A process for the production of a crude isocyanate having a low content of monobenzenic impurities

Abstract

The present application relates to a production process of crude isocyanate with low monobenzenoid impurity content, comprising the following steps: 1) taking low-boiling inert organic matter as stripping agent, stripping crude isocyanate in stripping tower to remove monobenzenoid impurity components, obtaining crude isocyanate product with low monobenzenoid impurity content; 2) separating monobenzenoid impurity components / stripping agent collected from the top of the stripping tower from crude isocyanate by rectification, recovering high-purity crude isocyanate at the bottom of the tower, and directly feeding the stripping agent vapor formed at the top of the tower into the bottom of the stripping tower for stripping, and collecting the concentrated monobenzenoid impurities at the middle of the tower as waste liquid. The production process of the present application effectively reduces the separation difficulty of monobenzenoid impurity components and crude isocyanate, realizes the simple recovery of high-purity crude isocyanate, and greatly reduces the loss of crude isocyanate.

Description

Technical Field

[0001] This invention relates to the field of isocyanate production technology, and specifically to a production process for crude isocyanate with low content of monobenzene ring impurities. Background Technology

[0002] Isocyanates are widely used in the synthesis and production of products such as shoe sole raw materials, polyurethane foam, thermal insulation materials, and artificial leather. Isocyanates are mainly prepared using the phosgene method. The corresponding amines are mixed with an inert solvent and then thoroughly mixed with phosgene to undergo a phosgenation reaction, yielding a photochemical reaction solution. After phosgene removal and solvent removal, crude isocyanate is obtained. In the production of polymethylene polyphenyl polyisocyanates, the raw material amines typically contain small amounts of monophenylcyclic amines, resulting in the presence of monophenylcyclic isocyanates or their derivatives in the crude isocyanate. Monophenylcyclic isocyanates or their derivatives have low boiling points and low reactivity, and are easily retained in the product during subsequent crude isocyanate separation. This results in the polymethylene polyphenyl polyisocyanate product containing a certain amount (usually 800-3000 ppm) of monophenylcyclic isocyanates or their derivatives, which may pose a health hazard to users.

[0003] CN107652208A discloses a method and system for removing solvents from isocyanate products obtained from phosgenation reactions. The method involves using towers with different types of internal components to remove solvents from the isocyanate products. Specifically, a random-packed tower is used to remove the solvent components from the isocyanate products. However, this patent does not describe the removal efficiency of monobenzene ring isocyanates and their derivatives from the isocyanate products during this process, and the stripping process leads to the loss of some isocyanates.

[0004] CN108586706 A discloses a method for purifying isocyanates, specifically relating to a method for preparing isocyanates with low chlorine content. The method involves stripping isocyanates with an inert gas under atmospheric pressure to remove chlorinated derivatives and solvents. This patent uses hot nitrogen to strip crude isocyanates to remove light components. However, because the content of monobenzene ring impurities is very low, if complete removal of monobenzene ring impurities is required, a significant amount of crude isocyanate will be entrained in the gas phase during stripping. This mixture of isocyanate and polybenzene ring impurities needs to be recycled back into the system to reduce the loss of crude isocyanate. This leads to the accumulation of monobenzene ring impurities within the system, forming a dynamic equilibrium and limiting the complete removal of monobenzene ring impurities from the crude isocyanate.

[0005] Monobenzene ring impurities, as a light component, exist in the light component at the top of the column and in the pure isocyanate product in the column during the subsequent separation process of crude isocyanate. This results in the need for further treatment of the waste liquid generated at the top of the column, a reduction in the quality of the pure isocyanate product in the column, and an impact on the reactivity of the pure isocyanate.

[0006] Because the content of this monobenzene ring impurity in crude isocyanate is low, the traditional inert gas stripping process results in a low separation degree between the monobenzene ring impurity and crude isocyanate. A large amount of inert gas is required to remove the impurity at a high temperature. During this process, a large amount of crude isocyanate is removed along with the monobenzene ring impurity. The loss of crude isocyanate accounts for 0.15%-0.3% of the total isocyanate feed, resulting in huge economic losses.

