Method for producing isocyanates

Abstract

The invention relates to a method for producing isocyanates, comprising the step (A) of reacting an amine with a stoichiometric excess of phosgene, (a) as a diluent during the reaction and / or (b) as a medium for cooling (so-called quenching) the reaction mixture formed by the reaction of the amine with phosgene, using an aromatic solvent of the formula C6H6-XClX, where X = 1 or 2, wherein a liquid product mixture comprising the isocyanate and the aromatic solvent used and a gaseous product mixture comprising phosgene and hydrogen chloride are obtained (optionally after the reduced pressure); then (B) separating the isocyanate from the liquid product mixture obtained in step (A), comprising a pure distillation step, in which the separated isocyanate forms a product stream, in which at least one stream comprising an aromatic solvent of formula C6H6-YClY is discharged (intermittently or continuously) in the pure distillation or in a distillation step upstream of the pure distillation, wherein Y = X + 1, such that the isolated isocyanate has a mass ratio of aromatic solvent of formula C6H6-YClY of from 0.0 ppm to 9.9 ppm, preferably from 0.0 ppm to 5.0 ppm, particularly preferably from 0.0 ppm to 3.0 ppm, based on its total mass.

Description

[0001] This patent application is a divisional application of a Chinese patent application with the application number 202080065277.6 and the filing date of September 14, 2020, with the same title. Technical Field

[0002] The present invention relates to a process for preparing isocyanates, comprising the step (A) of reacting an amine with a stoichiometric excess of phosgene using an aromatic solvent of the formula C 6 H 6–X Cl X where X = 1 or 2, (a) as a diluent during the reaction and / or (b) as a medium for cooling (so-called quenching) the reaction mixture formed by the reaction of the amine with phosgene, to obtain a liquid product mixture comprising an isocyanate and the aromatic solvent used and a gas product mixture comprising phosgene and hydrogen chloride; followed by (B) separating the isocyanate from the liquid product mixture obtained in step (A), including a pure distillation step, wherein the separated isocyanate is formed as a product stream, and wherein at least one stream comprising an aromatic solvent of the formula C 6 H 6–Y Cl Y is discharged (intermittently or continuously) in the pure distillation or in a distillation step upstream of the pure distillation, where Y = X + 1, such that the separated isocyanate has a mass proportion of the aromatic solvent of the formula C 6 H 6–Y Cl Y of 0.0 ppm to 9.9 ppm, preferably 0.0 ppm to 5.0 ppm, particularly preferably 0.0 ppm to 3.0 ppm, based on its total mass. Background Art

[0003] Isocyanates are prepared in large quantities and are mainly used as raw materials for the production of polyurethanes. Most of them are prepared by the reaction of the corresponding amines with phosgene, using a stoichiometric excess of phosgene. The reaction of amines with phosgene can be carried out in the gas phase or in the liquid phase, where the reaction can be carried out batchwise or continuously. The method for preparing organic isocyanates from primary amines and phosgene has been described many times. Here, of industrial interest are not only aromatic isocyanates, such as diamines of the diphenylmethane series (hereinafter referred to as MMDI - "monomeric MDI"), mixtures of MMDI, and polyamines of the diphenylmethane series (which are higher homologues of MMDI, hereinafter referred to as PMDI, "polymeric MDI"; the mixture of MMDI and PMDI is hereinafter collectively referred to as MDI) or toluene diisocyanate (TDI), but also aliphatic or cycloaliphatic isocyanates, such as 1,5 - pentane diisocyanate (PDI), 1,6 - hexamethylene diisocyanate (HDI), and isophorone diisocyanate (IPDI). In addition, isocyanates having a benzyl isocyanate group (araliphatic isocyanates) are also important, and in particular xylylene diisocyanate (XDI) may be mentioned here.

[0004] The modern industrial - scale preparation of isocyanates is carried out semi - continuously (part of the preparation steps are carried out batchwise, e.g., the reaction is carried out batchwise and the post - treatment is carried out continuously) or continuously (all steps are continuous).

[0005] The method carried out in the liquid phase, commonly known as liquid - phase phosgenation, is characterized in that the reaction conditions are selected such that at least the reaction components amine, crude isocyanate, and phosgene, but preferably all reactants, products, and reaction intermediates, are present as liquids in a suitable solvent under the selected conditions. After the reaction, the gas phase containing the by - product hydrogen chloride and unreacted (due to the use in excess of stoichiometry) phosgene is separated; here, the desired isocyanate is retained in the liquid phase to the greatest extent together with the solvent. The crude isocyanate is thus obtained as a liquid stream in a mixture with the solvent, and this liquid stream is post - treated to obtain the pure isocyanate (and to recover the solvent as well as the dissolved portions of phosgene and hydrogen chloride).

[0006] The method carried out in the gas phase, commonly known as gas - phase phosgenation, is characterized in that the reaction conditions are selected such that at least the reaction components amine, isocyanate, and phosgene, but preferably all reactants, products, and reaction intermediates, are gaseous under the selected conditions. The advantages of gas - phase phosgenation are especially the reduction of phosgene entrainment (so - called phosgene "hold - up"), the avoidance of difficult - to - phosgenate intermediates, the increase in the reaction yield, and the reduction of the energy requirement due to working with less solvent. The reaction mixture first obtained in the gas - phase phosgenation is cooled in a so - called quench by contact with a quench liquid consisting of a solvent or an isocyanate - solvent mixture, such that the desired isocyanate is mostly liquefied and a gas phase containing hydrogen chloride and phosgene is present.

[0007] Thus, in all methods for preparing isocyanates related to industrial scale, a liquid crude isocyanate stream is obtained which must be post-treated to obtain the desired isocyanate in pure form and to recover other valuable materials such as solvents. This post-treatment generally includes separating out the solvent, dissolved phosgene and dissolved hydrogen chloride. Subsequently, a fine purification of the isocyanate is carried out, which may also include isomer separation if required. Depending on the type of isocyanate, homolog separation can be carried out prior to fine purification. It should be particularly mentioned here that MMDI is partially separated from an isocyanate mixture containing MMDI and PMDI from which the solvent, phosgene and hydrogen chloride have been substantially removed, to obtain an MMDI fraction (crude MMDI) containing PMDI at most in negligible traces and a mixture composed of PMDI and MMDI.

[0008] The post-treatment of a crude isocyanate stream on an industrial scale is not an easy task because many different requirements must be considered simultaneously. In addition to obtaining the target product in as pure a form as possible, mention should also be made here of recovering phosgene, hydrogen chloride and solvents as loss-free as possible, especially in order to recycle them into the process (optionally after further conversion, e.g. hydrogen chloride into chlorine). All of this must be carried out under the most economical conditions possible, i.e. with the smallest possible energy consumption and the smallest possible loss of valuable products (especially isocyanates, where unwanted subsequent reactions may occur in the case of an unoptimally designed post-treatment). It goes without saying that here, the desired isocyanate must be freed of by-products, the aromatic solvents used, excess phosgene, etc. as much as possible. Since chlorinated aromatics (especially mono- or dichlorobenzene) have proven to be advantageous as solvents in isocyanate production, it may be mentioned here that the requirements regarding the residual content of such chlorinated aromatic solvents in the pure isocyanate are becoming increasingly strict, thus also increasing the challenge for distillation technology. The background for these increased requirements is that polyurethane products produced from isocyanates should not produce harmful vapours, which is particularly important for foam materials used in seat cushions or mattresses. In this regard, in addition to the aromatic solvents themselves used, the prior art has focused on their fully chlorinated reaction products (in the case of using mono- or dichlorobenzene as solvents, i.e. hexachlorobenzene). The aromatic solvents used in phosgenation and / or in the post-treatment of the crude isocyanate are chlorinated to form solvents with a higher chlorine content compared to the aromatic solvents used. According to current knowledge, the prior art has not particularly focused on the reaction products of the aromatic solvents used that contain only one additional chlorine substituent.

[0009] The post-treatment of crude isocyanates has been described many times: International patent application WO 2017 / 050776 A1 describes the work-up of a solvent-containing crude isocyanate stream of a liquid obtained by liquid-phase phosgenation by separating off phosgene and hydrogen chloride (“degassing”), then separating off the solvent, and subsequently further purifying the crude isocyanate, which has largely been freed from phosgene, hydrogen chloride and solvent, by distillation. In the case of MDI, this distillation also includes homolog separation, in which MMDI is separated off, leaving a PMDI / MMDI mixture depleted in MMDI (so-called “polymer separation”).

[0010] WO 2019 / 134909 A1 describes in detail the possibilities for MDI work-up (including homolog separation and isomer separation). WO 2018 / 114846 A1 describes the possibilities for work-up of crude TDI for liquid-phase and gas-phase phosgenation.

[0011] International patent application WO 2019 / 145380 A1 describes the preparation and work-up of aliphatic, cycloaliphatic and araliphatic isocyanates in the gas phase, followed by rapid partial liquefaction (“quenching”) of the resulting product gas mixture with a quench liquid containing an aromatic solvent. Here, in addition to a liquid product stream containing the desired isocyanate, a gas stream containing hydrogen chloride and phosgene is obtained. The isocyanate is separated from the liquid product stream by means of a plurality of distillation steps. The excess phosgene is recovered as a phosgene gas stream and recycled to the reaction. This document teaches that the content of benzene, chlorobenzene and dichlorobenzene in this recovered phosgene gas stream is limited to a value of 0.5 wt% or less. Thereby, the content of hexachlorobenzene in the isocyanate to be prepared is reduced (see the examples of this application). This document is based on the recognition that small amounts of benzene, chlorobenzene or dichlorobenzene in the gas stream introduced into the reaction zone lead to the formation of poly-chlorinated aromatic hydrocarbons, more precisely especially to the formation of interfering hexachlorobenzene. It should be understood that the term poly-chlorinated aromatic hydrocarbons clearly refers to the reaction products formed by multiple chlorination of benzene, chlorobenzene and dichlorobenzene, more precisely the reaction products formed in the reaction zone (see page 3, last paragraph).

[0012] According to the teachings of this document, the recovery of excess phosgene is carried out in such a way that first, the gas mixture of hydrogen chloride and phosgene generated in the reaction is separated into a gaseous hydrogen chloride stream and a liquid phosgene stream. Then, this liquid phosgene stream is partially evaporated, which is preferably carried out in a distillation column. This distillation column can have a feed point for liquid fresh phosgene at the top. The contents of benzene, chlorobenzene, and dichlorobenzene in the gaseous phosgene stream withdrawn from this distillation column can be adjusted to a value of 0.5 wt% or less as predetermined according to the invention by appropriately selecting the outlet temperature of this gaseous phosgene stream and optionally by appropriately selecting the amount of fresh phosgene additionally fed to this distillation column. This document does not mention other measures for achieving this value. In particular, this document does not teach discharging from the entire process an aromatic solvent stream that has been additionally monochlorinated in the reaction from a distillation step upstream of a pure distillation or a pure distillation step of the isocyanate. WO 2019 / 145380 A1 does not indicate that a reaction other than a complete chlorination reaction even occurs, nor does this document indicate that a chlorination reaction (any kind) may be important in the post-treatment (see page 4, first paragraph).

[0013] European Patent Application EP 1 717 223 A2 describes (see paragraph

[0015] ) a method for purifying isocyanates, in which a) separating a stream (1) containing isocyanate, higher-boiling and lower-boiling components, and non-vaporizable residues in a distillation including at least one theoretical plate into a sub-stream (2) containing non-vaporizable residues and isocyanate and a vapor stream (3) containing isocyanate and low-boiling components, b) keeping the non-vaporizable residues in sub-stream (2) separate from the vapor stream (3) and / or at least part of the stream containing the vapor stream (3), c) separating at least one other vapor stream (4) containing isocyanate and a stream (8) mainly containing non-vaporizable residues from sub-stream (2), and d) distilling and separating one or more vapor streams (4) containing isocyanate and the vapor stream (3) from a) into three separate streams (5, 6, 7) having different boiling ranges, where the lowest-boiling stream (5) contains the main part of the low-boiling components of the crude isocyanate stream (1), the highest-boiling stream (7) contains the main part of the high-boiling components of the crude isocyanate stream (1), and the middle-boiling stream (6) mainly contains the desired product.

