Isocyanate distillation method

Abstract

The present invention relates to a process for the distillative purification of crude isocyanates obtained by phosgenation of the corresponding isocyanates in a distillation apparatus under vacuum, characterized in that at least one liquid piston compressor is used to generate a vacuum in the distillation apparatus; a specific working fluid is used for the at least one liquid piston compressor; the pressure p on the suction side of the at least one liquid piston compressor is 10 to 200 mbar(a); the operating temperature of the at least one liquid piston compressor is -17°C to +15°C; and the working fluid at the outlet from the at least one liquid piston compressor has an AC value of less than 35,000 ppm. The present invention also relates to the corresponding use of at least one liquid piston compressor for generating a vacuum in an apparatus for distilling crude isocyanates, and to a corresponding method for operating a liquid piston compressor to provide a vacuum for the distillative purification of isocyanates obtained by phosgenation of the corresponding amine in a distillation apparatus to obtain the corresponding isocyanates.

Description

[Technical Field]

[0001] The present invention relates to a process for the distillation purification of crude isocyanates obtained by phosgenation of corresponding amines in a distillation apparatus under vacuum to obtain the corresponding isocyanates, characterized in that the vacuum is generated in the distillation apparatus using at least one liquid ring compressor, a specific working fluid is used for the at least one liquid ring compressor, the pressure p on the suction side of the at least one liquid ring compressor is 10 to 200 mbar(a), the working temperature of the at least one liquid ring compressor is -17°C to +15°C, and the working fluid at the outlet from the at least one liquid ring compressor has an AC value of less than 35,000 ppm. The present invention relates to a corresponding use of at least one liquid ring compressor for generating a vacuum in a distillation apparatus for crude isocyanates, and a corresponding process for operating a liquid ring compressor for providing a vacuum for the distillation purification of isocyanates obtained by phosgenation of the corresponding amines in a distillation apparatus to obtain the corresponding isocyanates.

[0002] According to the present invention, the term "isocyanate" should be understood to mean an organic compound having at least one isocyanate group obtained by phosgenation of the corresponding amine. [Background technology]

[0003] Various methods for isolating and purifying isocyanates from phosgenation reactions have been described in the prior art. In the most commonly used method, the crude solution from the phosgenation is first freed from excess phosgene. The solvent, usually chlorobenzene or dichlorobenzene, is then distilled off in a so-called solvent separation, after which the crude isocyanate thus obtained is then purified by distillation, usually in two or more stages. Many industrially important isocyanates are evaporated and removed from the final distillation column overhead as a vapor stream or, after condensation, as a liquid side stream.

[0004] The final distillation of various isocyanates obtained in each case by phosgenation of the corresponding amine is described, for example, in US Pat. No. 3,549,504. There, the distillation is carried out in the presence of an inert gas at a pressure at the top of the column of 5 to 200 mmHg, preferably 20 to 150 mmHg, so that the temperature at the bottom of the column can be kept as low as possible to avoid decomposition reactions. Exemplary embodiments cite a head pressure of 30 mmHg for the distillation of xylylene diisocyanate (XDI) and tolylene triisocyanate, 20 mmHg for diphenylmethane diisocyanate (MDI), 50 mmHg for hexamethylene diisocyanate (HDI), and 40 mmHg for tolylene diisocyanate (TDI).

[0005] WO2017 / 076551A1 describes a method for producing a diisocyanate containing TDI, MDI, HDI, pentamethylene diisocyanate (PDI), isophorone diisocyanate (IPDI), XDI, or dicyclohexylmethane diisocyanate (H 12 Dividing wall columns suitable for the distillation of isocyanates such as MDI are described, in which the desired isocyanate is removed in each case as a side stream, while low-boiling components are obtained at the top of the column. The distillation of a crude TDI mixture at a head pressure of 70 mbar is described as an exemplary embodiment.

[0006] Further description of a method for purifying isocyanates can be found in DE 10260092 A1. It describes first separating hydrogen chloride, phosgene, and solvent after isocyanate production to obtain a crude isocyanate stream. This still contains components with boiling points lower than those of the isocyanate, components with boiling points higher than those of the isocyanate, and non-evaporating residue. The crude isocyanate is subjected to crude distillation under vacuum at 1 to 120 mbar(a), followed by precision distillation in at least one column at 1 to 80 mbar(a). Additional isocyanate is recovered in vapor form from the residue obtained in the crude distillation at a pressure of 0.1 to 40 mbar(a) and is also fed to precision distillation. Distillation of TDI in a column with a head pressure of 18 mbar(a) is described as an exemplary embodiment.

[0007] DE 2242626 A1 describes a method for concentrating a phosgene-containing solution, in which an organic solvent is separated under vacuum from nitrogen- or oxygen-containing compounds with higher boiling points, such as isocyanates. The vacuum is generated using a liquid ring pump using a phosgene-containing solvent as the working fluid. In the described method, the solvent is separated from the isocyanates in an evaporator, with the isocyanates being obtained as a concentrate at the bottom of the evaporator. This is therefore a solvent separation as part of the workup of the crude product from the amine phosgenation mentioned earlier. The pressure range for operation of the evaporator is 5 to 2000 torr, preferably 50 to 500 torr, with an exemplary embodiment citing a pressure of 160 torr. It should be noted that a side stream of the working fluid can be recycled through a heat exchanger to remove the heat formed as a result of the drive and phosgene adsorption. It should also be noted that phosgene can accumulate in the working fluid, reducing the efficiency of the pump and therefore requiring continuous replacement. The absorption of phosgene into the working medium is simultaneously described as an advantage of this process, since it releases phosgene off-gassing. This document also describes the possibility of using the described process in the final distillation, i.e., the distillation of the isocyanate concentrate mentioned at the beginning, since in this distillation still bound phosgene can be separated and liberated.

[0008] However, in practice, the use of liquid ring compressors in the final distillation of isocyanates produced by amine phosgenation surprisingly often results in more or less pronounced cavitation, which manifests itself in the form of noise and even mechanical damage to the compressor. Lowering the operating temperature can counteract the problem, but this is only achievable at the expense of considerable equipment complexity and energy costs, and is limited at the latest by the freezing temperature of the respective working fluid. Cavitation problems arise particularly in the case of isocyanates, which have relatively low stability and therefore must be distilled as gently as possible; that is, their distillation requires the lowest possible pressure to largely avoid oligomerization and decomposition reactions, as is known from the prior art mentioned above.

[0009] Various approaches have been attempted in the past to prevent cavitation in liquid ring compressors. For example, DE 10019718 A1 describes using a cooling gas, preferably ambient air, to cool the suction gas before it enters the liquid ring compressor. While this procedure is advantageous for drawing in high-temperature vapors at high suction pressures, it is inadequate for the required low suction pressures, making it unsuitable for use in isocyanate distillation. The required operating temperature of a liquid ring compressor is always lower than ambient temperature, and the presence of volatile substances such as phosgene also exacerbates the cavitation problem.

