A process for the preparation of isocyanates by a gas phase method

Abstract

The invention relates to a process for the production of isocyanates by the gas phase method, in which a corresponding amine is phosgenated with a trace amount of hydrogen chloride and a stoichiometric excess of phosgenum in the presence or absence of an inert medium, the reaction conditions being chosen such that at least the reaction components amine, hydrogen chloride, isocyanate and phosgenum are gaseous under these conditions, and at least one gas stream comprising amine and one gas stream comprising hydrogen chloride are fed into a pre-reaction zone, in which the pre-reaction takes place, and the pre-reaction stream and at least one gas stream comprising phosgenum are fed into a reaction zone, which makes it possible to extend the operating period of the apparatus more effectively and to increase the operating efficiency.

Description

Technical Field

[0001] This invention relates to a method for preparing isocyanates, and more specifically to a gas-phase method for preparing isocyanates. Background Technology

[0002] It is well known that isocyanates can be prepared by gas-phase phosgenation of aliphatic, alicyclic, or aromatic amines. In this method, the amine feed stream needs to be gasified before entering the gas-phase phosgenation reactor to react with phosgene to generate isocyanates, and the gasification effect of the amine has a significant impact on the reaction results.

[0003] Under current technological levels, it is difficult to achieve 100% complete vaporization of amines in industrial production. The resulting amine gas stream contains trace amounts of unvaporized amine droplets. Without corresponding measures to eliminate these droplets, they will enter the gas-phase phosgenation reactor along with the amine gas stream, leading to adverse consequences. Firstly, the reaction of amine molecules on the surface of the droplets with phosgene to form isocyanates releases heat, which can easily cause the internal amine molecules to undergo carbonization and decompose at high temperatures, generating ammonia. Ammonia then reacts with hydrogen chloride (one of the products of the amine-phosgene reaction) to form solid ammonium chloride, easily causing blockages in the reactor and pipelines, requiring frequent cleaning and shortening the equipment's operating cycle. Secondly, the interaction between the amine molecules inside these droplets and the isocyanates formed on the droplet surface easily produces high-boiling-point urea byproducts. This not only increases the amount of heavy component impurities in the reaction products, but the high-boiling-point urea can also liquefy on the reactor wall and, upon heating, further generate solid blockages.

[0004] European patent EP1449826A1 discloses a method in which the reaction of phosgene with diamine (to form diisocyanate) and the subsequent reaction of diamine with diisocyanate (to form corresponding urea oligomers) compete during the gas-phase reaction of aromatic diamines and phosgene. An improved mixing of the reactants phosgene and diamine can increase the selectivity of diisocyanate and reduce urea formation while simultaneously avoiding backflow in a tubular reactor. This reduces the amount of condensate in the tubular reactor, decreases deposition on the reactor walls leading to a smaller unused cross-section of the tube and a gradual increase in reactor pressure, ultimately determining the run-time of the method. EP1449826A1 improves the mixing of the reactants phosgene and diamine by using multiple nozzles.

[0005] European patent EP1754698 discloses a special evaporation technique for the heat load of amines used in gas-phase phosgenation. Deposits in the reactor of the photochemical reaction are primarily caused by the decomposition of the amine during the reaction. Secondly, the long residence time and overheating during evaporation cause partial decomposition of the amine through ammonia removal. When using aliphatic amines, ammonia removal during evaporation not only reduces the yield but also forms ammonium chloride deposits in downstream pipes and equipment during subsequent phosgenation reactions, requiring frequent reactor cleaning, affecting reactor operating cycles, and causing corresponding production losses. EP1754698A1 discloses these disadvantages, particularly in cases where tube bundle heat exchangers, plate heat exchangers, or falling film evaporators are commonly used for amine evaporation and overheating. As a technical solution, the document discloses preventing ammonia removal during evaporation by using specific millimeter- or micrometer-scale heat exchangers for evaporation and overheating of aliphatic amines. However, a disadvantage of micrometer-scale heat exchangers is the extremely small channels, causing blockages due to the small amount of solids present in industrial processes, thus reducing evaporator operating time. Another drawback of the disclosed complete evaporation of amines is that amines cannot contain non-evaporable components, as these will inevitably deposit as solid residues on the evaporator surface, thus impairing heat transfer and eventually causing evaporator blockage. However, providing the desired quality of amines in industrial processes is very complex and expensive. Therefore, although reactor operating time has been improved by following the teachings of the aforementioned literature, evaporator operating time has significantly deteriorated, failing to advantageously improve the overall operating time of the production facility.

