Isocyanate distillation method

Abstract

The present invention relates to a method for distillatively purifying crude isocyanates obtained by phosgenation of corresponding amines under vacuum in a distillation apparatus to obtain the corresponding isocyanates, characterized in that at least one liquid ring compressor is used to generate the vacuum in the distillation apparatus, a specific working liquid 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 from 10 to 200 mbar (absolute), the working temperature of the at least one liquid ring compressor is from -17°C to +15°C, and the working liquid at the outlet of the at least one liquid ring compressor has an AC value of less than 35000 ppm, to the corresponding use of at least one liquid ring compressor for generating the vacuum in a distillation apparatus for crude isocyanates, and to the corresponding method for operating the liquid ring compressor in order to provide a vacuum for the distillative purification of isocyanates obtained by phosgenation of the corresponding amines in a distillation apparatus to obtain the corresponding isocyanates.

Description

[0001] The present invention relates to a method for distillatively purifying crude isocyanates obtained by phosgenation of corresponding amines under vacuum in a distillation apparatus to obtain the corresponding isocyanates, characterized in that at least one liquid ring compressor is used to generate the vacuum in the distillation apparatus, a specific working liquid 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 from 10 to 200 mbar (absolute), the working temperature of the at least one liquid ring compressor is from -17°C to +15°C, and the working liquid at the outlet of the at least one liquid ring compressor has an AC value of less than 35000 ppm, to the corresponding use of at least one liquid ring compressor for generating the vacuum in a distillation apparatus for crude isocyanates, and to the corresponding method for operating the liquid ring compressor to provide vacuum for the distillative purification of isocyanates obtained by phosgenation of corresponding amines in a distillation apparatus to obtain the corresponding isocyanates.

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

[0003] Various methods for separating and purifying isocyanates from phosgenation reactions are described in the prior art. In the most commonly used methods, 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, and the crude isocyanate obtained is then purified by distillation, usually in multiple stages. Many industrially important isocyanates are evaporated here and taken off from the purification distillation column as a vapor stream at the top of the column or as a liquid side stream after condensation.

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

[0005] WO 2017 / 076551 A1 describes a process suitable for distilling isocyanates such as TDI, MDI, HDI, pentamethylene diisocyanate (PDI), isophorone diisocyanate (IPDI), XDI or dicyclohexylmethane diisocyanate (HDI). 12The distillation of a crude TDI mixture at a top pressure of 70 mbar is described as an example.

[0006] A further description of the method for purifying isocyanates can be found in DE 10260092 A1. This describes separating hydrogen chloride, phosgene and solvent after preparing isocyanates to obtain a crude isocyanate stream. It still contains components with a boiling point lower than that of isocyanates, components with a boiling point higher than that of isocyanates and residues that cannot be evaporated. Crude isocyanates are subjected to a crude distillation under vacuum at 1 to 120 mbars (absolute) and subsequently subjected to a rectification at 1 to 80 mbars (absolute) in at least one tower. Other isocyanates are recovered from the residue obtained in the crude distillation in a vapor form under a pressure of 0.1 to 40 mbars (absolute) and are also supplied to rectification. The distillation of TDI is described as an example, wherein the tower has a tower top pressure of 18 mbars (absolute).

[0007] DE 2242626 A1 describes a method for evaporative concentration of a phosgene-containing solution, in which an organic solvent is separated from a nitrogen- or oxygen-containing compound with a higher boiling point, such as an isocyanate, under vacuum. The vacuum is generated with a liquid ring pump, which uses a phosgene-containing solvent as a working fluid. In the method, the solvent is separated from the isocyanate in an evaporator, wherein the isocyanate is obtained as a concentrate at the bottom of the evaporator. This is therefore a solvent separation as part of the post-treatment of the crude product from the phosgenation of amines mentioned at the beginning. The pressure range mentioned for operating the evaporator is 5 to 2000 Torr, preferably 50 to 500 Torr, and the embodiment mentions a pressure of 160 Torr. It should be noted that it is possible to circulate a substream of the working fluid through a heat exchanger to output the heat formed by the drive device and the absorption of phosgene. It should also be noted that phosgene accumulates in the working fluid and can reduce the efficiency of the pump, so it must be replaced continuously. Phosgene absorption in the working medium is simultaneously described as an advantage of the method because phosgene is removed from the exhaust gas in this way. The document also indicates that it is possible to use the process in the final distillation, ie in the distillation of the isocyanate concentrate mentioned at the outset, since in this distillation the still bound phosgene can be cleaved off and liberated.

[0008] In practice, however, the use of liquid ring compressors in the purification distillation of isocyanates produced by phosgenation of amines surprisingly always leads to more or less pronounced cavitation, which manifests itself in the form of noise up to mechanical damage to the compressor. This problem can be counteracted by lowering the operating temperature, but this can only be achieved with considerable equipment and energy costs on the one hand and is limited at the latest by the solidification temperature of the respective operating fluid on the other hand. Cavitation problems occur in particular in the case of isocyanates which have a relatively low stability and therefore have to be distilled as gently as possible, i.e., their distillation requires, as is known from the above-cited prior art, as low a pressure as possible in order to substantially avoid oligomerization and decomposition reactions.

[0009] Various methods have been tried in the past to avoid cavitation in liquid ring compressors. For example, DE 10019718 A1 describes cooling the suction gas before entering the liquid ring compressor using a cooling gas, preferably ambient air. This procedure is advantageous for the suction of hot vapors at high suction pressures, but it is not suitable for the present case of isocyanate distillation, because this measure is insufficient for the required low suction pressures. The required operating temperature of liquid ring compressors is often below ambient temperature, and the presence of volatile substances, such as phosgene, also exacerbates the cavitation problem.

[0010] Document 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 various disadvantages. Firstly, due to the ionic liquid, previously unnecessary and usually expensive substances are introduced into the method, which require procurement, storage, transportation and disposal. In addition, although the ionic liquid itself has no vapor pressure, it cannot be ruled out that volatile components of the suction liquid accumulate in the liquid. Even if it is assumed that no volatile substances accumulate, the method also has the disadvantage that the desired effect of absorbing phosgene from the suction gas does not apply. Finally, due to the high reactivity of the functional groups present in the ionic liquid and, for example, phosgene, it is also impossible to rule out undesirable reactions between the ionic liquid and the components of the suction liquid.

