Process and apparatus for the synthesis of mdi

Abstract

The application provides a synthesis method and device of MDI, and belongs to the technical field of isocyanate. The synthesis method comprises the following steps: introducing MDA solution and phosgene into a multi-stage jet flow-cyclone coupling reinforced mixing reactor to perform pre-reaction, so as to form a gas-liquid mixture; directly guiding the gas-liquid mixture to the topmost tray surface of a packed tower reactor, and flowing downwards along the tray step by step; further reacting under the catalysis of modified molecular sieve arranged on the tray, and collecting a target product. The application strengthens the mixing effect of liquid material through the multi-stage jet flow-cyclone coupling reinforced mixing reactor, synchronously removes HCl through MgAl2O4 modified ZSM-5 molecular sieve, and realizes the reflux of phosgene and the removal of HCl by combining with tower top condensation. The MDI ortho-dichlorobenzene mixed solution is continuously discharged from the tower kettle, so that the industrial continuous production demand is met.

Description

Technical Field

[0001] This invention belongs to the field of isocyanate technology, specifically relating to a method and apparatus for synthesizing MDI. Background Technology

[0002] Diphenylmethane diisocyanate (MDI) is a core raw material for the production of polyurethane. Polyurethane, as a high-performance polymer material, is widely used in many fields such as construction, automobiles, home appliances, and furniture. The production process of MDI is complex and technologically demanding. Its main raw materials include aniline, formaldehyde, chlorine, and carbon monoxide. Specifically, aniline and formaldehyde first react under the action of an acidic catalyst to produce diphenylmethane diamine (MDA). Then, MDA undergoes a phosgenation reaction with phosgene to produce crude MDI. Finally, through a series of processes such as distillation and purification, different types of MDI products are obtained. This production process not only requires precise control of reaction conditions but also places extremely high demands on the corrosion resistance and safety of the equipment.

[0003] Specifically, traditional MDI production processes often face two core problems:

[0004] Firstly, uneven mixing of materials leads to incomplete reaction. In traditional tower or batch reactors, the contact area between the MDA solution and phosgene is limited, which can easily lead to incomplete local reactions, thereby reducing the raw material conversion rate and affecting the product yield.

[0005] Secondly, the treatment of the byproduct HCl is complex. Traditional processes require an additional independent absorption device (such as an alkaline spray tower) after the reaction to treat the byproduct hydrogen chloride gas. This not only increases the complexity and energy consumption of the process, but may also affect the purity of the MDI product due to HCl residue. At the same time, the recovery rate of unreacted phosgene is low, resulting in raw material waste and environmental pressure. Summary of the Invention

[0006] In view of this, the present invention aims to solve the problems of uneven material mixing, low HCl removal efficiency, and poor phosgene utilization in traditional MDI production. It provides a highly efficient synthesis method with a compact process, high raw material conversion rate, high raw material utilization rate, and excellent product purity. This method enhances the mixing effect of liquid materials through a multi-stage jet-swirl coupled enhanced mixing reactor, combined with simultaneous HCl removal using magnesium aluminum spinel (MgAl2O4) modified ZSM-5 molecular sieve, and incorporates top condensation to achieve phosgene reflux and HCl removal. The o-dichlorobenzene mixed solution of MDI is continuously discharged from the bottom of the column, meeting the requirements of continuous industrial production.

[0007] To achieve this objective, the present invention adopts the following technical solution:

[0008] In a first aspect, the present invention provides a method for synthesizing MDI, comprising the following steps:

[0009] S1: MDA solution and phosgene are introduced into a multi-stage jet-swirl coupled enhanced mixing reactor to pre-react the two under jet and swirl conditions to form a gas-liquid mixture.

[0010] S2: The gas-liquid mixture obtained in step S1 is guided to the surface of the top tray of the packed tower reactor, so that it flows down the tray step by step; the tray is provided with magnesium aluminum spinel modified ZSM-5 molecular sieve; under the action of the magnesium aluminum spinel modified ZSM-5 molecular sieve, MDA reacts with phosgene to generate reaction products containing MDI and HCl.

[0011] The temperature of the top condenser of the packed tower reactor is below 8°C;

[0012] The bottom temperature of the packed tower reactor shall not be lower than 170°C.

[0013] Preferably, the interior of the multi-stage jet-swirl coupled enhanced mixing reactor is divided into a primary mixing zone, a secondary mixing zone, and a tertiary mixing zone arranged sequentially along the material flow direction.

[0014] Preferably, the primary mixing zone is used to mix the MDA solution and phosgene by impact, and is provided with two independent feed channels, which are designed with upper and lower double pipes and lateral oblique cuts, respectively, for conveying the MDA solution and phosgene; and the primary mixing zone is provided with a honeycomb guide plate as an impact zone.

[0015] Preferably, the secondary mixing zone is used to perform swirling shearing on the material that has passed through the primary mixing zone, and is provided with a swirling shearing chamber, in which spiral swirling blades are provided.

[0016] Preferably, the three-stage mixing zone is used to perform laminar flow mixing or micro-scale mixing of the material that has passed through the two-stage mixing zone, and is provided with a porous microchannel dispersion plate.

[0017] Preferably, the honeycomb baffle has a pore size of 6-15 mm and an opening rate of 70-90%.

[0018] Preferably, the number of spiral swirl blades is 4 to 8, the pitch is 130 to 170 mm, and the helix angle is 40 to 50°.

[0019] Preferably, the edge of the spiral swirl blade is provided with serrated protrusions, the tooth height of the serrated protrusions is 4~6 mm, and the tooth pitch is 8~12 mm.

[0020] Preferably, the porous microchannel dispersion plate has a thickness of 18-22 mm, a pore size of 40-60 μm, and an open porosity of 65-75%.

[0021] Preferably, a conical flow collector is provided below the porous microchannel dispersion plate to uniformly direct the material passing through the three-stage mixing zone to the outlet.

[0022] Preferably, the cone angle of the conical shroud is 25~35°.

[0023] Preferably, a coil-type buffer is provided inside the conical manifold.

[0024] Preferably, the inner diameter of the coil in the coil-type buffer is 18~22 mm, the length is 4.5~6 m, and the number of turns of the coil is 9~13.

[0025] Preferably, the flow rate of the material in the coil-type buffer is 0.5~0.8 m / s.

[0026] Preferably, the magnesium aluminum spinel-modified ZSM-5 molecular sieve is prepared according to the following method:

[0027] (1) Provide a mixed salt solution of magnesium and aluminum salts according to the target Mg / Al molar ratio;

[0028] (2) The mixed salt solution is mixed with ZSM-5 molecular sieve powder and impregnated with an equal volume;

[0029] (3) Seal the impregnated sample, age it at room temperature, dry it, and then calcine it to obtain the final product.

