Method for continuously preparing diphenylmethane diisocyanate and modified product thereof

Abstract

The invention belongs to the technical field of fine chemical engineering, and particularly relates to an integrated process for continuously producing diphenylmethane diisocyanate (MDI) and a modified product thereof based on triphosgene (BTC). The invention discloses an HG catalyst which is prepared by adopting a compound template agent and combining a microwave-hydrothermal combined method with a supercritical CO2 drying technology. The invention further discloses a method for continuously preparing the diphenylmethane diisocyanate and the modified product of the diphenylmethane diisocyanate, which comprises the following steps: preparing Ph-Amine by taking formaldehyde and aniline as raw materials under the action of an HG catalyst; ph-Amine and BTC are used as raw materials to carry out a three-stage continuous isocyanate esterification reaction, and finally, pure MDI and mixed MDI are respectively obtained through solvent recovery and purification, and the mixed MDI is used for preparing modified MDI.

Description

Technical Field

[0001] The present invention belongs to the technical field of fine chemical industry, and particularly relates to an integrated process for continuously producing diphenylmethane diisocyanate (MDI) and its modified products based on triphosgene (BTC). Background Art

[0002] Diphenylmethane diisocyanate (MDI), chemical formula: C 15 H 10 N 2 O 2 ; it is a white to light yellow solid; its application fields include being used as polyurethane materials, adhesives, etc.

[0003] In traditional MDI preparation processes, the synthesis of amine intermediates often has problems such as low catalyst efficiency, many by-products, and long reaction cycles. Moreover, in the MDI synthesis process, there are also problems such as high HCl residue and low solvent recovery rate. In addition, the existing processes have high requirements for equipment and it is difficult to achieve continuous production.

[0004] As the mainstream synthesis process of MDI, the traditional phosgene method has long faced multiple technical bottlenecks (US 4,010,198; CN103664806A). First, there are significant defects in process safety: the high corrosiveness and leakage risk of highly toxic phosgene (COCl 2 2) force enterprises to configure complex safety facilities, directly resulting in an increase in equipment investment cost by more than 40%. Second, the by-product control system is inefficient: in the batch reaction process, the local concentration gradient of amine substances and phosgene triggers uncontrollable side reactions, resulting in the content of trimers and biurets in the product being as high as 12% - 18%, and the content of NCO groups (isocyanate groups) being suppressed to 28% - 31%, seriously affecting the reaction activity of the product. Third, there is insufficient environmental compliance: the solvent recovery rate of traditional processes is generally lower than 80%, the absorption efficiency of hydrogen chloride (HCl) tail gas is less than 65%, and the chemical oxygen demand (COD) of wastewater exceeds 500 mg / L, presenting significant environmental risks.

[0005] To avoid the toxicity of phosgene, existing technologies have tried to use triphosgene (BTC, C 3 Cl 6 3O 3Alternative (CN114409543B; JP 2018-025634A), but its industrial application still has systematic defects. First, the problem of reaction kinetic imbalance is prominent: the one-time addition of amine solution causes intense exothermic reaction (ΔT > 25°C), the side reaction rate increases by 30%, resulting in a broadening of the product molecular weight distribution (PDI > 1.5). Second, not only is it difficult to meet the product purity standard, but the NCO content in the resulting mixed MDI fluctuates between 29.5% and 32.5%, and the residual HCl > 100 ppm, directly restricting downstream applications. That is, this mixed MDI needs to be modified by adding a certain proportion of external isocyanates such as 4,4-MDI, 2,4-MDI, polymeric MDI, etc. Summary of the Invention

[0006] The technical problem to be solved by the present invention is to provide a method for continuously preparing diphenylmethane diisocyanate and its modified products.

[0007] To solve the above problems, the present invention provides an HG catalyst used in a method for continuously preparing diphenylmethane diisocyanate and its modified products. The preparation method of the HG catalyst includes the following steps: Step 1: Mix silane coupling agent KH-550 with tetraethyl orthosilicate (TEOS) and ethanol, and heat and stir at 60 ± 10°C for 4 ± 0.5 hours to form a silylated precursor; The weight ratio of silane coupling agent KH-550 to tetraethyl orthosilicate (TEOS) = (0.1 ± 0.01):1; The dosage ratio of tetraethyl orthosilicate to ethanol is 1 g / 1 - 1.5 ml (preferably 1 g / 1.2 - 1.3 ml); Step 2: Add aluminum isopropoxide to the silylated precursor obtained in Step 1, and stir evenly (stir for 2 ± 0.5 hours) to obtain a homogeneous solution; The weight ratio of aluminum isopropoxide to tetraethyl orthosilicate (TEOS) in Step 1 = 0.23 - 0.24:1; Step 3: Mix an aqueous solution of tetraethylammonium hydroxide (TEAOH) with a mass concentration of 15 - 25% (preferably 20 ± 1%) and an aqueous solution of trimethylamine (TMA) with a mass concentration of 35 - 45% (preferably 40 ± 1%) according to the molar ratio of tetraethylammonium hydroxide (TEAOH):trimethylamine (TMA) = (4 ± 0.1):1 to obtain a template agent mixture; First, drop the template agent mixture (the dropping time is about 30 ± 10 minutes) into the homogeneous solution obtained in Step 2, and then add deionized water to form a gel; The volume ratio of the template agent mixture to ethanol in Step 1 = 48 - 52% (preferably 50%); The deionized water: the volume ratio of ethanol to that in Step 1 is 2.1 - 2.2:1; Step 4. Subject the gel obtained in Step 3 to microwave irradiation, hydrothermal treatment (hydrothermal crystallization), first washing, supercritical CO 2 drying, acid treatment, second washing, and drying to obtain the HG catalyst.

[0008] As an improvement to the preparation method of the HG catalyst of the present invention: in Step 4: The microwave irradiation is carried out at 80 ± 10°C and a power of 300 - 500 W for 30 - 60 minutes (which can be carried out in a microwave reaction kettle, corresponding to 300 - 400 g of tetraethyl orthosilicate); The hydrothermal treatment is carried out at 120 ± 10°C (atmospheric pressure static hydrothermal) for 12 ± 1 hour; Both the first washing and the second washing are carried out with ethanol; The supercritical CO 2 drying is carried out at 40 ± 5°C and a pressure of 10 ± 0.5 MPa for 4 ± 0.5 hours; The acid treatment is carried out by soaking in a (0.5 ± 0.1) mol / L oxalic acid solution at 60 ± 10°C for 5 ± 0.5 hours; The drying is carried out by vacuum drying at 60 ± 10°C for 3 ± 0.5 hours.

