Method for Producing Isocyanates

Abstract

The present invention relates to a process for the preparation of isocyanates by reacting at least one amine or its salt with a stoichiometric excess of phosgene in the presence of a solvent in a condensed phase, comprising the steps A) transferring to a thermal phosgenation process and B) draining off the residual phosgene and / or hydrogen chloride. The invention is characterized in that the reaction is carried out in batches in at least two reactors arranged parallel to one another, and step A) and / or step B) are carried out asynchronously in at least two of the reactors arranged parallel to one another.

Description

[Technical field]

[0001] The present invention relates to a process for preparing isocyanates by reacting the corresponding amines with phosgene in an inert solvent. [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 and / 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, which essentially consists of phosgene, hydrogen chloride and solvent, must be treated to avoid the release of phosgene.To increase the economic efficiency of the process, it is advantageous to treat the process off-gas, which essentially consists of phosgene, hydrogen chloride and solvent, so that at least a part of these components can be further used or reused.It is particularly advantageous if phosgene can be further used or reused.

[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 process off-gas stream is led to a distillation column, and a phosgene-containing stream is discharged from its bottom. At the head of the column, a stream consisting essentially of hydrogen chloride is discharged, 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] WO2008049783 describes a continuous process for producing isocyanates in which the mixing of reactant streams and / or the reaction of the resulting reaction mixture is carried out in at least two lines connected in parallel, thus making it possible to shut down individual lines in the event of operation at only partial load and keep the operating line running in its optimum range.

[0010] 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.

[0011] For batch production of isocyanates, the treatment of off-gas and, if possible, the recovery of phosgene from this off-gas is also desirable. 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 and thus ensure a uniform inlet stream for treatment, which cannot be easily achieved with the process off-gas due to the large volume and toxic and corrosive components. During the batch phosgenation process, periods occur when the mass flow rate of the process off-gas is very high. In order to ensure that incomplete absorption of phosgene in the phosgene absorption according to, for example, DE 102008009761 A1 or US 2007 / 0249859 A1 and thus the penetration of phosgene from the phosgene absorption into the HCl off-gas, the absorption column, and also the mass flow rate of the solvent, must be designed for these peaks where the mass flow rate of the process off-gas is very high. Outside these peak loads, the absorption column becomes oversized, which causes further problems.

[0012] 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. [Prior art documents] [Patent documents]

[0013] [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] International Publication No. 2008049783 [Patent Document 5] U.S. Pat. No. 4,233,267 Summary of the Invention [Problem to be solved by the invention]

[0014] It was therefore an object of the present invention to provide an improved process, even in the batchwise production of isocyanates by phosgenation of amines, which allows the reprocessing of process off-gases to be operated efficiently and whereby the recovered phosgene can be reused as far as possible in the phosgenation process. [Means for solving the problem]

[0015] The object is to provide a process for the preparation of isocyanates by reacting at least one amine or its salt with a stoichiometric excess of phosgene in the presence of a solvent in condensed phase, comprising the steps of: A) transferring to thermal phosgenation; B) venting residual phosgene and / or hydrogen chloride; Including, This is achieved by a method, characterized in that the reaction is carried out batchwise in at least two reactors arranged in parallel and step A) and / or step B) are carried out asynchronously in at least two of the parallel reactors. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0016] Both monoamines and polyamines, i.e. amines having two or more amino groups per molecule, can be used in the process of the present invention. Diamines are preferred.

[0017] 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.

[0018] 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), 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.

[0019] 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), 1,3-bis(aminomethyl)benzene (m-XDA), bis(aminomethyl)norbornane (NBDA) and mixtures thereof.

[0020] 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.

[0021] 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).

[0022] The reaction is carried out in the presence of a solvent. All solvents known to those skilled in the art that are inert or at least largely inert under 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.

[0023] The reaction is carried out batchwise. This should be understood to mean that in the present invention, the liquid reaction product is not continuously removed from the reactor, but is removed from the reactor 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 remove 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 batch phosgenation in the context of the present invention. However, the gas stream essentially comprising hydrogen chloride, phosgene, inert gas and solvent vapors may be removed 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.

