METHOD FOR THE PREPARATION OF ISOCYANATES

Description

[0001] The invention relates to a process for the production of isocyanates by reacting corresponding amines with phosgene in inert solvents.

[0002] Isocyanates are important basic materials in the chemical industry. They are often produced in large quantities. Di- and polyisocyanates, in particular, are primarily used as starting materials for the production of polyurethanes. Their large-scale production is generally achieved in good yields through the phosgenation of the corresponding amines, amine hydrochlorides, and / or amine carbonates with an excess of phosgene. The corresponding carbamic chloride is formed as an intermediate, which is then converted into the isocyanate with the elimination of hydrogen chloride.

[0003] In practice, chlorobenzene or o-dichlorobenzene have become established as solvents for the reaction, as they are largely inert and well-suited for recovering excess phosgene and separating it from hydrogen chloride. However, it is also possible to use other solvents that are inert under the reaction conditions.

[0004] The process exhaust gas, which consists primarily of phosgene, hydrogen chloride, and solvent, must be treated to prevent the release of phosgene. Furthermore, to increase the economic efficiency of the processes, it is advantageous to treat the process exhaust gas, which consists primarily of phosgene, hydrogen chloride, and solvent, so that at least some of these components can be reused or recycled. Reusing or recycling the phosgene is particularly beneficial.

[0005] US2007 / 0249859A1 describes a sequence of at least two absorption steps, wherein the sequence includes at least one isothermal absorption step and at least one adiabatic absorption step, and the phosgene obtained in this way is recycled into the phosgenation reaction.

[0006] WO2019 / 134909 discloses a process for the phosgenation of methylenedianiline (MDA). The components are mixed in the mixing zone and then fed to the reaction zone, where excess phosgene and HCl are separated as a gas stream. Several mixing zones and / or reaction zones and / or, if present, separation zones can be connected in series or in parallel. The phosgene-containing process exhaust gas stream obtained in this way is fed to a phosgene decomposition unit comprising two (or more) phosgene decomposition units connected in parallel, operated alternately, and regenerated.

[0007] In DE102008009761A1, the process exhaust gas stream is first partially condensed, and the liquid condensate is processed in a stripping column. A liquid solvent stream is obtained at the bottom of the column, and a gas stream at the top, which, together with the previously uncondensed components, is directed into an absorption unit. There, the phosgene contained in the gas is absorbed in a solvent, and the resulting phosgene solution, optionally after enrichment with further phosgene, is used again in the phosgenation reaction.

[0008] US2018 / 0044179A1 employs a distillative separation of the process exhaust gas. Here, the process exhaust gas stream is fed into a distillation column, from the bottom of which a phosgene-containing stream is drawn off. At the top of the column, a stream consisting primarily of hydrogen chloride is drawn off, compressed, and partially condensed. The condensed portion is then expanded and returned to the top of the column as reflux.

[0009] While phosgene is often recycled in the same process, hydrogen chloride offers other utilization options. For example, it can simply be used or marketed as an aqueous solution, i.e., as hydrochloric acid. Alternatively, it can be catalytically or electrochemically oxidized to chlorine or used in the oxychlorination of ethylene to ethylene dichloride.

[0010] WO2008049783A1 describes a continuous process for the production of isocyanates in which the mixing of the reactant streams and / or the conversion of the resulting reaction mixture takes place in at least two parallel strands, so that individual strands are switched off during partial load operation and the strands in operation continue to run in the optimal range.

[0011] Many isocyanates are produced in large quantities, and continuous processes are preferred for these. Isocyanates produced in smaller quantities are generally manufactured by batch liquid-phase phosgenation, as the effort required to convert to a continuous process is very high and not cost-effective given the small quantities produced. In some cases, hybrid processes are also used, combining batch reaction with continuous, downstream processing of the produced isocyanates. This requires the installation of buffer tanks into which the reaction products are discharged and from which the continuous processing unit is then fed.

[0012] For batch production of isocyanates, exhaust gas treatment and, if possible, the recovery of phosgene from this exhaust gas are also desirable. However, problems arise that have not yet been addressed in the prior art. The mass flow rates of process exhaust gas and its composition fluctuate considerably depending on the operating conditions. While it is relatively easy to install a buffer tank for liquid product streams to ensure a consistent feed flow for processing, this is not easily achieved for process exhaust gas due to its large volumes and toxic and corrosive components. During batch phosgenation, periods of very high process exhaust gas mass flow occur.To reliably prevent incomplete absorption of phosgene and thus phosgene breakthrough with the HCl exhaust gas in a phosgene absorption column according to DE102008009761A1 or US2007 / 0249859A1, the absorption column and the solvent mass flow rate would have to be designed to handle peaks with very high process exhaust gas mass flows. Consequently, the absorption column would be oversized outside of these peak loads, which in turn would lead to further problems.