[0014] Here, the material stream (1), the crude isocyanate stream to be purified, which is preferred in the case of preparing isocyanates by phosgenation, is a stream from which hydrogen chloride, phosgene, and solvent have been substantially separated. Thus, the content of hydrogen chloride and phosgene in the material stream (1) is each less than 1000 ppm, and the solvent content is less than 1% by weight, preferably less than 0.5% by weight, and particularly preferably less than 0.1% by weight. The crude isocyanate stream (1) usually contains, in addition to the isocyanate obtained as the desired product, 100 ppm to 5% of components with a boiling point lower than that of the isocyanate (low boilers), 100 to 5000 ppm of components with a boiling point higher than that of the isocyanate (high boilers), but whose boiling point under standard pressure is not more than 60 °C higher than that of the isocyanate, and contains 1 to 8% by weight of non-vaporizable residues of a polymeric nature, i.e., pyrolysis of the product before vaporization under standard pressure. Thus, solvents with a lower boiling point than the isocyanate are present in the crude isocyanate stream (1) at most in trace amounts, and thus, in this context, low boilers clearly refer to other components. Specifically mentioned are especially chlorine-containing by-products formed by further reaction of the isocyanate (not the solvent) (for example, in the preparation of 1,6-diisocyanatohexane, the two chlorine-containing minor components 1-isocyanato-6-chlorohexane or 1,6-dichlorohexane). There is no mention in the literature of the chlorination of the solvent, either partially or completely. In the case of aromatic isocyanates, methylphenyl isocyanate (for the preparation of TDI) and phenyl isocyanate (for the preparation of MDI) are mentioned as typical low boilers.

[0015] In a preferred embodiment of the method shown in Figure 2, the crude isocyanate stream (1) is fed laterally into a dividing-wall column and separated therein into three streams 5, 6, and 7, where the lowest-boiling stream (5) is taken off at the top of the column above the dividing wall, and in the column, the middle-boiling stream (6) (= the desired product stream) is taken off in the dividing-wall region above the feed, while the highest-boiling stream (7) is taken off at the bottom of the column. In the examples, the typical composition of these streams for the case of preparing TDI is mentioned: The lowest-boiling stream (5) (in addition to a substantial proportion of isocyanate, i.e., 99.4% by weight of TDI) contains 0.5% by weight of low boilers; the composition of the remaining 0.1% by weight is not given (see the table on page 7). The middle-boiling stream (6) contains 99.9% by weight of TDI, 10 ppm of low boilers, and 50 ppm of high-boiling components. Thus, the composition of the remaining 0.094% by weight (940 ppm) of the middle-boiling stream (6) is not given. The highest-boiling stream (7) consists of 30% by weight of TDI and 70% by weight of high-boiling components.

[0016] Relating to the set goal of obtaining isocyanates with as low a solvent residue ratio as possible to avoid harmful vapors in subsequent polyurethane products, it has now been found that other factors are important. Therefore, further improvements in this field are needed. Summary of the Invention

[0017] In view of this need, the present invention thus provides a method for preparing isocyanates, comprising the steps of: (A) using an aromatic solvent of the formula C 6 H 6–X Cl X wherein X = 1 or 2, (a) as a diluent during the reaction and / or (b) as a medium for cooling (so-called quenching) the reaction mixture formed by the reaction of an amine with phosgene, reacting an (aromatic, aliphatic, cycloaliphatic or araliphatic, preferably aromatic) amine with a stoichiometric excess of phosgene to thereby obtain (optionally after decompression) a liquid product mixture comprising an isocyanate and the aromatic solvent used and a gas product mixture comprising phosgene and hydrogen chloride; (B) separating the isocyanate from the liquid product mixture obtained in step (A), including a pure distillation step, wherein a separated isocyanate is produced as a product stream, wherein in said pure distillation or in a distillation step upstream of said pure distillation (intermittently or continuously) at least one stream comprising an aromatic solvent of the formula C 6 H 6–Y Cl Y is discharged (i.e., sent out of the method for preparing the isocyanate), wherein Y = X + 1, such that the separated isocyanate has a mass ratio of the aromatic solvent of the formula C 6 H 6–Y Cl Y based on its total mass of 0.0 ppm to 9.9 ppm, preferably 0.0 ppm to 5.0 ppm, particularly preferably 0.0 ppm to 3.0 ppm.

[0018] Because it has been found that, in addition to the aromatic solvent (C 6 H 6–X Cl X ) used, its subsequent product having only one additional chlorine substituent (the additionally monochlorinated aromatic solvent C 6 H 6–Y Cl Y ) must be sufficiently separated from the isocyanate target product, and since the solvent is usually recycled several times in the isocyanate preparation process in the prior art, this requires discharging the additionally monochlorinated aromatic solvent C 6 H 6–Y Cl Y. That is, when providing an isocyanate with sufficient purity, the monochlorinated solvent C 6 H 6–Y Cl Y also plays a decisive role in ensuring a polyurethane product without vapors harmful to health. If, in addition to restricting the use of aromatic solvents (C 6 H 6–X Cl X ) in the isocyanate to be prepared as is common in the art, the content of this additional monochlorinated aromatic solvent C 6 H 6–Y Cl Y is also restricted, more precisely restricted to 9.9 ppm or less, as provided by the present invention, the total content of chlorinated aromatic hydrocarbons (i.e., the solvents used and all their chlorination products) can be considered low enough.

[0019] The mass ratio of the aromatic solvent of formula C 6 H 6–Y Cl Y in the isocyanate based on the total mass can in principle be determined by all methods familiar to those skilled in the art for determining the concentration of low molecular weight organic compounds. Within the accuracy required for the purposes of the present invention, these generally provide consistent results. Preferably, measurement is carried out by gas chromatography using a flame ionization detector (FID detector) or an electron capture detector (ECD detector). In case of doubt, the value determined by gas chromatography using a flame ionization detector is authoritative.

[0020] The term "discharge" and the derived verbs and adjectives within the scope of the present invention refer to the sending out of a stream generated in the process (i.e., in the process of preparing the isocyanate) from the process (i.e., from the method of preparing the isocyanate); that is, the stream so discharged will no longer be recycled into the method, neither into the reaction section nor into the post-treatment section. The discharged stream is preferably disposed of, especially by incineration. However, it is not excluded to use such discharged streams materially in addition to recycling them into the method of preparing the isocyanate. The discharged stream contains the aromatic solvent of formula C 6 H 6–Y Cl Y , especially in a mass ratio of 1.0% to 10%, preferably 1.5% to 5.0%, particularly preferably 2.2% to 3.0% based on its total mass.

[0021] First, the various possible embodiments of the present invention will be described Brief summary below.

[0022] In a first embodiment of the present invention, which can be combined with all other embodiments, in step (B), before the pure distillation, the liquid product mixture obtained in step (A) is degassed to separate the dissolved phosgene.

[0023] In a second embodiment of the present invention, which can be combined with all other embodiments, in step (B), the gaseous product mixture obtained in step (A) is washed with an aromatic solvent of the formula C 6 H 6–X Cl X to separate the isocyanate.

[0024] In a third embodiment of the present invention, which can be combined with all other embodiments, in step (B), before the pure distillation, the liquid product mixture obtained in step (A) is subjected to solvent distillation to separate an aromatic solvent of the formula C 6 H 6–X Cl X (wherein the solvent distillation may include further distillation to purify the separated aromatic solvent of the formula C 6 H 6–X Cl X of the aromatic solvent).

[0025] In a fourth embodiment of the present invention, which is a specific design of the third embodiment, the aromatic solvent of the formula C 6 H 6–X Cl X separated in the solvent distillation is subjected to solvent purification to separate the phosgene contained therein (solvent degassing).

[0026] In a fifth embodiment of the present invention, which can be combined with all other embodiments as long as these embodiments do not exclude the use of a dividing wall column in the pure distillation, in step (B), the pure distillation is carried out in a dividing wall column, wherein an isocyanate product stream is obtained in the side stream discharge of the dividing wall column and an aromatic solvent of the formula C 6 H 6–X Cl X is obtained at the top of the dividing wall column (wherein the pure distillation may include another distillation column for purifying the aromatic solvent of the formula C 6 H 6–X Cl X obtained at the top of the dividing wall column).

[0027] In a sixth embodiment of the present invention, which can be combined with all other embodiments as long as these embodiments do not provide for the use of a dividing wall column in the pure distillation, in step (B), the pure distillation is carried out in two distillation columns in series without a dividing wall, wherein an aromatic solvent of the formula C 6 H 6-X Cl XAn aromatic solvent and obtaining an isocyanate product stream as the distillate of the second distillation column, wherein the pure distillation may include a further distillation column for purifying the aromatic solvent of the formula C 6 H 6–X Cl X obtained at the top of the first distillation column).

[0028] In a seventh embodiment of the present invention, which may be combined with all other embodiments provided that these embodiments include solvent distillation, the aromatic solvent of the formula C 6 H 6–X Cl X separated in the solvent distillation is obtained in a mixture with the aromatic solvent of the formula C 6 H 6–Y Cl Y (= the mixture obtained in the solvent distillation), wherein the first part of the mixture (optionally after solvent degassing) is recycled to step (A) and the second part of the mixture is not recycled to step (A) but is discharged (intermittently or continuously).

[0029] In an eighth embodiment of the present invention, which is an alternative to the seventh and the ninth embodiment mentioned below but may additionally be combined with all other embodiments provided that these embodiments include solvent distillation, the aromatic solvent of the formula C 6 H 6–X Cl X separated in the solvent distillation is obtained in a mixture with the aromatic solvent of the formula C 6 H 6–Y Cl Y (= the mixture obtained in the solvent distillation), wherein the first part of the mixture (optionally after solvent degassing) is recycled to step (A) and the second part of the mixture is purified in a further distillation, wherein the aromatic solvent of the formula C 6 H 6–X Cl X is separated from the second part of the mixture and subsequently (at least partially, in particular completely) recycled to step (A), wherein the part of the second part of the mixture remaining after separating the aromatic solvent of the formula C 6 H 6–X Cl X (enriched with the aromatic solvent of the formula C 6 H 6–Y Cl Y ) is discharged (intermittently or continuously).

[0030] In a ninth embodiment of the present invention, which is an alternative to the seventh and eighth embodiments but can additionally be combined with all other embodiments as long as these embodiments include solvent distillation, the aromatic solvent of formula C 6 H 6–X Cl X obtained in the solvent distillation is obtained in a mixture with the aromatic solvent of formula C 6 H 6–Y Cl Y (= the mixture obtained in the solvent distillation), wherein the mixture is purified in a further distillation, wherein the aromatic solvent of formula C 6 H 6–X Cl X is separated from the mixture and subsequently (at least partially, especially completely) recycled to step (A), and the portion of the mixture remaining after separating the aromatic solvent of formula C 6 H 6–X Cl X (enriched in the aromatic solvent of formula C 6 H 6–Y Cl Y ) is discharged (intermittently or continuously).

[0031] In a tenth embodiment of the present invention, which is a specific design of the fifth or sixth embodiment, the aromatic solvent of formula C 6 H 6–X Cl X obtained at the top of the dividing wall column or at the top of the first distillation column results in a mixture with the aromatic solvent of formula C 6 H 6–Y Cl Y (= the mixture obtained at the top of the dividing wall column or at the top of the first distillation column).

