Method for producing isocyanate

Abstract

To provide a method for producing isocyanate which can reduce a residual ratio of a solvent as much as possible.SOLUTION: A method for producing isocyanate includes: a step (A) of reacting amine and a stoichiometrically excess amount of phosgene as (a) a diluent during reaction and / or (b) means for cooling a reaction mixture formed by reaction of amine with phosgene, using an aromatic solvent represented by Formula: C6H6-XClX (where X=1 or 2) to obtain a mixture of a liquid product containing isocyanate and the aromatic solvent and a mixture of a gas product containing phosgene and hydrogen chloride; and a purification distillation step (B) of obtaining isolated isocyanate as a product flow, wherein the purification distillation or a distillation step that is an upstream step of the purification distillation includes a step of removing at least one flow containing an aromatic solvent represented by Formula: C6H6-YClY (where Y=X+1), so that the isolated isocyanate contains an aromatic solvent represented by Formula: C6H6-YClY having a mass fraction of 0.0 ppm to 9.9 ppm with respect to its total mass.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a process for producing isocyanates, comprising the step of reacting (A) an amine with a stoichiometric excess of phosgene with an isocyanate of formula CH as (a) a diluent during the reaction and / or (b) a means for cooling the reaction mixture formed from the reaction of the amine with phosgene (i.e., quenching). 6-X Cl X (wherein X=1 or 2), to obtain (optionally after decompression) a liquid product mixture comprising the isocyanate and the aromatic solvent used, and a gaseous product mixture comprising phosgene and hydrogen chloride, followed by (B) isolating the isocyanate from the liquid product mixture obtained in step (A), by a final distillation step in which the isolated isocyanate is obtained as a product stream, the isolated isocyanate being a compound of the formula CH in a mass fraction ranging 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, relative to the total mass of the isocyanate. 6-Y Cl Y to have an aromatic solvent of the formula CH 6-Y Cl Y wherein at least one stream comprising an aromatic solvent of formula (wherein Y=X+1) is withdrawn (at intervals or continuously) in a final distillation or in a distillation step upstream of the final distillation. [Background technology]

[0002] Isocyanates are produced in large quantities and mainly serve as starting materials for polyurethane production.Isocyanates are usually produced by reacting the corresponding amine with phosgene using a stoichiometric excess of phosgene.The reaction of amine with phosgene can be carried out in either gas phase or liquid phase, and the reaction can be carried out batchwise or continuously.The method for producing organic isocyanates from primary amines and phosgene has been described many times. Of interest on a large industrial scale are 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 (i.e., higher homologues of MMDI, hereinafter referred to as "PMDI" - "polymeric MDI"; mixtures of MMDI and PMDI are hereinafter abbreviated as "MDI") or tolylene diisocyanate (TDI), as well as aliphatic or cycloaliphatic isocyanates, such as pentane 1,5-diisocyanate (PDI), hexamethylene 1,6-diisocyanate (HDI), and isophorone diisocyanate (IPDI). Isocyanates containing benzyl isocyanate groups (aliphatic isocyanates), especially xylylene diisocyanate (XDI), are also important.

[0003] Modern industrial-scale production of isocyanates is carried out semi-continuously (with some preparative steps carried out batchwise, e.g., batchwise reaction and continuous workup) or continuously (all steps are carried out continuously).

[0004] This is characteristic of a liquid-phase process regime, typically referred to as liquid-phase phosgenation, in which reaction conditions are selected such that at least the amine, crude isocyanate, and phosgene reactants, but preferably all reactants, products, and reaction intermediates, are in liquid form in a suitable solvent under the selected conditions. Once the conversion is complete, the gas phase containing the hydrogen chloride by-product and unconverted phosgene (since it was used in superstoichiometric amounts) is separated. This leaves the desired isocyanate, along with the solvent, primarily in the liquid phase. The crude isocyanate is thus obtained in admixture with the solvent as a liquid stream that is worked up to obtain the pure isocyanate (and to recover the solvent and the dissolved fractions of phosgene and hydrogen chloride).

[0005] This is characteristic of gas phase process regimes, typically referred to as gas phase phosgenation, in which reaction conditions are selected such that at least the amine, isocyanate and phosgene reactants, but preferably all reactants, products and reaction intermediates, are gaseous under the selected conditions. The advantages of gas-phase phosgenation are, among others, a reduced "hold-up" of phosgene, the avoidance of difficult-to-phosgenate intermediates, improved reaction yields, and a lower energy requirement due to the use of less solvent. The reaction mixture, initially obtained in gaseous form in gas-phase phosgenation, is cooled in the so-called quench by contacting it with a quench liquid composed of a solvent or an isocyanate-solvent mixture, so that most of the desired isocyanate is liquefied, leaving behind a gas phase containing chlorides and phosphorus.

[0006] Therefore, all relevant industrial-scale isocyanate production processes generate a liquid crude isocyanate stream that must be worked up to obtain the desired isocyanate in pure form and recover other valuable materials, such as the solvent. This workup generally involves removal of the solvent, dissolved phosgene, and dissolved hydrogen chloride. This is followed by final purification of the isocyanate, which may include isomer separation, if necessary. Depending on the type of isocyanate, homolog separation may be performed before final purification. It is worth noting here that partial separation of MMDI from isocyanate mixtures containing MMDI and PMDI involves substantial removal of solvent, phosgene, and hydrogen chloride to obtain an MMDI fraction containing only traces of PMDI (crude MMDI) and a mixture of PMDI and MMDI.

[0007] The workup of crude isocyanate streams on an industrial scale is not easy, since many different requirements must be taken into account simultaneously. It is important to mention here not only the obtaining of the desired product in the purest form, but also the recovery of phosgene, hydrogen chloride, and solvent with minimal losses, especially in order to recycle them to the process (optionally, for example, after further conversion of hydrogen chloride to chlorine). All this must be done under conditions of maximum economic feasibility, i.e., with minimal energy consumption and with minimal loss of valuable products (especially isocyanates that may undergo undesirable further reactions in the case of an unoptimized workup). Needless to say, the desired isocyanate must be obtained as completely as possible free from by-products, aromatic solvents used, excess phosgene, etc. Since chlorinated aromatic hydrocarbons (especially monochlorobenzene or dichlorobenzene) have proven advantageous as solvents for isocyanate production, it is important to mention that in this context, the requirements for pure isocyanate are becoming increasingly stringent with regard to the residual content of such chlorinated aromatic solvents, which also poses challenges to distillation technology. These increased requirements are being introduced because polyurethane products made from isocyanates must not produce hazardous emissions, which is particularly important in the case of foams for seat cushions or mattresses. In addition to the aromatic solvents actually used, the prior art focuses on their fully chlorinated reaction products (i.e., hexachlorobenzene, when monochlorobenzene or dichlorobenzene is used as the solvent). Solvents with a higher chlorine content than the aromatic solvents used are formed by chlorination of the aromatic solvents used in the phosgenation and / or workup of the crude isocyanate. To the applicant's knowledge, the use of reaction products of aromatic solvents with only one additional chlorine substituent has not received particular attention in the prior art until now.

[0008] The workup of crude isocyanates has already been described many times.

[0009] Patent document 1 describes the workup of a crude isocyanate stream containing a liquid solvent obtained by liquid-phase phosgenation by separation of phosgene and hydrogen chloride ("dephosgenation") followed by separation of the solvent. This is followed by further purification by distillation of the crude isocyanate (from which phosgene, hydrogen chloride, and solvent have already been largely removed). In the case of MDI, this distillation also includes the separation of homologs, and MMDI is separated, leaving a PMDI / MMDI mixture depleted in MMDI (so-called "polymer separation").

[0010] Workup options, including the separation of homologues and isomers from MDI, are detailed in US Pat. No. 5,629,493. Crude TDI work-up options are described in US Pat. No. 5,623,999 for both liquid and gas phase phosgenation.

[0011] Patent Document 4 describes the production and workup 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. This produces not only a liquid product stream containing the desired isocyanate, but also a gas stream containing hydrogen chloride and phosgene. The isocyanate is isolated from the liquid product stream through multiple distillation steps. Excess phosgene is recovered as a phosgene gas stream and recycled to the reaction. This document teaches limiting the content of benzene, chlorobenzene, and dichlorobenzene in the recovered phosgene gas stream to a value of 0.5 wt.% or less. This reduces the hexachlorobenzene content in the isocyanate produced (see the examples in this application). This document is based on the finding that small amounts of benzene, chlorobenzene, or dichlorobenzene in the gas stream introduced into the reaction zone can lead to the formation of polychlorinated aromatic compounds, especially the problematic hexachlorobenzene. The term polychlorinated aromatics is understood to mean the reaction products formed primarily in the reaction zone by polychlorination of benzene, chlorobenzenes and dichlorobenzenes (see last paragraph on page 3).

[0012] According to the teaching of this document, the recovery of excess phosgene is first carried out by separating the gas mixture of hydrogen chloride and phosgene produced in the reaction into a gaseous hydrogen chloride stream and a liquid hydrogen chloride stream. The liquid phosgene stream is then partially evaporated. This is preferably carried out in a distillation column. The top of this distillation column can be provided with a point for feeding fresh liquid phosgene. The content of benzene, chlorobenzene, and dichlorobenzene in the gaseous phosgene stream removed from the distillation column can be reduced to the value of 0.5% by weight or less specified in accordance with the present invention by appropriately selecting the outlet temperature of this gaseous phosgene stream and, optionally, by appropriately selecting the amount of fresh phosgene additionally applied to the distillation column. This document does not specify any further means for achieving this value. In particular, this reference does not teach that a stream of aromatic solvent from a distillation step upstream of the final distillation step of isocyanate or further monochlorinated in the reaction is discharged from the overall process. The '491 patent does not indicate that reactions other than complete chlorination reactions occur, nor does the document indicate that chlorination reactions (of any kind) may be important in work-up situations (see first paragraph on page 4).

[0013] Patent Document 5 (see paragraph

[0015] ) describes a method for purifying isocyanates, which comprises: a) separating, in a distillation comprising at least one theoretical stage, a stream 1 comprising isocyanate, high-boiling and low-boiling components and non-evaporable residue into a split stream 2 comprising non-evaporable residue and isocyanate and a vapor stream 3 comprising isocyanate and low-boiling materials; b) keeping the non-evaporated residue of the divided stream 2 separate from the vapor stream 3 and / or the material stream at least partially comprising the vapor stream 3; c) separating from the divided stream 2 at least one further isocyanate-containing vapor stream 4 and a stream 8 containing substantially non-evaporable residue; d) separating the isocyanate-containing vapor stream(s) 4 and the vapor stream 3 from a) by distillation into three individual streams 5, 6, 7 having different boiling ranges; wherein minimum boiling stream 5 comprises a substantial proportion of the low boiling material of crude isocyanate stream 1, maximum boiling stream 7 comprises a substantial proportion of the high boiling material of crude isocyanate stream 1, and intermediate boiling stream 6 comprises substantially the desired product.

