JPH11300102A - Crystallization method - Google Patents

Abstract

(57) [Summary] [PROBLEMS] To remove scaling and obtain a crystal having a large particle size in a batch cooling crystallization method. SOLUTION: From the start of cooling to the end of crystallization,
A series of operations consisting of a temperature lowering step of lowering the mother liquor temperature to precipitate a part of the solute as crystals and a temperature increasing step of raising the mother liquor temperature after this step until the precipitated crystals are completely dissolved are performed a plurality of times. And further including a final temperature lowering step, and sequentially decreasing the minimum value of the mother liquor temperature and the maximum value of the mother liquor temperature in each time and the final temperature lowering step.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a batch cooling crystallization method.

[0002]

2. Description of the Related Art A batch cooling crystallization method using a crystallizer is performed by gradually cooling a mother liquor comprising pyromellitic acid as a solute and dilute nitric acid as a solvent from 100 ° C to 40 ° C in a crystallizer. Then, a cooling step in which part of pyromellitic acid is crystallized, and cooling is stopped when the mother liquor temperature reaches 40 ° C, and the temperature is raised to 80 ° C (before the precipitated crystals are completely dissolved). A crystallization method and the like comprising three steps of a heating step and a final cooling step of gradually cooling from 80 ° C. to 20 ° C. are known (JP-A-62-247802).

[0003]

However, in the method in which the heating step is performed only once, the crystal growth is insufficient, and fine crystals generated by stirring or the like remain as they are. There is a problem that the liquid content in the wet cake obtained by filtering the slurry is increased, and the purity of the product obtained by drying the wet cake is reduced. Further, in the cooling step, when scaling occurs on the cooling heat transfer surface, the crystallizing pot or the like cannot be cooled to a predetermined temperature based on a decrease in the overall heat transfer coefficient due to the occurrence of the scaling, and the production capacity is reduced. There is a problem that the temperature is significantly reduced.

[0004]

Means for Solving the Problems The present inventors have conducted intensive studies to solve the above problems, and as a result, when a series of operations including a temperature lowering step and a temperature raising step are repeated a plurality of times, the crystallization has occurred.
The inventors have found that the liquid content in the wet cake obtained by filtering the slurry is low, and that the scaling generated on the cooling heat transfer surface of the crystallization pot or the like can be efficiently removed, thereby completing the present invention. That is, the present invention is a method of cooling and crystallizing in a batch system, from the start of cooling to the end of crystallization,
A series of operations consisting of a temperature lowering step of lowering the mother liquor temperature to precipitate a part of the solute as crystals and a temperature increasing step of raising the mother liquor temperature after this step until the precipitated crystals are completely dissolved are performed a plurality of times. Performing, further comprising a final cooling step, and a crystallization method characterized by sequentially reducing the minimum value of the mother liquor temperature in each time and the final temperature lowering step and the maximum value of the mother liquor temperature in the temperature raising step, respectively. To provide. Hereinafter, the present invention will be described in detail.

[0005]

BEST MODE FOR CARRYING OUT THE INVENTION The crystallization method of the present invention is carried out batchwise, and a mother liquor to be used for crystallization is usually obtained by completely dissolving a solute in a solvent. In the method of the present invention, the combination of the solute and the solvent is not particularly limited, and may be any combination of organic substances, between inorganic substances, and any combination of organic and inorganic substances, but when the solute is an inorganic substance, Often insoluble in organic solvents, the solvent inevitably becomes an aqueous medium (water or a solution of water and an inorganic substance, or a solution of an organic solvent miscible with water and water), and the specific heat of the mother liquor increases. For this reason, a combination of organic substances that can quickly switch between the temperature lowering step and the temperature raising step is more preferable. Further, the method of the present invention, for example, an organic phosphorus compound having a melting point of 85 ° C. or less and a boiling point of 80
It is suitably used for a mother liquor consisting of a combination of an organic compound with a lower alkyl ester of acetic acid having a temperature of not more than ° C.

[0006] In the crystallization method of the present invention, in the first cooling step, the mother liquor in which the solute is completely dissolved in the solvent (the mother liquor temperature at this time is T ₀ ° C) is cooled and a part of the solute is cooled. In order to precipitate as a crystal. The cooling rate is appropriately set according to the combination of the solute and the solvent, but is preferably in the range of about 1 to 20 ° C. per hour industrially. or,
It is preferable to minimize the difference (ΔT) between the mother liquor temperature and the refrigerant temperature from the viewpoint of avoiding scaling. Industrially, it is more preferably at most 5 ° C, particularly preferably at most 3 ° C.

