Method for preparing isocyanates

Abstract

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

Description

[Technical field]

[0001] The present invention relates to a method for producing isocyanates by reacting the corresponding amines with phosgene. Furthermore, the present invention relates to a device for separating the off-gas stream from a phosgenation reaction. [Background technology]

[0002] Isocyanates are important raw materials in the chemical industry. They are often produced in large quantities. Diisocyanates and polyisocyanates in particular are primarily used as starting materials for the production of polyurethanes. Their large-scale production is usually achieved in good yields by phosgenation of the corresponding amines, amine hydrochlorides or amine carbonates with an excess of phosgene. In this reaction, the corresponding carbamoyl chlorides are formed as intermediates, which are then converted to isocyanates by elimination of hydrogen chloride.

[0003] The industrially established solvents for the reaction are chlorobenzene or o-dichlorobenzene, which are largely inert in their behavior and are very suitable for the recovery of excess phosgene and its separation from hydrogen chloride. However, it is also possible to use other solvents which are inert under the reaction conditions.

[0004] The process off-gas, consisting essentially of phosgene, hydrogen chloride and solvent, must be treated to avoid the release of phosgene. Also in order to improve the economic efficiency of the process, it is advantageous to treat the process off-gas, consisting essentially of phosgene, hydrogen chloride and solvent, so that at least a part of these components, in particular phosgene, can be further used or reused. Numerous variants are known for the separation of this process off-gas stream, which differ from one another depending on the main objective.

[0005] US 2007 / 0249859 describes at least a two-step sequence of absorption steps, which comprises at least one isothermal absorption step and at least one adiabatic absorption step, in which the phosgene thus obtained is returned to the phosgenation reaction.

[0006] In DE 102008009761 A1, the process off-gas stream is first subjected to partial condensation and the liquid condensate is treated in a stripping column. This process gives a liquid solvent stream at the bottom and a gas stream at the top of the column, which is led to absorption together with the previously uncondensed fraction. The phosgene present therein is absorbed in the solvent and the phosgene solution obtained is reused in the phosgenation reaction, optionally after being enriched with further phosgene.

[0007] US 2018 / 0044179 is based on the distillative separation of the process off-gas. In this case, the off-gas stream is led to a distillation column, and a phosgene-containing stream is withdrawn from its bottom. At the head of the column, a stream consisting essentially of hydrogen chloride is withdrawn, compressed and therefore partially condensed. The condensed fraction is depressurized and returned at the head of the column.

[0008] Thus, although phosgene is often recycled to the same process, there are other options for utilizing hydrogen chloride. For example, it can simply be used or sold as an aqueous solution, i.e., as hydrochloric acid. Alternatively, it can be catalytically or electrochemically oxidized to chlorine, or it can be used in the oxychlorination of ethylene to ethylene dichloride.

[0009] Many isocyanates are produced in large quantities, for which continuous processes are preferred. Isocyanates produced in smaller quantities are usually produced by batch liquid phase phosgenation, as the costs associated with switching to a continuous process are prohibitively high and unrecoverable given the small quantities produced. In some cases, hybrid processes are also operated in which a batch reaction regime is combined with continuous downstream processing of the produced isocyanates. This requires the installation of a buffer vessel into which the reaction product is discharged and then fed into the continuous process.

[0010] For batch production of isocyanates, it is also desirable to treat the off-gas and, if possible, to recover phosgene from this off-gas. However, in this regard, there are problems that have not yet been addressed in the prior art. For example, the mass flow rate of the process off-gas and its composition vary greatly depending on the operating conditions, whereas it is relatively easy to install a buffer tank for the liquid product stream, thus ensuring a uniform inlet stream for treatment, which cannot be easily achieved with the process off-gas due to its large volume and toxic and corrosive components.

[0011] US Patent No. 4,233,267 describes a system combining a batch reactor with a continuous distillation apparatus, in which the volatile reaction products are first condensed and temporarily stored in a condensate vessel, from which they are fed to a continuously operating column essentially independent of the operating parameters of the batch reactor. In the case of a phosgenation process, this approach has various disadvantages. First, the condensation of phosgene, let alone HCl, requires very low temperatures. Secondly, the need for buffering means that such an approach necessarily increases the amount of phosgene in the system.

[0012] For the operation of the absorption to separate phosgene from process off-gases from isocyanate production, the disclosures of DE 102008009761 A1 and US 2007 / 0249859 A1 both recommend adjusting the amount of solvent for absorption along with the phosgene mass flow rate. In said disclosures the weight ratio of solvent to phosgene at the inlet to the isothermal absorption is preferably in the range of 0.1:1 to 10:1, more preferably in the range of 1:1 to 3:1.

