A vacuum crystallization purification process of menthyl benzoate

Abstract

This invention discloses a vacuum crystallization purification process for menthyl benzoate, comprising the following steps: S100, feeding; S200, cooling; S300, centrifugation. The parameter control process in step S200, cooling, specifically includes the crystallization temperature range, vacuum degree, methanol reflux flow rate, reflux methanol temperature, condenser medium, temperature rebound time, two-stage slow cooling method, multi-stage cooling method, and purity control of the menthyl benzoate raw material. This invention utilizes a rapid cooling method involving vacuum absorption and methanol reflux. Through precise temperature control, the system enters a transient stable state of crystallization before crystallization, where supersaturation and dissolution saturation are in equilibrium. Through two-stage slow cooling and multi-stage cooling stages, the system gradually enters a supersaturated solution state. Due to the precise control of the saturation-supersaturation-crystallization state, impurities cannot crystallize, thus achieving deep purification and obtaining a high-purity crystalline product.

Description

Technical Field

[0001] This invention relates to the field of fine chemical product purification technology, and in particular to a vacuum crystallization purification process for menthyl benzoate. Background Technology

[0002] Menthyl benzoate is an important intermediate in the synthesis of L-menthol. The conventional purification method is distillation. However, due to the high boiling point of menthyl benzoate, it decomposes during high-temperature distillation. Therefore, it is difficult to achieve a high content of over 99.5% through distillation, and distillation also consumes a lot of energy, resulting in high production costs.

[0003] To avoid product loss due to high-temperature decomposition of menthyl benzoate, some factories use vacuum crystallization to purify it. Vacuum crystallization is a physical process that separates the solute and solvent in a solution by crystallizing under reduced pressure. The operating principle involves adding a heated saturated solution to a crystallizer, which is a sealed container insulated with heat-insulating material, maintaining a vacuum inside and being insulated from the outside environment.

[0004] However, in existing vacuum crystallization purification processes, the addition of the solution necessitates flash evaporation followed by adiabatic cooling to an equilibrium temperature corresponding to the internal pressure. This means that supersaturated crystallization is a result of the simultaneous action of adiabatic evaporation of the solvent and vaporization cooling of the solution. Consequently, crystallization process parameters are difficult to control. Even in a supersaturated crystallization state, a small amount of impurities also crystallizes, resulting in a purity of less than 99% or even lower for the produced menthyl benzoate, along with a low yield. Improvements are urgently needed. Summary of the Invention

[0005] In order to overcome the shortcomings of the prior art, this invention attempts to reduce the possibility of impurity crystallization through more precise temperature control, and thus proposes a vacuum crystallization purification process for menthyl benzoate.

[0006] To achieve the above objectives, the present invention adopts the following technical solution:

[0007] A vacuum crystallization purification process for menthyl benzoate includes the following steps:

[0008] S100, Feeding:

[0009] Menthyl benzoate and methanol were fed into the crystallization vessel at a mass ratio of 6:4, and stirring was started after feeding.

[0010] S200, Cooling:

[0011] Open the inlet and outlet valves of the jacket cooling water and control the temperature for rapid cooling through the circulating water regulating valve;

[0012] When the temperature inside the crystallizer is 3°C higher than the circulating water temperature, it is difficult to cool down. Close the circulating water regulating valve, open the chilled water valves of the first-stage condenser and the second-stage condenser, and at the same time start the vacuum pump of the vacuum system. Slowly adjust the vacuum degree from 0 to -50KPa, and cool down by refluxing methanol through the condenser.

[0013] Continue cooling until the temperature is 3°C above the crystallization temperature (i.e., T). c (+3℃), the rate of temperature drop decreases;

[0014] Continue adjusting the vacuum level, observe the amount of refluxed methanol, and raise the temperature to 2°C above the crystallization temperature (i.e., T). c After +2℃, add 100g of seed crystals, adding once for every 1℃ decrease. Stop adding seed crystals after observing the turbidity in the reactor or the temperature rebound, and reduce the stirring speed to 60% of the stirring speed in step S100.

