WO1997024303A1

WO1997024303A1 - Method for purifying crystalline substance

Info

Publication number: WO1997024303A1
Application number: PCT/JP1996/003669
Authority: WO
Inventors: Koichi Nagaoka; Koji Yamamoto; Keiko Moriya
Original assignee: Kabushiki Kaisha Kobe Seiko Sho
Priority date: 1995-12-26
Filing date: 1996-12-17
Publication date: 1997-07-10
Also published as: JPH09176055A

Abstract

A method for efficiently purifying a particular substance contained in a raw material mixture, comprising the steps of: (1) subjecting the raw material mixture in a liquid form or a slurry form wherein part of the particular substance has crystallized to crystallization under high pressure to precipitate crystals and settle them in a pressure vessel; (2) lowering the pressure in the pressure vessel to entirely or mostly dissolve the settled crystals, thereby forming a liquid phase or a slurry having a high concentration of the particular substance in the lower part of the pressure vessel; and then (3) enhancing the pressure in the pressure vessel to crystallize the particular substance in the lower part, followed by discharge of the mother liquor out of the pressure vessel.

Description

Specification

TECHNICAL FIELD The present invention relates to a purification method capable of obtaining a specific substance at a high yield and a high purity from a mixture containing a specific substance having a crystallinity. From a mixture of dimethylnaphthalene isomers obtained as a petroleum fraction or obtained by addition of a methyl group to naphthalene or cyclization of hydrocarbon, etc., a method for obtaining 2,6-dimethylnaphthalene of high purity It can be used effectively. BACKGROUND ART Conventional methods for separating and purifying a specific substance from a mixture containing the specific substance include a distillation method using a difference in boiling point and a cooling crystallization method using a difference in solubility in a solvent depending on temperature. Is widely used, and recently, a pressure crystallization method utilizing a pressure difference has been proposed. However, in any method, it is difficult to purify a specific substance from a raw material mixture having a low concentration of the specific substance to a high purity and a high yield in a single-stage treatment. A method is used in which the mixture is subjected to multiple stages of treatment to successively increase the concentration of the specified substance. In particular, a solid solution is formed with the specified substance, or the physical properties (such as the boiling point and solubility in solvents) are approximated. If other substances are included, various problems will arise in narrowing down or refining specific substances.

For example, the case of purifying 2.6-dimethylnaphthalene (hereinafter, dimethylnaphthalene is abbreviated as DMN) will be described as an example. 2,6-DMN can be obtained from coal or petroleum fractions by distillation or the like, or by addition of methyl groups to naphthalene or by cyclization of hydrocarbons. As a mixture containing the DMN isomers of DMN has 10 types of isomers, and the boiling points of these isomers are close to each other, so that there is a natural limit to the reduction by fractional distillation, and the concentration as 2,6-DMN is 10 ~ It can only be increased to about 40%.

In addition, the applicant has previously proposed a pressure crystallization method (JP-A-63-275528) as an efficient method for producing 2,6-DMN, but in order to carry out this method efficiently, a raw material mixture must be used. The 2,6-DMN concentration must be about 50% or more, and 2.6-DMN concentration is 10-40%. — Getting a DMN is difficult.

In addition, the present inventors have confirmed that it is difficult to purify 2,6-DMN from the above mixture. Another reason is that some of the other DMN isomers form a solid solution with 2.6-DMN. It was confirmed that it was difficult to remove from the 2,6-DMN crystallized product. That is, if 2.6-DMN does not form a solid solution at all with other DMN isomers, and if 2,6-DMN precipitates as a pure substance by crystallization, the surface of the 2.6-DMN crystallized product All impurities, including the DMN isomer attached to the mother liquor together with the mother liquor, should be present in the mother liquor, and the content ratio of the impurities (DMN isomer) in the raw material mixture and 2.6-DMN

BB

The content of impurities (DMN isomers) attached to and mixed with the mother liquor on the surface of the product should be the same for all DMN isomers. However, the results obtained by cooling crystallization are as shown in Table 1 below. table 1

