Continuous synthesis process and production unit for metaldehyde

Abstract

The invention discloses a metaldehyde continuous synthesis process and a production unit, and belongs to the field of metaldehyde synthesis, the process comprises the following steps: S1, uniformly mixing paraldehyde and acetaldehyde as a reaction mother solution; s2, continuously dropwise adding acetaldehyde and a catalyst into the reaction mother liquor, controlling the reaction temperature to be below-5 DEG C, continuously discharging the synthesized metaldehyde crystals and reaction feed liquid, and carrying out solid-liquid separation; s3, carrying out heat exchange and cooling on one part of the reaction material liquid obtained by separation, refluxing to the step S2 as a reaction mother solution, and depolymerizing the other part of the reaction material liquid to obtain acetaldehyde as an acetaldehyde reactant continuously dropwise added in the step S2; and purifying the separated metaldehyde to obtain the product. And S4, the step S2 and the step S3 are cyclically carried out. According to the scheme, continuous synthesis of metaldehyde is realized, the composition and the temperature of a reaction system are controlled through arrangement of the reaction mother liquor and reflux of the reaction feed liquid, the production efficiency and the product yield are improved, meanwhile, the production energy consumption is reduced, the process is simple, and large-scale production is facilitated.

Description

Technical Field

[0001] The present application relates to a continuous synthesis process and production unit of metaldehyde, belonging to the field of metaldehyde synthesis. Background Art

[0002] Metaldehyde is a cyclic tetramer of acetaldehyde (2,4,6,8-tetramethyl-1,3,5,7-tetraoxocane), with the molecular formula C8H 16 O4. Metaldehyde is a highly effective insecticide that kills soft-bodied pests such as snails and slugs, as well as snails that are susceptible to blood-sucking parasites (the active ingredient in over 80% of snail-killing pills worldwide is metaldehyde). It can also be used in solid fuels, plastic foaming agents, safety matches, fireworks, and artificial rainfall, among other applications.

[0003] Currently, the industrial production of metaldehyde is primarily based on a batch process using a stirred-tank reaction crystallizer. This process results in low industrial production efficiency. Furthermore, in actual production, it has been found that due to the stringent reaction and crystallization conditions (generally requiring a temperature control between -20°C and -10°C), the polymerization rate accelerates dramatically when the catalyst concentration exceeds a certain value, accompanied by the release of significant heat. This makes it difficult to control the reaction temperature and significantly reduces the yield of the target product. Therefore, efficiently removing the reaction heat is key to improving the yield of metaldehyde.

[0004] To solve the above problems, some technicians have used special catalysts to increase the yield of metaldehyde. For example, the invention patent with authorization publication number CN 102206200B proposes a method for producing metaldehyde, which proposes adding oxalyl chloride to increase the yield of metaldehyde. The invention patent with application publication number CN 105198857A proposes a method for synthesizing metaldehyde, which uses a polymeric acid catalyst to make H + During the reaction, the acid-binding agent is gradually released slowly, thereby slowing down the heat release during the reaction and increasing the selectivity of metaldehyde. The above two schemes have higher selectivity requirements for the catalyst.

[0005] Other technicians have slowed down the exothermicity of the reaction by adding materials in batches multiple times to increase the yield of metaldehyde. For example, the invention patent with application publication number CN 116987058A adds acetaldehyde twice to the polycondensation reactor to make the reaction gentle and facilitate temperature control, thereby increasing the polycondensation yield of metaldehyde. The invention patent with application publication number CN 114452920A proposes to intermittently add raw materials and catalysts and mix them under the stirring of a stirring roller to achieve a small amount of multiple polymerization reactions, so that the raw materials are mixed more fully, while reducing the accumulation of heat in the reactants, improving the reaction efficiency, and shortening the polymerization reaction time. Some researchers have also continuously removed the reaction heat by adding volatile solvents such as ether to keep the reaction temperature within a low range (Liu Guoliang, Qin Changkun. Research on the Synthesis Reaction of Metaldehyde [J]. Fine Chemicals, 1991, (3): 26-27.). The above production process is still an intermittent production method, with low production efficiency and the inability to continuously and stably control the reaction temperature.

