Method for producing polyisocyanates

Abstract

The present invention relates to a process for producing polyisocyanates by reacting an organic amine with phosgene in an inert solvent, the process comprising: (a) mixing an amine-containing feed stream and a phosgene-containing feed stream in a mixing device, thereby obtaining a reaction mixture; (b) feeding the reaction mixture to a residence device in which the reaction continues, thereby obtaining an intermediate reaction mixture; (c) feeding the intermediate reaction mixture to a reactor in which a crude reaction product and a gas phase are obtained, and (d) subjecting the crude reaction product to a post-treatment to remove phosgene, if still present, and a solvent, in which additional gaseous hydrogen chloride is fed to the reactor of stage (c) such that the additional gaseous hydrogen chloride is contacted with the crude reaction product, and removing excess phosgene and hydrogen chloride from the crude reaction product in the reactor by distillation.

Description

describe

[0001] This invention relates to a method for producing polyisocyanates by reacting an organic amine with phosgene in an inert solvent, the method comprising:

[0002] (a) A feed stream containing amine and a feed stream containing phosgene are mixed in a mixing device to obtain a reaction mixture;

[0003] (b) The reaction mixture is fed into a residence device in which the reaction continues to occur, thereby obtaining an intermediate reaction mixture;

[0004] (c) The intermediate reaction mixture is fed into a reactor to obtain the crude reaction product and the gas phase, and

[0005] (d) Post-process the crude reaction product to remove phosgene (if present) and solvent.

[0006] Polyisocyanates such as methylene diphenyl diisocyanate or toluene diisocyanate can be produced by the phosgenation of the corresponding polyamines in a solvent. This method is described, for example, in "Isocyanates, Organic, Ullmann's Encyclopedia of Industrial Chemistry, 7th Edition, Vol. 20, 2012, pp. 63-82." Phosgeneification can be performed in several ways. WO-A 2004 / 056756 provides a good overview of the method options and describes a three-stage method comprising a mixing nozzle in the first stage, a residence time reactor in the second stage, and at least one reaction tower in the final stage. In this method, the pressure is reduced in subsequent stages such that the pressure in the subsequent stages is lower than that in the previous stage. In the first stage, an amine-containing feed is mixed and reacted with a phosgene-containing feed to obtain a first reaction mixture containing intermediates such as carbamoyl chloride and amine hydrochloride. The major conversion of these intermediates is achieved in the second stage, while in the third stage, the remaining intermediates are converted to the final product, and phosgene and hydrogen chloride are at least partially removed in at least one reaction tower. After the final stage conversion is completed, conventional post-treatment such as solvent separation can be initiated to produce the final polyisocyanate.

[0007] Several byproducts formed by impurities in the feed or during the phosgenation reaction are known to affect the quality of the final polyisocyanate product. Therefore, numerous methods are known and described to improve quality by adjusting phosgenation and post-treatment processes. For example, WO-A 99 / 54289 describes a method for reducing the amount of chlorinated byproducts and the iodine color index. Furthermore, GB-A 1549294, DD-A 285593, US 4,465638, DE-A 45 00 774, DE-A 4021 712, DE-A 40 06 978, EP-A 445 602, or DE-A 43 18 018 describe methods in which additives are added to improve quality. US 4,876,380 describes a method in which the crude reaction product is post-treated by an extraction process to improve product quality.

[0008] Another known method for improving the quality of diisocyanates is to strip the final reaction mixture with hydrogen chloride. For this purpose, US 4,193,932 teaches the use of HCl gas to strip gaseous components such as excess phosgene and HCl from the final isocyanate-containing reaction mixture. In this way, acidic substances and hydrolyzable chlorine-containing substances are minimized.

[0009] The method is further improved by first removing phosgene from the reaction mixture and then performing heat treatment in the presence of hydrogen chloride gas, as described in US 5,364,958.

[0010] CN-A 111 961 185 also describes HCl stripping after phosgene removal, and further uses it to control the phosgene adduct in the reaction product within the range of 0.5 ppm to 40,000 ppm, preferably 0.5 ppm to 25,000 ppm.

[0011] In all known methods, the phosgenation reaction to form isocyanate is completed first, before the HCl treatment of the reaction product begins.

[0012] The object of the present invention is to further improve the method for producing polyisocyanates, specifically MDI, by reducing the number of process steps and thus the complexity of the method.

