GB2051793A - Process for producing sebacic acid - Google Patents

Abstract

In a process for producing sebacic acid from adipic acid comprising three steps, i.e. (1) half-esterification of adipic acid with methanol to yield monomethyl adipate, (2) electrolytic condensation of the monomethyl adipate to yield dimethyl sebacate, followed by separation and purification of the dimethyl sebacate and (3) hydrolysis of dimethyl sebacate to yield sebacic acid, the desired sebacic acid can be produced with easy recovery of an alkali metal salt of monomethyl adipate and easy separation of the dimethyl sebacate by removing the methanol from the electrolytic solution containing dimethyl sebacate after the electrolytic condensation after or at the same time that water is added to the electrolytic solution, separating the residual solution into oily and aqueous layers, obtaining dimethyl sebacate from the oily layer, recovering the alkali metal salt of monomethyl adipate as a monomethyl adipate solution from the aqueous layer by addition of monomethyl adipate to the aqueous layer before or after removal of water by evaporation from the aqueous layer, and recycling the monomethyl adipate solution to the electrolytic condensation step.

Description

SPECIFICATION Process for producing sebacic acid This invention relates to a process for producing sebacic acid from adipic acid. More particularly, it relates to a process for producing sebacic acid by the electrolytic condensation of monomethyl adipate.

The electrochemical condensation reaction of carboxylic acids is generally called as the Kolbe reaction and described in the literature by H. Kolbe [Ann., 69,257 (1849)], A.C. Brown [Ann. 261, 107 (1891)], and others.

By this reaction there is formed dimethyl sebacate from monomethyl adipate in methanol as solvent and in the presence of an alkali metal salt of monomethyl adipate.

In order to produce sebacic acid from adipic acid on a commercial scale, the above process must be carried out in three successive steps. The first step is preparation of monomethyl adipate, the second step comprises electrolytic condensation of the monomethyl adipate into dimethyl sebacate, separation of the latter from the electrolytic solution containing it and subsequent purification, and the third step is hydrolysis of the dimethyl sebacate to yield sebacic acid.

In carrying out conventional processes, the most important problem yet to be solved seems to be the separation of dimethyl sebacate in much purer form from the electrolytic solution containing it in the second step.

In order to solve the above problem, when the electrolytic solution contains substantially three ingredients of monomethyl adipate, an alkali metal salt thereof, and dimethyl sebacate, Japanese Patent Appln Kokai (Laid-Open) Nos. 89,317/75, 149,919178, 149,920/78 and 135,921/78 disclose as commercially practicable methods detailed procedures for separating purer dimethyl sebacate and for recovering monomethyl adipate and an alkali metal salt thereof from the electrolytic solution obtained after electrolytic condensation.

In another case where the electrolytic solution obtained by batchwise electrolytic condensation contains substantially no monomethyl adipate but contains substantially two ingredients of an alkali metal salt of monomethyl adipate and dimethyl sebacate, Japanese PatentAppln Kokoku (Post-Exam Publn) Nos.

37,564/71 and 51,327/72 and Japanese Patent Appln Kokai (Laid-Open) No.3,816/71 disclosed methods for separating and purifying dimethyl sebacate and recovering an alkali metal salt of monomethyl adipate from the electrolytic solution. These methods, however, are not satisfactory to be used on a commercial scale.

For instance, Japanese Patent Appin Kokoku (Post-Exam Publn) No. 51,327/72 discloses a process which can be applicable in industrial production and which is near to the process of this invention. According to the disclosure, the electrolytic solution is treated so as to remove methanol, then admixed with water to separate into an oily layer containing substantially dimethyl sebacate and an aqueous layer containing substantially an alkali metal salt of monomethyl adipate, and the aqueous layer is concentrated by evaporation to recover the alkali metal salt as a saturated aqueous solution.

In the above disclosure, however, no mention is made of the concentration of water in the solution in which the electrolytic condensation is carried out, except for the description in one Example that the initial water content of the electrolytic solution is 2.5 moles/liter. It is a well known fact that the water content of an electroytic solution is closely related to the results of electrolytic condensation. In Japanese Patent Appin Kokai (Laid-Open) No 22,317/79, it is described that both the current efficiency and the material yield will decline unless the water content of electrolytic solution is kept within the range of from 0.15 to 3.0% by weight during the electrolytic condensation.Accordingly, assuming that the electrolytic condensation is carried out while keeping the water content of electrolytic solution within the range of from 0.15 to 3.0% by weight, the following two problems seem to arise in carrying out the process disclosed in Japanese Patent Appln Kokoku (Post-Exam Pubin) No. 51,327/72.

The first problem is associated with the step of removing methanol from the electrolytic solution. Since in this case neither water nor the monomethyl adipate used as starting material is present in any appreciable amount, the alkali metal salt of monomethyl adipate should be precipitated in the methanol distillation column at the bottom if methanol has been removed to make the methanol concentration low. An example of such a case is shown in Comparative Example 1 mentioned later.

In order to prevent completely the precipitation of alkali metal salt of monomethyl adipate at the bottom of methanol distillation column, it seems necessary to leave behind a certain amount of methanol after the removal of methanol. But this seems tio make the subsequent step of extracting the alkali metal salt with water complicate, because if the amount of residual methanol in the electrolytic solution exceeds a certain limit, partition of the alkali metal salt into the oily layer will be increased during the water extraction treatment. The alkali metal 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 upon purification of dimethyl sebacate by distillation of the oily layer and contaminate the dimethyl sebacate purified.In order to prevent the alkali metal salt of monomethyl adipate from transfer into the oily layer, it will become necessary to use more water for the extraction or to repeat the extraction more times. The presence of methanol not only affects the partition of an alkali metal salt of monomethyl adipate into the oil layer but also will interfere with the phase separation between the oily layer and the aqueous layer.

The second problem is connected with the step of removing water by evaporation from the aqueous layer containing an alkali metal salt of monomethyl adipate. In the process described in Japanese Patent Appln Kokoku (Post-Exam Publn) No.51,327/72 cited above, a saturated aqueous solutoin of the alkali metal salt recovered by the evaporation of water is recycled to the electrolytic condensation step.In view of the convenience in controlling the residual water concentration in the water evaporator and the temperature decrease in the recycling line leading to the electrolytic condensation step, it seems necessary to allow a certain amount of water to remain unremoved in the evaporator so as to keep the concentration of the alkali metal salt of monomethyl adipate below the saturation point in order to prevent completely the alkali metal salt of monomethyl adipate from precipitation in the water evaporator and recycle line. This means that the recycled alkali metal salt of monomethyl adipate accompanies a pretty large amount of water so that it will be difficult to maintain the water content of the solution for electrolytic condensation within the necessary range of from 0.15 to 3.0% by weight.Consequently, both the current efficiency and the material yield of the electrolytic condensation will be inevitably sacrificed, unless the recycled amount of the alkali metal salt of monomethyl adipate is kept at a very low level.

