Process and apparatus for producing mda, process and apparatus for producing mdi

Abstract

This invention relates to a method and apparatus for preparing MDA and MDI. The MDA preparation method includes: first, mixing formaldehyde with a first aniline to react and obtain a first reaction product; separating the first reaction product to obtain an aqueous phase product and an oil phase product; mixing the oil phase product with a second aniline to react and obtain a second reaction product with a hydroxymethyl acetal concentration less than or equal to 0.01%; mixing the second reaction product with a mixture to react and obtain a third reaction product, wherein the mixture includes hydrochloric acid and a circulating liquid, a portion of the third reaction product is mixed with the hydrochloric acid as the circulating liquid to form the mixture, and the remaining portion of the third reaction product undergoes post-treatment to obtain the MDA product. The method of this invention can obtain MDA with low color, low N-methyl impurities, and low N-formyl impurities, and further can obtain MDI products with low hydrolytic chlorine and low color.

Description

Technical Field

[0001] This invention relates to the field of isocyanate technology, and in particular to methods and apparatus for preparing MDA and MDI. Background Technology

[0002] The synthesis of diphenylmethane diisocyanate (MDI) typically involves a two-step reaction: condensation and phosgenation. The condensation reaction uses aniline and formaldehyde as raw materials and hydrochloric acid as a catalyst, proceeding sequentially through reaction, neutralization, water washing, and stripping to obtain the intermediate product diphenylmethane diamine (MDA). MDA then undergoes cold and hot phosgenation reactions with phosgene to remove HCl, excess phosgene, and solvent, yielding MDI. This process inevitably results in MDI containing hydrolyzed chlorine and colored substances, affecting both the product's performance and appearance.

[0003] Currently, the main approach is to improve the post-processing technology following the phosgenation reaction to reduce the hydrolytic chlorine and color of MDI. For example, CN107324333A identifies chromogenic impurities in bicyclic MDI as aldehydes, ketones, chlorinated compounds, and nitrogen-containing substances, and uses an integral adsorbent to physically adsorb MDI to reduce product color. However, this method requires regeneration of the adsorption tower after adsorption, increasing the use of desorbents and raising operating costs. Frequent switching of the equipment also affects operating efficiency. In addition, the eluent used for regeneration inevitably enters the bicyclic MDI, affecting product quality. CN114787125A uses HCl to treat phosgenation products in a bubble cap or plate tower to reduce the color and hydrolytic chlorine of MDI, but does not explain the source of the chromogenic substances or the reaction principle of HCl treatment, which introduces certain uncertainties for industrial application. CN116063204A discloses a method for decomposing halogenated impurities in MDI with a catalyst to reduce hydrolytic chlorine and color by adding a precious metal catalyst bed in a nitrogen stripping tower. However, this method uses a precious metal catalyst, which is costly, and does not explain further treatment measures for the decomposition products.

[0004] In addition to further processing the phosgenation reaction products to improve product quality, the impurity content in MDA products can also be reduced by controlling the generation of secondary amines (N-methyl impurities, N-formyl impurities) and colored impurities during the condensation reaction stage, thereby reducing the color and hydrolytic chlorine content of MDI products.

[0005] The condensation reaction involves multiple intermediate states and is a complex process, typically consisting of a series of mixers or reactors. Existing technologies mainly focus on enhancing mixing and controlling process conditions (temperature, ratio, concentration) to reduce the generation of these impurities. For example, CN101279923A suggests that uneven formaldehyde distribution leads to increased concentrations of N-methyl compounds and colored substances. Therefore, it employs hypergravity to enhance formaldehyde mixing and reduce impurities in MDA products. However, it fails to disclose how N-methyl compounds are generated or what impurities affect the color of MDA, and neglects the control of N-formyl compounds. CN117417258A correlates the Reynolds number, reaction temperature, and formaldehyde concentration of the condensation reaction with the dispersion coefficient ω. By controlling ω within a certain range, it aims to ensure that N-methyl impurities in MDA are ≤0.25%. However, this method only considers control under steady-state conditions and does not consider how to dynamically adjust when fluctuations occur in actual production. CN1151119C discloses a semi-batch process for preparing MDA, where at least 50% formaldehyde is added to the circulation pipeline before mixing... Heating the compound to above 75°C can minimize the content of unwanted byproducts. However, this method uses an intermittent process, which leads to long batch processing times and low equipment utilization, and it cannot achieve dynamic adjustment of process conditions in a continuous process. EP4345088A1 discloses a method of adding formaldehyde in stages in multiple gradient-heated reactors to reduce color and N-methyl impurity concentration, thereby obtaining MDI products with low color and low hydrolytic chlorine. This patent divides the condensation reaction into 4-25 reaction zones. In the first reaction zone, formaldehyde is mixed with aniline and hydrochloric acid. In the second reaction zone, another part of formaldehyde is added, and the temperature is within 20°C higher than that of the first reaction zone. The temperature of the third reaction zone is 15-50°C higher than that of the second reactor, until a qualified reaction solution is obtained. This patent ignores the impact of N-formyl impurities on product quality and does not explain the potential relationship between N-methyl impurities and product color. Summary of the Invention

[0006] Therefore, it is necessary to provide a method and apparatus for preparing MDA and an apparatus for preparing MDI to address the above problems. The method can obtain MDA with low color, low N-methyl impurities, and low N-formyl impurities, thereby obtaining MDI products with low hydrolytic chlorine and low color.