[0009] A mesoporous zeolite catalyst with a specific surface area ≥ 650 m² / g and a mesoporous structure (pore diameter 2 - 5 nm) is obtained as the HG catalyst.

[0010] The present invention also simultaneously provides a method for continuously preparing diphenylmethane diisocyanate and its modified products (that is, provides a preparation method of MDI / modified MDI): using formaldehyde and aniline as raw materials, under the action of the HG catalyst, Ph-Amine is prepared; then, using Ph-Amine and BTC as raw materials, a three-stage continuous isocyanation reaction is carried out, and finally, through solvent recovery and purification, pure MDI and mixed MDI are obtained respectively, and the mixed MDI is used to prepare modified MDI; Ph-Amine represents phenyl polyamine, BTC represents bis(trichloromethyl) carbonate (that is, triphosgene), and MDI represents diphenylmethane diisocyanate.

[0011] As an improvement to the method for continuously preparing diphenylmethane diisocyanate and its modified products of the present invention, it includes the following steps: 1). Synthesize phenyl polyamine (Ph-Amine): Condensation reaction: Under the protection of an inert gas (such as nitrogen), aniline and formaldehyde react at 40 - 60 °C for 7 - 9 hours under the action of HG catalyst; then hydrochloric acid is added, and the reaction is first carried out at 60 - 70 °C for 4 ± 0.5 hours, and then at 90 - 100 °C for 7 ± 0.5 hours; the molar ratio of formaldehyde to aniline is 0.4 - 0.55:1, and the molar ratio of hydrogen chloride to aniline is 1.0 - 1.5:1; the weight ratio of HG catalyst to aniline is 0.01 - 0.1:1 (preferably 0.04 - 0.06:1); The obtained reaction product is neutralized and then allowed to stand for stratification. The organic phase (oil phase) obtained by stratification is distilled under reduced pressure to obtain a crude Ph-Amine product (the crude product contains HG catalyst, and the amine value is 8.5 - 9.5 mg KOH / g, the aniline residue and moisture are both < 100 ppb of Ph-Amine); 2), Preparation of crude MDI: Using the crude Ph-Amine product obtained in step 1) for preparation, the obtained reaction solution contains crude MDI; 3), Solvent recovery and purification: After the reaction solution obtained in step 2) is distilled under normal pressure (two-stage normal pressure distillation) to recover the solvent, and then pure MDI (with NCO content ≥ 33% and HCl residue ≤ 50 ppm) is obtained by short-path molecular distillation; mixed MDI is also obtained accordingly; The obtained mixed MDI is used to prepare modified MDI.

[0012] As a further improvement of the method for continuously preparing diphenylmethane diisocyanate and its modified product of the present invention, step 2) includes the following steps: 2.1) Raw material preparation: Preparation of BTC solution: BTC is mixed evenly with a solvent to obtain a BTC solution (which can be placed in a BTC solution storage tank for standby); the weight ratio of BTC to the solvent is 1:3 - 5 (preferably 1:4); the BTC is bis(trichloromethyl) carbonate (i.e., triphosgene); Preparation of mixed amine solution: The crude Ph-Amine product obtained in step 1) is mixed with a solvent (which can be placed in a transfer mixing kettle), filtered, and the obtained filter residue is HG catalyst (after drying, it can be used for the next batch); a tin-based catalyst is added to the filtrate and mixed evenly to obtain a mixed amine solution (which can be placed in a mixed amine solution storage tank for standby), the weight ratio of the crude Ph-Amine product to the solvent is 1:3 - 5 (preferably 1:4); the weight ratio of the tin-based catalyst to the crude Ph-Amine product is 0.01 - 0.1:1 (preferably 0.004 - 0.006:1); Note: The application times of the HG catalyst ≥ 5 times; The solvents used for the BTC solution and the mixed amine solution are preferably the same solvent; 2.2) Tertiary isocyanation reaction: According to the weight ratio of BTC to the crude Ph-Amine being 2.3 - 2.4:1, set the dosage ratio of the BTC solution and the mixed amine solution; and successively conduct the following tertiary reactions: Primary reaction: Mix the BTC solution with 1 / 3 of the mixed amine solution, and react at normal pressure, 30 - 40 °C, and a rotation speed of 250 ± 50 rpm for 2 ± 0.5 hours to obtain a primary reaction solution; Secondary reaction: Mix the primary reaction solution with 1 / 3 of the mixed amine solution, and under the protection of an inert gas, react at 60 - 70 °C, a pressure of 0.2 ± 0.05 Mpa, and a rotation speed of 250 ± 50 rpm for 2 ± 0.5 hours to obtain a secondary reaction solution; Tertiary reaction: Mix the secondary reaction solution with 1 / 3 of the mixed amine solution, and react at 90 ± 5 °C, a vacuum condition of -0.06 ± 0.01 Mpa, and a rotation speed of 250 ± 50 rpm for 2 ± 0.5 hours to obtain a tertiary reaction solution; the tertiary reaction solution contains crude MDI; That is, the set pressure gradient is normal pressure → slightly positive pressure → vacuum.

[0013] As a further improvement of the method for continuously preparing diphenylmethane diisocyanate and its modified products of the present invention, the following step 4) is also included: 4) Preparation of modified MDI: Mix the mixed MDI with polyether polyol, tin catalyst, and amine catalyst, and conduct a modification reaction at 80 - 100 °C under the protection of an inert gas (such as nitrogen); until the NCO content drops to 12% - 18% (preferably 15 ± 0.5%), to obtain modified MDI (modified MDI with a viscosity of about 1600 - 2000 mPa·s); The polyether polyol is 48 - 52% (preferably 50%) of the weight of the mixed MDI; The addition amount of the tin catalyst is 0.05 - 0.15% (preferably 0.08 - 0.15%, more preferably 0.08 - 0.1%) of the weight of the mixed MDI; the addition amount of the amine catalyst is 0.02 - 0.06% (preferably 0.05 - 0.06%) of the weight of the mixed MDI.

[0014] As a further improvement of the method for continuously preparing diphenylmethane diisocyanate and its modified products of the present invention, the tin catalysts in step 2.1) and step 4) are at least any one of the following (a combination of one or more): stannous octoate, stannous neodecanoate, dibutyltin dilaurate, methyltin mercaptide, butyltin mercaptide, octyltin mercaptide; The amine catalyst in step 4) is A33 catalyst; The polyether polyol in step 4) is PPG - 2000.