[0032] In an eleventh embodiment of the present invention, which is a first specific design of the tenth embodiment, the resulting mixture (i.e., the mixture obtained at the top of the dividing wall column or at the top of the first distillation column) is discharged.

[0033] In a twelfth embodiment of the present invention, which is a second specific design of the tenth embodiment, a first portion of the mixture obtained at the top of the dividing wall column or at the top of the first distillation column is recycled to step (A), and a second portion of the mixture is not recycled to step (A) but is discharged (intermittently or continuously).

[0034] In a thirteenth embodiment of the present invention, which is a third specific design of the tenth embodiment, more precisely for step (B) it includes using the formula C 6 H 6–X ClX In the case of the step of washing the gaseous product mixture obtained in step (A) with an aromatic solvent to separate the isocyanate (= the second embodiment above), a first part of the mixture obtained at the top of the dividing wall column or at the top of the first distillation column is recycled to the washing step, and a second part of the mixture is not recycled to the washing step but is (intermittently or continuously) discharged.

[0035] In the fourteenth embodiment of the present invention, which is the fourth specific design of the tenth embodiment, a first part of the mixture obtained at the top of the dividing wall column or at the top of the first distillation column is recycled to step (A), and a second part of the mixture is purified in a further distillation, wherein an aromatic solvent of the formula C 6 H 6–X Cl X is separated from the second part of the mixture and then (at least partially, especially completely) recycled to step (A), wherein the part (enriched with the aromatic solvent of the formula C 6 H 6–X Cl X remaining after separation of the aromatic solvent of the formula C 6 H 6–Y Cl Y from the second part of the mixture) is (intermittently or continuously) discharged.

[0036] In the fifteenth embodiment of the present invention, which is the fifth specific design of the tenth embodiment, more precisely for the case of the step (B) including washing the gaseous product mixture obtained in step (A) with an aromatic solvent of the formula C 6 H 6–X Cl X to separate the isocyanate (= the second embodiment above), a first part of the mixture obtained at the top of the dividing wall column or at the top of the first distillation column is recycled to the washing step, and a second part of the mixture is purified in a further distillation, wherein an aromatic solvent of the formula C 6 H 6–X Cl X is separated and then (at least partially, especially completely) recycled to the washing step or step (A), wherein the part (enriched with the aromatic solvent of the formula C 6 H 6–X Cl X remaining after separation of the aromatic solvent of the formula C 6 H 6– Y Cl Y from the second part of the mixture) is (intermittently or continuously) discharged.

[0037] In the sixteenth embodiment of the present invention, which is the sixth specific design of the tenth embodiment, the mixture obtained at the top of the dividing wall column or at the top of the first distillation column is purified in a further distillation, wherein an aromatic solvent of the formula C 6 H 6–X Cl X is separated from the mixture and then (at least partially, especially completely) recycled to step (A), wherein the portion of the mixture remaining after separating the aromatic solvent of the formula C 6 H 6–X Cl X (enriched with the aromatic solvent of the formula C 6 H 6–Y Cl Y ) is discharged (intermittently or continuously).

[0038] In the seventeenth embodiment of the present invention, which is the seventh specific design of the tenth embodiment, more precisely for the case where step (B) comprises the step of washing the gaseous product mixture obtained in step (A) with an aromatic solvent of the formula C 6 H 6–X Cl X to separate the isocyanate (= the above-mentioned second embodiment), the mixture obtained at the top of the dividing wall column or at the top of the first distillation column is purified in a further distillation, wherein an aromatic solvent of the formula C 6 H 6–X Cl X is separated from the mixture and (at least partially, especially completely) recycled to the washing step or step (A), wherein the portion of the mixture remaining after separating the aromatic solvent of the formula C 6 H 6–X Cl X (enriched with the aromatic solvent of the formula C 6 H 6–Y Cl Y ) is discharged (intermittently or continuously).

[0039] In the eighteenth embodiment of the present invention, it can be combined with all other embodiments as long as these embodiments include solvent distillation, wherein the aromatic solvent of the formula C 6 H 6–X Cl X obtained in the solvent distillation is obtained in a mixture with the aromatic solvent of the formula C 6 H 6–Y Cl Y (= the mixture obtained in the solvent distillation), wherein the mixture is partially or completely discharged, and the discharged portion of the mixture / the discharged mixture is incinerated.

[0040] In the nineteenth embodiment of the present invention, which can be combined with all other embodiments, the pipeline for connecting the tank container for receiving the liquid product mixture from step (A) to the distillation apparatus for performing step (B) and / or for connecting these distillation apparatuses to each other is made of stainless steel of type 2.4610, 1.4529 or 1.4539.

[0041] In the twentieth embodiment of the present invention, which is a specific design of the nineteenth embodiment, the pure distillation step for obtaining the separated isocyanate product stream is carried out in a distillation column having a container for receiving the liquid distillation column bottoms, which is made of stainless steel of type 2.4610, 1.4529 or 1.4539.

[0042] In the twenty - first embodiment of the present invention, which can be combined with all other embodiments as long as these embodiments do not involve liquid - phase phosgenation, the reaction of the amine with phosgene in step (A) is carried out in the gas phase, including (b).

[0043] In the twenty - second embodiment of the present invention, which is a specific design of the twenty - first embodiment, the amine is selected from diamines of the diphenylmethane series (to obtain diisocyanates of the diphenylmethane series), tolylene diamine (to obtain tolylene diisocyanate), xylylene diamine (to obtain xylylene diisocyanate), bis(aminomethyl)cyclohexane (to obtain bis(isocyanatomethyl)cyclohexane), bis(aminomethyl)norbornane (to obtain bis(isocyanatomethyl)norbornane), hexahydrotoluene diamine (to obtain hexahydrotoluene diisocyanate), 1,6 - hexamethylene diamine (to obtain hexamethylene diisocyanate), 1,5 - pentamethylene diamine (to obtain pentamethylene diisocyanate) and isophorone diamine (to obtain isophorone diisocyanate).

[0044] In the twenty - third embodiment of the present invention, which can be combined with all other embodiments as long as these embodiments do not involve gas - phase phosgenation, the reaction of the amine with phosgene in step (A) is carried out in the liquid phase, including (a).

[0045] In a twenty-fourth embodiment of the present invention which is a specific design of the twenty-third embodiment, the amine is selected from diamines and polyamines of the diphenylmethane series (to obtain di- and polyisocyanates of the diphenylmethane series), naphthalenediamine (to obtain naphthalene diisocyanate), tolylenediamine (to obtain tolylene diisocyanate), xylylenediamine (to obtain xylylene diisocyanate), bis(aminomethyl)cyclohexane (to obtain bis(isocyanatomethyl)cyclohexane), bis(aminomethyl)norbornane (to obtain bis(isocyanatomethyl)norbornane), hexahydrotoluenediamine (to obtain hexahydrotoluene diisocyanate), 4,4'-diaminodicyclohexylmethane (to obtain 4,4'-diisocyanatodicyclohexylmethane), 1,6-hexamethylenediamine (to obtain hexamethylene diisocyanate), 1,5-pentamethylenediamine (to obtain pentamethylene diisocyanate), and isophoronediamine (to obtain isophorone diisocyanate).

[0046] In a twenty-fifth embodiment of the present invention, which can be combined with all embodiments including liquid-phase or gas-phase phosgenation, especially those including gas-phase phosgenation, the amine used is tolylenediamine (to obtain tolylene diisocyanate).

[0047] In a twenty-sixth embodiment of the present invention, which is a specific design of the twenty-fifth embodiment, X = 2.

[0048] In a twenty-seventh embodiment of the present invention, which is a specific design of the twenty-third embodiment, the amine used is a mixture of diamines and polyamines of the diphenylmethane series (to obtain diisocyanates and polyisocyanates of the diphenylmethane series).

[0049] In a twenty-eighth embodiment of the present invention, which is a specific design of the twenty-seventh embodiment, X = 1.

[0050] In a twenty-ninth embodiment of the present invention, which is another specific design of the twenty-first embodiment, the amine is selected from hexamethylenediamine (to obtain hexamethylene diisocyanate), pentamethylenediamine (to obtain pentamethylene diisocyanate), and isophoronediamine (to obtain isophorone diisocyanate).

[0051] In a thirtieth embodiment of the present invention, which is a specific design of the twenty-ninth embodiment, X = 1.

[0052] In a thirty-first embodiment of the present invention, which can be combined with all other embodiments, a product mixture is supplied to pure distillation, and the product mixture contains an aromatic solvent having a mass ratio of 8% to 49% based on its total mass of C 6 H 6–X Cl X .

[0053] In the thirty-second embodiment of the present invention, which can be combined with all other embodiments, a product mixture is supplied to pure distillation, the product mixture containing an aromatic solvent having a mass proportion of 10% to 30% based on its total mass of C 6 H 6–X Cl X .

[0054] In the thirty-third embodiment of the present invention, which can be combined with all other embodiments, an effluent stream containing an aromatic solvent of the formula C 6 H 6– Y Cl Y contains the solvent (i.e., the aromatic solvent of the formula C 6 H 6–Y Cl Y ) in a mass proportion of 1.0% to 10% based on its total mass (i.e., the total mass of the effluent stream).

[0055] In the thirty-fourth embodiment of the present invention, which can be combined with all other embodiments, an effluent stream containing an aromatic solvent of the formula C 6 H 6– Y Cl Y contains the solvent (i.e., the aromatic solvent of the formula C 6 H 6–Y Cl Y ) in a mass proportion of 1.5% to 5.0% based on its total mass (i.e., the total mass of the effluent stream).

[0056] In the thirty-fifth embodiment of the present invention, which can be combined with all other embodiments, an effluent stream containing an aromatic solvent of the formula C 6 H 6– Y Cl Y contains the solvent (i.e., the aromatic solvent of the formula C 6 H 6–Y Cl Y ) in a mass proportion of 2.2% to 3.0% based on its total mass (i.e., the total mass of the effluent stream).

[0057] The embodiments of the present invention outlined above and further possible embodiments are described below Clarify more specifically . Unless otherwise stated or clearly evident from the context, all embodiments can be combined with each other arbitrarily.

[0058] According to step (A) of the process according to the invention, the amine reacts with a stoichiometric excess of phosgene to obtain a liquid product mixture comprising isocyanate and the aromatic solvent used (apart from the gaseous product mixture comprising phosgene and hydrogen chloride), which can be carried out within the scope of the invention as is in principle known from the prior art. As mentioned at the beginning, there are two principal process variants, liquid-phase phosgenation and gas-phase phosgenation. Liquid-phase phosgenation is always carried out in the presence of a solvent as a diluent (variant (a)). However, in gas-phase phosgenation, the reaction can also be carried out in the presence of solvent vapour as a diluent. When carrying out the reaction preferably in the gas phase, a solvent (or a mixture containing a certain proportion of the desired isocyanate in addition to the solvent) is added to rapidly cool the reaction mixture (variant (b)).

[0059] Irrespective of the process scheme, the reaction of the primary amine with phosgene in step (A) is preferably carried out continuously.

[0060] Examples of liquid-phase phosgenation are described in DE 37 44 001 C1, EP 0 314 985 A1, EP 1369 412 A1, DE-A-102 60 027, DE-A-102 60 093, DE-A 103 10 888, DE-A-10 2006 022 448, US-A2007 / 0299279 and the documents cited therein respectively. Liquid-phase phosgenation in step (A) includes variant (a). Variant (b), the so-called quench, is generally not necessary.

