A method and system for green production of hexamethylene diisocyanate

Abstract

The application provides a method and system for green preparation of hexamethylene diisocyanate, which comprises a first thermal decomposition reaction device and a second thermal decomposition reaction device connected in series, a two-component heat carrier composed of a low-boiling heat carrier and a high-boiling heat carrier, a first partial condenser and a second partial condenser. The application has the beneficial effects that: the product hexamethylene diisocyanate is prevented from staying at high temperature for too long, thereby avoiding further polymerization reaction; the by-product methanol generated in the thermal decomposition reaction is continuously carried out of the reaction system by the low-boiling carrier, thereby promoting the continuous forward progress of the thermal decomposition reaction; the isocyanate concentration is prevented from rising due to the loss of a large amount of low-boiling heat carrier, thereby preventing the side reaction from intensifying; the isocyanate concentration is effectively reduced, thereby preventing the occurrence of the polymerization side reaction.

Description

Technical Field

[0001] This invention belongs to the field of chemical synthesis, and in particular relates to a method and system for the green preparation of hexamethylene diisocyanate. Background Technology

[0002] Isocyanates are important organic synthesis intermediates, widely used in pharmaceuticals, dyes, adhesives, sealants, and other fields. They are also a crucial raw material for the synthesis of polyurethane, with approximately 90% of the isocyanates produced annually used in polyurethane materials. Currently, more than 50 types of isocyanates are industrially produced globally, with larger production volumes including phenyl diisocyanate (TDI), diphenylmethane diisocyanate (MDI), polyphenylene polymethyl isocyanate (PAPI), and hexamethylene diisocyanate. Compared to TDI, MDI, and PAPI, HDI, an aliphatic isocyanate, possesses a saturated molecular structure and less steric hindrance, resulting in products with excellent light stability, heat resistance, yellowing resistance, and weather resistance, giving it significant market application value.

[0003] Currently, HDI is mainly produced worldwide through the phosgene process. Although this method has a mature synthesis process and is economical, it also has the following problems: (1) The process is complex and the reaction conditions are harsh; (2) A large amount of highly toxic phosgene gas is required during the production process, which leads to strict restrictions on the storage, transportation and use of raw materials; (3) Highly corrosive hydrochloric acid is generated as a byproduct, which can easily corrode equipment; (4) The residual chlorine in the product is difficult to remove, which directly affects the performance of the product. Therefore, with the development of technology and the increasing environmental protection requirements, the phosgene process has gradually been restricted, and the simple, economical and environmentally friendly non-phosgene synthesis method has gained more favor.

[0004] Many green, non-phosgene methods for synthesizing HDI have been reported in patents and literature, such as the carbonylation of nitro compounds, the carbonylation of hexamethylenediamine, the cyanation method, and the thermal decomposition of carbamates. Among them, the thermal decomposition of carbamates has the advantages of a clean process and environmentally friendly raw materials, making it the process with the greatest potential for industrialization. This process is mainly achieved through two steps: (1) preparing the intermediate methyl hexamethylenedicarbamate (HDU); (2) catalytically thermally decomposing the synthesized intermediate HDU to prepare HDI and the byproduct methanol.

[0005] The thermal decomposition of HDU to prepare HDI is the most important and challenging step in the urethane thermal decomposition route for HDI production. The reaction conditions not only affect the reaction rate but, more importantly, the yield and quality of the HDI product. The thermal decomposition process of HDU mainly consists of two steps, with the latter requiring more energy than the former.

[0006] (1) HDU first pyrolyzes the methyl formate group at one end to generate the intermediate hexamethylene 1-(-6-carbamate) isocyanate (HMI).

[0007]

[0008] (2) The intermediate HMI undergoes thermal decomposition upon further heating to form HDI.

[0009]

[0010] Currently, the main problems with this step are as follows: (1) The generated isocyanate is very easy to react with the generated byproduct methanol to generate methyl carbamate; (2) The generated HDI reacts with the raw material HDU to form a polymer; (3) HDI self-polymerizes under high temperature conditions to generate trimer.

