Apparatus and method for separating a mixture containing a heat sensitive substance

Abstract

The present invention provides an apparatus and method for continuous purification of high boiling point thermally sensitive compounds by selective continuous distillation of the medium entering the apparatus to remove more volatile impurities therefrom. The apparatus is a columnar device comprising at least three sections arranged in series: a mass transfer section, an evaporation section and a cooling section, wherein the evaporation section is a self-priming vertical falling film evaporator and the residence time of the thermally sensitive mixture in the evaporator zone is fixed. In particular cases, examples of high boiling point thermally sensitive media include mixtures of aromatic polyamines, such as mixtures of methyldiphenylamine (MDA) and higher homologues thereof; and mixtures of aromatic polyisocyanates, such as mixtures of methyldiphenyl isocyanate (MDI) and higher homologues thereof. The method based on the apparatus provides the above purified medium with a residual volatile solvent (e.g. aniline or chlorobenzene) content of no more than a few tens of ppm and a residence time of the volatile medium in the evaporator zone of no more than 100 seconds, which does not result in a significant change in the color of the thermally sensitive medium.

Description

Technical Field

[0001] This invention relates to an apparatus for separating mixtures containing at least one heat-sensitive substance. Specifically, the invention relates to a tower-type apparatus comprising at least three sections arranged in series, stacked on top of each other, and connected by a fluid medium: a mass transfer section, an evaporation section, and a cooling section. The invention also relates to a method for separating mixtures containing at least one heat-sensitive substance, particularly a method for separating polyamines and polyisocyanates. Specifically, the invention proposes an apparatus and method for the continuous purification of high-boiling-point heat-sensitive compounds by selectively and continuously distilling the medium input to the apparatus, thereby removing more volatile impurities. Background Technology

[0002] Separating mixtures containing heat-sensitive substances, i.e., substances with thermal degradation properties, is a common process in industry. In this context, the industrial sector implements both direct separation processes, i.e., obtaining two or more different products, and purification processes, i.e., removing unwanted impurities from raw materials. Herein and below, the term "separation" also includes purification processes. Separation processes for heat-sensitive mixtures include: separation of water-glycol solutions, separation of high-boiling-point aromatic compounds and pharmacological substances, and removal of unreacted starting compounds or solvents from polyamines and polyisocyanates. It is precisely the sensitivity of heat-sensitive substances to temperature changes that leads to a series of limitations related to the structural characteristics of the equipment used and its operating mode.

[0003] Given current technology, most processes employing continuous distillation for media separation utilize evaporators. These evaporators are designed to ensure continuous heating and residence of the evaporated medium both within and outside the evaporator area under significant temperature loads. Examples of such evaporators include evaporators with steam chambers or heat recovery boilers, vertical tubular integral boiling evaporators, and thermosiphons. The combination of these evaporators with the bottom distillate accumulation space in the lower section of the distillation column causes high-boiling-point media to accumulate over periods ranging from tens of minutes to several hours under temperature loads.

[0004] Mixtures of polyamines and polyisocyanates are a major representative of heat-sensitive media. Polydiaminodiphenylmethane (pMDA) is a raw material, an unpurified mixture of diamines and polyamines, synthesized through the condensation reaction of aniline and formaldehyde. Subsequently, polymethyldiamine (pMDA) is used as a raw material to prepare polyisocyanates (pMDI) via phosphorescence. These materials are extremely sensitive to low-temperature heat loads not exceeding 200-250°C, and irreversible structural changes related to thermal degradation and coking processes occur under prolonged heating. During the phosgenation stage, it is crucial that the starting material pMDA is free of water, as water leads to the formation of a corrosive environment and shortens equipment lifespan; it must also be free of aniline, as aniline leads to the accumulation of diisocyanates and polyisocyanates (i.e., phenylisocyanates) that are difficult to separate from the mixture, and these substances are toxic to the subsequent preparation of polyurethane composites.

[0005] During distillation, in a distillable thermosensitive mixture, the component with higher volatility may be the component with even higher volatility relative to the thermosensitive component with the closest volatility. The most common thermosensitive media (such as pMDA and pMDI mixtures) contain volatile components aniline and monochlorobenzene or dichlorobenzene, respectively. These components have a relative volatility of over 3 in the mixture at residual pressures below 20 mbar.

[0006] For example, US Patent No. 7312362 (published December 25, 2007, Bayer MaterialScience AG [DE]) discloses various process variations for separating diamines and polyamines by distillation. This document suggests final aniline and water removal from the target components in a tower apparatus, and recommends using high-temperature steam as the heating medium. The separation process includes pre-evaporation, distillation, and cooling. A disadvantage of this method is the high residual water and aniline content in the target product, with water content exceeding 100 ppm, which, as mentioned earlier, adversely affects the subsequent phosgenation reaction in the amine medium.

[0007] Document WO2019115213 (published June 20, 2019, Sulzer Chemtech [CH]) describes a process and apparatus for distilling a mixture of highly heat-sensitive diisocyanates and polyisocyanates. Therefore, according to the proposed scheme, a falling film evaporator is used in the distillation column. However, this method requires forced circulation of the bottom product, and the medium accumulated at the bottom of the column needs to maintain a relatively long residence time at its boiling point temperature.

