Apparatus and method for distilling polyisocyanates

Abstract

The invention relates to a distillation apparatus for separating monomeric diisocyanates from polyisocyanate compositions, comprising at least one pre-evaporation apparatus comprising a first inlet for the polyisocyanate composition to be distilled, a pre-evaporator having a first inner chamber, a first gas outlet for the evaporated constituents, a first liquid outlet for the non-evaporated constituents of the polyisocyanate composition, and at least one first condenser assigned to the first gas outlet, at least one thin-film evaporation apparatus comprising a second inlet for the non-evaporated constituents of the polyisocyanate composition, a thin-film evaporator having a second inner chamber, a second gas outlet for the evaporated constituents, a second liquid outlet for the polyisocyanate composition which has been substantially freed of unconverted isocyanates, and at least one second condenser assigned to the second gas outlet, and a vacuum generation apparatus comprising at least one vacuum pump, which is configured to achieve a pressure P1 in the inner chamber of the pre-evaporator of the at least one pre-evaporation apparatus and a pressure P2 in the inner chamber of the thin-film evaporator of the at least one thin-film evaporation apparatus, wherein P2 < P1, characterized in that the first liquid outlet is connected to the second inlet by a connecting line which contains a liquid seal device which is arranged to establish a pressure seal between the first inner chamber and the second inner chamber.

Description

[0001] This invention relates to a method for preparing monomer-poor polyisocyanates and an apparatus for distilling polyisocyanates.

[0002] Modification reactions of aliphatic and cycloaliphatic diisocyanates are well-known. The resulting polyisocyanates are used as crosslinking agents in coating systems and adhesives. On one hand, common modification reactions involve the isocyanate reacting with itself, for example, to form biuret, isocyanurate, urea diketone, or iminooxadiazine diketone. On the other hand, isocyanates can react with polyols or polyamines to undergo oligomerization, generating urethane, urethane, and / or urea groups. The key is the formation of higher molecular weight adducts with vapor pressures lower than the monomeric diisocyanate itself. Unconverted diisocyanates are separated from the reaction mixture by, for example, thin-film distillation, leaving the polyisocyanate as a bottom product, which can be diluted with a solvent if necessary.

[0003] For example, WO2008 / 068198A1 describes in detail the catalytic preparation of polyisocyanates and the distillation removal of unconverted diisocyanates from polyisocyanate mixtures. Monomeric diisocyanates are preferably removed by multi-stage distillation under vacuum at temperatures of 90 to 220°C. The pressure is preferably decreased progressively, reaching 0.1 to 10 hPa in the final stage. The equipment used includes flash evaporators, falling film evaporators, thin-film evaporators, and / or short-path evaporators, with short-path evaporators being preferred for the final stage distillation due to their low pressure drop, thus achieving particularly low pressure. This allows the monomeric diisocyanate to be removed gently. It should be particularly noted that the heat load should be kept as low as possible, for example by omitting intermediate containers and tanks or by keeping the bottom volume small and the piping short. The distillate, i.e., the monomeric diisocyanate, is preferably returned to the reaction, and may be subjected to additional treatment, such as filtration, if necessary, to improve the color value. The condensation of the distillate and related issues are not discussed.

[0004] DE102004038784A1 also describes the removal of unconverted diisocyanates from a mixture of polyisocyanates by distillation. This distillation is carried out using at least one short-path evaporator, optionally in combination with other evaporators. The mentioned process conditions are 5 mbar to 10 mbar. -4 mbar, evaporator temperature is 30 to 230°C. In one embodiment, the monomeric diphenylmethane diisocyanate is separated by distillation at 0.05 mbar and 177°C, and the evaporated monomeric diphenylmethane diisocyanate is condensed at 50°C.

[0005] EP1426393A2 describes the preparation of monomer-poor polyisocyanates containing urea diketone groups. Examples describe the removal of hexamethylene diisocyanate from a mixture of polyisocyanates by distillation. This is carried out using a short-path evaporator with a pre-evaporator at a pressure of 0.1 to 0.5 mbar and a heating medium temperature of 140 to 150 °C. This separates the unconverted monomer and catalyst, and the distillate is reused in the reaction.

[0006] WO2021008908A1 also describes a method for distilling polyisocyanates. Here, the vapor is condensed in stages at at least two different condenser temperatures. The distillation itself is also performed in multiple stages, with the pressure preferably decreasing progressively in each stage. The pressure of the first stage is preferably 2 to 100 mbar, and the pressure of the last stage is preferably 0.005 to 10 mbar. This embodiment describes a three-stage vacuum system, wherein each distillation stage has one vacuum pump, which are connected in series. The connections are as follows: the first distillation stage at a pressure of 25 mbar is located between the second and third vacuum pumps; the second distillation stage at a pressure of 7 mbar is located between the first and second vacuum pumps; and the final distillation stage at a pressure of 1.4 mbar is located on the suction side of the first vacuum pump.

[0007] In summary, it can be seen that the distillation separation of monomeric diisocyanates from polyisocyanates requires extremely low pressure. Because the monomer content in the feedstock is sometimes quite high, industrial-scale distillation is typically multi-stage, with different pressures at each stage, especially the final stage, which is fed only a small amount of monomer and operates at extremely low pressure. Industrial-scale single-stage distillation is limited by the monomer vapor pressure and requires very large equipment, which is often difficult to achieve the required pressure levels, and is accompanied by extremely high investment costs.

