IL25898A

IL25898A - Removal of certain metallic and organic impurities from an aqueous quaternary ammonium salt solution

Info

Publication number: IL25898A
Application number: IL25898A
Authority: IL
Original assignee: Monsanto Co
Priority date: 1965-06-01
Filing date: 1966-06-01
Publication date: 1970-02-19
Also published as: LU51229A1; CH473095A; DE1594627A1; DE1594627C3; GB1130455A; GB1152699A; NL6607488A; DE1594627B2; DE1593034A1; BE681773A; BE681980A; LU51219A1; SE309773B; US3340190A; NL6607654A

Description

Patents Form No. 3 PATENTS AND DESIGNS ORDINANCE.

SPECIFICATION.

"EEHQV . OF GBBSfrXK MAII C AED ORGAHIC It!PURlTISS EOS AQUEOUS QUASEJB¾JRY ΑίΙΠΟΚΙϋΗ S *T SOLUTION" Πθ»ΟΛΒ 0*B*10D 0»3ΛΥ»Κ1 0»>ίΌ/¾3 0*tnn»r fnofl" HOlSAlfO ¾ existin states of America* of > o ¾ L e, ¥issoiu?»if' πϊϊίϊΐ^ do hereby declare the nature of this invention and in what manner the same is to be performed, to be particularly described and ascertained in and by the following s tatement : - The present invention pertains to removing certain metallic and organic impurities from an aqueous solution of a hydrotropic quaternary ammonium Adiponitrile can be produced by electro-hydrodimerizlng acrylonitrile. he eleotrohydrodimerization is carried out in en electrolytic cell having an anode chamber and cathode chamber separated by a cation permeable membrane. A dilute aqueous solution of sulfuric acid is continuously circulated through the anode chamber as an anolyte. Acrylonitrile is dissolved in an aqueous solution of a hydrotropic quaternary ammonium salt* This solution, i.e. catholyte, is continuously circulated through the cathode chamber. An electrical potential is established between the anode and the cathode sufficient to produce unidirectional current flow. Under the influence of such unidirectional current acrylonitrile is reductively dimerized to adiponitrile at the cathode. As initial charge the catholyte contains water, quaternary ammonium salt and acrylonitrile. Because of the recirculation feature, the catholyte contains these materials along with a substantial quantity of reaction products, primarily adiponitrile. In general, the operating pH of the catholyte is from 6.5 to 12. Temperature is usually maintained between O and 60°C. Based on the weight of the salt plus water the salt concentration in the catholyte generally ranges from US to 65 weight per cent.

To obtain adiponitrile from the circulating catholyte a portion of the catholyte, in continuous adiponitrile production, is withdrawn from the catholyte circulating system, remove adiponitriXe along with a small quantity of by-produots including propionltrile, bis-cyanoethylether and hydroxy-propionitrile* The acrylonitrile phase from the acrylonitrile extraction procedure, containing substantially all of the reaction products, is then water-washed to remove any dissolved quaternary ammonium salt. All aqueous quaternary ammonium salt recovered from this operation, which is relatively free of reaction products, is then passed to an evaporator to therein reduce the water content and remove any remaining acrylonitrile. The concentrated quaternary ammonium salt solution produced by this evaporation is then customarily recycled to the catholyte circulating system.

In general, the pH of the quaternary ammonium salt solution entering and leaving the evaporator remains substantially the same as it is in the catholyte, between 6.5 end 12.