[0015] During the temperature rebound, the temperature was kept constant for 2 hours. After the rebound ended, the temperature was decreased at a rate of 0.5°C per hour for the first 2 hours, and then at a rate of 1°C per hour until the temperature reached the crystallization temperature (i.e., T). c Maintain this temperature for crystallization;

[0016] S300, centrifugation:

[0017] When the temperature of the crystallizer is adjusted to the discharge temperature, prepare to discharge the material; shut off the vacuum system, open the nitrogen shut-off valve to break the vacuum in the vessel and maintain a slight positive pressure of 20 kPa, open the bottom valve of the crystallizer, and enter the centrifuge for centrifugation;

[0018] The mother liquor obtained from centrifugation enters the mother liquor storage tank, and the centrifuged crystals are melted in the melting tank and then enter the product tank.

[0019] Preferably, the stirring speed in step S100 is 50-60 r / min. At this speed, dissolution and heat exchange are easier, and the disturbance to the crystallization rate is smaller.

[0020] Preferably, in the crystallization vessel of step S100, the dissolution of menthyl benzoate and methanol is carried out by heating the heat transfer wall with 0.4 MPa steam, and in the melting vessel of step S300, the melting of menthyl benzoate is carried out by heating the heat transfer wall with 0.4 MPa steam.

[0021] Preferably, the seed crystal in step S200 is menthyl benzoate crystal with a purity greater than 99.8%.

[0022] Preferably, the centrifuged crystals obtained in step S300 are washed with methanol produced by a methanol washing tank, and the washing liquid is combined with the crystallization mother liquor and then enters the crystallization mother liquor storage tank.

[0023] Preferably, the discharge temperature in step S300 is the crystallization temperature ±0.3℃. This discharge temperature ensures the crystallization yield without affecting the purity.

[0024] Preferably, the product tank in step S300 is subjected to a purity test. If the purity is less than 99%, it can be transferred to the next stage crystallization reactor for secondary crystallization.

[0025] Preferably, the parameter control process in step S200, which involves cooling, specifically includes the following steps:

[0026] S201. Based on the solubility properties of menthyl benzoate and methanol, take menthyl benzoate raw material with a purity greater than 90%, and follow step S100 with a mass ratio of menthyl benzoate to methanol of 6:4. According to the optimal crystallization temperature of the saturated methanol solution of menthyl benzoate, the crystallization temperature can be preliminarily determined to be 10-12℃, which is the initial calibration crystallization temperature range of the temperature control of this invention.

[0027] S202. By controlling the vacuum degree, methanol is vaporized to promote the production of supersaturated solution. The vaporization absorbs heat and removes heat from the crystallization vessel. The vaporized methanol is processed by a primary condenser and a secondary condenser to obtain condensed reflux methanol. The quality and temperature of the reflux methanol affect the cooling rate. Based on the requirements of the cooling rate and production speed, it is initially determined that the methanol reflux flow rate is controlled at 5-15% of the initial methanol input, and the reflux methanol temperature is controlled at 8-13℃. Under these conditions, the cooling adjustment response is relatively rapid, thus the initial vacuum degree is determined to be -50KPa (i.e. 0.05MPa).