[0014] The crude isocyanate stream to be purified, stream 1, is preferably a stream from which hydrogen chloride, phosgene and solvent have been substantially separated when producing isocyanates by phosgenation. Stream 1 is a stream containing a mixture of isocyanates. As a result, the hydrogen chloride and phosgene contents in Stream 1 are in each case 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 not only contains the isocyanate obtained as the desired product, but also typically contains 100 ppm to 5% of components with a boiling point lower than that of the isocyanate (low boilers), 100 ppm to 5000 ppm of components with a boiling point higher than that of the isocyanate (high boilers) (provided that their boiling points are not more than 60°C higher than the boiling point of the isocyanate at standard pressure), and 1% to 8% by weight of polymeric non-evaporable residues (i.e., the product is thermally decomposed before evaporating at standard pressure). Therefore, solvents with a boiling point lower than that of the isocyanate are present in Stream 1 in only trace amounts, and in this context the term low boilers is clearly understood to mean the other components. Specifically, chlorine-containing by-products resulting from the further reaction of isocyanates (but not the solvent) are mentioned, inter alia (for example, 1,6-dichlorohexane in the case of producing the two chlorine-containing secondary components 1-isocyanato-6-chlorohexane or 1,6-diisocyanatohexane). The document does not mention partial or complete chlorination of the solvent. Typical low-boiling substances in the case of aromatic isocyanates are methylphenyl isocyanate (production of TDI) and phenyl isocyanate (production of MDI).

[0015] In a preferred embodiment of the described process shown in Figure 2, crude isocyanate stream 1 is fed laterally to a dividing distillation column where it is separated into three streams 5, 6, and 7, with lowest boiling stream 5 being removed above the dividing wall at the top of the column, intermediate boiling stream 6 (i.e., the desired product stream) being removed in the region of the dividing wall above the feed, and highest boiling stream 7 being removed at the bottom of the column. This example shows a typical configuration of these streams when producing TDI.

[0016] Lowest-boiling stream 5 contains 0.5 wt.% low-boiling materials (in addition to a significant proportion of isocyanates, i.e., 99.4 wt.% TDI), with the remaining 0.1 wt.% unspecified (see table at the top of page 7). Mid-boiling stream 6 contains 99.9 wt.% TDI, 10 ppm low-boiling materials, and 50 ppm high-boiling components. The remaining 0.094 wt.% (940 ppm) of mid-boiling stream 6 is unspecified. Highest-boiling stream 7 consists of 30 wt.% TDI and 70 wt.% high-boiling components. [Prior art documents] [Patent documents]

[0017] [Patent Document 1] International Publication No. 2017 / 050776 [Patent Document 2] International Publication No. 2019 / 134909 [Patent Document 3] International Publication No. 2018 / 114846 [Patent Document 4] International Publication No. 2019 / 145380 [Patent Document 5] European Patent Application Publication No. 1717223 Summary of the Invention [Problem to be solved by the invention]

[0018] With the objective of obtaining isocyanates with as low a residual solvent content as possible to avoid hazardous emissions in downstream polyurethane products, further factors have now been found to be important, and therefore further improvements were needed in this area. [Means for solving the problem]

[0019] Therefore, in view of this need, the present invention provides a method for producing an isocyanate, comprising: (A) an (aromatic, aliphatic, cycloaliphatic or araliphatic, preferably aromatic) amine and stoichiometric and a stoichiometric excess of phosgene, (a) as a diluent during the reaction; and / or (b) as a means for cooling (so-called quenching) the reaction mixture formed from the reaction of the amine with phosgene; Formula C6H 6-X Cl X reacting with an aromatic solvent of formula (wherein X=1 or 2) to obtain (optionally after decompression) a liquid product mixture comprising the isocyanate and the aromatic solvent used, and a gaseous product mixture comprising phosgene and hydrogen chloride; (B) isolating isocyanates from the liquid product mixture obtained in step (A), comprising a final distillation step in which the isolated isocyanates are obtained as a product stream, the isolated isocyanates being present in a mass fraction ranging 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 the total mass of the isocyanates. 6-Y Cl Y to have an aromatic solvent of the formula CH 6-Y Cl Y wherein at least one stream comprising an aromatic solvent of formula (wherein Y=X+1) is discharged (at intervals or continuously) in a final distillation or in a distillation step upstream of the final distillation (i.e., sent from a process for producing isocyanates); The present invention provides a method comprising:

[0020] In fact, the aromatic solvents used (CH 6-XCl X ), as well as reaction products with only one additional chlorine substituent (aromatic solvent containing one more chlorine, CH 6-Y Cl Y ) has also been found to need to be adequately separated from the desired isocyanate product. As a result of the typically multiple solvent recycle circuits within state-of-the-art isocyanate production processes, this also includes the aromatic solvent CH 6-Y Cl Y These additional chlorine-containing solvents, CH, must be pumped out of the process (i.e., discharged). 6-Y Cl Y In fact, this is crucial to being able to provide polyurethane products without hazardous emissions. 6-X Cl X In addition to limiting the content of aromatic solvents (CH) (as is customary in the art), one chlorine-containing aromatic solvent (CH) is also included. 6-Y Cl Y If the content of chloroaromatics (i.e., the solvent used and all its chlorinated products) is also limited, i.e., to 9.9 ppm or less as specified in the present invention, the total content of chloroaromatics (i.e., the solvent used and all its chlorinated products) can be assumed to be sufficiently low.

[0021] of formula CH in isolated isocyanate relative to total mass 6-Y Cl Y The mass fraction of aromatic solvents can in principle be determined by any method well known to those skilled in the art for determining the concentration of low molecular weight organic compounds. These generally provide consistent results with the precision required for the purposes of the present invention. Preference is given to measurements 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 decisive.

[0022] The term "discharge" and the verbs and adjectives derived therefrom relate in the context of the present invention to the discharge of a stream generated in a process (i.e. a process for producing isocyanates) from the process (i.e. from the process for producing isocyanates), i.e. the stream thus discharged is not recycled back into the process (neither to the reaction part nor to the work-up part). Preferably, the discharged stream is sent for disposal, in particular for incineration. However, the recovery of substances of such discharged streams, apart from their recycling into the process for producing isocyanates, is not excluded. Discharged streams in particular contain a compound of formula CH in a mass fraction in the range of 1.0% to 10%, preferably in the range of 1.5% to 5.0%, particularly preferably in the range of 2.2% to 3.0%, relative to their total mass. 6-Y Cl Y Contains aromatic solvents.

[0023] Below follows a brief summary of various possible embodiments of the present invention.

[0024] In a first embodiment of the invention, which can be combined with all other embodiments, the liquid product mixture obtained in step (A) undergoes dephosgenation in step (B) to separate the dissolved phosgene before final distillation.

[0025] In a second embodiment of the present invention, which can be combined with all other embodiments, the mixture of gaseous products obtained in step (A) is subjected in step (B) to a reaction with a compound of formula CH to separate the isocyanates. 6-X Cl X The mixture is subjected to scrubbing with an aromatic solvent.

[0026] In a third embodiment of the present invention, which can be combined with all other embodiments, the liquid product mixture obtained in step (A) is subjected to a step (B) of the formula CH 6-X Cl X The aromatic solvent is subjected to solvent distillation to separate the aromatic solvent (the solvent distillation is carried out by 6-X Cl X(This may include further distillation to purify the separated aromatic solvent).

[0027] In a fourth embodiment of the present invention, which is a specific configuration of the third embodiment, a compound of formula CH separated by solvent distillation is 6-X Cl X The aromatic solvent is sent to a solvent purification to separate the phosgene present therein (solvent dephosgenation).

[0028] In a fifth embodiment of the present invention, which can be combined with all other embodiments, provided that this does not exclude the use of a dividing column in the final distillation, in step (B) the final distillation is carried out using a side stream takeoff withdrawn from the dividing column. and a product stream of isocyanate of the formula CH 6-X Cl X (The final distillation is carried out in a dividing column to obtain an aromatic solvent of formula CH, obtained at the top of the dividing column.) 6-X Cl X (which may include a further distillation column for purifying the aromatic solvent).

[0029] In a sixth embodiment of the present invention, which can be combined with all other embodiments, provided that they do not provide for the use of a dividing distillation column in the final distillation, in step (B) the final distillation is carried out in two non-dividing distillation columns arranged in series, with a distillation product of formula CH at the top of the first distillation column. 6-X Cl X and a product stream of isocyanates as the distillate from the second distillation column (the final distillation is carried out using the aromatic solvent of formula CH obtained at the top of the first distillation column). 6-X Cl X (which may include a further distillation column for purifying the aromatic solvent).

[0030] In a seventh embodiment of the present invention, which can be combined with all other embodiments, provided that they include solvent distillation, a compound of formula CH separated by solvent distillation is 6-X Cl XThe aromatic solvents of formula CH 6-Y Cl Y of aromatic solvent (i.e. the mixture obtained by solvent distillation), a first part of this mixture (optionally after solvent dephosgenation) is recycled to step (A), and a second part of this mixture is discharged (at intervals or continuously) without being recycled to step (A).

[0031] In an eighth embodiment of the present invention, which is an alternative to the seventh embodiment and the ninth embodiment below, but which may otherwise be combined with all other embodiments, provided that these include solvent distillation, a compound of formula CH separated by solvent distillation is 6-X Cl X The aromatic solvents of formula CH 6-Y Cl Y of the formula CH 6-X Cl X is separated from the second portion of this mixture and then recycled (at least partially, in particular completely) to step (A), 6-X Cl X The portion of the second portion of the mixture (formula C6H6 -Y Cl Y (rich in aromatic solvent) is discharged (at intervals or continuously).

[0032] In a ninth embodiment of the present invention, which is an alternative to the seventh and eighth embodiments, but which may otherwise be combined with all other embodiments, provided that they include solvent distillation, a compound of formula CH separated by solvent distillation is 6-X Cl X The aromatic solvents of formula CH 6-Y Cl Y and an aromatic solvent of formula CH 6-XCl X is separated from this mixture and then recycled (at least partially, in particular completely) to step (A), 6-X Cl X A portion of this mixture (formula CH) remaining after separating the aromatic solvent from this mixture 6-Y Cl Y (rich in aromatic solvent) is discharged (at intervals or continuously).