[0024] Stirred tank reactors are particularly suitable as reactors, although other types of reactors, such as loop reactors, can in principle also be used. According to the invention, the reaction of amine with phosgene is carried out in at least two reactors arranged in parallel, preferably in 2 to 10 reactors arranged in parallel, more preferably in 2 to 6 reactors arranged in parallel, most preferably in 2 or 4 reactors arranged in parallel. The reaction is preferably carried out at an absolute pressure of 0.100 MPa to 2.000 MPa, preferably 0.105 MPa to 1.000 MPa, more preferably 0.110 MPa to 0.600 MPa. The reactor is therefore preferably equipped with a pressure-retaining device, for example a control valve, through which process off-gases can escape from the reactor.

[0025] In base phosgenation, i.e. the reaction of amine 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°C or higher and lower than 100°C. A suspension containing carbamoyl chloride, amine hydrochloride and a small amount of free isocyanate is formed. The preferred procedure here is that a solution of phosgene in an inert solvent is first charged, and then a solution or suspension of amine in the same solvent and optionally further phosgene are added. This keeps the concentration of free amine low, thus suppressing the undesired formation of urea.

[0026] In the second stage of hot phosgenation, the temperature is raised, preferably to within the range of 120°C to 200°C. This heating of the reaction mixture to a temperature above 100°C, which takes place at the beginning of the hot phosgenation, should in this case be understood to mean step (B) "step of transition to hot phosgenation". The temperature is then preferably kept within this range until the reaction to isocyanates is complete, i.e. until the evolution of HCl has ceased and / or until the reaction mixture becomes clear, preferably at least for the time that further phosgene is fed. Phosgene is advantageously used in excess for the reaction. 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.

[0027] 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] .

[0028] 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 hot phosgenation from the base phosgenation described above and is therefore also referred to below as hot phosgenation. Thus, again, the temperature is preferably kept in the range of 120-200°C while phosgene and optionally an inert gas are preferably introduced into the reaction mixture. The introduction is preferably continued until the reaction to the isocyanate is completed. Here again, phosgene is preferably used in excess to accelerate the reaction.

[0029] Phosgenation is usually carried out with a stoichiometric 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.

[0030] In both the base phosgenation and the amine hydrochloride or carbamate phosgenation, residual phosgene and hydrogen chloride gas are preferably removed at the end of the reaction by purging with an inert gas, preferably nitrogen. If necessary, the reaction mixture may be filtered to remove any solids present, such as unreacted amine hydrochloride.

[0031] During the course of batch phosgenation, at least 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.

[0032] According to the invention, in order to allow a more economical design of the equipment and operating conditions for process off-gas reprocessing, at least two of the reactors are operated asynchronously with respect to one another, i.e. at least one reactor is operated asynchronously with at least one of the other reactors. This means that step A) of the transition to thermal phosgenation, i.e. the heating of the reaction mixture above 100° C., and / or step B) of the discharge of residual phosgene and / or hydrogen chloride, which may include depressurization of the reactor to a lower pressure, is carried out in this at least one reactor with a time delay or under different conditions, in particular at a different speed, than in at least one of the other reactors. For example, during the transition to thermal phosgenation, it is possible to carry out the same temperature gradient with a time delay in the at least two asynchronously operated reactors, or to carry out in at least one reactor a different temperature gradient, in particular a flatter temperature gradient, with or without a time delay, 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 operated asynchronously carry out the same temperature gradient with a time delay, so that the temperature rise in the transition to thermal phosgenation does not occur simultaneously. 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. If the periods of temperature rise in at least two parallel-arranged reactors, in which the transition to thermal phosgenation occurs asynchronously, overlap, in each case the time of exceeding a temperature of 100 ° C in the reaction mixture is important, and the above-mentioned time delays relate to this time.

[0033] The same applies to step B) of discharging residual phosgene and / or 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 pressure profile and 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.

[0034] In a further preferred embodiment of the present invention, at least two of the parallel arranged reactors, in which step A) and / or step B) are performed asynchronously, operate 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.

