Separation and purification method for byproduct benzyl benzoate in process for preparing benzoic acid through toluene oxidation

Abstract

The invention relates to the technical field of benzyl benzoate preparation, and discloses a separation and purification method of benzyl benzoate as a byproduct of a process for preparing benzoic acid through toluene oxidation, the separation and purification method comprises distillation, rectification and melt crystallization, and the melt crystallization comprises primary crystallization and secondary crystallization. According to the scheme, kettle residues and tar in benzyl benzoate are removed through distillation, benzyl benzoate with the raw material external standard content of about 46% is purified to about 72%, the product content is increased to about 92% through rectification, and then the content is increased to 99% through primary crystallization, so that the purity of benzyl benzoate is improved. And finally, 99% of the product is subjected to secondary crystallization to obtain a perfume-grade product with the content being greater than or equal to 99.9% and the chromaticity being less than or equal to 20 (platinum cobalt), and the method has very high popularization and application value in the industry.

Description

Technical Field

[0001] The invention relates to the technical field of benzylic acid benzoate preparation, and in particular to a method for separating and purifying benzylic acid benzoate produced as a byproduct in a process of preparing benzoic acid by oxidizing toluene. Background Art

[0002] Benzyl Benzoate is a transparent, colorless crystalline or oily liquid with a faint aroma. It is widely used in the fragrance industry, mainly as a solvent for artificial musk, vanillin and other fragrances. It is also a fixative for floral flavors and a blending fragrance for flavors such as ylang-ylang.

[0003] At present, the industrial product of benzyl benzoate is mainly synthesized by the reaction of sodium benzoate and benzyl chloride. However, this method has two main problems: first, benzyl chloride has a strong pungent smell, and second, the chlorine element it contains cannot meet the production requirements of fragrance-grade benzyl benzoate. Therefore, this method is not suitable for the production of high-quality benzyl benzoate. Another production process is to use toluene as a raw material to produce benzoic acid by oxidation. In this process, a certain amount of benzyl benzoate will be produced as a by-product. Due to the high boiling point of benzyl benzoate, it will remain in the distillation residue of benzoic acid during the distillation of benzoic acid. According to statistics, the amount of benzoic acid distillation residue accounts for about 5% to 7% of the total production of benzoic acid, and its main components include benzoic acid, benzyl benzoate, catalyst cobalt salt and manganese salt, fluorenone, etc. It is worth noting that the content of benzyl benzoate in the distillation residue can reach 50% to 70%. Since benzyl benzoate, a by-product of benzoic acid produced by the toluene oxidation process, does not contain chlorine, it has a natural advantage as a fragrance-grade use.

[0004] However, the components in the benzoic acid distillation residue are complex, containing many substances similar to the structure and physical properties of benzyl benzoate, and the catalysts (such as cobalt salts and manganese salts) used in the toluene oxidation process are also present in the distillation residue, and these catalysts can continuously decompose low-boiling substances such as benzoic acid when heated. This makes it extremely difficult to directly extract high-purity benzyl benzoate from the benzoic acid distillation residue. Therefore, it is of great significance to develop a separation and purification method that can extract high-purity benzyl benzoate from the benzoic acid distillation residue. Summary of the invention

[0005] The present invention aims to provide a method for separating and purifying benzyl benzoate produced as a by-product in the process of preparing benzoic acid by oxidizing toluene, and aims to extract high-purity benzyl benzoate from the residual liquid of benzoic acid distillation.

[0006] To achieve the above-mentioned purpose, the present invention adopts the following technical scheme: a method for separating and purifying benzyl benzoate as a by-product of the process of preparing benzoic acid by oxidation of toluene, comprising distillation, rectification and melt crystallization performed in sequence, and the melt crystallization step includes primary crystallization and secondary crystallization.

[0007] Preferably, as an improvement, the distillation stage is negative pressure distillation, the heating temperature is 190-220°C, and the vacuum degree is 150pa.

[0008] Preferably, as an improvement, the distillation stage is negative pressure distillation, the negative pressure distillation stage is collected in sections, the distillation heating temperature is 162-170°C, the top temperature of the distillation tower is 118-122°C, and the vacuum degree is 170-600 Pa.