[0013] However, it has now surprisingly been found that the phosgene mass flow rate to the absorption device is not a good guide to the necessary or even optimal mass flow rate of fresh solvent to the absorption device. This is especially true in the case of batchwise production of isocyanates by phosgenation of the corresponding amines, where there are large variations in the composition of the process off-gas and in the overall mass flow rate. In fact, it has been observed that even if the preferred range of the solvent to phosgene ratio at the inlet to the isothermal absorption is maintained, stages of phosgene penetration into the absorption device off-gas occur, while in other stages unnecessarily large amounts of solvent are wasted. [Prior art documents] [Patent documents]

[0014] [Patent Document 1] US Patent Application Publication No. 2007 / 0249859 [Patent Document 2] DE 102008009761 A1 [Patent Document 3] US Patent Application Publication No. 2018 / 0044179 [Patent Document 4] U.S. Pat. No. 4,233,267 Summary of the Invention [Problem to be solved by the invention]

[0015] It was therefore an object of the present invention to provide an improved process for producing isocyanates, as well as a device for separating an off-gas stream from a phosgenation reaction, which allows phosgene to be separated from the process off-gas by absorption in a reliable and resource-saving manner, despite variations in the composition and mass flow rate of the process off-gas from the phosgenation. [Means for solving the problem]

[0016] The object is to provide a method for producing an isocyanate, comprising the steps of: A) reacting at least one amine or salt thereof with phosgene to obtain a liquid reaction product comprising an isocyanate and further a first gaseous process off-gas stream comprising hydrogen chloride and phosgene; B) separating the gaseous first process off-gas stream, comprising hydrogen chloride, phosgene, any solvent, any low boiling secondary components and any inert components, in at least one absorption device in which the first process off-gas stream is contacted with an absorbent to obtain a phosgene-depleted second off-gas stream having a first phosgene content and a first phosgene solution having a second phosgene content; 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 the amine; E) optionally isolating the isocyanate by distillative separation of the liquid reaction product obtained in step A. In a method comprising: a first phosgene content of the phosgene-depleted second off-gas stream obtained in step B) is compared with a first target value of said first phosgene content and / or a second phosgene content of the first phosgene solution obtained in step B) is determined and compared with a predetermined second target value of said second phosgene content, An absorbent having a mass flow rate M1 is introduced into the absorption device, this mass flow rate M1 is under closed-loop control, and if the first phosgene content and / or the second phosgene content deviate from the respective target values, a reference variable of the mass flow rate M1 is changed, preferably automatically. This is achieved by a method characterized in that DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0017] In accordance with the present invention, the terms "comprising" or "containing" preferably mean "consisting essentially of," and more preferably mean "consisting of. The additional embodiments described in the claims and the specification can be combined as desired, unless the context clearly indicates to the contrary.

[0018] As used herein, "at least one" refers to one or more, e.g., two, three, four, five, six, seven, eight, nine or more. In the context of the composition of compounds described herein, the number does not refer to the absolute number of molecules, but rather to the nature of the composition. Thus, "at least one amine" means, for example, that there can be only one type of amine or multiple different types of amines, without specifying the amount of each compound.

[0019] Numerical values ​​specified herein without decimal places refer in each case to the full value specified to one decimal place, e.g. "99%" means "99.0%".

[0020] Numerical ranges given in the format "in / x to y" are inclusive of the recited values. When two or more preferred numerical ranges are given in this format, it is understood that all ranges resulting from combining the various limit points are likewise encompassed.

[0021] In the present application, the "low boiling point secondary component" is 10 5 In step A), the boiling points are compounds which have a lower boiling point in Pa than the isocyanates produced in step A). ​​These can be, for example, chlorinated organic compounds or fragments of the isocyanates produced in step A).

[0022] In a preferred embodiment of the method of the invention, the target value of the first phosgene content is in the range of 0.00% to 2.00% by volume, preferably in the range of 0.01% to 1.00% by volume, more preferably in the range of 0.02% to 0.50% by volume, most preferably in the range of 0.02% to 0.2% by volume. A defined first target value in this range can be compared with the measured actual value of the first phosgene content of the phosgene-depleted second off-gas stream obtained in step B). Based on the variance between the target value and the actual value, the mass flow rate M1 of the absorbent into the absorption device can be adjusted, preferably automatically.

[0023] In a further preferred embodiment of the method of the invention, the target value of the second phosgene content is in the range of 20% by weight to 70% by weight, preferably in the range of 30% by weight to 68% by weight, more preferably in the range of 42% by weight to 66% by weight. A defined second target value in this range can be compared with the measured actual value of the second phosgene content of the first phosgene solution obtained in step B). Based on the variance between the target value and the actual value, the mass flow rate M1 of the absorbent into the absorption device can be adjusted, preferably automatically.

[0024] In a further preferred embodiment of the method of the invention, the above two embodiments are combined with each other. In normal operation, the second phosgene content is controlled in this embodiment by adjusting the mass flow rate M1 to a predetermined second target value, and in case of exceeding the first target value of the first phosgene content, which serves here as a threshold value, i.e. in case of deviation from normal operation, i.e. in case of penetration of an increased amount of phosgene into the second off-gas stream, the mass flow rate M1 is increased so that more absorbent is introduced into the absorption device than is required for the adjustment of the second phosgene content in the first phosgene solution. The increase in the mass flow rate M1 here is preferably progressively larger according to the degree of exceeding the first target value and remains active for a certain time or preferably until the measured actual value of the first phosgene content falls back below the first target value. This is known as an override concept, whereby the closed-loop control on the second phosgene content is inactive for the duration of the phosgene penetration, and instead the mass flow rate M1 is specified based on the first phosgene content. In this way, the desired second phosgene content of the first phosgene solution can be maintained as accurately as possible during normal operation, while at the same time phosgene permeation into the second off-gas stream can be reduced or prevented.