[0007] Therefore, a more efficient and economical technical solution is needed to remove monobenzene ring impurities from crude isocyanates, thereby improving product quality. Summary of the Invention

[0008] The purpose of this invention is to provide a production process for crude isocyanate with low content of monobenzene ring impurities, which can effectively reduce the difficulty of separating monobenzene ring impurity components from crude isocyanate, achieve simple recovery of high-purity crude isocyanate, and greatly reduce the loss of crude isocyanate.

[0009] To achieve the above objectives, the present invention adopts the following technical solution:

[0010] A process for producing crude isocyanate with low content of monobenzene ring impurities includes the following steps:

[0011] 1) Using low-boiling-point inert organic matter as stripping agent, crude isocyanate is stripped in stripping tower to remove monobenzene ring impurities. The bottom of the stripping tower yields crude isocyanate product with low monobenzene ring impurity content, and the top of the stripping tower yields monobenzene ring impurities containing a small amount of crude isocyanate and stripping agent.

[0012] 2) The monobenzene ring impurities, stripping agent and crude isocyanate collected from the top of the stripping tower are separated by a distillation tower. High-purity crude isocyanate is obtained at the bottom of the distillation tower, stripping agent vapor is collected at the top of the tower, and concentrated monobenzene ring impurities are collected in the tower.

[0013] In one specific embodiment, the monobenzene ring impurities in the crude isocyanate are halogenated monobenzene ring substances, selected from at least one of monochlorophenyl isocyanate, dichlorophenyl isocyanate, trichlorophenyl isocyanate, monobromophenyl isocyanate, dibromophenyl isocyanate, and tribromophenyl isocyanate; preferably, the content of the monobenzene ring impurities in the crude isocyanate is 800-2000 ppm.

[0014] In one specific implementation, the low-boiling-point inert organic compound mentioned in step 1) is selected from at least one of carbon tetrachloride, chloroform, dichloromethane, and monochloromethane; preferably, the amount of stripping agent used is 0.01-0.02 Nm. 3The preferred concentration of crude isocyanate is 0.013-0.017 Nm / h. 3 / h per ton of crude isocyanate.

[0015] In one specific implementation, the operating temperature of the stripping tower in step 1) is 160-220°C, preferably 170-210°C, more preferably 180-200°C; and the operating pressure is controlled at 10-100 kPa, preferably 30-80 kPa, more preferably 50-60 kPa.

[0016] In one specific implementation, the mass content of isocyanate in the top gas phase of the stripping tower in step 1) is 1-5%, preferably 2-4%; preferably, the content of monobenzene ring impurities in the crude isocyanate obtained at the bottom of the stripping tower in step 1) is 10-1000 ppm, preferably 100-500 ppm.

[0017] In one specific implementation, in step 2), the mixed gas phase extracted from the top of the stripping tower is cooled in a condenser to form a liquid phase, and the cooling temperature of the heat exchanger is controlled at 10-50°C, preferably 20-40°C.

[0018] In one specific implementation, the reboiler temperature of the distillation column in step 2) is 220-300℃, preferably 220-260℃; the operating pressure is controlled at 70-150kPa, preferably 80-110kPa.

[0019] In one specific implementation, the content of monobenzene ring impurities in the stripping agent vapor obtained from the top of the distillation column in step 2) is less than 500 ppm, preferably less than 200 ppm, and more preferably less than 150 ppm; preferably, the stripping agent vapor collected from the top of the distillation column is directly introduced into the stripping column in step 1) for use as stripping steam.

[0020] In a specific implementation, in step 2), the amount of monobenzene ring impurities collected from the tower accounts for 3.5%-4.2% of the tower feed by mass, preferably 3.9%-4.1%. The content of low-boiling-point inert organic matter in the collected product is less than 50,000 ppm, preferably less than 5,000 ppm; the crude isocyanate content is less than 3%, preferably less than 2%. Preferably, the concentrated monobenzene ring impurities collected from the tower are directly treated as waste liquid.