[0015] As a further improvement to the method for continuously preparing diphenylmethane diisocyanate and its modified products according to the present invention, step 1) includes the following steps: 1.1) Condensation reaction: Under the protection of an inert gas (such as nitrogen), aniline and an HG catalyst are added to a reaction kettle (polyamine reaction kettle), and an aqueous formaldehyde solution with a mass concentration of 35 - 37% is added dropwise (the dropping time is about 2 ± 0.5 h) in a water bath at 10 ± 2°C. After stirring evenly (stirring for 30 ± 10 minutes), the temperature is raised to 40 - 60°C for reaction for 7 - 9 hours; subsequently, an aqueous hydrochloric acid solution with a mass concentration of 35 - 37% is added dropwise (the dropping time is about 2 ± 0.5 h), and the reaction is carried out at 60 - 70°C for 4 ± 0.5 hours and at 90 - 100°C for 7 ± 0.5 hours in sequence; The molar ratio of formaldehyde to aniline is 0.4 - 0.55:1, and the molar ratio of hydrogen chloride (hydrogen chloride in hydrochloric acid) to aniline is 1.0 - 1.5:1; the weight ratio of the HG catalyst to aniline is 0.01 - 0.1:1; 1.2) Neutralization treatment: After cooling the product obtained in step 1.1) (transparent brown solution) to room temperature, it is transferred to a neutralization and separation kettle, and an NaOH solution with a mass concentration of 36 - 40% is added dropwise. After stirring (stirring for 30 - 45 minutes), neutralization and separation are carried out (pH is about 13 - 14). After standing, the upper organic phase is separated out; The molar ratio of NaOH to hydrogen chloride is 0.9 - 1.1:1 (preferably 1:1); 1.3) Vacuum distillation: The organic phase obtained in step 1.2) is distilled under a reduced pressure of 0.1 MPa, and the fraction at 60 - 220°C is collected to obtain an aniline solution (the aniline solution can be recycled and reused); the residue in the kettle is phenyl polyamine (Ph - Amine).

[0016] As a further improvement to the method for continuously preparing diphenylmethane diisocyanate and its modified products according to the present invention, The solvent in step 2.1) is at least any one of dichloromethane, dichloroethane, chlorobenzene, toluene, dimethyl isooctanoate, and dimethyl butanol (a combination of one or more); In step 2.2): The gases generated in the first - stage reaction, second - stage reaction, and third - stage reaction are condensed. The condensed liquid (containing trace phosgene) is returned to the corresponding reaction system for the corresponding reaction, and the condensed gas (HCl gas) enters the HCl absorption tower for absorption.

[0017] That is, the HCl gas is removed and recovered.

[0018] As a further improvement of the method for continuously preparing diphenylmethane diisocyanate and its modified products of the present invention, in the step 3): The tertiary reaction solution obtained in step 2) is subjected to two-stage atmospheric distillation: first, atmospheric distillation (for example, at 80-100 °C) to collect the solvent (which can be recycled and reused); then the temperature is raised to 120-150 °C for atmospheric distillation; Note: The fractions collected at 120-150 °C are over-fractions (fractions with an NCO content less than 33%); the over-fractions are waste; Subsequently, short-path molecular distillation (180 ± 5 °C, vacuum degree ≤ 3 kPa) is used for purification to obtain pure MDI (pure MDI with an NCO content ≥ 33% and HCl residue ≤ 50 ppm); the substances in the kettle are mixed MDI; the content of NCO in the mixed MDI product is about 26%-33% (preferably 30%-33%).

[0019] The present invention also simultaneously provides the application of MDI or modified MDI prepared by the above method in polyurethane foams, elastomers or coatings.

[0020] The present invention is a method for continuously preparing MDI and its modified products proposed in view of the problems in the existing MDI preparation process such as low catalyst efficiency, many by-products, and poor product stability. The present invention significantly improves the reaction efficiency and reduces the impurity residue by designing a new catalyst and a new BTC method preparation process. At the same time, triphosgene replacing phosgene is more environmentally friendly, reduces the safety hazards during large-scale production, and realizes continuous production.

[0021] The present invention has the following technical advantages: 1. An HG catalyst is provided for synthesizing phenyl polyamine (Ph-Amine); The HG catalyst of the present invention has a unique structure: a compound template agent (TEAOH + TMA) is used, and through the combined microwave-hydrothermal method and supercritical CO 2 drying technology, the prepared HG catalyst has a high specific surface area (specific surface area ≥ 650 m² / g) and a mesoporous structure (pore diameter 2-5 nm), with uniform distribution of acidic sites, significantly improving the aniline-formaldehyde condensation reaction efficiency, and can be reused more than 5 times, which can reduce the content of high-boiling substances in phenyl polyamine. It can effectively control the content of high-molecular polyamine in phenyl polyamine, and the amine value of the obtained intermediate phenyl polyamine is stable at 8.5-9.5 mg KOH / g, and the aniline residue and moisture are both less than 100 ppb.

[0022] Thus, the NCO in the finally obtained diphenylmethane diisocyanate (MDI) is high.

[0023] 2. In the preparation process of MDI: 1), Using triphosgene to replace phosgene is more environmentally friendly and safer.

[0024] 2), Adopting a three-stage continuous reaction system: Through segmented temperature control (30~40°C → 60~70°C → 90±5°C) and pressure gradient (atmospheric pressure → slightly positive pressure → vacuum) design, HCl is removed in stages, thereby achieving efficient separation of hydrochloric acid gas (HCl residue ≤ 50 ppm), while reducing energy consumption by more than 20%, and making the NCO content ≥ 33%; Combined with short-path molecular distillation technology (180°C / 3 kPa), it has significant differences compared with the existing BTC process (single reaction kettle + atmospheric distillation).

[0025] Precisely control the weight ratio of BTC / Ph-Amine to 2.3 - 2.4:1, and it is proved by examples that this dosage ratio has a critical control effect on the NCO content (≥33%) and HCl residue (≤50 ppm).

[0026] Note: When the HG catalyst is recycled, the NCO content ≥ 32.8%.

[0027] 3), In the present invention, by controlling the distillation amount of pure MDI, the mixed MDI in the kettle residue can directly reach the ratio requirement (that is, the NCO content of the mixed MDI is 26% - 33%), and then directly carry out subsequent modification applications.

[0028] 4), High solvent recovery rate: Combining two-stage atmospheric distillation with short-path molecular distillation technology, the comprehensive solvent recovery rate ≥ 98% (verified by GC-MS), significantly reducing the raw material cost.

[0029] 5), The present invention can prepare high-purity Ph-Amine: In the directional synthesis of phenyl polyamine Ph-Amine, HG catalyst and staged control of aniline-formaldehyde condensation and ring-closure reaction are adopted, and the amine value of the product Ph-Amine is precisely controllable (8.5 - 9.5 mgKOH / g), the aniline residue < 100 ppb, and the moisture < 100 ppb, meeting the synthesis requirements of high-end MDI.

