures to be effective.
USP 3,580,839 (Fuerst) entitled, "Recovery
of Aromatic Hydrocarbons from Mixtures of ~ydrocarbons
by Selective Extraction with a Substituted Piperazine
Solvent", describes the use of piperazine derivatives
having the general formula:
X-N N-R
. \ CH2-CH2 /
where X denotes formyl or acetyl and R denotes lower
alkyl, as extractants for recovering mono- or di-
nuclear aromatic hydrocarbons which may have lower
(1~4 C) aliphatic hydrocarbon radical substituen-ts
from hydrocarbon mixtures. Thus, this process
proposes the separation of a hydrocarbon mixture
into two fractions; one fraction containing predom-
inantly aromatic hydrocarbons such as benzene,
toluene, xylene, ethylbenzene and styrene; and
another fraction having a residue predominate in
29,291-F -4-
--5--
paraffins, olefins or cycloparaffins. The pipera-
zine derivatives may be used in a pure form or
mixed with other extractants such as tetramethyl-
sulfone, diethylene glcyol or triethyl glycol.
N-formyl-N'-methyl piperazine is the preferred
extractant. This process is directed toward a
gross separation of products.
- In USP 3,684,665, Abe et al, proposes a
method for separating styrene from hydrocarbon mix-
tures containing xylenes which comprises extractively
distilling a hydrocarbon mixture with a suitable
solvent causing the styrene to be concentrated in
- the solvent from which it can then be removed.
Suitable solvents include dialkyl acetamides such
as dimethylacetamide, as well as dialkylsulfoxides,
alkylene carbonates, lactones, lactums, phenol,
alkylphenols, salicyclic acid, alkyl esters,
aniline, alkyl anilines, phthalic acid alkyl esters,
tetraalkyl ureas, N,N-dialkyl carbamic esters and
glycol monoalkyl ethers such as diethylene glycol
monoalkyl ether and N-methyl-pyrrolidone. Use of
a polymerization inhibitor such as hydroquinone,
tert-butylcatechol, phenothiozine, sulfur or mix-
tures thereof, is urged to prevent polymerization
of the styrene. This process, however, suffers
from the disadvantages of polymerization losses
especially at temperatures above 100C and the
production of styrene that is undesirably yellow
in color.
In USP 3,763,015,- Morimoto et al, present
another extractive distillation process utilizing
a polar organic solvent in the presenfe of a nitrile
29,291-F -5-
~2~ t7
polymerization inhibitor. E`ollowing extractive
distillation, the solvent containing styrene is
treated with nitric acid and then again distilled
to remove impurities and separate the styrene from
the solvent. Suitable solvents for this process
are given as diet~ylacetamide, ~-methylpropionitrile,
butyl lactone, N-methylpyrrolidone, dimethylformamide
and dimethylsulfoxide. The preferred polymeriæation
inhibitors are sodium nitrite or potassium nitrite
used iIl combination with a compound having at least
one nitro, nitroso, quinoide, phenolic or hydroxy
group in the molecule. The preferred additives
are p-tert-butyl-catechol, hydroquinone, p-benzo-
quinone, p~dinitrosobenzene, ~-nitro-~-naphthol,
o-nitrosonaphthol and ~-naphthoquinone. This
process produces a yellow styrene product requiring
further treatment with nitric acid to remove the
colored impurities. Also, yield losses through
polymerization are a problem.
In view of the aforementioned deficiencies
of the prior processes, it is highly desirable to
provide a process for effectively separating a mono-
vinylidene aromatic or other ethylenically unsaturated
hydrocarbon from hydrocarbon mixtures containing
same while coincidently inhibiting polymerization
to produce a substantiaily pure, color-free product.