That is, among the DMN isomers, 2.7-DMN and 2,3-DMN have been mixed into the crystallized crystal at a higher ratio than the other DMN isomers. DMN and 2.3-DMN are more likely to form a solid solution with 2.6-DMN than other DMN isomers, and it is considered that crystallization removal as an impurity becomes difficult. As another impurity having such a tendency, 2-methylnaphthalene has been confirmed to be highly likely to form a solid solution with 2.6-DMN. As described above, the present applicant has proposed that a pressure crystallization method is effective as a method for obtaining a specific substance with high purity from a mixture containing impurities that form a solid solution (Japanese Patent Publication No. 6-161842). ). In this method, as shown in the flow chart of Fig. 2, for example, in step (1), the raw material mixture is pressurized to a high pressure to crystallize a specific substance. The impurities that crystallized and adhered to the surface of step (1) or the mother liquor that adhered to the surface were washed away, and the obtained crystal was once depressurized and melted in step (3). By purifying and separating and removing the liquid phase, a high-purity target substance is obtained as a crystal.

That is, in pressure crystallization, after pressure is applied to crystallize, pressure is reduced in the separation step. Sweating (In this step, the crystals of the target substance partially melt when depressurized, and (In order to increase the purity of the target substance by washing away the substance), the crystals with a high impurity concentration that have formed a solid solution are preferentially melted, and the purity of the target substance is reduced by removing the melt. If the system is then depressurized once to melt the crystals and then repressurized, the newly precipitated crystals will have even lower impurities and the target substance will be obtained as high purity crystals. You can do it.

However, in this method, when the content of a specific substance in the raw material mixture to be treated is high, that is, when the concentration is relatively high, the temperature rise due to pressurization is large, so that the melting and perspiration during separation is more effective However, when the concentration of a specific substance in the raw material mixture is low, that is, when the concentration is low, the temperature rise due to pressurization is small, and the purity increase due to perspiration is expected. Do not go to.

For example, Table 2 below shows that a mixture of DMN isomers whose specific substance is 2,6-DMN is treated with adiabatic treatment of a mixture with different concentrations (for each concentration, the ratio of solids in the raw material mixture at atmospheric pressure is 10%). % Temperature from 1.% to 1.500 kg / cm ² ). Table 2

That is, in the case of a raw material mixture with a concentration of 70%, the temperature rises at 40 by pressurization, and about 20% of the crystals are melted by the subsequent pressure reduction, so that the purification effect by perspiration is sufficiently enhanced. . However, when the concentration of the raw material mixture is 30%, the temperature rise due to pressurization is only about 2 CTC. However, at this temperature difference, only about 2% of the crystals are melted, and the effect of sweating is about 110% of that at a concentration of 70%, so that impurities and the like that form a solid solution cannot be sufficiently removed by sweating.

As described above, even if the pressure crystallization method is adopted, if the concentration of the specific substance in the raw material mixture is low and contains a large amount of impurity components that form a solid solution, the purification efficiency by sweating is not sufficiently improved. This tendency is the same when the raw material mixture contains an impurity component having a solubility / sensitivity temperature close to that of the target substance. The present invention has been made in view of the problems of the prior art as described above, and its object is to form a solid solution with the target substance at a relatively low content, or It is intended to provide a method that can efficiently purify a high-purity target substance with a simple operation, even from a raw material mixture that contains impurities whose solubility and melting temperature are close to the target substance. is there. DISCLOSURE OF THE INVENTION The method of purifying a crystalline substance according to the present invention is a method of purifying the intended substance by subjecting it to a crystallization operation under a high pressure from a raw material mixture containing a specific substance. The point is that the steps (3) to (3) are performed sequentially.

(1) The above raw material mixture in the form of a liquid or a slurry in which the specified substance is partially crystallized is subjected to a crystallization operation under a high pressure to crystallize the crystals under a high pressure, and the generated crystals are subjected to pressure. The process of settling in the vessel,

(2) The pressure in the pressure vessel is reduced, and all of the settled crystals Or forming a liquid phase or slurry enriched with a specific substance at the lower part of the pressure vessel by melting

(3) A step of increasing the pressure in the pressure vessel, crystallizing the specific substance in the lower part, and then discharging the mother liquor out of the pressure vessel. The above-mentioned purification method of the present invention is particularly applicable to the purification of a specific substance from a mixture containing, as an impurity, a substance forming a solid solution with a specific substance to be purified, for example, from a mixture of DMN isomers. 6—It can be effectively used as a method for separating and purifying DMN. In addition, since the mother liquor discharged to the outside of the pressure vessel in the step (3) contains an equivalent amount of a specific substance, the mother liquor is mixed into the raw material mixture, and is again used in the above (1) to (3). It is preferable to repeat the above step because the yield of the specific substance can be further increased.

