A method for separating mixed dichlorobenzene

Abstract

The present invention provides a method for separating mixed dichlorobenzenes. The method comprises: (a) contacting the mixed dichlorobenzenes with extractant A in an extractive distillation column for extractive distillation separation to obtain crude p-dichlorobenzene from the top of the extractive distillation column; (b) feeding the bottom material of the extractive distillation column and solvent B into a crystallization and precipitation unit to obtain crystals mainly composed of m-dichlorobenzene and o-dichlorobenzene, as well as a solvent phase of extractant A and solvent B; (c) heating the crystals mainly composed of m-dichlorobenzene and o-dichlorobenzene and feeding them into a rectification column for rectification separation, separating out the crude product containing solvent B from the top of the rectification column, and taking out the crude m-dichlorobenzene from the side line and the crude o-dichlorobenzene from the bottom of the column. The method of the present invention includes using extractive distillation technology, crystallization technology and rectification technology to separate the mixed dichlorobenzenes containing o-dichlorobenzene, m-dichlorobenzene and p-dichlorobenzene, and finally obtaining high-purity o-, m- and p-dichlorobenzene products.

Description

Technical Field

[0001] The present invention relates to a method for separating mixed dichlorobenzenes. Background Art

[0002] There are three isomers of dichlorobenzene, namely ortho-dichlorobenzene, meta-dichlorobenzene and para-dichlorobenzene. They are all important fine chemical intermediates and are widely used in the synthesis of fine chemical products in fields such as medicine, pesticides, dyes, and pigments. For example, para-dichlorobenzene is an excellent bactericide, disinfectant, and deodorant, and can be used to prepare lubricants, corrosion inhibitors, etc., and can also be used to produce polysulfide resins, norfloxacin, and dye intermediates, etc.; meta-dichlorobenzene can be used to synthesize broad-spectrum antibacterial drugs such as econazole, ketoconazole, and miconazole, and can also synthesize analgesics such as diclofenac and flurbiprofen; ortho-dichlorobenzene can be synthesized into 3,4-dichloroaniline, which is the main raw material for several herbicides.

[0003] However, in the traditional process of preparing dichlorobenzene by benzene chlorination, the three isomers of dichlorobenzene will inevitably exist. Therefore, the separation and purification of mixed dichlorobenzenes is particularly important. The boiling points of ortho-dichlorobenzene, meta-dichlorobenzene and para-dichlorobenzene are 180.4 °C, 172 °C and 174.1 °C respectively, and the melting points are -17.5 °C, -24 °C and 52 °C respectively. The boiling point differences and physical properties of the three isomers are very close, and their relative volatility is only 1.059, indicating that it is very difficult to cut them cleanly by using a pure distillation process. And simply using the crystallization method cannot break the eutectic point, resulting in waste of some products. There are also direct adsorption method and indirect adsorption method for separating mixed dichlorobenzenes, which can only separate para-dichlorobenzene and meta-dichlorobenzene, so it is necessary to remove ortho-dichlorobenzene before using this method.

[0004] In order to improve the quality of monomers in mixed dichlorobenzenes, researchers at home and abroad have conducted a large number of studies on the separation of their isomers. Foreign research started earlier, and by the 1980s, various separation methods could be used, mainly the combination of precision distillation and crystallization techniques, to mass-produce para-dichlorobenzene, meta-dichlorobenzene and ortho-dichlorobenzene. While domestic development started later, and dichlorobenzene chemistry began to develop successively in the 1980s.

[0005] The current mainstream method for separating dichlorobenzene isomers is the combination of extractive distillation and crystallization. By using an extractant to increase the relative volatility between the three isomers, it is possible to obtain as pure a dichlorobenzene monomer as possible under a smaller number of trays and a lower reflux ratio.

[0006] The extractants reported now include organic solvent extractants such as diphenylamine, 2-pyrrolidone (CN1623967A), phosphate esters (CN100391917C), sulfolane, decanol, and cresol. The separation efficiency of the above extractants is not high. The separation factor of sulfolane is only 1.15, that of decanol is 1.1, that of cresol is 1.07, and that of triethyl phosphate only reaches 1.23.

[0007] CN102405084B reported the use of ionic liquids as extractants to separate m-dichlorobenzene and p-dichlorobenzene. The ionic liquid combinations composed of imidazole cations and phosphate anions showed good separation characteristics. For example, 1,3-diethylimidazolium diethylphosphate has a separation factor for p-dichlorobenzene / m-dichlorobenzene that can reach 1.34. However, using ionic liquids as extractants, their cost is relatively expensive, which is not conducive to large-scale production and use.

[0008] Therefore, further developing green, efficient, and reasonably priced extractants is the key point to promote the separation of mixed dichlorobenzenes by extractive distillation technology, especially the overall separation and purification scheme in combination with such extractants. Summary of the Invention

[0009] The purpose of the present invention is to provide a green, efficient, and reasonably priced method for high-efficient separation and purification among dichlorobenzene isomers in view of the difficulties in dichlorobenzene separation and the problem of low extractant efficiency in the current extractive distillation technology.

[0010] According to the first aspect of the present invention, the present invention provides a method for separating mixed dichlorobenzenes, which includes:

[0011] (a) Mixing mixed dichlorobenzenes with extractant A in an extractive distillation column for extractive distillation separation to obtain crude p-dichlorobenzene from the top of the extractive distillation column;

[0012] (b) Feeding the bottom material of the extractive distillation column and solvent B into a crystallization and precipitation unit to obtain crystals mainly composed of m-dichlorobenzene and o-dichlorobenzene, and a solvent phase of extractant A and solvent B;

[0013] (c) Heating the crystals mainly composed of m-dichlorobenzene and o-dichlorobenzene and feeding them into a distillation column for distillation separation, separating out the solvent B mixed therein from the top of the distillation column, side-drawing crude m-dichlorobenzene, and bottom-drawing crude o-dichlorobenzene;

[0014] The extractant A is a deep eutectic solvent containing a hydrogen bond acceptor and a hydrogen bond donor, wherein the hydrogen bond acceptor is composed of quaternary ammonium chloride and / or imidazole chloride, and the chemical formula is as shown in Formula A and Formula B;

[0015]

[0016] Among them,

[0017] In Formula A and Formula B, each of R1, R2 and R3 is hydrogen, C1-C 12 alkyl, C5-C 14 aryl, C6-C 20 aralkyl or C2-C 12 alkylene, R4 is C1-C alkyl with or without a hydroxyl group, C5-C 12 aryl, C6-C 14 aralkyl or C2-C 20 alkylene; 12

[0018] The hydrogen bond donor is one or more of carboxylic acid, amide and urea. The chemical formulas of amide and urea are shown in Formula C and Formula D respectively:

[0019]

[0020]

[0021] Among them,

[0022] In Formula C and Formula D, each of R1, R2, R3 and R4 is independently hydrogen, C1-C 12 alkyl, C5-C 14 aryl, C6-C 20 aralkyl or C2-C 12 alkylene;

[0023] The extractant B is water.