[0025] Examples of preferred diamines are diaminotoluene (TDA), in particular 2,4-diaminotoluene and 2,6-diaminotoluene, diaminodimethylbenzene, diaminonaphthalene, in particular 1,5-diaminonaphthalene (NDA), diaminobenzene, in particular 1,4-diaminobenzene (pPDI), 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, 1-amino-3,5,5-trimethyl-5-aminomethylcyclohexane (IPDA), bis(p-aminocyclohexyl) methane (PACM), 1,5-diamino-2-methylpentane, 2,5-diamino-2,5-dimethylhexane, 1,4-diaminocyclohexane, hexahydrotolylenediamine (H6TDA), in particular 2,4-hexahydrotolylenediamine, 2,6-hexahydrotolylenediamine, 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.

[0026] Examples of particularly preferred diamines are 1,5-diaminonaphthalene (NDA), 1,4-diaminobenzene (pPDA), 1,5-diaminopentane (PDA), 1,6-diaminohexane (HDA), 1-amino-3,5,5-trimethyl-5-aminomethylcyclohexane (IPDA), bis(p-aminocyclohexyl)methane (PACM), 1,5-diamino-2-methylpentane, 2,5-diamino-2,5-dimethylhexane, 1,4-diaminocyclohexane, hexahydrotolylenediamine (H6TDA), in particular 2,4-hexahydrotolylenediamine and 2,6-hexahydrotolylenediamine, 1,3-bis(aminomethyl)benzene (m-XDA), 1,4-bis(aminomethyl)benzene (p-XDA), bis(aminomethyl)cyclohexane (H6-XDA), bis(aminomethyl)norbornane (NBDA), and mixtures thereof.

[0027] Examples of very particularly preferred diamines are 1,5-diaminonaphthalene (NDA), 1,4-diaminobenzene (pPDA), 1,5-diaminopentane (PDA), bis(p-aminocyclohexyl)methane (PACM), hexahydrotolylenediamine (H6TDA), in particular 2,4-hexahydrotolylenediamine and 2,6-hexahydrotolylenediamine, 1,3-bis(aminomethyl)benzene (m-XDA), bis(aminomethyl)norbornane (NBDA), and mixtures thereof.

[0028] Examples of the most preferred diamines are 1,5-diaminonaphthalene (NDA), 1,4-diaminobenzene (pPDA), bis(p-aminocyclohexyl)methane (PACM), 1,3-bis(aminomethyl)benzene (m-XDA), bis(aminomethyl)norbornane and mixtures thereof.

[0029] Instead of the amine, it is also possible to phosgenate its salt, in particular the hydrochloride or carbamate of the amine, preferably the hydrochloride. In that case, these salts are usually prepared in situ in a first step at low temperature, as described, for example, in DE 19510259 A1, which may allow the introduction of an inert gas to be omitted. It is preferred to directly phosgenate the amine (base phosgenation), in particular in a two-stage reaction at different reaction temperatures (cold-hot phosgenation).

[0030] The reaction can be carried out in the presence of a solvent. In the case of gas-phase phosgenation, the presence of a solvent is often omitted during the reaction itself; instead, a solvent is added only after the reaction in order to quickly cool the reaction product. This process is also called quenching. All solvents known to those skilled in the art that are inert to the general reaction conditions are suitable. Preferred solvents are selected from the group consisting of aromatic hydrocarbons, halogenated aromatic hydrocarbons, especially chlorinated aromatic hydrocarbons, esters, ethers, halogenated hydrocarbons, and mixtures thereof. Aromatic hydrocarbons that are particularly preferably used according to the invention are selected from the group consisting of toluene, bromobenzene, chlorobenzene, dichlorobenzene, especially o-dichlorobenzene, and mixtures thereof. According to the invention, it is particularly preferred to use chlorobenzene, o-dichlorobenzene or a mixture of these two solvents.

[0031] The reaction is preferably carried out in the condensed phase, ie, as known as liquid phase phosgenation, in which solids and gases may be present in the essentially liquid reaction mixture.

[0032] The process of the invention is particularly advantageous in the production of isocyanates for industrial processes in which the reaction of at least one amine or amine salt with phosgene is carried out batchwise. Batchwise production of isocyanates, also known as batch or semi-batch operating modes, is characterized by particularly significant fluctuations in both the overall mass flow rate and composition of the process off-gas.

[0033] In a further embodiment, the reaction is carried out batchwise in the condensed phase. Batchwise reaction should be understood in the present invention to mean that the liquid reaction product is not continuously withdrawn from one or more reactors, but is withdrawn from the reactor(s) and transferred to a crude product holding tank or sent directly for further processing only when the conversion to isocyanate in said reactor is essentially complete. The skilled person is familiar with various methods for determining the end point of the reaction. What is usually used is what is known as the "clearing point", which is the point at which a clear solution is formed from the suspension that is initially present in the thermal phosgenation. Alternatively, it is also possible to use the end of the evolution of HCl gas or the presence of a certain NCO content in the reaction mixture. However, even during the reaction, it is possible to withdraw a portion of the liquid reaction mixture from the reactor and return it to the reactor, for example to provide better mixing or to control the temperature of the reaction mixture without violating the conditions of batchwise phosgenation in the context of the present invention. However, the gas stream essentially comprising hydrogen chloride, phosgene, inert gas and solvent vapors may be withdrawn from the reactor at any time, including continuously, and reprocessed. Preferably, the reaction is carried out in what is known as a semi-batch process, also called a fed-batch process or a fed process, i.e. an initially partially filled reactor is fed during the reaction with the amine and / or phosgene and optionally an inert gas.