[0006] Therefore, it is necessary to invent a new continuous green synthesis process for metaldehyde that can remove the reaction heat in a timely manner, so as to improve the yield and production efficiency of metaldehyde and reduce energy consumption. Summary of the Invention

[0007] In order to solve the above problems, a continuous synthesis process of metaldehyde is provided, which can maintain the production stability of metaldehyde, improve production efficiency and yield, and reduce production costs and production energy consumption.

[0008] According to one aspect of the present application, a continuous synthesis process for metaldehyde is provided, comprising the following steps:

[0009] S1, mixing paraldehyde and acetaldehyde to form a reaction mother solution;

[0010] S2. Continuously add acetaldehyde and a catalyst to the reaction mother liquor, control the reaction temperature below -5°C, and continuously discharge the synthesized metaldehyde crystals and the reaction liquid for solid-liquid separation;

[0011] S3, heat-exchanging a portion of the separated reaction liquid for cooling and then refluxing to step S2 as a reaction mother liquor, and depolymerizing the other portion to obtain acetaldehyde as the acetaldehyde reactant continuously added dropwise in step S2; separating and purifying the separated metaldehyde crystals to obtain a metaldehyde product;

[0012] S4. Repeat steps S2 and S3.

[0013] In this embodiment, paraldehyde and acetaldehyde are mixed as a reaction mother liquor. On the one hand, this can increase the content of paraldehyde in the reaction system, inhibit the occurrence of side reactions during the synthesis of paraldehyde, reduce the formation of paraldehyde, promote the progress of the target reaction, and thus facilitate the formation of paraldehyde. On the other hand, paraldehyde can be used as a diluent. Since the specific heat capacity of paraldehyde is 258.8 J / (mol·K), which is larger than the specific heat capacity of acetaldehyde of 141.0 J / (mol·K), under the same reaction process and heat release, the mixed reaction mother liquor of paraldehyde and acetaldehyde can cause the temperature of the reaction system to rise more slowly than when acetaldehyde is used as the reaction mother liquor. At the same time, as the reaction mother liquor is in direct contact with the reaction system, it can directly absorb the reaction heat more quickly and transfer the heat generated by the reaction during the cyclic production process, thereby reducing the temperature of the reaction system.

[0014] This solution is a continuous production process, in which acetaldehyde and the catalyst are fed simultaneously, and the metaldehyde crystal product and the reaction liquid are discharged and separated. The continuous production process solves the problem of unstable product quality between batches, compared with the intermittent operation adopted in the existing technology, that is, after a batch of reactions is completed, the material is discharged and then re-fed for production. At the same time, the cooling process and time between each batch are saved, the reaction efficiency is improved, and the production energy consumption is reduced, which is conducive to industrial and large-scale production.

[0015] In this solution, the reaction liquid and metaldehyde are continuously removed from the reactor during the production process, and the crystallized metaldehyde is first separated from the reaction liquid. On the one hand, compared with conventional batch production in which a coolant is only passed through the jacket of the reactor or the cooling coil to indirectly contact the reaction liquid for cooling, which makes the generated metaldehyde easily scale on the heat exchange coil and the inner wall of the reactor, thereby reducing heat exchange efficiency, the problem of metaldehyde crystallization on the inner wall is effectively reduced, thereby ensuring heat transfer efficiency. On the other hand, after separating the reaction liquid from the metaldehyde, a portion of the reaction liquid is cooled and then refluxed as a reaction mother liquor. While controlling the concentrations of various components in the reaction liquid, the cooled circulating mother liquor can fully contact the reaction liquid. Direct convection heat transfer can improve heat transfer efficiency and better control the reaction temperature within a lower range, thereby facilitating the reaction toward the production of metaldehyde, increasing the yield of metaldehyde, and reducing production energy consumption. At the same time, this method does not introduce other impurities, facilitates subsequent separation and purification, and simplifies the production process.

[0016] Optionally, the mass ratio of paraldehyde to acetaldehyde in step S1 is 1:(5-20).

[0017] Optionally, before step S2, the reaction mother liquor is cooled to -20 to -10°C before step S2 is performed.

[0018] Optionally, the portion of the reaction liquid cooled and refluxed to step S2 as reaction mother liquid in step S3 accounts for 10% to 30% of the total volume of the discharged reaction liquid.