[0028] S203. After adding seed crystals, crystals gradually form as the temperature decreases. Crystallization releases heat (chemical entropy energy is converted into heat energy), causing a rapid increase in system temperature. The crystalline product dissolves again, and dissolution absorbs heat (heat energy is converted into chemical entropy energy), causing the system temperature to decrease. The continuous cooling of the refluxed methanol offsets the heat released by crystallization. At this point, the vacuum level is adjusted to match the cooling rate of the refluxed methanol with the alternating frequency of crystallization release and dissolution absorption. The vacuum level is determined to be -39 kPa, and the methanol reflux flow rate is controlled at 7.8% of the initial methanol input (taking the liquid level in the condenser at a specific instant). The total amount of methanol is controlled at 11.5-13.5℃, while the reflux methanol temperature can be controlled at 11.5-13.5℃. Therefore, both the primary and secondary condensers use 10℃ chilled water as the heat exchange medium for the heat transfer walls. The heat released during condensation is carried away by the heat exchange medium. During the heat exchange process of reflux methanol, the continuously generated heat released during crystallization is consumed (the peak height of the Tt curve decreases), and the heat absorbed during dissolution is also offset (the valley depth of the Tt curve decreases). From the Tt curve of the system temperature versus time, the temperature rebound curve gradually becomes flat, indicating that the system has entered a state of equilibrium between supersaturated crystallization and dissolution saturation, that is, the crystallization temporary steady state.

[0029] S204. Based on the requirements of production time and crystallization temperature, within the crystallization temperature range of 10-12℃, as the preset crystallization temperature decreases (e.g., preset T...), c As the temperature of the seed crystal is reduced from 12℃ to 10℃, the crystallization tendency within the system increases (chemical entropy energy increases), and the slope of the continuously generated exothermic crystallization peak and the resulting endothermic dissolution valley both increase. In other words, the process of the temperature rebound curve gradually flattening out is prolonged. Therefore, the crystallization temperature range is determined to be 10.5-11.5℃, and the set temperature rebound time is 2h. Since stirring disturbs the crystallization, the stirring speed is reduced to 60% of the initial stirring speed.

[0030] S205. Maintaining the vacuum level at -39 kPa, continue to lower the temperature of the reflux methanol to 10.5 ± 0.5 °C, entering the second stage of slow cooling. The system state gradually changes from the transient stable state of crystallization to the supersaturated solution state. Set the temperature to decrease by 0.5 °C every 1 hour, for a total decrease of 1 °C.

[0031] S206. Increase the vacuum level, continue to lower the temperature of the reflux methanol, and increase the methanol reflux flow rate to accelerate the cooling rate. Enter a multi-stage cooling stage, setting the temperature to decrease by 1℃ every 1 hour until the temperature reaches 10.5-11.5℃. Maintain this temperature for crystallization, and continuously increase the vacuum level and lower the temperature of the reflux methanol to 10.3±0.2℃ to keep the system crystallizing at this temperature.

[0032] S207. Take menthol benzoate raw material with a purity of less than 90%. After the experiments in S201-206, as the purity of menthol benzoate decreases, the possibility of impurity crystallization can no longer be offset by parameter control. The purity of the obtained crystalline product cannot reach 99%. Therefore, menthol benzoate with a purity of more than 90% is selected as the crystallization raw material.

[0033] Furthermore, in step S207, the menthol benzoate raw material with a purity of less than 90% undergoes differentiated processing. The menthol benzoate raw material with a purity of less than 70% needs to be purified by adsorption or vacuum distillation. The menthol benzoate raw material with a purity of 70-90% can have its purity increased to over 90% through rapid crystallization. Rapid crystallization includes the following steps:

[0034] Open the inlet and outlet valves of the jacket cooling water and control the temperature for rapid cooling through the circulating water regulating valve;

[0035] When the temperature inside the crystallizer is 3°C higher than the circulating water temperature, it is difficult to cool down. Close the circulating water regulating valve, open the chilled water valves of the first-stage condenser and the second-stage condenser, and at the same time start the vacuum pump of the vacuum system. Slowly adjust the vacuum degree from 0 to -50KPa, and cool down by refluxing methanol through the condenser.

[0036] Continue cooling until the temperature is 3°C above the crystallization temperature (i.e., T). cAdd seed crystals at +3℃, observe the turbidity in the reactor or the temperature rebound, then stop adding seed crystals and reduce the stirring speed to 60% of the stirring speed in step S100.

