Process for producing crystals of bis-2-hydroxyethyl terephthalate

Abstract

To provide a method for producing bis-2-hydroxyethyl terephthalate (BHET), by which the crystal of the BHET having a high purity can stably be obtained from an aqueous solution of the BHET while reducing the capacity of an apparatus, a construction cost, an installation area, etc., compared with those of a conventional method and reducing the frequency of the dissolving operation of scaling.SOLUTION: Using a crystallization tank 10 for performing a crystallization operation of an aqueous solution containing 20 to 30wt% of BHET and a vacuum generation device 32 for reducing a pressure inside the crystallization tank 10, A step of adiabatically cooling the aqueous solution by adiabatically expanding the water vapor and lowering the temperature of the water vapor to obtain a slurry 13 containing BHET crystals in a crystallization vessel 10, and a step of supplying the slurry 13 extracted from the crystallization vessel 10 to a solid-liquid separator 20 to obtain BHET crystals and a mother liquid, wherein the slurry 13 is continuously obtained in the crystallization vessel 10 under conditions of 3.7 to 8. 6kPaA and 30 to 45 °C.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a method for producing crystals of bis-2-hydroxyethyl terephthalate (hereinafter referred to as "BHET"). [Background technology]

[0002] Generally, methods for obtaining crystals from aqueous solutions include cooling crystallization, which utilizes the difference in the solubility of a solute in water due to the temperature difference between the start and end of crystallization. Among these methods, crystallization tanks equipped with cooling jackets are widely used industrially in indirect cooling crystallization. However, when attempting to apply an indirect cooling crystallization tank equipped with a cooling jacket to a large-scale facility, it is necessary to increase the diameter or height, or both, of the crystallization tank in order to ensure the heat transfer area required for cooling. As a result, the slurry capacity inside the crystallization tank becomes excessive compared to the heat transfer area, resulting in excessive equipment costs, construction costs, and installation area.

[0003] Furthermore, in the case of indirect cooling crystallization, in a configuration equipped with a circulation system having a cooler for cooling the process liquid outside the crystallization tank, it is necessary to keep the temperature difference between the refrigerant and the process liquid low in order to suppress scaling in the cooler. When attempting to apply this configuration to large-scale facilities, the cooler and the circulation pump for circulating the process liquid would be too large to obtain the amount of heat exchange required for cooling, resulting in excessive equipment costs, construction costs, and installation space.

[0004] In the case of indirect cooling crystallization, whether using a cooling jacket or a circulation system with a cooler, crystal scaling and crystal deposition on the heat transfer surface over time are unavoidable. Removing crystal scaling and crystal deposition requires dissolution operations while production is stopped, making it difficult to avoid production stoppages other than for scheduled maintenance.

[0005] In addition, both batch and continuous operations are widely used in cooling crystallization. For example, Patent Document 1 describes a method for crystallizing BHET crystals by cooling an aqueous BHET solution at a predetermined cooling rate using a batch operation that utilizes the difference in the solubility of BHET in water due to the difference in operating temperature between the start and end of crystallization.

[0006] Generally, compared to continuous operations, batch operations in industrial processes have the following disadvantages: (1) the need to start and stop each unit operation increases the labor burden, (2) there is less flexibility in responding to increases and decreases in production volume, and (3) the equipment tends to become larger. Crystallization processes are no exception to this rule. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Japanese Patent Application Laid-Open No. 2008-88096 Summary of the Invention [Problem to be solved by the invention]

[0008] The present invention has been made under these circumstances, and aims to provide a method for producing BHET crystals from an aqueous solution of BHET, which can reduce the equipment capacity, construction costs, installation area, etc. compared to conventional methods, reduce the frequency of scaling dissolution operations, and enable stable production of high-purity BHET crystals. [Means for solving the problem]

[0009] In order to solve the above problems and achieve the above object, the present invention proposes the following aspects.

[0010] A first aspect of the present invention is a method for producing BHET crystals, comprising the steps of: a crystallization tank for performing a crystallization operation on an aqueous solution containing 20 to 30 wt % BHET; and a vacuum generator for reducing the pressure inside the crystallization tank to adiabatically cool the aqueous solution by evaporating water, which serves as a solvent for the aqueous solution, thereby obtaining a slurry containing BHET crystals in the crystallization tank; and supplying the slurry removed from the crystallization tank to a solid-liquid separator to obtain BHET crystals and a mother liquor.