[0034] Stirred tank reactors are particularly suitable as reactors, although other types of reactors, such as loop reactors, can in principle also be used. 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), more preferably 1.1 bar(a) to 6 bar(a). At least one reactor is therefore preferably equipped with a pressure-retaining device, for example a control valve, by which process off-gases can escape from the reactor. In the case of a continuous reaction regime, in particular tubular reactors are particularly suitable.

[0035] In a further preferred embodiment of the process of the invention, the reaction is carried out in the form of a base phosgenation. In base phosgenation, i.e. the reaction of amines with phosgene, the reaction is preferably carried out in two stages in an inert solvent. Such reactions are described, for example, in W. Siefken, Liebigs Annalen der Chemie, 562 (1949) p. 96. In the first stage cold phosgenation, the temperature of the reaction mixture is preferably kept in the range of 0-100 ° C. A suspension is formed which contains carbamoyl chloride, amine hydrochloride and small amounts of free isocyanate. The preferred procedure here is that a solution of phosgene in an inert solvent is initially charged, and a solution or suspension of the amine in the same solvent and optionally further phosgene are then added. This keeps the concentration of free amines low and thus suppresses the undesired formation of urea.

[0036] In the second stage hot phosgenation, the temperature is raised, preferably to within the range of 120° C. to 200° C. It is kept within this range for a time during which further phosgene is fed in until the reaction to isocyanates is complete, i.e. until the evolution of HCl has ceased and / or the reaction mixture is clear. Phosgene is advantageously used in excess. If necessary, the reaction may be carried out with the introduction of an inert gas, both in the cold phosgenation and in the hot phosgenation.

[0037] When the amine to be converted is a high-melting amine that is poorly soluble in the solvent, it is also possible to use a suspension of the amine for the phosgenation, which is preferably prepared by dispersion using a dynamic mixing unit, as described in EP 2897933, paragraphs

[0020] to

[0024] .

[0038] In the amine hydrochloride or carbamate phosgenation, the amine is preferably initially 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 0-80°C. The initial reaction with phosgene can occur sequentially or already simultaneously. This is followed by a further phosgenation step, which is essentially similar to the thermal phosgenation from the base phosgenation described above and is therefore also referred to hereinafter as thermal phosgenation. Thus, here too, the temperature is preferably kept in the range of 120-200°C while phosgene and optionally an inert gas are introduced into the reaction mixture. The introduction is continued until the reaction to the isocyanate is completed. Here too, phosgene is preferably used in excess to accelerate the reaction.

[0039] Phosgenation is usually carried out with an excess of phosgene, i.e. more than 1 mole of phosgene is used per mole of amino group. The molar ratio of total phosgene to amino group used is preferably 1.02:1 to 20.0:1, more preferably 1.1:1 to 10.0:1, most preferably 1.2:1 to 5.0:1. If necessary, additional phosgene or phosgene solution can be fed to the reaction mixture during the reaction to maintain a sufficient excess of phosgene or to compensate for phosgene loss.

[0040] In both base phosgenation and amine hydrochloride or carbamate phosgenation, residual phosgene and hydrogen chloride gas are most often at least partially removed at the end of the reaction, preferably by purging with an inert gas, preferably nitrogen, or less preferably by removal under reduced pressure. If necessary, the reaction mixture may be filtered to remove any solids present, such as unreacted amine hydrochloride.

[0041] In the course of batch phosgenation, two maxima usually occur in the mass flow rate of the process off-gas for each batch. An increase in the mass flow rate of the process off-gas from the reactor occurs, for example, during the transition to thermal phosgenation. Heating the reaction mixture reduces the solubility of gases in the mixture, leading to outgassing of dissolved gases and a resulting increase in the off-gas stream from the reactor, which, depending on the process selected, contains, among other things, phosgene, hydrogen chloride and / or carbon dioxide. The second maximum occurs at the end of the reaction, when residual phosgene and hydrogen chloride are driven off from the reaction mixture and, if the reaction is carried out under pressure, the reactor is depressurized.

[0042] In order to reduce the occurrence of these fluctuations in the process off-gas stream in batch processes and to allow a more economical design of the equipment and operating conditions, the reaction of step A) is preferably carried out in at least 2 reactors arranged in parallel, more preferably in 2 to 10 reactors arranged in parallel, particularly preferably in 2 to 6 reactors arranged in parallel, most preferably in 2 or 4 reactors arranged in parallel, at least one reactor being operated asynchronously with at least one of the other reactors. This means that the heating of the reaction mixture to above 100° C. and / or the stripping off of residual phosgene and hydrogen chloride at the end of the reaction, which may involve depressurization of the reactors to a lower pressure, is not carried out in all reactors at the same time or under the same conditions.

[0043] For example, during the transition to thermal phosgenation, it is possible to carry out the same temperature gradient with a time delay, otherwise, if the reactor is not operated synchronously with the other reactors, it is possible to carry out a different temperature gradient, in particular a flatter temperature gradient, so that the associated maximum value of the off-gas mass flow rate is lower and / or does not coincide with the maximum value of the off-gas mass flow rate of the other reactor. Particularly preferably, the reactors carry out the same temperature gradient with a time delay, so that the temperature increase in the transition to thermal phosgenation occurs either before or after the temperature increase in the other reactor. Preferably, the time delay is at least 5 minutes, more preferably at least 10 minutes, most preferably at least 20 minutes. Also preferably, the time delay is not more than 24 hours, preferably not more than 8 hours, more preferably not more than 2 hours, most preferably not more than 1 hour.