[0006] European patent publication EP1935876A1 (CN101205197) teaches us to minimize the heat load of the amine during the amine evaporation process in a gas-phase phosgenation reaction. Before the reaction, the amine is evaporated and heated to 200°C to 600°C, optionally using an inert gas such as N2, He, Ar, or an inert solvent. This is done using a high-efficiency falling film evaporator, and to further minimize the heat load on the feedstock amine, the evaporation is optionally aided by injecting vapors of an inert gas and / or inert solvent, reducing unvaporized droplets in the vaporized amine. Preferably, the vaporization and superheating of the feedstock amine are carried out in several stages, with the addition of a suitable droplet separator. However, all these processes result in pressure loss.

[0007] Chinese patents CN101912751 and CN105214568 disclose a heater for heating and vaporizing droplets in an amine gas stream. The heater uses electric heating to reduce droplets in the amine gas stream. However, these electric heating methods can easily cause coking or thermal decomposition of the amine on locally overheated surfaces, while also causing pressure loss, ultimately affecting the efficiency and quality of amine vaporization.

[0008] Chinese patent CN107667089 introduces a pressure boosting unit between the amine evaporation space and the reaction space. This unit provides a lower evaporation pressure to the amine evaporation space, reducing the boiling point of the amine. The superheated amine overcomes the pressure difference through compression by the pressure boosting unit and enters the reaction zone. However, in industrial applications, the presence of this unit can lead to an increase in unvaporized droplets in the amine gas stream, and may even damage the unit itself. Furthermore, the overheating operation after the amine leaves the evaporator can increase its residence time at high temperatures and exacerbate its decomposition.

[0009] Therefore, in industrial gas-phase phosgenation devices, it is of great practical significance to adopt a more efficient method to solve the problem of tiny droplets in the amine vaporization process and extend the reaction cycle. Summary of the Invention

[0010] The purpose of this invention is to provide a method for preparing isocyanates using a gas-phase method. By adding a pre-reaction zone before the reaction zone, the method solves the problem of tiny droplets in the amine gas-phase process, improves the efficiency and effect of amine vaporization, effectively extends the reaction cycle, and improves operating efficiency.

[0011] According to publicly available information, the overall reaction of the phosgenation process in the preparation of isocyanates is as follows (organic amines are represented by the general formula RNH2).

[0012] RNH₂ + 2COCl₂ → RNCO + 4HCl

[0013] The stepwise reaction equations for this reaction are shown in equations ①-④ below:

[0014] RNH2 + COCl2 → RNHCOCl + HCl (Equation ①)

[0015] RNH2 + HCl → RNH2·HCl (Equation ②)

[0016] RNH2·HCl + COCl2 → RN=C=O + HCl (Equation ③)

[0017]

[0018] In the gas phase phosgenation process, a pre-reaction zone is introduced before the reaction zone. Compared with the traditional reaction, this allows part of the reaction in formula ② to occur before the gaseous amine enters the reaction zone, making full use of the heat released by the reaction, improving the amine gasification effect, solving the problem of entrained micro-droplets during the amine gasification process, and extending the reactor's operating cycle.

[0019] Pre-processing part of the reaction in Equation ② has the following advantages: First, Equation ② is a rapid, strongly exothermic reaction. The large amount of heat released in a very short time provides heat for further vaporization of the small droplets in the amine gas stream, fully utilizing the heat released during the phosgenesis reaction to achieve energy integration. Second, the amine hydrochloride produced in Equation ② is a high-boiling-point, highly stable substance. Under the temperature conditions of the amine gas stream, it is a solid. The gaseous amine in the amine gas stream transforms into solid amine hydrochloride, drastically reducing its volume and lowering the pressure of the amine gas stream, thus lowering the boiling point of the amine and further reducing the difficulty of vaporizing the small amine droplets. Third, when inert substances are present in the amine gas stream, after the gaseous amine transforms into solid amine hydrochloride, based on the second advantage, the molar ratio of gaseous amine to inert substances decreases. According to Dalton's law of partial pressures, the partial pressure of gaseous amine further decreases, further reducing the difficulty of vaporizing the small amine droplets and further improving the vaporization effect.

[0020] The object of the present invention is achieved by means of a pre-reaction zone added between the vaporization and evaporation space of the amine and the reaction space, wherein at least one gas stream containing amine and at least one gas stream containing hydrogen chloride are supplied to the pre-reaction zone for reaction, the molar amount of hydrogen chloride in the pre-reaction zone being 0.01%-10% of the molar amount of amine in the amine stream, preferably 0.05%-5%. A pre-reaction stream is obtained from the outlet of the pre-reaction zone, and the pre-reaction stream and a gas stream containing at least phosgene are supplied to the reaction zone, and, with or without an inert medium, the respective pre-reaction stream and a stoichiometric excess of phosgene undergo a phosgenation reaction, wherein the reaction conditions are selected such that at least the reactants amine, hydrogen chloride, isocyanate, and phosgene are in the gaseous state under these conditions, and the amine hydrochloride is in the solid state.