[0011] According to the first aspect, a BHET solution to be crystallized in a crystallization tank is adiabatically cooled by evaporating the water solvent under reduced pressure using a vacuum generator, resulting in the precipitation of BHET crystals. The resulting slurry is then extracted from the crystallization tank and separated into crystals and a mother liquor by solid-liquid separation, yielding high-quality purified BHET.

[0012] A second aspect of the present invention is the method for producing crystals of BHET according to the first aspect, characterized in that the aqueous solution having BHET dissolved therein at a temperature of 60°C or higher is continuously supplied to the crystallization tank, and the aqueous solution is adiabatically cooled under conditions of 3.7 to 8.6 kPaA and 30 to 45°C inside the crystallization tank, thereby continuously obtaining the slurry in the crystallization tank.

[0013] According to the second aspect, it is possible to crystallize BHET into fine needle-like crystals. Continuous production of BHET crystals is expected to reduce labor, improve productivity, improve product quality, and reduce the required facility space.

[0014] A third aspect of the present invention is the method for producing crystals of BHET according to the first or second aspect, characterized in that the solid-liquid separator is a centrifuge or a filter.

[0015] According to the third aspect, BHET crystals can be easily obtained from a BHET slurry.

[0016] A fourth aspect of the present invention is the method for producing BHET crystals according to any one of the first to third aspects, characterized in that a cake layer of BHET crystals formed in the solid-liquid separator by solid-liquid separation is washed with water to increase the purity of BHET.

[0017] According to the fourth aspect, the purity of the BHET crystals can be easily increased.

[0018] A fifth aspect of the present invention is an apparatus for producing BHET crystals, comprising: a crystallization tank for performing a crystallization operation on an aqueous solution containing 20 to 30 wt % BHET; a vacuum generator for reducing the pressure inside the crystallization tank; and a crystallization apparatus for obtaining a slurry containing BHET crystals in the crystallization tank by adiabatically cooling the aqueous solution through evaporation of water, which is a solvent for the aqueous solution; and a solid-liquid separator for obtaining BHET crystals and a mother liquor from the slurry withdrawn from the crystallization tank.

[0019] According to the fifth aspect, a BHET solution to be crystallized in a crystallization tank is adiabatically cooled by evaporating the water solvent under reduced pressure using a vacuum generator, resulting in the precipitation of BHET crystals. The resulting slurry is then extracted from the crystallization tank and separated into crystals and a mother liquor by solid-liquid separation, yielding high-quality purified BHET. [Effects of the Invention]

[0020] According to the present invention, it is possible to stably obtain high-purity BHET crystals from an aqueous solution of BHET by reducing the equipment capacity, construction costs, and installation area compared to conventional methods, and by reducing the frequency of scaling dissolution operations. More specifically, the aqueous solution of BHET can be cooled without increasing the diameter and / or height of the crystallization tank to ensure a sufficient heat transfer area, and the amount of energy required for cooling is reduced. Furthermore, since it is possible to suppress the formation of crystal scaling and crystal deposition on the heat transfer surface over time, production stoppages other than those for scheduled maintenance due to scaling dissolution operations can be minimized. [Brief explanation of the drawings]

[0021] [Figure 1] FIG. 1 is a schematic diagram illustrating the configuration of an apparatus used in an embodiment. [Figure 2] 1 is a graph showing the change in the amount of BHET dissolved with temperature. [Figure 3] FIG. 1 is a schematic diagram showing the configuration of an apparatus used in Example 1. [Figure 4] FIG. 1 is a schematic diagram showing the configuration of an apparatus used in Comparative Example 1. DETAILED DESCRIPTION OF THE INVENTION

[0022] Hereinafter, the present invention will be described based on preferred embodiments with reference to the drawings.

[0023] 1, the apparatus used in the embodiment generally comprises a crystallization tank 10, a solid-liquid separator 20, and a vacuum generator 32. Of these, a crystallization apparatus 100 used for adiabatic cooling crystallization operation comprises the crystallization tank 10 and the vacuum generator 32.

[0024] <Crystallization process> An aqueous solution containing BHET is supplied as a raw solution 1 to a crystallization tank 10 through a pipe 41. A jacket 12 through which a heating medium 2 circulates is provided around the crystallization tank 10. By circulating the heating medium 2 through the jacket 12, the temperature of the aqueous solution in the crystallization tank 10 can be maintained at a predetermined value.