Evaporation usually takes place at a temperature between 80 and 115°C. By this evaporation procedure the salt concentration is Increased from about 25- 0 weight per cent to about 50-85 weight per cent* In the above described process a given quantity of quaternary ammonium salt is reused many times. The acrylonitrile extraction of the diluted catholyte and subsequent water-washing of the acrylonitrile phase produced in the extraction are employed to effect a clean separation of quaternary ammonium salt from reaction products: the acrylonitrile phase, which is fed to a distillation train to remove the adiponitrlle, is substantially free of quaternary ammonium salt and the quaternary ammonium salt recycled to the catholyte for use in the electrohydrodiraeri- products. It is quit© obvious, however, that a perfect separation of reaction products from quaternary ammonium salt is impossible. Polar compounds in the catholyte with a high af inity for water will tend to accumulate in the aqueous phase of the acrylonitrile-water-washing extraction procedure. Metal salts are produced by corrosion of the materials of construction of the cell itself, by corrosion of piping and by corrosion of other processing equipment. Hetal salts are also introduced into the catholyte as impurities in the sulfuric acid, water and raw material for the electrohydrodimerization reaction itself. These metal salts especially tend, because of their polar nature, to accumulate in the recycled quaternary ammonium salt.

After many recycles these metallic salts accumulate to a highly undesirable level* In time they plate out by electro-deposition on the cathode. Other materials also accumulate on the cathode. By analysis this material collecting on the cathode was found to Include nickel, lead, chromium and silver as principal parts and minor quantities of aluminum, copper, zinc and manganese along with some unidentified organic material. These materials by accummulating on the cathode surface substantially reduce the efficiency of the cell, i.e., the production of adiponitrile is significantly reduced. By analysis of the precipitate produced in the instant procedure, it was learned that these same metals and unidentified organic material are removed by the process to be described hereinafter.

The salt used in the electrohydrodimerization as an electrolyte must serve two functions: it must, first, acrylonitrile in water, i.e., the salt must be hydrotropic. Quaternary ammonium salts have been found to be admirably suited for such purposes. Among the general class of acceptable compounds are included tetraalkylammonium aryl sulfonates and tetraalkylammonium alk l sulfates. Tetra-methylammonium toluene sulfonate, tetraethy1ammonium ethyl sulfate, tetraethylammonium benzene sulfonate, triethylmethyl-ammonium benzene sulfonate, triethylmethy1ammonium toluene sulfonate have been found to be especially suited for use in the catholyte.

The general object of this invention is to eliminate or substantially reduce cathode fouling in an electrolytic cell used for the electrohydrodimerization of acrylonitrile to adiponitrile.

Another object is to substantially remove certain metallic and organic impurities from the catholyte.

A still further object is to improve, by reducing cell eathode fouling, product yield in the electrohydrodimerization of acrylonitrile to adiponitrile.

Other objects are apparent and still others will become apparent from reading the following descriptive material.

In general, the above objects are achieved in the provision of a process wherein the catholyte having a pH from 6.5> to 12 and including an aqueous solution of a hydrotropic quaternary ammonium salt first has separated therefrom the aqueous solution of hydrotropic quaternary ammonium salt. The pH of this aqueous solution is increased to a value at least greater than 7 and substantially greater aqueous solution is adjusted to between 2 and 85 weight per cent and heated at a temperature not greater than 115°C. for a period of time sufficient to precipitate the impurities.

These precipitated impurities are then separated from the aqueous solution by customary liquid-solid separation methods including filtration, centrifuging and decanting, and the like.

Theoretically it would be possible to precipitate the metallic and organic impurities precipitable by this process directly from the catholyte prior to the point wherein adiponitrile is extracted from the catholyte. However, the adiponitrile and by-product materials tend to hydrolyze thus producing carboxylic acid salts. Therefore, it is considered best that the instant precipitation and separation procedure be employed at a point in the general electrohydrodimerization process where the presence of adiponitrile and other nitriles is reduced to its lowest point, i.e., in the recycled, concentrated quaternary ammonium salt solution exiting from the evaporator noted hereinabove.