[0011] Starting from the prior art, the object of the present invention is to generate a vacuum for distilling isocyanates by using a liquid ring compressor, wherein cavitation in the working liquid is avoided even at low suction pressures, in particular below 200 mbar (absolute). According to the invention, a method for generating a vacuum is also to be provided, in which the solvent which has already been used in the production process can be used as working liquid, in which the exhaust gas can be purified to remove phosgene and / or hydrogen chloride, and in which it is not a problem if liquid is possibly present in the form of droplets in the suction stream.

[0012] These objects are achieved according to the invention by a process for distillatively purifying the crude isocyanates obtained by phosgenation of the corresponding amines under vacuum in a distillation apparatus to give the corresponding isocyanates, wherein

[0013] - at least one liquid ring compressor for generating the vacuum in the distillation unit,

[0014] - 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,

[0015] - a pressure p on the suction side of the at least one liquid ring compressor of 10 to 200 mbar (absolute),

[0016] - the operating temperature of the at least one liquid ring compressor is -17°C to +15°C and

[0017] - The operating fluid at the outlet of said at least one liquid ring compressor has an AC value of less than 35000 ppm.

[0018] The present invention also provides a process for the distillative purification under vacuum in a distillation apparatus of crude isocyanates obtained by phosgenation of the corresponding amines to give the corresponding isocyanates, wherein

[0019] - at least one liquid ring compressor for generating the vacuum in the distillation unit,

[0020] - 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,

[0021] - a pressure p on the suction side of the at least one liquid ring compressor of 10 to 200 mbar (absolute),

[0022] - the operating temperature of the at least one liquid ring compressor is -17°C to 15°C and

[0023] - the operating fluid at the outlet of the at least one liquid ring compressor has an AC value of less than 35000 ppm, wherein the AC value is determined in accordance with ASTM D 5629 for AC values ​​<100 ppm and in accordance with ASTM D 6099 for AC values ​​>100 ppm, wherein the operating fluid replaces the isocyanate as matrix in each case.

[0024] According to the invention, these objects are furthermore achieved by the use of at least one liquid ring compressor for generating a vacuum in a distillation apparatus for a crude isocyanate prepared by phosgenation of the corresponding amine, wherein

[0025] - 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,

[0026] - a pressure p on the suction side of the at least one liquid ring compressor of 10 to 200 mbar (absolute),

[0027] - the operating temperature of the at least one liquid ring compressor is -17°C to +15°C and

[0028] - The operating fluid at the outlet of said at least one liquid ring compressor has an AC value of less than 35000 ppm.

[0029] According to the invention, these objects are furthermore achieved by the use of at least one liquid ring compressor for generating a vacuum in a distillation apparatus for a crude isocyanate prepared by phosgenation of the corresponding amine, wherein

[0030] - 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,

[0031] - a pressure p on the suction side of the at least one liquid ring compressor of 10 to 200 mbar (absolute),

[0032] - the operating temperature of the at least one liquid ring compressor is -17°C to +15°C and

[0033] - the operating fluid at the outlet of the at least one liquid ring compressor has an AC value of less than 35000 ppm, wherein the AC value is determined in accordance with ASTM D 5629 for AC values ​​<100 ppm and in accordance with ASTM D 6099 for AC values ​​>100 ppm, wherein the operating fluid replaces the isocyanate as matrix in each case.

[0034] The invention preferably relates to the use according to the invention, wherein the phosgene content c(COCl2) in the working fluid at the outlet of the liquid ring compressor is 0.001% to 4.5% by weight and the hydrogen chloride content c(HCl) is less than 1.6% by weight.

[0035] According to the invention, these objects are furthermore achieved by a process for operating a liquid ring compressor to provide a vacuum for the distillative purification of isocyanates obtained by phosgenation of the corresponding amines in a distillation apparatus to obtain the corresponding isocyanates, wherein

[0036] - 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,

[0037] - a pressure p on the suction side of the at least one liquid ring compressor of 10 to 200 mbar (absolute),

[0038] - the operating temperature of the at least one liquid ring compressor is -17°C to +15°C and

[0039] - The operating fluid at the outlet of said at least one liquid ring compressor has an AC value of less than 35000 ppm.

[0040] Finally, these objects are achieved according to the invention by a process for operating a liquid ring compressor to provide a vacuum for the distillative purification of isocyanates obtained by phosgenation of the corresponding amines in a distillation apparatus to obtain the corresponding isocyanates, wherein

[0041] - 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,

[0042] - a pressure p on the suction side of the at least one liquid ring compressor of 10 to 200 mbar (absolute),

[0043] - the operating temperature of the at least one liquid ring compressor is -17°C to +15°C and

[0044] - the operating fluid at the outlet of the at least one liquid ring compressor has an AC value of less than 35000 ppm, wherein the AC value is determined in accordance with ASTM D 5629 for AC values ​​<100 ppm and in accordance with ASTM D 6099 for AC values ​​>100 ppm, wherein the operating fluid replaces the isocyanate as matrix in each case.

[0045] The invention is described in detail below. The statements, details and preferred embodiments made below apply accordingly to the method according to the invention for distillative purification of crude isocyanates obtained by phosgenation of the corresponding amines, to 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 to the method according to the invention for operating a liquid ring compressor to provide a vacuum for distillative purification of isocyanates obtained by phosgenation of the corresponding amines in a distillation apparatus.

[0046] In 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 device, in particular a corresponding distillation device, wherein when more than one liquid ring compressor is present, one, some or all of these liquid ring compressors present can be operated according to the present invention.

[0047] The construction and operating principle of a liquid ring compressor are well known to those skilled in the art. This function is attributed to an impeller with blades fixed thereto, which is eccentrically arranged in a generally cylindrical housing that is partially filled with a working fluid. During operation, the impeller rotates, and the working fluid contained in the housing forms a liquid ring due to centrifugal force. Gas-filled chambers are thus formed between the blades, which are bounded on the outside by the liquid ring and have different sizes due to the eccentricity. Due to the rotation of the impeller, the size of the individual chambers changes continuously, so that gas is sucked in from the suction sleeve (Saugstutzen) through one or more suction openings on the suction side, then compressed, and finally ejected in the direction of the discharge sleeve (Druckstutzen) through one or more discharge openings on the discharge side. A portion of the working fluid that forms the ring is always ejected with the gas. It can be separated from the gas in a liquid separator and is referred to as "working fluid at the outlet of the liquid ring compressor" in the present invention. It can optionally be used as a working fluid again after cooling and / or mixing with another sufficiently cold liquid. Some construction forms of liquid ring compressors include additional extraction sleeves for working fluid, for example to construct a larger cooling circuit. Due to the strong intensive mixing in the compressor, the liquid removed here corresponds to the working fluid that leaves with the gas at the pressure outlet sleeve.

