A method for preparing erythritol crystals with uniform particle size

Abstract

The present invention relates to the field of crystallization technology and provides a method for preparing erythritol crystals with uniform particle size. First, the present invention prepares a highly concentrated erythritol solution, then places it in a crystallization tank and crystallizes it under the condition of gas purging. By controlling the concentration of the erythritol solution and the crystallization conditions, erythritol crystals with a particle size distribution concentrated in the range of 60 to 80 mesh can be obtained; moreover, the operation of the present invention is simple, the crystallization rate is fast, and for the same volume of crystallization tank, the production capacity of the method of the present invention is 5 to 8 times that of the ordinary cooling crystallization method, having broad application prospects.

Description

Technical Field

[0001] The present invention relates to the technical field of crystallization, and particularly relates to a method for preparing erythritol crystals with uniform particle size. Background Art

[0002] Erythritol is a four-carbon sugar alcohol that widely exists in nature. It is a white crystalline powder with a refreshing sweet taste, high temperature resistance, and is insensitive to pH. Due to the advantages of high temperature stability, non-participation in human metabolism, anti-cariogenicity, antioxidant property, and moisture retention of erythritol, it is widely used in the fields of beverages, baked foods, candies, desserts, pharmaceuticals, and cosmetics.

[0003] In the development process of the erythritol industry, domestic and foreign erythritol application enterprises not only have requirements for basic indicators such as the purity, moisture, light transmittance, and odor of erythritol products, but also increasingly have requirements for the particle size of erythritol. Common particle size specifications in the market include: 14 - 30 mesh, 30 - 60 mesh, 60 - 80 mesh, 18 - 60 mesh, 24 - 80 mesh, etc. Among them, the 60 - 80 mesh product has the best fluidity, dissolution rate and other indicators, and has the largest market demand. Currently, when manufacturers produce products with specific particle sizes, they usually screen out qualified particle size erythritol crystals through a vibrating sieve with different mesh specifications after the product is dried. The residues, that is, erythritol crystals larger and smaller than the required particle size, can only wait to see if they meet the needs of other merchants, and more likely can only be redissolved and recrystallized, which not only reduces the yield of erythritol crystals, but also increases the product cost.

[0004] Currently, when producing erythritol crystals, erythritol fermentation broth is first obtained through fermentation, and then high-purity erythritol finished products are obtained through impurity removal, concentration, and cooling crystallization. The crystallization process is the last process to obtain erythritol finished products, and at the same time is the most critical process directly affecting the purity and particle size of erythritol finished products. Currently, the crystallization equipment used by erythritol manufacturers is a crystallization tank with an external coil. During production, cold water in the external coil is used to cool the erythritol material, so that erythritol crystals can be generated or enlarged. The erythritol products obtained by this method have a large particle size distribution, usually from 12 mesh to 100 mesh.

[0005] In related technologies, by adding crystal seeds, controlling the flow field, stirring rate, cooling rate and other conditions, the particle size uniformity of erythritol crystals is improved. However, the particle size of the obtained crystals is mainly distributed between 20 - 60 mesh, and the particles are still relatively large, which does not meet the market demand for 60 - 80 mesh products. Another related technology provides a cooling crystallization method for erythritol, mainly by adding crystal seeds at the appropriate time to improve the particle size uniformity of the product. The particle size of the product obtained by this method is mainly distributed between 40 mesh and 80 mesh, and the content of products with larger particle sizes is still relatively large.

[0006] In summary, there is an urgent need to provide a method for preparing erythritol crystals with uniform particle size and a particle size distribution mainly concentrated in the range of 60 to 80 mesh. SUMMARY OF THE INVENTION

[0007] In view of this, the present invention provides a method for preparing erythritol crystals with uniform particle size. The erythritol crystals obtained by the preparation method provided by the present invention have uniform particle size, and the mass fraction of crystals with a particle size of 60 to 80 mesh in the product is more than 90%.

