A low free hydrazine content carbazide and a continuous production method thereof

Abstract

The application provides a kind of low free hydrazine content carbazide and its continuous production method, belongs to fine chemical synthesis technical field.The method will dimethyl carbonate and hydrazine hydrate be mixed in proportion after emulsification treatment to form uniform emulsion;Sequentially into low temperature section and medium temperature section microchannel reactor for grading reaction, respectively complete intermediate generation and condensation reaction;Reactant is removed under low temperature high vacuum condition by thin film evaporator quickly light component;Heavy component is obtained by gradient cooling crystallization, centrifugal separation, washing and drying to get finished product carbazide;Centrifugal mother liquor and washing liquid are combined and returned to premix tank for recycling, cumulative number of times is not more than 3 times.The process realizes the accurate control of reaction process, significantly reduces the free hydrazine content, improves the product purity and yield, and is suitable for industrial continuous production.

Description

Technical Field

[0001] This application relates to a carbazine with low free hydrazine content and its continuous production method, belonging to the field of fine chemical synthesis technology. Background Technology

[0002] Carbazide, also known as carbazide, or 1,3-diaminourea in Chinese, is a white, fine, short columnar crystal or white crystalline powder at room temperature. It is highly soluble in water, sparingly soluble in ethanol, and insoluble in alcohols, ethers, chloroform, and benzene. Carbazide is a derivative of hydrazine and possesses strong reducing properties. As a chemical raw material and intermediate, it is widely used in the pharmaceutical, herbicide, plant growth regulator, and dye industries. As a deoxygenating agent for boiler water, carbazide is currently the most advanced material used in the world for boiler water deoxygenation. It has low toxicity, a high melting point, and a deoxygenation efficiency far exceeding that of other materials, making it a safe and environmentally friendly product.

[0003] Currently, the mainstream production method for carbazide is the carbonate hydrazine hydrolysis method, which involves reacting dimethyl carbonate and hydrazine hydrate to produce methyl hydrazine formate, and then reacting methyl hydrazine formate and hydrazine hydrate to produce carbazide.

[0004] US Patent Application US4496761A discloses a two-step method for preparing carbazide by reacting dimethyl carbonate with hydrazine hydrate. Dimethyl carbonate and hydrazine hydrate are reacted in a specific ratio at 50°C for 20 minutes, then at 25°C for 20 hours. After the reaction, the mixture is distilled under reduced pressure to remove methanol, water, and unreacted dimethyl carbonate. Hydrazine hydrate is then added, and the mixture is heated at 70°C for 4 hours. The mixture is then cooled to 0°C to crystallize the carbazide. The crystals are obtained by filtration, washing, and drying. The mother liquor and washings are combined and reused in the next batch of reaction. It is reported that after two reuses, the carbazide content of the product is 99.8%–100%, with a yield of approximately 77% based on dimethyl carbonate.

[0005] The above methods are intermittent and convenient for small-scale production. However, with increasing product demand and the trend towards intensive chemical production, the drawbacks of intermittent production—such as large equipment footprint, low capacity, low efficiency, high energy consumption, and low safety—are becoming increasingly apparent. Continuous reaction processes are easier to control, have lower energy and labor costs, and offer higher safety, making them one of the development directions for chemical production.

[0006] Chinese patent application CN109574870A discloses a continuous preparation method for hydrazide. The example describes dissolving dimethyl carbonate in methanol to prepare a 3 mol / L solution, which is then injected into a microchannel reactor or tubular reactor at a flow rate of 30 ml / min using a metering pump. 80% hydrazine hydrate is injected into the microchannel reactor or tubular reactor at a flow rate of 11.26 ml / min using another metering pump. The reaction is carried out at 75°C for 3 seconds, and the reactants are collected after exiting the microchannel reactor or tubular reactor, yielding a white crystalline product, carbazine. This continuous preparation method is merely a simple exploratory experimental method. Repeating this method cannot achieve the quality and yield mentioned in the example, making it difficult to implement in actual continuous production. This method is still some distance from true continuous production. Furthermore, due to the involvement of a multiphase system and exothermic effects in the reaction process, temperature control becomes more difficult during batch operations, easily leading to localized overheating and exacerbated side reactions. Incomplete conversion and low product purity are also likely, and there is a lack of effective design for mother liquor recycling, resulting in low resource utilization.

