Method for continuously crystallizing p-aminophenol

Abstract

The invention provides a method for continuous crystallization of p-aminophenol, and relates to the technical field of fine chemical crystallization, and the method comprises the following steps: adding a p-aminophenol stock solution into an evaporator for concentration; continuously adding the concentrated mother liquor into a primary DTB crystallizer for cooling crystallization; crystal mush in the first-stage DTB crystallizer is continuously added into the second-stage DTB crystallizer to be continuously cooled and crystallized; crystal mush discharged from the secondary DTB crystallizer is continuously added into a thickener; feeding crystal mush at the bottom of the thickener into a centrifugal machine, cleaning and drying to obtain a product, and completing two-stage continuous crystallization of p-aminophenol; clear liquid at the upper part of the thickener and centrifugal filtrate are returned to the mother liquor tank, one part is discharged, and the other part is returned to be continuously concentrated. The continuous crystallization method of p-aminophenol solves the problems of product particle crushing, poor particle size distribution, much fine powder, low bulk density and product scaling. The continuous crystallization production of p-aminophenol is realized, and the problem of batch-to-batch difference of p-aminophenol crystallization products is solved.

Description

Technical Field

[0001] The present invention relates to the technical field of compound crystallization, and particularly to a continuous crystallization method for a product with good particle size distribution, few fine powders, and high bulk density. Background Art

[0002] p-Aminophenol (abbreviated as PAP) is an important dye intermediate. In the dye industry, it is mainly used for the production of acid dyes, sulfur dyes, azo dyes, and fur dyes, etc.; it can also be used in the pesticide industry to produce the insecticide phosalone; it is widely used in the pharmaceutical industry and is a raw material for synthesizing drugs such as paracetamol and clofibrate; it can also be used as a metal preservative, a polymerization inhibitor for formaldehyde storage, and the synthesis of photosensitive materials, etc.

[0003] Currently, single-effect or multi-effect evaporation is generally used for the evaporation and concentration of p-aminophenol, resulting in high evaporation costs and large equipment floor areas. The crystallization section generally uses batch operation, and the crystallizer is an ordinary stirring kettle, with long crystallization time, low production efficiency, high labor costs, and general batch-to-batch differences in solid products. At the same time, the crystallization process is poor, resulting in many fine powders in the crystallized product, poor particle size distribution, poor crystal shape, low bulk density, poor product fluidity, seriously affecting downstream operations. For example, it is easy to entrain solvents and impurities during centrifugal solid-liquid separation, reducing the purity of the product. At the same time, the product appearance is poor and it is easy to agglomerate, affecting the sales market of the product. Summary of the Invention

[0004] The purpose of the present invention is to provide a method for the continuous crystallization of p-aminophenol, which solves the technical problems of easy fouling of the crystallizer, poor product particle size distribution, crystal breakage, many fine powders, poor bulk density, low crystallization production efficiency, and high production costs in the crystallization of p-aminophenol.

[0005] The present invention provides a method for the continuous crystallization of p-aminophenol, and the method includes:

[0006] Step 1, passing a p-aminophenol stock solution with a temperature of 20 - 50 °C and a concentration of 5% wt - 30% wt through a heat exchanger and continuously pumping it into an evaporator, controlling the feed flow rate to be 1 m 3 / h - 30 m 3 / h. The solvent of the p-aminophenol stock solution is a mixture of ethanol and water, and the proportion of ethanol in the ethanol / water mixed solvent is 0% wt - 100% swt. Evaporating and concentrating the stock solution to 30% wt - 70% wt by means of Mechanical Vapor Recompression (MVR), controlling the evaporation rate to be 0 m 3 / h - 28 m 3 / h, and the temperature of the mother liquor in the evaporator is 60 - 100 °C;