[0013] This objective is achieved by a method for producing polyisocyanates by reacting an organic amine with phosgene in an inert solvent, the method comprising:

[0014] (a) A feed stream containing amine and a feed stream containing phosgene are mixed in a mixing device to obtain a reaction mixture;

[0015] (b) The reaction mixture is fed into a residence device in which the reaction continues to occur, thereby obtaining an intermediate reaction mixture;

[0016] (c) The intermediate reaction mixture is fed into a reactor to obtain the crude reaction product and the gas phase, and

[0017] (d) Post-process the crude reaction product to remove phosgene (if present) and solvent.

[0018] Additional gaseous hydrogen chloride is fed into the reactor of stage (c) so that the additional gaseous hydrogen chloride comes into contact with the crude reaction product, and excess phosgene and hydrogen chloride are removed from the crude reaction product in the reactor by distillation.

[0019] Surprisingly, it has been found that stripping with hydrogen chloride can be performed in the reactor simultaneously with the termination of the reaction, without the need for an additional stripping tower as known in the art. In this way, additional process steps can be saved compared to known methods with additional stripping, and thus energy and investment costs can be reduced. For stripping, additional gaseous hydrogen chloride is fed into the reactor of stage (c) so that the additional gaseous hydrogen chloride comes into contact with the crude reaction product.

[0020] Polyisocyanates can be any polyisocyanate that can be produced in the liquid phase. Typically, polyisocyanates are diisocyanates, preferably toluene diisocyanate (TDI) or methylene diphenyl diisocyanate, and specifically methylene diphenyl diisocyanate. Preferred polyisocyanates are polyphenylene-polymethylene polyisocyanates (PMDI). Typically, PMDI is a mixture of polyphenylene-polymethylene polyisocyanates (including the monomeric methylene diphenyl diisocyanate) with different numbers of phenylene groups. In the following description, the abbreviation MDI refers to polyphenylene-polymethylene polyisocyanate, methylene diphenyl diisocyanate, and mixtures thereof.

[0021] To produce polyisocyanates, the corresponding organic amines react with phosgene to form isocyanates and hydrogen chloride. Therefore, for PMDI production, the amine is polyphenylene-polymethylene polyamine, for TDI production, the amine is toluene diamine, and for MDI production, the amine is methylene diphenyl diamine.

[0022] In the first stage (a) of this method, an amine-containing feed stream and a phosgene-containing feed stream are mixed in a mixing device to obtain a reaction mixture. The amine-containing feed stream is typically a mixture of an amine and an inert solvent, and the phosgene-containing feed stream contains phosgene and an inert solvent. The inert solvent used in the amine-containing feed stream and the phosgene-containing feed stream is preferably the same. The inert solvent can be any solvent that is inert to the components used and formed in the reaction. Preferably, the inert solvent is an aromatic or aliphatic hydrocarbon, a chlorinated aromatic hydrocarbon, a ketone, an ester, a nitrile, or a mixture thereof. Chlorinated aromatic hydrocarbons can be, for example, dichlorobenzene, chlorobenzene, trichlorobenzene, or a mixture thereof. Suitable aromatic or aliphatic hydrocarbons are, for example, toluene, xylene, benzene, pentane, hexane, heptane, octane, cyclohexane, or biphenyl. Suitable ketones are, for example, 2-butanone or methyl isobutyl ketone, suitable esters are, for example, diethyl isophthalate, ethyl acetate, or butyl acetate, and suitable nitriles are, for example, acetonitrile. Additionally, the solvent can also be sulfolane. However, specifically, the inert solvent is toluene, chlorobenzene, dichlorobenzene, or a mixture of at least two of them.

[0023] The concentration of the amine in the inert solvent depends on the amine used and the solvent. In the case of methylene diphenyl diamine and / or polyphenylene-polymethylene polyamine or a mixture thereof, the concentration is preferably in the range of 5% to 50% by weight, specifically in the range of 25% to 45% by weight. In the case of toluene diamine, the concentration is preferably in the range of 5% to 50% by weight, and specifically in the range of 15% to 30% by weight. The concentration of phosgene in the inert solvent can be from 1% to 70% by weight, preferably from 10% to 50% by weight. However, the solvent can also be omitted entirely.

[0024] During mixing, the amine reacts to produce carbamoyl chloride and amine hydrochloride, such that the reaction mixture specifically contains carbamoyl chloride, amine hydrochloride, hydrogen chloride, phosgene, and a solvent, wherein an excess of phosgene is used.