[0007] Therefore, how to construct an integrated production process for carbazide that can achieve stable and continuous operation, effectively suppress side reactions, and ensure high purity and high yield has become an urgent technical problem to be solved. Summary of the Invention

[0008] To address the aforementioned issues, a method for producing carbazine with low free hydrazine content and its continuous production is provided. This method not only yields carbazine with low free hydrazine content but also achieves high production yield. Furthermore, the continuous production process is easy to control, reducing energy consumption, improving production efficiency, and ensuring safe and controllable production.

[0009] According to one aspect of this application, a continuous production method for carbazine with low free hydrazine content is provided, characterized by comprising the following steps:

[0010] S1. Raw material premixing: Dimethyl carbonate and hydrazine hydrate are added to a premixing tank at a weight ratio of 1:(2.2-2.4) and stirred to form a mixture;

[0011] S2. Emulsification treatment: The mixture is fed into an emulsification device and processed to form a uniform emulsion;

[0012] S3. Low-temperature microchannel reaction: The emulsion is pumped into the first microchannel reactor, which is composed of multi-stage reaction modules connected in series. After the reaction is completed, the first reactant is obtained.

[0013] S4. Medium-temperature microchannel reaction: The first reactant is pumped into the second microchannel reactor, which is composed of multi-stage series reaction modules. After the reaction is completed, the second reactant is obtained.

[0014] S5. Thin-film evaporation separation: The second reactant is continuously fed into a thin-film evaporator, and methanol, water and unreacted dimethyl carbonate are evaporated under reduced pressure. The concentrated liquid of the heavy components after separation is collected.

[0015] S6. Gradient cooling crystallization: The concentrated solution of heavy components is continuously fed into the crystallization kettle, and the temperature is gradually reduced by stirring to form a kettle liquid containing carbazine crystals;

[0016] S7. Centrifugal separation: The liquid in the vessel is sent to a centrifuge for solid-liquid separation. The filter cake obtained by separation is washed and dried to obtain wet carbazine. After drying, the finished carbazine is obtained. At the same time, the centrifugal mother liquor and washing liquid are collected.

[0017] S8. Mother liquor recycling: The centrifuged mother liquor and washing liquid are combined and transported to the premix tank, mixed with newly added dimethyl carbonate, and the above operations S2 to S7 are repeated. The cumulative number of times the centrifuged mother liquor and washing liquid are combined shall not exceed 3 times, so as to realize the continuous production of carbazide.

[0018] Preferably, the hydrazine hydrate used in step S1 is hydrazine hydrate with a mass fraction of 80%.

[0019] Specifically, by limiting the weight ratio of dimethyl carbonate to hydrazine hydrate, the molar requirements of the two main reactions (dimethyl carbonate → methyl hydrazine carbamate → carbazine) are matched, so that the concentration of active components in the reaction system is in the optimized range of "sufficient main reaction and suppressed side reaction", avoiding excessive hydrazine hydrate leading to residual free hydrazine, or insufficient amount leading to incomplete reaction.

[0020] Optionally, in step S2, the emulsification equipment consists of a high-shear emulsification delivery pump and a static mixer. The mixture is first sheared and dispersed by the high-shear emulsification delivery pump, and then enters the static mixer for further mixing to form a uniform emulsion.

[0021] Specifically, the high-shear emulsification pump has a rotational speed of 3000 r / min and a shear rate of 3.0 × 10⁻⁶. 4 s -1 The mixture then enters a static mixer for further mixing. The residence time of the material in the static mixer is 40 seconds, ultimately forming a uniform emulsion with an average droplet diameter of 5 μm.

[0022] Optionally, in step S3, the number of reaction modules in the first microchannel reactor is not less than 10, each reaction module is connected in series, and the reaction pressure is controlled at 0.1 MPa, the reaction temperature at 20-25℃, and the reaction time at 10-12 min.

[0023] Specifically, the first microchannel reactor employs a multi-stage series structure to generate methyl hydrazine formate at a low temperature of 20–25℃. This reaction is exothermic; if the temperature is too high or the residence time is insufficient, self-polymerization of dimethyl carbonate or excessive hydrolysis can easily occur, generating byproducts. By setting up no fewer than 10 series modules, the total residence time is extended to 10–12 minutes, and the system pressure is maintained at 0.1 MPa to prevent solvent evaporation, ensuring the reaction proceeds fully. The microchannel reactor possesses extremely high specific surface area and excellent thermal conductivity, enabling rapid removal of reaction heat, precise temperature control, and suppression of hot spot formation, thereby effectively reducing the probability of side reactions and obtaining high-purity intermediates.

[0024] Optionally, in step S4, the second microchannel reactor has no fewer than 15 reaction modules, each connected in series, and the reaction pressure is controlled at 0.2-0.3 MPa, the reaction temperature at 65-70℃, and the reaction time at 15-20 min.