[0037] Continue cooling until the system temperature reaches the crystallization temperature (i.e., T). c ), and maintain this temperature for crystallization.

[0038] Compared with the prior art, the beneficial effects of the present invention are:

[0039] 1. This invention controls parameters, including temperature, to bring the system into a state of equilibrium between supersaturated crystallization and dissolution saturation before crystallization, i.e., a crystallization quasi-stable state. This crystallization quasi-stable state avoids the sudden exothermic reaction during the addition of seed crystals and avoids the possibility of impurities being directly encapsulated during crystallization. Through two stages of slow cooling and multiple stages of cooling, the system gradually enters a supersaturated solution state and crystallizes uniformly after reaching the crystallization point. Due to the precise control of the saturation-supersaturation-crystallization state, impurities cannot crystallize, thereby achieving a deep purification effect.

[0040] 2. This invention utilizes a rapid cooling method involving vacuum absorption and methanol reflux. Through precise temperature control, the crystallization system sequentially undergoes adjustments to the vacuum level, temperature rebound time, a two-stage slow cooling phase, and multiple cooling phases before reaching the crystallization temperature. This process can purify menthyl benzoate raw material with a purity greater than 90% to approximately 99.5%, achieving a crystallization yield of over 46% and yielding a high-purity crystalline product.

[0041] 3. This invention utilizes vacuum and nitrogen low-temperature protection to prevent oxidation loss of menthyl benzoate raw material, resulting in low energy consumption and compliance with green and environmentally friendly production standards. Attached Figure Description

[0042] Figure 1 The flowchart (horizontally placed) shows a vacuum crystallization purification process for menthyl benzoate proposed in this invention.

[0043] Figure 2 This is a Tt curve diagram of the parameter control process in the vacuum crystallization purification process of menthyl benzoate proposed in this invention. Detailed Implementation

[0044] The technical solution of the present invention will be clearly and completely described below with reference to the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0045] Example 1:

[0046] Reference Figure 1-2A vacuum crystallization purification process for menthyl benzoate includes the following steps:

[0047] S100, Feeding:

[0048] In the crystallization vessel, 30 kg of menthyl benzoate with a purity of 94.5% and 20 kg of methanol (purity of 99.9%) were added, and the stirring speed was turned on at 50 r / min after the materials were added.

[0049] S200, Cooling:

[0050] Predetermine the crystallization temperature T c The temperature is 11.5℃. Open the inlet and outlet valves of the jacket cooling water and control the temperature rapidly through the circulating water regulating valve.

[0051] When the temperature inside the crystallizer reaches 28℃, close the circulating water regulating valve, open the chilled water valves of the first-stage condenser and the second-stage condenser, and simultaneously start the vacuum pump of the vacuum system. Slowly adjust the vacuum degree from 0 to -50KPa, and cool down by refluxing methanol through the condenser.

[0052] Continue cooling to 14.5℃, the rate of temperature drop slows down; continue adjusting the vacuum level and observe the amount of refluxed methanol. After the temperature reaches 13.5℃, add 100g of seed crystals, adding one every time the temperature drops by 1℃. After adding 100g of seed crystals when the temperature drops to 11.5℃, observe the turbidity in the reactor. The temperature rebounds instantly to around 14.1℃. Stop adding seed crystals and reduce the stirring speed to 30r / min.

[0053] During the temperature rebound, the temperature was kept constant for 2 hours. After the rebound ended, the temperature was reduced by 0.5°C per hour for the first 2 hours, and then by 1°C per hour until the temperature reached 11.5°C. This temperature was then maintained for crystallization.

[0054] S300, centrifugation:

[0055] Maintain the temperature of the crystallizer at 11.5±0.3℃ and prepare to discharge the material; shut off the vacuum system, open the nitrogen shut-off valve to break the vacuum inside the vessel and maintain a slight positive pressure of 20KPa, open the bottom valve of the crystallizer, and enter the centrifuge for centrifugation;

[0056] The mother liquor obtained from centrifugation enters the mother liquor storage tank, and the centrifuged crystals are melted in the melting tank and then enter the product tank.