The present inventors conducted extensive studies to solve the above problems on the premise that the electrolytic condensation is carried out while keeping the water content of the electrolytic solution within the range of from 0.15 to 3.0% by weight. As a result, it was found that the first problem can be solved by adding water to the electrolytic solution prior to or at the same time the removal of methanol and the second problem can be solved by the addition of monomethyl adipate before or after the removal of water from the aqueous layer by evaporation.

This invention provides a process for producing sebacic acid by half-esterifying adipic acid with methanol to form monomethyl adipate, subjecting the resulting monomethyl adipate to batchwise electrolytic condensation in methanol solution in the presence of an alkali metal salt of monomethyl adipate while maintaining the water content of the solution within the range of from 0.15 to 3.0% by weight, thus obtaining an electrolytic solution containing dimethyl sebacate, then separating dimethyl sebacate from said electrolytic solution, and hydrolyzing the dimethyl sebacate to yield sebacic acid, which process is characterized by removing methanol from said electrolytic solution after or at the same time that water is added to the electrolytic solution, whereby the residual solution is separated into two layers, an oily one containing substantially dimethyl sebacate and an aqueous one containing substantially the alkali metal salt of monomethyl adipate, collecting dimethyl sebacate from the oily layer, while recovering the alkali metal salt of monomethyl adipate as a monomethyl adipate solution from the aqueous layer with addition of monomethyl adipate to the aqueus layer before or after water is removed by evaporation from the aqueous layer to such an amount as to maintain the level of the water content of the electrolytic solution at the electrolytic condensation, and recycling the monomethyl adipate solution to the electrolytic condensation step.

The preparation of monomethyl adipate by the half-esterification of adipic acid in the first step according to this invention is carried out as described below in detail.

The starting solution can be of the simple composition comprising adipic acid and methanol. However, in order to prepare advantageously the monomethyl adipate by suppressing the formation of di methyl adipate, the starting solution is preferably composed 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 carried out in the presence of acidic catalysts such as, for example, sulfuric acid, hydrochloric acid, nitric acid, other inorganic acids, p-toluenesulfonic acid, and other organic sulfonic acids.

Although not subject to any particular limitation, the reaction conditions are preferably such that the pressure is atmospheric or superatmospheric and the temperature is 50"C or higher. It is also possible to carry out the reaction in the absence of catalysts under such higher temperature and pressure conditions that the pressure is 2 to 15 atm. and the temperature is 120 to 200"C.

Strongly acidic cation exchange resins can also be used as the acidic catalyst. The use of strongly acidic cation exchange resin is disadvantageous in catalytic capacity but advantageous in corrosion of the reactor, etc. and separation of the reaction product from the catalyst, compared with the case of using a mineral acid or an inorganic sulfonic acid as a catalyst. Further, comparing with the case of using no catalyst, it may be disadvantageous in using a catalyst but disadvantageous from the viewpoint of the reaction conditions and formation of by-products. It has been known that when the half-esterification is conducted in the absence of a catalyst under higher temperature, higher pressure, and longer reaction time conditions, the formation of by-products such as cyclopentanone and high-boiling compounds is enhanced.By taking into account the above situation, it is most preferable to use a strongly acidic cation exchange resin as the catalyst in producing monomethyl adipate on a commercial scale.

Examples of the strongly acidic cation exchange resin suiable for use are a polystyrene series resin having sulfonic acid grops, which may be either of the gel type or of the porous type. The resin should be used as a fixed bed so that the esterification may be carried out continuously and the resin may manifest its activity effectively. When used continually for a long period of time, the strongly acidic cation exchange resin becomes deteriorated in catalytic activity for the esterification owing to accumulation of adsorbed metal ions. If necessary, the resin can be regenerated for reuse in a customary way, for example, by treating with an aqueous nitric acid. For this purpose, it is desirable to carry out the adsorption of metal ions and the esterification reaction separately in two ion exchange resin towers.

Although a higher reaction temperature in the fixed bed of a strongly acidic cation exchange resin favors the rate of reaction, it is more practical in view of the heat resistance of the resin to maintain the temperature at 60t to 90"C.

In passing the starting solution through the fixed bed of a strongly acidic cation exchange resin, the deposition of adipic acid in the fixed resin bed at the operating temperature is undesirable. To avoid this, it is necessary to maintain the amount of solvents such as, for example, methanol, water and dimethyl adipate in excess of a certain limit. However, because of the necessity of removing the solvent in separating substantially pure monomethyl adipate from the reaction mixture, it is undesirable to use a large excess of solvent. In order to prevent the deposition of adipic acid without using a large excess of solvent, it becomes necessary to replenish the deficiency of solvent by recycling a portion of the effluent solution from the fixed bed.The amount of recycle solution is not predeterminable, because it depends upon the amounts of adipic acid and solvent in the starting solution and the operation temperature of the fixed bed, but it is sufficient to keep the recycling amount so as to avoid the deposition of adipic acid. Also, the rate of feeding the starting solution to the fixed bed is not subject to any particular limitation, but it is preferably set so that the reaction in the fixed bed may proceed neraly to the equilibrium point.

The procedure of converting the monomethyl adipate obtained in the above step into dimethyl sebacate by electrolytic condensation in the second step of the present process is described below in detail.

To prepare a charge solution for electrolytic condensation, the monomethyl adipate obtained in the previous step and the alkalki metal salt of monomethyl adipate recycled from the purification step of the electrolytic solution, described later, are dissolved in the methanol recycled also from the purification step of the electrolytic solution.

As for the water content of the charge solution thus prepared, if it is below 0.15% by weight, the current efficiency becomes extremely low, while if it exceeds 3.0% by weight, both the current efficiency and the material yield become considerably decreased. It is prerequisite, therefore, to maintain the water content at a level in the range of from 0.15 to 3.0% by weight in order to conduct the electrolytic condensation while maintaining both the current efficiency and the material yield at high levels.

The electrolytic condensation is carried out batchwise until the monomethyl adipate has been substantially exhausted. The residual monomethyl adipate in the electrolytic solution after the end of electrolytic condensation moves into the oily layer in considerable amounts when the electrolytic solution is treated with water, as described later, for the purpose of purification. The amount of monomethyl adipate moved into the oil layer should be kept small by using a large quantity of water, because monomethyl adipate forms an azeotrope with dimethyl sebacate and cannot be removed by distillation, thus remaining in the purified diemthyl sebacate as an impurity.It is, therefore, preferable to reduce the residual amount of monomethyl adipate as small as possible by continued electrolytic condensation until the residual amount becomes at most 0.1% by weight of the charged monomethyl adipate in view of the purity of dimethyl sebacate.