[0025] Specifically, the second microchannel reactor continues to employ a multi-stage series design, promoting the condensation reaction of methyl hydrazine formate and hydrazine hydrate to generate carbazine under mesophilic conditions of 65-70℃. This reaction has a high activation energy and requires sufficient temperature to drive it; however, excessively high temperatures will accelerate a series of side reactions. By controlling the reaction time to 15-20 minutes, the reaction is ensured to be fully completed while avoiding degradation caused by prolonged high temperatures. The multi-module series structure ensures the plug flow characteristics of the material, eliminates backmixing, improves reaction selectivity and conversion rate, and is beneficial for obtaining high-purity carbazine.

[0026] The first microchannel is a rectangular channel (0.8mm × 0.5mm) made of 316L; the second microchannel is a pulsed circular variable diameter channel (200μm in diameter).

[0027] Optionally, in step S5, the second reactant is continuously fed into a thin-film evaporator, and the evaporation temperature is controlled to be ≤70℃ and the evaporation pressure is controlled to be ≤-0.095MPa.

[0028] Specifically, a thin-film evaporator is used to separate the light components of the reaction products. Operating at temperatures ≤70℃ and evaporation pressures ≤-0.095MPa, rapid evaporation under low temperature and high vacuum is achieved. The thin-film evaporator is equipped with a high-speed rotating scraping device, which forms an extremely thin liquid film on the heating surface, resulting in high heat transfer efficiency and a short residence time. This allows for the efficient removal of methanol, water, and unreacted dimethyl carbonate without causing the decomposition of heat-sensitive carbazine. Compared to traditional atmospheric distillation or rotary evaporation, this method significantly reduces the possibility of byproduct regeneration, and in particular, prevents the risk of reverse decomposition of carbazine, leading to the generation of free hydrazine, due to high-temperature retention.

[0029] Optionally, in step S6, the temperature is first lowered to 25-35°C by stirring, and then the stirring speed is slowed down to continue lowering the temperature to 5-10°C.

[0030] The crystallization process employs a phased temperature control method, first lowering the temperature to 25-35℃ and then slowly cooling it to 5-10℃. The initial rapid cooling promotes crystal nucleation, while the subsequent slowing of the cooling rate and reduction of stirring intensity facilitates orderly crystal growth and prevents the aggregation of small crystals that may carry away the mother liquor. This gradient cooling method yields crystals with larger particle sizes and regular morphologies, significantly reducing the content of free hydrazine and other impurities adsorbed on the crystal surface and embedded within, thereby improving product purity and filtration performance, and reducing the washing burden.

[0031] Optionally, the temperature is first cooled to 30°C by stirring at a rate of 5-8°C / h, and then the stirring rate is reduced from 120-130 r / min to 50-65 r / min, while the temperature is further cooled to 5-10°C by stirring at a rate of 1-3°C / h.

[0032] Furthermore, low-speed stirring reduces the impact on growing crystals, preventing breakage and secondary nucleation; slow cooling controls supersaturation to a moderate level, avoiding explosive crystallization. Through synergistic regulation, mother liquor entrainment is minimized, solid phase purity is improved, and the free hydrazine content in the product is reduced.

[0033] Optionally, the centrifugation parameters in step S7 are: centrifugation speed 3000-5000 r / min, centrifugation temperature 5-10℃, and centrifugation time 10-20 min;

[0034] The water temperature for cold washing is 5-10℃, and the number of washing cycles is 1-2. The amount of water used for each washing cycle is 0.5-1 times the mass of the filter cake.

[0035] Specifically, by adjusting centrifugation parameters, efficient solid-liquid separation can be achieved at low temperatures, preventing crystals from dissolving or absorbing moisture at high temperatures. Low-temperature centrifugation also helps maintain system stability and reduces the release of free hydrazine. Using cold water for washing and controlling the amount of water used each time effectively removes impurities adhering to the mother liquor on the surface without causing product dissolution and loss due to excessive water volume. Low-temperature washing also inhibits the encapsulation effect caused by crystal dissolution and reprecipitation, further reducing impurity residue.

[0036] Optionally, in step S8, when applying it for the first time, the amount of newly added dimethyl carbonate is 30-32% of the amount of dimethyl carbonate used in the first feeding.

[0037] When using it a second time, the amount of newly added dimethyl carbonate should be 20-22% of the amount of dimethyl carbonate used in the first feeding.

[0038] When applying it for the third time, the amount of newly added dimethyl carbonate should be 8-10% of the amount of dimethyl carbonate used in the first feeding.