[0009] In this technical solution, the top temperature of the distillation tower refers to the gas temperature.

[0010] Preferably, as an improvement, when the negative pressure distillation is collected for the first time, when the top temperature of the distillation tower reaches 122° C., full reflux is started, and the reflux time is 20 min.

[0011] Preferably, as an improvement, during the secondary collection of negative pressure distillation, when the top temperature of the distillation tower reaches 134°C, full reflux is started, the reflux time is 10 minutes, and the reflux ratio is 3:1 to 5:1.

[0012] Preferably, as an improvement, in the primary crystallization stage, the internal temperature is controlled at 12-14° C. and the crystallization time is 3-4 h.

[0013] Preferably, as an improvement, during the sweating stage of the primary crystallization, the internal temperature is controlled at 15-16° C. and the sweating time is 9-10 h.

[0014] Preferably, as an improvement, in the secondary crystallization stage, the internal temperature is controlled at 12-14° C. and the crystallization time is 3-4 h.

[0015] Preferably, as an improvement, during the sweating stage of the secondary crystallization, the internal temperature is controlled at 19-21° C. and the sweating time is 3-4 hours.

[0016] Preferably, as an improvement, a material chemical step is further included after sweating, and the temperature of the chemical materials for primary crystallization and secondary crystallization is 25°C.

[0017] The principle and advantage of this scheme are: in actual application, the components in the benzoic acid rectification residue are complex, containing many substances similar to the structure and physical properties of benzyl benzoate, and because the catalyst of toluene oxidation also remains in the rectification residue, the substance in the benzoic acid rectification residue will be continuously decomposed into low-boiling substances such as benzoic acid when heated. Therefore, it is difficult to directly extract high-purity benzyl benzoate from the benzoic acid rectification residue, and there are also few related reports about extracting high-purity benzyl benzoate from the residue in the prior art. Based on this, in the technical scheme, for the problem of difficult operation of extracting high-purity benzyl benzoate from the benzoic acid rectification residue in the prior art, the purification process of benzyl benzoate has been comprehensively developed and upgraded. In the technical research and development stage, the inventor has tried to directly purify by the method of combining distillation with rectification, but still cannot obtain a high-purity product. Finally, after a long period of continuous research and development and verification, distillation, rectification and two-stage static crystallization operations were used to remove the kettle residue and tar in benzyl benzoate by distillation, and the benzyl benzoate with an external standard content of about 46% in the raw material was purified to about 72%, and then the product content was increased to about 92% by distillation, and then the content was increased to 99% through primary crystallization. The 99% product was then subjected to secondary crystallization to obtain a fragrance-grade product with a content ≥99.9% and a chroma ≤20 (platinum cobalt). BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Figure 1 It is the gas chromatogram of the benzyl benzoate crystalline product of the present invention.

[0019] Figure 2 This is a diagram of the raw material of benzyl benzoate in the embodiment of the present invention (before purification).

[0020] Figure 3 This is a picture of the benzyl benzoate crystal product of the present invention.

[0021] Figure 4 This is a picture of the stirred crystallization product of benzyl benzoate of the present invention.

[0022] Figure 5 This is a diagram of the finished product of direct distillation of benzyl benzoate of the present invention. DETAILED DESCRIPTION

[0023] The following is further described in detail through specific implementations, but the implementation of the present invention is not limited thereto. Unless otherwise specified, the technical means used in the following implementations are conventional means well known to those skilled in the art; the experimental methods used are all conventional methods; the materials, reagents, etc. used can all be obtained from commercial channels.

[0024] Example 1

[0025] A method for separating and purifying benzyl benzoate produced as a by-product in a process for preparing benzoic acid by oxidizing toluene, comprising the following steps:

[0026] Step 1: Distillation:

[0027] ① Add crude benzyl benzoate to the distillation device, turn on the magnetic heating jacket to start heating, and at the same time turn on the vacuum pump system to start pumping negative pressure. The heating temperature is 190-200°C and the vacuum is 150pa. In this stage, the light group of benzoic acid is mainly collected;

[0028] ② When no fraction is distilled out, gradually increase the temperature of the magnetic heating mantle to 220°C, vacuum 150Pa, collect benzyl benzoate, and stop distillation when no fraction is distilled out.

