JP2013136523A - METHOD FOR PRODUCING ε-CAPROLACTAM - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for producing ε-caprolactam, capable of obtaining high quality ε-caprolactam from crude ε-caprolactam by using evaporative crystallization in good yield.SOLUTION: This production method of ε-caprolactam comprises: a purification step of allowing ε-caprolactam to be crystallized from a mixed solution prepared by mixing crude ε-caprolactam produced by Beckmann rearrangement of cyclohexanoneoxime with an organic solvent, and then subjecting the resulted solution to solid/liquid separation to produce the ε-caprolactam and a drop crystallization collected mother liquor; and a collection step of evaporating an evaporative crystallization mother liquor including the drop crystallization-collected mother liquor to cause crystallization, and then subjecting the resultant solution to solid/liquid separation to obtain collected ε-caprolactam and an evaporative crystallization-collected mother liquor. In the method, prior to the collection step, the drop crystallization-collected mother liquor is mixed with at least a portion of an evaporative crystallization-collected mother liquor, at least a portion of the evaporative crystallization mother liquor extracted from a vessel in which evaporative crystallization is performed, or both of them, to prepare a mixed solution; and the mixed solution is introduced into the vessel to mix the mixed solution with the evaporative crystallization mother liquor stored in the vessel.

Description

  The present invention relates to a method for producing ε-caprolactam, and more particularly to a method for obtaining high-quality ε-caprolactam in high yield from crude ε-caprolactam containing impurities obtained by Beckmann rearrangement of cyclohexanone oxime.

 ε-Caprolactam is an important compound as a production intermediate of nylon-6, and various production methods are known. For example, it is produced in large quantities by subjecting cyclohexanone oxime to Beckmann rearrangement in the presence of an acidic medium such as fuming sulfuric acid. However, this method has a problem that a large amount of ammonium sulfate with a small added value is by-produced.

  As a method for improving this, a method for producing ε-caprolactam by a gas phase Beckmann rearrangement reaction using a solid catalyst is known. As the solid catalyst used in the gas phase Beckmann rearrangement reaction, a boric acid catalyst, a silica / alumina catalyst, a solid phosphoric acid catalyst, a composite metal oxide catalyst, a zeolite catalyst, and the like have been proposed. However, since ε-caprolactam obtained by this method contains various impurities, it is usually purified by various methods such as crystallization, extraction, distillation and hydrogenation.

  Patent Document 1 discloses a method for producing purified ε-caprolactam by solid-liquid separation by crystallizing ε-caprolactam by co-mixing a molten crude ε-caprolactam and a cooled organic solvent. It is disclosed. In addition, as a method for increasing the recovery rate of ε-caprolactam, the effect of further crystallization from the mother liquor after crystallization is also mentioned.

JP 2002-3472 A

  As a method of crystallizing the mother liquor after crystallization that occurs when ε-caprolactam crystallizes by solid-liquid separation by co-mixing the molten crude ε-caprolactam and a cooled organic solvent, There is known “evaporation crystallization” in which the solvent is evaporated and concentrated to increase the supersaturation degree of the target product to precipitate crystals of the target product dissolved in the mother liquor. While evaporative crystallization can cool the mother liquor with the heat of vaporization and can suppress energy for cooling, it is placed on the heat transfer surface, the inner wall surface of the crystallization tank, and the crystallization tank. It tends to cause crystal adhesion (scaling) to internals such as baffles and stirring blades. Therefore, when recovering crystals by further evaporating crystallization from the mother liquor after crystallization, the crystals that should be obtained may not scale, resulting in difficulty in increasing the recovery rate. Driving is difficult. Therefore, a method for suppressing scaling in evaporation crystallization has been demanded.

  The present invention has been made in view of such circumstances, and yields high-quality ε-caprolactam from a crude ε-caprolactam containing impurities obtained by the Beckmann rearrangement of cyclohexanone oxime using evaporation crystallization. It aims at providing the manufacturing method of the epsilon caprolactam which can be obtained well.

In order to solve the above problems, the present invention crystallizes ε-caprolactam from a solution obtained by mixing a crude ε-caprolactam obtained by Beckmann rearrangement of cyclohexanone oxime and an organic solvent, and the ε-caprolactam is obtained by solid-liquid separation. A purification step of obtaining caprolactam and a drop crystallization recovery mother liquor;
A recovery step of evaporating and crystallizing the evaporation crystallization mother liquor containing the dripping crystallization recovery mother liquor and then obtaining recovered ε-caprolactam and evaporation crystallization recovery mother liquor by solid-liquid separation, and
Prior to the recovery step, the dripping crystallization recovery mother liquor is mixed with at least a part of the evaporation crystallization recovery mother liquor or at least a part of the evaporation crystallization mother liquor extracted from a container for performing evaporation crystallization, or both. Thus, there is provided a method for producing ε-caprolactam, which is made into a mixed solution, and the mixed solution is introduced into the container and mixed with the evaporation crystallization mother liquor stored in the container.

  According to this method, when the drop crystallization recovery mother liquor is mixed with the evaporation crystallization mother liquor or the evaporation crystallization recovery mother liquor in advance and diluted, the dripping crystallization recovery mother liquor is introduced into the vessel for performing the evaporation crystallization. In addition, it is possible to suppress the precipitation of abrupt crystals that cause scaling. Therefore, it can be set as the manufacturing method of (epsilon) -caprolactam which raised the recovery rate of (epsilon) -caprolactam and improved productivity by continuous operation.

  In the present invention, it is desirable to supply the mixed liquid to a liquid phase part in a container for performing the evaporation crystallization.

  According to this method, the mixed liquid to be introduced into the container for evaporating crystallization is mixed with the evaporating crystallization mother liquor stored in the container and then crystallized. Therefore, crystallization can be easily controlled and scaling can be suppressed.

  In the present invention, in the recovery step, it is desirable to supply the mixed solution in a circumferential direction of the container.

  According to this method, a stirring blade for stirring the inside of the container becomes unnecessary, and scaling to the stirring blade can be eliminated.

  In the present invention, it is desirable that at least a part of the inner wall of the vessel for performing the evaporation crystallization is heated to a temperature higher than the operation temperature of the evaporation crystallization.