The concentration of monomethyl adipate in the charge solution is 10 to 60% by weight. If the concentration exceeds 60% by weight, the cell voltage becomes higher, whereas a concentration below 10% by weight results in a decrease in volume utilization factor and in current efficiency.

To increase the conductivity of electrolytic solutoin for the electrolytic condensation, neutralizing bases such as hydroxides, carbonates, bicarbonates, methylates or ethylates of lithium, potassium and sodium or amines can be used. However, amines ae oxidized at the anode, accelerating the consumption of anode while lithium compounds act to decrease the current efficiency. Therefore, it is preferable to use hydroxides, carbonates, bicarbonates and methylates of sodium or potassium. Further, since an alkali metal salt of monomethyl adipate from the purification treatment of the electrolytic solution is recycled in the form of solution in monomethyl adipate, as described later, it is most preferable in view of the solubility to use hydroxides, carbonates or bicarbonates of potassium.The neutralization degree of monomethyl adipate (the neutralization degree is defined as mole percent of monomethyl adipate neutralized with a base) in the charge solution is preferably 5 to 50% by mole. If it is below 5% by mole, the cell voltage becomes high, whereas, neutralization above 50% by mole results in a decrease in current efficiency.

As the electrolytic cell, there can be used one commonly used in organic electrolytic reactions, which allows the electrolytic solution to flow at a high flow rate between both electrodes. As an example, in a electrolytic cell there are placed a cathode plate and an anode plate facing each other with an intervening polypropylene plate which determines the distance between both electrodes. The polypropylene plate has an aperture at its central part to allow the electrolytic solution to pass through. The effective surface area of the electrode is determined by the area of the aperture and the electrode distance is determined by the thickness of the polypropylene plate.The circulating electrolytic solution from an electrolytic solution tank enters the cell through a feeding port provided in the electrolytic cell, then flows between the electrodes to undergo the reaction and leaves the electrolytic cell from an exit port to return to the tank.

The electrode materials for the anode are platinum, rhodium, ruthenium, iridium and the like which are used each alone or as alloys and usually plated on a substrate metal plate such as titanium or tantalum. The cathode materials are preferably those having a low hydrogen overvoltage but not limited to any particular metals. Platinum, iron, stainless steel and titanium may be used.

The flow rate of electrolytic solution in the electrolytic cell is preferably 1 to 4 m/sec, because if it is below 1 m/sec, the current efficiency will be lower while if it exceeds 4 m/sec, the pressure loss in the cell will become increased. 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 electrolytic cell will be larger, while if it exceeds 3 mm, the cell voltage will become higher. 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 electrolytic solution is preferably 45 to 65"C. If the temperature is decreased below 45"C, the current efficiency will be lower and the cell voltage becomes higher, while the upper temperature limit depends on the boiling point of electrolytic solution.

Below are described in detail the procedures in the next step of purifying the electrolytic solution, wherein dimethyl sebacate is separated from the electrolytic solution obtained by the electrolytic condensation in the second step of the present process, followed by purification and the alkali metal salt of monomethyl alipate is recovered.

The step of purifying the electrolytic solution comprises the removal of methanol, the separation of dimethyl sebacate from an alkali metal salt of monomethyl adipate, the purification of separated dimethyl sebacate and the recovery and recycling (to the electrolytic condensation step) of the separated alkali metal salt of monomethyl adipate.

Methanol is removed from the electrolytic solution after or at the same time water is added to the electrolytic soltuion. Water can be added directly to a methanol distillation column or to the electrolytic solution which is fed to the methanol distillation column. The amount of water to be added is that sufficient to keep the alkali metal salt of monomethyl adipate from precipitating at the bottom of methanol distillation column and is preferably 0.5 to 5 parts by weight per part by weight of the alkali metal salt of monomethyl adipate If the water added is below 0.5 part by weight, an increased amount of methanol must be left unremoved, while if it is greater than 5 parts by weight, hydrolysis of the ester linkage may possibly take place.

The removal of methanol in the methanol distillation column is conducted under atmospheric pressure.

The residual amount of methanol in the distillation residue after the removal of a greater portion of methanol should be as small as possible in order to minimize the amount of an alkali metal salt of monomethyl adipate which tends to transfer into an oily layer in the subsequent phase separation treatment. In a large-scale operation, the residual methanol is preferably 6% by weight or less, most preferably 3% by weight or less. In removing the methanol, the temperature of distillation column will become 80"C or higher at the bottom and at such a temperature hydrolysis of ester linkage may take place when the residue is retained at the bottom for a long period of time. The residence time should be less than 2 hours, although the shorter, the better.

To separate dimethyl sebacate from the alkali metal salt of monomethyl adipate, the residue after removal of methanol is cooled and left standing to separate into an oily layer and an aqueous layer. As previously described, with the increase of remaining methanol exceeding 6% by weight in the residue after removal of methanol, more of the alkali metal salt of monomethyl adipate moves into the oily layer and, at the same time, the separation into oily and aqueous layers becomes less favorable, although the extent of transfer of the salt depends upon the content of the salt and water in the residue. With an increase in the methanol content of the residue, the oily phase tends to be the lower layer, though depending also upon the alkali metal salt of monomethyl adipate content and the water content of the residue.Consequently, it is necessary to avoid a methanol content of the residue in the critical region where reversal of the relative position of oily and aqueous layers takes place.

The purification of dimethyl sebacate is accomplished by customary distillation of the separated oily layer.

The recovery of the alkali metal salt of monomethyl adipate and the recycling of recovered salt to the electrolytic condensation step are carried out in the following manner. Prior to the removal of water by evaporation, monomethyl adipate is added to the separated aqueous layer or, alternatively, when the amount of alkali metal salt of monomethyl adipate to be recycled is small, water is removed from the aqueous layer until a saturated aqueous solutoin of the alkali metal salt is obtained and then monomethyl adipate is added.

In the former case where monomethyl adipate is added to the aqueous layer prior to the removal of water by evaporation, the water content of the residue can be freely controlled, because there occurs no precipitation of the alkali metal salt of the monomethyl adipate. Consequently, it becomes possible to regulate very easily the water content in the aforementioned second step of the process. The monomethyl adipate being added to the separated aqueous layer can be substituted by the starting solution for the electrolytic condensation. The amount of monomethyl adipate to be added should be just sufficient for the prevention of the deposition of alkali metal salt of monomethyl adipate from the residue when the water has been removed as far as possible. The addition of monomethyl adipate in a large excess will require a water evapoator of an increased capacity.It is preferable for a large-scale operation to add 1 to 10 parts by weight of monomethyl adipate per part by weight of the alkali metal salt of monomethyl adipate. To avoid a chemical change due to thermal effect, the residence time of the residue in the evaporator is preferably up to 10 minutes for a large-scale operation, though the shorter, the better. Since the water vapor from the evaporator accompanies monomethyl adipate, though slight in the quantity, it is better to provide a column to recover the accompanied monomethyl adipate.