[0013] US4233267A describes a combined system consisting of a batch reactor and a continuous distillation apparatus, in which the volatile reaction products are first condensed and temporarily stored in a condensate tank. From there, they are fed into the continuously operated column, essentially independent of the operating parameters of the batch reactor. This approach presents several disadvantages for phosgenation processes. On the one hand, the condensation of phosgene or even HCl requires very low temperatures. On the other hand, due to the necessary buffering, this approach inevitably increases the amount of phosgene present in the system.

[0014] The object of the invention was therefore to provide an improved method with which process exhaust gas treatment can be efficiently operated even in batch isocyanate production by phosgenation of the amines, and the recovered phosgene can be used again in the phosgenation process if possible.

[0015] This problem could be solved by a process for the preparation of isocyanates by reacting at least one amine or its salt with a stoichiometric excess of phosgene in condensed phase in the presence of a solvent, comprising the steps: A) Transition to hot phosgenation and B) Expulsion of remaining phosgene and / or hydrogen chloride, characterized in that the conversion takes place batchwise in at least two parallel reactors and that step A) and / or step B) take place asynchronously in at least two of the parallel reactors.

[0016] For the process according to the invention, both mono- and polyamines, i.e., amines with two or more amino groups per molecule, can be used. Diamines are preferred.

[0017] Examples of preferred diamines are diaminotoluene (TDA), in particular 2,4-diaminoluene and 2,6-diaminotoluene, diaminodimethylbenzene, diaminonaphthalene, in particular 1,5-diaminonaphthalene (NDA), diaminobenzene, in particular 1,4-diaminobenzene (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, Hexahydrotoluylenediamine (H6TDA), in particular 2,4-Hexahydrotoluylenediamine, 2,6-Hexahydrotoluylenediamine, 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 favored amines 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, hexahydrotoluylenediamine (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 particularly favored amines are 1,5-diaminonaphthalene (NDA), 1,4-diaminobenzene (pPDA), 1,5-diaminopentane (PDA), bis(p-aminocyclohexyl)methane (PACM), hexahydrotoluenediamine (H6TDA), 1,3-bis(aminomethyl)benzene (m-XDA), bis(aminomethyl)norbornane (NBDA) and mixtures thereof.

[0020] Examples of the most preferred amines 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 amines themselves, their salts, in particular the hydrochlorides or carbamates, preferably the hydrochlorides of the amines, can also be phosgenated. These salts are then generally produced in situ at low temperature in a first step, as described, for example, in DE19510259A1, whereby the introduction of the inert gas can optionally be omitted. Preferably, the amines are phosgenated directly (base phosgenation) in a two-step reaction at different reaction temperatures (cold-hot phosgenation).

[0022] The reaction takes place in the presence of a solvent. All solvents known to those skilled in the art are suitable, provided they are inert or at least largely inert under the prevailing reaction conditions. Preferred solvents are selected from the group consisting of aromatic hydrocarbons, halogenated aromatic hydrocarbons, in particular chlorinated aromatic hydrocarbons, esters, ethers, and halogenated hydrocarbons, and mixtures thereof. Aromatic hydrocarbons particularly preferred according to the invention are selected from the group consisting of toluene, bromobenzene, chlorobenzene, dichlorobenzene, in particular o-dichlorobenzene, and mixtures thereof. According to the invention, chlorobenzene, o-dichlorobenzene, or mixtures of these two solvents are particularly preferred.

[0023] The reaction is carried out batchwise. In this context, this means that the liquid reaction product is not continuously withdrawn from the reactors, but rather that the liquid reaction product is only removed from a reactor and transferred to a crude product reservoir or directly to further processing once the reaction to isocyanate in that reactor is complete. Various methods are known to those skilled in the art for determining the endpoint of the reaction. Most commonly, the so-called clear point is used, i.e., the point at which a clear solution has been obtained from the initially present suspension during hot phosgenation. Alternatively, the end of the evolution of HCl gas or a constant NCO content in the reaction mixture can also be used.Even during the reaction, a portion of the liquid reaction mixture can be withdrawn from the reactor and returned to it, for example, to improve mixing or to temper the reaction mixture, without violating the condition of batch phosgenation as defined in the present invention. Gaseous streams comprising essentially hydrogen chloride, phosgene, inert gas, and solvent vapors, on the other hand, can be withdrawn from the reactors at any time, even continuously, and processed. Preferably, the reaction is carried out using a so-called semi-batch process, also known as a fed-batch process or feed-in process, meaning that amine and / or phosgene and, optionally, inert gas are fed into the reactor during the reaction, starting from a partial filling.