The present invention provides a process
for separating an ethylenically unsaturated hydro-
carbon from a hydrocarbon mixture characterized by
(a) contacting aJhydrocarbon mixture containing
the unsaturated hydrocarbon with a soluble amine
containing more than one amine group and which (1)
29,291-F -6-
-7-
is sufficiently electronegative.to allow a selective
separation of the unsaturated hydrocarbon from the
hydrocarbon mix-ture, (2) inhibits polymerization
of the unsaturated hydrocarbon and (3) has a boiling
- 5 point sufficiently different from the unsaturated
hydrocarbon to allow separation of the amine there-
from following a selective separation of the unsatu
rated hydrocarbon from.the hydrocarbon mixture;
- (b) separating said amine/hydrocarbon mixture
into at least two fractions, one of which contains
amine and the unsaturated hydrocarbon.
Subse~uently the unsaturated hydrocarbon
can be removed from the amine fraction or processed
further without said removal.
By soluble amine (hereinafter termed amine)
is meant amine soluble in a styrene/hydrocarbon
mixture over the range of operating temperatures.
Surprisingly, in an extractive distilla~ion
process using an amine as described, an ethylenically
unsaturated hydrocarbon can effectively be separated
from a hydrocarbon mixture containing same without
the addition of other solvents and/or polymerization
inhibitors such as thos~ described by US Patent
Nos. 3,763,015 and 3,684,665. Using said process,
an unsaturated aliphatic hydrocarbon such as butadiene
or acetylene or monovinylidene aromatic such as
styrene or vinyltoluene having unexpectedly high
purity and/or desirable color can be obtained
without significant losses due to polymerization.
In a preferred embodiment of the present invention,
the amine is N-(aminoethyl)piperazine (AEP). In a
29,291-F -7-
-8- ~ 2~7
particularly preferred embodiment, AEP is employed
as the amine solvent or extractant in an extractive
distillation process for the separation of styrene
from a hydrocarbon mixture containing styrene. Due
to the removal of color bodies by the AEP, a styrene
having a commercially desirable color can be recovered
in purities often exceeding 99 percent.
~ Understanding of the present invention
is facilitated by reference to the accompanying
drawings, in which
.
Figure 1 is a schematic representation
depicting an embodiment of the present invention
which is particularly usual in the separation of
vinyltoluene from a mixture compxising vinyltoluene
and other C-9 aromatics, or the separation of acet-
ylene from a mixture containing butadiene and butene;
and
Figure 2 is a schematic representation
depicting an alternative and more preferred embodi-
ment of the present invention which embodim~nt isparticularly useful for separating styrene from a
hydrocarbon mixture comprised primariiy of hydro-
carbon having from 6 to 10 carbon atoms.
The process of the invention is best revealed
through a basic discussion of extractive distillation.
Extractive distillation refers to processes where a
high boiling material t often referred to as an
extractive distillation solvent, is added to change
the relative volatility of a hydrocarbon mixture
to be subsequently separated using extractive dis-
tillation techniques. In general, there are two
primary reasons for adding such a solvent.
29,291-F -8-
_9~ 7
The first reason is to alter the relative
volatility of two or more components in the feed
stream. Specifically, one or more components may
have similar vapor pressures and would distill off
together unless the relative volatilities of the
components are changed. For example the solvent
will change the relative volatilities of an unsatu-
- rated compound and also change the relative vola-
- tilities of a material having an acetylenic unsatu-
ration as compared to a material having an ethylenic
unsaturation.
The second reason for using the solvent
is to prevent azeotropic formation of the feed mix-
t~re components. The solvent added generally has
a boiling point much higher than the components
of the feed mixture thereby preventing azeotrope
formation.
~ Referring more particularly to the drawings,
Figure 1 schematically illustrates an embodi-
ment of the present invention wherein an ethylenicallyunsaturated hydrocarbon, e.g. vinyltoluene is separated
- from a hydrocarbon mixture containing same utilizing
an extractive distillation column. In the embodiment
illustrated, the hydrocarbon mixture containing the
~ 25 unsaturated hydrocarbon is supplied to an extractive
distillation column 101 by means of line 10. An
amine solvent, preferably N-aminoalkylpiperazine,
is added to the column 101 by means of line 11.