[0044] The same applies to the removal of residual phosgene and hydrogen chloride. In this case, for example, the decompression and / or the introduction of the inert gas, if necessary, can be carried out asynchronously, i.e. with a time delay and / or at different speeds, in at least two of the parallel reactors in order to ensure that the relevant maximum value of the off-gas mass flow rate is lower and / or does not coincide with the maximum value of the off-gas mass flow rate of the other reactor. Particularly preferably, the decompression and / or the introduction of the inert gas, if necessary, is carried out with the same specifications as the other reactor in terms of the pressure profile and the inert gas mass flow rate over time, but with a time delay. Preferably, the removal of phosgene and / or hydrogen chloride is carried out with a time delay of at least 5 minutes, more preferably at least 10 minutes, most preferably at least 20 minutes. Also preferably, the time delay is not more than 24 hours, preferably not more than 8 hours, more preferably not more than 2 hours, most preferably not more than 1 hour.

[0045] In a further preferred embodiment of the invention, the asynchronously operated reactors are operated according to the same operating instructions, but the individual process steps are carried out with a time delay of at least 5 minutes, preferably at least 10 minutes, more preferably at least 20 minutes. It does not matter in which direction the time delay occurs, i.e. which reactor is operating behind or ahead of the other reactors.

[0046] In a further preferred embodiment, none of the parallel reactors are operated synchronously with any of the other reactors. Preferably, all reactors are operated according to the same operating instructions, but the individual process steps are performed with a time delay with respect to each other.

[0047] The first process off-gas typically contains phosgene, hydrogen chloride and any solvent vapors, inert gases and traces of other volatile components of the reaction mixture, such as amines or isocyanates. This process off-gas is separated in step B) into a second off-gas stream depleted in phosgene and a phosgene-containing absorbent, i.e. the first phosgene solution, in at least one absorption device. Preferably, the separation in step B) is carried out in only one absorption device.

[0048] Suitable absorption devices are, for example, gas scrubbers in which the first process off-gas stream is contacted with a liquid stream, also called absorbent or scrubbing medium, examples being submerged scrubbers, spray scrubbers, packed columns with random packing, packed columns with structured packing, tray columns or falling film absorbers. For efficient absorption of phosgene from the first process off-gas stream, it is also possible to combine different types of gas scrubbers. Here, it is preferred to combine two or more, more preferably two gas scrubbers in the absorption device. In a further preferred embodiment, the absorption device comprises a gas scrubber selected from the group consisting of packed columns with random packing, packed columns with structured packing and tray columns, preferably selected from the group consisting of packed columns with random packing and packed columns with structured packing.

[0049] Particularly suitable absorption devices comprise falling film absorbers suitable for performing isothermal absorption and tray columns suitable for performing adiabatic absorption, packed columns with random packing or packed columns with structured, preferably with random packing or packed columns with structured, more preferably with structured packing. The falling film absorbers are preferably vertically arranged shell-and-tube heat exchangers, in which the process off-gas and the absorbent are conducted either at the tube side or at the shell side of the heat exchanger, preferably at the tube side of the heat exchanger, while at the other side, in each case, the cooling medium is conducted cocurrently or countercurrently, preferably countercurrently, to the process off-gas stream. The process off-gas and the absorbent can equally be conducted cocurrently or countercurrently, preferably countercurrently, to each other. The tubes may be equipped with random or structured packing or other internal materials to improve mass transfer and / or heat exchange. The tray columns, packed columns with random packing or packed columns with structured packing are preferably set up so that the process off-gas from the isothermal gas scrubber comes into contact with the absorbent in countercurrent. The absorbent is preferably introduced at the top of the column via a liquid distributor. The packed column is preferably a packed column with structured packing. Particularly preferably, the falling film absorber and the tray column, the packed column with random packing or the packed column with structured packing are combined in a columnar apparatus which comprises a feed line for the process off-gas at the bottom end. The falling film absorber is arranged above it and above the tray column, the packed column with random packing or the packed column with structured packing. A liquid distributor is preferably present above the falling film absorber and at the head of the columnar apparatus so that the absorbent is evenly distributed on the tubes / columns. At the head of the absorption device, there may be an internal or external condenser for further cooling the phosgene-depleted off-gas stream.

[0050] The second off-gas stream, depleted as far as possible of phosgene, usually consists essentially of hydrogen chloride, i.e. to at least 80% by volume, preferably to at least 95% by volume, more preferably to at least 98% by volume. It also contains residual solvent, inert gases, if used, and traces of phosgene. The phosgene content of this off-gas stream is preferably 1.0% by volume or less, more preferably 0.5% by volume or less, most preferably 0.2% by volume or less. The natural and generally desirable lower limit of the phosgene content in this off-gas stream is 0.00% by volume. However, not only can adhering to this limit lead to increased consumption of absorbent, but it may also be advantageous for the design of the closed-loop control unit to tolerate a low phosgene content in the off-gas, for example in order to use this as a closed-loop control variable. The phosgene content in the second off-gas stream is therefore preferably at least 0.01% by volume, more preferably at least 0.02% by volume, most preferably at least 0.05% by volume.

[0051] To achieve maximum efficiency of absorption, it is advantageous to initially cool the first process off-gas to a temperature in the range of 5 ° C to -25 ° C, preferably in the range of 0 ° C to -20 ° C, more preferably in the range of -5 ° C to -15 ° C. The cooling can be carried out simply by one or more heat exchangers or by quenching, i.e. by contacting the first process off-gas 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 an irrigation heat exchanger. A suitable cooled liquid is, for example, an absorbent. Preferably, the absorbent here is deposited as a liquid after contact with the process off-gas, which liquid is recycled to the quenching device, the liquid loop or the quenching device itself comprising a cooler for cooling the liquid to the desired temperature. During cooling, partial condensation / absorption of phosgene occurs.