[0008] Pressure fluctuations in the distillation system are detrimental to operation. Firstly, droplets of higher-boiling-point polyisocyanates are heavily entrained by the distilled monomers. This not only leads to yield losses but also creates stubborn deposits at the top of the respective distillation units and in subsequent piping and equipment, such as condensers. Reusing the distillate for further polyisocyanate production also results in quality losses. Secondly, monomer separation from the polyisocyanates is affected, leading to excessively high residual monomer content in the polyisocyanates under otherwise identical conditions.

[0009] It has been found that pressure fluctuations observed when polyisocyanates are distilled in a multi-stage distillation system consisting of one or more pre-evaporators and a thin-film evaporator are often attributable to insufficient separation of the pressure stages. In principle, there are many methods to achieve separation, but many of these methods either fail to avoid the aforementioned problems or introduce other drawbacks.

[0010] Therefore, the objective is to provide a method and apparatus for multi-stage distillation of polyisocyanates, wherein reliable and low-maintenance pressure stage separations are provided, enabling constant operation. Various considerations and experiments have been conducted to this end in this invention.

[0011] As a simple and commonly used measure, installing a spring-loaded pressure-holding valve in the connecting pipe between the last pre-evaporator and the thin-film evaporator can be considered. However, the typically small pressure differential below the common operating range of such valves causes problems with this measure. Even if a pressure-holding valve with a matching operating range is found, it will cause fluctuations in the feed flow to the thin-film evaporator during operation, and the thin-film evaporator is sensitive to such fluctuations. In addition, due to the high molecular weight components in the polyisocyanate composition, the moving parts of the pressure-holding valve are at risk of sticking together in the long run, thus affecting its function.

[0012] Another approach is to implement level regulation using a regulating valve in the separator of the pre-evaporator. This allows for the adjustment of the outflow of unevaporated components from the polyisocyanate composition to maintain a certain level at the bottom of the separator, ensuring the required pressure seal, while reducing fluctuations compared to using a pressure-holding valve. However, this method is more expensive due to the need for corresponding sensors and actuators; it is also particularly prone to error. During continuous operation, level measurement can repeatedly fail due to the higher molecular weight components in the polyisocyanate composition causing the regulating valve to stick or become clogged.

[0013] This also applies to methods such as using gear pumps for level regulation, which are commonly used for uniform and low-pulsation delivery and can form a pressure seal. However, the delivered medium may cause moving parts to stick. Since it is a forced-flow pump, there is also a risk of uncontrolled pressure increases, requiring appropriate measures to ensure safety, which will further increase investment costs and maintenance needs. Furthermore, it is necessary, for example, to regulate the level upstream of the pump to ensure it does not run dry.

[0014] Orifice plates can also be considered to reduce the outflow of unevaporated components from polyisocyanate compositions. The advantage of this approach is that it requires no moving parts or complex conditioning techniques. However, the disadvantages are that even small pressure differentials can make orifice plate design difficult, and its functionality depends on throughput, being effective only within a very narrow load range, which is not conducive to operational practice.

[0015] Surprisingly, this can be achieved by integrating a liquid seal device into the connecting pipe between the last pre-evaporator and the thin-film evaporator. This ensures the free and constant outflow of the unevaporated components of the polyisocyanate composition, while simultaneously allowing it to act as a sealing fluid and establish a pressure seal. The operation of this liquid seal device is largely unaffected by load and requires no moving parts. The required pressure differential is simply achieved by the height provided to the sealing fluid column within the liquid seal device.

[0016] Different types of liquid seal devices can be used. Suitable liquid seal devices include, for example, tubular liquid seal devices, bottle-type liquid seal devices, baffle-type liquid seal devices, or bell-shaped liquid seal devices, which are typically designed in a U-shape. Compared to the other configurations mentioned, tubular liquid seal devices have the advantage of being less prone to deposit accumulation, but this comes with a larger space requirement. Regardless of the form of the liquid seal device used, it is advantageous to have a vent valve at or at least near the lowest point.

[0017] Therefore, the first subject of the present invention is a distillation apparatus for separating monomeric diisocyanates from polyisocyanate compositions, comprising or consisting of the following components: a. At least one pre-evaporation device, the pre-evaporation device comprising a first inlet for the polyisocyanate composition to be distilled, a pre-evaporator having a first inner chamber, a first gas outlet for the evaporated component, a first liquid outlet for the unevaporated component in the polyisocyanate composition, and at least one first condenser distributed to the first gas outlet. b. At least one thin-film evaporation device, the thin-film evaporation device comprising a second inlet for unevaporated components in a polyisocyanate composition, a thin-film evaporator having a second inner chamber, a second gas outlet for evaporated components, a second liquid outlet for a polyisocyanate composition from which monomeric diisocyanates have been substantially removed, and at least one second condenser distributed to the second gas outlet, and c. A vacuum generating device including at least one vacuum pump, configured to reach pressure P1 in the inner chamber of the pre-evaporator of the at least one pre-evaporation device and pressure P2 in the inner chamber of the thin-film evaporator of the at least one thin-film evaporation device, wherein P2 <P1, The feature is that the first liquid outlet is connected to the second inlet via a connecting pipe, the connecting pipe including a liquid seal device configured to establish a pressure seal between the first inner chamber and the second inner chamber.

