A process for the preparation of low-chlorinated impurity isocyanates

Abstract

The application provides a method for preparing low-chlorinated impurity isocyanate, and belongs to the technical field of isocyanate. The method is characterized in that two pretreatments are performed before isocyanate refining. On the one hand, naphthenes are used to extract the phosgenation reaction liquid, and insoluble substances such as urea and carbodiimide are removed before phosgene removal, thereby effectively reducing the generation of phosgene adducts. On the other hand, phosgene synthesis tail gas, phosgenation reaction tail gas and isocyanate refining tail gas are collected and mixed as stripping agents according to the target gas composition, and stripping treatment is performed in the phosgene removal stage. By controlling the source and gas composition of the stripping agent, phosgene can be quickly removed, the high-temperature polymerization of isocyanate and the generation of phosgene adducts can be inhibited, the generation of chlorinated isocyanate impurities can be reduced from the root, and the product quality is significantly improved.

Description

Technical Field

[0001] This invention relates to the field of isocyanate technology, and more specifically to a method for preparing isocyanates with low chlorinated impurities. Background Technology

[0002] Isocyanates are one of the most important raw materials in the synthesis of polyurethane materials, and they have a wide range of applications in polyurethane foams, rubber, fibers, coatings, adhesives, and synthetic leather. Currently, most industrial-scale isocyanate production methods employ phosgenation, which involves reacting organic primary amines with phosgene in an inert solvent, followed by a series of post-processing and separation steps. During the phosgenation reaction, some chlorinated impurities in the isocyanate are inevitably generated. Because chlorinated isocyanates have very similar chemical properties to isocyanates, they are difficult to remove and separate using conventional methods, affecting the product's color, acidity, and hydrolytic chlorine content, which is extremely detrimental to downstream applications.

[0003] In order to obtain isocyanates with low chlorination impurities and improve product quality, various treatment methods from the source or the back end have been introduced in the currently disclosed technical solutions.

[0004] CN114315648A discloses a method for reducing the hydrolytic chlorine content in isocyanates, including adding an azide reagent to isocyanates with high hydrolytic chlorine content, followed by adding an adsorbent, stirring, and filtering to convert acyl chloride impurities in the isocyanate into isocyanates and separating the generated chlorides. This technical solution involves back-end conversion of chlorinated isocyanates, which presents problems such as increased processing equipment, complex process operation, and the potential adverse effects of adding reagents to the isocyanate on product quality.

[0005] CN111630027A discloses a method for preparing low-chlorinated isocyanates, comprising providing an amine reactant and reacting the amine reactant with a phosgene stream in a reaction zone to form a product including the corresponding isocyanate, wherein the phosgene stream has an average CO content of 0.5 wt% or more. This technical solution, by increasing the CO content in the phosgene stream, essentially increases the CO excess rate of phosgene synthesis and reduces the chlorine content carried in the phosgene stream, preventing the reaction of chlorine with isocyanates to generate chlorinated impurities. However, it does not consider other factors that lead to the formation of chlorinated products.

[0006] CN111718282A discloses a method for preparing isocyanates with low chlorinated impurity content based on salt formation phosgenation. The salt particles obtained by this method have a particle size distribution where the proportion within ±30% of the average particle size accounts for more than 70% of the total particle size distribution, and the average residence time without stirring is less than 60 min. Compared with traditional methods, the product obtained by this method has a lower chlorinated impurity content. This technical solution reduces the formation of chlorinated impurities by limiting the particle size range of the amine salt; however, in actual production, the salt particle size is difficult to control and monitor, relying heavily on equipment and process methods, and long-term operation makes it difficult to achieve the desired results.

[0007] In summary, existing technologies still have shortcomings such as complex operation, impact on product quality, and difficulty in process monitoring. There is a need to develop a new process that can obtain isocyanates with low chlorinated impurity content without affecting product quality and with easier operation. Summary of the Invention

[0008] In view of the problems existing in the prior art, the present invention provides a method for preparing isocyanates with low chlorinated impurities. Compared with the existing methods, it is easier to obtain isocyanates with low chlorinated impurity content without affecting product quality or increasing process complexity, thereby improving product quality.

[0009] Through continuous research, the inventors have discovered that in the preparation of isocyanates, chlorinated isocyanates are mostly acyl chlorides or heavy chlorinated compounds. The main source is phosgene in the phosgenation reaction and isocyanate refining process, where phosgene reacts with urea or carbodiimide to form phosgene adducts. These phosgene adducts decompose or polymerize at high temperatures, causing an increase in acidity and hydrolyzed chlorine in the isocyanate product, resulting in a pale yellow color and affecting the application of downstream products.

[0010] To achieve this objective, the present invention adopts the following technical solution:

[0011] The purpose of this invention is to provide a method for preparing low-chlorinated impurity isocyanates, the method comprising the following steps:

[0012] a. Phosgene synthesis reaction: Chlorine and excess carbon monoxide are reacted to generate mixed phosgene, and the mixed phosgene is condensed to obtain fresh phosgene and phosgene synthesis tail gas;

[0013] b. Phosgene reaction: The liquid amine stream is mixed with the fresh phosgene described in step a to carry out the phosgene reaction, and the phosgene reaction liquid and phosgene reaction tail gas are obtained.

[0014] c. Pretreatment of reaction solution: The phosgenation reaction solution described in step b is extracted with cycloalkanes and then removed by the extractant to obtain a pretreated solution;

[0015] d. Phosgene removal: Phosgene is removed from the pretreatment liquid described in step c using a stripping agent to obtain a phosgene-removed liquid.