[0021] In one specific implementation, the crude isocyanate obtained from the bottom of the distillation column in step 2) has a mass content of monobenzene ring impurities of less than 5%, preferably less than 3%; preferably, the crude isocyanate obtained from distillation can be directly recycled into the crude isocyanate product for use.

[0022] Compared with the prior art, the present invention has the following beneficial effects:

[0023] The production process of this invention uses low-boiling-point inert organic matter as stripping gas. In the stripping tower, crude isocyanate is stripped to remove monobenzene ring impurities. The removed monobenzene ring impurities, the low-boiling-point organic matter used for stripping, and the entrained crude isocyanate are condensed in the condenser at the top of the stripping tower to form a mixture. Then, the monobenzene ring impurities / stripping agent are separated from the crude isocyanate at the bottom of the distillation tower. The high-purity crude isocyanate is recovered, and the stripping agent and monobenzene ring impurities are separated at the top of the tower. The stripping agent vapor formed can be directly fed into the bottom of the stripping tower for stripping. The concentrated monobenzene ring impurities in the tower are treated as waste liquid.

[0024] The low-boiling-point inert organic compound used in this invention for stripping has good solubility for monobenzene ring impurities. In mixtures formed by monobenzene ring impurities and isocyanates, it can improve the separation degree between monobenzene ring impurity components and crude isocyanates, effectively reducing the difficulty of separating monobenzene ring impurity components from crude isocyanates and achieving the recovery of high-purity crude isocyanates. The low-boiling-point inert stripping agent and monobenzene ring impurity components can be recovered by distillation. By controlling the operating conditions of the distillation column, high-purity inert stripping agent vapor is obtained directly from the top of the distillation column. The pressure of the obtained inert stripping agent vapor is suitable and can be directly used for stripping crude isocyanates, forming an inert gas agent that can be recycled within the system.

[0025] The production process of this invention is simple and efficient, and is particularly suitable for industrial applications. Attached Figure Description

[0026] Figure 1 This is a schematic diagram of the production process of crude isocyanate with low content of monobenzene ring impurities according to the present invention.

[0027] Among them, 1 is crude isocyanate raw material; 2 is stripping agent; 3 is mixed gas phase; 4 is crude isocyanate with low content of monobenzene ring impurities after stripping; 5 is mixed liquid phase; 6 is stripping agent vapor; 7 is monobenzene ring impurities; 8 isocyanate; 9 is final crude isocyanate product; 10 is stripping tower; 11 is condenser; 12 is distillation tower. Detailed Implementation

[0028] To better understand the technical solution of the present invention, the following embodiments will further illustrate the method provided by the present invention. However, the present invention is not limited to the listed embodiments, but should also include any other well-known modifications within the scope of the claims of the present invention.

[0029] like Figure 1 As shown, a process for producing crude isocyanate with low content of monobenzene ring impurities includes the following steps:

[0030] 1) In stripping column 10, crude isocyanate feedstock 1 flows from top to bottom from the top of stripping column 10, and low-boiling-point inert organic vapor flows from bottom to top as stripping agent 2. Monobenzene ring impurities in crude isocyanate are stripped and transferred to the gas phase. The mixed gas phase 3 collected from the top of the stripping column is cooled in condenser 11 to form a condensed mixed liquid phase 5, which is sent to distillation column 12. Crude isocyanate 4 with low monobenzene ring impurity content after stripping is obtained at the bottom of the stripping column.

[0031] 2) In distillation column 12, a mixed liquid phase 5 composed of monobenzene ring impurities, low-boiling-point inert organic matter, and crude isocyanate after condensation is added from the middle of the column and distilled in the column. At the top of the column, stripping agent vapor 6 with low monobenzene ring impurities is obtained, and at the bottom of the column, refined crude isocyanate 8 is obtained. High-purity monobenzene ring impurities 7 are collected from the column and treated as waste liquid. The refined crude isocyanate 8 obtained at the bottom of the column is recovered into crude isocyanate product 9. The low-boiling-point inert organic gas phase (stripping agent vapor 6) at the top of the column can be directly used as stripping vapor in stripping column 10.