[0061] By the process according to the invention, it is possible to preferably phosgenate amines selected from the following: diamines and polyamines of the diphenylmethane series (to obtain di- and polyisocyanates of the diphenylmethane series), naphthalenediamine (to obtain naphthalene diisocyanate), toluenediamine (to obtain toluene diisocyanate), xylylenediamine (to obtain xylylene diisocyanate), bis(aminomethyl)cyclohexane (to obtain bis(isocyanatomethyl)cyclohexane), bis(aminomethyl)norbornane (to obtain bis(isocyanatomethyl)norbornane), hexahydrotoluenediamine (to obtain hexahydrotoluene diisocyanate), 4,4'-diaminodicyclohexylmethane (to obtain 4,4'-diisocyanatodicyclohexylmethane), 1,6-hexamethylenediamine (to obtain hexamethylene diisocyanate), 1,5-pentamethylenediamine (to obtain pentamethylene diisocyanate) and isophoronediamine (to obtain isophorone diisocyanate). Toluenediamine and diamines and polyamines of the diphenylmethane series are particularly preferred, and diamines and polyamines of the diphenylmethane series are particularly preferred. For this purpose, a mixture of diamines and polyamines of the diphenylmethane series is phosgenated in a manner known per se to the corresponding mixture of diisocyanates and polyisocyanates of the diphenylmethane series (= MDI), wherein preferably monochlorobenzene (X = 1) is used as the solvent in step (A) (however, dichlorobenzene (X = 2) can also be used in principle). In the case of MDI, the work-up of the product mixture obtained in step (A) (step (B)) preferably also includes the step of separating the fraction of diisocyanates of the diphenylmethane series (= MMDI; "monomeric MDI"), leaving the MDI mixture depleted of diisocyanates of the diphenylmethane series, and subsequently further purely distilling the separated monomeric MDI fraction (in the case of the remaining MDI mixture depleted of diisocyanates of the diphenylmethane series, the separation of monomeric MDI is regarded as "pure distillation"). The requirements according to the invention regarding the residual content of the solvent in the product stream relate to the MDI mixture depleted of monomeric MDI and the separated MMDI itself.

[0062] Liquid-phase phosgenation In a preferred embodiment, it is processed as follows: The reactants, the primary amine and phosgene, are dissolved separately in a solvent. Suitable solvents for this purpose are monochlorobenzene (X = 1) and dichlorobenzene (X = 2), the latter in the ortho- or para-isomer form, preferably in the ortho-isomer form. Based on the total mass of the solution, the primary amine is preferably used at a concentration of 10% to 40% by mass, preferably 10% to 20% by mass. Based on the total mass of the solution, phosgene is preferably used at a concentration of 10% to 40% by mass, preferably 25% to 35% by mass.

[0063] The effective mixing of primary amines with phosgene is of great significance in liquid-phase processes. In the prior art, static mixing devices (preferably nozzles) and dynamic mixing devices (comprising mechanical moving parts) are used for this purpose. After mixing, the mixed reaction participants pass through the reaction zone to complete the reaction. The mixing device and the reaction zone can also be arranged in a common reactor. Phosgene is used in a stoichiometric excess relative to the primary amino groups of the amine, in particular in a molar ratio of phosgene to primary amino groups of from 4.0:1 to 1.1:1, particularly preferably from 3.0:1 to 1.1:1, very particularly preferably from 2.0:1 to 1.1:1.

[0064] Liquid-phase phosgenation can be carried out at various temperature- and pressure levels. Thus, liquid-phase phosgenation can be carried out, for example, at a temperature of from 0 °C to 250 °C, preferably from 20 °C to 200 °C and at a pressure of (abs.) from 1.0 bar (abs.) to 70 bar, (abs.) preferably from 1.0 bar (abs.) to 50 bar.

[0065] In a preferred embodiment, part of the hydrogen chloride formed as a by-product in the reaction is dissolved in the liquid phase and part is degassed. The proportion of the hydrogen chloride present in the gaseous state that is dissolved depends on the selected temperature- and pressure levels.

[0066] In another embodiment, the temperature- and pressure levels are selected such that the hydrogen chloride is first substantially completely dissolved or present in liquefied form and only forms a gas phase after a targeted reduction in pressure (for example in a gas / liquid separator). In the terminology of the present invention, this reduction in pressure is part of step (A).

[0067] Thus, in any case, at the end of step (A), a liquid stream containing the isocyanate to be prepared and the solvent, and a gas stream containing hydrogen chloride and optionally evaporated solvent are obtained. Since phosgene is used in superstoichiometric amounts, both streams furthermore contain phosgene. Both streams can be taken directly from the reaction zone. It is also possible to take out the two-phase process product (comprising a liquid phase and a gas phase) from the reaction zone and transfer it to a phase separation device. This phase separation can be carried out in all devices suitable for separating a gas phase and a liquid phase known to the person skilled in the art. Preference is given to using gas-liquid separators, such as cyclone separators, deflection separators and / or gravity separators, with or without static separation aids. Phase separation can likewise be assisted by reducing the pressure compared to the pressure prevailing in the reaction zone, due to the enhanced degassing of hydrogen chloride (and optionally other gaseous constituents). The liquid stream taken out from the reaction zone, or - if present - the liquid stream taken out from the phase separation device downstream of the reaction zone, is in this embodiment the starting material for the work-up to be carried out in step (B), i.e., in the terms of the present invention, this liquid phase is the "liquid product mixture comprising isocyanate and the aromatic solvent used".

[0068] Taking the primary amine TDA as an example, the phosgenation in the liquid phase is illustrated in more detail below: In the liquid-phase process, TDA dissolved in one of the solvents defined above is fed at a temperature of -10 °C to 220 °C, preferably 0 °C to 200 °C, particularly preferably 20 °C to 180 °C, to be mixed with phosgene. Phosgene is likewise dissolved in one of the solvents defined above and is fed at a temperature of -40 °C to 200 °C, preferably -30 °C to 170 °C, particularly preferably -20 °C to 150 °C, to be mixed with TDA. The mixing of the TDA-solution and the phosgene solution is preferably carried out using a static mixer or a dynamic mixer in the liquid-phase process. Examples of suitable static mixers are in particular nozzles or nozzle arrangements, such as those described in DE 17 92 660 A, US 4,289,732 or US 4,419,295. Examples of suitable dynamic mixers are in particular pump-like assemblies, such as centrifugal pumps (see US 3,713,833) or specific mixer-reactors (see EP 0 291 819 A, EP 0 291 820 A, EP 0 830894 A).

[0069] In the liquid-phase process, the reaction in the subsequent reaction zone is carried out at a temperature of 0 °C to 250 °C, preferably 20 °C to 200 °C, particularly preferably 20 °C to 180 °C, the average residence time of the reaction mixture in the reaction zone being from 10 seconds to 5 hours, preferably from 30 seconds to 4 hours, particularly preferably from 60 seconds to 3 hours, and at up to 100 bar (abs.) , preferably 1.0 bar (abs.) to 70 bar (abs.) , particularly preferably 1.0 bar(abs.) to 50 bar (abs.) Under a pressure of. For the reactions in the reaction zone, examples of process variants that can be used according to the invention are described, for example, in US - A 2007 / 0299279 (especially page 7, paragraphs

[0070] ,

[0071] ,

[0089] ) and DE - A 103 10 888 (especially page 5, paragraphs

[0038] ,

[0039] ) and the documents cited in these documents.

[0070] Examples of gas - phase phosgenation are described in EP 0 570 799 A1, EP 1 555 258 A1, EP 1 526 129 A1 and DE 101 61 384 A1, and especially for aliphatic isocyanates, in EP 0 289 840 B1, EP 1 754698 B1, EP 1 319 655 B1 and EP 1 362 847 B1. The advantage of this method over other conventional liquid - phase phosgenations is the energy savings achieved by minimizing the expensive and complex solvent - and phosgene circuits. Gas - phase phosgenation includes variant (b) in step (A). Variant (a), in the case of gas - phase phosgenation, using the vapors of the aromatic solvent used in step (A) as a diluent, is also possible but not necessary.

[0071] Using the process according to the invention, the amine can preferably be selected from diamines of the diphenylmethane series (to obtain diisocyanates of the diphenylmethane series), tolylene diamine (to obtain tolylene diisocyanate), xylylenediamine (to obtain xylylene diisocyanate), bis(aminomethyl)cyclohexane (to obtain bis(isocyanatomethyl)cyclohexane), bis(aminomethyl)norbornane (to obtain bis(isocyanatomethyl)norbornane), hexahydrotoluene diamine (to obtain hexahydrotoluene diisocyanate), 1,6 - hexamethylene diamine (to obtain hexamethylene diisocyanate), 1,5 - pentamethylene diamine (to obtain pentamethylene diisocyanate) and isophorone diamine (to obtain isophorone diisocyanate). Tolyene diamine is particularly preferred, and for this, dichlorobenzene (X = 2), especially the ortho - isomer, is preferably used as the solvent in step (A). For this purpose, tolylene diamine, which is present as a mixture of different isomers (especially 2,4 - and 2,6 - tolylene diamine (m - tolylene diamine)), is phosgenated in a manner known per se to the corresponding mixture of tolylene diisocyanate isomers (= TDI), where, as described above, dichlorobenzene (X = 2), especially the ortho - isomer, is preferably used as the solvent in step (A) (however, in principle, monochlorobenzene (X = 1) can also be used).

[0072] In Gas-phase phosgenation a preferred embodiment of First, a gas stream of a primary amine is provided. Methods suitable for this are in principle known to those skilled in the art. Preferred embodiments are described in detail below.

[0073] The primary amine can be converted into the gas phase in all evaporation apparatuses known in the prior art, in particular in falling film evaporators. Those evaporation apparatuses are preferably used in which a small working content is guided through the falling film evaporator with a high circulation power.

[0074] In order to minimize the thermal load on the amine, regardless of the exact embodiment of the evaporation apparatus, it is preferred to assist the evaporation process by adding an inert gas such as N 2 , He, Ar (in particular N 2 ) or the vapor of a solvent. Suitable solvents for this purpose are monochlorobenzene (X = 1) and dichlorobenzene (X = 2), the latter in the ortho- or para-isomer form, preferably in the ortho-isomer form.

[0075] The evaporation of the starting amine (optionally with overheating) (in particular to a temperature of 200 °C to 430 °C, preferably 250 °C to 420 °C, particularly preferably 250 °C to 400 °C) is carried out in one or more stages, preferably in multiple stages, in order to avoid unevaporated droplets in the gaseous amine stream as much as possible. Particularly preferred are the multi-stage evaporation step and the overheating step, in which a droplet separator is installed between the evaporation system and the overheating system (at at least one site, i.e., between at least one evaporation system and the subsequent overheating system, or between each evaporation system and the subsequent overheating system) and / or the evaporation device also has the function of a droplet separator. Suitable droplet separators are known to those skilled in the art.

[0076] In a further step, a gaseous phosgene stream is provided. The molar ratio of phosgene to the primary amine group is preferably set to 1.1:1 to 20:1, particularly preferably 1.2:1 to 5.0:1. As described above for the primary amine, it is also preferred to heat the phosgene to a temperature of 200 °C to 430 °C, preferably 250 °C to 420 °C, particularly preferably 250 °C to 400 °C, and optionally dilute it with an inert gas such as N 2 , He, Ar (in particular N 2 ) or with the vapor of an inert solvent as defined above for the amine.

[0077] The gaseous reaction participants, a primary amine and phosgene, are mixed in a mixing zone and react in a subsequent reaction zone. The separately heated reaction participants, the amine and phosgene, are preferably fed into the mixing and reaction by means of a nozzle arrangement. The nozzle arrangement for introducing the reactant gas streams, the amine and phosgene, can be designed in various ways known to the person skilled in the art; examples can be found in EP 2 199 277 B1, paragraphs

[0017] to

[0019] , EP 1 449 826 B1, paragraphs

[0011] to

[0012] , EP 1 362 847 B1, paragraphs

[0011] to

[0012] , EP 1 526 129 B1, paragraphs

[0009] to

[0011] and EP 1 555 258 B1, paragraphs

[0008] to

[0011] .