[0025] The mixing device used in the first stage (a) is preferably a static stirrer, specifically a mixing nozzle. The pressure upstream of the nozzle is preferably in the range of 3 bar to 70 bar, specifically 15 bar to 45 bar. The pressure difference above the nozzle is at least 0.5 bar. The temperature of the reaction mixture leaving the first stage (a) is preferably in the range of 80°C to 190°C, specifically 90°C to 150°C.

[0026] The first stage (a) is followed by a second stage (b), in which the reaction mixture is fed into a residence device where the reaction continues to obtain an intermediate reaction mixture. The residence device may include one or more residence units, preferably two to seven, and specifically three to five. In the second stage (b), the amine hydrochloride formed in the first stage (a) reacts to produce carbamoyl chloride, and a portion of the carbamoyl chloride further dissociates into isocyanate and hydrogen chloride. Typically, in stage (b), 50 wt% to 99 wt% of the carbamoyl chloride dissociates into isocyanate and hydrogen chloride, preferably 70 wt% to 98 wt% and specifically 80 wt% to 95 wt% of the carbamoyl chloride dissociates into isocyanate and hydrogen chloride.

[0027] The residence device of the second stage (b) preferably operates at a pressure in the range of 2 to 35 bar, specifically in the range of 15 to 35 bar. Typically, in methods for producing MDI, the pressure in the second stage (b) is in the range of 2 to 10 bar, and more preferably, the second stage (b) includes more than one residence device with the pressure decreasing sequentially from the first residence device to the last. In methods for producing TDI, the first residence device is a phase separator or plug flow reactor operating at a pressure in the range of 10 to 20 bar, and the second residence device is a reaction tower operating at a pressure in the range of 3 to 8 bar. Furthermore, the pressure in the second stage (b) is lower than the pressure in the first stage (a). To depressurize the reaction mixture downstream of the mixing device, a regulating valve or any other suitable device for reducing the pressure to the pressure of the residence device can be used. However, if the mixing device is a mixing nozzle, it is preferable to use the pressure drop of the nozzle for pressure reduction.

[0028] The mixing device of the first stage (a) can be a separate device or can be integrated into the residence device of the second stage (b). Specifically, if the mixing device is a mixing nozzle, the mixing nozzle can be immersed in the gas phase of the residence device or preferably in the liquid phase. If the residence device includes more than one residence device, in this case, the mixing nozzle is immersed in the gas phase of the first residence device or preferably in the liquid phase. Immersion in the gas phase or immersion in the liquid phase means that the mixing nozzle can be completely or partially positioned in the respective phase. Alternatively, the reaction mixture leaving the mixing device can also be delivered to the gas phase of the residence device or preferably to the liquid phase using pipes, submerged pipes, or insert pipes.

[0029] The temperature in the second stage (b) is preferably in the range of 80°C to 190°C, specifically in the range of 80°C to 150°C. The residence device in the second stage preferably comprises at least one of a tubular reactor, a stirred vessel, a non-stirred residence device, or a phase separation device for the gas and liquid phases. Heating of each residence device can be achieved by any suitable heating method known to those skilled in the art. For example, each residence device may be provided with a pumping loop, which may in turn have a heat exchanger for setting the reaction temperature. Furthermore, specifically if the residence device is a stirred tank, heating is preferably achieved by a double jacket. In a reaction tower, such as that used in methods for producing TDI, heat is preferably introduced via a bottom evaporator.

[0030] In the case of stirred vessels, non-stirred residence equipment, or possibly phase separation equipment, the liquid phase is preferably depressurized under level control and the gas phase is depressurized under pressure control to a subsequent residence equipment or from the last residence equipment to the reactor of the third stage (c). However, the gas phase, which mainly contains phosgene, hydrogen chloride, and may contain solvent, can also be directly post-processed, for example, fractionated into phosgene, hydrogen chloride, and solvent, or fractionated into mixtures thereof.

[0031] Depending on the desired residence time and facility capacity, the residence devices of the second level (b) can have relatively large size and volume, which may be considered disadvantageous from a cost or safety perspective, such as phosgene retention under high pressure. In this case, the residence devices can be implemented as two or more similar or different residence devices or types, which can be connected in parallel or, if appropriate, in series to affect the residence time spectrum.

