US3616322A - Process for purifying a catholyte used for electrolytic hydrodimerization of acrylonitrile - Google Patents

Abstract

In a process for producing adiponitrile by electrolyzing a mixture containing acrylonitrile, a supporting electrolyte and water, the improvement characterized in that said electrolysis is carried out by removing acrylonitrile from a catholyte contaminated with metal ions, reacting the metal ions contained in the catholyte with carbon dioxide in an alkaline region of a pH value of not lower than 10 thereby precipitating the contaminant metal ions as carbonates thereof, removing thus precipitated carbonates therefrom and recycling the resulting purified solution of the supporting electrolyte as a catholyte.

Description

United States Patent Maomi Seko Tokyo;

Akira Yomiyama, Nobeokashi, Miyazakiken; Shinsaku Ogawa, Nobeoka-shi, Miyazaki-ken; Ryozo Komori, Nobeokashi, Miyazaki-ken; Muneo Yoshida, Nobeoka-shi, Miyazaki-ken, all of Japan [21] Appl. No. 835,193

[72] Inventors [22] Filed June 20, 1969 [45] Patented Oct. 26, 1971 [73] Assignee Asahi Kasei Kogyo Kabushiki Kaisha Osaka, Japan [32] Priority June 26, 1968 Japan [54] PROCESS FOR PURIFYING A CATHOLYTE USED FOR ELECTROLYTIC HYDRODIMERIZATION OF ACRYLONITRILE 7 Claims, 1 Drawing Fig.

[52] U.S. Cl 20fi/73A 51 in. CI c071 29/06 50 Field of Search 204/7244 [56] References Cited FOREIGN PATENTS 1,548,304 12/1968 France Primary Examiner-F. C. Edmundson Attorney-Flynn & Frishauf PROCESS FOR PURIFYING A CATHOLYTE USED FOR ELECTROLYTIC HYDRODIMERIZATION OF ACRYLONITRILE BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to a process for purifying a catholyte containing metal ions as contaminants obtained in the electrolytic hydrodimerization of acrylonitrile for producing adiponitrile.

2. Description of the Prior Art There have been known heretofore various processes for obtaining adiponitrile by electrolytically hydrodimerizing acrylonitrile, for example, as disclosed in U.S. Pat. No. 3,193,481 and others in which an aqueous solution containing acrylonitrile and adiponitrile in high concentrations is electrolyzed using quaternary ammonium salts having oleophilic anions as a supporting electrolyte. There have also been known processes as disclosed in Dutch Pat. Specification No. 6,708,254 and others in which acrylonitrile and adiponitrile in a suspension state are electrolyzed using quaternary ammonium salts having nonoleophilic anions as a supporting electrolyte.

It has been proposed heretofore as disclosed in U.S. Pat. No. 3,193,480 that adiponitrile be produced by the electrolysis of acrylonitrile in a electrolytic cell divided by an ion exchange membrane to form an anode compartment and a cathode compartment using solutions as described above as a catholyte.

However, when the electrolytic hydrodimerization processes as proposed heretofore are practiced on a commercial scale for a prolonged period of time, there are intermixed into a catholyte a variety of metals derived from materials constituting an anode, electrolytic cell, pipes, pumps and appendant equipment as well as from an anolyte and water being employed, or ever from equipment used for the preparation of supporting electrolytes.

These metals primarily include, for example, lead, iron, calcium, magnesium, silver, nickel, chromium and zinc. When these metals are accumulated in a catholyte, they tend to be converted into hydroxides thereof by a layer of concentrated alkali being present on a cathode surface to precipitate thereon, or, in other occasion, electrolytically reduced to a metallic condition with consequential precipitation of metals on the cathode surface.

When the precipitation of these metals on the cathode surface occurs, however small in amounts, e.g., in the vicinity of 50 mgJdm, the supply of acrylonitrile to the surface of the electrode is hindered, leading to inconveniences in byproduct formation of propionitrile and in generation of hydrogen gas. This is quite undesirable because, by hinderances described above, the yield of adiponitrile is lowered and so is the electric current efficiency.

In order to prevent such phenomenon as mentioned above from occurring, there has been known a process for electrolytically purifying a catholyte as disclosed in the specification of Dutch Pat. Application No. 6,515,510, or a process described in that of Dutch Pat. Application No. 6,607,654 in which a catholyte is heattreated after being made to alkaline.

