Method for preparing isocyanates

Abstract

The invention relates to a method for preparing isocyanates by reacting corresponding amines with phosgene. The invention further relates to an apparatus for separating exhaust gas flows from a phosgenation reaction.

Description

Process for the production of isocyanates The invention relates to a process for producing isocyanates by reacting corresponding amines with phosgene. Furthermore, the invention relates to a device for separating off-gas streams from a phosgenation reaction. Isocyanates are important raw materials in the chemical industry. They are often produced in large quantities. Diisocyanates and polyisocyanates, in particular, are used primarily as starting materials for the production of polyurethanes. Their large-scale production is generally achieved in good yields by phosgenating the corresponding amines, amine hydrochlorides, and amine carbonates with an excess of phosgene. The corresponding carbamic acid chloride is formed as an intermediate, which is then converted into the isocyanate by elimination of hydrogen chloride. In practice, chlorobenzene or o-dichlorobenzene have been established as solvents for the reaction. These are largely inert and well suited for recovering excess phosgene and separating it from hydrogen chloride. However, other solvents that are inert under the reaction conditions can also be used. The process offgas, which essentially consists of phosgene, hydrogen chloride, and solvent, must be treated to prevent its release. To improve the economic efficiency of the process, it is also advisable to treat the process offgas, which essentially consists of phosgene, hydrogen chloride, and solvent, so that at least some of these components, especially the phosgene, can be further processed or reused. Numerous variants are known for separating this process offgas stream, which differ depending on the specific application. US2007 / 0249859A1 describes a sequence of absorption steps comprising at least two stages, wherein the sequence comprises at least one isothermal absorption step and at least one adiabatic absorption step, and the phosgene obtained in this way is recycled to the phosgenation reaction. In DE102008009761A1, the process offgas stream is first partially condensed, and the liquid condensate is treated in a stripping column. There, a liquid solvent stream is obtained at the bottom and a gas stream at the top, which is passed into an absorption column together with the previously uncondensed fractions. There, the phosgene contained is absorbed in a solvent, and the resulting phosgene solution, optionally after enrichment with further phosgene, is reused in the phosgenation reaction. US2018 / 0044179A1 relies on distillative separation of the process off-gas. Here, the off-gas stream is fed into a distillation column, from whose bottom a phosgene-containing stream is withdrawn. At the top of the column, a phosgene-containing stream consisting essentially of hydrogen chloride is The stream is extracted, compressed, and partially condensed. The condensed portion is expanded and fed to the top of the column as reflux. While phosgene is often recycled in the same process, hydrogen chloride offers other recycling options. For example, it can be used or marketed simply as an aqueous solution, i.e., as hydrochloric acid. Alternatively, it can be catalytically or electrochemically oxidized to chlorine or used in the oxychlorination of ethylene to produce ethylene dichloride. Many isocyanates are produced in large quantities, and continuous processes are preferred. Isocyanates produced in smaller quantities are typically produced by batch liquid-phase phosgenation, as the effort required to convert to a continuous process is very high and not worthwhile given the small quantities produced. In some cases, hybrid processes are also used, combining a batch reaction with continuous, downstream processing of the produced isocyanates. This requires the installation of buffer vessels into which the reaction products are discharged and from which the continuous processing is then fed. For the batch production of isocyanates, treatment of the offgas and, if possible, the recovery of phosgene from this offgas is also desirable. However, this presents problems that have not yet been addressed in the prior art. For example, the mass flow of process offgas and its composition fluctuate greatly depending on the operating conditions. While it is relatively easy to install a buffer tank for liquid product streams, thus ensuring a consistent feed flow for processing, this is not easily possible for process offgas due to its large volumes and its toxic and corrosive components. US4233267A describes a combined system consisting of a batch reactor and a continuous distillation apparatus, in which the volatile reaction products are first condensed and temporarily stored in a condensate tank, from where they are fed into the continuously operating column, essentially independent of the operating parameters of the batch reactor. This approach presents several disadvantages for phosgenation processes. On the one hand, the condensation of phosgene or even HCl requires very low temperatures. On the other hand, such a procedure inevitably increases the amount of phosgene in the plant due to the necessary buffering. For the operation of an absorption system for the separation of phosgene from a process off-gas from isocyanate production, DE102008009761A1 and US2007 / 0249859A1 each recommend adjusting the amount of solvent for absorption to the phosgene mass flow. For the weight ratio of solvent to phosgene at the inlet to the For isothermal absorption, the range from 0.1:1 to 10:1 is disclosed as preferred and the range from 1:1 to 3:1 as particularly preferred. Surprisingly, however, it has now been discovered that the phosgene mass flow to the absorption device does not represent a good guideline for the required or even optimal mass flow of fresh solvent to the absorption device. This is particularly true for the batch production of isocyanates by phosgenation of the corresponding amines, which results in significant fluctuations in the composition and total mass flow of the process offgas. Rather, it has been observed that even when adhering to the preferred ranges for the solvent-to-phosgene ratio at the inlet to the isothermal absorption, phosgene breaks through into the offgas of the absorption device in phases, while in other phases an unnecessarily large amount of solvent is added. The object of the invention was therefore to provide an improved process for the production of isocyanates and a device for the separation of exhaust gas streams from a phosgenation reaction, with which phosgene can be separated from the process exhaust gas reliably and in a resource-saving manner by absorption, despite fluctuations in the composition and mass flow of the process exhaust gas from a phosgenation. This problem was solved by a process for producing an isocyanate comprising the steps: A) reacting at least one amine or its salt with phosgene to obtain a liquid reaction