[0029] Step 2: Distillation: The distillation stage is negative pressure distillation, combined with segmented collection. The specific steps are as follows:

[0030] ① Add the benzyl benzoate distillation product obtained in step 1 into the distillation device, turn on the magnetic heating jacket to start heating, and evacuate at the same time. The heating temperature in the pre-distillation stage is 162-170° C., the top temperature (gas temperature) of the distillation tower is 118-122° C., and the pressure is 170-600 Pa;

[0031] ② When the top temperature of the distillation tower (gas temperature) reaches 122°C, white liquid begins to condense at the top and full reflux begins. After reflux for 20 minutes, the distillation is stopped, the cooling temperature is set to 120°C, and all the benzoic acid on the distillation head is melted and extracted.

[0032] ③ Restart negative pressure distillation. When the top temperature (gas temperature) of the distillation tower reaches 134°C, light yellow liquid begins to condense on the top and full reflux begins. After reflux for 10 minutes, start to extract while reflux. During the distillation process, adjust the reflux ratio to 3:1-5:1.

[0033] ④ Gradually increase the temperature of the magnetic heating jacket to 260℃ and continue to extract. When no fraction is distilled out, stop the distillation.

[0034] Step 3: Primary crystallization:

[0035] ① Feeding: feeding the benzyl benzoate distillation product obtained in step 2 into a crystallizer.

[0036] ②Crystallization: First open the jacket circulating liquid, lower the temperature of the feed liquid, add seed crystals when the internal temperature reaches 15℃, start crystal growth at the internal temperature of 11.5℃, slowly cool down, control the internal temperature at 12-14℃, crystallize for 3-4h, and then discharge the mother liquor.

[0037] ③ Sweating: Increase the temperature of the circulating fluid appropriately, control the internal temperature at 15-16°C, slowly increase the temperature to induce sweating, and sweat for 9-10 hours.

[0038] ④Chemical processing: Increase the circulating liquid temperature to 25℃ to melt the product.

[0039] Step 4: Secondary crystallization:

[0040] ① Feeding: The primary crystalline product of benzyl benzoate (≥99%) obtained in step 3 is fed into a crystallizer.

[0041] ②Crystallization: First open the jacket circulating liquid, lower the feed liquid temperature, add seed crystals when the internal temperature reaches 15°C, start crystal growth at an internal temperature of 12.2°C, slowly cool down, control the internal temperature at 12-14°C, crystallize for 3-4 hours, and then discharge the mother liquor.

[0042] ③ Sweating: Increase the temperature of the circulating fluid appropriately, control the internal temperature at 19-21°C, slowly increase the temperature to induce sweating, and sweat for 3-4 hours.

[0043] ④Chemical processing: Increase the circulating liquid temperature to 25℃ to melt the product.

[0044] Example 1: Data analysis of samples at each stage

[0045] 1. The fractions and still residues in each distillation stage were weighed and sampled for analysis. The experimental results are shown in Table 1.

[0046] Table 1

[0047]

[0048] From the data in Table 1, it can be seen that in the distillation stage of the technical scheme, the crude benzyl benzoate distillation mass yield is 64.12%, the kettle residue is 34.78%, and the percent yield is 99.09%.

[0049] 2. The fractions and still residues in each distillation stage were weighed and sampled for analysis. The results are shown in Table 2.

[0050] Table 2

[0051]

[0052]

[0053] From the data in Table 2, it can be seen that the mass yield of the distillation fraction of benzyl benzoate is 92.14%, the still residue is 7.17%, and the percent yield is 98.96%; the total percent yield of distillation + rectification is 98.06%.

[0054] 3. Data analysis was performed on the mother liquor, sweat solution and product in the primary crystallization stage. The results are shown in Table 3.