[0010] WO 2006 / 029884 A1 describes a method for operating a liquid ring compressor in which an ionic liquid is used as the working fluid. This procedure also has several drawbacks. First, the method introduces ionic liquids, which are previously unnecessary and usually expensive substances, requiring procurement, storage, transportation, and disposal. Furthermore, although ionic liquids themselves have no vapor pressure, the accumulation of volatile components of the aspirated liquid in the liquid cannot be ruled out. Even assuming that volatile substances do not accumulate, this method has the disadvantage that the desired effect of absorbing phosgene from the aspirated gas no longer applies. Finally, due to the functional groups present in ionic liquids and the high reactivity of, for example, phosgene, undesired reactions between the ionic liquid and components of the aspirated liquid cannot be ruled out. [Prior art documents] [Patent documents]

[0011] [Patent Document 1] US3,549,504 [Patent Document 2] WO2017 / 076551A1 [Patent Document 3] DE10260092A1 [Patent Document 4] DE2242626A1 [Patent Document 5] DE10019718A1 [Patent Document 6] WO2006 / 029884A1 Summary of the Invention

[0012] Starting from this prior art, the object of the present invention is to distill isocyanates using a liquid ring compressor for vacuum generation, in which cavitation in the working liquid is avoided even at low suction pressures, in particular below 200 mbar(a).A further object of the present invention is to provide a method for generating vacuum, in which a solvent already used in the production process can be used as the working liquid, the off-gas can be purified from phosgene and / or hydrogen chloride, and in which there are no problems when liquid may be present in the suction stream in the form of droplets.

[0013] According to the invention, these objects are achieved by a process for the distillative purification of crude isocyanates obtained by phosgenation of the corresponding amines in a distillation apparatus under vacuum to obtain the corresponding isocyanates, wherein: - at least one liquid ring compressor is used to generate a vacuum in the distillation apparatus; a working fluid selected from the group consisting of chlorobenzene, dichlorobenzene, xylene, 1-chloro-2,4-dimethylbenzene, chlorotoluene and mixtures thereof is used for said at least one liquid ring compressor; - the pressure p on the suction side of the at least one liquid ring compressor is between 10 and 200 mbar(a); - the operating temperature of the at least one liquid ring compressor is between -17°C and +15°C; and the working fluid at the outlet from said at least one liquid ring compressor has an AC value of less than 35000 ppm.

[0014] The present invention further provides a process for the distillative purification of crude isocyanates obtained by phosgenation of the corresponding amines in a distillation apparatus under vacuum to obtain the corresponding isocyanates, comprising: - at least one liquid ring compressor is used to generate a vacuum in the distillation apparatus; a working fluid selected from the group consisting of chlorobenzene, dichlorobenzene, xylene, 1-chloro-2,4-dimethylbenzene, chlorotoluene and mixtures thereof is used for said at least one liquid ring compressor; - the pressure p on the suction side of the at least one liquid ring compressor is between 10 and 200 mbar(a); - the operating temperature of the at least one liquid ring compressor is between -17°C and +15°C; and the working fluid at the outlet from said at least one liquid ring compressor has an AC value of less than 35,000 ppm, wherein the determination of the AC value is carried out according to ASTM D 5629 for AC values ​​< 100 ppm and according to ASTM D 6099 for AC values ​​> 100 ppm, the working fluid replacing in each case isocyanate as matrix.

[0015] The above object is further achieved according to the invention by the use of at least one liquid ring compressor for generating a vacuum in a distillation apparatus for crude isocyanates produced by phosgenation of the corresponding amines, wherein: a working fluid selected from the group consisting of chlorobenzene, dichlorobenzene, xylene, 1-chloro-2,4-dimethylbenzene, chlorotoluene and mixtures thereof is used for said at least one liquid ring compressor; - the pressure p on the suction side of the at least one liquid ring compressor is between 10 and 200 mbar(a); - the operating temperature of the at least one liquid ring compressor is between -17°C and +15°C; and the working fluid at the outlet from said at least one liquid ring compressor has an AC value of less than 35000 ppm.

[0016] The above object is further achieved according to the invention by the use of at least one liquid ring compressor for generating a vacuum in a distillation apparatus for crude isocyanates produced by phosgenation of the corresponding amines, wherein: a working fluid selected from the group consisting of chlorobenzene, dichlorobenzene, xylene, 1-chloro-2,4-dimethylbenzene, chlorotoluene and mixtures thereof is used for said at least one liquid ring compressor; - the pressure p on the suction side of the at least one liquid ring compressor is between 10 and 200 mbar(a); - the operating temperature of the at least one liquid ring compressor is between -17°C and +15°C; and the working fluid at the outlet from said at least one liquid ring compressor has an AC value of less than 35,000 ppm, wherein the determination of the AC value is carried out according to ASTM D 5629 for AC values ​​< 100 ppm and according to ASTM D 6099 for AC values ​​> 100 ppm, the working fluid replacing in each case isocyanate as matrix.

[0017] The present invention preferably provides a use according to the invention, in which in the working fluid at the outlet from the liquid ring compressor the content of phosgene c(COCl2) is 0.001 to 4.5% by weight and the content of hydrogen chloride c(HCl) is less than 1.6% by weight.

[0018] The above object is further achieved according to the present invention by a method for operating a liquid ring compressor to provide a vacuum for the distillative purification of isocyanates obtained by phosgenation of the corresponding amine in a distillation apparatus to obtain the corresponding isocyanates, wherein: a working fluid selected from the group consisting of chlorobenzene, dichlorobenzene, xylene, 1-chloro-2,4-dimethylbenzene, chlorotoluene and mixtures thereof is used for said at least one liquid ring compressor; - the pressure p on the suction side of the at least one liquid ring compressor is between 10 and 200 mbar(a); - the operating temperature of the at least one liquid ring compressor is between -17°C and +15°C; and the working fluid at the outlet from said at least one liquid ring compressor has an AC value of less than 35000 ppm.

[0019] Finally, these objects are further achieved, according to the invention, by a method in which a liquid ring compressor is operated to provide a vacuum for the distillative purification of isocyanates obtained by phosgenation of the corresponding amine in a distillation apparatus to obtain the corresponding isocyanates, wherein: a working fluid selected from the group consisting of chlorobenzene, dichlorobenzene, xylene, 1-chloro-2,4-dimethylbenzene, chlorotoluene and mixtures thereof is used for said at least one liquid ring compressor; - the pressure p on the suction side of the at least one liquid ring compressor is between 10 and 200 mbar(a); - the operating temperature of the at least one liquid ring compressor is between -17°C and +15°C; and the working fluid at the outlet from the at least one liquid ring compressor has an AC value of less than 35,000 ppm, wherein the determination of the AC value is carried out according to ASTM D 5629 for AC values ​​<100 ppm and according to ASTM D 6099 for AC values ​​>100 ppm, the working fluid replacing in each case isocyanate as matrix.

[0020] The present invention will now be described in more detail, with the following enumerated references, details and preferred embodiments correspondingly applying to the method according to the invention for the distillative purification of crude isocyanates obtained by phosgenation of the corresponding amines, the use according to the invention of at least one liquid ring compressor for generating a vacuum in a distillation apparatus for crude isocyanates and the method according to the invention for operating a liquid ring compressor for providing a vacuum for the distillative purification of isocyanates obtained by phosgenation of the corresponding amines in a distillation apparatus.