Since it is generally desirable to maintain a high
concentration of the amine throughout the extraction
distillatiQn column and the amine is generally less
.
29,291-F -9-
-10- ~2~ 7
volatile than the components of the hydrocarbon
feed, the amine i5 prefexably added to the column
above the point where the hydrocarbon feed enters
the column. It is also preferable to add the
solvent a sufficient distance below the top of
the distillation column to reduce the concentration
of the amine in the ascending vapors to a negligible
amount before the overhead product is withdrawn.
The vapor exiting from the top of column 101 is
passed to an overhead condenser 12 which condenses
the vapor. A portion of the condensed vapor is
returned to the column 101 as liquid overflow.
- A line 13 carries off the distillate, which distil-
late, in the separation of vinyltoluene from a
hydrocarbon mixture containing vinyltoluene and
~-methyl styrene, dicyclopentadiene or 1,2,4- or -
1,2,3-trimethyl styrene generally contains the
~-methyl styrene and/or dicyclopentadiene. The
extractant, i.e., the ethylenically unsaturated
hydrocarbon such as the vinyl'toluene, and high
boiling components (bottoms) of the hydrocarbon
feed, are removed from the bottom of the column 101
via a line 14. ~wing-to the relatively high boiling
point of the amine, the bottoms removed by line 14
generally contain a hiyh concentration of the amine.
The material removed through line 14 generally is
'passed through a~reboiler 15 which returns a portion
of these components as reboil vapor to the column 101.
The portion of the bottoms material not
returned to the column 101 enters a stripping
column 202. As illustrated in the embodiment shown
in Figure 1, additional amine is introduced into
the stripping column 202 via line 21 above the
29,291-F -10-
~L22~
entrance of the bottoms material to the column.
Although this further addition of amine is not
necessary for the process of the present inven--
tion, it is preferred, particularly when the
bottoms exiting from column 101 contain phenyl
acetylene or high boiling materials other than
the amine. The unsaturated hydrocarbon is passed
from column 202 as a vapor to a condensor 22 which
condenses the gaseous material. A portion of the
condensed material is returned to the column 202
and the remainder removed through line 23. The
material in line 23 is essentially pure, e.g., at
least 98 percent pure, preferably at least 99 per-
cent pure, and of excellent color.
The amine, with any impurities it may
contain, exits from the bottom of the stripping
column 202, and a portion of this material is
passed through a reboiler 24. Since the remaining
portion generally contains some high boiling
materials and impurities, e.g., polymeric materials
or the like, ~he amine is advantageously purified
prior to reuse. Such purification is generally
effected by means of a subseguent distillation.
In Figure 1, this purification is performed by
introducing the amine into a purification column
303, distilling off the more volatile impurities
via line 30 and returning the amine to both the
extractive distillation column and the stripping
column via line 11. Heavier or less volatile
impurities may be removed via line 31.
Figure 2 illustrates a preferred embodi-
ment of the present invention particularly useful
29,291-F -11-
~12~ 42~7
for the recovery of styrene from the hydrocarbon
products obtained in the cracking of petroleum
products such as naphtha, gas oil, light or crude
oil and the like. The preferred embodiment illu-
strated in Figure 2 includes:
a) a first ~tractive distillation column 111for separating styrene from close boiling li~uids
such as xylenes, particularly o-xylene, and ethyl-
benzene;
b) a solvent stripper column 222 for the
removal of styrene, high boiling materials such
as phenyl acetylene and high boiling impurities
from the,amine solvent;
c) a second extractive distillation column
333 for the separation of the high boiling materials
and impurities from the styrene;
d) a solvent recovery column 444 for the
removal of high boiling materials and impuri-ties
such as phenyl acetylene from the amine; and
e) a solvent purification columns 555, for
th~ removal of heavy impurities such as polymers
from the amine.