[0024] The method of the present invention includes using extractive distillation technology, crystallization technology and distillation technology to separate the mixed dichlorobenzenes containing ortho-dichlorobenzene, meta-dichlorobenzene and para-dichlorobenzene, and finally obtaining high-purity ortho-, meta- and para-dichlorobenzene products. Among them, extractive distillation uses the selected deep eutectic solvent (DES) extractant, and the separation efficiency of the mixed dichlorobenzenes is high. The recovery of the extractant adopts the crystallization precipitation recovery process, which is simple, green and efficient, and greatly saves equipment investment. Both the extractant and the solvent can form a closed-loop operation, greatly improving the use efficiency of the extractant and the solvent and reducing the production cost. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 Shows a process flow diagram according to a preferred embodiment of the present invention.

[0026] DESCRIPTION OF THE REFERENCE NUMERALS

[0027] ① is the extractive distillation column; ② is the crystallization precipitation unit;

[0028] ③ is the flash tank; ④ is the distillation column;

[0029] ⑤ is the crystallization and purification unit for p-dichlorobenzene; ⑥ is the crystallization and purification unit for m-dichlorobenzene;

[0030] ⑦ is the crystallization and purification unit for o-dichlorobenzene;

[0031] The specific fraction information is as follows:

[0032] 1 is mixed dichlorobenzene; 2 is extractant A;

[0033] 3 is the bottom product of the extractive distillation column; 4 is the top product of the extractive distillation column;

[0034] 5 is solvent B; 6 is the solvent phase of the crystallization unit;

[0035] 7 is the crystal phase of the crystallization unit; 8 is the side stream product at the top of the rectification unit;

[0036] 9 is the bottom product of the rectification column; 10 is the top product of the rectification unit;

[0037] 11 is the crystallization mother liquor of p-dichlorobenzene; 12 is the p-dichlorobenzene product;

[0038] 13 is the o-dichlorobenzene product; 14 is the m-dichlorobenzene product;

[0039] 15 is the reflux liquid of the mixed solvent. Detailed implementation mode

[0040] In the ranges disclosed herein, the endpoints and any values are not limited to the exact ranges or values, and these ranges or values should be understood to include values close to these ranges or values. For numerical ranges, between the endpoint values of each range, between the endpoint values of each range and individual point values, and between individual point values, they can be combined with each other to obtain one or more new numerical ranges, and these numerical ranges should be regarded as specifically disclosed herein.

[0041] The present invention provides a method for separating mixed dichlorobenzene, and the method includes:

[0042] (a) Mixing dichlorobenzene and extractant A are contacted in an extractive distillation column for extractive distillation separation, and a crude product of p-dichlorobenzene is obtained from the top of the extractive distillation column;

[0043] (b) The bottom product of the extractive distillation column and solvent B enter the crystallization unit to obtain crystals mainly composed of m-dichlorobenzene and o-dichlorobenzene, and a solvent phase of extractant A and solvent B;

[0044] (c) Heating the crystals mainly composed of m-dichlorobenzene and o-dichlorobenzene and feeding them into a rectification column for rectification separation, separating out solvent B from the top of the rectification column, side-stream extracting a crude product of m-dichlorobenzene, and bottom-stream extracting a crude product of o-dichlorobenzene;

[0045] The extractant A is a deep eutectic solvent containing a hydrogen bond acceptor and a hydrogen bond donor. Among them, the hydrogen bond acceptor is composed of quaternary ammonium chloride and / or imidazole chloride, and their chemical formulas are shown in Formula A and Formula B respectively;

[0046]

[0047] Among them,

[0048] In Formula A and Formula B, R1, R2 and R3 are each hydrogen, C1-C 12 alkyl, C5-C 14 aryl, C6-C 20 aralkyl or C2-C 12 alkylene, and R4 is C1-C 12 alkyl with or without a hydroxyl group, C5-C 14 aryl, C6-C 20 aralkyl or C2-C 12 alkylene;

[0049] The hydrogen bond donor is one or more of carboxylic acids, amides and ureas. The chemical formulas of the amide and urea are shown in Formula C and Formula D respectively:

[0050]

[0051] Among them,

[0052] In Formula C and Formula D, R1, R2, R3 and R4 are each independently hydrogen, C1-C 12 alkyl, C5-C 14 aryl, C6-C 20 aralkyl or C2-C 12 alkylene;

[0053] The extractant B is water.

[0054] The method of the present invention includes using extractive distillation technology, crystallization technology and distillation technology to separate the mixed dichlorobenzenes containing ortho-dichlorobenzene, meta-dichlorobenzene and para-dichlorobenzene, and finally obtaining high-purity ortho-, meta- and para-dichlorobenzene products. Among them, extractive distillation uses the selected deep eutectic solvent (DES) extractant, and the separation efficiency of the mixed dichlorobenzenes is high. The recovery of the extractant adopts a crystallization precipitation recovery process, which is simple, green and efficient, and greatly saves equipment investment.

[0055] According to a preferred embodiment of the present invention, the solvent B is water. Water is a green solvent with strong hydrogen bond acceptor and donor properties. Its function is to break the hydrogen bond between the hydrogen bond donor and the hydrogen bond acceptor in the extractant A, so that the hydrogen bond donor and the hydrogen bond acceptor form new hydrogen bonds with water respectively, reducing the solubility of dichlorobenzene in the extractant A, so that dichlorobenzene precipitates out.

[0056] According to a preferred embodiment of the present invention, among the hydrogen bond acceptors, the preferred imidazole chlorides are one or more of 1-methyl-3-methylimidazole chloride, 1-ethyl-3-methylimidazole chloride, and 3-methylimidazole chloride.

[0057] According to a preferred embodiment of the present invention, among the hydrogen bond acceptors, the preferred quaternary ammonium salts are one or more of choline chloride, tetramethylammonium chloride, tetraethylammonium chloride, and tetrabutylammonium chloride, preferably 1-ethyl-3-methylimidazole chloride and / or choline chloride.