[0021] In the context of the present invention, at least one liquid ring compressor is used. According to the present invention, it is possible to use one or more, for example 2 to 8, liquid ring compressors in a corresponding apparatus, in particular in a corresponding distillation apparatus, where, if two or more liquid ring compressors are present, one, some or all of the liquid ring compressors present may be operated according to the present invention.

[0022] The structure and operating principle of a liquid ring compressor are generally known to those skilled in the art. Its function is based on an impeller with blades attached to it, eccentrically positioned within a generally cylindrical housing partially filled with working fluid. During operation, the impeller rotates, and the working fluid present in the housing forms a liquid ring due to centrifugal force. This results in the formation of gas-filled chambers between the blades, which are separated on the outside by the liquid ring and have different sizes due to the eccentricity. The rotation of the impeller causes a continuous change in the size of the individual chambers, resulting in gas being drawn in from the suction port through one or more suction openings on the suction side, then compressed, and finally discharged through one or more pressure openings toward the pressure port on the pressure side. A portion of the working fluid forming the ring is always discharged along with the gas. This can be separated from the gas in a liquid separator and, in the context of the present invention, is referred to as the "working fluid at the outlet from the liquid ring compressor." This liquid can be reused as working fluid, optionally after cooling and / or mixing with additional sufficiently cold liquid. Some liquid ring compressor designs include additional withdrawal ports for working fluid, for example to create larger cooling circuits. Due to the intense mixing in the compressor, the liquid withdrawn here corresponds to the working fluid leaving with the gas at the pressure port.

[0023] A special form of liquid ring compressor is represented by the side channel pump, in which the impellers are arranged concentrically and the pumping effect is achieved by channels of various depths arranged transversely to them. However, these are rarely used as vacuum pumps.

[0024] The isocyanates / crude isocyanates which can be purified by the process according to the invention generally include all aliphatic, cycloaliphatic, araliphatic or aromatic isocyanates known to those skilled in the art.

[0025] The present invention particularly preferably provides a process according to the invention, in which the isocyanate is a monoisocyanate, in particular R,S-1-phenylethyl isocyanate, 1-methyl-3-phenylpropyl isocyanate, pentyl isocyanate, 6-methyl-2-heptaisocyanate, cyclopentyl isocyanate or 3-(methylthio)phenyl isocyanate, a diisocyanate, in particular hexamethylene diisocyanate (HDI), pentamethylene diisocyanate (PDI), 1,4-butane diisocyanate, anate, 1,8-diisocyanatooctane, 1,9-diisocyanatononane, 2-methylpentamethylene diisocyanate, 2,2-dimethylpentamethylene diisocyanate, neopentane diisocyanate, 2,2,4-trimethylhexamethylene diisocyanate and 2,4,4-trimethylhexamethylene diisocyanate, 3-isocyanatomethyl-3,5,5-trimethylcyclohexyl isocyanate (IPDI), 2,4'- and 4,4'-diisocyanatodicyclohexylmethane (H 12MDI), 4,4'-diisocyanato-3,3'-dimethyldicyclohexylmethane, 2,4- and 2,6-diisocyanatomethylcyclohexane (H6TDI), 1,3-cyclohexane diisocyanate, 1,4-cyclohexane diisocyanate, 1,3-diisocyanato-2-methylcyclohexane, 1,3-diisocyanato-4-methylcyclohexane, 1,4-diisocyanato-3,3,5-trimethylcyclohexane, 1,3-bis(isocyanatomethyl)cyclohexane (H6XDI), 1,4-bis(isocyanatomethyl)cyclohexane, isomers of bis(isocyanatomethyl)bicyclo[2.2.1]heptane (NBDI), xylylene diisocyanate cyanates (XDI), in particular 1,3-xylylene diisocyanate (m-XDI) or 1,4-xylylene diisocyanate (p-XDI), 1,3-bis(1-isocyanato-1-methylethyl)benzene (m-TMXDI), 1,4-bis(1-isocyanato-1-methylethyl)benzene (p-TMXDI), monomeric diphenylmethane diisocyanate, 2,4-tolylene diisocyanate, 2,6-tolylene diisocyanate, 1,5-naphthalene diisocyanate or 1,4-phenylene diisocyanate, triisocyanates, in particular 4-isocyanatomethyl-1,8-octane diisocyanate, and mixtures thereof.

[0026] The method according to the invention is particularly preferably used to purify isocyanates selected from the group consisting of HDI, PDI, IPDI, H12MDI, XDI, H6-XDI, H6-TDI, MDI, NDI, and mixtures thereof. This method is very particularly preferably used to purify HDI, XDI, PDI, or TDI. PDI and XDI, especially PDI, are obtained by phosgenation, which contains a high proportion of chlorinated by-products, which can be removed under thermal stress by removing volatile compounds such as phosgene or hydrogen chloride. Therefore, the method according to the invention is particularly advantageous for the distillation of PDI and XDI, and most advantageously for the distillation of PDI.

[0027] The methods for producing the isocyanates used according to the present invention are known to those skilled in the art. The organic isocyanates produced by the phosgenation of the corresponding amines always contain different low-boiling and high-boiling, partially chlorine-containing by-products. The terms "low-boiling" and "high-boiling" should be considered in relation to the isocyanate. It is not important for the process according to the present invention whether the phosgenation is carried out in the gas phase or in the liquid phase, and whether the amine reacts with phosgene directly, for example, in the base phosgenation known to those skilled in the art, or whether it is first converted into a salt with an acidic compound such as hydrogen chloride, as in the hydrochloride phosgenation known to those skilled in the art, or with an acidic compound such as carbon dioxide, as in the carbamate phosgenation known to those skilled in the art, and then reacted with phosgene. Industrial processes for all variants are well known and are briefly outlined below.

[0028] The present invention therefore provides a process according to the invention, wherein the crude isocyanate originates from gas phase or liquid phase phosgenation, preferably from liquid phase phosgenation.

[0029] When the phosgenation is carried out in the gas phase, the amine is preferably evaporated and heated to a temperature in the range of 200°C to 600°C. Optionally, the evaporation and use of the amine vapor generated during the evaporation are carried out in the presence of an inert gas and / or vapor of an inert solvent. The inert gas is preferably nitrogen. Suitable inert solvents are, for example, chlorobenzene, dichlorobenzene, especially o-dichlorobenzene, xylene, 1-chloro-2,4-dimethylbenzene, chlorotoluene, especially o-chlorotoluene, or mixtures thereof. In particular, chlorobenzene and o-dichlorobenzene or mixtures thereof have proven technically advantageous and are preferred because, on the one hand, they are very inert under the reaction conditions, and on the other hand, due to their physical properties, especially their boiling point, they are generally easily separated from the isocyanate.

[0030] The phosgene used in the phosgenation is preferably used in excess relative to the amine. An amount of phosgene corresponding to 150% to 350% of the theoretical amount based on the ongoing phosgenation reaction is generally sufficient. The phosgene stream is preferably heated to a temperature in the range of 200°C to 600°C before the reaction.