[0007] Step 2: Continuously pump the concentrated p-aminophenol mother liquor into the first-stage DTB crystallizer through a centrifugal pump, and control the feed flow rate to be 1 m 3 / h - 30 m 3 / h. The first-stage DTB crystallizer is preferably a self-developed internal draft tube crystallizer (DTB crystallizer). The temperature of the first-stage DTB crystallizer is set to be constant, and the temperature range is 10 - 80 °C. At this time, the decrease in temperature causes p-aminophenol to precipitate crystals in the ethanol / water mixed solvent, and at the same time, the crystals grow in the crystallizer. Control the solid-liquid ratio in the first-stage DTB crystallizer to be 1% wt - 30% wt;

[0008] Step 3: Pump the crystal slurry containing crystals in the first-stage DTB crystallizer into the second-stage DTB crystallizer through a centrifugal pump, and control the feed flow rate to be 1 m 3 / h - 30 m 3 / h. The second-stage DTB crystallizer is preferably a self-developed internal draft tube crystallizer (DTB crystallizer). The temperature of the second-stage DTB crystallizer is set to be constant, and the temperature range is 5 - 30 °C. At this time, the crystals in Step 2 continue to grow. Control the solid-liquid ratio in the second-stage DTB crystallizer to be 10% - 60%;

[0009] Step 4: Continuously pump the crystal slurry in the second-stage DTB crystallizer into the thickener. The thickener is a conical kettle with stirring. The upper part of the kettle is the feed inlet, and the bottom of the kettle is the crystal slurry discharge outlet. Control the stirring rate of the thickener to be 0 - 250 rpm; Due to gravity, the crystals continuously sink and accumulate at the bottom of the thickener. Control the solid-liquid ratio at the bottom of the thickener to be 20% - 80%. At the same time, continuously pump the clear liquid at the upper part of the thickener into the mother liquor recovery tank;

[0010] Step 5: The crystal slurry at the bottom of the thickener flows by gravity into the centrifuge. The centrifuge is preferably a double-push centrifuge or a vertical centrifuge. After solid-liquid separation, the separated solid is washed with ethanol at 0 - 5 °C, and then sent into the drying system. The drying system is preferably an automatic air-blast drying system. The separated solid is bagged after drying. The separated filtrate is continuously pumped into the mother liquor recovery tank;

[0011] Step 6: Part of the mother liquor in the mother liquor recovery tank is drained from the lower part of the tank by a pump at 0 m 3 / h - 5 m 3 / h, and the other part of the mother liquor is pumped back to the heat exchanger in Step 1 to continue to participate in concentration and crystallization.

[0012] Beneficial effects: In this continuous crystallization method of p-aminophenol, through evaporation and concentration by MVR technology, passing through a primary cooling crystallizer, a secondary cooling crystallizer, a thickener, etc., compared with the existing method, the cost of evaporation and concentration is reduced. The two-stage crystallization process prevents excessive supersaturation in the crystallization of p-aminophenol and a large number of nuclei from bursting out. The obtained product crystals have good appearance, uniform particle size, good fluidity, no fouling of the product, and high bulk density. Among them, part of the mother liquor is discharged externally, and part of the mother liquor is returned for crystallization, which not only improves the product purity but also increases the total product yield. Description of the Drawings

[0013] Figure 1 It is a process flow diagram for a continuous crystallization method of p-aminophenol.

[0014] Figure 2 It is a schematic diagram of the connection of process equipment for a continuous crystallization method of p-aminophenol

[0015] In the figure: 1. Heat exchanger; 2. Evaporator; 3. Primary DTB crystallizer; 4. Secondary DTB crystallizer; 5. Thickener; 6. Centrifuge; 7. Mother liquor recovery tank; 8. Drying system. Detailed Embodiments

[0016] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are some but not all of the embodiments of the present invention. Usually, the components of the embodiments of the present invention described and shown in the accompanying drawings here can be arranged and designed in various different configurations.

[0017] Therefore, the following detailed description of the embodiments of the present invention provided in the accompanying drawings is not intended to limit the scope of the claimed present invention, but merely represents the selected embodiments of the present invention. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts fall within the scope of protection of the present invention.