[0024] Stirred tank reactors are particularly suitable, but other reactor designs, such as loop reactors, can also be used in principle. According to the invention, the reaction of amine with phosgene takes place in at least two parallel reactors, preferably in two to ten parallel reactors, particularly preferably in two to six parallel reactors, and most preferably in two or four parallel reactors. The reaction preferably takes place at an absolute pressure of 0.100 MPa to 2.000 MPa, more preferably 0.105 MPa to 1.000 MPa, and particularly preferably from 0.110 MPa to 0.600 MPa. Accordingly, the reactors are preferably equipped with a pressure maintenance device, for example, a control valve, through which process exhaust gas can escape from the reactor.

[0025] 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, the cold phosgenation, the temperature of the reaction mixture is preferably kept in the range ≥ 0 °C and < 100 °C. A suspension is formed containing carbamic chloride, amine hydrochloride, and small amounts of free isocyanate. Preferably, a solution of phosgene in an inert solvent is placed first, and then a solution or suspension of the amine in the same solvent, as well as optionally further phosgene, is added. In this way, the concentration of free amine is kept low, thus suppressing the undesired formation of urea.

[0026] In the second stage, hot phosgenation, the temperature is increased and preferably remains in the range of 120 °C to 200 °C. This heating of the reaction mixture to a temperature above 100 °C at the beginning of hot phosgenation is referred to here as step (B) "transition to hot phosgenation". The temperature is then preferably maintained in this range while further phosgene is preferably added at least until the conversion to the isocyanate is complete, i.e., until the evolution of HCl ceases and / or the reaction mixture becomes clear. Phosgene is expediently used in excess for the reaction. If necessary, the reaction can be carried out in both cold and hot phosgenation with the introduction of an inert gas.

[0027] If the amines to be reacted are high-melting amines and poorly soluble in the solvent, a suspension of the amine can also be used for phosgenation. This is preferably produced by dispersion with a dynamic mixing unit, as described in EP2897933B1, paragraphs

[0020] to

[0024] .

[0028] In the amine hydrochloride or carbamate phosgenation, the amine is preferably first reacted with hydrogen chloride gas or carbon dioxide in an inert liquid medium to produce the corresponding salt. The reaction temperature during this salt formation is preferably in the range of 0 to 80 °C. Subsequently or concurrently, a first reaction with phosgene can take place. This is followed by a further phosgenation step, which is essentially similar to the hot phosgenation described above and is therefore also referred to as hot phosgenation in the following. Here, too, the temperature is preferably maintained in the range of 120 to 200 °C, and during this time phosgene and optionally an inert gas are preferably introduced into the reaction mixture. The introduction preferably continues until the reaction to the isocyanate is complete.Here too, phosgene is preferably used in excess to accelerate the reaction.

[0029] Phosgenation is typically carried out with a stoichiometric excess of phosgene, meaning that more than one mole of phosgene is used per mole of amino groups. Preferably, the molar ratio of total phosgene to amino groups is 1.02:1 to 20.0:1, particularly preferably 1.1:1 to 10.0:1, and most preferably 1.2:1 to 5.0:1. If necessary, additional phosgene or phosgene solution can be added to the reaction mixture during the reaction to maintain a sufficient excess of phosgene or to compensate for a loss of phosgene.

[0030] In both base phosgenation and amine hydrochloride or carbamate phosgenation, the remaining phosgene and hydrogen chloride gas are preferably blown out with an inert gas, preferably nitrogen, after the reaction has ceased. If necessary, the reaction mixture can be filtered to remove any solids that may be present, such as unreacted amine hydrochlorides.

[0031] During batch phosgenation, at least two maxima in the mass flow rate of process exhaust gas typically occur for each batch. An increased mass flow rate of process exhaust gas from a reactor occurs, for example, during the transition to hot phosgenation. The heating of the reaction mixture reduces the solubility of gases in the mixture, leading to the release of dissolved gases and thus an increased exhaust gas flow from the reactor, which, depending on the chosen process, primarily includes phosgene, hydrogen chloride, and / or carbon dioxide. A second maximum occurs after the reaction is complete, when remaining phosgene and hydrogen chloride are driven off from the reaction mixture and, if the reaction took place under pressure, the reactor is depressurized.

