Method for producing aliphatic diisocyanates in gas phase

Abstract

The invention relates to a method for producing aliphatic diisocyanates by reacting the corresponding aliphatic diamines with phosgene in the gas phase.

Description

[0001] The present invention relates to a process for the preparation of aliphatic diisocyanates by reacting the respective aliphatic diamine with phosgene in the gas phase.

[0002] Isocyanates are produced in large quantities and are mainly used as starting materials for the preparation of polyurethanes. Since the commonly used monomeric diisocyanates usually have a high vapor pressure, polyisocyanates prepared therefrom are used, inter alia in the field of paints, for reasons of labor hygiene. These polyisocyanates are, for example, uretdiones, isocyanurates, iminooxadiazinediones, biurets, carbamates, allophanates or ureas, which are prepared from monomeric diisocyanates, usually in the presence of a catalyst. However, the purity of the monomers is extremely high, since the secondary components usually contained therein sometimes severely reduce the activity of the catalyst. It is therefore necessary to use higher catalyst concentrations or longer reaction times, which significantly reduce the quality of the resulting polyisocyanates, for example in terms of color and storage stability.

[0003] It has therefore already been desirable in the preparation of monomeric diisocyanates to form as few secondary components as possible, in particular chlorine-containing secondary components, in order to subsequently limit the effort required to remove or minimize these secondary components, for example by fractional distillation.

[0004] Examples of such secondary components, which cause an increase in the effort for the purification of monomeric diisocyanates, are in the case of aliphatic diisocyanates the respective chloroalkyl isocyanates, in which one NCO group is replaced by a CI atom. For example, in the case of pentane-1,5-diisocyanate (hereinafter simply 1,5-PDI or PDI) this is, for example, 5-chloropentyl isocyanate (hereinafter simply CPI), and in the case of hexane-1,6-diisocyanate (hereinafter simply 1,6-HDI or HDI) it is 6-chlorohexyl isocyanate (hereinafter simply CHI). On the one hand, the formation of these chloroalkyl isocyanates reduces the yield of diisocyanate in the phosgenation process; on the other hand, due to their monofunctionality, they act as chain terminators in subsequent oligomerization or polymerization reactions. A high content of so-called hydrolyzable chlorine (HC value) also interferes with the further processing of monomeric aliphatic diisocyanates to polyisocyanates, since, as described above, it leads to deactivation of the commonly used catalysts used in further processing, so that monomeric aliphatic diisocyanates with an HC value < 100 ppm, preferably < 50 ppm, are usually used.

[0005] It is difficult to separate chloroalkyl isocyanates (for example CPI or CHI) from the respective diisocyanate by distillation, since their vapor pressure is usually similar to that of the respective diisocyanate. It is therefore desirable to achieve as low a content of chloroalkyl isocyanates in the crude product as possible, in order to keep the subsequent purification effort and thus the thermal stress on the product low.

[0006] The preparation of aliphatic diisocyanates from the corresponding amines is known per se and can be carried out without phosgene (T. Lesiak, K. Seyda, Journal fur Praktische Chemie (Leipzig), 1979, 321 (1), 161-163) or by reaction with phosgene (for example W. Siefken, Justus Liebigs Ann. Chem. 562, 1949, page 25 ff, (page 122) or DE 2 625 075 A1).

[0007] In the phosgene-free preparation described above, the amine is first reacted with formic acid to form a formamide, which is then oxidized with halogen in the presence of a tertiary amine to form the isocyanate. The disadvantage of this method is that it is a complex two-step process, in which a large amount of by-products is produced. The resulting yield losses and the high purification costs required reduce the economic viability of the method.

[0008] DE 2 625 075 A1 claims a process for the preparation of carbamoyl chlorides and isocyanates, characterized in that a primary amine salt in solid form is reacted with phosgene in the presence of a liquid at elevated temperature in a rotary oven, paddle dryer or fluidized bed reactor. The disadvantage of this method is that it is also a multi-step process, in which the amine salt is first prepared in a solvent in the first step, which solvent must then be removed again, for example by filtration or centrifugation and subsequent drying, before the reaction with phosgene. This is both time-consuming and expensive, reducing the economic viability of the method.