Particularly, in the process of Dutch Pat. Application No. 6,607,654 referred to above, when producing adiponitrile by electrolytic hydrodimerization of acrylonitrile using a catholyte of an aqueous solution of a hydrotropic quaternary ammonium salt having a pH ranging from 6 to l2l, impurities are removed from the catholyte by a combination of steps comprising:

a. firstly, separating the solution of the hydrotropic quaternary ammonium salt from the catholyte,

b. secondly, adjusting thus separated solution of ammonium salt to have a pH of at least 7 or higher and a concentration of the ammonium salt in the range of from 28 to 85 percent, heating thus adjusted solution at a temperature not higher than 115 C., and

c. lastly, separating impurities from the solution of ammonium salt.

The prior an process known heretofore such as referred to above is accompanied by a drawback in that since a catholyte is heated necessarily under a strong alkaline condition, there occur undesirable side reactions such as, e.g., thermal decomposition of quaternary ammonium salts due to Hoffmann decomposition, hydrolysis of acrylonitrile, adiponitrile or other byproducts, and polymerization of acrylonitrile.

SUMMARY OF THE INVENTION It is, accordingly, an object of the present invention to provide a process for purifying a contaminated catholyte obtained in the electrolytic hydrodimerization of acrylonitrile, free from the drawbacks of the prior art processes mentioned above.

It has been found that the object of the present invention mentioned above can be preferably accomplished by removing metal ions accumulated in a catholyte due to the electrolytic hydrodimerization of acrylonitrile from a catholyte in the form of carbonates thereof. Thus, the present invention has its basis on this novel finding.

More particularly, the present invention relates to a process for purifying a catholyte used for electrolytically hydrodimerizing acrylonitrile in an electrolytic cell divided by a diaphragm to form an anode compartment and a cathode compartment, which comprises blowing a gaseous carbon dioxide into the catholyte containing metal ions thereby making the catholyte alkaline, particularly at a pH of not lower than 10, precipitating the metal ions in the form of carbonates thereof, and removing thus precipitated carbonates therefrom.

While the object of the present invention may also be accomplished by the addition of compounds which produce C0,," in water, such as, e.g., sodium carbonate or potassium carbonate, together with or in place of C0,, in the following explanation of the present invention, specific reference will be made to the process in which the blowing of carbon dioxide is adopted.

In general, when removing metals by precipitation as carbonates thereof, there is a possibility that the formation of precipitation of metals occurs only insufficiently in a low-pH region, due to the formation of bicarbonates having higher solubilities than those of carbonates.

In order to prevent this from occurring, it is preferable in the process of this invention that the alkalinity of the catholyte be adjusted specifically to have a pH value of not lower than 10, before, during, or after blowing a gaseous carbon dioxide thereinto.

However, when a catholyte containing acrylonitrile is made alkaline, particularly at a pH higher than l0, acrylonitrile contained therein tends to be converted into biscyanoethyl ether or polymerized or hydrolyzed. Thus, it is preferable that a catholyte from which acrylonitrile has been removed beforehand by distillation or the like operation be employed in the process of this invention.

Particularly preferable catholytes for use in the process of the present invention are ones obtained in the electrolysis of acrylonitrile and adiponitrile in a suspension state as disclosed in Dutch Pat. Specification No. 6,708,254, having been freed from not only acrylonitrile but also adiponitrile according to such means as quiescence, centrifuge, heat condensation, filteration-condensation, etc. Because, when such catholytes as referred to above are used, the undesirable hydrolysis of adiponitrile and byproducts formed by electrolysis such as propionitrile and Z-cyanoethyl adiponitirle can be prevented, even if pH values thereof are increased to l0 or higher in the process of this invention.

As described above, the process of this invention is preferably applicable to metal ion-containing catholytes free from acrylonitrile,

Now, the mode of working of a series of operations involving the purification process of the present invention will be ex plained in the following:

First, procedures for making a catholyte alkaline by electrolysis thereof will be explained as follows:

A metal ion-containing catholyte having been freed from acrylonitrile is introduced to a cathode compartment of an electrolytic cell divided by an anion exchange membrane to form an anode compartment and the cathode compartment. As hydrogen gas is generated from a cathode, pH value of the catholyte is elevated.