Another possible alternative procedure includes operating the electrohydrodimerization process at an extremely high catholyte pH. Thus because of the inherent nature of evaporation, i.e., application of heat, the generally required conditions of the instant inventive procedure would be achieved, i.e., the pH would be alkaline, salt concentration would be proper and the temperature would be elevated; therefore, precipitation would occur. The precipitated material could then be filtered from the recycled quaternary ammonium However, because of practical considerations it is generally best that the procedure of this invention, as noted before, be exercised upon an aqueous quaternary ammonium salt stream containing small quantities of extraneous materials, such as nitriles and the like. In its most basic form the concept herein consists of increasing the pH of the aqueous quaternary ammonium salt solution to a value substantially higher than that obtaining in the catholyte and further that this increased value be at least greater than 7· That is, it is primarily necessary that the increased pH be alkaline, thus if the catholyte were operating at a pH value below 7, e.g. 6.5» an increase in pH would not necessarily mean that the solution would be made alkaline. Thus the requirements of the present inventive processes are that the pH first be made at least alkaline and that it be substantially higher than the pH obtaining in the catholyte. Some precipitation is obtained when the pH is increased as little as 1/2 a pH unit; although, best precipitation occurs when the pH is Increased at least two units. It is necessary that the temperature of the aqueous solution be maintained at an elevated temperature until precipitation occurs no matter what the pH or the concentration. When the conditions of pH increase and concentration are set, then the amount of time necessary to obtain precipitation at a particular elevated temperature can be determined by the simple expedient of measuring time lapse until a precipitate is observed. In general, however, between 1 and hours of heating at a temperature not greater than about 115°C is necessary for proper precipitation. Preferably, the temperature is main- ° A concentration range within which precipitation can advantageously take place is between lj.5> and Q$ weight per cent based on the total quaternary ammonium salt solution weight.

The preferred pH range for the catholyte is between ?.5> and 10. Therefore, because of obvious economic reasons the smallest possible increase in pH to obtain proper precipitation is preferred because a smaller amount of readjustment is necessary to return the pH of the solution to its proper value for use in the catholyte.

Another feature of this invention is that is is not necessary to treat the quaternary ammonium salt each time it is recycled. In practical operation a small quantity of the quaternary ammonium salt solution is withdrawn, treated and then returned to the recycle stream. A further alternative would be to treat all the quaternary ammonium salt being recycled at prescribed intervals during operation and at times when adiponitrile yield is reduced to an undesirable level. A concentration of metal ions in the range of as little as 0.5-10 parts per million based on the quaternary ammonium salt solution can reduce the yield of adiponitrile.

The following examples are set forth to illustrate the invention. It is not intended that this illustrative matter limit the invention in any way whatsoever.

EXAMPLE I A tetramethylammonium toluene sulfonate solution that had been recycled through the electrohydrodimerization cell about 20 times was extracted with acrylonitrile. The pH thereof was then adjusted to II. with tetramethylammonium hydroxide. The aqueous solution was concentrated to 70 per - 0 thus formed, after cooling the solution, was then filtered from the aqueous solution. Thereafter, the pH was readjusted to 8.7 and the salt concentration reduced to about 60 weight per cent. This purified aqueous salt solution was again used as an electrolyte for 12 days. In the table below the average of the product analyses obtained with impure and purified salt are set forth. All percentages are given as weight per cent based on the total product.

TABLE 1 Product Composition for Impure and Purified Electrolyte OAS Electrolyte HQPN(5) Impure 80.0 13.3 2.0 0.1 i+.6 Purified 85.7 6.9 1.7 0.1 5.6 (1) adiponitrile, (2) propionitrile, (3) bis-cyanoethylether, (ij.) high boilers, and (5) hydroxypropionitrile It can be quite easily seen that the quantity of adiponitrile produced with purified aqueous salt is increased by greater than 5 per cent over the product yield of adiponitrile when impure salt is used as the supporting electrolyte.