[0078] In addition to the possibilities of diluting the gaseous primary amine stream and the gaseous phosgene stream already mentioned, a separate dilution gas stream (inert gas such as N 2 , He, Ar, especially N 2 ) or the vapour of an inert solvent as defined above for the amine can be fed directly into the mixing. In this case, the dilution gas stream is preferably heated to a temperature of from 100 °C to 500 °C, preferably from 150 °C to 450 °C, particularly preferably from 150 °C to 400 °C.

[0079] The further conversion in the reaction zone of the reaction participants, the primary amine and phosgene, which have already been mixed in the mixing zone, preferably takes place adiabatically. Adiabatic conversion means forgoing the targeted removal of the heat of reaction generated by means of a heat transfer medium. Thus, apart from unavoidable heat losses, the enthalpy of reaction quantitatively reflects the temperature difference between the product gas stream and the reactant gas streams.

[0080] In the reaction zone, the amine and phosgene are rapidly converted into the corresponding isocyanate, and more preferably adiabatically. The reaction is preferably carried out in such a way that the amine is completely converted before entering the quench described in more detail below.

[0081] In the so-called quenching, the isocyanate formed in the quenching zone is rapidly cooled and (except for the trace amounts remaining in the gas phase) liquefied (quenched) by contact with the quenching liquid in the quenching zone. Suitable quenching liquids include the above-mentioned solvents and mixtures composed of the isocyanate to be prepared and the solvent. The contact is preferably carried out by injecting the quenching liquid into the gaseous stream of the reaction product mixture. The construction and operating possibilities of the quenching zone are in principle known from the prior art. Devices and methods of the prior art can also be used within the scope of the present invention. For example, possible embodiments of the quenching zone are disclosed in EP 1 403 248 A1 and EP 1 935 875 A1. The temperature of the quenching liquid used in the quenching is preferably selected such that, on the one hand, it is high enough to crack the carbamyl chloride corresponding to the isocyanate back into the isocyanate and hydrogen chloride. (Although it is not possible to determine whether the intermediate carbamyl chloride known from liquid-phase phosgenation will also be formed in gas-phase phosgenation. However, since it can be conceived independently here that the isocyanate liquefied in the quenching reacts with the existing hydrogen chloride gas fraction to form carbamyl chloride, the temperature of the quenching liquid should be high enough to inhibit this reaction.) On the other hand, the isocyanate and the optional solvent used as a diluent in the gaseous amine stream and / or gaseous phosgene stream should be substantially condensed or substantially dissolved in the solvent, while the excess phosgene, hydrogen chloride and the optional inert gas used as a diluent pass through the quenching zone substantially without condensation and without dissolution. Therefore, the temperature of the selected quenching liquid is not allowed to be too high. A quenching liquid maintained at a temperature of 50 °C to 200 °C, preferably 80 °C to 180 °C, is particularly well-suited for selectively obtaining isocyanate from the gaseous reaction mixture. The mixture obtained in the quenching zone contains a gas part and a liquid part, i.e., it is biphasic. This biphasic mixture is sent to a collection zone for phase separation. The liquid phase and the gas phase are preferably continuously withdrawn from the collection zone. In this embodiment, the liquid phase thus obtained is the starting material for the post-treatment to be carried out in step (B), i.e., the liquid phase is a liquid product mixture containing the isocyanate and the aromatic solvent used.

[0082] In a preferred embodiment, the mixing zone, the reaction zone, the quenching zone and the collection zone are arranged in the above order from top to bottom in a vertical, especially conical or cylindrical or conical-cylindrical reactor. In this embodiment, the mixture produced in the quenching flows into the collection zone due to gravity. In another arrangement of the collection zone, in some cases it is necessary to pump the mixture of the reaction product mixture and the quenching liquid into the collection zone.

[0083] After phosgenation in step (A), in step (B) the post-treatment of the liquid product mixture from (A) is carried out, i.e., the isocyanate is separated from the liquid product mixture obtained in step (A), obtaining a formula C based on its total mass 6 H 6–Y ClY The mass ratio of the aromatic solvent is 0.0 ppm to 9.9 ppm, preferably 0.0 ppm to 5.0 ppm, and particularly preferably 0.0 ppm to 3.0 ppm, of the isocyanate. According to the invention, step (B) comprises at least Pure distillation steps and preferably at least one distillation step upstream of said pure distillation, in particular for separating out the formula C 6 H 6–X Cl X of the aromatic solvent Solvent distillation . In certain embodiments, step (B) further comprises for separating out the dissolved phosgene from the liquid product mixture obtained in step (A) Dephosphorization Gas , where the degassing does not necessarily have to be designed as a distillation (see below). In certain cases, it may also be necessary to preface the pure distillation with its own distillation step ( Residue separation) , for separating out the so-called residues (by-products with very high boiling points of the phosgenation). The steps outlined only briefly here will be elucidated in more detail below.

[0084] Preferably, the step of the pure distillation and, if carried out, the distillation step upstream thereof are carried out continuously, so as to continuously produce a stream of separated isocyanate. Then the mass ratio of the aromatic solvent of the formula C 6 H 6–Y Cl Y is determined in the continuously produced stream of separated isocyanate, and more precisely especially at intervals of 1 hour to 16 hours, preferably 4 hours to 12 hours. In the case of a discontinuous embodiment of step (B), the separated isocyanate is produced discontinuously, i.e. batchwise. Then preferably the mass ratio of the aromatic solvent of the formula C 6 H 6–Y Cl Y is determined in each batch. If the operating conditions of step (B) have once been optimized such that the mass ratio of the aromatic solvent of the formula C 6 H 6–Y Cl Y is low enough, the measurement frequency can also be reduced, regardless of whether the separated isocyanate is produced continuously or batchwise.

[0085] Likewise, regardless of whether the separated isocyanate is produced continuously or batchwise, for the deviation found from the mass ratio of C 6 H 6–Y Cl Y of the aromatic solvent predetermined according to the invention, by increasing the discharge of the aromatic solvent of the formula C 6 H 6–Y Cl Y is offset.

[0086] Examples of the work-up of the phosgenation products are described in EP-A-1 413 571 (TDI), US 2003 / 0230476 A1 (TDI) and EP 0289 840 B1 (HDI, IDPI and H12-MDI). The basic mode of operation described there can in principle also be used in the process according to the invention. This applies if the stream to be discharged containing at least one additionally monochlorinated aromatic solvent C 6 H 6–Y Cl Y can be incorporated into these modes of operation in such a way that the purity requirements are met with respect to the concentration of C 6 H 6– Y Cl Y in the desired isocyanate.

[0087] As already mentioned above, optionally dissolved phosgene (and dissolved hydrogen chloride) is first separated off from the liquid product mixture obtained in step (A) in a separate step. This process variant is preferred, especially when the phosgenation in step A) is carried out in the liquid phase, since the liquid crude process product obtained in liquid-phase phosgenation tends to contain a significantly higher proportion of dissolved phosgene and dissolved hydrogen chloride than that obtained in gas-phase phosgenation. This so-called degassing step can in principle be carried out in any manner known to the person skilled in the art, in particular by distillation, absorption or a combination of both.

[0088] After the degassing step or - especially when step a) is carried out in the gas phase - immediately after step (A), as described above, the aromatic solvent C 6 H 6–X Cl X (more precisely: its main part, i.e. sufficient to obtain a liquid product mixture depleted in the aromatic solvent of the formula C 6 H 6–X Cl X whose content of the formula C 6 H 6–X Cl XThe mass proportion of the aromatic solvent is 8% to 49%, preferably 10% to 30%). This solvent separation is carried out by distillation (solvent distillation). If desired, the solvent separated in this way can be subjected to solvent purification to separate the phosgene present therein (= solvent dephosgenation). This is preferably done by distillation in a solvent-dephosgenation column, whereby a purified solvent is produced as the bottom product, which contains at most trace amounts of phosgene and isocyanate. However, this distillation for dephosgenation cannot effect the separation of the aromatic solvent used from the additionally monochlorinated aromatic solvent, since both are obtained together in the bottom product of the dephosgenation column. For economic reasons, there is a search for recycling the recovered, optionally dephosgenated solvent to step (A). The recovered, optionally dephosgenated solvent of formula C 6 H 6–X Cl X can also contain components of a solvent of formula C 6 H 6–Y Cl Y while its other components remain in the liquid product mixture depleted of the solvent of formula C 6 H 6–X Cl X obtained from the bottom product of the solvent distillation). If this is the case, the solvent of formula C 6 H 6–X Cl X is thus obtained in the solvent distillation in a mixture with the solvent of formula C 6 H 6–Y Cl Y and thus, in one embodiment of the invention, the entire mixture can be recycled to step (A). In this case, the discharge of the stream containing the aromatic solvent of formula C 6 H 6–Y Cl Y which is essential for the present invention is then completely transferred to a downstream step of the fine purification (= pure distillation) of the isocyanate. However, it is also possible to effect at least partially this discharge essential for the present invention already in this solvent separation step. Embodiments suitable for this are described below: In this embodiment of the invention, the solvent distillation is thus designed such that the aromatic solvent of formula C 6 H 6–X Cl X separated in the solvent distillation is in a mixture with the solvent of formula C 6 H 6–Y Cl Yobtained from a mixture of aromatic solvents, where a first part of the mixture (optionally after solvent degassing; see above) is recycled to step (A), and a second part of the mixture is not recycled to step (A), but is (intermittently or continuously) discharged and then incinerated or otherwise used, but not recycled back into the process. Incineration is preferred. In this embodiment of the invention, the second part of the separated mixture (intentionally not returned to the process) is an essential discharge of the stream of aromatic solvent containing formula C 6 H 6–Y Cl Y or a part thereof (if there are other discharge points) that is essential for the present invention.

[0089] The corresponding design of the solvent distillation is achieved by setting appropriate conditions for temperature, pressure, reflux ratio, and / or number of trays in the distillation. It goes without saying that the conditions to be precisely selected depend on the aromatic solvent used in step (A), the type of amine, and the concentrations of the isocyanate and the additionally monochlorinated solvent in the starting mixture to be distilled, which can be easily determined by those skilled in the art for each application case. Optionally, simple preliminary experiments may be required to determine the optimal distillation parameters.

[0090] Instead of discharging the second part of the mixture obtained in the solvent distillation, the second part of the mixture can be purified in a further distillation (which is considered a component of the solvent distillation in the terms of the present invention and thus a distillation step upstream of the pure distillation), where the aromatic solvent of formula C 6 H 6–X Cl X is separated from the second part of the mixture and then (optionally after solvent degassing) at least partially, especially completely, recycled to step (A). Of course, it is also possible to purify all the mixtures separated in the solvent distillation in a further distillation and separate the aromatic solvent of formula C 6 H 6–X Cl X from this mixture, and then (optionally after solvent degassing) at least partially separated, especially completely, and recycled to step (A).

[0091] Regardless of whether only the second part of the mixture or all of it is sent to the above-mentioned further distillation, due to its relatively high boiling point, the additionally monochlorinated aromatic solvent C 6 H 6–Y Cl Y is enriched in the bottoms of this further distillation and thus leaves the distillation through the bottoms stream, while a stream of aromatic solvent of formula C 6 H 6–X Cl X is obtained as the tops stream (which contains at most a negligible amount of the additionally monochlorinated aromatic solvent C6 H 6–Y Cl Y )。Thus, the bottoms stream from the further distillation is withdrawn and incinerated or otherwise used, but is no longer recycled to the process. Incineration is preferred. In this embodiment of the invention, the separation of the bottoms stream from the further distillation (intentionally forgoing its return to the process) is an essential discharge of the stream of aromatic solvent containing C 6 H 6–Y Cl Y or a part thereof (if there are other discharge points) that is essential for the present invention.