product containing the isocyanate and a first gaseous process off-gas stream containing hydrogen chloride and phosgene, B) Separation of the gaseous first process off-gas stream containing hydrogen chloride, phosgene, optionally solvents, optionally low-boiling secondary components and optionally inerts in at least one absorption device in which the first process off-gas stream is contacted with an absorption medium, whereby a phosgene-depleted second off-gas stream having a first phosgene content and a first phosgene solution having a second phosgene content are obtained, C) optionally mixing the first phosgene solution obtained in step B) with additional phosgene and / or solvent to obtain a conditioned second phosgene solution, D) optionally using the first phosgene solution from step B) or the conditioned second phosgene solution from step C) for the phosgenation of an amine, E) optionally isolating the isocyanate by distillative separation of the liquid reaction product obtained in step A, characterized in that • the first phosgene content of the phosgene-depleted second offgas stream obtained in step B) is compared with a first target value for this first phosgene content and / or the second phosgene content of the first phosgene solution obtained in step B) is determined and is compared with a previously determined second target value for this second phosgene content and • the absorption medium is introduced into the absorption device with a mass flow Ml and this mass flow Ml is subject to control, wherein the reference variable for the mass flow Ml is changed, preferably automatically, when the first phosgene content and / or the second phosgene content deviate from their respective target value. According to the invention, the terms "comprising" or "containing" preferably mean "consisting essentially of," and particularly preferably "consisting of." The further embodiments mentioned in the claims and in the description can be combined in any desired way, unless the context clearly indicates otherwise. "At least one," as used herein, refers to 1 or more, for example, 2, 3, 4, 5, 6, 7, 8, 9, or more. In the context of components of the compounds described herein, this statement does not refer to the absolute amount of molecules, but rather to the type of component. "At least one amine" therefore means, for example, that only one type of amine or several different types of amines may be present, without specifying the amount of the individual components. Numerical values ​​stated without decimal places refer to the full specified value with one decimal place. For example, "99%" stands for "99.0%." Numerical ranges specified in the format "in / from x to y" include the specified values. If multiple preferred numerical ranges are specified in this format, it is understood that all ranges resulting from the combination of the different endpoints are also included. In this context, “low-boiling secondary components” are understood to mean compounds which, at 10 5 Pa have a lower boiling point than the isocyanate produced in step A). ​​These can be, for example, chlorinated organic compounds or fragments of the isocyanate produced in step A). In a preferred embodiment of the process according to the invention, the target value for the first phosgene content is in the range from 0.00 vol% to 2.00 vol%, preferably in the range from 0.01 vol% to 1.00 vol%, particularly preferably in the range from 0.02 vol% to 0.50 vol%, and very particularly preferably in the range from 0.02 vol% to 0.2 vol%. A fixed first target value in this range can be compared with a determined actual value for the first phosgene content of the phosgene-depleted second offgas stream obtained in step B). The mass flow M1 of the absorption medium into the absorption device can then be adjusted, preferably automatically, based on the deviation between the target value and the actual value. In a further preferred embodiment of the process according to the invention, the target value for the second phosgene content is in the range from 20 wt.% to 70 wt.%, preferably in the range from 30 wt.% to 68 wt.%, and particularly preferably in the range from 42 wt.% to 66 wt.%. A fixed second target value in this range can be compared with a determined actual value for the second phosgene content of the first phosgene solution obtained in step B). Based on the deviation between the target value and the actual value, the mass flow M1 of the absorbent into the absorption device can then be adjusted, preferably automatically. In a further preferred embodiment of the process according to the invention, the two previously described embodiments are combined. In this embodiment, during normal operation, the second phosgene content is regulated by adjusting the mass flow M1 to the previously defined second setpoint. If the first setpoint for the first phosgene content, which now serves as the threshold value, is exceeded, i.e., if there is a deviation from normal operation, namely the penetration of an increased amount of phosgene into the second exhaust gas stream, the mass flow M1 is increased, so that more absorbent is introduced into the absorption device than would be required to adjust the second phosgene content in the first phosgene solution.The increase in mass flow Ml is preferably greater the greater the exceedance of the first setpoint, and remains active either for a specific time or preferably until the determined actual value for the first phosgene content has fallen below the first setpoint again. This is a so-called override concept in which, during the time of phosgene breakthrough, the control relating to the second phosgene content is inactive, and the mass flow Ml is instead specified based on the first phosgene content. In this way, the desired second phosgene content of the first phosgene solution can be maintained as precisely as possible during normal operation, while at the same time reducing or preventing phosgene from breaking through into the second exhaust gas stream. Examples of preferred diamines are diaminotoluene (TDA), in particular 2,4-diaminoluene and 2,6-diaminotoluene, diaminodimethylbenzene, diaminonaphthalene, in particular 1,5-diaminonaphthalene (NDA), diaminobenzene, in particular 1,4-diaminobenzene (pPDl), diaminodiphenylmethane (MDA), in particular 2,2'-diaminodiphenylmethane, 2,4'-diaminodiphenylmethane and 4,4'-diaminodiphenylmethane, 1,4-diaminobutane, 1,5-diaminopentane (PDA), 1,6-diaminohexane (HDA), 1,11-diaminoundecane, l-amino-3,5,5-trimethyl-5-aminomethylcyclohexane (1PDA), bis(p-aminocyclohexyl)methane (PACM), l,5-diamino-2-methylpentane, 2,5-Diamino-2,5-dimethylhexane, 1,4-Diaminocyclohexane, Hexahydrotoluenediamine (H6TDA), especially 2,4-Hexahydrotoluenediamine, 2,6-Hexahydrotoluenediamine, 1,3-Bis(aminomethyl)benzene (m-XDA), 1,4-Bis(aminomethyl)benzene (p-XDA), Bis(aminomethyl)cyclohexane (H6-XDA), tetramethylxylylenediamine (TMXDA), bis(aminomethyl)norbornane (NBDA), neopentanediamine, 2,4,4-trimethylhexamethylenediamine, 2,2,4-trimethylhexamethylenediamine and mixtures thereof. Examples of particularly