[0008] The inventors of DE202013003950 (published June 25, 2013, Huntsman LLC) proposed using a heated nitrogen stream to additionally purge volatile components during the final purification of pMDA in a tower-type apparatus. This approach not only suffers from all the aforementioned drawbacks, but also increases the load on the vacuum system during the extraction of the purge gas.

[0009] Therefore, the question of how to organize the separation process of mixtures containing heat-sensitive substances in order to obtain high-purity separation products without triggering the negative effects of thermal decomposition remains of practical significance. Summary of the Invention

[0010] The purpose of this invention is to develop a process for separating mixtures containing high-boiling-point heat-sensitive substances, characterized by the absence of a negative thermal decomposition process.

[0011] The technological achievement lies in obtaining a separated product containing high-boiling-point heat-sensitive substances, with the residual content of volatile substances not exceeding 100 ppm, while the change in the original color is small.

[0012] The technical problem is solved by using an apparatus in the process of separating a mixture containing a heat-sensitive substance. This apparatus includes: a mass transfer section with a mixture inlet; an evaporation section located below the mass transfer section and connected to it via a fluid medium; and a cooling section located below the evaporation section, connected to it via a fluid medium, and having an outlet for the liquid containing the heat-sensitive substance. The evaporation section includes a vertical film evaporator with a distribution device at its top for forming a thin film composed of the mixture from the mass transfer section on the inner surface of the vertical film evaporator. The vertical film evaporator is designed to heat the film layer, thereby causing the liquid containing the heat-sensitive substance to form a vapor phase of the mixture. The distribution device is also designed to allow the vapor phase of the mixture from the vertical film evaporator to enter the mass transfer section. The mass transfer section is designed to ensure contact between the mixture entering the mass transfer section and the vapor phase of the mixture from the evaporation section. Simultaneously, the vertical film evaporator is designed to transport the liquid containing the heat-sensitive substance from the evaporation section to the cooling section for cooling.

[0013] While the inventors of this invention do not wish to be confined to a particular theory, they believe that using the above-mentioned device can shorten the residence time of the heat-sensitive substance in the high-temperature region of the evaporation section, thereby avoiding undesirable decomposition reactions, and at the same time reducing the content of undesirable volatile impurities (such as water, unreacted starting compounds, reaction byproducts, etc.) in the separated products.

[0014] By arranging three key components sequentially, efficient separation of mixtures containing high-boiling-point thermosensitive substances can be achieved without adverse thermal decomposition processes. These three components include: a mass transfer section, which separates the input mixture through mass transfer between the liquid and gas phases; an evaporation section, which generates a vapor phase from the thermosensitive liquid film, which then enters the mass transfer section; and a cooling section, which limits the high-temperature residence time of the medium after the evaporation section.

[0015] Effective separation of the mixture is achieved through a mass exchange process occurring within the mass transfer section of the device, between the downstream feed liquid flowing into the device (the spray flow at the top of the mass transfer section) and the upstream saturated vapor generated in the evaporation section. Furthermore, the vertical film evaporator in the evaporation section ensures that the film flows naturally along its inner surface, thereby shortening the residence time of heat-sensitive substances in high-temperature regions and preventing thermal decomposition. For amine and isocyanate media, the residence time in the evaporation chamber does not exceed 100 seconds.

[0016] According to one embodiment of the present invention, the membrane evaporator comprises vertically arranged tubes. The tube length is preferably:

[0017] L[м]=τ_Film*wср,

[0018] Where τ_Film is the average residence time of the thin film flow on the inner wall of the heat exchange tube, ranging from 1 to 100 seconds; wср is the average velocity of the thin film flowing along the inner wall of the tube.

[0019] The pipe length should preferably be selected to maintain the residence time τ_Film of the film flow between 20 and 80 seconds. More preferably, the pipe length should ensure that the residence time τ_Film of the film flow is between 30 and 60 seconds.

[0020] Furthermore, the design of this distribution device enables a thin film to form on the inner surface of the membrane evaporator tubes.

[0021] According to one embodiment of the invention, a distribution device for forming a thin film of a mixture from a mass transfer section on the inner surface of a vertical film evaporator is a blind plate.

[0022] The apparatus according to the invention is preferably used for separating mixtures of diamines or polyamines, or mixtures of diisocyanates or polyisocyanates.

[0023] Preferably, the mass transfer section is a packed tower equipped with an internal contact device based on mesh or sheet-like structured packing elements.

[0024] Preferably, the specific surface area of ​​the internal contact device is 400-1000 m² / m³, and the porosity exceeds 80%.

[0025] In the apparatus described in this invention, the cooling section is or includes an inclined, self-flowing "tube-in-tube" type tubular heat exchanger or a heat recovery heat exchanger. Preferably, the inclination angle of the heat exchange axis of the self-flowing tubular heat exchanger relative to the overall horizontal plane of the equipment is 1-90°.

