DE1593034A1 - Process for removing contaminants from a catholyte - Google Patents

Description

Wc 12 624/66 13 / Ko

Monsanto Company
Sto Louis, Missouri (V ₀ St, A.)

Process for removing contaminants from a catholyte

The invention relates to a method for removing certain metallic and organic impurities from an aqueous solution of a hydrotropic, quaternary ammonium salt _e

Adipic acid dinitrile can be produced by the electrohydrodimerization of acrylonitrile. The electrohydrodimerization is in an electrolyte cell with an anode compartment and one of them separated by a cation-permeable membrane Cathode compartment. Executed. A dilute aqueous solution sulfuric acid is continuously circulated through the anode space as an anolyte. Acrylonitrile comes in a aqueous solution of a hydrotropic quaternary ammonium salt solved. This solution, i.e. the catholyte, is kept circulating continuously through the cathode chamber The anode and the cathode will have a sufficiently high level of electricity

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BATH ORIGINAL

Potential established in order to obtain a unidirectional current flow. Under the influence of such a unidirectional current, acrylonitrile is reductively dimerized to adipic acid dinitrile at the cathode. The catholyte contains water, quaternary ammonium salt and acrylonitrile as the starting charge. Due to the circulation or circulation, the catholyte contains these materials together with a considerable amount of reaction products, mainly adipic acid dinitrile. In general, the pH of the catholyte is in the range from 6.5 to 12. The temperature is usually maintained between 40 unc 6O ⁰ C. Based on the weight of the salt and water, the salt concentration in the catholyte is generally in the range from 45 to 65% by weight.

To obtain adipic dinitrile from the circulating catholyte, part of the catholyte is used in the continuous Adipic acid dinitrile production from the catholyte circulation system peeled off, diluted with water and then combined with acrylonitrile to remove adipic dinitrile with a small amount of by-products including propononitrile, bis-cyanoethyl ether and hydroxypropiononitrile extracted. The acrylonitrile phase obtained from the acrylonitrile extraction treatment, which in the Containing essentially all of the reaction products, water is then used to remove any dissolved quaternary Ammonium salt washed. The entire aqueous quaternary

009830/1878

Ammonium salt _t ausidieser the working stage is obtained, and is relatively free of reaction products, an evaporator is then fed to the water content therein to
and remove any remaining acrylonitrile., The concentrated quaternary ammonium salt solution obtained from this evaporation is then usually recycled to the catholyte circulation system »
In general, the pH of the quaternary ammonium salt solution entering and leaving the evaporator remains essentially the same as in the catholyte, namely between 6.5 and 12 ". The evaporation usually takes place at a temperature between 80 and 115 ° C
this evaporation process increases the salt concentration from about 25-40% by weight to about 5 ° to 85% by weight »

In the method described above vri.rd a
given amount of a quaternary ammonium salt reused many times o The acrylonitrile extraction of the diluted
Catholytes and the subsequent water washing of the acrylonitrile phase, which is formed during the extraction, are
designed to effect a pure and clean separation of the quaternary ammonium salt from reaction products;
the acrylonitrile phase, which is fed to a distillation chain to remove the adipic acid dinitrile, is essentially free of quaternary ammonium salt and the quaternary ammonium salt, which is returned to the catholyte in the circuit.

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to be used in the electrohydrodimerization of acrylonitrile is essentially free of reaction products ,, It is clearly seen that, however, perfect separation of reaction products from the quaternary ammonium salt is impossible »Polar compounds in the catholyte with a high affinity for Water tends to accumulate in the aqueous phase of the acrylonitrile water scrubbing extraction processa Metal salts are formed by corrosion of the construction materials of the cell itself, as well as by corrosion of the pipe system and by corrosion of other treatment devices.Metal salts are also found in the catholyte as impurities in the sulfuric acid in water and in the raw material for the electrohydrodimerization reaction itself These metal salts to a very undesirable Asju measure of _0. Over time, they plate out by electrodeposition on the cathode. Other materials also accumulate on the cathode · Analysis has determined that these cathode-accumulating material are mainly nickel, lead, chromium and silver and minor amounts of aluminum, copper, zinc and manganese along with some unidentified organic material includes. These

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Due to their accumulation on the cathode surface, materials considerably reduce the effectiveness of the cell, ie _o the production of adipic acid dinitrile is significantly reduced.

Present rigto by analyzing the process of awgeitteiie-teiarefe & H The precipitate formed was found to be the same metals and unidentified organic material by the method according to the invention, which is given is described, to be removed «

(1.) it must convert the electric current or forward, and (2) it must dissolve acrylonitrile in water, d "h _e the salt must be hydrotropic": -the in Elektrohydrodimerisation as an electrolyte for use reaching salt two functions must satisfy quaternary ammonium salts proved insbesonder ³ suitable for these purposes "to the general class of useful compounds include Tetraalkylammoniumarylsulfonate and Eetraalkylammoniumalkylsulfate. Tetramethylammonium toluenesulfonate, Tetraäthylammoniumäthylsulfat, TetraäthylammoniumbenzolBulfonat, Triäthylmethylammoniumbenzenesulfonat, Triäthylmethylammoniumtoluenesulfonat have been found to be particularly suitable for use in the catholyte

The object of the invention is generally to provide a Process for the elimination or substantial reduction of the cathode pollution in an electrolytic cell which is used for Electrohydrodimerization of acrylonitrile see Adipineäuredini «

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009830/1870

tril is used, in particular certain metallic and removing organic contaminants from the catholyte. According to the invention, by the lowering the contamination of the cell cathode, the product yield in the Electrohydro ^ inerization of acrylonitrile to adipic acid dinitrile be improved.