[0016] e. Isocyanate refining: The decolorized gas-liquid from step d is solvent-removed and refined to obtain product isocyanate and isocyanate refining tail gas;

[0017] Wherein, the stripping agent in step d is a combined gas obtained by mixing the phosgene synthesis tail gas in step a, the phosgenation reaction tail gas in step b, and the isocyanate refining tail gas in step e.

[0018] The method described in this invention involves two pretreatments before isocyanate purification. First, the phosgenation reaction liquid is extracted with cycloalkanes to remove insoluble substances such as urea and carbodiimide before phosgene removal, effectively reducing the formation of phosgene adducts. Second, the tail gas from phosgene synthesis, phosgenation reaction, and isocyanate purification is collected and mixed according to the target gas composition as a stripping agent. This agent is used for stripping during the phosgene removal stage. By controlling the source and composition of the stripping agent, phosgene can be rapidly removed, inhibiting the high-temperature polymerization of isocyanates and the formation of phosgene adducts. This reduces the formation of chloroisocyanate impurities at the source, significantly improving product quality.

[0019] It is worth noting that the remaining tail gases from the phosgene synthesis tail gas, phosgenation reaction tail gas, and isocyanate refining tail gas that were not used to mix to obtain a combined gas, as well as the phosgene removal tail gas generated in the phosgene removal stage, are all processed in the tail gas treatment stage.

[0020] As a preferred embodiment of the present invention, the reaction is carried out under catalytic conditions, namely, light irradiation and / or the use of a catalyst.

[0021] Preferably, the catalyst is activated carbon and / or a palladium catalyst.

[0022] Preferably, the catalyst is loaded into a fixed-bed reactor.

[0023] As a preferred technical solution of the present invention, in step a, the molar ratio of carbon monoxide to chlorine is (1.03-1.12):1, for example, 1.03:1, 1.04:1, 1.05:1, 1.06:1, 1.07:1, 1.08:1, 1.09:1, 1.10:1, 1.11:1 or 1.12:1, etc., but is not limited to the listed values. Other unlisted values ​​within the above range are also applicable, preferably (1.05-1.1):1.

[0024] Preferably, in step a, the reaction pressure is 0.2-0.8 MPaA, such as 0.2 MPaA, 0.25 MPaA, 0.3 MPaA, 0.35 MPaA, 0.4 MPaA, 0.45 MPaA, 0.5 MPaA, 0.55 MPaA, 0.6 MPaA, 0.65 MPaA, 0.7 MPaA, 0.75 MPaA, or 0.8 MPaA, but is not limited to the listed values. Other unlisted values ​​within the above range are also applicable, preferably 0.3-0.5 MPaA.

[0025] Preferably, in step a, the termination temperature of the reaction is 50-100℃, such as 50℃, 60℃, 65℃, 70℃, 75℃, 80℃, 85℃, 90℃, 95℃ or 100℃, but it is not limited to the listed values. Other unlisted values ​​within the above range are also applicable, preferably 60-80℃.

[0026] Preferably, in step a, the mass ratio of free chlorine in the fresh phosgene is less than 500 ppm.

[0027] As a preferred technical solution of the present invention, in step b, the liquid phase amine stream is a solution formed by an amine with the molecular structure expression R(NH2)n and an inert solvent; wherein R is an aliphatic or aromatic hydrocarbon group of C4-C15, and n is an integer from 1 to 10.

[0028] Preferably, the amine in the liquid-phase amine stream is any one of aniline, cyclohexylamine, 1,6-hexanediamine, 1,4-diaminocyclohexane, 1-amino-3,3,5-trimethyl-5-aminomethylcyclohexane, 4,4'-diaminodicyclohexylmethanediamine, p-phenylenediamine, m-phenylenediethylenediamine, 2,4-toluenediamine, 2,6-toluenediamine, diphenylmethanediamine, or polymethylene polyphenyl polyamine.

[0029] Preferably, the inert solvent in the liquid-phase amine stream includes any one or a combination of at least two of aromatic hydrocarbons, chloroaromatic hydrocarbons, dialkyl terephthalate or diethyl phthalate, preferably aromatic hydrocarbons and / or chloroaromatic hydrocarbons, more preferably any one or a combination of at least two of chlorobenzene, dichlorobenzene, toluene or xylene.

[0030] Preferably, the mass ratio of amine to inert solvent in the liquid amine stream is 1:(1.5-8), such as 1:1.5, 1:2, 1:2.5, 1:3, 1:3.5, 1:4, 1:4.5, 1:5, 1:6, 1:7 or 1:8, but it is not limited to the listed values. Other unlisted values ​​within the above range are also applicable, preferably 1:(2-5).

[0031] Preferably, the mass ratio of amine to fresh phosgene in the liquid amine stream is 1:(1.5-15), such as 1:1.5, 1:2, 1:2.5, 1:3, 1:3.5, 1:4, 1:4.5, 1:5, 1:6, 1:7, 1:8, 1:9, 1:10, 1:11, 1:12, 1:13, 1:14 or 1:15, etc., but not limited to the listed values. Other unlisted values ​​within the above range are also applicable, preferably 1:(2-5).

[0032] As a preferred technical solution of the present invention, in step b, the termination temperature of the phosgenation reaction is 60-150℃, such as 60℃, 70℃, 80℃, 90℃, 100℃, 110℃, 120℃, 130℃, 140℃ or 150℃, but it is not limited to the listed values. Other unlisted values ​​within the above range are also applicable.