[0055] Table 3

[0056] Serial number Sample name Weight(g) Proportion (%) Purity (GC) / % 1 Crude distillation product 459.5 100％ 91.96％ 2 Primary mother liquor 96.50 21％ 75.16％ 3 One-time sweat 82.71 18％ 91.92％ 4 One-time product 275.7 60％ 99.01％

[0057] 4. Data analysis was performed on the mother liquor, sweat solution and product in the secondary crystallization stage. The results are shown in Table 4.

[0058] Table 4

[0059] Serial number Sample name Weight(g) Proportion (%) Purity (GC) / % 1 One-time product 275.7 100％ 99.15％ 2 Secondary mother liquor 41.355 15％ 92.79％ 3 Secondary sweat solution 17.9205 6.5％ 99.17％ 4 Secondary products 215.046 78％ 99.94％

[0060] From the results in Tables 3 and 4 above, it can be seen that 91.96% of the crude benzyl benzoate can obtain more than 99% of the product after primary crystallization, with a single-pass yield of 60%; after secondary crystallization, more than 99.90% of the product can be obtained, with a single-pass yield of 78%; after primary crystallization, the mother liquor is returned to the distillation process, the primary crystallization sweating liquid is returned to the primary crystallization, and the primary crystallization product goes to the secondary crystallization; the secondary crystallization mother liquor is applied to the primary crystallization, the secondary crystallization sweating liquid is returned to the secondary crystallization, and the secondary crystallization product is the final product (purity 99.94%, meeting the requirements of fragrance-grade products), and the entire crystallization system has a calculated cycle yield of >98%.

[0061] Example 1 Gas Chromatography Analysis

[0062] The benzyl benzoate product (secondary crystal product) was subjected to gas chromatography analysis to obtain a gas phase spectrum of the product as shown in FIG. Figure 1 As shown in FIG. 1 , the absorption peak of benzyl benzoate appears at 12.151 min, with a peak area of ​​99.94%. In addition, the product diagrams of benzyl benzoate before and after purification are shown in FIG. Figure 2 , Figure 3 As shown: Before purification, the raw material of the product is a black viscous liquid containing a lot of insoluble matter ( Figure 2 ); after distillation, rectification and crystallization, a colorless liquid is obtained ( Figure 3 ), purity ≥99.9%, chromaticity ≤20 (platinum-cobalt color number).

[0063] Comparative Example 1

[0064] In this comparative example, steps 1 and 2 are the same as those in Example 1, and dynamic crystallization is subsequently used instead of static crystallization. The specific steps are as follows:

[0065] Step 1, distillation: (same as Example 1)

[0066] ① Add crude benzyl benzoate to the distillation device, turn on the magnetic heating jacket to start heating, and at the same time turn on the vacuum pump system to start pumping negative pressure. The heating temperature is 190-200°C and the vacuum is 150pa. In this stage, the light component benzoic acid is mainly collected;

[0067] ② When no fraction is distilled out, gradually increase the temperature of the magnetic heating mantle to 220°C, vacuum 150Pa, collect benzyl benzoate, and stop distillation when no fraction is distilled out.

[0068] Step 2: Distillation:

[0069] ① Add the benzyl benzoate distillation product obtained in step 1 into the distillation device, turn on the magnetic heating jacket to start heating, and evacuate at the same time. The heating temperature in the pre-distillation stage is 162-170° C., the top temperature (gas temperature) of the distillation tower is 118-122° C., and the pressure is 170-600 Pa;

[0070] ② When the top temperature of the distillation tower (gas temperature) reaches 122°C, white liquid begins to condense at the top and full reflux begins. After reflux for 20 minutes, the distillation is stopped, the cooling temperature is set to 120°C, and all the benzoic acid on the distillation head is melted and extracted.

[0071] ③ Restart negative pressure distillation. When the top temperature (gas temperature) of the distillation tower reaches 134°C, light yellow liquid begins to condense on the top and full reflux begins. After reflux for 10 minutes, start to extract while reflux. During the distillation process, adjust the reflux ratio to 3:1-5:1.

[0072] ④ Gradually increase the temperature of the magnetic heating jacket to 260℃ and continue to extract. When no fraction is distilled out, stop the distillation.