In general, these purposes are achieved by the process according to the invention in which the catholyte having a pH of 6.5 to 12 including an aqueous solution of a hydrotropic quaternary ammonium salt is first subjected to a separation treatment in which the aqueous solution of the hydrotropic, quaternary ammonium salt is separated from the catholyte ₀ The pH value of this aqueous solution is increased to a value of at least above 7 and significantly above the pH value of the catholyte. ew .- $ and the solution is heated at a temperature up to 115 ° 0 for a period of time sufficient to precipitate the impurities. These precipitated impurities are then separated from the aqueous solution by conventional separation methods for liquid-solid mixtures, including Filtration, centrifugation and decanting or the like, separated

Theoretically it would be possible to do that through the -it procedure precipitable metallic and organic deposits precipitate directly from the catholyte before the time

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at which adipic dinitrile is extracted from the catholyte will. However, adipic dinitrile and the by-product materials tend to be for hydrolysis, wodfjuch carboxylic acid salts are formed. It is therefore considered most convenient that the precipitation and separation process according to the invention at a point in the ordinary electrohydrodimerization process is used in which the presence of adiponitrile and other nitriles is reduced to the lowest value, i.e. in the im Recycled concentrated quaternary ammonium salt solution obtained from the above Evaporator leaks ",

Another alternative process option involves performing the electrohydrodimerization process with a particularly high pH value of the catholyte. Due to the inherent nature of the evaporation, i. the application of heat, the generally required conditions of the process according to the invention were thereby achieved, that is to say

-Value
pH would be alkaline, the salt concentration would be appropriate and the temperature would be elevated and therefore precipitation would take place The precipitated material could then be filtered off from the recirculated quaternary ammonium salt solution «

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However, based on practical considerations, it is generally best to proceed as described above according to the invention in an aqueous quaternary ammonium salt stream with a content of small amounts of Premd materials such as nitriles or the like. to execute. According to the procedure with the highest basicity, the process consists in that one the pH of the aqueous solution the quaternary ammonium salt to a significantly higher value than that obtained in the catholyte, increases and wherein this increased value is further at least above 7. This means it in the first place what is required is that the increased pH is alkaline, so that when the catholyte is operated at a pH below 7, e.g. at 6.5, an increase in pH would not necessarily mean that the solution was rendered alkaline will. Thus, the conditions of the method according to the invention are that the pH is made at least alkaline must be and that it is significantly above the resulting pH in the catholyte. Some precipitation is obtained when the pH is increased by as little as 1/2 pH unit, with optimum precipitation with an increase in pH of at least 2 units takes place. It is necessary that the temperature of the aqueous solution be at a is maintained at an elevated temperature until precipitation takes place, regardless of the size of the pH or the concentration. When the conditions of pH increase and

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Concentration is determined, then the amount of time required to achieve precipitation at a particular elevated temperature can be determined by the simple means of determining the time elapsed before precipitation is observed. In general, however, heating between 1 and 4 hours at a temperature of up to about 115 ^° C. is necessary for adequate precipitation. The temperature is preferably maintained between 80 and 115 ° O until a precipitation is observed. A concentration range within which the precipitation can advantageously take place is between 45 and 85% by weight, based on the total weight of the quaternary ammonium salt solution

The preferred pH range for the catholyte is between 7.5 and 10o. Therefore, for obvious economic and technical reasons, the low & possible? Increase in pH is preferred to achieve proper precipitation as it requires less readjustment to bring the pE v / value of the solution back to its appropriate value for use in the catholyte.

Sin further feature of the invention resides in that ea lot not necessary to treat the quaternary ammonium salt each in its recirculation in the circuit. In practice, a small amount of the quaternary ammonium salt solution is drawn off and treated

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and then fed back into the power returned in the circuit ο According to a further alternative, the total amount the recirculated quaternary ammonium salt at prescribed intervals during operation and at times when the adipic dinitrile yield is reduced to an undesirably low level, one treated Concentration of metal ions in the range of as little as 0.5 to 10 parts per million based on the quaternary Ammonium as solution can reduce the yield of adiponitrile to decrease,.

The invention is explained in more detail below with the aid of examples

example 1

iane Tetramethylammoniumtoiuolsulfonatlösung, which about 20 times in the circuit through the 31ektroh; ^r drodimerisationszelle was performed »was extracted with acrylonitrile * The pH of the solution was then adjusted with tetramethylammonium hydroxide in H The wäiSrige solution was concentrated to 70 $ and then heated during a Sturide at 112 to 114 ⁰ G," The precipitate which formed was Then, after cooling the solution, it was filtered off from the aqueous solution. Then the pH was again adjusted to - .7 and the salt concentration was lowered to about 60% by weight of 12 days used «Ir, in the following table

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belle I are the mean values of the -troduktanaly ^ eii, the carried out with impure and purified salt ^ official percentages are given as percentages by weight, "based on the entire product.