preferred amines are 1,5-diaminonaphthalene (NDA), 1,4-diaminobenzene (pPDA), 1,5-diaminopentane (PDA), 1,6-diaminohexane (HDA), l-amino-3,5,5-trimethyl-5-aminomethylcyclohexane (1PDA), bis(p-aminocyclohexyl)methane (PACM), l,5-diamino-2-methylpentane, 2,5-diamino-2,5-dimethylhexane, 1,4-diaminocyclohexane, hexahydrotoluenediamine (H6TDA), in particular 2,4-hexahydrotoluenediamine and 2,6-hexahydrotoluenediamine, 1,3-bis(aminomethyl)benzene (m-XDA), 1,4-bis(aminomethyl)benzene (p-XDA), Bis(aminomethyl)cyclohexane (H6-XDA) and bis(aminomethyl)norbornane (NBDA) and mixtures thereof. Examples of particularly preferred amines are 1,5-diaminonaphthalene (NDA), 1,4-diaminobenzene (pPDA), 1,5-diaminopentane (PDA), bis(p-aminocyclohexyl)methane (PACM), hexahydrotoluenediamine (H6TDA), in particular 2,4-hexahydrotoluenediamine and 2,6-hexahydrotoluenediamine, 1,3-bis(aminomethyl)benzene (m-XDA) and Bis(aminomethyl)norbornane (NBDA) and mixtures thereof. Examples of most preferred amines are 1,5-diaminonaphthalene (NDA), 1,4-diaminobenzene (pPDA), bis(p-aminocyclohexyl)methane (PACM), 1,3-bis(aminomethyl)benzene (m-XDA) and bis(aminomethyl)norbornane and mixtures thereof. Instead of the amines, their salts, especially the hydrochlorides or carbamates, preferably the hydrochlorides of the amines, can also be phosgenated. These salts are then generally prepared in a first step in situ at low temperature, as described, for example, in DE19510259A1, in which case the introduction of an inert gas can be omitted if necessary. It is preferred to phosgenate the amines directly (base phosgenation) in a two-stage reaction at different reaction temperatures (cold-hot phosgenation). The reaction can take place in the presence of a solvent. In the case of gas-phase phosgenation, the presence of a solvent is often dispensed with during the reaction itself and instead added after the reaction in order to rapidly cool the reaction products. This process is also referred to as quenching. All solvents known to the person skilled in the art that are inert to the prevailing reaction conditions are suitable. Preferred solvents are selected from the group consisting of aromatic hydrocarbons, halogenated aromatic hydrocarbons, in particular chlorinated aromatic hydrocarbons, esters, ethers and halogenated hydrocarbons, and mixtures thereof. Aromatic hydrocarbons used with particular preference according to the invention are selected from the group consisting of toluene, bromobenzene, chlorobenzene, dichlorobenzene, in particular o-dichlorobenzene, and mixtures thereof.According to the invention, chlorobenzene, o-dichlorobenzene or mixtures of these two solvents are particularly preferably used. The reaction is preferably carried out in the condensed phase, i.e. as a so-called liquid-phase phosgenation, whereby solids and gases may also be present in the essentially liquid reaction mixture. The process according to the invention for the production of isocyanates is particularly advantageous for industrial processes in which the reaction of at least one amine or amine salt with phosgene takes place batchwise. Batchwise production of isocyanates, also known as batch or semi-batch operation, is characterized by particularly strong fluctuations in both the total mass flow and the composition of the process offgas. In a further embodiment, the reaction takes place batchwise in the condensed phase. In this case, batchwise reaction means that the liquid reaction product is not continuously withdrawn from the reactor or reactors, but rather the liquid reaction product is essentially only withdrawn from a reactor and transferred to a crude product receiver or directly for further processing once the conversion to isocyanate in this reactor has ended. Various methods for determining the end point of the reaction are known to those skilled in the art. In most cases, the so-called clear point is used, i.e. the time at which a clear solution has formed from the initially present suspension during hot phosgenation. Alternatively, the end of the evolution of HCl gas or a constant NCO content in the reaction mixture can also be used.However, even during the reaction, a portion of the liquid reaction mixture can be removed from the reactor and returned to it, for example, to achieve better mixing or to control the temperature of the reaction mixture, without violating the condition of batchwise phosgenation within the meaning of the present invention. Gaseous streams comprising essentially hydrogen chloride, phosgene, inert gas, and solvent vapors, however, can be removed from the reactors at any time, even continuously, and processed. The reaction is preferably carried out in a so-called semi-batch process, also referred to as a fed-batch process or feed process, i.e., amine and / or phosgene and optionally inert gas are fed in starting from a partial fill of the reactor during the reaction. Stirred tank reactors are particularly suitable, but other reactor types, such as loop reactors, can also be used in principle. The reaction is preferably carried out at a pressure of 1.0 bar(a) to 20 bar(a), preferably 1.05 bar(a) to 10 bar(a), and particularly preferably 1.1 bar(a) to 6 bar(a). Accordingly, the at least one reactor is preferably equipped with a pressure-maintaining device, such as a control valve, through which process offgas can escape from the reactor. Tubular reactors are particularly suitable for continuous reaction operations. In a further preferred embodiment of the process according to the invention, the reaction takes place in the form of a base phosgenation. In a base phosgenation, i.e. the reaction of the amines with phosgene, the reaction is preferably carried out in two stages in the inert solvent. Such reactions are described, for example, in Siefken, Liebigs Annalen der Chemie, 562 (1949), p. 96. In the first stage, the cold phosgenation, the temperature of the reaction mixture is preferably kept in a range between 0 and 100 °C. A suspension is formed which contains carbamic acid chloride, amine hydrochloride and small amounts of free isocyanate. Preferably, a solution of phosgene in an inert solvent is initially charged and then a solution or suspension of the amine in the same solvent and, if desired, further phosgene is added. In this way, the concentration of free amine is kept low and the undesired formation of ureas is suppressed. In the second stage, hot phosgenation, the temperature is increased and then preferably lies in a range of 120 to 200 °C. It is maintained in this range while additional phosgene is added until the conversion to isocyanate is complete, i.e., the evolution of HCl ceases and / or the reaction mixture clears. Phosgene is preferably used in excess. If necessary, the reaction can be carried out in both cold and hot phosgenation with the introduction of an inert gas. If the amines to be reacted are high-melting amines that are poorly soluble in the solvent, a suspension of the amine can also be used for phosgenation. This is preferably prepared by dispersion with a dynamic mixing unit, as described in EP2897933B1, paragraph