[0007] A method for preparing MDA includes the following steps:

[0008] S1, formaldehyde and first aniline are mixed and reacted to obtain the first reaction product;

[0009] S2, the first reaction product is separated into an aqueous phase product and an oil phase product;

[0010] S3, the oil phase product is mixed with the second aniline and reacted to obtain a second reaction product with a mass concentration of hydroxymethyl acetalamine less than or equal to 0.01%;

[0011] S4, the second reaction product is mixed with the mixture to react and obtain a third reaction product, wherein the mixture includes hydrochloric acid and a circulating liquid, a portion of the third reaction product is mixed with the hydrochloric acid as the circulating liquid to form the mixture, and the remaining portion of the third reaction product is post-processed to obtain the MDA product.

[0012] In one embodiment, the mass concentrations of aniline and hydroxymethyl acetal in the oil phase product are detected online, and the amount of the second aniline used is controlled according to the following formula, where Q is the mass flow rate of the oil phase product, x is the mass concentration of aniline in the oil phase product, y is the mass concentration of hydroxymethyl acetal in the oil phase product, and q is the mass flow rate of the second aniline.

[0013]

[0014] In one embodiment, the pH of the mixture and the mass concentrations of dihydroazobenzene and azobenzene in the third reaction product are detected online, and the proportion of the third reaction product used as the circulating liquid is controlled according to the following formula in conjunction with the reaction temperature, where pH is the pH value of the reaction liquid, z is the sum of the mass concentrations of dihydroazobenzene and azobenzene in the third reaction product, and T is the reaction temperature.

[0015] Cycle ratio = z 1 / 4 ×10 7-pH ×e -1 / T % 。

[0016] In one embodiment, steps S1 to S4 are all performed under a protective atmosphere.

[0017] An apparatus for preparing MDA, comprising:

[0018] The first reaction vessel is equipped with a formaldehyde feed pipe and a first aniline feed pipe for mixing formaldehyde and first aniline and reacting them.

[0019] A phase separator, which is connected to the first reactor, is used to separate the first reaction product obtained from the first reactor into phases.

[0020] The second reaction vessel is connected to the phase separator and is also equipped with a second aniline feed pipe for mixing the oil phase product obtained by the phase separator with the second aniline for reaction.

[0021] The third reaction vessel is connected to the second reaction vessel and is equipped with a mixture feed pipe. The third reaction vessel is used to mix the second reaction product obtained from the second synthesis vessel with the mixture for reaction. The third reaction vessel is also equipped with a circulation pipe connected to the mixture feed pipe, which is used to circulate the third reaction product, which serves as a circulating liquid, to the mixture feed pipe. The mixture feed pipe is also connected to a hydrochloric acid feed pipe, which is used to mix the hydrochloric acid and the circulating liquid into a mixture and transport it to the third reaction vessel.

[0022] In one embodiment, the outlet of the phase separator is provided with a first online detector for detecting the concentrations of aniline and hydroxymethyl acetal in the oil phase product obtained from the phase separator.

[0023] In one embodiment, the outlet of the second reactor is equipped with a second online detector for detecting the concentration of hydroxymethyl acetal in the second reaction product obtained from the second reactor.

[0024] In one embodiment, the mixture feed pipe is further provided with a pH detector for detecting the pH value of the mixture in the mixture feed pipe;

[0025] And / or, the third reaction vessel is further equipped with a temperature detector for detecting the reaction temperature of the second reaction product and the mixture;

[0026] And / or, the outlet of the third reactor is equipped with a third online detector for detecting the concentration of dihydroazobenzene and azobenzene in the third reaction product obtained from the third reactor.

[0027] In one embodiment, the third reactor is further provided with a coolant circulation pipe for controlling the temperature of the third reactor.

[0028] In one embodiment, the first reactor, the phase separator, the second reactor, and the third reactor are all provided with an inlet pipe and an outlet pipe for introducing protective gas and discharging exhaust gas.

[0029] A method for preparing MDI, comprising the method for preparing MDA described above.

[0030] An apparatus for preparing MDI, comprising the apparatus for preparing MDA.

[0031] In the preparation method of MDA, formaldehyde and aniline first react to generate acetal amine, and then undergo an isomerization reaction under the catalysis of hydrochloric acid to generate MDA. In the preparation method of this invention, formaldehyde is first reacted with a first aniline, and then the oil phase product containing acetal amine is reacted with a second aniline. By controlling the mass concentration of hydroxymethyl acetal amine, the mass concentration of N-methyl impurities and N-formyl impurities can be significantly reduced. At the same time, during the isomerization reaction under the catalysis of hydrochloric acid, a portion of the product is used as a circulating liquid to mix with hydrochloric acid, which can avoid the hydrolysis of acetal amine due to excessively high local concentration or strong acidity, thereby reducing the generation of N-methyl impurities and colored azobenzene. As a result, MDA with low color, low N-methyl impurities, and low N-formyl impurities can be obtained, and thus MDI products with low hydrolytic chlorine and low color can be obtained. Attached Figure Description

[0032] To more clearly illustrate the technical solutions in the embodiments of this application or the conventional technology, the drawings used in the description of the embodiments or the conventional technology will be briefly introduced below. Obviously, the drawings described below are only some embodiments of this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0033] Figure 1 This is a schematic diagram of the MDA preparation apparatus of the present invention.