[0033] In a tenth embodiment of the present invention, which is a specific configuration of the fifth embodiment / sixth embodiment, a distillation column having the formula CH obtained at the top of the dividing distillation column or the top of the first distillation column is 6-X Cl X The aromatic solvents of formula CH 6-Y Cl Y The aromatic solvent is obtained in a mixture with the aromatic solvent (i.e., the mixture obtained at the top of the dividing distillation column or at the top of the first distillation column).

[0034] In an eleventh embodiment of the present invention, which is a first specific configuration of the tenth embodiment, the resulting mixture (i.e., the mixture obtained at the top of the dividing distillation column or the top of the first distillation column) is discharged.

[0035] In a twelfth embodiment of the present invention, which is a second specific configuration of the tenth embodiment, a first portion of the mixture obtained at the top of the dividing distillation column or at the top of the first distillation column is recycled to step (A), and a second portion of this mixture is discharged (at intervals or continuously) without being recycled to step (A).

[0036] In a thirteenth embodiment of the present invention, which is a third specific configuration of the tenth embodiment, namely, in step (B), the mixture of gaseous products obtained in step (A) is treated with a compound of formula CH to separate isocyanates. 6-X Cl XIn the case where the process includes a step of scrubbing with an aromatic solvent (i.e., the second embodiment described above), a first portion of the mixture obtained at the top of the dividing distillation column or at the top of the first distillation column is recycled to the scrubbing step, and a second portion of this mixture is discharged (at intervals or continuously) without being recycled to the scrubbing step.

[0037] In a fourteenth embodiment of the present invention, which is a fourth specific configuration of the tenth embodiment, a first portion of the mixture obtained at the top of the dividing distillation column or at the top of the first distillation column is recycled to step (A), and a second portion of this mixture is purified by further distillation to produce a compound of formula CH 6-X Cl X is separated from the second portion of the mixture and then recycled (at least partially, in particular completely) to step (A), and an aromatic solvent of formula CH ... 6-X Cl X A portion of the second portion of this mixture (formula CH) remaining after separating the aromatic solvent 6-Y Cl Y (rich in aromatic solvents) is discharged (at intervals or continuously).

[0038] In a fifteenth embodiment of the present invention, which is a fifth specific configuration of the tenth embodiment, namely, step (B) comprises treating the mixture of gaseous products obtained in step (A) with a compound of formula CH to separate the isocyanate. 6-X Cl X In the case where the step of scrubbing with an aromatic solvent of formula CH is included (i.e., the second embodiment described above), a first portion of the mixture obtained at the top of the dividing distillation column or at the top of the first distillation column is recycled to the scrubbing step, and a second portion of this mixture is purified by further distillation to produce an aromatic solvent of formula CH 6-X Cl X is separated from the second portion of this mixture and then recycled (at least partially, in particular completely) to the scrubbing step or step (A), and the aromatic solvent of formula CH 6-X Cl X A portion of the second portion of this mixture (formula CH) remaining after separating the aromatic solvent 6-Y Cl Y (rich in aromatic solvents) is discharged (at intervals or continuously).

[0039] In a sixteenth embodiment of the present invention, which is a sixth specific configuration of the tenth embodiment, the mixture obtained at the top of the dividing distillation column or at the top of the first distillation column is purified by further distillation to produce a product of the formula CH 6-X Cl X is separated from this mixture and then recycled (at least partially, in particular completely) to step (A), and from this mixture is 6-X Cl X The portion of this mixture (formula CH) that remains after separating the aromatic solvent 6-Y Cl Y (rich in aromatic solvents) is discharged (at intervals or continuously).

[0040] In a seventeenth embodiment of the present invention, which is a seventh specific configuration of the tenth embodiment, namely, step (B) comprises treating the mixture of gaseous products obtained in step (A) with a compound of formula CH to separate the isocyanate. 6-X Cl X In the case where the step of scrubbing with an aromatic solvent of formula CH is included (i.e., the second embodiment described above), the mixture obtained at the top of the dividing distillation column or at the top of the first distillation column is purified by further distillation to obtain an aromatic solvent of formula CH 6-X Cl X is separated from this mixture and recycled (at least partially, in particular completely) to the scrubbing step or to step (A), and from this mixture is 6-X Cl X The portion of this mixture (formula CH) that remains after separating the aromatic solvent 6-Y Cl Y The aromatic solvent-rich mixture is discharged (at intervals or continuously).

[0041] In an eighteenth embodiment of the present invention, which can be combined with all other embodiments, the compounds of formula CH separated by solvent distillation are 6-X Cl X Aromatic solvents of the formula CH 6-Y Cl Yprovided that the solvent distillation is included in the process, in which the aromatic solvent is obtained in a mixture with the aromatic solvent (i.e., the mixture obtained by solvent distillation), the mixture is partially or completely discharged, and the discharged part of the mixture / discharged mixture is incinerated.

[0042] In a nineteenth embodiment of the present invention, which can be combined with all other embodiments, the piping used to connect the tank vessel for receiving the liquid product mixture from step (A) with the distillation means for carrying out step (B) and / or to connect these distillation means with each other is made of stainless steel of type 2.4610, 1.4529 or 1.4539.

[0043] In a twentieth embodiment of the present invention, which is a specific configuration of the nineteenth embodiment, the final distillation step to obtain the isolated isocyanate product stream is carried out in a distillation column having a vessel for receiving the liquid distillation bottoms fabricated from type 2.4610, 1.4529, or 1.4539 stainless steel.

[0044] In a 21st embodiment of the present invention, which can be combined with all other embodiments, the reaction of the amine with phosgene in step (A) is carried out in the gas phase, provided that it does not involve liquid phase phosgenation, and (b) is included.

[0045] In a 22nd embodiment of the present invention, which is a specific configuration of the 21st embodiment, the amine is selected from the group consisting of diamines of the diphenylmethane series (to obtain diisocyanates of the diphenylmethane series), 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), hexahydrotolylenediamine (to obtain hexahydrotolylene diisocyanate), 1,6-hexamethylenediamine (to obtain hexamethylene diisocyanate), 1,5-pentamethylenediamine (to obtain pentamethylene diisocyanate) and isophoronediamine (to obtain isophorone diisocyanate).

[0046] In a 23rd embodiment of the present invention, which can be combined with all other embodiments, the reaction of the amine with phosgene in step (A) is carried out in the liquid phase, provided that it does not involve gas phase phosgenation, and (a) is included.

[0047] In a 24th embodiment of the present invention, which is a specific configuration of the 23rd embodiment, the amine is selected from the group consisting of diamines and polyamines of the diphenylmethane series (to obtain diisocyanates and polyisocyanates of the diphenylmethane series), naphthalenediamine (to obtain naphthalenediisocyanate), tolylenediamine (to obtain tolylenediisocyanate), xylylenediamine (to obtain xylylenediisocyanate), bis(aminomethyl)cyclohexane (to obtain bis(isocyanatomethyl)cyclohexane), bis(aminomethyl ) norbornane (to obtain bis(isocyanatomethyl)norbornane), hexahydrotolylenediamine (to obtain hexahydrotolylene 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).

[0048] In a 25th embodiment of the present invention, which can be combined with all embodiments involving liquid or gas phase phosgenation, in particular gas phase phosgenation, the amine used is tolylenediamine (to obtain tolylene diisocyanate).

[0049] In a twenty-sixth embodiment of the present invention, which is a specific configuration of the twenty-fifth embodiment, X=2.

[0050] In a 27th embodiment of the invention, which is a specific configuration of the 23rd embodiment, the amine used is a mixture of diamines and polyamines of the diphenylmethane series (to obtain di- and polyisocyanates of the diphenylmethane series).

[0051] In a twenty-eighth embodiment of the present invention, which is a specific configuration of the twenty-seventh embodiment, X=1.

[0052] In a 29th embodiment of the present invention, which is a more specific configuration of the 21st embodiment, the amine is selected from the group consisting of hexamethylenediamine (to obtain hexamethylene diisocyanate), pentamethylenediamine (to obtain pentamethylene diisocyanate) and isophoronediamine (to obtain isophorone diisocyanate).

[0053] In a 30th embodiment of the present invention, which is a specific configuration of the 29th embodiment, X=1.

[0054] In a 31st embodiment of the present invention, which can be combined with all other embodiments, the final distillation contains a compound of formula CH in a mass fraction ranging from 8% to 49% relative to its total mass. 6-X Cl X A product mixture comprising:

[0055] In a 32nd embodiment of the present invention, which can be combined with all other embodiments, the final distillation contains a compound of formula CH in a mass fraction ranging from 10% to 30% relative to its total mass. 6-X Cl X A product mixture comprising:

[0056] In a 33rd embodiment of the present invention which can be combined with all other embodiments, The formula is C6H 6-Y Cl Y The effluent stream containing the aromatic solvent of formula CH is added with a mass fraction of the said solvent (i.e., a compound of formula CH) in the range of 1.0% to 10% relative to its total mass (i.e., the total mass of the effluent stream). 6-Y Cl Y aromatic solvents).

[0057] In a thirty-fourth embodiment of the present invention, which may be combined with all other embodiments, 6-Y Cl Y The effluent stream containing the aromatic solvent of formula CH is added with a mass fraction of the said solvent (i.e., a compound of formula CH) in the range of 1.5% to 5.0% relative to its total mass (i.e., the total mass of the effluent stream). 6-Y ClY aromatic solvents).

[0058] In a thirty-fifth embodiment of the present invention, which may be combined with all other embodiments, 6-Y Cl Y The effluent stream containing the aromatic solvent of formula CH has a mass fraction ranging from 2.2% to 3.0% of the said solvent (i.e., a compound of formula CH) relative to its total mass (i.e., the total mass of the effluent stream). 6-Y Cl Y aromatic solvents). DETAILED DESCRIPTION OF THE INVENTION

[0059] The embodiments briefly outlined above, as well as further possible embodiments of the invention, are described in more detail below. The embodiments can be combined with each other as appropriate, unless otherwise stated or apparent from the context.

[0060] Step (A) of the process according to the invention, i.e., the reaction of an amine with a stoichiometric excess of phosgene to obtain a liquid product mixture containing an isocyanate and the aromatic solvent used (in addition to a gaseous product mixture containing phosgene and hydrogen chloride), can be carried out in the context of the present invention in principle as known from the prior art. As explained at the beginning, there are two basic process variants: liquid-phase 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 vapors as a diluent. The addition of a solvent (or a mixture containing not only the solvent but also a portion of the desired isocyanate) to rapidly cool the reaction mixture (variant (b)) is preferably used in the gas-phase reaction regime.