[0019] The reaction liquid directly cooled and refluxed is controlled to account for 10% to 30% of the total volume of the discharged reaction liquid. On the one hand, since the content of paraldehyde in the reaction liquid is higher than that in the reaction mother liquor, controlling the amount of direct cooling and reflux can ensure that the content of paraldehyde in the reaction is relatively stable, thereby stabilizing the reaction rate and reaction ratio. On the other hand, the reaction liquid is depolymerized into acetaldehyde as a reactant to participate in the reaction, which improves the utilization rate of raw materials, saves production costs, and makes the production process more energy-efficient and efficient.

[0020] Optionally, the cooling temperature of the part of the reaction liquid that is cooled and refluxed to step S2 as the reaction mother liquid in step S3 is below -10°C.

[0021] Optionally, the catalyst in step S2 is one of pyridine-hydrobromic acid, hydrochloric acid-sulfuric acid composite catalyst, phosphoric acid-sulfuric acid composite catalyst, and hydrochloric acid-hydrobromic acid composite catalyst.

[0022] Specifically, the above catalysts can be obtained through commercial channels.

[0023] According to another aspect of the present application, a production unit for continuous synthesis of metaldehyde is provided, comprising:

[0024] A slurry bed reactor, comprising, from bottom to top, a connected riser and a gas-liquid separation zone; a downcomer is further provided between the riser and the gas-liquid separation zone, the upper end of the downcomer being connected to the gas-liquid separation zone, and the lower end of the downcomer being connected to the riser via a solid-liquid separation device; the slurry bed reactor is provided with an acetaldehyde feed port, a circulating mother liquor inlet, and a catalyst feed port; and a gas distributor is further provided at the bottom of the slurry bed reactor;

[0025] A solid-liquid separator, wherein the feed port of the solid-liquid separator is connected to the solid discharge port of the solid-liquid separation device, and the liquid discharge port above the solid-liquid separator is connected to the depolymerization unit, and the solid discharge port below the solid-liquid separator is connected to the separation and purification unit.

[0026] Specifically, the solid-liquid separation device is a hydrocyclone; the metaldehyde continuous synthesis unit further includes a circulating sedimentation-negative pressure separation device, which includes a separator, a vacuum pump, a clean liquid intermediate tank, and a circulating mother liquid heat exchanger. The circulating sedimentation-negative pressure separation device can extract the reaction liquid from the gas-liquid separation zone of the slurry bed reactor into the clean liquid intermediate tank under the action of the vacuum pump, and return part of the reaction liquid to the slurry bed reactor through the circulating mother liquid heat exchanger, and transport the other part of the reaction liquid to the depolymerization unit, depolymerize and cool it, and then transport it to the slurry bed reactor to participate in the reaction as a reactant;

[0027] The liquid outlet of the circulating mother liquid heat exchanger is communicated with the feed inlet of the slurry bed reactor.

[0028] Optionally, a gas outlet is provided at the upper end of the slurry bed reactor, and the gas outlet is connected to the gas distributor through a gas heat exchanger.

[0029] Specifically, an inert gas, such as nitrogen or argon, is used as a circulating gas to circulate in a slurry bed reactor and a gas heat exchanger.

[0030] Optionally, a mist separator is further provided between the gas outlet and the gas heat exchanger.

[0031] Optionally, a heat exchange cavity is provided on the periphery of the slurry bed reactor, and a coolant flows through the heat exchange cavity.

[0032] The shell-side discharge port of the slurry bed reactor is connected to the shell-side feed port through a heat exchanger. The heat exchanger continuously cools the coolant that absorbs heat to reduce the temperature of the coolant.

[0033] The heat exchange chamber provided on the periphery of the slurry bed reactor is provided with a coolant, which can not only cool the reaction mother liquor initially added, but also absorb and transfer the heat generated during the reaction in time, ensuring that the temperature in the slurry bed reactor is constantly maintained below -5°C, ensuring that the reaction proceeds in the forward direction and increasing the reaction rate.

[0034] Specifically, the coolant is ethylene glycol or ethanol.

[0035] The beneficial effects of this application include but are not limited to:

[0036] 1. According to the continuous synthesis process of metaldehyde of the present application, by using paraldehyde and acetaldehyde as reaction mother liquors, on the one hand, the occurrence of side reactions is suppressed, the target reaction is promoted, and thus the production of paraldehyde is facilitated; on the other hand, paraldehyde can be used as a diluent to increase the specific heat capacity of the reaction system, reduce the reaction heating rate, promote the forward reaction, and improve the yield of paraldehyde.