[0032] To enable a more economical design of the equipment and operating conditions for process exhaust gas treatment, according to the invention at least two of the reactors are operated asynchronously with respect to each other, that is, at least one reactor is operated asynchronously with respect to at least one of the other reactors. This means that step A) Transition to hot phosgenation, i.e., heating the reaction mixture to over 100 °C and / or step B) Expulsion of remaining phosgene and / or hydrogen chloridewhich may also include reducing the reactor pressure, and in at least one reactor, this occurs with a time offset or under different conditions, in particular at a different rate, than in at least one of the other reactors. For example, during the transition to hot phosgenation, it is possible to perform the same temperature ramp with a time offset in the at least two asynchronously operated reactors, or alternatively, to perform a different, in particular a shallower, temperature ramp with or without a time offset in at least one reactor, so that the associated maximum exhaust gas mass flow is lower and / or does not coincide with the maximum exhaust gas mass flow of the other reactors. It is particularly preferred that the asynchronously operated reactors perform the same temperature ramp with a time offset, so that the temperature increase during the transition to hot phosgenation does not occur simultaneously.Preferably, the time offset is at least 5 minutes, particularly preferably at least 10 minutes, and most preferably at least 20 minutes. Also preferably, the time offset is a maximum of 24 hours, more preferably a maximum of 8 hours, more preferably a maximum of 2 hours, and most preferably a maximum of 1 hour. If the periods in which the temperature increases in the at least two of the parallel reactors, in which the transition to hot phosgenation takes place asynchronously, overlap, the decisive factor is the point in time at which the temperature of 100 °C in the reaction mixture is exceeded in each case, and the aforementioned time offsets refer to this point in time.

[0033] The same applies to step B). Expulsion of remaining phosgene and / or hydrogen chloride.For example, the pressure expansion and / or the optional introduction of inert gas into at least two of the parallel reactors can be carried out asynchronously, i.e., with a time delay and / or at different speeds, in order to ensure that the associated maximum exhaust gas mass flow is lower and / or does not coincide with the maximum exhaust gas mass flow of the other reactors. Particularly preferably, the pressure expansion and / or the optional introduction of inert gas is carried out with the same specifications regarding the pressure profiles and inert gas mass flow rates as for the other reactors, but with a time offset. Preferably, the expulsion of phosgene and / or hydrogen chlorine occurs with a time offset of at least 5 minutes, particularly preferably at least 10 minutes, and most preferably at least 20 minutes.Preferably, the time offset is a maximum of 24 hours, preferably a maximum of 8 hours, particularly preferably a maximum of 2 hours, and most preferably a maximum of 1 hour.

[0034] In a further preferred embodiment of the invention, the at least two of the parallel reactors, in which step A) and / or step B) are carried out asynchronously, are operated according to the same operating instructions, but the individual process steps are carried out with a time offset of at least 5 minutes, preferably at least 10 minutes and particularly preferably at least 20 minutes.

[0035] In a further preferred embodiment, at least three, preferably at least four, and particularly preferably all of the parallel-arranged reactors are operated asynchronously. Preferably, all reactors are operated according to the same operating instructions, but the individual process steps are staggered in time. Preferably, the time stagger is at least 5 minutes, particularly preferably at least 10 minutes, and most preferably at least 20 minutes. Also preferably, the time stagger is a maximum of 24 hours, more preferably a maximum of 8 hours, particularly preferably a maximum of 2 hours, and most preferably a maximum of 1 hour.

[0036] Furthermore, it is also advantageous to time the reactors in such a way that one of the reactors is not in the transition to hot phosgenation, i.e., undergoing a temperature increase to over 100 °C, while in another reactor phosgene and hydrogen chloride are being driven off, or the reactor is being depressurized.

[0037] The process exhaust gas typically contains phosgene, hydrogen chloride, solvent vapors, and optionally inert gas and traces of other volatile components of the reaction mixture, such as amines or isocyanates. In a further preferred embodiment, process exhaust gas streams are extracted from the reactors and at least partially combined to form at least one process exhaust gas stream from which phosgene is recovered. Particularly preferably, the recovered phosgene is reused to convert amines to isocyanates. Preferably, the process exhaust gas streams from the various reactors are combined in a manifold and then jointly fed to a processing unit in which phosgene is recovered from the collected process exhaust gas.

[0038] In a preferred embodiment of the invention, the recovery of phosgene from the at least one process exhaust gas stream comprises at least one absorption step in which the phosgene is washed out of the process exhaust gas stream by means of at least one absorbent in one or more absorption devices, yielding an exhaust gas stream and a phosgene solution. Here, the process exhaust gas is brought into contact with an absorbent to wash out the phosgene and to produce an exhaust gas stream containing hydrogen chloride with as little phosgene as possible.

[0039] In general, such an absorption step can be carried out in one or more suitable absorption devices.