[0021] In the method of this invention, the reaction of phosgene with an amine is carried out in the gas phase. During gas-phase phosgenation, this invention aims to ensure that the reaction raw materials (amine, phosgene), the final product (isocyanate), and any inert compounds that may have been introduced remain in the gas phase under the reaction conditions, and that the amine hydrochloride is a solid. Excess phosgene and the formed hydrogen chloride gas are separated from the basic reaction mixture obtained from the reaction, and at least part of the separated excess phosgene is recycled back into the reaction.

[0022] Regarding the setup of the pre-reaction zone, the pre-reaction zone only needs to meet the conceptual design. The form and shape of the pre-reaction zone are not important, as long as the process objective of this invention can be achieved. It can be a separate independent reaction zone, or it can be achieved by modifying the original process equipment, such as by modifying the connecting pipe between the evaporation zone and the reaction zone.

[0023] Hydrogen chloride gas added to the pre-reaction zone can be added alone or mixed with an inert medium. The molar amount of hydrogen chloride added to the pre-reaction zone is 0.01%-10% of the molar amount of amino groups in the amine stream, preferably 0.05%-5%. Hydrogen chloride reacts with gaseous amine in the pre-reaction zone to form solid hydrochloride particles, and the pre-reaction stream obtained from the pre-reaction zone is in a gas-solid suspension state (forming an aerosol). If too little hydrogen chloride is added, the heat generated by the reaction of the trace amount of hydrogen chloride with the amine has a very limited effect on improving the vaporization of the amine droplets. If too much hydrogen chloride is added, the hydrochloric acid particles generated in the reaction will become larger or denser, causing the hydrochloric acid particles to settle in the gaseous amine, causing blockage of the pre-reaction zone. It can also cause incomplete reaction of the hydrochloride particles in the reaction zone, resulting in blockage of the reaction zone and causing the opposite reaction effect.

[0024] The temperature of the hydrogen chloride stream added to the pre-reaction zone should be ≥ (temperature of the amine stream - 5℃), and preferably ≥ the temperature of the amine stream; the pressure of the hydrogen chloride stream added should not be lower than the pressure of the amine stream. Too low a hydrogen chloride addition temperature may cause a decrease in the temperature of the amine stream, which not only has limited improvement on the vaporization of small droplets, but may also cause the amine, which was originally vaporized, to generate more droplets due to the lower temperature.

[0025] The hydrogen chloride stream should be added to the pre-reaction zone in a way that is as uniform as possible to reduce uneven reaction distribution or local over-discharge, so that the heat generated by the reaction is evenly distributed in the pre-reaction zone.

[0026] The reaction between gaseous amine and hydrogen chloride in the pre-reaction zone is carried out under an absolute pressure of 0.01 to 0.5 MPa, and the average reaction time in the pre-reaction zone is 0.01 to 2 s, preferably 0.05 to 1 s, and more preferably 0.1 to 0.5 s.

[0027] In the method of the invention, the evaporation temperature of the raw material amine is 150-500°C, preferably 170-450°C, more preferably 180-400°C. The evaporation temperature needs to be higher than the boiling point of the amine at the corresponding evaporation pressure. An inert gas, such as N2, He, or Ar, or an inert solvent, such as a halogen-substituted aromatic hydrocarbon, such as chlorobenzene or o-dichlorobenzene vapor dilution, can be used. The raw material amine can be converted into a gaseous state using all types of evaporators known in the art, preferably by injecting inert gas and / or inert solvent vapors to assist the evaporation process. After the gaseous amine leaves the evaporator, the temperature of the amine stream can be increased using known superheating methods to ensure that the temperature of the gaseous amine is significantly higher than its dew point. Before the amine stream is supplied to the pre-reaction zone and the reaction zone, sufficient engineering measures, such as adequate thermal insulation or electric heating, are preferably adopted to counteract the risk of new micro-droplets forming in the amine stream due to heat radiation loss from equipment or pipelines.

[0028] Various phosgene-containing sub-streams (e.g., recycled phosgene and fresh phosgene) can be combined to form a total phosgene-containing feed stream before being introduced into the reaction space. Multiple sub-streams (in each case, recycled phosgene, fresh phosgene, or a mixture thereof) can be introduced into the reaction space at the same or different locations, thereby further introducing phosgene during the reaction.

[0029] For the purposes of this invention, the term "fresh phosgene" refers to a phosgene-containing stream that has not been recycled from the phosgenation process and has not undergone any reaction stages involving phosgene reactions after phosgene is typically synthesized from chlorine and carbon monoxide.

[0030] For the purposes of this invention, the method of converting phosgene from the reaction mixture to recycled phosgene is well known in the industry (e.g., GB737442A) and is not the focus of this invention.