[0030] 6), High product consistency, suitable for large-scale production.

[0031] 3, The modified MDI has excellent performance: In the in-situ modification process of polyether polyol, by precisely blocking the polyether polyol (preferably controlling the NCO content to 15±0.5%), the obtained modified MDI has low viscosity (1900±200 mPa·s) and high reaction activity, and is suitable for polyurethane foam, high-precision polyurethane injection molding and spraying processes. Description of the Drawings

[0032] The following further elaborates on the specific implementation manners of the present invention in conjunction with the drawings.

[0033] Figure 1 Schematic diagram of the device for the three-stage isocyanation reaction of the present invention. Specific embodiments

[0034] The present invention will be further described below in conjunction with specific embodiments, but the protection scope of the present invention is not limited thereto: BTC: Triphosgene, that is, bis(trichloromethyl) carbonate.

[0035] Ph-Amine: Phenyl polyamine, MDI: Diphenylmethane diisocyanate.

[0036] Preparation example 1 of catalyst, preparation of HG catalyst, the following steps are carried out in sequence: 1). Add 36 g of silane coupling agent KH-550 to a mixed solution of 360 g of tetraethyl orthosilicate (TEOS) and 450 mL of ethanol, stir at 60 °C for 4 hours to form a silylated precursor; 2). Add 85.5 g of aluminum isopropoxide to the silylated precursor obtained in step 1), and continue to stir for 2 hours to obtain a homogeneous solution; 3). According to the molar ratio of tetraethylammonium hydroxide: trimethylamine = 4:1, mix an aqueous solution of tetraethylammonium hydroxide with a mass concentration of 20% and an aqueous solution of trimethylamine with a mass concentration of 40% to obtain a templating agent mixture; Dropwise add 221 mL of the templating agent mixture (the dropping time is about 30 minutes) to the homogeneous solution obtained in step 2); after the addition of the templating agent mixture is completed, add 960 mL of deionized water (at this time, the pH is about 10-11) to form a gel; 4). Transfer the gel to a microwave reaction kettle, irradiate it by microwave at 80 °C and a power of 500 W for 60 minutes, and then carry out hydrothermal treatment at 120 °C under normal pressure for 12 hours; 5). Filter the hydrothermal product obtained in step 4), wash the obtained filter cake with ethanol 3 times (the purpose of washing is to remove the templating agent residues - tetraethylammonium hydroxide and trimethylamine)), and transfer it to a supercritical CO 2 Drying device, dry at 40 °C and a pressure of 10 MPa for 4 hours; 6). Immerse the dried product in a 0.5 mol / L oxalic acid solution at 60 °C for 5 hours, wash it with ethanol until neutral (that is, after secondary washing), and then dry it in vacuum at 60 °C for 3 hours to obtain HG catalyst (about 100 g).

[0037] The specific surface area of the HG catalyst is 650 m² / g, and there is a mesoporous structure with pore diameters of 2-5 nm.

[0038] Catalyst Comparative Example 1: The use of "trimethylamine (TMA)" in Step 3) of Catalyst Preparation Example 1 was cancelled, that is, only TEAOH was used as the template agent; an aqueous solution of tetraethylammonium hydroxide with a mass concentration of 20% was used to replace the "template agent mixture", and the rest was the same as Catalyst Preparation Example 1. The obtained catalyst was named HG Catalyst A. Its specific surface area was about 520 m² / g, and there was a mesoporous structure with pore diameters of 4 - 10 nm.

[0039] Catalyst Comparative Example 2: The "microwave irradiation for 60 minutes under the conditions of 80°C and a power of 500W" in Step 4) of Catalyst Preparation Example 1 was cancelled, that is, the gel was directly hydrothermally treated statically at 120°C under normal pressure for 13 hours; the rest was the same as Catalyst Preparation Example 1. The obtained catalyst was named HG Catalyst B. Its specific surface area was about 550 m² / g, and there was a mesoporous structure with pore diameters of 1 - 3 nm.

[0040] Catalyst Comparative Example 3: The "supercritical CO 2 drying" in Step 5) of Catalyst Preparation Example 1 was changed to ordinary drying: drying at 40°C under normal pressure for 4 hours; the rest was the same as Catalyst Preparation Example 1. The obtained catalyst was named HG Catalyst C. Its specific surface area was about 400 m² / g, and there was a bimodal pore structure with pore diameters of 1 - 2 nm and >10 nm.

[0041] Example 1: A preparation method of diphenylmethane diisocyanate (MDI) and its modified products, which were carried out in the following steps: I. Synthesis of phenyl polyamine (Ph-Amine) 1.1) Condensation reaction: Under nitrogen protection, 1019 g of aniline (10.9 mol) and 50 g of HG catalyst (obtained from Catalyst Preparation Example 1) were added to a polyamine reaction kettle. An aqueous solution of 37% formaldehyde (containing 4.54 mol of formaldehyde) was slowly added dropwise (the dropping time was about 2 h) in a 10°C water bath, and after stirring for 30 minutes, the temperature was raised to 40 - 60°C and reacted for 8 hours; then an aqueous solution of 37% hydrochloric acid (containing 10.95 mol of hydrogen chloride) was added dropwise (the dropping time was about 2 h), and the reaction was carried out at 60 - 70°C for 4 hours and at 90 - 100°C for 7 hours to obtain a transparent brown solution; That is, in this case, the molar ratio of formaldehyde:aniline = 0.42:1; the molar ratio of hydrogen chloride:aniline = 1.005:1; 1.2) Neutralization treatment: After the transparent brown solution obtained in Step 1.1) was cooled to room temperature, it was transferred to a neutralization and separation kettle, and a 38% NaOH solution (containing 10.95 mol of sodium hydroxide) was added dropwise to adjust the pH to about 13 - 14, stirred for 30 minutes, and after standing, the upper organic phase was separated; That is, in this case, the molar ratio of NaOH to hydrogen chloride in step 1) is 1:1. 1.3) Vacuum distillation: The organic phase is distilled under a reduced pressure of 0.1 MPa until no more distillate is produced, and the distillate at 60 - 220 °C is collected to obtain an aniline solution, which can be recycled and reused. 924 g of crude phenyl polyamine (Ph-Amine) is obtained in the kettle. The crude phenyl polyamine (Ph-Amine) contains the HG catalyst. The amine value of Ph-Amine is 8.9 mg KOH / g, and the residual aniline and moisture are both less than 100 ppb.