[0031] To carry out the phosgenation, the preheated amine-containing stream and the similarly preheated phosgene stream are preferably passed continuously through a cylindrical reaction space and mixed therein. Suitable cylindrical reaction spaces are, for example, tubular reactors, typically made of steel, glass, alloy, and / or enameled steel. They generally have a length sufficient to allow complete reaction of the amine with phosgene under the process conditions. Furthermore, the dimensions of the reaction space are preferably selected so that a turbulent flow with a Reynolds number of at least 2500 prevails in the reaction space. This is generally ensured when the flow rate exceeds 90 m / s. Such a flow rate can be ensured by adjusting the appropriate pressure difference between the product line to the reaction space and the outlet from the reaction space. Typically, the pressure in the feed line is 200-300 mbar(g) and at the outlet from the reaction space is 150-200 mbar(g).

[0032] Once the phosgenation reaction in the reaction space is complete, the formed isocyanate is preferably removed from the mixture continuously leaving the reaction space. This can be done, for example, by selective condensation in an inert solvent such as chlorobenzene or dichlorobenzene. If the amine-containing stream already contains an inert solvent, it is preferable to use the same inert solvent here. The solvent temperature is preferably selected so that, on the one hand, it is higher than the decomposition temperature of the carbamic acid chloride corresponding to the isocyanate, and, on the other hand, the isocyanate condenses or dissolves in the solvent, while phosgene, hydrogen chloride, and any inert gases pass through the condensation stage in gaseous form. Solvent temperatures of 120°C to 200°C are particularly preferred.

[0033] The gas mixture passing through the condensation stage to recover at least one isocyanate is then preferably freed of excess phosgene in a manner known per se. This can be carried out by cold trapping, absorption in an inert solvent (e.g., chlorobenzene, MCB or dichlorobenzene, ODB) at temperatures between -10°C and 8°C, or by adsorption on activated carbon and hydrolysis. The hydrogen chloride gas passing through the phosgene recovery stage can preferably be recycled in a manner known per se to recover the chlorine required for the phosgene synthesis.

[0034] Another embodiment of phosgenation is the phosgenation of amines in a liquid phase. The reaction can then be carried out in a variety of ways, each of which is known to those skilled in the art. The amine is directly reacted with phosgene in an inert liquid medium, known as base phosgenation, or the amine is first converted to the corresponding salt by reacting with hydrogen chloride gas or carbon dioxide in an inert liquid medium, and then reacted with phosgene, known as hydrochloride or carbamate phosgenation. Suitable liquid media for all phosgenations include, in particular, chlorobenzene and / or dichlorobenzene.

[0035] In base phosgenation, the reaction is carried out in two stages in an inert liquid medium. Such reactions are described, for example, in W. Siefken, Liebigs Annalen der Chemie, 562 (1949) p. 96. In the first stage, low-temperature phosgenation, the temperature of the reaction mixture is preferably maintained in the range of 0°C to 100°C. This forms a suspension containing carbamic acid chloride, amine hydrochloride, and a small amount of free isocyanate. It is preferred to first charge a solution of phosgene in an inert solvent, followed by a solution or suspension of the amine in the same solvent, and optionally additional phosgene. This keeps the concentration of free amine low, thus suppressing the undesired formation of urea.

[0036] In the second stage, high-temperature phosgenation, the temperature is increased, preferably in the range of 120°C to 200°C. It is maintained in this range while feeding additional phosgene until the reaction to give the isocyanate is complete, i.e., until the evolution of HCl has ceased. Phosgene is advantageously used in excess. If necessary, the reaction can be carried out by introducing an inert gas in both the low-temperature and high-temperature phosgenation.

[0037] In hydrochloride or carbamate phosgenation, an amine is preferably first reacted with hydrogen chloride gas or carbon dioxide in an inert liquid medium to produce the corresponding salt. The reaction temperature during this salt formation is preferably in the range of 0°C to 80°C. The phosgenation step follows as a second step substantially similar to the high-temperature phosgenation from the base phosgenation described above. Thus, again, the temperature is preferably maintained in the range of 120°C to 200°C, and phosgene and, optionally, an inert gas are introduced into the reaction mixture until the reaction to form the isocyanate is complete. Again, it is preferable to use an excess of phosgene to promote the reaction.

[0038] After quenching the reaction in both the base phosgenation and the hydrochloride or carbamate phosgenation, it is preferred to blow off the remaining phosgene and hydrogen chloride gas with an inert gas, preferably nitrogen. If necessary, filtration can be carried out to remove solids, such as unconverted hydrochloride salt.

[0039] The reaction product initially produced in such a phosgenation reaction contains not only the desired isocyanate but also hydrogen chloride, phosgene, solvent residues, as well as impurities and non-evaporable residues. The crude isocyanate to be purified according to the present invention is preferably such a reaction product from which phosgene and hydrogen chloride have already been separated in one or more steps, preferably by distillation.

[0040] It is particularly preferred that the crude isocyanate to be purified by the present invention is a crude isocyanate from which most of the solvent has already been separated.Therefore, the crude isocyanate preferably contains only trace amounts of phosgene and hydrogen chloride and a small amount of solvent.It also contains other low-boiling and high-boiling secondary components and non-evaporative residues.At least some of the secondary components and / or non-evaporative residues are preferably chlorine-containing compounds.

[0041] The content of hydrolyzable chlorine (HC value) in the isocyanate can be determined in accordance with ISO 15028:2014 and is preferably 200 to 20,000 ppm based on the crude isocyanate used in accordance with the present invention.

[0042] According to the present invention, the crude isocyanate is distilled under vacuum to minimize yield losses due to oligomer formation and / or isocyanate decomposition. The distillation can, in principle, be carried out in any suitable distillation apparatus known to those skilled in the art. This generally includes at least one evaporator for evaporating at least a portion of the crude isocyanate and a condenser for at least partially condensing the vapor stream thus produced. The distillation apparatus preferably further includes at least one distillation column, preferably with or without a dividing wall. For example, to improve separation performance, it is preferable to use internals known to those skilled in the art, such as random packing, bubble cap trays, sieve trays, dual flow trays, and structured packing. According to the present invention, it is particularly preferable to use internals that result in a low pressure drop, such as structured packing, random packing, or dual flow trays.

[0043] The distillation apparatus used in accordance with the present invention is preferably fluidly connected to a device for generating a negative pressure in the distillation apparatus. According to the present invention, this device includes a liquid ring compressor. Liquid ring compressors have significant advantages over other compressors, such as dry compressors for distilling isocyanates from phosgenation. For example, they are less sensitive to liquid droplets entrained in the liquid and tend not to suffer from deposits due to the decomposition of organic compounds introduced with the vapor stream. They can simultaneously function as gas scrubbers, resulting in, for example, at least partial removal of phosgene present in the vapor stream, resulting in a less interesting off-gas stream. However, the negative pressure (suction pressure) achievable with liquid ring compressors is limited by their operating principle.

[0044] Therefore, according to the present invention, when particularly low pressures, e.g., 0.1 to 30 mbar(a), are established in the distillation apparatus, it is possible to use one or more pre-compressors with the same or different operating principles between the distillation apparatus and at least one liquid ring compressor present. These can include, for example, gas ejectors, screw compressors, gas ring pumps, or rotary blowers. In such an embodiment, it is advantageous to perform condensation between the pre-compressor and the liquid ring compressor, thereby minimizing the transport flow for the liquid ring compressor.

