CH646201A5 - METHOD FOR PRODUCING SEBACINIC ACID FROM ADIPINIC ACID. - Google Patents

Description

The invention relates to a process for the production of sebacic acid from adipic acid, in particular a process for the production of sebacic acid by electrolytic condensation of monomethyl adipate.

The electrochemical condensation reaction of carboxylic acids is commonly referred to as the Kolbe reaction and is described in the literature by H. Kolbe (Ann. 69 (1849) 257, AC Brown (Ann. 261 (1891) 107) and other authors. This reaction uses dimethyl sebacate formed from monomethyl adipate in methanol as solvent and in the presence of an alkali salt of monomethyl adipate.

To produce sebacic acid from adipic acid on an industrial scale, the above-mentioned process must be carried out in three successive stages. The first step is to produce monomethyl adipate. In the second stage, the electrolytic condensation of the monomethyl adipate to give dimethyl sebacate, the separation of the latter from the electrolyte solution containing it and the subsequent purification, and in the third stage the hydrolysis of the dimethyl sebacate takes place with the formation of sebacic acid.

When carrying out the usual processes, the most important problem that still has to be solved seems to be the separation of the dimethyl sebacate in a much purer form from the electrolyte solution containing it in the second stage.

For the solution of the above-mentioned problem in the case where the electrolytic solution essentially contains three constituents, namely monomethyl adipate, an alkali salt of monomethyl adipate and dimethyl sebacate, Japanese laid-open publications 89 317 / 75.149 919 / 78.149 920/78 and 135 921 / 78 as measures that can be carried out on an industrial scale, detailed measures for the separation of purer dimethyl sebacate and for the recovery of monomethyl adipate and its alkali salt from the electrolytic solution which is obtained after the electrolytic condensation.

For the other case, in which the electrolyte solution which is obtained by batch electrolytic condensation contains essentially no monomethyl adipate but essentially two constituents, namely an alkali salt of monomethyl adipate and dimethyl sebacate, Japanese Patent Specifications 37 564/71 and 51 327 / 72 and Japanese Laid-Open Publication 3816/71. Process for the separation and purification of dimethyl sebacate and for the production of an alkali salt of monomethyl adipate from the electrolyte solution. However, these processes are unsatisfactory when used on an industrial scale.

For example, Japanese Laid-Open Specification 51 327/72 describes a process which can be used for large-scale production and which comes close to the process according to the invention. According to the description, the Eleks

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subjected to a treatment to remove methanol, then mixed with water to separate it into an oily layer essentially containing dimethyl sebacate and an aqueous layer essentially containing an alkali salt of monomethyl adipate, followed by evaporation of the aqueous layer is concentrated, whereby the alkali salt is obtained as a saturated aqueous solution.

In this description, however, the water concentration in the solution in which the electrolytic condensation is carried out is not mentioned, except that in one example it is found that the original water content of the electrolytic solution is 2.5 mol / l. It is well known that the water content of an electrolytic solution is closely related to the results of the electrolytic condensation. Japanese laid-open specification 22 317/79 states that both the current yield and the product yield decrease if the water content of the electrolytic solution is not kept in the range of 0.15 to 3.0% by weight during the electrolytic condensation. Accordingly, assuming that the electrolytic condensation is carried out while the water content of the electrolytic solution is kept in the range of 0.15 to 3.0% by weight, there are two problems mentioned below when performing the process described in Japanese Laid-Open Specification 51,327 / 72 procedure described.

The first problem is related to the step of removing methanol from the electrolytic solution. In this case, since there is neither a substantial amount of water nor the monomethyl adipate used as the starting material, the alkali salt of monomethyl adipate in the methanol distillation column would have to be separated off at the bottom when the methanol has been removed in order to lower the methanol concentration. An example of such a case is described in Comparative Example 1 which follows later. In order to completely prevent the precipitation of the alkali salt of monomethyl adipate at the bottom of the methanol distillation column, it appears to be necessary to leave a certain amount of methanol behind after the removal of the methanol. However, this seems to complicate the subsequent measure of extraction of the alkali salt with water, because if the amount of the remaining methanol in the electrolyte solution exceeds a certain limit, the distribution or the transition of the alkali salt into the oily layer is increased during the water extraction treatment. The alkali salt of monomethyl adipate in the oily layer not only causes a loss of material, but also tends to be converted into dimethyl adipate and other compounds in the purification of dimethyl sebacate by distillation of the oily layer and to contaminate the purified dimethyl sebacate. In order to prevent the transition of the alkali salt from monomethyl adipate into the oily layer, it is necessary to use a larger amount of water for the extraction or to repeat the extraction several times. The presence of methanol not only causes an alkali salt of monomethyl adipate to pass into the oily layer, but also interferes with the phase separation between the oily layer and the aqueous layer.

The second problem is related to the removal of water by evaporation from the aqueous layer containing an alkali salt of monomethyl adipate. In the process described in the above-mentioned Japanese patent application 51 327/72, a saturated aqueous solution of the alkali salt obtained by evaporating water is returned to the electrolytic condensation stage. Given the simplicity of regulating the concentration of residual

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Water in the water evaporator and lowering of the temperature in the circuit line leading to the electrolytic condensation, it appears necessary not to remove a certain amount of water and to leave it in the evaporator in order to keep the concentration of the alkali salt of the monomethyl adipate below the saturation point, so that the alkali salt of the monomethyl adipate is precipitated in the water evaporator and is completely prevented in the circulation line. This means that the circulated alkali salt of the monomethyl adipate carries with it a fairly large amount of water, so that it is difficult to keep the water content of the electrolytic condensation solution within the necessary range of 0.15 to 3.0% by weight . As a result, both the current yield and the product yield of the electrolytic condensation are inevitably reduced unless the amount of the alkali salt of the monomethyl adipate that is recycled is kept very low.

In an effort to solve the above problems provided that the electrolytic condensation is carried out while keeping the water content of the electrolytic solution within the range of 0.15 to 3.0% by weight, it has now been found that the first problem can be solved by adding water to the electrolytic solution before or simultaneously with the removal of the methanol, and that the second problem can be solved by adding monomethyl adipate before or after the removal of water from the aqueous layer by evaporation. The invention is based on these findings.

The object of the invention is therefore to make a process for the production of sebacic acid from adipic acid available on an industrial scale in an advantageous manner in the second stage with easy removal of dimethyl sebacate and recovery of an alkali salt from monomethyl adipate.