[0032] In step 1), the monobenzene ring impurities in the crude isocyanate refer to halogenated monobenzene ring substances, including at least one of monochlorophenyl isocyanate, dichlorophenyl isocyanate, trichlorophenyl isocyanate, monobromophenyl isocyanate, dibromophenyl isocyanate, and tribromophenyl isocyanate, with a content typically between 800-2000 ppm, such as 800 ppm, 1000 ppm, 1500 ppm, and 1800 ppm. The crude isocyanate typically consists of 92 wt%-95 wt% bicyclic and polycyclic isocyanates, with the remainder being impurities.

[0033] In step 1), the low-boiling-point inert organic compound used includes at least one of carbon tetrachloride, chloroform, dichloromethane, and monochloromethane, and the amount of stripping agent used is 0.01-0.02 Nm. 3 The preferred concentration of crude isocyanate is 0.013-0.017 Nm / h. 3 / h per ton of crude isocyanate.

[0034] In step 1), the operating temperature of the stripping tower is 160-220℃, including but not limited to 160℃, 170℃, 180℃, 190℃, 200℃, 210℃, and 220℃, preferably 170-210℃, and more preferably 180-200℃; the operating pressure is controlled at 10-100 kPa, including but not limited to 10 kPa, 20 kPa, 30 kPa, 40 kPa, 50 kPa, 60 kPa, 70 kPa, 80 kPa, 90 kPa, and 100 kPa, preferably 30-80 kPa, and more preferably 50-60 kPa. Crude isocyanate is introduced into the liquid distributor inlet line at the top of the stripping tower and evenly distributed into the packing material, forming a thin film that flows uniformly downwards on the packing surface. Vapors of low-boiling-point inert organic compounds are introduced from the bottom of the stripping tower and evenly distributed upwards through the gas distributor. In this process, low-boiling-point inert organic vapors come into contact with crude isocyanate on the packing surface, and low-level monobenzene ring impurities are transferred from the crude isocyanate to the low-boiling-point organic vapors. The packing material can be any commonly used type, such as ring packing, saddle packing, corrugated metal packing, or wire mesh packing.

[0035] In step 1), the low-boiling-point inert organic matter, monobenzene ring impurities, and crude isocyanate in the top gas phase of the stripping column form an organic mixture, wherein the mass percentage of crude isocyanate is 1-5%, preferably 2-4%; the gas phase of this mixture is cooled in a condenser to form a liquid phase, and then sent to the distillation column in step 2) for separation. The cooling temperature of the condenser is controlled at 10-50°C, for example, including but not limited to 15°C, 20°C, 25°C, 30°C, 35°C, 40°C, 45°C, and 50°C, preferably 20-40°C.

[0036] In step 1), the crude isocyanate obtained from the bottom of the stripping tower contains 10-1000 ppm of monobenzene ring impurities, for example, including but not limited to 10 ppm, 30 ppm, 50 ppm, 75 ppm, 100 ppm, 150 ppm, 200 ppm, 300 ppm, 400 ppm, 500 ppm, 600 ppm, 700 ppm, 800 ppm, 900 ppm, and 1000 ppm, preferably 100-500 ppm.

[0037] In step 2), the reboiler operating temperature of the distillation column is controlled at 220-300℃, such as including but not limited to 220℃, 230℃, 240℃, 250℃, 260℃, 270℃, 280℃, 290℃, and 300℃, preferably 220-260℃; the operating pressure is controlled at 70-150kPa, such as including but not limited to 70kPa, 80kPa, 90kPa, 100kPa, 110kPa, 120kPa, 130kPa, 140kPa, and 150kPa, preferably 80-100kPa.

[0038] In step 2), the content of monobenzene ring impurities in the low-boiling-point inert organic vapor obtained at the top of the column is less than 500 ppm, preferably less than 200 ppm, more preferably less than 150 ppm, for example 100 ppm, 50 ppm, 10 ppm or lower, and this organic vapor is directly introduced into the stripping column of step 1) for use as stripping steam.