[0026] Another aspect of the present invention, for achieving the above-mentioned technical effects, is a method for separating a mixture containing at least one heat-sensitive substance. This separation method utilizes a mixture separation apparatus comprising: a mass transfer section located below and connected thereto via a fluid medium; an evaporation section including a vertical film evaporator; and a cooling section located below and connected thereto via a fluid medium. The mixture containing at least one heat-sensitive substance is introduced into the mass transfer section, then enters the vertical film evaporator of the evaporation section and forms a thin film. This thin film moves by gravity along the inner surface of the film evaporator. Simultaneously, the mixture film moving by gravity in the vertical film evaporator is heated to obtain a vapor phase of the mixture and a liquid stream containing the heat-sensitive substance. The vapor phase of the mixture rises to the mass transfer section for mass transfer, achieved through contact between the vapor phase and the mixture entering the mass transfer section. Simultaneously, the liquid stream containing the heat-sensitive substance flows from the evaporation section to the cooling section, where it is cooled and discharged as a separation product containing the heat-sensitive substance.

[0027] This method is preferably used to separate mixtures of diamines or polyamines, or mixtures of diisocyanates or polyisocyanates. It is also preferred to use this method to separate (poly)diaminodiphenylmethane (pMDA) or polydiaminodiphenylisocyanate (pMDI) to obtain two or more streams of pMDA or pMDI.

[0028] According to the present invention, the residence time of the thin film flow in the vertical film evaporator is 1 to 100 seconds. Attached Figure Description

[0029] Figure 1 The apparatus shown is that of the present invention, wherein: Ⅰ-mass transfer section, Ⅱ-evaporation section, Ⅲ-cooling section, 1-vapor phase outlet, 2-droplet separator or enhanced section of the mass transfer part of the apparatus, 3-inlet of the mixture to be separated / purified, 4-gas-liquid flow collector and distributor, 5-contact device, 6-support element, 7-gas and liquid phase distribution device, 8-self-flowing vertical film evaporator, 9-heat transfer medium, 10-cooler, 11-separation product outlet. Detailed Implementation

[0030] The main aspects of the invention will be described in detail below.

[0031] The device of the present invention is a tower-type device, which consists of at least three sections connected in series: a mass transfer section (I), an evaporation section (II), and a cooling section (III).

[0032] The mass transfer section (I) of the tower apparatus refers to a section within the tower in which a mixture containing heat-sensitive substances is fed through inlet 3 for separation. This section is equipped with an internal contact device 5 and supporting elements 6 for fixation. The contact device 5 of the mass transfer section can be selected from any known contact device in the prior art, including mass transfer discs, structured packing, etc. Most preferably, a mesh or sheet-like structured packing with a specific surface area of ​​400-1000 m² / m³ and a porosity exceeding 80% is used as the contact device because such a contact device can achieve the optimal number of separation stages per unit tower height while maintaining a reasonable hydraulic pressure drop under normal distillation process conditions with low residual pressure.

[0033] Depending on the physicochemical properties and gas-liquid equilibrium of the components in the mixture to be separated, the specific structure of the internal contact device and the optimal number of separation stages in the mass transfer section of the device vary. For example, when separating a mixture of polyamine with water and aniline, or a mixture of polyisocyanate with a chloroaromatic solvent (e.g., monochlorobenzene), the optimal number of trays is 3 to 20, more preferably 4 to 10, and most preferably 5 to 8. Furthermore, the optimal material for the internal contact device is austenitic stainless steel, such as AISI 304 / 316.

[0034] To improve efficiency and optimize equipment operation, a droplet separator 2 and a flow distributor 4 can be optionally installed in the mass transfer section of the tower unit. The droplet separator 2 refers to any device that can be used to prevent droplet escaping, such as a structured packing element with a specific surface area of ​​100-2000 m² / m³.

[0035] As an alternative, the droplet separator 2 can be replaced or modified to create an enhanced section of the tower and reduce the content of high-boiling-point substances in the upper product stream. However, even without major additional modifications, this area may still facilitate mass transfer between the retained droplets and the rising vapor phase of the more volatile components in the medium when the equipment uses amine or isocyanate media as feed.

[0036] The vapor phase can be discharged through outlet 1 located at the top of the mass transfer section.

[0037] The evaporation section (II) of this device is a gravity-flow vertical film evaporator 8 located below the mass transfer section. The film evaporator 8 is a single-pass tubular evaporator, with geometry similar to tubular heat exchangers conforming to the international TEMA standard. The vapor phase used in the mass transfer process is generated in a thin-film mode on the inner surface of the vertical tube bundle. Unlike typical bulk boiling devices, where the vapor phase is generated as bubbles within the liquid, in thin-film boiling mode, the equilibrium vapor phase forms on the continuously heated film surface flowing down the vertical surface. In this case, the liquid and vapor phases of the evaporator move in countercurrent flow.

[0038] According to the present invention, the self-flowing vertical film evaporator 8 is a film evaporator that does not require forced circulation of the mixture. This self-flowing vertical film evaporator ensures that mixtures containing heat-sensitive substances have a fixed residence time.

[0039] The evaporation section is equipped with a distribution device 7, which can be any available distribution device in the prior art, capable of uniformly distributing the liquid phase on the inner surface of the evaporator or the evaporator tube wall. A distribution device that directly wets the perimeter of the tubes is most recommended, in which the liquid flowing into the evaporator wets the tubes by overflow under gravity.