The objects of the invention are achieved by a process for the production of sebacic acid, in which adipic acid with methanol is subjected to the semi-esterification to form monomethyl adipate, the monomethyl adipate obtained is batch-electrolytically condensed in methanol solution in the presence of an alkali metal salt of monomethyl adipate, while maintains the water content of the solution in the range of 0.15 to 3.0% by weight and thereby receives an electrolytic solution containing dimethyl sebacate, which separates dimethyl sebacate from the electrolytic solution and hydrolyzed to form sebacic acid. The method is characterized in that, after or simultaneously with the addition of water to the electrolytic solution, methanol is removed from the electrolytic solution, and thereby the remaining solution in two layers, namely an oily layer which essentially contains dimethyl sebacate, and an aqueous layer which in essentially contains the alkali salt of monomethyl adipate, the dimethyl sebacate is isolated from the oily layer, while on the other hand the alkali salt of monomethyl adipate as the monomethyl adipate solution is separated from the aqueous layer with the addition of monomethyl adipate to the aqueous layer before or after evaporating water from the aqueous layer removed to such an extent that the water content of the electrolytic solution is maintained in the electrolytic condensation and returns the monomethyl adipate solution to the electrolytic condensation.

The figure shows a flow diagram illustrating one of the embodiments of the invention.

The preparation of monomethyl adipate by semi-esterification of adipic acid in the first stage of the process

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according to the invention is described in detail below.

The starting solution can simply be composed of adipic acid and methanol. However, in order to advantageously produce the monomethyl adipate by suppressing the formation of dimethyl adipate, the starting solution preferably consists of 1 mole of adipic acid, 0.2 to 2 moles of dimethyl adipate, 0.5 to 5 moles of methanol and 1 to 10 moles of water.

The reaction is generally carried out in the presence of acidic catalysts, for example sulfuric acid, hydrochloric acid, nitric acid, other inorganic acids, p-toluenesulfonic acid and other organic sulfonic acids. The reaction conditions are not subject to any particular limitation, but the process is preferably carried out at normal pressure or elevated pressure and at a temperature of 50.degree. It is also possible to carry out the reaction in the absence of catalysts at a higher temperature and pressure, namely a pressure of 2 to 14.7 bar and a temperature of 120 ° C. to 200 ° C.

Strongly acidic cation exchange resins can also be used as acidic catalysts. Compared to the use of a mineral acid or an organic sulfonic acid as a catalyst, the use of strongly acidic cation exchange resins has the disadvantage in terms of the catalytic effect, but advantages in terms of corrosion of the reactor etc. and separation of the reaction product from the catalyst. Furthermore, working without a catalyst is disadvantageous compared to using a catalyst, namely from the standpoint of the reaction conditions and the formation of by-products. It is known that when semi-esterification is carried out in the absence of a catalyst at a higher temperature, pressure and with a longer reaction time, the formation of by-products, e.g. Cyclopentanone and high-boiling compounds, is increased. Taking these aspects into account, it is particularly advantageous to use a strongly acidic cation exchange resin as a catalyst in the production of monomethyl adipate on an industrial scale.

Examples of strongly acidic cation exchange resins which are suitable for the purposes of the invention are polystyrene resins containing sulfonic acid groups, which may be of the gel type or of the porous type. The resin should be used in a fixed bed so that the esterification can be carried out continuously and the resin can develop its activity effectively. With continuous use for a long period of time, the catalytic activity of the strongly acidic cation exchange resin for esterification deteriorates as a result of the accumulation of adsorbed metal ions. If necessary, the resin can be regenerated for reuse in a conventional manner, for example by treatment with aqueous nitric acid. For this purpose, it is appropriate to carry out the adsorption of the metal ions and the esterification reaction separately in two ion exchange resin columns.

Although the reaction rate is favored by a higher reaction temperature in the fixed bed of a strongly acidic cation exchange resin, it is more expedient in view of the heat resistance of the resin to keep the temperature at 60 ° to 90 ° C.

When the starting solution is passed through the fixed bed of the strongly acidic cation exchange resin, the deposition of adipic acid in the fixed resin bed at the working temperature is undesirable. To avoid this, an excess of the amount of solvents, e.g. Methanol, water and dimethyl adipate, required up to a certain limit. However, since the solvent has to be removed from the reaction mixture to remove essentially pure monomethyl adipate, the use of the solvent in a large excess is undesirable. In order to prevent the deposition of adipic acid in large excess without using the solvent, the solvent deficit must be replenished by recirculating part of the solution running out of the fixed bed. The amount of the circulating solution cannot be determined beforehand because it depends on the amounts of adipic acid and the solvent in the starting solution and on the working temperature of the fixed bed, but it is sufficient to keep the amount of the circulating so high that deposition of adipic acid is avoided. Likewise, the feed rate of the starting solution to the fixed bed is not particularly limited, but it is preferably set so that the reaction in the fixed bed can proceed close to the equilibrium point.

The conversion of the monomethyl adipate obtained in the above-described step to dimethyl sebacate by electrolytic condensation in the second step of the process according to the invention is described in detail below.

To prepare a feed solution for the electrolytic condensation, the monomethyl adipate obtained in the previous step and the alkali metal salt circulated from the cleaning step of the electrolytic solution described later are dissolved in the methanol, which is also recycled from the cleaning step of the electrolytic solution becomes.

If the water content of the feed solution prepared in this way is less than 0.15% by weight, the current yield becomes extremely low, while if the water content is above 3.0% by weight, both the current yield and the product yield become considerably lower. It is therefore necessary to keep the water content in the range of 0.15 to 3.0% by weight in order to carry out the electrolytic condensation while maintaining both a high current yield and a high product yield.

The electrolytic condensation is carried out in batches until the monomethyl adipate is essentially used up. The remaining monomethyl adipate in the electrolytic solution after the completion of the electrolytic condensation is largely transferred to the oily layer when, as will be described later, the electrolytic solution is treated with water for the purpose of cleaning. The amount of monomethyl adipate that passes into the oily layer must be kept low by using a large amount of water because the monomethyl adipate forms an azeotrope with dimethyl sebacate and cannot be removed by distillation and therefore remains as an impurity in the purified dimethyl sebacate. The residual amount of monomethyl adipate is therefore preferably kept as low as possible by continuing the electrolytic condensation until the residual amount, with regard to the purity of the dimethyl sebacate, is at most 0.1% by weight of the monomethyl adipate used.

The concentration of the monomethyl adipate in the feed solution is expediently 10 to 60% by weight. When the concentration exceeds 60% by weight, the cell voltage becomes higher, while a concentration below 10% by weight results in a deterioration in the volume utilization factor and the current efficiency.