In the present invention, in the evaporation crystallization, the solvent evaporation load is desirably 100 kg / m ² / hour or more and 2000 kg / m ² / hour or less per liquid surface area.

In the present invention, it is desirable that the operation supersaturation degree is 0.1 kg / m ³ or more and 20 kg / m ³ or less in the evaporation crystallization.

  In the present invention, it is desirable that the operation pressure is 10 to 500 torr in the evaporative crystallization.

  In the present invention, in the evaporation crystallization, the crystallization temperature is preferably 0 ° C. or more and 60 ° C. or less.

  In the present invention, in the recovery step, a part of the evaporation crystallization mother liquor is withdrawn from the container, and a part of the evaporation crystallization mother liquor is supplied again to the container, so that the outside of the container The evaporation crystallization mother liquor is circulated, and a part of the evaporation crystallization mother liquor withdrawn to the outside of the vessel is 10 times or more and 10,000 times the evaporation amount of the solvent evaporated from the evaporation crystallization mother liquor in the evaporation crystallization. It is desirable that the amount is twice or less.

  In the present invention, the crude ε-caprolactam is desirably obtained by gas phase Beckmann rearrangement using cyclohexanone oxime as a raw material and a solid catalyst.

  In the present invention, the purification step comprises the liquid crude ε-caprolactam and a cooled aliphatic hydrocarbon solvent, or a cooled aliphatic hydrocarbon and a small amount of an organic solvent having a higher polarity than the aliphatic hydrocarbon. It is desirable to perform crystallization by mixing and mixing the mixed solvent in the container under conditions lower than the liquid crude ε-caprolactam.

  In the present invention, the liquid crude ε-caprolactam is molten crude ε-caprolactam, a mixed solution of molten crude ε-caprolactam and an aliphatic hydrocarbon solvent, or molten crude ε-caprolactam and aliphatic. It is desirable to be a mixed solution with a mixed solvent in which a small amount of an organic solvent having a polarity higher than that of the hydrocarbon and the aliphatic hydrocarbon is mixed.

  According to the present invention, scaling in evaporation crystallization can be suppressed, and high-quality ε-caprolactam can be stably obtained with high yield from crude ε-caprolactam containing impurities obtained by Beckmann rearrangement of cyclohexanone oxime. It becomes possible.

It is a flowchart in the case of implementing the manufacturing method of this embodiment by a continuous process. It is a flowchart of the process including a crystallization tank and a solid-liquid separator. It is sectional drawing of a crystallization tank.

In the production method of ε-caprolactam according to this embodiment, ε-caprolactam is crystallized from a solution obtained by mixing crude ε-caprolactam obtained by Beckmann rearrangement of cyclohexanone oxime and an organic solvent, and the ε-caprolactam is obtained by solid-liquid separation. A purification step of obtaining caprolactam and a drop crystallization recovery mother liquor;
A recovery step of evaporating and crystallizing the evaporation crystallization mother liquor containing the dripping crystallization recovery mother liquor and then obtaining recovered ε-caprolactam and evaporation crystallization recovery mother liquor by solid-liquid separation, and
Prior to the recovery step, the dripping crystallization recovery mother liquor is mixed with at least a part of the evaporation crystallization recovery mother liquor or at least a part of the evaporation crystallization mother liquor extracted from a container for performing evaporation crystallization, or both. The mixed liquid is introduced into the container and mixed with the evaporation crystallization mother liquor stored in the container.

In the present specification, “dropped crystallization recovery mother liquor” refers to a liquid phase portion produced as a result of crystallization of ε-caprolactam in a purification step and solid-liquid separation.
The “evaporative crystallization recovery mother liquor” refers to a liquid phase portion that is generated as a result of evaporative crystallization and solid-liquid separation in the recovery process. In the following specification, it may be referred to as “recycle liquid”.
The “evaporation crystallization mother liquor” refers to the entire target to be subjected to evaporation crystallization in the recovery process, and is a liquid stored in a container (crystallization tank 7 described later) that performs evaporation crystallization. The evaporation crystallization mother liquor includes a dripping crystallization recovery mother liquor generated in the purification step, which is a step preceding the recovery step, and an evaporation crystallization recovery mother liquor generated in the recovery step and used in circulation.

  In the present specification, “crude ε-caprolactam” means ε-caprolactam which is a target product obtained in the production process of ε-caprolactam, and impurities (reaction solvent, unreacted raw materials, side reaction products, etc.) And a reaction mixture. The following explanation uses a zeolite-based catalyst (solid catalyst) such as silica-alumina, metallosilicate, silicalite, etc. as crude ε-caprolactam (hereinafter sometimes abbreviated as “crude lactam”), and cyclohexanone oxime The reaction mixture obtained by vapor phase Beckmann rearrangement is exemplified, but the crude lactam to which the present invention can be applied is not limited thereto.

FIG. 1 is a flowchart showing an example of the case where the manufacturing method of the present embodiment is carried out in a continuous process. The steps described in the figure include a crystallization tank 1, a solid-liquid separator 2, a crystallization tank 3, a solid-liquid separator 4, a crystal washing machine 5, a dissolution tank 6, a crystallization tank 7, a solid-liquid separator 8, A cooling facility 9 is included.
Hereinafter, it demonstrates in order.

(Purification process)
First, in the dissolution tank 6, the melted crude lactam is mixed with an organic solvent, and a solution in which the melted crude lactam and the organic solvent are mixed is prepared.
The crude lactam is not uniquely defined depending on the production method, but in addition to methanol and unreacted cyclohexanone oxime used as a solvent, cyclohexanone, cyclohexenone, n-hexonitrile, 5-hexonitrile, methyl lactam, 1,3,4, 5-tetrahydroazepin-2-one, 1,5,6,7-tetrahydroazepin-2-one, 1,2,3,4,6,7,8,9-octahydrophenazine (hereinafter abbreviated as OHP) And by-products such as amines such as 3-N-methyl-4,5,6,7-tetrahydrobenzimidazole (hereinafter sometimes abbreviated as MTHI).