[0058] According to a preferred embodiment of the present invention, among the hydrogen bond donors, the preferred ones are one or more of oxalic acid, citric acid, urea, tetramethylurea, formamide, acetamide, N-methylformamide, and N-methylacetamide, preferably one or more of oxalic acid, citric acid, urea, and formamide.

[0059] According to a preferred embodiment of the present invention, the preferred extractant A is one or more of the combinations of choline chloride / oxalic acid, choline chloride / citric acid, choline chloride / urea, choline chloride / formamide, 1-ethyl-3-methylimidazole chloride / oxalic acid, 1-ethyl-3-methylimidazole chloride / citric acid, 1-ethyl-3-methylimidazole chloride / urea, 1-ethyl-3-methylimidazole chloride / formamide, tetraethylammonium chloride / oxalic acid, tetraethylammonium chloride / citric acid, tetraethylammonium chloride / urea, and tetraethylammonium chloride / formamide.

[0060] In the present invention, imidazole and quaternary ammonium chlorides are two common organic salts with a wide range of uses and low prices. Chlorine, as an anion, has a relatively small structure and certain hydrogen bond acceptor ability, and can form deep eutectic solvents with some organic solvents having hydrogen bond donor ability.

[0061] In the present invention, carboxylic acids, amides, or ureas all contain carbon-oxygen double bonds in their molecular structures. The presence of the double bond will increase the separation of mixed dichlorobenzenes. When there are some independent hydrogens or hydroxyl groups in R1, R2, R3, and R4 in the molecular structure, it is possible to form deep eutectic solvents with imidazole chlorides or quaternary ammonium chlorides.

[0062] The extractant of the present invention can increase the relative volatility between the three isomers of dichlorobenzene, making the separation effect better.

[0063] According to a preferred embodiment of the present invention, the mass ratio of the hydrogen bond acceptor to the hydrogen bond donor in the extractant A is 0.2 to 20. A higher ratio or a lower ratio will make the formation of the deep eutectic solvent less stable and the melting point decrease less significantly, resulting in some deep eutectic solvents existing in solid form within the feed range of the extractive distillation column, which is not conducive to extractive distillation operation.

[0064] According to a preferred embodiment of the present invention, the mass flow rate ratio of extractant A to mixed dichlorobenzene is 0.4-20; the amount of extractant A is mainly based on two points: the first point is the separation efficiency of the selected extractant for o-dichlorobenzene and m-dichlorobenzene / p-dichlorobenzene. The higher the separation efficiency, the smaller the amount of extractant A, otherwise the more the amount required; the second point is the content of m-dichlorobenzene and o-dichlorobenzene in the mixed dichlorobenzene. When the content of m-dichlorobenzene and o-dichlorobenzene is higher than that of p-dichlorobenzene, the amount of extractant A needs to be more, otherwise the amount can be reduced. Under the above mass flow rate ratio, extractant A has a better separation effect on the separation of mixed dichlorobenzene.

[0065] According to a preferred embodiment of the present invention, the amount of solvent B added is 0.2 to 10 times the amount of the extractive distillation tower kettle. If the amount added is too small, the extractant A cannot be completely dissolved, especially the effect between the hydrogen bond donor and the acceptor in the extractant A is destroyed, while if the amount added is too large, the load of subsequent flash evaporation will be increased. Within the above ratio range, a better effect is achieved.

[0066] According to a preferred embodiment of the present invention, the mixed dichlorobenzene contains 20-60% by weight, preferably 30-50% by weight, and more preferably 40-45% by weight of p-dichlorobenzene, 1-30% by weight, preferably 5-25% by weight, and more preferably 10-20% by weight of m-dichlorobenzene, and 20-60% by weight, preferably 30-50% by weight, and more preferably 40-45% by weight of o-dichlorobenzene.

[0067] In the method of the present invention, both the extractant and the solvent can be used in a closed loop, which greatly improves the use efficiency of the extractant and the solvent and reduces the production cost.

[0068] According to a preferred embodiment of the present invention, the operating conditions of the extractive distillation tower include: a reflux ratio of 1 to 20, an operating pressure of 5 kPa to 100 kPa, and a tower top temperature of 81°C to 174°C. In order to better separate meta-dichlorobenzene, ortho-dichlorobenzene and para-dichlorobenzene, in addition to the effect of extractant A, the reflux ratio also has a great influence. In order to achieve better separation, it is necessary to operate at a higher reflux ratio. When the reflux ratio is relatively small, para-xylene and ortho- and meta-xylene cannot be separated well, and when the reflux ratio is relatively large, the impact on purity will become smaller, but the energy consumption of the distillation tower will increase rapidly. The above reflux ratio range is a better range. The extractive distillation tower is operated under negative pressure conditions, which can effectively reduce the operating temperature of the entire tower. Controlling the tower top temperature of the extractive distillation tower at 81°C to 174°C can obtain a crude product of para-dichlorobenzene, and the crude product also contains a certain amount of meta-dichlorobenzene. Subsequent methods are required for further purification.

[0069] According to some embodiments of the present invention, the rectification column is a common packed rectification column, and a draw-off port is provided at the side draw position. The packed rectification column has the advantage of a high number of theoretical plates at the same height. When separating meta-dichlorobenzene and ortho-dichlorobenzene isomers, a high number of theoretical plates is required to meet the requirements. Since the meta-dichlorobenzene and ortho-dichlorobenzene crystals from the crystallization precipitation unit will contain a part of water, there will be some water at the top of the rectification unit. The meta-dichlorobenzene drawn from the side draw is relatively pure. Therefore, a draw-off port needs to be provided at the side draw position.

[0070] According to a preferred embodiment of the present invention, the rectification column is a packed rectification column, and preferably a draw-off port is provided at the side draw position.

[0071] According to a preferred embodiment of the present invention, the operating conditions of the rectification column include: the reflux ratio is 1 to 20, the operating pressure is 50 kPa to 100 kPa, and the bottom temperature of the column is 155 °C to 180 °C. To better separate meta-dichlorobenzene and ortho-dichlorobenzene, a higher reflux ratio operation is required. When the reflux ratio is small, the product purity of the meta-dichlorobenzene drawn from the side draw and the ortho-dichlorobenzene drawn from the bottom of the column will be affected. When the reflux ratio is large, the influence on the purity will become smaller, but the energy consumption of the rectification column will increase rapidly. Therefore, within the above range, it is a better reflux ratio range. The operation pressure is carried out under negative pressure, which can reduce the operating temperature of the whole column. By controlling the temperature of the bottom of the column within a certain range, crude meta-dichlorobenzene and crude ortho-dichlorobenzene can be obtained at the side draw and the bottom of the column respectively.