In the recovery of styrene from a hydro-
carbon mixture resulting from a cracking operation,
~ 25 a pre-distillation column (not shown) for removing
the light hydrocarbons, e.g., hydrocarbons generally
having 5 or less carbon atoms, and a predistillation
column (not shown) for removing the heavy hydro-
carbons, e.g., hydrocarbons generally having 11 or
more carbon atoms, are often advantageously employed
prior to the introduction of the hydrocarbons to
column 111. In such a case, the removal of the
29,291-F . -12-
-13 ~ Z ~ 7
light and heavy hydrocarbons is advantageously
effected in conditions productive o~ a hydrocarbon
fraction having a boiling point at atmospheric
pressure of from 120 to 160C, preferably from
125 to 155C.
In such an operation, the hydrocarbon
mixture containlng styrene is introduced into the
first extractive distillation column through line
2. An amine is introduced into this first extractive
distillation column 111 at a point above the point
where the hydrocarbon mixture is introduced into
the column lll. This amine-feed is also located
a few trays (either actual or theoretical) belcw
the top of the column ill to reduce the concentration
of the amine in the overhead product. The vapor
exiting from the top of column 111 is passed to
an overhead condenser which condenses the vapor
and returns a portion of it to the column. A
line 4 carries off a distillate for further pro-
cessing and/or subsequent use. Generally speaking,the distillate comprises primarily xylene and ethyl-
benzene. A portion of the material exiting from
the bottom of column 111, which generally comprises
the separated styrene, amine, phenyl acetylene and
higher boiling hydrocarbons and impurities, e.g.,
polymeric materials, is trans~erred to the solvent
stripping column 222 through line 5. The remaining
portion of this material passes through a reboiler
which returns the material to column 111.
In solvent stripper 222, the amine is
separated from the styrene. The separated amine is
passed from the btoom of column 222 and introduced near
29,291-F -13-
-14- ~22~
the top of the extractive distillation column 333.
The separated styrene, which generally contain~ phenyl
acetylene and other impurities, exits from the top of
column 222, is condensed and a portion of the condense~
material passed to column 333 through line 7 at a point
below the introduction of the amine. In this second
extractive distillation column 333, the styrene is
separated from the phenyl acetylene and high boiling
materials, with a relatively pure, preferably at least
99 percent pure, styrene exiting from the -top of column
333. The relatively pure material is condensed and
removed through line 8 for subsequent use. The amine
containing the phenyl acetylene and/or high boiling
materials exits from column 333 and a portion thereof
is transferred to the solvent recovery column 444
- through line 9. In the solvent recovery column 444,
the amine and phenyl acetylene, are separated. The
phenyl acetylene exits from the top of column 444, is
condensed and a portion thereof removed by means of
line 41 for subsequent use. The amine which now con-
tains the higher boiling impurities, e.g., the polymeric
materials, is removed from column 444 by means of line
42. A portion of this material is passed through a
reboiler and returned to the column 444. A second and,
generally, relatively large proportion of the material
is returned by means of line 43 to line 3 for reuse in
the first extractive distillation column 111. The
remainder of the amine and the impurities having the
higher boiling points are introduced into a solvent
purification column 555 by means of line 42. In column
555, the amine is separated from the high boiling
impurities. The separated amine, which is now rela-
tively pure, exits from the top of column 555 and is
returned to line 3 for subsequent reuse in column 111.
29,291-F -14-
, :
-15- ~'2~2~
The heavy impurities ar removed fxom the bottom of
column 555 by means of line 44.
It will be apparent to those skilled in
the art that certain process features might be
modified, deleted or added according to the par-
ticular process parameters chosen and that such
choice depends upon many diverse practical con-
- siderations such as economy, convenience and
energy conservation.
