Process for the purification of liquid sorbents that comprise bimetallic salt complexes

Liquid sorbents that are solutions in an aromatic hydrocarbon or a halogenated aromatic hydrocarbon of a bimetallic salt complex having the generic formula M.sub.I M.sub.II X.sub.n .multidot.Aromatic, wherein M.sub.I is a Group I-B metal, M.sub.II is a Group III-A metal, X is halogen, n is the sum of the valences of M.sub.I and M.sub.II, and Aromatic is a monocyclic aromatic hydrocarbon or halogenated aromatic hydrocarbon having 6 to 12 carbon atoms and that contain alkylated aromatic compounds as impurities are purified by a process in which a portion of the liquid sorbent is subjected to two successive flash distillations, the first of which is carried out under atmospheric pressure to separate a distillate that is rich in the aromatic hydrocarbon or halogenated aromatic hydrocarbon and the second of which is carried out under subatmospheric pressure to separate a distillate that is rich in alkylated aromatic compounds, the residue of the distillation is reconstituted by adding aromatic hydrocarbon or halogenated aromatic hydrocarbon to it, and the reconstituted portion of the sorbent is combined with the remainder of the sorbent.

This invention relates to a process for the purification of liquid sorbents 
that are used to separate olefins from gas streams. More particularly, it 
relates to a process for the separation of alkylated aromatic compounds 
from liquid sorbents that contain certain bimetallic salt complexes. 
Bimetallic salt complexes that have the generic formula M.sub.I M.sub.II 
X.sub.n .multidot.Aromatic, wherein M.sub.I is a Group I-B metal, M.sub.II 
is a Group III-A metal, X is halogen, n is the sum of the valences of 
M.sub.I and M.sub.II, and Aromatic is a monocyclic aromatic hydrocarbon or 
halogenated aromatic hydrocarbon having 6 to 12 carbon atoms, are known to 
be useful in the separation from gas mixtures of such complexible ligands 
as olefins, acetylenes, aromatics, and carbon monoxide. For example, in 
U.S. Pat. No. 3,651,159, Long et al. disclosed a process in which a 
sorbent solution of a cuprous aluminum tetrahalide in toluene was used to 
separate ethylene, propylene, and other complexible ligands from a gas 
feedstream. The complexed ligands were recovered by ligand exchange with 
toluene. The resulting solution of cuprous aluminum tetrahalide in toluene 
was recycled and used to separate additional quantities of the complexible 
ligands from the gas feedstream. 
In processes such as that disclosed by Long et al. in which a ligand 
sorbent containing a bimetallic salt complex is recycled without 
purification and is used for long periods of time, there is a gradual 
increase in the amounts of reaction by-products and other impurities in it 
until sufficient impurities are present to interfere with the efficient 
operation of the process. For example, when the ligand sorbent is 
contacted with a gas stream that contains an olefin having 2 to 4 carbon 
atoms, some of the olefin reacts with the aromatic hydrocarbon or 
halogenated aromatic hydrocarbon in the sorbent to form alkylated aromatic 
compounds and some undergoes polymerization to form olefin oligomers. 
Attempts to inhibit the polymerization and alkylation reactions, for 
example, by the addition of ammonia or another basic compound to the 
sorbent, have been unsuccessful because they failed to reduce the amounts 
of by-products formed or because they interfered with the efficient 
operation of the process. 
In U.S. Pat. No. 4,014,950, Keyworth et al. disclosed a process in which 
the polyalkylated aromatic compounds and olefin oligomers that are present 
as impurities are removed from a liquid sorbent by contacting the sorbent 
with an organic solvent in which the impurities are soluble and with which 
the liquid sorbent is immiscible and separating a solution of the 
impurities in the sorbent-immiscible organic solvent from the liquid 
sorbent. 