[0092] The discharge of a part (second part) of the mixture obtained in the solvent distillation and / or the purification of this part or the entire mixture in the further distillation, and then the discharge of the bottoms stream from the further distillation are carried out in such a way that the additionally monochlorinated aromatic solvent C 6 H 6–Y Cl Y is discharged from the process to a sufficient extent. The exact conditions (such as the quantity ratio of the first part to the second part of the mixture and / or the exact embodiment of the further distillation and / or the frequency of discharge at intervals during its conduct) depend on the framework conditions of the individual case and can be readily determined by a person skilled in the art, optionally with preliminary experiments. The relevant framework conditions of course also include whether there are additional sites in the process for discharging the additionally monochlorinated aromatic solvent C 6 H 6–Y Cl Y .

[0093] Although it is in principle possible to remove the additionally monochlorinated aromatic solvent in the solvent distillation to an extent sufficient for the purposes of the present invention, it is expensive. Therefore, it is preferred to discharge the additionally monochlorinated aromatic solvent mainly or completely in the following fine purification step.

[0094] The product mixture to be supplied to the fine purification (= pure distillation) contains the aromatic solvent of formula C 6 H 6–X Cl XAn aromatic solvent, regardless of whether the solvent separation is upstream of the fine purification (i.e., the value of the solvent content within the said range is set by selecting the conditions in the solvent separation or automatically adjusted as a result of the reaction conditions in step (A). The fine purification of the isocyanate to be prepared is carried out by distillation to separate out the still-present low-boiling and high-boiling organic minor components (= low boilers and high boilers) and the still-present solvent and optionally the still-present residual components of chlorides and phosgene. The still-present solvent, hydrogen chloride, and phosgene are also more volatile than the isocyanate to be prepared and thus are mainly separated together with the low-boiling organic minor components until complete separation. The pure distillation can in turn consist of sub-steps such that the separation of the low boilers and high boilers is carried out in two distillation columns connected in series. However, these separation operations can also be carried out in one step (i.e., in a single distillation column) using a dividing-wall column.

[0095] Step (B) preferably further includes (especially when step (A) is carried out in the gas phase) post-treating the gaseous product mixture obtained in step (A) to separate out any isocyanate optionally contained therein. This is preferably achieved by washing the gaseous product mixture with the aromatic solvent C 6 H 6–X Cl X used. The solvent of the formula C 6 H 6–X Cl X for this purpose does not have to be 100% pure for the additionally monochlorinated aromatic solvent C 6 H 6–Y Cl Y The isocyanate-solvent mixture obtained in the washing can be sent to the post-treatment of the liquid product mixture from step (A).

[0096] The present invention includes the following aspects: 1. A method for preparing an isocyanate, comprising the steps of: (A) Using an aromatic solvent of the formula C 6 H 6–X Cl X where X = 1 or 2, (a) as a diluent during the reaction and / or (b) as a medium for cooling the reaction mixture formed by the reaction of an amine with phosgene, reacting an amine with a stoichiometric excess of phosgene, thereby obtaining a liquid product mixture containing an isocyanate and the aromatic solvent used and a gas product mixture containing phosgene and hydrogen chloride; (B) Separating the isocyanate from the liquid product mixture obtained in step (A), including a pure distillation step, where a separated isocyanate is produced as a product stream, and at least one stream containing an aromatic solvent of the formula C 6 H 6–Y Cl Y is discharged in such a way in the pure distillation or in a distillation step upstream of the pure distillation, where Y = X + 1, such that the separated isocyanate has a mass proportion of the aromatic solvent of the formula C 6 H 6–Y Cl Y ranging from 0.0 ppm to 9.9 ppm based on its total mass.

[0097] 2. The method according to aspect 1, wherein the liquid product mixture obtained in step (A) is subjected to solvent distillation in step (B) before the pure distillation to separate out the aromatic solvent of the formula C 6 H 6–X Cl X .

[0098] 3. The method according to aspect 1, wherein in step (B), the pure distillation is carried out in a dividing wall column, where an isocyanate product stream is obtained in the side stream discharge of the dividing wall column and an aromatic solvent of the formula C 6 H 6–X Cl X is obtained at the top of the dividing wall column, or is carried out in two distillation columns without a dividing wall connected in series, where an aromatic solvent of the formula C 6 H 6–X Cl X is obtained at the top of the first distillation column and an isocyanate product stream is obtained as the distillate of the second distillation column.

[0099] 4. The method according to aspect 2, wherein in step (B), the pure distillation is carried out in a dividing wall column, where an isocyanate product stream is obtained in the side stream discharge of the dividing wall column and an aromatic solvent of the formula C 6 H 6-X Cl X is obtained at the top of the dividing wall column, or is carried out in two distillation columns without a dividing wall connected in series, where an aromatic solvent of the formula C 6 H 6-X Cl X is obtained at the top of the first distillation column and an isocyanate product stream is obtained as the distillate of the second distillation column.

[0100] 5. The method according to aspect 4, wherein the aromatic solvent of formula C 6 H 6–X Cl X is obtained in a mixture with an aromatic solvent of formula C 6 H 6–Y Cl Y wherein a first portion of the mixture obtained in the solvent distillation is recycled to step (A), and a second portion of the mixture obtained in the solvent distillation is not recycled to step (A) but is discharged, or wherein the aromatic solvent of formula C 6 H 6–X Cl X is obtained in a mixture with an aromatic solvent of formula C 6 H 6–Y Cl Y wherein a first portion of the mixture obtained in the solvent distillation is recycled to step (A), and a second portion of the mixture obtained in the solvent distillation is purified in a further distillation, wherein the aromatic solvent of formula C 6 H 6–X Cl X is separated from the second portion of the mixture obtained in the solvent distillation and then recycled to step (A), wherein the remainder of the second portion of the mixture obtained in the solvent distillation after separating the aromatic solvent of formula C 6 H 6–X Cl X from the second portion of the mixture obtained in the solvent distillation is discharged, or wherein the aromatic solvent of formula C 6 H 6–X Cl X is obtained in a mixture with an aromatic solvent of formula C 6 H 6–Y Cl Y wherein the mixture obtained in the solvent distillation is purified in a further distillation, wherein the aromatic solvent of formula C 6 H 6–X Cl X is separated from the mixture obtained in the solvent distillation and then recycled to step (A), wherein the remainder of the mixture obtained in the solvent distillation after separating the aromatic solvent of formula C 6 H 6–X Cl X from the mixture obtained in the solvent distillation is discharged.

[0101] 6. The method according to aspect 2, wherein the aromatic solvent of formula C6 H 6–X Cl X The aromatic solvent of is obtained in a mixture with an aromatic solvent of the formula C 6 H 6–Y Cl Y wherein a first portion of the mixture obtained in the solvent distillation is recycled to step (A), and a second portion of the mixture obtained in the solvent distillation is not recycled to step (A) but is discharged, or wherein the aromatic solvent of the formula C 6 H 6–X Cl X obtained by separation in the solvent distillation is obtained in a mixture with an aromatic solvent of the formula C 6 H 6–Y Cl Y wherein a first portion of the mixture obtained in the solvent distillation is recycled to step (A), and a second portion of the mixture obtained in the solvent distillation is purified in a further distillation, wherein the aromatic solvent of the formula C 6 H 6–X Cl X is separated from the second portion of the mixture obtained in the solvent distillation and then recycled to step (A), and the remainder of the second portion of the mixture obtained in the solvent distillation after separation of the aromatic solvent of the formula C 6 H 6–X Cl X from the second portion of the mixture obtained in the solvent distillation is discharged, or wherein the aromatic solvent of the formula C 6 H 6–X Cl X obtained by separation in the solvent distillation is obtained in a mixture with an aromatic solvent of the formula C 6 H 6–Y Cl Y wherein the mixture obtained in the solvent distillation is purified in a further distillation, wherein the aromatic solvent of the formula C 6 H 6–X Cl X is separated from the mixture obtained in the solvent distillation and then recycled to step (A), and the remainder of the mixture obtained in the solvent distillation after separation of the aromatic solvent of the formula C 6 H 6–X Cl X from the mixture obtained in the solvent distillation is discharged.

[0102] 7. The method according to any one of aspects 3 to 5, wherein the formula C obtained at the top of the dividing wall column or at the top of the first distillation column6 H 6–X Cl X The aromatic solvent of is obtained in a mixture with an aromatic solvent of the formula C 6 H 6–Y Cl Y and is obtained in a mixture with an aromatic solvent of the formula C

[0103] 8. The method according to aspect 7, wherein the mixture obtained at the top of the dividing wall column or at the top of the first distillation column is discharged, or wherein the mixture obtained at the top of the dividing wall column or at the top of the first distillation column is purified in a further distillation, wherein an aromatic solvent of the formula C 6 H 6–X Cl X is separated from the mixture obtained at the top of the dividing wall column or at the top of the first distillation column and then recycled to step (A), wherein the remainder of the mixture obtained at the top of the dividing wall column or at the top of the first distillation column after separating the aromatic solvent of the formula C 6 H 6– X Cl X is discharged, or wherein a first portion of the mixture obtained at the top of the dividing wall column or at the top of the first distillation column is recycled to step (A), and a second portion of the mixture obtained at the top of the dividing wall column or at the top of the first distillation column is not recycled to step (A) but is discharged, or wherein a first portion of the mixture obtained at the top of the dividing wall column or at the top of the first distillation column is recycled to step (A), and a second portion of the mixture obtained at the top of the dividing wall column or at the top of the first distillation column is purified in a further distillation, wherein an aromatic solvent of the formula C 6 H 6–X Cl X is separated from the second portion of the mixture obtained at the top of the dividing wall column or at the top of the first distillation column and then recycled to step (A), wherein the remainder of the second portion of the mixture obtained at the top of the dividing wall column or at the top of the first distillation column after separating the aromatic solvent of the formula C 6 H 6–X Cl X is discharged.

[0104] 9. The method according to aspect 7, which includes in step (B) using an aromatic solvent of the formula C 6 H6–X Cl X Step of washing the gaseous product mixture obtained in step (A) with an aromatic solvent to isolate the isocyanate, wherein a first portion of the mixture obtained at the top of the dividing wall column or at the top of the first distillation column is recycled to the washing step, and a second portion of the mixture obtained at the top of the dividing wall column or at the top of the first distillation column is not recycled to the washing step but is discharged, or wherein a first portion of the mixture obtained at the top of the dividing wall column or at the top of the first distillation column is recycled to the washing step, and a second portion of the mixture obtained at the top of the dividing wall column or at the top of the first distillation column is purified in a further distillation, wherein an aromatic solvent of the formula C 6 H 6–X Cl X is separated from the second portion of the mixture obtained at the top of the dividing wall column or at the top of the first distillation column and then recycled to the washing step or step (A), wherein the remainder of the second portion of the mixture obtained at the top of the dividing wall column or at the top of the first distillation column after separating the aromatic solvent of the formula C 6 H 6–X Cl X is discharged, or wherein the mixture obtained at the top of the dividing wall column or at the top of the first distillation column is purified in a further distillation, wherein an aromatic solvent of the formula C 6 H 6–X Cl X is separated from the mixture obtained at the top of the dividing wall column or at the top of the first distillation column and then recycled to the washing step or step (A), wherein the remainder of the mixture obtained at the top of the dividing wall column or at the top of the first distillation column after separating the aromatic solvent of the formula C 6 H 6–X Cl X is discharged.

[0105] 10. The method according to any one of the preceding aspects, wherein the pipeline for connecting the tank container for receiving the liquid product mixture from step (A) to the distillation device for performing step (B) and / or for connecting these distillation devices to each other is made of stainless steel of type 2.4610, 1.4529 or 1.4539.