In another case where the water is removed from the aqueous layer without prior addition of monomethyl adipate, it is necessary to leave behind the water in an amount sufficiently enough for completely dissolving the alkali metal salt of monomethyl adipate, in order to facilitate the control of water content of the residual solution in removing water and to prevent the alkali metal salt of monomethyl adipate from deposition. It is also necessary to add monomethyl adipate as soon as the water has been removed, in order to prevent the residual solution from deposition of the alkali metal salt of monomethyl adipate due to a temperature decrease on the way to recycle to the electrolytic condensation step. A preferable amount of monomethyl adipate to be added in a large-scale operation is 0.5 to 5 parts by weight per part by weight of the alkali metal salt of monomethyl adipate.Because of a high temperature of the residual solutoin after the removal of water by evaporation, it is undesirable to add methanol in place of monomethyl adipate. The alkali metal salt of monomethyl adipate accompanying a certain amount of water is recycled to the electrolytic condensation step. Accordingly, in order to keep the water content of the charge solution for electrolytic condensation 3.0% by weight or lower, it is necessary to control the alkali metal salt of monomethyl adipate content in the recycling solution at a certain level, preferably 4.5% by weight or lower. The residence time of the aqueous solution in the water evaporator can be 10 minutes or less for a large-scale operation.

As described above in detail, according to this invention it becomes possible to maintain the water content of electrolytic solution at a level in the range of 0.15 to 3.0% by weight throughout the batchwise electrolytic condensation by the following improved procedures: In purifying the electrolytic solution after the electrolytic condensation, methanol is removed after addition of water. In removing water by evaporation from the aqueous layer containing the alkali metal salt of monomethyl adipate, monomethyl adipate is added to the aqueous solution prior to the removal of water or, alternatively, when the amount of alkali metal salt of monomethyl adipate being recycled to the electrolytic condensation step is small, water is removed from the aqueous layer until a saturated aqueous solution of the alkali metal salt has been obtained and then immediately monomethyl adipate is added.The alkali metal salt of monomethyl adipate thus recovered in the form of solution in monomethyl adipate is recycled to the electrolytic condensation step to allow the electrolytic solution to maintain the water content at a level in the range of from 0.15 to 3.0% by weight. Thus, it is possible to establish a process according to which the batchwise electrolytic condensation is conducted with high current efficiency and high material yield and the electrolytic solution can be purified with great easiness.

In the third step of the process of this invention, dimethyl sebacate separated and purified in the preceding step is hydrolyzed in a manner as described below in detail.

In order to carry out the hydrolysis to completion and to increase the purity of sebacic acid by avoiding contamination with unreacted components such as, for example, monomethyl sebacate, a batch-type reaction process is used because of its economical advantage over a continuous process.

The hydrolysis can be effected by use of catalysts such as 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 preferable in view of the corrosion of materials of reactor, etc. and the rate of reaction. The reaction can be rapidly completed by adding to an aqueous solutoin containing 10 to 20% by weight of an acid catalyst dimethyl sebacate in an amount not exceeding the solubility at the reaction temperature and carrying out the reaction while removing the liberated methanol. It is also possible to conduct the hydrolysis at first in water and then in an aqueous nitric acid.In this case, the hydrolysis of dimethyl sebacate can be conducted by adding water four times as large as the weight of dimethyl sebacate at 220" to 280 C, under a pressure of 35 to 50 atm, for 2 to 5 hours and then in an aqueous solutoin containing 12 to 25% by weight of nitric acid, at 80 to 1 00'C for 0.5 to 1.5 hours. It is further possible to effect the hydrolysis by adding sebacic acid, which is the intended product, to accelerate the reaction and continually removing the liberated methanol.This method has advantages over the hydrolysis in the presence of nitric acid in that the reaction can be completed in one step at a relatively low temperature and at a tolerable rate without causing a chemical change in methanol and without the need of subsequent removal of the catalyst, although disadvantageous from the viewpoint of reaction rate. When nitric acid is used as the catalyst, it reacts with a part of liberated methanol, forming methyl nitrate even if the reaction is carried out while removing the methanol. Moreover, the method of using sebacic acid has an advantage in the color of the product over the above-said method of carrying out the hydrolysis without catalyst in the first half of the reaction time.By taking the above facts into consideration, it is most desirable for a large-scale operation to cary out the hydrolysis of dimethyl sebacate by adding sebacic acid and removing the methanol liberated by the reaction out of the reaction system.

The accelerating effect of sebacic acid is marked in the first half of the reaction period, in which two phases of dimethyl sebacate and water coexist. The amount of sebacic acid to be added is 1 to 20% by weight based on the weight of dimethyl sebacate.

Beside the addition of sebacic acid, which is a reaction product, it is advantageous for the acceleration of hydrolysis to remove continually from the reaction system the methanol formed by the reaction and, at the same time, continually adding water to replenish the deficiency caused by the removal of water together with the methanol. The hydrolysis can be completed 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 form a heterogeneous mixture, it can preferably be removed in the second half of the reaction, in which both phases form a homogeneous solution, because the effect of methanol removal is more marked in this stage. The removal of methanol out of the reaction system by evaporation is accompanied by the evaporation of water.

The amounts of methanol and water being removed are preferably 2 to 6 parts by weight per part by weight of the charged dimethyl sebacate. It is preferable to maintain the water content of the reactant mixture at 10 to 75% by weight throughout the reaction period.

Although beneficial for the rate of reaction, a high reactoin temperature results in increased reaction pressure and coloration of the reaction product. Especially when the temperature exceeds 220"C, the coloration becomes marked. It is preferable to conduct the reaction at 180 to 220"C for the first half of the reaction, in which the reactant mixture still remains in the form of heterogeneous solution and then at 150 to 220"C for the second half, in which the mixture forms a homogeneous solution.

One of the embodiments of this invention is illustrated below with reference to a flow sheet shown in the accompanying drawing.