For example, while a high concentration
of the ethylenically unsaturated hydrocarbon being
separated in the hydrocarbon feed is preerred,
lower concentrations are suitable and fall within
the scope of the invention. However, a predistil-
lation may be advantageously carried out in order
to obtain a hydrocarbon feed having a higher con-
centration of the unsaturated hydrocarbon and
thereby improve the overall efficiency of the
separation process. Moreover, components which
exhibit similar volatility to the amine may decrease
the performance of the amine and preferably should
he removed. It should be clear from the above
discussion that a plurality of separation columns
utilized at different points in the overall process
such as introduction of the hydrocarbon feed, amine
recovery, etc. and/or technigues may be used for
removal and/or purification of the various streams.
Also the extractive distillation part of the pro-
cess in which use is made of an amine as solvent
may take place in one or more columns with the
possibility of the amine solvent recovery operation
interspersed between said columns. ,
.
29,291-F -15-
-16-~2 ~
,
With regard to the materials employed in
the practice of the present invention, the hydro-
carbon mixture from which the ethylenically unsat-
urated hydrocarbon is separated is a mixture com-
prising two or more hydrocarbons at least one ofwhich is the ethylenically unsaturated hydrocarbon
to be subsequently separated. The ethylenically
unsaturated hydrocarbon is a hydrocarbon having
an ethylenic unsaturation between two carbon atoms,
which atoms are not part of an aromatic ring,
including the alkadienses such as butadiene, the
acetylenes and the monovinylidene aromatics such
as styrene, alkyl-substituted styrenes such as
vinyltoluene and ethylvinylbenzene and vinylnaphthalene.
The other hydrocarbon component(s) may contain an
ethylenic unsaturation, e.g., vinyltoluene is often
present in the separation of styrene from a thermally
cracked petroleum product, but, in general, the
other components are predominantly hydrocarbons
having no ethylenic unsaturation and include satu-
rated aliphatics and cycloaliphatics, and aromatics.
In addition, the hydrocarbon misture may optionally
contain one ore more inorganic components or sub-
stituted hydrocarbon components. The process of the
present inventlon is particularly useful for the
separation of monovinylidene aromatic, in particular
styrene or vinyltoluene, from a hydrocarbon mixture
containing one or more aromatic components in addition
to the styrene and/or vinyltoluene. Such other
aromatic component(s) generally included benzene,
toluene ethylbenzene, o-, p- and m-xylene and/or
indene. More particularly, the process of the
present invention is preferably employed in the
separation of styrene from pyrolysis gas and the
29,291-E' -16-
-17-
separation of vinyltoluene from a mixture of vinyl-
toluene and ~-methyl styrene, dicyclopentadiene
or 1,2,4- or 1,2,3-trlmethylbenzene.
The amine employed in the practice of the
present invention is a soluble amine containing
more than one amine group and which is (1) suf~
ficiently electronegative relative to the ethyl-
enically unsaturated hydrocarbon to allow the
selective separation of the unsaturated hydrocarbon
from the hydrocarbon mixture, (2) inhibits the
polymerization of the ethylenically unsaturated
hydrocarbon and (3) has a boiling point sufficiently
high to allow separation of the amine from the
unsaturated hydrocarbon following the selective
separation of the unsaturated hydrocarbon from the
hydrocarbon mixture~ By the term "soluble" is
meant that the amine forms a solution with the
hydrocarbon mixture (i.e., the .combination of the
amine and hydrocarbon mixtures appears as a uniformly
dispersed or homogeneous li~uid under visual inspection
- at.no magnification). Generally speaking, the
boiling point of the amine is sufficiently high if
it exceeds the boiling point of the unsaturated
hydrocarbon to be separated by at least 20, more
preferably 30C, ak the pressures employed in the
- process. Of course, the greater the differences
-. in boiling points, the easier is the subsequent
separation.