[0040] According to the present invention, in the self-flowing vertical film evaporator 8, the thin film mode can be arbitrarily established at a specific spray density, which is achieved by the liquid medium flowing naturally to the inner surface of the evaporator 8 (especially the inner surface of the evaporator tubes) via the distribution device 7.

[0041] According to the present invention, the evaporator 8 does not employ any forced means (e.g., pumps, comb devices, etc.) to form and / or maintain a thin film state on the inner surface of the evaporator or the evaporator tube bundle. This feature is defined as self-flowing. In the evaporator 8, no other measures are employed besides the natural barrier of the distribution device 7, which forms a sufficient spray density to ensure the stability of the thin film state.

[0042] The advantage of this type of evaporator is that the mixture containing heat-sensitive substances has a short residence time on the heat exchange tube surface, which is limited to the gravity flow time of the liquid film from the evaporator's distribution elements to the subsequent part of the tower unit (i.e., the cooling section). Another advantage of this type of evaporator is that it eliminates the need for high-temperature and expensive circulating pumps, which are typically used to achieve separation processes but are also the most prone to depressurization and can cause the mixture containing heat-sensitive and / or highly reactive substances to come into contact with oxygen in the ambient air.

[0043] Those skilled in the art will readily recognize that the specific geometric parameters of the apparatus depend on the characteristics of the mixture to be separated. Here, to describe the geometric characteristics of the evaporation section of the apparatus, the following set of formulas should be used:

[0044] (1) Re=(4*Gор [кг / (м*с)]) / (μ [Па*с]) ,

[0045] Where Re is the Reynolds number; G_(or) is the spray density calculated according to formula (2); and μ is the dynamic viscosity of the mixture being separated flowing through the evaporator.

[0046] (2) G_ор=(G (N-1 liq) [кг / (м*с)]) / (∑〖P тр〗 [м]),

[0047] Where G (N-1 liq) represents the amount of liquid used for evaporator spraying; Ptr represents the inner circumference of the wet heat exchange tubes of the evaporator.

[0048] (3) δср= ∛((3*Gор*μ) / (ρ^2*g)) [м],

[0049] Where δср is the average thickness of the liquid film formed on the inner surface of the heat exchange tube, ρ is the density (kg / m³) of the separated medium flowing through the evaporator; and g=9.8 is the free acceleration constant.

[0050] (4) w_aver= ∛((Gор 2 *g) / (3* μ* ρ)),

[0051] Where w_aver is the average velocity [m / s] of the thin film moving along the inner surface of the tube by gravity.

[0052] (5) τ_Film=(L [м]) / wср, where L is the length of the heat exchange tube of the film evaporator and τ_Film is the average residence time of the liquid on the inner surface of the heat exchange tube.

[0053] When the device is powered by a polyamine or polyisocyanate medium, the preferred range of the Reynolds number is 0.1-10000, more preferably 1-1000, and most preferably 1-50.

[0054] The tube length can be selected from the standard size series of heat exchangers conforming to the TEMA BEM type standard. When the equipment is fed by amine or isocyanate media, the tube length should allow the liquid film to flow by gravity in the range of 1 to 100 seconds, preferably 20 to 80 seconds, and more preferably 30 to 60 seconds.

[0055] The proposed membrane evaporator can be heated using any available high-efficiency heat transfer medium, such as steam at the appropriate pressure, or a high-boiling-point organic heat transfer medium with suitable thermophysical properties, such as organosilicon or polyalkylphenyl heat transfer media.

[0056] The third part (III) of the device is a cooler 10 for cooling the liquid stream containing the heat-sensitive substance flowing out from the lower part of the film evaporator 8. The cooler 10 can be any cooling device available in the prior art, including but not limited to: gravity-flow inclined tube heat exchangers and heat recovery heat exchangers. A typical example of such a device is a "tube-in-tube" inclined tube heat exchanger, whose axis is inclined at an angle between 1 and 90 degrees relative to the horizontal direction, more preferably between 15 and 60 degrees. The advantage of this section is that it minimizes the residence time of the fluid containing the heat-sensitive substance in the high-temperature region surrounding the film evaporator 8, thereby reducing the residence time of the heat-sensitive medium at high temperatures to the time required for the fluid to flow naturally down the inner wall of the evaporator 8. This minimizes the risk of degradation of the heat-sensitive substance due to heat while correspondingly maintaining the physical and mechanical properties and color index of the medium.

[0057] The product flowing out from the bottom of the equipment can be used as a raw material for subsequent chemical transformations, or it can be used to prepare commercial products.

[0058] The steam generated at the top of the equipment can be condensed using any method known in the prior art. The most preferred condensation method is vapor-phase condensation on the outer surface of the horizontally arranged tube bundles in a shell-and-tube heat exchanger. The advantage of this method is the lower hydraulic pressure drop in the vapor-phase condensation system.

[0059] Specific examples of separating mixtures containing heat-sensitive substances include the separation of diamines and polyamines, and water and aniline. This description is for illustrative purposes only and does not constitute a limitation thereof, as will be apparent to those skilled in the art.