[0045] In a further embodiment of the present invention, a plurality of distillation apparatuses are similarly, preferably fluidly, connected to at least one liquid ring compressor, and may be selected, for example, from the group consisting of distillation apparatuses for separating solvents from the reaction output from which phosgene and hydrogen chloride have already been removed, distillation apparatuses for working up the reaction solvent, distillation apparatuses for concentrating the residue, distillation apparatuses for removing further low boilers from the crude isocyanate concentrate from which phosgene, hydrogen chloride and solvent have already been removed, and apparatuses for the final distillation of the crude isocyanate.

[0046] In a preferred embodiment, the off-gases from multiple distillation units are combined in a recovery conduit, from which they are passed together into the suction port of at least one liquid ring compressor. The off-gas streams from the various distillation units can each be subjected to gas scrubbing after condensation of the vapors. The off-gas streams can be individually or jointly scrubbed in any desired combination. Preferably, the individual gas streams are scrubbed before the off-gases are combined in the recovery conduit.

[0047] Suitable gas scrubbers include, in principle, all devices capable of contacting a gas stream with a liquid stream, i.e., for example, submerged scrubbers, spray scrubbers, randomly packed columns, fluidized bed scrubbers, or rotary scrubbers. Preference is given to those with a low pressure drop for the gas flowing through, for example, randomly packed columns or spray scrubbers. The uncondensed off-gas preferably passes through the gas scrubber from bottom to top, while the solvent passes through the scrubber countercurrently, i.e., from top to bottom.

[0048] Preferred solvents used for scrubbing the gas stream are those also mentioned above for the production of isocyanates, such as chlorobenzene, dichlorobenzene, especially o-dichlorobenzene, xylene, 1-chloro-2,4-dimethylbenzene, chlorotoluene, especially o- or m-chlorotoluene, or mixtures thereof, preferably chlorobenzene and / or o-dichlorobenzene. It is very particularly preferred to use the same solvent for scrubbing the gas stream also used for the production of isocyanates. The temperature of the solvent is preferably between -20°C and +10°C, particularly preferably below the operating temperature of at least one liquid ring compressor. In this preferred mode, volatile organic compounds and phosgene are at least partially scrubbed from the off-gas stream to obtain a pre-purified off-gas stream. In contrast, hydrogen chloride can only be removed to a very limited extent from the off-gas stream under these conditions and therefore remains largely in the off-gas stream. In fact, it is often the major component.

[0049] According to the invention, it is also possible to reduce the hydrogen chloride content by washing with a suitable inorganic washing medium, for example a solution of silver sulfate in concentrated sulfuric acid.

[0050] According to the invention, the at least one liquid ring compressor is generally operated at a pressure p (suction pressure) of 10 to 200 mbar(a), preferably 20 to 150 mbar(a), particularly preferably 30 to 120 mbar(a).

[0051] The pressure ranges described are sufficient for many distillation operations in the purification of crude isocyanate and allow distillation while largely avoiding decomposition reactions. According to the present invention, if lower pressures are required, this can be achieved as described above by using precompressors with different or the same operating principles.

[0052] In the method according to the present invention, suitable working fluids for generating the liquid rings in the at least one liquid ring compressor include solvents that can also be used in the production of isocyanates, such as chlorobenzene, dichlorobenzene, especially o-dichlorobenzene, xylene, 1-chloro-2,4-dimethylbenzene, chlorotoluene, especially o- or m-chlorotoluene, or mixtures thereof, preferably chlorobenzene and / or o-dichlorobenzene. These working fluids are largely inert under the operating conditions and have melting and boiling points that are favorable for use as working fluids. The working fluid is preferably the same solvent as that used in the production of isocyanates. This allows for the use of the same solvent in the production process, eliminating the need for additional post-treatment of the working fluid.

[0053] The operating temperature of the at least one liquid ring compressor should be understood to mean the temperature of the liquid discharged at the pressure port. The operating temperature of the at least one liquid ring compressor in the method according to the present invention is generally between -17°C and +15°C. Higher temperatures increase the vapor pressure of the working fluid and therefore reduce the possible suction pressure when operating the at least one liquid ring compressor. Lower operating temperatures increase the energy costs and complexity of the equipment for cooling the working fluid as well as its viscosity. The latter factor further increases the already high energy consumption for transporting the working fluid. The operating temperature of the liquid ring compressor is preferably between -15°C and +12°C, particularly preferably between -12°C and +10°C.

[0054] Even if the reaction product from the phosgenation has already been previously freed of hydrogen chloride and phosgene, the off-gas from the distillation apparatus obtained in the distillation purification of the crude isocyanate in the process according to the invention still typically contains a small amount of phosgene. This can be due to incomplete separation of phosgene in the previous distillation step, but it can also be formed, for example, by removing phosgene from chlorine-containing low-boiling secondary components. This phosgene can be at least partially removed, preferably by using at least one liquid ring compressor according to the invention to generate a vacuum in the distillation apparatus. This reduces the amount of phosgene in the off-gas, thus making it safer to handle and easier to dispose of.

[0055] Therefore, phosgene is preferably at least partially dissolved in the working fluid so that the working fluid at the outlet of the at least one liquid ring compressor has a phosgene content of preferably 0.001% to 4.5% by weight, more preferably 0.005% to 2% by weight, particularly preferably 0.01% to 1% by weight, and most preferably 0.05% to 0.5% by weight. A low phosgene content requires a disproportionately high use of fresh low-phosgene or phosgene-free working fluid, which then requires post-treatment. Concentration to a higher content leads to an undesirable increase in the total amount of the harmful substance phosgene present in the plant and increases the risk of cavitation occurring in the at least one liquid ring compressor.

[0056] However, according to the present invention, it has been found that even if the phosgene content in the hydraulic fluid is kept low, cavitation problems often occur when the content of acidic chlorine compounds, also called AC value, AC content or simply acidity, increases. The determination of the AC value is carried out according to ASTM D 5629 for AC values ​​< 100 ppm and according to ASTM D 6099 for AC values ​​> 100 ppm, where the hydraulic fluid replaces isocyanate as the matrix.

[0057] For reliable operation of the at least one liquid ring compressor, it is necessary according to the invention that the working fluid at the outlet from the at least one liquid ring compressor has an AC value of less than 35,000 ppm, preferably less than 25,000 ppm, particularly preferably less than 15,000 ppm.

[0058] The presence of hydrogen chloride particularly increases the risk of cavitation. Therefore, in a preferred embodiment of the present invention, the method is operated so that the working fluid leaving the at least one liquid ring compressor at the pressure port has an HCl content of ≦1.6 wt.%, preferably ≦1 wt.%, particularly preferably ≦0.5 wt.%, and very particularly preferably ≦0.2 wt.%. The HCl content is preferably at least 1 ppm, particularly preferably at least 10 ppm, very particularly preferably at least 20 ppm, and most preferably at least 50 ppm.

[0059] This allows cavitation to be significantly reduced or even completely avoided in the method according to the present invention. Those skilled in the art will recognize that factors that contribute to the occurrence of cavitation include the operating temperature, suction pressure, and any gas ballast containing an inert gas. Higher operating temperatures, lower suction pressures, and low or insufficient gas ballast have negative consequences, and therefore, in such cases, it is desirable to more strictly limit the HCl content. Furthermore, as mentioned above, the phosgene content in the working fluid also plays a role. The tendency for cavitation increases with increasing phosgene content in the working fluid, and therefore, it is advantageous to establish a lower hydrogen chloride content.