[0039] Specifically, the mother liquor and washing liquid are combined and reused a maximum of 3 times. The amount of dimethyl carbonate added is dynamically adjusted according to the number of cycles. The decreasing feeding method can gradually reduce the concentration of reactants, balance the contradiction between conversion rate and side reaction inhibition, and ultimately effectively control the free hydrazine content and purity in the carbazine product.

[0040] According to another aspect of this application, a carbazine with low free hydrazine content is provided, which is prepared by the above-described continuous production method; the carbazine has a free hydrazine content of <100ppm and a purity of ≥99.1%.

[0041] The beneficial effects of this application include, but are not limited to:

[0042] 1. The continuous production method for carbazine with low free hydrazine content of this application establishes a complete continuous production process of "raw material premixing - emulsification treatment - two-stage microchannel reaction - thin film evaporation separation - gradient cooling crystallization - centrifugal separation - mother liquor recycling", which changes the traditional batch production mode and realizes the industrial continuous and stable production of carbazine. This continuous operation mode eliminates the need for frequent batch feeding, unloading and equipment cleaning operations, which greatly reduces the number of production equipment and the floor space occupied, significantly shortens the production cycle and increases the production capacity. At the same time, it avoids the fluctuation of process parameters such as temperature and pressure in batch operation, reduces energy consumption and labor costs, and improves the safety factor of the production process.

[0043] 2. The continuous production method of carbazine with low free hydrazine content in this application effectively solves the problem of uneven mixing caused by the difference in miscibility between dimethyl carbonate and hydrazine hydrate in the emulsification step. Through the synergistic effect of high-shear emulsification and static mixing, a uniform emulsion is formed, which increases the contact area of ​​raw materials and ensures that the reaction conditions of each unit of material are consistent in the subsequent reaction, thus laying the foundation for the stability and thoroughness of the reaction.

[0044] The two-stage microchannel reactor adopts a multi-stage series reaction module design. The low-temperature microchannel reaction can avoid the decomposition of the intermediate product methyl hydrazine carbamate, ensuring the purity of the intermediate. The medium-temperature microchannel reaction can promote the full progress of the main reaction and inhibit the formation of free hydrazine, effectively solving the problem of high free hydrazine content in the existing process.

[0045] 3. The continuous production method for carbazine with low free hydrazine content in this application adopts low temperature and reduced pressure conditions in the thin film evaporation separation step, which can quickly separate methanol, water and unreacted raw materials, avoiding the problem of carbazine decomposition or increase of free hydrazine caused by prolonged high temperature heating in conventional distillation, and further ensuring product quality.

[0046] The gradient cooling crystallization process effectively avoids the phenomenon of impurities and free hydrazine being trapped during rapid crystal formation by controlling the cooling rate and stirring rate in stages, thereby improving crystal purity and morphological uniformity. The optimized centrifugal separation parameters and cold water washing process can efficiently remove residual impurities and free hydrazine from the surface of the filter cake, further reducing the free hydrazine content in the product and improving product purity.

[0047] 4. The continuous production method of carbazine with low free hydrazine content in this application adopts a mother liquor recycling design and limits the cumulative recycling time to no more than 3 times. At the same time, the dimethyl carbonate replenishment ratio is matched for each recycling. This not only realizes the efficient recovery and utilization of unreacted raw materials in the mother liquor, which greatly improves the raw material utilization rate and product yield, but also avoids the accumulation of impurities caused by excessive recycling, ensuring that the product quality is stable at a high purity (≥99.1%) and low free hydrazine (<100ppm) level.

[0048] The overall process is stable and controllable, and the product quality is consistent. It can meet the application needs of high-quality carbazide in multiple fields such as pharmaceuticals, herbicides, plant growth regulators, dyes, and boiler water deoxygenation, and has significant economic value and industrial application prospects. Detailed Implementation

[0049] The present application is described in detail below with reference to the embodiments, but the present application is not limited to these embodiments.

[0050] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of skill in the art. The reagents and raw materials used in this invention are readily available through conventional means, and unless otherwise specified, they shall be used in accordance with conventional methods in the art or as per the product instructions. Furthermore, any methods and materials similar to or equivalent to those described herein may be applied to the methods of this invention. The preferred embodiments and materials described in this patent are for illustrative purposes only.

[0051] In this application, the dimethyl carbonate is an industrial-grade product with a purity of ≥99.5%, and the hydrazine hydrate is commercially available 80% hydrazine hydrate; the high-shear emulsification transfer pump is a Yoshida Machinery HSM15 model from Japan; the microchannel reactor reaction module is made of 316L stainless steel; the thin-film evaporator is a Nanjing Zhengyuan ZZ 1200-150 model; and the centrifuge is an LGZ-1250 model centrifuge made of 304L stainless steel.