[0034] In the diagram: 10. First reactor; 11. Phase separator; 12. Second reactor; 13. Third reactor; 14. First online detector; 15. Second online detector; 16. Third online detector; 17. pH meter; 18. Temperature meter; 101. Formaldehyde feed pipe; 102. First aniline feed pipe; 111. Aqueous phase product discharge pipe; 121. Second aniline feed pipe; 131. Mixture feed pipe; 132. Hydrochloric acid feed pipe; 133. Circulation pipe; 134. Mixer; 135. Coolant circulation pipe; 20. Inlet pipe; 30. Outlet pipe. Detailed Implementation

[0035] To facilitate understanding of the present invention, it will be described in more detail below. However, it should be understood that the present invention can be implemented in many different forms and is not limited to the embodiments or examples described herein. Rather, these embodiments or examples are provided to make the disclosure of the present invention more thorough and complete.

[0036] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. The terminology used herein in the specification of this invention is for the purpose of describing particular embodiments or examples only and is not intended to limit the invention. The optional scope of the term "and / or" as used herein includes any one of two or more of the related listed items, as well as any and all combinations of the related listed items, including any two related listed items, any more related listed items, or a combination of all related listed items.

[0037] In this invention, numerical ranges are involved. Unless otherwise specified, the numerical ranges are considered continuous and include the minimum and maximum values ​​of the range, as well as every value between the minimum and maximum values. Furthermore, when the range refers to integers, it includes every integer between the minimum and maximum values ​​of the range. Additionally, when multiple ranges are provided to describe features or characteristics, the ranges may be merged. In other words, unless otherwise specified, all ranges disclosed herein should be understood to include any and all subranges to which they are included.

[0038] It is currently known that the color of MDI and the process conditions for preparing MDA by hydrolysis chlorine are closely related to the condensation reaction. In the process of preparing MDA by condensation reaction, formaldehyde and aniline first react to generate acetal amine, and then undergo an isomerization reaction under the catalysis of hydrochloric acid to generate MDA. However, it is not yet clear how the color molecules, N-methyl impurities, and N-formyl impurities in MDA are generated.

[0039] Regarding the MDA preparation process, the applicant, through long-term research, discovered that, firstly, formic acid in formaldehyde has a relatively small impact on the generation of N-formyl MDA impurities. Most N-formyl MDA impurities actually originate from the disproportionation of intermediates, and this disproportionation also generates an equal amount of N-methyl MDA impurities. The specific disproportionation mechanism is as follows: acetal amines react with formaldehyde to generate hydroxymethyl acetal amines. Hydroxymethyl acetal amines undergo an intramolecular disproportionation reaction (similar to the Cannizzaro reaction), where the hydroxyl group is oxidized to a carbonyl group, and the methylene group between the two nitrogen atoms is reduced to a methyl group. Meanwhile, N-methyl MDA impurities and N-formyl MDA impurities react with phosgene in the photochemical stage to generate corresponding formyl chlorides, significantly increasing the hydrolysis chlorine content of MDI.

[0040] The specific reaction equation for the disproportionation process is as follows:

[0041]

[0042] Secondly, excessively high local concentrations or strong acidity of hydrochloric acid can promote the hydrolysis of acetal amines, leading to the simultaneous generation of N-methyl impurities and the colored substance azobenzene. The specific hydrolysis mechanism is as follows: acetal amines undergo hydrolysis under the action of hydrochloric acid, simultaneously yielding N-methylaniline and phenylhydroxylamine. Phenylhydroxylamine then reacts with aniline to generate dihydroazobenzene. Dihydroazobenzene reduces hydroxymethylaniline formed by the condensation of aniline and formaldehyde to yield N-methylaniline and azobenzene. Since the boiling point of azobenzene is close to that of bicyclic MDI, it is difficult to remove during the purification process. Even a concentration at the ppm level can significantly deepen the color of MDI. In addition, N-methylaniline will continue to react to generate N-methylMDA.

[0043] The specific reaction equations for the hydrolysis process are as follows:

[0044]

[0045] Having clarified the generation mechanism of N-methyl impurities, N-formyl impurities, and colored impurities in the preparation of MDA via condensation reaction, this invention provides a method for preparing MDA, comprising the following steps:

[0046] S1, formaldehyde and first aniline are mixed and reacted to obtain the first reaction product;

[0047] S2, the first reaction product is separated into an aqueous phase product and an oil phase product;

[0048] S3, the oil phase product is mixed with the second aniline and reacted to obtain a second reaction product with a mass concentration of hydroxymethyl acetalamine less than or equal to 0.01%;

[0049] S4, the second reaction product is mixed with the mixture to react and obtain a third reaction product, wherein the mixture includes hydrochloric acid and a circulating liquid, a portion of the third reaction product is mixed with the hydrochloric acid as the circulating liquid to form the mixture, and the remaining portion of the third reaction product is post-processed to obtain the MDA product.

[0050] In step S1, formaldehyde and first aniline are first mixed and reacted to generate a first reaction product consisting of a mixture of acetal amine and water. The reaction temperature is preferably 75℃-95℃, and the molar ratio of formaldehyde to first aniline is preferably 0.43:1-0.47:1.