[0011] To overcome the above problems, the following measures can be taken: (1) Remove the methanol generated by the reaction from the thermal decomposition system as soon as possible; (2) Use a highly efficient catalyst to reduce the thermal decomposition temperature and time; (3) Add a heat carrier (i.e. an inert solvent that does not react with the raw materials and products) to the reaction system to reduce the concentration of isocyanate and avoid the occurrence of side reactions.

[0012] Patent CN101530785A discloses a method for preparing HDI using a two-component inert solvent (composed of an inert solvent with a higher boiling point and an inert solvent with a lower boiling point). The method utilizes the evaporation of the low-boiling-point solvent to remove byproduct alcohols, thus promoting the thermal decomposition reaction. However, the evaporation of the low-boiling-point solvent increases the concentration of substances, leading to side reactions.

[0013] Patent CN102964272A discloses a method for preparing HDI by catalytic thermal decomposition of HDU using a composite catalyst. This process achieves a high HDI yield, greater than 92%, but because the reaction uses only a high-boiling-point heat transfer medium, the generated alcohols are difficult to separate from the system in a timely manner, resulting in low HDI purity.

[0014] Patent CN114456091A discloses an apparatus for preparing HDI by pyrolyzing HDU in a mixed solvent. This patent achieves the separation and collection of products, by-products and medium-boiling-point heat carriers by adding a product separator. However, the products, by-products and medium-boiling-point heat carriers need to enter the product separator in a certain order, so the production efficiency is low and the product purity is low, only about 40%. Summary of the Invention

[0015] In view of this, the present invention aims to provide a green and efficient preparation system for hexamethylene diisocyanate to solve the problems existing in the prior art.

[0016] To achieve the above objectives, the technical solution of the present invention is implemented as follows:

[0017] A green preparation system for hexamethylene diisocyanate includes a mixing tank, a first thermal decomposition reaction device, a second thermal decomposition reaction device, a distillation device, and a product tank. The mixing tank is equipped with a methyl hexamethylene dicarboxylate inlet, a heat carrier inlet, and a catalyst inlet. The outlet of the mixing tank is connected to the inlet of the first thermal decomposition reaction device. The liquid phase outlet of the first thermal decomposition reaction device is connected to the inlet of the second thermal decomposition reaction device. The liquid phase outlet of the second thermal decomposition reaction device is connected to the inlet of the distillation device. The outlet of the distillation device is connected to the inlet of the product tank.

[0018] Furthermore, the first thermal decomposition reaction device and the second thermal decomposition reaction device are one or two of the following: a closed reaction vessel, a fluidized bed reactor, a falling film evaporator, and a thin film evaporator.

[0019] Furthermore, the preparation system also includes a first condenser, a second condenser, and a by-product tank. The feed inlet of the first condenser is connected to the mixed steam outlet of the first pyrolysis reaction device, the feed inlet of the second condenser is connected to the mixed steam outlet of the second pyrolysis reaction device, and the outlets of both the first and second condensers are connected to the feed inlet of the by-product tank.

[0020] Furthermore, the preparation system also includes a by-product condenser, which is located between the first and second condensers and the by-product tank. The outlets of the first and second condensers are connected to the inlet of the by-product condenser, and the outlet of the by-product condenser is connected to the inlet of the by-product tank.

[0021] Furthermore, the heat carrier outlet of the distillation unit is connected to the heat carrier inlet of the mixing tank.

[0022] Furthermore, the preparation system also includes a product condenser, which is located between the distillation unit and the product tank. The outlet of the distillation unit is connected to the inlet of the product condenser, and the outlet of the product condenser is connected to the inlet of the product tank.