[0039] In step 2), the amount of monobenzene ring impurity product obtained in the tower is 3.5%-4.2% of the tower feed mass, preferably 3.9%-4.1%. Among the monobenzene ring impurity product obtained in the tower, the content of low-boiling-point inert organic matter is less than 50,000 ppm, preferably less than 5,000 ppm, more preferably less than 1,000 ppm, for example 500 ppm, 100 ppm, 10 ppm; the content of crude isocyanate is less than 3%, preferably less than 2%, for example 1%, 0.5%, 0.1%, 10 ppm. This portion of pure product is directly discharged as waste liquid.

[0040] In step 2), the content of monobenzene ring impurities in the crude isocyanate obtained from the distillation column is less than 5%, preferably less than 3%, for example, 1%, 100 ppm, 10 ppm or lower. This crude isocyanate obtained through distillation can be directly recycled into the crude isocyanate product. While there is some enrichment and concentration of monobenzene ring impurities in the crude isocyanate obtained from the distillation column in this step, the total flow rate of this recovered crude isocyanate is very small, and its impact on the content of monobenzene ring impurities in the crude isocyanate product is minimal.

[0041] The present invention will be further illustrated by the following examples, but the present invention is not limited thereto.

[0042] In the examples and comparative examples, the crude isocyanate loss ratio refers to the proportion of unrecoverable crude isocyanate due to stripping to the feed crude isocyanate. Specifically, in the examples, it is the proportion of crude isocyanate collected as waste liquid from the middle of the distillation column in step 2 to the feed crude isocyanate; in the comparative examples, it is the proportion of crude isocyanate collected as waste liquid from the top of the stripping column to the feed crude isocyanate. The isocyanate content was determined using conventional gas chromatography methods, which are well known to those skilled in the art.

[0043] Example 1

[0044] The crude isocyanate feed contains 850 ppm (mass ratio) of monobenzene ring impurities. In the stripping tower, the crude isocyanate flows from top to bottom within the packing, while chloroform vapor flows from bottom to top, with a stripping ratio of 0.015 Nm³. 3 The stripping column produces crude isocyanate at a rate of / h / ton. Monobenzene ring impurities in the crude isocyanate are stripped and transferred to the gas phase. The stripping column operates at 165°C and 20 kPa. The top of the stripping column yields a gas-phase mixture with a crude isocyanate content of 1.9%, which is cooled to a liquid phase in a condenser at an operating temperature of 20°C. The bottom of the stripping column yields a crude isocyanate product with a low monobenzene ring impurity content of 165 ppm.

[0045] In the distillation column, the condensate obtained from the condensate is added from the middle of the column and distilled. The operating temperature of the column bottom is 255℃, and the operating pressure is 75kPa. The top of the distillation column yields a low-purity trichloromethane vapor phase with monobenzene ring impurities of 133ppm, which can be directly used as stripping steam in the stripping column. High-purity monobenzene ring impurities are collected from the column, with a collection rate of 3.56% of the feed. The low-boiling-point inert organic matter content is 5200ppm, and the crude isocyanate content is 1.6%. The collected monobenzene ring impurities are treated as waste liquid. The purified crude isocyanate obtained from the bottom of the column has a monobenzene ring impurity content of 3.2%, which can be directly recovered into the aforementioned crude isocyanate product. The final monobenzene ring impurity content in the crude isocyanate product is 231ppm.

[0046] Example 2

[0047] In this example, the content of monobenzene ring impurities in the crude isocyanate is 1350 ppm (mass ratio). In the stripping tower, the crude isocyanate flows from top to bottom in the packing, while dichloromethane vapor flows from bottom to top, with a stripping ratio of 0.019 Nm. 3 The stripping column produces crude isocyanate at a rate of / h / ton. Monobenzene ring impurities in the crude isocyanate are stripped and transferred to the gas phase. The stripping column operates at a temperature of 205°C and a pressure of 75 kPa. The top of the stripping column yields a gas-phase mixture with a crude isocyanate content of 3.6%, which is cooled to a liquid phase in a condenser at an operating temperature of 35°C. The bottom of the stripping column yields a crude isocyanate product with a low monobenzene ring impurity content of 542 ppm.