[0008] In order to achieve the above-mentioned invention object, the present invention provides the following technical solutions:

[0009] A method for preparing erythritol crystals with uniform particle size, comprising the following steps:

[0010] Perform pretreatment on the erythritol fermentation broth to obtain wet crystals;

[0011] Using water as a solvent, dissolve the wet crystals to obtain an erythritol solution; the temperature of the crystal dissolution is above 85°C; the mass concentration of the erythritol solution is 80 to 95%;

[0012] Place the erythritol solution in a crystallization tank, introduce a gas into the crystallization tank, and perform crystallization under the condition of gas purging to obtain erythritol crystals with uniform particle size; the mass fraction of crystals with a particle size of 60 to 80 mesh in the erythritol crystals is ≥90%;

[0013] The conditions for the crystallization include: the temperature of the crystallization tank wall is 80 to 90°C, and the gas flow rate is 80 to 200 times the volume of the erythritol solution per hour; the gas is a dry gas that does not react with erythritol.

[0014] Preferably, the temperature of the crystal dissolution is 95 to 110°C.

[0015] Preferably, the crystallization is carried out under stirring conditions, and the rotation speed of the stirring is 30 to 200 rpm.

[0016] Preferably, the crystallization tank includes a tank body and a jacket provided outside the tank body; an air inlet valve and an exhaust valve are provided on the tank body; a stirrer is provided inside the tank body; a steam inlet valve is provided on the jacket;

[0017] The gas is introduced through the air inlet valve and flows out through the exhaust valve;

[0018] The method for controlling the temperature of the crystallization tank wall during crystallization is: introducing steam into the jacket through the steam inlet valve.

[0019] Preferably, the crystallization tank is conical; the stirrer is a double-helix stirrer.

[0020] Preferably, the gas is air and / or carbon dioxide.

[0021] Preferably, the water content of the erythritol crystals is 0.2 wt% or less.

[0022] Preferably, the pretreatment includes: filtering the erythritol fermentation broth, followed by desalting and decolorization, concentration, cooling crystallization, and solid-liquid separation to obtain the wet crystals.

[0023] Preferably, the filtration includes ceramic membrane filtration and nanofiltration membrane filtration in sequence; the desalting and decolorization are carried out using ion exchange resins.

[0024] Preferably, the pressure for concentration is -0.09 to 0.095 MPa, and the temperature is 70 to 80 °C; the initial temperature for cooling crystallization is from 70 to 80 °C, the end temperature is from 20 to 30 °C, and the cooling rate is 1.5 to 15 °C / h.

[0025] The present invention provides a method for preparing erythritol crystals with uniform particle size, comprising the following steps: subjecting an erythritol fermentation broth to pretreatment to obtain wet crystals; dissolving the wet crystals with water as a solvent to obtain an erythritol solution; the temperature for crystal dissolution is above 85 °C; the mass concentration of the erythritol solution is 80 to 95%; placing the erythritol solution in a crystallization tank, introducing a gas into the crystallization tank, and carrying out crystallization under the condition of gas purging to obtain erythritol crystals with uniform particle size; the mass fraction of the crystals with a particle size of 60 to 80 mesh in the erythritol crystals is ≥90%; the crystallization conditions include: the temperature of the crystallization tank wall is 80 to 90 °C, and the gas flow rate is 80 to 200 times the volume of the erythritol solution per hour; the gas is a dry gas that does not react with erythritol. The present invention first prepares an erythritol solution with a relatively high concentration, then places it in a crystallization tank and carries out crystallization under the condition of gas purging. Because the concentration of the erythritol solution is relatively high, as the temperature decreases and the gas purging removes water, the supersaturation of the material increases, and a large number of crystallization nuclei are rapidly generated. The subsequent crystallization grows on the nuclei, and secondary nucleation is less, thus obtaining erythritol crystals with uniform particle size. And because a large number of primary nuclei are generated, the overall crystallization process has a short duration, resulting in a relatively fine overall particle size; by controlling the concentration of the erythritol solution and the crystallization conditions, the present invention can obtain erythritol crystals with a particle size distribution concentrated in the range of 60 to 80 mesh. Moreover, through continuous gas purging, the present invention can also improve the caking phenomenon of the crystals; due to a large number of material nuclei, the surface area available for crystal growth is large, the crystallization rate is fast, and the small amount of water in the erythritol solution is also conducive to accelerating the crystallization rate; in addition, the high concentration of the erythritol solution and the low water content cause the water in the initial material to evaporate under the heat released during the crystallization process and be carried away by the introduced gas, so subsequent special drying is not required, and the operation is simple.