[0033] Preferably, in step b, the reaction pressure of the phosgenation reaction is 0.2-3.0 MPaA, such as 0.2 MPaA, 0.5 MPaA, 0.7 MPaA, 1.0 MPaA, 1.3 MPaA, 1.5 MPaA, 1.8 MPaA, 2.0 MPaA, 2.2 MPaA, 2.5 MPaA, 2.7 MPaA, or 3.0 MPaA, but is not limited to the listed values. Other unlisted values ​​within the above range are also applicable.

[0034] Preferably, in step b, the reaction residence time of the phosgenation reaction is 0.5-3h, such as 0.5h, 0.7h, 1h, 1.3h, 1.5h, 1.7h, 2h, 2.3h, 2.5h, 2.8h or 3h, but it is not limited to the listed values. Other unlisted values ​​within the above range are also applicable.

[0035] Preferably, in step b, the reactor type for the phosgenation reaction is any one or a combination of at least two of the following: a dynamic mixer, a tubular reactor, a tower reactor, or a batch reactor.

[0036] As a preferred embodiment of the present invention, in step c, the cycloalkane is a cycloalkane containing one alicyclic ring without substituted alkyl groups on the ring, with the general molecular formula C1. n H 2n And n is an integer greater than or equal to 3.

[0037] Preferably, in step c, the cycloalkane is cyclopentane and / or cyclohexane.

[0038] Preferably, in step c, the mass ratio of the cycloalkane to the phosgenation reaction liquid is (1-3):1, for example, 1:1, 1.3:1, 1.5:1, 1.8:1, 2:1, 2.3:1, 2.5:1, 2.8:1 or 3:1, but it is not limited to the listed values. Other unlisted values ​​within the above range are also applicable.

[0039] As a preferred technical solution of the present invention, in step c, the extraction time is 5-20 min, such as 5 min, 7 min, 10 min, 11 min, 13 min, 15 min, 16 min, 18 min or 20 min, etc., but is not limited to the listed values. Other unlisted values ​​within the above range are also applicable.

[0040] Preferably, in step c, filtration is performed after extraction to remove insoluble matter.

[0041] Preferably, in step c, the extractant removal is performed using a distillation process to recover the extractant.

[0042] As a preferred embodiment of the present invention, the stripping agent in step d comprises CO, CO2, and HCl, and may optionally include an inert carrier gas such as N2; wherein the molar ratio of CO2 to CO is (0.5-8):1, for example 0.5:1, 0.8:1, 1:1, 1.5:1, 2:1, 2.5:1, 3:1, 4:1, 5:1, 6:1, 7:1, or 8:1, preferably (0.8-5):1, and more preferably... The molar ratio of HCl to CO is selected as (1-3):1; the molar ratio of HCl to CO is (1-20):1, such as 1:1, 2:1, 3:1, 4:1, 5:1, 6:1, 7:1, 8:1, 9:1, 10:1, 13:1, 15:1, 17:1 or 20:1, etc., preferably (2-15):1, more preferably (3-10):1; but it is not limited to the listed values, other unlisted values ​​within the above range are also applicable.

[0043] This invention, by adjusting the mixing ratio of the phosgene synthesis tail gas, the phosgenation reaction tail gas, and the isocyanate refining tail gas in the stripping agent, so that the molar amounts of CO, CO2, and HCl meet the above-mentioned ratio range, can quickly remove phosgene, inhibit the high-temperature polymerization of isocyanates and the formation of phosgene adducts, reduce the formation of chloroisocyanate impurities from the source, and significantly improve product quality.

[0044] It is worth noting that the main component of the phosgene synthesis tail gas in step a is CO, the main component of the phosgenation reaction tail gas in step b is HCl, and the main component of the isocyanate refining tail gas in step e is CO2. During the phosgene removal process, CO in the combined gas inhibits the high-temperature decomposition of phosgene to generate chlorine, preventing chlorine from directly reacting with isocyanate to generate chlorinated impurities. CO2 in the combined gas inhibits the high-temperature self-polymerization of isocyanate, reducing the formation of phosgene adducts. HCl in the combined gas can effectively convert phosgene adducts into hydrogen chloride adducts, which can be rapidly decomposed in the subsequent high-temperature stage, preventing them from being converted into chlorinated impurities.

[0045] As a preferred technical solution of the present invention, in step d, the phosgene removal is carried out in a phosgene removal tower, which is a two-section design; the upper section is a packed tower or a plate tower with a theoretical number of 3-8 plates; the lower section is a plate tower with a theoretical number of 10-20 plates; the pretreatment liquid is fed from the middle of the phosgene removal tower, and the combined gas is countercurrently stripped from the bottom of the phosgene removal tower to remove phosgene.

[0046] Preferably, the operating pressure of the deluminating tower is 0.05-0.3 MPaA, such as 0.05 MPaA, 0.1 MPaA, 0.12 MPaA, 0.15 MPaA, 0.17 MPaA, 0.2 MPaA, 0.23 MPaA, 0.25 MPaA, or 0.3 MPaA, but it is not limited to the listed values. Other unlisted values ​​within the above range are also applicable, preferably 0.12-0.2 MPaA.

[0047] Preferably, the bottom temperature of the deluminated gas tower is 120-200℃, such as 120℃, 140℃, 150℃, 160℃, 170℃, 180℃ or 200℃, but is not limited to the listed values. Other unlisted values ​​within the above range are also applicable, preferably 140-180℃.