[0060] In a preferred embodiment, the invention provides a method according to the invention, in which the content of phosgene c(COCl2) in the working liquid at the outlet from the at least one liquid ring compressor is 0.001% by weight to 4.5% by weight, preferably 0.005% by weight to 2% by weight, particularly preferably 0.01% by weight to 1% by weight and most preferably 0.05% by weight to 0.5% by weight and the content of hydrogen chloride c(HCl) is ≦1.6% by weight, preferably ≦1% by weight, particularly preferably ≦0.5% by weight and very particularly preferably ≦0.2% by weight.

[0061] Therefore, in a particularly preferred embodiment, the content of hydrogen chloride in the working fluid upon leaving the compressor is such that the maximum content c max(HCl), where the result of the formula is less than 0.01%, the maximum content c of 0.01% by weight max (HCl) applies:

number

[0062] According to the present invention, the phosgene and HCl contents in the working fluid can be influenced in different ways, possibly independently of each other. As already mentioned above, for example, at least one off-gas stream from a distillation apparatus connected to the liquid ring compressor, preferably fluidically, i.e., contributing to the generation of vacuum in the distillation apparatus, can be subjected to gas washing in a gas scrubber. This allows the introduction of phosgene and / or hydrogen chloride into the liquid ring compressor to be influenced by the washing liquid used and operating conditions such as temperature, saturation, and liquid amount. Alternatively, the working fluid itself can be treated, for example, by stripping with an inert gas. This can be carried out in the at least one liquid ring compressor itself or externally in a separate device.

[0063] In a further preferred embodiment of the method according to the invention, the content of hydrogen chloride and / or phosgene in the working fluid is influenced by at least partially replacing the used working fluid with fresh / post-treated working fluid. A special case of this embodiment represents a method in which the working fluid is heated after leaving the liquid ring compressor to release and separate the previously dissolved volatile compounds, and then cooled again and reintroduced as working fluid into the liquid ring compressor.

[0064] Also, a portion of the working liquid is ejected together with the compressed gas at the pressure port. In a preferred embodiment of the method according to the invention, the liquid leaving the pressure port, preferably a liquid comprising gas and working liquid, is separated in a liquid separator after leaving the at least one liquid ring compressor. It is preferred that at least a portion of the separated working liquid is recycled via a recirculation conduit, i.e., to the inlet port for the working liquid, preferably after being cooled in a cooler. The cooling preferably performed according to the invention advantageously removes heat resulting, for example, from compression, condensation, and absorption, making it possible to establish an operating temperature in the desired range.

[0065] Preferably, a side stream of working fluid is continuously or discontinuously removed from the system and replaced with fresh or post-treated working fluid (hereinafter referred to as replacement working fluid). This advantageously allows the phosgene content in the working fluid at the outlet from the at least one liquid ring compressor to be adjusted to the range described herein. The replacement working fluid can be introduced into the system at various times. For example, the liquid can be introduced into a recirculation line, supplied via a separate working fluid port in the at least one liquid ring compressor itself, or introduced into a liquid separator. Preferably, before being introduced into the working fluid, the replacement working fluid is brought to a temperature that is 20°C or less, preferably 15°C or less, particularly preferably 10°C or less, and very particularly preferably 5°C or less higher than the operating temperature of the at least one liquid ring compressor. The temperature of the replacement working fluid is particularly preferably below the operating temperature of the liquid ring compressor and above the freezing temperature of the replacement working fluid. The replacement working fluid preferably has a phosgene content of ≦1000 ppm, preferably ≦100 ppm, particularly preferably ≦20 ppm. Furthermore, the exchange working fluid preferably contains ≦250 ppm, preferably ≦100 ppm, particularly preferably ≦50 ppm of hydrogen chloride. A hydrogen chloride content of <1 ppm, preferably <5 ppm, particularly preferably <10 ppm, does not substantially contribute to the improvement of the process and is therefore not necessary.

[0066] The invention therefore preferably provides a method according to the invention, in which at least a portion of the working liquid is continuously withdrawn from the at least one liquid ring compressor, cooled to a temperature that is at most 20°C, preferably at most 15°C, particularly preferably at most 10°C, very particularly preferably at most 5°C below the operating temperature of the at least one liquid ring compressor, and then continuously recycled to the at least one liquid ring compressor.

[0067] In a preferred embodiment, the exchange working fluid contains a solvent, preferably a solvent that has already been used in phosgenation and preferably has been post-treated by distillation. In this case, the solvent preferably has a phosgene content of ≦1000 ppm, preferably ≦100 ppm, particularly preferably ≦20 ppm, but also preferably ≧1 ppm, particularly preferably ≧5 ppm, particularly preferably ≧10 ppm. Further reduction of the phosgene content requires disproportionately high costs and complexity for solvent purification.

[0068] The invention therefore preferably provides a method according to the invention, in which the working fluid in at least one liquid ring compressor is at least partially, preferably continuously, replaced by a working fluid having a phosgene content of ≦1000 ppm, preferably ≦100 ppm, particularly preferably ≦20 ppm.

[0069] The present invention also provides isocyanates obtainable, preferably by the process according to the invention, and isocyanates obtainable. [Example]

[0070] How to determine the AC value (acidity) of a hydraulic fluid: The determination of the AC value (acidity) is carried out according to ASTM D 5629 for AC values ​​< 100 ppm and according to ASTM D 6099 for AC values ​​> 100 ppm, where the respective working fluid replaces the isocyanate referred to in the method as the matrix.

[0071] Method for determining the phosgene content in hydraulic fluids: The phosgene content of a working fluid can be determined by infrared spectroscopy. For this purpose, the absorption spectra of the corresponding pure working fluid and working fluid samples are measured. After subtracting the spectrum of the pure working fluid, the absorption spectrum of the working fluid at approximately 1806 cm -1 The intensity of the carbonyl band of phosgene is measured. The phosgene content is determined by calibration with samples each having a known phosgene content, taking into account their origin. Measurements are carried out in CaF2 cuvettes of appropriate path length, and the absorbance is measured at 4 cm after dilution, if necessary. -1 The resolution should be within the range of 0.01-1.50 for at least four scans.

[0072] Determination of HCl content in hydraulic fluid: In this case, the HCl content (hydrogen chloride content) of the working fluid is the difference between the phosgene content and the AC value (acidity) of the working fluid.

[0073] Example 1: Hexamethylene diisocyanate, obtained from the vapor-phase phosgenation of hexamethylenediamine, from which hydrogen chloride, phosgene, chlorobenzene, and low-boiling secondary components had already been largely removed in a multi-stage distillation process, was subjected to a continuous final distillation under vacuum as crude isocyanate. The crude isocyanate had an HC content of approximately 8000 ppm according to ISO 15028:2014. Distillation was carried out in a column equipped with an internal condenser at a head pressure of 50 mbar(a). The vapor leaving the top of the column was washed with chlorobenzene in a gas scrubber. The washed vapor stream was cooled to -11 °C in a condenser and suctioned into a liquid ring compressor to generate a negative pressure. A pressure sensor installed just upstream of the suction port indicated a suction pressure of 30 mbar. The liquid ring compressor was operated in recirculation mode at an operating temperature of -2 °C using chlorobenzene as the working fluid. That is, the outgoing working fluid was recycled via a condenser back to the working fluid inlet. A portion of the working fluid was continuously discharged and replaced with redistilled chlorobenzene having a phosgene content of 10 ppm and a hydrogen chloride content of 2 ppm. The amount of working fluid exchanged was adjusted at regular intervals so as to establish an AC value of 2150 ppm in the working fluid leaving on the pressure side. The phosgene content was determined to be 0.25% by weight, and the hydrogen chloride content calculated accordingly was 0.03% by weight.