  For example, in the crude lactam obtained by the gas phase Beckmann rearrangement method, the concentration of cyclohexanone oxime is usually 10 ppm or more with respect to ε-caprolactam, 1,2,3,4,6,7,8,9-octa. The concentration of hydrophenazine is 10 ppm or more, the concentration of 3-N-methyl-4,5,6,7-tetrahydrobenzimidazole is 25 ppm or more, and 1,3,4,5-tetrahydroazepin-2-one, The concentration of 6,7-tetrahydroazepin-2-one and their structural isomers (hereinafter abbreviated as caprenolactams) is contained at 25 ppm or more. Among these impurities, caprenolactams are included. Except for this, it is possible to remove and purify all at once by applying the crystallization method of the present invention.

  The crude lactam obtained by the gas phase Beckmann rearrangement reaction using a zeolite-based catalyst contains a solvent such as methanol, various low-boiling by-products, and various high-boiling by-products. Therefore, for the purpose of increasing the crystallization efficiency, it is preferable to carry out preliminary distillation or the like to remove some or all of these solvents and by-products as necessary before dissolution in the dissolution tank 6.

  As the organic solvent used in the dissolution tank 6, a hydrocarbon solvent having a low polarity is recommended in order to increase the recovery rate by preventing a large amount of lactam from remaining in the solution after crystallization.

  Examples of such solvents include aliphatic straight chain hydrocarbons having 6 to 12 carbon atoms, aliphatic side chain hydrocarbons, and alicyclic hydrocarbons. More specifically, aliphatic side chain hydrocarbons such as n-hexane, n-heptane, n-octane, n-nonane, n-decane, etc., aliphatic hexanes such as methylhexane, isooctane, neohexane, Examples thereof include alicyclic hydrocarbons such as methylcyclopentane, cyclohexane, and methylcyclohexane, and mixtures of aliphatic hydrocarbons such as petroleum benzine (specified in JIS K 8594. Mixtures such as hexane and isohexane). Among them, aliphatic hydrocarbons having a boiling point not lower than the melting point (69 ° C.) of ε-caprolactam and lower than the boiling point of ε-caprolactam (267 ° C.), such as n-heptane, isooctane, and petroleum benzine, are preferable, and the boiling point is ε-caprolactam. An aliphatic hydrocarbon (for example, cyclohexane, etc.) having a melting point or higher and about 150 ° C. or lower is more preferable.

  These may be used alone or in combination of two or more or more polar organic solvents that can be mixed uniformly with these. That is, as long as impurities that are the object of the present invention can be removed, a small amount of aromatic hydrocarbon such as benzene, toluene and xylene, halogenated hydrocarbon such as trichlene, ether such as propyl ether and isopropyl ether, etc. , Esters such as ethyl acetate and isopropyl acetate, alcohols such as ethanol and isopropanol, etc. are mixed, the boiling point is higher than the melting point of ε-caprolactam and lower than the boiling point of ε-caprolactam, preferably the boiling point is higher than the melting point of ε-caprolactam. And it can also be used as a mixed solvent of about 150 ° C. or lower.

  Of these solvents, combinations of solvents with low polarity and close boiling points are recommended. In this case, in continuous operation, even if there is a change in the solvent composition, the change in the solubility of lactam is small and the influence on the crystallization yield is small, so that the concentration control of the solvent becomes easy. As such a solvent combination, an aliphatic linear hydrocarbon and an alicyclic hydrocarbon are preferable, and a combination of n-heptane and cyclohexane is particularly preferable.

  In addition, it is good also as performing crystallization of a back | latter stage only using a melt | fusion rough | crude lactam, without mixing a melt | fusion rough | crude lactam and an organic solvent in the dissolution tank 6 as mentioned above. However, it is preferable to perform crystallization using a solution obtained by mixing a molten crude lactam and an organic solvent in the dissolution tank 6 because a crystalline lactam having a low oxime content can be obtained.

  The solution adjusted in the dissolution tank 6 is supplied to the crystallization tank 1. In the crystallization tank 1, the solution prepared in the dissolution tank 6 and the organic solvent (hereinafter sometimes referred to as a cold solvent) cooled in the cooling equipment 9 are supplied in parallel to perform crystallization. I do.

  As the said cold solvent, what cooled the thing similar to the organic solvent used for mixing with a crude lactam in the above-mentioned dissolution tank 6 can be used conveniently. In addition, as long as the solubility of ε-caprolactam is not high (a solvent having high polarity), the organic solvent used for mixing with the crude lactam and the cold solvent may be different types.

  The amount and temperature of the cold solvent supplied to the crystallization tank 1 are defined in accordance with a crystallization temperature determined in advance in order to realize a target crystal recovery rate. That is, the cold solvent to be supplied includes the amount of heat of the difference between the temperature of the crude lactam (molten crude lactam or a mixed solution of molten crude lactam and an organic solvent) supplied to the crystallization tank 1 and the crystallization temperature, A heat removal capability capable of offsetting the heat of crystallization of the precipitated crystals is required. Accordingly, the amount and temperature of the cold solvent are defined such that the sensible heat of the cold solvent cancels out these amounts of heat.

  The amount of the solvent applied for crystallization is about 1/2 to about 5 times by mass, preferably about 1 to about 4 times by mass, with respect to the crude lactam used for purification. Even if it is used beyond the above range, a crystallization effect commensurate with the amount used cannot be obtained, and the cost required for solvent recovery increases. On the other hand, if the amount is small, it may not be possible to recover crystals of sufficient quality.

  The crystallization temperature is about 10 ° C. to less than the melting point of lactam, preferably about 30 ° C. to about 60 ° C., more preferably about 40 ° C. to about 60 ° C. The crystallization temperature takes into account the lactam solubility of the liquid phase at the crystallization temperature in order to achieve both the target crystal recovery rate and the reduction of heat removal requirements (reduction of process operating energy). It is preferable to set the temperature close to the upper limit temperature at which a predetermined crystal recovery rate can be set. Further, such a crystallization temperature is preferable because impurities can be easily separated.

  Furthermore, in order not to cause scaling on the outer wall of the crystallization tank, the crystallization temperature is set to a temperature slightly higher than the crystallization tank temperature to the extent that the crystallization product is not excessively dissolved again. Good. Specifically, by setting the temperature to a temperature several degrees higher than the crystallization temperature, more specifically, the lactam crystallization temperature + about 5 ° C or lower, more preferably + 1 ° C or lower, the generation of scaling on the vessel wall is suppressed. I can do it. Alternatively, the outer wall of the crystallization tank may be a heat insulating structure.