[0018] In this invention, when the term "one / kind" is associated with a countable parameter, it should be understood only as an indefinite article, and the term is only indicated when explicitly stated (e.g., with the addition of "exactly one / kind"). Therefore, expressions such as "one pipe" or "one vacuum pump" do not preclude the possibility of the existence of more pipes or vacuum pumps.

[0019] Preferably, the expression "comprising" or "containing" according to the present invention means "consistent with...", more preferably "consisting with...".

[0020] In this document, "polyisocyanate" refers to isocyanates obtained by modifying diisocyanates, wherein at least two diisocyanate molecules are incorporated into the polyisocyanate. Such polyisocyanates are also referred to below as polyisocyanate compositions, and are commonly known as paint polyisocyanates. Therefore, "polyisocyanate" is not explicitly defined as isocyanates directly obtained from the phosgenation reaction of diamines or polyamines. Isocyanates obtained by phosgenation are referred to herein as monomeric isocyanates or monomeric diisocyanates.

[0021] The pre-evaporation unit, thin-film evaporation unit, vacuum generator, and connecting pipelines may independently include other equipment useful for the operation of the distillation unit, such as storage tanks, heat exchangers, mixing units, conveying units, inerting units, discharge containers, or regulating technologies.

[0022] As those skilled in the art will understand, the thin-film evaporation apparatus described herein refers to an evaporation apparatus in which the liquid film is distributed on a heated wall not only by gravity but also by a mechanical scraping system. This thin-film evaporation apparatus preferably includes one or more heating sections, i.e., externally heated pipes, within which the scraping system rotates. This scraping system preferably consists of a rotor and fixed or movable scrapers fixed thereon. One can distinguish between a wall-mounted rotor where the scrapers directly scrape along the heated wall and a non-wall-mounted rotor where the distance between the scrapers and the heated wall is very small, preferably 0.2 mm to 3 mm.

[0023] In a preferred embodiment of the distillation apparatus of the present invention, the liquid seal device is designed as a tubular liquid seal device, a bottle-type liquid seal device, a baffle-type liquid seal device, or a bell-shaped liquid seal device; more preferably, the liquid seal device is designed as a tubular liquid seal device or a bell-shaped liquid seal device, and most preferably, it is designed as a tubular liquid seal device. The liquid seal device is preferably configured to ensure that the sealing liquid height h is at least 25 mm, preferably at least 50 mm, more preferably at least 100 mm, and most preferably at least 150 mm. In principle, multiple liquid seal devices (liquid seal device 1, liquid seal device 2, ..., up to liquid seal device n) can also be integrated in series into a connecting pipe, which may be advantageous, for example, in situations where space is limited. In this case, the sum of the respective sealing liquid heights h(liquid seal device 1) + h(liquid seal device 2) + ... + h(liquid seal device n) is considered as the relevant sealing liquid height h. Preferably, the liquid seal device and / or other parts of the connecting pipe are designed to be heatable, for example, having a double jacket or electric heating.

[0024] Alternatively, instead of designing the connecting pipe as a single connecting pipe, two or more pipes arranged side by side can be used. In this case, in this invention, each pipe should be equipped with at least one liquid seal device for pressure sealing.

[0025] A preferred embodiment has exactly one connecting pipe, which has exactly one liquid seal device for pressure sealing.

[0026] Pre-evaporators are, for example, falling film evaporators, circulating evaporators, flash evaporators, plate evaporators, or tank evaporators. Falling film evaporators are particularly preferred due to their mild operating conditions, in which evaporation occurs by a thin film and therefore under conditions of small temperature difference between the evaporator surface and the polyisocyanate composition. Particularly suitable falling film evaporators are, for example, tube bundle downcomer evaporators having multiple evaporation tubes arranged substantially vertically and extending substantially in parallel. Multiple pre-evaporators can also be combined in a pre-evaporation unit. This can optionally be carried out in parallel or in at least two stages in series, wherein the liquid drawn from the bottom of one pre-evaporator then serves as the feed for another pre-evaporator.

[0027] Pre-evaporators typically have a distributor that ensures the polyisocyanate composition to be distilled is evenly distributed on the heating surface of the pre-evaporator. Optionally, the first gas outlet of the pre-evaporator may be equipped with a droplet separator to reduce the entrainment of polyisocyanate droplets by the distillate.

[0028] The first gas outlet is provided with at least one condenser configured for the primary condensation of vapors containing monomeric diisocyanate from the pre-evaporator. In a preferred embodiment, the first gas outlet is provided with at least two condensers, more preferably exactly two, which are connected to the pre-evaporator and the vacuum generator such that vapors from the pre-evaporator pass sequentially through these condensers in their journey to the vacuum system. Suitablely, the temperature of each subsequent condenser is equal to or lower than, preferably lower than, the temperature of the upstream condenser. Suitable condensers are, in particular, shell-and-tube heat exchangers. Typically, condensation occurs outside the pre-evaporator, and is therefore referred to herein as an external or externally located condenser. However, it is also conceivable to integrate one or more condensers as internal (or “internal”) condensers into the pre-evaporator, which in this case has a separate liquid drain for the condensed vapors.

[0029] Suitable thin-film evaporators include rigid rotor thin-film evaporators, scraped thin-film evaporators, or thin-film short-path evaporators.

[0030] Thin-film evaporators typically have a distributor that ensures the polyisocyanate composition is uniformly distributed onto the heating surface of the evaporator. Optionally, a second gas outlet, i.e., the gas outlet of the thin-film evaporator, can be equipped with a droplet separator to reduce the entrainment of polyisocyanate droplets by the distillate.