[0074] The liquid ring compressor was able to operate for extended periods without cavitation problems.

[0075] Example 2: The distillation was carried out as in Example 1, except that this time the off-gas stream was combined with the off-gas stream from the column for solvent separation from the reaction product in a recovery line. The off-gas stream from the solvent separation had hydrogen chloride as the main component, and also contained chlorobenzene and traces of phosgene. The combined off-gas stream from the recovery line was then subjected to the same further treatment as the scrubbed vapor stream from Example 1.

[0076] The exchange rate of the working fluid of the liquid ring compressor was selected so that an AC value of 9000 ppm was established in the working fluid leaving the pressure side. The phosgene content was determined to be 0.007 wt. % and the corresponding calculated hydrogen chloride content was 0.9 wt. %.

[0077] The liquid ring compressor was able to operate for a long time, but a slight cavitation noise was heard periodically.

[0078] Example 3: Starting from the operating conditions of Example 2, in the test run, the exchange rate of the working fluid was reduced and the AC value of the working fluid leaving the pressure side was increased to 18100 ppm. The phosgene content was determined to be 0.014 wt. % and the corresponding calculated hydrogen chloride content was 1.8 wt. %.

[0079] Operating the liquid ring compressor in this manner resulted in more intense cavitation noise, vibration and increased pressure measured at the inlet.

[0080] This problem could be overcome by lowering the operating temperature to -12°C and increasing the pressure in the suction port to 80 mbar(a).

[0081] Example 4: Tolylene diisocyanate, obtained from the vapor-phase phosgenation of tolylenediamine and from which hydrogen chloride, phosgene, and o-dichlorobenzene had already been largely removed in a multi-stage distillation process, was subjected to a continuous final distillation under vacuum as crude isocyanate. According to ISO 15028:2014, the crude isocyanate had an HC content of approximately 12,000 ppm. The distillation was carried out in a dividing-wall column with an internal condenser at a head pressure of 135 mbar (a). A stream containing low-boiling secondary components and o-dichlorobenzene was removed from the top of the column, and purified tolylene diisocyanate was obtained as a side stream. The vapor exiting the column top was washed with o-dichlorobenzene in a gas scrubber. The washed vapor stream was cooled to 2 °C in a condenser and suctioned through a liquid-ring compressor to generate a negative pressure. A pressure sensor installed just upstream of the suction port indicated a suction pressure of 120 mbar. The liquid ring compressor was operated in recirculation mode with o-dichlorobenzene as working fluid at an operating temperature of 7°C. That is, the outgoing working fluid was recycled via a cooler back to the working fluid inlet. A portion of the working fluid was continuously discharged and replaced with redistilled o-dichlorobenzene having a phosgene content of 10 ppm and a hydrogen chloride content of 2 ppm. The amount of working fluid exchanged was adjusted at regular intervals so that an AC value of 950 ppm was established in the working fluid leaving on the pressure side. The phosgene content was determined to be 0.02% by weight, and the hydrogen chloride content calculated accordingly was 0.08% by weight.

[0082] The liquid ring compressor was able to operate for extended periods without cavitation problems.

[0083] Comparative Example 5: Pentamethylene diisocyanate, obtained from the phosgenation of pentamethylenediamine and from which hydrogen chloride, phosgene, chlorobenzene, and low-boiling secondary components had already been largely removed in a multi-stage distillation process, was subjected to a continuous final distillation under vacuum as crude isocyanate. The crude isocyanate had an HC content of approximately 13,000 ppm according to ISO 15028:2014. Distillation was carried out in a column equipped with an internal condenser at a head pressure of 45 mbar(a). The vapor exiting the column top was cooled to -12 °C in a condenser without the use of a gas scrubber as described above and suctioned into a liquid ring compressor to generate a negative pressure. A pressure sensor installed just upstream of the suction port indicated a suction pressure of 30 mbar. The liquid ring compressor was operated in recirculation mode at an operating temperature of -9 °C using chlorobenzene as the working fluid. That is, the exiting working fluid was recycled via a condenser back to the working fluid inlet. A portion of the hydraulic fluid was continuously drained and replaced with redistilled chlorobenzene containing 10 ppm phosgene and 2 ppm hydrogen chloride.

[0084] There was severe cavitation noise, vibration and an increase in suction pressure. Analysis of the hydraulic fluid leaving the pressure side showed an AC value of 38,600 ppm. The phosgene content was determined to be 2.8 wt. % and the corresponding calculated hydrogen chloride content was 1.8 wt. %. Lowering the operating temperature to -14°C did not correct the problem.

[0085] The hydraulic fluid replacement rate was then tripled, resulting in a drop in the AC value to approximately 12900 ppm, normal operation returning, and therefore the operating temperature being able to be returned to -9°C without further problems.

Claims

1. 1. A process for the distillative purification of crude isocyanates obtained by phosgenation of the corresponding amines in a distillation apparatus under vacuum to obtain the corresponding isocyanates, comprising: at least one liquid ring compressor is used to generate a vacuum in the distillation apparatus; a working fluid selected from the group consisting of chlorobenzene, dichlorobenzene, xylene, 1-chloro-2,4-dimethylbenzene, chlorotoluene, and mixtures thereof is used for the at least one liquid ring compressor; the pressure p on the suction side of the at least one liquid ring compressor is 10 to 200 mbar(a); the operating temperature of the at least one liquid ring compressor is between −17° C. and +15° C.; and The method of claim 1, wherein the working fluid at the outlet from the at least one liquid ring compressor has an AC value of less than 35,000 ppm, wherein the determination of the AC value (acidity) is carried out according to ASTM D 5629 for AC values ​​< 100 ppm and according to ASTM D 6099 for AC values ​​> 100 ppm, wherein the respective working fluid substitutes for the isocyanate referred to in that test method as the matrix.

2. In the working liquid at the outlet from the at least one liquid ring compressor, the content of phosgene c(COCl ２ 2. The process according to claim 1, characterized in that the content of hydrogen chloride c(HCl) is 0.001 to 4.5% by weight and the content of hydrogen chloride c(HCl) is ≦1.6% by weight.

3. 3. The method according to claim 1 or 2, characterized in that at least a portion of the working liquid is continuously withdrawn from the at least one liquid ring compressor, cooled to a temperature not more than 20°C below the operating temperature of the at least one liquid ring compressor, and then continuously recycled to the at least one liquid ring compressor.

4. 4. The process according to claim 1, wherein the crude isocyanate is derived from a gas phase phosgenation or a liquid phase phosgenation.