[0035] In a further preferred embodiment, at least three, preferably at least four, more preferably all of the parallel reactors are operated asynchronously with each other. Preferably, all reactors are operated according to the same operating instructions, but the individual process steps are carried out with a time delay with respect to each other. 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 24 hours or less, preferably 8 hours or less, more preferably 2 hours or less, most preferably 1 hour or less.

[0036] It may further be advantageous to stall a reactor while stripping off phosgene and hydrogen chloride or depressurization is taking place in another reactor, so that transition to hot phosgenation, i.e., temperature increase above 100° C., does not proceed in either reactor.

[0037] The process off-gas typically contains phosgene, hydrogen chloride, solvent vapors, as well as any inert gases, and traces of other volatile components of the reaction mixture, such as amines or isocyanates. In a further preferred embodiment, a process off-gas stream is removed from the reactor and at least partially combined with at least one process off-gas stream from which phosgene is recovered. The recovered phosgene is particularly preferably reused in the reaction of amines to isocyanates. The process off-gas streams of the various reactors are preferably combined in one collection line and then fed together to a reprocessing where phosgene is recovered from the collected process off-gas.

[0038] In a preferred embodiment of the invention, the recovery of phosgene from at least one process off-gas stream comprises at least one absorption step in which phosgene is scrubbed from the process off-gas stream by at least one absorbent in one or more absorption devices to obtain an off-gas stream and a phosgene solution, in which the process off-gas is contacted with the absorbent in order to scrub phosgene from the process off-gas and to produce a hydrogen chloride-containing off-gas stream that is as depleted as possible of phosgene.

[0039] Such an absorption step may generally be carried out in one or more suitable absorption devices.

[0040] Suitable absorption devices are, for example, gas scrubbers in which the process off-gas stream is contacted with a liquid stream, also called absorbent or scrubbing medium, examples of said scrubbers are 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 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. Preferably, 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, more preferably selected from the group consisting of packed columns with random packing and packed columns with structured packing.

[0041] 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.

[0042] The off-gas stream, which is as depleted 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 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.

[0043] To achieve maximum efficiency of absorption, it is advantageous to initially cool the 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. Cooling can be carried out simply by one or more heat exchangers or by quenching, i.e. by contacting the 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 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.

[0044] The residual gas phase is then contacted with the absorbent, preferably in countercurrent flow. At the same time, the ascending gas phase is preferably contacted with the descending liquid absorbent in countercurrent flow. 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 an isothermal absorption and then through an adiabatic absorption. The volumetric flow rate of the absorbent is preferably variable and can be adapted in each case to the prevailing process conditions and the resulting requirements. The absorbent used is preferably the same solvent that was also present during the reaction of the amine or its salt with phosgene. It is particularly preferred to use chlorobenzene, o-dichlorobenzene or a mixture of these two as absorbent. In a preferred embodiment with two absorption steps, fresh or reprocessed absorbent is preferably introduced into the second absorption step, i.e. the absorption step through which the gas passes afterwards, whereas in the first absorption step it is preferred to use already loaded absorbent from the second absorption step, possibly together with fresh absorbent and / or together with likewise 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.

[0045] The loaded absorbent can be used as a quench medium to initially cool the process off-gas stream, as described above. A portion of the absorbent is continuously or discontinuously withdrawn and reused, optionally after addition of further phosgene and / or solvent, for isocyanate production, preferably in one of the reactors for the phosgenation of amines or their salts. 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 process off-gas, the amount of fresh absorbent for absorption and the amount of absorbent withdrawn.

[0046] In a particularly preferred embodiment of the method of the invention, the mass flow rate of the absorbent to the absorption device is preferably controlled automatically, preferably by closed-loop control. For this purpose, a setpoint value of the mass flow rate of the absorbent to the absorption device is determined and converted into a control variable of an actuator that influences the mass flow rate. The determination of the setpoint value of the mass flow rate is based on the following measured variables a) to d): a) the phosgene content of the off-gas stream obtained downstream of at least one absorption step b) Mass flow rate of process off-gas to the absorption device c) The concentration of the phosgene solution obtained in the absorption step d) Hydrogen chloride content in the process off-gas stream to the absorption device or changes therein over time.