[0051] According to the disproportionation mechanism, to suppress the formation of N-methylMDA and N-formylMDA impurities, the content of hydroxymethyl acetalamine intermediate should be controlled to prevent internal disproportionation reactions. To control the formation of hydroxymethyl acetalamine, in step S2 of this invention, the first reaction product is separated into an aqueous phase and an oil phase. The aqueous phase product undergoes post-processing, while the oil phase product proceeds to step S3, reacting with a second aniline. By controlling the concentration of hydroxymethyl acetalamine using the reaction between hydroxymethyl acetalamine and aniline, the concentration of N-methylMDA and N-formylMDA impurities can be significantly reduced. The reaction equation is as follows:

[0052]

[0053] Furthermore, online infrared detectors and other detection instruments can be used to detect the mass concentrations of aniline and hydroxymethyl acetal in the oil phase product online, and the amount of the second aniline used can be controlled according to the following formula, where Q is the flow rate of the oil phase product, x is the mass concentration of aniline in the oil phase product, generally between 5% and 30%, y is the mass concentration of hydroxymethyl acetal in the oil phase product, generally between 0.01% and 0.2%, q is the flow rate of the second aniline, and the mass flow rate ratio of q / Q is preferably less than or equal to 0.30.

[0054]

[0055] In the step of reacting the oil phase product with the second aniline, the preferred temperature is 75℃-95℃.

[0056] Therefore, by controlling the amount of second aniline online, it is possible to accurately obtain the second reaction product with a mass concentration of hydroxymethyl acetalamine less than or equal to 0.01%, and to avoid the waste of aniline and the increase in energy consumption and process time for distilling aniline in the post-processing.

[0057] According to the hydrolysis mechanism, when the isomerization reaction is carried out under the catalysis of hydrochloric acid, in order to reduce the production of N-methylaniline and the colored substance azobenzene, the concentration of phenylhydroxylamine can be controlled. Therefore, in step S4 of this invention, a portion of the product is used as a circulating liquid to mix with hydrochloric acid, thereby reducing the concentration of hydrochloric acid when mixed with the second reaction product. This can avoid the hydrolysis of acetal amine due to excessively high local hydrochloric acid concentration or strong acidity.

[0058] Furthermore, the pH of the mixture can be detected online using a pH meter, and the concentrations of dihydroazobenzene and azobenzene in the third reaction product can be detected online using an online infrared detector. The concentration of phenylhydroxylamine produced can be inferred from the concentrations of dihydroazobenzene and azobenzene, and the mass or volume ratio of the third reaction product used as the circulating liquid can be controlled according to the following formula in conjunction with the reaction temperature to improve control efficiency. Here, pH is the pH value of the mixture, preferably 2.5-5.5; z is the sum of the mass concentrations of dihydroazobenzene and azobenzene in the third reaction product, generally 0.08%-0.12%; and T is the reaction temperature, preferably 30℃-60℃.

[0059] Cycle ratio = z 1 / 4 ×10 7-pH ×e -1 / T % 。

[0060] By adjusting the cyclic ratio, the sum of the mass concentrations of dihydroazobenzene and azobenzene in the third reaction product is controlled below 0.02%, which ensures the effectiveness of the third reaction.

[0061] Therefore, in the MDA preparation method of the present invention, the concentration of hydroxymethyl acetal is controlled by reacting the oil phase product with the second aniline, and the concentration of dihydroazobenzene and azobenzene is controlled by using a portion of the third reaction product as a circulating liquid, thereby indirectly controlling the concentration of phenylhydroxylamine. Furthermore, by simultaneously and online adjusting the amount of second aniline used and the circulating ratio of the third reaction product, MDA with low color, low N-methyl impurities, and low N-formyl impurities can be obtained. Then, by reacting this MDA with phosgene, an MDI product with low hydrolytic chlorine and low color can be obtained. Moreover, when fluctuations occur in actual production, the method of the present invention can achieve dynamic adjustment to ensure the stability of the process.

[0062] In addition to the hydrolysis of acetal amine to obtain phenylhydroxylamine, aniline may also be oxidized by air to produce phenylhydroxylamine. Since phenylhydroxylamine is more reactive, it will react rapidly with aniline to produce dihydroazobenzene and azobenzene. Therefore, it is preferable that steps S1 to S4 are carried out under a protective atmosphere, such as nitrogen or argon.

[0063] The present invention does not limit the post-processing steps of the third reaction product. Preferably, in the remaining part of the step of obtaining MDA product by post-processing the third reaction product, the post-processing steps include: first, heating the third reaction product to 100℃-130℃ and holding it at that temperature for 1h-2h to carry out a high-temperature isomerization reaction to ensure that the intermediate is completely converted into MDA; then, neutralizing the HCl in the third reaction product with an alkaline compound such as liquid alkali; after neutralization, performing phase separation again; washing the oil phase with water to remove residual NaCl; and then distilling or stripping the washed oil phase at 6kPa-20kPa (absolute pressure) and 200℃-240℃ to remove aniline and water, thereby obtaining a qualified MDA product.

[0064] like Figure 1 The apparatus shown is for preparing MDA according to the present invention, which is applicable to the above-mentioned method for preparing MDA, and includes a first reaction vessel 10, a phase separator 11, a second reaction vessel 12 and a third reaction vessel 13.

[0065] The first reaction vessel 10 is equipped with a formaldehyde feed pipe 101 and a first aniline feed pipe 102. Formaldehyde and the first aniline enter the first reaction vessel 10 through the formaldehyde feed pipe 101 and the first aniline feed pipe 102, respectively. The first reaction vessel 10 is used to mix formaldehyde and the first aniline and react to obtain the first reaction product.

[0066] The phase separator 11 of the present invention is connected to the first reaction vessel 10 through a pipeline. The first reaction product in the first reaction vessel 10 enters the phase separator 11 through the pipeline. The phase separator 11 is used to separate the first reaction product into an aqueous phase product and an oil phase product. The aqueous phase product is discharged through the aqueous phase product discharge pipe 111 and then subjected to post-processing.