[0030] Preferably, the roasting includes a first stage and a second stage.

[0031] Preferably, the first stage is: heating to 500-600°C at a rate of 2-5 °C / min and holding at that temperature for 2-4 hours.

[0032] Preferably, the second stage is: heating to 700-900°C at a rate of 2-5 °C / min and holding at that temperature for 4-6 hours.

[0033] Preferably, in step S1, the molar ratio of MDA to phosgene in the MDA solution is 1:2 to 1:2.8.

[0034] Preferably, the temperature of the pre-reaction in step S1 is 40~70°C and the residence time is 3~6 s.

[0035] Preferably, the reaction temperature in step S2 is: decreasing stepwise from the bottom to the top of the column, with the bottom temperature not lower than 170°C, the top temperature not lower than 110°C, and the residence time being 20~40 min.

[0036] In a second aspect, the present invention provides an MDI synthesis apparatus for carrying out the above-described synthesis method, comprising:

[0037] The primary reaction unit is a multi-stage jet-swirl coupled enhanced mixing reactor used to pre-react the MDA solution with phosgene.

[0038] The secondary reaction unit is a packed tower reactor, located below the primary reaction unit;

[0039] The discharge port of the primary reaction unit is in direct contact with the top surface of the top tray of the secondary reaction unit, so that the pre-reacted material enters the secondary reaction unit.

[0040] The secondary reaction unit includes at least one tray and a magnesium aluminum spinel-modified ZSM-5 molecular sieve disposed on the tray.

[0041] Preferably, the tray is a sieve tray, a bubble cap tray, or a floating valve tray.

[0042] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0043] (1) Liquid materials have high mixing efficiency and more complete reaction.

[0044] This invention uses liquid phosgene and MDA solution as raw materials and designs a multi-stage jet-swirl coupled enhanced mixing reactor specifically for the MDI synthesis system. Through a three-stage mixing mechanism of "jet impact + swirl shear + microchannel dispersion", it breaks through the limitations of the traditional single mixing mode and achieves a mixing uniformity of more than 98% for phosgene and MDA solution.

[0045] Among them, jet impact (primary mixing): using double nozzles staggered counter-jet to make two materials collide directly under high pressure, breaking droplets and initially mixing them, solving the problem of uneven mixing in traditional single jet;

[0046] Swirling shear (two-stage mixing): The centrifugal and shear forces generated by the high-speed rotating flow field break up droplet aggregation, expand the contact area, and improve the mixing efficiency by 30% compared to a single jet.

[0047] Microchannel dispersion (three-stage mixing): Turbulent pulsation as nano-sized droplets pass through microchannels achieves molecular-level mixing, ultimately resulting in a mixing uniformity exceeding 98%.

[0048] (2) The reaction is completed in a plate-type packed tower.

[0049] Abandoning the traditional staged temperature control of continuous batch reactors, this invention conducts the thermal reaction in a packed tower, precisely controlling the temperature gradient in the packed tower, that is, decreasing stepwise from the bottom to the top of the tower, with the bottom temperature not lower than 170°C and the top temperature not lower than 110°C, thus realizing the synthesis of MDI in a single reactor.

[0050] (3) HCl is removed by condensation at the top of the tower, resulting in high phosgene utilization.

[0051] Precise temperature control at the top of the column ensures timely discharge of HCl, while phosgene returns to the column to continue participating in the reaction, reducing phosgene consumption by 12-15% compared to traditional processes; at the same time, partial solvent reflux is achieved, reducing solvent loss.

[0052] (4) The modified molecular sieve further absorbs HCl, resulting in a product with superior purity.

[0053] Magnesium aluminum spinel modified ZSM-5 molecular sieve has a large adsorption capacity and strong stability. It can remove HCl in real time and does not adsorb phosgene and MDI, avoiding side reactions between HCl and MDI. The final MDI product has a purity of ≥99.6% and a chlorinated impurity content of ≤0.07%, which meets the requirements of high-end polyurethane products. Attached Figure Description

[0054] Figure 1 This is a schematic diagram of the multi-stage jet-swirl coupled enhanced mixing reactor in this invention;

[0055] Among them, 1 is the MDA / ODCB solution inlet, 2 is the liquid phosgene inlet, 3 is the primary mixing zone, 4 is the secondary mixing zone, 5 is the tertiary mixing zone, and 6 is the discharge buffer unit;

[0056] Figure 2 This is an enlarged schematic diagram of the structure of the primary mixing zone in a multi-stage jet-swirl coupled enhanced mixing reactor.

[0057] Among them, 7 is the honeycomb baffle in the primary mixing zone;

[0058] Figure 3 This is an enlarged schematic diagram of the secondary mixing zone in a multi-stage jet-swirl coupled enhanced mixing reactor;

[0059] Among them, 8 is the swirl fixing plate in the secondary mixing zone; 9 is the spiral swirl blade;

[0060] Figure 4 This is a schematic diagram of a porous microchannel dispersion plate in the three-stage mixing zone of a multi-stage jet-swirl coupled enhanced mixing reactor.

[0061] Among them, 10 is a porous microchannel dispersion plate;

[0062] Figure 5 This is a schematic diagram of a plate tower packed reactor.

[0063] Figure 6 The chromatogram is of the MDI prepared in Example 1. Detailed Implementation

[0064] The technical solution of the present invention will be clearly and completely described below with reference to the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0065] This invention aims to solve the problems of uneven material mixing, low HCl removal efficiency, and poor phosgene utilization in traditional MDI production. It provides a highly efficient synthesis process with a compact flow, high raw material conversion rate, high raw material utilization rate, and excellent product purity. The process enhances the mixing effect of liquid materials through a multi-stage jet-swirl coupled enhanced mixing reactor, combined with simultaneous HCl removal using magnesium aluminum spinel (MgAl2O4) modified ZSM-5 molecular sieve, and top condensation to achieve phosgene reflux and HCl removal. The MDI ODCB mixed solution is continuously discharged from the bottom of the column, meeting the requirements of continuous industrial production.

[0066] Specifically, the present invention includes the following technical solutions.

[0067] In a first aspect, the present invention provides a method for synthesizing MDI, comprising the following steps:

[0068] S1: MDA solution and phosgene are introduced into a multi-stage jet-swirl coupled enhanced mixing reactor to pre-react the two under jet and swirl conditions to form a gas-liquid mixture.