In the first step, numeral 2 is a dissolving tank which is fed with adipic acid and methanol through an inlet 1. Methanol and water withdrawn from the top of distillation column 5, dimethyl adipate from the top of distillation column 7, adipic acid and monomethyl adipatefrom the bottom of distillation column 8, and a portion of the reaction solutoin from branch outlet 9 are recycled to the dissolving tank 2. In the dissolving tank 2, adipic acid is dissolved and the resulting solutoin is fed as the starting solution to the top of an ion exchange resin column 3. In the column 3, minute amounts of metal ions in the starting solution are adsorbed and the esterification reaction partly takes place. The starting solution freed from metal ions flows out of the bottom of the column 3 and is sent to the top of an ion exchange resin column 4.The esterification reaction proceeds mainly in the column 4. As soon as the effluent from the bottom of the column 3 shows the presence of metal ions in a concentration exceeding the predetermined limit, the ion exchange resin should be regenerated in a customary manner by treating with, for example, aqueous nitric acid. A portion of the esterification reaction solution discharged from the bottom of the column 4 is recycled to the dissolving tank 2 through the branch outlet 9 and the remainder is sent to the distillation column 5. Methanol and water distilled out of the top of column 5are recycled to the dissolving tank 2, while the residue discharged from the bottom is sent to a distillation column 6. In the column 6, water and cyclopentanone are distilled off and the residue discharged from the bottom is sent to a distillation column 7.In the column 7, dimethyl adipate is distilled off and the residue discharged from the bottom is sent to a distillation column 8. The dimethyl adipate obtained from the top of the distillation column 7 is recycled to the dissolving tank2. Monomethyl adipate is obtained from the top of the distillation column 8 and the residue containing adipic acid and monomethyl adipate is discharged from the bottom to be recycled to the dissolving tank 2. The monomethyl adipate obtained from the top of the column 8 is sent to the second step of the process.

In the second step, numeral 12 is an electrolytic solutoin tank, to which is fed the monomethyl adipate obtained from the top of distillation column 8 in the first step. The methanol obtained from the top of distillation column 14 and the monomethyl adipate solutoin, discharged from the bottom of recovery column 20, containing the alkali metal salt of monomethyl adipate are recycled to the electrolytic solution tank 12. The solution formed in the tank 12 is circulated through an electrolytic cell 13, in which the electrolytic condensation takes place. After completion of the electrolytic condensation, the electrolytic solution is discharged from the tank 12 and mixed with the water sent from the top of recovery column 20 via a feed inlet 21. The resulting electrolytic solution containing water is sent to the distillation column 14.In the distillation column 14, methanol is distilled out of the top and recycled to the electrolytic solution tank 12, while the residue discharged from the bottom is sent to a decanter 15 where it is left standing to separate into two layers. The oil layer is sent to a distillation column 16, while the aqueous layer discharged from the bottom is mixed with monomethyl adipate sent from a branch outlet 11 via a feed inlet 22. The resulting mixture is sent to the bottom of an evaporater 19 where water is evaporated. The evaporated water together with the residue is fed to the recovery column 20. Water is withdarwn from the top of the recovery column 20 and recycled to the feeding inlet21, while a monomethyl adipate solutoin containing the alkali metal salt of monomethyl adipate is discharged from the bottom of the column 20 and recycled to the electrolytic solution tank 12.In the distillation column 16, low-boiling impurities are distilled off and the residue is sent to a distillation column 17. The residue is further stripped of low-boiling impurities in the column 17 and sent to a distillation column 18. Dimethyl sebacate is obtained from the top of the column 18 and sent to the third step of the process. High-boiling impurities are discharged from the bottom of column 18.

In the third step, numeral 23 is a reactor to which is fed the dimethyl sebacate obtained from the top of the distillation column 18 in the second step. Sebacic acid and water are introduced through a feed inlet 27. After the reaction has been initiated, heating is continued until a homogeneous solution is formed. When the homogeneous solution is formed, methanol and water are continuously removed and sent to a distillation column 24. In the column 24, methanol is distilled out of the top and sent to the feed inlet 10 in the first step, while water is discharged from the bottom as the residue and recycled continuously to the reactor 23. After completion of the hydrolysis in the reactor 23, activated carbon is added through a feed inlet 27to decolorize the reaction solution.An aqueous solution containing the activated carbon and sebacic acid is discharged from the reactor 23 and sent to a filter 25to remove the activated carbon. The filtrate is sent to an evaporator 26. In the evaporator26, water is evaporated and recovered. Molten sebacic acid is obtained from the evaporator26 and sent to a flaker or granulator to obtain flaky or granular sebacic acid.

The invention is illustrated below in further detail with reference to Examples.

Example 1 Sebacic acid was prepared from adipic acid according to the flow sheet shown in the accompanying drawing.

Preparation of monomethyl adipate in the first step was performed in the following way: Adipic acid and methanol were fed to the dissolving tank2throughthe inlet 1 at the rates of13.8 kg/hour and 3.0 kg/hour, respectively. To the tank 2 were also recycled methanol and water from the top of distillation column 5, dimethyl adipate from the top of distillation column 7, adipic acid and monomethyl adipate from the bottom of distillation column 8 and a portion of the reaction solution from the branch outlet 9. While maintaining the temperature of the tank 2 at 80"C, the starting material solution formed in the tank was withdrawn at a rate of 240 kg/hour and fed to the top of ion exchange resin column 3.The starting material solution was of the composition : 28.0% by weight of adipic acid, 20.3% by weight of dimethyl adipate, 30.8% by weight of monomethyl adipate, 5.7% by weight of methanol, 14.9% by weight of water, and 0.3% by weight of cyclopentanone. This solution at 80C was passed through the ion exchange resin column 3 packed with 15 liters of a regenerated H±type strongly acidic cation exchange resin (Amberlite 200C; trademark, manufactured by Rohm and Haas Co.). The resin was regenerated every 200 hours of continuous operation, using 1 N nitric acid as regenerant.The effluent from the bottom of the column 3 was fed to the top of another ion exchange resin column 4 packed with 60 liters of regenerated H±type strongly acidic cation exchange resin, Amberlite 200 C. The temperature ofthe solution was maintained at 80"C throughout. The reaction solution was withdrawn from the bottom of the ion exchange resin column 4 at a rate of 240 kg/hour. One-fourth of the reaction solution was sent to the distillation column 5 and three-fourths was recycled to the dissolving tank 2 via the branch outlet 9.The reaction solution withdrawn from the bottom of the ion exchange resin column 4 was of the composition: 22.0% by weight of adipic acid, 20.2% by weight of dimethyl adipate, 37.4% by weight of monomethyl adipate, 4.3% by weight of methanol, 15.7% by weight of water, and 0.3 by weight of cyclopentanone. The variations with time in iron ion and cyclopentanone contents of the solutoin withdrawn from the ion exchange resin column 4were as shown in Table 1. The temperature at the bottom of distillation column 5 operating at atmospheric pressure was set at 140"C.