[0067] In this invention, the second reaction vessel 12 is connected to the phase separator 11 via a pipeline. The oil phase product obtained by the phase separator 11 enters the second reaction vessel 12 through the pipeline. The second reaction vessel 12 is also provided with a second aniline feed pipe 121. The second aniline enters the second reaction vessel 12 through the second aniline feed pipe 121. The second reaction vessel 12 is used to mix the oil phase product with the second aniline to react and obtain the second reaction product.

[0068] The third reaction vessel 13 of this invention is connected to the second reaction vessel 12 via a pipeline. The second reaction product obtained from the second reaction vessel 12 enters the third reaction vessel 13 through the pipeline. The third reaction vessel 13 is provided with a mixture feed pipe 131, through which the mixture enters the third reaction vessel 13. The third reaction vessel 13 is used to mix the second reaction product with the mixture for reaction. The third reaction vessel 13 is also provided with a circulation pipe 133, which is connected to the mixture feed pipe 131 and is used to circulate the third reaction product (as a circulating liquid) to the mixture feed pipe 131. The mixture feed pipe 131 is also connected to a hydrochloric acid feed pipe 132, used to mix the hydrochloric acid and the circulating liquid into a mixture and transport it to the third reaction vessel 13.

[0069] Optionally, a mixer 134 can be added. The hydrochloric acid feed pipe 132, the circulation pipe 133 and the mixture feed pipe 131 are all connected to the mixer 134. The hydrochloric acid and the circulating liquid enter the mixer 134 through the hydrochloric acid feed pipe 132 and the circulation pipe 133, respectively. After being mixed in the mixer 134, they enter the third reaction vessel 13 through the mixture feed pipe 131.

[0070] Optionally, the outlet of the phase separator 11 is equipped with a first online detector 14. Specifically, the first online detector 14 can be installed on the pipeline connecting the phase separator 11 and the second reaction vessel 12 to detect the mass concentration of aniline and hydroxymethyl acetal in the oil phase product obtained from the phase separator 11, and adjust the feed flow rate of second aniline in the second aniline feed pipe 121 based on the detection results. The first online detector 14 can be an online infrared detector, etc.

[0071] Optionally, the outlet of the second reactor 12 is equipped with a second online detector 15. Specifically, the second online detector 15 can be installed on the pipeline connecting the second reactor 12 and the third reactor 13 to detect the concentration of hydroxymethyl acetal in the second reaction product obtained from the second reactor and monitor the reaction effect of the second reactor 12.

[0072] Optionally, the mixture feed pipe 131 is further equipped with a pH detector 17 for detecting the pH value of the mixture in the mixture feed pipe 131. The third reaction vessel 13 is further equipped with a temperature detector 18 for detecting the reaction temperature of the second reaction product and the mixture in the third reaction vessel 13. The outlet of the third reaction vessel 13 is equipped with a third online detector 16 for detecting the concentration of dihydroazobenzene and azobenzene in the third reaction product obtained from the third reaction vessel 13, so as to adjust the ratio used as the circulating liquid and indirectly control the concentration of phenylhydroxylamine. After the circulation ratio is adjusted, the sum of the mass concentrations of dihydroazobenzene and azobenzene in the third reaction product is controlled below 0.02%, which can ensure the reaction effect of the third reaction vessel 13.

[0073] Optionally, the third reactor 13 is also equipped with a cooling device, which may be a coolant circulation pipe 135, such as a cooling water circulation pipe, to control the temperature of the third reactor 13.

[0074] Optionally, the first reactor 10, the phase separator 11, the second reactor 12, and the third reactor 13 are all equipped with an inlet pipe 20 and an outlet pipe 30. Protective gases such as nitrogen and argon enter the equipment through the inlet pipe 20 to form a protective atmosphere in the equipment, and the exhaust gas is discharged from the equipment through the outlet pipe 30, thereby ensuring that steps S1 to S4 of the present invention are carried out under a protective atmosphere.

[0075] The present invention also provides a method for preparing MDI, including the method for preparing MDA, and then reacting the obtained MDA with phosgene to obtain the MDI product.

[0076] This invention does not limit the steps of the reaction between MDA and phosgene. Optionally, in the reaction step, the MDA and phosgene solution first enter a cold reactor for cold phosgenation. The phosgene-amine molar ratio is preferably controlled at 3:1-8:1, the temperature is preferably 30℃-90℃, and the pressure is preferably 1 bar-20 bar (absolute pressure). The cold reactor can be a static mixer or a dynamic mixer; a jet mixer is preferred for the static mixer, and a high-shear pump is preferred for the dynamic mixer. The material exiting the cold reactor enters a hot reactor for thermal phosgenation. The temperature is preferably controlled at 90℃-150℃, and the pressure is preferably 2 bar-10 bar (absolute pressure). The hot reactor can be a tower reactor, a stirred tank reactor, a stirredless reactor, or a combination thereof. The number of hot reactors is preferably 1-6, arranged in series. The material from the thermal reactor undergoes phosgene removal and solvent removal processes to finally obtain crude MDI. The temperature for phosgene removal is preferably controlled at 150-200℃, and the pressure is preferably 2-10 bar (absolute pressure). Solvent removal is carried out using 1-3 stage distillation / rectification or a combination thereof, with the temperature preferably controlled at 120℃-230℃ and the pressure preferably controlled at 0.01 bar-0.9 bar (absolute pressure).