[0020] to paragraph

[0024] , described. In amine hydrochloride or carbamate phosgenation, the amine is preferably first reacted with hydrogen chloride gas or carbon dioxide in an inert liquid medium to produce the corresponding salt. The reaction temperature during this salt formation is preferably in the range of 0 to 80 °C. Subsequently or simultaneously, an initial reaction with phosgene can take place. This is followed by a further phosgenation step, which is essentially similar to the hot phosgenation from the base phosgenation described above and is therefore also referred to below as hot phosgenation. Here, too, the temperature is preferably kept in the range of 120 to 200 °C while phosgene and optionally an inert gas are passed into the reaction mixture. The introduction continues until the conversion to the isocyanate is complete. Here, too, phosgene is preferably used in excess to accelerate the reaction. Phosgenation is usually carried out with an excess of phosgene, i.e., more than one mole of phosgene is used per mole of amino groups. The molar ratio of total phosgene used to amino groups is preferably 1.02:1 to 20.0:1, particularly preferably 1.1:1 to 10.0:1, and most preferably 1.2:1 to 5.0:1. If desired, additional phosgene or phosgene solution can be added to the reaction mixture during the reaction. to maintain a sufficient excess of phosgene or to compensate for a loss of phosgene. In both base phosgenation and amine hydrochloride or carbamate phosgenation, the remaining phosgene and hydrogen chloride gas are in most cases at least partially removed after the reaction has ended, preferably by purging with an inert gas, preferably nitrogen, or, less preferably, under reduced pressure. If necessary, the reaction mixture can be filtered to remove any solids present, such as unreacted amine hydrochlorides. During batch phosgenation, two maximum values ​​generally occur for each batch in terms of the mass flow of process offgas. An increased mass flow of process offgas from a reactor occurs, for example, during the transition to hot phosgenation. Heating the reaction mixture decreases the solubility of gases in the mixture, resulting in the outgassing of dissolved gases and thus an increased offgas flow from the reactor, which, depending on the process selected, includes phosgene, hydrogen chloride, and / or carbon dioxide. A second maximum value occurs after the reaction has ended, when remaining phosgene and hydrogen chloride are expelled from the reaction mixture and, if the reaction was carried out under pressure, the reaction vessel is depressurized. In order to reduce these fluctuations in the process offgas stream that occur in the batch process and to enable a more economical design of the apparatus and operating conditions, it is preferable to carry out the reaction in step A) in at least two reactors arranged in parallel, more preferably in two to ten reactors arranged in parallel, most preferably in two to six reactors arranged in parallel, and most preferably in two or four reactors arranged in parallel, with at least one reactor being operated asynchronously to at least one of the other reactors. This means that the heating of the reaction mixture to above 100°C and / or the expulsion of remaining phosgene and hydrogen chloride after the end of the reaction, which optionally also includes depressurizing the reactor to a lower pressure, does not take place simultaneously or under the same conditions in all reactors. For example, during the transition to hot phosgenation, it is possible to implement the same temperature ramp with a time delay, or to implement a different, in particular flatter, temperature ramp for the reactor not operated synchronously with the other reactors, so that the associated maximum of the exhaust gas mass flow is lower and / or does not coincide with the maximum of the exhaust gas mass flows of the other reactors. Particularly preferably, the reactor implements the same temperature ramp with a time delay, so that the temperature increase in the transition to hot phosgenation occurs either before or after the temperature increase of the other reactors. The time delay is preferably The time offset is at least 5 minutes, particularly preferably at least 10 minutes, and most preferably at least 20 minutes. The time offset is also preferably a maximum of 24 hours, preferably a maximum of 8 hours, particularly preferably a maximum of 2 hours, and most preferably a maximum of 1 hour. The same applies to the expulsion of remaining phosgene and hydrogen chloride. Here, for example, the expansion and / or the optional introduction of inert gas in at least two of the parallel reactors can take place asynchronously, i.e. with a time offset and / or at different speeds, in order to ensure that the associated maximum off-gas mass flow is lower and / or does not coincide with the maximum off-gas mass flow rates of the other reactors. Particularly preferably, the expansion and / or the optional introduction of inert gas takes place with the same specifications regarding the temporal pressure profiles or inert gas mass flow rates as for the other reactors, but with a time offset. The expulsion of phosgene and / or hydrogen chloride preferably takes place with a time offset of at least 5 minutes, more preferably at least 10 minutes, and most preferably at least 20 minutes.Also preferably, the time offset is a maximum of 24 hours, preferably a maximum of 8 hours, particularly preferably a maximum of 2 hours and most particularly preferably a maximum of 1 hour. In a further preferred embodiment of the invention, the asynchronously operated reactors are operated according to the same operating instructions, but run through the individual process steps with a time offset of at least 5 minutes, preferably at least 10 minutes, and particularly preferably at least 20 minutes. The direction of the time offset, i.e., which reactor runs first or second, is irrelevant. In a further preferred embodiment, none of the parallel reactors is operated synchronously with any of the other reactors. Preferably, all reactors are operated according to the same operating instructions, but run through the individual process steps with a time offset from one another. The first process offgas generally contains phosgene, hydrogen chloride, and optionally solvent vapors, inert gas, and traces of other volatile components of the reaction mixture, such as amines or isocyanates. In step B), this process offgas is separated in at least one