[0047] Such a procedure is particularly suitable in batch reaction regimes, where fluctuations in the amount and composition of the process off-gas inevitably occur. A recommendation from the prior art is that the amount of absorbent is guided by the mass flow rate of phosgene to the absorption device. See, for example, paragraph DE 102008009761 A1.

[0048] and US Patent Application Publication No. 2007 / 0249859, paragraph

[0048] , disclose that the weight ratio of solvent (in this case absorbent) to phosgene at the inlet to the absorption is 0.1:1 to 3:1:1, preferably 1:1 to 10. How to deal with the fluctuations in the amount of phosgene at the inlet to the absorption and the total mass flow rate of the process off-gas is not discussed, since these documents mainly relate to continuous processes in which such fluctuations do not occur or occur to a much lesser extent. However, operational practice shows that applying this criterion alone, especially in batch operation of a single reactor or multiple synchronously operating reactors, is not sufficient to operate the process with satisfactory effectiveness and efficiency. In batch operation, the penetration of phosgene into the hydrogen chloride-containing off-gas stream downstream of the absorption occurs many times. Basing the design on the absorption device on peak load is also problematic, since even during periods of lower load, a large amount of absorbent must still be expended to maintain a sufficient liquid load for good mass transfer. This in turn leads to significant fluctuations in the phosgene concentration in the absorbent, as a consequence of which it becomes difficult to reuse the resulting phosgene solution in the phosgenation reaction or makes reprocessing of the absorbent more complicated.

[0048] In order to achieve maximum constancy of the quality of the phosgene-depleted off-gas stream downstream of the absorption, it is preferable to take into account the phosgene content of the off-gas stream obtained downstream of at least one absorption step when determining the target value of the mass flow rate of the absorbent. This can be done, for example, by determining the content of phosgene in said off-gas stream and comparing it with a target value. If the content is above the target value, the target value of the mass flow rate of the absorbent to the absorption device is increased. If the content is below the target value, the mass flow rate of the absorbent to the absorption device is decreased. Methods for determining the phosgene content in the off-gas stream are known to those skilled in the art. The determination can be carried out, for example, by IR filter photometry, for example using a multicomponent process photometer.

[0049] The present invention therefore relates to a computer-implemented method for regulating the mass flow rate of an absorbent in an absorption device for reprocessing a phosgene-containing process off-gas stream, in which a target value of the mass flow rate of the absorbent is determined and converted into control variables of actuators influencing the mass flow rate, and in determining the target value of the mass flow rate, the following measured variables are taken into account: the phosgene content of the off-gas stream obtained downstream of at least one absorption step; the mass flow rate of the process off-gas to the absorption device; the concentration of the phosgene solution obtained in the absorption step; Hydrogen chloride content in the process off-gas stream to the absorption device The present invention further provides a method, wherein at least one of or a change over time therein is taken into account. In a preferred embodiment of the computer-implemented method, the phosgene content of the off-gas stream obtained downstream of the at least one absorption step is taken into account when determining the target value of the absorbent mass flow rate.

[0050] [Example] Comparative Example 1: A solution of 60 kg of phosgene in 150 kg of chlorobenzene was poured into two parallel 0.5 m 200 rpm reactors equipped with temperature control units and reflux condensers. 3 In each of the stirred tank reactors, the temperature was adjusted to 0°C. After passing through a reflux condenser, the off-gas from the reactors was collected in a collection line. To the phosgene solution in each of the two reactors, a suspension of 25 kg of 1,5-diaminonaphthalene in 75 kg of chlorobenzene was added simultaneously, while stirring. The suspension was stirred for about 120 minutes without further cooling before passing from the cold phosgenation stage to the hot phosgenation stage. For this, the reaction mixtures in the two reactors were heated simultaneously and kept under reflux for about 6 hours. The pressure during the reaction was about 2.9 bar(a). After the reaction mixture became clear, which signals the end of the phosgenation reaction, the reactors were first depressurized, the liquid crude product was poured off and the isocyanate was isolated in a distillation sequence.

