METHOD FOR THE PRODUCTION OF 1,5-PENTANEDIISOCYANATE IN THE GAS PHASE

Description

[0001] The invention relates to a process for the preparation of 1,5-pentane diisocyanate (PDI) by reacting 1,5-pentanediamine (PDA) with phosgene in the gas phase.

[0002] Isocyanates are produced in large quantities and are mainly used as starting materials for the production of polyurethanes. Since the usual monomeric diisocyanates have a relatively low molecular weight and generally a correspondingly high vapor pressure, polyisocyanates made from them are used, particularly in paint applications, for reasons of occupational hygiene. These include, for example, uretdiones, isocyanurates, iminooxadiazinediones, biurets, urethanes, allophanates or ureas, which are produced from the monomeric diisocyanates by di- and trimerization, usually in the presence of catalysts. However, this places particularly high demands on the purity of the monomers, since the secondary components usually contained in them can sometimes significantly reduce the activity of the catalysts. Higher catalyst concentrations or longer reaction times must then be used, which can affect the quality of the resulting polyisocyanates, for example.significantly worsened in terms of color and storage stability.

[0003] It is therefore desirable that as few secondary components as possible are formed during the production of the monomeric diisocyanates in order to limit the effort required to subsequently remove or minimize them, e.g. by fractional distillation.

[0004] In the case of 1,5-pentane diisocyanate, the chlorine-containing secondary components 5-chloropentyl isocyanate (CPI), N-carbamoylpiperidine ("C6-Im"), and the two isomeric N-carbamoyltetrahydropyridines ("C6-Az") are formed. While the formation of CPI reduces the yield and interferes with further processing due to its monofunctionality as a chain terminator, the components C6-Im and C6-Az, which contribute to the so-called HC value (hydrolyzable chlorine), particularly impair catalysis during the further processing of PDI into polyisocyanates. Therefore, the HC value of the monomers used to produce the polyisocyanates should always be < 100, preferably < 50 ppm.

[0005] In PDI used to produce polyisocyanates, the CPI concentration should be < 0.3%, and the sum of the C6-Im and C6-Az concentrations should not exceed 400 ppm, preferably 200 ppm. Since the separation of C6-Im and C6-Az from PDI, e.g., by distillation, is very difficult and laborious, their concentration in the raw materials should not be significantly higher.

[0006] The preparation of 1,5-pentane diisocyanate (PDI) from 1,5-pentanediamine (PDA) is known per se and can be carried out without phosgene (T. Lesiak, K. Seyda, Journal für Praktische Chemie (Leipzig), 1979, 321 (1), 161 - 163) or by reaction with phosgene (e.g. W. Siefken, Justus Liebigs Ann. Chem. 562, 1949, p. 25 ff., (p. 122) or DE 2 625 075 A1 ).

[0007] In the phosgene-free preparation cited above, PDA is first reacted with formic acid to form formamide and then oxidized with halogens in the presence of tertiary amines to form PDI. A disadvantage of this process is that it is a complex two-stage process, resulting in the formation of considerable byproducts. The resulting yield losses and the required high purification effort reduce the economic viability of this process. DE 2 625 075 A1 claims a process for the preparation of carabamic acid chlorides and isocyanates, characterized in that salts of primary amines in solid form are reacted with phosgene in the presence of a liquid at elevated temperature in a rotary kiln, a paddle dryer, or in a fluidized-bed reactor.The disadvantage of this process is that it is also a multi-stage process, in which, in the first stage, an amine salt is initially prepared in a solvent, which then has to be removed before reaction with phosgene, e.g., by filtration or centrifugation followed by drying. This is time-consuming and costly and reduces the economic viability of this process.

[0008] DE 1 900 514 A1 describes the two-step production of PDI from caprolactam by conversion to hydroxamic acids and their subsequent phosgenation. The yield reported for the conversion of caprolactam to PDI is only approximately 32%.

[0009] WO 2008 / 015134 A1 claims a process for producing PDI in which bio-based lysine is converted into PDA, which is subsequently converted into PDI. The conversion of PDA to PDI can be carried out phosgene-free or in the presence of phosgene, with the latter variant being possible in the liquid phase or the gas phase. Any interfering impurities present in PDI and measures to avoid or minimize them are not mentioned.