To increase the conductivity of the electrolytic solution for the electrolytic condensation, neutralizing bases, e.g. Hydroxides, carbonates, bicarbonates, methylates or ethylates of lithium, potassium and sodium or amines can be used. However, amines are oxidized at the anode, which speeds up anode consumption

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will, while lithium compounds cause a reduction in electricity yield. Therefore hydroxides, carbonates, bicarbonates and methylates of sodium or potassium are preferably used. Further, since, as will be described later, an alkali salt of monomethyl adipate from the cleaning treatment of the electrolytic solution is circulated in the form of a solution in monomethyl adipate, the use of hydroxides, carbonates or bicarbonates of potassium is particularly preferred in view of the solubility. The degree of neutralization of the monomethyl adipate (the degree of neutralization is defined as the amount of monomethyl adipate neutralized with a base in mol%) in the feed solution is preferably 5 to 50 mol%. If it is below 5 mol%, the cell voltage becomes high, while a degree of neutralization above 50 mol% results in a reduction in the current efficiency. For the purposes of the invention, an electrolysis cell which is normally used for organic electrolytic reactions and which allows the electrolyte solution to flow between the two electrodes at a high flow rate is suitable. For example, a cathode plate and an anode plate, which face each other, and a polypropylene plate, which determines the distance between the two electrodes, are inserted into the electrolysis cell. The central part of the polypropylene plate is provided with an opening that allows the electrolyte solution to pass through. The effective surface area of the electrodes is determined by the area of the opening, and the electrode spacing depends on the thickness of the polypropylene plate. The circulating electrolytic solution from a reservoir for the electrolytic solution enters the cell through an inlet opening provided in the electrolysis cell, then flows between the electrodes to initiate the reaction and leaves the electrolysis cell through an outlet opening to enter the reservoir to return.

Platinum, rhodium, ruthenium, iridium and the like are used as electrode materials for the anode. Like. Which are used alone or as alloys and are usually plated on a metal sheet serving as a base, for example made of titanium or tantalum. Materials which have a low hydrogen overvoltage but are not limited to special metals are preferably used for the cathode. Platinum, iron, stainless steel and titanium are suitable.

The flow rate of the electrolyte solution in the electrolysis cell is preferably 1 to 4 m / sec. If it is less than 1 m / sec. the current yield is lower, while when exceeding 4 m / sec. the pressure loss in the cell increases. The distance between the electrodes is preferably 0.5 to 3 mm. If the distance is less than 0.5 mm, the pressure loss in the electrolysis cell increases, while if it exceeds 3 mm, the cell voltage increases. The current density is preferably 5 to 40 A / dm2. If it is less than 5 A / dm2, the current efficiency becomes lower. The temperature of the electrolytic solution is preferably 45 ° to 65 ° C. If the temperature falls below 45 ° C, the current efficiency will decrease and the cell voltage will increase. The upper temperature limit depends on the boiling point of the electrolyte solution.

The measures in the next stage, i. H. described in the cleaning of the electrolyte solution, in which dimethyl sebacate is separated from the electrolyte solution obtained in the electrolytic condensation in the second stage of the process and then purified, and the alkali salt of monomethyl adipate is obtained.

The step of cleaning the electrolyte solution includes the

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Removal of methanol, the separation of dimethyl sebacate from the alkali azate of monomethyl adipate, the purification of the separated dimethyl sebacate and the recovery and recycling (for electrolytic condensation) of the separated alkali salt from monomethyl adipate.

Methanol is removed from the electrolyte solution after or simultaneously with the addition of water to the electrolyte solution. The water can be added directly to the methanol distillation column or to the electrolyte solution which is fed to the methanol distillation column. The water is added in an amount sufficient to prevent the alkali salt of monomethyl adipate from being separated at the bottom of the methanol distillation column. It is preferably 0.5 to 5 parts by weight per part by weight of alkali metal salt of monomethyl adipate. If the amount of water added is less than 0.5 part by weight, the amount of methanol that will not be removed must be increased, while if the amount of water is more than 5 parts by weight, hydrolysis of the ester linkage may take place.

The removal of methanol in the methanol distillation column is conveniently carried out under normal pressure. The residual amount of methanol in the distillation residue after removal of a large proportion of the methanol must be as small as possible in order to keep the amount of the alkali salt of monomethyl adipate which passes into the oily layer in the subsequent phase separation treatment as low as possible. In industrial operation, the remaining methanol is preferably 6% by weight or less, with a residual content of 3% by weight or less being particularly preferred. When the methanol is removed, the temperature of the distillation column at the bottom rises to 80 ° C. or higher, and at such a temperature hydrolysis of the ester linkage can take place if the residue is retained on the bottom for a long time. The dwell time should be less than 2 hours. The shorter it is, the better.

To remove dimethyl sebacate from the alkali salt of the monomethyl adipate, the residue is cooled after the removal of methanol and left to stand, separating into an oily layer and an aqueous layer. As already described, when the amount of methanol remaining exceeds 6% by weight in the residue after removal of methanol, a larger amount of the alkali salt of monomethyl adipate passes into the oily layer, and at the same time, the separation becomes an oily layer and an aqueous layer Make the layer less smooth, although the extent of the transition of the salt depends on the content of salt and water in the residue. As the methanol content of the residue increases, the oily phase tends to be the lower layer, although this also depends on the alkali salt content of monomethyl adipate and the water content of the residue. Accordingly, a methanol content of the residue in the critical area, in which a reversal of the relative position of the oily layer and the aqueous layer takes place, must be avoided.

The dimethyl sebacate can be purified by conventional distillation of the separated oily layer.

The recovery of the alkali salt of monomethyl adipate and the return of the recovered salt for electrolytic condensation are carried out as follows: Before the water is removed by evaporation, monomethyl adipate is added to the separated aqueous layer. Alternatively, if the amount of the alkali salt of monomethyl adipate to be recycled is small, water is removed from the aqueous layer until a saturated aqueous solution of the alkali salt is obtained, after which monomethyl adipate is added.

In the former case, in the monomethyl adipate the aqueous

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If the layer is added by evaporation before the removal of water, the water content of the residue can be easily regulated because there is no precipitation of the alkali salt of monomethyl adipate. It is therefore possible to regulate the water content very easily in the above-mentioned second stage of the process. The monomethyl adipate added to the separated aqueous layer can be replaced by the starting solution for the electrolytic condensation. The amount of monomethyl adipate to be added must be sufficient to prevent the separation of alkali salt of monomethyl adipate from the residue when the water has been removed as much as possible. The addition of monomethyl adipate in large excess requires a water evaporator with an increased capacity.

In industrial operation, 1 to 10 parts by weight of monomethyl adipate are preferably added per part by weight of alkali metal salt of monomethyl adipate. In order to avoid a chemical change as a result of a thermal effect, the dwell time of the residue in industrial operation is preferably up to 10 minutes, the shorter the dwell time, the greater the advantage. Since the water vapor from the evaporator accompanies the monomethyl adipate, even in small quantities, it is more expedient to provide a column for recovering the entrained monomethyl adipate.