[0025] In the initial stage of the crystallization process, when the crystallization tank 10 is filled with an aqueous solution containing BHET, the pressure inside the crystallization tank 10 is reduced using a vacuum generator 32. The water contained in the solution as a solvent evaporates, and the water vapor expands adiabatically, lowering the temperature. This adiabatic cooling operation cools the solution in the crystallization tank 10. By performing adiabatic cooling crystallization by adiabatic cooling of the solution in the crystallization tank 10, the heat of the supplied raw solution 1 and the heat of crystallization of BHET are removed as the water evaporates, resulting in the deposition of BHET crystals in the crystallization tank 10, producing a slurry 13 containing a mixture of the aqueous solution and BHET crystals. Only a small amount of external heat input is required. Therefore, hot water, steam, or the like can be used as the heating medium 2. After the slurry 13 is retained in the crystallization tank 10, a new aqueous solution is supplied to continuously carry out adiabatic crystallization, and a portion of the slurry 13 is withdrawn from the crystallization tank 10.

[0026] During the crystallization operation, crystals are entrained in droplets during the evaporation of water, and may adhere as scaling near the liquid surface of the slurry 13, for example, to the sidewall of the crystallization tank 10 or, if an agitator 14 is installed, to the drive shaft of the agitator 14. In the case of indirect cooling, if scaling occurs on the cooled heat transfer surface, it becomes difficult to perform the dissolving operation during the crystallization operation. In the case of adiabatic cooling crystallization, even if the dissolving operation is performed near the liquid surface of the slurry 13, it is difficult to prevent water evaporation due to reduced pressure, so it is possible to carry out crystal production operation by crystallization while performing the scaling dissolution process.

[0027] To dissolve scaling, for example, dissolving equipment such as a shower ring or spray nozzle is installed in the space 15 above the surface of the slurry 13 inside the crystallization tank 10, and a scaling dissolving solution 4 such as water is supplied to the location where scaling has occurred, such as the side wall surface of the crystallization tank 10, the surface of the slurry 13, or the drive shaft of the agitator 14.

[0028] Furthermore, in the case of adiabatic cooling crystallization, the slurry 13 can be cooled by the latent heat of evaporation of water, so when the apparatus of the embodiment is applied to a large-scale facility, there is no need to increase the diameter and / or height of the crystallization tank 10 in order to ensure the required heat transfer area.In addition, the capacity of the slurry 13 held inside the crystallization tank 10 does not become excessively large.

[0029] This reduces equipment and construction costs. It also makes it possible to compact the area required for equipment installation. Furthermore, there is no need to stop production to perform dissolution operations except for scheduled maintenance, which is caused by crystal scaling and crystal accumulation over time, as is the case with indirect cooling systems. Continuous crystal generation saves labor. Continuous automatic process monitoring is also easy. This is expected to improve productivity, product quality and safety, and reduce facility space.

[0030] The temperature of the raw solution 1 supplied to the crystallization tank 10 and the temperature of the aqueous solution or slurry 13 contained in the crystallization tank 10 are preferably set according to the amount of BHET dissolved. 3 The amount of BHET dissolved in the solution is as follows:

[0031] 5℃:0.28g 40℃: 1.34g 60℃: 11.26g 80℃: 53.42g

[0032] A plot of the amount of BHET dissolved (g / 100 cc-water) is shown in Figure 2. If the temperature of the aqueous solution is below or near 40°C, the amount of BHET dissolved in water is as low as about 1 wt%, and a high recovery rate of BHET is expected. Furthermore, when the crystallization operation is performed at an internal temperature of the crystallization tank 10 of 40°C or below, it is preferable to use the starting material stock solution 1 as an aqueous solution containing 20 to 30 wt% BHET so that the concentration of BHET in the slurry 13 obtained in the crystallization tank 10 is 20 to 30 wt%.

[0033] The aqueous solution supplied to the crystallization tank 10 as the stock solution 1 is preferably heated to a temperature of 65°C or higher, preferably 65 to 70°C, to dissolve all of the BHET. The step of supplying the stock solution 1 to the crystallization tank 10 may be continuous or batchwise. The aqueous solution is adiabatically cooled in the crystallization tank 10, and the temperature of the cooled aqueous solution is controlled to around 40°C, preferably 30 to 45°C, and more preferably 35 to 40°C, as described above.