[0077] The present invention also provides an MDI preparation apparatus, including the aforementioned MDA preparation apparatus, and an apparatus for preparing MDI by MDA phosgenation reaction, such as a cold reactor and a hot reactor.

[0078] Since the MDA preparation method of the present invention can obtain MDA with low color intensity, low N-methyl impurities, and low N-formyl impurities, the MDA can be reacted with phosgene to prepare MDI, resulting in an MDI product with low hydrolytic chlorine and low color intensity.

[0079] The following specific embodiments will further illustrate the preparation method and apparatus for MDA and MDI.

[0080] Comparative Example 1

[0081] Utilize Figure 1 The MDA preparation apparatus shown involves introducing 37% formaldehyde into a 10L first reactor at a flow rate of 100 g / min, simultaneously introducing first aniline at a flow rate of 250 g / min. The temperature of the first reactor is 88°C. The second aniline feed pipe and the circulation pipe are closed. The resulting first reaction product is directly transferred from the second reactor to the third reactor. 30% hydrochloric acid is introduced into the 10L third reactor through a mixture feed pipe at a flow rate of 46 mL / min. The reaction temperature is controlled at 50°C through a cooling liquid circulation pipe. Furthermore, high-purity nitrogen is not introduced into the first reactor, the phase separator, the second reactor, or the third reactor.

[0082] After 3 hours of stable operation, the online mass concentration of hydroxymethyl acetal in the oil phase product obtained after phase separation was 0.145%, the online mass concentration of aniline was 7.6%, the online mass concentration of hydroxymethyl acetal in the second reaction product was 0.145%, the online pH was -1.0, and the online mass concentration of dihydroazobenzene and azobenzene in the third reaction product was 0.10%.

[0083] The product from the third reaction was further heated to 120°C and held at that temperature for 1.5 hours to carry out a high-temperature isomerization reaction. After the isomerization reaction was completed, it was neutralized with 32% liquid alkali to remove HCl. The neutralization temperature was maintained at 95°C. After neutralization, the MDA was washed with water to remove residual NaCl from the MDA phase. After washing with water, it was distilled at 10 kPa and 210°C for 1 hour to remove water and aniline, thus obtaining the MDA product. At this point, the mass concentration of N-methyl MDA in the MDA product was 0.37%, and the mass concentration of N-formyl MDA was 0.12%.

[0084] The MDA obtained above was first subjected to a cold phosgene reaction. MDA and chlorobenzene were first mixed in a static mixer, and then reacted with a phosgene solution in a dynamic mixer. The phosgene-to-amine molar ratio was controlled at 5:1, the temperature at 80℃, and the pressure at 16 bar (absolute pressure). The material obtained from the cold reactor was then fed into a hot reactor for a thermal phosgene reaction, with the temperature controlled at 110℃ and the pressure at 3 bar (absolute pressure). The resulting material contained phosgene and the solvent chlorobenzene in addition to MDI. Phosgene was first removed at 180℃ and 2.5 bar (absolute pressure), followed by three stages of solvent removal. The first stage solvent removal tower had a temperature and pressure of 160℃ and 0.8 bar (absolute pressure), the second stage at 170℃ and 0.09 bar (absolute pressure), and the third stage at 210℃ and 0.01 bar (absolute pressure). Finally, crude MDI was obtained, with a hydrolyzed chlorine content of 596 ppm and a platinum-cobalt color of 151.

[0085] Comparative Example 2

[0086] The only difference between Comparative Example 2 and Comparative Example 1 is that, after 1 hour of stable operation, the online mass concentration of hydroxymethyl acetal in the oil phase product obtained after phase separation (flow rate of 272 g / min) was 0.142%, and the online mass concentration of aniline was 7.6%. Upon opening the second aniline feed pipe, according to the equation... The second aniline was introduced into the second reactor at a flow rate of 52 g / min, and the temperature of the second reactor was 88 °C.

[0087] After 3 hours of stable operation, the online mass concentration of hydroxymethyl acetal in the second reaction product was 0.009%, and the online pH was -1.0. The online mass concentration of dihydroazobenzene in the third reaction product was 0.08%.

[0088] The third reaction product was post-processed using the method described in Comparative Example 1, resulting in an MDA concentration of 0.24% N-methyl MDA and 0.014% N-formyl MDA. MDI was then prepared by reacting MDA with phosgene using the method described in Comparative Example 1, yielding crude MDI with a hydrolyzed chlorine concentration of 309 ppm and a platinum-cobalt color of 135.

[0089] Comparative Example 3

[0090] The only difference between Comparative Example 3 and Comparative Example 1 is that high-purity nitrogen gas was introduced into the first reactor, the phase separator, the second reactor, and the third reactor for deoxygenation, with a flow rate of 100 mL / min.

[0091] After 1 hour of stable operation, the online mass concentration of hydroxymethyl acetal in the oil phase product obtained after phase separation was 0.146%, the online mass concentration of aniline was 7.4%, the online mass concentration of hydroxymethyl acetal in the second reaction product was 0.145%, the online pH was -1.0, and the online mass concentration of dihydroazobenzene and azobenzene in the third reaction product was 0.09%.

[0092] Then open the valve of the circulation pipe, setting the valve opening to 5%, and mix part of the third reaction product as a circulating liquid with hydrochloric acid to form a mixture. After 2 minutes, the pH rises to 2.9. Then, control the third reaction vessel online, according to the equation: circulation ratio = z 1 / 4 ×10 7-pH ×e -1 / T % ， The mass cycle ratio is automatically set to 21.4.