[0032] The reactor in the third stage (c) is preferably operated at a pressure in the range of 1.5 bar to 10 bar, specifically 2 bar to 5 bar. Downstream of the residence device in the second stage (b), the intermediate reaction mixture is depressurized to the pressure of the third stage (c) using a regulating valve or any other device suitable for this purpose. Pressure drop from the equipment or connecting pipes used can also be utilized.

[0033] In any case, as described above, the pressure in the next stage is selected such that it is lower than the pressure in the previous stage, that is, the pressure in stage (b) is lower than the pressure in stage (a) and the pressure in stage (c) is lower than the pressure in stage (b).

[0034] In the third stage (c), the carbamoyl chloride formed in the first stage (a) (which is not dissociated in the second stage (b)) dissociates into isocyanate and hydrogen chloride.

[0035] The temperature in the third stage is preferably between 80°C and 190°C. The reactor used in the third stage is specifically a reaction tower, such as those described in WO-A 99 / 54289. The reaction tower preferably has more than 10 theoretical plates. Plate towers are preferred. However, other internal tower fittings can also be used, ensuring the residence time required for the dissociation of carbamoyl chloride and the rapid and efficient removal of hydrogen chloride and phosgene. Suitable trays for plate towers include, for example, valve trays with relatively high liquid weirs, sieve trays, bubble cap trays, dual-flow trays, or distillation trays.

[0036] The temperature at the bottom of the reaction tower is preferably in the range of 120°C to 220°C, and the temperature at the top of the reaction tower is preferably in the range of 50°C to 120°C.

[0037] The reactor in stage (c) can also be used to remove phosgene adducts formed during the reaction from the intermediate reaction mixture. As with residence units, the reactor can be disadvantageously large. In this case, the reactor in stage (c) can also be implemented as two or more similar columns connected in series. The crude reaction product obtained in the reactor (i.e., the output from the bottom if a reaction column is used) is post-treated by conventional methods to remove any remaining phosgene and to separate the solvent in stage (d).

[0038] Phosgene, hydrogen chloride, and possible solvents can be separated from the gas phase in known ways and, if appropriate, recycled, the gas phase exiting the reactor of the third stage (c) and, if appropriate, the residence device of the second stage (b). Specifically, it is preferred to recycle a portion of the hydrogen chloride as additional hydrogen chloride back to the third stage (c).

[0039] Each of the first to third levels (a), (b), and (c) can be operated adiabatically or isothermally.

[0040] The rapid reaction for forming carbamoyl chloride and hydrogen chloride, as well as amine hydrochloride, between amines and phosgene requires high pressure in both the first stage (a) and the second stage (b) to obtain a high phosgene concentration in the liquid phase, and thus to achieve a high phosgene excess to minimize the production of byproducts. Furthermore, good mixing is essential. Therefore, using a mixing nozzle as the mixing device is preferred. The high inlet pressure upstream of the mixing nozzle allows for a high pressure drop above the nozzle, and thus allows for the introduction of high mixing energy.

[0041] To compensate for the temperature drop that may be caused by the endothermic decomposition of carbamoyl chloride, the heatable residence device in the second stage (b) is preferred.

[0042] The reaction in stage (c), namely the dissociation of carbamoyl chloride into isocyanate and hydrogen chloride, is a pressure-dependent equilibrium reaction. Preferably, it is shifted to the desired isocyanate side by a low pressure. Since this reaction does not require any phosgene, a low phosgene concentration in the liquid phase is preferred.

[0043] By means of the present invention, additional gaseous hydrogen chloride is fed into the gas phase of the third-stage (c) reactor. By feeding additional hydrogen chloride into the gas phase of the third-stage (c) reactor, the stripping of phosgene is improved, and thus the concentration of phosgene in the liquid phase can be further reduced. Specifically, all excess phosgene is stripped in the third-stage (c) reactor so that an additional stripping tower for removing phosgene from the crude reaction products can be omitted.

[0044] Specifically, when the chlorine used to synthesize the required intermediate phosgene contains bromine, it has been surprisingly shown that the degrading effect of bromine on the color of the isocyanate can be mitigated by feeding additional gaseous hydrogen chloride into the third stage (c) reactor. This effect specifically occurs when the isocyanate is MDI.