[0023] A green preparation method for hexamethylene diisocyanate, utilizing any of the above-described green preparation systems for hexamethylene diisocyanate, includes the following steps:

[0024] S1: Under nitrogen protection, the raw material hexamethylene dicarboxylate, heat carrier and catalyst are added to the mixing tank, mixed evenly and preheated to obtain a mixture;

[0025] S2: The mixed solution obtained in step S1 is transported to the first thermal decomposition reaction device to carry out a thermal decomposition reaction to obtain a mixed solution;

[0026] S3: The mixture obtained in step S2 is transported to the second thermal decomposition reaction device and heated to carry out a secondary thermal decomposition reaction to obtain a mixture containing the target product and the heat carrier.

[0027] S4: The mixture obtained in step S3 is transported to a distillation unit for distillation. The distilled product is transported to a product tank, and the distilled heat carrier is transported back to the batching tank for recycling via pipeline.

[0028] Furthermore, in S1, the mass ratio of hexamethylene dicarboxylate, heat carrier, and catalyst is 1:1-20:0.01-0.5, and the preheating temperature is 70-120℃.

[0029] Furthermore, the heat transfer medium in S1 is composed of a low-boiling-point heat transfer medium and a high-boiling-point heat transfer medium, and the mass ratio of the low-boiling-point heat transfer medium to the high-boiling-point heat transfer medium is 1:1-20.

[0030] Preferably, the low-boiling-point heat transfer fluid is a solvent with a normal boiling point of 100-200℃ and that does not react with isocyanates and alcohols, including toluene, xylene, o-chlorotoluene, p-chlorotoluene, and p-dichlorobenzene;

[0031] Preferably, the high-boiling-point heat transfer fluid is a solvent with a normal-pressure boiling point greater than 290°C that does not react with isocyanates and alcohols, including diethyl phthalate, diethyl terephthalate, dioctyl terephthalate, and dioctyl phthalate.

[0032] Furthermore, the reaction temperature of the first thermal decomposition reaction in S2 is 150-200℃, the reaction pressure is -0.05-1.0MPa, and the reaction time is 2-6h;

[0033] Preferably, the reaction temperature of the second thermal decomposition reaction in S3 is 200-250℃, the reaction pressure is -0.05-1.0MPa, and the reaction time is 0.5-2.5h.

[0034] Compared with existing technologies, the green preparation method and system for hexamethylene diisocyanate described in this invention has the following advantages:

[0035] (1) The present invention uses a first thermal decomposition reaction device and a second thermal decomposition reaction device connected in series to allow the raw material hexamethylene dicarboxylate to undergo preliminary decomposition at a lower temperature, and then further decompose it at a higher temperature to completely prepare the target product hexamethylene diisocyanate. This can avoid the product hexamethylene diisocyanate from staying at high temperature for too long, thereby avoiding further polymerization reaction.

[0036] (2) The heat carrier described in this invention is a two-component heat carrier composed of a low-boiling-point heat carrier and a high-boiling-point heat carrier, which enables the methanol by-product generated by the thermal decomposition reaction to be continuously carried out of the reaction system by the low-boiling-point heat carrier, thereby promoting the thermal decomposition reaction to proceed in the forward direction.

[0037] (3) By setting up a first condenser and a second condenser, the present invention allows the low-boiling-point heat carrier with a higher boiling point in the mixed steam of the pyrolysis reactor to flow back to the pyrolysis reactor. The methanol, a byproduct with a lower boiling point, is collected by the byproduct tank through the condenser. This avoids the increase in isocyanate concentration due to the large loss of low-boiling-point heat carrier, which would exacerbate the side reactions. At the same time, the high-boiling-point heat carrier group that is always present in the reaction system can also effectively reduce the isocyanate concentration and avoid the occurrence of polymerization side reactions. Attached Figure Description

[0038] The accompanying drawings, which form part of this invention, are used to provide a further understanding of the invention. The illustrative embodiments of the invention and their descriptions are used to explain the invention and do not constitute an undue limitation of the invention. In the drawings:

[0039] Figure 1 This is a schematic diagram of the connection structure of a green preparation system for hexamethylene diisocyanate according to an embodiment of the present invention;

[0040] Figure 2 This is a gas chromatogram of a hexamethylene diisocyanate product prepared by a green preparation method for hexamethylene diisocyanate as described in an embodiment of the present invention.