[0040] Suitable absorption devices include, for example, gas scrubbers in which the process exhaust gas stream is brought into contact with a liquid stream, also called an absorbent or scrubbing medium, such as immersion scrubbers, spray scrubbers, packed columns, stacked columns, tray columns, or falling film absorbers. Different types of gas scrubbers can also be combined for efficient absorption of phosgene from the process exhaust gas stream. It is preferred to combine two or more, and particularly preferably two, gas scrubbers in one absorption device. Preferably, the absorption device comprises a gas scrubber selected from the group consisting of packed columns, stacked columns, and tray columns, and more preferably from a group consisting of packed columns and a group of stacked columns.

[0041] A particularly suitable absorption device comprises a falling film absorber suitable for isothermal absorption and a tray, packed, or filled column, preferably a packed or filled column, and particularly preferably a packed column, suitable for adiabatic absorption. The falling film absorber is preferably a vertically arranged shell-and-tube heat exchanger, wherein the process exhaust gas and the absorbene are guided on both the tube and shell sides of the heat exchanger, preferably on the tube side, while a cooling medium is guided on the opposite side in co-current or counter-current flow, preferably counter-current flow, to the process exhaust gas stream. The process exhaust gas and the absorbene can also be guided in co-current or counter-current flow, preferably counter-current flow, to each other.The tubes can optionally be fitted with packings, fills, or other internals to improve mass and / or heat transfer. The tray, packed, or filled column is preferably configured such that the process exhaust gas from the isothermal gas scrubber is in countercurrent contact with the absorbent. The absorbent is preferably fed into the column at the top via a liquid distributor. Preferably, it is a packed column with a structured packing. Particularly preferably, the falling film absorber and the tray, packed, or filled column are combined in a column-shaped apparatus that includes a feed line for the process exhaust gas at the bottom. The falling film absorber is arranged above this feed line, and the tray, packed, or filled column is positioned above the absorber.Liquid distributors are preferably located above the falling film absorber and at the top of the column-shaped apparatus to distribute the absorbent evenly onto the tubes or the column. An internal or external condenser is optionally located at the top of the absorption device for further cooling of the exhaust gas stream, which is now depleted of phosgene.

[0042] The exhaust gas stream, which should ideally contain as little phosgene as possible, typically consists essentially of hydrogen chloride, i.e., at least 80 vol%, preferably at least 95 vol%, and particularly preferably at least 98 vol%. It also contains residual solvent, optionally inert gases, and traces of phosgene. The phosgene content of this exhaust gas stream is preferably a maximum of 1.0 vol%, particularly preferably a maximum of 0.5 vol%, and most preferably a maximum of 0.2 vol%. The natural and generally desirable lower limit for the phosgene content in this exhaust gas stream is 0.00 vol%. However, adhering to this limit can lead to increased absorbent consumption, and it can also be advantageous for the design of a control system to allow a low phosgene content in the exhaust gas, for example, to use it as a control variable. Therefore, the phosgene content in the exhaust gas stream is preferably at least 0.01 vol%, and particularly preferably at least 0.02 vol%.-% and especially preferred at a minimum of 0.05 vol-%.

[0043] To achieve the most efficient absorption possible, it is advantageous to first cool the process exhaust gas to a temperature in the range of 5 °C to -25 °C, preferably in the range of 0 °C to -20 °C, and particularly preferably in the range of -5 °C to -15 °C. Cooling can be achieved simply by means of one or more heat exchangers or by quenching, i.e., by contacting the process exhaust gas with a pre-cooled liquid in a quench device. This can be done, for example, in a gas scrubber, preferably a spray scrubber, or in a sprinkler heat exchanger. The absorbent is suitable, for example, as the cooled liquid. Preferably, the absorbent is separated as a liquid after contact with the process gas, and this liquid is recirculated into the quench device, wherein the liquid circuit or the quench device itself contains a cooler to cool the liquid to the desired temperature.During cooling, partial condensation or absorption of phosgene occurs.

[0044] The remaining gas phase is then brought into contact with the absorbent, preferably in countercurrent flow. Preferably, the rising gas phase is brought into contact with the descending liquid absorbent in countercurrent flow. Suitable devices for this purpose are known to those skilled in the art. Preferably, the absorption is a combination of two absorption steps, wherein the gas preferably first undergoes isothermal absorption and then adiabatic absorption. The flow rate of the absorbent is preferably variable and can be adapted to the prevailing process conditions and the resulting requirements. Preferably, the same solvent is used as the absorbent that is also present during the reaction of the amine or its salt with phosgene. Chlorobenzene, o-dichlorobenzene, or a mixture of both is particularly preferred as the absorbent.In the preferred embodiment with two absorption steps, fresh or processed absorbent is preferably introduced in the second absorption step, i.e., the absorption step through which the gas subsequently passes, while in the first absorption step, already loaded absorbent from the second absorption step is preferably used, optionally together with fresh absorbent and / or a recycled, also already loaded absorbent from the first absorption step and / or the preceding cooling process. Preferably, after contact with the absorbent, the gas stream passes through a condenser to condense the absorbed absorbent and separate it from the gas stream as completely as possible.