[0072] According to a preferred embodiment of the present invention, the method further includes: subjecting the crude p-dichlorobenzene in step (a) to freeze crystallization separation to obtain a p-dichlorobenzene product, and returning the crystallization mother liquor as the raw material of the extraction agent for the extractive distillation column.

[0073] According to a preferred embodiment of the present invention, the method further includes: subjecting the solvent phase of the extraction agent A and the solvent B in step (b) to vacuum flashing separation to obtain the extraction agent A and the solvent B.

[0074] According to a preferred embodiment of the present invention, the method further includes: subjecting the crude meta-dichlorobenzene and the crude ortho-dichlorobenzene in step (c) to freeze crystallization respectively to obtain a meta-dichlorobenzene product and an ortho-dichlorobenzene product.

[0075] According to a preferred embodiment of the present invention, the bottom product of the rectification column needs to be subjected to freeze crystallization operation, and the temperature is controlled to finally obtain an ortho-dichlorobenzene product. According to the purity requirement of the ortho-dichlorobenzene product, the ortho-dichlorobenzene product can also be further purified by using the method of multiple freeze crystallization, that is, the method of recrystallization.

[0076] According to a preferred embodiment of the present invention, the side stream product of the rectification column needs to be subjected to freeze crystallization operation to finally obtain m-dichlorobenzene product; according to the purity requirement of the m-dichlorobenzene product, multiple freeze crystallization methods, that is, recrystallization method, can also be adopted to crystallize the mother liquor multiple times to precipitate o-dichlorobenzene and obtain m-dichlorobenzene with higher purity.

[0077] According to a preferred embodiment of the present invention, the mother liquor and crystals other than the o-dichlorobenzene and m-dichlorobenzene products re-enter the rectification column for rectification separation. The above distillate fraction is a mixture of o-dichlorobenzene and m-dichlorobenzene, which can be returned to the rectification column for reuse and separation.

[0078] According to a preferred embodiment of the present invention, the method further includes: returning the extractant A obtained by vacuum flash distillation separation as the extractant raw material of the extractive distillation column, and returning the solvent B obtained by vacuum flash distillation separation as the solvent raw material of the crystallization precipitation unit.

[0079] According to a preferred embodiment of the present invention, the freeze crystallization temperature of the p-dichlorobenzene crude product in step (a) is controlled within the range of -24°C to 52°C. In order to obtain a high-purity p-dichlorobenzene product, the recrystallization method, that is, multi-step crystallization, can be adopted to purify the purity of p-dichlorobenzene in the crystal. The freeze crystallization needs to control the temperature within the above range. If the temperature is too high, p-dichlorobenzene cannot crystallize out, while if the temperature is too low, m-dichlorobenzene will also crystallize out simultaneously, failing to achieve effective separation and purification. After crystallization / recrystallization, the mother liquor is a mixture containing m-dichlorobenzene and p-dichlorobenzene, which is returned to the extractive distillation column for re-separation.

[0080] According to a preferred embodiment of the present invention, the vacuum flash distillation is carried out in a vacuum flash tank. The purpose of the vacuum flash tank is to remove the solvent B so that the solvent B and the extractant A can be reused, which belongs to the regeneration unit of the extractant. An ordinary flash tank can achieve the above purpose.

[0081] According to a preferred embodiment of the present invention, the operating pressure of the vacuum flash tank is 0.5 kPa to 50 kPa. Operating at a lower pressure can reduce the operating temperature of the vacuum flash tank and reduce the requirement for the feed temperature. Before the feed liquid enters the vacuum flash tank from the crystallization precipitation unit, the feed liquid must be heated to reach the bubble point feed temperature.

[0082] According to a preferred embodiment of the present invention, the method further includes: returning the solvent B inclusions for vacuum flash distillation.

[0083] According to a preferred embodiment of the present invention, the method further includes: returning the mother liquor and / or crystals other than the product obtained by the freeze crystallization separation of each step to the rectification column for rectification separation. This is specifically carried out according to needs. For example, the mother liquor obtained by the freeze crystallization separation of the crude p-dichlorobenzene can be returned to the extractive rectification column, while the crystals and mother liquor obtained by the freeze crystallization of the crude m-dichlorobenzene and the crude o-dichlorobenzene respectively are returned to the rectification column.

[0084] According to a preferred embodiment of the present invention, the obtained extractant A re-enters the extractive rectification column, and the solvent B enters the crystallization and precipitation separation unit. The above operations can realize the closed-loop recycling of extractant A and solvent B.

[0085] On the basis of not violating the common knowledge in the art, the above preferred conditions can be arbitrarily combined to obtain various preferred examples of the present invention.

[0086] The whole separation method of the mixed dichlorobenzenes by the selected extractant A and the mixed dichlorobenzenes formed for the extractant A of the present invention can effectively achieve the high-efficiency and green separation of the mixed dichlorobenzenes. The closed-loop use of extractant A and solvent B reduces the loss of the solvent. The extractant A has high extraction efficiency, the overall process is simple, and the operation range is large.

[0087] The present invention will be further described below by way of examples, but the present invention is not limited to the scope of the described examples. The experimental methods without specific conditions noted in the following examples are carried out according to conventional methods and conditions, or selected according to the product specifications.

[0088] The reagents and raw materials used in the present invention are all commercially available.

[0089] In the present invention, the product purity of all examples is obtained by HPLC detection.