[0031] At least one condenser is assigned to the second gas outlet, which is configured to substantially condense the vapor containing monomeric diisocyanate from the thin-film evaporator. In a preferred embodiment, at least two condensers, more preferably exactly two or three condensers, are assigned to the second gas outlet, and these condensers are connected to the thin-film evaporator and the vacuum generating device such that the vapor from the thin-film evaporator sequentially passes through these condensers on its way to the vacuum system. Suitably, the temperature of each subsequent condenser is equal to or lower than, preferably lower than the temperature of the upstream condenser. Suitable condensers are in particular shell-and-tube heat exchangers. Usually, the condensation takes place outside the pre-evaporator, and is therefore referred to herein as an external or externally located condenser. However, it is also conceivable to integrate one or more condensers as internal (or "internally located") condensers into the pre-evaporator, which then has a separate liquid discharge port for condensing the vapor. For example, a short-path evaporator has such an internal (or "internally located") condenser, which therefore has a particularly low pressure drop and is preferably used for applications with particularly high vacuum requirements.

[0032] The vacuum generating device is connected to the pre-evaporation device and the thin-film evaporation device via a pipe or hose, preferably a pipe. This device comprises at least one vacuum pump. The vacuum generating device is configured to extract gas from the first inner chamber of the pre-evaporator through the first gas outlet, and extract gas from the second inner chamber of the thin-film evaporator through the second gas outlet. According to the present invention, the vacuum system is configured to achieve a pressure P1 in the first inner chamber of the pre-evaporator and a pressure P2 in the second inner chamber of the thin-film evaporator, where P2 < P1. The device of the present invention is preferably configured to achieve a pressure P1 in the inner chamber of the pre-evaporator that is from 100 Pa to 12000 Pa, preferably from 200 Pa to 4000 Pa, more preferably from 300 Pa to 2500 Pa, with the proviso that the achieved pressure P1 is higher than the pressure P2 in the second inner chamber of the thin-film evaporator. The device of the present invention is likewise preferably configured to achieve a pressure P2 in the second inner chamber of the thin-film evaporator that is from 0.1 Pa to 2000 Pa, preferably from 5 Pa to 1000 Pa, more preferably from 10 Pa to 100 Pa, with the proviso that the achieved pressure P2 is lower than the pressure P1 in the inner chamber of the pre-evaporator. The device of the present invention is most preferably configured to make P1 and P2 fall within the above ranges, with the proviso that P2 < P1, and the difference dP = P1 – P2 to be achieved is from 50 Pa to 10000 Pa, preferably from 80 Pa to 2500 Pa.

[0033] In principle, a suitable vacuum pump refers to any pump or compressor known to those skilled in the art for this application, such as a diaphragm pump, reciprocating piston pump, rotary vane pump, choke vane pump, rotary piston pump, screw compressor, or liquid ring compressor. Rotary piston pumps and / or liquid ring compressors are preferred as vacuum pumps. Particularly preferred is a combination of a pre-vacuum pump and one or more main vacuum pumps, wherein the pre-vacuum pump is preferably a liquid ring compressor, and the one or more main vacuum pumps are one or more rotary piston pumps.

[0034] In another preferred embodiment of the apparatus of the present invention, a heat exchanger is located downstream of the second liquid outlet of the thin-film evaporator (i.e., the outlet of the polyisocyanate composition for which monomeric diisocyanate has been substantially removed by distillation). This heat exchanger is configured to cool the composition, preferably rapidly. The size of the heat exchanger is preferably such that the polyisocyanate composition for which monomeric diisocyanate has been substantially removed cools to a temperature below 100°C, preferably below 98°C, and more preferably below 96°C, within less than 5 minutes, more preferably less than 1 minute, after leaving the thin-film evaporator. Temperature regulation ensures that the polyisocyanate composition for which monomeric diisocyanate has been substantially removed is not overcooled. Preferably, the heat exchanger and temperature regulation are configured to cool the polyisocyanate composition for which monomeric diisocyanate has been substantially removed to a temperature of 50°C to 100°C, more preferably 60°C to 98°C, and most preferably 70°C to 96°C. Suitable heat exchangers for cooling include, for example, plate heat exchangers, shell-and-tube heat exchangers, or two-tube heat exchangers.

[0035] Another subject of the invention is a method for separating monomeric diisocyanates from polyisocyanate compositions by at least two-stage distillation in a distillation apparatus comprising a pre-evaporator and a thin-film evaporator, characterized in that: • The pressure present in the inner chamber of the pre-evaporator is P1 and • The pressure inside the chamber of the thin-film evaporator is P2, where P2 <P1, The polyisocyanate composition of the depleted monomeric diisocyanate, which is the bottom product of the pre-evaporator, enters the thin-film evaporator through a connecting pipe containing a liquid seal device.

[0036] In a preferred embodiment of the method of the present invention, the outflowing depleted monomeric diisocyanate polyisocyanate composition itself serves as the sealing fluid in the liquid sealing device.