[0009] EP 0 100 047 A1 describes that the formation of CHI is a problem in the phosgenation of HDA and proposes a solution to the problem, namely the preparation of hexamethylene diisocyanate by thermal cleavage of the corresponding hexamethylene dialkylcarbamates instead of by phosgenation of HDA. Although no CHI is formed due to the non-use of chlorine gas in this method, this method requires additional steps for the synthesis of these dialkylcarbamates. This method is susceptible to disturbances and ultimately unsatisfactory both in terms of the expenditure of the method and in terms of the yield, so that it cannot replace the phosgenation of HDA on an industrial scale.

[0010] WO 2008 / 015134 A1 claims a process for the preparation of PDI, in which biobased lysine is converted into PDA, which is subsequently converted into PDI. The conversion of PDA into PDI can be carried out without phosgene or in the presence of phosgene, wherein the latter variant can be carried out in the liquid phase or in the gas phase. No mention is made of interfering impurities that can be present in PDI and measures to avoid or minimize them.

[0011] WO 2016 / 042125 A1 describes a method for preparing 1,5-PDI in the gas phase, wherein the reactants are at a gas temperature of 230-320°C before entering the reactor, and two reactant streams are supplied to the reactor through an annular gap nozzle. An inert gas stream is introduced into the annular gap of the nozzle and thus between the two reactant streams. Using this method, a crude product with a considerably low CPI content, i.e., <0.5% by weight, preferably <0.3% by weight, can be obtained, where the calculation does not include the solvent. For example, the lowest content value disclosed in GC analysis is 0.286% by area.

[0012] EP 3 533 785 A1 describes a method for preparing 1,5-PDI, where the primary objective is not to suppress the formation of the chlorinated minor component during the reaction, but rather to remove it later through a series of purification steps (particularly including a heat treatment step). The HC value of the crude product after degassing and solvent removal is disclosed to be typically between 2000 ppm and 20000 ppm. However, the presence and removal of the minor component CPI are not mentioned. Nevertheless, given the high HC value in the crude product, it can be inferred that a significant amount of CPI must also be generated. Furthermore, this method requires additional steps (at least in the form of heat treatment), which is detrimental to its economic feasibility.

[0013] WO 2010 / 115908 A1 describes a method for preparing isocyanates in the gas phase, wherein the reactant gas is immediately cooled in a quenching process after the reaction by the addition of a liquid quenching medium to reduce or avoid the formation of byproducts. The quenching liquid contains a portion of the product stream and has been desorbed, optionally containing solid particles, to avoid deposits in pipes, particularly atomizer nozzles. The composition of the quenching liquid is disclosed as any composition between 0% and 100% isocyanate and solvent. However, no crude product concentration or flow rate that can be used to derive this concentration is mentioned. The presence of chloroalkyl isocyanates is also not disclosed, and therefore the effect of different compositions and flow rates of the quenching liquid on the formation of minor components is not explained.

[0014] WO 2018 / 224530 A1 describes a method for preparing isocyanates, wherein a mixture of reaction products is cooled by contact with a quenching liquid, wherein the quenching liquid comprises an organic solvent comprising up to 50% by weight of its total mass and the balance being composed of the isocyanate to be prepared, to a total weight of 100% by weight. The crude product stream thus cooled is separated into a liquid phase and a gas phase, and the liquid stream is introduced into a collection tank together with another liquid solvent stream containing more than 50% by weight of organic solvent. The solvent content in the crude isocyanate used as the quenching liquid is controlled via this other solvent stream. The solvent flow rate is set such that the desired solvent concentration in the crude product returned as the quenching liquid to the quenching zone is set to a maximum of 50%. No necessary correlation is established between the formation of chloroalkyl isocyanates or byproducts and the concentration of the crude isocyanate. It has only been observed that eliminating the upper quenching stage, which mainly contains solvent, and introducing another solvent stream into the collection tank downstream of the quenching zone does not negatively impact product quality. Specific embodiments are not disclosed.

[0015] WO 2018 / 224529 A1 describes a method for preparing isocyanates, wherein the reaction product mixture is also cooled in a quenching zone after passing through the reaction zone. The quenching liquid used contains up to 25% by weight of an organic solvent. For example, a method for preparing toluene diisocyanate (TDI) is disclosed, wherein the TDI content in the crude isocyanate is 98% by weight at the bottom of the reactor where it is initially produced. A comparative example describes a TDI concentration of 35% by weight at the same location, in which case the yield of this method is 97.6%, slightly lower than the yield of the method of the present invention. Since this document mainly concerns aromatic diisocyanates (TDI), it is not surprising that the issue of chloroalkyl isocyanates is not discussed further.