[0090] According to Figure 1The process is carried out as follows. The mixed dichlorobenzene raw material 1 enters from the middle of the extractive distillation column ①, and the extractant A2 enters from the upper middle part of the extractive distillation column. In the extractive distillation column, the ortho-dichlorobenzene and meta-dichlorobenzene are carried and come out from the bottom of the extractive distillation column, such as the bottom material 3 of the extractive distillation column. The top material 4 of the extractive distillation column is the crude para-dichlorobenzene, which enters the para-dichlorobenzene crystallization and purification unit ⑤, and finally obtains the pure para-dichlorobenzene product 12 and the para-dichlorobenzene crystallization mother liquor 11. The para-dichlorobenzene crystallization mother liquor 11 is combined with the mixed dichlorobenzene raw material and sent back to the extractive distillation column ①. The bottom material fraction 3 of the extractive distillation column ① enters the crystallization and precipitation unit ②, and the solvent B of the fraction 5 is added during the process to precipitate the meta-dichlorobenzene and ortho-dichlorobenzene dissolved in the extractant A. The extractant A and the solvent B are in a homogeneous phase, which is the solvent phase 6 of the crystallization and precipitation unit, and enters the flash tank ③. The gas-phase fraction 5 is the recovered solvent B, which is sent back to the crystallization and precipitation unit ②, and the liquid phase is the extractant A, which is combined with the fraction 2 and enters the extractive distillation column. The crystals obtained in the crystallization and precipitation unit, such as the crystallization phase 7 of the crystallization and precipitation unit, are mainly meta-dichlorobenzene and ortho-dichlorobenzene, and enter from the middle of the distillation column ④. In the distillation unit, the meta-dichlorobenzene and ortho-dichlorobenzene are separated. A small amount of the solvent B, such as the fraction 10, is obtained at the top of the column and sent back to the flash tank ③. The top side-line discharge 8 of the distillation unit is obtained at the top side-line position of the column, which is mainly meta-dichlorobenzene and also contains a small amount of ortho-dichlorobenzene. After passing through the meta-dichlorobenzene crystallization and purification unit ⑥, the high-purity meta-dichlorobenzene product 14 is obtained. The crystals are a mixture of ortho-dichlorobenzene and meta-dichlorobenzene, which enter the fraction mixed solvent reflux liquid 15 and are sent back to the distillation unit, and are combined with the crystallization phase 7 of the crystallization and precipitation unit and enter. The bottom discharge 9 of the distillation column ④ is mainly ortho-dichlorobenzene, which also enters the ortho-dichlorobenzene crystallization and purification unit ⑦ for crystallization / recrystallization, and finally obtains the ortho-dichlorobenzene product 13 and the mixed solvent reflux liquid (a mixture containing ortho-dichlorobenzene and meta-dichlorobenzene) 15, which are also sent to the distillation column. Thus, three products of ortho-dichlorobenzene, meta-dichlorobenzene and para-dichlorobenzene are obtained respectively.

[0091] Example 1

[0092] The mixed dichlorobenzene, upon detection, contains ortho-dichlorobenzene, meta-dichlorobenzene, and para-dichlorobenzene in a mass ratio of 0.45:0.1:0.45. Choline chloride / oxalic acid is used as the extractant, and the mass ratio of choline chloride to oxalic acid is 0.8. The mixed dichlorobenzene is fed into the middle of the extractive distillation column at a rate of 100 kg / h, and the extractant is fed into the upper middle part of the extractive distillation column at a rate of 600 kg / h. The top pressure of the extractive distillation column is 5 kPa, the reflux ratio is 4, and the distillation temperature at the top is controlled at 81 °C. The crude para-dichlorobenzene is obtained at the top of the column, and after freezing crystallization / recrystallization at -24 °C, the para-dichlorobenzene product is finally obtained. Its purity is detected to be 99.82%, and the overall recovery rate is 96.3%. The bottom product of the extractive distillation column enters the crystallization and precipitation unit. Water is used as the solvent, and the addition amount of water is 3 times the mass ratio of the bottom product of the extractive distillation column. After a certain amount of stirring and standing, the solvent phase and crystals are obtained. The solvent phase enters the vacuum flash tank, and the pressure of the flash tank is 3 kPa. Finally, water and the regenerated extractant are obtained. The water re-enters the crystallization and precipitation unit, and the extractant enters the extractive distillation column. The obtained crystals are heated and then enter the distillation column for further separation. The distillation column operates under the condition of 50 kPa, the reflux ratio is set to 6, and the temperature at the bottom is controlled at 155 °C. Finally, the crude meta-dichlorobenzene and the crude ortho-dichlorobenzene are obtained at the side line and the bottom of the column respectively. After further crystallization / recrystallization, the meta-dichlorobenzene product and the ortho-dichlorobenzene product are obtained respectively. Upon detection, the purity of meta-dichlorobenzene is 99.52% and the recovery rate is 93.2%, and the purity of ortho-dichlorobenzene is 99.65% and the recovery rate is 97.4%.

[0093] Example 2

[0094] The mixed dichlorobenzene, upon detection, contains ortho-dichlorobenzene, meta-dichlorobenzene, and para-dichlorobenzene with a mass ratio of 0.45:0.1:0.45. Choline chloride / citric acid is used as the extractant, and the mass ratio of choline chloride to citric acid is 2. The mixed dichlorobenzene is fed into the middle of the extractive distillation column at a rate of 100 kg / h, and the extractant is fed into the upper middle part of the extractive distillation column at a rate of 800 kg / h. The top pressure of the extractive distillation column is 20 kPa, the reflux ratio is 6, and the distillation temperature at the top is controlled at 118 °C. The crude para-dichlorobenzene is obtained at the top of the column, and after freezing crystallization / recrystallization at 0 °C, the para-dichlorobenzene product is finally obtained. Its purity is detected to be 99.88%, and the overall recovery rate is 94.8%. The bottom product of the extractive distillation column enters the crystallization and precipitation unit. Water is used as the solvent, and the addition amount of water is 5 times the mass ratio of the bottom product of the extractive distillation column. After a certain amount of stirring and standing, a solvent phase and crystals are obtained. The solvent phase enters a vacuum flash tank, and the pressure of the flash tank is 5 kPa. Finally, water and the regenerated extractant are obtained. The water re-enters the crystallization and precipitation unit, and the extractant enters the extractive distillation column. The obtained crystals are heated and then enter a distillation column for further separation. The distillation column operates under the condition of 100 kPa, the reflux ratio is set to 3, and the temperature at the bottom is controlled at 180 °C. Finally, the crude meta-dichlorobenzene and the crude ortho-dichlorobenzene are obtained at the side line and the bottom of the column respectively. After further crystallization / recrystallization, the meta-dichlorobenzene product and the ortho-dichlorobenzene product are obtained respectively. Upon detection, the purity of meta-dichlorobenzene is 99.37% and the recovery rate is 93.6%, and the purity of ortho-dichlorobenzene is 99.49% and the recovery rate is 97.6%.