[0048] In the distillation column, the condensate obtained from the condensate is added from the middle of the column and distilled. The operating temperature of the column bottom is 276℃, and the operating pressure is 90kPa. The top of the distillation column yields dichloromethane with low monobenzene ring impurities, containing 181ppm of monobenzene ring impurities, which can be directly used as stripping steam in the stripping column. High-purity monobenzene ring impurities are collected from the column, with a collection rate of 4.05% of the distillation column feed. The content of low-boiling-point inert organic matter is 658ppm, and the crude isocyanate content is 2.5%. The collected monobenzene ring impurities are treated as waste liquid. The purified crude isocyanate obtained from the bottom of the column contains 2.8% monobenzene ring impurities and can be directly recovered into the aforementioned crude isocyanate product. The final monobenzene ring impurity content in the crude isocyanate product is 614ppm.

[0049] Example 3

[0050] In this example, the content of monobenzene ring impurities in the crude isocyanate is 1950 ppm (mass ratio). In the stripping tower, the crude isocyanate flows from top to bottom in the packing, while carbon tetrachloride vapor flows from bottom to top, with a stripping ratio of 0.011 Nm. 3 The stripping column produces crude isocyanate at a rate of / h / ton. Monobenzene ring impurities in the crude isocyanate are stripped and transferred to the gas phase. The stripping column operates at 195°C and 55 kPa. The top of the stripping column yields a gas-phase mixture with a crude isocyanate content of 2.2%, which is cooled to a liquid phase in a condenser at an operating temperature of 45°C. The bottom of the stripping column yields a crude isocyanate product with a low monobenzene ring impurity content of 741 ppm.

[0051] In the distillation column, the condensate obtained from the condensate is added from the middle of the column and distilled. The operating temperature of the distillation column bottom is 287℃, and the operating pressure is 125kPa. The top of the distillation column yields a carbon tetrachloride vapor phase with low monobenzene ring impurities, containing 436ppm of monobenzene ring impurities, which can be directly used as stripping steam in the stripping column. High-purity monobenzene ring impurities are collected from the column, with a collection rate of 4.08% of the distillation column feed. The content of low-boiling-point inert organic matter is 30800ppm, and the crude isocyanate content is 2.14%. The collected monobenzene ring impurities are treated as waste liquid. The purified crude isocyanate obtained from the bottom of the column contains 3.04% monobenzene ring impurities, which can be directly recovered into the aforementioned crude isocyanate product. The final monobenzene ring impurity content in the crude isocyanate product is 804ppm.

[0052] Comparative Example 1

[0053] In this example, the content of monobenzene ring impurities in the crude isocyanate was 850 ppm (mass ratio). Nitrogen was used for stripping in a stripping tower operating at 165°C and 20 kPa, with a stripping ratio of 0.015 Nm³. 3 / h per ton of crude isocyanate. The crude isocyanate product obtained from the bottom of the stripping tower contains 568 ppm of monobenzene ring impurities, and the isocyanate content in the gas phase mixture at the top of the tower is 4.8%.

[0054] Comparative Example 2

[0055] In this example, the content of monobenzene ring impurities in the crude isocyanate was 1350 ppm (mass ratio). Nitrogen was used for stripping in a stripping tower operating at 205°C and 75 kPa, with a stripping ratio of 0.019 Nm³. 3 / h per ton of crude isocyanate. The crude isocyanate product obtained from the bottom of the stripping tower contains 864 ppm of monobenzene ring impurities, and the isocyanate content in the gas phase mixture at the top of the tower is 5.6%.

[0056] Comparative Example 3

[0057] In this example, the content of monobenzene ring impurities in the crude isocyanate was 1950 ppm (mass ratio). Nitrogen was used for stripping in a stripping tower operating at 195°C and 20 kPa, with a stripping ratio of 0.015 Nm³. 3 / h per ton of crude isocyanate. The crude isocyanate product obtained from the bottom of the stripping tower contains 1470 ppm of monobenzene ring impurities, and the isocyanate content in the gas phase mixture at the top of the tower is 2.8%.