[0010] EP 2 684 867 A1 claims 1,5-pentane diisocyanate (PDI) with a content of 5-400 ppm of the compounds (1) and (2) by cold-hot phosgenation of bio-based 1,5-pentanediamine (PDA) or its salt, a process for its preparation and polyisocyanates prepared therewith.

[0011] It describes the phosgenation of 1,5-pentanediamine salts, such as hydrochlorides, in inert solvents such as o-dichlorobenzene. The crude PDI thus obtained is conditioned to 180-245°C in the presence of an inert gas, such as nitrogen, and optionally a phosphorus-containing compound, such as tris(tridecyl)phosphite, prior to distillation to reduce the content of compounds (1) and (2). Nothing is reported about the presence of the minor component CPI or its removal. This process also involves several steps and requires long reaction times, which has a negative impact on its economic viability.

[0012] WO 2009 / 027232 A1 generally describes a process for producing isocyanates by reacting the corresponding amines with phosgene in the presence of at least one inert medium in the gas phase, wherein an inert medium is metered between the fluid streams of amine and phosgene. This is intended to suppress solid deposition at the junction of the amine and phosgene streams. The process is specifically described using 1,6-hexamethylene diisocyanate as an example. WO 2009 / 027232 A1 provides no indication that the process therein is still disadvantageous due to the resulting proportions of C6-Im and C6-Az, and certainly no indication of how the described process could be carried out to avoid high proportions of these compounds.

[0013] There is therefore still a great need for a simple and cost-effective process for the production of PDI with sufficiently low contents of CPI, C6-Im and the two isomeric C6-Az, which avoids the disadvantages of the prior art processes.

[0014] Surprisingly, it has now been found that PDI raw materials with very low contents of CPI, C6-Im and the two isomeric C6-Az can be obtained by reacting PDA with phosgene in the gas phase above its boiling temperature under special conditions as described below.

[0015] The present invention relates to a process for the preparation of 1,5-pentane diisocyanate (PDI) by reacting 1,5-pentanediamine (PDA) with phosgene in the gas phase, characterized in that a) the gas temperatures of both phosgene and 1,5-pentanediamine (PDA) are in the range of 230-320°C before entering the reactor and b) the two reactant streams and an inert gas stream are fed to the reactor via an annular gap nozzle, whereby the inert gas stream is fed via the annular gap and thus between the two reactant streams, c) the reactant streams and the inert gas stream mix after entering the reactor and d) the amine and the phosgene then react.

[0016] The phosgenation of amines in the gas phase is known per se and can be carried out, for example, as described in EP 0 289 840 B1, EP 1 319 655 A2, EP 1 555 258 A1, EP 1 275 639 A1, EP 1 275 640 A1, EP 1 449 826 A1, EP 1 754 698 B1, DE 10 359 627 A1 or in the German patent application DE 10 2005 042392 A1.

[0017] Technical-grade PDA with a purity of > 99% and a water content of < 500 ppm is used. It can be obtained using known processes from both petrochemical-based and bio-based production, e.g., by decarboxylation of lysine. PDA from bio-based production is preferred.

[0018] Before carrying out the process according to the invention, the PDA is evaporated, heated to 230°C to 320°C, preferably 270°C to 310°C, and fed to the reactor, preferably a tubular reactor. An inert gas such as N2, He, Ar, or vapors of an inert solvent, e.g., aromatic hydrocarbons with or without halogen substitution, can be admixed with the PDA.

[0019] The phosgene used in the phosgenation is also heated to 230°C to 320°C, preferably 270°C to 310°C, before being fed into the reactor.

[0020] The two reactant streams and the inert gas stream are fed to the reactor via an annular gap nozzle, as described, for example, in EP 1 555 258 A1. This patent application claims a process for producing di- and triisocyanates in the gas phase in a tubular reactor having a double-walled guide tube arranged centrally in the direction of its rotational axis, wherein a concentric annular gap is formed between the inner and outer walls of this double-walled guide tube, the vaporous di- and / or triamines and phosgene are heated separately to temperatures of 200-600°C, and the amine stream is fed to the tubular reactor via the concentric annular gap, while phosgene is fed to the tubular reactor over the remaining cross-sectional area of ​​the tubular reactor.The production of 1,5-pentane diisocyanate (PDI) is not described, nor are there any indications of the formation of chlorine-containing by-products or measures to avoid or minimize them.