[0045] The loaded absorbent can be used, as previously described, as a quench medium to initially cool the process exhaust gas stream. A portion of the absorbent is withdrawn either continuously or batchwise and, optionally after the addition of further phosgene and / or solvent, reused for isocyanate production, preferably in one of the reactors for the phosgenation of the amine or its salt. Preferably, the withdrawn loaded absorbent is a solution containing 20 wt.% to 70 wt.%, particularly preferably 30 wt.% to 68 wt.%, and most preferably 42 wt.% to 66 wt.% phosgene in chlorobenzene. The concentration of the solution depends in particular on the phosgene content of the process exhaust gas, the amount of fresh absorbent available for absorption, and the amount of absorbent withdrawn.

[0046] In a particularly preferred embodiment of the method according to the invention, the mass flow rate of absorbent into the absorption device is preferably automatically controlled or, more preferably, regulated. For this purpose, a setpoint for the mass flow rate of absorbent into the absorption device is determined and translated into a control variable for an actuator that influences the mass flow rate. At least one of the following measured variables a) to d), or their change over time, is taken into account in determining the setpoint for the mass flow rate: a) the phosgene content of the exhaust gas stream obtained after at least one absorption step b) the mass flow rate of process exhaust gas into the absorption device c) the concentration of the phosgene solution obtained in the absorption step d) the hydrogen chloride content in the process exhaust gas stream into the absorption device

[0047] Such a procedure is particularly suitable for batch reactions, where fluctuations in the quantity and composition of the process exhaust gas are inevitable. Prior art recommends adjusting the amount of absorbent to the mass flow rate of phosgene to the absorption device. For example, DE102008009761 A1, paragraph

[0048] , and US2007 / 0249859 A1, paragraph

[0048] , disclose weight ratios of solvent (in this case, absorbent) and phosgene at the absorption inlet of 0.1:1 to 10:1, preferably 1:1 to 3:1. How to deal with fluctuations in the amount of phosgene and the total mass flow rate of process exhaust gas at the absorption inlet is not discussed further, as these documents primarily concern continuous processes where such fluctuations do not occur or occur only to a significantly lesser extent.In practical operation, however, it has become apparent that applying this criterion alone, particularly in batch operation of a single reactor or multiple synchronously operated reactors, is insufficient for satisfactory process effectiveness and efficiency. In batch operation, phosgene repeatedly leaches into the hydrogen chloride-containing exhaust gas stream after absorption. Designing the absorption device for peak load is also problematic, as a large quantity of absorbent would then have to be added during periods of lower load to maintain sufficient liquid loading for effective mass transfer. This, in turn, leads to significant fluctuations in the phosgene concentration in the absorbent, consequently complicating the reuse of the recovered phosgene solution in the phosgenation reaction and / or increasing the effort required for absorbent processing.

[0048] To achieve the most consistent possible quality of the phosgene-deficient exhaust gas stream after absorption, it is preferred to include the phosgene content of the exhaust gas stream obtained after at least one absorption step in determining the target value for the mass flow rate of absorbene. This can be done, for example, by determining the phosgene content in the exhaust gas stream and comparing it to a target value. If the content is above the target value, the target value for the mass flow rate of absorbene to the absorption device is increased. If the content is below the target value, the mass flow rate of absorbene to the absorption device is decreased. Methods for determining the phosgene content in the exhaust gas stream are known to those skilled in the art. The determination can be carried out, for example, by IR filter photometry, such as with a multi-component process photometer.

[0049] A computer-implemented method for adjusting a mass flow rate of absorbent in an absorption device for the processing of phosgene-containing process exhaust gas streams is also disclosed, wherein a setpoint for the mass flow rate of absorbent is determined and translated into a control variable for an actuator that influences the mass flow rate, wherein at least one of the following measured variables, or their change over time, is taken into account in the determination of the setpoint for the mass flow rate: the phosgene content of the exhaust gas stream obtained after at least one absorption step, the mass flow rate of process exhaust gas into the absorption device, the concentration of the phosgene solution obtained in the absorption step, and the content of hydrogen chloride in the process exhaust gas stream into the absorption device.