[0037] 2. According to the continuous synthesis process of metaldehyde of the present application, by removing the metaldehyde and reaction liquid generated by the reaction, the problem of metaldehyde crystals scaling on the inner wall of the reactor is reduced, thereby improving heat transfer efficiency and reducing energy loss. At the same time, the reaction liquid is removed and a portion of it is directly cooled and refluxed, so that the low-temperature reaction liquid is in direct contact with the reaction system, thereby improving heat exchange efficiency and saving energy consumption.

[0038] 3. According to the production unit for continuous synthesis of metaldehyde of the present application, a gas distributor is provided at the bottom of the slurry bed reactor and is connected to a gas heat exchanger. A low-temperature inert gas is directly introduced into the reaction system, where it comes into direct contact with the reaction system, thereby lowering the temperature of the reaction system and shifting the reaction equilibrium toward the formation of metaldehyde. This improves product yield and heat exchange efficiency, thereby saving production energy consumption. BRIEF DESCRIPTION OF THE DRAWINGS

[0039] The drawings described herein are used to provide a further understanding of the present application and constitute a part of the present application. The illustrative embodiments of the present application and their descriptions are used to explain the present application and do not constitute an improper limitation on the present application. In the drawings:

[0040] Figure 1 This is a schematic diagram of the process flow involved in Example 1 of the present application.

[0041] Figure 2 This is a schematic diagram of the process flow involved in Example 3 of this application.

[0042] Figure numerals: 1. slurry bed reactor, 2. riser, 3. gas-liquid separation zone, 4. downcomer, 5. solid-liquid separation device, 6. gas distributor, 7. solid-liquid separator, 8. circulating mother liquor heat exchanger, 9. liquid discharge port, 10. solid discharge port, 11. acetaldehyde feed port, 12. catalyst feed port, 13. circulating mother liquor inlet, 14. separator, 15. vacuum pump, 16. cleaning liquid intermediate tank, 17. gas outlet, 18. gas heat exchanger, 19. mist-entrainment separator, 20. heat exchange chamber, 21. depolymerization unit, 22. separation and purification unit, 23. enamel stirred tank, 24. filter, 25. coil. DETAILED DESCRIPTION

[0043] The present application is described in detail below with reference to embodiments, but the present application is not limited to these embodiments.

[0044] Unless otherwise specified, the methods used in the examples and comparative examples of the present application are conventional methods in the prior art.

[0045] Unless otherwise defined, all professional and scientific terms used herein have the same meanings as those familiar to those skilled in the art. The reagents and raw materials used in the present invention can be purchased through conventional channels. Unless otherwise specified, the reagents and raw materials used in the present invention are used in accordance with conventional methods in the art or in accordance with the product instructions. In addition, any methods and materials similar to or equivalent to those described herein can be applied to the present invention. The preferred embodiments and materials described in this patent are for illustrative purposes only.

[0046] In order to more clearly understand the above-mentioned objects, features and advantages of the present application, the present application is further described in detail below in conjunction with the accompanying drawings and specific embodiments. It should be noted that the embodiments of the present application and the features therein can be combined with each other in the absence of conflict.

[0047] In the following description, many specific details are set forth to facilitate a full understanding of the present application. However, the present application may also be implemented in other ways different from those described herein. Therefore, the scope of protection of the present application is not limited to the specific embodiments disclosed below.

[0048] In addition, in the description of the present application, it should be understood that the terms "center", "up", "down", "front", "back", "left", "right", "vertical", "horizontal", "top", "bottom", "inside", "outside", "axial", "radial", "circumferential", etc., indicating the orientation or position relationship, are based on the orientation or position relationship shown in the accompanying drawings, and are only for the convenience of describing the present application and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore cannot be understood as a limitation on the present application.

[0049] In this application, unless otherwise expressly specified and limited, a first feature "above" or "below" a second feature may be that the first and second features are in direct contact, or the first and second features are in indirect contact through an intermediate medium. In the description of this specification, the description with reference to the terms "one embodiment", "some embodiments", "example", "specific example", or "some examples" means that the specific features, structures, materials or characteristics described in conjunction with the embodiment or example are included in at least one embodiment or example of the present application. In this specification, the schematic representation of the above terms does not necessarily refer to the same embodiment or example. Moreover, the specific features, structures, materials or characteristics described may be combined in an appropriate manner in any one or more embodiments or examples.