[0021] When carrying out the process according to the invention, a tubular reactor is used which also has a double-walled guide tube arranged centrally in the direction of its rotational axis, with a concentric annular gap formed between the inner and outer walls. In contrast to the procedure described in EP 1 555 258 A1, the preheated PDA stream, optionally diluted by an inert medium, is fed to the tubular reactor through the inner jacketed tube at an average velocity of 20-150 m / s, preferably 20-100 m / s, while the preheated phosgene is fed to the reactor over the remaining cross-sectional area between the outer jacketed tube and the inner wall of the tubular reactor at an average flow velocity of at least 1 m / s, preferably 5-15 m / s.In addition, the two reactant streams are separated upon entry into the reactor by a cylindrical jacket-shaped inert gas stream which, after preheating to 230°C to 320°C, preferably 270°C to 310°C, is fed to the tubular reactor via the concentric annular gap of the double-jacketed tube at an average velocity of 20-150 m / s, preferably 20-100 m / s.

[0022] The annular gap nozzle to be used according to the invention is also referred to below as a separating gap nozzle or nitrogen separating gap nozzle (using nitrogen as inert gas) because the inert gas stream separates the two reactant streams.

[0023] The inert gas stream can consist, for example, of nitrogen, noble gases such as helium or argon, or vapors of inert solvents. Nitrogen is preferred. Suitable solvents include aromatic hydrocarbons with or without halogen substitution, such as chlorobenzene, o-dichlorobenzene, toluene, chlorotoluene, xylene, chloronaphthalene, or decahydrodronaphthalene.

[0024] The flow rates of gaseous PDA and phosgene are selected such that the molar phosgene excess based on an amino group is 30 to 300%, preferably 60 to 200%.

[0025] The process according to the invention preferably uses tubular reactors without internals and without moving parts inside the reactor. The tubular reactors are generally made of steel, glass, alloyed, or enameled steel and are dimensioned to enable complete reaction of the PDA with the phosgene under the process conditions. The gas streams are introduced into the tubular reactor via a separating gap nozzle at one end of the tubular reactor, as described above. The mixing zone is preferably maintained at a temperature within the range of 230°C to 320°C, preferably 270°C to 310°C, although this temperature can optionally be maintained by heating the tubular reactor.

[0026] When carrying out the process according to the invention, the pressure in the feed lines to the reaction chamber is generally 200-3,000 mbar abs., preferably 800-1,500 mbar abs., and at the outlet from the reaction chamber, 150-2,000 mbar abs., preferably 750-1,440 mbar abs., whereby a flow velocity within the reaction chamber of 3 to 120 m / s, preferably 5 to 75 m / s, is maintained by maintaining a suitable differential pressure. Under these conditions, turbulent flow conditions generally prevail within the reaction chamber.

[0027] The residence time of the reaction mixture in the reactor is 0.1 to 1 s, preferably 0.2 to 0.5 s. The residence time is calculated from the temporal throughput of the reactant streams, the reactor dimensioning and the reaction parameters pressure and temperature.

[0028] After the phosgenation reaction in the reaction chamber has taken place, the gaseous mixture continuously leaving the reaction chamber is freed of the PDI formed. This can be done, for example, with the aid of an inert solvent whose temperature is selected such that it is, on the one hand, above the decomposition temperature of the carbamic acid chloride corresponding to the PDI and, on the other hand, below the condensation temperature of the PDI and preferably also of any solvent used in vapor form as a diluent. This allows PDI and the auxiliary solvent to condense or dissolve in the solvent, while excess phosgene, hydrogen chloride, and any inert gas used as a diluent pass through the condensation stage or the solvent in gaseous form.Solvents of the type exemplified above, especially technical mono- (MCB) and dichlorobenzene (ODB), maintained at a temperature of 60 to 200°C, preferably 90 to 170°C, are particularly suitable for the selective recovery of PDI from the gaseous mixture leaving the reaction chamber. MCB is preferred. Possible methods for the selective condensation of the isocyanate formed from the gas mixture leaving the reactor using such solvents include, for example, passing the gas mixture through the solvent mentioned or injecting the solvent (solvent mist) into the gas stream (quench).