[0018] It should be noted that: Similar reference numerals and letters denote similar items in the following drawings. Therefore, once an item is defined in one drawing, it does not need to be further defined and explained in subsequent drawings.

[0019] In the description of the present invention, it should be noted that the orientation or positional relationship indicated by the terms "center", "upper", "lower", "left", "right", "vertical", "horizontal", "inner", "outer", etc. is based on the orientation or positional relationship shown in the drawings, or the orientation or positional relationship in which the inventive product is customarily placed during use. It is only for the convenience of describing the present invention and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and thus should not be construed as a limitation on the present invention. In addition, the terms "first", "second", "third", etc. are only used for differential description and should not be construed as indicating or implying relative importance.

[0020] In addition, terms such as "horizontal" and "vertical" do not mean that the components are required to be absolutely horizontal or hanging vertically, but may be slightly inclined. For example, "horizontal" only means that its direction is more horizontal relative to "vertical", and does not mean that the structure must be completely horizontal, but may be slightly inclined.

[0021] In the description of the present invention, it should also be noted that unless otherwise clearly specified and defined, the terms "set", "installed", "connected", "connected" should be understood in a broad sense. For example, it may be a fixed connection, a detachable connection, or an integral connection; it may be a mechanical connection or an electrical connection; it may be directly connected or indirectly connected through an intermediate medium, and may be the communication inside two elements. For those of ordinary skill in the art, the specific meanings of the above terms in the present invention can be understood according to specific circumstances.

[0022] The following will describe in detail some embodiments of the present invention with reference to the drawings. Without conflict, the following embodiments and the features in the embodiments can be combined with each other.

[0023] This embodiment provides a method for continuous crystallization of p-aminophenol. Please refer to Figure 1 , Figure 2 as shown, including: Step 1, pumping the p-aminophenol stock solution with a temperature of 20 - 50°C and a concentration of 5%wt - 30%wt continuously into the evaporator 2 through the heat exchanger 1, controlling the feed flow rate to be 1 m 3 / h - 30 m 3 / h. The solvent of the p-aminophenol stock solution is a mixture of ethanol and water, and the proportion of ethanol in the ethanol / water mixed solvent is 0%wt - 100%wt. Evaporate and concentrate the stock solution to 30%wt - 70%wt by means of Mechanical Vapor Recompression (MVR), and control the evaporation rate to be 0 m 3 / h - 28 m 3 / h. The temperature of the mother liquor in the evaporator 2 is 60 - 100°C. The whole process of this step is protected by nitrogen;

[0024] Step 2: Continuously pump the concentrated p-aminophenol mother liquor into the first-stage DTB crystallizer 3 through a centrifugal pump, and control the feed flow rate to be 1 m 3 / h - 30 m 3 / h. The first-stage DTB crystallizer 3 is preferably a self-developed inner draft tube crystallizer with stirring (DTB crystallizer). The temperature of the first-stage DTB crystallizer 3 is set to be constant, and the temperature range is 10 - 80°C. At this time, the decrease in temperature causes p-aminophenol to precipitate crystals in the ethanol / water mixed solvent, and at the same time, the crystals grow in the crystallizer. Control the solid-liquid ratio in the first-stage DTB crystallizer 3 to be 1% wt - 30% wt. The whole process of this step is protected by nitrogen;

[0025] Step 3: Pump the crystal slurry containing crystals in the first-stage DTB crystallizer 3 into the second-stage DTB crystallizer 4 through a centrifugal pump, and control the feed flow rate to be 1 m 3 / h - 30 m 3 / h. The second-stage DTB crystallizer 4 is preferably a self-developed inner draft tube crystallizer with stirring (DTB crystallizer). The temperature of the second-stage DTB crystallizer 4 is set to be constant, and the temperature range is 5 - 30°C. At this time, the crystals in Step 2 continue to grow. Control the solid-liquid ratio in the second-stage DTB crystallizer 4 to be 10% - 60%. The whole process of this step is protected by nitrogen;