[0060] It is recommended to supply the equipment (inlet 3) with a wet mixture of diamine and polyamine (pMDA) with water and aniline. The moisture content in the pMDA feedstock can be 0-15% (by mass), more preferably 0-8% (by mass). To reduce the steam load in the mass transfer section (I) of the equipment, the pMDA feedstock can be pre-dehydrated using any existing technology. The most preferred pre-dehydration method is to perform single-stage evaporation of the mixture in an evaporator (flash evaporator), under mild conditions of temperature not exceeding 150°C and pressure not less than 200 mbar, in which a considerable portion of the moisture in the mixture can be removed in the form of a water-aniline azeotropic mixture vapor. The aniline content in the feedstock mixture can be 0-60% (by mass), more preferably 0-30% (by mass). The distillation process of the diamine and polyamine mixture is carried out in the range of 1-50 mbar residual pressure, more preferably 2-20 mbar, and most preferably 3-10 mbar. The temperatures of the upper and lower sections of the mass transfer section (I) should be set according to the oligomer composition of the polyamine, the feed temperature, and the requirements for extracting the oligomer components. Based on the oligomer composition of the polyamide, the temperature of the evaporation section (II) should be 200-280°C, more preferably 220-250°C. The temperature of the cooling section (III) should be set sufficient to cool the amine solution to no more than 150°C, more preferably no more than 100-120°C. Given the high viscosity of the resulting medium, the storage temperature of the amine medium before final transportation should not be lower than 60-100°C.

[0061] The achievable residual volatile component content varies depending on the specific thermosensitive medium and the components it releases. The specific performance of this equipment is illustrated in detail through the following example: distillation of thermosensitive media such as pMDA and pMDI mixed with aniline and monochlorobenzene, respectively.

[0062] For specific thermosensitive media fed into the equipment, the color and viscosity of the thermosensitive product output from the bottom of the device (through outlet 11) can be improved respectively. Given that the properties of the thermosensitive media change over time under specific temperature stress, it is clear that such a device with a shorter contact residence time can largely avoid the negative effects caused by thermal damage.

[0063] In specific examples, the performance of the device is demonstrated in more detail, such as the distillation of thermosensitive substances such as pMDA and pMDI with a mixture of aniline and monochlorobenzene.

[0064] Embodiments of the present invention

[0065] Analysis method:

[0066] 1. The mass fractions of oligomers and aniline in pMDA samples were determined by high-performance liquid chromatography (HPLC) on an Agilent 1260 Infinity II system equipped with a 1260 DAD WR diode array detector and a Kinetex C18 column, at 250 × 4.6, 5 μm, 100 Å. The samples were pre-dissolved in acetonitrile with added dimethyl sulfoxide. Calculations were performed using an absolute calibration method.

[0067] 2. The residual moisture content in the pMDA sample was determined by Karl Fischer coulometric titration on an automated titrator conforming to GOST 24614, "Liquids and gases that do not react with Fischer reagents." The determination was performed using the coulometric method. The following reagents were used for moisture determination: Hydranal-Coulomat AK and Hydranal-Coulomat CK.

[0068] 3. The mass fraction of water in aniline was determined by gas chromatography (GC) on an Agilent 7890A gas chromatograph equipped with a thermal conductivity detector and a DB-FFAP capillary column (30 m × 0.32 mm × 0.25 μm). The mass fraction of water was calculated using the absolute calibration method.

[0069] 4. The viscosity of pMDA was measured (plate / plate type) on an Anton Paar MCR 102 modular compact rheometer equipped with a PP 25 measurement system. The shear stress and dynamic viscosity were calculated by measuring the torque generated on the turntable of the measuring device by the sample at different rotation speeds (shear rates).

[0070] 5. The colorimetry of pMDA and pMDI samples was determined using a Lovibond PFXi-995 spectrophotometer and prestressed optical glass cuvettes with a path length of 10 mm. This method is based on measuring the color coordinates and colorimetry of solvent-diluted samples (ethyl acetate for pMDA, monochlorobenzene for pMDI; mass concentration 20-25%) in the wavelength range of 420 to 710 nm. The results are expressed as iodine values ​​on an iodine scale.

[0071] 6. The mass fraction of oligomers in pMDI samples was determined by liquid chromatography. Specifically, the samples were first pre-derivatized with methanol, and then the resulting reaction products were analyzed using an Agilent 1260 Infinity II liquid chromatograph equipped with a 1260 DAD WR diode array detector and a Kinetex C18 column (250 × 4.6, 5 μm, 100 Å pore size). The oligomer content in the samples was determined by peak area internal normalization.

[0072] 7. The mass fractions of phenyl isocyanate and 4,4'-methylene diphenyl isocyanate impurities in the MHB fraction were determined by gas chromatography using an Agilent 7890A gas chromatograph equipped with a flame ionization detector and a DB-35 (30 m × 0.25 mm × 0.25 μm) capillary column. Calculations were performed using the absolute calibration method.

[0073] 8. The dynamic viscosity of pMDI was determined by calculation using kinematic viscosity and density.