Amines which are sufficiently electronegative
for the purposes of the present invention are those
amines which sufficiently change the relative vola-
tility of the unsaturated hydrocarbon being separated
29,291-F -17-
~2~ 7
-18-
with respect to the other compnents in the mix-ture,
particularly the components having boiling points
close to that of the unsaturated hydrocarbon (gen-
erally by reducing the relative volatility of the
unsaturated hydrocarbon) to allow the selec-tive
separation of the unsaturated hydrocarbon from the
hydrocarbon mixture.
In addition to changing the volatility of
the unsaturated hydrocarbon, the amine inhibits the
polymerization of the unsaturated hydrocarbon so
that less of the unsaturated hydrocarbon is polymerized
during its separation in th~ presence of the amine
than if the same hydrocarbon were subjected to an
identical treatment except no amine employed.
Advantageously, less than 1, preferably less than
0.5, weight percent of the unsaturated hydrocarbon
is polymerized during its separation by the method
of the present invention.
In general, the amines useful in the process
of the present invention contain at least two amino
- or substituted amino groups, with one amino group
preferably being unsubstituted, which amino or sub-
stituted amino groups are separated. by two or more,
preferably two carbon atoms. Representative amines
are diethylene triamine and N-aminoalkyl piperazine.
While the amines most advantageously employed will
vary depending on the hydrocarbon to be separated,
the amines which, in general, are preferably employed
herein are N-aminoalkyl piperazines having an alkyl
group of from 1 to 4 carbon atoms. -The most preferred
amine is N-aminoethyl piperazine.
29,291-F -18-
~2~'7
.
The conditions under which the separation
of the unsaturated hydrocarbon is most advantageollsly
~ conducted will vary depending on many factors including
the specific amine employed and the composition
of the hydrocarbon mixture and the unsaturated
hydrocarbon to be separated therefrom. As a specific
example, in the preferred embodiment wherein styrene
is separated from a hydrocarbon mixture containing
o-xylene using an e~tractive distillation column,
the extractive distillation column or columns advan-
- tageously have from 90 to 130 theoretical stages,
and preferably from 115 to 125 theoretical stages.
The stages are advantageously effectuated in pac3~ed
columns utilizing a packing such~as a regular sheet
packing or dumped packing. The extractive distil-
lation step is conducted under a combination of
pressure and temperature conditions sufficient to
effect a separation yet not promote undesirable
polymerization. Generally, this means that the
bottom temperatures from 120C to 140C are advan-
ageously empolyed with temperatures from 120C
- to 135C being preferred.
Higher temperatures of-ten promote undesirable
reactions and lower economic efficiency of the process.
To save energy costs and help reduce undesirable
reactions, the process is generally operated at reduced
pressure. Advantageously, bottom pressures are main-
tained at from 70 to 125 mm Hg, with a range from
110 to 120 mm Hg being preferred. The same considera-
tions of efficiency and minimizing undesirable reactionsapply here. Temperatures at the top of the column
advantageously range from 45C to 70C, preferably
from 45C to 55C. Advantageously, pressures at the
29,291-F -19-
-20~ 7
top range from about 30 to 45 ~n Hg, wi~h 35 to
45 mm Hg being preferred. The amine/styrene ratio
of the hydrocarbon feed with advantageously range
from about 5 to about 9, with a ratio of about 8
being preferred.