[0052] The relevant chemical reaction equations in this application are as follows:

[0053] 1. The reaction of dimethyl carbonate and hydrazine hydrate first produces methyl hydrazine carbamate.

[0054]

[0055] 2. Methyl hydrazinoate reacts with hydrazine hydrate to form carbazine:

[0056]

[0057] Overall reaction equation:

[0058]

[0059] A series of side reactions:

[0060]

[0061] Example 1: Carbazine #1

[0062] A continuous production method for carbazine 1# with low free hydrazine content includes the following steps:

[0063] S1. Raw material premixing: Dimethyl carbonate and hydrazine hydrate are added to a premixing tank at a weight ratio of 1:2.3 and stirred to form a mixture;

[0064] S2. Emulsification treatment: The mixture is fed into the emulsification equipment, which consists of a high-shear emulsification conveying pump and a static mixer. The mixture is first sheared and dispersed by the high-shear emulsification conveying pump, and then enters the static mixer for further mixing to form a uniform emulsion.

[0065] S3. Low-temperature microchannel reaction: The emulsion is pumped into the first microchannel reactor. The first microchannel reactor has 10 reaction modules connected in series. The reaction pressure is controlled at 0.1 MPa, the reaction temperature at 23°C, and the reaction time at 10 min.

[0066] S4. Medium-temperature microchannel reaction: The first reactant is pumped into the second microchannel reactor. In step S4, the second microchannel reactor has 16 reaction modules connected in series. The reaction pressure is controlled at 0.3 MPa, the reaction temperature at 68°C, and the reaction time at 18 min.

[0067] S5. Thin-film evaporation separation: The second reactant is continuously fed into a thin-film evaporator, and the evaporation temperature is controlled to be ≤70℃ and the evaporation pressure is controlled to be ≤-0.095MPa. Methanol, water and unreacted dimethyl carbonate are evaporated under reduced pressure, and the concentrated liquid of the heavy components after separation is collected.

[0068] S6. Gradient cooling crystallization: The concentrated heavy component solution is continuously fed into the crystallization kettle, and the temperature is reduced in stages by stirring. First, the temperature is reduced to 30°C by stirring at a rate of 6°C / h, and then the stirring rate is reduced from 130r / min to 65r / min. At the same time, the temperature is reduced to 8°C by stirring at a rate of 2°C / h to form a kettle solution containing carbazide crystals.

[0069] S7. Centrifugal separation: The liquid in the vessel is sent to a centrifuge for solid-liquid separation. The separated filter cake is washed and dried. The centrifugation parameters are: centrifugation speed 4000 r / min, centrifugation temperature 8℃, centrifugation time 15 min. The water temperature for cold water washing is 6℃, and the number of washings is 2. The amount of water used for each washing is 1 times the mass of the filter cake. Wet carbazine is obtained. After drying, the finished product carbazine 1# is obtained. At the same time, the centrifugation mother liquor and washing liquid are collected.

[0070] Example 2 Carbazine #2

[0071] A continuous production method for carbazine 2# with low free hydrazine content includes the following steps:

[0072] S1. Raw material premixing: The centrifuged mother liquor and washing liquid obtained in Example 1 are combined and pumped to a premixing tank. At the same time, dimethyl carbonate is added. The amount of dimethyl carbonate used here is 30% of the dimethyl carbonate used in the first batch of production. The mixture is stirred and mixed to form a mixture.

[0073] S2. Emulsification treatment: The mixture is fed into the emulsification equipment, which consists of a high-shear emulsification conveying pump and a static mixer. The mixture is first sheared and dispersed by the high-shear emulsification conveying pump, and then enters the static mixer for further mixing to form a uniform emulsion.

[0074] S3. Low-temperature microchannel reaction: The emulsion is pumped into the first microchannel reactor. The first microchannel reactor has 10 reaction modules connected in series. The reaction pressure is controlled at 0.1 MPa, the reaction temperature at 20℃, and the reaction time at 12 min.

[0075] S4. Medium-temperature microchannel reaction: The first reactant is pumped into the second microchannel reactor. In step S4, the second microchannel reactor has 16 reaction modules connected in series. The reaction pressure is controlled at 0.2 MPa, the reaction temperature at 65°C, and the reaction time at 20 min.