Although carbonates of metals may be formed by flowing a gaseous carbon dioxide into the catholyte thus made alkaline and discharged from the cell, it is preferable that carbon dioxide gas be blown into the catholyte entering the cell followed by making the same alkaline, since the concentration of alkali formed at the boundary surface of the cathode is quite high so that the rate of precipitation of carbonates may be greatly accelerated.

It is preferable to employ an anion exchange membrane as a diaphragm in an electrolytic cell, since by so doing hydrogen ions derived from an anode compartment and other ions are prevented from migrating into a cathode compartment. Any anion exchange membranes conventionally used may be conveniently employed in the process of this invention. For example, membranes having quaternary ammonium groups obtained by chloromethylating a styrene-divinylbenzene copolymer matrix followed by quaternizing, or those obtained by copolymerizing styrene, vinyl pyridine and divinylbenzene followed by quatemizing, inclusive of uniform membranes and nonuniform membranes having an inserted core material, may be conveniently used.

Although anion exchange membranes are most preferable as a diaphragm as described above, other diaphragms than anion exchange membranes may also be used. For example, it is possible to use a cation exchange membrane as a diaphragm, if a neutral or alkaline solution of a supporting electrolyte is employed as an anolyte.

Any materials having a low hydrogen overvoltage and a resistance to corrosion may be used for a cathode. Preferable materials for a cathode include, for example, iron, stainless steel, nickel plate and nickel-plated iron plate.

In order to elevate an alkalinity of a metal ion-containing catholyte in the process of the present invention, there may be adopted another method in which the catholyte is passed through an anion exchange membrane of hydroxy group type. In this method, the anion exchange membrane after used may be regenerated by using sodium hydroxide or an aqueous ammonia. In this instance, however, the use of a supporting electrolyte having an oleophilic anion such as, e.g., aryl sulfonate, is not preferable, because it makes the regeneration of the anion exchange membrane difficult.

There is still another method of making a catholyte alkaline in which the alkalinity is elevated by the incorporation of sodium hydroxide, slaked lime or the like thereinto. When quaternary ammonium sulfates are used as a supporting electrolyte, the incorporation of slaked lime is particularly preferable since the alkalinity is elevated with the precipitation of calcium sulfate.

in order to elevate the alkalinity of a catholyte in the process of the present invention, there may be adopted various methods other than described above.

When applying the process of this invention to a catholyte containing a supporting electrolyte having sulfate ions, the alkalinity may be elevated to a pH value of higher than 10, after removing acrylonitrile therefrom. As for a catholyte in a suspension state, it is preferable that first acrylonitrile be removed therefrom, second the catholyte be phase separated into an oil phase and an aqueous phase, third the alkalinity of thus separated aqueous phase alone be elevated to a pH value higher than according to the methods as described above and last a gaseous carbon dioxide be blown thereinto.

Although a high alkalinity of the catholyte is desirable in the process of this invention since the higher the alkalinity becomes the more the carbonates precipitate, a pH value of from about 11 to about 13 is most preferable from the economical point of view. Also, since the lower the concentration of a supporting electrolyte in the catholyte to be treated becomes, the higher the rate of precipitation of the carbonate becomes, it is preferable that the concentration of the supporting electrolyte is not higher than 25 percent.

In order to expedite the removal of iron ions from a catholyte, it is suggested that first the alkalinity of the catholyte be elevated and subsequently an operation of exposing the catholyte to air thereby oxidizing iron ions being present therein and the operation of blowing a gaseous carbon dioxide be combined together.

Carbonates and/or hydroxides thus formed according to the process of the present invention are then removed from a catholyte by sedimentation, filteration or centrifuge.

The catholyte thus treated in accordance with the present invention has a pH value of higher than 10. However, a catholyte used in the production of adiponitrile by the electrolytic hydrodimerization of acrylonitrile is required to have a pH value of lower than 10 in order to prevent the byformation of biscyanoethyl ether. Thus, the treated catholyte is adjusted to have the desired pH value by adding an acid thereto.