In the other case, in which the water is removed from the aqueous layer without the prior addition of monomethyl adipate, it is necessary to leave the water in an amount sufficient to completely dissolve the alkali salt of monomethyl adipate in order to regulate the water content of the remaining solution upon removal of the water and to prevent the separation of the alkali salt of monomethyl adipate. It is also necessary to add monomethyl adipate once the water has been removed to prevent the deposition of the alkali salt of monomethyl adipate from the remaining solution as a result of a drop in temperature on the way back to electrolytic condensation. In industrial operation, the monomethyl adipate is preferably added in an amount of 0.5 to 5 parts by weight per part by weight of alkali metal salt of monomethyl adipate. Due to the high temperature of the residual solution after removal of water by evaporation, it is impractical to add methanol instead of monomethyl adipate. The alkali salt of monomethyl adipate, accompanied by a certain amount of water, is returned to the electrolytic condensation stage. Accordingly, the alkali salt content of monomethyl adipate in the circulating solution must be kept at a certain concentration, preferably 4.5% by weight or lower, in order to keep the water content of the feed solution for electrolytic condensation at 3.0% by weight or lower . The residence time of the aqueous solution in the water evaporator can be 10 minutes or less in industrial operation.

As described in detail above, according to the invention it is possible to keep the water content of the electrolytic solution at a concentration in the range of 0.15 to 3.0% by weight during the entire batch-wise electrolytic condensation by the following improved methods: When cleaning the Electrolytic solution after the electrolytic condensation, methanol is removed after adding water. When water is removed by evaporation from the aqueous layer containing the alkali salt of monomethyl adipate, monomethyl adipate is added to the aqueous solution prior to the removal of water or, alternatively, if the amount of alkali salt returned to the electrolytic condensation of

Monomethyl adipate is small, water is removed from the aqueous layer until a saturated solution of the alkali salt is formed, whereupon monomethyl adipate is added immediately. The alkali salt of monomethyl adipate recovered in this way in the form of a solution in monomethyl adipate is returned to the electrolytic condensation so that the water content of the electrolyte solution can be kept at a concentration in the range from 0.15 to 3.0% by weight. It is thus possible to design a method in which the batch electrolytic condensation is carried out with a high current yield and high product yield and the electrolyte solution can be cleaned very easily.

In the third step of the process according to the invention, the dimethyl sebacate separated and purified in the previous step is hydrolyzed in the manner described in detail below.

In order to bring the hydrolysis to completion and to increase the purity of the sebacic acid by avoiding contamination with unreacted components, for example monomethyl sebacate, the reaction is carried out in batches on account of the economic advantage over a continuous process.

The hydrolysis can be carried out using catalysts, e.g. inorganic acids, e.g. Nitric acid, sulfuric acid and hydrochloric acid, or organic sulfonic acids, e.g. p-toluenesulfonic acid. Of these, nitric acid is preferred in view of the corrosion of the materials of the reactor, etc., and the reaction rate. The reaction can be rapidly completed by adding to an aqueous solution containing 10 to 20% by weight of an acid catalyst, dimethyl sebacate in an amount not exceeding the solubility at the reaction temperature, and the reaction to remove the released methanol is carried out. It is also possible to carry out the hydrolysis first in water and then in aqueous nitric acid. In this case, the hydrolysis of the dimethyl sebacate can be carried out by adding water in 4 times the amount by weight of the dimethyl sebacate for 2 to 5 hours at 220 to 280 ° C under a pressure of 34.3 to 49 bar and then in aqueous solution containing 12 to 25 wt. % Contains nitric acid, be carried out at 80 to 100 ° C for 0.5 to 1.5 hours. It is also possible to carry out the hydrolysis by adding sebacic acid, the desired product, in order to accelerate the reaction and to continuously remove the methanol released. This method has the advantages over hydrolysis in the presence of nitric acid that the reaction can be carried out in one step at a relatively low temperature at an acceptable rate without causing a chemical change in the methanol and without the subsequent removal of the catalyst being necessary , although the process is disadvantageous from the point of view of the reaction rate. When using nitric acid as a catalyst, it reacts with a part of the released methanol to form methyl nitrate, even if the reaction is carried out with removal of the methanol. Furthermore, the process using sebacic acid has an advantage over the color of the product over the process described above in which the hydrolysis is carried out in the first half of the reaction time without a catalyst. Taking into account the above facts, it is particularly advantageous in industrial operation to carry out the hydrolysis of dimethyl sebacate by adding sebacic acid with removal of the methanol released by the reaction from the reaction system.

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The accelerating effect of sebacic acid is particularly pronounced in the first half of the reaction time, in which two phases, namely dimethyl sebacate and water, are present simultaneously. The amount of sebacic acid to be added is 1 to 20% by weight, based on the weight of dimethyl sebacate. In addition to the addition of sebacic acid, which is a reaction product, to accelerate the hydrolysis, it is advantageous to continuously withdraw the methanol formed by the reaction from the reaction system and, at the same time, to continuously add water in order to increase the deficit caused by the removal of water together with the methanol complete. The hydrolysis can be carried out to completion in this way. Although the methanol can be removed immediately after the beginning of the first half of the reaction in which dimethyl sebacate and water are formed as a heterogeneous mixture, it is preferably removed in the second half of the reaction in which the two phases form a homogeneous solution because the effect of removing methanol is stronger in this phase. The removal of methanol from the reaction system by evaporation is accompanied by the evaporation of water. The amounts of methanol and water which are removed are preferably 2 to 6 parts by weight per part by weight of the dimethyl sebacate used. The water content of the mixture of the reactants is preferably kept at 10 to 75% by weight during the entire reaction time.

A high reaction temperature is advantageous for the reaction rate, but leads to an increased reaction pressure and discoloration of the reaction product. The discoloration becomes particularly severe when the temperature exceeds 220 ° C. The reaction is preferably at 180 to 220 ° C. during its first half, in which the mixture of the reactants remains in the form of a heterogeneous solution, and then at 150 to 220 ° during the second half, in which the mixture forms a homogeneous solution C performed.

One of the embodiments of the invention is described below with reference to the flow diagram in the figure.