EXAMPLE II In an electrolytic cell through which an aqueous solution of tetramethylammonium toluene sulfonate as electrolyte had been recirculated many times adiponitrile electrohydrodimerized from acrylonitrile had a yield of 80 weight per cent. The acrylonitrile, adiponitrile, propionitrile, and other like products were extracted from the electrolyte. The aqueous solution of tetramethylammonium tolulene sulfonate was concentrated from I ..8 weight per cent sulfonate solution was then made alkaline with tetramethyl-ammonium hydroxide to adjust the pH thereof to about 11+ and then heated until a precipitate appeared. Hie resulting precipitate was then filtered from the aqueous solution. The 60 weight per cent aqueous filtrate was diluted to a concentration of 39.1 weight per cent. The thus produced aqueous tetramethylammonium toluene sulfonate diluted filtrate was then reused for an extensive period of time, i.e., about 12 days, as the electrolyte for electrohydrodimerizing acrylonitrile to adiponitrile. In this period, using the purified electrolyte, the adiponitrile yield was 86.1 weight per cent, a significant improvement over untreated salt. The table set forth on the following page comparatively summarizes the operating conditions and results of the two tests using in the first test a non-alkaline, untreated aqueous electrolyte, treating such electrolyte by the instant process and then reusing such treated electrolyte for a second test.

TABLE 2 Alkaline Treatment Alkaline Untreated Treated Composition of Catholyte Tetramethylammonium toluene sulfonate 1+1.8 39.1 Acrylonitrile 16.2 Water 29.7 27.9 Products 13.7 15.6 Catholyte pH 8.1+ 8.7 Yield Adiponitrile 80.0 86.1 Propionitrile 13.3 6.9 bis-Cyanoethylether 2.0 1.7 High boilers 5.3 The improvement in adiponitrile yield achieved The advantages of removing the deleterious impurities including numerous metals and some organic matter is quite readily apparent; product yield is substantially increased. When the quantity of impurities deposited on the cathode is substantially reduced, the cell operates at greater efficiency for a longer period of time thus increasing the useful life of a particular cell assembly.

The invention has been described by reference to particular examples, illustrations and embodiments. However, because certain modifications of the procedure should be readily apparent to those skilled in the art, the invention should be broadly construed and should only be limited by the appended claims.

Claims

HA ISiG WS particularly described and ascertained the nature of oar ©aid invention end in tshat Banner the seme ie to he performed* we declare that what we claim is $

1. A process for removing impurities including copper, nickel, silver, lead and chromium from a catholyte employed in electrohydrodimerizing acrylonitrile to adiponi- trile, said catholyte having a pH from 6.5 to 12 and including an aqueous solution of a hydrotropxc quaternary ammonium salt, characterized by: b (a) separating said aqueous solution of a hydrotropic quaternary ammonium salt from said catholyte, (b) increasing the pH of said aqueous solution to a value at least greater than 7 and substantially greater than the pH of said catholyte, (c) adjusting the salt concentration of said aqueous solution to between 25 and 85 percent, (d) heating said aqueous solution at a temperature not greater than 115°C. for a period of time sufficient to precipitate said impurities, and (e)j separating said impurities from said aqueous solution.

2. The. process of Claim 1, characterized in that said quaternary ammonium salt is tetramethylammonium sulfonate, tetraethylammonium ethyl sulfate, tetraethyl- ammonium, benzene sulfonate, triethylmethylammonium benzene sulfonate, or triethylmethylaramoniura toluene sulfonate.

3. The process of Claim 1 or 2, characterized in that said salt concentration is adjusted to between $ and 25898/2 85 weight er cen and said aqueous solution is heated at 80 to 115°C

4. , . The process of Glaim 3, characterized in that the time for precipitation is from 1 to k hours.

5. · The process of Claim kt characterized in that said pH of said aqueous solution is increased by at least 2.

6. The process of Claim 5, characterized in that the catholyte pH is between 7.5 and 10. 7· A process for removing impurities including copper, nickel, silver, lead and chromium from a catholyte employed in electrohydrodimerizing acry-lonltrile to adiponitrile substantially as described in the herein Examples.