[0031] In this invention, the phosgene stream may contain 0-10 wt% HCl gas, preferably 0.1-5 wt%.

[0032] An additional inert medium can be used in the method of the present invention. An inert medium is a gaseous medium in the reaction space at the reaction temperature that does not react with compounds present during the reaction. The inert medium is typically mixed with the amine and / or phosgene before the reaction, but it can also be introduced separately from the feed stream. For example, nitrogen, rare gases such as helium or argon, or aromatic compounds such as chlorobenzene, dichlorobenzene, xylene, or carbon monoxide can be used. Nitrogen and / or chlorobenzene are preferred as the inert medium. The inert medium is added to the gas stream containing amine or hydrogen chloride or phosgene such that the volume ratio of the inert medium to the amine or phosgene is 0 to 20:1.

[0033] The reaction of phosgene with amines is carried out in the reaction space at an absolute pressure of 0.01 to 0.5 MPa. Preferably, it is carried out at an absolute pressure of 0.07 to 0.3 MPa, more preferably at an absolute pressure of 0.09 to 0.2 MPa.

[0034] In the method of this invention, the temperature in the reaction zone is selected to be higher than the boiling point of the amine used, based on the prevailing pressure within the reaction zone. The temperature of the reaction zone is generally controlled at 200–600°C, preferably 250–450°C.

[0035] In this invention, phosgene is used in excess of the amino group, and the molar ratio of phosgene to the amino group of the amine is 2.2–20:1, preferably 4–10:1, more preferably 6–8:1. The pre-reaction stream and the phosgene stream can be introduced into the reaction space along with an inert medium. The flow rate of the pre-reaction stream and the phosgene stream into the reaction zone is 5–100 m / s, preferably 10–80 m / s. The average reaction time between the pre-reaction stream and the phosgene in the reaction zone is 0.01–15 s, preferably 0.04–10 s, more preferably 0.08–5 s. The average contact time is the duration from the start of mixing of the raw materials to the time the reaction mixture leaves the reaction space and enters the post-processing stage.

[0036] A high-temperature mixture of phosgene, an inert medium, hydrogen chloride, and isocyanate is obtained at the outlet of the reaction zone. The reaction is terminated by cooling the reaction mixture to the condensation temperature of the isocyanate. Preferably, the reaction of the pre-reaction stream and phosgene to generate isocyanate is terminated by injecting one or more suitable liquid streams (quenching liquids) into the outlet of the reaction zone, thereby rapidly cooling the gas mixture. Purified isocyanate product is obtained through further post-treatment and purification steps. The quenching medium is an inert solvent or a reaction solution containing isocyanate. The inert solvent is selected from one or more of chlorobenzene, dichlorobenzene, toluene, xylene, and decahydronaphthalene. The amount of quenching medium used is 1 to 60 times the mass of the amine-containing stream, preferably 2 to 30 times.

[0037] The isocyanate of this invention has the general formula R(NCO). n Aliphatic, alicyclic, or aromatic isocyanates, wherein R is an aliphatic, alicyclic, or aromatic hydrocarbon group having 4 to 15 carbon atoms, and n is an integer from 1 to 10.

[0038] Preferably, the general formula R(NH2) n The amines shown are aniline, cyclohexylamine, 1,4-butanediamine, 1,3-cyclohexanedimethylamine, 1,6-hexanediamine, 1,4-diaminocyclohexane, 1-amino-3,3,5-trimethyl-5-aminomethylcyclohexane, 4,4'-diaminodicyclohexylmethanediamine, p-phenylenediamine, m-phenylenediethylenediamine, 2,4 or 2,6-toluenediamine, 1,8-diamino-4-(aminomethyl)octane or triaminononane.

[0039] Preferably, the general formula is R(NCO). n The isocyanates shown are phenyl isocyanate, cyclohexyl isocyanate, 1,4-butanediisocyanate, 1,3-dimethylisocyanate cyclohexane, 1,6-hexanediisocyanate, 1,4-cyclohexanediisocyanate, isophorone diisocyanate, 4,4'-dicyclohexylmethane diisocyanate, terephthalic diisocyanate, isophthalimide diisocyanate, toluene diisocyanate, 1,8-diisocyanate-4-isocyanate methyl octane, or nonane triisocyanate.

[0040] The positive effects of this invention are as follows: In the process of preparing isocyanate by gas-phase phosgene, a pre-reaction zone is introduced before the reaction zone. By adding a hydrogen chloride stream to the pre-reaction zone, the heat released by the reaction is fully utilized, the pressure of the amine stream is reduced, and the partial pressure of the amine is effectively reduced, which greatly reduces the difficulty of amine vaporization and effectively improves the amine vaporization effect. This reduces the probability of reactor contamination and significantly reduces the amount of solid pollutants generated by reactor blockage, thus significantly improving the reactor's operational stability and operating cycle.