[0026] Step 4: Continuously pump the crystal slurry in the second-stage DTB crystallizer 4 into the thickener 5. The thickener 5 is a conical kettle with a jacket and stirring. The upper part of the kettle is the feed inlet, and the bottom of the kettle is the crystal slurry discharge outlet. The stirring rate of the thickener 5 is controlled to be 0 - 250 rpm. The temperature of the thickener 5 is set to be constant, and the temperature range is 5 - 30°C. Due to gravity, the crystals continuously sink and accumulate at the bottom of the thickener 5. Control the solid-liquid ratio at the bottom of the thickener 5 to be 20% - 80%. At the same time, continuously pump the clear liquid at the upper part of the thickener 5 into the mother liquor recovery tank 7;

[0027] Step 5: The crystal slurry at the bottom of the thickener 5 flows by gravity into the centrifuge 6. The centrifuge 6 is preferably a double-pushing centrifuge 6 or a vertical centrifuge 6. After solid-liquid separation, the solid is washed with ethanol at 0 - 5°C and then sent into the drying system 8. The drying system 8 is preferably an automatic air-blast drying system. After the solid is dried, it is bagged. The centrifuged filtrate is continuously pumped into the mother liquor recovery tank 7;

[0028] Step 6: Part of the mother liquor in the mother liquor recovery tank 7 is drained from the lower part of the tank by a pump at a rate of 0 m 3 / h - 5 m 3 / h, and the other part of the mother liquor is pumped back to the heat exchanger 1 in Step 1 to continue to participate in concentration and crystallization.

[0029] Example 1: A p-aminophenol stock solution with a purity of 94%, a temperature of 35 °C, and a concentration of 10% wt is continuously pumped into the evaporator 2 through the heat exchanger 1, and the feed flow rate is controlled to be 5 m 3 / h. The solvent of the p-aminophenol stock solution is a mixture of ethanol and water, and the proportion of ethanol in the ethanol / water mixed solvent is 55% wt. Under nitrogen protection, the stock solution is evaporated and concentrated to 20% wt by means of Mechanical Vapor Recompression (MVR), and the evaporation rate is controlled to be 2.5 m 3 / h. The temperature of the mother liquor in the evaporator 2 is 90 °C; the concentrated p-aminophenol mother liquor is continuously pumped into the first-stage DTB crystallizer 3 through a centrifugal pump, and the feed flow rate is 2.5 m 3 / h. The first-stage crystallization temperature is 50 °C, crystals start to precipitate in the first-stage crystallization, and the solid-liquid ratio is 11% wt. The crystal slurry containing crystals in the first-stage DTB crystallizer 3 is pumped into the second-stage DTB crystallizer 4 through a centrifugal pump, and the feed flow rate is 2.5 m 3 / h. The temperature of the second-stage DTB crystallizer 4 is 20 °C, and the solid-liquid ratio is 17% wt. The crystal slurry in the second-stage DTB crystallizer 4 is continuously pumped into the thickener 5, and the stirring rate of the thickener 5 is 50 rpm. The solid-liquid ratio of the clear liquid at the top of the thickener 5 is close to 0% wt, the top mother liquor is pumped into the mother liquor tank, the solid-liquid ratio at the bottom is 46% wt, and the crystal slurry flows by gravity into the vertical centrifuge 6. After solid-liquid separation, the solid is washed with ethanol at 0 °C, then dried by blowing and encapsulated. The centrifuged filtrate is continuously pumped into the mother liquor recovery tank 7. 0.7 m 3 / h of the mother liquor in the mother liquor recovery tank 7 is drained by a pump from the lower part of the tank, and the remaining mother liquor is pumped back to the heat exchanger 1 to continue participating in concentration and crystallization. The p-aminophenol product has good particles, the average particle size D50 is 117 μm, the weight proportion of particles with a size of 80 μm - 160 μm exceeds 60%, the product purity is 99.7%, and the bulk density is 0.9. The overall product yield is 92%.