[0093] After 3 hours of stable operation, the mass concentrations of dihydroazobenzene and azobenzene decreased to 0.016%, the pH was 3.2, the mass circulation ratio was automatically adjusted to 7.0, and remained stable.

[0094] The third reaction product after stable operation was post-processed according to the method in Comparative Example 1. The mass concentration of N-methyl MDA in MDA was 0.14%, and the mass concentration of N-formyl MDA was 0.10%. MDI was prepared by reacting MDA with phosgene using the method in Comparative Example 1. The hydrolyzed chlorine content of the crude MDI was 256 ppm, and the platinum-cobalt color was 18.

[0095] Example 1

[0096] The only difference between Example 1 and Comparative Example 1 is that high-purity nitrogen gas was introduced into the first reactor, the phase separator, the second reactor, and the third reactor for deoxygenation, and the flow rate was 100 mL / min.

[0097] After 1 hour of stable operation, the online mass concentration of hydroxymethyl acetal in the oil phase product obtained after phase separation (flow rate of 272 g / min) was 0.146%, and the online mass concentration of aniline was 7.6%. The second aniline feed pipe was then opened, and according to the equation... The second aniline was introduced into the second reactor at a flow rate of 54 g / min, and the temperature of the second reactor was 88 °C.

[0098] After 3 hours of stable operation, the online mass concentration of hydroxymethyl acetal in the second reaction product was 0.008%, and the online pH was -1.0. The online mass concentration of dihydroazobenzene and azobenzene in the third reaction product was 0.08%.

[0099] Then open the valve of the circulation pipe, setting the valve opening to 5%, and mix part of the third reaction product as a circulating liquid with hydrochloric acid to form a mixture. After 2 minutes, the pH rises to 3.1. Then, control the third reaction vessel online, according to the equation: circulation ratio = z 1 / 4 ×10 7-pH ×e -1 / T % ， The mass cycle ratio is automatically set to 12.2.

[0100] After 3 hours of stable operation, the mass concentrations of dihydroazobenzene and azobenzene decreased to 0.011%, the pH was 3.4, the mass circulation ratio was automatically adjusted to 4.0, and remained stable.

[0101] The third reaction product after stable operation was post-processed according to the method in Comparative Example 1. The concentration of N-methyl MDA in MDA was 0.038%, and the concentration of N-formyl MDA was 0.013%. MDI was prepared by reacting MDA with phosgene using the method in Comparative Example 1. The hydrolyzed chlorine of the crude MDI was 62 ppm, and the platinum-cobalt color was 15.

[0102] Example 2

[0103] The difference between Example 2 and Comparative Example 1 is that high-purity nitrogen gas was introduced into the first reactor, the phase separator, the second reactor, and the third reactor to remove oxygen, and the flow rate was 100 mL / min; at the same time, the aniline flow rate in the first reactor was reduced to 244 g / min.

[0104] After 1 hour of stable operation, the online mass concentration of hydroxymethyl acetal in the oil phase product obtained after phase separation (flow rate of 266 g / min) was 0.279%, and the online mass concentration of aniline was 5.6%. The second aniline feed pipe was then opened, and according to the equation... The second aniline was introduced into the second reactor at a flow rate of 79 g / min, and the temperature of the second reactor was 88 °C.

[0105] After 3 hours of stable operation, the online mass concentration of hydroxymethyl acetal in the second reaction product was 0.007%, and the online pH was -1.0. The online mass concentration of dihydroazobenzene and azobenzene in the third reaction product was 0.07%.

[0106] Then open the valve of the circulation pipe, setting the valve opening to 5%, and mix part of the third reaction product as a circulating liquid with hydrochloric acid to form a mixture. After 2 minutes, the pH rises to 3.0. Then, control the third reaction vessel online, according to the equation: circulation ratio = z 1 / 4 ×10 7-pH ×e -1 / T % ， The mass cycle ratio is automatically set to 14.7.

[0107] After 3 hours of stable operation, the mass concentrations of dihydroazobenzene and azobenzene decreased to 0.010%, the pH was 3.4, the mass circulation ratio was automatically adjusted to 3.9, and remained stable.

[0108] The third reaction product after stable operation was post-processed according to the method in Comparative Example 1. The concentration of N-methyl MDA in MDA was 0.039%, and the concentration of N-formyl MDA was 0.011%. MDI was prepared by reacting MDA with phosgene using the method in Comparative Example 1. The hydrolyzed chlorine of the crude MDI was 65 ppm, and the platinum-cobalt color was 14.

[0109] Example 3

[0110] The difference between Example 3 and Comparative Example 1 is that high-purity nitrogen gas was introduced into the first reactor, phase separator, second reactor, and third reactor for deoxygenation, with a flow rate of 100 mL / min. Simultaneously, the aniline flow rate in the first reactor was increased to 264 g / min.

[0111] After 1 hour of stable operation, the online mass concentration of hydroxymethyl acetal in the oil phase product obtained after phase separation (flow rate of 286 g / min) was 0.052%, and the online mass concentration of aniline was 12%. The second aniline feed pipe was then opened, and according to the equation... The second aniline was introduced into the second reactor at a flow rate of 33 g / min, and the temperature of the second reactor was 88 °C.

[0112] After 3 hours of stable operation, the online mass concentration of hydroxymethyl acetal in the second reaction product was 0.010%, and the online pH was -1.0. The online mass concentration of dihydroazobenzene and azobenzene in the third reaction product was 0.100%.