[0052] The residual gas phase is then contacted with the absorbent, preferably in countercurrent. At the same time, the ascending gas phase is preferably contacted with the descending liquid absorbent in countercurrent. Devices suitable for this purpose are known to the person skilled in the art. Preferably, the absorption comprises a combination of two absorption steps, the gas preferably first passes through isothermal absorption and then through adiabatic absorption. The volumetric flow rate of the absorbent is variable and can be adapted in each case 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, it is particularly preferred to use chlorobenzene, o-dichlorobenzene or a mixture of these two solvents as the absorbent. Very particularly preferably, the absorbent is chlorobenzene.

[0053] In a further preferred embodiment, the solvent used as absorbent in at least one absorption step is preferably a solvent also present in the reaction of at least one amine or its salt. In a preferred embodiment with two absorption steps, a fresh or reprocessed absorbent is preferably introduced into the second absorption step, i.e. the absorption step through which the gas passes afterwards, while in the first absorption step it is preferred to use an already loaded absorbent from the second absorption step, possibly together with a fresh absorbent and / or together with a similarly already loaded absorbent recycled from the first absorption step and / or the preceding cooling process. Preferably, the gas stream after contact with the absorbent passes through a condenser in order to condense the absorbed absorbent and separate it as completely as possible from the gas stream.

[0054] The loaded absorbent can be used as a quench medium to initially cool the first process off-gas stream, as described above. A portion of the absorbent is withdrawn continuously or discontinuously and, optionally after enrichment with further phosgene, is used in one of the reactors for the phosgenation of the amine or its salt. The withdrawn loaded absorbent is preferably a solution containing 20% ​​to 70% by weight, more preferably 30% to 68% by weight, most preferably 42% to 66% by weight of phosgene in chlorobenzene. The content of the solution here depends, inter alia, on the phosgene content of the first process off-gas, the amount of fresh absorbent for absorption and the amount of absorbent withdrawn.

[0055] The closed-loop control system of the device described according to the invention can be implemented, for example, with the aid of a computer.Thus, the present invention further provides a device for separating an off-gas stream from a phosgenation reaction, comprising at least one, preferably only one, absorption device for contacting a first process off-gas stream comprising hydrogen chloride and phosgene with an absorbent, and also comprising a computer for closed-loop control of the separation, characterized in that the computer is set up for controlling the process of the invention.

[0056] Alternatively or preferably, the present invention provides a device for separating an off-gas stream from a phosgenation reaction, comprising: at least one absorption device for contacting a first process off-gas stream comprising hydrogen chloride and phosgene with an absorbent M1 to obtain a second phosgene-depleted off-gas stream having a first phosgene content and a first phosgene solution having a second phosgene content; an apparatus associated with the absorption device for determining a first phosgene content in the second off-gas stream and / or an apparatus associated with the absorption device for determining a second phosgene content of the first phosgene solution; a device for varying the absorbent mass flow rate M1 at which the absorbent is introduced into the absorption device; Optionally, further comprising a computer for closed-loop control of the separation, characterized in that the computer is configured for controlling the method of the present invention. The present invention further provides a device comprising:

[0057] The device used for determining the first phosgene content of the aforementioned device of the present invention can be, for example, a sampling station or an online analyzer. The sampling station can collect samples from the off-gas stream, which can then be analyzed, for example, by spectroscopy, gas chromatography or wet chemical analysis. Preferably, the sampling station is an online analyzer capable of determining the content of phosgene in the off-gas stream by IR spectroscopy. Preferably, the device of the present invention comprises at least one device for off-gas post-treatment, and the device for determining the first phosgene content in the second off-gas stream is arranged between the absorption device and the at least one device for off-gas post-treatment. The term "between" should be understood in this context to mean between process steps, and not necessarily as a spatial arrangement. The off-gas post-treatment device is suitable for removing any residual phosgene from the off-gas stream. Such off-gas post-treatment devices are known to the person skilled in the art. For example, they may be alkaline scrubbers or activated carbon towers. The advantage of arranging the device for determining the first phosgene content upstream of such an off-gas after-treatment device is that this not only allows a more direct closed-loop control of the absorption device, but also makes it possible to omit after-treatment, the off-gas being fed only to the off-gas after-treatment device or said device being operated only if phosgene is detected in the off-gas stream.

[0058] The device used to change the absorbent mass flow rate M1 of the device of the present invention may be, for example, a control valve, a delivery pump with an adjustable delivery rate, or a bypass line with an adjustable flow rate. Preferably, the device for changing the absorbent mass flow rate M1 is a closed-loop control valve that can limit the flow of absorbent in the corresponding supply line.

[0059] [Example] Comparative Example 1: A solution containing 120 kg of phosgene in 300 kg of chlorobenzene was poured into a 1 m 3 It was prepared in a stirred tank reactor at 0°C. To the phosgene solution was added, with stirring, a suspension of 50 kg 1,5-diaminonaphthalene in 150 kg chlorobenzene. The suspension was stirred for about 120 minutes without further cooling before passing from the cold to the hot phosgenation stage. For this, the reaction mixture was heated and then kept under reflux for about 6 hours. The pressure during the reaction was about 2.9 bar(a). The reactor was first depressurized after the reaction mixture became clear, which signals the end of the phosgenation reaction. The liquid crude product was then poured off and the isocyanate was isolated by a distillation sequence.