This invention relates to an improved process for the purification of 
liquid sorbents. In this process, which is more efficient and more 
economical to operate than that disclosed in U.S. Pat. No. 4,014,950, a 
portion of the liquid sorbent that contains the alkylated aromatic 
compounds that are formed as reaction by-products when a gas feedstream 
that contains at least one olefin having 2 to 4 carbon atoms is contacted 
with a liquid sorbent that is a solution of the bimetallic salt complex 
M.sub.I M.sub.II X.sub.n .multidot.Aromatic in an aromatic hydrocarbon or 
an aromatic halogenated hydrocarbon, is removed from the system in which 
the liquid sorbent is being used to remove olefins from a gas feedstream, 
purified by a two-stage distillation that removes from it substantial 
amounts of aromatic hydrocarbons or halogenated aromatic compounds and 
alkylated aromatic compounds, reconstituted by the addition to it an 
aromatic hydrocarbon or a halogenated aromatic hydrocarbon, and returned 
to the system. Liquid sorbent which has been purified in this way can be 
used for long periods of time without danger of build-up of amounts of 
reaction by-products that interfere with the operation of the olefin 
separation procedure and make necessary the replacement of the liquid 
sorbent and the cleaning of the apparatus. 
The liquid sorbents that are purified by the process of this invention are 
solutions of a bimetallic salt complex in an aromatic hydrocarbon or a 
halogenated aromatic hydrocarbon that contain alkylated aromatic 
hydrocarbons, alkylated halogenated aromatic hydrocarbons, and/or olefin 
oligomers. The bimetallic salt complexes in the liquid sorbents have the 
generic formula M.sub.I M.sub.II X.sub.n .multidot.Aromatic. M.sub.I is a 
Group I-B metal; that is, copper, silver, or gold. Copper (I) is the 
preferred metal. M.sub.II is Group III-A metal; that is boron, aluminum, 
gallium, indium, or thallium. Boron and aluminum are the preferred metals, 
aluminum being particularly preferred. X is halogen, i.e., fluorine, 
chlorine, bromine, or iodine; it is preferably chlorine or bromine. The 
sum of the valences of M.sub.I and M.sub.II is represented by n. Aromatic 
is a mnocyclic aromatic hydrocarbon or halogenated aromatic hydrocarbon 
having 6 to 12 carbon atoms, and preferably 6 to 9 carbon atoms, such as 
benzene, toluene, ethylbenzene, xylene, mesitylene, chlorobenzene, 
bromobenzene, iodobenzene, dichlorobenzene, dibromobenzene, chlorotoluene, 
bromotoluene, iodotoluene, or chloroxylene. It is preferably benzene or 
toluene. Illustrative of these bimetallic salt complexes are the 
following: CuBF.sub.4 .multidot.benzene, CuBCl.sub.4 .multidot.benzene, 
AgBF.sub.4 .multidot.mesitylene, AgBCl.sub.4 .multidot.xylene, 
AgAlCl.sub.4 .multidot.xylene, AgAlBr.sub.4 .multidot.bromobenzene, 
CuGaCl.sub.4 .multidot.toluene, CuInI.sub.4 .multidot.1,2-dichlorobenzene, 
CuTlI.sub.4 .multidot.p-chlorotoluene, and the like. The preferred 
bimetallic salt complexes are CuAlCl.sub.4 .multidot.benzene, CuAlCl.sub.4 
.multidot.toluene, and CuAlBr.sub.4 .multidot.benzene. The aromatic 
hydrocarbon or halogenated aromatic hydrocarbon in which the bimetallic 
salt complex is dissolved is usually and preferably the same as that used 
in the preparation of the bimetallic salt complex, but if desired it may 
be a different one. The total amount of aromatic hydrocarbon or 
halogenated aromatic hydrocarbon in the liquid sorbent, that is, the 
amount in the bimetallic salt complex plus the amount used as solvent, is 
at least 10 mole percent of the amount of the bimetallic salt M.sub.I 
M.sub.II X.sub.n that is present. It is preferred that the amount of 
aromatic hydrocarbon or halogenated aromatic hydrocarbon be 100 to 450 
mole percent of the amount of the bimetallic salt. The particularly 
preferred liquid sorbents contain 25 to 75 percent by weight of 
CuAlCl.sub.4 .multidot.benzene in benzene. 
When a gas feedstream that contains at least one olefin having 2 to 4 
carbon atoms and optionally one or more other complexible ligands is 
brought into contact with one of the aforementioned liquid sorbents, the 
olefin and any other complexible ligands in the feedstream react with the 
liquid sorbent to form a reaction mixture that comprises complexes of 
these ligands with the bimetallic salt complex. The reaction mixture is 
then heated or treated with another complexible ligand to displace the 
olefin and other complexible ligands from it. The stripped liquid sorbent 
is recycled to the system where it is used to remove additional amounts of 
olefins and other complexible ligands from the gas feedstream. 