[0095] Example 3

[0096] The mixed dichlorobenzene, upon detection, contains ortho-dichlorobenzene, meta-dichlorobenzene and para-dichlorobenzene with a mass ratio of 0.4:0.2:0.4. 1-Ethyl-3-methylimidazolium chloride / citric acid is used as the extractant, and the mass ratio of 1-ethyl-3-methylimidazolium chloride to citric acid is 1.2. The mixed dichlorobenzene is fed into the middle of the extractive distillation column at a rate of 100 kg / h, and the extractant is fed into the upper middle part of the extractive distillation column at a rate of 2000 kg / h. The top pressure of the extractive distillation column is 50 kPa, the reflux ratio is 6, and the distillation temperature at the top is controlled at 147 °C. The crude para-dichlorobenzene is obtained at the top of the column, and after freezing crystallization / recrystallization at -10 °C, the para-dichlorobenzene product is finally obtained. Its purity is detected to be 99.95%, and the overall recovery rate is 94.5%. The bottom product of the extractive distillation column enters the crystallization and precipitation unit. Water is used as the solvent, and the addition amount of water is 5 times the mass ratio of the bottom product of the extractive distillation column. After a certain amount of stirring and standing, a solvent phase and crystals are obtained. The solvent phase enters the vacuum flash tank, and the pressure of the flash tank is 1 kPa. Finally, water and the regenerated extractant are obtained. The water re-enters the crystallization and precipitation unit, and the extractant enters the extractive distillation column. The obtained crystals are heated and then enter the distillation column for further separation. The distillation column operates under the condition of 80 kPa, the reflux ratio is set at 10, and the temperature at the bottom of the column is controlled at 165 °C. Finally, the crude meta-dichlorobenzene and the crude ortho-dichlorobenzene are obtained at the side line and the bottom of the column respectively. After further crystallization / recrystallization, the meta-dichlorobenzene product and the ortho-dichlorobenzene product are obtained respectively. Upon detection, the purity of meta-dichlorobenzene is 99.60% and the recovery rate is 94.1%, and the purity of ortho-dichlorobenzene is 99.71% and the recovery rate is 98.1%.

[0097] Example 4

[0098] The mixed dichlorobenzene, upon detection, contains ortho-dichlorobenzene, meta-dichlorobenzene, and para-dichlorobenzene with a mass ratio of 0.4:0.2:0.4. 1-Ethyl-3-methylimidazolium chloride / urea is used as the extractant, and the mass ratio of 1-ethyl-3-methylimidazolium chloride to urea is 5. The mixed dichlorobenzene is fed into the middle of the extractive distillation column at a rate of 100 kg / h, and the extractant is fed into the upper middle part of the extractive distillation column at a rate of 1200 kg / h. The top pressure of the extractive distillation column is 20 kPa, the reflux ratio is 20, and the distillation temperature at the top is controlled at 118°C. A crude para-dichlorobenzene product is obtained at the top of the column, and it is frozen and recrystallized at -10°C to finally obtain a para-dichlorobenzene product with a detected purity of 99.91% and an overall recovery rate of 94.6%. The bottom product of the extractive distillation column enters the crystallization and precipitation unit. Water is used as the solvent, and the addition amount of water is 10 times the mass ratio of the bottom product of the extractive distillation column. After a certain amount of stirring and standing, a solvent phase and crystals are obtained. The solvent phase enters a vacuum flash tank with a pressure of 0.5 kPa, and finally water and the regenerated extractant are obtained. The water re-enters the crystallization and precipitation unit, and the extractant enters the extractive distillation column. The obtained crystals are heated and then enter a distillation column for further separation. The distillation column operates under the condition of 50 kPa, the reflux ratio is set at 20, and the temperature at the bottom of the column is controlled at 155°C. Finally, a crude meta-dichlorobenzene product and a crude ortho-dichlorobenzene product are obtained at the side line and the bottom of the column respectively. After further crystallization / recrystallization, a meta-dichlorobenzene product and an ortho-dichlorobenzene product are obtained respectively. Upon detection, the purity of meta-dichlorobenzene is 99.65% and the recovery rate is 93.8%, and the purity of ortho-dichlorobenzene is 99.77% and the recovery rate is 97.6%.

[0099] Example 5

[0100] The mixed dichlorobenzene, upon detection, contains ortho-dichlorobenzene, meta-dichlorobenzene, and para-dichlorobenzene with a mass ratio of 0.4:0.2:0.4. Choline chloride / formamide is used as the extractant, and the mass ratio of choline chloride to formamide is 0.5. The mixed dichlorobenzene is fed into the middle of the extractive distillation column at a rate of 100 kg / h, and the extractant is fed into the upper middle part of the extractive distillation column at a rate of 200 kg / h. The top pressure of the extractive distillation column is 20 kPa, the reflux ratio is 8, and the distillation temperature at the top is controlled at 118 °C. The crude para-dichlorobenzene is obtained at the top of the column, and after freezing crystallization / recrystallization at -10 °C, the para-dichlorobenzene product is finally obtained. Its purity is detected to be 99.51%, and the overall recovery rate is 97.0%. The bottom product of the extractive distillation column enters the crystallization precipitation unit. Water is used as the solvent, and the addition amount of water is 10 times the mass ratio of the bottom product of the extractive distillation column. After a certain amount of stirring and standing, a solvent phase and crystals are obtained. The solvent phase enters a vacuum flash tank, and the pressure of the flash tank is 0.5 kPa. Finally, water and the regenerated extractant are obtained. The water re-enters the crystallization precipitation unit, and the extractant enters the extractive distillation column. The obtained crystals are heated and then enter a distillation column for further separation. The distillation column operates under the condition of 50 kPa, the reflux ratio is set at 20, and the temperature at the bottom of the column is controlled at 155 °C. Finally, the crude meta-dichlorobenzene and the crude ortho-dichlorobenzene are obtained at the side line and the bottom of the column respectively. After further crystallization / recrystallization, the meta-dichlorobenzene product and the ortho-dichlorobenzene product are obtained respectively. Upon detection, the purity of meta-dichlorobenzene is 99.57%, the recovery rate is 94.0%, the purity of ortho-dichlorobenzene is 99.69%, and the recovery rate is 97.9%.