[0050] Example 1

[0051] refer to Figure 1 The embodiments of the present application disclose a production unit for continuous synthesis of metaldehyde, comprising:

[0052] A slurry bed reactor 1 includes, from bottom to top, a connected riser 2 and a gas-liquid separation zone 3; a downcomer 4 is further provided between the riser 2 and the gas-liquid separation zone 3; the upper end of the downcomer 4 is connected to the gas-liquid separation zone 3, and the lower end of the downcomer 4 is connected to the riser 2 via a liquid discharge port 9 of a solid-liquid separation device 5; the slurry bed reactor 1 is provided with an acetaldehyde feed port 11, a circulating mother liquor inlet 13, and a catalyst feed port 12; and a gas distributor 6 is further provided at the bottom of the slurry bed reactor 1;

[0053] The solid-liquid separator 7 has a feed port connected to the solid discharge port 10 of the solid-liquid separation device 5 , and the liquid discharge port above the solid-liquid separator 7 is connected to the depolymerization unit 21 , and the solid discharge port below the solid-liquid separator 7 is connected to the separation and purification unit 22 .

[0054] By arranging a gas distributor 6 at the bottom of the slurry bed reactor 1, recycled inert gas can be introduced as a flow carrier to directly contact the reaction liquid and form a countercurrent with the reaction liquid, which can promote material mixing and improve the yield of tetraacetaldehyde.

[0055] The metaldehyde continuous synthesis unit further includes a circulating sedimentation-negative pressure separation device, which includes a separator 14, a vacuum pump 15, a clean liquid intermediate tank 16, and a circulating mother liquid heat exchanger 8. The circulating sedimentation-negative pressure separation device can extract the reaction liquid from the gas-liquid separation zone 3 of the slurry bed reactor 1 into the clean intermediate tank under the action of the vacuum pump 15, and return part of the reaction liquid to the slurry bed reactor 1 through the circulating mother liquid heat exchanger 8. The remaining part of the reaction liquid is transported to the depolymerization unit 21 for depolymerization and cooling, and then transported to the slurry bed reactor 1 as a reactant to participate in the reaction.

[0056] The liquid outlet of the circulating mother liquid heat exchanger 8 is communicated with the circulating mother liquid inlet 13 of the slurry bed reactor 1 .

[0057] The circulation sedimentation-negative pressure separation device extracts the reaction liquid and cools it for circulation, allowing the low-temperature reaction liquid to directly contact the reaction system, thereby improving heat exchange efficiency and saving energy.

[0058] As an embodiment, a gas outlet 17 is provided at the upper end of the slurry bed reactor 1 , and the gas outlet 17 is connected to the gas distributor 6 through a gas heat exchanger 18 .

[0059] The recycled gas is cooled in the gas heat exchanger 18, and the low-temperature circulating gas directly enters the slurry bed reactor 1 and comes into direct contact with the material, which can enhance the heat exchange effect and further reduce the temperature of the reaction liquid.

[0060] As an embodiment, a mist separator 19 is further provided between the gas outlet 17 and the gas heat exchanger 18 .

[0061] As an embodiment, a heat exchange chamber 20 is provided on the periphery of the polycondensation reactor, and a cooling liquid flows through the heat exchange chamber 20 .

[0062] The heat exchange chamber 20 works together with the cooled reflux reaction liquid and the introduced low-temperature circulating gas to reduce the temperature in the reaction system, which is beneficial to ensure the temperature stability of the reaction system, improve the stability of continuous production, and ensure the yield of tetraacetaldehyde.

[0063] Example 2

[0064] In this example, the synthesis processes 1# to 5# and D1# and D2# of metaldehyde are carried out in the production unit described in Example 1.

[0065] Synthesis process 1#

[0066] The continuous synthesis process of metaldehyde comprises the following steps:

[0067] S1. Mix paraldehyde and acetaldehyde in a mass ratio of 1:9 as a reaction mother solution and cool to -20°C;

[0068] S2. Continuously adding acetaldehyde and a hydrochloric acid-sulfuric acid composite catalyst to the reaction mother liquor, controlling the reaction temperature below -5°C, and continuously discharging the synthesized metaldehyde crystals and the reaction liquid and performing solid-liquid separation;

[0069] S3, cooling 30% of the separated reaction liquid to below -10°C by heat exchange and then refluxing to step S2 as the reaction mother liquor, and depolymerizing the remaining portion to obtain acetaldehyde as the acetaldehyde reactant continuously added dropwise in step S2; separating and purifying the separated metaldehyde crystals to obtain the metaldehyde product;

[0070] S4. Repeat steps S2 and S3.