absorption device into a phosgene-depleted second offgas stream and a phosgene-containing absorbent, i.e., a first phosgene solution. The separation in step B) preferably takes place in exactly one absorption device. Suitable absorption devices are, for example, gas scrubbers, in which the first process exhaust gas stream is mixed with a liquid stream, also called absorbent or scrubbing medium, in contact is brought, such as immersion scrubbers, spray scrubbers, packed columns, structured packing columns, tray columns, or falling-film absorbers. For efficient absorption of the phosgene from the first process offgas stream, different types of gas scrubbers can also be combined. It is preferred to combine two or more, particularly preferably two, gas scrubbers in one absorption device. In a further preferred embodiment, the absorption device comprises a gas scrubber selected from the group consisting of packed columns, structured packing columns, and tray columns, preferably consisting of packed columns and structured packing columns. A particularly suitable absorption device comprises a falling-film absorber suitable for carrying out isothermal absorption and a tray, packed, or structured column, preferably a packed or structured column, particularly preferably a structured column, suitable for carrying out adiabatic absorption. The falling-film absorber is preferably a vertically arranged tube bundle heat exchanger, wherein the process exhaust gas and the absorbent are conducted on both the tube and shell sides of the heat exchanger, preferably on the tube side of the heat exchanger, while on the other side a cooling medium is conducted in cocurrent or countercurrent, preferably countercurrent, to the process exhaust gas stream. The process exhaust gas and the absorbent can also be conducted in cocurrent or countercurrent to one another, preferably countercurrent to one another.The tubes can optionally be provided with random packing, packings, or other internals to improve mass and / or heat transfer. The tray, packed, or packed column is preferably designed such that the process offgas from the isothermal gas scrubber is contacted with the absorbent in countercurrent. The absorbent is preferably introduced at the top of the column via a liquid distributor. Preferably, the column is a structured packing column. Particularly preferably, the falling-film absorber and the tray, packed, or packed column are combined in a column-shaped apparatus that includes a feed line for the process offgas at the bottom end. The falling-film absorber is arranged above it, and above it, the tray, packed, or packed column.Liquid distributors are preferably located above the falling-film absorber and at the top of the column-shaped apparatus to distribute the absorbent evenly among the tubes or column. An internal or external condenser is optionally located at the top of the absorption device for further cooling of the now phosgene-depleted offgas stream. The second offgas stream, which is as low in phosgene as possible, usually consists essentially of hydrogen chloride, i.e. to at least 80 vol%, preferably to at least 95 vol% and particularly preferably to at least 98 vol%. It also contains residues of the solvent, optionally inert gases and traces of phosgene. The phosgene content of this offgas stream is preferably at most 1.0 vol%, particularly preferably at most 0.5 vol% and very particularly preferably at most 0.2 vol%. The natural and fundamentally desirable lower limit for the phosgene content in this offgas stream is 0.00 vol%. However, compliance with this limit can On the one hand, this can lead to increased absorbent consumption, and on the other hand, it can be advantageous for the design of a control system to allow a low phosgene content in the exhaust gas, e.g., to use this as a control variable. Therefore, the phosgene content in the second exhaust gas stream is preferably at least 0.01 vol%, more preferably at least 0.02 vol%, and most preferably at least 0.05 vol%. In order to achieve the most efficient absorption possible, it is advantageous to first cool the first process exhaust gas to a temperature in the range from 5 °C to -25 °C, preferably in the range from 0 °C to -20 °C, particularly preferably in the range from -5 °C to -15 °C. The cooling can be carried out simply by means of one or more heat exchangers or by quenching, i.e. by bringing the first process exhaust gas into contact with an already cooled liquid in a quenching device. This can be done, for example, in a gas scrubber, preferably a spray scrubber, or in a sprinkled heat exchanger. The absorbent, for example, is a suitable cooled liquid. Preferably, the absorbent is separated as a liquid after contact with the process exhaust gas, and this liquid is recirculated to the quenching device, wherein the liquid circuit or the quenching device itself contains a cooler to cool the liquid to the desired temperature.During cooling, partial condensation or absorption of phosgene occurs. The remaining gas phase is then contacted with the absorbent, preferably in countercurrent. Preferably, the ascending gas phase is contacted in countercurrent with the descending liquid absorbent. Suitable devices for this are known to those skilled in the art. The absorption preferably involves a combination of two absorption steps, with the gas preferably first undergoing isothermal absorption and then adiabatic absorption. The mass flow of the absorbent is variable and can be adapted to the prevailing process conditions and the resulting requirements. The absorbent is preferably selected from the group consisting of aromatic hydrocarbons, halogenated aromatic hydrocarbons, in particular chlorinated aromatic hydrocarbons, esters, ethers, and halogenated hydrocarbons, or mixtures thereof.According to the invention, chlorobenzene, o-dichlorobenzene, or mixtures of these two solvents are particularly preferably used as the absorbent. The absorbent is most preferably chlorobenzene. In a further preferred embodiment, a solvent is used as the absorption medium in the at least one absorption step, which is preferably the solvent which is also present in the reaction of the at least one amine or its salt. In the preferred embodiment with two absorption steps, fresh or processed absorbent is preferably introduced in the second absorption step, i.e. the absorption step through which the gas subsequently passes, while in the first absorption step, preferably already loaded absorbent from the second absorption