The crystallization operation under high pressure is carried out at 500 to 3,000 kg / cm ² , more preferably 1,000, from the viewpoint of efficiently performing pressure crystallization while minimizing temperature rise due to pressurization. to 2. it is preferably performed at a pressure in the range of 000 kg / cm ^2. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic process explanatory view illustrating a purification method according to the present invention, FIG. 2 is a flowchart showing a conventional concentration / purification method, and FIG. 3 is a purification procedure employed in Examples. FIG. BEST MODE FOR CARRYING OUT THE INVENTION As described above, the purification method employed in the present invention is basically a pressure crystallization method. This method is common to the conventional purification method in that it adopts a method of purifying a specific substance efficiently from a raw material mixture that has a low concentration of a specific substance and contains a considerable amount of impurities that form a solid solution. As a means of pressure, pressure operation during pressure crystallization is divided into two stages, a high concentration phase of a specific substance is formed in a pressure vessel, and crystallization is performed by pressurizing the specific substance in the high concentration phase. As a result, we succeeded in efficiently obtaining high-purity specific substances.

FIG. 1 is a schematic explanatory diagram of a process when a purification method according to the present invention is performed. First, a raw material mixture A is charged into a pressure vessel 1 as shown in (a). The raw material mixture A at this time may be in the form of a solution in which the target substance is completely dissolved, or may be in the form of a slurry in which a part of the specific substance is crystallized by cooling to an appropriate temperature. Next, when the pressure in the pressure vessel 1 is increased as shown in (b), the dissolved specific substance crystallizes out in the entire area of the vessel 1.

If the pressure is maintained while maintaining the pressure at this time, the crystal that has crystallized in the pressure vessel 1 as shown in (c) settles below the vessel 1 by its own gravity, and the crystal C that has settled out Contains a considerable amount of impurities that form a solid solution with the specific substance to be purified, but most of the impurities are contained in the supernatant S in a liquid state.

Thereafter, when stepping down the pressure vessel 1 as shown (d), the precipitated crystals C to melt down, the high liquidus S _c of a particular substance concentration made form the lower part, on the high impurity concentration above A two-phase state having a specific substance concentration different from that of the clear portion S is formed. Then, when the pressure in the pressure vessel 1 is increased again, the specific substance is crystallized in the liquid phase S c having a high specific substance port as shown in (e), and the crystallization system is converted into the raw material mixture as described above. compared specific substance concentration is increased significantly, since the concentration of impurities is decreased, the crystal C _s to out crystallization in comparison to crystallization C which crystallized in step (c) above certain Very high material purity. Therefore, when the liquid phase is discharged out of the pressure vessel 1 from this state and pressed to sufficiently remove the mother liquor adhering around the crystal, a high-purity target substance can be obtained as a crystal cake.

Since the mother liquor discharged in this step contains a considerable amount of a specific substance to be purified, if necessary, the mother liquor may be returned to the first step and mixed with the raw material mixture to perform the above treatment. This is preferable because the yield of the specific substance as a whole can be further increased.

The operating pressure when performing the crystallization operation, 500~3, 000 kg / cm, more preferably 1, 000~2, 000 kg / cm 2 range is desirable, 5 0 0 kg / cm ² less than the pressure of the In this case, the crystallization of the specified substance does not proceed sufficiently and the yield is difficult to increase.On the other hand, if the pressure is excessively high, exceeding 3,000 kg / cm ² , the temperature increases significantly due to the pressure and the crystallization occurs. The crystallization equipment must be equipped with a cooling mechanism, which imposes a heavy burden on equipment.