[0034] Crystals of BHET are precipitated in the crystallization tank 10, forming a slurry 13 in which the crystals are mixed with the aqueous solution. In this case, the temperature of the slurry 13 can be considered to be the same as the temperature of the aqueous solution contained in the slurry 13. The temperature of the slurry 13 may temporarily rise as the raw solution 1 is supplied to the crystallization tank 10. It is preferable to withdraw the slurry 13 from the crystallization tank 10 while the interior of the crystallization tank 10 is being operated at the above-mentioned temperature by adiabatic cooling of the slurry 13. It is also preferable to control the temperature of the slurry 13 so that it is continuously maintained within the above-mentioned temperature range.

[0035] The pressure (absolute pressure: kPaA) inside the crystallization tank 10 during the adiabatic cooling operation is preferably within an appropriate range relative to the saturated water vapor pressure. For example, if the temperature of the slurry 13 inside the crystallization tank 10 is 30°C, the pressure is preferably about 3.7 kPaA; if it is 35°C, the pressure is preferably about 5.0 kPaA; if it is 40°C, the pressure is preferably about 6.6 kPaA; and if it is 45°C, the pressure is preferably about 8.6 kPaA. For example, the pressure inside the crystallization tank 10 may be about 80 to 95% of the saturated water vapor pressure.

[0036] The pressure inside the crystallization tank 10 during the crystallization step may be controlled by controlling the pressure of the gas phase in the space 15 above the liquid surface of the slurry 13 inside the crystallization tank 10, or by controlling the pressure generated by the vacuum generator 32 outside the crystallization tank 10. When measuring the pressure outside the crystallization tank 10, the pressure may be measured in the pipe 46 between the crystallization tank 10 and the vapor condenser 30, or in the pipe 47 between the gas-liquid separation tank 31 and the vacuum generator 32.

[0037] It is preferable to continuously supply a crystallization tank 10 with an aqueous solution of BHET dissolved at a temperature of 60°C or higher as the stock solution 1, and adiabatically cool the slurry 13 under conditions of 3.7 to 8.6 kPaA and 30 to 45°C inside the crystallization tank 10, thereby continuously obtaining a BHET slurry 13 in the crystallization tank 10. It is also preferable to continuously obtain a BHET slurry 13 by performing adiabatic cooling crystallization under conditions of 5.0 to 6.6 kPaA, 35 to 40°C, and 30 to 45°C.

[0038] A slurry 13 containing an aqueous solution of BHET and crystals is obtained in the crystallization tank 10. The step of withdrawing the slurry 13 from the crystallization tank 10 may be continuous or batchwise. As will be described in detail later, at least a portion of the mother liquor obtained by solid-liquid separation of the crystals and the mother liquor from the slurry 13 may be returned to the crystallization tank 10. The step of returning the mother liquor 44 to the crystallization tank 10 may be continuous or batchwise.

[0039] The supply of stock solution 1 to crystallization tank 10, the withdrawal of slurry 13, and the return of mother liquor 44 may each be carried out continuously. By performing continuous crystallization, BHET can be continuously crystallized while maintaining the temperature and BHET concentration of slurry 13 in crystallization tank 10 within predetermined ranges. The ratio of stock solution 1 to mother liquor 44 may be adjusted as appropriate. Although not shown, the slurry 13 in crystallization tank 10 may be stirred by installing an agitator such as an agitator blade inside crystallization tank 10 or by installing a circulation means for withdrawing the mother liquor to the outside and then resupplying it.

[0040] <Solid-liquid separation process> The produced slurry 13 is extracted from the crystallization tank 10 via a pipeline 42 by a pump 11 and transferred to a solid-liquid separator 20 via a pipeline 43. The slurry 13 is supplied to the solid-liquid separator 20 and subjected to a solid-liquid separation operation, whereby the slurry is separated into BHET crystals and a mother liquor. This allows high-quality purified BHET to be obtained.

[0041] The solid-liquid separator 20 is not particularly limited, but is preferably a centrifuge or a filter. This allows BHET crystals to be easily obtained from the BHET slurry. Specific examples of the solid-liquid separator 20 include a bottom-discharge batch centrifuge having a filter material such as a filter cloth or a metal screen, a continuous basket centrifuge having a metal screen, and a horizontal centrifuge. Specific examples of the filter include a vacuum filter, a pressure filter, a suction filter, a centrifugal filter, or a sedimentation separator, in which the slurry is supplied onto a continuously running filter cloth and continuously vacuum-filtered by suction from a vacuum tray that reciprocates along the direction of the filter cloth.