[0051] The off-gas from the collection line was fed simultaneously to a two-stage absorption column, where it was washed with chlorobenzene in countercurrent. The process off-gas here 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 36 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 MCB solution of phosgene was finally taken off at 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 taken-off 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. At the head of the absorption column, a hydrogen chloride stream almost free of phosgene was obtained. However, during the reaction there was brief penetration of small amounts of phosgene into the off-gas of the absorption device, making further treatment of the process off-gas more cumbersome.

[0052] During the production run, a maximum process off-gas mass flow rate of about 70 kg / h was observed in the collection line. This occurred during the transition from the cold to the hot phosgenation stage, when the process off-gas consisted of about 90% phosgene and about 10% hydrogen chloride. After passing this maximum, the overall mass flow rate of the process off-gas decreased and the composition shifted towards a higher hydrogen chloride content. 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, which was much less pronounced than the first maximum.

[0053] Comparative Example 2: The synthesis from Comparative Example 1 was repeated under the same reaction conditions. Unlike 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 time when the phosgene mass flow rate in the process off-gas was at a maximum, was 82 kg / h. During the hot phosgenation, about 1 hour after the process off-gas mass flow rate reached a maximum, a penetration of phosgene into the off-gas of the absorption device occurred, which intensified over the following 1.5 hours. Over the entire operating time of the batch production, the consumption of chlorobenzene was higher than in Example 1, and as a result, the concentration established in the phosgene solution obtained at the bottom of the column was on average lower. Moreover, it turned out that the penetration of phosgene into the off-gas of the absorption column was more pronounced than in Comparative Example 1.

[0054] Example 1 (according to the present invention): The synthesis from example 1 was repeated with the difference that the second reactor was operated with a time delay of 3 hours, i.e. the addition of the amine suspension and the heating for the transition from cold to hot phosgenation in the second reactor were not carried out synchronously with the first reactor, but with said 3 hour time delay. Furthermore, the mass flow rate of fresh chlorobenzene to the absorption column was set to a constant 30 kg / h.

[0055] During the transition from cold to hot phosgenation in the first reactor, a first maximum of the process off-gas mass flow rate in the collection line was observed. The mass flow rate at this point was about 35 kg / h, the process off-gas again consisted of about 90% phosgene and about 10% hydrogen chloride. In the further course of production, the process off-gas mass flow rate passed a second, higher maximum of about 46 kg / h. This occurred during the transition from the cold to the hot phosgenation stage in the second reactor, the process off-gas consisted of about 79% phosgene and about 21% hydrogen chloride. After passing this maximum, the overall mass flow rate of the process off-gas decreased and the composition shifted towards a higher hydrogen chloride content. Since the depressurization of the two reactors after the reaction was also carried out with a corresponding time delay, the process off-gas mass flow rate in the collection line likewise showed not one, but two further short, less pronounced peaks.

[0056] Despite the reduced use of chlorobenzene in the absorption column compared to Comparative Examples 1 and 2, no phosgene penetration into the absorption column off-gas was observed at any time during this experiment. The phosgene content in the phosgene solution removed at the bottom of the absorption column varied, but was on average higher than in the previous Examples 1 and 2. Both the maximum gas and liquid loading of the absorption column were reduced compared to Examples 1 and 2, thereby allowing the use of smaller diameter and, therefore, lower capital cost absorption columns.

[0057] Example 2 (according to the present invention): The synthesis from Example 1 was repeated with a time-delay operation of the reactor. However, unlike Example 1, in this case the absorption device was not charged with a constant mass flow rate of chlorobenzene. Instead, the phosgene content in the off-gas from the absorption device was tracked by online 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 a closed-loop control circuit, and finally on a control valve in a 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.

[0058] Over the entire run time of the batch production, the chlorobenzene consumption of this example was again lower than in Example 1, resulting in a higher concentration on average being established in the phosgene solution obtained at the bottom of the column, while the off-gas from the absorption column was only minimally contaminated with phosgene over the entire run time. The phosgene content here was low and constant, so that further reprocessing to destroy residual phosgene is simple.