[0057] The final product yielded 14 kg of menthyl ester (purity 99.6%) and 36 kg of crystallization mother liquor, with a yield of 46.7% and a menthyl ester purity of 88.2% in the crystallization mother liquor.

[0058] Example 2:

[0059] The mother liquor from multiple batches of Example 1 was collected, methanol was added, and the mass ratio of menthyl benzoate (including the total mass of raw materials containing impurities) to methanol was adjusted to 6:4. The following rapid crystallization steps were then performed:

[0060] After feeding, start the stirring speed at 50 r / min and pre-determine the crystallization temperature T. c The temperature is 11.5℃. Open the inlet and outlet valves of the jacket cooling water and control the temperature rapidly through the circulating water regulating valve.

[0061] When the temperature inside the crystallizer reaches 28℃, close the circulating water regulating valve, open the chilled water valves of the first-stage condenser and the second-stage condenser, and simultaneously start the vacuum pump of the vacuum system. Slowly adjust the vacuum degree from 0 to -50KPa, and cool down by refluxing methanol through the condenser.

[0062] Continue cooling to 14.5℃, add seed crystals every 100g, and stop adding seed crystals after observing the turbidity in the reactor or the temperature rebounds. Reduce the stirring speed to 30r / min.

[0063] Continue cooling until the system temperature reaches 11.5℃, then maintain this temperature for crystallization;

[0064] Maintain the temperature of the crystallizer at 11.5±0.3℃ and prepare to discharge the material; shut off the vacuum system, open the nitrogen shut-off valve to break the vacuum inside the vessel and maintain a slight positive pressure of 20KPa, open the bottom valve of the crystallizer, and enter the centrifuge for centrifugation;

[0065] The mother liquor obtained from centrifugation enters the mother liquor storage tank, and the centrifuged crystals are melted in the melting tank and then enter the product tank.

[0066] The crystallization yield was 75.6%, and the purity increased from 88.2% to 92.1%. After drying, the crystals were melted and fed back to the crystallization vessel. After undergoing the crystallization step of Example 1 again, a product with a purity of 99.5% was obtained.

[0067] Example 3:

[0068] Reference Figure 1-2 A vacuum crystallization purification process for menthyl benzoate includes the following steps:

[0069] S100, Feeding:

[0070] In the crystallization kettle, 30 kg of menthyl benzoate with a purity of 90.6% and 20 kg of methanol (purity of 99.9%) were added, and the stirring speed was turned on at 60 r / min after the materials were added.

[0071] S200, Cooling:

[0072] Predetermine the crystallization temperature T cAt 11℃, open the inlet and outlet valves of the jacket cooling water and control the temperature rapidly through the circulating water regulating valve;

[0073] When the temperature inside the crystallizer reaches 28℃, close the circulating water regulating valve, open the chilled water valves of the first-stage condenser and the second-stage condenser, and simultaneously start the vacuum pump of the vacuum system. Slowly adjust the vacuum degree from 0 to -50KPa, and cool down by refluxing methanol through the condenser.

[0074] Continue cooling down to 14℃, the rate of temperature drop slows down; continue adjusting the vacuum level and observe the amount of refluxed methanol. After the temperature reaches 13℃, add 100g of seed crystals, adding once for every 1℃ decrease. After adding 100g of seed crystals when the temperature drops to 12℃, observe the turbidity in the reactor. The temperature rebounds instantly to around 15.3℃, stop adding seed crystals, and reduce the stirring speed to 36r / min.

[0075] During the temperature rebound, the temperature was kept constant for 2 hours. After the rebound ended, the temperature was reduced by 0.5°C per hour for the first 2 hours, and then by 1°C per hour until the temperature reached 11°C. This temperature was then maintained for crystallization.