[0048] A special form of liquid ring compressor is the side channel pump, in which the impeller is arranged concentrically and the pumping effect is achieved by means of a channel-like space of varying depth which is present laterally relative to it. However, these are only rarely used as vacuum pumps.

[0049] Isocyanates or crude isocyanates which can be purified by the process according to the invention are generally all aliphatic, cycloaliphatic, araliphatic or aromatic isocyanates known to the person skilled in the art.

[0050] The present invention particularly preferably relates to the process according to the invention, wherein the isocyanate is selected from the group consisting of monoisocyanates, in particular R,S-1-phenylethyl isocyanate, 1-methyl-3-phenylpropyl isocyanate, amyl isocyanate, 6-methyl-2-heptane isocyanate, cyclopentyl isocyanate or 3-(methylthio)phenyl isocyanate, diisocyanates, in particular hexamethylene diisocyanate (HDI), pentamethylene diisocyanate (PDI), 1,4-butane diisocyanate, esters, 1,8-octane diisocyanate, 1,9-nonane diisocyanate, 2-methylpentane diisocyanate, 2,2-dimethylpentane diisocyanate, neopentane diisocyanate, 2,2,4-trimethylhexane diisocyanate and 2,4,4-trimethylhexane diisocyanate, 3-isocyanatomethyl-3,5,5-trimethylcyclohexyl isocyanate (IPDI), 2,4'- and 4,4'-dicyclohexylmethane diisocyanate (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-methyl-cyclohexane, 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) The invention also includes monomers, xylylene diisocyanate (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-toluene diisocyanate, 2,6-toluene diisocyanate, 1,5-naphthalene diisocyanate or 1,4-phenylene diisocyanate, triisocyanates, in particular 4-isocyanatomethyl-1,8-octane diisocyanate and mixtures thereof.

[0051] The process according to the invention is particularly preferably used for purifying isocyanates selected from the group consisting of HDI, PDI, IPDI, H12MDI, XDI, H6-XDI, H6-TDI, MDI, NDI and mixtures thereof. The process is very particularly preferably used for purifying HDI, XDI, PDI or TDI. Specifically PDI and XDI, especially PDI, are obtained in the phosgenation together with a high proportion of chlorinated by-products, from which volatile compounds such as phosgene or hydrogen chloride can be cleaved under thermal stress, so that the process according to the invention is particularly advantageous for the distillation of PDI and XDI, and most advantageous for the distillation of PDI.

[0052] The method for preparing the isocyanates used according to the invention is known to those skilled in the art. The organic isocyanates prepared by the phosgenation of the corresponding amines always contain various by-products with low boiling points and high boiling points, partly containing chlorine. The terms "low boiling point" and "high boiling point" should be considered relative to the isocyanates. It is not important for the method according to the invention whether the phosgenation is carried out in the gas phase or the liquid phase and whether the amine reacts directly with phosgene (for example, in the alkali phosgenation known to those skilled in the art) or first with an acidic compound, such as hydrogen chloride (such as in the hydrochloride phosgenation known to those skilled in the art) or carbon dioxide (such as in the carbamate phosgenation known to those skilled in the art) to be converted into salts and then react with phosgene. The industrial methods for all variants are well known and should be briefly summarized below.

[0053] The present invention therefore relates to the process according to the invention, wherein the crude isocyanate originates from gas-phase phosgenation or liquid-phase phosgenation, preferably from liquid-phase phosgenation.

[0054] If the phosgenation is carried out in the gas phase, the amine is preferably evaporated and heated to a temperature in the temperature range of 200° C. to 600° C. Optionally, the evaporation and the use of the amine vapor generated in 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. Chlorobenzene and o-dichlorobenzene or mixtures thereof have proven to be technically advantageous and preferred in particular, since under the reaction conditions, they behave very inertly on the one hand and, on the other hand, due to their physical properties, in particular due to their boiling point, are generally easy to separate from the isocyanate.

[0055] The phosgene used in the phosgenation is preferably used in excess, based on the amine. A phosgene amount corresponding to 150% to 350% of the theoretical value, 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.

[0056] In order to carry out phosgenation, preferably the preheated amine-containing stream and the same preheated phosgene stream are continuously fed into the cylindrical reaction space and mixed therein with each other. Suitable cylindrical reaction space is, for example, a tubular reactor usually composed of steel, glass, alloy steel or enamel steel. They usually have a length sufficient to enable amine and phosgene to react completely under process conditions. In addition, it is preferred to select the size of the reaction space so that there is a turbulent flow with a Reynolds number of at least 2500 in the reaction space. When the flow velocity is greater than 90 m / s, this is usually guaranteed. Such a flow velocity can be guaranteed by adjusting the corresponding pressure difference between the product conduit leading to the reaction space and the outlet of the reaction space. Generally speaking, the pressure in the feed conduit is 200 to 300 millibars (gauge pressure), and the pressure at the outlet of the reaction space is 150 to 200 millibars (gauge pressure).

[0057] After the phosgenation reaction has been completed in the reaction space, the mixture continuously leaving the reaction space is preferably freed from the isocyanate formed. This can be achieved, for example, by selective condensation in an inert solvent, such as chlorobenzene or dichlorobenzene. If the amine-containing stream already contains an inert solvent, the same inert solvent is preferably used here. The temperature of the solvent is preferably selected here so that, on the one hand, it is above the decomposition temperature of the carbamoyl chloride corresponding to the isocyanate, and on the other hand, the isocyanate condenses or dissolves in the solvent, while phosgene, hydrogen chloride and optionally an inert gas pass through the condensation stage in gaseous form. Solvent temperatures of 120° C. to 200° C. are particularly suitable.

[0058] The gas mixture which has passed through the condensation stage for recovering the at least one isocyanate is subsequently preferably freed from excess phosgene in a manner known per se. This can be achieved by using cold traps, absorption in an inert solvent (e.g. chlorobenzene MCB or dichlorobenzene ODB) at a temperature of -10°C to 8°C or adsorption and hydrolysis on activated carbon. The hydrogen chloride gas which has passed through the phosgene recovery stage can preferably be recycled in a manner known per se to recover the chlorine required for the phosgene synthesis.

[0059] Another embodiment of the phosgenation is the phosgenation of amines in the liquid phase. The reaction can be carried out in various ways known to the person skilled in the art. The amine is reacted directly with phosgene in an inert liquid medium (so-called base phosgenation), or the amine is first converted into the corresponding salt by reaction with hydrogen chloride gas or carbon dioxide in an inert liquid medium and then reacted with phosgene (so-called hydrochloride or carbamate phosgenation). Suitable liquid media for all phosgenations are in particular chlorobenzene and / or dichlorobenzene.