[0074] 8.1 Kinematic viscosity was determined using a VPZ-1 capillary viscometer. This method involves using a calibrated glass viscometer at a constant temperature to measure the time (in seconds) required for a given volume of the test liquid to flow under gravity. Kinematic viscosity is the product of the measured flow time and the viscometer constant.

[0075] 8.2 The density of pMDI was determined at 25 °C according to GOST 18995.1 "Liquid Chemicals" standard. (See "Method for Determination of Density").

[0076] Example 1 (Invention)

[0077] like Figure 1 As shown, the device consists of a mass transfer section, which is an AISI 316 stainless steel tube with an inner diameter of 50 mm, filled with Sulzer CY structured packing, with a total packing height of 0.8 m; a droplet separation section, also composed of AISI 316 stainless steel tubes with an inner diameter of 50 mm, filled with Sulzer CY structured packing, with a total packing layer height of 160 mm; and a multi-point liquid distributor with a distribution capacity of approximately 1000 points / m². To minimize heat loss, a localized area of ​​the reboiler is electrically heated to approximately 200°C and insulated with mineral wool.

[0078] like Figure 1 As shown, the device consists of an evaporation section, which is a vertical shell-and-tube heat exchanger with an inner diameter of 80 mm. It is equipped with three 28×1.5 mm, 400 mm long heat exchange tubes, heated by Thermolan Lab5 thermal silicone oil. The distribution device is a closed, disc-shaped unit that forms a thin film by the liquid flowing through the wetting area around the tube wall. The calculated hydrodynamic characteristics of the thin film flow are a Reynolds number of approximately 30 and a residence time of approximately 30 seconds for pMDA.

[0079] like Figure 1 As shown, the device consists of a cooling section, which is an inclined, self-flowing "tube-in-tube" type heat exchanger with an inner tube diameter of 25mm. The entire cooling section is inclined at 105 degrees relative to the tower's mounting plane. оPMS10 silicone oil was injected into the gap between the tubes.

[0080] The pMDA feedstock used for equipment testing was prepared according to the method described in patent CN100422239. The initial color of the pMDA feedstock was iodine value 23. The pMDA feedstock fed into the device had the following average composition: water content of 0.25-0.5% (mass fraction), aniline content of 16.33% (mass fraction), and the remainder being MDA and its oligomer mixture. The feedstock consumption was 1.1 to 1.15 kg / h, and the feed stream temperature was maintained at 80°C by electric heating.

[0081] In the upper part of the mass transfer section of the device, a residual pressure of 6-8 mbar is maintained, while in the evaporation section, the heat transfer medium temperature is maintained at 245-250°C. The pressure difference from the top of the mass transfer section to the bottom of the evaporation section is less than 1.5 mbar. The exhaust temperature is 55-65°C, while the steam temperature in the lower section is approximately 235°C.

[0082] At the evaporator outlet, pMDA at a flow rate of 950-1000 g / h flows through the cooling section and is cooled to 100-120°C before flowing into a 12-liter receiving tank equipped with an external electric heating device.

[0083] A vapor phase of 100-200 g / h is condensed in a shell-and-tube heat exchanger-condenser. The condensation site is located on the outer surface of the finned tube bundle, with a specific surface area of ​​approximately 1 m². 2 A water-ethylene glycol heat transfer medium at a temperature of 10-25°C is fed into the condenser cavity. The condensed aniline fraction is collected in a 12-liter receiving tank with an outer jacket, and the water-ethylene glycol heat transfer medium is introduced into the annular gap of the outer jacket of this receiving tank after passing through the steam condenser.

[0084] After the equipment has been running continuously for 4 to 6 hours, the accumulated upper and lower layer products are periodically discharged. Samples are then taken from these products and quantitative and qualitative analyses are performed using high-performance liquid chromatography.

[0085] The condensate fraction mainly consists of a mixture of aniline and water, with an MDA content not exceeding 0.001% (mass fraction). The purified pMDA from the bottom product of this apparatus has a water content of less than 10 ppm and an aniline content of less than 20 ppm. The purified pMDA has an iodine value of 43 on the iodine scale. The viscosity of the purified pMDA at 100°C is 34 mPa*s.

[0086] Example 2 (Invention)

[0087] The apparatus described in Example 1 is used to extract a substance with the following average composition from a pMDA stream of 950-980 g / h: water content less than 10 ppm, aniline content less than 40 ppm, MDA content of 48% (mass fraction), and the remainder being MDA polymer; the MDA stream flow rate accounts for 2-25% of the feed mass flow rate. The pMDA donor stream has an iodine value of 43 according to the iodine scale. The temperature of the feed stream is maintained at 150-200°C by electric heating. The design hydrodynamic characteristics of this membrane evaporator are a Reynolds number of approximately 20 and a pMDA residence time of approximately 30 seconds.

[0088] In the upper part of the mass transfer section of the device, a residual pressure of 3-4 mbar is maintained, while in the evaporation section, the heat transfer medium temperature is maintained at 225-235°C. The pressure difference from the top of the mass transfer section to the bottom of the evaporation section is less than 1.5 mbar. The exhaust temperature is 200-210°C, while the steam temperature in the lower section is 220-230°C.