When the presence of carbon dioxide ions in a catholyte is undesirable, carbon dioxide gas may be removed therefrom by blowing air thereinto after lowering the pH to value of lower than 4.

As described above, in accordance with the present invention, the concentration of metal ions in a catholyte obtained in the production of adiponitrile by the electrolytic hydrodimerization of acrylonitrile can be maintained at a low level, e.g. lower than about 2 ppm, with consequential decrease in the precipitation of metals on the surface of the cathode. As a result, adiponitrile can be obtained in high yields for a prolonged period of time. 4

The process of the present invention is applicable to the process as described in the US. Pat. No. 3,l93,48l referred to above in which an aqueous solution containing acrylonitrile and adiponitrile in high concentrations is electrolyzed to effect the electrolytic hydrodimerization using a quaternary ammonium salt having oleophilic anions as a supporting electrolyte.

It also is applicable to the process as disclosed in the Dutch Pat. Specification No. 6,708,254 referred to above in which the electrolysis is carried out using a quaternary ammonium salt having nonoleophilic anions as a supporting electrolyte while maintaining acrylonitrile and adiponitrile in the form of a suspension.

in addition, the process of the present invention is widely applicable to other processes for producing adiponitrile from acrylonitrile by the electrolysis thereof than referred to above.

In general, in practicing the process of the present invention, no restriction is imposed on the concentration of acrylonitrile and adiponitrile in the catholyte, type and concentration of the supporting salt employed.

in accordance with the present invention, a catholyte need not be heated to any particularly high temperatures and the purification of the catholyte, i.e. removal of metals therefrom, can be accomplished at about room temperature or at a temperature at which the electrolysis is effected. Thus, the process of this invention is not accompanied by the drawbacks of the prior art referred to hereinbefore.

Another advantage of the process of this invention is that since, in general, metal carbonates have smaller solubilities in water than those of metal hydroxides, the removal of precipitates of metal carbonates can be achieved easier than those of metal hydroxides.

The working of the process of this invention will be explained by referring to the accompanying drawing which illustrates a typical flow sheet of the present process. It should not be construed, however, that the present invention is restricted by the following description.

Numeral 2 designates an electrolytic cell divided by a cation exchange membrane to form a cathode compartment and an anode compartment employed for the production of adiponitrile by electrolytically hydrodimerizing acrylonitrile. An anolyte in the cell 2 is recycled between the anode compartment of the cell 2 and an anolyte tank 1. Between the cathode compartment of the cell 2 and a catholyte tank 3 is recycled a catholyte.

The catholyte consists of an aqueous phase mainly comprising an aqueous solution of a quaternary ammonium salt and an oil phase mainly comprising acrylonitrile and adiponitrile suspended therein.

A part of the catholyte of the catholyte tank 3 is supplied to an acrylonitrile stripper 4 where acrylonitrile is distilled and thus distilled acrylonitrile is recycled to the catholyte tank 3. Suspension, i.e. catholyte, from which acrylonitrile has been removed is separated into two phases in quiescence in a decanter 5, and an oil phase mainly comprising adiponitrile is fed to a purification process through line 6. An aqueous phase is supplied to a tank 7.

Numeral l0 designates an electrolytic cell divided by an anion exchange membrane to form an anode compartment and a cathode compartment provided for increasing an alkalinity of the catholyte fed from the tank 7. Between the anode compartment of the cell and an anolyte tank 11 is recycled an anolyte. A catholyte in the cell 10 is supplied from the tank 7 and is recycled between the cathode compartment of the cell 10 and the tank 7. Into the tank 7 is blown carbon dioxide gas from a line 8 and hydrogen gas generated in the cell 10 is discharged from a line 9.

A part of liquid in the tank 7 is fed to a packed tower 12 where the liquid is brought into contact with air 13 in a countercurrent fashion to oxidize iron ions present in the catholyte.

Subsequently, carbonates and/or hydroxides of metals formed are filtered off in a filter l4 and the catholyte freed from contaminants is stored in a tank 15 where the pH thereof is adjusted at about 4 by the addition of an acid from a line 16. Then, the adjusted catholyte is supplied to a packed tower 18 where the same is countercurrently contacted with air supplied from a line 17 thereby removing carbon dioxide therefrom. The resulting purified catholyte thus obtained is recycled to the catholyte tank 3.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The following examples will serve to illustrate the present invention more fully.