In the mother liquor draining step, the purity of the obtained crystal can be further improved by performing the sweating treatment by slightly lowering the operating pressure if necessary. Further,該桔crystal C _s when the lowered to the operating pressure to atmospheric pressure after emitting perspiration process because ¾ solution, it is possible to perform discharge of the container 1 out easier and smoothly. In discharging the final mother liquor, it is desirable that the liquid phase S having a high impurity concentration be extracted from above the pressure vessel 1. Thus, when trying to extract the liquid phase S having a high impurity concentration from below the pressure vessel 1, the liquid phase S having a high impurity concentration has to pass between the crystals C _s having a high purity. As a result, the liquid Cs is contaminated by the liquid phase S having a high impurity concentration. However, if the liquid phase S is extracted from the upper side as described above, such contamination does not occur, and the specific substance is not contained. times of high purity桔晶C _s which crystallized out from the high liquidus S _c density remains high purity Because it can be collected.

As described above, in the present invention, after the pressure crystallization, the crystal is once settled by gravity and then melted to form a liquid phase having an increased specific substance concentration. This method has a remarkable effect when crystallizing and refining a specific substance from a mixture system that forms a solid solution or a mixture system with similar crystallization temperatures and pressures. I do.

In other words, when a specific substance is crystallized from a mixture system with different crystallization temperature and pressure without forming a solid solution, the specific substance is obtained as almost pure crystals by pressing and cooling regardless of the concentration of the raw material mixture. However, as described above, in a mixed system containing impurities forming a solid solution as described above, the lower the concentration of the specific substance in the raw material, the higher the impurity content in the precipitated crystal. In other words, the degree to which the purity can be increased by one crystallization operation largely depends on the specific substance concentration of the original raw material mixture.To obtain high-purity crystals from a low-concentration raw material mixture, once the purity is low, High-purity crystals cannot be obtained unless solid crystals and solid-liquid separation are performed after crystallization of the crude crystals, and the crystallization and solid-liquid separation are repeated one or more times.

However, in the present invention, a high-concentration region of a specific substance is formed in the pressure vessel 1 by the first pressure crystallization, gravity sedimentation, and pressure-down melting without the need for solid-liquid separation during crystallization as described above. Since the specific substance can be crystallized from the high concentration region by the subsequent pressure increase, the purification operation is greatly simplified, and the loss of the specific substance by at least two solid-liquid separations is suppressed, and the recovery rate is reduced. Is also enhanced.

In practicing the present invention, as described above, it is preferable to perform a sweating treatment when finally obtaining a high-purity crystal, but the crystal finally deposited in the container 1 is a specific substance. Crystallized from the concentrated system with increased concentration, and the impurity concentration of the mother liquor attached around Since the degree is sufficiently low, crystals of sufficiently high purity can be obtained without intentionally performing perspiration.

In the above description, the main feature of the present invention is described as “purification from a low-concentration raw material mixture to a high-purity substance”. However, if the present invention is applied to a raw material mixture originally having a high concentration of a specific substance, it can be further improved. It is clear that a high-purity target substance can be obtained, and in this case also, compared with the conventional method, it is possible to obtain a higher-purity target substance with a simpler operation and a high recovery rate. Examples Hereinafter, the present invention will be described in more detail with reference to examples.However, the present invention is not limited to the following examples, and may be appropriately changed within a range that can conform to the spirits described above and below. It is also possible to implement the present invention, and all of them are included in the technical scope of the present invention. In the following, “%” means “% by weight”.

2, 6 — Pressure crystallization was performed using the mixture containing other DMN isomers and monomethylnaphthalene (MMN) as impurities as a raw material with a DMN content of around 30%, as shown in Fig. 3. Was. That is, after using a pressure crystallization apparatus with an internal capacity of 2 O ml, the raw material mixture (sample) is charged into the pressure vessel 1a, and the pressure crystallization is performed while maintaining the isothermal state in the thermostatic chamber 2. An appropriate pressure difference is provided between the drainage vessel 1b and the mother liquor to be separated to the vessel 1b side through the filter F. In the embodiment of the present invention, the mother liquor is left for 10 minutes after pressurized crystallization. After the crystallized crystals are sedimented by gravity at the bottom of the vessel 1a, the pressure is once released to melt the crystals, and then the pressure crystallization is performed again, and then the mother liquor is separated in the direction of the vessel 1b. Adopt the method Was. In the comparative example, after the pressure crystallization, the mother liquor was immediately separated in the direction of the container 1b by a pressure difference without leaving and releasing the pressure.