[0073] Step 3: Primary dynamic crystallization:

[0074] ① Put the benzyl benzoate distillation product obtained in step 2 into the crystallizer, turn on the agitator and the high and low temperature integrated machine, and set the medium temperature to 10°C.

[0075] ②Crystallization: When the internal temperature reaches 15℃, add seed crystals. Crystals begin to precipitate after 15 minutes. Control the internal temperature at 12-14℃ and crystallize for 0.5h.

[0076] ③ Filtration: Adjust the temperature of the funnel jacket to 12°C, pump the liquid in the crystallizer into the funnel, maintain the internal temperature at 13°C and filter under reduced pressure.

[0077] ④Melting materials: Raise the medium temperature of the funnel jacket to 25°C to melt the product.

[0078] Step 4: Secondary dynamic crystallization:

[0079] ① Feeding: The first-level dynamic crystallization product (≥95%) of benzyl benzoate obtained in step 3 is fed into a crystallizer.

[0080] ②Crystallization: When the internal temperature reaches 15℃, add seed crystals. Crystals will begin to precipitate after 15 minutes. Control the internal temperature at 15-16℃ and allow crystallization to take 1 hour.

[0081] ③ Filtration: Adjust the temperature of the funnel jacket to 14°C, pump the liquid in the crystallizer into the funnel, maintain the internal temperature at 15°C and filter under reduced pressure.

[0082] ④Melting materials: Raise the medium temperature of the funnel jacket to 25°C to melt the product.

[0083] Step 5: Three-level dynamic crystallization:

[0084] ① Feeding: The secondary dynamic crystallization product of benzyl benzoate (≥99%) obtained in step 4 is fed into a crystallizer.

[0085] ②Crystallization: When the internal temperature reaches 15℃, add seed crystals. Crystals begin to precipitate after 15 minutes. Control the internal temperature at 17-18℃ and allow crystallization to take 2 hours.

[0086] ③ Filtration: Adjust the temperature of the funnel jacket to 16°C, pump the liquid in the crystallizer into the funnel, maintain the internal temperature at 17°C and filter under reduced pressure.

[0087] ④Melting materials: Raise the medium temperature of the funnel jacket to 25°C to melt the product.

[0088] The dynamic crystallization results of Comparative Example 1 are as follows:

[0089] Table 5 Primary dynamic crystallization

[0090] Serial number Material Name Weight(g) content(%) Proportion (%) 1 Crude distillation product 1542.1 91.96％ 100％ 2 First-stage crystallization mother liquor 747.92 86.80％ 48.50％ 3 First-class crystal products 778.76 95.41％ 50.50％

[0091] Table 6 Secondary dynamic crystallization

[0092]

[0093]

[0094] Table 7 Three-stage dynamic crystallization

[0095] Serial number Material Name Weight(g) content(%) Proportion (%) 1 Secondary crystallization products 379.96 99.12％ 100％ 2 Tertiary crystallization mother liquor 200.24 98.10％ 52.70％ 3 Three-stage crystallization product 177.82 99.91％ 46.80％

[0096] From the data in Tables 5 to 7, it can be seen that in this comparative example, dynamic crystallization is adopted. After one dynamic crystallization, 91.96% of the crude benzyl benzoate can obtain more than 95% of the product, and the single-pass yield is 50.50%; after two dynamic crystallizations, more than 99% of the product can be obtained, and the single-pass yield is 48.8%; after three dynamic crystallizations, more than 99.9% of the product can be obtained, and the single-pass yield is 46.80%. The chromaticity of the product is still 200, and the chromaticity cannot meet the standard.

[0097] Compared with the static crystallization in Example 1, the single-pass cycle of dynamic crystallization is shorter, but three stages of crystallization are required to obtain qualified products, the yield is low and the product color does not meet the standard.

[0098] Comparative Example 2

[0099] The steps in this comparative example are the same as those in Example 1. After distillation to obtain 71.82% of the crude product, rectification is performed to observe whether it can be directly distilled to 99.9% purity, and the chroma is ≤20.