[0041] Explanation of reference numerals in the attached figures:

[0042] 1. Batching tank; 2. First pyrolysis reaction apparatus; 3. Second pyrolysis reaction apparatus; 4. Distillation apparatus; 5. Product tank; 6. First condenser; 7. Second condenser; 8. By-product tank. Detailed Implementation

[0043] It should be noted that, unless otherwise specified, the embodiments and features described in the present invention can be combined with each other.

[0044] The present invention will now be described in detail with reference to the accompanying drawings and embodiments.

[0045] A green preparation system for hexamethylene diisocyanate includes a mixing tank 1, a first thermal decomposition reaction device 2, a second thermal decomposition reaction device 3, a distillation device 4, and a product tank 5. The mixing tank 1 is provided with a hexamethylene dicarboxylate inlet, a heat carrier inlet, and a catalyst inlet. The outlet of the mixing tank 1 is connected to the inlet of the first thermal decomposition reaction device 2. The liquid phase outlet of the first thermal decomposition reaction device 2 is connected to the inlet of the second thermal decomposition reaction device 3. The liquid phase outlet of the second thermal decomposition reaction device 3 is connected to the inlet of the distillation device 4. The outlet of the distillation device 4 is connected to the inlet of the product tank 5.

[0046] The first thermal decomposition reaction device 2 and the second thermal decomposition reaction device 3 are one or two of the following: closed reactor, fluidized bed reactor, falling film evaporator, and thin film evaporator.

[0047] A green preparation system for hexamethylene diisocyanate further includes a first condenser 6, a second condenser 7, and a by-product tank 8. The inlet of the first condenser 6 is connected to the mixed steam outlet of the first thermal decomposition reaction device 2, the inlet of the second condenser 7 is connected to the mixed steam outlet of the second thermal decomposition reaction device 3, and the outlets of both the first condenser 6 and the second condenser 7 are connected to the inlet of the by-product tank 8.

[0048] A green preparation system for hexamethylene diisocyanate also includes a by-product condenser, which is located between a first condenser 6 and a second condenser 7 and a by-product tank 8. The outlets of the first condenser 6 and the second condenser 7 are connected to the inlet of the by-product condenser, and the outlet of the by-product condenser is connected to the inlet of the by-product tank 8.

[0049] The heat carrier outlet of the distillation unit 4 is connected to the heat carrier inlet of the batching tank 1.

[0050] A green preparation system for hexamethylene diisocyanate also includes a product condenser, which is located between a distillation unit 4 and a product tank 5. The outlet of the distillation unit 4 is connected to the inlet of the product condenser, and the outlet of the product condenser is connected to the inlet of the product tank 5.

[0051] A green preparation method for hexamethylene diisocyanate, utilizing the aforementioned green preparation system for hexamethylene diisocyanate, includes the following steps:

[0052] S1: Under nitrogen protection, the raw material hexamethylene dicarboxylate, heat carrier and catalyst are added to the mixing tank 1, mixed evenly and preheated to a certain temperature to obtain a mixed solution;

[0053] S2: The mixed solution obtained in step S1 is transported to the first thermal decomposition reaction device 2 and heated to carry out a thermal decomposition reaction to obtain a mixed solution;

[0054] S3: The mixture obtained in step S2 is transported to the second thermal decomposition reaction device 3 and heated to carry out a secondary thermal decomposition reaction to obtain a mixture containing the target product and the heat carrier.

[0055] S4: The mixture obtained in step S3 is transported to the distillation unit 4 for distillation, and the distilled product is transported to the product tank 5. The distilled heat carrier is transported back to the batching tank 1 through the pipeline for recycling.