[0061] The reaction of the primary amine with phosgene in step (A) is preferably carried out continuously, regardless of the process regime.

[0062] Examples of liquid-phase phosgenation are described in German Patent No. 3744001, European Patent Application Publication No. 0314985, European Patent Application Publication No. 1369412, German Patent Application Publication No. 10260027, German Patent Application Publication No. 10260093, German Patent Application Publication No. 10310888, German Patent Application Publication No. 102006022448, US Patent Application Publication No. 2007 / 0299279, and the references cited therein. Liquid-phase phosgenation includes variant (a) of step (A). Variant (b), the so-called quenching, is generally not required.

[0063] The process according to the invention preferably involves the use of diamines and polyamines of the diphenylmethane series (to obtain di- and polyisocyanates of the diphenylmethane series), naphthalenediamine (to obtain naphthalenediisocyanate), tolylenediamine (to obtain tolylenediisocyanate), xylylenediamine (to obtain xylylenediisocyanate), bis(aminomethyl)cyclohexane (to obtain bis(isocyanatomethyl)cyclohexane), bis(aminomethyl)norbornane (to obtain bis(isocyanatomethyl)norbornane), hexahydrotolylenediamine (to obtain hexahydrotolylenediisocyanate), 4,4'-diaminodicyclohexylmethane (to obtain 4,4'-diisocyanatodicyclohexylmethane), 1,6-hexamethylenediamine ( This allows the phosgenation of amines selected from hexamethylene diisocyanate, 1,5-pentamethylene diamine (to obtain pentamethylene diisocyanate), and isophorone diamine (to obtain isophorone diisocyanate). Toluene diamines and diamines and polyamines of the diphenylmethane series are particularly preferred, with diamines and polyamines of the diphenylmethane series being particularly preferred. For this purpose, mixtures of diamines and polyamines of the diphenylmethane series are phosgenated in a manner known per se to obtain the corresponding mixtures of di- and polyisocyanates of the diphenylmethane series (i.e., MDI), preferably with monochlorobenzene (X=1) being used as solvent in step (A) (although in principle dichlorobenzene (X=2) can also be used). The work-up (step (B)) of the product mixture obtained in step (A) in the case of MDI preferably also comprises a step of separating a fraction of diisocyanates of the diphenylmethane series (i.e., MMDI; "monomeric MDI") to obtain an MDI mixture depleted in diisocyanates of the diphenylmethane series, followed by a further final distillation of the separated monomeric MDI fraction (in the case of the remaining MDI mixture depleted in diisocyanates of the diphenylmethane series, the separation of monomeric MDI is considered as the "final distillation"). The requirement according to the invention regarding the residual content of this solvent in the product stream relates to the MDI mixture depleted in monomeric MDI and to the isolated MMDI itself.

[0064] In a preferred embodiment of liquid phase phosgenation, the procedure is as follows.

[0065] The reaction products of the primary amine and phosgene are dissolved separately in solvents. Suitable solvents for this purpose are monochlorobenzene (X=1) and dichlorobenzene (X=2), the latter in the form of the ortho- or para-isomer, preferably the ortho-isomer. The primary amine is preferably used in 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 in a concentration of 10% to 40% by mass, preferably 25% to 35% by mass, based on the total mass of the solution.

[0066] Efficient mixing of the primary amine and phosgene is very important in liquid-phase processes. In the prior art, static mixing devices (preferably nozzles) and dynamic mixing devices (including mechanically moving parts) have been used for this purpose. After mixing, the mixed reactants pass through a reaction zone to complete the conversion. The mixing device and reaction zone may also be located in a common reactor. Phosgene is used in stoichiometric excess over the primary amino groups of the amine, with the molar ratio of phosgene to primary amino groups typically ranging from 4.0:1 to 1.1:1, particularly preferably from 3.0:1 to 1.1:1, and even more preferably from 2.0:1 to 1.1:1.

[0067] Liquid phase phosgenation can be carried out at various temperature and pressure levels. Thus, for example, liquid phase phosgenation can be carried out at temperatures ranging from 0° C. to 250° C., preferably from 20° C. to 200° C., and at a pressure of 1.0 bar. (絶対) ~70bar (絶対) , preferably 1.0 bar (絶対) ~50bar (絶対) The reaction may be carried out at a pressure ranging from 0.1 to 1000 psi.

[0068] In a preferred embodiment, the hydrogen chloride formed as a by-product in the reaction is present in a partially dissolved form in the liquid phase and is partially outgassed. The proportion of dissolved hydrogen chloride compared to that in the gas phase depends on the temperature and pressure levels selected.

[0069] In another embodiment, the temperature and pressure levels are selected so that the hydrogen chloride is initially present in substantially fully dissolved or liquefied form, and only after deliberate decompression (e.g., in a gas-liquid separator) does the gas phase form, which, in the terms of the present invention, is an integral part of step (A).

[0070] Therefore, in either case, at the end of step (A), the isocyanates produced and A liquid stream containing the solvent and a gas stream containing hydrogen chloride and, optionally, evaporated solvent are obtained. Since phosgene is used in superstoichiometric amounts, both streams additionally contain phosgene. Both streams can be removed directly from the reaction zone. It is also possible to remove the two-phase product of the process (containing a liquid phase and a gas phase) from the reaction zone and transfer it to an apparatus for phase separation. This phase separation can be carried out in any apparatus known to those skilled in the art that is suitable for separating gas and liquid phases. For example, it is preferable to use gas-liquid separators such as cyclone separators, deflection separators, and / or gravity separators, with or without the aid of static separation. Similarly, the phase separation can be assisted by reducing the pressure compared to the pressure in the reaction zone, since this will promote the outgassing of hydrogen chloride (and any other gaseous components). The liquid stream removed from the reaction zone or, if present, from the apparatus for phase separation arranged downstream of the reaction zone, is in this embodiment the starting material for the work-up carried out in step (B), i.e., in the terms of the present invention, this liquid phase is a "liquid product mixture comprising isocyanate and the aromatic solvent used."

[0071] Liquid phase phosgenation is illustrated in more detail below using TDA as an example of a primary amine.

[0072] In the liquid phase process, TDA dissolved in one of the solvents defined above is fed to mix with phosgene at a temperature in the range from -10°C to 220°C, preferably from 0°C to 200°C, particularly preferably from 20°C to 180°C. Phosgene is fed to mix with TDA, also in one of the solvents defined above, at a temperature in the range from -40°C to 200°C, preferably from -30°C to 170°C, particularly preferably from -20°C to 150°C. The mixing of the TDA and phosgene solutions in the liquid phase process is preferably carried out using static or dynamic mixers. Examples of suitable static mixers include nozzles or nozzle arrangements such as those described, for example, in DE-A-1 792 660, U.S. Pat. No. 4,289,732 or U.S. Pat. No. 4,419,295. Examples of suitable dynamic mixers include pump-like assemblies such as centrifugal pumps (see U.S. Pat. No. 3,713,833) or certain mixer reactors (see EP-A-0291819, EP-A-0291820, EP-A-0830894).

[0073] In the liquid phase method, the reaction in the downstream reaction zone is carried out at a temperature in the range of 0°C to 250°C, preferably 20°C to 200°C, particularly preferably 20°C to 180°C, with an average residence time of the reaction mixture in the reaction zone in the range of 10 seconds to 5 hours, preferably 30 seconds to 4 hours, particularly preferably 60 seconds to 3 hours, and at a maximum pressure of 100 bar. (絶対) , preferably 1.0 bar (絶対) ~70bar (絶対) , particularly preferably 1.0 bar (絶対) ~50bar (絶対) With regard to the reaction in the reaction zone, examples of process regimes that can be used according to the present invention are described, for example, in US Patent Application Publication No. 2007 / 0299279 (in particular page 7, paragraphs

[0070] ,

[0071] ,

[0089] ) and DE Patent Application Publication No. 10310888 (in particular page 5, paragraphs

[0038] ,

[0039] ), and the references cited therein, respectively.

[0074] Examples of gas-phase phosgenation are described in EP-A-0570799, EP-A-1555258, EP-A-1526129, and DE-A-10161384, and, in the case of aliphatic isocyanates in particular, EP-A-0289840, EP-A-1754698, EP-A-1319655, and EP-A-1362847. The advantages of this process over otherwise conventional liquid-phase phosgenation include energy savings achieved by minimizing expensive and complex solvent and phosgene circuits. Gas-phase phosgenation involves variant (b) of step (A). In variant (a), in the case of gas-phase phosgenation, it is also possible, but not necessary, to use vapors of the aromatic solvent used in step (A) as diluent.

[0075] The process according to the invention allows the phosgenation of amines, preferably selected from diamines of the diphenylmethane series (to give diisocyanates of the diphenylmethane series), tolylenediamine (to give tolylenediisocyanate), xylylenediamine (to give xylylenediisocyanate), bis(aminomethyl)cyclohexane (to give bis(isocyanatomethyl)cyclohexane), bis(aminomethyl)norbornane (to give bis(isocyanatomethyl)norbornane), hexahydrotolylenediamine (to give hexahydrotolylenediisocyanate), 1,6-hexamethylenediamine (to give hexamethylene diisocyanate), 1,5-pentamethylenediamine (to give pentamethylene diisocyanate), and isophoronediamine (to give isophorone diisocyanate). Particular preference is given to tolylenediamine, and dichlorobenzene (X=2), especially the ortho-isomer, is preferably used as the solvent in step (A). For this purpose, tolylenediamine in the form of a mixture of various isomers, in particular 2,4- and 2,6-tolylenediamine (meta-tolylenediamine), is phosgenated in a manner known per se to give the corresponding mixture of tolylene diisocyanate isomers (i.e. TDI), in which, as mentioned above, preferably dichlorobenzene (X=2), in particular the ortho isomer, is used as solvent in step (A) (although in principle monochlorobenzene (X=1) can also be used).

[0076] In a preferred embodiment of gas phase phosgenation, the procedure is as follows.

[0077] First, a gas stream of a primary amine is provided. Methods suitable for this purpose are known in principle to those skilled in the art. Preferred embodiments are specified below.

[0078] The primary amine can be converted into the gas phase in all evaporators known from the prior art, in particular in falling film evaporators.Rather than falling film evaporators with high circulation power, it is preferred to use evaporators in which a small amount of working material is introduced.