[0100] Step 1, distillation: (same as Example 1)

[0101] ① Add crude benzyl benzoate to the distillation device, turn on the magnetic heating jacket to start heating, and at the same time turn on the vacuum pump system to start pumping negative pressure. The heating temperature is 190-200°C and the vacuum is 150pa. In this stage, the light group of benzoic acid is mainly collected;

[0102] ② When no fraction is distilled out, gradually increase the temperature of the magnetic heating mantle to 220°C, vacuum 150Pa, collect benzyl benzoate, and stop distillation when no fraction is distilled out.

[0103] Step 2: Distillation:

[0104] ① Add 484 g (content 71.82%) of the distillate obtained in step 1, turn on the magnetic heating jacket to start heating, and at the same time turn on the vacuum pump system to start vacuuming.

[0105] ② When the top temperature of the distillation tower (gas temperature, Tgas) reaches 122°C, white liquid begins to condense on the top and full reflux begins. After reflux for 20 minutes, the distillation is stopped and the cooling temperature (Tcold) is set to 120°C to melt and extract all the benzoic acid on the distillation head.

[0106] ③ Restart negative pressure distillation. When the top temperature of the distillation tower (gas temperature, Tgas) reaches 134.7℃, light yellow liquid begins to condense on the top and full reflux is started. After reflux for 30 minutes, extraction is started at a ratio of 10:1.

[0107] ④ After 16 hours of distillation, the reflux ratio was adjusted to 20:1 for extraction because the content of two impurities in the fraction did not change much. After continuing the distillation for 13 hours, the distillation was stopped because the product was still unqualified.

[0108] ⑤Weigh each distillate and still residue and take samples for analysis.

[0109] Comparative Example 2 Experimental Data Table:

[0110] Table 8

[0111]

[0112] Table 9 Detailed analysis data table

[0113]

[0114] From the data in Tables 8 and 9, it can be seen that even at a relatively large reflux ratio, the product still cannot reach 99.9% when 71.82% of the product is directly distilled, and the maximum content is only 94.91%, and the mass yield is 43.97%, which is also relatively low. There are 5 impurities within the retention time of 14.7 to 18.08 minutes in GC, and the separation effect of the two impurities with retention times of 15.07 minutes and 16.0 minutes is poor.

[0115] Comparative Example 3

[0116] In this comparative example, the fractions with a content of ≥90% obtained in comparative example 2 were mixed and then distilled again. The specific steps are as follows:

[0117] ① Add 140 g (90.78%) of the mixed distillation fraction of Comparative Example 2, turn on the magnetic heating jacket to start heating, and at the same time turn on the vacuum pump system to start pumping negative pressure.

[0118] ② When the top temperature of the distillation tower (gas temperature, Tgas) reaches 130°C, light yellow liquid begins to condense on the top and full reflux begins. After reflux for 40 minutes, extraction begins while refluxing.

[0119] ③ When no fraction is distilled out, gradually increase the temperature of the magnetic heating jacket to 260℃, and stop distillation when no fraction flows out.

[0120] ④Weigh each distillate and still residue and take samples for analysis.

[0121] Experimental results and analysis data table of comparative example 3:

[0122] Table 10

[0123]

[0124] Table 11 Detailed analysis data table

[0125]

[0126]

[0127] The results in Table 10-11 show that the maximum content of the 90.78% distillation fraction can reach 96.69% after distillation again, which is still difficult to reach 99.9%. The distillation yield is 66.0%, and the distillation product is yellow. There are 5 impurity peaks within the retention time of 14.7-18.08min, and the two impurities at retention times of 15min and 16min are still difficult to separate. In summary, it is impossible to directly obtain a product with a content of ≥99.9% and a color that meets the requirements through distillation + distillation.

[0128] In order to analyze and compare the quality of products obtained by different methods, the chromaticity and purity of the products prepared in Example 1 and Comparative Examples 1-3 were compared. The results are shown in Table 12 and Table 13. Figure 4-5As shown in the figure: the black viscous benzyl benzoate raw material (content 46.48%) can obtain a fragrance-grade product with a content of ≥99.9% and a chromaticity of ≤20 (platinum cobalt) after distillation, rectification and two-stage static crystallization. However, using dynamic crystallization instead of static crystallization, although the content can reach more than 99.9%, the product color is light yellow and the chromaticity does not meet the standard. Using distillation + rectification operation to process benzyl benzoate, even if the number of rectifications is increased, the content of the obtained product still cannot reach more than 99.9%, and the chromaticity does not meet the standard.