In addition to reacting with the liquid sorbent to form complexes, the 
olefins in the gas feedstream react with the aromatic hydrocarbon or 
halogenated aromatic hydrocarbon in the liquid sorbent to form alkylated 
aromatic compounds that have the structural formula 
##STR1## 
wherein X represents halogen; R represents hydrogen, methyl, or ethyl; b 
represents 1 or 2; and a and m each represents 0, 1, or 2. When the 
aromatic hydrocarbon in the liquid sorbent is benzene, the alkylated 
aromatic compounds have the structural formula 
##STR2## 
When the aromatic hydrocarbon is benzene and the olefin is ethylene, the 
alkylated aromatic compounds have the structural formula 
##STR3## 
Polyalkylated benzenes that have more than three alkyl substituents on the 
aromatic ring have only slight solubility in the liquid sorbent, and they 
tend to form deposits in the cooler parts of the apparatus. Unlike the 
mono-, di-, and trialkylated compounds, the polyalkylated benzenes are too 
high boiling to be useful as the stripping gas that separates the olefins 
from the liquid sorbent. 
The olefins also undergo polymerization in the liquid sorbent to form small 
amounts of olefin oligomers that have molecular weights in the range of 
about 100 to 1000 and that have only limited solubility in the liquid 
sorbent. In most cases, the liquid sorbent contains at least 10 parts by 
weight of alkylated aromatic compounds per part by weight of olefin 
oligomers formed as a reaction by-product. 
The alkylation and polymerization reactions that yield the reaction 
by-products are catalyzed by the small amounts of aluminum chloride and 
other acidic compounds that are present in the liquid sorbent. They are 
also promoted by the elevated temperatures that are often used to 
decomplex the relatively-stable complexes formed by the olefins and the 
bimetallic salt complex. 
In the practice of this invention, a portion of the stripped liquid 
sorbent, which contains the aforementioned bimetallic complex, aromatic 
hydrocarbon or halogenated aromatic hydrocarbon, and impurities that 
comprise alkylated aromatic compounds, is removed from the system, 
purified by removing alkylated aromatic compounds from it, reconstituted, 
and returned to the system where it is used to remove additional amounts 
of olefins and other complexible ligands from the gas feedstream. The 
portion of the liquid sorbent that is treated in this way may constitute 
from about 5% to 50% of the volume of liquid sorbent in the system. It is 
generally preferred that 10% to 20% by volume of the liquid sorbent be 
removed from the system and purified. 
The impurity-containing stripped sorbent that has been removed from the 
system is purified by subjecting it first to flash distillation at 
atmospheric pressure to a temperature in the range of 80.degree. 
C.-135.degree. C. to remove from it a fraction that is rich in aromatic 
hydrocarbon or halogenated aromatic hydrocarbon and then to a flash 
distillation at a pressure in the range of 10 torr to 100 torr to a 
temperature in the range of 100.degree. C. to 160.degree. C. to remove 
from it a fraction that is rich in mono- and/or dialkylated aromatic 
compounds. 
Following the flash distillation steps, sufficient aromatic hydrocarbon or 
halogenated aromatic hydrocarbon is added to the distillation residue to 
replace the materials that have been removed from it by distillation. The 
reconstituted purified liquid sorbent is then returned to the system where 
it is combined with the unpurified portion of the liquid sorbent. Sorbent 
that has been purified in this way can be used for long periods of time 
without build-up of those amounts of polyalkylated aromatic compounds that 
interfere with the efficient operation of the process and necessitate the 
replacement of the liquid sorbent. 
The purification of the liquid sorbent may be carried out as a continuous 
or batch process. 