[0058] Comparative Example 4

[0059] The crude isocyanate feed contains 850 ppm (mass ratio) of monobenzene ring impurities. In the stripping tower, the crude isocyanate flows from top to bottom in the packing, while chloroform vapor flows from bottom to top, with a stripping ratio of 0.03 Nm. 3 The stripping column operates at a rate of 165°C and 20 kPa per ton of crude isocyanate per hour. Monobenzene ring impurities in the crude isocyanate are stripped and transferred to the gas phase. The top of the stripping column yields a gas-phase mixture with a crude isocyanate content of 1.8%, while the bottom yields a crude isocyanate product with a low monobenzene ring impurity content of 128 ppm.

[0060] Comparative Example 5

[0061] The crude isocyanate feed contains 850 ppm (mass ratio) of monobenzene ring impurities. In the stripping tower, the crude isocyanate flows from top to bottom within the packing, while chloroform vapor flows from bottom to top, with a stripping ratio of 0.015 Nm³. 3The stripping column operates at a rate of 140°C and 20 kPa per ton of crude isocyanate per hour. Monobenzene ring impurities in the crude isocyanate are stripped and transferred to the gas phase. The top of the stripping column yields a gas phase mixture with a crude isocyanate content of 0.56%, while the bottom yields a crude isocyanate product with a low monobenzene ring impurity content of 479 ppm.

[0062] Table 1. Effects of Examples 1-3 and Comparative Examples 1-3

[0063]

[0064] Table 2 Effects of Example 1 and Comparative Examples 4-5

[0065]

[0066] As can be seen from the table above, the single-benzene-ring impurity removal process described in this case, compared with the conventional nitrogen stripping process, has a better removal effect on single-benzene-ring impurities, and the loss of crude isocyanate is significantly reduced, demonstrating good implementation results and significance. Deviating from the stripping tower operating parameters in this case may lead to a significant increase in the proportion of MDI content in the gas phase at the top of the stripping tower to the feed MDI content, resulting in a significant decrease in economic efficiency, or a significant decrease in the removal rate of single-benzene-ring impurities in the crude isocyanate at the bottom of the stripping tower.

[0067] Although the present invention has been described in detail through the preferred embodiments described above, it should be understood that the above description should not be considered as a limitation of the present invention. Those skilled in the art will understand that modifications or adjustments can be made to the present invention based on the teachings of this specification. These modifications or adjustments should also be within the scope defined by the claims of the present invention.

Claims

1. A process for producing crude isocyanate with low content of monobenzene ring impurities, characterized in that, Includes the following steps: 1) Using low-boiling-point inert organic matter as stripping agent, crude isocyanate is stripped in stripping tower to remove monobenzene ring impurities. The bottom of the stripping tower yields crude isocyanate product with low monobenzene ring impurity content, and the top of the stripping tower yields monobenzene ring impurities containing a small amount of crude isocyanate and stripping agent. 2) The monobenzene ring impurities, stripping agent and crude isocyanate collected from the top of the stripping tower are separated by a distillation tower. High-purity crude isocyanate is obtained at the bottom of the distillation tower, stripping agent vapor is collected at the top of the tower, and concentrated monobenzene ring impurities are collected in the tower. The low-boiling-point inert organic compound mentioned in step 1) is selected from at least one of carbon tetrachloride, chloroform, dichloromethane, and monochloromethane; The crude isocyanate is obtained by removing phosgene and solvent from the photochemical reaction solution; the crude isocyanate consists of 92wt%-95wt% of bicyclic and polycyclic isocyanates, with the remainder being impurities; The monobenzene ring impurities in the crude isocyanate are halogenated monobenzene ring substances, selected from at least one of monochlorophenyl isocyanate, dichlorophenyl isocyanate, trichlorophenyl isocyanate, monobromophenyl isocyanate, dibromophenyl isocyanate, and tribromophenyl isocyanate; the content of the monobenzene ring impurities in the crude isocyanate is 800-2000 ppm. The operating temperature of the stripping tower in step 1) is 160-220℃; the operating pressure is controlled at 10-100kPa; and the dosage of the stripping agent is 0.01-0.02Nm. 3 / h per ton of crude isocyanate.