Claims

1. 1. A process for producing isocyanates by reacting at least one amine or salt thereof with a stoichiometric excess of phosgene in the presence of a solvent in a condensed phase, comprising: A) transferring to thermal phosgenation; B) venting residual phosgene and / or hydrogen chloride; Including, 13. The process according to claim 12, wherein the reaction is carried out batchwise in at least two reactors arranged in parallel and step A) and / or step B) are carried out asynchronously in at least two of the parallel reactors.

2. 2. The method according to claim 1, characterized in that the at least one amine is at least one diamine, preferably 1,5-diaminonaphthalene (NDA), 1,4-diaminobenzene (pPDA), 1,5-diaminopentane (PDA), bis(p-aminocyclohexyl)methane (PACM), hexahydrotolylenediamine (H6TDA), 1,3-bis(aminomethyl)benzene (m-XDA) or bis(aminomethyl)norbornane (NBDA), or a mixture thereof.

3. 3. The process according to claim 1 or 2, characterized in that the reaction is carried out in a semi-batch process.

4. 4. The process according to any one of claims 1 to 3, characterized in that the reaction is carried out in 2 to 10, preferably 2 to 6, more preferably 2 to 4 reactors arranged in parallel.

5. 5. The method according to any one of claims 1 to 4, characterized in that at least three, preferably at least four and more preferably all of said parallel arranged reactors are operated asynchronously with respect to each other.

6. 5. The process according to claim 1, wherein the step A) of transferring to thermal phosgenation is carried out asynchronously in at least two of the reactors.

7. 7. The process according to claim 1, wherein a process off-gas stream is withdrawn from the reactor and is at least partially combined with at least one process off-gas stream from which phosgene is recovered.

8. 8. The method according to claim 1, wherein a process off-gas stream is withdrawn from the reactor and at least partially combined with at least one process off-gas stream in which phosgene is recovered, and wherein said recovery of phosgene from said at least one process off-gas stream comprises at least one absorption step in which said phosgene is scrubbed from said process off-gas stream by at least one absorbent in one or more absorption devices to obtain an off-gas stream and a phosgene solution.

9. 9. The method according to claim 8, characterized in that the solvent used as absorbent in said at least one absorption step is preferably the solvent also present in the reaction of said at least one amine or salt thereof.

10. 10. The method according to claim 8 or 9, 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.

11. The absorbent is added at a mass flow rate, the setpoint of which is preferably determined automatically by the following measured variables a) to d): a) the phosgene content of the off-gas stream obtained downstream of the at least one absorption step; b) the mass flow rate of process off-gas to the absorption device; c) the hydrogen chloride content in the process off-gas stream to the absorption device; d) the concentration of the phosgene solution obtained in the absorption step; The method according to any one of claims 8 to 10, characterized in that the method is determined taking into consideration at least one of the following or changes over time:

12. 12. The process according to claim 8, wherein the phosgene solution obtained in the absorption device is reused for isocyanate production, optionally after addition of further phosgene and / or solvent.

13. 13. The method according to claim 11 or 12, characterized in that the phosgene content of the off-gas stream obtained downstream of the at least one absorption step is taken into account when determining the target value of the mass flow rate of absorbent.

14. 1. A computer-implemented method for regulating a mass flow rate of an absorbent in an absorption device for reprocessing a phosgene-containing process off-gas stream, comprising: determining a target value of the mass flow rate of an absorbent and converting it into control variables for actuators that affect the mass flow rate, and, in determining the target value of the mass flow rate, measuring the following measured variables: the phosgene content of said off-gas stream obtained downstream of at least one absorption step; the mass flow rate of the process off-gas to the absorption device; the concentration of the phosgene solution obtained in the absorption step; Hydrogen chloride content in the process off-gas stream to the absorption device. The computer-implemented method of claim 1, wherein at least one of the following is taken into account:

15. 15. The method according to claim 14, characterized in that the phosgene content of the off-gas stream obtained downstream of the at least one absorption step is taken into account when determining the target value of the mass flow rate of absorbent.

Images