[0030] Example 2: A p-aminophenol stock solution with a purity of 89%, a temperature of 32 °C, and a concentration of 14% wt is continuously pumped into the evaporator 2 through the heat exchanger 1, and the feed flow rate is controlled to be 26 m 3 / h. The solvent of the p-aminophenol stock solution is a mixture of ethanol and water, and the proportion of ethanol in the ethanol / water mixed solvent is 70% wt. Under nitrogen protection, the stock solution is evaporated and concentrated to 27% wt by means of Mechanical Vapor Recompression (MVR), and the evaporation rate is controlled to be 13.5 m 3 / h, the temperature of the mother liquor in the evaporator 2 is 85°C; the concentrated p-aminophenol mother liquor is continuously pumped into the first-stage DTB crystallizer 3 by a centrifugal pump, and the feeding flow rate is 12.5 m 3 / h, the first-stage crystallization temperature is 50°C, crystals start to precipitate in the first-stage crystallization, and the solid-liquid ratio is 14% wt. The crystal slurry containing crystals in the first-stage DTB crystallizer 3 is pumped into the second-stage DTB crystallizer 4 by a centrifugal pump, and the feeding flow rate is 12.5 m 3 / h, the temperature of the second-stage DTB crystallizer 4 is 15°C, and the solid-liquid ratio is 22% wt. The crystal slurry in the second-stage DTB crystallizer 4 is continuously pumped into the thickener 5, and the stirring rate of the thickener 5 is 80 rpm. The solid-liquid ratio of the clear liquid at the top of the thickener 5 is close to 0.4% wt, the mother liquor at the top is pumped into the mother liquor tank, the solid-liquid ratio at the bottom is 52% wt, and the crystal slurry flows by gravity into the double-push centrifuge 6. After solid-liquid separation, the solid is washed with ethanol at 0°C, dried by blowing, and then packaged. The centrifuged filtrate is continuously pumped into the mother liquor recovery tank 7. 3 m 3 / h of the mother liquor in the mother liquor recovery tank 7 is discharged from the lower part of the tank by a pump, and the remaining mother liquor is pumped back into the heat exchanger 1 to continue participating in concentration and crystallization. The p-aminophenol product has good particles, the average particle size D50 is 105 μm, the weight ratio of particles with a size of 65 μm - 160 μm exceeds 65%, the product purity is 99.2%, and the bulk density is 0.8. The overall product yield is 91%.

[0031] The DTB crystallizer mentioned in this patent has the patent number: 202422957510.X, which is the crystallizer in the continuous crystallization device.

[0032] Finally, it should be noted that: the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit it; although the present invention has been described in detail with reference to the foregoing embodiments, those of ordinary skill in the art should understand that: they can still modify the technical solutions recorded in the foregoing embodiments, or perform equivalent replacements on some or all of the technical features; and these modifications or replacements do not make the essence of the corresponding technical solutions deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for continuous crystallization of p-aminophenol, characterized in that, It includes the following steps: Step 1: Evaporate and concentrate the p-aminophenol stock solution to obtain the concentrated mother liquor; Step 2: Continuously add the concentrated mother liquor obtained in Step 1 into a first-stage DTB crystallizer for the first crystallization to obtain a first-stage crystallization slurry; Step 3: Continuously add the first-stage crystallization slurry obtained in Step 2 into a second-stage DTB crystallizer for the second crystallization; Obtain a second-stage crystallization slurry; Step 4: Continuously add the second-stage crystallization slurry obtained in Step 3 into a thickener. The slurry is layered in the thickener, and the supernatant liquid on the upper layer is sent to the mother liquor tank; Step 5: The lower-layer liquid in the thickener in Step 4 is sent to a centrifuge for separation. The separated solid is dried and bagged, and the separated liquid is sent to the mother liquor tank; Step 6: For the mother liquor in the mother liquor recovery tank, a part is drained by a pump from the lower part of the tank, and another part of the mother liquor is pumped back to the heat exchanger in Step 1 to continue to participate in concentration and crystallization.