[0056] In S1, the mass ratio of hexamethylene dicarboxylate, heat carrier, and catalyst is 1:1-20:0.01-0.5, and the preheating temperature is 70-120℃.

[0057] The heat transfer medium in S1 consists of a low-boiling-point heat transfer medium and a high-boiling-point heat transfer medium, with a mass ratio of 1:1-20 between the low-boiling-point heat transfer medium and the high-boiling-point heat transfer medium.

[0058] The low-boiling-point heat transfer fluid is a solvent with a boiling point of 100-200℃ at normal pressure that does not react with isocyanates and alcohols, including toluene, xylene, o-chlorotoluene, p-chlorotoluene, and p-dichlorobenzene;

[0059] The high-boiling-point heat transfer fluid is a solvent with a boiling point greater than 290°C at normal pressure that does not react with isocyanates and alcohols, including diethyl phthalate, diethyl terephthalate, dioctyl terephthalate, and dioctyl phthalate.

[0060] The reaction temperature of the first thermal decomposition reaction in S2 is 150-200℃, the reaction pressure is -0.05-1.0MPa, and the reaction time is 2-6h.

[0061] The reaction temperature for the second thermal decomposition reaction in S3 is 200-250℃, the reaction pressure is -0.05-1.0MPa, and the reaction time is 0.5-2.5h.

[0062] Example 1

[0063] The experimental steps are as described above, wherein:

[0064] S1: The mass ratio of raw material hexamethylene dicarboxylate, heat carrier and catalyst is 1:10:0.1, the preheating temperature is 100℃, and the mass ratio of low boiling point heat carrier to high boiling point heat carrier in the heat carrier is 1:4.

[0065] S2: The temperature of the first thermal decomposition reaction is 170℃, the reaction time is 2.5h, and the pressure is 0.2MPa;

[0066] S3: The second thermal decomposition reaction temperature is 230℃, the reaction time is 1h, and the pressure is 0.6MPa.

[0067] The reaction substrate and product were qualitatively and quantitatively detected by gas chromatography. The conversion rate of the raw material hexamethylene dicarboxylate was 100%, the selectivity of the product hexamethylene diisocyanate was 97.3%, and the purity of the product was 98.5%.

[0068] Example 2

[0069] The experimental steps are as described above, wherein:

[0070] S1: The mass ratio of raw material hexamethylene dicarboxylate, heat carrier and catalyst is 1:5:0.05, the preheating temperature is 120℃, and the mass ratio of low boiling point heat carrier to high boiling point heat carrier in the heat carrier is 1:20.

[0071] S2: The temperature of the first thermal decomposition reaction is 180℃, the reaction time is 2h, and the pressure is 0.2MPa;

[0072] S3: The second thermal decomposition reaction temperature is 240℃, the reaction time is 0.5h, and the pressure is 0.8MPa.

[0073] The reaction substrate and product were qualitatively and quantitatively detected by gas chromatography. The conversion rate of the raw material hexamethylene dicarboxylate was 100%, the selectivity of the product hexamethylene diisocyanate was 96.8%, and the purity of the product was 99.1%.

[0074] Example 3

[0075] The experimental steps are as described above, wherein:

[0076] S1: The mass ratio of raw material hexamethylene dicarboxylate, heat carrier and catalyst is 1:20:0.25, the preheating temperature is 100℃, and the mass ratio of low boiling point heat carrier to high boiling point heat carrier in the heat carrier is 1:9.

[0077] S2: The temperature of the first thermal decomposition reaction is 150℃, the reaction time is 5h, and the pressure is atmospheric pressure;

[0078] S3: The second thermal decomposition reaction temperature is 220℃, the reaction time is 1.5h, and the pressure is 0.4MPa.