[0060] In the alkaline phosgenation, the reaction is carried out in two stages in an inert liquid medium. Such a reaction is described, for example, in W. Siefken, Liebigs Annalen der Chemie, 562 (1949) p. 96. In the first stage, the cold phosgenation, the temperature of the reaction mixture is preferably kept at 0° C. to 100° C. This forms a suspension containing carbamoyl chloride, amine hydrochloride and a small amount of free isocyanate. Preferably, a solution of phosgene in an inert solvent is initially loaded, and then a solution or suspension of an amine in the same solvent and optionally further phosgene are added. The concentration of free amine is thus kept low, thus suppressing undesirable urea formation.

[0061] In the second stage, the hot phosgenation, the temperature is increased and is preferably between 120° C. and 200° C. It is maintained in this range while further supplying phosgene until the reaction providing the isocyanate is terminated, i.e. the formation of HCl ceases. Phosgene is advantageously used in excess here. If desired, the reaction can be carried out under introduction of an inert gas both in the cold phosgenation and in the hot phosgenation.

[0062] In the hydrochloride or carbamate phosgenation, the 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 0° C. to 80° C. This phosgenation step is followed by a second step, which is essentially analogous to the thermal phosgenation in the above-described alkaline phosgenation. Here, the temperature is therefore also preferably maintained at 120° C. to 200° C., while phosgene and optionally an inert gas are introduced into the reaction mixture. This introduction is carried out until the reaction to form the isocyanate has terminated. Here, too, it is preferred to use phosgene in excess to accelerate the reaction.

[0063] After the reaction has terminated, both in the base phosgenation and in the hydrochloride or carbamate phosgenation, the remaining phosgene and hydrogen chloride gas are preferably blown off with an inert gas, preferably nitrogen. If necessary, filtration can be carried out to remove any solids present, such as the hydrochloride which has not been further converted.

[0064] The reaction product initially formed in such a phosgenation contains not only the desired isocyanate, but also residual hydrogen chloride, phosgene, solvents and impurities and residues that cannot be evaporated. The crude isocyanate to be purified according to the invention is preferably a reaction product from which phosgene and hydrogen chloride have been separated in one or more steps, preferably by distillation.

[0065] Particularly preferably, the crude isocyanate to be purified according to the invention is a crude isocyanate from which the major amount of solvent has been separated. Accordingly, the crude isocyanate preferably contains only traces of phosgene and hydrogen chloride and a small amount of solvent. In addition, it contains other low-boiling and high-boiling secondary components and non-evaporable residues. At least a portion of the secondary components and / or the non-evaporable residues are preferably chlorine-containing compounds.

[0066] The content of hydrolyzable chlorine in the isocyanate (HC value) can be determined in accordance with ISO 15028:2014 and is preferably from 200 to 20 000 ppm for the crude isocyanates used according to the invention.

[0067] According to the present invention, the crude isocyanate is distilled under vacuum to minimize the yield loss caused by oligomer formation and / or isocyanate decomposition. The distillation can be carried out in various suitable distillation apparatuses known to those skilled in the art in principle. It 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 made. The distillation apparatus preferably further includes at least one distillation column with or without a dividing wall. It is known to those skilled in the art and preferably used to be the internals in the distillation column, such as random packing, bubble cap tray, sieve plate, dual-flow tray, structured packing, etc., to, for example, improve separation performance. It is particularly preferred according to the present invention to use internals that cause a low pressure drop, such as structured packing, random packing or dual-flow tray.

[0068] The distillation apparatus used according to the invention is preferably connected in a fluidic manner to a device for generating a negative pressure in the distillation apparatus. According to the invention, such an apparatus comprises a liquid ring compressor. For distilling isocyanates from phosgenation, liquid ring compressors have great advantages over other compressors, such as dry-running compressors. For example, they are insensitive to droplets entrained in the liquid and do not tend to produce agglutinants, which are formed, for example, by the decomposition of organic compounds introduced with the vapor stream. They can also act as gas scrubbers at the same time, so that, for example, the phosgene contained in the vapor stream is at least partially washed out and a less worrying exhaust stream is obtained. However, the negative pressure (suction pressure) that can be achieved by the liquid ring compressor is limited by the operating principle.

[0069] If, according to the invention, a particularly low pressure of, for example, 0.1 to 30 mbar (absolute) is to be established in the distillation apparatus, it is therefore possible to use at least one or more precompressors with the same or different operating principles between the distillation apparatus and the at least one liquid ring compressor present. Mention may be made here, for example, of gas ejectors, screw compressors, gas ring pumps or rotary blowers. In such an embodiment, it may be advantageous to carry out the condensation between the precompressor and the liquid ring compressor in order to minimize the delivery flow of the liquid ring compressor.

[0070] In another embodiment of the invention, a plurality of distillation apparatuses are connected to the at least one liquid ring compressor in a similar manner, preferably in a fluidic manner. These distillation apparatuses can be selected, for example, from distillation apparatuses for separating off solvent from the reaction output from which phosgene and hydrogen chloride have been removed, distillation apparatuses for working up the reaction solvent, distillation apparatuses for concentrating residues, distillation apparatuses for separating off further low boilers from the crude isocyanate concentrate from which phosgene, hydrogen chloride and solvent have been removed, and apparatuses for the purification distillation of crude isocyanates.

[0071] In a preferred embodiment, the exhaust gases of a plurality of distillation units are combined in a collecting conduit and introduced from there together into the suction jacket of the at least one liquid ring compressor. The exhaust gas streams from the various distillation units can each be subjected to a gas wash after condensation of the vapors. The exhaust gas streams can be washed individually or together in any combination. Preferably, the individual gas streams are washed before the exhaust gases are combined in the collecting conduit.

[0072] Suitable gas scrubbers include in principle all devices in which a gas stream can be brought into contact with a liquid stream, i.e., for example, submerged scrubbers, spray scrubbers, random packing towers, fluidized bed scrubbers or rotary scrubbers. Preference is given to those which have a low pressure drop on the gas flowing through, such as random packing towers or spray scrubbers. The uncondensed exhaust gas preferably passes through the gas scrubber from bottom to top, while the solvent passes through the scrubber in countercurrent, i.e., from top to bottom.