[0041] Attached image description: Figure 1 This is a schematic diagram of a process used in a specific embodiment of the present invention, wherein 1 is an amine stream, 2 is an inert gas stream, 3 is a vaporized amine stream, 4 is a hydrogen chloride stream or a mixed stream of hydrogen chloride and inert gas, 5 is a pre-reaction stream, 6 is a phosgene stream or a mixed stream of phosgene and inert gas, 7 is a preheated phosgene stream or a mixed stream of phosgene and inert gas, 8 is a cooled reaction stream, 9 is a spray stream, 100 is an amine preheating zone, 200 is a pre-reaction zone, 300 is a phosgene and / or inert gas preheating zone, 400 and 500 are reaction zones, 400 is a mixing zone in the reaction zone, and 500 is a reaction spray zone. Detailed implementation method:

[0042] The present invention will be further explained through the following embodiments, but is not limited thereto.

[0043] The effectiveness of amine vaporization directly affects the amount of agglomerates and deposits on the reactor inner wall and in the cold zone nozzle area. Excessive deposits increase the pressure loss of the reactor. Therefore, the effectiveness of amine vaporization can be evaluated by the quality of the deposits and the pressure loss of the reactor.

[0044] Example 1

[0045] Inert stream 2 (nitrogen) enters the amine preheating zone 100 at a flow rate of 400 kg / h, and 1,6-hexanediamine stream 1 enters at a flow rate of 2000 kg / h (17.24 kmol / h). A mixed stream 3 of nitrogen and amine enters the pre-reaction zone 200 at a temperature of 340°C. Hydrogen chloride stream 4 enters at a flow rate of 0.08 Nm³. 3Phosgene stream 4 enters the pre-reaction zone 200 at a flow rate of 3.57 mol / h. The temperature of stream 4 entering the pre-reaction zone is 340℃. The amount of hydrogen chloride added to stream 4 is approximately 0.010% of the molar amount of amino in stream 1. After the reaction of stream 3 and stream 4 in the pre-reaction zone 200, pre-reaction stream 5 is obtained. The absolute pressure of the pre-reaction zone is 0.09 MPa, and the pre-reaction time is 0.02 s. Phosgene stream 6 enters the phosgene preheater 300 at a flow rate of 8500 kg / h. The hydrogen chloride content in the phosgene stream is 2%, resulting in a preheated phosgene stream 7 at a temperature of 330℃. Phosgene stream 7 and pre-reaction stream 5 enter the mixing zone 400 of the tubular reactor for continuous reaction. The absolute pressure of the reaction zone is 0.085 MPa, slightly lower than atmospheric pressure. The reaction reaches a reaction temperature of 450℃ and ends after 1.5 seconds. In the reaction spray zone 500, the reaction gas mixture is cooled (rapidly cooled) by spraying in a solution of chlorobenzene and the isocyanate produced in the reaction. The total volume of the spray liquid is 70m³. 3 The isocyanate formed is condensed and washed to obtain an isocyanate reaction liquid. The process gas leaving the reactor, containing hydrogen chloride, unreacted phosgene, and trace amounts of isocyanate, is recovered from the isocyanate component through a scrubbing tower. Phosgene is then recycled through a phosgene absorption and desorption system. The liquid material from the reactor undergoes post-processing steps such as phosgene removal, solvent removal, and distillation to obtain the final 1,6-hexanediisocyanate product.

[0046] After 240 days of reactor operation, the total pressure drop in the reaction zone remained at around 5 kPa without significant change. The reactor was shut down for maintenance to determine the working status of the pre-reaction zone and the reaction zone. Aggregates and deposits on the inner wall of the reaction zone and in the cooling nozzle area were collected, yielding approximately 18 kg of solid material, which did not affect the continued operation of the reactor.