[0076] S5. Thin-film evaporation separation: The second reactant is continuously fed into a thin-film evaporator, and the evaporation temperature is controlled to be ≤70℃ and the evaporation pressure is controlled to be ≤-0.095MPa. Methanol, water and unreacted dimethyl carbonate are evaporated under reduced pressure, and the concentrated liquid of the heavy components after separation is collected.

[0077] S6. Gradient cooling crystallization: The concentrated heavy component solution is continuously fed into the crystallization kettle, and the temperature is reduced in stages by stirring. First, the temperature is reduced to 30°C by stirring at a rate of 5°C / h, and then the stirring rate is reduced from 130r / min to 65r / min. At the same time, the temperature is reduced to 10°C by stirring at a rate of 3°C / h to form a kettle solution containing carbazide crystals.

[0078] S7. Centrifugal separation: The liquid in the vessel is sent to a centrifuge for solid-liquid separation. The separated filter cake is washed and dried. The centrifugation parameters are: centrifugation speed 3000 r / min, centrifugation temperature 10℃, centrifugation time 20 min. The water temperature for cold water washing is 10℃, and the number of washings is 2. The amount of water used for each washing is 1 times the mass of the filter cake. Wet carbazine is obtained. After drying, the finished product carbazine 2# is obtained. At the same time, the centrifugation mother liquor and washing liquid are collected.

[0079] Example 3 Carbazine #3

[0080] A continuous production method for carbazine 3# with low free hydrazine content includes the following steps:

[0081] S1. Raw material premixing: The centrifuged mother liquor and washing liquid obtained in Example 2 are combined and pumped to the premixing tank. At the same time, dimethyl carbonate is added. The amount of dimethyl carbonate used here is 20% of the dimethyl carbonate used in the first batch of production. The mixture is stirred and mixed to form a mixture.

[0082] S2. Emulsification treatment: The mixture is fed into the emulsification equipment, which consists of a high-shear emulsification conveying pump and a static mixer. The mixture is first sheared and dispersed by the high-shear emulsification conveying pump, and then enters the static mixer for further mixing to form a uniform emulsion.

[0083] S3. Low-temperature microchannel reaction: The emulsion is pumped into the first microchannel reactor. The first microchannel reactor has 10 reaction modules connected in series. The reaction pressure is controlled at 0.1 MPa, the reaction temperature at 25°C, and the reaction time at 10 min.

[0084] S4. Medium-temperature microchannel reaction: The first reactant is pumped into the second microchannel reactor. In step S4, the second microchannel reactor has 16 reaction modules connected in series. The reaction pressure is controlled at 0.2 MPa, the reaction temperature at 70°C, and the reaction time at 15 min.

[0085] S5. Thin-film evaporation separation: The second reactant is continuously fed into a thin-film evaporator, and the evaporation temperature is controlled to be ≤70℃ and the evaporation pressure is controlled to be ≤-0.095MPa. Methanol, water and unreacted dimethyl carbonate are evaporated under reduced pressure, and the concentrated liquid of the heavy components after separation is collected.

[0086] S6. Gradient cooling crystallization: The concentrated solution of heavy components is continuously fed into the crystallization kettle, and the temperature is reduced in stages by stirring. First, the temperature is reduced to 30°C by stirring at a rate of 8°C / h, and then the stirring rate is reduced from 130r / min to 65r / min. At the same time, the temperature is reduced to 5°C by stirring at a rate of 2°C / h to form a kettle solution containing carbazide crystals.

[0087] S7. Centrifugal separation: The liquid in the vessel is sent to a centrifuge for solid-liquid separation. The separated filter cake is washed and dried. The centrifugation parameters are: centrifugation speed 5000 r / min, centrifugation temperature 5℃, centrifugation time 10 min. The water temperature for cold water washing is 10℃, and the washing is performed once. The amount of water used for each washing is 1 times the mass of the filter cake. Wet carbazine is obtained. After drying, the finished product carbazine 3# is obtained. At the same time, the centrifugation mother liquor and washing liquid are collected.

[0088] Example 4: Carbazine #4

[0089] A continuous production method for carbazine #4 with low free hydrazine content includes the following steps:

[0090] S1. Raw material premixing: The centrifuged mother liquor and washing liquid obtained in Example 3 are combined and pumped to the premixing tank. At the same time, dimethyl carbonate is added. The amount of dimethyl carbonate used here is 10% of the dimethyl carbonate used in the first batch of production. The mixture is stirred and mixed to form a mixture.

[0091] S2. Emulsification treatment: The mixture is fed into the emulsification equipment, which consists of a high-shear emulsification conveying pump and a static mixer. The mixture is first sheared and dispersed by the high-shear emulsification conveying pump, and then enters the static mixer for further mixing to form a uniform emulsion.