  In the crystallization tank 1, liquid crude ε-caprolactam (molten crude lactam or a mixed solution of molten crude lactam and an organic solvent) is continuously mixed (mixed and mixed) at a constant rate together with a cold solvent to form crystal precipitation. With this, the crystallization temperature is maintained.

  In this case, even if the ratio of the crude lactam used for crystallization and the solvent is the same, a part of the solvent used as described above is added to the crude lactam in advance to make a diluted solution, and the solution does not precipitate crystals. A crystallization method is preferred in which the cooling is performed within the range and the remaining cooling solvent is added together to perform the crystallization treatment. This makes it possible to reduce the content of specific impurities, particularly the oxime, remaining in the obtained crystal as compared with a method not previously applied as a diluted solution.

  In the crystallization tank 1, the concentration of the slurry in which the crystallized crystals are dispersed can be stabilized by securing the time necessary for crystallization (that is, by having a ripening time). The time for stabilizing the slurry concentration is preferably 5 minutes or more.

  In the case of adopting a continuous crystallization process in the crystallization tank 1, it is necessary to increase the capacity of the crystallization tank 1 in order to extend the average residence time of the crystallization tank 1. Therefore, an appropriate time is selected from the viewpoint of crystal quality and economy. Since the time required for stabilizing the concentration of the slurry is sufficient from 5 minutes to 60 minutes, it is preferably managed in such a range, more preferably from 20 minutes to 40 minutes.

  The slurry containing ε-caprolactam crystals obtained in the crystallization tank 1 is subjected to sedimentation separation of crystals contained in the slurry using a solid-liquid separator 2 such as a liquid cyclone, and a part of the slurry containing fine crystals is contained. After separating the liquid phase, crystallization may be performed again from the remaining liquid phase containing relatively large crystals. Specifically, the remaining liquid phase may be introduced into the crystallization tank 3, and a cold solvent may be poured again into the remaining portion to cool the remaining portion to crystallize the lactam. In this case, the yield (precipitation amount) can be increased as compared with the case where ε-caprolactam is recovered by crystallization only in the crystallization tank 1 without using the solid-liquid separator 2 and the crystallization tank 3. It becomes possible.

  In this case, since the yield in crystallization in the crystallization tank 1 can be lowered, the crystal obtained in the crystallization tank 1 can be made less likely to contain impurities. In addition, since the cold solvent is added again in the crystallization tank 3, the impurity concentration of the solution to be crystallized can be reduced, which is effective in improving the quality of the ε-caprolactam crystals obtained in the crystallization tank 3. It is.

  In the solid-liquid separator 2, the amount of the organic solvent to be removed from the slurry is not particularly limited, but is usually 10% to 70% of the liquid phase, and the amount is determined by the crystal quality. The crystallization method in the crystallization tank 3 is not different from that of the crystallization tank 1, and for example, the crystallization tank may be kept warm and the cold solvent may be mixed and mixed. The crystallization temperature is set in a range of about 30 ° C. to 60 ° C., but is set lower than the temperature condition in the crystallization tank 1.

  The slurry obtained in the crystallization tank 1 and the crystallization tank 3 is introduced into the solid-liquid separator 4. In the solid-liquid separator 4, the introduced slurry can be separated into a solid phase containing ε-caprolactam crystals and a liquid phase containing impurities by solid-liquid separation. In this case, the steps from the crystallization tank 1 to the solid-liquid separator 4 are operated while keeping the temperature of the slurry not lowered, but may have a heating function or a cooling function as necessary. .

  Solid-liquid separation can be performed using a filter used for normal crystallization, such as vacuum filtration, pressure filtration, etc. In order to perform continuous filtration, centrifugal filtration, continuous scraping centrifugal A filter, a centrifugal decanter, or the like may be used. Especially in continuous scraping centrifugal filters and centrifugal decanters, crystals can be rinsed continuously, and the liquid phase containing impurities attached to the crystals can be washed to further improve the quality of the crystals. Is preferable.

  The crystals (solid phase) of ε-caprolactam separated by the solid-liquid separator 4 are supplied to the crystal washer 5 for washing the crystals and washed. Thereby, impurities adhering to the crystal surface can be removed, and ε-caprolactam (purified lactam) with higher purity can be obtained as compared with those not washed. The crystals (solid phase) of ε-caprolactam separated by the solid-liquid separator 4 are preferably supplied to the crystal washing machine 5 for washing the crystals and washed, but crystals of ε-caprolactam are obtained only by solid-liquid separation. If the purity of the crystal is satisfactory, the crystal washer 5 may be omitted.

  The organic solvent used for the washing is preferably a solvent having a low solubility of ε-caprolactam, and the above-described hydrocarbon solvent having a low polarity can be used.

  In the present embodiment, the solid-liquid separator 4 and the crystal washer 5 are described as separate devices, but the same device functions as the solid-liquid separator 4 and the crystal washer 5. It is good also as having (a solid-liquid separator has a crystal washing part).

  Further, in order to obtain a higher purity ε-caprolactam (purified lactam), the solid-liquid separation by the solid-liquid separator 4 and the crystal washing by the crystal washing machine 5 may be repeated a plurality of times. A step of repulp washing the crystals by adding a crystal washing solvent between the separation and the crystal washing cycles may be added.

  Moreover, each process from melt | dissolution of the rough | crude lactam in the dissolution tank 6 to the washing | cleaning of the crystal | crystallization in the crystal washing machine 5 may be performed by a continuous type, and is good also as a batch type (batch type). Since production efficiency can be improved, it is preferable to carry out by a continuous type.

  The liquid phase containing the organic solvent separated by the solid-liquid separator 4 and the liquid phase containing the organic solvent discharged from the crystal washer 5 are partially used as an organic solvent for dissolving the crude lactam in the dissolution tank 6. May be.