In a preferred embodiment of this invention, a liquid sorbent that is a 
solution of cuprous aluminum tetrachloride.multidot.benzene in benzene and 
that has been used in a process for the removal of ethylene from a gas 
feedstream until it contains more than 15% by weight, and in many cases 
more than 25% by weight, of ethylbenzenes is purified by removing from it 
a portion that constitutes about 10% to 20% of the volume of the liquid 
sorbent; subjecting the separated portion of the liquid sorbent to flash 
distillation under atmospheric pressure to a temperature in the range of 
80.degree.-135.degree. C., preferably 120.degree.-135.degree. C., to 
distill off a fraction that is rich in benzene and other volatile 
compounds; flash distilling the residue of the first flash distillation at 
a pressure of 10 torr to 100 torr, preferably 40 torr to 75 torr, to a 
temperature in the range of 100.degree.-160.degree. C., preferably in the 
range of 130.degree.-150.degree. C. to distill off a fraction that is rich 
in ethylbenzene; and adding to the residue of the second flash 
distillation a volume of benzene that is sufficient to replace the volumes 
of benzene, ethylbenzene, and other organic compounds removed in the two 
flash distillations. The reconstituted purified liquid sorbent is returned 
to the system where it is combined with the unpurified portion of the 
liquid sorbent to form a sorbent that contains about 2.4 moles of benzene, 
0.6 mole of ethylbenzene, and less than 0.3 mole of other organic 
compounds per mole of cuprous aluminum tetrachloride. 
The gases that distill from the liquid sorbent during the flash 
distillation at atmospheric pressure are passed through a cold water 
condenser to yield a first distillate that generally contains 90% to 95% 
by weight of benzene, 4% to 6% by weight of ethylbenzene, and small 
amounts of diethylbenzenes, triethylbenzenes, and toluene. This fraction 
may be used in combination with fresh benzene to bring the volume of the 
residue of the second flash distillation to the volume of the portion of 
impurity-containing liquid sorbent that was removed from the system and 
purified. 
The gases that distill from the liquid sorbent during the flash 
distillation under subatmospheric pressure are passed through a cold water 
condenser to yield a second distillate that generally contains 25% to 40% 
by weight of benzene, 50% to 65% by weight of ethylbenzene, 5% to 10% by 
weight of diethylbenzenes, and small amounts of polyethylbenzenes, 
toluene, and ethyltoluenes. This distillate may be redistilled to separate 
ethylbenzene from its other components, or it may be used without further 
treatment as a reactant or solvent in various chemical processes. 
During the flash distillation under subatmospheric pressure, the di-, tri-, 
tetra-, penta-, and hexaethylbenzenes that are in the liquid sorbent 
undergo transalkylation and/or dealkylation reactions that reduce the 
total amount of these compounds in the liquid sorbent from about 8%-15% to 
less than 5%.

The invention is further illustrated by the following examples. 
EXAMPLE 1 
A series of runs was carried out using the following procedure: 
A. A liquid sorbent that contained 28.6 mole percent of cuprous aluminum 
tetrachloride and 71.4 mole percent of benzene was prepared by adding 1.1 
moles of cuprous chloride to 1 mole of anhydrous aluminum chloride in 
benzene. The resulting solution was filtered to remove unreacted cuprous 
chloride and insoluble impurities from it. It then had a specific gravity 
of 1.30. 
B. A gas feedstream that had the following composition: 
______________________________________ 
Ethylene 19.6 mole percent 
Carbon monoxide 2.3 
Nitrogen 70.7 
Benzene 7.4 
______________________________________ 
was fed at ambient temperature and 19 psia into a column in which it was 
contacted with 73 gallons of the liquid sorbent whose preparation is 
described above. The ethylene and carbon monoxide in the feedstream 
reacted with the liquid sorbent as it traveled through the column to form 
a reaction mixture that was a solution of the ethylene-cuprous aluminum 
tetrachloride complex and the carbon monoxide-cuprous aluminum 
tetrachloride complex in the liquid sorbent. This solution was fed to a 
stripping column where it was brought into contact with benzene vapor at 
95.degree. C. The mixture of gases leaving the stripping column, which 
contained 90.4 mole percent of ethylene and 9.6 mole percent of benzene, 
was cooled to separate the benzene from the ethylene. More than 97% of the 
ethylene in the gas feedstream was recovered in this way. The stripped 
liquid sorbent was returned to the column where it reacted with additional 
amounts of ethylene and carbon monoxide in the gas feedstream. 