[0048] As a preferred technical solution of the present invention, in step d, the mass ratio of phosgene in the dephosgene-free liquid is less than 2000 ppm, preferably less than 200 ppm; the mass ratio of phosgene adducts in the dephosgene-free liquid is less than 500 ppm, preferably less than 50 ppm.

[0049] It is worth noting that the solvent removal and purification described in step e of this invention are well known in the art and can be carried out by any known process method in the prior art, such as gas stripping, distillation, rectification and other separation methods.

[0050] The method of this invention can prepare aliphatic or aromatic low-chlorinated impurity isocyanates with an R(NCO)n structure, wherein R is a C4-C15 aliphatic or aromatic hydrocarbon group, and n is an integer from 1 to 10; preferably, the prepared isocyanate is one or more of phenyl isocyanate, cyclohexyl isocyanate, 1,6-hexane diisocyanate, 1,4-cyclohexane diisocyanate, isophorone diisocyanate, 4,4'-dicyclohexylmethane diisocyanate, terephthalic diisocyanate, isophthalic dimethyl diisocyanate, 2,4-toluene diisocyanate, 2,6-toluene diisocyanate, dimethylene diphenyl diisocyanate (MDI), and polymethylene polyphenyl polyisocyanate. The chlorinated impurity refers to a structure formed by the substitution of one or more NCO groups in the isocyanate molecule with chlorine atoms.

[0051] It should be noted that the method of the present invention reduces the content of chlorinated impurities in the same isocyanate, but since the absolute values ​​of chlorinated impurity content in different isocyanates vary greatly, the relevant comparison is limited to between the same isocyanates.

[0052] Compared with existing technical solutions, the present invention has at least the following beneficial effects:

[0053] (1) The method of the present invention performs two pretreatments before isocyanate purification. On the one hand, the phosgenation reaction liquid is extracted with cycloalkanes to remove insoluble substances such as urea and carbodiimide before phosgen removal, which effectively reduces the formation of phosgen adducts. On the other hand, the tail gas of phosgen synthesis, tail gas of phosgenation reaction, and tail gas of isocyanate purification are collected and mixed according to the target gas composition as a stripping agent. The stripping treatment is carried out in the phosgen removal stage. By controlling the source of the stripping agent and the gas composition, phosgen can be removed quickly, inhibiting the high-temperature polymerization of isocyanate and the formation of phosgen adducts, reducing the formation of chloroisocyanate impurities from the source, and significantly improving the product quality.

[0054] (2) The method described in this invention can more easily obtain isocyanates with low chlorinated impurity content without affecting product quality or increasing process complexity, thereby improving product quality. Attached Figure Description

[0055] Figure 1 This is a schematic diagram of the overall process flow of the method for preparing low-chlorinated impurity isocyanates according to the present invention;

[0056] In the diagram: 1-phosgene synthesis reactor; 2-phosgenation reactor; 3-extraction mixer; 4-extractant recovery tower; 5-phosgene removal tower; 6-solvent removal tower and product purification tower; 7-combined gas mixer. Detailed Implementation

[0057] The present invention will be further illustrated below with specific embodiments. These embodiments are merely illustrative and do not limit the scope of the invention.

[0058] The main sources of raw materials involved in the examples and comparative examples are as follows:

[0059] Carbon monoxide: Produced by gasification equipment in Ningbo Wanhua Industrial Park; industrial product.

[0060] Chlorine: Produced in the chlorine and hydrogen workshop of Ningbo Wanhua Chlor-Alkali Company; industrial product.

[0061] MDA: Industrial products produced by the MDI unit in Ningbo Wanhua Industrial Park;

[0062] Toluene diamine (TDA): Produced by the TDI unit in Yantai Wanhua Industrial Park; industrial product.

[0063] Determination of phosgene or phosgene adduct content in samples: Accurately weigh 0.5 g of sample, dissolve in 10 g of dichloromethane solvent, derivatize with 2 g of 10% diphenylurea for 60 min, and perform quantitative analysis of sample composition using gas chromatography. The gas chromatograph was an Agilent 7890A, the detector was FID, and the column was an Agilent 19091J-413HP-5, 0℃-325℃ (350℃): 30 m × 320 μm × 0.25 μm.

[0064] Chlorinated isocyanate content analysis: Isocyanates were diluted 5-fold with dichloromethane, and the diluted solution was analyzed by gas chromatography. The gas chromatograph was an Agilent 7890A, the detector was FID, and the column was DB-5 (30m × 0.53mm × 1.5μm).

[0065] [Drive Start-up Phase]

[0066] according to Figure 1 The schematic diagram of the overall process flow shows that a CO stream and a Cl2 stream undergo phosgene synthesis in a phosgene synthesis reactor 1. The resulting mixed phosgene is condensed to obtain fresh phosgene and phosgene synthesis tail gas. A liquid amine stream and the fresh phosgene are then introduced into a phosgenation reactor 2 for phosgenation reaction, yielding a phosgenation reaction liquid and a phosgenation reaction tail gas. The phosgenation reaction liquid and cycloalkanes (stream not shown) are introduced into an extraction mixer 3 for extraction, and then into an extractant recovery tower 4 for extractant removal. The resulting pretreated liquid enters a phosgene removal tower 5 from the middle, and a stripping agent enters the phosgene removal tower 5 from the bottom for countercurrent stripping to remove phosgene. The resulting dephosgene-removed liquid enters a solvent removal tower and a product purification tower 6 for solvent removal and purification, yielding isocyanate and isocyanate purification tail gas.