[0073] Preferred solvents for washing the gas stream are the solvents mentioned above that can also be used for the preparation of isocyanates, such as chlorobenzene, dichlorobenzene, in particular o-dichlorobenzene, xylene, 1-chloro-2,4-dimethylbenzene, chlorotoluene, in particular o- or m-chlorotoluene, or mixtures thereof, preferably chlorobenzene and / or o-dichlorobenzene. Very particularly preferably, the same solvent that is also used to prepare isocyanates is used to wash the gas stream. The temperature of the solvent is preferably -20°C to +10°C, and it is particularly preferred that the temperature is less than or equal to the operating temperature of the at least one liquid ring compressor. In this preferred manner, volatile organic compounds and phosgene are at least partially washed out from the exhaust stream to obtain a pre-purified exhaust stream. In contrast, hydrogen chloride can only be removed from the exhaust stream to a very limited extent under these conditions and therefore remains essentially in the exhaust gas. It is often even its main component.

[0074] According to the invention, the hydrogen chloride content can also be reduced by washing with a suitable inorganic washing medium, for example a solution of silver sulfate in concentrated sulfuric acid.

[0075] 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 (absolute), preferably 20 to 150 mbar (absolute), particularly preferably 30 to 120 mbar (absolute).

[0076] The pressure ranges mentioned are sufficient for many distillation tasks in the purification of crude isocyanates and enable distillation to be achieved with substantial avoidance of decomposition reactions. If lower pressures are required according to the invention, this can be achieved as described above by using a precompressor with a different or identical operating principle.

[0077] In the method according to the invention, suitable working fluids for producing a liquid ring in the at least one liquid ring compressor generally include solvents that can also be used to prepare isocyanates, i.e., for example, chlorobenzene, dichlorobenzene, in particular o-dichlorobenzene, xylene, 1-chloro-2,4-dimethylbenzene, chlorotoluene, in particular o- or m-chlorotoluene, or mixtures thereof, preferably chlorobenzene and / or o-dichlorobenzene. On the one hand, these working fluids are substantially inert under operating conditions and have melting points and boiling points that are favorable for use as working fluids. The working fluid used is particularly preferably the same solvent that can also be used to prepare isocyanates. Additional post-treatment of the working medium can thus be omitted, since this can be carried out together with the solvent used in the preparation.

[0078] The operating temperature of the at least one liquid ring compressor is understood here to mean the temperature of the liquid discharged at the extrusion sleeve. The operating temperature of the at least one liquid ring compressor is usually -17°C to +15°C in the method according to the invention. Higher temperatures increase the vapor pressure of the working fluid and thus reduce the possible suction pressure when operating the at least one liquid ring compressor. Lower operating temperatures increase the energy costs and equipment costs for cooling the working fluid and the viscosity of the working fluid. The latter factor leads to a further increase in the already high energy consumption for conveying the working fluid. The operating temperature of the liquid ring compressor is preferably -15°C to +12°C, particularly preferably -12°C to +10°C.

[0079] The offgas from the distillation apparatus obtained in the distillative purification of the crude isocyanate in the process according to the invention usually still contains small amounts of phosgene, even if the reaction product from the phosgenation has been previously freed of hydrogen chloride and phosgene. This can be due on the one hand to the fact that the phosgene was not completely separated off in the preceding distillation step, but can also be formed, for example, by the cracking off of phosgene from chlorine-containing low-boiling secondary components. By using at least one liquid ring compressor according to the invention to generate the vacuum in the distillation apparatus, this phosgene can advantageously be at least partially washed out. This reduces the amount of phosgene in the offgas, making it safer to handle and easier to dispose of.

[0080] Phosgene is preferably at least partially dissolved in the working liquid, so that the liquid preferably has a phosgene content of 0.001 to 4.5% by weight, 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 at the outlet of the at least one liquid ring compressor. Lower phosgene contents require disproportionately high use of fresh working liquid that is depleted in phosgene or phosgene-free, which requires subsequent post-treatment. Enrichment to higher contents leads to an undesirable increase in the total amount of the hazardous substance phosgene present in the device and increases the risk of cavitation at the at least one liquid ring compressor.

[0081] However, according to the invention, it has now been found that even with low phosgene contents in the operating fluid, cavitation problems often occur when the content of acidic chlorine compounds (also referred to as AC value, AC content or simply acidity) increases. The AC value is determined according to ASTM D 5629 for AC values ​​<100 ppm and according to ASTM D6099 for AC values ​​>100 ppm, wherein the operating fluid replaces the isocyanate as a matrix.

[0082] For reliable operation of the at least one liquid ring compressor, it is necessary according to the invention that the operating fluid at the outlet of 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.

[0083] The presence of hydrogen chloride increases the risk of cavitation in particular. Therefore, in a preferred embodiment of the invention, the method is operated so that the operating fluid leaving the at least one liquid ring compressor at the outlet sleeve has an HCl content of ≤ 1.6% by weight, preferably ≤ 1% by weight, particularly preferably ≤ 0.5% by weight, very particularly preferably ≤ 0.2% by weight. The HCl content is preferably at least 1 ppm, particularly preferably at least 10 ppm, very particularly preferably at least 20 ppm, most preferably at least 50 ppm.

[0084] As a result, cavitation can be significantly reduced or even completely avoided in the process according to the invention. It is known to the person skilled in the art that factors that also play a role in the occurrence of cavitation are optionally the operating temperature, the suction pressure and possible gas ballast using inert gases. Higher operating temperatures, lower suction pressures and low or absent gas ballast have adverse effects, so in these cases it is advisable to limit the HCl content more strictly. In addition, as mentioned above, the phosgene content in the working fluid also plays a role. As the phosgene content in the working fluid increases, the cavitation tendency increases, so it is again advantageous to establish a lower hydrogen chloride content.

[0085] In a preferred embodiment, the present invention relates to a process according to the invention, wherein the phosgene content c(COCl2) in the working fluid at the outlet of the at least one liquid ring compressor is from 0.001% by weight to 4.5% by weight, preferably from 0.005% by weight to 2% by weight, particularly preferably from 0.01% by weight to 1% by weight, most preferably from 0.05% by weight to 0.5% by weight, and the hydrogen chloride content 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.

[0086] In a particularly preferred embodiment of the invention, the hydrogen chloride content of the working fluid when leaving the compressor is therefore below the maximum content c according to general formula (I): max (HCl), where for formula results below 0.01%, a maximum content of 0.01% by weight applies max (HCl):

[0087] c max (HCl) = p saug / 10^(6.115 - 900 / (T+279)) – c(COCl2) / 100 (I),

[0088] where c max (HCl), p saug , T and c(COCl2) are defined as follows

[0089] c Max(HCl) Maximum content of hydrogen chloride in the working fluid in % by weight

[0090] p saug Suction pressure of liquid ring compressors in mbar (absolute)

[0091] T is the operating temperature of the liquid ring compressor in °C

[0092] c(COCl2) Phosgene content in the working solution in wt%.