2. The method for continuous crystallization of p-aminophenol according to claim 1, characterized in that: In Step 1, the p-aminophenol stock solution is prepared from p-aminophenol crude product and a solvent; The temperature of the p-aminophenol stock solution is 20 - 50 °C; The p-aminophenol concentration is 5 wt% - 30 wt%; The feed flow rate of p-aminophenol is 1 m 3 / h - 30 m 3 / h; The solvent of the p-aminophenol stock solution is a mixed solvent of ethanol and water; the proportion of ethanol in the mixed solvent is 0 wt% - 100 wt%.

3. A method for continuous crystallization of p-aminophenol according to any one of claims 1 to 2, characterized in that: In Step 1, the evaporation and concentration method is Mechanical Vapor Recompression (MVR); The p-aminophenol stock solution is evaporated and concentrated to 30 wt% - 70 wt%; The evaporation rate is 0 m 3 / h - 28 m 3 / h; The temperature of the mother liquor in the evaporator is 60 - 100 °C.

4. A method for continuous crystallization of p-aminophenol according to claim 1, characterized in that: In Step 2, the temperature of the first-stage DTB crystallizer is set to be constant; The first-stage DTB crystallizer is a self-developed inner draft tube type crystallizer with stirring (DTB crystallizer); The temperature range of the first-stage DTB crystallizer is 10 - 80 °C; The solid-liquid ratio in the first-stage DTB crystallizer is 1 wt% - 30 wt%; Control the feed flow rate to be 1 m 3 / h - 30 m 3 / h.

5. A method for continuous crystallization of p-aminophenol according to claim 1, characterized in that: In Step 3, the temperature of the second-stage DTB crystallizer is set to be constant; The second-stage DTB crystallizer is a self-developed inner draft tube type crystallizer with stirring (DTB crystallizer); the temperature range of the second-stage DTB crystallizer is 5 - 30 °C, The solid-liquid ratio of the second-stage DTB crystallizer is 10% - 60%; Control the feed flow rate at 1 m 3 / h - 30 m 3 / h.

6. A method for continuous crystallization of p-aminophenol according to claim 1, characterized in that: In Step 4, the thickener is a conical kettle with a jacket and stirring. The upper part of the kettle is the feed inlet, and the bottom of the kettle is the slurry discharge outlet; The temperature of the thickener is set to be constant; The stirring rate of the thickener is controlled at 0 - 250 rpm; The temperature range of the thickener is 5 - 30 °C; The solid-liquid ratio at the bottom of the thickener is controlled at 20% - 80%; The clear liquid at the upper part of the thickener is continuously pumped into the mother liquor recovery tank.

7. A method for continuous crystallization of p-aminophenol according to claim 1, characterized in that: In Step 5, the material at the bottom of the thickener is separated by a centrifuge; The centrifuge is a double-push centrifuge or a vertical centrifuge.

8. A method for continuous crystallization of p-aminophenol according to claim 1, characterized in that: In Step 5, the separated solid is washed with ethanol and then sent to a drying system for drying; The temperature of the washing ethanol is 0 - 5 °C; The drying system is an automatic air-blowing drying system; The separated liquid is continuously pumped into the mother liquor recovery tank.

9. A method for continuous crystallization of p-aminophenol according to claim 1, characterized in that: In the sixth step, a part of the mother liquor in the mother liquor recovery tank is discharged by a pump from the lower part of the tank; The sewage discharge flow rate is 0 m 3 / h - 5 m 3 / h.

10. A method for continuous crystallization of p-aminophenol according to claim 1, characterized in that: In the sixth step, a part of the mother liquor in the mother liquor recovery tank is pumped back to the heat exchanger in the first step to continue to participate in concentration and crystallization.

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