[0079] To minimize thermal stress on the amine, regardless of the exact configuration of the evaporation apparatus, it is preferable to support the evaporation operation by introducing an inert gas such as N2, He, Ar (especially N2) or solvent vapor. Suitable solvents for this purpose are monochlorobenzene (X=1) and dichlorobenzene (X=2), the latter in the form of the ortho or para isomer, preferably the ortho isomer.

[0080] The evaporation (and, if necessary, superheating) of the starting amine (especially to temperatures in the range of 200°C to 430°C, preferably 250°C to 420°C, particularly preferably 250°C to 400°C) is preferably carried out in a single-stage or multi-stage, preferably multi-stage, manner in order to avoid non-evaporated droplets in the gaseous amine stream to the greatest extent possible. Particularly preferred are multi-stage evaporation and superheating processes in which a droplet separator is installed between the evaporation system and the superheating system (at least at one point, i.e., between at least one evaporation system and a subsequent superheating system or between each evaporation system and a subsequent superheating system) and / or the evaporation device also functions as a droplet separator. Suitable droplet separators are known to those skilled in the art.

[0081] In a further step, a gaseous phosgene stream is provided. It is preferable to establish a molar ratio of phosgene to primary amine groups of 1.1:1 to 20:1, particularly preferably 1.2:1 to 5.0:1. As described above for primary amines, the phosgene is also preferably heated to a temperature in the range of 200°C to 430°C, preferably 250°C to 420°C, particularly preferably 250°C to 400°C, and optionally diluted with an inert gas such as N2, He, Ar (especially N2), or with the vapors of an inert solvent as defined above for amines.

[0082] The gaseous reactants, primary amine and phosgene, are mixed in a mixing zone, and a The reactants react in a reaction zone separated by a gas barrier. The separately heated amine and phosgene reactants are preferably mixed and fed to the reaction via a nozzle arrangement. The nozzle arrangement for introducing the gas streams of the amine and phosgene reactants can be configured in various ways known to those skilled in the art, examples of which can be found, for example, in EP 2199277, paragraphs

[0017] to

[0019] , EP 1449826, paragraphs

[0011] to

[0012] , EP 1362847, paragraphs

[0011] to

[0012] , EP 1526129, paragraphs

[0009] to

[0011] , and EP 1555258, paragraphs

[0008] to

[0011] .

[0083] In addition to the above-mentioned options for diluting the gas stream of primary amine and the gaseous phosgene stream, it is also possible to introduce a separate dilution gas stream (an inert gas such as N, He, Ar, especially N) or the vapors of an inert solvent as defined above for the amine directly into the mixture. In this case, this dilution gas stream is preferably heated to a temperature in the range from 100°C to 500°C, preferably from 150°C to 450°C, particularly preferably from 150°C to 400°C.

[0084] The further conversion of the primary amine and phosgene reactant mixed in the mixing zone in the reaction zone is preferably carried out in an adiabatic manner. Adiabatic conversion means that the controlled removal of the generated reaction heat by a heat transfer medium is not required. Therefore, the reaction enthalpy is quantitatively reflected in the temperature difference between the product gas stream and the reactant gas stream (apart from unavoidable heat loss).

[0085] In the reaction zone, the amine and phosgene are rapidly converted, preferably adiabatically, to the corresponding isocyanate. The reaction is preferably conducted such that the amine is completely converted before entering a quench, which is described in more detail herein below.

[0086] The so-called quenching involves the liquefaction (quenching) of the formed isocyanates in a quenching zone by rapid cooling and contact with a quenching liquid in the quenching zone (except for traces remaining in the gas phase). Suitable quenching liquids include the above-mentioned solvents and mixtures of the isocyanates produced with the solvents mentioned. The contacting is preferably carried out by injecting the quenching liquid into the gas stream of the reaction product mixture. The construction and operation options for the quenching zone are known in principle from the prior art. Prior art devices and methods can also be used in the context of the present invention. Possible configurations of the quenching zone are disclosed, for example, in EP-A-1 403 248 and EP-A-1 935 875. The temperature of the quench liquid used for the quench is preferably selected, on the one hand, so that it is high enough to cleave the carbamoyl chloride corresponding to the isocyanate into isocyanate and hydrogen chloride (it is by no means certain whether the carbamoyl chloride intermediate known from liquid-phase phosgenation is also formed in gas-phase phosgenation. However, it is independently conceivable that the liquefied isocyanate partially reacts with the hydrogen chloride gas present during the quench to form carbamoyl chloride, and therefore the temperature of the quench liquid must be high enough to inhibit this reaction). On the other hand, the isocyanate and any solvent additionally used as a diluent in the gaseous amine stream and / or gaseous phosgene stream must be condensed as much as possible or dissolved in the solvent as much as possible, while excess phosgene, hydrogen chloride, and any inert gas additionally used as a diluent pass through the quench zone as uncondensed and undissolved as possible, so the temperature of the selected quench liquid must also not be too high. Quench liquids particularly suitable for selectively obtaining isocyanates from the gaseous reaction mixture are those maintained at a temperature between 50°C and 200°C, preferably between 80°C and 180°C. The mixture obtained in the quench zone comprises a gaseous portion and a liquid portion, i.e., is biphasic. This biphasic mixture is sent to a collection zone for phase separation. The liquid and gaseous phases are preferably continuously removed from the collection zone. In this embodiment, the liquid phase thus obtained is the starting material for the work-up carried out in step (B), i.e., this liquid phase contains the isocyanates and The resulting mixture of liquid products contains the aromatic solvent and the aromatic solvent used.

[0087] In a preferred embodiment, the mixing zone, reaction zone, quench zone, and collection zone are arranged in the above order from top to bottom in an upright, particularly conical or cylindrical or conical-cylindrical, reactor. In this embodiment, the mixture produced in the quench flows into the collection zone by gravity. In other collection zone configurations, it may be necessary in some circumstances to pump the mixture of reaction products and quench liquid into the collection zone.

[0088] After the phosgenation in step (A), step (B) comprises the work-up of the liquid product mixture from (A), i.e. the isolation of isocyanates from the liquid product mixture obtained in step (A) to obtain isocyanates of formula CH in a mass fraction ranging 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, relative to its total mass. 6-Y Cl Y According to the invention, step (B) comprises at least one final distillation step, preferably at least one distillation step arranged upstream of the final distillation step, in particular a distillation step of the formula CH 6-X Cl X In certain embodiments, step (B) also comprises a dephosgenation step for separating dissolved phosgene from the liquid product mixture obtained in step (A), the dephosgenation not necessarily having to be constituted as a distillation (see below). In certain cases, it may be necessary to arrange a dedicated distillation step upstream of the final distillation (residue separation) for separating the so-called residues (very high-boiling by-products of the phosgenation). The steps briefly outlined here will be explained in more detail below.

[0089] The final distillation step, and, if carried out, the distillation steps located upstream thereof, are preferably carried out continuously so as to obtain a continuous stream of isolated isocyanate. 6-Y ClY The mass fraction of aromatic solvents is determined in this continuously obtained stream of isolated isocyanate, in particular at intervals of 1 hour to 16 hours, preferably 4 hours to 12 hours. In the case of discontinuous step (B), the isolated isocyanate is obtained discontinuously, i.e., batch by batch. Then, a compound of formula CH 6-Y Cl Y It is preferred that the mass fraction of aromatic solvent of formula CH is determined for each batch. 6-Y Cl Y If the mass fraction of aromatic solvent is optimized to be sufficiently low, the measurement frequency can also be reduced, regardless of whether the isolated isocyanate is obtained continuously or batchwise.

[0090] Similarly, the isolated isocyanates of formula CH provided in accordance with the present invention, whether obtained continuously or batchwise, 6-Y Cl Y Any deviation from the mass fraction of aromatic solvents in 6-Y Cl Y This is countered by an increase in aromatic solvent emissions.

[0091] Examples of workup of phosgenation products are described in EP 1 413 571 (TDI), US 2003 / 0230476 (TDI), and EP 0 289 840 (HDI, IDPI, and H12-MDI). The basic procedures described therein can in principle also be employed in the process according to the invention. This is because the phosgenation product is prepared from an aromatic solvent CH 6 containing one additional chlorine atom. 6-Y Cl Y This is the case when the necessary discharge according to the invention of at least one stream containing CH can be incorporated into these procedures, so that 6-Y Cl Y The purity requirements regarding the concentration of isocyanate are met in the desired isocyanate.

[0092] As already mentioned above, optionally dissolved phosgene (and dissolved hydrogen chloride) is first separated in a separate step from the liquid product mixture obtained in step (A). This process regime is preferred, especially when the phosgenation in step A) is carried out in the liquid phase, since the liquid crude products obtained in liquid-phase phosgenation tend to contain significantly higher proportions of dissolved phosgene and dissolved hydrogen chloride than those obtained in gas-phase phosgenation. This so-called dephosgenation step is This can in principle be carried out in any way known to those skilled in the art, in particular by distillation, absorption or a combination of the two.

[0093] After the dephosgenation step, or immediately after step (A), in particular when step a) is carried out in the gas phase, the aromatic solvent CH used, as already mentioned above, is added in a separate step. 6-X Cl X (More precisely, most of them, i.e., the compounds of the formula CH 6-X Cl X is sufficient to obtain a liquid product mixture depleted of aromatic solvent, 6-X Cl X This is preferably followed by the separation of the aromatic solvent (whose mass fraction, relative to its total mass, ranges from 8% to 49%, preferably from 10% to 30%). This solvent separation is carried out by distillation (solvent distillation). The solvent thus separated can, if necessary, be subjected to solvent purification (i.e., solvent dephosgenation) to separate the phosgene present therein. This is preferably carried out by distillation in a solvent dephosgenation column, which gives a purified solvent containing at most traces of phosgene and isocyanate as the bottom product. However, such distillation carried out for dephosgenation does not achieve the separation of the solvent used from the aromatic solvent containing one more chlorine, since both are obtained together at the bottom of the dephosgenation column. For economic reasons, it is desirable to recycle the recovered, optionally dephosgenated, solvent to step (A). 6-X Cl X The recovered, optionally dephosgenated solvent is a compound of formula CH 6-Y Cl Y(Other proportions may be obtained from the bottom of the solvent distillation, 6-X Cl X (Leading in the liquid product mixture after depletion of the solvent). 6-X Cl X The solvent has the formula CH 6-Y Cl Y In one embodiment of the present invention, the entire mixture can be recycled to step (A), in which case the compound of formula CH, which is essential for the present invention, is obtained by solvent distillation in a mixture with a solvent of formula CH 6-Y Cl Y The discharge of the stream containing the aromatic solvent is completely transferred to the downstream step of the final purification of the isocyanate (i.e. the final distillation). However, it is also possible to carry out this discharge, which is essential for the invention, at least in part beforehand in this step of solvent separation. Suitable embodiments for this are described below.