Example and Comparative Example I

2, 6—DMN using a raw material mixture of 29.9% port strength, and subject to pressure crystallization at 1,000 ° C / cm ² at 60 ° C according to the present invention to crystallize crude crystals. after allowed to stand crystals 0 min to gravity settling, then melted most of the crystals by lowering the pressure to a 200 kg / cm ² (sample of this concentration is completely at 60 ° C in 0.99 kg / cm ² Then, the pressure was increased again to 1,000 kg / cm 'to perform pressure crystallization, and the mother liquor was separated in the direction of the container 1b by a differential pressure (within 250 kg / cm ² ). Table 3 shows the purity of the obtained crystals, the amount of impurities mixed therein, and the content ratio of the target substance and impurities contained in the crystals to the raw materials.

For comparison, Table 3 also shows the results when pressure crystallization was performed only once under the conditions of 60 ° C x 1,000 kg / cra ^{2 and} the mother liquor was separated using the same raw material mixture as above. did.

Table 3 Concentration of each isomer (%) and 2,6-DMN content ratio (1)

2, 6-DMN 2, 7-paint 2, 3-DMN 1, 6- 麵 1, 3-DMN 1-MMN 2-paint Raw material concentration 29.94 11.04 4.91 11.53 22.88 1.73 1.16 Concentration 73.20 8.41 3.48 3.26 6.12 0.0 0.59 to

Comparative example

Ratio to raw material 1445 0.762 0.709 0.283 0.267 0.231 0.509 is degree 81.64 7.39 2.68 1.81 3.36 0.26 0.46 The present invention

2.73 0.67 0.55 0.16 0.15 0.15 0.40

As is clear from Table 3, in the comparative example, the purity of the obtained crystal was 2.6%, whereas the purity of the obtained crystal was 81.6% in the method of the present invention. You can see that it is rising.

Example 2. Comparative Example 2

Using a raw material mixture having a concentration of 2, 6 -01 ^ 1 ^ 32.5%, the crude crystals were once subjected to pressure crystallization at 60 ° C X 1,000 kg / cm ² in accordance with the method of the present invention. After allowing the crystals to crystallize and standing for 10 minutes to settle the crystals by gravity, the internal pressure was lowered to ²⁰ kg / m ² to melt most of the crystals (a sample with this concentration was 60.C. Then, the solution was completely melted at 15 Okg / cm ² ), the pressure was increased again to 1.000 kg / cm ² , pressure crystallization was performed, and the mother liquor was separated in the direction of the vessel 1 b by the differential pressure (within 250). Table 4 shows the purity of the obtained crystals, the amount of impurities mixed therein, and the ratio of the target substance and impurities contained in the crystals to the raw materials.

Table The results of the case also for comparison, using the same raw material mixture as above, to release the 6 0 ° Cx 1, only once in a 000 kg / cm ² Conditions mother liquor pressure crystallization from a row of connexion minute Also described in 4.

Table 4 Concentration of each isomer (%) and ratio of 2.6-DMN (1)

2.6-DMN 2, 7-DMN 2, 3-DM 1.6-Paint 1.3-DMN Marauder 2-Orchid Raw material concentration 32.5 16.8 9.3 9.5 18.6 0.71 0.55 Macau 79.4 10.9 6.0 0.9 1.5 0.06 0.15 Comparative example

Ratio to raw material 2.44 0.65 0.65 0.09 0.08 0.08 0.27 Concentration 83.8 7.7 4.5 0.9 1.7 0.06 0.13 Invention

2.58 0.46 0.48 0.10 0.09 0.08 0.24

As is clear from Table 4, in the comparative example, the purity of the obtained crystal was 69.4%, whereas in the method of the present invention, the purity was 83.8%. Has risen to. When comparing the impurity content to the raw material ratio, there is no significant difference in the numerical values of components other than 2,7-DMN and 2.3-DMN, but a solid solution is formed with 2.6-DMN. For 2,7-DMN and 2,3-DMN, the value of 0.65 in the comparative example was reduced to 0.47 in the example, and the content of the solid solution-forming impurities in the obtained crystals was reduced. It can be seen that it has decreased significantly.