[0067] The phosgene synthesis tail gas, the phosgenation reaction tail gas, and the isocyanate refining tail gas are mixed in the combined gas mixer 7 according to the target gas composition. The combined gas obtained in the combined gas mixer 7 is used as the stripping agent for phosgene removal. The remaining tail gas from the phosgene synthesis tail gas, the phosgenation reaction tail gas, and the isocyanate refining tail gas that is not used to obtain the combined gas, as well as the phosgene removal tail gas generated in the phosgene removal stage, are all processed in the tail gas treatment stage.

[0068] The above process is repeated until a cyclic phosgene balance is established, which serves as the operating mode for the start-up phase in the following embodiments and comparative examples. The relevant parameters are further adjusted as the reaction conditions in the various embodiments and comparative examples below.

[0069]

Example 1

[0070] This embodiment provides a method for preparing low-chlorinated MDI, the method comprising the following steps:

[0071] a. Phosgene synthesis reaction: 156 kmol / h of carbon monoxide and 152 kmol / h of chlorine are mixed at a molar ratio of 1.03:1 and then fed into a phosgene synthesis reactor to generate mixed phosgene. The reaction pressure is 0.2 MPaA, and the reaction termination temperature is 50°C. The reaction is carried out under catalytic conditions, specifically using a palladium catalyst, which is packed into a fixed-bed reactor. The mixed phosgene is condensed to obtain fresh phosgene and phosgene synthesis tail gas. The mass ratio of free chlorine in the fresh phosgene is less than 500 ppm.

[0072] b. Phosgene reaction: 10 t / h MDA and the fresh phosgene described in step a are thoroughly mixed under the action of 15 t / h chlorobenzene and then reacted in a dynamic and a batch reactor. The dynamic and batch reactors are designed in series. The termination temperature of the phosgene reaction is 60℃, the reaction pressure is 0.2 MPaA, and the reaction residence time is 0.5 h, to obtain phosgene reaction liquid and phosgene reaction tail gas.

[0073] c. Pretreatment of reaction solution: The phosgenation reaction solution described in step b is mixed with 15t / h cyclohexane for extraction. The insoluble substances such as urea and carbodiimide are filtered out by the extraction filter. The extraction time is 5min. The extract is then removed by conventional distillation to remove the extractant and reuse the extractant to obtain the pretreated solution.

[0074] d. Phosgene Removal: The phosgene synthesis tail gas from step a, the phosgenation reaction tail gas from step b, and the MDI refining tail gas from step e are collected and mixed according to the target gas composition to obtain a combined gas as a stripping agent, which contains 4.5 kmol / h CO, 2.25 kmol / h CO2, and 4.5 kmol / h HCl. This combined gas is sent to the phosgene removal tower for phosgene removal by stripping pretreatment liquid to obtain phosgene-removed gas-liquid. The phosgene removal tower is a two-stage design, with 3 theoretical plates in the upper packing layer and 10 theoretical plates in the lower tray. The operating pressure of the phosgene removal tower is 0.3 MPaA, and the bottom temperature is 200℃.

[0075] e. MDI Refining: The deluminated gas-liquid mixture from step d is subjected to solvent removal and refining to obtain product MDI and MDI refining tail gas; wherein, the solvent removal conditions are 3 kPaA and 145°C; and the refining conditions are 3 kPaA and 160°C.

[0076]

Example 2

[0077] This embodiment provides a method for preparing low-chlorinated MDI, the method comprising the following steps:

[0078] a. Phosgene synthesis reaction: 1131 kmol / h of carbon monoxide and 1010 kmol / h of chlorine are mixed at a molar ratio of 1.12:1 and then fed into a phosgene synthesis reactor to generate mixed phosgene. The reaction pressure is 0.8 MPaA, and the reaction termination temperature is 100°C. The reaction is carried out under catalytic conditions, specifically using a palladium catalyst, which is packed into a fixed-bed reactor. The mixed phosgene is condensed to obtain fresh phosgene and phosgene synthesis tail gas. The mass ratio of free chlorine in the fresh phosgene is less than 500 ppm.

[0079] b. Phosgene reaction: 10 t / h MDA and the fresh phosgene described in step a are thoroughly mixed under the action of 80 t / h chlorobenzene and then reacted in a dynamic and a batch reactor. The dynamic and batch reactors are designed in series. The termination temperature of the phosgene reaction is 1500℃, the reaction pressure is 3.0 MPaA, and the reaction residence time is 3 h, to obtain phosgene reaction liquid and phosgene reaction tail gas.

[0080] c. Pretreatment of reaction solution: The phosgenation reaction solution described in step b is mixed with 240 t / h cyclohexane for extraction. The insoluble substances such as urea and carbodiimide are filtered out by the extraction filter. The extraction time is 20 min. The extract is then removed by conventional distillation to remove the extractant and reuse the extractant to obtain the pretreated solution.

[0081] d. Phosgene Removal: The phosgene synthesis tail gas from step a, the phosgenation reaction tail gas from step b, and the MDI refining tail gas from step e are collected and mixed according to the target gas composition to obtain a combined gas as a stripping agent, which contains 4.5 kmol / h CO, 36 kmol / h CO2, and 90 kmol / h HCl. This combined gas is sent to the phosgene removal tower for stripping pretreatment liquid to remove phosgene, resulting in a phosgene-removed gas-liquid. The phosgene removal tower is a two-stage design, with 8 theoretical plates in the upper packing layer and 20 theoretical plates in the lower tray. The operating pressure of the phosgene removal tower is 0.05 MPaA, and the bottom temperature is 120℃.