[0079] The reaction substrate and product were qualitatively and quantitatively detected by gas chromatography. The conversion rate of the raw material methyl hexamethylene dicarboxylate was 100%, the selectivity of the product hexamethylene diisocyanate was 98.1%, and the purity of the product was 98.9%.

[0080] Comparative Example 1

[0081] The experimental steps are as described above, wherein:

[0082] S1: The mass ratio of raw material hexamethylene dicarboxylate, heat carrier and catalyst is 1:10:0.1, the preheating temperature is 100℃, and the heat carrier is a single-component high-boiling-point heat carrier.

[0083] S2: The temperature of the first thermal decomposition reaction is 170℃, the reaction time is 2.5h, and the pressure is 0.2MPa;

[0084] S3: The second thermal decomposition reaction temperature is 230℃, the reaction time is 1h, and the pressure is 0.6MPa.

[0085] The reaction substrate and product were qualitatively and quantitatively detected by gas chromatography. The conversion rate of the raw material hexamethylene dicarboxylate was 100%, the selectivity of the product hexamethylene diisocyanate was 86.5%, and the purity of the product was 93.3%.

[0086] Comparative Example 2

[0087] The experimental steps are as described above, wherein:

[0088] S1: The mass ratio of raw material hexamethylene dicarboxylate, heat carrier and catalyst is 1:10:0.1, the preheating temperature is 100℃, and the mass ratio of low boiling point heat carrier to high boiling point heat carrier in the heat carrier is 1:4.

[0089] S3: The mixture in S1 is directly fed into the second thermal decomposition reaction device without passing through the first thermal decomposition reaction device. The second thermal decomposition reaction temperature is 220℃, the reaction time is 3.5h, and the pressure is 0.6MPa.

[0090] The reaction substrate and product were qualitatively and quantitatively detected by gas chromatography. The conversion rate of the raw material hexamethylene dicarboxylate was 98.4%, the selectivity of the product hexamethylene diisocyanate was 82.6%, and the purity of the product was 92.8%.

[0091] Comparative Example 3

[0092] The experimental steps are as described above, wherein:

[0093] S1: The mass ratio of raw material hexamethylene dicarboxylate, heat carrier and catalyst is 1:10:0.1, the preheating temperature is 100℃, and the mass ratio of low boiling point heat carrier to high boiling point heat carrier in the heat carrier is 1:4.

[0094] S2: The temperature of the first thermal decomposition reaction is 170℃, the reaction time is 2.5h, the pressure is 0.2MPa, and the connection between the first thermal decomposition reaction device and the first condenser is closed.

[0095] S3: The second thermal decomposition reaction temperature is 230℃, the reaction time is 1h, the pressure is 0.6MPa, and the connection between the second thermal decomposition reaction device and the second condenser is closed.

[0096] The reaction substrate and product were qualitatively and quantitatively detected by gas chromatography. The conversion rate of the raw material methyl hexamethylene dicarboxylate was 96.5%, the selectivity of the product hexamethylene diisocyanate was 84.5%, and the purity of the product was 89.7%.

[0097] In the above examples and comparative examples, the conversion rate of hexamethylene dicarboxylate, the selectivity of hexamethylene diisocyanate, and the purity of the product were all determined by gas chromatography in accordance with the requirements of Appendix A of the national standard GB / T37042-2018. The conversion rate of hexamethylene dicarboxylate, the selectivity of hexamethylene diisocyanate, and the purity of the product were calculated by the area normalization method.

[0098] Table 1

[0099] Conversion rate % Selectivity % purity% Example 1 100 97.3 98.5 Example 2 100 96.8 99.1 Example 3 100 98.1 98.9 Comparative Example 1 100 86.5 93.3 Comparative Example 2 98.4 82.6 92.8 Comparative Example 3 96.5 84.5 89.7

[0100] Comparing Example 1 and Comparative Example 1, it can be seen that using a two-component heat transfer fluid can significantly improve the selectivity and quality of the product.