In the first stage, 2 adipic acid and methanol are fed to inlet 1 through a dissolving tank. Methanol and water, which are withdrawn from the top of the distillation column 5, dimethyl adipate from the top of the distillation column 7, adipic acid and monomethyl adipate from the bottom of the distillation column 8 and part of the reaction solution from the branched outlet 9 are returned to the dissolving tank 2. Adipic acid is dissolved in the dissolving container 2, and the solution formed is applied to the top of an ion exchange resin column 3 as a starting solution. Small amounts of metal ions are adsorbed in the starting solution in column 3. Furthermore, the esterification reaction takes place partially. The starting solution freed from metal ions emerges at the bottom of the column 3 and is applied to the top of the ion exchange resin column 4. The esterification reaction mainly takes place in column 4. As soon as the discharge from the bottom of the column 3 shows the presence of metal ions in a concentration which exceeds a predetermined limit, the ion exchange resin must be regenerated in the usual way by treatment with, for example, aqueous nitric acid. A part of the esterification reaction solution running from the bottom of the column 4 is returned to the dissolving tank 2 through the branched outlet 9, and the rest is fed to the distillation column 5. Methanol and water, which pass at the top of column 5, are returned to dissolving tank 2, while the residue taken off at the bottom is fed to a distillation column 6. Be in column 6

Distilled water and cyclopentanone. The residue drawn off from the bottom is fed to a distillation column 7. In the column 7, dimethyl adipate is distilled off, and the residue taken off from the bottom of the column is fed to a distillation column 8. The dimethyl adipate drawn off from the top of the distillation column 7 is returned to the dissolving tank 2. Monomethyl adipate is withdrawn from the top of the distillation column 8, and the residue containing adipic acid and monomethyl adipate is withdrawn from the bottom of the column and circulated to the dissolving tank 2. The monomethyl adipate drawn off from the top of column 8 is fed to the second stage of the process.

In the second stage, the monomethyl adipate drawn off from the top of the distillation column 8 in the first stage is fed to an electrolyte solution container 12. The methanol obtained from the top of the distillation column 14 and the monomethyl adipate solution drawn off from the bottom of the recovery column 20 and containing the alkali salt of monomethyl adipate are supplied to the electrolyte solution tank 12. The solution formed in the container 12 is circulated through an electrolysis cell 13 in which the electrolytic condensation takes place. After the electrolytic condensation has ended, the electrolyte solution is drawn off from the container 12 and mixed with water which is fed from the top of the recovery column 20 via an inlet 21. The water-containing electrolyte solution thus formed is fed to the distillation column 14. In the distillation column 14, methanol is distilled off overhead and returned to the electrolyte solution tank 12, while the residue withdrawn from the bottom of the column is fed to a decanter 15 where it is left to stand, separating into two layers.

The oil layer is fed to a distillation column 16, while the aqueous layer drawn off from the bottom is mixed with monomethyl adipate, which is fed from a branched outlet 11 via an inlet 22. The resulting mixture is led to the bottom of an evaporator 19, where water is evaporated. The evaporated water is fed to the recovery column 20 together with the residue. The water is withdrawn from the top of the recovery column 20 and returned to the entry port 21, while a monomethyl adipate solution containing the alkali salt of monomethyl adipate is withdrawn from the bottom of the column 20 and returned to the electrolyte solution tank 12. In the distillation column 16, the low-boiling impurities are distilled off, while the residue is fed to a distillation column 17. Low-boiling impurities are stripped off from the residue in column 17, whereupon the residue is fed to a distillation column 18. Dimethyl sebacate is obtained from the top of column 18 and fed to the third stage of the process. High-boiling impurities are drawn off from the bottom of column 18.

In the third stage, dimethyl sebacate, which is drawn off from the top of the distillation column 18 in the second stage, is fed to a reactor 23. Sebacic acid and water are introduced through an inlet 27. After the reaction has been initiated, heating is continued until a homogeneous solution has been formed. After formation of the homogeneous solution, methanol and water are continuously drawn off and fed to a distillation column 24. In column 24, methanol is distilled off at the top and fed to inlet 10 to the first stage, while water is withdrawn from the bottom as a residue and is continuously recycled to reactor 23. After the hydrolysis in the reactor 23 has ended, activated carbon is replaced by an

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occurs 27 added to decolorize the reaction solution. An aqueous solution, which contains the activated carbon and sebacic acid, is discharged from the reactor 23 and fed to a filter 25 for separation, the activated carbon. The filtrate is passed to an evaporator 26. Water is evaporated and drawn off in the evaporator 26. Melted sebacic acid is withdrawn from the evaporator 26 and fed to a flaking device or a granulator to form flaky or granular sebacic acid.

The invention is further illustrated by the following examples.

example 1

Sebacic acid was prepared from adipic acid according to the flow diagram in the figure. The preparation of monomethyl adipate in the first stage was carried out as follows:

the column 7 passing dimethyl adipate was returned to the dissolving tank 2, while the residue, which contained adipic acid and monomethyl adipate, was drawn off from the bottom and fed to the distillation column 8, s The temperature of the columns 8 operating at the bottom under reduced pressure of 27 mbar became 210 ° C. The monomethyl adipate passing at the top of column 8 in an amount of 15 kg / h was used to prepare the electrolytic solution for the electrolytic condensation. A solution containing adipic acid and monomethyl adipate was withdrawn from the bottom of column 8 and returned to dissolving tank 2.

Table 1

15

20th

35

40

13.8 kg of adipic acid and 3.0 kg of methanol were fed to the dissolving tank 2 through the inlet 1 per hour. Into the container 2, circulating methanol and water were also introduced from the top of the distillation column 5, dimethyl adipate from the top of the distillation column 7, adipic acid and monomethyl adipate from the bottom of the distillation column 8 and part of the reaction solution from the branched outlet 9. While the temperature of the container 2 was kept at 80 ° C for 2 seconds, the solution of the raw material formed in the container was charged in an amount of 240 kg / hour. withdrawn and placed on top of the ion exchange resin column 3. The solution of the starting materials had the following composition: 28.0% by weight adipic acid, 20.3 30% by weight dimethyl adipate, 30.8% by weight monomethyl adipate, 5.7% by weight methanol, 14.9% by weight water and 0.3% by weight cyclopentanone. This solution was passed at 80 ° C. through an ion exchange resin column 3 which was filled with 151 of a regenerated strongly acidic cation exchange resin of the H + type (Amberlite 200 C, manufacturer Rohm and Haas Co.). The resin was continuously regenerated in operation every 200 hours using N-nitric acid as a regenerant. The drain from the base of column 3 was placed on top of another ion exchange resin column 4, which was filled with 601 regenerated strongly acidic cation exchange resin of the H + type "Amberlite 200 C". The temperature of the solution was kept constantly at 80 ° C. The reaction solution was from the base of the ion exchange resin column 4 in an amount of 240 kg / hour. 45 deducted. A quarter of the reaction solution was fed to the distillation column 5, and three-quarters were fed to the dissolving tank 2 via the branched outlet 9. The reaction solution drawn off from the base of the ion exchange resin column 4 had the following composition: 22.0% by weight adipic acid, 20.2% by weight dimethyl adipate, 37.4% by weight monomethyl adipate, 4.3% by weight methanol, 15. 7% by weight of water and 0.3% by weight of cyclopentanone. The changes in the iron ion and cyclopentanone content of the solution withdrawn from the ion exchange resin column 4 with S5 of time are shown in Table 1. The temperature at the bottom of the distillation column 5 operating at normal pressure was set to 140.degree. Methanol and water which passed at the top of column 5 were returned to dissolving tank 2, while the residue taken off from the bottom was fed to distillation column 6. The temperature at the bottom of the column 6 operating under reduced pressure of 266 mbar was set to 175 ° C. Water and cyclopentanone were distilled off from the top of column 6, while the residue withdrawn from the bottom was fed to distillation column 7. The temperature at the bottom of the column 7 operating under normal pressure of 27 mbar was set at 200 ° C. That on the head