[0082] e. MDI Refining: The deluminated gas-liquid mixture from step d is subjected to solvent removal and refining to obtain product MDI and MDI refining tail gas; wherein, the solvent removal conditions are 3 kPaA and 145°C; and the refining conditions are 3 kPaA and 160°C.

[0083]

Example 3

[0084] This embodiment provides a method for preparing low-chlorinated MDI, the method comprising the following steps:

[0085] a. Phosgene synthesis reaction: 382 kmol / h of carbon monoxide and 354 kmol / h of chlorine are mixed at a molar ratio of 1.08:1 and then fed into a phosgene synthesis reactor to generate mixed phosgene. The reaction pressure is 0.4 MPaA, and the reaction termination temperature is 70°C. The reaction is carried out under catalytic conditions, specifically using a palladium catalyst, which is packed into a fixed-bed reactor. The mixed phosgene is condensed to obtain fresh phosgene and phosgene synthesis tail gas. The mass ratio of free chlorine in the fresh phosgene is less than 500 ppm.

[0086] b. Phosgene reaction: 10 t / h MDA and the fresh phosgene described in step a are thoroughly mixed under the action of 35 t / h chlorobenzene and then reacted in a dynamic and a batch reactor. The dynamic and batch reactors are designed in series. The termination temperature of the phosgene reaction is 100℃, the reaction pressure is 1.0 MPaA, and the reaction residence time is 1.5 h, to obtain phosgene reaction liquid and phosgene reaction tail gas.

[0087] c. Pretreatment of reaction solution: The phosgenation reaction solution described in step b is mixed with 70t / h cyclohexane for extraction. The insoluble substances such as urea and carbodiimide are filtered out by the extraction filter. The extraction time is 10min. The extract is then removed by conventional distillation to remove the extractant and reuse the extractant to obtain the pretreated solution.

[0088] d. Phosgene Removal: The phosgene synthesis tail gas from step a, the phosgenation reaction tail gas from step b, and the MDI refining tail gas from step e are collected and mixed according to the target gas composition to obtain a combined gas as a stripping agent, which contains 4.5 kmol / h CO, 9 kmol / h CO2, and 36 kmol / h HCl. This combined gas is sent to the phosgene removal tower for stripping pretreatment liquid to remove phosgene, resulting in a phosgene-removed gas-liquid. The phosgene removal tower is a two-stage design, with 5 theoretical plates in the upper packing layer and 15 theoretical plates in the lower tray. The operating pressure of the phosgene removal tower is 0.16 MPaA, and the bottom temperature is 160℃.

[0089] e. MDI Refining: The deluminated gas-liquid mixture from step d is subjected to solvent removal and refining to obtain product MDI and MDI refining tail gas; wherein, the solvent removal conditions are 3 kPaA and 145°C; and the refining conditions are 3 kPaA and 160°C.

[0090]

Example 4

[0091] This embodiment provides a method for preparing low-chlorinated impurity TDI, the method comprising the following steps:

[0092] a. Phosgene synthesis reaction: 382 kmol / h of carbon monoxide and 354 kmol / h of chlorine are mixed at a molar ratio of 1.08:1 and then fed into a phosgene synthesis reactor to generate mixed phosgene. The reaction pressure is 0.4 MPaA, and the reaction termination temperature is 70°C. The reaction is carried out under catalytic conditions, specifically using a palladium catalyst, which is packed into a fixed-bed reactor. The mixed phosgene is condensed to obtain fresh phosgene and phosgene synthesis tail gas. The mass ratio of free chlorine in the fresh phosgene is less than 500 ppm.

[0093] b. Phosgene reaction: 10 t / h TDA and the fresh phosgene described in step a are thoroughly mixed under the action of 35 t / h chlorobenzene and then reacted in a dynamic and a batch reactor. The dynamic and batch reactors are designed in series. The termination temperature of the phosgene reaction is 100℃, the reaction pressure is 1.0 MPaA, and the reaction residence time is 1.5 h, to obtain phosgene reaction liquid and phosgene reaction tail gas.

[0094] c. Pretreatment of reaction solution: The phosgenation reaction solution described in step b is mixed with 70t / h cyclohexane for extraction. The insoluble substances such as urea and carbodiimide are filtered out by the extraction filter. The extraction time is 10min. The extract is then removed by conventional distillation to remove the extractant and reuse the extractant to obtain the pretreated solution.

[0095] d. Phosgene Removal: The phosgene synthesis tail gas from step a, the phosgenation reaction tail gas from step b, and the TDI refining tail gas from step e are collected and mixed according to the target gas composition to obtain a combined gas as a stripping agent, which contains 4.5 kmol / h CO, 9 kmol / h CO2, and 36 kmol / h HCl. This combined gas is sent to the phosgene removal tower for stripping pretreatment liquid to remove phosgene, resulting in a phosgene-free gas-liquid. The phosgene removal tower is a two-stage design, with 5 theoretical plates in the upper packing layer and 15 theoretical plates in the lower tray. The operating pressure of the phosgene removal tower is 0.05 MPaA, and the bottom temperature is 145℃.

[0096] e. TDI Refining: The deluminated gas-liquid mixture from step d is subjected to solvent removal and refining to obtain product TDI and TDI refining tail gas; wherein, the solvent removal conditions are 3 kPaA and 145°C; and the refining conditions are 3 kPaA and 150°C.