[0026] In summary, the erythritol crystals obtained by the method provided by the present invention have uniform particle size and are not prone to caking. The particle size of the product is mainly distributed between 60 and 80 mesh, which better meets the market demand. Moreover, the operation of the present invention is simple, the crystallization rate is fast, and for the same volume of crystallization tank, the production capacity of the method of the present invention is 5 to 8 times that of the ordinary cooling crystallization method, having broad application prospects. Description of the Drawings

[0027] Figure 1 It is a microscope photograph of the erythritol crystals obtained in Example 1;

[0028] Figure 2 It is a microscope photograph of the erythritol crystals obtained in Example 2;

[0029] Figure 3 It is a microscope photograph of the erythritol crystals obtained in Example 3;

[0030] Figure 4 It is a microscope photograph of the erythritol crystals obtained in Comparative Example 1;

[0031] Figure 5 It is a microscope photograph of the erythritol crystals obtained in Comparative Example 2. Detailed Embodiments

[0032] The present invention provides a method for preparing erythritol crystals with uniform particle size, comprising the following steps:

[0033] Pretreat the erythritol fermentation broth to obtain wet crystals;

[0034] Using water as a solvent, dissolve the wet crystals to obtain an erythritol solution; the temperature of the crystal dissolution is above 85 °C; the mass concentration of the erythritol solution is 80-95%;

[0035] Place the erythritol solution in a crystallization tank, introduce a gas into the crystallization tank, and carry out crystallization under the condition of gas purging to obtain erythritol crystals with uniform particle size; the mass fraction of the crystals with a particle size of 60-80 mesh in the erythritol crystals ≥ 90%;

[0036] The conditions for the crystallization include: the temperature of the crystallization tank wall is 80-90 °C, and the gas flow rate is 80-200 times the volume of the erythritol solution per hour; the gas is a dry gas that does not react with erythritol.

[0037] The present invention pretreats an erythritol fermentation broth to obtain wet crystals. There are no special requirements for the erythritol fermentation broth of the present invention, and those well-known to those skilled in the art can be used. In the present invention, the pretreatment preferably includes: sequentially filtering, desalting and decolorizing, concentrating, cooling and crystallizing, and solid-liquid separation of the erythritol fermentation broth to obtain the wet crystals; the filtration preferably includes sequentially performing ceramic membrane filtration and nanofiltration membrane filtration; the pore size of the membrane core used for the ceramic membrane filtration is preferably 50 nm; the cut-off molecular weight of the nanofiltration membrane filtration is preferably 200 Dal, and the nanofiltration membrane is preferably a spiral wound membrane; the desalting and decolorizing is preferably carried out using an ion exchange resin; the conditions for the desalting and decolorizing preferably include: the material temperature is 30-40 °C, and the feed flow rate is 1-1.5 BV / h; during the desalting and decolorizing, the conductivity of the discharged material is preferably monitored, and when the conductivity of the discharged material rises to 200 μs / cm, it indicates that the resin has failed, and at this time, the feeding needs to be stopped and a new resin column needs to be switched; the pressure for the concentration is preferably -0.09-0.095 MPa, and the temperature is preferably 70-80 °C; the initial temperature for the cooling and crystallization is preferably 70-80 °C, and the end temperature is preferably 20-30 °C, specifically it can be 25 °C, and the cooling rate is preferably 1.5-15 °C / h; in a specific embodiment of the present invention, the cooling and crystallization is preferably gradient cooling crystallization, specifically preferably: first cooling at a rate of 7-15 °C / h to 60-65 °C, then cooling at a rate of 3.5 °C / h to 50 °C, then cooling at a rate of 3.5 °C / h to 40 °C, and finally cooling at a rate of 1.5 °C / h to room temperature; the method for the solid-liquid separation is preferably centrifugation.

[0038] After obtaining the wet crystals, the present invention uses water as a solvent to dissolve the wet crystals to obtain an erythritol solution. In the present invention, the temperature for the dissolution of crystals is preferably above 85 °C, more preferably 95-110 °C, specifically it can be 90 °C, 100 °C or 110 °C; the mass concentration of the erythritol solution is preferably 80-95%, specifically it can be 80%, 83%, 85%, 87%, 90% or 95%; in a specific embodiment of the present invention, the higher the concentration of the erythritol solution, the higher the temperature for the dissolution of crystals; the dissolution of crystals is preferably carried out in a dissolution tank, and the volume of the dissolution tank is preferably 11.5 m 3 .