[0007] Switching from the cooling step to the first heating step preferably reaches a mother liquor temperature about 10 ° C lower than the mother liquor temperature (T ₁ ° C) at which part of the solute began to precipitate as crystals. The time point, more preferably, the time point when the mother liquor temperature reaches about 7 ° C. lower than T ₁ ° C., particularly preferably, the time point when the mother liquor temperature reaches ₁ to 5 ° C. lower than T ₁ ° C. can be used as a guide. The mother liquor temperature at the time of switching from the first temperature lowering step to the first temperature raising step is defined as T ₂ ° C. The highest mother liquor temperature (referred to as T ₃ ° C) in the first heating step is a temperature lower than the above-mentioned T ₀ ° C, preferably T ₁ ° C.
When the mother liquor temperature reaches T ₃ ° C, a heat-retaining step at the same temperature may be added if necessary. The heat retention time in this step is preferably 5 to 30 minutes, more preferably 15 to 30 minutes. After the end of this step, the first series of operations is completed.

[0008] When the first series of operations is completed, cooling is started. The cooling rate in the second cooling step is the same as that in the first cooling step. Lowest mother liquor temperature in this second cooling step (T ₄ ° C)
Is a temperature lower than the above T ₂ ° C, and preferably T ₄
° C. is 2 to 8 ° C. temperature lower than T ₂ ° C..

When the mother liquor temperature reaches T ₄ ° C, a second heating step is performed. Maximum mother liquor temperature T ₅ ° C at this time
Is a temperature lower than the above T ₃ ° C, preferably T ₃ ° C
1 to 8 ° C. lower than the temperature. After the second heating process,
If necessary, a step of keeping the temperature at T ₅ ° C may be included. The warming time is the same as in the first series of operations.

After the second series of operations is completed, a third series of operations is preferably performed. Incidentally, the mother liquor temperature at the time of switching from the third cooling step to the third heating step, when assumed as T ₆ ℃, T ₆ ℃ is temperature lower than the T ₄ ° C., preferably, T ₆ ℃ is 2-8 ℃ than T ₄ ℃
Low temperature. Assuming that the highest mother liquor temperature in the third heating step is T ₇ ° C, T ₇ ° C is lower than the above T ₅ ° C, and preferably, T ₇ ° C is 1 temperature lower than T ₅ ° C. ８8 ° C. lower temperature. In the third series of operations, if necessary, a step of keeping the temperature at T ₇ ° C may be included. The heat retention time is the same as in the first and second times. When a series of operations after the fourth time are performed,
This is performed according to a series of operations from the third to the third.

Then, a final temperature lowering step is performed. The minimum mother liquor temperature at this time is defined as T _i ° C.

The method of the present invention is preferably used particularly when crystallization is carried out in a device in which a crystallizer, a circulation pump, and an external circulation heat exchanger are connected in a loop by piping. In this case, the crystallization slurry passes from the crystallizer K to the circulation pump P via the pipe A, and from the pump P to the external circulation heat exchanger E via the pipe B, where the heat exchanger It is circulated from E to the crystallization pot K via the pipe C. In the known crystallization method, the crystals were crushed in the crystallization apparatus, particularly in the pump P, and it was difficult to obtain large crystals. Since a series of operations is performed a plurality of times, it becomes possible to dissolve fine crystals generated by crushing and to grow relatively large crystals that remain undissolved even further. The product purity can be improved based on the decrease in the liquid rate. Further, in the crystallization method of the present invention, scaling generated on a heat transfer surface such as an external circulation heat exchanger can be removed.

[0013]

Next, the present invention will be described in more detail with reference to examples, but the present invention is not limited to these examples. Parts and% in the examples are parts by weight and% by weight, respectively.

Example 1 Crystallization was carried out under the following conditions using a crystallizer equipped with a crystallizer made of glass lining, a circulation pump and an external heat exchanger made of stainless steel as shown in FIG. Solute: N-acetyl O, S-dimethylphosphoramidothioate 100 parts Solvent: Ethyl acetate 185 parts Mother liquor temperature T ₀ : 40 ° C Mother liquor volume / crystallizer capacity: 0.83 ΔT (difference between mother liquor temperature and refrigerant temperature ): 2 to 3 ° C. (constant during crystallization operation) Crystal deposition temperature T ₁ : 35 ° C. Series of operations: 4 times Mother liquor temperature T ₂ : 32 ° C. (minimum temperature in the first cooling step) Mother liquor temperature T ₃ : 35 ° C (the highest temperature in the first heating step) Mother liquor temperature T ₄ : 29 ° C (the lowest temperature in the second cooling step) Mother liquor temperature T ₅ : 33 ° C (the highest temperature in the second heating step) Mother liquor Temperature T ₆ : 26 ° C. (minimum temperature in the third cooling step) Mother liquor temperature T ₇ : 28 ° C. (maximum temperature in the third heating step) Mother liquor temperature T ₈ : 20 ° C. (minimum temperature in the fourth cooling step) ) Mother liquor temperature T ₉ : 22 ° C (maximum temperature in the fourth heating step) Mother liquor temperature T _i : 12 to 13 ° C (minimum temperature in the final cooling step)