step, optionally together with fresh absorbent and / or a recycled, also already loaded absorbent from the first absorption step and / or the preceding cooling process. Preferably, after contact with the absorbent, the gas stream passes through a condenser to condense the absorbed absorbent and separate it as completely as possible from the gas stream. The loaded absorbent can, as described above, be used as a quench medium to initially cool the first process offgas stream. A portion of the absorbent is withdrawn either continuously or discontinuously and, optionally after enrichment with further phosgene, used in one of the reactors for phosgenating the amine or its salt. The withdrawn, loaded absorbent is preferably a solution containing 20 wt. % to 70 wt. %, more preferably 30 wt. % to 68 wt. % and most preferably 42 wt. % to 66 wt. % of phosgene in chlorobenzene. The content of the solution depends in particular on the phosgene content of the first process offgas, on the amount of fresh absorbent for absorption and on the amount of absorbent withdrawn. The controls in the device described according to the invention can be carried out, for example, with computer support. Therefore, a further subject of the invention is a device for separating off-gas streams from a phosgenation reaction, comprising at least one, preferably precisely one, absorption device for contacting a first process off-gas stream containing hydrogen chloride and phosgene with an absorbent, and comprising a computer for controlling the separation, characterized in that the computer is configured to control the process according to the invention. Alternatively or preferably, a further subject of the invention is a device for separating exhaust gas streams from a phosgenation reaction comprising • at least one absorption device for contacting a first process off-gas stream containing hydrogen chloride and phosgene with an absorption medium M1 to obtain a phosgene-depleted second off-gas stream having a first phosgene content and a first phosgene solution having a second phosgene content, • a device associated with the absorption device for determining the first phosgene content in the second exhaust gas stream and / or a device associated with the absorption device for determining the second phosgene content of the first phosgene solution, • a device for changing the absorption medium mass flow Ml with which the absorption medium is introduced into the absorption device • and optionally a computer for controlling the separation, characterized in that the computer is set up to control the method according to the invention. A sampling point or an online analyzer, for example, can be used as a device for determining the first phosgene content for the aforementioned devices according to the invention. The sampling point is suitable for taking samples from the exhaust gas stream, which can then be analyzed, for example, by means of spectrometric methods, gas chromatography or wet-chemical methods. It is preferably an online analyzer which is suitable for determining the phosgene content in the exhaust gas stream with the aid of IR spectrometry. The device according to the invention preferably comprises at least one device for exhaust gas aftertreatment, wherein the device for determining the first phosgene content in the second exhaust gas stream is arranged between the absorption device and the at least one exhaust gas aftertreatment device. The term “between” in this context is to be understood as between the method steps and not necessarily as a spatial arrangement. The device for exhaust gas aftertreatment is suitable for removing any remaining phosgene from the exhaust gas stream. Such devices for exhaust gas aftertreatment are known to the person skilled in the art.These can be, for example, alkaline scrubbers or activated carbon towers. The advantage of arranging the device for determining the first phosgene content upstream of such an exhaust gas aftertreatment device is that, on the one hand, this allows for more direct control of the absorption device and, on the other hand, the aftertreatment can be omitted if necessary. The exhaust gas is only fed to the exhaust gas aftertreatment device, or the aftertreatment device is only activated, if phosgene is detected in the exhaust gas stream. For the device according to the invention, a control valve, a feed pump with a controllable flow rate, or even a bypass line with a controllable flow rate can be used, for example. Preferably, the device for changing the absorption medium mass flow rate Ml is a control valve capable of throttling the flow of the absorption medium in the corresponding supply line. Examples Comparison example 1: In a 1 m 3A solution of 120 kg of phosgene in 300 kg of chlorobenzene was prepared at 0°C in a stirred-tank reactor equipped with a temperature control device and a reflux condenser. A suspension of 50 kg of 1,5-diaminonaphthalene in 150 kg of chlorobenzene was added to the phosgene solution with stirring. The suspension was stirred without further cooling for approximately 120 minutes before transitioning from the cold phosgenation phase to the hot phosgenation phase. For this purpose, the reaction mixture was heated and then held at reflux for approximately 6 hours. The pressure during the reaction was approximately 2.9 bar(a). After the reaction mixture had cleared, indicating the end of the phosgenation reaction, the reactors were first depressurized. The liquid crude product was then drained off, and the isocyanate was isolated in a distillation sequence. During the production run, the process offgas was fed to a two-stage absorption column, where it was scrubbed with chlorobenzene in countercurrent. The process offgas first passed through an isothermal absorption stage at approximately -15 °C and then an adiabatic absorption stage before leaving the process as offgas. Fresh chlorobenzene was added at -5 °C to the top of the adiabatic absorption stage. The mass flow of fresh chlorobenzene to the absorption column was approximately 20 kg / h. After passing through the adiabatic absorption stage, the solution of phosgene in chlorobenzene obtained there was passed on to the isothermal absorption stage as a scrubbing solution. Ultimately, a solution of phosgene in MCB was withdrawn from the bottom of the two-stage absorption column. After adjusting the phosgene concentration, this solution served as the starting material for further phosgenation reactions.The concentration of the withdrawn phosgene solution decreased during the course of the reaction, so that an increasing amount of fresh phosgene had to be added to adjust the phosgene concentration for reuse. The process offgas mass flow from the reactor reached a maximum of 70 kg / h during the transition to hot phosgenation, and the process offgas consisted of approximately 90% phosgene and approximately 10% hydrogen chloride. The ratio of chlorobenzene to phosgene at the inlet to the absorption column at this time was accordingly approximately 0.3:1, and a phosgene-free hydrogen chloride stream was obtained at the outlet of the absorption column. As the batch progressed, the total mass flow of process offgas decreased, and the composition shifted toward higher hydrogen chloride contents. Surprisingly, phosgene breakthrough into the offgas from the absorption column occurred during this phase. 