[0047] Example 2

[0048] Inert stream 2, containing nitrogen at a flow rate of 400 kg / h, and stream 1, containing 1-amino-3,3,5-trimethyl-5-aminomethylcyclohexane at a flow rate of 2500 kg / h (14.71 kmol / h), enter the amine preheating zone 100. A mixed stream 3 of nitrogen and amine enters the pre-reaction zone 200 at a temperature of 340°C. Hydrogen chloride stream 4 flows at a flow rate of 0.70 Nm³ / h. 3Phosgene stream 4 enters the pre-reaction zone 200 at a flow rate of 31.25 mol / h. The temperature of stream 4 entering the pre-reaction zone is 335℃. The amount of hydrogen chloride added to stream 4 is approximately 0.100% of the molar amount of amino in stream 1. After the reaction of stream 3 and stream 4 in the pre-reaction zone 200, pre-reaction stream 5 is obtained. The absolute pressure of the pre-reaction zone is 0.11 MPa, and the pre-reaction time is 0.04 s. Phosgene stream 6 enters the phosgene preheater 300 at a flow rate of 8750 kg / h. The hydrogen chloride content in the phosgene stream is 1%, resulting in a preheated phosgene stream 7 at a temperature of 330℃. Phosgene stream 7 and pre-reaction stream 5 enter the mixing zone 400 of the tubular reactor for continuous reaction. The absolute pressure of the reaction zone is 0.105 MPa, slightly higher than atmospheric pressure. The reaction reaches a reaction temperature of 440℃ and ends after 2.0 seconds. In the reaction spray zone 500, the reaction gas mixture is cooled (rapidly cooled) by spraying in a solution of chlorobenzene and isocyanate produced in the reaction. The total volume of the spray liquid is 100m³. 3 / h, the formed isocyanate is condensed and washed, and the reaction solution and process gas generated by the reaction are treated in the same way as in Example 1, to obtain the final isophorone diisocyanate product.

[0049] After 240 days of operation, the pressure drop of the reactor remained at around 5 kPa without significant change. The reactor was shut down for maintenance to determine the working status of the pre-reaction zone and the reaction zone. Aggregates and deposits on the inner wall of the reaction zone and the cooling nozzle area were collected, yielding a total of about 16 kg of solid material, which did not affect the continued operation of the reactor.

[0050] Example 3

[0051] Hydrogen chloride flow 4 at 8.00 Nm 3 A flow rate of 357.1 mol / h was introduced into the pre-reaction zone 200. The temperature of stream 4 entering the pre-reaction zone was 341°C. The amount of hydrogen chloride added to stream 4 was approximately 1.04% of the molar amount of amino group in stream 1. The hydrogen chloride content in the phosgene stream was 5%. The absolute pressure of the pre-reaction zone was 0.13 MPa, and the pre-reaction time was 0.5 s. The absolute pressure of the reaction zone was 0.12 MPa, slightly higher than atmospheric pressure. The reaction reached a reaction temperature of 460°C and ended after 2.5 seconds. Other reaction conditions were the same as in Example 1, yielding the final 1,6-hexamethylene diisocyanate product.

[0052] After 240 days of operation, the pressure drop of the reactor remained at around 5 kPa without significant change. The reactor was shut down for maintenance to determine the working status of the pre-reaction zone and the reaction zone. Aggregates and deposits on the inner wall of the reaction zone and the cooling nozzle area were collected, yielding a total of about 14 kg of solid material, which did not affect the continued operation of the reactor.

[0053] Example 4

[0054] Amine stream 1, consisting of 2,4- and 2,6-toluenediamine (in a ratio of 80 / 20), enters the amine preheating zone 100 at a flow rate of 2000 kg / h (16.37 kmol / h), while hydrogen chloride stream 4 enters at a flow rate of 73.13 Nm. 3 A flow rate of 3258.9 mol / h enters the pre-reaction zone 200. The temperature of stream 4 entering the pre-reaction zone is 344℃. The amount of hydrogen chloride added in stream 4 is approximately 9.97% of the molar amount of amino in stream 1. The absolute pressure of the pre-reaction zone is 0.12 MPa, and the pre-reaction time is 1.0 s. Phosgene stream 6 enters the phosgene preheater 300 at a flow rate of 10500 kg / h. The hydrogen chloride content in the phosgene stream is 0.5%. The absolute pressure of the reaction zone is 0.11 MPa, slightly higher than atmospheric pressure. The reaction reaches a reaction temperature of 470℃ and ends after 1.5 seconds. In the reaction spray zone 500, the reaction gas mixture is cooled (quenched) by spraying o-dichlorobenzene and the isocyanate solution produced in the reaction. The total amount of spray liquid is 70 m³. 3 / h, the formed isocyanate is condensed and washed, and other reaction conditions are the same as in Example 1 to obtain the final toluene diisocyanate product.

[0055] After 240 days of operation, the pressure drop of the reactor increased from 10 kPa to 11 kPa. The reactor was shut down for maintenance to determine the working status of the pre-reaction zone and the reaction zone. Aggregates and deposits on the inner wall of the reaction zone and the cooling nozzle area were collected. A total of about 20 kg of solid material was obtained, which did not affect the continued operation of the reactor.