[0129] Table 12 Comparison of chromaticity and purity of products obtained by different methods

[0130] Serial number color Chroma(Platinum-Cobalt) content(%) raw material Black sticky >500 (exceeds the upper limit of platinum and cobalt) 46.48％ Example 1 Product colorless 12 99.94％ Comparative Example 1 Product Light yellow 180 99.91％ Comparative Example 2 Product yellow 300 94.91％ Comparative Example 3 Product yellow 220 96.69％

[0131] The above is only an embodiment of the present invention, and the common knowledge such as the known specific technical solutions and / or characteristics in the solution is not described in detail here. It should be pointed out that for those skilled in the art, without departing from the technical solution of the present invention, several modifications and improvements can be made, which should also be regarded as the protection scope of the present invention, and these will not affect the effect of the implementation of the present invention and the practicality of the patent. The scope of protection required by this application shall be based on the content of its claims, and the specific implementation methods and other records in the specification can be used to interpret the content of the claims.

Claims

1. A method for separating and purifying benzyl benzoate produced as a by-product in the process of preparing benzoic acid by oxidation of toluene, characterized in that: The method comprises distillation, rectification and melt crystallization performed in sequence, wherein the melt crystallization step comprises primary crystallization and secondary crystallization.

2. The method for separating and purifying the by-product benzyl benzoate produced by the process of preparing benzoic acid by oxidation of toluene according to claim 1, characterized in that: The distillation stage is negative pressure distillation, the heating temperature is 190-220° C., and the vacuum degree is 150 Pa.

3. The method for separating and purifying the by-product benzyl benzoate produced by the process of preparing benzoic acid by oxidation of toluene according to claim 2, characterized in that: The distillation stage is negative pressure distillation, and the negative pressure distillation stage is collected in sections. The distillation heating temperature is 162-170° C., the top temperature of the distillation tower is 118-122° C., and the vacuum degree is 170-600 Pa.

4. The method for separating and purifying the by-product benzyl benzoate from the process of preparing benzoic acid by oxidation of toluene according to claim 3, characterized in that: When the negative pressure distillation was first collected, when the top temperature of the distillation tower reached 122°C, full reflux was started and the reflux time was 20 min.

5. The method for separating and purifying benzyl benzoate produced as a by-product of a process for preparing benzoic acid by oxidation of toluene according to claim 4, characterized in that: During the secondary collection of negative pressure distillation, when the top temperature of the distillation tower reaches 134°C, full reflux begins, the reflux time is 10 minutes, and the reflux ratio is 3:1 to 5:

1.

6. The method for separating and purifying benzyl benzoate produced as a by-product in the process of preparing benzoic acid by oxidation of toluene according to claim 5, characterized in that: In the primary crystallization stage, the internal temperature is controlled at 12-14°C and the crystallization time is 3-4 hours.

7. The method for separating and purifying benzyl benzoate produced as a by-product of a process for preparing benzoic acid by oxidation of toluene according to claim 6, characterized in that: During the sweating stage of primary crystallization, the internal temperature is controlled at 15-16°C and the sweating time is 9-10 hours.

8. The method for separating and purifying benzyl benzoate produced as a by-product in the process of preparing benzoic acid by oxidation of toluene according to claim 7, characterized in that: In the secondary crystallization stage, the internal temperature is controlled at 12-14°C and the crystallization time is 3-4h.

9. The method for separating and purifying benzyl benzoate produced as a by-product of a process for preparing benzoic acid by oxidation of toluene according to claim 8, characterized in that: During the sweating stage of secondary crystallization, the internal temperature is controlled at 19-21°C and the sweating time is 3-4 hours.

10. The method for separating and purifying benzyl benzoate produced as a by-product in the process of preparing benzoic acid by oxidation of toluene according to claim 9, characterized in that: After sweating, the process also includes a material transformation step, and the temperature of the materials for the primary crystallization and the secondary crystallization is both 25°C.

Images