5. 5. The method according to claim 1, wherein the working fluid in the at least one liquid ring compressor is at least partially replaced by a working fluid having a phosgene content of ≦1000 ppm.

6. The isocyanate may be a monoisocyanate, 1-methyl-3-phenylpropyl isocyanate, pentyl isocyanate, 6-methyl-2-heptane isocyanate, cyclopentyl isocyanate or 3-(methylthio)phenyl isocyanate, a diisocyanate, pentamethylene diisocyanate (PDI), 1,4-butane diisocyanate, 1,8-diisocyanatooctane, 1,9-diisocyanatononane, 2-Methylpentamethylene diisocyanate, 2,2-dimethylpentamethylene diisocyanate, neopentane diisocyanate, 2,2,4-trimethylhexamethylene diisocyanate and 2,4,4-trimethylhexamethylene diisocyanate, 3-isocyanatomethyl-3,5,5-trimethylcyclohexyl isocyanate (IPDI), 2,4'- and 4,4'-diisocyanatodicyclohexylmethane (H １２ MDI), 4,4'-diisocyanato-3,3'-dimethyldicyclohexylmethane, 2,4- and 2,6-diisocyanatomethylcyclohexane (H ６ TDI), 1,3-cyclohexane diisocyanate, 1,4-cyclohexane diisocyanate, 1,3-diisocyanato-2-methylcyclohexane, 1,3-diisocyanato-4-methyl-cyclohexane, 1,4-diisocyanato-3,3,5-trimethylcyclohexane, 1,3-bis(isocyanatomethyl)cyclohexane (H ６ 6. The method according to claim 1, wherein the isocyanate is selected from the group consisting of 1,4-bis(isocyanatomethyl)cyclohexane, isomeric bis(isocyanatomethyl)bicyclo[2.2.1]heptanes (NBDI), xylylene diisocyanate (XDI), 1,3-bis(1-isocyanato-1-methylethyl)benzene (m-TMXDI), 1,4-bis(1-isocyanato-1-methylethyl)benzene (p-TMXDI), monomeric diphenylmethane diisocyanate, 2,4-tolylene diisocyanate, 2,6-tolylene diisocyanate, 1,5-naphthalene diisocyanate or 1,4-phenylene diisocyanate, triisocyanates, and mixtures thereof.

7. 7. The method according to claim 1, wherein a pressure p of 10 to 200 mbar(a) is generated.

8. 1. Use of at least one liquid ring compressor for generating a vacuum in a distillation unit for crude isocyanates produced by phosgenation of the corresponding amines, comprising: a working fluid selected from the group consisting of chlorobenzene, dichlorobenzene, xylene, 1-chloro-2,4-dimethylbenzene, chlorotoluene, and mixtures thereof is used for the at least one liquid ring compressor; the pressure p on the suction side of the at least one liquid ring compressor is 10 to 200 mbar(a); the operating temperature of the at least one liquid ring compressor is between −17° C. and +15° C.; and The use, characterized in that the working fluid at the outlet from said at least one liquid ring compressor has an AC value of less than 35,000 ppm, wherein the determination of the AC value (acidity) is carried out according to ASTM D 5629 for AC values ​​< 100 ppm and according to ASTM D 6099 for AC values ​​> 100 ppm, wherein the respective working fluid substitutes for the isocyanate referred to in that test method as matrix.

9. In the working liquid at the outlet from the liquid ring compressor, the phosgene content c (COCl ２ 9. Use according to claim 8, characterized in that the content of hydrogen chloride c(HCl) is between 0.001 and 4.5% by weight and the content of hydrogen chloride c(HCl) is less than 1.6% by weight.

10. 10. Use according to claim 8 or 9, characterized in that a pressure p of 10 to 200 mbar(a) is generated.

11. 1. A process for the distillation purification of isocyanates obtained by phosgenation of corresponding amines in a distillation apparatus, comprising the steps of: operating a liquid ring compressor to provide a vacuum for the distillation purification of isocyanates obtained by phosgenation of corresponding amines in a distillation apparatus to obtain the corresponding isocyanates; a working fluid selected from the group consisting of chlorobenzene, dichlorobenzene, xylene, 1-chloro-2,4-dimethylbenzene, chlorotoluene, and mixtures thereof is used for at least one liquid ring compressor; the pressure p on the suction side of the at least one liquid ring compressor is 10 to 200 mbar(a); the operating temperature of the at least one liquid ring compressor is between −17° C. and +15° C.; and The method of claim 1, wherein the working fluid at the outlet from the at least one liquid ring compressor has an AC value of less than 35,000 ppm, wherein the determination of the AC value (acidity) is carried out according to ASTM D 5629 for AC values ​​< 100 ppm and according to ASTM D 6099 for AC values ​​> 100 ppm, wherein the respective working fluid substitutes for the isocyanate referred to in that test method as the matrix.

12. In the working liquid at the outlet from the liquid ring compressor, the phosgene content c (COCl ２ 12. The process according to claim 11, characterized in that the content of hydrogen chloride c(HCl) is between 0.001 and 4.5% by weight and the content of hydrogen chloride c(HCl) is less than 1.6% by weight.

13. The isocyanate may be a monoisocyanate, 1-methyl-3-phenylpropyl isocyanate, pentyl isocyanate, 6-methyl-2-heptane isocyanate, cyclopentyl isocyanate or 3-(methylthio)phenyl isocyanate, a diisocyanate, pentamethylene diisocyanate (PDI), 1,4-butane diisocyanate, 1,8-diisocyanatooctane, 1,9-diisocyanatononane, 2-Methylpentamethylene diisocyanate, 2,2-dimethylpentamethylene diisocyanate, neopentane diisocyanate, 2,2,4-trimethylhexamethylene diisocyanate and 2,4,4-trimethylhexamethylene diisocyanate, 3-isocyanatomethyl-3,5,5-trimethylcyclohexyl isocyanate (IPDI), 2,4'- and 4,4'-diisocyanatodicyclohexylmethane (H １２ MDI), 4,4'-diisocyanato-3,3'-dimethyldicyclohexylmethane, 2,4- and 2,6-diisocyanatomethylcyclohexane (H ６ TDI), 1,3-cyclohexane diisocyanate, 1,4-cyclohexane diisocyanate, 1,3-diisocyanato-2-methylcyclohexane, 1,3-diisocyanato-4-methyl-cyclohexane, 1,4-diisocyanato-3,3,5-trimethylcyclohexane, 1,3-bis(isocyanatomethyl)cyclohexane (H ６ 13. The method of claim 11 or 12, characterized in that the isocyanate is selected from the group consisting of 1,3-bis(1-isocyanato-1-methylethyl)benzene (m-TMXDI), 1,4-bis(isocyanatomethyl)cyclohexane, isomeric bis(isocyanatomethyl)bicyclo[2.2.1]heptanes (NBDI), xylylene diisocyanate (XDI), 1,3-bis(1-isocyanato-1-methylethyl)benzene (m-TMXDI), 1,4-bis(1-isocyanato-1-methylethyl)benzene (p-TMXDI), monomeric diphenylmethane diisocyanate, 2,4-tolylene diisocyanate, 2,6-tolylene diisocyanate, 1,5-naphthalene diisocyanate or 1,4-phenylene diisocyanate, triisocyanates, and mixtures thereof.

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