[0093] According to the invention, the content of phosgene and HCl in the working liquid can be influenced in different ways and in some cases independently of each other. As already described above, for example, at least one exhaust stream from a distillation device preferably connected in a fluid manner to a liquid ring compressor (i.e., it contributes to the generation of the vacuum in this distillation device) can be subjected to gas washing in a gas scrubber. Depending on the washing liquid used and the operating conditions, such as temperature, saturation, liquid amount, etc., this can affect the phosgene and / or hydrogen chloride input to the liquid ring compressor. Alternatively, the working liquid itself can be treated by stripping with, for example, an inert gas. This can be carried out either in the at least one liquid ring compressor itself or in an external separate device.

[0094] In another 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 or retreated working fluid. A special case of this embodiment is a method in which the working fluid is heated after leaving the liquid ring compressor, wherein previously dissolved volatile compounds are discharged and separated off, and then cooled again for reintroduction into the liquid ring compressor as working fluid.

[0095] A portion of the working fluid is also ejected with the compressed gas at the outlet sleeve. In a preferred embodiment of the method according to the invention, the liquid leaving the outlet sleeve, which preferably contains gas and working fluid, is separated in a liquid separator after leaving the at least one liquid ring compressor. Preferably, at least a portion of the separated working fluid is then circulated via a circulation conduit, preferably after cooling in a cooler, i.e. to the inlet sleeve for the working fluid. By means of the cooling preferably carried out according to the invention, it is advantageously possible to discharge heat, for example due to compression, condensation and absorption, and to establish a working temperature within the desired range.

[0096] Preferably, a substream of the working liquid is continuously or discontinuously removed from the system and replaced with a fresh or post-treated working liquid, hereinafter referred to as the replacement working liquid. The phosgene content in the working liquid at the outlet of the at least one liquid ring compressor can thereby be advantageously adjusted to the range according to the present invention. The replacement working liquid can be introduced into the system at various locations. It can be introduced, for example, into a circulation conduit, supplied via a separate working liquid pipe sleeve in the at least one liquid ring compressor itself, or introduced into a liquid separator. Preferably, before being introduced into the working liquid, the replacement working liquid is brought to a temperature that is at most 20°C higher than the working temperature of the at least one liquid ring compressor, preferably at most 15°C, particularly preferably at most 10°C, and very particularly preferably at most 5°C. The temperature of the replacement working liquid is particularly preferably less than or equal to the working temperature of the liquid ring compressor and higher than the solidification temperature of the replacement working liquid. The replacement working liquid preferably has a phosgene content of ≤ 1000 ppm, preferably ≤ 100 ppm, particularly preferably ≤ 20 ppm. Furthermore, the replacement 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, no longer contributes significantly to improving the process and is therefore not necessary.

[0097] The present invention therefore preferably relates to a method according to the invention, wherein at least a portion of the working fluid 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 lower than the working temperature of the at least one liquid ring compressor, and then continuously resupplied to the at least one liquid ring compressor.

[0098] In a preferred embodiment, the replacement working fluid contains a solvent, which preferably consists of a solvent, which has been used for phosgenation and preferably worked up by distillation. In this case, its phosgene content is preferably ≤ 1000 ppm, preferably ≤ 100 ppm, particularly preferably ≤ 20 ppm, but at the same time preferably ≥ 1 ppm, particularly preferably ≥ 5 ppm, particularly preferably ≥ 10 ppm. A further reduction in the phosgene content would require disproportionately high costs for purifying the solvent.

[0099] The invention therefore preferably relates to a method according to the invention, wherein the operating fluid in the at least one liquid ring compressor is at least partially, preferably continuously, replaced with an operating fluid having a phosgene content of ≤ 1000 ppm, preferably ≤ 100 ppm, particularly preferably ≤ 20 ppm.

[0100] The present invention also relates to isocyanates obtainable, preferably obtained, by the process according to the invention.

[0101] Embodiment:

[0102] Method for determining the AC value (acidity) of the working fluid:

[0103] The AC value (acidity) was determined in accordance with ASTM D 5629 for AC values ​​<100 ppm and in accordance with ASTM D 6099 for AC values ​​>100 ppm, the respective working fluid replacing the isocyanate mentioned as substrate in the method.

[0104] Method for determining the phosgene content in the working solution:

[0105] The phosgene content of the working solution can be determined by infrared spectroscopy. For this purpose, the extinction spectra of the corresponding pure working medium and the working solution sample are measured. The phosgene content of the working solution at approximately 1806 cm is then measured after subtracting the spectrum of the pure working medium. -1 The intensity of the carbonyl band of phosgene at 4 cm 2 is measured by calibration with samples having a known phosgene content and taking their origin into account. The measurement is carried out in a CaF2 cuvette with a suitable layer thickness so that the extinction (if necessary after dilution) is at 4 cm 2. -1 The resolution is 0.01 to 1.50 with at least 4 scans.

[0106] Determination of HCl content in working solution:

[0107] In the present case, the HCl content (hydrogen chloride content) of the working liquid is the difference between the phosgene content and the AC value (acidity) of the working liquid.

[0108] Embodiment 1:

[0109] Hexamethylene diisocyanate obtained by gas phase phosgenation of hexamethylene diamine and from which hydrogen chloride, phosgene, chlorobenzene and low-boiling secondary components have been substantially removed in a multi-stage distillation process is subjected to continuous purification distillation under vacuum as a crude isocyanate. The crude isocyanate has an HC content of about 8000 ppm according to ISO 15028:2014. The distillation is carried out at a top pressure of 50 mbar (absolute) in a tower with an internal condenser. The vapor leaving the top of the tower is washed with chlorobenzene in a gas scrubber. The washed vapor stream is cooled to -11°C in a cooler and sucked in with a liquid ring compressor to generate a negative pressure. The pressure sensor directly installed upstream of the suction sleeve shows a suction pressure of 30 mbar. The liquid ring compressor is operated at a working temperature of -2°C with chlorobenzene as the working fluid in circulating liquid operation, that is, the working fluid leaving is recycled in a circulating manner via a cooler to the inlet sleeve for the working fluid. Here, part of the working fluid is continuously discharged and replaced by redistilled chlorobenzene with a phosgene content of 10 ppm and a hydrogen chloride content of 2 ppm. The amount of working fluid to be replaced is adjusted regularly so that an AC value of 2150 ppm is established in the working fluid leaving on the pressure side. The phosgene content is determined to be 0.25% by weight, and the calculated hydrogen chloride content is correspondingly 0.03% by weight.