[0042] After washing the crystals in the solid-liquid separator 20, it is preferable to separate them into a crystal stream Cr and a mother liquor 44. While the method for washing the crystals is not particularly limited, it is preferable to wash the cake layer consisting of BHET crystals formed in the solid-liquid separator 20 with water. This easily increases the purity of the BHET crystals. For efficient washing of the cake layer, it is preferable to use a solid-liquid separator with a mechanism that can fully automatically wash the cake layer with water. The wash water used for washing the crystals may be combined with the mother liquor or may be separated from the mother liquor.

[0043] The BHET crystals obtained by the solid-liquid separator 20 are transferred to a subsequent process as a crystal stream Cr. The crystal stream Cr can be dried to remove adhering water, thereby obtaining a higher quality BHET. The drying method is not particularly limited, but examples include vacuum drying and heat drying.

[0044] The mother liquor 44 discharged from the solid-liquid separator 20 is temporarily stored in the mother liquor receiving tank 21, and then extracted by the mother liquor pump 22 and returned to the crystallization tank 10 through a pipeline 45. In this way, the mother liquor contained in the slurry 13 discharged from the crystallization tank 10 is returned to the crystallization tank 10 after undergoing solid-liquid separation and is circulated. If the accumulation or concentration of impurities in the mother liquor affects the properties or quality of the crystals, a portion of the mother liquor 44 may be purged and discharged to the outside of the system as a purge stream Pu.

[0045] <Condensation process> The evaporated vapor generated by the adiabatic cooling operation in the crystallization tank 10 passes through a pipe 46, is condensed in the vapor condenser 30, and is introduced into the gas-liquid separation tank 31. The vapor condenser 30 is disposed between the crystallization tank 10 and the vacuum generator 32. The gas-liquid separation tank 31 is disposed between the vapor condenser 30 and the vacuum generator 32.

[0046] There are no limitations on the equipment or process of the vapor condenser 30, but examples include a shell-and-tube heat exchanger, a spiral heat exchanger, and a plate heat exchanger. The evaporated vapor is introduced into the vapor condenser 30 through which the refrigerant 3 flows, to condense the water. As the water in the evaporated vapor condenses, a liquid phase containing condensed water and a gas phase containing gas such as air are mixed together.

[0047] In the gas-liquid separation tank 31, the gas phase passes through a pipe 47 and is sucked into a vacuum generator 32. A suitable vacuum generator 32 is one that minimizes the impact on operation even if uncondensed water is present on the intake side. The liquid phase is extracted by a pump 33 through a pipe 48 and is discarded or reused as a condensed water stream Cd. The condensed water is composed of water evaporated in the crystallization tank 10, and can be reused as industrial water such as cleaning water. The condensed water may be reused within the system or outside the system. The resulting condensed water can be supplied as a cleaning liquid for the solid-liquid separator 20 or as a scaling solution 4 for the crystallization tank 10.

[0048] The present invention has been described above based on preferred embodiments, but the present invention is not limited to the above-described embodiments, and various modifications are possible within the scope of the gist of the present invention.

[0049] The BHET used as a raw material is not particularly limited, but can be obtained by an esterification reaction between terephthalic acid and ethylene glycol, a transesterification reaction between a terephthalic acid ester such as dimethyl terephthalate and ethylene glycol, an addition reaction between terephthalic acid and ethylene oxide, or a depolymerization reaction of polyethylene terephthalate (PET) with ethylene glycol. [Example]

[0050] The present invention will now be described with reference to specific examples.

[0051] Example 1 3 shows the configuration of the apparatus used in Example 1. The crystallization tank 101 and the vapor condenser 102 are connected via a pipe 121. The vapor condenser 102 and the vacuum pump Vc are connected via a pipe 122. A gas-liquid separation tank 103 is disposed between the vapor condenser 102 and the vacuum pump Vc. The temperatures of the constant temperature baths 104 and 105 are controlled by a temperature indicating controller (TIC).

[0052] First, 1.5 kg of an aqueous solution (raw material) containing 28 wt% BHET was heated to 70°C. At 70°C, it was visually confirmed that the solids in the raw material had completely dissolved in the water. Next, the entire amount of raw material was charged into crystallization tank 101. After the raw material supply solution was charged into crystallization tank 101, the agitator was started using a motor (M) and the agitation state was maintained. Warm water was passed from thermostatic bath 104 through the jacket of crystallization tank 101 and circulated through pipes 131 and 132.