[0016] There is still a need for an efficient and cost-effective method for preparing aliphatic diisocyanates, which have a lower chloroalkyl isocyanate content and preferably a low overall HC value in the crude product, avoiding the disadvantages of existing methods.

[0017] The present invention achieves this objective by preparing aliphatic diisocyanates by gas-phase phosgenation of the corresponding aliphatic diamine, wherein at least one aliphatic diamine is reacted with phosgene at a certain flow rate to generate a reaction mixture, and the reaction mixture is contacted at least partially in a quenching zone with a quenching liquid containing at least one organic solvent to obtain a crude product solution, characterized in that the quenching liquid is introduced into the quenching zone at such a flow rate and solvent content that the solvent content of the resulting crude product solution is 68 to 88 by weight.

[0018] The method of this invention is particularly suitable for preparing aliphatic diisocyanates having up to 17 carbon atoms, preferably straight-chain aliphatic or branched straight-chain aliphatic diisocyanates having 4 to up to 11 carbon atoms. Examples of such straight-chain aliphatic or branched straight-chain aliphatic diisocyanates include butane-1,4-diisocyanate, pentane-1,5-diisocyanate (PDI), hexane-1,6-diisocyanate (HDI), 2-methylpentane-1,5-diisocyanate, 2,2,4-trimethylhexane-1,6-diisocyanate, 2,4,4-trimethylhexane-1,6-diisocyanate, and octane-1,8-diisocyanate. Particularly preferred aliphatic diisocyanates are PDI and HDI, with PDI being particularly preferred, wherein the difference in vapor pressure between the diisocyanate PDI and the corresponding chloroalkyl isocyanate, namely 5-chloropentyl isocyanate (CPI), is very small. For PDI, the advantages in purifying crude PDI to pure PDI are obvious because the crude product has a low CPI content.

[0019] Phosgenesis of diamines in the gas phase is known, for example, and can be carried out as described in EP 0 289 840 B1, EP 1 319655 A2, EP 1 555 258 A1, EP 1 275 639 A1, EP 1 275 640 A1, EP 1 449 826 A1, EP 1754 698 B1, DE 10 359 627 A1, or DE 10 2005 042392 A1. Specifically, the phosgenesis of 1,5-pentanediamine in the gas phase has been described, for example, in WO 2016 / 042125 A1, and can be carried out as described therein.

[0020] For example, industrial diamines having a purity >99% and a water content <500 ppm can be used as diamines in the methods of this invention. The respective diamines can be derived from petrochemical-based preparations, or from bio-based preparations and / or sources by known methods. Particularly for the preparation of PDI and HDI, the respective diamines or precursors of said diamines are preferably derived from bio-based preparations and / or sources. Bio-based preparations and / or sources of diamines are currently understood to refer to those diamines whose preparation is carried out through fermentation, i.e., by means of bacterial, yeast, or enzymatic reactions, or in which at least one of the starting materials for the preparation of the diamine is a renewable raw material. Bio-based preparations and sources of diamines are particularly preferred, i.e., those diamines prepared, for example, from sugars or amino acids through fermentation.

[0021] Prior to implementing the method of the present invention, the diamine is typically evaporated, preferably heated to 230°C to 320°C, more preferably 270°C to 310°C, and then fed to a reactor, preferably a tubular reactor. An inert gas, such as N2, He, Ar, or vapor of an inert solvent, such as vapor of an aromatic hydrocarbon with or without halogen substitution, may be mixed into the diamine. However, it should be noted that the solvent vapor is not always completely inert; therefore, N2, He, Ar, and more preferably N2, are preferred as the inert gas feed.

[0022] Phosgene used for phosgenation is preferably heated to 230°C to 320°C, more preferably 270°C to 310°C, before being fed into the reactor. Inert gases, such as N2, He, Ar, or vapors of inert solvents, such as vapors of halogen-substituted aromatic hydrocarbons like chlorobenzene, o-dichlorobenzene, toluene, chlorotoluene, xylene, chloronaphthalene, or decahydronaphthalene, may also be mixed into the phosgene. However, it should be noted that solvent vapors are not always completely inert; therefore, N2, He, and Ar, more preferably N2, are preferred as the inert gas feed.