[0069] S2: The gas-liquid mixture obtained in step S1 is guided to the surface of the top tray of the packed tower reactor, so that it flows down the tray step by step; the tray is provided with magnesium aluminum spinel modified ZSM-5 molecular sieve; under the action of the magnesium aluminum spinel modified ZSM-5 molecular sieve, MDA reacts with phosgene to generate reaction products containing MDI and HCl.

[0070] The temperature of the top condenser of the packed tower reactor is below 8°C;

[0071] The bottom temperature of the packed tower reactor shall not be lower than 170°C.

[0072] According to this invention, an MDA solution and phosgene, preferably an ODCB solution of MDA and liquid phosgene, are first introduced into a multi-stage jet-swirl coupled enhanced mixing reactor, allowing them to pre-react under jet and swirl conditions to form a gas-liquid mixture. The molar ratio of MDA to phosgene in the MDA solution is 1:2 to 1:2.8, such as 1:2, 1:2.1, 1:2.2, 1:2.3, 1:2.4, 1:2.5, 1:2.6, 1:2.7, or 1:2.8. The pre-reaction temperature is 40 to 70°C, such as 40°C, 45°C, 50°C, 55°C, 60°C, 65°C, or 70°C; the residence time is 3 to 6 seconds, such as 3 seconds, 4 seconds, 5 seconds, or 6 seconds.

[0073] In some embodiments of the present invention, the interior of the multi-stage jet-swirl coupled enhanced mixing reactor is divided into a primary mixing zone, a secondary mixing zone, and a tertiary mixing zone arranged sequentially along the material flow direction.

[0074] The primary mixing zone is used to mix the MDA solution with phosgene by impact. It has two independent feed channels, each employing a double-pipe design with a side-sloping cut (e.g., ...). Figure 1 As shown, this is used to separately transport MDA solution and phosgene. The upper oblique feed channel carries liquid phosgene, while the lower oblique feed channel carries MDA solution (preferably MDA-ODCB solution). Both feed channels are laterally obliquely angled for opposing jets. This differentiated upper-lower arrangement with lateral oblique angles utilizes the density difference between the media. Liquid phosgene has a higher density than MDA-ODCB solution. With upper feed and downward oblique jet, the high-density phosgene impacts from top to bottom, while the low-density MDA solution obliquely counteracts from bottom to top. This gravity difference facilitates active mixing of the two phases, avoiding the material stratification and segregation problems that occur with single-direction feed.

[0075] Furthermore, a honeycomb-shaped guide plate is provided within the primary mixing zone as an impact zone. The pore size of the honeycomb-shaped guide plate is 6-15 mm, such as 6 mm, 7 mm, 8 mm, 9 mm, 10 mm, 11 mm, 12 mm, 13 mm, 14 mm, or 15 mm; the opening ratio is 70-90%, such as 70%, 72%, 75%, 78%, 80%, 82%, 85%, 88%, or 90%.

[0076] The secondary mixing zone is used to perform swirling shearing on the material passing through the primary mixing zone. It is equipped with a swirling shearing chamber containing helical swirling blades. The number of helical swirling blades is 4 to 8, such as 4, 5, 6, 7, or 8 blades, with a pitch of 130 to 170 mm, such as 130 mm, 135 mm, 140 mm, 145 mm, 150 mm, 155 mm, 160 mm, 165 mm, or 170 mm; the helix angle is 40 to 50°, such as 40°, 41°, 42°, 43°, 44°, 45°, 46°, 47°, 48°, 49°, or 50°.

[0077] In some preferred embodiments of the present invention, the edge of the spiral swirl blade is provided with serrated protrusions, the tooth height of the serrated protrusions is 4~6 mm, such as 4 mm, 4.2 mm, 4.5 mm, 4.8 mm, 5 mm, 5.2 mm, 5.5 mm, 5.8 mm or 6 mm, etc.; the tooth pitch is 8~12 mm, such as 8 mm, 8.5 mm, 9.0 mm, 9.5 mm, 10 mm, 10.5 mm, 11 mm, 11.5 mm or 12 mm, etc.

[0078] The third-stage mixing zone is used to perform laminar flow mixing or micro-scale mixing of the material that has passed through the second-stage mixing zone, and is equipped with a porous microchannel dispersion plate.

[0079] In this invention, the thickness of the porous microchannel dispersion plate is 18~22 mm, such as 18 mm, 18.5 mm, 19 mm, 19.5 mm, 20 mm, 20.5 mm, 21 mm, 21.5 mm or 22 mm, etc.; the pore size is 40~60 μm, such as 40 μm, 42 μm, 45 μm, 48 μm, 50 μm, 52 μm, 55 μm, 58 μm or 60 μm, etc.; the porosity is 65~75%, such as 65%, 66%, 67%, 68%, 69%, 70%, 71%, 72%, 73%, 74% or 75%, etc.

[0080] In some embodiments of the present invention, a conical flow collector is further provided below the porous microchannel dispersion plate to uniformly direct the material passing through the three-stage mixing zone to the outlet. The cone angle of the conical flow collector is 25~35°, such as 25°, 26°, 27°, 28°, 29°, 30°, 31°, 32°, 33°, 34° or 35°, etc.

[0081] In some preferred embodiments of the present invention, the conical flow collector is provided with a coil-type buffer inside, which has the following functions: First, it stabilizes and homogenizes the flow, effectively maintaining the molecular-level uniform mixing state formed by multi-stage mixing, eliminating pulse fluctuations and instantaneous concentration deviations caused by high-pressure jet feeding, and preventing gas-liquid two-phase stratification, material agglomeration, and local flow deviation; Second, it protects and stabilizes the pressure, buffering the impact force of high-speed material falling, preventing high-velocity material from directly scouring the top plate of the packed tower, preventing plate wear, loosening and falling off of molecular sieve packing, and ensuring the structural stability of the reaction system inside the tower. Specifically, the inner diameter of the coil in the coil-type buffer is 18~22 mm, such as 18 mm, 18.5 mm, 19 mm, 19.5 mm, 20 mm, 20.5 mm, 21 mm, 21.5 mm or 22 mm, etc.; the length is 4.5~6 m, such as 4.5 m, 4.8 m, 5 m, 5.2 m, 5.5 m, 5.8 m or 6 m, etc.; the number of turns of the coil is 9~13, such as 9 turns, 10 turns, 11 turns, 12 turns or 13 turns.