In another preferred embodiment of the
invention wherein styrene is separated from phenyl
acetylene by extractive distillation, the extractive
distillation column or columns advantageously have
10 from 70 to 80 theoretical stages, with 70 to 75
stages being preferred. The column(s) used in the
separation contain trays, typically of the bubble
cap type, sieve type or valve type. The stages
may also be effectuated by packed column utilizing
a packing such as a sheet or dumped packing. Again,
the extractive distillation step is conducted under
a combination of pressure and temperature conditions
sufficient to effect a separation yet not promote
undesirable polymerization. Generally speaking,
this means that bottom temperatures from 140C to
160C are advantageously employed with temperatures
from 145C to 150C being preferred. Higher tempera-
tures often promote undesirable reactions and lower
temperatures result in lower economic efficiency
of the process. The process is generally operated
at reduced pressure to save energy costs and to
prevent undesirable reactions. Advantageously,
bottom pressures from 100 to 120 mm Hg are employed
with pressures of about 100 mm Hg preferred. The
same considerations of efficiency and minimizing
undesirable reactions apply here. Temperatures at
the top of the column advantageously range from
29,291-F -20-
, :
-21- ~ 2~7
45C to 70C, preferably from 45C to 55C. Pres-
sures at the top advantageously range from 30 -to
45 mm Hg, with pressures from 35 to 40 mm Hg being
preferred. The amine/styrene ratio in the hydro~
carbon feed will advantageously vary from 7 to 9,
and is preferably about 8.
Example 1
The total.mixture obtained by the thermal
cracking of a petroleum fraction such as naphtha
10 is separated in one of the usual ways, e.g., by
distillation either at atmospheric or reduced pres-
sure and a fraction boiling between about 125C
and 155C is-obtained. This fraction contains
styrene, xylen~s, ethylbenzene, paraffins, naphtha-
ler~e, polyalkyl-substituted aromatics, phenylakynes
and dienes having from ~-9 carbon atoms and typically
has a APXA color of 65. A typical example of such
a mixture is:
Component % by wei~ht
20 light hydrocarbons 0.6
- benzene and toluene 6.0
ethylbenzene 8.0
m- and p-xylene 32.2
o-xylen~ 14.0
25 styrene 37.8
C-9's 0.7
C-8's and other aromatics 0.7
This mixture is fed to a packed column
about 50 meters high and having from about 100 to
about 125 theoretical plates, and is introduced
at about 20 meters below the top of the tower.
29,291-F -21-
22~ t7
AEP is fed to the same column at a point situated
near the top,`e.g., about 2 meters below the -top,
in a weight'ratio of AEP to hydrocarbon feed of
about 3:1. The reflux ratlo is about 7.5. The
top pressure and temperature are about 40 mm Hg
and about 60C, the reboiler temperature is about
130C. Even with this high,temperature the loss
due to polymerization is less than 0.2 percent
relative to the styrene present in the feed. The
top product of this column contains only about 1.0
percent styrene and the bottoms product contains
almost all the styrene originally present in the
' feed. The bottoms product containing styrene, AEP
and phenyl acetylene is passed to a stripping column
where the AEP is recovered substantially free of
styrene, phenyl acetylene and any colored bodies.
The styrene/phenyl acetylene containing hydrocarbon
mixture is passed from the top of this column to a
second packed distilling column about 30 meters
high and h~ving about 70 to 80 theoretical plates
and is introduced about 30 meters below the top of
the second distilling column. The AEP recovered
from the bottom of the stripping column acts as a
solvent and is ~ed into the second distillation
column about 2.5 meters below the top, in a weight
ratio of AEP to hydrocarbon feed of about 8:1.
The reflux'ratio is about 4. The top pressure and
temperature are about 35 mm Hg and about 50C,
respectively, and the reboiler temperature is about
55C.
The top product of this second packed
distillation column contains styrene of about 99.6
percent purity and is colorless. Since the AEP acts
.
29,291-F -22-
-23-
as a very efficient inhibitor, polymerization of
styrene is practically nil in this second dlstillation
column. Any colored compounds present stay in the
AEP, so that a high-purity, low-color styrene is
obtained from the top of the second column. The
APHA color is less than 5 for the styrene product
from the second extractive distillation column.
Note that the higher the reflu~ ratio in the extractive
distillation column, the greater the dilution of the
extractant by increasing the amount of non-extractan-t
- material in the li~uid overflow with the effect of
this dilution generally being a ~ecrease in relative
volatilities. The optimum reflux ratio is readily
determined for a particular process set up by a
skilled operator.