[0042] II. Preparation of MDI: 2.1) Raw material preparation: Preparation of BTC solution: 2177 g of BTC is mixed evenly with 8708 g of dichloroethane and placed in the BTC solution storage tank for standby. Preparation of mixed amine solution: 924 g of the crude Ph-Amine obtained in step one and 3696 g of dichloroethane are placed in the transfer mixing kettle. After filtration, a solid is obtained. The solid is dried (dried to constant weight at 100 °C) to obtain the recoverable HG catalyst for the next batch application. 4.62 g of thiol butyltin is added to the filtrate and mixed evenly, and then placed in the mixed amine solution storage tank for standby.

[0043] That is, BTC:crude Ph-Amine = 2.356:1.

[0044] 2.2) Three-stage reaction: The device used for the three-stage reaction is as Figure 1 shown: The outlet of the first mixing storage tank M01 is hermetically connected to the inlet of the first reaction kettle R0201. The first reaction kettle R0201 is equipped with an N 2 inlet (which can be used as a protective gas in case of an accident); the top of the first reaction kettle R0201 is equipped with a gas outlet, which is connected to the first HCl absorption tower T0201 through the first condenser C0201; the bottom of the first reaction kettle R0201 is equipped with a material outlet; the material outlet of the first reaction kettle R0201 is hermetically connected to the inlet of the second mixing storage tank M02 through a peristaltic pump; The outlet of the second mixing storage tank M02 is hermetically connected to the inlet of the second reaction kettle R0202. The second reaction kettle R0202 is equipped with an N 2 inlet; the top of the second reaction kettle R0202 is equipped with a gas outlet, which is connected to the second HCl absorption tower T0202 through the second condenser C0202; the bottom of the second reaction kettle R0202 is equipped with a material outlet; the material outlet of the second reaction kettle R0202 is hermetically connected to the inlet of the third mixing storage tank M03 through a peristaltic pump; The outlet of the mixing storage tank M03 is hermetically connected to the feed inlet of the third-stage reaction kettle R0203, and there is an N 2 inlet (used as a protective gas in case of an accident); the top of the third-stage reaction kettle R0203 is provided with a gas outlet, and this gas outlet is connected to the HCl absorption tower T0203 after passing through the condenser C0203; a material outlet is provided at the bottom of the third-stage reaction kettle R0203; the third-stage reaction kettle R0203 is provided with a vacuum extraction port, and for the sake of simplicity of the drawing, this vacuum extraction port is Figure 1 omitted in the figure.

[0045] The specific process of the three-stage reaction is as follows: A. First-stage reaction: Transfer all the BTC solution in the BTC solution storage tank into the mixing storage tank M01 through a peristaltic pump. At the same time, pump 1 / 3 of the mixed amine solution in the mixed amine solution storage tank into the mixing storage tank M01 and mix for 30 minutes. The mixed material flows into the first-stage reaction kettle R0201 for the first-stage reaction (atmospheric pressure), and reacts at 30 °C and a rotation speed of 250 ± 10 rpm for 2 hours. The HCl gas generated by the reaction is discharged from the gas outlet at the top of the first-stage reaction kettle R0201, condensed by the condenser C0201, and then enters the HCl absorption tower T0201, so as to realize the recovery of HCl gas; the first-stage reaction liquid obtained from the reaction is discharged from the material outlet at the bottom of the first-stage reaction kettle R0201 and transferred into the mixing storage tank M02 through a peristaltic pump; B. Second-stage reaction: After the first-stage reaction is completed, pump 1 / 3 of the mixed amine solution in the mixed amine solution storage tank into the first-stage reaction liquid in the mixing storage tank M02 and mix for 30 minutes. The mixed material flows into the second-stage reaction kettle R0202 for the second-stage reaction. Nitrogen is introduced into the second-stage reaction kettle R0202, and the reaction is carried out at 60 °C, a slightly positive pressure of 0.2 ± 0.05 MPa, and a rotation speed of 250 ± 10 rpm for 2 hours. The HCl gas generated by the reaction is discharged from the gas outlet at the top of the second-stage reaction kettle R0202, condensed by the condenser C0202, and then enters the HCl absorption tower T0202, so as to realize the recovery of HCl gas; the second-stage reaction liquid obtained from the reaction is discharged from the material outlet at the bottom of the second-stage reaction kettle R0202 and transferred into the mixing storage tank M03 through a peristaltic pump; C. Third-stage reaction: After the secondary reaction is completed, the remaining 1 / 3 of the mixed amine solution in the mixed amine solution storage tank is pumped into the secondary reaction solution in the mixed storage tank M03 and mixed for 30 minutes. The mixed material flows into the tertiary reaction kettle R0203 and reacts at 90 °C, -0.06 ± 0.01 MPa vacuum condition, and 250 ± 10 rpm rotation speed for 2 hours. The generated HCl gas is discharged from the gas outlet at the top of the tertiary reaction kettle R0203, condensed by the condenser C0203 and then enters the HCl absorption tower T0203, thus realizing the recovery of HCl gas; the obtained tertiary reaction solution is discharged from the material outlet at the bottom of the tertiary reaction kettle R0203.

[0046] The tertiary reaction solution contains crude MDI.

[0047] Note: The gases generated in the primary reaction, secondary reaction, and tertiary reaction are condensed. The condensed liquid (containing trace amounts of phosgene) is returned to the corresponding reaction system for the corresponding reaction, and the condensed gas (HCl gas) enters the HCl absorption tower for absorption.

[0048] 2.3) Solvent recovery and purification: The tertiary reaction solution discharged from the material outlet at the bottom of the tertiary reaction kettle R0203 enters the solvent recovery tower through a peristaltic pump and is subjected to atmospheric distillation at 80 - 100 °C. The fraction with a tower temperature of 80 - 85 °C is collected as the solvent dichloroethane, and the solvent recovery rate is 98.8%. Subsequently, it is reused (i.e., the main solvent is recovered).

[0049] Then, atmospheric distillation is carried out at 120 - 150 °C; the fraction collected at 120 - 150 °C is the over - fraction (the fraction with an NCO content less than 33%). The over - fraction is waste.

[0050] The substance obtained in the solvent recovery tower is named crude MDI.

[0051] The crude MDI discharged from the bottom of the solvent recovery tower enters the product distillation tower through a peristaltic pump. Subsequently, in the product distillation tower, the crude MDI is purified by short - path molecular distillation (180 ± 5 °C, ≤ 3 kPa vacuum degree) to obtain 300 g of pure MDI. In the pure MDI, the NCO content ≥ 33% and the HCl residue ≤ 50 ppm. The substance in the product distillation tower is the mixed MDI product, and the NCO content of the mixed MDI is 32.55% and the weight is 700 g.