[0082] In this invention, the flow rate of the material in the coil-type buffer is preferably controlled to be 0.5~0.8 m / s by adjusting the frequency and valve opening. For example, it can be 0.5 m / s, 0.55 m / s, 0.6 m / s, 0.65 m / s, 0.7 m / s, 0.75 m / s, or 0.8 m / s. This can maintain the steady flow of the material, keep the molecular-level mixing state from decaying, and ensure that the material is always in a uniform gas-liquid mixing state before entering the packed tower, thus providing stable feed conditions for efficient reaction in the tower.

[0083] To facilitate understanding, the movement state of the material after its introduction in step S1 is explained below:

[0084] Two feedstocks are fed independently from the upper and lower lateral oblique feed channels of the reactor. Liquid phosgene is injected downwards from the upper oblique channel, while the MDA-ODCB solution is injected upwards from the lower oblique channel. The two feedstocks form a cross-flow jet at a lateral oblique angle and enter the primary mixing zone simultaneously. The feedstock first reaches the honeycomb baffle plate at the center of the primary mixing zone. The high-speed jet feedstock undergoes impact, shearing, and splitting within the honeycomb channels. Large liquid agglomerates are initially torn apart and broken into micron-sized droplets. At the same time, the gravity difference and cross-flow effect are used to achieve initial uniform mixing of the two phases, eliminating initial feed deviation and stratification, and completing the primary impact premixing. The continuous fluid, having completed the initial mixing in the first stage, flows smoothly downwards into the swirling shear chamber of the second-stage mixing zone under the pressure difference of the equipment. Upon entering the chamber, the material undergoes a circular swirling motion with the high-speed rotating helical blades. Under the combined shearing, tearing, and disturbance effects of the blade surfaces, serrated edges, and the inner wall of the chamber, the droplet agglomeration structure is further broken down, continuously reducing the particle size and enhancing the gas-liquid two-phase contact area, thus achieving overall fluid homogenization and completing the second-stage swirling enhanced mixing. The homogeneous fluid after the second-stage swirling shearing continues to be conveyed downwards at a uniform speed, passing through the porous microchannel dispersion plate of the third-stage mixing zone. Under the turbulent micro-sized mixing action of the micropores, the material is further stripped, refined, and dispersed, forming nanoscale uniform droplets, achieving precise molecular-level mixing. Finally, the homogeneous gas-liquid mixture, which has undergone three-stage progressive mixing, is collected and regulated by the lower conical flow collector and then enters the coil-type buffer. After pressure and flow stabilization, it is uniformly delivered to the top plate of the packed tower reactor at the rear end. The material flow is continuous throughout the process, with no dead zones, no concentration stratification, and no instantaneous segregation, providing highly uniform feed material for the efficient, stable, and controllable phosgenation reaction in the tower.

[0085] According to the present invention, the gas-liquid mixture obtained in step S1 is then guided to the surface of the top tray of the packed tower reactor, allowing it to flow downwards along the tray. The tray is equipped with a magnesium aluminate spinel-modified ZSM-5 molecular sieve. Under the action of the magnesium aluminate spinel-modified ZSM-5 molecular sieve, MDA reacts with phosgene to generate reaction products containing MDI and HCl. The byproducts generated by the reaction, HCl gas, a small amount of uncondensed phosgene, and ODCB solvent vapor are light gaseous components that escape upwards and converge at the top of the tower, where they undergo low-temperature condensation in the top condenser. When the temperature is controlled below 8°C, the phosgene and ODCB solvent vapor are fully condensed into liquid and refluxed back into the tower to continue participating in the reaction. Only the low-boiling-point, non-condensable, high-purity HCl gas is continuously collected from the top gas phase outlet and sent to the subsequent recovery and treatment system. The target product MDI generated in the reaction is completely dissolved in ODCB solvent to form a homogeneous liquid system. Under the action of gravity, it continuously converges downwards along the tray, and is continuously enriched and purified as the reaction proceeds. Finally, it is stably collected in the bottom of the tower to form a high-purity MDI-ODCB mixed solution, which can be continuously collected through the bottom outlet. This realizes an integrated continuous production mode of continuous product discharge, precise separation of by-products, and recycling of unreacted raw materials, effectively avoiding the problems of carbamoyl chloride decomposition, product discoloration, and excessive chlorine impurities caused by HCl retention in the tower.

[0086] The reaction temperature in this step is as follows: decreasing stepwise from the bottom of the column to the top of the column, with the bottom temperature not lower than 170°C and the top temperature not lower than 110°C, and the residence time is 20~40 min, such as 20 min, 22 min, 25 min, 30 min, 32 min, 35 min, 38 min or 40 min, etc.

[0087] For example, the temperature at the top of the column can be 110°C, 115°C, or 120°C; the temperature at the bottom of the column can be 170°C, 175°C, or 180°C, etc.

[0088] The reason for setting the above-mentioned temperature gradient in this invention is that the temperature gradient from top to bottom in the tower can accurately match the entire process characteristics of the material, namely "initial reaction of feed at the top of the tower and deep enrichment reaction in the bottom of the tower", and specifically solve the process defects of the isothermal reaction system: maintaining a high temperature of above 110°C at the top of the tower can effectively avoid the tower blockage problem caused by the low-temperature condensation and accumulation of phosgene and ODCB solvent, ensuring normal reflux of gaseous feedstock and smooth escape and separation of by-product HCl, and stabilizing the material circulation system; maintaining a high temperature of above 170°C in the bottom of the tower can provide sufficient activation energy for the deep phosgenation reaction of residual amino intermediates in the tower, thoroughly improving the feedstock conversion rate and product purity; at the same time, gradient temperature control can avoid the drawbacks of high-temperature carbonization of materials in the upper section and incomplete low-temperature reaction of products in the lower section, suppressing the occurrence of side reactions throughout the process and adapting to the needs of continuous and stable production.

[0089] In some embodiments of the present invention, the packed tower is preferably a plate packed tower, and the tower diameter can be designed to be 1.5~2.0 m according to the production capacity, such as 1.5 m, 1.6 m, 1.7 m, 1.8 m, 1.9 m or 2 m, etc.; the tower height is 12~15 m, such as 12 m, 12.5 m, 13 m, 13.5 m, 14 m, 14.5 m or 15 m, etc.; the tower plates are filled with magnesium aluminum spinel (MgAl2O4) modified ZSM-5 molecular sieve.

[0090] In this invention, the structure of the tray is not limited to a single type and can be adapted to conventional plate tower structures such as sieve trays, bubble cap trays, or valve trays. The selection can be flexible based on production capacity, material load, and process conditions. Among these, the guided valve structure is the optimal preferred solution, as the valve plates in this structure open flexibly.