[0094] Thus, in such an embodiment of the invention, solvent distillation is carried out to remove hydroxyl groups of the formula CH 6-X Cl X The aromatic solvent of formula CH 6-Y Cl Y and an aromatic solvent of formula CH, wherein a first part of the mixture (optionally after dephosgenation of the solvent, see above) is recycled to step (A), and a second part of the mixture is discharged (at intervals or intermittently) without being recycled to step (A) and is then incinerated or otherwise used, but is not recycled to the process. Incineration is preferred. In this embodiment of the invention, the described separation of the second part of the mixture (intentionally avoiding its recycling to the process) is achieved by the separation of a compound of formula CH, which is essential to the invention or parts thereof. 6-Y Cl Y constitutes the discharge of the stream containing aromatic solvents (if there are other discharge points).

[0095] The corresponding configuration of the solvent distillation is achieved by establishing the appropriate conditions of temperature, pressure, reflux ratio, and / or number of theoretical stages in this distillation. Of course, the exact conditions selected will depend on the type of aromatic solvent used in step (A), the amine, and the concentration of both the isocyanate and one additional chlorine-containing aromatic solvent in the starting mixture to be distilled, and can be easily determined by those skilled in the art for each application. In some cases, simple preliminary experiments may be necessary to identify the optimal distillation parameters.

[0096] Instead of discharging the second part of the mixture obtained in the solvent distillation, the second part of the mixture can be purified by further distillation (in the terms of the present invention, an integral part of the solvent distillation, and therefore a distillation step located upstream of the final distillation), to give a compound of formula CH 6-X Cl X is separated from the second portion of the mixture and then (optionally after dephosgenation of the solvent) at least partially, in particular completely, recycled to step (A). Naturally, all mixtures separated in the solvent distillation can be purified by further distillation to obtain aromatic solvents of formula CH 6-X Cl X of aromatic solvents from the mixture It is also possible to separate it from the phosgenated product and then (optionally after solvent dephosgenation) at least partly, in particular completely, to recycle it to step (A).

[0097] Regardless of whether only the second portion of the mixture or the entire mixture is sent to the further distillation, one additional chlorine-containing aromatic solvent CH 6-Y Cl Y has a relatively high boiling point and therefore accumulates in the bottom of this further distillation and leaves the distillation via the bottom stream, while 6-X Cl X of aromatic solvent stream (which contains at most an insignificant proportion of one more chlorine-containing aromatic solvent CH 6-Y Cl Y(including ) is taken as an overhead stream. This under-stream of further distillation is therefore discharged, incinerated or used in another way, but is not recycled to the process. Incineration is preferred. In this embodiment of the invention, the described separation of the under-stream of further distillation (intentionally avoiding its recycling to the process) is carried out by separating the under-stream of further distillation from the compound of formula CH, which is essential to the invention. 6-Y Cl Y constitutes the discharge of a stream containing aromatic solvents or a part thereof (if there are other discharge points).

[0098] Following the discharge of a part (second part) of the mixture obtained in the solvent distillation and / or the purification of a part / total of this mixture in a further distillation, the discharge of the bottom stream of this further distillation is carried out, resulting in the production of one further chlorine-containing aromatic solvent CH 6-Y Cl Y is discharged from the process to a sufficient extent. The exact conditions (e.g., the molar ratio of the first part to the second part of the mixture, and / or the exact configuration of the further distillation and / or the frequency of the discharges, if they are spaced apart, etc.) depend on the boundary conditions of each individual case and can be easily determined by one skilled in the art, optionally by carrying out preliminary experiments. Relevant process conditions include the addition of one chlorine-containing aromatic solvent CH 6-Y Cl Y This also includes whether there are further points in the process that discharge

[0099] This is because, although it is possible in principle to discharge the aromatic solvent containing one more chlorine to a sufficient extent for the purposes of the present invention in solvent distillation alone, this is costly and inconvenient, and therefore it is preferred to discharge the aromatic solvent containing one more chlorine mainly or entirely in the final purification step described below.

[0100] In particular, the product mixture fed to the final purification (i.e. final distillation) is preferably a mixture of products of formula CH with a mass fraction ranging from 8% to 49%, preferably from 10% to 30%, relative to its total mass, regardless of whether a solvent separation is arranged upstream of the final purification. 6-X Cl X(i.e., the solvent content in the stated range is established by selecting the conditions for solvent separation or is automatically established as a result of the reaction conditions of step (A)). This final purification of the isocyanate produced is carried out by distillation to separate the remaining low- and high-boiling organic secondary components (i.e., low-boiling and high-boiling substances) and any residual solvent, and optionally any remaining residual components of hydrogen chloride and phosgene. Any remaining solvent, hydrogen chloride, and phosgene also have lower boiling points than the isocyanate produced and are therefore separated mainly or entirely together with the low-boiling organic secondary components. The final distillation can then be composed of substeps, so that the separation of low-boiling and high-boiling substances is carried out in two distillation columns arranged in series. However, it is also possible to carry out these separation operations in one step (i.e., in a single distillation column) using a dividing distillation column.

[0101] Step (B) preferably also comprises (especially when step (A) is carried out in the gas phase) a work-up of the gaseous product mixture obtained in step (A) to separate any isocyanates present therein. This is done by separating the gaseous product mixture from the aromatic solvent CH 6-X Cl X It is preferable to scrub with a solution of formula C6H 6-X Cl X The solvent is an aromatic solvent CH6H6, which contains one more chlorine. 6-Y Cl Y The isocyanate-solvent mixture obtained by scrubbing can be sent to workup of the liquid product mixture from step (A).

[0102] Possible embodiments of step (B) are described in detail below using the example of TDI, a particularly preferred isocyanate, and first only the basic configuration of a preferred distillation sequence is clarified with reference to various variants (1-4). Needless to say, this basic configuration is not limited to the workup of TDI. Subsequently, it is explained how the process according to the invention can be carried out in a specific distillation sequence.

[0103] Variation 1 Variant 1, which is particularly suitable when step A) is carried out in the liquid phase, is in principle This is described in the PERP report on TDI / MDI (Chem Systems, Process Evaluation Research Planning TDI / MDI 98 / 99 S8, Tarrytown, NY, USA: Chem Systems 1999, pp. 27-32). In this variant, after the distillative separation of hydrogen chloride and phosgene is complete, the liquid reaction mixture still contains a solvent content of more than 50% by weight, preferably 51% to 85% by weight, particularly preferably 55% to 65% by weight, based on its total mass. This mixture is sent to solvent separation, where the solvent-TDI mixture is first distilled off in a pre-evaporator into a solvent distillation column. In the solvent distillation column, the solvent is distilled off and recycled to the front-end section of the process. The bottom stream from this solvent distillation contains not only TDI, but also, particularly preferably, 15% to 25% by weight of solvent, based on the total mass of this bottom stream. This stream is transferred to a so-called intermediate column, where further solvent is distilled off, and the solvent-free bottom product is sent to a final distillation column for TDI purification. This column is operated under negative pressure and provides a purified, saleable isocyanate TDI as a distillate stream. A portion of the TDI remains in the distillation bottom stream of this final distillation column. It is also possible, as described in particular in EP 1 371 635, to combine the functions of the intermediate column for TDI purification and the distillation column in a divided distillation column, resulting in an overhead stream of low boilers and solvent, pure TDI in the region of the dividing wall, and a product stream comprising TDI and high boilers (distillation residue) as a distillation bottom stream.

[0104] Variation 2 In contrast to variant 1 of this embodiment, the liquid reaction mixture after complete distillative separation of hydrogen chloride and phosgene retains a solvent content of only 50.0% by weight or less, based on its total mass. This mixture is sent to a pre-evaporator, from which the solvent-TDI mixture is distilled and enters a distillation column. In this variant, since the TDI has already been freed of solvent in a subsequent distillation column, the bottom stream from this distillation column can be transferred to a TDI purification column, resulting in this variant having one less column than variant 1. The TDI purification column is operated at negative pressure and provides purified isocyanate TDI, which can be sold as a distillate stream. It is also possible to combine the functions of the TDI purification column and an upstream distillation column in a divided distillation column, as described in particular in EP 1 413 571 A1, to obtain a product stream containing low-boiling materials and solvent overhead, pure TDI in the region of the dividing wall, and TDI and high-boiling components (distillation residue) as a distillation bottom stream.

[0105] Variation 3 Variant 3 comprises the distillation sequences described in variants 2 and 1, but without the pre-evaporators mentioned in each case. In this case, the fraction of the distillation residue in the described distillation sequence is included in the liquid material flow to the respective final TDI purification column. This process is also known in principle (Patent Document 5).

[0106] Variation 4 This variant is particularly applicable when step (A) is carried out in the gas phase. Since the liquid crude process product obtained in gas phase phosgenation contains relatively little dissolved phosgene and dissolved hydrogen chloride (i.e., compared to liquid phase phosgenation), it is preferable to use phosgene and chloride. Separate removal of hydrogen is avoided. Therefore, in this embodiment, the liquid product mixture is sent directly to a solvent separation, where the solvent, as well as any dissolved hydrogen chloride and any dissolved phosgene, are separated by distillation at the top, or, if the solvent content is sufficiently low, directly to a TDI purification column for final distillation. In either case, the TDI purification column is preferably configured as a dividing distillation column. Low-boiling materials (i.e., by-products boiling lower than TDI, any hydrogen chloride still present, any phosgene and solvent still present, and inert gases) are removed from the top of the TDI purification column. The overhead stream may further contain small amounts of entrained TDI. The purified TDI is removed as a distillate stream in the region of the dividing wall. The resulting distillation bottom stream contains the so-called distillation residue and a certain amount of TDI, which is not distilled off in order to keep the distillation bottom stream processable (flowable), and possibly traces of solvent. It goes without saying that, instead of a dividing distillation column, two non-dividing distillation columns arranged in series can also be used. In this latter embodiment, low boiling materials are removed at the top of the first distillation column and the bottoms stream from the first distillation column forms the feed to the second distillation column. Purified TDA is removed from the second distillation column as a distillate stream, while the bottoms stream from the second distillation column comprises distillation residue in admixture with TDI.

[0107] In this variant 4, the solvent separation, if carried out, is preferably carried out at a temperature ranging from 160°C to 200°C and at a pressure ranging from 160 mbar to 220 mbar, both ranges relating to the bottom of the distillation column used.