[0045] Additional gaseous hydrogen is fed into the reactor of stage (c) so that it contacts the crude reaction product. Low-boiling-point compounds are stripped from the crude reaction product by this method. Furthermore, phosgene adducts that may form in the reaction and potentially cause undesirable isocyanate coloration are converted into corresponding hydrogen chloride adducts (e.g., carbodiimide hydrochloride) and phosgene, and then the phosgene is stripped from the crude reaction product. If the reactor is a reaction tower, the low-boiling-point compounds stripped by hydrogen chloride enter the gas phase and can be removed at the top of the reaction tower. In this respect, the reaction tower operates not only as a stripping tower but also simultaneously as a distillation tower, where low-boiling-point compounds are distilled off at the top of the tower, and high-boiling-point compounds (specifically, the crude reaction product) remain liquid and can be removed at the bottom of the tower. If the reaction tower also operates as a distillation tower, it is preferable to recycle solvent reflux from the top condenser or external reflux from solvent separation or solvent / phosgene separation to the top of the tower.

[0046] To ensure adequate contact between the additional gaseous hydrogen chloride and the liquid phase in the reaction column, it is preferable to feed the additional gaseous hydrogen chloride below the facility in the reactor. If the reactor includes trays as facilities, such as valve trays, bubble cap trays, or distillation trays with relatively high weirs as described above, it is particularly preferable to feed the additional gaseous hydrogen chloride below the lowest tray.

[0047] Additional hydrogen chloride can be fed into the gas phase below the reactor or directly into the liquid phase. If hydrogen chloride is fed into the liquid phase, it is preferable to feed the additional hydrogen chloride into the liquid phase at the bottom of the reactor. Alternatively, the additional hydrogen chloride can be fed into the bottom loop upstream or downstream of an external heat exchanger.

[0048] Typically, an external heat exchanger in the bottom loop is used to heat and partially evaporate the liquid phase at the bottom to a desired bottom temperature. To do this, a portion of the liquid phase is taken out at the bottom of the reaction column, passed through a heat exchanger where that portion of the liquid phase is heated and evaporated, and then the heated and partially evaporated portion of the liquid phase is recycled back to the bottom of the reaction column, preferably above the bottom liquid level.

[0049] Since hydrogen chloride is also formed during the reaction, only a small amount of additional hydrogen chloride is needed. The amount of additional gaseous hydrogen chloride fed into the reactor of the third stage (c) is preferably in the range of 0.0015 kg / kg polyisocyanate to 0.32 kg / kg polyisocyanate, more preferably in the range of 0.004 kg / kg polyisocyanate to 0.15 kg / kg polyisocyanate, and specifically in the range of 0.0079 kg / kg polyisocyanate to 0.063 kg / kg polyisocyanate.

[0050] To avoid cooling of the components in the reaction tower, it is preferable that the additional gaseous hydrogen chloride fed into the third-stage reactor has a temperature in the range of 20°C to 250°C, more preferably in the range of 50°C to 200°C, and specifically in the range of 100°C to 180°C. If the additional gaseous hydrogen chloride is fed into the bottom loop upstream of the heat exchanger, the temperature of the additional gaseous hydrogen chloride may be lower because the entire feed stream containing the additional hydrogen chloride is heated in the heat exchanger. However, even in this case, it is preferable to feed the hydrogen chloride into the bottom loop at a temperature within the previously defined range. Specifically, it is preferable to feed the additional hydrogen chloride into the third-stage reactor at a temperature corresponding to the bottom temperature in the reaction tower. Example

[0051] In the pilot plant, 50 kg / h of polymethylene polyaniline was diluted in 60 kg / h of chlorobenzene. The mixture, at approximately 130°C, was fed into a mixing nozzle to react with a 192 kg / h feed stream containing 60 wt% phosgene, 38 wt% chlorobenzene, and 2 wt% hydrogen chloride, at a temperature of 5°C. The pressure downstream of the mixing nozzle was maintained at 8 bar. The reaction mixture was fed into a residence unit comprising a series of stirred vessels operating at a decreasing pressure, starting at 4.5 bar and ending at approximately 1.6 bar. The gas phase separated from the vessels was collected and fed, along with the liquid product from the last vessel, into a downstream reaction column. In the reaction column, the final cracking of the remaining carbamoyl chloride was achieved. Simultaneously, low-boiling compounds such as phosgene and hydrogen chloride were removed by distillation. Distillation was performed by partially evaporating the bottom product in an external evaporator and externally refluxing the liquid chlorobenzene solvent to the top of the column. The reaction column operated at a top pressure of 1.4 bar and a bottom temperature of 174°C. A gaseous hydrogen chloride feed stream was introduced into the column between the lowest tray and the surface of the liquid at the bottom. The reaction product containing approximately 63.1 kg / h of MDI was removed from the bottom of the column, and the solvent was removed by multi-step distillation to obtain crude MDI product. The color of the product was measured using a Dr. Lange LICO 690 instrument in a 1:5 dilution with chlorobenzene and described according to the CIELAB system by values ​​L*, a*, and b*. Here, L* describes the perceived transparency of the sample. The effect of the hydrogen chloride feed stream to the bottom of the reaction column on the L* value of the final crude MDI sample is shown in Table 1.