[0071] Synthesis process 2#

[0072] The continuous synthesis process of metaldehyde comprises the following steps:

[0073] S1. Mix paraldehyde and acetaldehyde in a mass ratio of 1:5 as a reaction mother solution and cool to -15°C;

[0074] S2. Continuously adding acetaldehyde and pyridine-hydrobromic acid to the reaction mother liquor, controlling the reaction temperature below -5°C, and continuously discharging the synthesized metaldehyde crystals and the reaction liquid for solid-liquid separation;

[0075] S3, cooling 20% ​​of the separated reaction liquid to below -10°C by heat exchange and then refluxing to step S2 as the reaction mother liquor, and depolymerizing the remaining portion to obtain acetaldehyde as the acetaldehyde reactant continuously added dropwise in step S2; separating and purifying the separated metaldehyde crystals to obtain the metaldehyde product;

[0076] S4. Repeat steps S2 and S3.

[0077] Synthesis process 3#

[0078] The continuous synthesis process of metaldehyde comprises the following steps:

[0079] S1. Mix paraldehyde and acetaldehyde in a mass ratio of 1:20 as a reaction mother solution and cool to -10°C;

[0080] S2, continuously adding acetaldehyde and phosphoric acid-sulfuric acid composite catalyst to the reaction mother liquor, controlling the reaction temperature below -5°C, and continuously discharging the synthesized metaldehyde crystals and the reaction liquid and performing solid-liquid separation;

[0081] S3, cooling 10% of the separated reaction liquid to below -10°C by heat exchange and then refluxing to step S2 as the reaction mother liquor, and depolymerizing the remaining portion to obtain acetaldehyde as the acetaldehyde reactant continuously added dropwise in step S2; separating and purifying the separated metaldehyde crystals to obtain the metaldehyde product;

[0082] S4. Repeat steps S2 and S3.

[0083] Synthesis process 4#

[0084] The difference from the synthesis process 1 is that the mass ratio of paraldehyde to acetaldehyde is 1:50.

[0085] Synthesis process 5#

[0086] The difference from the synthesis process 1 is that 80% of the reaction liquid separated in step S3 is directly cooled and refluxed.

[0087] Synthesis process D1#

[0088] The difference from the synthesis process 1 is that the reaction mother liquor is paraldehyde.

[0089] Synthesis process D2#

[0090] The difference from the synthesis process 1 is that all the reaction liquid separated in step S3 participates in the depolymerization as a reactant.

[0091] Example 3

[0092] The difference from the synthesis process 1 is that an enamel stirred tank 21 with a jacket and a cooling coil is used as the polycondensation reactor. The process is as follows: Figure 2 As shown:

[0093] Acetaldehyde and a hydrochloric acid-hydrobromic acid composite catalyst are added dropwise to an enameled stirred kettle 23 through an acetaldehyde feed port 11 and a catalyst feed port 12, respectively. Simultaneously, the reacted liquid is continuously discharged into a solid-liquid separator 7, which has a built-in filter 24. Metaldehyde crystals and the reaction liquid are continuously separated by the filter 24. The metaldehyde crystals are continuously discharged from the solid-liquid separator 7 and separated and purified to obtain the product. 10% of the reaction liquid is directed to a circulating mother liquid heat exchanger 8 for cooling to -10°C before being returned to the enameled stirred kettle 21. The remaining reaction liquid is sent to a depolymerization unit 21 for depolymerization and cooling before being transferred to a polycondensation reactor as a reactant for the reaction. Low-temperature ethylene glycol is introduced into the heat exchange chamber 20 and coil 25 of the enameled stirred kettle 23 to help control the reaction liquid temperature below -5°C.

[0094] Example 4

[0095] The difference from the synthesis process 1 is that an enameled stirred kettle with a jacket and a cooling coil is used as the polycondensation reactor, and an intermittent operation mode is adopted. 1000 parts of acetaldehyde are added to the reactor, stirred and cooled to -15°C, and then a hydrochloric acid-sulfuric acid composite catalyst is added dropwise. The catalyst addition rate is controlled to ensure that the temperature of the mixed solution of the polymerization reaction does not exceed 0°C. The reaction time is 3 hours. After the catalyst addition is completed, the reaction temperature is controlled to not exceed 5°C and stirred for 3 hours.