[0101] Example 6

[0102] The mixed dichlorobenzene, upon detection, contains ortho-dichlorobenzene, meta-dichlorobenzene and para-dichlorobenzene with a mass ratio of 0.45:0.1:0.45. Choline chloride / citric acid is used as the extractant, and the mass ratio of choline chloride to citric acid is 0.2. The mixed dichlorobenzene is fed into the middle of the extractive distillation column at a rate of 100 kg / h, and the extractant is fed into the upper middle part of the extractive distillation column at a rate of 40 kg / h. The top pressure of the extractive distillation column is 100 kPa, the reflux ratio is 20, and the distillation temperature at the top is controlled at 174 °C. The crude para-dichlorobenzene is obtained at the top of the column, and it is frozen and crystallized / recrystallized at 52 °C to finally obtain the para-dichlorobenzene product, whose purity is detected to be 99.21% and the overall recovery rate is 92.1%. The bottom product of the extractive distillation column enters the crystallization and precipitation unit. Water is used as the solvent, and the addition amount of water is 1 times the mass ratio of the bottom product of the extractive distillation column. After a certain amount of stirring and standing, a solvent phase and crystals are obtained. The solvent phase enters the vacuum flash tank, and the pressure of the flash tank is 50 kPa. Finally, water and the regenerated extractant are obtained. The water re-enters the crystallization and precipitation unit, and the extractant enters the extractive distillation column. The obtained crystals are heated and then enter the distillation column for further separation. The distillation column operates under the condition of 50 kPa, the reflux ratio is set to 1, and the temperature at the bottom of the column is controlled at 155 °C. Finally, the crude meta-dichlorobenzene and the crude ortho-dichlorobenzene are obtained at the side line and the bottom of the column respectively. After further crystallization / recrystallization, the meta-dichlorobenzene product and the ortho-dichlorobenzene product are obtained respectively. Upon detection, the purity of meta-dichlorobenzene is 99.05% and the recovery rate is 90.5%, and the purity of ortho-dichlorobenzene is 99.16% and the recovery rate is 92.7%.

[0103] Example 7

[0104] The mixed dichlorobenzene, upon detection, contains ortho-dichlorobenzene, meta-dichlorobenzene and para-dichlorobenzene with a mass ratio of 0.45:0.1:0.45. Choline chloride / citric acid is used as the extractant, and the mass ratio of choline chloride to citric acid is 2. The mixed dichlorobenzene is fed into the middle of the extractive distillation column at a rate of 100 kg / h, and the extractant is fed into the upper middle part of the extractive distillation column at a rate of 2000 kg / h. The top pressure of the extractive distillation column is 100 kPa, the reflux ratio is 1, and the distillation temperature at the top is controlled at 174 °C. The crude para-dichlorobenzene is obtained at the top of the column, and is frozen and recrystallized at 52 °C to finally obtain the para-dichlorobenzene product, whose purity is detected to be 99.14%, and the overall recovery rate is 91.3%. The bottom product of the extractive distillation column enters the crystallization and precipitation unit. Water is used as the solvent, and the addition amount of water is 1 times the mass ratio of the bottom product of the extractive distillation column. After a certain amount of stirring and standing, a solvent phase and crystals are obtained. The solvent phase enters the vacuum flash tank, and the pressure of the flash tank is 50 kPa. Finally, water and the regenerated extractant are obtained, where the water re-enters the crystallization and precipitation unit, and the extractant enters the extractive distillation column. The obtained crystals are heated and then enter the distillation column for further separation. The distillation column operates under the condition of 50 kPa, the reflux ratio is set to 1, and the temperature of the bottom of the column is controlled at 155 °C. Finally, the crude meta-dichlorobenzene and the crude ortho-dichlorobenzene are obtained at the side line and the bottom of the column respectively, and the meta-dichlorobenzene product and the ortho-dichlorobenzene product are obtained through further crystallization / recrystallization respectively. Upon detection, the purity of meta-dichlorobenzene is 99.01% and the recovery rate is 90.2%, and the purity of ortho-dichlorobenzene is 99.09% and the recovery rate is 91.9%.

[0105] Example 8

[0106] The mixed dichlorobenzene, upon detection, contains ortho-dichlorobenzene, meta-dichlorobenzene, and para-dichlorobenzene with a mass ratio of 0.45:0.1:0.45. Choline chloride / citric acid is used as the extractant, and the mass ratio of choline chloride to citric acid is 20. The mixed dichlorobenzene is fed into the middle of the extractive distillation column at a rate of 100 kg / h, and the extractant is fed into the upper-middle part of the extractive distillation column at a rate of 200 kg / h. The top pressure of the extractive distillation column is 20 kPa, the reflux ratio is 15, and the distillation temperature at the top is controlled at 118 °C. The crude para-dichlorobenzene is obtained at the top of the column, and after freezing crystallization / recrystallization at 0 °C, the para-dichlorobenzene product is finally obtained. Its purity is detected to be 99.25%, and the overall recovery rate is 92.8%. The bottom product of the extractive distillation column enters the crystallization and precipitation unit. Water is used as the solvent, and the addition amount of water is 0.2 times the mass ratio of the bottom product of the extractive distillation column. After a certain amount of stirring and standing, a solvent phase and crystals are obtained. The solvent phase enters a vacuum flash tank, and the pressure of the flash tank is 50 kPa. Finally, water and the regenerated extractant are obtained. The water re-enters the crystallization and precipitation unit, and the extractant enters the extractive distillation column. The obtained crystals are heated and then enter a distillation column for further separation. The distillation column operates under the condition of 50 kPa, the reflux ratio is set to 8, and the temperature of the bottom of the column is controlled at 155 °C. Finally, the crude meta-dichlorobenzene and the crude ortho-dichlorobenzene are obtained at the side line and the bottom of the column respectively. After further crystallization / recrystallization, the meta-dichlorobenzene product and the ortho-dichlorobenzene product are obtained respectively. Upon detection, the purity of meta-dichlorobenzene is 99.33% and the recovery rate is 92.2%, and the purity of ortho-dichlorobenzene is 99.46% and the recovery rate is 93.8%.

[0107] It should be noted that the above-described embodiments are only used to explain the present invention and do not constitute any limitation to the present invention. The present invention has been described by referring to typical embodiments, but it should be understood that the words used therein are descriptive and explanatory words rather than limiting words. Modifications can be made to the present invention within the scope of the claims of the present invention as stipulated, and revisions can be made to the present invention without departing from the scope and spirit of the present invention. Although the present invention described therein relates to specific methods, materials, and embodiments, it does not mean that the present invention is limited to the specific examples disclosed therein. On the contrary, the present invention can be extended to all other methods and applications with the same functions.