[0097] Comparative Example 1

[0098] Comparative Example 1 provides a method for preparing low-chlorinated impurity MDI. Compared with the method described in Example 3, the only difference is that the pretreatment of the reaction solution in step c is omitted, and the combined gas stripping in the phosgene removal in step d is omitted. That is, the phosgenation reaction solution in step b is directly subjected to the solvent removal and purification in step e.

[0099] Comparative Example 2

[0100] Comparative Example 1 provides a method for preparing low-chlorinated impurity MDI. Compared with the method described in Example 3, the only difference is that the combined gas stripping in the phosgene removal step d is omitted, that is, the pretreatment liquid in step c is directly subjected to the solvent removal and purification in step e.

[0101] Comparative Example 3

[0102] Comparative Example 1 provides a method for preparing low-chlorinated impurity MDI. Compared with the method described in Example 3, the only difference is that the pretreatment of the reaction solution in step c is omitted, and the HCl component of the combined gas in the phosgene removal in step d is omitted. That is, in step d, the phosgene synthesis tail gas in step a and the MDI purification tail gas in step e are collected and mixed according to the target gas composition to obtain a combined gas as a stripping agent, which contains 4.5 kmol / h CO and 9 kmol / h CO2.

[0103] Comparative Example 4

[0104] Comparative Example 1 provides a method for preparing low-chlorinated impurity MDI, which differs from the method described in Example 3 only in that the pretreatment of the reaction solution in step c is omitted.

[0105] Comparative Example 5

[0106] This comparative example provides a method for preparing low-chlorinated impurity MDI, which differs from the method described in Example 3 only in that carbon tetrachloride is used as the extractant in step c.

[0107] In the preparation processes of the above embodiments and comparative examples, the contents of phosgene and phosgene adducts in the dephosgene gas-liquid mixture, as well as the contents of chlorinated MDI in the product MDI, were tested respectively. The specific test results are shown in Table 1.

[0108] Table 1

[0109]

[0110]

[0111] In summary, the method of this invention performs two pretreatments before isocyanate purification. First, the phosgenation reaction liquid is extracted with cycloalkanes to remove insoluble substances such as urea and carbodiimide before phosgene removal, effectively reducing the formation of phosgene adducts. Second, the tail gas from phosgene synthesis, phosgenation reaction, and isocyanate purification is collected and mixed according to the target gas composition as a stripping agent. This stripping process is then performed during the phosgene removal stage. By controlling the source and composition of the stripping agent, phosgene can be rapidly removed, inhibiting high-temperature polymerization of isocyanates and the formation of phosgene adducts. This fundamentally reduces the formation of chloroisocyanate impurities, significantly improving product quality. The method of this invention can more easily obtain isocyanates with low chloro impurity content without affecting product quality or increasing process complexity, thus improving product quality.

[0112] The present invention has been illustrated with the above embodiments to illustrate its detailed structural features. However, the present invention is not limited to the above detailed structural features, that is, it does not mean that the present invention must rely on the above detailed structural features to be implemented. Those skilled in the art should understand that any improvements to the present invention, equivalent substitutions for the components used in the present invention, additions of auxiliary components, and selection of specific methods, etc., all fall within the protection scope and disclosure scope of the present invention.

[0113] The preferred embodiments of the present invention have been described in detail above. However, the present invention is not limited to the specific details of the above embodiments. Within the scope of the technical concept of the present invention, various simple modifications can be made to the technical solution of the present invention, and these simple modifications all fall within the protection scope of the present invention.

[0114] It should also be noted that the various specific technical features described in the above specific embodiments can be combined in any suitable way without contradiction. In order to avoid unnecessary repetition, the present invention will not describe the various possible combinations separately.

[0115] Furthermore, various different embodiments of the present invention can be combined in any way, as long as they do not violate the spirit of the present invention, they should also be regarded as the content disclosed by the present invention.

Claims

1. A method for preparing isocyanates with low chlorinated impurities, characterized in that, The method includes the following steps: a. Phosgene synthesis reaction: Chlorine and excess carbon monoxide are reacted to generate mixed phosgene, and the mixed phosgene is condensed to obtain fresh phosgene and phosgene synthesis tail gas; b. Phosgene reaction: The liquid amine stream is mixed with the fresh phosgene described in step a to carry out the phosgene reaction, and the phosgene reaction liquid and phosgene reaction tail gas are obtained. c. Pretreatment of reaction solution: The phosgenation reaction solution described in step b is extracted with cycloalkanes and then removed by the extractant to obtain a pretreated solution; d. Phosgene removal: Phosgene is removed from the pretreatment liquid described in step c using a stripping agent to obtain a phosgene-removed liquid. e. Isocyanate refining: The decolorized gas-liquid from step d is solvent-removed and refined to obtain product isocyanate and isocyanate refining tail gas; Wherein, the stripping agent in step d is a combination gas obtained by mixing the phosgene synthesis tail gas in step a, the phosgenation reaction tail gas in step b, and the isocyanate refining tail gas in step e; the composition of the stripping agent includes CO, CO2, and HCl; wherein, the molar ratio of CO2 to CO is (0.5-8):1, and the molar ratio of HCl to CO is (1-20):

1. In step b, the liquid-phase amine stream is a solution formed by an amine with the molecular structure expression R(NH2)n and an inert solvent; wherein R is a C4-C15 aliphatic or aromatic hydrocarbon group, and n is an integer from 1 to 10. In step c, the cycloalkane is cyclopentane and / or cyclohexane.

2. The method according to claim 1, characterized in that, In step a, the reaction is carried out under catalytic conditions, namely, light irradiation and / or the use of a catalyst.