5.2 vol% phosgene was detected in the offgas stream by IR measurement. The ratio of hydrogen chloride to phosgene at the inlet to the absorption column at this time was approximately 1.3:1. At the end of the reaction, the process offgas flow ceased. Only during the depressurization of the reactors did an increased mass flow of process offgas occur again briefly in the manifold, but this was far less pronounced than the first peak. Comparative Example 1 clearly shows that with a constant MCB mass flow into the absorption device, problems arise during operation of this device. These include, on the one hand, a strong fluctuation in the composition of the resulting phosgene solution and, on the other hand, the occasional penetration of phosgene into the exhaust gas of the absorption device. This applies even though the ratios of phosgene to MCB at the inlet to the absorption device corresponded to the ratios recommended in the prior art. Comparison example 2: The synthesis from Comparative Example 1 was repeated under the same reaction conditions. Unlike in Comparative Example 1, the absorption device was not supplied with a constant mass flow of chlorobenzene. Instead, this flow was varied depending on the phosgene mass flow in the process offgas, so that the mass flow of fresh hydrogen chloride into the absorption column always corresponded to 1.3 times the mass flow of phosgene into the absorption column, unless the minimum spray density of the absorption column was exceeded. Thus, the maximum mass flow of chlorobenzene during the transition from cold to hot phosgenation, i.e., at the time of the highest phosgene mass flow in the process offgas, was 82 kg / h. During hot phosgenation, there was again a significant breakthrough of phosgene into the exhaust gas from the absorption device. In this experiment, over 5 vol.% phosgene was also detected in the exhaust gas. Over the entire batch production run, the chlorobenzene consumption was higher than in Example 1, resulting in a lower average concentration of the phosgene solution obtained at the bottom of the column, and correspondingly more phosgene had to be added to achieve the desired phosgene concentration. Example 1 (according to the invention): The synthesis from Comparative Example 1 was repeated, but a closed-loop control system was installed to operate the absorption column, controlling the mass flow of absorbent. For this purpose, the phosgene content in the offgas from the absorption device was monitored using an online 1R measurement. The target value for the phosgene content was set at 0.05 vol%, and the deviation of the measured value from this target value ultimately acted via a control loop on a control valve in the fresh chlorobenzene feed line to adjust the chlorobenzene mass flow to the absorption column accordingly and keep the deviation as small as possible. Over the entire batch production runtime, with minimal use of chlorobenzene, it was possible to ensure that the offgas from the absorption column was only minimally contaminated with phosgene. The observed phosgene content fluctuated between 0.03 vol% and 0.08 vol%. Further post-treatment to destroy the residual phosgene was thus easily possible. The concentration of the phosgene solution at the bottom fluctuated, but on average decreased. the maximum possible value that could be achieved without large amounts of phosgene penetrating into the exhaust gas. Example 2 (according to the invention): The synthesis from Example 1 was repeated using a different control concept. In this control system, the primary focus was on maintaining a constant phosgene content in the phosgene solution obtained at the bottom of the absorption column (bottom concentration control). For this purpose, this concentration was determined as a controlled variable and compared with the desired setpoint. This control difference was evaluated, and the mass flow of MCB to the absorption column was automatically adjusted to minimize the difference. An override concept was also implemented. For this purpose, a threshold of 0.05 vol.% was set for the phosgene content in the offgas downstream of the absorption column. If this threshold was exceeded, the bottom concentration control was deactivated, and the MCB mass flow to the absorption column was increased by a predefined value until the phosgene content of the offgas fell below the threshold again. This operating mode resulted in a very constant phosgene content of approximately 55 wt.% in the phosgene solution obtained at the bottom. During hot phosgenation, the override mode was activated for a few minutes, so that the increased mass flow of absorbent led to a decrease in the phosgene content, which was compensated by adding more phosgene to the phosgene solution. In contrast to Comparative Example 2, the override mode made it possible to keep the phosgene content in the offgas below 0.2 vol.%. Example 3 (according to the invention): In contrast to the previously described examples and comparative examples, the reaction in Example 3 was carried out in two identically equipped reactors arranged in parallel. The total batch size was divided equally between the two reactors, and the reaction was carried out in both reactors with a time offset of 3 hours. The process offgas was combined in a collecting line and fed to the absorption device, which was otherwise operated with a control system as in Example 1. Since the maximum mass flows of process offgas from the two reactors did not occur simultaneously, the maximum gas loading of the absorption column was also lower. The observed phosgene content in the offgas fluctuated between 0.04 vol% and 0.07 vol%.