[0108] In particular when carried out in a dividing distillation column, the TDI final distillation is preferably carried out at a temperature in the range of 160°C to 200°C and a pressure in the range of 50 mbar to 100 mbar, both ranges relating to the bottom of the distillation column used.

[0109] Thus, in all the variants described, the final distillation of TDI provides an overhead stream of low boilers, i.e., an overhead stream from a fractional distillation column or from a distillation column located upstream of the non-fractional TDI purification column. The main component of this overhead stream of low boilers is the aromatic solvent CH used. 6-X Cl X The aromatic solvent (CH 6-X Cl X ) and aromatic solvents containing one more chlorine (CH 6-Y Cl Y ) is in particular 51% to 99%, particularly preferably 60% to 99%, based on the total mass (the remainder mainly or entirely consisting of TDI). This naturally applies not only to TDI, but also to all isocyanates suitable for the final purification described (separation of low-boiling and high-boiling substances in two distillation columns arranged in series or in one divided distillation column). In the prior art, it is customary to condense this low-boiling overhead stream obtained in the final distillation (initially in gaseous form) and to partially recycle the condensate obtained after separation of the non-condensable fraction to the final distillation as reflux, which is then partially discharged from the final distillation before being recycled to another point in the process. However, the low-boiling overhead stream also contains a significant proportion of one more chlorine-containing aromatic solvent, CH 6-Y Cl Y If the compound contains the formula CH, then the compound is 6-X Cl X The aromatic solvent of formula CH 6-Y Cl Y Surprisingly, as has been found in the context of the present invention, recycling of this solvent poses problems when the solvent CH is obtained in a mixture with an aromatic solvent containing one more chlorine. 6-Y Cl Y Because of its relatively high boiling point, it gradually accumulates in the condensate of the low-boiling overhead stream (e.g., it is not appreciably emitted as a gas phase along with non-condensable gases), thus contaminating the process and, ultimately, the isolated isocyanate as a result of solvent recycle. This can be countered in various ways in the context of the present invention.

[0110] The requirement according to the invention with regard to the purity of the isolated isocyanate is most easily achieved in this case if the mixture obtained is (completely) discharged and sent for disposal, in particular for incineration, in contrast to the usual prior art which provides for the recycle of the low-boiling overhead stream to another point in the process. However, since the desired product can only be recovered thermally and not physically, Therefore, it is preferred to recycle only a first portion of the mixture obtained at the top of the dividing distillation column or at the top of the first distillation column to another point in the process (i.e., the process for producing isocyanates), discharge (at intervals or continuously), and then send a second portion for disposal, in particular for incineration (see also Example 2).

[0111] Recycling to another point in the process may be to step (A) or to a reaction of formula CH to remove the isocyanate. 6-X Cl X In embodiments involving the above-described scrubbing of the gaseous product mixture obtained in step (A) with an aromatic solvent, it may be preferable to perform this scrubbing.

[0112] Instead of discharging the second portion of the mixture obtained at the top of the dividing distillation column or at the top of the first distillation column, this second portion can also be purified by further distillation (in the context of the terminology of the present invention, an integral part of the final distillation), to give a product of the formula CH 6-X Cl X The aromatic solvent is separated from the second portion of the mixture and then at least partially, in particular completely, recycled to another point in the process. Step (A) and, optionally, scrubbing of the gaseous product mixture obtained in step (A) (see Example 4) are suitable therefor, as described above. With regard to the further distillation, reference is made to what has been said above in relation to the solvent distillation. Therefore, what has been said above also applies here. In particular, here too, the bottom stream from the further distillation is discharged and incinerated or used in another way, but is not recycled to the process. Incineration is preferred.

[0113] Of course, similarly, any mixture obtained at the top of the dividing distillation column or at the top of the first distillation column may be further purified by distillation to produce a product of the formula CH 6-X Cl X The aromatic solvent can be separated from the mixture and then at least partially, in particular completely, recycled to another point in the process. Here, scrubbing of the mixture of step (A) and, optionally, the gaseous products obtained in step (A) is likewise suitable.

[0114] Following the discharge of a portion (second portion) of the mixture obtained at the top of the dividing distillation column or at the top of the first distillation column and / or the purification of a portion / the entire mixture in further distillation, the discharge of the bottom stream of this further distillation is now carried out, resulting in the production of one more chlorine-containing aromatic solvent CH 6-Y Cl Y is discharged from the process to a sufficient extent. The exact conditions (e.g., the molar ratio of the first part to the second part of the mixture, and / or the exact configuration of the further distillation and / or the frequency of the discharges in case of intervals, etc.) depend on the boundary conditions in each case and can be easily determined by the skilled person, optionally by carrying out preliminary experiments. Of course, the relevant boundary conditions include the addition of one chlorine-containing aromatic solvent CH 6-Y Cl Y This also includes whether it has already been partially discharged at other points (e.g., solvent refining).

[0115] The aromatic solvent used, CH, is missing as a result of the procedure according to the invention. 6-X Cl X (The discharge of one more chlorine-containing aromatic solvent always results in the simultaneous discharge of a proportion of the aromatic solvent used) with the freshly supplied solvent CH 6-X Cl X Of course, this is independent of the exact point at which the aromatic solvent containing one more chlorine is discharged.

[0116] Iron chlorides, especially iron(III) chloride, are known or feared to cause chlorination of the aromatic solvents used, and it is possible to reduce this undesired chlorination reaction (and thus the content of one more chlorine-containing aromatic solvent in the isolated isocyanate) by using particularly corrosion-resistant stainless steels at all key points where removal of corrosive substances is known or feared, and where significant solvent concentration still occurs. The following types are particularly suitable for this purpose: 1) Low carbon austenitic nickel-molybdenum alloy having the following mass fractions relative to the total mass: Nickel-chromium alloy: Carbon: 0.01%~0.015%, Silicon: 0%~0.08%, Manganese: 0% to 1.00%, Phosphorus: 0% to 0.025%, Sulfur: 0%~0.015%, Chromium: 14.0%~18.0%, Molybdenum: 14.0%~17.0%, Titanium: 0% to 0.70%, Copper: 0%~0.50%, Cobalt: 0% to 2.00%, Iron: 0%~3.00%, Nickel: balance up to 100% Type 2.4610 Stainless Steel (also known as "Hastelloy C4") 2) Austenitic special stainless steel having the following mass fractions relative to the total mass: Carbon: 0%~0.02%, Sulfur: 0%~0.010%, Nitrogen: 0.15%~0.25%, Chromium: 20.0%~21.0%, Nickel: 24.0%~26.0%, Manganese: 0% to 1.0%, Silicon: 0%~0.5%, Molybdenum: 6.0%~7.0%, Copper: 0.5%~1.5%, Phosphorus: 0% to 0.03%, Iron: balance up to 100%; Type 1.4529 stainless steel. 3) High-alloy, low-carbon austenitic stainless steels having the following mass fractions relative to the total mass: Carbon: 0%~0.2%, Manganese: 2%, Nickel: 23%~28%, Chromium: 19%~23%, Sulfur: 0%~0.3%, Molybdenum: 4% to 5%, Nitrogen: 0%~0.1%, Copper: 1%~2%, Phosphorus: 0% to 0.03%, Silicon: 0%~0.7%, Iron: balance up to 100%; Type 1.4539 stainless steel.

[0117] In any case, since particularly high-quality stainless steel is already used in the actual reaction (i.e. in the reactor), stainless steel must be used in the downstream workup, especially in the crude product vessel (tank vessel), to receive the mixture of crude liquid products of the reaction. Particular attention must also be paid to the piping used to connect the crude product vessel to the distillation means used and / or to connect these distillation means to each other and to the bottom vessel of the final distillation column and its bottom piping.

[0118] The present invention will now be described in more detail with reference to examples. [Example]

[0119] Amounts reported in percent and ppm are mass fractions relative to the total mass of the respective stream.

[0120] Examples 1 to 4 illustrate the work-up (step (B)) of the TDI product mixture obtained in the gas-phase reaction (step (A)) according to variant 4 (no separate dephosgenation, but separate separation of most of the solvent in a solvent column upstream of the final distillation; final distillation in a split distillation column). The solvent CH used in the gas-phase reaction was used as a means of cooling the reaction mixture. 6-X Cl X was ortho-dichlorobenzene (ODB). The ODB-containing stream removed overhead from the fractional distillation column was condensed and then partially recycled to the fractional distillation column as reflux and partially recycled to the process (i.e., for scrubbing the gaseous reaction products remaining in step (A) after quenching).

[0121] Example 1 (Comparative, Formula CH 6-Y Cl Y (Do not discharge streams containing aromatic solvents): The purified TDI taken as a side stream from the fractional distillation column had a trichlorobenzene (TCB) concentration of 11 ppm. The low-boiling stream taken overhead contained 8.6% TDI, 88.6% ODB, and 2.2% TCB. More highly chlorinated chlorobenzenes such as tetrachlorobenzene, pentachlorobenzene, and hexachlorobenzene were not detectable by the gas chromatography analytical method used.

[0122] Example 2 (present invention): To achieve depletion of TCB, 1.9 t of a stream containing 8.6% TDI, 88.6% ODB, and 2.2% TCB from the portion destined for recycle to the process was discharged and sent to a one-time external incineration. 6-Y Cl Y This corresponds to the discharge of a stream containing aromatic solvents. A total of 42 kg of TCB was depleted in the solvent recycle loop.

[0123] This one-time depletion of TCB throughout the process reduced the TCB concentration in the TDI sidestream from the dividing distillation column from 11 ppm to 6 ppm over a 12-hour period.

[0124] Example 3 (Comparative, Formula CH 6-Y Cl Y No aromatic solvent-containing stream is discharged; Process Simulation (VTPlan): The composition of the overhead stream of the TDI dividing column is as follows: 92.8% ODB, 5.1% TDI, 2.1%TCB.

[0125] The side stream (TDI product) from the TDI dividing column contains 15 ppm TCB.

[0126] Example 4 (invention; simulation experiment similar to Example 3): A 300 kg / h TCB-rich overhead stream from the portion intended for recycle to the process is diverted to a downstream distillation column. This is a packed column with 24 theoretical plates operating at a head pressure of 70 mbar. ODBs with a TCB content of less than 1 ppm are separated as the overhead product in this downstream distillation column. The ODBs thus purified are then likewise recycled to the process. As the bottom product from this downstream distillation column, a 14.5 kg / h TCB-rich stream containing 3.0% TCB, 96.9% TDI, and 0.1% ODBs is removed, discharged, and sent to external waste recovery. This is a TCB-rich stream of the formula CH according to the present invention. 6-Y Cl Y of aromatic solvent-containing stream discharged.