[0052] Note: The NCO content can be detected by a conventional chemical titration method.

[0053] Description: In the present invention, the distillation amount of pure MDI is controlled by controlling the parameters of short-path molecular distillation, so that the proportion of mixed MDI in the still residue meets the requirements (i.e., the NCO content of mixed MDI is 26% - 33%), and then it can be directly used for subsequent modification applications; III. Preparation of modified MDI: Add 350 g of polyether polyol (such as PPG - 2000), 0.7 g of stannous octoate, and 0.35 g of A33 catalyst to 700 g of the mixed MDI product. The addition amount of stannous octoate is 0.1% of the mixed MDI, and the addition amount of A33 catalyst is 0.05% of the mixed MDI. React at 80 - 100 °C under nitrogen protection until the NCO content drops to 15% to obtain modified MDI with a viscosity of 1800 mPa·s (25 °C).

[0054] Table 1 Key data of Example 1

[0055] Example 2: Recycling of HG catalyst: First recycling: Change the "50 g of HG catalyst" in step 1.1) of Example 1 to the recyclable HG catalyst obtained in step 2.1) of Example 1. Since there may be slight damage to the HG catalyst, fresh HG catalyst is supplemented until the total amount is 50 g; the rest is the same as Example 1.

[0056] And so on, that is, for subsequent batches, the recyclable HG catalyst from the previous batch is used, and fresh HG catalyst is supplemented until the total amount is 50 g.

[0057] A total of 5 recyclings are carried out, and the obtained results are shown in Table 2 below.

[0058] Table 2 Key data of Example 2

[0059] In summary, it can be seen that the "recyclable HG catalyst" obtained in step 2.1) of the present invention can indeed be recycled.

[0060] Example 3 - 1: In step 1.1) of Example 1 involving "synthesis of phenyl polyamine (Ph - Amine)", change the "37% formaldehyde aqueous solution containing 4.54 mol of formaldehyde" to "37% formaldehyde aqueous solution containing 6.06 mol of formaldehyde", that is, change the molar ratio of formaldehyde:aniline to about 0.55:1; the rest is the same as Example 1.

[0061] Example 3-2: In step 1.1) of "synthesizing phenyl polyamine (Ph-Amine)" in Example 1, change the "37% aqueous formaldehyde solution containing 4.54 mol of formaldehyde" to "37% aqueous formaldehyde solution containing 4.36 mol of formaldehyde", that is, change the molar ratio of formaldehyde to aniline to 0.4:1; the rest is the same as Example 1.

[0062] Comparative Example 1-1: In step 1.1) of "synthesizing phenyl polyamine (Ph-Amine)" in Example 1, change the "37% aqueous formaldehyde solution containing 4.54 mol of formaldehyde" to "37% aqueous formaldehyde solution containing 6.81 mol of formaldehyde", that is, change the molar ratio of formaldehyde to aniline to approximately 0.62:1; the rest is the same as Example 1.

[0063] Comparative Example 1-2: In step 1.1) of "synthesizing phenyl polyamine (Ph-Amine)" in Example 1, change the "37% aqueous formaldehyde solution containing 4.54 mol of formaldehyde" to "37% aqueous formaldehyde solution containing 3.89 mol of formaldehyde", that is, change the molar ratio of formaldehyde to aniline to approximately 0.36:1; the rest is the same as Example 1.

[0064] Example 4-1: In step 1.1) of "synthesizing phenyl polyamine (Ph-Amine)" in Example 1, change the "37% hydrochloric acid containing 10.95 mol of hydrogen chloride" to "37% hydrochloric acid containing 10.90 mol of hydrogen chloride", that is, the molar ratio of aniline to hydrogen chloride is 1:1; and extend the stirring time for neutralization treatment in step 1.2) from "30 minutes" to "45 minutes", the rest is the same as Example 1.

[0065] Example 4-2: In step 1.1) of "synthesizing phenyl polyamine (Ph-Amine)" in Example 1, change the "37% hydrochloric acid containing 10.95 mol of hydrogen chloride" to "37% hydrochloric acid containing 16.35 mol of hydrogen chloride", that is, the molar ratio of aniline to hydrogen chloride is 1:1.5; and increase the amount of sodium hydroxide used for neutralization treatment in step 1.2), that is, still ensure the molar ratio of NaOH to hydrogen chloride in step 1) is 1:1; the rest is the same as Example 1.

[0066] The comparison of the above cases is shown in Table 3 below: Table 3

[0067] Example 5-1: In step 2.1) of "Preparation of MDI" in Example 1, change "2177 g of BTC and 8708 g of dichloroethane" to "2124 g of BTC and 8496 g of dichloroethane", and keep the amount of crude Ph-Amine unchanged, still 924 g. Therefore, the weight ratio of BTC:crude Ph-Amine = 2.30:1; the rest is the same as in Example 1.

[0068] Example 5-2: In step 2.1) of "Preparation of MDI" in Example 1, change "2177 g of BTC and 8708 g of dichloroethane" to "2218 g of BTC and 8872 g of dichloroethane", and keep the amount of crude Ph-Amine unchanged, still 924 g. Therefore, the weight ratio of BTC:crude Ph-Amine = 2.40:1; the rest is the same as in Example 1.

[0069] Comparative Example 2-1: In step 2.1) of "Preparation of MDI" in Example 1, change the amount of crude Ph-Amine from "924 g" to "990 g", and keep the amount of BTC unchanged, that is, BTC:crude Ph-Amine = 2.20:1 (similar to the existing usage ratio); the rest is the same as in Example 1.

[0070] Comparative Example 2-2: Change the amount of crude Ph-Amine in step 2.1) of "Preparation of MDI" in Example 1, and keep the amount of BTC unchanged, so that BTC:crude Ph-Amine = 2.50:1 (similar to the existing usage ratio). The rest is the same as in Example 1.

[0071] The comparison of the above cases is shown in Table 4 below: Table 4

[0072] Comparative Example 3: Compared with Example 1, referring to Example 8 in Patent CN 112292413 B, change "The three-stage reaction system realizes the staged removal of HCl through a temperature-pressure gradient (atmospheric pressure → slightly positive pressure → vacuum)" in Example 1 of the present invention to "Single reaction kettle + atmospheric distillation"; Specifically as follows: Add the mixed amine solution into the reaction kettle, keep the temperature at 145 °C, introduce the BTC solution in 28 h, and react at 145 °C to 150 °C for 3 - 4 h. Distill at 120 °C, and the obtained product is a mixture. This mixture cannot accurately design the application formula, and the yield and conversion rate data of the mixture cannot be determined, and the process is not rigorous.