[0037] In another preferred embodiment of the method of the present invention, the pressure P1 is 100 Pa to 12000 Pa, preferably 200 Pa to 4000 Pa, more preferably 300 Pa to 2500 Pa, and / or the temperature of the pre-evaporator is 110°C to 200°C, preferably 120°C to 190°C, more preferably 135°C to 175°C; and the pressure P2 is 0.1 Pa to 2000 Pa, preferably 5 Pa to 1000 Pa, more preferably 10 Pa to 100 Pa, and / or the temperature of the thin-film evaporator is 120°C to 220°C, preferably 135°C to 190°C, more preferably 140°C to 175°C, wherein the difference dP = P1 – P2 is 50 Pa to 10000 Pa, preferably 80 Pa to 2500 Pa.

[0038] A liquid seal device in the connecting pipe is used to separate the vacuum stages in the pre-evaporator and the thin-film evaporator. Because it requires no moving parts, it is particularly suitable for current applications of polyisocyanate distillation. To operate substantially independently of load conditions, pressure and temperature fluctuations, or changes in feed composition, it is preferable that the sealing liquid in the liquid seal device forms a sealing liquid height h of at least 25 mm, preferably at least 50 mm, more preferably at least 100 mm, and most preferably at least 150 mm of pressure seal. The greater the pressure difference between the two distillation stages, the higher the sealing liquid height of the one or more liquid seal devices should be designed. Preferably, the value of the sealing liquid height h (in millimeters) corresponds to at least 10 times the value of the pressure difference dP (in Pascals). If this condition is met, other parameters (e.g., pressure drop in the connecting pipe) become less important in the design of the distillation apparatus. Insufficient separation between vacuum stages can lead to pressure fluctuations and thus problems. On the one hand, uncontrolled boiling may occur in the pre-evaporator, in which liquid is more severely entrained into the condenser; on the other hand, monomeric diisocyanate may permeate from the pre-evaporator into the thin-film evaporator, overloading the thin-film evaporator and thus failing to reliably achieve the desired product quality.

[0039] Polyisocyanates or polyisocyanate compositions are, for example, crude products of isocyanate oligomerization, still containing unconverted monomeric diisocyanate components. Oligomerization can be achieved by reacting the monomeric diisocyanate with itself or with other compounds (e.g., polyols, polythiols, or polyamines). The reaction is preferably carried out in the presence of at least one catalyst and terminated by deactivating the catalyst upon reaching a certain conversion. This deactivation can be achieved in various ways. Preferably, it is achieved by adding compounds that deactivate their respective catalysts. Some catalysts can also be deactivated by heat or removed from the reaction mixture to terminate the reaction. Depending on the reaction being carried out, the polyisocyanate composition preferably comprises urea dione groups, isocyanurate groups, carbamate groups, urea groups, biuret groups, iminooxadiazine dione groups, urethane groups, or combinations of these functional groups; more preferably, isocyanurate groups, carbamate groups, biuret groups, iminooxadiazine dione groups, or combinations of these functional groups; most preferably, isocyanurate groups and / or carbamate groups. These groups can exist in a single oligomer molecule and / or as a mixture of molecules with different functional groups.

[0040] The monomeric diisocyanates to be separated typically include all industrially available aliphatic, cycloaliphatic, aryliphatic, or aromatic diisocyanates with a boiling point lower than that of the polyisocyanate from which it should be separated, or a vapor pressure higher at a given temperature than that polyisocyanate. Preferably, the monomeric diisocyanate to be separated is a diisocyanate from which a polyisocyanate is generated through oligomerization, such that the polyisocyanate composition comprises primarily the polyisocyanate and unconverted monomeric diisocyanate, and optionally residual auxiliaries, such as catalysts, solvents, and / or terminators, or preferably consists thereof. More preferably, the diisocyanate is selected from 1,5-diisocyanate pentane (PDI), 1,6-diisocyanate hexane (HDI), 1-isocyanate-3,3,5-trimethyl-5-isocyanate methylcyclohexane (isophorone diisocyanate, IPDI), 4,4'-diisocyanate dicyclohexylmethane, 2,4'-diisocyanate dicyclohexylmethane, bis(isocyanate methyl)norbornene, 1,3- and 1,4-bis(isocyanate methyl)benzene (XDI), 2,4- and 2,6-toluene diisocyanate (hereinafter individually or collectively referred to as toluene diisocyanate or TDI), 2,4'- and 4,4'-diisocyanate diphenylmethane (MDI), or a mixture of two or more diisocyanates from the group thereof. More preferably, the monomeric diisocyanate to be separated is a diisocyanate selected from 1,5-diisocyanate pentane, 1,6-diisocyanate hexane, isophorone diisocyanate, and toluene diisocyanate, or a mixture of two or more diisocyanates from the group thereof. Most preferably, it is toluene diisocyanate, or a mixture of toluene diisocyanate and 1,6-diisocyanate hexane, or a mixture of toluene diisocyanate and 1,5-diisocyanate pentane.

[0041] The method of the present invention is particularly suitable for separating monomeric diisocyanates from polyisocyanate compositions, wherein the polyisocyanate composition before distillation preferably has a monomeric diisocyanate content (monomer content) of at least 40% by weight, preferably at least 50% by weight, more preferably at least 55% by weight, based on the entire polyisocyanate composition.