[0029] The gas mixture passing through the condensation stage to obtain PDI is then freed of excess phosgene in a conventional manner. This can be achieved using a cold trap, absorption in an inert solvent maintained at a temperature of -10°C to 8°C (e.g., chlorobenzene, MCB, or dichlorobenzene, ODB), or by adsorption and hydrolysis on activated carbon. The hydrogen chloride gas passing through the phosgene recovery stage can be recycled in a conventional manner to recover the chlorine required for phosgene synthesis.

[0030] The purification of PDI is preferably carried out by distillation of the crude PDI solution in the solvent used for isocyanate condensation.

[0031] The advantages of the method according to the invention are: a) Low by-product formation and thus low levels of chlorine-containing by-products even in the raw materials. Excluding the solvent, the CPI concentrations are < 0.5 wt.%, preferably < 0.3 wt.%, and the sum of C6-Im and C6-Az is < 400 ppm, preferably < 350 ppm. This minimizes the effort required for subsequent distillation. b) Avoidance of solid deposits on the reactor wall and in the quench.

[0032] The present invention relates to a process for the preparation of 1,5-pentane diisocyanate (PDI) by reacting 1,5-pentanediamine (PDA) with phosgene in the gas phase, characterized in that a) the gas temperatures of both phosgene and 1,5-pentanediamine (PDA) are in the range of 230-320°C before entering the reactor and b) the two reactant streams and an inert gas stream are fed to the reactor via an annular gap nozzle, whereby the inert gas stream is fed via the annular gap and thus between the two reactant streams, c) the reactant streams and the inert gas stream mix after entering the reactor and d) the amine and the phosgene then react.

[0033] In a second embodiment of the method, the temperature of the PDA in a) is in the range of 270°C to 310°C

[0034] In a third embodiment, the process according to embodiment 1 or 2 is conducted such that the temperature of the phosgene in a) is in the range from 270°C to 310°C.

[0035] In a third embodiment, the process according to any one of embodiments 1 to 3 is conducted such that the flow rates of gaseous PDA and phosgene are selected such that the molar phosgene excess based on an amino group is 30 to 300%.

[0036] In a fourth embodiment, the process according to any one of embodiments 1 to 3 is conducted such that the flow rates of gaseous PDA and phosgene are selected such that the molar phosgene excess based on an amino group is 60 to 200%.

[0037] In a fifth embodiment, the process according to any one of embodiments 1 to 4 is conducted such that the pressure in the feed lines to the reaction chamber is 200-3,000 mbar abs. and the pressure at the outlet from the reaction chamber is 150-2,000 mbar abs.

[0038] In a sixth embodiment, the process according to any one of embodiments 1 to 4 is conducted such that the pressure in the feed lines to the reaction chamber is 800 to 1,500 mbar abs. and the pressure at the outlet from the reaction chamber is 750 to 1,440 mbar abs.

[0039] In a seventh embodiment, the process according to any one of embodiments 1 to 6 is conducted such that the residence time of the reaction mixture in the reactor is 0.1 to 1 s.

[0040] In an eighth embodiment, the process according to any one of embodiments 1 to 6 is conducted such that the residence time of the reaction mixture in the reactor is 0.2 to 0.5 s. Examples: GC method of PDI analysis:

[0041]

[0042] A 2-liter four-necked flask equipped with a stirrer, thermometer, reflux condenser, dropping funnel, and gas inlet tube was charged with 463 g of MCB, and 437 g of phosgene was condensed into it at -5°C. A solution of 75 g of PDA in 416 g of MCB was added dropwise over a period of 30 minutes with stirring and cooling, while maintaining a temperature between 0 and 8°C. After the addition was complete, the cooling was removed, and the reaction mixture was gradually heated to reflux over a period of 2 hours while further phosgene was introduced, with more significant gas evolution occurring in the temperature range of 40–80°C. Phosgenation was then continued under reflux for a further 12 hours. The reaction mixture was freed of phosgene by purging with nitrogen, and the filter residue was washed several times with MCB. The filter residue was dried and weighed while the combined filtrates were largely freed from the solvent by vacuum distillation using a Rotavapor.The following were obtained: Solid: 17.3 g; crude solution: 97.7 g containing 9% MCB. Yield: 56.7% of the theoretical PDI.

[0043] GC analysis (MCB excluded, area percentage (fl%)): CPI 1,432 C6-Az 0,409 C6-Im 0,000 PDI 98,159 Comparison example 2: Liquid-phase phosgenation of PDA in ODB (not according to the invention)

[0044] Analogously to Comparative Example 1, 75 g of PDA were reacted with ODB, keeping the solvent quantity, reaction time, and temperature the same. The following were obtained: Solid: 14.8 g; Crude solution: 85.5 g containing 17% ODB; Yield: 44.0% of the theoretical PDI.