Elapsed time, hours

Cyclopentanone content,% by weight

Iron ion content, ppm

50

0.3

5

100

0.3

5

150

0.3

5

200

0.3

5

The following construction materials were used for the above-mentioned apparatus: Dissolving tank 2: SUS 304 (JIS); Ion exchange resin columns 3 and 4: SUS 304; Distillation columns 5 and 6: SUS 304; Distillation column 7: SUS 316; Distillation column 8: Titan (reboiler) and SUS 316 (other parts).

In the second stage, dimethyl sebacate was produced as follows:

In the electrolytic solution tank 12, the monomethyl adipate produced in the first stage, methanol passing from the top of the distillation column 14 and a monomethyl adipate solution drawn off from the bottom of the recovery column 20, which contained the potassium salt of monomethyl adipate. The feed solution had a total weight of 500 kg and consisted of a methanol solution containing 35.7% by weight of monomethyl adipate, 5.0% by weight (the degree of neutralization corresponded to 10 mol%) of the potassium salt of monomethyl adipate and 1.8% by weight of water contained. The feed solution was circulated through the electrolyte solution tank 12 and the electrolysis cell 13. The electrolysis cell was provided with an anode, a cathode and three intermediate electrodes. Each electrode consisted of a 100 cm x 100 cm titanium sheet 3 mm thick. The anode and the three intermediate electrodes were plated on one side with platinum in a thickness of 3 μm. The electrodes were held at a distance of 1 mm by inserting polypropylene plates of 1 mm thickness between adjacent electrodes. The effective area of the electrodes was 277 dm2 in total The electrolysis cell was provided with an inlet and an outlet for the solution The electrolysis was carried out for 13.7 hours, the electrolyte solution being passed between the electrodes at a linear flow rate of 2.0 m / sec and the Current density at 10.1 A / dm2 and the temperature of the solution at 53 to 56 ° C. The cell voltage per pair of electrodes changes from 7.5 V to 5.7 V. The amount of the electrolyte solution after the electrolytic reaction was complete was 445 It had the following composition, determined by gas chromatography: 23.0% by weight of dimethyl sebacate, 0.01% by weight of monomethyl adipate, 1.8% by weight of water The yield for the formation of dimethyl sebacate was 62.2% and the material yield 79.8%.

After the electrolytic reaction was completed, the electrolytic solution was withdrawn from the electrolytic solution tank 12 and mixed with 46 kg of water, which was withdrawn from the top of the recovery column 20 and supplied to the electrolytic solution through the outlet 21. The electrolytic solution mixed with the water was then supplied to the distillation column 14. The temperature at the bottom of the column 14 operating at normal pressure was kept at 98 ° C. Methanol was distilled off from the top of column 14 while a mixture of oil and water was withdrawn from the bottom. The average residence time at the bottom of column 14 was 1 hour and the average methanol content of the mixture drawn off from the bottom was 1.1% by weight. After cooling, this mixture was fed to the decanter 15 and left to stand for 1 hour,

whereby it separated into two layers, an oil layer and a water layer. The oily layer weighed 139 kg and contained 0.01% by weight of potassium salt of monomethyl adipate. The oily layer was fed to the distillation column 16. In the column 16 operating at normal pressure, the bottom temperature was set to 170 ° C. Low-boiling by-products were distilled off from column 16 overhead, and the residue was sent to distillation column 17. In the column 17 operating under reduced pressure of 27 mbar, the tray temperature was set to 185 ° C. Low-boiling by-products were distilled off from column 17 overhead, while the residue was fed to distillation column 18. In the column 18 operating under reduced pressure of 27 mbar, the tray temperature was set to 230 ° C. Dimethyl sebacate was withdrawn from the top of column 18 and fed to the third stage. The dimethyl sebacate had a purity of 99.9% or more (the purity as a concentration in% by weight was determined by gas chromatography). The aqueous layer withdrawn from the decanter 15 was mixed with 71 kg of monomethyl adipate withdrawn from the outlet 11 and introduced through the inlet 22. The resulting mixture was fed to the bottom of the evaporator 19. In the evaporator 19 operating at normal pressure, the temperature was set to 120 ° C. The water vapor emerging at the top of the evaporator 19 and the residue were fed to the recovery column 20. Water containing no monomethyl adipate was withdrawn from the top of the recovery column 20 and returned to the inlet 21. A monomethyl adipate solution containing the potassium salt of monomethyl adipate was withdrawn from the bottom of column 20 and returned to the electrolysis plant. The total residence time of the solution in the evaporator 19 and in the recovery column 20 was 3 minutes. The water content of the monomethyl adipate solution containing the potassium salt of monomethyl adipate and withdrawn from the bottom of the recovery column 20 was 6.8% by weight.

In the third stage, sebacic acid was produced as follows:

210 kg of the dimethyl sebacate obtained in the second stage, 9.8 kg of sebacic acid and 130 kg of water were introduced into the reactor 23. With vigorous stirring, the feed mixture was heated to a reaction temperature of 180 ° C. to allow the reaction to take place. The reaction mixture was initially heterogeneous, but became a homogeneous solution after a reaction time of 5.0 hours. As soon as the mixture became homogeneous, methanol and water were continuously distilled off from the reactor 23 and fed to the distillation column 24. In the column 24 working at normal pressure, the soil temperature was set to 100 ° C.

9 646 201

and the overhead methanol was fed to inlet 10 to the first stage. The water freed from the methanol was withdrawn from the bottom of the column 24 and returned to the reactor 23. The reaction in the reactor for 23 seconds was carried out for a further 10 hours while continuously removing methanol and water and the missing water by means of circulating water from the distillation column

24 and a small amount of fresh water was added through the inlet 27. A total of 956 kg of methanol and water

10 were distilled off, and the total amount of water supplied was 996 kg. The pressure in reactor 23 was kept at 6.9 to 8.8 bar (manometer pressure) throughout the reaction. The degree of hydrolysis after the completion of the reaction was 99.8 mol%. The degree of hydrolysis was

15 dete sebacic acid in mol%, based on the dimethyl sebacate expressed.