[0089] At the evaporator outlet, pMDA at a flow rate of 850-950 g / h flows through the cooling section and is cooled to 120°C before flowing into a 12-liter receiving tank equipped with an external electric heating device.

[0090] A vapor phase of 50-150 g / h is condensed in a shell-and-tube heat exchanger-condenser. The condensation site is located on the outer surface of the finned tube bundle, with a specific surface area of ​​approximately 1 m². 2 The silicone heat transfer fluid at 110°C is fed into the condenser cavity. The condensate is collected in a 12-liter receiving tank with an outer jacket, into which the silicone heat transfer fluid is introduced from the steam condenser into the annular gap of the receiving tank.

[0091] Depending on the temperature of the feed stream and the heat load of the evaporator, the proportion of MDA extracted from the feed stream is 2% to 25% of the total feed.

[0092] After the equipment has been running continuously for 4 to 6 hours, the accumulated upper and lower layer products are periodically discharged. Samples are then taken from these products and quantitative and qualitative analyses are performed using high-performance liquid chromatography.

[0093] The condensate fraction (i.e., MDA melt) contains up to 0.3% (mass fraction) of MDA. The bottom product of the unit—purified pMDA with reduced MDA content—has a water content of less than 1 ppm and an aniline content of less than 10 ppm. After MDA removal, the color change of pMDA is less than 10 iodine values ​​on the iodine scale. At 100°C, the viscosity of pMDA is 48 mPa*s.

[0094] Example 3 (Invention)

[0095] The purified pMDA prepared in Example 2 was subjected to phosgenation according to the method described in WO2023 / 063852. After the phosgenation reaction was completed and the isocyanate solution was pre-filtered and purified, the resulting product was fed into the tower apparatus described in Example 1 for separation and removal of excess solvent.

[0096] Therefore, the equipment consumes 1100-1150 g / h of crude polyisocyanate, with the following average composition: 10-15% monochlorobenzene (mass fraction), and the remainder being polyisocyanate. The temperature of the feed stream is maintained at 50-80°C by electric heating. The design hydrodynamic characteristics of this membrane evaporator are a Reynolds number of approximately 20 and a residence time of approximately 30 seconds for pMDI.

[0097] In the upper part of the mass transfer section of the device, a residual pressure of 5-10 mbar is maintained, while in the evaporation section, the heat carrier temperature is maintained at approximately 200°C. The pressure difference from the top of the mass transfer section to the bottom of the evaporation section is less than 1.5 mbar. The exhaust temperature is 40-45°C, while the steam temperature in the lower section reaches as high as 200°C.

[0098] At the evaporator outlet, pMDI at a flow rate of 1000 g / h flows through the cooling section and is cooled to 60-70°C before flowing into a 12-liter receiving tank equipped with an external electric heating device.

[0099] A vapor phase of 100-150 g / h is condensed in a shell-and-tube heat exchanger-condenser. The condensation site is located on the outer surface of the finned tube bundle, with a specific surface area of ​​approximately 1 m². 2 The silicone heat transfer medium at a temperature of -20°C is delivered into the piping space of the condenser. The condensate is collected in a 12-liter receiving tank with an outer jacket, and the silicone heat transfer medium is introduced into the annular gap of the receiving tank after the steam condenser.

[0100] After the equipment has been running continuously for 4 to 6 hours, the accumulated upper and lower layer products are periodically discharged. Samples are then taken from these products and quantitative and qualitative analyses are performed using high-performance liquid chromatography.

[0101] The condensate fraction of MHB contains trace amounts of MDI isomers. The bottom product of this equipment—purified pMDI—has an MHB content of less than 30 ppm. The color of polymethyl methacrylate (pMDI) determined by the iodine value method does not exceed 110 units, and its viscosity at 25°C is 180 mPa*s.

[0102] Example 4 (Comparative Example)

[0103] The pMDA raw material prepared according to the method described in patent document CN100422239 has the following average composition: water content 0.25-0.5 (mass fraction), aniline content 16.33 (mass fraction), and MDA and its oligomer mixture content as the balance. The water and aniline are removed sequentially by periodic distillation in a device that serves as a 500 mL volume laboratory distillation flask, and a MDA recovery fraction of 25% (mass fraction) of the initial mass of the polyamine medium is extracted. The device is equipped with a magnetic stirrer and is placed in an electrically heated conical flask. It is connected to a vapor fraction receiver by a "trap to trap" method and is equipped with a corresponding air heater and vapor flow cooler.

[0104] The distillation process was carried out at temperatures and pressures close to those described in Examples 1 and 2. During the sequential completion of all media distillation and purification stages, the residence time of the product at the bottom of the vessel was 45 to 90 minutes.

[0105] During the fractionation process, using 150g of crude p-MDA as raw material, approximately 26g of water-aniline fraction and approximately 13g of MDA fraction were obtained. After fractionation, samples were taken from the upper fraction and the bottom product, and quantitative and qualitative analyses were performed using high-performance liquid chromatography (HPLC).

[0106] The bottom product obtained during fractionation, namely purified pMDA, has an aniline content of less than 10 ppm, an iodine value (on the iodine scale) of 66, and a viscosity of 56 mPa*s at 100°C.