[0050] In one version of the disclosed computer-implemented method, the phosgene content of the exhaust gas stream obtained after at least one absorption step is included in the determination of the target value for the mass flow rate of absorbene. Examples Comparative example 1:

[0051] In two parallel 0.5 m³ stirred tank reactors with temperature control and attached reflux condensers, a solution of 60 kg of phosgene in 150 kg of chlorobenzene was prepared at 0°C in each reactor. The exhaust gas from the reactors was collected in a manifold after passing through the reflux condensers. While stirring, a suspension of 25 kg of 1,5-diaminonaphthalene in 75 kg of chlorobenzene was simultaneously added to the phosgene solution in both reactors. Without further cooling, the suspension was stirred for approximately 120 minutes before the transition from the cold phosgenation phase to the hot phosgenation phase. For this, the reaction mixture in both reactors was heated simultaneously and held under reflux for approximately 6 hours. The pressure during the reaction was approximately 2.9 bar(a).After the reaction mixture cleared, indicating the end of the phosgenier reaction, the reactors were first depressurized and the liquid crude product was drained off, and the isocyanate was isolated in a distillation sequence.

[0052] The exhaust gas from the collection line was fed into a two-stage absorption column, where it was scrubbed countercurrently with chlorobenzene. The process exhaust gas first passed through an isothermal absorption stage at approximately -15 °C and then an adiabatic absorption stage before exiting the process as exhaust gas. Fresh chlorobenzene at -5 °C was introduced at the top of the adiabatic absorption stage. The mass flow rate of fresh chlorobenzene to the absorption column was approximately 36 kg / h. After passing through the adiabatic absorption stage, the resulting phosgene-in-chlorobenzene solution was fed into the isothermal absorption stage as a scrubbing solution. Ultimately, a solution of phosgene in MCB was drawn from the bottom of the two-stage absorption column. After adjusting the phosgene concentration, this solution served as the starting material for further phosgenation reactions.The concentration of the extracted phosgene solution decreased during the reaction, necessitating the addition of increasing amounts of fresh phosgene to adjust the concentration for reuse. A largely phosgene-free hydrogen chloride stream was obtained at the top of the absorption column. However, a small amount of phosgene briefly leaked into the exhaust gas from the absorption unit during the reaction, requiring further treatment of the process exhaust.

[0053] During the production run, a maximum process exhaust gas mass flow of approximately 70 kg / h was observed in the manifold. This occurred during the transition from the cold to the hot phosgenation phase, and the process exhaust gas consisted of approximately 90% phosgene and approximately 10% hydrogen chloride. After passing this maximum, the total mass flow of process exhaust gas decreased, and the composition shifted towards higher hydrogen chloride concentrations. At the end of the reaction, the process exhaust gas flow ceased. Only during reactor depressurization did a brief increase in the mass flow of process exhaust gas occur in the manifold, but this was far less pronounced than the initial maximum. Comparative example 2:

[0054] The synthesis from Comparative Example 1 was repeated under identical reaction conditions. Unlike in Comparative Example 1, the absorption apparatus was not supplied with a constant mass flow of chlorobenzene. Instead, this flow was varied depending on the phosgene mass flow in the process exhaust gas, so that the mass flow of fresh hydrogen chloride into the absorption column always corresponded to 1.3 times the mass flow of phosgene into the absorption column, unless the minimum spray density of the absorption column was undershot. Thus, the maximum mass flow of chlorobenzene at the transition from cold to hot phosgenation, i.e., at the time of the highest phosgene mass flow in the process exhaust gas, was 82 kg / h. During hot phosgenation, approximately 1 h after the occurrence of the maximum in the process exhaust gas mass flow, phosgene breakthrough into the exhaust gas of the absorption apparatus occurred, which intensified over the following 1.5 h.Over the entire duration of the batch production, the consumption of chlorobenzene was higher than in Example 1, resulting in a lower average concentration of the phosgene solution obtained at the bottom of the column. Furthermore, a greater breakthrough of phosgene into the exhaust gas of the absorption column was observed than in comparative Example 1. Example 1 (according to the invention):

[0055] The synthesis from Example 1 was repeated with the difference that the second reactor was operated with a time offset of 3 hours. This means that the addition of the amine suspension, as well as the heating for the transition from cold to hot phosgenation in the second reactor, did not occur synchronously with the first reactor, but rather with this 3-hour time offset. Furthermore, the mass flow rate of fresh chlorobenzene to the absorption column was set to a constant 30 kg / h.

[0056] During the transition of the first reactor from cold to hot phosgenation, an initial maximum in the process exhaust gas mass flow was observed in the manifold. At this point, the mass flow was approximately 35 kg / h, and the process exhaust gas again consisted of approximately 90% phosgene and approximately 10% hydrogen chloride. Later in the production process, the process exhaust gas mass flow reached a second, higher maximum of approximately 46 kg / h. This occurred during the transition of the second reactor from the cold to hot phosgenation phase, and the process exhaust gas consisted of approximately 79% phosgene and approximately 21% hydrogen chloride. After passing this maximum, the total mass flow of process exhaust gas decreased, and its composition shifted towards higher hydrogen chloride concentrations.Since the depressurization of the two reactors after the reaction also occurred with a corresponding time offset, not one, but two further short and less pronounced peaks were also visible in the process exhaust gas mass flow in the collecting line.