[0106] 11. The method according to any one of the preceding aspects, wherein a product mixture is supplied to the pure distillation, the product mixture containing the formula C 6H 6–X Cl X The mass ratio of the aromatic solvent based on its total mass is 8% to 49%.

[0107] 12. The method according to any one of the foregoing aspects, wherein the effluent stream comprising the aromatic solvent of formula C 6 H 6–Y Cl Y comprises the aromatic solvent of formula C 6 H 6–Y Cl Y in a mass ratio of 1.0% to 10% based on its total mass. Detailed Embodiments

[0109] Possible embodiments of step (B) are shown in detail below using the particularly preferred isocyanate TDI as an example, wherein first the basic structure of the preferred distillation sequence is illustrated only with the aid of different variants (1 to 4). Of course, this basic structure is not limited to the work-up of TDI. Subsequently, it will be explained how the method according to the invention can be implemented in said distillation sequence.

[0110] Variant 1 Variant 1, which is particularly suitable if step A) is carried out in the liquid phase, is described in principle in ChemSystem’s PERP Report for TDI / MDI (Chem Systems, Process Evaluation Research Planning TDI / MDI 98 / 99 S8, Tarrytown, N.Y., USA: Chem Systems 1999, pages 27 to 32). In this variant, the liquid reaction mixture after the distillative separation of hydrogen chloride and phosgene still contains a solvent proportion of > 50% by mass, preferably 51% to 85% by mass, particularly preferably 55% to 65% by mass, based on its total mass. This mixture is sent to solvent separation, where first the solvent-TDI mixture is distilled into a solvent distillation column in a pre-evaporator. In the solvent distillation column, the solvent is distilled off and fed back into the previous stage of the process. The bottom stream of this solvent distillation contains, based on the total mass of the bottom stream, in addition to TDI, particularly preferably 15% to 25% by mass of solvent, based on the total mass of this bottom stream. This stream is introduced into a so-called intermediate column, in which the solvent is further distilled off, and the bottom product from which the solvent has been removed is fed into a final distillation column for the purification of TDI. The column operates under negative pressure and provides a marketable purified isocyanate TDI as the distillate stream. A part of the TDI remains in the bottom stream of the final distillation column. The tasks of the intermediate column and the distillation column for TDI purification can also be combined in a dividing wall column, particularly as described in EP 1 371 635 A1, where a top stream consisting of low boilers and solvent, pure TDI in the dividing wall region, and a product stream containing TDI and high-boiling components (distillation residues) as the bottom stream are obtained.

[0111] Variant 2 Compared with Variant 1, in this embodiment, the liquid reaction mixture after the distillative separation of hydrogen chloride and phosgene also contains a solvent proportion of only ≤ 50.0% by mass, based on its total mass. This mixture is fed into a pre-evaporator, from which the solvent-TDI mixture is distilled into a distillation column. In this variant, the solvent has already been removed from the TDI in the latter-mentioned distillation column, so that the bottom stream of this distillation column can be introduced into a TDI purification column, and thus one column less is required in this variant than in Variant 1. The TDI purification column operates under negative pressure and provides a marketable purified isocyanate TDI as the distillate stream. The functions of the TDI purification column and the distillation column upstream thereof can also be combined in a dividing wall column, particularly as described in EP 1 413 571 A1, where a top stream consisting of low boilers and solvent, pure TDI in the dividing wall region, and a product stream containing TDI and high-boiling components (distillation residues) as the bottom stream are obtained.

[0112] Variant 3 Variant 3 includes the distillation sequence described in Variants 2 and 1, but without the pre-evaporators mentioned separately. In this case, the proportion of the distillation residue is carried via the liquid volume flow in the described distillation sequence until the respective last TDI purification column. This method is likewise known in principle (EP 1 717 223 A2).

[0113] Variant 4 This variant is used particularly when step (A) is carried out in the gas phase. Since the liquid crude process product obtained in gas-phase phosgenation contains dissolved phosgene and dissolved hydrogen chloride at most in relatively small amounts (i.e., compared to liquid-phase phosgenation), it is preferred to dispense with the separate separation of phosgene and hydrogen chloride. In this embodiment, the liquid product mixture is thus either fed directly to the solvent separation, in which the solvent and optionally dissolved hydrogen chloride and optionally dissolved phosgene are separated by distillation at the top of the column, or - if the solvent proportion is low enough - directly to the TDI purification column for pure distillation. In both cases, the TDI purification column is preferably designed as a dividing-wall column. The low boilers (i.e., by-products with a boiling point lower than that of TDI, optionally still present hydrogen chloride, optionally still present phosgene, the solvent, and optionally inert gases) are taken off from the TDI purification column at the top of the column. The overhead stream may also contain a small amount of entrained TDI. The purified TDI is withdrawn as a distillate stream in the dividing-wall region. The obtained bottoms stream of the distillation column contains the so-called distillation residue and a certain amount of TDI, which have not been distilled off in order to keep the bottoms stream processable (flowable), and optionally contains trace proportions of the solvent. Of course, two distillation columns without a dividing wall connected in series can also be used instead of the dividing-wall column. In the latter-mentioned embodiment, the low boilers are taken off at the top of the first distillation column, and the bottoms stream of the first distillation column forms the feed of the second distillation column. The purified TDA is withdrawn as a distillate stream from the second distillation column, while the bottoms stream of the second distillation column contains the distillation residue mixed with TDI.

[0114] In Variant 4, the solvent separation - if carried out - is preferably carried out at a temperature of 160 °C to 200 °C and a pressure of 160 mbar to 220 mbar, where both of these data relate to the bottom of the distillation column used.

[0115] The pure distillation of TDI, particularly when carried out in a dividing-wall column, is preferably carried out at a temperature of 160 °C to 200 °C and a pressure of 50 mbar to 100 mbar, where both of these data relate to the bottom of the distillation column used.

[0116] In all the variants described, the pure distillation of TDI thus gives a low-boiling overhead stream, i.e. the overhead stream of the dividing wall column or the overhead stream of the distillation column upstream of the TDI purification column without a dividing wall. The main component of this low-boiling overhead stream is the aromatic solvent C 6 H 6–X Cl X . The total mass proportion of the aromatic solvent (C 6 H 6–X Cl X ) used and the additionally monochlorinated aromatic solvent (C 6 H 6–Y Cl Y ) in this overhead stream, based on its total mass, is in particular 51% to 99%, particularly preferably 60% to 99% (where the remainder consists mainly to completely of TDI). Of course, this applies not only to TDI, but also to all isocyanates that can be purified in this way (separating low boilers and high boilers in two distillation columns in series or in a dividing wall column). In the prior art, this low-boiling overhead stream obtained in pure distillation (first gaseous) is usually condensed, and the condensate portion obtained after separating the non-condensable portion is recycled as reflux back into the pure distillation and partially withdrawn from the pure distillation and then recycled to another point in the process. However, if the low-boiling overhead stream contains a significant proportion of the additionally monochlorinated aromatic solvent C 6 H 6–Y Cl Y , i.e. the aromatic solvent of the formula C 6 H 6–X Cl X separated here is produced in a mixture with the aromatic solvent of the formula C 6 H 6–Y Cl Y , then this solvent-return is problematic, as has surprisingly been found within the scope of the present invention. This additionally monochlorinated solvent C 6 H 6–Y Cl Y gradually accumulates in the condensate of the low-boiling overhead stream due to its higher boiling point (without significant discharge as a gas phase (e.g. together with non-condensable gases)) and is thus enriched in the process after the solvent return and ultimately contaminates the isocyanate to be separated in this way. Within the scope of the present invention, this can be countered in various ways: Compared with the conventional prior art that provides for recycling the low-boiling top stream to another point in the process, the requirement for the purity of the separated isocyanate according to the present invention can be most easily achieved in this case if the resulting mixture is (completely) discharged and disposed of, in particular incinerated. However, since the desired product here can only be utilized thermally and not materially, it is preferred to recycle only the first part of the mixture obtained at the top of the dividing wall column or at the top of the first distillation column to another point in the process (i.e., the process for preparing isocyanate) and (intermittently or continuously) discharge the second part and then dispose of it, in particular incinerate it (see also Example 2).

[0117] Return to another point in the process can be carried out in step (A), or in an embodiment for separating the isocyanate from the gaseous product mixture obtained in step (A) including the above-mentioned washing with an aromatic solvent of formula C 6 H 6–X Cl X Preferably in said washing.

[0118] Instead of discharging the second part of the mixture obtained at the top of the dividing wall column or at the top of the first distillation column, this second part can also be purified in a further distillation (within the scope of the terms of the present invention, which is also considered a part of pure distillation), where an aromatic solvent of formula C 6 H 6–X Cl X is separated from the second part of the mixture and then at least partially, in particular completely, recycled to another point in the process. As described above, step (A) and optionally washing the gaseous product mixture obtained in step (A) (see Example 4) are suitable for this. Regarding the further distillation, reference can be made to the content described above regarding the solvent distillation. The content described there applies accordingly here. In particular, the bottom stream of the further distillation is also discharged here and incinerated or otherwise used, but is no longer recycled to the process. Preferably incinerated.

[0119] Of course, it is also possible to purify all of the mixture obtained at the top of the dividing wall column or at the top of the first distillation column in a further distillation and separate an aromatic solvent of formula C 6 H 6–X Cl X from this mixture and then at least partially, especially completely, recycle it to another point in the process. Step (A) and optionally washing the gaseous product mixture obtained in step (A) are again suitable for this.

[0120] A part (second part) of the mixture obtained at the top of the dividing wall column or at the top of the first distillation column and / or the bottoms stream of the further distillation after purifying a part or the entire mixture in a further distillation is discharged in such a way that the additionally monochlorinated aromatic solvent C 6 H 6–Y Cl Y is discharged from the process to a sufficient extent. The exact conditions (such as the quantitative ratio of the first part to the second part of the mixture and / or the exact embodiment of the further distillation and / or the discharge frequency when carried out at intervals) depend on the framework conditions of the individual case and can be readily determined by a person skilled in the art, optionally with preliminary experiments. The relevant framework conditions of course also include whether the additionally monochlorinated aromatic solvent C 6 H 6–Y Cl Y has already been partially discharged at another site (such as solvent purification).

[0121] The proportion of the aromatic solvent C 6 H 6–X Cl X used, which is optionally missing in accordance with the invention (a certain proportion of the aromatic solvent used is always discharged together when discharging the additionally monochlorinated aromatic solvent) is compensated for by newly fed-in solvent C 6 H 6–X Cl X . Of course, this does not depend on the exact site at which the additionally monochlorinated aromatic solvent is discharged.

[0122] Since the chlorination of the aromatic solvent used is caused by iron chlorides such as especially iron(III) chloride, one possibility of reducing this unwanted chlorination reaction (and thus reducing the content of additionally monochlorinated aromatic solvent in the separated isocyanate) consists in using a particularly corrosion-resistant stainless steel at all critical sites where wear damage is known to occur or is feared and where a significant solvent concentration is still present. The following types are particularly suitable for this: 1) Low-carbon austenitic nickel-molybdenum-chromium alloy having the following mass proportions based on the total mass: ● Carbon: 0.01% to 0.015%, ● Silicon: 0% to 0.08%, ● Manganese: 0% to 1.00%, ● Phosphorus: 0% to 0.025%, ● Sulfur: 0% to 0.015%, ● Chromium: 14.0% to 18.0%, ● Molybdenum: 14.0% to 17.0%, ● Titanium: 0% to 0.70%, ● Copper: 0% to 0.50%, ● Cobalt: 0% to 2.00%, ● Iron: 0% to 3.00%, ● Nickel: the balance up to 100%.

[0123] - Type 2.4610 stainless steel, also known as "Hastelloy C4".