[0053] Next, chiller water was passed through vapor condenser 102 from thermostatic bath 105 and circulated through pipes 133 and 134. Vacuum pump Vc was started to adjust the pressure and temperature. Next, adiabatic cooling of crystallization tank 101 was performed while adjusting the degree of vacuum, to precipitate BHET crystals. The cooling operation was terminated when thermometer 111, which indicates the raw material temperature, read 38.6°C, thermometer 112, which indicates the temperature of the gas phase above the raw material, read 36.6°C, and pressure gauge 113, which indicates the degree of vacuum of vacuum pump Vc, read 6.6 kPaA.

[0054] Next, the slurry was removed from the crystallization tank 101 and deliquored for 5 minutes using a top-discharge centrifuge (centrifugal effect 750 G) while maintaining the operating temperature at 38.6°C at the end of crystallization. Next, a crystal washing operation was carried out by spraying room temperature water with an atomizer onto the cake remaining on the filter cloth of the centrifuge. In the crystal washing operation, water was used in a ratio of 10 parts by weight per 100 parts by weight of solids equivalent to the dried product of the cake layer. The cake layer after the crystal washing operation was deliquored for 5 minutes using a top-discharge centrifuge (centrifugal effect 750 G).

[0055] (Comparative Example 1) FIG. 4 shows the configuration of the apparatus used in Comparative Example 1. First, about 1.9 kg of a BHET aqueous solution (raw material) having the same composition as in Example 1 was heated to 70°C. At 70°C, it was visually confirmed that the solids in the raw material had completely dissolved in the water. Next, the entire amount of raw material was charged into crystallization tank 201. After the raw material was charged, the agitator was started and the agitation state was maintained. Next, warm water was passed through the jacket of crystallization tank 201 from thermostatic tank 202 and circulated through pipes 231 and 232.

[0056] Next, an indirect cooling operation was performed by adjusting the set temperature of the thermostatic bath 202. The temperature inside the crystallization tank 201 was monitored using a thermometer 211 that indicates the raw material temperature. The temperature was rapidly cooled from 70°C to 65°C, and then gradually cooled from 65°C to 35°C over 3 hours. Once the temperature reached 35°C, the operation state was maintained for 30 minutes. After the cooling operation was completed, the slurry was removed from the crystallization tank 201, and thereafter, a solid-liquid separation operation and a crystal washing operation were performed under the same conditions as in Example 1.

[0057] (Analysis results) Table 1 shows the analysis results of the washed crystals of Example 1 and Comparative Example 1. The analysis items were the crystal moisture content measured with a Karl Fischer moisture meter and color (L value, a value, b value) measured by whiteness measurement.

[0058] [Table 1]

[0059] As can be seen from Table 1, Example 1 (adiabatic cooling type) has a lower moisture content than Comparative Example 1 (indirect cooling type), which is expected to reduce the load on the subsequent drying process. In addition, the washed crystals of BHET obtained in Example 1 exhibited a color that was equal to or higher than that of Comparative Example 1.

[0060] Tables 2 and 3 show the material balances based on the test results of Example 1 and Comparative Example 1, respectively. The total amount of the supplied liquid is converted to 100.

[0061] [Table 2]

[0062] [Table 3]

[0063] Comparing the material balances of Example 1 and Comparative Example 1, it can be seen that in Example 1 (adiabatic cooling type), a portion of the water in the feed liquid is evaporated as vapor, resulting in a corresponding decrease in the amount of mother liquor in the slurry. Because the condensate contains the water evaporated in the crystallization tank, it can be reused as industrial water for cleaning and other purposes, leading to a reduction in the plant's consumption rate. Furthermore, the small amount of mother liquor reduces the load on the regeneration process and wastewater treatment facilities when recycling the mother liquor.