(Recovery process)
Among the liquid phases discharged from the solid-liquid separator 2, the solid-liquid separator 4, and the crystal washing machine 5, the liquid phase that is not used in the dissolution tank 6 is obtained by recovering crystals by recrystallizing the liquid phase. The yield can be easily increased. Of course, the crystals may be recovered by a combination of known methods such as distillation, extraction, chemical treatment, activated carbon treatment and the like other than crystallization. The recovered crystals can be efficiently recovered from the crude lactam by adopting a method of recycling the recovered crystals to the above-described purification step.

  More specifically, the liquid phase (dropped crystallization recovery mother liquor) not used in the dissolution tank 6 is introduced into the crystallization tank 7. The crystallization method in the crystallization tank 7 employs evaporation crystallization under reduced pressure. From the viewpoint of energy saving, the solvent may be distilled off to some extent from the drop crystallization recovery mother liquor using a multi-effect can etc. before evaporative crystallization.

  In the case of evaporation crystallization, since the solvent is evaporated and separated, the organic solvent discharged from the crystallization tank 7 is a purified organic solvent from which impurities have been removed. It can be reused as a cold solvent, a solvent used in the crystallization tank 3, a washing solvent applied to the crystal washing machine 5, or the like.

  The slurry after the crystallization treatment is introduced into a solid-liquid separator 8 and separated into ε-caprolactam crystals (recovered lactam) as a solid phase and a liquid phase in which impurities are concentrated. The recovered lactam discharged from the solid-liquid separator 8 is introduced into the dissolution tank 6 and reused.

  On the other hand, the liquid phase discharged from the solid-liquid separator 8 is distilled in a solvent recovery step (not shown) to recover the organic solvent and the impurities are separated as a residue. Although not shown, the lactam yield is further improved by recovering the lactam in the liquid phase discharged from the solid-liquid separator 8. The recovery method may be recovered by combining known methods such as crystallization, distillation, extraction, chemical treatment, activated carbon treatment and the like. The recovered lactam can be efficiently recovered from the crude lactam by adopting a method of recycling to the above-described purification step.

  Note that, when mixed solvents having different boiling points are used, the composition of the recovered solvent is different. However, it is preferable to reuse the mixture after adjusting the concentration by mixing the recovered solvent including the solvent recovery step.

  The recovered solvent may be circulated as a solvent used in the crystallization tank 1 and the like, and the discharge from this process is only the residue in the solvent recovery step, resulting in a production process with very little waste.

  Next, the recovery process will be described in more detail with reference to FIGS.

FIG. 2 is a flowchart of a process including the crystallization tank 7 and the solid-liquid separator 8.
The dripping crystallization recovery mother liquor introduced into the crystallization tank 7 is cooled to such an extent that it is crystallized in the crystallization tank 1 in the previous stage, and in the state of being kept cold, the solid-liquid separation is performed in the solid-liquid separator 4 shown in FIG. Therefore, it is a highly concentrated solution that is almost saturated. Therefore, when such a high concentration dropped crystallization recovery mother liquor is introduced into the crystallization tank 7 under a reduced pressure environment, the solvent evaporates from the dropped crystallization recovered mother liquor, so that the crystal is precipitated immediately. Become. If such rapid crystal precipitation is performed, it causes scaling in the crystallization tank 7, which tends to cause problems such as a decrease in crystal recovery rate and a failure to continuously operate due to blockage of a line or the like.

  Therefore, in the crystallization tank 7 of the present embodiment, scaling in the crystallization tank 7 is suppressed by adopting the following configuration.

  First, the inside of the crystallization tank 7 includes a portion (liquid phase portion 7a) in which slurry (evaporated crystallization mother liquor) containing crystallized crystals is stored, and a space (gas phase portion) above the liquid phase portion 7a. 7b). The crystallization tank 7 has a circular shape in cross section, but the diameter of the gas phase part 7b is larger than the diameter of the liquid phase part 7a. The liquid phase portion 7a and the gas phase portion 7b have a jacket structure, and are heated by supplying a heat medium to the jacket portion, thereby suppressing the scaling of ε-caprolactam on the inner wall.

  In the crystallization tank 7, at least a part of the evaporated crystallization mother liquor in the crystallization tank 7 is extracted and introduced into the crystallization tank 7 again, so that at least a part of the evaporated crystallization mother liquid is crystallized in the crystallization tank 7. Is provided with an external circulation line 15 for circulation outside. By circulating the evaporation crystallization mother liquor through the external circulation line 15 as described above, the dripping crystallization recovery mother liquor is diluted, the concentration of the evaporation crystallization mother liquor is made uniform, and scaling is suppressed.

  In the following description, the circulation of the evaporation crystallization mother liquor using the external circulation line 15 may be referred to as “external circulation”. Further, the evaporation crystallization mother liquor flowing in the external circulation line 15 may be referred to as “external circulation slurry”.

  Among the liquid phases discharged from the solid-liquid separator 2, the solid-liquid separator 4, and the crystal washer 5 in FIG. 1, the liquid phase not used in the dissolution tank 6 (dropped crystallization recovery mother liquor) is passed through the pipe 11. It is introduced into the external circulation line 15 and introduced into the crystallization tank 7 together with the external circulation slurry flowing in the external circulation line 15. The pipe 11 is connected to the suction side of the pump 14 a of the external circulation line 15, and the dropped crystallization recovery mother liquor joins with the external circulation slurry and is mixed by the pump 14 a before being supplied to the crystallization tank 7. .

  In the external circulation line 15, a heat exchanger 17 for heating the slurry flowing inside is arranged. The external circulation slurry and dripping crystallization recovery mother liquor flowing in the external circulation line 15 are preheated by the heat exchanger 17 and then introduced laterally (direction intersecting the direction of gravity) with respect to the crystallization tank 7. .

  FIG. 3 is a cross-sectional view of the crystallization tank 7 at a height position where the external circulation line 15 is connected. As shown in FIG. 3A, the external circulation line 15 is arranged so that the tip 15a of the external circulation line 15 does not face the direction of the center 7x of the crystallization tank 7 (crystallization tank). 7) (toward the circumferential direction of 7). Or as shown in FIG.3 (b), it connects to the crystallization tank 7 toward the side surface direction (tangential direction of the outer periphery of the crystallization tank 7) of the crystallization tank 7. FIG. In the case of the configuration shown in FIG. 3B, the tip portion corresponding to the tip 15a shown in FIG.