2. The production process according to claim 1, characterized in that, The stripping agent is used in a proportion of 0.013-0.017 Nm. 3 / h per ton of crude isocyanate.

3. The production process according to claim 1, characterized in that, The operating temperature of the stripping tower in step 1) is 170-210℃; the operating pressure is controlled at 30-80kPa.

4. The production process according to claim 3, characterized in that, The operating temperature of the stripping tower in step 1) is 180-200℃; the operating pressure is controlled at 50-60kPa.

5. The production process according to claim 1, characterized in that, The mass content of isocyanate in the top gas phase of the stripping tower described in step 1) is 1-5%.

6. The production process according to claim 5, characterized in that, The isocyanate content in the top gas phase of the stripping tower in step 1) is 2-4% by mass.

7. The production process according to claim 6, characterized in that, The crude isocyanate obtained from the bottom of the stripping tower in step 1) contains 10-1000 ppm of monobenzene ring impurities.

8. The production process according to claim 7, characterized in that, The crude isocyanate obtained from the bottom of the stripping tower in step 1) contains 100-500 ppm of monobenzene ring impurities.

9. The production process according to claim 1, characterized in that, In step 2), the gaseous mixture extracted from the top of the stripping tower is cooled into a liquid phase in a condenser, and the cooling temperature of the heat exchanger is controlled at 10-50℃.

10. The production process according to claim 9, characterized in that, In step 2), the gaseous mixture extracted from the top of the stripping tower is cooled into a liquid phase in a condenser, and the cooling temperature of the heat exchanger is controlled at 20-40℃.

11. The production process according to claim 9, characterized in that, In step 2), the reboiler temperature of the distillation column is 220-300℃; the operating pressure is controlled at 70-150kPa.

12. The production process according to claim 11, characterized in that, In step 2), the reboiler temperature of the distillation column is 220-260℃; the operating pressure is controlled at 80-110kPa.

13. The production process according to claim 11, characterized in that, In step 2), the content of monobenzene ring impurities in the stripping agent vapor obtained from the top of the distillation column is less than 500 ppm.

14. The production process according to claim 13, characterized in that, In step 2), the content of monobenzene ring impurities in the stripping agent vapor obtained from the top of the distillation column is less than 200 ppm.

15. The production process according to claim 14, characterized in that, In step 2), the content of monobenzene ring impurities in the stripping agent vapor obtained from the top of the distillation column is less than 150 ppm.

16. The production process according to claim 13, characterized in that, The stripping agent vapor extracted from the top of the distillation column is directly introduced into the stripping column in step 1) and used as stripping steam.

17. The production process according to claim 13, characterized in that, In step 2), the amount of monobenzene ring impurities extracted from the tower accounts for 3.5%-4.2% of the tower feed by mass, and the content of low-boiling-point inert organic matter in the extracted product is less than 50,000 ppm; the content of crude isocyanate is less than 3%.

18. The production process according to claim 17, characterized in that, In step 2), the amount of monobenzene ring impurities extracted from the tower accounts for 3.9%-4.1% of the tower feed by mass, and the content of low-boiling-point inert organic matter in the extracted product is less than 5000 ppm; the content of crude isocyanate is less than 2%.

19. The production process according to claim 17, characterized in that, In step 2), the concentrated monobenzene ring impurities extracted from the tower are directly treated as waste liquid.

20. The production process according to claim 17, characterized in that, In step 2), the crude isocyanate obtained from the bottom of the distillation column contains less than 5% by mass of monobenzene ring impurities.

21. The production process according to claim 20, characterized in that, In step 2), the crude isocyanate obtained from the bottom of the distillation column contains less than 3% by mass of monobenzene ring impurities.

22. The production process according to claim 20, characterized in that, The crude isocyanate obtained from distillation in step 2) can be directly recycled into the crude isocyanate product for use.

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