A part of the obtained slurry was subjected to solid-liquid separation with a small centrifugal filter (6 inches; centrifugal effect was 350 G), and the liquid content and average particle size of the wet cake were determined.
And 450 μm. The overall heat transfer coefficient on the cooling surface of the external circulation heat exchanger was 460 Kcal / m ² · Hr · ° C. Similarly, when the liquid content and the average particle size of the wet cake were determined when the number of operations in the series was set to three times, they were 6.2% and 430 μm, respectively. Rate and average particle size are about 7% and 370 μm, respectively.
Met. The overall heat transfer coefficient on the cooling surface of the external circulating heat exchanger is 380 Kc when a series of operations is performed three times.
al / m ² · Hr · ° C, 170 Kca when the number of operations is 2
l / m ² · Hr · ° C.

Comparative Example 1 A crystallization operation was carried out in the same manner as in Example 1. However, when a series of operations was performed once, the wet cake had a liquid content and an average particle size of 8.6% and 300 μm, respectively. Met. The overall heat transfer coefficient on the cooling surface of the external circulation heat exchanger is 160 Kcal /
m ² · Hr · ° C.

Comparative Example 2 In the same manner as in Example 1, first, crystals were precipitated from the mother liquor. The crystal deposition temperature T ₁ was 35 ° C. At this time, the overall heat transfer coefficient on the cooling surface of the external circulation heat exchanger was reduced to 60 to 80 Kcal / m ² · Hr · ° C. due to the occurrence of scaling. After crystallization, the temperature of the refrigerant is set to 5
While maintaining the temperature at a low temperature of 10 ° C. (ΔT), the solution was gradually cooled to a final mother liquor temperature of 12 to 13 ° C. to complete the crystallization. In the same manner as in Example 1, a part of the obtained slurry was centrifugally filtered to determine the liquid content and the average particle size of the wet cake.
They were about 13% and about 200 μm, respectively. Also, ΔT is 2
Crystallization was performed at 3 ° C., but scaling occurred and the production capacity was significantly reduced. Further, crystallization was attempted by adding a seed crystal, but scaling also occurred.

[0018]

According to the present invention, the wet cake obtained by filtering the slurry obtained by crystallization has a small liquid content, and the product purity can be improved. In addition, scaling during crystallization can be easily removed, the overall heat transfer coefficient of the cooling heat transfer surface increases, and the production capacity can be greatly improved.

[Brief description of the drawings]

FIG. 1 is a schematic diagram of a crystallizer.

[Explanation of symbols]

 A, B, C ... piping K ... crystallizer pot P ... circulation pump E ... external circulation heat exchanger

──────────────────────────────────────────────────の Continued on the front page (51) Int.Cl. ⁶ Identification symbol FI B01D 9/02 625 B01D 9/02 625A C07F 9/24 C07F 9/24

Claims (4)

[Claims] 1. A method of cooling and crystallizing in a batch system, comprising a step of cooling a mother liquor and lowering a temperature of a mother liquor to crystallize a part of a solute by the time the cooling is started and the crystallization is completed. Thereafter, until the precipitated crystals are completely dissolved, a series of operations consisting of a temperature raising step of raising the temperature of the mother liquor is performed a plurality of times, and further includes a final temperature lowering step, and the mother liquor in each time and the final temperature lowering step A crystallization method, wherein the minimum value of the temperature and the maximum value of the mother liquor temperature in the temperature raising step are sequentially reduced. 2. The method according to claim 1, wherein the crystallization is performed in a crystallizer in which a crystallizer, a circulation pump, and an external circulation heat exchanger are connected in a loop by piping. 3. The method according to claim 1, wherein a series of operations including a temperature lowering step and a temperature raising step is performed three times or more. 4. The method according to claim 1, wherein the solute is an organic phosphorus compound, and the solvent is a lower alkyl acetate.