[0127] As a result of the continuous discharge, workup, and recycle of purified solvent ODB, the composition of the overhead stream from the dividing column changes as follows: 95.3% ODB, 4.56% TDI, 0.14%TCB.

[0128] This continuous removal of trichlorobenzene from the process reduces the trichlorobenzene contamination in the TDI product from the previous 15 ppm (Example 3) to less than 3 ppm.

Claims

1. 1. A method for producing an isocyanate, comprising: (A) an amine and a stoichiometric excess of phosgene, (a) as a diluent during the reaction; and / or (b) as a means for cooling the reaction mixture formed from the reaction of the amine with phosgene; Formula C ６ H ６－Ｘ Cl Ｘ wherein X=1 or 2, to obtain a liquid product mixture comprising the isocyanate and the aromatic solvent used, and a gaseous product mixture comprising phosgene and hydrogen chloride; (B) isolating isocyanate from the liquid product mixture obtained in step (A), comprising a final distillation step to obtain isolated isocyanate as a product stream, wherein the isolated isocyanate has a mass fraction of from 0.0 ppm to 9.9 ppm based on the total mass of the isocyanate, of formula C ６ H ６－Ｙ Cl Ｙ In the final distillation or in a distillation step upstream of the final distillation, an aromatic solvent of formula C ６ H ６－Ｙ Cl Ｙ wherein Y=X+1, and A method comprising:

2. The liquid product mixture obtained in step (A) is subjected in step (B) to a distillation step of the formula C ６ H ６－Ｘ Cl Ｘ 2. The process of claim 1, wherein the aromatic solvent is passed through a solvent distillation to separate the aromatic solvent.

3. In step (B), A final distillation is carried out in a fractional distillation column to obtain a product stream of isocyanate in a side stream withdrawn from the fractional distillation column and a product stream of isocyanate of formula C at the top of the fractional distillation column. ６ H ６－Ｘ Cl Ｘ or The final distillation is carried out in two successively arranged non-dividing distillation columns, and at the top of the first distillation column, a distillate of formula C is produced. ６ H ６－Ｘ Cl Ｘ and obtaining a product stream of isocyanate as a distillate from the second distillation column.

4. In step (B), A final distillation is carried out in a fractional distillation column to obtain a product stream of isocyanate in a side stream withdrawn from the fractional distillation column and a product stream of isocyanate of formula C at the top of the fractional distillation column. ６ H ６－Ｘ Cl Ｘ or The final distillation is carried out in two successively arranged non-dividing distillation columns, and at the top of the first distillation column, a distillate of formula C is produced. ６ H ６－Ｘ Cl Ｘ and obtaining a product stream of isocyanate as a distillate from the second distillation column.

5. Formula C separated by solvent distillation ６ H ６－Ｘ Cl Ｘ The aromatic solvent of formula C ６ H ６－Ｙ Cl Ｙ a first portion of the mixture obtained by solvent distillation is recycled to step (A), and a second portion of the mixture obtained by solvent distillation is discharged without being recycled to step (A), or Formula C separated by solvent distillation ６ H ６－Ｘ Cl Ｘ The aromatic solvent of formula C ６ H ６－Ｙ Cl Ｙ a first portion of the mixture obtained by solvent distillation is recycled to step (A) and a second portion of the mixture obtained by solvent distillation is purified by further distillation to give a product of formula C ６ H ６－Ｘ Cl Ｘ is separated from the second portion of the mixture obtained by solvent distillation and thereafter recycled to step (A), and an aromatic solvent of formula C is obtained from the second portion of the mixture obtained by solvent distillation. ６ H ６－Ｘ Cl Ｘ a portion of the second portion of the mixture obtained by the solvent distillation remaining after separating the aromatic solvent is discharged, or Formula C separated by solvent distillation ６ H ６－Ｘ Cl Ｘ The aromatic solvent of formula C ６ H ６－Ｙ Cl Ｙ and the mixture obtained by solvent distillation is purified by further distillation to obtain a compound of formula C ６ H ６－Ｘ Cl Ｘ is separated from the mixture obtained by the solvent distillation and then recycled to step (A), and an aromatic solvent of formula C is obtained from the mixture obtained by the solvent distillation. ６ H ６－Ｘ Cl Ｘ 5. The method according to claim 4, wherein the part of the mixture obtained in the solvent distillation remaining after separating the aromatic solvent is discharged.

6. Formula C separated by solvent distillation ６ H ６－Ｘ Cl Ｘ The aromatic solvent of formula C ６ H ６－Ｙ Cl Ｙ a first portion of the mixture obtained by solvent distillation is recycled to step (A), and a second portion of the mixture obtained by solvent distillation is discharged without being recycled to step (A), or Formula C separated by solvent distillation ６ H ６－Ｘ Cl Ｘ The aromatic solvent of formula C ６ H ６－Ｙ Cl Ｙ a first portion of the mixture obtained by solvent distillation is recycled to step (A) and a second portion of the mixture obtained by solvent distillation is purified by further distillation to give a product of formula C ６ H ６－Ｘ Cl Ｘ is separated from the second portion of the mixture obtained by solvent distillation and thereafter recycled to step (A), and an aromatic solvent of formula C is obtained from the second portion of the mixture obtained by solvent distillation. ６ H ６－Ｘ Cl Ｘ a portion of the second portion of the mixture obtained by the solvent distillation remaining after separating the aromatic solvent is discharged, or Formula C separated by solvent distillation ６ H ６－Ｘ Cl Ｘ The aromatic solvent of formula C ６ H ６－Ｙ Cl Ｙ and the mixture obtained by solvent distillation is purified by further distillation to obtain a compound of formula C ６ H ６－Ｘ Cl Ｘ is separated from the mixture obtained by the solvent distillation and then recycled to step (A), and an aromatic solvent of formula C is obtained from the mixture obtained by the solvent distillation. ６ H ６－Ｘ Cl Ｘ 3. The method of claim 2, wherein the portion of the mixture obtained in the solvent distillation remaining after separating the aromatic solvent is discharged.

7. The distillation column is formed by distilling a mixture of the distillation column and the first distillation column at a temperature of 1000° C. ６ H ６－Ｘ Cl Ｘ The aromatic solvent of formula C ６ H ６－Ｙ Cl Ｙ 6. The process according to claim 3, wherein the aromatic hydrocarbons are obtained in a mixture of 100% with an aromatic solvent.

8. The mixture obtained at the top of the dividing distillation column or the top of the first distillation column is discharged; or The mixture obtained at the top of the dividing distillation column or at the top of the first distillation column is purified by further distillation to produce a product of formula C ６ H ６－Ｘ Cl Ｘ is separated from the mixture obtained at the top of the dividing distillation column or the top of the first distillation column and then recycled to step (A), and an aromatic solvent of formula C is obtained from the mixture obtained at the top of the dividing distillation column or the top of the first distillation column. ６ H ６－Ｘ Cl Ｘ A portion of the mixture obtained at the top of the dividing distillation column or at the top of the first distillation column remaining after separating the aromatic solvent is discharged, or a first portion of the mixture obtained at the top of the dividing distillation 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 distillation column or at the top of the first distillation column is discharged without being recycled to step (A); or a first portion of the mixture obtained at the top of the fractional distillation 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 fractional distillation column or at the top of the first distillation column is purified by further distillation to produce a product of formula C ６ H ６－Ｘ Cl Ｘ is separated from the top of the fractional distillation column or the second portion of the mixture obtained at the top of the first distillation column and then recycled to step (A), and an aromatic solvent of formula C is separated from the top of the fractional distillation column or the second portion of the mixture obtained at the top of the first distillation column and then recycled to step (A), ６ H ６－Ｘ Cl Ｘ 8. The method according to claim 7, wherein a portion of the second portion of the mixture obtained at the top of the dividing distillation column or at the top of the first distillation column remaining after separating the aromatic solvent is discharged.

9. In step (B), in order to separate the isocyanate, the mixture of gaseous products obtained in step (A) is subjected to a reaction of the formula C ６ H ６－Ｘ Cl Ｘ scrubbing the mixture with an aromatic solvent of A first portion of the mixture obtained at the top of the divided distillation column or the top of the first distillation column is recycled to a scrubbing step, and a second portion of the mixture obtained at the top of the divided distillation column or the top of the first distillation column is discharged without being recycled to the scrubbing step, or a first portion of the mixture obtained at the top of the fractional distillation column or at the top of the first distillation column is recycled to a scrubbing step, and a second portion of the mixture obtained at the top of the fractional distillation column or at the top of the first distillation column is purified by further distillation to produce a product of formula C ６ H ６－Ｘ Cl Ｘ is separated from the top of the fractional distillation column or the second portion of the mixture obtained at the top of the first distillation column and then recycled to the scrubbing step or step (A), and an aromatic solvent of formula C is separated from the top of the fractional distillation column or the second portion of the mixture obtained at the top of the first distillation column and then recycled to the scrubbing step or step (A), ６ H ６－Ｘ Cl Ｘ a portion of the second portion of the mixture obtained at the top of the dividing distillation column or at the top of the first distillation column remaining after separating the aromatic solvent is discharged; or The mixture obtained at the top of the dividing distillation column or at the top of the first distillation column is purified by further distillation to produce a product of formula C ６ H ６－Ｘ Cl Ｘ is separated from the mixture obtained at the top of the fractional distillation column or at the top of the first distillation column and then recycled to the scrubbing step or step (A), and an aromatic solvent of formula C is obtained from the mixture obtained at the top of the fractional distillation column or at the top of the first distillation column. ６ H ６－Ｘ Cl Ｘ 8. The process according to claim 7, wherein a portion of the mixture obtained at the top of the dividing distillation column or at the top of the first distillation column remaining after separating the aromatic solvent is discharged.

10. 10. The process of any one of claims 1 to 9, wherein the piping used to connect the tank vessel for receiving the liquid product mixture from step (A) with the distillation means for carrying out step (B) and / or to connect these distillation means with each other is made from stainless steel of type 2.4610, 1.4529 or 1.4539.

11. The final distillation is provided with a mass fraction of the formula C ranging from 8% to 49% relative to its total mass. ６ H ６－Ｘ Cl Ｘ 11. The process of claim 1, wherein a product mixture comprising the aromatic solvent is provided.

12. Formula C ６ H ６－Ｙ Cl Ｙ 12. The process according to any one of claims 1 to 11, wherein the effluent stream comprising aromatic solvent contains a mass fraction of said solvent in the range of 1.0% to 10% relative to its total mass.