[0091] In some embodiments of the present invention, the trays can adopt a guided floating valve structure, with flexible valve opening that can automatically adjust the opening degree according to the liquid material flow rate to ensure stable contact of the reaction liquid; each tray is equipped with a molecular sieve replacement port to facilitate the periodic replacement and regeneration of the modified molecular sieve, reducing equipment maintenance costs; the maximum temperature of the reboiler reaches 150°C to ensure the smooth progress of the thermal reaction; an anti-clogging discharge port is set at the bottom of the reboiler, which is equipped with a magnetically driven conveying pump to prevent material pipeline blockage and avoid media leakage during the conveying process.

[0092] In addition, the condenser at the top of the column is circulated with cooling water to strictly control the temperature below 8°C. At this temperature, phosgene and ODCB vapors are condensed and refluxed into the column to continue participating in the reaction, achieving efficient internal circulation of phosgene. Meanwhile, HCl gas, due to its extremely low boiling point and insolubility in the condensate, is discharged from the top of the column with high purity and enters the hydrochloric acid recovery or alkaline washing system.

[0093] This invention explains that the magnesium-aluminum spinel-modified ZSM-5 molecular sieve has the following advantages: strong adsorption selectivity, with Mg-O sites on its surface serving as excellent HCl trapping sites to generate MgCl2, and exhibiting better chemical inertness to phosgene than single oxides; through magnesium-aluminum spinel modification, its thermal stability is significantly improved, achieving a service life of over 4000 hours under continuous use at 150℃, and after failure, it can be regenerated by calcination at 400℃ for 4 hours, with a regeneration adsorption capacity recovery rate of over 90%. Therefore, this invention uses magnesium-aluminum spinel-modified ZSM-5 molecular sieve to fill the trays.

[0094] In some embodiments of the present invention, the magnesium aluminum spinel-modified ZSM-5 molecular sieve is prepared according to the following method:

[0095] (1) Provide a mixed salt solution of magnesium and aluminum salts according to the target Mg / Al molar ratio;

[0096] (2) The mixed salt solution is mixed with ZSM-5 molecular sieve powder and impregnated with an equal volume;

[0097] (3) Seal the impregnated sample, age it at room temperature, dry it, and then calcine it to obtain the final product.

[0098] Preferably, step (1) is as follows: accurately weigh the magnesium salt and aluminum salt according to the target Mg / Al molar ratio (usually 1:2, to conform to the stoichiometric ratio of MgAl2O4). Dissolve them in an appropriate amount of deionized water and stir until completely clear. The volume of the solution should be close to or slightly larger than the total pore volume of the ZSM-5 molecular sieve to ensure uniform impregnation without excess liquid phase.

[0099] Preferably, step (2) involves placing the ZSM-5 molecular sieve powder into a mortar or beaker. The prepared mixed salt solution is then slowly added dropwise to the ZSM-5 molecular sieve powder while vigorously stirring or grinding to ensure that the solution is uniformly absorbed by the molecular sieve, forming a moist but non-free slurry.

[0100] According to the present invention, after step (2), the impregnated sample is sealed and aged and dried at room temperature. As a preferred embodiment, aging at room temperature for 12-24 h, preferably 15-20 h, allows the metal ions to diffuse fully inside the molecular sieve. Then the sample is placed in a drying oven and dried at 100-120°C for 10-15 h, preferably at 105-110°C for 12 h, to completely remove moisture.

[0101] Then, according to the present invention, roasting is performed. The roasting preferably includes a first stage and a second stage.

[0102] The first stage involves a programmed temperature increase to 500-600°C at a rate of 2-5 °C / min, followed by a holding time of 2-4 h. The second stage involves a programmed temperature increase to 700-900°C at a rate of 2-5 °C / min, followed by a holding time of 4-6 h.

[0103] The above 2~5 °C / min can be 2 °C / min, 2.5 °C / min, 3 °C / min, 3.5 °C / min, 4 °C / min, 4.5 °C / min, or 5 °C / min, etc.

[0104] The above 500~600℃ can be 500, 510, 520, 530, 540, 550, 560, 570, 580, 590 or 600, etc.

[0105] The above 2~4 h can be 2 h, 2.2 h, 2.5 h, 2.8 h, 3 h, 3.2 h, 3.5 h, 3.8 h or 4 h, etc.

[0106] The aforementioned 700~900℃ can be 700℃, 720℃, 750℃, 780℃, 800℃, 820℃, 850℃, 880℃, or 900℃, etc.

[0107] The above 4~6 h can be 4 h, 4.2 h, 4.5 h, 4.8 h, 5 h, 5.2 h, 5.5 h, 5.8 h or 6 h, etc.

[0108] In some embodiments of the invention, the dried sample is preferably placed in a muffle furnace or tube furnace. Under air atmosphere, the temperature is programmed to rise to 500-600°C at a rate of 2-5 °C / min and held for 2-4 hours to completely decompose the nitrates. Then, the temperature is further increased to a higher temperature (700-900°C) and calcined at this temperature for 4-6 hours. This high-temperature step is crucial for the formation of the magnesium-aluminum spinel crystalline phase. The high temperature induces a solid-state reaction between the magnesium and aluminum oxides to form MgAl₂O₄.

[0109] Secondly, the present invention also provides an MDI synthesis apparatus for carrying out the above-described synthesis method, comprising:

[0110] The primary reaction unit is a multi-stage jet-swirl coupled enhanced mixing reactor used to pre-react the MDA solution with phosgene.

[0111] The secondary reaction unit is a packed tower reactor, located below the primary reaction unit;

[0112] The discharge port of the primary reaction unit is in direct contact with the top surface of the top tray of the secondary reaction unit, so that the pre-reacted material enters the secondary reaction unit.

[0113] The secondary reaction unit includes at least one tray and a magnesium aluminum spinel-modified ZSM-5 molecular sieve disposed on the tray.

[0114] The tray is a sieve tray, a bubble cap tray, or a floating valve tray.

[0115] A schematic diagram of the above MDI synthesis apparatus can be seen in [the diagram]. Figures 1-5 .

[0116] To further illustrate the present invention, the following embodiments provide a detailed description. Unless otherwise specified, all experimental materials used in the following embodiments of the present invention are commercially available products.

[0117] The following embodiments involve a schematic diagram of the structure of a multi-stage jet-swirl coupled enhanced mixing reactor, as shown in the figure. Figure 1As shown, it includes MDA / ODCB solution inlet 1, liquid phosgene inlet 2, primary mixing zone 3, secondary mixing zone 4, tertiary mixing zone 5, and discharge buffer unit 6.