[0092] S3. Low-temperature microchannel reaction: The emulsion is pumped into the first microchannel reactor. The first microchannel reactor has 10 reaction modules connected in series. The reaction pressure is controlled at 0.1 MPa, the reaction temperature at 25°C, and the reaction time at 10 min.

[0093] S4. Medium-temperature microchannel reaction: The first reactant is pumped into the second microchannel reactor. In step S4, the second microchannel reactor has 16 reaction modules connected in series. The reaction pressure is controlled at 0.2 MPa, the reaction temperature at 65°C, and the reaction time at 20 min.

[0094] S5. Thin-film evaporation separation: The second reactant is continuously fed into a thin-film evaporator, and the evaporation temperature is controlled to be ≤70℃ and the evaporation pressure is controlled to be ≤-0.095MPa. Methanol, water and unreacted dimethyl carbonate are evaporated under reduced pressure, and the concentrated liquid of the heavy components after separation is collected.

[0095] S6. Gradient cooling crystallization: The concentrated solution of heavy components is continuously fed into the crystallization vessel and cooled in stages with stirring. First, the temperature is lowered to 30°C with stirring at a rate of 5°C / h. Then, the stirring rate is reduced from 130r / min to 65r / min, while the temperature is further lowered to 5°C with a rate of 2°C / h, to form a vessel solution containing carbazide crystals.

[0096] S7. Centrifugal separation: The liquid in the vessel is sent to a centrifuge for solid-liquid separation. The separated filter cake is washed and dried. The centrifugation parameters are: centrifugation speed 4000 r / min, centrifugation temperature 10℃, centrifugation time 20 min. The water temperature for cold water washing is 5℃, and the number of washings is 2. The amount of water used for each washing is 1 times the mass of the filter cake. Wet carbazine is obtained. After drying, the finished carbazine 4# is obtained. At the same time, the centrifugation mother liquor and washing liquid are collected and are not sent to the premixing tank. They are directly sent to the mother liquor distillation.

[0097] Comparative Example 1

[0098] The difference between Comparative Example 1 and Example 1 is that the weight ratio of dimethyl carbonate to hydrazine hydrate in Comparative Example 1 is 1:1.5, while all other conditions are the same as in Example 1.

[0099] Comparative Example 2

[0100] The difference between Comparative Example 2 and Example 1 is that the first microchannel reactor is omitted in Comparative Example 2, and the mixture is directly introduced into the second microchannel reactor and reacted at 70°C for 15 minutes, while the other conditions remain the same.

[0101] Comparative Example 3

[0102] The difference between Comparative Example 3 and Example 1 is that a thin-film evaporator was not used in Comparative Example 3. Instead, atmospheric distillation was used to remove light components. The operating temperature was 90°C and the distillation time was 40 min. The other process parameters remained unchanged.

[0103] Comparative Example 4

[0104] The difference between Comparative Example 4 and Example 1 is that the crystallization process in Comparative Example 4 adopts a one-time rapid cooling to 10°C, the cooling rate is 10°C / h, and the stirring rate is kept constant at 300r / min, while the other steps are the same.

[0105] Comparative Example 5

[0106] The difference between Comparative Example 5 and Example 1 is that in Comparative Example 5, the centrifuged mother liquor and washing liquid are combined and reused without limitation on the number of times, and can be reused continuously up to the 5th time. The amount of material added each time remains the same, and the rest of the operation is the same as in Example 1.

[0107] Experimental Example

[0108] The carbazine prepared in Examples 1-4 and Comparative Examples 1-5 were subjected to detection experiments. The detection methods or reference standards and technical indicators are shown in Table 1. The experimental data of each example and comparative example are shown in Table 2.

[0109] Table 1

[0110]

[0111] Table 2 Experimental data for each embodiment and comparative example

[0112]

[0113] As can be seen from the above, the carbazine prepared by the production method specified in this application has better performance indicators than the industry standard. The free hydrazine content is as low as 87 ppm (far below the requirement of ≤250 ppm), the purity reaches 99.8%, and the melting point, drying loss, and clarity all meet the requirements of high-purity applications. This shows that the process has outstanding advantages in suppressing side reactions and improving product quality.