Methanol and water distilled out of the top of column 5were recycled to the dissolving tank 2, while the residue withdrawn from the bottom was sent to the distillation column 6. The temperature at the bottom of column 6operating under a reduced pressure of 200 mmHg was set at 1750C. Water and cyclopentanone were distilled off from the top of the column 6, while the residue withdrawn from the bottom was sent to the distillation column 7. The temperature at the bottom of column 7 operating under a reduced pressure of 20 mmHg was set at 200 C. The dimethyl adipate distilled out of the top of the column 7 was recycled to the dissolving tank 2, while the residue containing adipic acid and monomethyl adipate was withdrawn from the bottom and sent to the distillation column 8.The temperature at the bottom of the column 8, operating under a reduced pressure of 20 mmHg was set at 210"C. The monomethyl adipate obtained at a rate of 15 kg/hour from the top of the column 8was used in preparing the electrolytic solution for electrolytic condensation. A solutoin containing adipic acid and monomethyl adipate was withdrawn from the bottom of the column 8 and recycled to the dissolving tank 2.

The construction materials for the above equipments were as follows: the dissolving tank 2: SUS 304 (JIS); the ion exchange resin columns 3 and 4: SUS 304; the distillation columns 5 and 6: SUS 304; the distillation column 7: SUS 316; the distillation column 8: titanium (reboiler) and SUS 316 (other parts).

TABLE 1 Time elapsed Cyclopentanone Iron ion content (hours) content (% by wt) (ppm) 50 0.3 5 100 0.3 5 150 0.3 5 200 0.3 5 Preparation of dimethyl sebacate in the second step was performed in the following way: To the electrolytic solution tank 12, were charged monomethyl adipate prepared in the first step, methanol from the top of distillation column 14, and a monomethyl adipate solution from the bottom of recovery column 20, which contained potassium salt of monomethyl adipate. The charge solution was 500 kg in total and was a methanol solution containing 35.7% by weight of monomethyl adipate, 5.0% by weight (the neutralization degree corresponded to 10% by mole) of potassium salt of monomethyl adipate, and 1.8% by weight of water.The charge solution was circulated through the electrolytic solution tank 12 and the electrolytic cell 13. The electrolytic cell had an anode, a cathode, and three intermediate electrodes. Each electrode was a titanium plate, 100 cm x 100 cm in size and 3 mm in thickness. The anode and three intermediate electrodes were plated on one side with platinum, 3 ti in thickness. The electrodes were held 1 mm apart from each other by inserting polypropylene plates, each 1 mm in thickness, between adjoining electrodes. The effective area of the electrodes was 277 dm2 in total. The electrolytic cell was provided with an inlet and an outlet for the solution.The electrolysis was continued for 13.7 hours, while passing the electrolytic solution between the electrodes at a linear flow rate of 2.0 misecond and maintaining the current density and the solution temperature at 10.1 A/dm2 and 53 to 56on, respectively. The cell voltage per pair of electrodes changed from 7.5 V to 5.7 V. The electrolytic solution after completion of the electrolytic reaction was 445 kg in quantity and of the following composition, as determined by gas chromatography: dimethyl sebacate, 23.0% by weight; monomethyl adipate.0.01 by weight; water, 1.8% by weight. The current efficiency for the formation of dimethyl sebacate was 62.2% and the material yield was 79.8%.

After completion of the electrolytic reaction, 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 recovery column 20 and supplied through the feed outlet 21 to the electrolytic solution. The electrolytic solution mixed with the water was then sent to the distillation column 14. The temperature at the bottom of column 14, operating at atmospheric pressure was maintained at 98"C. Methanol was distilled out of the top of the column 14, while a mixture of oil and water was withdrawn from the bottom. The mean residence time at the bottom of the column 14 was one hour and the mean methanol content of the mixture withdrawn from the bottom was 1.1% by weight.After cooling, this mixture was sent to the decanter 15 and left standing for one hour to separate into two layers of oil and water. The oily layer weighed 139 kg and contained 0.01% by weight of potassium salt of monomethyl adipate. The oily layer was sent to the distillation column 16. In the column 16 operating at atmospheric pressure, the bottom temperature was set at 170"C. Low-boiling by-products were distilled offfrom the top of the column 16 and the residue was sent to distillation column 17. In column 17 operating under a reduced pessure of 20 mmHg, the bottom temperature was set at 185 C. Low-boiling by-products were distilled off from the top of the column 17, while the residue was sent to the distillation column 18.In the column 18 operating under a reduced pressure of 20 mmHg, the bottom temperature was set at 230 . Dimethyl sebacate was obtained from the top of the column 18 and sent to the third step. The dimethyl sebacate showed a purity of 99.9% or higher (the purity in concentration by weight was determined by gas chromatography). The aqueous layer from decanter 15 was mixed with 71 kg of monomethyl adipate withdrawn from the branch outlet 11 and supplied through the branch inlet 22. The resulting mixture was sent to the bottom of the evaporator 19. In the evaporator 19, operating at atmospheric pressure, the temperature was set at 120 C. The water vapor from the top of the evaporator 19 and the residue were sent to the recovery column 20.Water containing no monomethyl adipate was withdrawn from the top of recovery column 20 and recycled to the feed inlet 21. A monomethyl adipate solution containing potassium salt of monomethyl adipate was withdrawn from the bottom of the column 20 and recycled to the electrolysis unit.

The residence times of the solution in the evapoartor 19 and in the recovery column 20 were 3 minutes in total. The water content of the monomethyl adipate solution containing potassium salt of monomethyl adipate withdrawn from the bottom of recovery column 20 was 6.8% by weight.

Preparation of sebacic acid in the third step was performed in the following way: Into the reactor 23, were charged 210 kg of the dimethyl sebacate obtained in the second step,9.8 of sebacic acid and 130 kg of water. While being vigorously stirred, the charge mixture was heated at a reaction temperature of 1 80"C to allow the reaction to proceed. The reactant mixture was heterogeneous in the beginning, but became a homogeneous solution after 5.0 hours of reaction. When the mixture became homogeneous, methanol and water were allowed continually to distil from the reactor 23 and sent to the distillation olumn 24.In the column 24 operating at atmospheric pressure, the bottom temperature was set at 100"C and the methanol distilled out of the top was sent to the feed inlet 10 in the first step. Water freed from methanol was withdrawn from the bottom of the column 24 and recycled to the reactor 23. The reaction in the reactor23 was continued for additional 10 hours, while continually removing methanol and water and replenishing the deficiency of water with recycled water from the distillation column 24 and a little fresh water fed through the feed inlet 27. The sum total of methanol and water distilled out was 956kg and the supplied water was 996 in total. The pressure in the reactor23was 7 to kg/cm2 (gauge) throughout the reaction. The degree of hydrolysis after completion of the reaction was 99.8% by mole.The degree of hydrolysis was expressed as the molar ratio (in percent) of sebacic acid, which was formed, to the charged dimethyl sebacate.