[0056] Comparative Example 1

[0057] Inert nitrogen stream 2, flowing at a rate of 400 kg / h, and 1,6-hexanediamine stream 1, flowing at a rate of 2000 kg / h (17.24 kmol / h), enter the amine preheating zone. No pre-reaction zone is set up. The mixed nitrogen and amine stream 3 directly enters the reaction mixing zone 400. The temperature of stream 3 is 340°C. Phosgene stream 6, flowing at a rate of 8500 kg / h, enters the phosgene preheater 300. The hydrogen chloride content in the phosgene stream is 2%. Phosgene stream 7, preheated to 330°C, is obtained. Phosgene stream 7 and stream 3 enter the mixing zone 400 of the tubular reactor for continuous reaction. The absolute pressure of the reaction zone is 0.085 MPa, slightly lower than atmospheric pressure. Other conditions are the same as in Example 1, to obtain the final 1,6-hexanediisocyanate product.

[0058] After 180 days of operation, the pressure drop of the reactor increased from 5 kPa to 12 kPa, indicating that the reactor was blocked. The reactor was shut down for maintenance to determine the working status of the pre-reaction zone and the reaction zone. Agglomerates and deposits on the inner wall of the reaction zone and the cooling nozzle area were collected, and a total of about 60 kg of solid material was obtained.

[0059] Comparative Example 2

[0060] Hydrogen chloride flow 4 at 120 Nm 3 A flow rate of 5357.1 mol / h was introduced into the pre-reaction zone 200. The temperature of stream 4 entering the pre-reaction zone was 341°C. The amount of hydrogen chloride added to stream 4 was approximately 15.5% of the molar amount of amino group in stream 1. The absolute pressure of the pre-reaction zone was 0.10 MPa, and the pre-reaction time was 0.5 s. The absolute pressure of the reaction zone was 0.09 MPa, slightly lower than atmospheric pressure. The reaction reached a reaction temperature of 450°C and ended after 1.5 seconds. Other reaction conditions were the same as in Example 1, yielding the final 1,6-hexamethylene diisocyanate product.

[0061] After 156 days of operation, the pressure drop of the reactor increased from 10 kPa to 18 kPa, indicating that the reactor was blocked. The reactor was shut down for maintenance to determine the working status of the pre-reaction zone and the reaction zone. Agglomerates and deposits on the inner wall of the reaction zone and the cooling nozzle area were collected, and a total of about 59 kg of solid material was obtained.

[0062] Comparative Example 3

[0063] Amine stream 1, consisting of 2,4- and 2,6-toluenediamine (in a ratio of 80 / 20), enters the amine preheating zone 100 at a flow rate of 2000 kg / h (16.37 kmol / h), while hydrogen chloride stream 4 enters at a flow rate of 0.02 Nm³. 3 A stream of phosgene enters the pre-reaction zone 200 at a flow rate of 0.89 mol / h. The temperature of stream 4 entering the pre-reaction zone is 344℃. The amount of hydrogen chloride added to stream 4 is approximately 0.003% of the molar amount of amino in stream 1. The absolute pressure of the pre-reaction zone is 0.12 MPa, and the pre-reaction time is 1.1 s. Phosgene stream 6 enters the phosgene preheater 300 at a flow rate of 10500 kg / h. The absolute pressure of the reaction zone is 0.11 MPa, slightly lower than atmospheric pressure. The reaction reaches a reaction temperature of 470℃ and ends after 1.5 seconds. In the reaction spray zone 500, the reaction gas mixture is cooled (quenched) by spraying o-dichlorobenzene and the isocyanate solution produced in the reaction. The total volume of the spray liquid is 70 m³. 3 / h, other reaction conditions are the same as in Example 1, to obtain the final toluene diisocyanate product.

[0064] After 182 days of operation, the pressure drop of the reactor increased from 10 kPa to 20 kPa, indicating that the reactor was blocked. The reactor was shut down for maintenance to determine the working status of the pre-reaction zone and the reaction zone. Agglomerates and deposits on the inner wall of the reaction zone and the cooling nozzle area were collected, and a total of about 65 kg of solid material was obtained.

[0065] As can be seen from the above embodiments and comparative examples, the preparation method of the present invention can effectively reduce the generation of clogging substances in the reaction zone and effectively improve the operating cycle of the reactor.

Claims

1. A method for preparing isocyanate by gas phase reaction, comprising reacting a corresponding amine with hydrogen chloride and a stoichiometric excess of phosgene in the presence or absence of an inert medium, wherein the reaction conditions are selected such that at least the reactants amine, hydrogen chloride, isocyanate, and phosgene are in the gaseous state under these conditions, characterized in that, At least one stream containing an amine and a stream containing hydrogen chloride are fed into a pre-reaction zone to react and obtain a pre-reaction stream. The pre-reaction stream and a stream containing at least phosgene are fed into the reaction zone. The molar amount of hydrogen chloride in the pre-reaction zone is 0.01%-10% of the molar amount of amine groups in the amine stream.

2. The method according to claim 1, characterized in that, The molar amount of hydrogen chloride in the pre-reaction zone is 0.05%-5% of the molar amount of amine groups in the amine feed stream.