  As a result, the dripping crystallization recovery mother liquor introduced from the pipe 11 forms a flow in the circumferential direction of the evaporation crystallization mother liquor (indicated by a curved arrow in the figure) inside the crystallization tank 7, and the crystallization is caused by the flow. The evaporation crystallization mother liquor in the crystallization tank 7 can be stirred, and the dropped crystallization recovery mother liquor can be efficiently diluted with the evaporation crystallization mother liquor in the crystallization tank 7. Therefore, in the crystallization tank 7 shown in the figure, a stirring blade for stirring the inside becomes unnecessary, scaling to the stirring blade can be eliminated, and scaling to other portions in the crystallization tank 7 is also reduced.

  Note that when the dropped crystallization recovery mother liquor is introduced into the liquid phase part 7 a, the direction of the tip 15 a of the external circulation line 15 may be the direction of the center 7 x of the crystallization tank 7. Further, the direction of the tip 15a may be oriented vertically (gravitational direction) with respect to the crystallization tank 7.

  Further, the dripping crystallization recovery mother liquor can be used as a configuration in which it is introduced into the gas phase part 7b of the crystallization tank 7 and falls from the gas phase part 7b to the liquid phase part 7a in the crystallization tank 7. In order to suppress scaling at the tip of the circulation line 15, it is preferable to introduce the dripping crystallization recovery mother liquor into the liquid phase part 7a as described above.

  Returning to FIG. 2, the bottom of the crystallization tank 7 is connected to a pipe 12 for discharging the evaporated crystallization mother liquor obtained in the crystallization tank 7 and introducing it into the solid-liquid separator 8. The pipe 12 is provided with a discharge pump 14b on the way, and sends the evaporation crystallization mother liquor downstream. In the solid-liquid separator 8, the sent evaporation crystallization mother liquor is subjected to solid-liquid separation to obtain a recovered lactam and a liquid phase.

  Further, at least a part of the liquid phase generated in the solid-liquid separator 8 (evaporation crystallization recovery mother liquor, recycle liquid) is introduced into the external circulation line 15 via the pipe 16 and again introduced into the crystallization tank 7. It has been reused. The pipe 16 is connected to the suction side of the pump 14a of the external circulation line 15, and the evaporated crystallization recovery mother liquor joins with the external circulation slurry and the dropped crystallization recovery mother liquor before being supplied to the crystallization tank 7, It mixes with the pump 14a.

  In such a configuration, in the external circulation line 15, a drop crystallization recovery mother liquor introduced into the crystallization tank 7 from the pipe 11, and an evaporation crystallization mother liquor (external circulation slurry) circulated through the external circulation line 15, And the recycle liquid recycled through the pipe 16 is mixed. As a result, the dropped crystallization recovery mother liquor to be introduced becomes a mixed liquid diluted with the evaporated crystallization mother liquor and the recycle liquid, and crystals are deposited as soon as the high concentration dropped crystallization recovery mother liquor is introduced into the crystallization tank 7. The risk of doing so is reduced.

  As shown in the pump 14a in the figure, a pump for feeding the drop crystallization recovery mother liquor may be provided on the downstream side of the position where the pipes 11 and 16 are connected in the external circulation line 15. In such an arrangement, when passing through the pump 14a, the drop crystallization recovery mother liquor, the evaporation crystallization mother liquor, and the recycle liquid are mixed and introduced into the crystallization tank 7 as a uniform slurry (mixed liquid). . In the case of a non-uniform slurry, if a thick slurry is introduced into the crystallization tank 7, crystals may precipitate as soon as possible, which may cause scaling, but by introducing a uniform slurry, Scaling can be suppressed.

In addition, as an operating condition of the crystallization tank 7, the solvent evaporation load in the crystallization tank 7 is preferably 100 kg / m ² / hour or more and 2000 kg / m ² / hour or less per liquid surface area.
The slurry concentration of the evaporated crystallization mother liquor stored in the liquid phase part 7a in the crystallization tank 7 is preferably 10% by mass or more and 40% by mass or less.
The operation supersaturation degree of the evaporation crystallization is preferably 0.1 kg / m ³ or more and 20 kg / m ³ or less.
The operation pressure for evaporation crystallization is preferably 10 torr (1.3 kPa) or more and 500 torr (66.7 kPa) or less.
The crystallization temperature for evaporation crystallization is preferably 0 ° C. or higher and 60 ° C. or lower.
The amount of the evaporation crystallization mother liquor circulating through the external circulation line 15 is preferably 10 times or more and 10,000 times or less with respect to the evaporation amount of the solvent evaporated from the evaporation crystallization mother liquor in the evaporation crystallization. Thereby, since the supersaturation degree of the evaporation crystallization mother liquor is lowered, it becomes difficult to scale. In addition, since a large crystal can be obtained by crystallization and the surface area becomes small, the amount of impurities attached to the obtained crystal can also be reduced.

  In the crystallization tank 7 of the present embodiment, by adopting the above-described configuration, it is possible to suppress the scaling in the crystallization tank 7 and improve the productivity and the crystal recovery rate. .

  The recovered lactam thus obtained is purified together with the crude lactam in the above purification step. The ε-caprolactam (purified lactam) obtained in the purification step is preferably managed so that the basic compound such as cyclohexanone oxime, MTHI, or OHP is not more than a specific concentration. More specifically, the concentration of cyclohexanone oxime contained in the purified lactam is less than 10 ppm with respect to ε-caprolactam, and the concentration of 1,2,3,4,6,7,8,9-octahydrophenazine is less than 10 ppm. It is preferable to manage the operating conditions of the crystal washer 5 so that the concentration of 3-N-methyl-4,5,6,7-tetrahydrobenzimidazole is less than 25 ppm.

  The degree of treatment of the crude lactam is controlled by the above impurity content index. The degree of removal of impurities by the crystallization treatment can be adjusted by crystallization conditions such as the type of solvent used, the amount of solvent, and the crystallization temperature, and may be confirmed by preliminary experiments.