Table I.

Product composition for impure and cleaned electrolyte BGi (3) mW 4.6 Product composition ($) 2,
1, 0
7th 0.1
0.1 electrolyte (D impure 80,
cleaned 85, 0
7th 13,
6, ! 2) i3
, 9

(1) adipic acid dinitrile, (2) propionic acid nitrile, (3) biscyanoethyl ether, (4) high boiling material, (5) hydroxypicppionic acid nitrile ; QAS = natural ammonium salt,

It is clear "there ^r: the liii fc purified salt produced amount of adiponitrile to $ 5 is opposite the product yield of adiponitrile using impure salt increases as supporting electrolyte _o

Example 2

In an electrolytic cell through which an aqueous Solution of tetramethylammonium toluenesulfonate electrolyte was circulated many times, the yield was on adipic dinitrile electrohydrodimerized from acrylonitrile 80 Gev .- # o acrylonitrile> Adipic acid dinitrile, propionitrile and other similar products were made from the Electrolyte extracted ,, the aqueous solution of tetramethylammonium feulüoisulfonat was from 41.8 Grew "- $ to 60 fi

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BATH ORIGINAL

concentrated. The aqueous tetramethylammonium toluenesulfonate solution was then made alkaline with tetramethylammonium hydroxide to adjust its pH to about 14, and then heated until precipitation occurred. The resulting precipitate was then filtered off from the aqueous solution. The aqueous filtrate of 60 wt. - $ » was then diluted to a concentration of 39» 1% by weight
Ge / tetramethylammonium toluenesulfonate purge was then used again for an extended period of time, ie "about 12 days, as an electrolyte for the electrohydrodimerization of acrylonitrile to adipic dinitrile". During this period, the adipic dinitrile yield using the purified electrolyte was 86.1 wt Represents improvement over the untreated salt. In Table II below, the working conditions and results of two experiments are comparatively summarized, whereby in the first experiment a non-alkaline, untreated, aqueous electrolyte is used, this electrolyte is treated according to the method according to the invention and then the treated one Electrolyte is used again for a second attempt.

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untreated alkaline
treated 4 pounds, 8 39.1 15.4 16.2 29.7 27.9 13.7 15.6 8.4 8.7 80.0 86.1 13.3 6.9 2, o «, 7 4.7 5.3

Table II

Alkaline treatment Composition of the catholyte

Tetramethylammonium toluenesulfo-

nat

Acrylonitrile
water
Products
Ka% olyt pH value

yield

Adipic acid dinitrile
Propionic acid nitrile
Bis-oyanoethyl ether
high boiling material

The improvement in the adipic dinitrile content achieved by removing the metallic impurities. prey is clearly visible.

The benefits of removing harmful contaminants, including numerous metals and some organic materials are readily apparent; the product yield is significantly increased »If the amount on impurities that are deposited on the cathode, is significantly reduced, the cell works with larger Effectiveness over a longer period of time, thereby increasing the useful life of a particular cell arrangement will. __

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Claims (1)

! ¥ 93034 -H- Patent application 1 »Methods for removing contaminants, including, in particular copper, nickel, silver, lead and chromium, from a catholyte that is used in electrohydrodimerization from acrylonitrile to adipic acid dinitrile is used, the catholyte having a pH in the range from 6.5 to 12 and includes an aqueous solution of a hydrotropic, quaternary ammonium salt, characterized in that that he (a) the aqueous solution of a hydrotropic, quaternary Separating the ammonium salt from the catholyte, (b) the pH of the aqueous solution to a value at least above 7, which is significantly above -Value
half the pH / Ie s catholyte increases, (c) the salt concentration of the aqueous solution is adjusted to a range between 25 and 85 ° ß , (d) the aqueous solution is ^{heated to a temperature of at most 115 °} C. for a sufficient period of time for the impurities to be precipitated, and (e) separating the impurities from the aqueous solution, 2. The method according to claim 1 _f, characterized in that the quaternary ammonium salt used is tetramethylanimonium sulfonate, tetraethylammonium ethyl sulfate, tetraäthylainmoniurabenzol sult'onat, Träthy line thylainmoniumbenzolsulfonat or Trietnylmethylammoniumtoluolaulfonat, 0Q9830 / 1Ö76 3o method according to claim 1 or 2, characterized in that that you can adjust the salt concentration to a value in the range between 45 and 85 wt .-? 6 and the aqueous solution heated at 80 to 115 ° C, 4 «Method according to one of claims 1 to 3» characterized in that that the precipitation is carried out for a period of 1 to 4 hours » 5 ο method according to one of claims 1 to 4, characterized in that that the pH of the aqueous solution is increased by at least 2 units » 6. The method according to any one of claims 1 to 5 »characterized in that that the pH of the catholyte is set between 7.5 and 10 * BATH ORIGINAL 009830/1876