[0060] During the production run, the process off-gas was fed to a two-stage absorption column, where it was washed with chlorobenzene in countercurrent. In said run, the process off-gas first passed through an isothermal absorption stage at about -15 ° C, then through an adiabatic absorption stage, before leaving the process as off-gas. Fresh chlorobenzene was added at -5 ° C at the top of the adiabatic absorption stage. The mass flow rate of fresh chlorobenzene to the absorption column was about 20 kg / h. After passing through the adiabatic absorption stage, the chlorobenzene solution of phosgene obtained there was sent to the isothermal absorption stage as a washing solution, so that a solution of phosgene in MCB was finally withdrawn from the bottom of the two-stage absorption column, which served as starting material for further phosgenation after adjusting the phosgene concentration. The concentration of the withdrawn phosgene solution decreased in the course of the reaction, which meant that an increasing amount of fresh phosgene had to be added to adjust the phosgene concentration for further use.

[0061] In the transition to the thermal phosgenation, the process off-gas mass flow rate from the reactor passed up to 70 kg / h, the process off-gas consisted of about 90% phosgene and about 10% hydrogen chloride. Thus, the ratio of chlorobenzene to phosgene at the inlet to the absorption column at this point was about 0.3:1, and a phosgene-free hydrogen chloride stream was obtained at the outlet of the absorption column. In the second half of the batchwise process, the overall mass flow rate of the process off-gas decreased and the composition shifted towards a higher hydrogen chloride content. Surprisingly, a penetration of phosgene into the absorption column off-gas occurred during this stage. 5.2 vol. % phosgene was detected in the off-gas stream by IR measurements. The ratio of hydrogen chloride to phosgene at the inlet to the absorption column at this point was about 1.3:1.

[0062] At the end of the reaction, the process off-gas stream was stopped. Only when the pressure in the reactor was released was there another brief increase in the mass flow rate of the process off-gas in the collection line, but this was much less pronounced than the first maximum.

[0063] Comparative Example 1 clearly shows that at a constant MCB mass flow rate to the absorption device, problems arise in the operation of said device, which are, firstly, significant fluctuations in the composition of the resulting phosgene solution and, secondly, periods of phosgene penetration into the off-gas of the absorption device, even if the amount ratio of phosgene to MCB at the inlet to the absorption device corresponds to the amount ratio recommended in the prior art.

[0064] Comparative Example 2: The synthesis from Comparative Example 1 was repeated under the same reaction conditions. In contrast to Comparative Example 1, in this case the absorption device was not charged with a constant mass flow rate of chlorobenzene. Instead, it was varied as a function of the phosgene mass flow rate in the process off-gas in such a way that the mass flow rate of fresh hydrogen chloride to the absorption column always corresponded to 1.3 times the mass flow rate of phosgene to the absorption column, as long as the perfusion density of the absorption column did not fall below a minimum value. The maximum mass flow rate of chlorobenzene during the transition from cold to hot phosgenation, i.e. at the point when the phosgene mass flow rate in the process off-gas was at a maximum, was 82 kg / h.

[0065] During the hot phosgenation, there was again a significant phosgene penetration into the off-gas of the absorption device. In this experiment too, more than 5% by volume of phosgene was detected in the off-gas. Over the entire run time of the batch production, the consumption of chlorobenzene was higher than in Example 1, so that the concentration established in the phosgene solution obtained at the bottom of the column was on average lower and correspondingly larger amounts of phosgene had to be added to obtain the desired phosgene concentration.

[0066] Example 1 (according to the present invention): The synthesis from Comparative Example 1 was repeated during operation, with the closed-loop control unit that controlled the mass flow rate of absorbent for the operation of the absorption column set.For this purpose, the phosgene content in the off-gas from the absorption device was tracked by online IR measurement.The target value of the phosgene content was set to 0.05% by volume, and the deviation of the measured value from this target value was acted upon by the closed-loop control circuit, and finally on the control valve in the new chlorobenzene supply line, so as to adjust the mass flow rate of chlorobenzene to the absorption column accordingly and to keep the deviation as small as possible.

[0067] Over the entire production time of the batch, this made it possible to minimize the use of chlorobenzene and ensure that only a minimum of phosgene was mixed into the off-gas from the absorption column. The observed phosgene content varied from 0.03% to 0.08% by volume. Further work-up to destroy the residual phosgene was therefore easily possible. The concentration of the bottom phosgene solution varied, but on average took the maximum possible value achievable without larger amounts of phosgene penetrating into the off-gas.

[0068] Example 2 (according to the present invention): The synthesis from example 1 was repeated using a different closed-loop control concept. The main focus of said closed-loop control was a constant phosgene content in the phosgene solution obtained at the bottom of the absorption column (closed-loop control of the bottom concentration). This was done by determining said concentration as a closed-loop control variable and comparing it with a desired target value. This closed-loop control evaluated the difference and automatically adjusted the mass flow rate of MCB to the absorption column to minimize the difference. Furthermore, an override concept was implemented. For this, in this case a threshold value of 0.05% by volume was set for the phosgene content in the off-gas downstream of the absorption column, and if this was exceeded, the closed-loop control of the bottom concentration was deactivated and the MCB mass flow rate to the absorption column was increased by a predefined value until the phosgene content of the off-gas was again below the threshold value.