  Each crystallization treatment is preferably performed under crystallization treatment conditions so that a satisfactory refined lactam quality can be obtained at one time. However, if a high degree of purification is required, crystallization treatment should be performed multiple times. Is recommended.

For caprenolactams that are difficult to remove by crystallization, ε-caprolactam obtained by the crystal washer 5 can be removed by contacting with hydrogen in the presence of a hydrogenation catalyst. Such treatment can usually reduce caprenolactams to less than 25 ppm.
The method for producing ε-caprolactam according to this embodiment is performed as described above.

  According to the method for producing ε-caprolactam having the above-described structure, scaling in evaporation crystallization can be suppressed, and high-quality ε- Caprolactam can be obtained with good yield.

  In this embodiment, the pipe 11 and the pipe 16 are connected to the external circulation line 15, and the dropped crystallization recovery mother liquor is evaporated in the external circulation line 15 with a part of the evaporation crystallization mother liquor (external circulation slurry). Although mixed with both of the crystallization recovery mother liquor (recycled liquid), it is not limited to this.

  As a first modification, only the pipe 11 is connected to the external circulation line 15, and the dropping crystallization recovery mother liquor and the external circulation slurry are mixed and supplied to the crystallization tank 7 in the external circulation line 15. The recycle liquid may be supplied into the crystallization tank 7 by connecting the pipe 16 to the crystallization tank 7.

  Alternatively, as a second modification, the pipe 11 and the pipe 16 are connected to each other without connecting the pipe 11 and the pipe 16 to the external circulation line 15, and then the joined pipe is connected to the crystallization tank 7. It is good as well. In this case, when the pipe 11 and the pipe 16 are connected, the dropped crystallization recovery mother liquor and the recycle liquid are mixed and supplied to the crystallization tank 7 in the combined pipe. In this case, if a pump for feeding the mixed liquid of the dripping crystallization recovery mother liquor and the recycle liquid is disposed downstream of the place where the pipe 11 and the pipe 16 are connected, the mixed liquid becomes a uniform slurry and crystallizes. Since it introduce | transduces into the analysis tank 7, it is preferable.

  The pipe 16 in the first modified example and the joined pipe in the second modified example are provided with a heat exchanger similar to the heat exchanger 17 provided in the external circulation line 15 and crystallized after preheating. It is preferable to supply the mixed liquid to the tank 7.

  The preferred embodiments of the present invention have been described above with reference to the accompanying drawings, but it goes without saying that the present invention is not limited to such examples. Various shapes, combinations, and the like of the constituent members shown in the above-described examples are examples, and various modifications can be made based on design requirements and the like without departing from the gist of the present invention.

  EXAMPLES The present invention will be described below with reference to examples, but the present invention is not limited to these examples.

Example 1
(Purification process)
Using a high silica type zeolite catalyst, a gas phase Beckmann rearrangement reaction of cyclohexanone oxime was carried out at 380 ° C. in the presence of methanol to obtain a reaction product. From the reaction product, impurities having a boiling point lower than that of ε-caprolactam and impurities having a boiling point higher than that of ε-caprolactam were removed by distillation to obtain crude ε-caprolactam.

  Next, the crude ε-caprolactam was heated to 75 ° C., and the obtained melt and a mixed solvent of n-heptane: cyclohexane = 3: 1 (weight ratio) cooled to 5 ° C. were kept at a jacket temperature of 56 ° C. The crystallization tank was poured simultaneously and continuously. The crystallization temperature was 55 ° C., and the average residence time was 30 minutes.

  The slurry obtained in the crystallization tank was sent to a centrifugal decanter (solid-liquid separator) kept at 55 ° C. for solid-liquid separation, and then the mixed solvent kept at about 50 ° C. was added to the resulting crystal. Crystals of ε-caprolactam were obtained by spraying and washing continuously. Moreover, what combined the liquid phase isolate | separated in a centrifugal decanter and the mixed solvent used for the washing | cleaning of a crystal | crystallization was obtained as dripping crystallization collection | recovery mother liquor.

(Recovery process)
Next, the dropping crystallization recovery mother liquor is supplied to a crystallization tank that performs evaporation crystallization in the recovery step, and is mixed with the slurry stored in the crystallization tank and the following evaporation crystallization recovery mother liquid to obtain an evaporation crystallization mother liquid. Evaporative crystallization was performed.

  In evaporative crystallization, a part of the evaporative crystallization mother liquor is extracted from the crystallization tank and supplied to the crystallization tank again via a pipe provided outside the crystallization tank, thereby circulating outside the crystallization tank. A flow of liquid (external circulation liquid) was formed. The evaporation crystallization mother liquor in the crystallization tank was stirred by the flow of the liquid, and no stirrer was provided in the crystallization tank.

  Further, the evaporation crystallization mother liquor from which ε-caprolactam crystals are precipitated by evaporating crystallization is extracted from the crystallization tank, and the liquid phase (evaporation crystallization recovery mother liquor, recycle liquid) obtained by solid-liquid separation is recrystallized. It was recycled by supplying it to the tank.

When supplying the drop crystallization recovery mother liquor, the external circulation liquid and the recycle liquid to the crystallization tank, these liquids are separately supplied to the crystallization tank through a supply nozzle installed toward the center of the crystallization tank. The solution was fed into the evaporation crystallization mother liquor stored inside. The supply rate of the drop crystallization recovery mother liquor was 0.5 m ³ / hour, the circulation rate of the external circulation liquid was 9.6 m ³ / hour, and the supply speed of the recycle liquid was 1.0 m ³ / hour.

  When the operation temperature of evaporation crystallization was 39 ° C., the residence time of the evaporation crystallization mother liquor in the crystallization tank was 266 seconds, and the operation pressure was 140 torr (18.7 kPa), operation for several days was possible.

  However, when the operating pressure was set at 110 torr (14.7 kPa), scaling to the inner wall of the crystallization tank and the piping for extracting the evaporated crystallization mother liquor from the crystallization tank increased, and operation became impossible in several hours.