After the reaction had ended, 1 kg of activated carbon (special quality S-C, manufacturer Nihon Jescoal Industry Co.) was introduced into the reactor 23 through the inlet 27, whereupon the reaction mixture was stirred at 130 ° C. for 1 hour. After the decolorization treatment with activated carbon was completed, the reaction mixture was removed from the reactor and fed to the filter 25 to filter out the activated carbon. The filtrate was fed to the evaporator 26 where it was heated to a final temperature of 180 ° C to remove the water by evaporation. The molten sebacic acid was fed to a flaking device to form sebacic acid in flake form. The sebacic acid produced in this way had a melt color of 70, an ash content of 50 ppm, a moisture content of 0.05% by weight and an acid number of 552 mg KOH / g. For measuring the color of the melt

25 g sebacic acid heated at 150 ° C for 20 minutes. The color of the sebacic acid in the molten state was changed after the

35 American Public Health Association (APHA) method.

Example 2

The experiment described in Example 1 was repeated, 40 but the dimethyl sebacate in the second stage was prepared as follows:

In the electrolytic solution tank 12, monomethyl adipate, methanol and a monomethyl adipate solution containing 45 the potassium salt of monomethyl adipate, which was returned from the bottom of the recovery column 20, were charged. The feed solution weighed a total of 500 kg and contained 33.0% by weight of monomethyl adipate, 10.0% by weight (the degree of neutralization corresponded to 20 mol%) of the potassium 50 salt of monomethyl adipate and 1.2% by weight of water. Using a similar electrolytic cell as in Example 1, the solution was circulated through the electrolytic solution tank 12 and the electrolytic cell 13. The feed solution was at a linear flow rate of 2 m / sec. passed between the electrodes. The electrolysis was carried out for 6.3 hours while the current density was kept at 20 A / dm2 and the temperature of the solution at 55 to 58 ° C. The cell voltage per pair of electrodes changed from 9.2 to 7.6 V. After the end of the 60 reaction, the electrolyte solution weighed 442 kg. It had the following composition, determined by gas chromatography: 21.5% by weight of dimethyl sebacate, 0.02% by weight of monomethyl adipate, 1.0% by weight of water. The current yield was 63.5% and the material yield 80.1%. After completion of the reaction, the electrolytic solution was withdrawn from the electrolytic solution tank 12 in the same manner as described in Example 1, mixed with 80 kg of water, and supplied to the distillation column 14. The solution was in the

646201

10th

Column treated in the manner described in Example 1. A mixture of oil and an aqueous solution was drawn off from the bottom. The average residence time at the bottom of the column was 0.8 hours and the average methanol content of the mixture drawn off from the bottom was 2.1% by weight. The mixture separated into two layers in the decanter 15 as in Example 1. The oily layer weighed 130 kg and contained 0.03% by weight of the potassium salt of monomethyl adipate. The aqueous layer weighed 141 kg. As in Example 1, the oily layer was cleaned in the distillation columns 16, 17 and 18. The dimethyl sebacate drawn off from the top of the distillation column 18 was fed to the third stage. The purity of the dimethyl sebacate obtained was 99.9% or more. The aqueous layer from the decanter 15 was mixed with 150 kg of monomethyl adipate, which was fed through the inlet 22, and passed to the evaporator 19 and the recovery column 20 as in Example 1. In the evaporator 19 operating at normal pressure, the temperature was set at 128 ° C. The residence time of the solution in the evaporator 19 and in the recovery column 20 was 4 minutes in total. The water content of the monomethyl adipate solution containing the potassium salt of monomethyl adipate and withdrawn from the bottom of the recovery column 20 was 2.3% by weight.

Example 3

The experiment described in Example 1 was repeated, but the dimethyl sebacate was prepared in the second stage in the manner described below.

In the electrolytic solution tank 12, monomethyl adipate, methanol and the monomethyl adipate solution returned from the evaporator 19 containing the potassium salt of monomethyl adipate were put. The feed solution weighed a total of 500 kg and contained 25.0% by weight of monomethyl adipate, 2.3% by weight (the degree of neutralization corresponded to 6.9 mol%) of the potassium salt of monomethyl adipate and 2.9% by weight of water. The feed solution was electrolyzed for 9.6 hours in the manner described in Example 1. The cell voltage per pair of electrodes changed from 8.8 to 6.2 V.

After the electrolysis reaction was complete, the electrolyte solution weighed 446 kg and contained 16.1% by weight of dimethyl sebacate and 0.01% by weight of monomethyl adipate. The current yield was 62.3% and the material yield 79.9%. After the electrolytic reaction had ended, as in Example 1, the solution was mixed with water, methanol was removed in the distillation column 14 and separated into two layers, an oil layer and an aqueous layer, in the decanter 15. The oily layer weighed 98 kg and contained 0.01% by weight of the potassium salt of monomethyl adipate. In the manner described in Example 1, the oily layer in the distillation columns 16, 17 and 18 was cleaned, with dimethyl sebacate being drawn off from the top of the column 18. The dimethyl sebacate was 99.9% or more pure. The aqueous layer coming from the decanter, weighing 70 kg, was fed directly to the evaporator 19. In the evaporator 19 operating at normal pressure, the temperature was set to 110 ° C. Water vapor passed overhead and an aqueous solution containing 47% by weight of the potassium salt of monomethyl adipate was obtained as a side stream. The aqueous solution was quenched by adding 100 kg of monomethyl adipate to obtain a monomethyl adipate solution containing the potassium salt of monomethyl adipate.

Example 4

The experiment described in Example 1 was repeated, but the dimethyl sebacate in the second stage was prepared as follows:

The electrolytic condensation was based on that in Example

1 described manner, but using the sodium salt of monomethyl adipate instead of the potassium salt. The cell voltage per pair of electrodes s was changed from 7.6 V to 5.6 V. The current yield was 61.3% and the material yield 79.1%. After the reaction had ended, as in Example 1, the electrolyte solution was mixed with water, then methanol was removed in the distillation column 14 and separated into an oily layer and an aqueous layer in the decanter 15. The oily layer containing 0.01% by weight of the sodium salt of monomethyl adipate was purified in the distillation columns 16, 17 and 18 as described in Example I. The dimethylse-bacat drawn off from the top of column 18 showed a purity of 99.9% or more. The aqueous layer from the decanter 15 was mixed with 150 kg of monomethyl adipate, which was supplied through the inlet 22. The mixture was passed to evaporator 19 as in Example 1. A monomethyl adipate solution containing the sodium salt of monomethyl adipate was withdrawn from the recovery column 20 as a bottom stream. The water content of this solution was 4.1% by weight.