Example 2
The procedure of Example 1 is repeated,
but now using as the amine diethylene triamine.
Here again, a styrene-rich fraction was obtained.
Example 3
Using generally the extractive distillation
method embodied in Figure 1 and generally the techniques
employed in Example 1 for separating styrene, vinyl-
toluene is removed from a hydrocarbon mixture containing
primarily aromatic hydrocarbons having 9 carbon atoms
including ~-methylstyrene, 1,2,4-trimethylbenzene
and 1,2,3-trimethylbenzene in the presence of N-amino-
ethyl piperazine. The top product from the extractive
distillation column contains less than about 2 percent
vinyltoluene whereas the bottom product contains
vinyltoluene, indene and aminoethyl piperazine. The
vinyltoluene, which possesses excellent color, is
29,291-F -23-
24 ~2~2~Z~'7
easily separated from the indene and/or aminoethyl
piperazine using additional ext:ractive distillation
columns.
Example 4
A hydrocarbon stream containing prlmarily
- hydrocarbons having 4 carbon atoms including acetylene,
butadiene, butene, isobutene and butane is feed to
an extractive distillation column and subsequently
distilled in the presence of N-aminoethyl piperazine.
The material exiting from the top of the extractive
distillation column contains the majority of the
butadiene, butene, isobutene and butane contained
in the hydrocarbon feed. The bottoms from the
columns consist essentially of AEP and acetylenes
lS with only minor amounts of the other C4 hydrocarbons
being present.
Example 5
A hydrocarbon stream of the following
composition:
20 Component % by weight
C4 hydrocarbons 2.2
isopentane 12
1-pentene 4
isoprene 18
25 n-pentane 23
methyl substituted butenes 8.4
other pentenes 3.2
cyclopentadiene 5.6
pentadienes 10.5
30 other C5 hydrocarbons 4.6
other hydrocarbons 8.5
29,291-F -24-
.:
L'7
-25-
is fed to a packed distillation column to separate
the stream into a top produc-t containing the lightex
components (e.g., C4 hydrocarbons) and a bottoms
product comprising the remaining components of the
hydrocarbon stream. The separated hydrocarbon
stream containing the heavier components is fed
to a second distillation column to separate the
stream into a bottoms product containing the poly-
butadienes and other heavier components and a top
product containing, among other components, the
isopentane, n-pentane and isoprene. The top product'
from this column is admixed with N-~aminoethyl)
piperazine (AEP) in a weight ratio of 3 parts A~P
for each part of.the top product and the resulting
admixture is fed to a thi'rd distillation column.
The relative volatilities o~ the hydrocarbon com-
ponents in the stream are now rendered sufficiently
different from isoprene such that isoprene having
a purity higher than-98 percent can be obtained.
One method for obtaining the isoprene is by removing,
from the side of the column, the pure isoprene
- fraction. In another method, no side stream is
employed and the bottoms product contains a mixture
of isoprene, AEP and cyclopentadiene. In this
method, the isoprene is easily separated from the
AEP and/or cyclopentadiene using additional extractive
distillation columns.
Alternatively, if no AEP is employed,
isoprene can only be recovered as a mixture with
not insubstantial amounts of isopentane, n-pentane,
cyclopentadiene and various other components.
29,291-F -25-
-26- ~a2~ 7
By repeating the described process using
a variety of weight ratios of AEP to the isoprene
containing hydrocarbon stream, isoprene of differing
purities can be obtained. Specifically, as the
relative concentration of AEP to the isopr~ne con-
taining hydrocarbon stream is reduced, the isoprene
- recovered will generally contain greater amounts
- of the other components.
- The above description and example serve
to illustrate the invention and its advantages and
should not be interpreted as limiting since further
modifications of the disclosed invention will be
-- apparent to those skilled in the art. All such
modifications are deemed to be within the scope of
the invention as defined by the following claims.
29,291-F -26-