A stream that consisted of 10% of the stripped liquid sorbent was passed 
continuously at the rate of 3.0 gallons per hour into a flash distillation 
vessel where it was heated by means of a steam coil to 135.degree. C. at 
atmospheric pressure. The gases leaving the vessel were taken through a 
cooling water condenser and a fraction rich in benzene was collected. The 
hot flashed sorbent was fed at the rate of 1.86 gallons per hour to a 
vacuum flash vessel evacuated to 50 torr by means of a vacuum pump where 
it was heated to 145.degree. C. Gases leaving the vacuum vessel were 
passed over chilled water coils to condense all of the ethylbenzene and 
benzene in them. The hot vacuum flashed sorbent was mixed with distillate 
collected during the atmospheric distillation and fresh benzene to dilute 
it and to replace the benzene, ethylbenzene, and other volatile compounds 
that had distilled off. The reconstituted purified sorbent was returned to 
the system where it was combined with the 90% of the stripped liquid 
sorbent that had not been purified. The resulting liquid sorbent contained 
2.4 moles of benzene, 0.6 mole of ethylbenzene, and less than 0.3 mole of 
other organic compounds per mole of cuprous aluminum tetrachloride. 
This purified liquid sorbent was found to be as effective as 
freshly-prepared liquid sorbent in removing ethylene and other complexible 
ligands from gas feedstreams. 
EXAMPLE 2 
The procedure described in Example 1C was used to purify a stripped liquid 
sorbent that was a solution of cuprous aluminum tetrachloride in benzene 
that contained as impurities alkylated benzenes. The liquid sorbent had a 
specific gravity of 1.31. Its organic phase had the following composition: 
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% by Weight 
______________________________________ 
Benzene 71.2 
Ethylbenzene 16.9 
Diethylbenzene 3.6 
Triethylbenzene 0.8 
Tetraethylbenzene 
0.2 
Pentaethylbenzene 
0.5 
Hexaethylbenzene 
3.4 
Toluene 2.2 
Ethyltoluene 0.9 
Diethyltoluene 0.2 
Triethyltoluene 0.1 
______________________________________ 
A 20% portion (154 pounds) of the liquid sorbent was heated under 
atmospheric pressure to 135.degree. C. in 2 hours to distill off a 
benzene-rich fraction which weighed 41 pounds and then under the pressure 
of 20 torr to 132.degree. C. in 2 hours to distill off an 
ethylbenzene-rich fraction which weighed 24 pounds. These distillates had 
the following composition: 
______________________________________ 
% by Weight 
1st 2nd 
(atmospheric) 
(vacuum) 
Distillate Distillate 
______________________________________ 
Benzene 93.6 33.8 
Ethylbenzene 5.2 51.9 
Diethylbenzene 
0.2 8.2 
Triethylbenzene 
&lt;0.1 0.3 
Tetraethylbenzene 
&lt;0.1 &lt;0.1 
Pentaethylbenzene 
&lt;0.1 &lt;0.1 
Hexaethylbenzene 
&lt;0.1 &lt;0.1 
Toluene 1.0 3.8 
Ethyltoluene &lt;0.1 2.0 
Diethyltoluene 
&lt;0.1 &lt;0.1 
Triethyltoluene 
&lt;0.1 &lt;0.1 
______________________________________ 
Sixty-five pounds of benzene was added to the residue of the vacuum flash 
distillation to restore the liquid sorbent to its original volume. The 
reconstituted purified sorbent weighed 158 pounds and had a specific 
gravity of 1.24. Its organic phase had the following composition: 
______________________________________ 
% by Weight 
______________________________________ 
Benzene 91.3 
Ethylbenzene 3.2 
Diethylbenzene 4.0 
Triethylbenzene 1.1 
Tetraethylbenzene 
&lt;0.1 
Pentaethylbenzene 
&lt;0.1 
Hexaethylbenzene 
&lt;0.1 
Toluene 0.1 
Ethyltoluene 0.3 
Diethyltoluene &lt;0.1 
______________________________________ 
The reconstituted purified liquid sorbent was returned to the system where 
it was combined with the 80% of the stripped sorbent that had not been 
purified. The resulting purified liquid sorbent was used to remove 
additional amounts of ethylene and other complexible ligands from the gas 
feedstream.