[0073] Example 6-1: In step 3 of "modified MDI preparation" involved in Example 1, change the "stannous octoate addition amount (weight ratio of stannous octoate to mixed MDI)" from "0.1%" to "0.08%", and change the "A33 catalyst addition amount (weight ratio of A33 catalyst to mixed MDI)" from "0.05%" to "0.06%"; the rest is the same as Example 1.

[0074] Example 6-2: In step 3 of "modified MDI preparation" involved in Example 1, change the "stannous octoate addition amount (weight ratio of stannous octoate to mixed MDI)" from "0.1%" to "0.15%", and change the "A33 catalyst addition amount (weight ratio of A33 catalyst to mixed MDI)" from "0.05%" to "0.06%"; the rest is the same as Example 1.

[0075] Comparative Example 4-1: In step 3 of "modified MDI preparation" involved in Example 1, change the "stannous octoate addition amount (weight ratio of stannous octoate to mixed MDI)" from "0.1%" to "0.20%", and change the "A33 catalyst addition amount (weight ratio of A33 catalyst to mixed MDI)" from "0.05%" to "0.08%"; the rest is the same as Example 1.

[0076] Comparative Example 4-2: In step 3 of "modified MDI preparation" involved in Example 1, change the "stannous octoate addition amount (weight ratio of stannous octoate to mixed MDI)" from "0.1%" to "0.03%", and change the "A33 catalyst addition amount (weight ratio of A33 catalyst to mixed MDI)" from "0.05%" to "0.01%"; the rest is the same as Example 1.

[0077] Comparative Example 5: In step 3 of "modified MDI preparation" involved in Example 1, cancel the use of A33 catalyst, and keep the stannous octoate addition amount unchanged; the rest is the same as Example 1.

[0078] Experiment 1. A low-density flexible foam formulation is as shown in Table 5 below. The modified MDI used in the formulation is the modified MDI prepared in Example 1, Example 6-1, Example 6-2, Comparative Example 4-1, Comparative Example 4-2, and Comparative Example 5.

[0079] Table 5 Low-density flexible foam formulation table

[0080] Foam preparation operation process: Add polyether polyol PPG-2000, latex-like polyether HF-22, deionized water, silicone oil HGD-858H, and dimethyl methylphosphonate into a stainless steel stirring kettle, stir at 1200 rpm for 20 minutes until the system is transparent, add amine catalyst A33, and stir at 500 rpm for 5 minutes to obtain Component A.

[0081] Stannous octoate was added to the modified MDI, and the mixture was stirred at a low speed of 300 rpm for 10 minutes under nitrogen protection to obtain component B.

[0082] Pour component B into component A and mix for 15 seconds using a mechanical stirrer (1500 rpm, Φ50 mm anchor blade). Immediately pour into a 30×30×15 cm mold and allow to foam freely for 5 minutes at 45°C and 70% humidity. Then cure at 60°C and 50% humidity for 20 minutes, and then demould at 80°C and 30% humidity for 180 minutes to obtain foam.

[0083] Finally, the foam was placed in a 23°C / 50%RH environment for 48 hours to obtain a low-density soft foam product. Various performance tests were performed according to ASTM D3574 standard.

[0084] Table 6 Performance comparison table

[0085] According to Table 6 above, it can be seen that the pore structure of Example 1 is the best.

[0086] Comparative Experiment 2: The “HG catalyst obtained by using catalyst preparation example 1” in Example 1 is respectively changed to “HG catalyst A obtained by catalyst comparison example 1”, “HG catalyst B obtained by catalyst comparison example 2”, and “HG catalyst C obtained by catalyst comparison example 3”; the rest is the same as Example 1.

[0087] The results are shown in Table 7 below: Table 7

[0088] Finally, it should be noted that the above examples are only some specific embodiments of the present invention. Obviously, the present invention is not limited to the above embodiments, and there are many variations. All variations that can be directly derived or associated with the content disclosed by a person skilled in the art should be considered as the protection scope of the present invention.

Claims

1. The HG catalyst used in the method for continuously preparing diphenylmethane diisocyanate and its modified product is characterized in that The preparation method of the HG catalyst comprises the following steps: Step 1: Mix silane coupling agent KH-550, ethyl orthosilicate and ethanol, and heat and stir at 60±10° C. for 4±0.5 hours to form a silanization precursor; Silane coupling agent KH-550:ethyl orthosilicate = (0.1±0.01):1 weight ratio; The usage ratio of ethyl orthosilicate to ethanol is 1g / 1~1.5ml; Step 2: adding aluminum isopropoxide to the silylation precursor obtained in step 1, stirring evenly to obtain a homogeneous solution; Aluminum isopropoxide:ethyl orthosilicate in step 1 = 0.23-0.24:1 weight ratio; Step 3, according to the molar ratio of tetraethylammonium hydroxide: trimethylamine = (4±0.1): 1, a tetraethylammonium hydroxide aqueous solution with a mass concentration of 15-25% and a trimethylamine aqueous solution with a mass concentration of 35-45% are mixed to obtain a template agent mixed solution; Firstly, the template mixed solution is added dropwise to the homogeneous solution obtained in step 2, and then deionized water is added to form a gel; The template mixed solution: ethanol in step 1 = 48-52% by volume; The volume ratio of the deionized water to the ethanol in step 1 is 2.1-2.2:1; Step 4: The gel obtained in step 3 is subjected to microwave irradiation, hydrothermal treatment, primary washing, supercritical CO2 drying, acid treatment, secondary washing, and drying to obtain a HG catalyst.

2. The HG catalyst used in the method for continuously preparing diphenylmethane diisocyanate and its modified product according to claim 1, characterized in that In the step 4: Microwave irradiation: 80±10℃, 300~500W power, 30~60 minutes; The hydrothermal method is: 120±10°C for 12±1 hours; The first washing and the second washing are both ethanol washing; The supercritical CO2 drying is carried out at 40±5°C and 10±0.5MPa pressure for 4±0.5 hours; The acid treatment is soaking in a (0.5 ± 0.1) mol / L oxalic acid solution at 60 ± 10 ° C for 5 ± 0.5 hours; The drying is performed by vacuum drying at 60±10° C. for 3±0.5 hours.

3. A method for continuously preparing diphenylmethane diisocyanate and its modified product, characterized in that: Using formaldehyde and aniline as raw materials, under the action of the HG catalyst described in claim 1 or 2, Ph-Amine is prepared; then using Ph-Amine and BTC as raw materials, a three-stage continuous isocyanate reaction is carried out, and finally, after solvent recovery and purification, pure MDI and mixed MDI are obtained respectively, and the mixed MDI is used to prepare modified MDI; Ph-Amine stands for phenyl polyamine, BTC stands for bis(trichloromethyl) carbonate, and MDI stands for diphenylmethane diisocyanate.