[0064] When the crystallization tank is sized based on the results of Example 1 and Comparative Example 1, the slurry volume in the crystallization tank in Table 2 (Example 1) can be reduced to one-fourth or less of that of the crystallization tank in Table 3 (Comparative Example 1). The advantages of a smaller crystallization tank capacity include a reduction in the time required for miscellaneous operations such as filling with liquid at the start of operation and withdrawing liquid at the end of operation, a reduction in the capacity of the motor installed in the crystallization tank, and reduced equipment and construction costs due to the reduced size of the crystallization tank. [Explanation of symbols]

[0065] 100 Crystallizer 10,101,201 Crystallization tank 13 Slurry 20 Solid-liquid separator 30,102 Vapor condenser 31,103 Gas-liquid separation tank 32 Vacuum generator

Claims

1. a step of evaporating water, which is a solvent, from the aqueous solution using a crystallization tank for performing a crystallization operation on an aqueous solution containing 20 to 30 wt % of bis-2-hydroxyethyl terephthalate and a vacuum generator for reducing the pressure inside the crystallization tank, and adiabatically expanding the water vapor to lower the temperature of the water vapor, thereby adiabatically cooling the aqueous solution, thereby obtaining a slurry containing crystals of bis-2-hydroxyethyl terephthalate in the crystallization tank; supplying the slurry extracted from the crystallization tank to a solid-liquid separator to obtain crystals of bis-2-hydroxyethyl terephthalate and a mother liquor; and A method for producing crystals of bis-2-hydroxyethyl terephthalate, comprising continuously supplying the aqueous solution, in which bis-2-hydroxyethyl terephthalate is dissolved at a temperature of 60°C or higher, to the crystallization tank, and adiabatically cooling the aqueous solution under conditions of 3.7 to 8.6 kPaA and 30 to 45°C inside the crystallization tank, thereby continuously obtaining the slurry in the crystallization tank.

2. The method for producing crystals of bis-2-hydroxyethyl terephthalate according to claim 1, wherein the aqueous solution in which bis-2-hydroxyethyl terephthalate is dissolved at a temperature of 60°C or higher is continuously supplied to the crystallization tank, and the aqueous solution is adiabatically cooled under conditions of 5.0 to 6.6 kPaA and 35 to 40°C inside the crystallization tank, thereby continuously obtaining the slurry in the crystallization tank.

3. 3. The method for producing crystals of bis-2-hydroxyethyl terephthalate according to claim 1, wherein the solid-liquid separator is a centrifuge or a filter.

4. The method for producing crystals of bis-2-hydroxyethyl terephthalate according to any one of claims 1 to 3, characterized in that the purity of bis-2-hydroxyethyl terephthalate is increased by washing with water a cake layer of bis-2-hydroxyethyl terephthalate crystals formed in the solid-liquid separator by solid-liquid separation.

5. The method for producing crystals of bis-2-hydroxyethyl terephthalate according to any one of claims 1 to 4, characterized in that a vapor condenser is disposed between the crystallization tank and the vacuum generator, and water vapor in the crystallization tank is introduced into the vapor condenser to be condensed.

6. a crystallization apparatus comprising: a crystallization tank for performing a crystallization operation on an aqueous solution containing 20 to 30 wt % of bis-2-hydroxyethyl terephthalate; and a vacuum generator for reducing the pressure inside the crystallization tank, wherein water as a solvent is evaporated from the aqueous solution, and the water vapor is adiabatically expanded to lower the temperature of the water vapor, thereby adiabatically cooling the aqueous solution, thereby obtaining a slurry containing crystals of bis-2-hydroxyethyl terephthalate in the crystallization tank; a solid-liquid separator for separating bis-2-hydroxyethyl terephthalate crystals and a mother liquor from the slurry discharged from the crystallization tank; and An apparatus for producing crystals of bis-2-hydroxyethyl terephthalate, comprising: continuously supplying the aqueous solution, in which bis-2-hydroxyethyl terephthalate is dissolved, at a temperature of 60°C or higher to the crystallization tank; and adiabatically cooling the aqueous solution under conditions of 3.7 to 8.6 kPaA and 30 to 45°C inside the crystallization tank, thereby continuously obtaining the slurry in the crystallization tank.

7. The apparatus for producing crystals of bis-2-hydroxyethyl terephthalate according to claim 6, wherein the aqueous solution in which bis-2-hydroxyethyl terephthalate is dissolved at a temperature of 60°C or higher is continuously supplied to the crystallization tank, and the aqueous solution is adiabatically cooled under conditions of 5.0 to 6.6 kPaA and 35 to 40°C inside the crystallization tank, thereby continuously obtaining the slurry in the crystallization tank.

8. The apparatus for producing crystals of bis-2-hydroxyethyl terephthalate according to claim 6 or 7, further comprising a vapor condenser between the crystallization tank and the vacuum generator, and the water vapor in the crystallization tank is introduced into the vapor condenser to be condensed.

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