[0039] After obtaining the erythritol solution, the present invention places the erythritol solution in a crystallization tank, introduces a gas into the crystallization tank, and crystallizes under the condition of gas purging to obtain erythritol crystals with uniform particle size. In the present invention, the mass fraction of the crystals with a particle size of 60-80 mesh in the erythritol crystals ≥ 90%, preferably ≥ 92%.

[0040] In the present invention, the crystallization tank preferably includes a tank body and a jacket disposed outside the tank body; a steam inlet valve is preferably provided on the jacket; an air inlet valve and an exhaust valve are provided on the tank body; a stirrer is preferably disposed inside the tank body, and the stirrer is preferably a double - helix stirrer; the crystallization tank is preferably conical, specifically a cone with a larger upper part and a smaller lower part.

[0041] In the present invention, the crystallization tank further includes a feed valve and a discharge valve; the discharge valve of the crystal dissolution tank is preferably communicated with the feed valve of the crystallization tank. In a specific embodiment of the present invention, after the crystal dissolution is completed, it is preferred to first introduce steam into the jacket of the crystallization tank to control the temperature of the crystallization tank wall at the temperature required for crystallization, and then discharge the material from the crystal dissolution tank so that the erythritol solution enters the crystallization tank.

[0042] In the present invention, the conditions for crystallization include: maintaining the temperature of the crystallization tank wall at 80 - 90 °C, specifically it can be 80 °C, 83 °C, 85 °C or 90 °C; at the same erythritol concentration, the temperature of the crystallization tank wall is lower than the temperature of crystal dissolution; the gas flow rate is 80 - 200 times / hour of the volume of the erythritol solution, preferably 100 - 150 times / hour of the volume of the erythritol solution; in a specific embodiment of the present invention, the volume of the crystallization tank is 11.5 m 3 。

[0043] In the present invention, the crystallization is preferably carried out under stirring conditions, and the rotation speed of the stirring is preferably 30 - 200 rpm, more preferably 100 - 150 rpm, specifically it can be 80 rpm, 100 rpm or 150 rpm.

[0044] In the present invention, the gas is a dry gas that does not react with erythritol, preferably air and / or carbon dioxide, more preferably air; the gas is preferably dried before inlet; in a specific embodiment of the present invention, the gas can be a normal - temperature gas or a hot gas, and the temperature of the hot gas is preferably 80 °C - 100 °C; the gas is introduced through the air inlet valve and flows out through the exhaust valve, so as to carry away the moisture in the material; in a specific embodiment of the present invention, it is preferred to introduce the gas after solids precipitate in the erythritol solution in the crystallization tank to prevent the solution in the tank from splashing due to the upward rush of the gas. In the present invention, the exhaust valve is preferably connected to a cyclone separator and a tail gas treatment device in sequence, so as to recycle the light erythritol fine powder and prevent environmental pollution.

[0045] In the present invention, the preferred way to control the crystallization temperature is as follows: steam is introduced into the jacket through the steam inlet valve. Specifically, the amount of steam introduced can be controlled by adjusting the opening degree of the steam inlet valve, so as to control the temperature of the crystallization tank wall within the required range. There are no special requirements for the steam in the present invention. Industrial steam well-known in the art can be used as long as it can achieve the purpose of heating and heat preservation and controlling the temperature of the crystallization tank wall.

[0046] The method provided by the present invention has a fast crystallization rate and high efficiency, and the crystallization time is only 2 - 3 h.

[0047] In the present invention, the water content of the erythritol crystals is 0.2 wt% or less. Preferably, after the water content of the erythritol crystals in the crystallization tank meets the requirements, the steam inlet valve is closed, and cooling water is introduced into the jacket to cool the crystals (when the gas used in crystallization is a hot gas, the hot gas needs to be replaced with a normal temperature gas after crystallization). After 15 minutes, the inlet valve is closed, and then the discharge valve is opened to collect the erythritol crystals. Specifically, they can be placed in a finished product bin for collection, and then obtained as qualified erythritol crystal products after screening and packaging.

[0048] Next, the technical solutions in the present invention will be clearly and completely described in conjunction with the embodiments in the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. Based on the embodiments in the present invention, all other embodiments obtained by those of ordinary skill in the art without making creative efforts fall within the protection scope of the present invention.