[0057] Despite the reduced use of chlorobenzene in the absorption column compared to Examples 1 and 2, no phosgene breakthrough into the exhaust gas of the absorption column was observed at any time in this experiment. While the phosgene content in the phosgene solution taken from the bottom of the absorption column fluctuated, it was on average higher than in the previously described Examples 1 and 2. Both the maximum gas and liquid loading of the absorption column were reduced compared to Examples 1 and 2, allowing the use of an absorption column with a smaller diameter and thus lower investment costs. Example 2 (according to the invention):

[0058] The synthesis from Example 1, with staggered reactor operation, was repeated. However, unlike in Example 1, the absorption unit was not supplied with a constant mass flow of chlorobenzene. Instead, the phosgene content in the exhaust gas from the absorption unit was monitored online. The target value for the phosgene content was set to 0.05 vol%, and the deviation of the measured value from this target value, via a control loop, ultimately acted on a control valve in the fresh chlorobenzene supply line to adjust the chlorobenzene mass flow to the absorption column accordingly and keep the deviation as small as possible.

[0059] Over the entire duration of the batch production, the consumption of chlorobenzene in this example was even lower than in Example 1, resulting in a higher average concentration of the phosgene solution obtained at the bottom of the column, while the exhaust gas from the absorption column was only minimally contaminated with phosgene throughout the entire operation. The phosgene content was low and constant, making further post-treatment to destroy the residual phosgene straightforward.

Claims

1. Process for producing isocyanates by reacting at least one amine or salt thereof with a stoichiometric excess of phosgene in the condensed phase in the presence of a solvent, comprising the steps of: A) transitioning to a hot phosgenation and B) expelling residual phosgene and / or hydrogen chloride, characterized in that the reaction is carried out in a batchwise manner in at least two reactors arranged in parallel and in that step A) and / or step B) are carried out asynchronously in at least two of the reactors arranged in parallel.

2. Process 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 mixtures thereof.

3. Process according to either of Claims 1 or 2, characterized in that the reaction is carried out in a semi-batchwise process.

4. Process according to any of Claims 1 to 3, characterized in that the reaction is carried out in 2 to 10, preferably in 2 to 6, and more preferably in 2 to 4, reactors arranged in parallel,5. Process according to any of Claims 1 to 4, characterized in that at least three, preferably at least four, and more preferably all, of the reactors arranged in parallel are operated asynchronously to one another.

6. Process according to any of Claims 1 to 4, characterized in that step A) transitioning to a hot phosgenation is carried out asynchronously in at least two of the reactors.

7. Process according to any of Claims 1 to 6, characterized in that process offgas streams are withdrawn from the reactors and at least partially combined into at least one process offgas stream from which phosgene is recovered.

8. Process according to any of Claims 1 to 7, characterized in that process offgas streams are withdrawn from the reactors and at least partially combined into at least one process offgas stream from which phosgene is recovered, wherein the recovery of phosgene from the at least one process offgas stream comprises at least one absorption step in which the phosgene is scrubbed from the process offgas stream by means of at least one absorbent in one or more absorption devices, affording an offgas stream and a phosgene solution.

9. Process according to Claim 8, characterized in that a solvent employed as absorbent in the at least one absorption step is preferably the solvent that is also present in the reaction of the at least one amine or salt thereof.

10. Process according to either of Claims 8 or 9, characterized in that the absorption device comprises a gas scrubber selected from the group consisting of packed columns with a random packing, packed columns with a structured packing, and tray columns, preferably consisting of packed columns with a random packing and packed columns with a structured packing.

11. Process according to any of Claims 8 to 10, characterized in that the absorbent is added in a mass flow, the target value for which is determined, preferably automatically, taking account of at least one of the following measured variables a) to d) or its change over time: a) the phosgene content of the offgas stream obtained downstream of the at least one absorption step, b) the mass2021P30223WE flow of process offgas into the absorption device, c) the content of hydrogen chloride in the process offgas flow into the absorption device, d) the concentration of the phosgene solution obtained in the absorption step.

12. Process according to any of Claims 8 to 11, characterized in that the phosgene solution obtained in the absorption device, optionally after addition of further phosgene and / or solvent, is reused for isocyanate production.

13. Process according to either of Claims 11 or 12, characterized in that the phosgene content of the offgas stream obtained downstream of the at least one absorption step is taken into account in determining the target value for the mass flow of absorbent.