3. The method according to claim 2, characterized in that, The catalyst is activated carbon and / or palladium catalyst.

4. The method according to claim 3, characterized in that, The catalyst is loaded into a fixed-bed reactor.

5. The method according to claim 1, characterized in that, In step a, the molar ratio of carbon monoxide to chlorine is (1.03-1.12):

1.

6. The method according to claim 5, characterized in that, In step a, the molar ratio of carbon monoxide to chlorine is (1.05-1.1):

1.

7. The method according to claim 1, characterized in that, In step a, the reaction pressure is 0.2-0.8 MPaA.

8. The method according to claim 7, characterized in that, In step a, the reaction pressure is 0.3-0.5 MPaA.

9. The method according to claim 1, characterized in that, In step a, the reaction is terminated at a temperature of 50-100°C.

10. The method according to claim 9, characterized in that, In step a, the reaction is terminated at a temperature of 60-80°C.

11. The method according to claim 1, characterized in that, In step a, the mass ratio of free chlorine in the fresh phosgene is less than 500 ppm.

12. The method according to claim 1, characterized in that, The amine in the liquid-phase amine stream is any one of aniline, cyclohexylamine, 1,6-hexanediamine, 1,4-diaminocyclohexane, 1-amino-3,3,5-trimethyl-5-aminomethylcyclohexane, 4,4'-diaminodicyclohexylmethanediamine, p-phenylenediamine, m-phenylenediethylenediamine, 2,4-toluenediamine, 2,6-toluenediamine, diphenylmethanediamine, or polymethylene polyphenyl polyamine.

13. The method according to claim 1, characterized in that, The inert solvent in the liquid-phase amine stream includes any one or a combination of at least two of aromatic hydrocarbons, chloroaromatic hydrocarbons, dialkyl terephthalate or diethyl phthalate.

14. The method according to claim 13, characterized in that, The inert solvent in the liquid-phase amine stream is an aromatic hydrocarbon and / or a chlorinated aromatic hydrocarbon.

15. The method according to claim 14, characterized in that, The inert solvent in the liquid phase amine stream is any one or a combination of at least two of chlorobenzene, dichlorobenzene, toluene, or xylene.

16. The method according to claim 1, characterized in that, The mass ratio of amine to inert solvent in the liquid amine stream is 1:(1.5-8).

17. The method according to claim 16, characterized in that, The mass ratio of amine to inert solvent in the liquid amine stream is 1:(2-5).

18. The method according to claim 1, characterized in that, The mass ratio of amine to fresh phosgene in the liquid amine stream is 1:(1.5-15).

19. The method according to claim 18, characterized in that, The mass ratio of amine to fresh phosgene in the liquid amine stream is 1:(2-5).

20. The method according to claim 1, characterized in that, In step b, the termination temperature of the phosgenation reaction is 60-150℃.

21. The method according to claim 1, characterized in that, In step b, the reaction pressure of the phosgenation reaction is 0.2-3.0 MPaA.

22. The method according to claim 1, characterized in that, In step b, the reaction residence time of the phosgenation reaction is 0.5-3h.

23. The method according to claim 1, characterized in that, In step b, the reactor type for the phosgenation reaction is any one or a combination of at least two of the following: dynamic mixer, tubular reactor, tower reactor, or batch reactor.

24. The method according to claim 1, characterized in that, In step c, the mass ratio of the cycloalkane to the phosgenation reaction liquid is (1-3):

1.

25. The method according to claim 1, characterized in that, In step c, the extraction time is 5-20 minutes.

26. The method according to claim 1, characterized in that, In step c, filtration is performed after extraction to remove insoluble matter.

27. The method according to claim 1, characterized in that, In step c, the extractant is removed using a distillation process.

28. The method according to claim 1, characterized in that, The stripping agent described in step d comprises CO, CO2, and HCl; wherein the molar ratio of CO2 to CO is (0.8-5):1; and the molar ratio of HCl to CO is (2-15):

1.

29. The method according to claim 28, characterized in that, The stripping agent described in step d comprises CO, CO2, and HCl; wherein the molar ratio of CO2 to CO is (1-3):1, and the molar ratio of HCl to CO is (3-10):

1.

30. The method according to claim 1, characterized in that, In step d, the phosgene removal is carried out in a phosgene removal tower, which is a two-section design; the upper section is a packed tower or a plate tower with a theoretical number of 3-8 plates; the lower section is a plate tower with a theoretical number of 10-20 plates; the pretreatment liquid is fed from the middle of the phosgene removal tower, and the combined gas is countercurrently stripped from the bottom of the phosgene removal tower to remove phosgene.

31. The method according to claim 30, characterized in that, The operating pressure of the deluminating gas tower is 0.05-0.3 MPaA.

32. The method according to claim 31, characterized in that, The operating pressure of the deluminating tower is 0.12-0.2 MPaA.

33. The method according to claim 30, characterized in that, The bottom temperature of the deluminated gas tower is 120-200℃.

34. The method according to claim 33, characterized in that, The bottom temperature of the deluminated gas tower is 140-180℃.

35. The method according to claim 1, characterized in that, In step d, the mass ratio of phosgene in the deluminated gas-liquid mixture is less than 2000 ppm; the mass ratio of phosgene adducts in the deluminated gas-liquid mixture is less than 500 ppm.

36. The method according to claim 35, characterized in that, In step d, the mass ratio of phosgene in the deluminated gas-liquid mixture is less than 200 ppm; the mass ratio of phosgene adducts in the deluminated gas-liquid mixture is less than 50 ppm.

Images