[0042] In the pre-evaporator, monomeric isocyanates are partially removed from the polyisocyanate composition by distillation, thereby obtaining a liquid, monomer-depleted polyisocyanate composition at the bottom of the pre-evaporator. This monomer-depleted polyisocyanate composition leaves the pre-evaporator as a bottom product, preferably having a monomeric diisocyanate content of 1.0% to 18.0% by weight, more preferably 3.0% to 15.0% by weight, and more preferably 5.0% to 12.0% by weight, based on the monomer-depleted polyisocyanate composition. This range of residual monomeric diisocyanate content is particularly advantageous because, on the one hand, it ensures that the viscosity of the monomer-depleted polyisocyanate composition is not too high, thereby facilitating better replacement of the sealing fluid in the liquid seal device and preventing the formation of high molecular weight deposits; on the other hand, it prevents the thin-film evaporator from being overloaded with excessive monomers, as the large pressure drop during distillation to remove residual monomers in the thin-film evaporator would adversely affect the vacuum in the inner chamber of the thin-film evaporator.

[0043] The resulting polyisocyanate composition, depleted of monomeric isocyanates, is introduced as a feed stream into the thin-film evaporator via a connecting pipe. It flows through a liquid seal device, which is preferably designed to retain a portion of the outflowing polyisocyanate composition within the liquid seal device, forming a pressure seal with a sealing liquid height of h. Therefore, the polyisocyanate composition preferably acts as the sealing liquid within the liquid seal device itself, establishing a pressure seal between the first inner chamber of the pre-evaporator and the second inner chamber of the thin-film evaporator. The sealing liquid here is continuously and at least partially replaced to maintain its substantially constant quality and is only minimally affected by aging processes (e.g., further increase in the molecular weight of the polyisocyanate component). If gas does bubble through the sealing liquid towards the thin-film evaporator, it passes in a controlled, uniform, and low-level manner, resulting in minimal pressure fluctuations in both inner chambers with negligible impact on the quality of the distillate.

[0044] In the thin-film evaporator, the separation of monomeric diisocyanate from the polyisocyanate composition is substantially completed. Preferably, the polyisocyanate composition with substantially removed monomeric diisocyanate produced as a bottom product contains up to 0.5% by weight of monomeric diisocyanate, based on the total mass of the polyisocyanate composition. More preferably, this content is up to 0.3% by weight, and most preferably up to 0.1% by weight.

[0045] In a preferred embodiment of the invention, the bottom discharge is cooled, preferably rapidly, after leaving the thin-film evaporator. The bottom discharge is preferably cooled to 50°C to 100°C, more preferably 60°C to 98°C, and most preferably 70°C to 96°C. This yields a polyisocyanate composition with only very low monomeric diisocyanate content. While not adhering to any particular theory, it is presumed that this method prevents or at least minimizes the reverse cracking of the polyisocyanate component and thus the regeneration of monomeric diisocyanates. Preferably, the cooling is achieved actively, for example in a heat exchanger, or immediately after discharge from the thin-film evaporator by quenching with the cooled product. To obtain a particularly monomer-poor product, the residence time of the product after leaving the thin-film evaporator at a temperature above 100°C, preferably above 98°C, more preferably above 96°C, should preferably be less than 5 minutes, more preferably less than 1 minute.

[0046] The method of the present invention can also be designed as a method for preparing monomer-poor polyisocyanate compositions, which includes further steps of transferring to a product storage tank, filling device, or mixing container for example, mixing the product with a solvent.

[0047] The distillation apparatus and method of the present invention can efficiently provide polyisocyanate compositions with diverse further applications. Another subject of the invention is the use of the distillation apparatus for preparing adhesive compositions or one- or two-component polyurethane coatings. The advantages of the invention are even more pronounced when used with high-viscosity polyisocyanate compositions because the key liquid-sealing device significantly reduces the risk of adhesion of moving parts without their presence, thereby achieving improved safety and equipment efficiency.

[0048] When used as a crosslinking agent in adhesives or two-component coatings, the polyisocyanate composition is typically combined with OH and / or NH components known per se in two-component polyurethane systems, such as hydroxyl-functionalized polyesters, polyacrylates, polycarbonates, polyethers, polyurethanes, and polyfunctionalized amines. However, they can also be used as single components in the preparation of (partially) moisture-curing plastics and coatings.

[0049] Preferably, the distillation apparatus of the present invention is used for preparing a composite material comprising at least one cured polyisocyanate composition in direct contact with at least one substrate selected from metals, plastics, wood, or mixtures thereof.

[0050] The present invention will now be described with reference to the accompanying drawings and the following embodiments, but is not limited thereto. The accompanying drawings show: Figure 1 A schematic diagram showing one embodiment of the distillation apparatus of the present invention; Figure 2 A, Figure 2 B. Figure 2 C and Figure 2 D shows schematic diagrams of various implementation schemes of the liquid sealing device 5.

[0051] Figure 1 A schematic diagram of one embodiment of the distillation apparatus 1 of the present invention for separating monomeric diisocyanates from a polyisocyanate composition is shown.