[0049] Example 1

[0050] The erythritol fermentation broth is successively subjected to ceramic membrane filtration and nanofiltration. The pore size of the membrane core used for ceramic membrane filtration is 50 nm; the membrane core used for the nanofiltration is a spiral wound membrane with a molecular weight cut-off of 200 Dal. The filtrate obtained from the nanofiltration is passed into an ion exchange resin for desalting and decolorization. The operating conditions for desalting and decolorization are as follows: the material temperature is 30 °C, the feed flow rate is 1 BV / h, and the feed is stopped when the conductivity of the discharge rises to 200 μs / cm, and a new resin column is switched until the desalting and decolorization are completed. The desalted and decolorized liquid is concentrated at -0.09 MPa and 70 °C until the solution concentration reaches 56%. The concentrated liquid is in a common vertical crystallization tank with a stirring rate of 30 rpm. First, it is cooled at a rate of 15 °C / h to 60 °C, then at a rate of 3.5 °C / h to 50 °C, then at a rate of 3.5 °C / h to 40 °C, and finally at a cooling rate of 1.5 °C / h to room temperature, and then centrifuged to obtain wet crystals.

[0051] The wet crystals are added to 11.5 m 3In the crystal dissolution tank, using water as the solvent, crystal dissolution is carried out at 100 °C to obtain a 9 m erythritol solution with a mass concentration of 87% 3 .

[0052] Steam is introduced into the jacket of the 11.5 m 3 crystallization tank through the steam inlet valve, and the temperature of the crystallization tank wall is controlled at 85 °C. Then, the erythritol solution in the crystal dissolution tank is transferred to the crystallization tank. The air inlet valve and exhaust valve are opened, and normal temperature air is introduced into the air inlet valve of the crystallization tank. The double - helix stirring paddle is started for crystallization, where the stirring speed is 100 rpm and the air flow rate is 1000 m 3 / h.

[0053] After the water content of the erythritol crystals in the crystallization tank drops below 0.2 wt%, the steam inlet valve is closed to stop heating, and cooling water is introduced into the jacket to cool the crystals; the air inlet valve is closed 15 min after stopping heating. Then, the discharge valve is opened to collect the erythritol crystals. The content, water content, and particle size distribution of the obtained product are tested, and the results are shown in Table 1.

[0054] Example 2

[0055] Other conditions are the same as in Example 1, only changing the crystal dissolution temperature to 90 °C, and the mass concentration of the obtained erythritol solution is 83%; the air flow rate during crystallization is 1500 m 3 / h, and the stirring rate is 80 rpm. The test results of the content, water content, and particle size distribution of the obtained product are shown in Table 1.

[0056] Example 3

[0057] Other conditions are the same as in Example 1, only changing the crystal dissolution temperature to 110 °C, and the mass concentration of the obtained erythritol solution is 90%; the air flow rate during crystallization is 800 m 3 / h, and the stirring rate is 150 rpm. The test results of the content, water content, and particle size distribution of the obtained product are shown in Table 1.

[0058] Comparative Example 1

[0059] Other conditions are the same as in Example 1, only changing the crystal dissolution temperature to 80 °C, and the mass concentration of the obtained erythritol solution is 58%; a vertical crystallization tank with a cooling coil is used for step - by - step cooling crystallization. The cooling crystallization process is as follows: the stirring rate is 30 rpm, first cooling from 80 °C to 63 ± 1 °C within 1 h, then cooling at a rate of 5 °C / h to 50 °C, then cooling at a rate of 3.5 °C / h to 40 °C, then cooling at a rate of 2.2 °C / h to 30 °C, and finally cooling at a rate of 1.5 °C / h to 20 °C. Crystallization stops, and the product is discharged to a centrifuge for centrifugation, and the wet crystals are dried using a fluidized bed. The test results of the content, water content, and particle size distribution of the obtained product are shown in Table 1.

[0060] Comparative Example 2

[0061] Other conditions were the same as those in Example 1, except that the crystal melting temperature was changed to 90 °C, and the mass concentration of the obtained erythritol solution was 75%; the air flow rate during crystallization was 2000 m 3 / h, and the stirring rate was 150 rpm. The test results of the content, moisture, and particle size distribution of the obtained product are shown in Table 1.