After completion of the reaction, 1 kg of an activated carbon (special grade S-C of Nihon Jescoal Industry Co.) was added to the reactor23 through the feed inlet27 and the reaction mixture was stirred for one hour at 130"C. After completin of the decoloring treatment with activated carbon, the reaction mixture was removed from the reactor and sent to the filter 25 to filter off the activated carbon. The filtrate was sent to the evaporator26where the filtrate was heated to an ultimate temperature of 180"C to remove the water by evaporation. The molten sebacic acid was sent to a flakes to obtain sebacic acid in flake form.The sebacic acid thus obtained showed a melt color of 70, an ash content of 50 ppm, a moisture content of 0.05% by weight and an acid value of 552 mg KOH/g. For the measurement of melt color, 25 g of sebacic acid was heated at 150C for 20 minutes. The color of sebacic acid in molten state was measured according to the method of American Public Health Association (the APHA method).

Example 2 The procedures of Example 1 were repeated, except that-the procedure for preparing dimethyl sebacate in the second step was as described below.

Into the electrolytic solution tank 12, were charged monomethyl adipate, methanol, and a monomethyl adipate solution containing potassium salt of monomethyl adipate which was recycled from the bottom of recovery column 20. The charge solution was 500 kg in total and contained 33.0% by weight of monomethyl adipate, 10.0% by weight (the neutralization degree corresponding to 20% by mole) of potassium salt of monomethyl adipate, and 1.2% by weight of water. Using an electrolytic cell similar to that in Example 1, the charge solution was circulted through the electrolytic solution tank 12 and the electrolytic cell 13.The charge solution was passed between electrodes at a linear flow rate of 2 m/second and the electrolysis was carried out for 6.3 hours, while maintaining the current density and the temperature of the solution at 20 A/dm2 and at 55"C to 58"C, respectively. The cell voltage per pair of electrodes changed from 9.2 V to 7.6 V. After completion of the reaction, the electrolytic solution was 442 kg in quantity and of the following composition, as determined by gas chromatography: dimethyl sebacate, 21.5% by weight; monomethyl adipate, 0.02% by weight; water, 1.0% by weight. The current efficiency was 63.5% and the material yield was 80.1%.After completion of the reaction, the electrolytic solution was withdrawn from the electrolytic solution tank 12, mixed with 80 kg of water, and sent to the distillation column 14 in the same manner as described in Example 1. The solution was treated in the column similarly to Example 1 and a mixture of oil and an aqueous solution was obtained from the bottom. The mean residence time at the bottom of the column was 0.8 hour and the mean methanol content of the mixture obtained from the bottom was 2.1% by weight. The mixture was separated into two layers of oil and aqueous solutoin in the decanter 15, as in Example 1. The oily layer weighed 130 kg and contained 0.3% by weight of potassium salt of monomethyl adipate. The aqueous layer weighed 141 kg. Similar to Example 1, the oily layer was purified in the distillation columns 16, 17 and 18.

The dimethyl sebacate obtained from the top of distillation column 18 was sent to the third step. The purity of the dimethyl sebacate obtained was 99.9% or higher; The aqueous layer from the decanter 15 was mixed with 150 kg of monomethyl adipate supplied through the feed inlet 22 and, as in Example 1, sent to the evaporator 19 and the recovery column 20. In the evaporator 19, operating at atmospheric pressure, the temperature was set at 1 28"C. The residence times of the solution in the evaporator 19 and in the recovery column 20 were 4 minutes in total. The water content of the monomethyl adipate solution containing potassium salt of monomethyl adipate withdrawn from the bottom of the recovery column 20 was 2.3% by weight.

Example 3 The procedures of Example 1 were repeated exceptthatthe procedure for preparing dimethyl sebacate in the second step was as described below.

Into the electrolytic solution tank 12, were charged monomethyl adipate, methanol, and the monomethyl adipate solution containing potassium salt of monomethyl adipate which was recycled from the evaporator 19. The charge solution was 500 kg in total and contained 25.0% by weight of monomethyl adipate, 2.3% by weight (the neutralization degree corresponding to 6.9% by mole) of potassium salt of monomethyl adipate, and-2.9% by weight of water. The charge solution was electrolized for 9.6 hours in the same manner as in Example 1. The cell voltage per pair of electrodes changed from 8.8 V to 6.2 V. After completion of the electrolytic reaction, the electrolytic solution weighed 446 kg and contained 16.1% by weight of dimethyl sebacate and 0.01% by weight of monomethyl adipate.The current efficiency was 62.3% and the material yield was 79.9%. After completion of the electrolytic reaction, similar to Example 1, the solution was admixed with water, stripped of methanol in the distillation column 14, and separated into two layers of oil and aqueous solution in the decanter 15. The oily layer weihed 98 kg and contained 0.01% by weight of potassium salt of monomethyl adipate. Similar to Example 1, the oily layer was purified in the distillation columns 16, 17 and 18to obtain dimethyl sebacate from the top of column 18. The purity of the dimethyl sebacate was 99.9% or higher. The aqueous layer from the decanter, weighing 70 kg, was sent directly to the evaporator 19. In the evaporator 19 operating at atmospheric pressure, the temperature was set at 11 0'C.

Water vapor was released from the top and an aqueous solution containing 47% by weight of potassium salt of monomethyl adipate was obtained as side-stream. The aqueous solution was quenched by adding 100 kg of monomethyl adipate to obtain a monomethyl adipate solution containing potassium salt of monomethyl adipate.

Example 4 The procedures of Example 1 were repeated, except that the procedure for preparing dimethyl sebacate in the second step was as described below.

Electrolytic condensation was carried out in the same manner as in Example 1, except that sodium salt of monomethyl adipate was used in place of the potassium salt. The cell volage per pair of electrodes was changed from 7.6 V to 5.6 V. The current efficiency was 61.3% and the material yield was 79.1 %. After completion of the reaction, similarly to Example 1, the electrolytic solution was mixed with water, then stripped of methanol in the distillation column 14, then separated into oily and aqueous layers in the decanter 15. The oily layer containing 0.01% by weight of sodium salt of monomethyl adipate was purified, as in Example 1, in the distillation columns 16, 17 and 18. Dimethyl sebacate obtained from the top of column 18 showed a purity of 99.9% or more.The aqueous layer from the decanter 15was mixed with 150 kg of monomethyl adipate supplied through feed inlet 22 and, as in Example 1, sent to the evaporator 19. A monomethyl adipate solution containing sodium salt of monomethyl adipate was obtained as the bottom stream of recovery column 20. The water content of said solution was 4.1% by weight.