[0052] The distillation apparatus 1 consists of a pre-evaporator 2 with a pre-evaporator 101, a feed pipe 11 for the polyisocyanate composition, and a connecting pipe 12 containing a liquid seal device 5 that exits at a liquid outlet. Downstream of the liquid seal device 5, the connecting pipe 12 leads to a thin-film evaporator 3 and therein to the thin-film evaporator 102. The thin-film evaporator 102 has a liquid outlet from which a pipe 21, representing the product with the basic removal of the monomeric diisocyanate, exits. The pipe 21 may lead to, for example, a product storage tank, a filling device, or a mixing vessel for mixing the product, for example, with a solvent. On the gas side, the pre-evaporator 101 is connected to a condenser 111 via a vapor pipe 13, and the condenser 111 is in turn connected to a vacuum generator 4 via a pipe 15. The pipe 15 is installed between two vacuum pumps 121 and 122. A pipe 14 for condensing the distillate exits from the condenser 111. The thin-film evaporator 102 is equipped with an internally located condenser 112. Condenser 112 is supplied with a discharge pipe 25 for uncondensed vapor via a gas outlet. Discharge pipe 25 leads on the suction side to vacuum pump 122 of vacuum generator 4. Vacuum generator 4 is supplied with an exhaust pipe 26. The condensate distillate from condenser 112 exits through pipe 24. For example, the condensate distillate discharged through pipes 14 and 24 can be used as a raw material for further preparation of polyisocyanate compositions.

[0053] Figure 1 In the operation of the distillation apparatus 1 shown, in order to separate the monomeric diisocyanate from the polyisocyanate composition, the polyisocyanate composition is first introduced into the pre-evaporator 101 of the pre-evaporation apparatus 2 through the feed pipe 11, and separated into gas phase and liquid phase therein.

[0054] The liquid phase exits the pre-evaporator 101 via connecting pipe 12 and flows to the thin-film evaporator 102 of the thin-film evaporator 3. In the liquid seal device 5 of connecting pipe 12, the liquid phase forms a pressure seal as it flows through. The gaseous phase from the pre-evaporator 101 exits and flows to the condenser 111 via vapor pipe 13, where it is at least partially condensed. The condensed distillate is discharged via pipe 14, and the remaining gas flows through pipe 15 into the vacuum generator 4. The liquid phase entering the thin-film evaporator 102 via connecting pipe 12 is further purified therein and exits the thin-film evaporator via pipe 21 as a polyisocyanate composition with the basic removal of the monomer diisocyanate. For example, this product can be collected in a product storage tank, filling device, or mixing container where the product is mixed, for example, with a solvent. Uncondensed vapor in the thin-film evaporator 3 exits the condenser 112 via exhaust pipe 25 and flows to the suction side of the vacuum pump 122. The exhaust gas from the vacuum pump 121 exits the vacuum generator 4 via exhaust pipe 26. A distillate is obtained in condenser 112, and the condensed distillate is discharged from thin-film evaporator 3 through pipe 24. The condensed distillate from pipes 14 and 24 can, for example, be used as a feedstock for further preparation of polyisocyanate compositions.

[0055] Figure 2 Schematic diagrams of various implementation schemes of the liquid sealing device 5 are shown.

[0056] For example Figure 2 A shows a tubular liquid seal device. Figure 2 B shows a bottle-type liquid sealing device. Figure 2 C shows a baffle-type liquid seal device. Figure 2 Figure D shows a bell-shaped liquid seal device. Example

[0057] Unless otherwise stated, all percentage data are based on weight.

[0058] The residual monomer content was measured by gas chromatography with an internal standard, in each case based on the sum of HDI and TDI.

[0059] Example 1: Trimerization of a mixture of toluene-2,4-diisocyanate (TDI) and hexane-1,6-diisocyanate (HDI) occurs in the presence of a tributylphosphine catalyst. After the reaction is complete, a crude product is given, which still contains 60% by weight of monomeric diisocyanate (the sum of TDI and HDI).

[0060] Then, the crude product is... Figure 1Distillation is carried out in the distillation apparatus shown, where a downcomer evaporator with a downstream gas-liquid separator is used in the first stage. The pre-evaporator operates at 145°C with a pressure of approximately 300 Pa inside the chamber. The polyisocyanate obtained from the bottom outlet still contains approximately 10% by weight of residual monomeric diisocyanate (the sum of TDI and HDI). The product is discharged through a connecting pipe containing a bottle-type liquid seal device, which is structurally specified to have a liquid level of 25 mm. The connecting pipe leads to a thin-film evaporator with a mechanical scraper system. Here, distillation is completed at 162°C, where the pressure measured inside the thin-film evaporator fluctuates only slightly from 10 Pa to 100 Pa. Distillation can be run for several months without any problems with the bottom outlet from the pre-evaporator, requiring no downtime for maintenance. The time-averaged residual monomer content in the bottom product of the distillation is approximately 0.45% by weight.

[0061] Example 2 The distillation apparatus in Example 1 was improved. For this purpose, the connecting pipe containing the bottle-type liquid seal device was replaced with a new connecting pipe containing a tubular liquid seal device. The tubular liquid seal device is designed with a sealing liquid height of 150 mm.

[0062] The slight pressure fluctuations observed in Example 1 can be further reduced in the thin-film evaporator of the improved apparatus, so the pressure measured in the thin-film evaporator is now 10 Pa to 20 Pa. This also has a positive impact on product quality, so the residual monomer content in the bottom product of distillation is now constant at 0.30 wt% to 0.40 wt%.

[0063] Example 3 As a further improvement to Example 2, the temperature of the pre-evaporator was increased to 153°C. This further improves product quality, and the time-averaged residual monomer content in the bottom product of distillation can be reduced to 0.15% by weight.

[0064] Example 4 As a further improvement to Example 3, a heat exchanger (product cooler) was installed at the product outlet of the thin-film evaporator to rapidly cool the discharged resin to a temperature of 94°C in less than 1 minute. This further improves product quality, and the residual monomer content in the bottom product of distillation can be reduced to less than <0.1% by weight.