[0076] S300, centrifugation:

[0077] Maintain the temperature of the crystallizer at 11±0.3℃ and prepare to discharge the material; shut off the vacuum system, open the nitrogen shut-off valve to break the vacuum in the vessel and maintain a slight positive pressure of 20KPa, open the bottom valve of the crystallizer, and enter the centrifuge for centrifugation;

[0078] The mother liquor obtained from centrifugation enters the mother liquor storage tank, and the centrifuged crystals are melted in the melting tank and then enter the product tank.

[0079] The final yield was 13.8 kg of menthol product (purity 99.3%) and 36.2 kg of crystallization mother liquor, with a yield of 46% and a menthol purity of 83.2% in the crystallization mother liquor.

[0080] Example 4:

[0081] Reference Figure 1-2 A vacuum crystallization purification process for menthyl benzoate includes the following steps:

[0082] S100, Feeding:

[0083] In the crystallization kettle, 30 kg of menthyl benzoate with a purity of 96.3% and 20 kg of methanol (purity of 99.9%) were added, and the stirring speed was turned on at 55 r / min after the materials were added.

[0084] S200, Cooling:

[0085] Predetermine the crystallization temperature T c At 10.5℃, open the inlet and outlet valves of the jacket cooling water and control the temperature rapidly through the circulating water regulating valve;

[0086] When the temperature inside the crystallizer reaches 28℃, close the circulating water regulating valve, open the chilled water valves of the first-stage condenser and the second-stage condenser, and simultaneously start the vacuum pump of the vacuum system. Slowly adjust the vacuum degree from 0 to -50KPa, and cool down by refluxing methanol through the condenser.

[0087] Continue cooling to 13.5℃, the rate of temperature drop slows down; continue adjusting the vacuum level and observe the amount of refluxed methanol. After the temperature reaches 12.5℃, add 100g of seed crystals, adding one every time the temperature drops by 1℃. After adding 100g of seed crystals at 11.5℃, observe the turbidity in the reactor. The temperature rebounds instantly to around 14.4℃. Stop adding seed crystals and reduce the stirring speed to 33r / min.

[0088] During the temperature rebound, the temperature was kept constant for 2 hours. After the rebound ended, the temperature was reduced by 0.5°C per hour for the first 2 hours, and then by 1°C per hour until the temperature reached 10.5°C. This temperature was then maintained for crystallization.

[0089] S300, centrifugation:

[0090] Maintain the temperature of the crystallizer at 10.5±0.3℃ and prepare to discharge the material; shut off the vacuum system, open the nitrogen shut-off valve to break the vacuum inside the vessel and maintain a slight positive pressure of 20KPa, open the bottom valve of the crystallizer, and enter the centrifuge for centrifugation;

[0091] The mother liquor obtained from centrifugation enters the mother liquor storage tank, and the centrifuged crystals are melted in the melting tank and then enter the product tank.

[0092] The final product yielded 14.5 kg of menthyl ester (purity 99.8%) and 35.5 kg of crystallization mother liquor, with a yield of 48.3% and a menthyl ester purity of 93.02% in the crystallization mother liquor.

[0093] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any equivalent substitutions or modifications made by those skilled in the art within the scope of the technology disclosed in the present invention, based on the technical solution and inventive concept of the present invention, should be covered within the scope of protection of the present invention.