[0124] 2) Austenitic special stainless steel having the following mass ratios based on the total mass: ● Carbon: 0% to 0.02%, ● Sulfur: 0% to 0.010%, ● Nitrogen: 0.15% to 0.25%, ● Chromium: 20.0% to 21.0%, ● Nickel: 24.0% to 26.0%, ● Manganese: 0% to 1.0%, ● Silicon: 0% to 0.5%, ● Molybdenum: 6.0% to 7.0%, ● Copper: 0.5% to 1.5%, ● Phosphorus: 0% to 0.03%, ● Iron: the balance up to 100%.

[0125] - Type 1.4529 stainless steel.

[0126] 3) Highly alloyed carbon - lean austenitic stainless steel having the following mass ratios based on the total mass: ● Carbon: 0% to 0.2%, ● Manganese: 2%, ● Nickel: 23% to 28%, ● Chromium: 19% to 23%, ● Sulfur: 0% to 0.3%, ● Molybdenum: 4% to 5%, ● Nitrogen: 0% to 0.1%, ● Copper: 1% to 2%, ● Phosphorus: 0% to 0.03%, ● Silicon: 0% to 0.7%, ● Iron: the balance up to 100%.

[0127] - Type 1.4539 stainless steel.

[0128] These stainless steels should be used in particular for the work-up starting from the feed vessel (tank vessel) for the crude liquid product mixture for the reaction, since in the actual reaction (i.e., in the reactor) particularly high-quality stainless steels are generally already used anyway. Particular attention should also be paid to the pipelines for connecting the feed vessel to the distillation equipment used and / or for connecting these distillation equipment to one another, as well as to the bottoms vessel of the pure distillation column and its bottoms piping.

[0129] The invention is illustrated in more detail below with the aid of examples.

[0130] Examples: The percentage and ppm data are mass ratios based on the total mass of the respective material streams.

[0131] Examples 1 to 4 describe the work-up (step (B)) of the TDI product mixture obtained in a gas-phase reaction (step (A)) according to variant 4 (without separate degassing, but with most of the solvent separated off separately in the solvent column before the pure distillation; pure distillation in a dividing-wall column). The solvent C 6 H 6–X Cl X used as the medium for cooling the reaction mixture in the gas-phase reaction is o-dichlorobenzene (ODB). The ODB-containing stream distilled off via the top of the dividing-wall column is, after condensation, partly returned as reflux to the dividing-wall column and partly recycled to the process (i.e., after the washing of the gaseous reaction products remaining after quenching in step (A)).

[0132] Example 1 (comparative, without discharging a stream of aromatic solvent containing formula C 6 H 6–Y Cl Y ): The purified TDI taken off as a side stream from the dividing-wall column has a trichlorobenzene (TCB) concentration of 11 ppm. The low-boiling stream withdrawn via the top contains 8.6% TDI, 88.6% ODB, and 2.2% TCB. Higher-chlorinated chlorobenzenes such as tetrachlorobenzene, pentachlorobenzene, and hexachlorobenzene cannot be detected using the gas chromatography analysis method used.

[0133] Example 2 (according to the present invention): In order to deplete the TCB, a one-time stream of 1.9 tons containing 8.6% TDI, 88.6% ODB, and 2.2% TCB is withdrawn from a part of the process for return and fed to external incineration. This corresponds to the discharge of the stream containing the aromatic solvent of formula C 6 H 6–Y Cl Y according to the invention. Here, the solvent return circuit is depleted of a total of 42 kg of TCB.

[0134] This one-time depletion of the TCB in the entire process causes the TCB concentration in the TDI side stream of the dividing-wall column to drop from 11 ppm to 6 ppm within 12 hours.

[0135] Example 3 (comparative, without discharging the stream of aromatic solvent containing formula C 6 H 6–Y Cl Y ; process simulation (VTPlan): The composition of the overhead stream of the TDI dividing wall column is as follows: ● 92.8% ODB, ● 5.1% TDI, ● 2.1% TCB.

[0136] The side stream (TDI product) of the TDI dividing wall column contains 15 ppm TCB.

[0137] Example 4 (according to the present invention; simulation as in Example 3): 300 kg / h of the overhead stream rich in TCB is diverted from a part for returning to the process and introduced into a downstream distillation column. This is a packed column with 24 trays, operating at an overhead pressure of 70 mbar. In this downstream distillation column, ODB with a TCB content of less than 1 ppm is separated at the top. The thus purified ODB is then recycled to the process in the same way. A stream rich in TCB with 3.0% TCB, 96.9% TDI, and 0.1% ODB is withdrawn from the bottom of the downstream distillation column at 14.5 kg / h, discharged, and fed for external waste utilization. This corresponds to the discharge inclusive formula C according to the present invention 6 H 6–Y Cl Y of the aromatic solvent stream.

[0138] Due to the continuous discharge, post-treatment, and return of the purified solvent ODB, the composition of the overhead stream of the dividing wall column changes as follows: ● 95.3% ODB, ● 4.56% TDI, ● 0.14% TCB.

[0139] By continuously removing trichlorobenzene from the process, the trichlorobenzene contamination in the TDI product is reduced from 15 ppm (Example 3) to less than 3 ppm.

Claims

1. A process for preparing isocyanates, comprising the steps: (A) Using formula C 6 H 6–X Cl X aromatic solvent, where X = 1 or 2, (a) as a diluent during the reaction, or (b) as a medium for cooling the reaction mixture formed by the reaction of an amine with phosgene, or (c) as a diluent during the reaction and as a medium for cooling the reaction mixture formed by the reaction of an amine with phosgene, reacting an amine with a stoichiometric excess of phosgene, wherein the amine is toluenediamine such that toluene diisocyanate is obtained as the isocyanate, thereby obtaining a liquid product mixture comprising the isocyanate and the aromatic solvent used and a gas product mixture comprising phosgene and hydrogen chloride; (B) Separating the isocyanate from the liquid product mixture obtained in step (A), optionally additionally including washing the gaseous product mixture obtained in step (A) with an aromatic solvent of the formula C 6 H 6– X Cl X to separate out the isocyanate. including a pure distillation step in which a separated isocyanate is produced as a product stream, in which at least one stream containing an aromatic solvent of the formula C 6 H 6–Y Cl Y is discharged in the pure distillation or in a distillation step in the pure distillation and upstream of the pure distillation, where Y = X + 1, such that the separated isocyanate has a mass proportion of the aromatic solvent of the formula C 6 H 6–Y Cl Y of 0.0 ppm to 9.9 ppm based on its total mass, wherein the pure distillation It is carried out in a dividing wall column, wherein an isocyanate product stream is obtained in the side stream discharge of the dividing wall column and an aromatic solvent of the formula C 6 H 6–X Cl X is obtained at the top of the dividing wall column, wherein the aromatic solvent of the formula C 6 H 6–X Cl X obtained at the top of the dividing wall column is obtained in a mixture with the aromatic solvent of the formula C 6 H 6–Y Cl Y ​ or It is carried out in two series-connected distillation columns without dividing walls, wherein an aromatic solvent of formula C 6 H 6–X Cl X is obtained at the top of the first distillation column and an isocyanate product stream is obtained as the distillate of the second distillation column, wherein the aromatic solvent of formula C 6 H 6–X Cl X obtained at the top of the first distillation column is obtained in a mixture with the aromatic solvent of formula C 6 H 6–Y Cl Y and wherein the mixture obtained at the top of the dividing wall column or at the top of the first distillation column is discharged and not returned to the process for preparing isocyanates, or The mixture obtained at the top of the dividing wall column or at the top of the first distillation column is purified in a further distillation, where an aromatic solvent of the formula C 6 H 6–X Cl X is separated from the mixture obtained at the top of the dividing wall column or at the top of the first distillation column and subsequently recycled to step (A) or to the washing step, where the remainder of the mixture obtained at the top of the dividing wall column or at the top of the first distillation column after separation of the aromatic solvent of the formula C 6 H 6–X Cl X is discharged and not returned to the process for preparing the isocyanate. or wherein a first part of the mixture obtained at the top of the dividing wall column or at the top of the first distillation column is recycled to step (A) or the washing step, and a second part of the mixture obtained at the top of the dividing wall column or at the top of the first distillation column is not recycled to step (A) or the washing step but is discharged and not returned to the process for preparing isocyanates, or A first portion of the mixture obtained at the top of the dividing wall column or at the top of the first distillation column is recycled to step (A) or to the washing step, and a second portion of the mixture obtained at the top of the dividing wall column or at the top of the first distillation column is purified in a further distillation, wherein an aromatic solvent of the formula C 6 H 6–X Cl X is separated from the second portion of the mixture obtained at the top of the dividing wall column or at the top of the first distillation column and subsequently recycled to step (A) or to the washing step, wherein the remainder of the second portion of the mixture obtained at the top of the dividing wall column or at the top of the first distillation column, after separation of the aromatic solvent of the formula C 6 H 6–X Cl X from the second portion of the mixture obtained at the top of the dividing wall column or at the top of the first distillation column, is discharged and not returned to the process for preparing the isocyanate.

2. The method according to claim 1, wherein the liquid product mixture obtained in step (A) is subjected to solvent distillation in step (B) prior to said pure distillation to separate out an aromatic solvent of the formula C 6 H 6–X Cl X .

3. The process according to claim 2, Among them, the aromatic solvent of the formula C 6 H 6–X Cl X is obtained in a mixture with the aromatic solvent of the formula C 6 H 6–Y Cl Y wherein a first portion of the mixture obtained in the solvent distillation is recycled to step (A), and a second portion of the mixture obtained in the solvent distillation is not recycled to step (A) but is discharged and not returned to the method for preparing the isocyanate. or Among them, the aromatic solvent of formula C 6 H 6–X Cl X is obtained in a mixture with the aromatic solvent of formula C 6 H 6–Y Cl Y A first portion of the mixture obtained in the solvent distillation is recycled to step (A), and a second portion of the mixture obtained in the solvent distillation is purified in a further distillation, wherein the aromatic solvent of formula C 6 H 6–X Cl X is separated from the second portion of the mixture obtained in the solvent distillation and then recycled to step (A), wherein the remaining portion of the second portion of the mixture obtained in the solvent distillation after separating the aromatic solvent of formula C 6 H 6–X Cl X from the second portion of the mixture obtained in the solvent distillation is discharged and not returned to the method for preparing the isocyanate, or Among them, the aromatic solvent of the formula C 6 H 6–X Cl X is obtained in a mixture with the aromatic solvent of the formula C 6 H 6–Y Cl Y The mixture obtained in the solvent distillation is purified in a further distillation. The aromatic solvent of the formula C 6 H 6–X Cl X is separated from the mixture obtained in the solvent distillation and then recycled to step (A). The remaining part of the mixture obtained in the solvent distillation after separating the aromatic solvent of the formula C 6 H 6–X Cl X from the mixture obtained in the solvent distillation is discharged and not returned to the method for preparing isocyanate.

4. The process according to any one of claims 1 - 3, wherein the pipes for connecting the tank container for receiving the liquid product mixture from step (A) to the distillation means for carrying out step (B) and / or for connecting these distillation means to each other are made of stainless steel of type 2.4610, 1.4529 or 1.4539.

5. The method according to any one of claims 1 - 3, wherein a product mixture is supplied to the pure distillation, and the mass ratio of the aromatic solvent of the formula C 6 H 6–X Cl X in the product mixture based on its total mass is 8% to 49%.

6. The method according to any one of claims 1 - 3, wherein the effluent stream containing the aromatic solvent of formula C 6 H 6–Y Cl Y contains the aromatic solvent of formula C in a mass proportion of 1.0% to 10% based on its total mass. 6 H 6–Y Cl Y of the aromatic solvent.

7. The process according to any one of claims 1 - 3, comprising (a) or (c).