[0101] Comparing Example 1 and Comparative Example 2, it can be seen that the two-step thermal decomposition process is better than the one-step thermal decomposition process.

[0102] Comparing Example 1 and Comparative Example 3, it can be seen that timely removal of by-product methanol through a condenser can achieve higher feed conversion rate, product selectivity, and product purity.

[0103] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A process for the green preparation of hexamethylene diisocyanate, characterized in that: The method comprises the following steps: S1: under the protection of nitrogen, raw material methyl hexamethylene diurea, heat carrier and catalyst are added into a batching tank, mixed uniformly and preheated to obtain a mixed solution; S2: the mixed solution obtained in step S1 is transported to a first thermal decomposition reaction device to perform a first thermal decomposition reaction to obtain a mixed solution; S3: the mixed solution obtained in step S2 is transported to a second thermal decomposition reaction device to perform a second thermal decomposition reaction to obtain a mixed solution containing target product and heat carrier; S4: the mixed solution obtained in step S3 is transported to a rectification device to perform distillation, the distilled product is transported to a product tank, and the distilled heat carrier is transported back to the batching tank through a pipeline for recycling; In step S1, the mass ratio of methyl hexamethylene diurea, heat carrier and catalyst is 1:1-20:0.01-0.5, and the preheating temperature is 70-120℃; In step S1, the heat carrier is composed of a low-boiling heat carrier and a high-boiling heat carrier, and the mass ratio of the low-boiling heat carrier to the high-boiling heat carrier is 1:1-20; In step S2, the reaction temperature of the first thermal decomposition reaction is 150-200℃, the reaction pressure is-0.05-1.0 MPa, and the reaction time is 2-6 h; In step S3, the reaction temperature of the second thermal decomposition reaction is 200-250℃, the reaction pressure is-0.05-1.0 MPa, and the reaction time is 0.5-2.5 h The above-mentioned green preparation system for hexamethylene diisocyanate comprises a batching tank, a first thermal decomposition reaction device, a second thermal decomposition reaction device, a rectification device, a product tank, a first condenser, a second condenser and a by-product tank; The first condenser feed port is in communication with the mixed vapor outlet of the first thermal decomposition reaction device, the second condenser feed port is in communication with the mixed vapor outlet of the second thermal decomposition reaction device, and the outlet ports of the first condenser and the second condenser are in communication with the feed port of the by-product tank.

2. A process for the green preparation of hexamethylene diisocyanate according to claim 1, characterized by: The batching tank is provided with a methyl hexamethylene diurea feed port, a heat carrier feed port and a catalyst feed port, the outlet port of the batching tank is in communication with the feed port of the first thermal decomposition reaction device, the liquid phase outlet port of the first thermal decomposition reaction device is in communication with the feed port of the second thermal decomposition reaction device, the liquid phase outlet port of the second thermal decomposition reaction device is in communication with the feed port of the rectification device, and the outlet port of the rectification device is in communication with the feed port of the product tank; The first thermal decomposition reaction device and the second thermal decomposition reaction device are one or both of a closed reaction kettle, a fluidized bed reactor, a falling film evaporator and a thin film evaporator.

3. A process for the green preparation of hexamethylene diisocyanate according to claim 2, characterized in that: A by-product condenser is further included, which is arranged between the first condenser, the second condenser and the by-product tank, the outlet ports of the first condenser and the second condenser are in communication with the feed port of the by-product condenser, and the outlet port of the by-product condenser is in communication with the feed port of the by-product tank.

4. A process for green preparation of hexamethylene diisocyanate as claimed in claim 2, wherein: The heat carrier outlet port of the rectification device is in communication with the heat carrier feed port of the batching tank.

5. The green preparation method of hexamethylene diisocyanate according to claim 2, characterized in that: The product condenser is arranged between the rectifying device and the product tank, the outlet of the rectifying device is communicated with the inlet of the product condenser, and the outlet of the product condenser is communicated with the inlet of the product tank.

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