[0096] The polymerization reaction mixture was centrifuged, filtered, washed with water, and dried to obtain about 96 parts of metaldehyde.

[0097] The yields of the synthesis processes 1# to 5#, D1# to D2# in Example 2 and the synthesis of metaldehyde in Examples 3 and 4 were calculated, and the following results were obtained:

[0098] Table 1

[0099]

[0100] As can be seen from Table 1, the use of a method of circulating and cooling part of the reaction liquid can reduce the reaction temperature during the polycondensation reaction, thereby significantly improving the yield of metaldehyde.

[0101] The foregoing is merely an embodiment of the present application, and the scope of protection of the present application is not limited by these specific embodiments, but is determined by the claims of the present application. For those skilled in the art, the present application may have various modifications and variations. Any modifications, equivalent substitutions, improvements, etc. made within the technical ideas and principles of the present application should be included in the scope of protection of the present application.

Claims

1. A continuous synthesis process for metaldehyde, characterized in that: The following steps are involved: S1, mixing paraldehyde and acetaldehyde to form a reaction mother solution; S2. Continuously add acetaldehyde and a catalyst to the reaction mother liquor, control the reaction temperature below -5°C, and continuously discharge the synthesized metaldehyde crystals and the reaction liquid for solid-liquid separation; S3, heat-exchanging a portion of the separated reaction liquid for cooling and then refluxing to step S2 as a reaction mother liquor, and depolymerizing the other portion to obtain acetaldehyde as the acetaldehyde reactant continuously added dropwise in step S2; separating and purifying the separated metaldehyde crystals to obtain a metaldehyde product; S4. Repeat steps S2 and S3.

2. The continuous synthesis process of metaldehyde according to claim 1, wherein The mass ratio of paraldehyde to acetaldehyde in step S1 is 1:(5-20).

3. The continuous synthesis process of metaldehyde according to claim 1, wherein Before step S2, the reaction mother liquor is cooled to -20 to -10°C and then step S2 is performed.

4. The continuous synthesis process of metaldehyde according to claim 1, wherein The portion of the reaction liquid cooled and refluxed in step S3 to serve as reaction mother liquid in step S2 accounts for 10% to 30% of the total volume of the discharged reaction liquid.

5. The continuous synthesis process of metaldehyde according to claim 1, characterized in that: The temperature of the reaction liquid cooled and refluxed in step S3 to serve as the reaction mother liquid in step S2 is lowered to below -10°C.

6. The continuous synthesis process of metaldehyde according to claim 1, characterized in that: The catalyst in step S2 is one of pyridine-hydrobromic acid, hydrochloric acid-sulfuric acid composite catalyst, phosphoric acid-sulfuric acid composite catalyst, and hydrochloric acid-hydrobromic acid composite catalyst.

7. A production unit for continuous synthesis of metaldehyde, characterized in that: include: A slurry bed reactor, comprising, from bottom to top, a connected riser and a gas-liquid separation zone; a downcomer is further provided between the riser and the gas-liquid separation zone, the upper end of the downcomer being connected to the gas-liquid separation zone, and the lower end of the downcomer being connected to the riser via a solid-liquid separation device; the slurry bed reactor is provided with an acetaldehyde feed port, a circulating mother liquor inlet, and a catalyst feed port; and a gas distributor is further provided at the bottom of the slurry bed reactor; A solid-liquid separator, wherein the feed port of the solid-liquid separator is connected to the solid discharge port of the solid-liquid separation device, and the liquid discharge port above the solid-liquid separator is connected to the depolymerization unit, and the solid discharge port below the solid-liquid separator is connected to the separation and purification unit.

8. The metaldehyde continuous synthesis unit according to claim 7, characterized in that: A gas outlet is provided at the upper end of the slurry bed reactor, and the gas outlet is connected to the gas distributor through a gas heat exchanger.

9. The metaldehyde continuous synthesis unit according to claim 8, characterized in that: A mist separator is also provided between the gas outlet and the gas heat exchanger.

10. The metaldehyde continuous synthesis unit according to claim 7, characterized in that: A heat exchange cavity is provided on the periphery of the polycondensation reactor, and a cooling liquid is passed through the heat exchange cavity.

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