Comparative Example 1 The electrolytic condensation was carried out in the same manner as in Example 2. After completion of the condensation, the electrolytic solution was directly sent to the distillation column 14 without prior addition of water. Methanol was removed in the column 14 under the same conditions as in Example 2. When the methanol content of the bottom stream from the column 14 was adjusted to 9.2% by weight, partial deposition of potassium salt of monomethyl adipate was observed.

Referential Example 1 In the same manner as described in Example 1 methanol, monomethyl adipate, and a monomethyl adipate solution containing potassium salt of monomethyl adipate were charged into the electrolytic solution tank 12. The charge solution, 500 kg in total weight, contained 35.5% by eight of monomethyl adipate, 5.5% by weight (the neutralization degree corresponding to 11% by mole) of potassium salt of monomethyl adipate, and 4.5% by weight of water, The electrolysis was conducted in a manner similar to that in Example 1.The cell voltage per pair of electrodes changed from 6,8 V to 5.3 V, After completion of the electrolytic reaction, the electrolytic solution, 443 kg in weight, contained 2D.5% by weight of dimethyl sebacate and 0.01% by weight of monomethyl adipate. The current efficiency was 55.2% and the material yield was 71.1%.

Example 5 The procedures of Example 2 were repeated except that the procedure for preparing dimethyl sebacate was as described below.

In a manner similar to that described in Example 2,500keg of a solution containing 33.0% by weight of monomethyl adipate, 10.0% by weight of potassium salt of monomethyl adipate, and 0.7% by weight of water was prepared. The solution was charged into the electrolytic solution tank 12 and electrolytic condensation was carried out as in Example 2. The cell voltage changed from 9.3 V to 7.6 V. After completion of the electrolytic reaction, electrolytic solution, 443 kg in weight, contained 21.6% by weight of dimethyl sebacate, 0.01% by weight of monomethyl adipate, and 0.6%% by weight of water. The current efficiency for the formation of dimethyl sebacate was 63.8% and the material yield was 80.5%.Similarly to Example 2, the electrolytic solution after completion of the reaction was mixed with 40 kg of water supplied through the feed inlet 21 and stripped of methanol in the distillation column 14. The mean residence time in the column 14 was 0.6 hour and the methanol content of the bottom stream was 2.9 / by weight. The bottom stream was separated into oily and aqueous layers in the decanter 15. The oily layer containing 0.D6% by weight of potassium salt of monomethyl adipate was purified, as in Example 2, in the distillation columns 16, 17 and 18. The purity of the dimethyl sebacate obtained from the top of column 18 was 99.9% or more. The aqueous layer from the decanter 15 was mixed with 100 kg of monomethyl adipate supplied through the feed inlet 22 and, as in Example 2, sent to the evaporator 19. A monomethyl adipate solution containing potassium salt of monomethyl adipate was obtained as the bottom stream from recovery column 20. The water content of said solution was 2.1% by weight.

Claims (1)

1. A process for producing sebacic acid by half-esterifying adipic acid with methanol to form monomethyl adipate, which is subjected to batchwise electrolytic condensation in methanol medium in the presence of an alkali metal salt of monomethyl adipate, while maintaining the water content of the electrolytic solution at a level in the range of from 0.15 to 3.0% by weight, to obtain an electrolytic solution containing dimethyl sebacate, recovering the dimethyl sebacate from the electrolytic solution, and hydrolyzing the dimethyl sebacate to yield sebacic acid, which comprises removing the methanol from said electrolytic solution containing dimethyl sebacate after the electrolytic condensation after or at the same time water is added to the electrolytic solution, whereby the residual solution is separated into two layers, an oily one containing substantially dimethyl sebacate and an aqueous one containing substantially the alkali metal salt of monomethyl adipate, collecting the dimethyl sebacate from the oily layer, while recovering the alkali metal salt of monomethyl adipate as a monomethyl adipate solution from the aqueous layer by addition of monomethyl adipate to the aqueous layer before or after water is removed by evaporation from the aqueous layer to such an amount as to maintain the level of the water content of the electrolytic solution at the electrolytic condensation, and recycling the morlomethyl adipate solution to the electrolytic condensation step.

2. A process according to Claim 1, wherein the half esterification of adipic acid is carried out by passing a starting material solution containing adipic acid and methanol through a fixed bed packed with a strongly acidic cation exchage resin, 3, A process according to Claim 1, wherein the batchwise electrolytic condensation of monomethyl adipate is carried out until the residual amount of monomethyl adipate in the electrolytic solution decreases to at most 0.1% by weight of the charged monomethyl adipate.

4. A process according to Claim 1, wherein the batchwise electrolytic condensation of monomethyl adipate is carried out by using the charge solution containing 10 to 60% by weight of monomethyl adipate under the condition of a flow rate of the electrolytic solution in an electrolytic cell of 1 to 4 m/sec, an electrode distance of 0.5 to 3 mm, current density of 5 to 40 A/dm2 and an electrolytic solution temperature of 45 to 65"C, said monomethyl adipate in the charged solution being neutralized at 5 to 50% by mole in its neutralization degree by at least one base selected from the group consisting of hydroxides, carbonatqs, bicarbonates, methylates and ethylates of potassium and sodium.

5. A process according to Claim 4, wherein the base for neutralizing monomethyl adipate is a hydroxide, carbonate or bicarbonate of potassium.

6. A process according to Claim 1, wherein the amount of water added to the electrolytic solution is 0.5 to 5.0 parts by weight per part by weight of the alkali metal salt of monomethyl adipate.

7. A process according to Claim 1, wherein methanol is removed from the electrolytic solution until the methanol content of the residual solution becomes 6% by weight or less.

8. A process according to Claim 7, wherein the methanol content of the residual solution is 3% by weight or less.

9. A process according to Claim 1, wherein addition of monomethyl adipate to the aqueous layer is carried out before removal of water by evaporation.

10. A process according to Claim 9, wherein the amount of monomethyl adipate to be added to the aqueous layer containing substantially an alkali metal salt of monomethyi adipate is 1 to 10 parts by weight per part by weight of the alkali metal salt of monomethyl adipate.

11. A process according to Claim 1, wherein the hydrolysis of dimethyl sebacate is carried out by adding sebacic acid and removing methanol produced by the hydrolysis out of the system during the hydrolysis.

12. A process according to Claim 1 substantially as described herein with reference to the accompanying drawing or in any one of the Examples.

13. Sebacic acid obtained buy a process according to anyone of Claims 1 to 12.