(Example 2)
The purification step was performed in the same manner as in Example 1.
In the recovery process, before supplying the drop crystallization recovery mother liquor, the external circulation liquid and the recycle liquid to the crystallization tank, they are mixed in advance to form a mixed liquid, and the supply liquid is installed in the circumferential direction of the crystallization tank. The solution was directly fed into the evaporation crystallization mother liquor stored in the crystallization tank. The supply rate of the drop crystallization recovery mother liquor was 0.5 m ³ / hour, the circulation rate of the external circulation liquid was 9.6 m ³ / hour, and the supply speed of the recycle liquid was 1.0 m ³ / hour.

  When the operation temperature of evaporation crystallization was 39 ° C., the residence time of the evaporation crystallization mother liquor in the crystallization tank was 266 seconds, and the operation pressure was 90 torr (12.0 kPa), continuous operation for 5 days was achieved.

(Comparative Example 1)
The purification step was performed in the same manner as in Example 1.
In the recovery step, the evaporative crystallization mother liquor was stirred without external circulation of the evaporative crystallization mother liquor by providing a stirrer in the crystallization tank.

Moreover, the dripping crystallization collection | recovery mother liquor and the recycle liquid were separately supplied to the gaseous-phase part in a crystallization tank, respectively. The supply rate of the drop crystallization recovery mother liquor was 0.5 m ³ / hour, and the supply rate of the recycle liquid was 1.0 m ³ / hour.

  The operation temperature of evaporation crystallization was 39 ° C., the residence time of the evaporation crystallization mother liquor in the crystallization tank was 266 seconds, and the operation pressure was 300 torr (40.0 kPa). A large amount of scale adhered to the supply line, the inner wall of the crystallization tank, and the piping for extracting the evaporated crystallization mother liquor from the crystallization tank, and operation became impossible in a few hours.

  From these results, it was found that according to the production method of the present invention, scaling is difficult even if the operation pressure is reduced in evaporation crystallization, and continuous operation can be performed satisfactorily, and its usefulness has been confirmed.

DESCRIPTION OF SYMBOLS 1 ... Crystallization tank, 2 ... Solid-liquid separator, 3 ... Crystallization tank, 4 ... Solid-liquid separator, 5 ... Crystal washing machine, 6 ... Dissolution tank, 7 ... Crystallization tank, 7a ... Liquid phase part, 7b ... gas phase part, 7x ... center of crystallization tank, 8 ... solid-liquid separator, 9 ... cooling equipment, 11 ... piping, 12 ... piping, 13 ... piping, 14a, 14b ... pump, 15 ... external circulation line, 15a ... Piping tip, 16 ... Piping, 17 ... Heat exchanger

Claims (12)

Purification from which ε-caprolactam is crystallized from a mixed solution of crude ε-caprolactam obtained by Beckmann rearrangement of cyclohexanone oxime and an organic solvent, and the ε-caprolactam and dripping crystallization recovery mother liquor are obtained by solid-liquid separation. Process,
A recovery step of evaporating and crystallizing the evaporation crystallization mother liquor containing the dripping crystallization recovery mother liquor and then obtaining recovered ε-caprolactam and evaporation crystallization recovery mother liquor by solid-liquid separation, and
Prior to the recovery step, the dripping crystallization recovery mother liquor is mixed with at least a part of the evaporation crystallization recovery mother liquor or at least a part of the evaporation crystallization mother liquor extracted from a container for performing evaporation crystallization, or both. A method for producing ε-caprolactam, wherein the mixed solution is introduced into the vessel and mixed with the evaporated crystallization mother liquor stored in the vessel.   The manufacturing method of the epsilon caprolactam of Claim 1 which supplies the said liquid mixture to the liquid phase part in the container which performs the said evaporation crystallization.   The manufacturing method of the epsilon caprolactam of Claim 2 which supplies the said liquid mixture toward the circumferential direction of the said container in the said collection | recovery process.   The manufacturing method of the epsilon caprolactam of any one of Claim 1 to 3 which heats at least one part among the inner walls of the container which performs the said evaporative crystallization to the temperature higher than the operation temperature of the said evaporative crystallization. 5. The method for producing ε-caprolactam according to claim 1, wherein, in the evaporation crystallization, a solvent evaporation load is 100 kg / m ² / hour or more and 2000 kg / m ² / hour or less per liquid surface area. . 6. The method for producing ε-caprolactam according to claim 1, wherein in the evaporative crystallization, an operation supersaturation degree is 0.1 kg / m ³ or more and 20 kg / m ³ or less.   The method for producing ε-caprolactam according to any one of claims 1 to 6, wherein, in the evaporative crystallization, an operating pressure is 10 to 500 torr.   In the said evaporative crystallization, the crystallization temperature is 0 degreeC or more and 60 degrees C or less, The manufacturing method of the epsilon caprolactam of any one of Claim 1 to 7. In the recovery step, a part of the evaporation crystallization mother liquor is withdrawn from the container, and a part of the evaporation crystallization mother liquor is supplied again to the container, so that the evaporation crystallization mother liquor is supplied outside the container. Circulate,
The part of the evaporation crystallization mother liquor extracted outside the container is an amount that is 10 times or more and 10,000 times or less with respect to the evaporation amount of the solvent that evaporates from the evaporation crystallization mother liquor in the evaporation crystallization. The manufacturing method of the epsilon caprolactam of any one of 1-8.   The method for producing ε-caprolactam according to any one of claims 1 to 9, wherein the crude ε-caprolactam is obtained by gas phase Beckmann rearrangement using cyclohexanone oxime as a raw material and a solid catalyst.   The refining step comprises mixing the liquid crude ε-caprolactam and a cooled aliphatic hydrocarbon solvent, or a cooled aliphatic hydrocarbon and a small amount of an organic solvent having a higher polarity than the aliphatic hydrocarbon, The method for producing ε-caprolactam according to any one of claims 1 to 10, wherein crystallization is carried out by mixing and mixing in a container at a temperature lower than that of the liquid crude ε-caprolactam.   The liquid crude ε-caprolactam is molten crude ε-caprolactam, a mixed solution of molten crude ε-caprolactam and an aliphatic hydrocarbon solvent, or molten crude ε-caprolactam and an aliphatic hydrocarbon, and the fat. The method for producing ε-caprolactam according to claim 11, which is a mixed solution with a mixed solvent obtained by mixing a small amount of an organic solvent having a polarity higher than that of the group hydrocarbon.

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