[0114] In Comparative Example 1, the actual yield decreased, but the product purity was not significantly affected. However, dimethyl carbonate was distilled off, resulting in a low actual reaction conversion rate and a loss of raw materials. In Comparative Example 2, the first microchannel reactor was omitted, leading to a surge in side reactions and free hydrazine reaching 220 ppm. In Comparative Example 3, atmospheric distillation was used, but the high temperature caused product degradation and excessive loss on drying. In Comparative Example 4, rapid cooling caused impurities to be encapsulated in crystals, and free hydrazine rose to 225 ppm. In Comparative Example 5, the mother liquor was reused five times, resulting in a high free hydrazine content.

[0115] The above description is merely a preferred embodiment of this application and is not intended to limit this application. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of this application should be included within the protection scope of this application.

Claims

1. A continuous production method for carbazine with low free hydrazine content, characterized in that, Includes the following steps: S1. Raw material premixing: Dimethyl carbonate and hydrazine hydrate are added to a premixing tank at a weight ratio of 1:(2.2-2.4) and stirred to form a mixture; S2. Emulsification treatment: The mixture is fed into an emulsification device and processed to form a uniform emulsion; S3. Low-temperature microchannel reaction: The emulsion is pumped into the first microchannel reactor, which is composed of multi-stage reaction modules connected in series. After the reaction is completed, the first reactant is obtained. The reaction pressure is controlled at 0.1 MPa, the reaction temperature at 20-25℃, and the reaction time at 10-12 min. S4. Medium-temperature microchannel reaction: The first reactant is pumped into the second microchannel reactor, which is composed of multi-stage reaction modules connected in series. After the reaction is completed, the second reactant is obtained. The reaction pressure is controlled at 0.2-0.3 MPa, the reaction temperature at 65-70℃, and the reaction time at 15-20 min. S5. Thin-film evaporation separation: The second reactant is continuously fed into a thin-film evaporator, and methanol, water and unreacted dimethyl carbonate are evaporated under reduced pressure. The concentrated liquid of the heavy components after separation is collected. S6. Gradient cooling crystallization: The concentrated liquid of heavy components is continuously fed into the crystallization kettle, and the temperature is gradually reduced by stirring to form a kettle liquid containing carbazine crystals; S7. Centrifugal separation: The liquid in the vessel is sent to a centrifuge for solid-liquid separation. The filter cake obtained by separation is washed and dried to obtain wet carbazine. After drying, the finished carbazine is obtained. At the same time, the centrifugal mother liquor and washing liquid are collected. S8. Mother liquor recycling: The centrifuged mother liquor and washing liquid are combined and transported to the premix tank, mixed with newly added dimethyl carbonate, and the above operations S2 to S7 are repeated. The cumulative number of times the centrifuged mother liquor and washing liquid are combined shall not exceed 3 times, so as to realize the continuous production of carbazide. In step S5, the second reactant is continuously fed into a thin-film evaporator, and the evaporation temperature is controlled to be ≤70℃ and the evaporation pressure is controlled to be ≤-0.095MPa. In step S6, first stir and cool down to 25-35℃, then slow down the stirring speed and continue to cool down to 5-10℃.

2. The production method according to claim 1, characterized in that, In step S2, the emulsification equipment consists of a high-shear emulsification delivery pump and a static mixer. The mixture is first sheared and dispersed by the high-shear emulsification delivery pump, and then enters the static mixer for further mixing to form a uniform emulsion.

3. The production method according to claim 1, characterized in that, In step S3, the number of reaction modules in the first microchannel reactor is no less than 10, and each reaction module is connected in series.

4. The production method according to claim 1, characterized in that, In step S4, the second microchannel reactor has no fewer than 15 reaction modules, and each reaction module is connected in series.

5. The production method according to claim 1, characterized in that, First, stir at a rate of 5-8℃ / h to cool down to 30℃, then reduce the stirring rate from 120-130r / min to 50-65r / min, while continuing to cool down to 5-10℃ at a rate of 1-3℃ / h.

6. The production method according to claim 1, characterized in that, The centrifugation parameters in step S7 are: centrifugation speed 3000-5000 r / min, centrifugation temperature 5-10℃, and centrifugation time 10-20 min; The water temperature for cold washing is 5-10℃, and the number of washing cycles is 1-2. The amount of water used for each washing cycle is 0.5-1 times the mass of the filter cake.

7. The production method according to claim 1, characterized in that, In step S8, when applying it for the first time, the amount of newly added dimethyl carbonate is 30-32% of the amount of dimethyl carbonate used in the first feeding. When using it a second time, the amount of newly added dimethyl carbonate should be 20-22% of the amount of dimethyl carbonate used in the first feeding. When applying it for the third time, the amount of newly added dimethyl carbonate should be 8-10% of the amount of dimethyl carbonate used in the first feeding.

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