EXAMPLE 1 Referring to the fiowsheet of the accompanying drawing, the anolyte tank 1 was filled with a 2N sulfuric acid and the catholyte tank 3 was filled with an emulsion having the following composition:

Oil phase by weight) Acrylonitrile 20 Adiponitrile 70 Electrolysis byproducts 5 Water 5 Aqueous phase Tetraethylammonlum sulfate l0 Acrylonitrile. adiponitrile and electrolysis byproducts 8 Water Balance pH 4 Volume ratio ofoil phase to aqueous phase 2:8

A part of the emulsion was supplied to the acrylonitrile stripper 4 to remove acrylonitrile therefrom and the bottom was separated in quiescence in the decanter 5. At this point, the concentration of acrylonitrile remaining in the aqueous phase was lower than 0.05 percent.

The liquid, i.e. the aqueous phase free from acrylonitrile, is supplied to the electrolysis cell 10 as a catholyte. The electrolytic cell 10 included a cathode made of stainless steel, an anode made of lead alloy and a diaphragm of an anion exchange membrane. A 2N sulfuric acid was used as an anolyte. In the electrolytic cell, the electrolysis was conducted with an electric current of IO ampere/dm. and alkali was formed with an electric current efficiency of about 50 percent. Then, carbon dioxide gas was blown into the catholyte from the line 8 while maintaining the pH of the catholyte in the tank 7 at a value of 12.

A part of the resulting catholyte thus obtained was fed to the packed tower 12 where the same is subjected to aeration. Subsequently, impurities were removed from the catholyte in the filter 14 and the pH of the catholyte is adjusted at a value of 4 in the tank 15 by adding sulfuric acid thereto. By conducting the aeration in the packed tower l8, residual carbon dioxide gas is removed therefrom and the resulting purified catholyte is recycled to the catholyte tank 3.

The following effects were brought about by the treatment described above.

In the flowsheet referred to above, when no electrolytic cell 10 was employed and no blowing of carbon dioxide was conducted, the concentrations of calcium ions, magnesium ions, lead ions and iron ions in the aqueous phase of the catholyte in the catholyte tank 3 were 15 p.p.m., 3 p.p.m., 5 p.p.m. and 3 p.p.m., respectively.

Electrolytic hydrodimerization of acrylonitrile was conducted in the electrolytic cell 2 under such conditions as described above for about 400 hours with a result that the conversion of acrylonitrile to adiponitrile was lowered to about percent.

In contradistinction, when the electrolytic cell 10 and the blowing of carbon dioxide were employed thereby reducing the concentration of total metal ions such as calcium, magnesium, lead and iron, in the catholyte tank 3 to lower than about 2 p.p.m., the conversion of acrylonitrile to adiponitrile was maintained as high as 92 percent even after the continuous operation for about 400 hours.

In the instant example, the same effect as described above was obtained by using a cation exchange membrane as a diaphragm in the electrolytic cell and tetraethylammonium chloride as an anolyte.

EXAMPLE 2 Example 1 was repeated according to the same procedures as described therein except that an anion exchange tower was provided instead of the electrolytic cell 10 and the anolyte tank 11. An anion exchange resin having been converted to hydroxy group type by sodium hydroxide was used after a thorough washing with water. The liquid from the tank 7 was passed through a layer of the resin with blowing of carbon dioxide gas thereinto thereby maintaining the pH of the liquid at a value of 12.

The same effect as in example I was obtained.

EXAMPLE 3 In the operation of the example l, the pH of the catholyte was adjusted at about 13 by adding slaked lime to the aqueous phase of the decanter 5 in an amount of 5 kg. per m. of the aqueous phase, instead of employing the electrolytic cell 10, anolyte tank 11 and tank 7.

After filtering off calcium sulfate which had precipitated, carbon dioxide gas was blown into the liquid to make the pH at a value of 12. The resulting liquid was supplied to the packed tower 12.

By the treatment described above, the same effect as in example l was obtained. In this example, the same effect was obtained by adding sodium carbonate to the liquid instead of blowing carbon dioxide thereinto.