4. The method for continuously preparing diphenylmethane diisocyanate and its modified product according to claim 3, characterized in that The following steps are involved: 1) Synthesis of phenyl polyamines: Under the protection of inert gas, aniline and formaldehyde react at 40-60°C for 7-9 hours under the action of HG catalyst; then add hydrochloric acid, first react at 60-70°C for 4±0.5 hours, and then react at 90-100°C for 7±0.5 hours; the molar ratio of formaldehyde to aniline is 0.4-0.55:1, the molar ratio of hydrogen chloride to aniline is 1.0-1.5:1; the weight ratio of HG catalyst to aniline is 0.01-0.1:1; The obtained reaction product is neutralized and allowed to stand for separation, and the organic phase obtained by separation is distilled under reduced pressure to obtain a crude Ph-Amine product; 2) Preparation of crude MDI: The crude Ph-Amine obtained in step 1) is used for preparation, and the obtained reaction solution contains crude MDI; 3) Solvent recovery and purification: The reaction solution obtained in step 2) is subjected to atmospheric pressure distillation to recover the solvent, and then subjected to short-range molecular distillation to obtain MDI; mixed MDI is also obtained accordingly; The mixed MDI is used to prepare modified MDI.

5. The method for continuously preparing diphenylmethane diisocyanate and its modified product according to claim 4, characterized in that Step 2) includes the following steps: 2.1) Raw material preparation: Preparation of BTC solution: BTC and solvent are mixed evenly to obtain BTC solution; the weight ratio of BTC to solvent is 1:3-5; the BTC is bis(trichloromethyl) carbonate; Preparation of mixed amine solution: Step 1) The obtained crude Ph-Amine product is mixed with a solvent and filtered, and the obtained filter residue is the HG catalyst; a tin catalyst is added to the filtrate and mixed evenly to obtain a mixed amine solution, wherein the weight ratio of the crude Ph-Amine product to the solvent is 1:3-5; the weight ratio of the tin catalyst to the crude Ph-Amine product is 0.01-0.1:1; 2.2) Tertiary isocyanate reaction: According to the weight ratio of BTC to Ph-Amine crude product of 2.3~2.4:1, the dosage ratio of BTC solution and mixed amine solution is set; the following three-stage reactions are carried out in sequence: Primary reaction: Mix the BTC solution with 1 / 3 of the mixed amine solution, and react at 30~40℃ and 250±50 rpm for 2±0.5 hours to obtain the primary reaction solution; Secondary reaction: After mixing the primary reaction liquid with 1 / 3 of the mixed amine solution, react under inert gas protection at 60~70°C, 0.2±0.05 Mpa pressure, and 250±50 rpm speed for 2±0.5 hours to obtain a secondary reaction liquid; Tertiary reaction: After mixing the secondary reaction liquid with 1 / 3 of the mixed amine solution, react at 90±5°C, -0.06±0.01 MPa vacuum conditions, and 250±50 rpm for 2±0.5 hours to obtain a tertiary reaction liquid; the tertiary reaction liquid contains crude MDI.

6. The method for continuously preparing diphenylmethane diisocyanate and its modified product according to claim 5, characterized in that Also includes the following step 4): 4) Preparation of modified MDI: Mix MDI with polyether polyol, tin catalyst and amine catalyst under inert gas protection at 80-100°C for modification reaction until the NCO content drops to 12%-18% to obtain modified MDI; The polyether polyol is 48-52% of the weight of the mixed MDI; The amount of tin catalyst added is 0.05~0.15% of the weight of mixed MDI; the amount of amine catalyst added is 0.02~0.06% of the weight of mixed MDI.

7. The method for continuously preparing diphenylmethane diisocyanate and its modified product according to claim 5 or 6, characterized in that: The tin catalyst in step 2.1) and the tin catalyst in step 4) are at least any one of the following: stannous octoate, stannous neodecanoate, dibutyltin dilaurate, methyltin mercaptan, butyltin mercaptan, octyltin mercaptan; The amine catalyst in step 4) is A33 catalyst; The polyether polyol in step 4) is PPG-2000.

8. The method for continuously preparing diphenylmethane diisocyanate and its modified product according to claim 7, characterized in that The step 1) comprises the following steps: 1.1) Condensation reaction: Under the protection of inert gas, add aniline and HG catalyst into the reactor, add formaldehyde aqueous solution with a mass concentration of 35-37% at 10±2°C, stir evenly and heat to 40-60°C for reaction for 7-9 hours; then add hydrochloric acid with a mass concentration of 35-37%, react at 60-70°C for 4±0.5 hours and 90-100°C for 7±0.5 hours; The molar ratio of formaldehyde to aniline is 0.4-0.55:1, the molar ratio of hydrogen chloride to aniline is 1.0-1.5:1; the weight ratio of HG catalyst to aniline is 0.01-0.1:1; 1.2) Neutralization treatment: After the product obtained in step 1.1) is cooled to room temperature, it is transferred to a neutralization and stratification kettle, and a 36-40% NaOH solution is added dropwise and stirred for neutralization and stratification. After standing, the upper organic phase is separated; The molar ratio of NaOH to hydrogen chloride is 0.9~1.1:1; 1.3) Vacuum distillation: The organic phase obtained in step 1.2) is distilled under reduced pressure to collect the aniline solution; the contents of the kettle are phenylpolyamines.

9. The method for continuously preparing diphenylmethane diisocyanate and its modified product according to claim 8, characterized in that: The solvent in step 2.1) is at least one of dichloromethane, dichloroethane, chlorobenzene, toluene, dimethyl isooctanoate, and dimethyl butanol; In the step 2.2), the gases generated in the primary reaction, the secondary reaction and the tertiary reaction are condensed, and the condensed liquid is returned to the corresponding reaction system for the corresponding reaction, and the condensed gas enters the HCl absorption tower for absorption.

10. The method for continuously preparing diphenylmethane diisocyanate and its modified product according to claim 9, characterized in that: In the step 3): Step 2) The obtained tertiary reaction liquid is subjected to two-stage atmospheric distillation: first, atmospheric distillation is performed to collect the solvent; then, the temperature is raised to 120-150° C. for atmospheric distillation; Then, short-range molecular distillation at 180±5℃ and ≤3 kPa vacuum is used for purification to obtain MDI; the substance in the kettle is mixed MDI.

11. Use of MDI or modified MDI prepared according to any one of claims 3 to 10 in polyurethane foam, elastomer or coating.

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