[0045] GC analysis (MCB excluded, area%): CPI 4,047 C6-Az 0,000 C6-Im 0,508 PDI 95,445 Comparison example 3: Gas-phase phosgenation of PDA at 340°C with coaxial nozzle (simple smooth jet nozzle) - (not according to the invention)

[0046] In a gas-phase phosgenation plant with an amine evaporation stage, a tubular reactor (length: 1770 mm, internal diameter: 37 mm) with a coaxial nozzle (internal diameter: 6.5 mm) arranged on the reactor axis, and a downstream isocyanate condensation stage, 6.88 kg / h of PDA were continuously evaporated at a pressure of 1300 mbar abs., measured at the end of the isocyanate condensation stage, while introducing a nitrogen stream of 0.138 kg / h, superheated to 340°C, and fed to the reactor via the coaxial nozzle. At the same time, 36.6 kg / h of phosgene were heated to 340°C and continuously fed to the reactor via the annular space left by the nozzle, where the two reactant streams were mixed and reacted. The velocity of the gas flow in the reactor is approximately 6.8 m / s and the velocity ratio of amine / nitrogen to phosgene flow is 5.9.After an average residence time in the reactor of 0.26 s, the gas stream containing the reaction product PDI was cooled and condensed by injection cooling with monochlorobenzene, with the temperature of the liquid phase in the quench being approximately 90°C. After just 4 hours, the plant had to be shut down due to pressure increases caused by fouling at the nozzle and in the reactor.

[0047] The GC analysis of the crude solution obtained showed the following composition (MCB excluded, area%): CPI 0,499 C6-Az 0,195 C6-Im 3,893 PDI 95,414 Comparison example 4: Gas-phase phosgenation of PDA at 310°C with coaxial nozzle (not according to the invention)

[0048] Phosgenation was carried out as described in Example 3, whereby both the nitrogen-diluted gaseous PDA and the phosgene were heated to 310°C before entering the reactor. The gas flow velocity in the reactor was approximately 6.5 m / s, the velocity ratio of amine / nitrogen to phosgene flow was 6.0, and the average residence time in the reactor was 0.27 s. In this case, the plant had to be shut down after 7 h due to a pressure increase caused by fouling at the nozzle and in the reactor.

[0049] The GC analysis of the crude solution obtained showed the following composition (MCB excluded, area%): CPI 0,823 C6-Az 0,098 C6-Im 1,060 PDI 98,019 Comparison example 5: Gas-phase phosgenation of PDA at 340°C with nitrogen separation nozzle (not according to the invention)

[0050] In a gas-phase phosgenation plant with an amine evaporation stage, a tubular reactor (length: 1770 mm, inner diameter: 37 mm) with a separating gap nozzle arranged on the reactor axis (inner diameter: 6.5 mm, separating gap: inner diameter: 6.5 mm, outer diameter: 8.5 mm), and a downstream isocyanate condensation stage, 8.46 kg / h of PDA were continuously evaporated at a pressure of 1300 mbar abs., measured at the end of the isocyanate condensation stage, superheated to 340°C, and fed to the reactor via the inner central nozzle. At the same time, 1.48 kg of nitrogen and 45 kg of phosgene were heated to 310°C and continuously fed to the reactor via the separating gap (nitrogen) or the annular space left open by the nozzle (phosgene), where the two reactant streams were mixed and reacted. The velocity of the gas flow in the reactor was approximately 8.9 m / s and the velocity ratio of amine / nitrogen to phosgene flow was 5.59.After an average residence time in the reactor of 0.20 s, the gas stream containing the reaction product PDI was cooled and condensed by injection cooling with monochlorobenzene, with the temperature of the liquid phase in the quench being approximately 90°C. The plant operated smoothly for a period of 60 h. Thereafter, the pressure gradually increased, so that the plant had to be shut down after 66 h due to fouling in the reactor.