We claim:

l. A process for the continuous production of adiponitrile by subjecting to electrolysis a catholyte comprising acrylonitrile, an electrolyte and water and being an emulsion of an oil phase and an aqueous phase, wherein the catholyte is cycled through an electrolytic cell and a reservoir vessel, part of the catholyte is continuously or intermittently removed from the reservoir vessel, acrylonitrile and said oil phase containing adiponitn'le are removed from said catholyte, subsequently treated with carbon dioxide under alkaline condition at a pH not lower than 10 whereupon contaminant metal ions contained therein are precipitated in the form of the corresponding separating the precipitated carbonate and recycling the resulting purified solution of the supporting electrolyte as a catholyte, and the separated acrylonitrile is also returned to the reservoir.

2. Process according to claim 1 wherein said aqueous phase of the catholyte contains a quaternary ammonium salt having nonoleophilic anions as a supporting electrolyte.

3. Process according to claim 1, wherein said alkali is selected from sodium hydroxide and slaked lime.

4. Process according to claim 1, wherein said aqueous phase of the catholyte treated with carbon dioxide has a pH ranging from I] to 13 and the concentration of the electrolyte contained in said aqueous phase is not higher than 25 percent.

5. Process according to claim 1, wherein acrylonitrile and said oil phase are separated from said aqueous phase of said catholyte and said aqueous phase is electrolyzed as a catholyte in an electrolyzer having an anode compartment and a cathode compartment separated from each other with a diaphragm whereupon the pH of said aqueous phase is increased to not lower than 10, prior to, simultaneously with, or after the treatment of said aqueous phase with carbon dioxide therein.

6. Process according to claim I, wherein acrylonitrile and said oil phase are separated from said aqueous phase of said catholyte and said aqueous phase is passed through a hydroxy group type anion exchange resin whereupon the pH of said aqueous phase is increased to not lower than 10, prior to, simultaneously with, or after the treatment of said aqueous phase with carbon dioxide.

7. Process according to claim 1, wherein an alkali is added to said aqueous phase of the catholyte in sufi'icient quantity to raise the pH thereof to not lower than 10, prior to, simultaneously with, or after the treatment of said aqueous phase with carbon dioxide.

Dated October 26 197].

Pat ent No. 3 6 322 Inventor(s) NAOMI SEKO et al It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 7, lines 12-15 "separating...reservoir should be deleted and replaced with ---metal carbonate,

metal carbonate so precipitated is returned to the reservoir for reuse, and the separated Signed and sealed this 9th day of April 197A.

(SEAL) Attest:

C. MARSHALL DANN EDWARD I LFLETGHERJR.

Commissioner of Patents Attesting Officer DRM PO-1D50 (10-69) 9 us. GOVERNMENT PRINTING omc: nu o-sn-su.

Claims (6)

1. 2. Process according to claim 1, wherein said aqueous phase of the catholyte contains a quaternary ammonium salt having nonoleophilic anions as a supporting electrolyte.
2. 3. Process according to claim 1, wherein said alkali is selected from sodium hydroxide and slaked lime.
3. 4. Process according to claim 1, wherein said aqueous phase of the catholyte treated with carbon dioxide has a pH ranging from 11 to 13 and the concentration of the electrolyte contained in said aqueous phase is not higher than 25 percent.
4. 5. Process according to claim 1, wherein acrylonitrile and said oil phase are separated from said aqueous phase of said catholyte and said aqueous phase is electrolyzed as a catholyte in an electrolyzer having an anode compartment and a cathode compartment separated from each other with a diaphragm whereupon the pH of said aqueous phase is increased to not lower than 10, prior to, simultaneously with, or after the treatment of said aqueous phase with carbon dioxide therein.
5. 6. Process according to claim 1, wherein acrylonitrile and said oil phase are separated from said aqueous phase of said catholyte and said aqueous phase is passed through a hydroxy group type anion exchange resin whereupon the pH of said aqueous phase is increased to not lower than 10, prior to, simultaneously with, or after the treatment of said aqueous phase with carbon dioxide.
6. 7. Process according to claim 1, wherein an alkali is added to said aqueous phase of the catholyte in sufficient quantity to raise the pH thereof to not lower than 10, prior to, simultaneously with, or after the treatment of said aqueous phase with carbon dioxide.

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