[0069] This operating mode resulted in a very constant phosgene content of about 55% by weight in the phosgene solution obtained at the bottom. During the thermal phosgenation, the override mode was activated for a few minutes, and the resulting increase in the mass flow rate of the absorbent led to a decrease in the phosgene content, which was compensated for by adding more phosgene to the phosgene solution. In contrast to comparative example 2, the override made it possible to keep the phosgene content in the off-gas below 0.2% by volume.

[0070] Example 3 (according to the present invention): In contrast to the above 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 split in half between the two reactors, and the reactions were carried out in the two reactors with start-up times separated by 3 hours. The process off-gases were combined in a collection line and fed to an absorption device, otherwise operated with a closed-loop control unit as in Example 1. The maximum mass flow rates of the process off-gases from the two reactors did not occur simultaneously, so the maximum gas loading of the absorption column was also lower. The observed phosgene content in the off-gases varied from 0.04% to 0.07% by volume.

Claims

1. 1. A method for producing an isocyanate, comprising the steps of: A) reacting at least one amine or salt thereof with phosgene to obtain a liquid reaction product comprising an isocyanate and further a first gaseous process off-gas stream comprising hydrogen chloride and phosgene; B) separating the gaseous first process off-gas stream, comprising hydrogen chloride, phosgene, any solvent, any low boiling secondary components and any inert components, in at least one absorption device in which the first process off-gas stream is contacted with an absorbent to obtain a phosgene-depleted second off-gas stream having a first phosgene content and a first phosgene solution having a second phosgene content; 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 said isocyanate by distillative separation of said liquid reaction product obtained in step A. In a method comprising: the first phosgene content of the phosgene-depleted second off-gas stream obtained in step B) is compared with a first target value of the first phosgene content and / or the second phosgene content of the first phosgene solution obtained in step B) is determined and compared with a predetermined second target value of the second phosgene content, The absorbent having a mass flow rate M1 is introduced into the absorption device, the mass flow rate M1 is under closed-loop control, and if the first phosgene content and / or the second phosgene content deviate from their respective target values, a reference variable for the mass flow rate M1 is changed, preferably automatically. A method comprising:

2. 2. The method according to claim 1, characterized in that the target value of 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.%, more 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.%.

3. 3. The method according to claim 1 or 2, characterized in that the target value of the second phosgene content is in the range of 20 wt.% to 70 wt.%, preferably in the range of 30 wt.% to 68 wt.%, more preferably in the range of 42 wt.% to 66 wt.%.

4. 4. The method according to claim 1, characterized in that in normal operation the second phosgene content is controlled by regulating the mass flow rate M1 to the predetermined target value and, if the threshold value of the first phosgene content is exceeded, i.e. if there is a deviation from normal operation, the mass flow rate M1 is increased.

5. 5. The process according to claim 1, wherein 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), hexahydrotolylenediamine (H6TDA), in particular 2,4-hexahydrotolylenediamine and 2,6-hexahydrotolylenediamine, 1,3-bis(aminomethyl)benzene (m-XDA) or bis(aminomethyl)norbornane (NBDA), and mixtures thereof.

6. 6. The process according to claim 1, characterized in that the reaction of the at least one amine or salt thereof with phosgene is carried out in the presence of a solvent.

7. 7. The process according to claim 1, characterized in that the reaction of the at least one amine or salt thereof with phosgene is carried out in the condensed phase.

8. 8. The process according to claim 1, wherein the reaction of the at least one amine or salt thereof with phosgene is carried out batchwise.

9. 9. The process according to claim 1, characterized in that the reaction in step A) is a basic phosgenation of at least one amine with phosgene.

10. 10. The method according to claim 8 or 9, characterized in that the reaction in step A) is carried out in at least two reactors arranged in parallel, at least one reactor being operated asynchronously with at least one of the other reactors.

11. 11. The method according to any one of claims 1 to 10, characterized in that chlorobenzene, o-dichlorobenzene or a mixture of these two solvents, preferably chlorobenzene, is used as absorbent in said at least one absorption step.

12. 12. The method according to any one of claims 6 to 11, characterized in that a solvent is used as absorbent in the at least one absorption step, said solvent being preferably the solvent which is also present during the reaction of the at least one amine or salt thereof.

13. 13. The 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 a packed column with random packing, a packed column with structured packing and a tray column, preferably selected from the group consisting of a packed column with random packing and a packed column with structured packing.

14. 1. A device for separating an off-gas stream from a phosgenation reaction, comprising: at least one absorption device for contacting a first process off-gas stream comprising hydrogen chloride and phosgene with an absorbent M1 to obtain a second phosgene-depleted off-gas stream having a first phosgene content and a first phosgene solution having a second phosgene content; an apparatus associated with the absorption device for determining the first phosgene content in the second off-gas stream and / or an apparatus associated with the absorption device for determining the second phosgene content of the first phosgene solution; a device for varying the absorbent mass flow rate M1 at which the absorbent is introduced into the absorption device; Optionally, a computer for closed-loop control of the separation, characterized in that it is configured for controlling the method according to any one of claims 1 to 13. A device comprising:

15. 15. The device according to claim 14, characterized in that it comprises at least one device for off-gas after-treatment, and the apparatus for determining the first phosgene content in the second off-gas stream is arranged between the absorption device and the at least one device for off-gas after-treatment.

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