[0118] The enlarged structural diagram of the primary mixing zone 3 is shown below. Figure 2 As shown, a honeycomb-shaped guide plate 7 is provided. The honeycomb-shaped guide plate 7 is made of polytetrafluoroethylene, with a pore size of 6~15 mm and an opening ratio of 70~90%. The honeycomb-shaped guide plate 7 is arranged vertically along the material jet direction and is located in front of the outlet of the MDA solution feed nozzle and the phosgene feed nozzle; the vertical distance between the honeycomb-shaped guide plate 7 and the outlet end face of the two nozzles is 40~50 mm.

[0119] In this invention, the primary mixing zone 3 is a cross-jet impact zone. Its working principle is as follows: the fan-shaped jet of MDA-ODCB solution and the annular jet of liquid phosgene cross-collide on the surface of the honeycomb guide plate 7, forming a "film tearing effect" that breaks the material into micron-sized droplets (particle size ≤ 50 μm), and the initial mixing uniformity reaches more than 85%.

[0120] The enlarged structural diagram of the secondary mixing zone 4 is shown below. Figure 3 As shown, a swirl fixing plate 8 and a spiral swirl blade 9 are provided. The swirl fixing plate 8 fixes the central axis of the swirl blade and regulates the swirl cavity. The spiral swirl blade 9 is made of Hastelloy C-276, with 4 to 8 blades, a pitch of 130 to 170 mm, and a helix angle of 40 to 50°. The blades are fixed on the central axis, which is coaxial with the outer shell.

[0121] In a preferred embodiment, the edge of the spiral swirl blade 9 is provided with "serrated protrusions" (tooth height 4~6 mm, tooth pitch 8~12 mm) to enhance flow field disturbance and prevent droplet aggregation.

[0122] In this invention, the secondary mixing zone 4 is actually a swirling shear chamber. Its working principle is as follows: After the material from the primary mixing zone 3 enters the swirling shear chamber, it forms a high-speed rotating flow field (speed ≥ 1000 r / min) driven by the helical swirling blades 9. Through the "centrifugal shearing effect," the droplets are further broken up, and a radial pressure gradient is generated, ensuring sufficient contact between the materials during rotation, thus increasing the mixing uniformity to over 95%. The swirling fixing plate 8 is used to position and fix the swirling blade shaft, ensuring coaxial operation and preventing component wear. It also regulates the material flow pattern, eliminates flow deviation, withstands operating loads, and separates the chambers to prevent material cross-flow and pressure interference, ensuring stable operation of the secondary mixing zone. Simultaneously, the guiding flow within the secondary mixing zone, under the effects of shearing, tearing, and disturbance, further breaks down the droplet aggregation structure, continuously reduces the material particle size, strengthens the gas-liquid two-phase contact area, and achieves overall fluid homogenization.

[0123] Among them, the three-stage mixing zone 5 is equipped with a porous microchannel dispersion plate 10, such as Figure 4 As shown, its material is a ceramic matrix composite material with a thickness of 18~22 mm, a pore size of 40~60 μm, and an open porosity of 65~75%. The porous microchannel dispersion plate 10 is sealed to the outer shell, and more than 1,000 microchannels are evenly distributed on the plate.

[0124] In this invention, the working principle of the three-stage mixing zone 5 is as follows: when the material mixed by the two-stage swirl passes through the microchannel, it is subjected to the "capillary dispersion effect", and the droplets are further refined to the nanoscale (particle size ≤10 μm). At the same time, "turbulent pulsation" is formed in the microchannel to ensure molecular-level contact of the material, and the final mixing uniformity reaches more than 98%.

[0125] In this invention, a "conical flow collector" (cone angle 25~35°) is provided below the porous microchannel dispersion plate 10 to guide the mixed material to flow evenly to the outlet and avoid local accumulation.

[0126] Meanwhile, a coiled buffer (material: Hastelloy C-276, coil inner diameter of 18~22 mm, length of 4.5~6 m, coil number of 9~13 turns) is installed inside the conical manifold. The coiled buffer is coaxial with the outer shell, and its outlet is connected to the feed distributor of the top plate of the plate packed tower.

[0127] The working principle of the coil-type buffer: The material flow rate in the buffer is controlled at 0.5~0.8 m / s. The mixing state is maintained by the "secondary disturbance" of the coil, while ensuring that the material is evenly distributed to the tray at a stable flow rate, avoiding impact on the packing in the tower.

[0128] Temperature control: The outer side of the coil-type buffer is connected to the outer shell insulation layer to maintain the material temperature at 35~40℃, preventing incomplete liquefaction of phosgene or precipitation of MDA.

[0129] Preparation Example 1

[0130] The preparation method of the modified molecular sieve involved in the following examples is as follows:

[0131] 1) Raw materials and reagents:

[0132] ZSM-5 molecular sieve (usually in hydrogen form, H-ZSM-5)

[0133] Magnesium precursors: magnesium nitrate (Mg(NO3)2·6H2O) or magnesium chloride (MgCl2·6H2O) (nitrates are preferred because they decompose without residue during calcination);

[0134] Aluminum precursors: aluminum nitrate (Al(NO3)3·9H2O) or boehmite (AlO(OH));

[0135] Deionized water.

[0136] 2) Synthesis steps:

[0137] ①Preparation of mixed salt solution:

[0138] Weigh the magnesium and aluminum salts precisely according to the target Mg / Al molar ratio (1:2, to conform to the stoichiometric ratio of MgAl2O4).

[0139] ② Dissolve it in an appropriate amount of deionized water and stir until completely clear. The volume of the solution should be close to or slightly larger than the total pore volume of the ZSM-5 molecular sieve to ensure uniform impregnation without excess liquid phase.

[0140] ③ Equal volume impregnation:

[0141] Place the ZSM-5 molecular sieve powder into a mortar or beaker.

[0142] The prepared mixed salt solution is slowly added drop by drop to the ZSM-5 powder while stirring or grinding vigorously to ensure that the solution is uniformly absorbed by the molecular sieve, forming a moist but non-free slurry.

[0143] ④ Aging and drying:

[0144] The impregnated sample was sealed and aged at room temperature for 16 hours to allow the metal ions to fully diffuse inside the molecular sieve. The sample was then placed in a drying oven and dried at 120°C for 12 hours to completely remove moisture.

[0145] ⑤ High-temperature roasting (key step):

[0146] The dried sample was placed in a muffle furnace or tube furnace. Under air atmosphere, the temperature was programmed to rise to 600°C at a rate of 5 °C / min and held for 4 h to completely decompose the nitrates. Then, the temperature was further increased to 800°C and calcined at this temperature for 6 h. The resulting magnesium aluminate spinel-modified ZSM-5 molecular sieve was obtained.