[0113] Then open the valve of the circulation pipe, setting the valve opening to 5%, and mix part of the third reaction product as a circulating liquid with hydrochloric acid to form a mixture. After 2 minutes, the pH rises to 3.1. Then, control the third reaction vessel online, according to the equation: circulation ratio = z1 / 4 ×10 7-pH ×e -1 / T % ， The mass cycle ratio is automatically set to 13.1.

[0114] After 3 hours of stable operation, the mass concentrations of dihydroazobenzene and azobenzene decreased to 0.012%, the pH was 3.5, the mass circulation ratio was automatically adjusted to 3.2, and remained stable.

[0115] The third reaction product after stable operation was post-processed according to the method in Comparative Example 1. The concentration of N-methyl MDA in MDA was 0.042% and the concentration of N-formyl MDA was 0.012%. MDI was prepared by reacting MDA with phosgene using the method in Comparative Example 1. The hydrolyzed chlorine content of the crude MDI was 67 ppm and the platinum-cobalt color was 17.

[0116] The technical features of the above embodiments can be combined in any way. For the sake of brevity, not all possible combinations of the technical features in the above embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0117] The embodiments described above are merely illustrative of several implementations of the present invention, and while the descriptions are relatively specific and detailed, they should not be construed as limiting the scope of the invention patent. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention, and these all fall within the protection scope of the present invention. Therefore, the protection scope of this invention patent should be determined by the appended claims.

Claims

1. A method for preparing diphenylmethane diamine, characterized in that, Includes the following steps: S1, formaldehyde and first aniline are mixed and reacted to obtain the first reaction product; S2, the first reaction product is separated into an aqueous phase product and an oil phase product; S3, the oil phase product is mixed with a second aniline and reacted to obtain a second reaction product with a hydroxymethyl acetal concentration of less than or equal to 0.01%. The mass concentrations of aniline and hydroxymethyl acetal in the oil phase product are monitored online, and the amount of the second aniline used is controlled according to the following formula, where Q is the mass flow rate of the oil phase product, x is the mass concentration of aniline in the oil phase product, y is the mass concentration of hydroxymethyl acetal in the oil phase product, and q is the mass flow rate of the second aniline. ； S4, the second reaction product is mixed with the mixture to obtain a third reaction product. The mixture includes hydrochloric acid and a circulating liquid. A portion of the third reaction product is used as the circulating liquid and mixed with the hydrochloric acid to form the mixture. The remaining portion of the third reaction product undergoes post-processing to obtain a diphenylmethane diamine product. The pH of the mixture and the mass concentrations of dihydroazobenzene and azobenzene in the third reaction product are monitored online. The proportion of the third reaction product used as the circulating liquid is controlled according to the following formula, based on the reaction temperature: pH is the pH value of the mixture, z is the sum of the mass concentrations of dihydroazobenzene and azobenzene in the third reaction product, and T is the reaction temperature. 。 2. The method for preparing diphenylmethane diamine according to claim 1, characterized in that, Steps S1 to S4 are all performed under a protective atmosphere.

3. An apparatus for preparing diphenylmethane diamine, characterized in that, include: The first reaction vessel is equipped with a formaldehyde feed pipe and a first aniline feed pipe for mixing formaldehyde and first aniline and reacting them. A phase separator is connected to the first reaction vessel and is used to separate the first reaction product obtained from the first reaction vessel into phases. The outlet of the phase separator is equipped with a first online detector for detecting the concentration of aniline and hydroxymethyl acetal in the oil phase product obtained from the phase separator. The second reaction vessel is connected to the phase separator. The second reaction vessel is also equipped with a second aniline feed pipe for mixing the oil phase product obtained by the phase separator with the second aniline for reaction. The outlet of the second reaction vessel is equipped with a second online detector for detecting the concentration of hydroxymethyl acetal in the second reaction product obtained from the second reaction vessel. A third reaction vessel, connected to the second reaction vessel, is provided with a mixture feed pipe. The third reaction vessel is used to mix the second reaction product obtained from the second reaction vessel with the mixture for reaction. The third reaction vessel is also provided with a circulation pipe connected to the mixture feed pipe, used to circulate the third reaction product (as a circulating liquid) to the mixture feed pipe. The mixture feed pipe is also connected to a hydrochloric acid feed pipe, used to mix the hydrochloric acid and the circulating liquid into a mixture and transport it to the third reaction vessel. The mixture feed pipe is also equipped with a pH meter for detecting the pH value of the mixture in the mixture feed pipe. The third reaction vessel is also equipped with a temperature meter for detecting the reaction temperature of the second reaction product and the mixture. The outlet of the third reaction vessel is equipped with a third online detector for detecting the concentrations of dihydroazobenzene and azobenzene in the third reaction product obtained from the third reaction vessel.

4. The apparatus for preparing diphenylmethane diamine according to claim 3, characterized in that, The third reactor is also equipped with a coolant circulation pipe for controlling the temperature of the third reactor.

5. The apparatus for preparing diphenylmethane diamine according to claim 3, characterized in that, The first reactor, the phase separator, the second reactor, and the third reactor are all equipped with inlet pipes and outlet pipes for introducing protective gas and discharging exhaust gas.

6. A method for preparing diphenylmethane diisocyanate, characterized in that, The method for preparing diphenylmethane diamine according to any one of claims 1-2.

7. An apparatus for preparing diphenylmethane diisocyanate, characterized in that, The apparatus for preparing diphenylmethane diamine according to any one of claims 3-5.

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