Claims

1. A distillation apparatus for separating monomeric diisocyanates from a polyisocyanate composition, comprising: • At least one pre-evaporation device, the pre-evaporation device comprising a first inlet for the polyisocyanate composition to be distilled, a pre-evaporator having a first inner chamber, a first gas outlet for the evaporated component, a first liquid outlet for the unevaporated component in the polyisocyanate composition, and at least one first condenser distributed to the first gas outlet. • At least one thin-film evaporation device, comprising a second inlet for unevaporated components in a polyisocyanate composition, a thin-film evaporator having a second inner chamber, a second gas outlet for evaporated components, a second liquid outlet for the polyisocyanate composition from which unconverted isocyanates have been substantially removed, and at least one second condenser distributed to the second gas outlet. • A vacuum generating device including at least one vacuum pump, configured to reach pressure P1 in the inner chamber of the pre-evaporator of the at least one pre-evaporation device and pressure P2 in the inner chamber of the thin-film evaporator of the at least one thin-film evaporation device, wherein P2 <P1, Its features are, The first liquid outlet is connected to the second inlet via a connecting pipe, which includes a liquid seal device configured to establish a pressure seal between the first and second inner chambers.

2. The distillation apparatus as described in claim 1, characterized in that, The liquid sealing device is designed as a tubular liquid sealing device, a bottle-type liquid sealing device, a baffle-type liquid sealing device, or a bell-shaped liquid sealing device, preferably as a tubular liquid sealing device or a bell-shaped liquid sealing device, and more preferably as a tubular liquid sealing device.

3. The distillation apparatus as described in any one of claims 1 or 2, characterized in that, The liquid sealing device is configured to achieve a sealing liquid height h of at least 25 mm, preferably at least 50 mm, more preferably at least 100 mm, and most preferably at least 150 mm.

4. The distillation apparatus according to any one of claims 1 to 3, characterized in that, The pre-evaporator is a falling film evaporator, a circulating evaporator, a flash evaporator, a plate evaporator, or a tank evaporator, preferably a tube bundle downcomer evaporator.

5. The distillation apparatus according to any one of claims 1 to 4, characterized in that, The thin-film evaporator is a rigid rotor thin-film evaporator, a scraped thin-film evaporator, or a thin-film short-path evaporator.

6. A method for separating monomeric diisocyanates from a polyisocyanate composition by at least two stages of distillation in a distillation apparatus comprising a pre-evaporator and a thin-film evaporator, characterized in that: • The pressure present in the inner chamber of the pre-evaporator is P1 and • The pressure inside the chamber of the thin-film evaporator is P2, where P2 <P1, The polyisocyanate composition of the depleted monomeric diisocyanate, which is the bottom product of the pre-evaporator, enters the thin-film evaporator through a connecting pipe containing a liquid seal device.

7. The method as described in claim 6, characterized in that, The polyisocyanate composition of the depleted monomeric diisocyanate flows out and acts as a sealing fluid in the liquid sealing device.

8. The method as described in any one of claims 6 or 7, characterized in that: • Pressure P1 is 100 Pa to 12000 Pa, and / or the temperature in the pre-evaporator is 110°C to 200°C. • Pressure P2 is from 0.1 Pa to 2000 Pa, and / or temperature is from 120°C to 220°C, and • The difference dP = P1 – P2 is between 50 Pa and 10000 Pa.

9. The method according to any one of claims 6 to 8, characterized in that, The sealing fluid in the liquid sealing device forms a pressure seal with a sealing fluid height h of at least 25 mm, preferably at least 50 mm, more preferably at least 100 mm, and most preferably at least 150 mm.

10. The method according to any one of claims 6 to 9, characterized in that, The polyisocyanate composition contains a urea dione group, an isocyanurate group, a carbamate group, a urea group, a biuret group, an iminooxadiazine dione group, a urea carbamate group, or a combination of these functional groups.

11. The method according to any one of claims 6 to 10, characterized in that, The monomeric diisocyanate to be separated is selected from: 1,5-diisocyanate pentane (PDI), 1,6-diisocyanate hexane (HDI), 1-isocyanate-3,3,5-trimethyl-5-isocyanate methylcyclohexane (isophorone diisocyanate, IPDI), 4,4'-diisocyanate dicyclohexylmethane, 2,4'-diisocyanate dicyclohexylmethane, bis(isocyanate methyl)norbornene, 1,3- and 1,4-bis(isocyanate methyl)benzene (XDI), toluene diisocyanate (TDI), 2,4'- and 4,4'-diisocyanate diphenylmethane (MDI), or a mixture of two or more diisocyanates in this group.

12. The method according to any one of claims 6 to 11, characterized in that, The monomer content of the polyisocyanate composition before distillation is at least 40% by weight, based on the whole polyisocyanate composition.

13. The method according to any one of claims 6 to 12, characterized in that, The monomer content of the polyisocyanate composition, which is a depleted monomer leaving the pre-evaporator as a bottom product, is from 1.0% to 18.0% by weight, based on the polyisocyanate composition.

14. The method according to any one of claims 6 to 13, characterized in that, The product taken from the bottom of the thin-film evaporator is cooled, wherein preferably, the bottom product remains at a temperature above 100°C, more preferably above 98°C, more preferably above 96°C for a maximum of 5 minutes, and more preferably a maximum of 1 minute after leaving the thin-film evaporator.

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