Claims

1. A vacuum crystallization purification process for menthyl benzoate, characterized in that, Includes the following steps: S100, Feeding: Menthyl benzoate and methanol were fed into the crystallization vessel at a mass ratio of 6:4, and stirring was started after feeding. S200, Cooling: Open the inlet and outlet valves of the jacket cooling water and control the temperature for rapid cooling through the circulating water regulating valve; When the temperature inside the crystallizer is 3°C higher than the circulating water temperature, close the circulating water regulating valve, open the chilled water valves of the first-stage condenser and the second-stage condenser, and simultaneously start the vacuum pump of the vacuum system, adjusting the vacuum degree from 0 to -50KPa, and cooling the methanol by refluxing through the condenser. As the temperature continues to decrease to 3°C above the crystallization temperature, the rate of temperature decrease slows down. Continue to adjust the vacuum level and observe the amount of refluxed methanol. After the temperature reaches 2°C above the crystallization temperature, add 100g of seed crystals. Add seed crystals once for every 1°C decrease. Stop adding seed crystals after observing the turbidity in the reactor or the temperature rebound. Reduce the stirring speed to 60% of the stirring speed in step S100. During the temperature rebound, the temperature was kept constant for 2 hours. After the rebound ended, the temperature was reduced by 0.5°C per hour for the first 2 hours, and then by 1°C per hour until the temperature reached the crystallization temperature. The temperature was then maintained for crystallization. S300, centrifugation: When the temperature of the crystallizer is adjusted to the discharge temperature, prepare to discharge the material; shut off the vacuum system, open the nitrogen shut-off valve to break the vacuum in the vessel and maintain a slight positive pressure of 20 kPa, open the bottom valve of the crystallizer, and enter the centrifuge for centrifugation; The mother liquor obtained from centrifugation enters the mother liquor storage tank, and the centrifuged crystals are melted in the melting tank and then enter the product tank.

2. The vacuum crystallization purification process for menthyl benzoate according to claim 1, characterized in that, The stirring speed in step S100 is 50-60 r / min.

3. The vacuum crystallization purification process for menthyl benzoate according to claim 1, characterized in that, In the crystallization vessel of step S100, the dissolution of menthyl benzoate and methanol is carried out by heating the heat transfer wall with 0.4 MPa steam. In the melting vessel of step S300, the melting of menthyl benzoate is carried out by heating the heat transfer wall with 0.4 MPa steam.

4. The vacuum crystallization purification process for menthyl benzoate according to claim 1, characterized in that, The seed crystal in step S200 is specifically menthyl benzoate crystal with a purity greater than 99.8%.

5. The vacuum crystallization purification process for menthyl benzoate according to claim 1, characterized in that, The centrifuged crystals obtained in step S300 are washed with methanol produced by a methanol washing tank, and the washing liquid is combined with the crystallization mother liquor and then enters the crystallization mother liquor storage tank.

6. The vacuum crystallization purification process for menthyl benzoate according to claim 1, characterized in that, The discharge temperature in step S300 is the crystallization temperature ±0.3℃.

7. The vacuum crystallization purification process for menthyl benzoate according to claim 1, characterized in that, The purity of the product tank in step S300 is tested. If the purity is less than 99%, it enters the next stage crystallization kettle for secondary crystallization.

8. The vacuum crystallization purification process for menthyl benzoate according to claim 1, characterized in that, The parameter control process in step S200 cooling specifically includes the following steps: S201. Based on the solubility properties of menthyl benzoate and methanol, take menthyl benzoate raw material with a purity greater than 90%, and follow step S100 with a mass ratio of menthyl benzoate to methanol of 6:

4. Then, based on the optimal crystallization temperature of the saturated methanol solution of menthyl benzoate, the crystallization temperature is initially determined to be 10-12℃. S202. By controlling the vacuum degree, methanol is vaporized to promote the production of supersaturated solution. The vaporization absorbs heat and removes heat from the crystallization kettle. The vaporized methanol is processed by a primary condenser and a secondary condenser to obtain condensed reflux methanol. Based on the requirements of the rate of decrease and production speed, the methanol reflux flow rate is initially determined to be 5-15% of the initial methanol input, while the reflux methanol temperature is controlled at 8-13℃, and the initial vacuum degree is determined to be -50KPa. S203. After adding seed crystals, crystals gradually form. The exothermic crystallization causes the system temperature to rise rapidly. The crystalline product dissolves again, and the endothermic dissolution causes the system temperature to drop. Under the continuous cooling of refluxed methanol, the exothermic heat generated by continuous crystallization is offset. The vacuum degree is adjusted so that the cooling rate of refluxed methanol matches the alternating frequency of exothermic crystallization and endothermic dissolution. The vacuum degree is determined to be -39 kPa. The methanol reflux flow rate is controlled at 7.8% of the initial methanol input, and the reflux methanol temperature is controlled at 11.5-13.5℃. Therefore, 10℃ chilled water is used as the heat exchange medium for the heat transfer walls of both the primary and secondary condensers. The exothermic condensation is carried away by the heat exchange medium. During the heat exchange process of refluxed methanol, the exothermic crystallization is consumed, and the endothermic dissolution is also offset. The system enters an equilibrium state of supersaturated crystallization and saturated dissolution, i.e., the crystallization quasi-steady state. S204. Based on the requirements of production time and crystallization temperature, within the crystallization temperature range of 10-12℃, as the preset crystallization temperature decreases, the seed crystal input temperature decreases, the crystallization tendency within the system increases, and the slope of the continuously generated crystallization exothermic peak and the resulting dissolution endothermic valley both increase. That is, the process of the temperature rebound curve gradually becoming flat is prolonged. Therefore, the crystallization temperature range is determined to be 10.5-11.5℃, the set temperature rebound time is 2h, and since stirring disturbs the crystallization, the stirring speed is reduced to 60% of the initial stirring speed. S205, maintaining the vacuum level at -39KPa, continue to lower the temperature of the reflux methanol to 10.5±0.5℃, entering the second stage of slow cooling. The system state gradually changes from the crystallization metastable state to the supersaturated solution state. Set the temperature to decrease by 0.5℃ every 1h, for a total decrease of 1℃. S206. Increase the vacuum level, continue to lower the temperature of the reflux methanol, and increase the methanol reflux flow rate to accelerate the cooling rate. Enter a multi-stage cooling stage, setting the temperature to decrease by 1℃ every 1 hour until the temperature reaches 10.5-11.5℃. Maintain this temperature for crystallization, and continuously increase the vacuum level and lower the temperature of the reflux methanol to 10.3±0.2℃ to keep the system crystallizing at this temperature. S207. Take menthol benzoate raw material with a purity of less than 90%. After the experiments in S201-206, as the purity of menthol benzoate decreases, the possibility of impurity crystallization can no longer be offset by parameter control. The purity of the obtained crystalline product cannot reach 99%. Therefore, menthol benzoate with a purity of more than 90% is selected as the crystallization raw material.

9. The vacuum crystallization purification process for menthyl benzoate according to claim 8, characterized in that, In step S207, menthol benzoate raw materials with a purity of less than 90% undergo differentiated processing. Menthol benzoate raw materials with a purity of less than 70% require purification treatment through adsorption or vacuum distillation. Menthol benzoate raw materials with a purity of 70-90% are purified to over 90% through rapid crystallization. Rapid crystallization includes the following steps: Open the inlet and outlet valves of the jacket cooling water and control the temperature for rapid cooling through the circulating water regulating valve; When the temperature inside the crystallizer is 3°C higher than the circulating water temperature, close the circulating water regulating valve, open the chilled water valves of the first-stage condenser and the second-stage condenser, start the vacuum pump of the vacuum system, and gradually increase the vacuum degree from 0 to -50KPa. Cool the methanol by refluxing through the condenser. Continue cooling to 3°C above the crystallization temperature, add seed crystals, and stop adding seed crystals after observing the turbidity in the reactor or the temperature rebound. Reduce the stirring speed to 60% of the stirring speed in step S100. Continue cooling until the system temperature reaches the crystallization temperature, and maintain that temperature to allow crystallization to proceed.

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