[0110] Liquid ring compressors are able to operate for long periods of time without cavitation problems.

[0111] Embodiment 2:

[0112] The distillation was carried out analogously to Example 1, but this time the offgas stream was combined in a collecting conduit with the offgas stream from the column for separating the solvent from the reaction product. The offgas stream from the solvent separation had hydrogen chloride as the main component and additionally contained chlorobenzene and traces of phosgene. The combined offgas stream from the collecting conduit was subsequently subjected to the same further manipulations as the washed vapor stream from Example 1.

[0113] The replacement rate of the operating fluid of the liquid ring compressor was selected so that an AC value of 9000 ppm was established in the operating fluid leaving on the pressure side. The phosgene content was determined to be 0.007% by weight, and the calculated hydrogen chloride content was correspondingly 0.9% by weight.

[0114] Liquid ring compressors are capable of running for long periods of time, but slight cavitation noise may sometimes be heard during operation.

[0115] Embodiment 3:

[0116] Starting from the operating conditions in Example 2, the replacement rate of the operating fluid was reduced in the operating experiment, wherein the AC value of the operating fluid leaving on the pressure side was increased to 18100 ppm. The phosgene content was determined to be 0.014% by weight, and the calculated hydrogen chloride content was correspondingly 1.8% by weight.

[0117] Operating a liquid ring compressor in this manner results in increased cavitation noise, vibration and an increase in the pressure measured at the suction casing.

[0118] These problems can be overcome by lowering the operating temperature to -12°C and increasing the pressure in the suction jacket to 80 mbar (absolute).

[0119] Embodiment 4:

[0120] Toluene diisocyanate obtained from the gas phase phosgenation of toluenediamine and substantially removed from it in a multi-stage distillation process is subjected to continuous purification distillation under vacuum as a crude isocyanate. The crude isocyanate has an HC content of about 12000 ppm according to ISO 15028:2014. Distillation is carried out at a top pressure of 135 mbar (absolute) in a dividing wall column with an internal condenser, wherein a stream containing low-boiling secondary components and orthodichlorobenzene is taken out at the top of the column, and purified toluene diisocyanate is obtained as a side stream. The vapor leaving at the top of the column is washed with orthodichlorobenzene in a gas scrubber. The washed vapor stream is cooled to 2 ° C in a cooler and sucked with a liquid ring compressor to generate a negative pressure. The pressure sensor directly installed upstream of the suction sleeve shows a suction pressure of 120 mbar. The liquid ring compressor is operated in circulating liquid operation with o-dichlorobenzene as the working fluid at an operating temperature of 7°C, i.e. the leaving working fluid is recirculated in a circulating manner via a cooler back to the inlet jacket for the working fluid. Here, a portion of the working fluid is continuously discharged and replaced by redistilled o-dichlorobenzene with a phosgene content of 10 ppm and a hydrogen chloride content of 2 ppm. The amount of working fluid to be replaced is regularly adjusted so that an AC value of 950 ppm is established in the working fluid leaving on the pressure side. The phosgene content was determined to be 0.02% by weight, and the calculated hydrogen chloride content was correspondingly 0.08% by weight.

[0121] Liquid ring compressors are able to operate for long periods of time without cavitation problems.

[0122] Comparative Example 5:

[0123] Pentamethylene diisocyanate, obtained from the phosgenation of pentamethylenediamine and from which hydrogen chloride, phosgene, chlorobenzene and low-boiling secondary components have been substantially removed in a multi-stage distillation process, is subjected to continuous purification distillation under vacuum as crude isocyanate. The crude isocyanate has an HC content of about 13000 ppm according to ISO 15028:2014. The distillation is carried out in a column with an internal condenser at a column top pressure of 45 mbar (absolute). The vapor leaving at the top of the column is cooled to -12°C in a cooler without the use of a gas scrubber in front and sucked in with a liquid ring compressor to generate a negative pressure. The pressure sensor installed directly upstream of the suction sleeve shows a suction pressure of 30 mbar. The liquid ring compressor is operated at a working temperature of -9°C with chlorobenzene as the working fluid in circulating liquid operation, that is, the working fluid leaving is recycled in a circulating manner via a cooler back to the inlet sleeve for the working fluid. Here, a portion of the working fluid is continuously discharged and replaced by redistilled chlorobenzene with a phosgene content of 10 ppm and a hydrogen chloride content of 2 ppm.

[0124] There were severe cavitation noises, vibrations and an increase in suction pressure. Analysis of the operating fluid leaving on the pressure side showed an AC value of 38600 ppm. The phosgene content was determined to be 2.8% by weight and the calculated hydrogen chloride content was correspondingly 1.8% by weight. These problems could not be corrected even by lowering the operating temperature to -14°C.

[0125] The rate of fluid replacement was then tripled, causing the AC value to drop to approximately 12900 ppm, and operation resumed normally, allowing the operating temperature to be raised back to -9°C without further problems.

Claims

1. A process for distillatively purifying the crude isocyanates obtained by phosgenation of the corresponding amines under vacuum in a distillation apparatus to obtain the corresponding isocyanates, characterized in that - at least one liquid ring compressor for generating the vacuum in the distillation apparatus, - a working fluid selected from 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 from 10 to 200 mbar absolute, - the operating temperature of the at least one liquid ring compressor is between -17°C and 15°C and the operating fluid at the outlet of the at least one liquid ring compressor has an AC value of less than 35000 ppm, wherein the AC value is determined according to ASTM D 5629 for AC values ​​<100 ppm and according to ASTM D 6099 for AC values ​​>100 ppm, The phosgene content c(COCl2) in the working fluid at the outlet of the at least one liquid ring compressor is 0.001 wt% to 4.5 wt%, and the hydrogen chloride content c(HCl) is ≤1.6 wt%.

2. The method as claimed in claim 1, characterized in that In the working fluid at the outlet of the at least one liquid ring compressor, the phosgene content c(COCl2) is 0.005% by weight to 2% by weight, and the hydrogen chloride content c(HCl) is ≤1% by weight.

3. The method as claimed in claim 1, characterized in that In the working fluid at the outlet of the at least one liquid ring compressor, the phosgene content c(COCl2) is 0.01% by weight to 1% by weight, and the hydrogen chloride content c(HCl) is ≤0.5% by weight.

4. The method according to claim 1, characterized in that In the working fluid at the outlet of the at least one liquid ring compressor, the phosgene content c(COCl2) is 0.05% to 0.5% by weight, and the hydrogen chloride content c(HCl) is ≤0.2% by weight.