Claims

A process for the production of an isocyanate comprising the following steps: A) Reaction of at least one amine or its salt with phosgene, yielding a liquid reaction product containing the isocyanate and a first gaseous process exhaust stream containing hydrogen chloride and phosgene, B) Separation of the gaseous first process exhaust gas stream containing hydrogen chloride, phosgene, optionally solvents, optionally low-boiling by-products and optionally inerts in at least one absorption device in which the first process exhaust gas stream is contacted with an absorbent, wherein a second exhaust gas stream depleted in phosgene with a first phosgene content and a first phosgene solution with a second phosgene content are obtained, C) Optionally, mix the first phosgene solution obtained in step B) with additional phosgene and / or solvent to obtain a conditioned second phosgene solution, D) optionally, use of the first phosgene solution from step B) or the conditioned second phosgene solution from step C) for the phosgenation of an amine, E) optionally isolation of the isocyanate by distillative separation of the liquid reaction product obtained in step A), characterized in that • the first phosgene content of the second phosgene-depleted exhaust gas stream obtained in step B) is compared with a first target value for this first phosgene content and / or the second phosgene content of the first phosgene solution obtained in step B) is determined and compared with a previously defined second target value for this second phosgene content and • The absorbent is introduced into the absorption device with a mass flow rate Ml, and this mass flow rate Ml is subject to control, wherein the setpoint for the mass flow rate Ml is changed, preferably automatically, when the first phosgene content and / or the second phosgene content deviates from their respective setpoints. A method according to claim 1, characterized in that the setpoint for the first phosgene content is in the range of 0.00 vol% to 2.00 vol%, preferably in the range of 0.01 vol% to 1.00 vol%, and particularly preferably in the range of 0.02 vol% to 0.50 vol%, and most preferably in the range of 0.02 vol% to 0.2 vol%. A method according to claim 1 or 2, characterized in that the setpoint for the second phosgene content is in the range of 20 wt% to 70 wt%, preferably in the range from 30 wt.% to 68 wt.% and particularly preferably in the range of 42 wt.% to 66 wt.%.

4. Method according to one of claims 1 to 3, characterized in that in normal operation the second phosgene content is controlled by adjusting the mass flow rate Ml to the previously determined setpoint and that if a threshold value for the first phosgene content is exceeded, i.e. a deviation from normal operation, the mass flow rate Ml is increased.

5. A method according to any one of claims 1 to 4, characterized in that the at least one amine is a diamine, preferably 1,5-diaminonaphthalene (NDA), 1,4-diaminobenzene (pPDA), 1,5-diaminopentane (PDA), bis(p-aminocyclohexyl)methane (PACM), hexahydrotoluenediamine (H6TDA), in particular 2,4-hexahydrotoluenediamine and 2,6-hexahydrotoluenediamine, 1,3-bis(aminomethyl)benzene (m-XDA) as well as bis(aminomethyl)norbomane (NBDA) and mixtures thereof.

6. Method according to one of claims 1 to 5, characterized in that the reaction of the at least one amine or its salt with phosgene takes place in the presence of a solvent.

7. Method according to one of claims 1 to 6, characterized in that the reaction of the at least one amine or its salt with phosgene takes place in condensed phase.

8. Method according to any one of claims 1 to 7, characterized in that the reaction of the at least one amine or its salt with phosgene is carried out batchwise.

9. Method according to one of claims 1 to 8, characterized in that the reaction in step A) is a base phosgenation of at least one amine with phosgene.

10. Method according to one of claims 8 to 9, characterized in that the conversion in step A) takes place in at least 2 reactors arranged in parallel, wherein at least one reactor is operated asynchronously to at least one of the other reactors.

11. Method according to one of claims 1 to 10, characterized in that in the at least one absorption step chlorobenzene, o-dichlorobenzene or mixtures of these two solvents, preferably chlorobenzene, is used as the absorbent.

12. Method according to one of claims 6 to 11, characterized in that in the at least one absorption step a solvent is used as an absorbent, which is preferably the solvent which is also present in the reaction of the at least one amine or its salt.

13. Method according to any one of claims 1 to 12, characterized in that the absorption device comprises a gas scrubber selected from the group consisting of packed columns, packed columns and tray columns, preferably consisting of packed columns and packed columns.

14. Device for separating exhaust gas streams from a phosgenation reaction comprising • at least one absorption device for contacting a first process exhaust gas stream containing hydrogen chloride and phosgene with an absorbent Ml to obtain a second exhaust gas stream depleted of phosgene with a first phosgene content and a first phosgene solution with a second phosgene content, • a device associated with the absorption device for determining the first phosgene content in the second exhaust gas stream and / or a device associated with the absorption device for determining the second phosgene content of the first phosgene solution, • a device for changing the absorbent mass flow rate Ml with which the absorbent is introduced into the absorption device • and optionally a computer for controlling the separation, characterized in that the computer is set up to control the method according to one of claims 1 to 13.

15. Device according to claim 14, characterized in that it comprises at least one exhaust gas aftertreatment device, wherein the device for determining the first phosgene content in the second exhaust gas stream is arranged between the absorption device and the at least one exhaust gas aftertreatment device.