3. The method according to claim 1, characterized in that, The phosgene contains 0-10 wt% HCl gas.

4. The method according to claim 3, characterized in that, The phosgene contains 0.1 to 5 wt% HCl gas.

5. The method according to claim 1, characterized in that, Hydrogen chloride is added to the pre-reaction zone alone or mixed with an inert medium before being added to the pre-reaction zone.

6. The method according to claim 1, characterized in that, The temperature of the amine-containing stream entering the pre-reaction zone is 150-500℃, the temperature of the hydrogen chloride-containing stream added to the pre-reaction zone is ≥ (amine stream temperature - 5℃); and / or, the pressure of the hydrogen chloride stream added is not lower than the pressure of the amine stream.

7. The method according to claim 1, characterized in that, An inert medium is added to a stream containing an amine, hydrogen chloride, or phosgene, such that the molar ratio of the inert medium to the amine is 0 to 20:1, and / or the molar ratio of phosgene to the amino group of the amine is 2.2 to 20:

1.

8. The method according to claim 7, characterized in that, The molar ratio of phosgene to the amino group of amine is 4–10:

1.

9. The method according to claim 8, characterized in that, The molar ratio of phosgene to the amino group of amine is 6–8:

1.

10. The method according to any one of claims 1-9, characterized in that, The reaction between amine and hydrogen chloride in the pre-reaction zone was carried out under an absolute pressure of 0.01–0.5 MPa, with an average reaction time of 0.01–2 s.

11. The method according to any one of claims 1-9, characterized in that, The reaction between phosgene and the pre-reacting stream in the reaction zone is carried out at an absolute pressure of 0.01–0.5 MPa; the temperature of the reaction zone is 200–600 °C.

12. The method according to claim 11, characterized in that, The reaction between phosgene and the pre-reacting stream in the reaction zone is carried out at an absolute pressure of 0.07–0.3 MPa, and the temperature of the reaction zone is 250–450 °C.

13. The method according to claim 12, characterized in that, The reaction between phosgene and the pre-reacting stream in the reaction zone is carried out under an absolute pressure of 0.09–0.2 MPa.

14. The method according to any one of claims 1-9, characterized in that, The flow rates of the pre-reacting stream and the stream containing phosgene entering the reaction zone are 5–100 m / s; and / or, the average reaction time between the pre-reacting stream and phosgene in the reaction zone is 0.01–15 s.

15. The method according to claim 14, characterized in that, The flow rates of the pre-reacting stream and the stream containing phosgene entering the reaction zone are 10–80 m / s.

16. The method according to claim 14, characterized in that, The average reaction time between the pre-reacting stream and phosgene in the reaction zone is 0.04–10 s.

17. The method according to claim 16, characterized in that, The average reaction time between the pre-reacting stream and phosgene in the reaction zone is 0.08–5 s.

18. The method according to any one of claims 1-9, characterized in that, The reaction mixture obtained from the reaction zone is terminated by a quenching medium, which is an inert solvent or a reaction solution containing isocyanate. The inert solvent is selected from one or more of chlorobenzene, dichlorobenzene, toluene, xylene, and decahydronaphthalene, and / or the amount of quenching medium used is 1 to 60 times the mass of the stream containing amine.

19. The method according to claim 18, characterized in that, The amount of quenching medium used is 2 to 30 times the mass of the stream containing amine.

20. The method according to any one of claims 1-9, characterized in that, Isocyanates are compounds with the general formula R(NCO). n Aliphatic, alicyclic, or aromatic isocyanates, wherein R is an aliphatic, alicyclic, or aromatic hydrocarbon group having 4 to 15 carbon atoms, and n is an integer from 1 to 10; amines are compounds with the general formula R(NH2). n The structure.

21. The method according to claim 20, characterized in that, The isocyanate is phenyl isocyanate, cyclohexyl isocyanate, 1,4-butanediisocyanate, 1,3-dimethylisocyanate cyclohexane, 1,6-hexanediisocyanate, 1,4-cyclohexanediisocyanate, isophorone diisocyanate, 4,4'-dicyclohexylmethane diisocyanate, terephthalic diisocyanate, isophthalimide diisocyanate, toluene diisocyanate, 1,8-diisocyanate-4-isocyanate methyl octane, or nonane triisocyanate.

22. The method according to claim 20, characterized in that, The amine is aniline, cyclohexylamine, 1,4-butanediamine, 1,3-cyclohexanedimethylamine, 1,6-hexanediamine, 1,4-diaminocyclohexane, 1-amino-3,3,5-trimethyl-5-aminomethylcyclohexane, 4,4'-diaminodicyclohexylmethanediamine, p-phenylenediamine, m-phenylenediethylenediamine, 2,4 or 2,6-toluenediamine, 1,8-diamino-4-(aminomethyl)octane or triaminononane.

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