[0051] The GC analysis of the crude solution obtained showed the following composition (MCB excluded, area%): CPI 0,423 C6-Az 0,098 C6-Im 0,360 PDI 99,119 Example 1: Gas-phase phosgenation of PDA at 310°C with nitrogen separation nozzle (according to the invention)

[0052] Phosgenation was carried out as described in Comparative Example 5, with PDA, nitrogen, and phosgene heated to 310°C before entering the reactor. The gas flow velocity in the reactor was approximately 8.5 m / s, the velocity ratio of amine / nitrogen to phosgene flow was 6.0, and the average residence time in the reactor was 0.21 s. The plant operated smoothly for a period of 100 hours. After shutting down and reopening the plant, the nozzle and reactor showed no contamination.

[0053] The GC analysis of the crude solution obtained showed the following composition (MCB excluded, area%): CPI 0,286 C6-Az 0,032 C6-Im 0,004 PDI 99,678

[0054] In the liquid phase phosgenation of PDA (base phosgenation) in MCB and ODB (comparative examples 1 and 2), very poor yields are obtained and very high amounts of CPI are formed.

[0055] During gas-phase phosgenation with a coaxial nozzle (simple smooth jet nozzle), prohibitively high levels of C6-Im are produced at 340°C (Comparative Example 3). Even at 310°C (Comparative Example 4), the C6-Im levels are still relatively high. Furthermore, only very short plant operating times are achieved in Comparative Examples 3 and 4 due to fouling of the nozzle and reactor.

[0056] Gas-phase phosgenation with the nitrogen-separating nozzle still produces high C6-Im contents at 340°C (Comparative Example 5). Although the runtime is improved, it is still unsatisfactory.

[0057] Example 1 shows that by combining the use of a nitrogen separation nozzle and reactant temperatures of 310°C PDA, nitrogen and phosgene before entering the reactor, the proportions of chlorine-containing by-products can be significantly reduced and fouling no longer occurs even after a longer plant operation.

Claims

1. Process for preparing pentane 1,5-diisocyanate (PDI) by reacting pentane-1,5-diamine (PDA) with phosgene in the gas phase, characterized in that a) the gas temperatures both of phosgene and of pentane-1,5-diamine (PDA) prior to entry into the reactor are in the range of 230-320°C and b) the two reactant streams, and also an inert gas stream, are supplied to the reactor by means of an annular gap nozzle, the inert gas stream being supplied through the annular gap and hence between the two reactant streams, c) the reactant streams and the inert gas stream mix after entry into the reactor and d) then the amine and the phosgene react.

2. Process for preparing pentane 1,5-diisocyanate (PDI) according to Claim 1, wherein technical grade PDA having a purity of > 99% and a water content of < 500 ppm is used.

3. Process for preparing pentane 1,5-diisocyanate (PDI) according to Claim 2, wherein the PDA originates from biobased production.

4. Process for preparing pentane 1,5-diisocyanate (PDI) according to any of Claims 1 to 3, wherein the temperature of the PDA in a) is in the range from 270°C to 310°C.

5. Process for preparing pentane 1,5-diisocyanate (PDI) according to any of Claims 1 to 4, wherein the temperature of the phosgene in a) is in the range from 270°C to 310°C.

6. Process for preparing pentane 1,5-diisocyanate (PDI) according to any of Claims 1 to 5, wherein the flow rates of gaseous PDA and phosgene are chosen such that the molar phosgene excess based on one amino group is 30% to 300%.

7. Process for preparing pentane 1,5-diisocyanate (PDI) according to any of Claims 1 to 6, wherein the annular gap nozzle is formed by a tubular reactor having a twin-wall guide tube arranged centrally in the direction of its axis of rotation, wherein a concentric annular gap is formed between the inner and outer walls.

8. Process for preparing pentane 1,5-diisocyanate (PDI) according to any of Claims 1 to 7, wherein the inert gas stream in c) consists of nitrogen, a noble gas, vapours of an inert solvent or mixtures of these inert gases.

9. Process for preparing pentane 1,5-diisocyanate (PDI) according to any of Claims 1 to 8, wherein the pressure in the inlets to the reaction space is 200-3000 mbar abs.

10. Process for preparing pentane 1,5-diisocyanate (PDI) according to any of Claims 1 to 9, wherein the pressure at the outlet from the reaction space is 150-2000 mbar abs.

11. Process for preparing pentane 1,5-diisocyanate (PDI) according to any of Claims 1 to 10, wherein the dwell time of the reaction mixture in the reactor is 0.1 to 1 s.