Example 3, Comparative example 3

2. 6 -0 \ 11 ^ Using a raw material mixture of 32.5¾, a crude crystal is crystallized by pressure crystallization once under the conditions of 1,000 kg / era ² at 60 ° C according to the present invention. After allowing the crystals to settle by gravity for 10 minutes, the internal pressure was reduced to 200 _kg / cm ² to melt most of the crystals (the sample with this concentration was 60. 0 kg / cm ² to completely melt), pressurized again to 1,000 kg / cm ² , perform pressure crystallization, increase the pressure difference from container 1 b to 500 kg / cm ² , and transfer mother liquor to container 1 b Then, the pressure in the pressure vessel 1a was set to 80 Okg / cm and the pressure in the separation vessel 1b was set to the atmospheric pressure, and the mother liquor in the crystal gap was squeezed out by the differential pressure. Table 5 shows the purity of the crystals obtained, the amount of impurities mixed therein, and the ratio of the target substance and impurities contained in the crystals to the raw materials.

In addition, the same raw material mixture as above was used for comparison. Table 5 also shows the results when pressure crystallization was performed only once under the condition of C x 1.000 kg / on ² and the mother liquor was separated and squeezed out under the same conditions as above. Table 5 Concentrations of each isomer (%) and 2,6-DMN content ratio (-)

2, 6-DMN 2, 7-DMN 2.3-substitute 1, 6-DMN 1, 3-DMN 1- MN 2- 腳 Raw material concentration 32.5 16.8 9.3 9.5 18.6 0.71 0.55 Concentration 82.5 9.0 6.0 0.55 0.83 0.03 0.15 Comparative example

2.54 0.54 0.65 0.06 0.04 0.04 0.27 Concentration 85.9 7.3 4.4 0.58 0.89 0.03 0.13

Ratio to raw material 2.64 0.43 0.47 0.06 0.05 0.04 0.24

As is evident from Table 5, the purity of 2,6-DMN of the obtained crystal is 82.5% in the comparative example, while the purity of the obtained crystal is 85.9% in the method of the present invention. It is rising. Also, when comparing the ratio of each impurity content to the raw material, there is no significant difference in the numerical values of components other than 2.7-DMN and 2,3-DMN, but a solid solution is formed with 2.6-DMN. When comparing 2.7-DMN and 2.3-DMN, it can be seen that the example is significantly lower than the comparative example, and the content of solid solution-forming impurities is significantly reduced. Effects of the Invention The present invention is configured as described above, and forms a liquid phase having a high concentration of a specific substance by gravity-sedimenting a crude crystal generated by pressure crystallization and then reducing the pressure to melt the crystal. And then perform pressure crystallization again, which can effectively increase the purity of the target substance in a single solid-liquid separation, and in particular, a raw material mixture containing impurities that form a solid solution with the specific substance to be purified Therefore, the impurities can be effectively removed, and high purification efficiency can be obtained by a simple operation.

Claims

The scope of the claims

1. A method for purifying a specific substance from a raw material mixture containing the specific substance by subjecting the specific substance to a crystallization operation under high pressure, comprising the following steps (1) to (3): Method for the purification of toxic substances.

(1) The above-mentioned raw material mixture in the form of a liquid or a slurry in which the specific substance is partially crystallized is subjected to a crystallization operation under high pressure to precipitate crystals under high pressure, and the generated crystals are placed in a pressure vessel. Settling process within

(2) a step of lowering the pressure in the pressure vessel and dissolving all or most of the settled crystals to form a liquid phase or slurry having a high concentration of the specified substance in the lower part of the pressure vessel;

(3) increasing the pressure in the pressure vessel, crystallizing the specific substance in the lower part of the upper queen, and then discharging the mother liquor out of the pressure vessel;

2. The purification method according to claim 1, wherein the impurities contained in the raw material mixture are substances that form a solid solution with the specified substance.

3. Purification of a crystalline substance characterized in that the mother liquor discharged to the outside of the pressure vessel in the step (3) in claim 1 is subjected to the steps (1) to (3) in claim 1. Method.

4. The purification method according to claim 1, wherein the specific substance is 2.6-dimethylnaphthalene, and the mixture contains a dimethyl naphthalene isomer.

5. The purification method according to claim 1, wherein the crystallization operation under high pressure is performed at a pressure of 500 to 3.000 kg / cm ² .