5. The method according to claim 1, characterized in that At least a portion of the working fluid is continuously withdrawn from the at least one liquid ring compressor, cooled to a temperature that is at most 20° C. lower than the operating temperature of the at least one liquid ring compressor, and then continuously re-supplied to the at least one liquid ring compressor.

6. The method as claimed in claim 5, characterized in that At least a portion of the working fluid is continuously withdrawn from the at least one liquid ring compressor, cooled to a temperature that is at most 10° C. lower than the operating temperature of the at least one liquid ring compressor, and then continuously re-supplied to the at least one liquid ring compressor.

7. The method as claimed in claim 1, characterized in that The crude isocyanate originates from gas phase phosgenation or liquid phase phosgenation.

8. The method as claimed in claim 1, characterized in that The operating fluid in the at least one liquid ring compressor is at least partially replaced with an operating fluid having a phosgene content of ≤1000 ppm.

9. The method as claimed in claim 8, characterized in that The operating fluid in the at least one liquid ring compressor is at least partially replaced with an operating fluid having a phosgene content of ≤20 ppm.

10. The method according to claim 1, characterized in that The isocyanate is selected from the group consisting of monoisocyanate, diisocyanate, triisocyanate, and mixtures thereof.

11. The method as claimed in claim 10, characterized in that The isocyanate is selected from hexane diisocyanate (HDI), pentamethylene diisocyanate (PDI), 1,4-butane diisocyanate, 1,8-octane diisocyanate, 1,9-nonane diisocyanate, 2-methylpentane diisocyanate, 2,2-dimethylpentane diisocyanate, neopentane diisocyanate, 2,2,4-trimethylhexane diisocyanate and 2,4,4-trimethylhexane diisocyanate, 3-isocyanatomethyl-3,5,5-trimethylcyclohexyl isocyanate (IPDI), 2,4'- and 4,4'-dicyclohexylmethane diisocyanate (HDI), 2,4'- and 4,4'-dicyclohexylmethane diisocyanate (HDI), 2,2,4-trimethylhexane diisocyanate and 2,4,4-trimethylhexane diisocyanate. 12 MDI), 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-methyl-cyclohexane, 1,4-diisocyanato-3,3,5-trimethylcyclohexane, 1,3-bis(isocyanatomethyl)cyclohexane (H6XDI), 1,4-bis(isocyanatomethyl)cyclohexane 1,4-bis(1-isocyanato-1-methylethyl)benzene (p-TMXDI), monomeric diphenylmethane diisocyanate, 2,4-toluene diisocyanate, 2,6-toluene diisocyanate, 1,5-naphthalene diisocyanate or 1,4-phenylene diisocyanate, and mixtures thereof.

12. The method as claimed in claim 10, characterized in that The isocyanate is selected from 1,3-xylylenediisocyanate (m-XDI), 1,4-xylylenediisocyanate (p-XDI) and a mixture thereof.

13. The method of claim 1, wherein A pressure p of 10 to 150 mbar absolute is generated.

14. The method of claim 1, wherein A pressure p of 10 to 120 mbar absolute is generated.

15. Use of at least one liquid ring compressor for generating a vacuum in a distillation apparatus for crude isocyanates prepared by phosgenation of the corresponding amines, characterized in that - a working fluid selected from 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 from 10 to 200 mbar absolute, - the operating temperature of the at least one liquid ring compressor is between -17°C and +15°C and the operating fluid at the outlet of the at least one liquid ring compressor has an AC value of less than 35000 ppm, wherein the AC value is determined according to ASTM D 5629 for AC values ​​<100 ppm and according to ASTM D 6099 for AC values ​​>100 ppm, The phosgene content c(COCl2) in the working fluid at the outlet of the liquid ring compressor is 0.001 wt% to 4.5 wt%, and the hydrogen chloride content c(HCl) is less than 1.6 wt%.

16. The use as claimed in claim 15, characterized in that A pressure p of 10 to 150 mbar absolute is generated.

17. The use as claimed in claim 15, characterized in that A pressure p of 10 to 120 mbar absolute is generated.

18. A process for operating a liquid ring compressor to provide a vacuum for the distillative purification of an isocyanate obtained by phosgenation of the corresponding amine in a distillation apparatus to obtain the corresponding isocyanate, characterized in that - a working fluid selected from 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 from 10 to 200 mbar absolute, - the operating temperature of the at least one liquid ring compressor is between -17°C and +15°C and the operating fluid at the outlet of the at least one liquid ring compressor has an AC value of less than 35000 ppm, wherein the AC value is determined according to ASTM D 5629 for AC values ​​<100 ppm and according to ASTM D 6099 for AC values ​​>100 ppm, The phosgene content c(COCl2) in the working fluid at the outlet of the liquid ring compressor is 0.001 wt% to 4.5 wt%, and the hydrogen chloride content c(HCl) is less than 1.6 wt%.

19. The method as claimed in claim 18, characterized in that The isocyanate is selected from the group consisting of monoisocyanate, diisocyanate, triisocyanate, and mixtures thereof.

20. The method of claim 19, wherein The isocyanate is selected from hexane diisocyanate (HDI), pentamethylene diisocyanate (PDI), 1,4-butane diisocyanate, 1,8-octane diisocyanate, 1,9-nonane diisocyanate, 2-methylpentane diisocyanate, 2,2-dimethylpentane diisocyanate, neopentane diisocyanate, 2,2,4-trimethylhexane diisocyanate and 2,4,4-trimethylhexane diisocyanate, 3-isocyanatomethyl-3,5,5-trimethylcyclohexyl isocyanate (IPDI), 2,4'- and 4,4'-dicyclohexylmethane diisocyanate (HDI), 2,4'- and 4,4'-dicyclohexylmethane diisocyanate (HDI), 2,2,4-trimethylhexane diisocyanate and 2,4,4-trimethylhexane diisocyanate. 12 MDI), 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-methyl-cyclohexane, 1,4-diisocyanato-3,3,5-trimethylcyclohexane, 1,3-bis(isocyanatomethyl)cyclohexane (H6XDI), 1,4-bis(isocyanatomethyl)cyclohexane 1,4-bis(1-isocyanato-1-methylethyl)benzene (p-TMXDI), monomeric diphenylmethane diisocyanate, 2,4-toluene diisocyanate, 2,6-toluene diisocyanate, 1,5-naphthalene diisocyanate or 1,4-phenylene diisocyanate, and mixtures thereof.

21. The method of claim 19, wherein The isocyanate is selected from 1,3-xylylenediisocyanate (m-XDI), 1,4-xylylenediisocyanate (p-XDI) and a mixture thereof.