[0147] Example 1

[0148] This embodiment provides a plate tower packed reactor, the schematic diagram of which is shown below. Figure 5 As shown, it includes a plate-type packed tower body 12, a multi-stage jet-swirl coupled enhanced mixing reactor 11, a tower plate filled with modified molecular sieves 13, a tower top cooling water condenser 14, a phosgene / ODCB reflux liquid 15, and an HCl gas outlet 16.

[0149] Based on the above-mentioned reactor, the preparation method of this invention is as follows:

[0150] 1. Raw material preparation:

[0151] Add ODCB solvent to the preparation tank, heat to 60°C, slowly add solid MDA, stir until completely dissolved, and prepare a 30% (w / w) MDA-ODCB solution; maintain the temperature of the liquid phosgene storage tank at -3°C and the pressure at 0.2 MPa.

[0152] 2. Pretreatment:

[0153] Turn on the reboiler heating device to raise the reboiler temperature to 170℃; start the top cooling water condenser, introduce 6℃ cooling water, and adjust the flow rate to 65 m³ / h. 3 / h, control the temperature at the top of the column to be stable at 32~35℃; introduce nitrogen into the column and adjust the pressure inside the column to 0.14 MPa; turn on the stirring device in the column bottom and set the speed to 40 r / min.

[0154] 3. Feeding and mixing:

[0155] Turn on the MDA-ODCB solution metering pump and set the feed rate to 300 kg / h (of which 90 kg / h is pure MDA); open the liquid phosgene metering valve and adjust the phosgene feed rate to 98 kg / h (the molar ratio of MDA to phosgene is 1:2.2). After the two liquid materials are mixed in the multi-stage jet-swirl coupled enhanced mixing reactor, they are uniformly fed into the top tray of the tower.

[0156] The parameters of the multi-stage jet-swirl coupled enhanced mixing reactor are set as follows: guide plate aperture 10 mm, opening ratio 80%; swirl blades 6, pitch 150 mm, helix angle 45°; dispersion plate aperture 50 μm, opening ratio 70%; coil inner diameter 20 mm, length 5 m, coil number of 11 turns.

[0157] 4. Reaction and Separation:

[0158] The material flows through more than 10 trays in the tower, which are filled with magnesium aluminum spinel modified molecular sieves (the filling rate of magnesium aluminum spinel modified molecular sieves is 45%). The material reacts with the magnesium aluminum spinel modified molecular sieves to generate an MDI-ODCB mixed solution, and HCl is adsorbed. Unreacted phosgene is condensed and refluxed through the condenser at the top of the tower, and HCl enters the hydrochloric acid absorption tower.

[0159] 5. Product discharge and purification:

[0160] When the bottom liquid level reaches 2 / 3, open the discharge valve and the transfer pump to send the MDI-ODCB mixed solution (MDI mass fraction of about 20%) into the ODCB solvent removal unit at a flow rate of 160 kg / h (ODCB recovery rate of 99.5%).

[0161] Continuous testing data over 15 days showed that the average MDA conversion rate was 99.6%, the phosgene utilization rate was 98.7%, the MDI mass fraction in the MDI-ODCB mixed solution remained stable at 19.8%–20.2%, and the purity of the purified MDI product was 99.7%. The chromatogram is shown below. Figure 6 As shown (where RT 2.6 min: dichloromethane; RT 5.4 min: ODCB; RT 13.4 min: 2,4-MDI; RT 14 min: 4,4-MDI).

[0162] Example 2

[0163] The plate tower packed reactor involved in this embodiment is the same as that in Embodiment 1, and will not be described again here.

[0164] The preparation method is as follows:

[0165] 1. Raw material preparation: ODCB solvent is heated to 65℃ to dissolve MDA and prepare a 28wt% MDA-ODCB solution; liquid phosgene temperature is -4℃ and pressure is 0.22 MPa.

[0166] 2. Pretreatment: Referring to Example 1, the bottom temperature of the column was 175℃, and the cooling water at the top of the column was 7℃ with a flow rate of 70 m³ / h. 3 / h, internal pressure of the tower is 0.15 MPa, and stirring speed is 45 r / min.

[0167] 3. Feed mixing: Referring to Example 1, the MDA-ODCB solution is 320 kg / h (MDA purity 89.6 kg / h), and the liquid phosgene is 102 kg / h (molar ratio 1:2.3).

[0168] Reactor parameters: guide plate aperture 8 mm, opening ratio 85%; swirl blades 8, pitch 160 mm, helix angle 42°; dispersion plate aperture 45 μm, opening ratio 72%.

[0169] Steps 4 and 5 refer to Example 1.

[0170] Results after 15 days of continuous operation: MDA conversion rate 99.7%, phosgene utilization rate 98.9%, and purified MDI purity 99.72%.

[0171] Example 3

[0172] The plate tower packed reactor involved in this embodiment is the same as that in Embodiment 1, and will not be described again here.

[0173] The preparation method is as follows:

[0174] 1. Raw material preparation: ODCB solvent is heated to 55℃ to dissolve MDA and prepare a 32wt% MDA-ODCB solution; liquid phosgene temperature is -2℃ and pressure is 0.18 MPa.

[0175] 2. Pretreatment: Referring to Example 1, the bottom temperature of the column was 170℃, and the cooling water at the top of the column was 7℃ with a flow rate of 60 m³ / h. 3 / h, internal pressure of the tower is 0.13 MPa, and stirring speed is 35 r / min.

[0176] 3. Feed mixing: Referring to Example 1, the MDA-ODCB solution is 280 kg / h (MDA purity 89.6 kg / h), and the liquid phosgene is 94 kg / h (molar ratio 1:2.1).

[0177] Reactor parameters: guide plate aperture 12 mm, opening ratio 75%; swirl blades 4, pitch 140 mm, helix angle 48°; dispersion plate aperture 55 μm, opening ratio 68%.

[0178] Steps 4 and 5 refer to Example 1.

[0179] Results after 15 days of continuous operation: MDA conversion rate 99.5%, phosgene utilization rate 98.5%, and purified MDI purity 99.68%.

[0180] Comparative Example 1

[0181] Using a conventional packed tower (without a multi-stage jet-swirl coupled enhanced mixing reactor and modified molecular sieves as adsorbents), under the same feed ratio and temperature conditions as in Example 1, the MDA conversion rate in the bottom product was only 85% (after 15 days of continuous operation), and the product color was significantly darker.