[0062] Table 1 Detection results of the heating crystallization conditions and product indexes of Examples 1 to 3 and Comparative Examples 1 to 2

[0063]

[0064] Figures 1 to 3 are the microscope photos of the erythritol crystals obtained in Examples 1 to 3, Figures 4 to 5 are the microscope photos of the erythritol crystals obtained in Comparative Examples 1 to 2.

[0065] According to Table 1 and Figures 1 to 5 the test results in it, it can be seen that the erythritol crystals prepared by the present invention have uniform particle size, and the particle size is mainly distributed in the range of 60 to 80 mesh, which better meets the market demand. In Comparative Example 1, the conventional stepwise cooling method was used to prepare erythritol crystals, and the obtained product had uneven particle size and more large-particle crystals; in Comparative Example 2, the concentration of the erythritol solution was low. Although the air intake was increased to remove more moisture in the material, under this concentration condition, the crystallization particle size was not uniform, and there were many secondary nucleation situations, resulting in a wide particle size distribution, proving that the erythritol concentration has a great influence on the crystal particle size.

[0066] In addition, the total arsenic, lead, heavy metals, molds and yeasts, total number of colonies, Escherichia coli, and pathogenic bacteria (Salmonella, Shigella, and Staphylococcus aureus) of the erythritol crystals obtained in Examples 1 to 3 were detected. Among them, total arsenic and lead were not detected, heavy metals (calculated as Pb) were less than 5 mg / kg, molds and yeasts were less than 10 CFU / g, the total number of colonies was less than 10 CFU / g, Escherichia coli was less than 3 MPN / g, and pathogenic bacteria were not detected.

[0067] The above are only the preferred embodiments of the present invention. It should be noted that for those of ordinary skill in the art, without departing from the principle of the present invention, several improvements and refinements can still be made, and these improvements and refinements should also be regarded as the protection scope of the present invention.

Claims

1. A method for preparing erythritol crystals with uniform particle size, characterized in that, The preparation steps are as follows: Perform pretreatment on the erythritol fermentation broth to obtain wet crystals; Use water as a solvent to dissolve the wet crystals to obtain an erythritol solution; the temperature for crystal dissolution is 95 - 110 °C; the mass concentration of the erythritol solution is 80 - 95%; Place the erythritol solution in a crystallization tank, introduce gas into the crystallization tank, and perform crystallization under the condition of gas purging to obtain erythritol crystals with uniform particle size; the mass fraction of crystals with a particle size of 60 - 80 mesh in the erythritol crystals is ≥90%; The conditions for the crystallization include: the temperature of the crystallization tank wall is 80 - 90 °C, the gas flow rate is 80 - 200 times the volume of the erythritol solution per hour; the gas is a dry gas that does not react with erythritol; the crystallization is carried out under stirring conditions, and the rotation speed of the stirring is 30 - 200 rpm; The crystallization tank includes a tank body and a jacket arranged outside the tank body; an air inlet valve and an exhaust valve are arranged on the tank body; a stirrer is arranged inside the tank body; a steam inlet valve is arranged on the jacket; The gas is introduced through the air inlet valve and flows out through the exhaust valve; The method for controlling the temperature of the crystallization tank wall during crystallization is: introduce steam into the jacket through the steam inlet valve.

2. The preparation method according to claim 1, characterized in that, The crystallization tank is conical; the stirrer is a double - helix stirrer.

3. The preparation method according to claim 1, characterized in that, The gas is air and / or carbon dioxide.

4. The preparation method according to claim 1, characterized in that, The water content of the erythritol crystals is below 0.2 wt%.

5. The preparation method according to claim 1, characterized in that, The pretreatment includes: sequentially filtering, desalting and decolorizing, concentrating, cooling crystallization, and solid - liquid separation of the erythritol fermentation broth to obtain the wet crystals.

6. The preparation method according to claim 5, characterized in that, The filtration includes sequentially performing ceramic membrane filtration and nanofiltration membrane filtration; the desalting and decolorizing are carried out using ion - exchange resin.

7. The preparation method according to claim 5, characterized in that, The pressure for concentration is - 0.09 - 0.095 MPa, and the temperature is 70 - 80 °C; the initial temperature for cooling crystallization is 70 - 80 °C, the end - point temperature is 20 - 30 °C, and the cooling rate is 1.5 - 15 °C / h.

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