[0108] The preferred embodiments of the present invention have been described in detail above. However, the present invention is not limited thereto. Within the scope of the technical concept of the present invention, various simple modifications can be made to the technical solutions of the present invention, including any other suitable combination of each technical feature. These simple modifications and combinations should also be regarded as the content disclosed by the present invention and fall within the protection scope of the present invention.

Claims

1. A method for separating mixed dichlorobenzene, characterized in that, The method includes: (a) Mixing dichlorobenzene with extractant A and contacting them in an extractive distillation column for extractive distillation separation to obtain crude p-dichlorobenzene from the top of the extractive distillation column; (b) Feeding the bottom material of the extractive distillation column and solvent B into a crystallization and precipitation unit to obtain crystals mainly composed of m-dichlorobenzene and o-dichlorobenzene, as well as a solvent phase of extractant A and solvent B; (c) Feeding the crystals mainly composed of m-dichlorobenzene and o-dichlorobenzene into a distillation column for distillation separation after heating. Separate the crude product mixed with solvent B from the top of the distillation column, side-draw the crude m-dichlorobenzene, and bottom-draw the crude o-dichlorobenzene; The extractant A is a deep eutectic solvent containing a hydrogen bond acceptor and a hydrogen bond donor. Among them, the hydrogen bond acceptor is a quaternary ammonium chloride and / or an imidazole chloride, and their chemical formulas are shown in Formula A and Formula B respectively; Among them, In Formula A and Formula B, R1, R2, and R3 are each hydrogen, C1-C 12 alkyl, C5-C 14 aryl, C6-C 20 aralkyl, R4 is C1-C with or without a hydroxyl group 12 alkyl, C5-C 14 aryl, C6-C 20 aralkyl, and R is nitrogen; The hydrogen bond donor is one or more of carboxylic acid, amide, and urea. The chemical formulas of amide and urea are shown in Formula C and Formula D respectively: Among them, In Formula C and Formula D, R1, R2, R3 and R4 are each independently hydrogen, C1-C 12 alkyl, C5-C 14 aryl, C6-C 20 aralkyl, and in Formula C, at least one of R1, R2 and R3 is hydrogen, and in Formula D, at least one of R1, R2, R3 and R4 is hydrogen; The extractant B is water.

2. The method according to claim 1, wherein The hydrogen bond acceptor is selected from one or more of 1-methyl-3-methylimidazolium chloride, 1-ethyl-3-methylimidazolium chloride, 3-methylimidazolium chloride, choline chloride, tetramethylammonium chloride, tetraethylammonium chloride, and tetrabutylammonium chloride; and / or The hydrogen bond donor is selected from one or more of oxalic acid, citric acid, urea, formamide, acetamide, N-methylformamide, and N-methylacetamide.

3. The method according to claim 1 or 2, wherein The hydrogen bond acceptor is selected from 1-ethyl-3-methylimidazolium chloride and / or choline chloride; and / or The hydrogen bond donor is selected from one or more of oxalic acid, citric acid, urea, and formamide.

4. The method according to claim 1 or 2, wherein The extractant A is one or more of the combinations of choline chloride / oxalic acid, choline chloride / citric acid, choline chloride / urea, choline chloride / formamide, 1-ethyl-3-methylimidazolium chloride / oxalic acid, 1-ethyl-3-methylimidazolium chloride / citric acid, 1-ethyl-3-methylimidazolium chloride / urea, 1-ethyl-3-methylimidazolium chloride / formamide, tetraethylammonium chloride / oxalic acid, tetraethylammonium chloride / citric acid, tetraethylammonium chloride / urea, and tetraethylammonium chloride / formamide.

5. The method according to claim 1 or 2, wherein The mass ratio of the hydrogen bond acceptor to the hydrogen bond donor in the extractant A is 0.2 - 20; and / or The mass flow ratio of the extractant A to the mixed dichlorobenzene is 0.4 - 20; and / or The addition amount of the solvent B is 0.2 - 10 times the amount of the bottom material of the extractive distillation column; and / or The mixed dichlorobenzene contains 20 - 60 wt% of p-dichlorobenzene, 1 - 30 wt% of m-dichlorobenzene, and 20 - 60 wt% of o-dichlorobenzene.

6. The method according to claim 5, wherein The mixed dichlorobenzene contains 30 - 50 wt% of p-dichlorobenzene, 5 - 25 wt% of m-dichlorobenzene, and 30 - 50 wt% of o-dichlorobenzene.

7. The method according to claim 6, wherein The mixed dichlorobenzene contains 40 - 45 wt% of p-dichlorobenzene, 10 - 20 wt%, and 40 - 45 wt%.

8. The method according to claim 1, wherein, The operating conditions of the extractive distillation column include: the reflux ratio is 1 - 20, the operating pressure is 5 kPa - 100 kPa, and the top temperature is 81 °C - 174 °C.

9. The method according to claim 1, wherein The distillation column is a packed distillation column, with a draw-off port provided at the side line position, the reflux ratio is 1 - 20, the operating pressure is 50 kPa - 100 kPa, and the bottom temperature is 155 °C - 180 °C.

10. The method according to claim 1, wherein The method further includes: crystallizing and separating the crude p-dichlorobenzene obtained in step (a) to obtain a p-dichlorobenzene product, and returning the crystallization mother liquor as the raw material of the extractant for the extractive distillation column, with the crystallization temperature controlled within the range of -24 °C - 52 °C; and / or performing vacuum flashing separation on the solvent phase of extractant A and solvent B described in step (b) to obtain extractant A and solvent B; and / or performing freeze crystallization on the crude m-dichlorobenzene and crude o-dichlorobenzene described in step (c) respectively to obtain an m-dichlorobenzene product and an o-dichlorobenzene product.

11. The method according to claim 10, wherein, The method further includes: returning the extractant A obtained by vacuum flashing separation as the raw material of the extractant for the extractive distillation column, and returning the solvent B obtained by vacuum flashing separation as the raw material of the solvent for the crystallization precipitation unit.

12. According to the method of claim 10, wherein, the vacuum flashing is carried out in a vacuum flash tank, and the operating pressure of the vacuum flash tank is 0.5 kPa - 50 kPa.

13. The method according to claim 10, wherein The method further includes: returning the entrained solvent B for vacuum flashing.

14. The method according to claim 10, wherein The method further includes: returning the mother liquor and / or crystals except the product obtained by freeze crystallization separation in each step to the distillation column for distillation separation.

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