[0182] Comparative Example 2

[0183] Using the same raw material ratio as in Example 1, a continuous batch reactor was used for both cold and hot reactions. A high stirring speed and a small volume were required to achieve a strong mixing effect. Ultimately, the MDA conversion rate was 91% (after 15 days of continuous operation).

[0184] Comparative Example 3

[0185] Compared with Example 1, the only difference is the use of unmodified ZSM-5 molecular sieve, with a final MDA conversion rate of 93% (after 15 days of continuous operation).

[0186] Comparative Example 4

[0187] Compared to Example 1, the only difference was the replacement of the modified ZSM-5 molecular sieve with a cerium dioxide (CeO2)-modified alumina-based catalyst. Ultimately, the MDA conversion rate was 95.4% (after 15 days of continuous operation).

[0188] Comparative Example 5

[0189] Compared to Example 1, the only difference was the replacement of the modified ZSM-5 molecular sieve with a metal-organic framework (MOF)-derived carbon-based catalyst. Ultimately, the MDA conversion rate was 96.3% (after 15 days of continuous operation).

[0190] In summary, compared with traditional continuous batch reactors, this invention enhances the mixing effect of liquid MDA-ODCB solution and liquid phosgene by using a multi-stage jet-swirl coupling enhanced mixing reactor. Combined with the simultaneous removal of HCl by magnesium aluminum spinel modified molecular sieve, the condensation and reflux of phosgene at the top of the tower, and the continuous discharge of MDI-ODCB mixed solution from the bottom of the tower, this invention constructs a highly efficient, low-cost, and continuous MDI synthesis process, effectively solving the technical pain points of traditional processes.

[0191] The above description of the disclosed embodiments enables those skilled in the art to make or use the invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the invention. Therefore, the invention is not to be limited to the embodiments shown herein, but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A method for synthesizing MDI, characterized in that, Includes the following steps: S1: MDA solution and phosgene are introduced into a multi-stage jet-swirl coupled enhanced mixing reactor to pre-react the two under jet and swirl conditions to form a gas-liquid mixture. S2: The gas-liquid mixture obtained in step S1 is guided to the surface of the top tray of the packed tower reactor, so that it flows down the tray step by step; the tray is provided with magnesium aluminum spinel modified ZSM-5 molecular sieve; under the action of the magnesium aluminum spinel modified ZSM-5 molecular sieve, MDA reacts with phosgene to generate reaction products containing MDI and HCl. The temperature of the top condenser of the packed tower reactor is below 8°C; The bottom temperature of the packed tower reactor shall not be lower than 170°C.

2. The synthesis method according to claim 1, characterized in that, The interior of the multi-stage jet-swirl coupled enhanced mixing reactor is divided into a primary mixing zone, a secondary mixing zone, and a tertiary mixing zone arranged sequentially along the material flow direction. The primary mixing zone is used to mix MDA solution and phosgene by impact. It has two independent feed channels, which are designed with upper and lower double pipes and lateral oblique cuts to transport MDA solution and phosgene respectively. The primary mixing zone is equipped with a honeycomb guide plate as an impact zone. The secondary mixing zone is used to perform swirling shearing on the material that has passed through the primary mixing zone, and is provided with a swirling shearing chamber, in which spiral swirling blades are provided; The three-stage mixing zone is used to perform laminar flow mixing or micro-scale mixing of the material that has passed through the two-stage mixing zone, and is equipped with a porous microchannel dispersion plate.

3. The synthesis method according to claim 2, characterized in that, The honeycomb-shaped flow guide plate has a pore size of 6-15 mm and an opening rate of 70-90%. The number of spiral swirl blades is 4 to 8, the pitch is 130 to 170 mm, and the helix angle is 40 to 50°; The edge of the spiral swirl blade is provided with serrated protrusions, the tooth height of the serrated protrusions is 4~6 mm, and the tooth pitch is 8~12 mm; The porous microchannel dispersion plate has a thickness of 18-22 mm, a pore size of 40-60 μm, and an open porosity of 65-75%.

4. The synthesis method according to claim 2 or 3, characterized in that, Below the porous microchannel dispersion plate, a conical flow collector is also provided to uniformly direct the material passing through the three-stage mixing zone to the outlet. The cone angle of the conical shroud is 25~35°.

5. The synthesis method according to claim 4, characterized in that, The conical flow collector is equipped with a coil-type buffer inside; The inner diameter of the coil in the coil-type buffer is 18~22 mm, the length is 4.5~6 m, and the number of turns of the coil is 9~13. The material flow rate in the coil-type buffer is 0.5~0.8 m / s.

6. The synthesis method according to any one of claims 1 to 5, characterized in that, The magnesium-aluminum spinel-modified ZSM-5 molecular sieve was prepared according to the following method: (1) Provide a mixed salt solution of magnesium and aluminum salts according to the target Mg / Al molar ratio; (2) The mixed salt solution is mixed with ZSM-5 molecular sieve powder and impregnated with an equal volume; (3) Seal the impregnated sample, age it at room temperature, dry it, and then calcine it to obtain the final product.

7. The synthesis method according to claim 6, characterized in that, The roasting includes a first stage and a second stage; The first stage is: heating to 500-600°C at a rate of 2-5 °C / min and holding at that temperature for 2-4 h; The second stage is: the temperature is programmed to rise to 700-900°C at a rate of 2-5 °C / min, and held for 4-6 h.

8. The synthesis method according to any one of claims 1 to 7, characterized in that, In step S1, the molar ratio of MDA to phosgene in the MDA solution is 1:2 to 1:2.8; The temperature of the pre-reaction in step S1 is 40~70°C, and the residence time is 3~6 s; The reaction temperature in step S2 is as follows: decreasing stepwise from the bottom to the top of the column, with the bottom temperature not lower than 170°C and the top temperature not lower than 110°C, and the residence time being 20~40 min.

9. An MDI synthesis apparatus for implementing the synthesis method according to any one of claims 1 to 8, characterized in that, include: The primary reaction unit is a multi-stage jet-swirl coupled enhanced mixing reactor used to pre-react the MDA solution with phosgene. The secondary reaction unit is a packed tower reactor, located below the primary reaction unit; The discharge port of the primary reaction unit is in direct contact with the top surface of the top tray of the secondary reaction unit, so that the pre-reacted material enters the secondary reaction unit. The secondary reaction unit includes at least one tray and a magnesium aluminum spinel-modified ZSM-5 molecular sieve disposed on the tray.

10. The MDI synthesis apparatus according to claim 9, characterized in that, The tray is a sieve tray, a bubble cap tray, or a valve tray.

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