[0052] It can be seen that, specifically, using a small amount of hydrogen chloride, the color number can be improved from approximately 93 to over 96. When the hydrogen chloride feed exceeds 4 kg / h, the color number begins to decrease again, and above 8 kg / h, it again achieves the same value as without hydrogen chloride stripping. This shows that the optimal range for hydrogen chloride feed is between 0.5 kg / h and 4 kg / h, corresponding to specific values ​​of 0.0079 kg hydrogen chloride / kg MDI to 0.063 kg hydrogen chloride / kg MDI.

[0053] Table 1: L* values ​​depending on the amount of hydrogen chloride fed into the reaction tower

[0054] Example HCl volume [kg / h] L* color code 1 0 92.7 2 0.5 97 3 1 96.5 4 1.5 96.4 5 2 97.2 6 2.5 97.6 7 3 96 8 4 96.2 9 6.5 94.7 10 9.5 91.5 11 13 92.3

Claims

1. A method for producing polyisocyanates by reacting an organic amine with phosgene in an inert solvent, the method comprising: (a) A feed stream containing amine and a feed stream containing phosgene are mixed in a mixing device to obtain a reaction mixture; (b) The reaction mixture is fed into a residence device, where the reaction continues to occur, thereby obtaining an intermediate reaction mixture; (c) The intermediate reaction mixture is fed into a reactor to obtain crude reaction products and a gas phase, and (d) Post-treatment of the crude reaction product to remove phosgene, and if still present, solvent. Additional gaseous hydrogen chloride is fed into the reactor of stage (c) so that the additional gaseous hydrogen chloride comes into contact with the crude reaction product, and excess phosgene and hydrogen chloride are removed from the crude reaction product in the reactor by distillation.

2. The method according to claim 1, wherein the polyisocyanate is methylene diphenyl diisocyanate and / or polyphenylene-polymethylene polyisocyanate.

3. The method according to claim 1 or 2, wherein the mixing device is a mixing nozzle.

4. The method according to any one of claims 1 to 3, wherein the additional gaseous hydrogen chloride is fed below the facility in the reactor.

5. The method according to any one of claims 1 to 4, wherein the additional gaseous hydrogen chloride is fed into the bottom loop upstream or downstream of the external heat exchanger.

6. The method according to any one of claims 1 to 5, wherein the amount of additional gaseous hydrogen chloride is in the range of 0.0079 kg / kg polyisocyanate to 0.063 kg / kg polyisocyanate.

7. The method according to any one of claims 1 to 6, wherein the pressure in stage (b) is lower than the pressure in stage (a) and the pressure in stage (c) is lower than the pressure in stage (b).

8. The method according to any one of claims 1 to 7, wherein the residence device comprises at least one of a tubular reactor, a stirred vessel, a non-stirred residence device, or a phase separation device for gas and liquid phases.

9. The method according to any one of claims 1 to 8, wherein the pressure upstream of the mixing device is in the range of 3 bar to 70 bar.

10. The method according to any one of claims 1 to 9, wherein the temperature in each stage (b) to (c) is in the range of 80°C to 190°C.

11. The method according to any one of claims 1 to 10, wherein in stage (b), 50 wt% to 99 wt% of carbamoyl chloride dissociates into isocyanate and hydrogen chloride.

12. The method according to any one of claims 1 to 11, wherein the inert solvent is an aromatic or aliphatic hydrocarbon, a chlorinated aromatic hydrocarbon, a ketone, an ester, a nitrile, or a mixture thereof.

13. The method according to any one of claims 1 to 12, wherein the gaseous hydrogen chloride has a temperature in the range of 20°C to 250°C.