[0107] Example 5 (Comparative Example)

[0108] The pMDI feedstock obtained in Example 3 was periodically fractionated in an apparatus similar to that described in Comparative Example 4, except that the distillation flask and the distillate receiver were connected via a straight-through condenser cooled by a circulating organosilicon heat carrier.

[0109] The distillation process was carried out at temperatures and pressures close to those described in Example 3. During fractionation, the residence time of the product at the bottom of the vessel was 45 to 90 minutes.

[0110] During the fractionation process, approximately 16 g of MHB fraction was obtained from 150 g of pMDI feedstock. After fractionation, samples were taken from the upper fraction and the bottom product for quantitative and qualitative analysis using high-performance liquid chromatography (HPLC).

[0111] The bottom fraction obtained during fractionation, namely purified pMDI, has an MHB content of less than 30 ppm, an iodine value (on the iodine scale) of 305, and a viscosity of 233 mPa*s at 25°C.

[0112] Table 1 Summary of Features of Examples

[0113]

Claims

1. An apparatus for separating a mixture containing at least one heat-sensitive substance, the apparatus comprising: A mass transfer section with a mixed feed; An evaporation section located below the mass transfer section and connected to it via a fluid medium; The evaporation section includes a cooling section located below and connected to the evaporation section via a fluid medium. The cooling section has an outlet for discharging a liquid stream containing a heat-sensitive substance. The evaporation section includes a vertical film evaporator with a distribution device at its top for forming a thin film composed of a mixture from the mass transfer section on the inner surface of the vertical film evaporator. The vertical film evaporator is designed to heat the film layer, thereby forming a vapor phase of the mixture and a liquid stream containing the heat-sensitive substance. The distribution device is also designed to allow the vapor phase of the mixture from the vertical film evaporator to enter the mass transfer section. The mass transfer section is designed to ensure contact between the mixture entering the mass transfer section and the vapor phase of the mixture from the evaporation section. Simultaneously, the vertical film evaporator is designed to transport the liquid stream containing the heat-sensitive substance from the evaporation section to the cooling section for cooling.

2. The apparatus according to claim 1, characterized in that, A membrane evaporator consists of vertically arranged tubes.

3. The apparatus according to claim 2, characterized in that, The pipe length is: L[м]=τ_Film*wср, Where τ_Film is the average residence time of the thin film flow on the inner wall of the heat exchange tube, ranging from 1 to 100 seconds; wср is the average velocity of the thin film flowing along the inner wall of the tube.

4. The apparatus according to claim 3, characterized in that, The pipe length was selected to ensure that the residence time τ_Film of the film flow is between 20 and 80 seconds.

5. The apparatus according to claim 4, characterized in that, The pipe length was selected to ensure that the residence time τ_Film of the film flow is between 30 and 60 seconds.

6. The apparatus according to claim 2, characterized in that, The design of the distribution device allows a thin film to be formed on the inner surface of the thin film evaporator pipes.

7. The apparatus according to claim 1, characterized in that, The dispensing device is a blind plate.

8. The apparatus according to claim 1, for separating a mixture of diamines or polyamines, or a mixture of diisocyanates or polyisocyanates.

9. The apparatus according to claim 1, characterized in that, The mass transfer section is a packed tower equipped with internal contact devices based on mesh or sheet-like structured packing elements.

10. The apparatus according to claim 9, characterized in that, The specific surface area of ​​the internal contact device is 400-1000 m2 / m3, and the porosity exceeds 80%.

11. The apparatus according to claim 1, characterized in that, The cooling section is or includes an inclined, self-flowing "tube-in-tube" type tubular heat exchanger or a heat recovery heat exchanger.

12. The apparatus according to claim 11, characterized in that, The tilt angle of the heat exchanger axis of the gravity-flow tubular heat exchanger relative to the overall horizontal plane of the device is 1-90 degrees.

13. A method for separating a mixture containing at least one heat-sensitive substance using a mixture separation apparatus, the apparatus comprising: One mass transfer section; An evaporation section located below the mass transfer section and connected to it via a fluid medium, the evaporation section comprising a vertical film evaporator; The mixture includes a cooling section located below the evaporation section and connected to it via a fluid medium. A mixture containing at least one heat-sensitive substance is introduced into the mass transfer section, then enters a vertical film evaporator in the evaporation section and forms a thin film. This film moves by gravity along the inner surface of the film evaporator. Simultaneously, the mixture film moving by gravity in the vertical film evaporator is heated to obtain a vapor phase of the mixture and a liquid stream containing the heat-sensitive substance. The vapor phase of the mixture rises to the mass transfer section for mass transfer, achieved through contact between the vapor phase and the mixture entering the mass transfer section. Meanwhile, the liquid stream containing the heat-sensitive substance flows from the evaporation section to the cooling section, where it is cooled and discharged as a separation product containing the heat-sensitive substance.

14. The method according to claim 13, characterized in that, The residence time of the thin film flow in the vertical membrane evaporator is 1 to 100 seconds.

15. The method according to claim 13, characterized in that, Separation of diamines or polyamine mixtures, or diisocyanates or polyisocyanates.

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