Comparative Example 1 25 The electrolytic condensation was based on that in Example

2 performed way described. After the condensation had ended, the electrolyte solution was fed to the distillation column 14 immediately without adding water beforehand. In column 14, methanol was among the in

30 Example 2 conditions removed. When the methanol content of the bottom stream from column 14 was adjusted to 9.2% by weight, partial decomposition of the sodium salt of monomethyl adipate was found.

35

Reference Example 1

In the manner described in Example 1, methanol, monomethyl adipate and a monomethyl adipate solution containing the potassium salt of monomethyl adipate were added to the electrolyte solution container 12. The feed solution weighing a total of 500 kg contained 35.5% by weight of monomethyl adipate, 5.5% by weight (the degree of neutralization corresponded to 11 mol%) of the potassium salt of monomethyl adipate and 4.5% by weight of water. The electrolysis was carried out in the manner described in Example 1 45. The cell voltage per pair of electrodes changed from 6.8 V to 5.3 V. After the electrolytic reaction had ended, the electrolyte solution weighing 443 kg contained 20.5% by weight of dimethyl sebacate and 0.01% by weight of monomethyl adipate. The current yield was 55.2% and the material yield 71.1%.

Example 5

The experiment described in Example 2 was repeated, but the dimethyl sebacate was prepared in the manner described below 55.

500 kg of a solution which contained 33.0% by weight of monomethyl adipate, 10.0% by weight of potassium salt of monomethyl adipate and 0.7% by weight of water were prepared in the manner described in Example 2. The solution was placed in the electrolyte-60 solution tank 12, whereupon the electrolytic condensation was carried out in the manner described in Example 2. The cell voltage changed from 9.3 V to 7.6 V. After the electrolysis reaction had ended, the electrolyte solution weighing 443 kg contained 21.6% by weight of dimethyl 65 sebacate, 0.01% by weight of monomethyl adipate and 0.6% by weight .% Water. The current yield for the formation of dimethyl sebacate was 63.8% and the product yield 80.5%. In the manner described in Example 2, the electrolytic solution

11

646201

after the reaction has ended, mixed with 40 kg of water which was fed in through inlet 21 and freed of methanol in distillation column 14. The mean residence time in column 14 was 0.6 hours and the methanol content of the bottom stream was 2.9% by weight. The bottom stream was separated into an oily layer and an aqueous layer in the decanter 15. The oily layer which contained 0.06% by weight of potassium salt of monomethyl adipate was, in the manner described in Example 2, in the distillation columns 16,

17 and 18 cleaned. The purity of the column head

18 dimethyl sebacate withdrawn was 99.9% or more. The aqueous layer from the decanter 15 was treated with 100 kg of monomethyl adipate, which was added through the inlet 22.

s was mixed and mixed as in Example 2 to the evaporator

19 led. A monomethyl adipate solution containing the potassium salt of monomethyl adipate was obtained as a bottom stream from the recovery column 20. The water content of this solution was 2.1% by weight.

B

1 sheet of drawings

Claims (11)

646201 1. A process for the production of sebacic acid from adipic acid, in which adipic acid is subjected to the semi-esterification with methanol to form monomethyl adipate, the monomethyl adipate is condensed in batches electrolytically in methanol as the medium in the presence of an alkali metal salt of monomethyl adipate, while the water content of the electrolyte solution is in the range from 0 , 15 to 2. The method according to claim 1, characterized in that one carries out the semi-esterification of the adipic acid by passing a solution containing adipic acid and methanol as a starting material through a solid bed filled with a strongly acidic cation exchange resin. 2nd PATENT CLAIMS 3. The method according to claim 1, characterized in that the batchwise electrolytic condensation of monomethyl adipate is continued until the residual amount of monomethyl adipate in the electrolyte solution is reduced to at most 0.1% by weight of the total mono-methyl adipate added. 3.0% by weight and thereby receives an electrolytic solution containing dimethyl sebacate, which separates dimethyl sebacate from the electrolytic solution and hydrolyzed to form sebacic acid, characterized in that after the electrolytic condensation, methanol from the electrolytic solution containing dimethyl sebacate after or simultaneously with the addition of Water is removed to the electrolytic solution, thereby separating the remaining solution in two layers, namely an oily layer which essentially contains dimethyl sebacate and an aqueous layer which essentially contains the alkali salt of monomethyl adipate, which isolates dimethyl sebacate from the oily layer while on the other hand, the alkali salt of monomethyl adipate as a monomethyl adipate solution is recovered from the aqueous layer with the addition of monomethyl adipate to the aqueous layer before or after water is removed by evaporation from the aqueous layer to such an extent that the amount of water old of the electrolytic solution is maintained during the electro-lytic condensation, and the monomethyl adipate solution is returned to the electrolytic condensation. 4. The method according to claim 1, characterized in that the batchwise electrolytic condensation of monomethyl adipate on a solution containing 10-60% by weight of monomethyl adipate, which is fed to an electrolytic cell at an inflow rate of 1-4 m / s, with an electrode spacing of 0.5-3 mm, a current density of 5-40 A / dm2 and a temperature of the electrolytic solution of 45-65 ° C, the monomethyl adipate in the solution supplied by at least one base from the group of hydroxides, carbonates, bicarbonates, Methylates and ethylates of potassium and sodium, is neutralized to a degree of neutralization of 5-50 mol%. 5. The method according to claim 4, characterized in that one uses potassium hydroxide, carbonate or bicarbonate for the neutralization of the monomethyl adipate. 6. The method according to claim 1, characterized in that water is added to the electrolytic solution in an amount of 0.5-5.0 parts by weight per part by weight of alkali metal salt of monomethyl adipate. 7. The method according to claim 1, characterized in that removing methanol from the electrolytic solution until the methanol content of the remaining solution is 6 wt.% Or less. 8. The method according to claim 7, characterized in that methanol is removed to a methanol content of the remaining solution of 3 wt.% Or less. 9. The method according to claim 1, characterized in that the aqueous layer, the monomethyl adipate is added before the removal of water by evaporation. 10. The method according to claim 9, characterized in that the monomethyl adipate is added to the aqueous layer, which essentially contains an alkali salt of monomethyl adipate, in an amount of 1-10 parts by weight per part by weight of alkali metal salt of monomethyl adipate. 11. The method according to claim 1, characterized in that the hydrolysis of dimethyl sebacate is carried out by adding sebacic acid and removing the methanol formed by the hydrolysis from the system during the hydrolysis.