[0023] The optional separately supplied inert gas stream is preferably heated to 230°C to 320°C, more preferably 270°C to 310°C. It can be an inert gas, such as N2, He, Ar, or vapor of an inert solvent, such as vapor of an aromatic hydrocarbon with or without halogen substitution. However, it should be noted that solvent vapors are not always completely inert; therefore, N2, He, and Ar are preferred, with N2 being more preferred as the inert gas stream.

[0024] In a preferred embodiment of the invention, two reactant streams and an inert gas stream are supplied to the reactor via a preferably concentrically arranged annular gap nozzle, wherein the inert gas stream is supplied via the annular gap and thus between the two reactant streams, while the diamine is preferably supplied via the concentric inner channel of the annular gap nozzle, and the phosgene is supplied via the remaining cross-section of the reactor. The diamine stream, optionally diluted with an inert gas, preferably flows at an average velocity of 20-150 m / s, more preferably 20-100 m / s upon entering the reactor, while the preheated and optionally inert gas-diluted phosgene preferably flows at an average velocity of at least 1 m / s, more preferably 5-15 m / s, across the remaining cross-section between the outer wall of the annular gap nozzle and the inner wall of the reactor. The inert gas stream, which briefly separates the diamine and phosgene upon entering the reactor, can, for example, be supplied to the reactor at an average velocity of 20-150 m / s, preferably 20-100 m / s.

[0025] The preferred method of use is the annular gap nozzle, also known as the separation gap nozzle, because the inert gas flow separates the two reactant flows.

[0026] The flow rates of gaseous diamine and phosgene are preferably selected such that the phosgene molar excess based on the amino group is 30% to 300%, preferably 60% to 200%.

[0027] Preferably, the method of the present invention uses a tubular reactor with no internal components and no moving parts. The tubular reactor is typically made of steel, glass, alloy steel, or enamel-lined steel, and its dimensions should ensure that the diamine and phosgene can react substantially completely, preferably completely, under the process conditions. As described above, the gas stream is introduced into the tubular reactor at one end through, for example, a separation gap nozzle. The mixing zone is preferably maintained at a temperature between 230°C and 320°C, more preferably between 270°C and 310°C, wherein this temperature can optionally be maintained by heating the tubular reactor as needed.

[0028] The co-reactants, namely phosgene and aliphatic diamine, which are heated separately, are mixed and reacted, and the reactor region where the reaction occurs is defined as the reaction zone. The reaction within the reaction zone is preferably carried out adiabatically, i.e., without the need for targeted removal of the released heat of reaction. However, preferably, the temperature in the reaction zone is maintained below 450°C, more preferably below 425°C. This can be controlled by the flow rates of the diamine, phosgene, and optionally the inert gas, and their inlet temperatures.

[0029] Mixing and reaction are preferably carried out in a common technical device for chemical reactions, namely a reactor. The area where mixing occurs is called the mixing zone, and the area where reaction occurs is called the reaction zone. Here, the mixing zone and the reaction zone are not clearly separated in space, but rather merge and overlap accordingly.

[0030] In implementing the method of the present invention, the pressure in the feed line leading to the reaction space is typically 200 to 3000 mbar absolute pressure, preferably 800 to 1500 mbar absolute pressure, and the pressure at the outlet of the reaction space is 150 to 2000 mbar absolute pressure, preferably 750 to 1440 mbar absolute pressure, wherein the flow velocity within the reaction space is maintained at 3 to 120 m / s, preferably 5 to 75 m / s, by maintaining a suitable pressure differential. Under these conditions, the reaction space is typically in a turbulent state.

[0031] For example, the average residence time of the reaction mixture in the reactor is 0.05 to 4 s, preferably 0.1 to 2 s, and more preferably 0.2 to 0.5 s. The average residence time is calculated from the throughput of the reactant flow over time, the reactor size, and reaction parameters such as pressure and temperature.

[0032] After passing through the reaction zone, at least a portion, but preferably all, of the resulting reaction mixture is rapidly cooled and at least partially condensed to yield a crude product solution. This is achieved by introducing it into a quenching zone and contacting it with a quenching liquid. The structure and operation of the quenching zone are well known to those skilled in the art in the preparation of diisocyanates by phosgenation of the corresponding diamine in the gas phase. Possible configurations are disclosed, for example, in EP 1 403 248 A1 and EP 1 935 875 A1.

[0033] In a preferred embodiment, the mixing zone, reaction zone, and quenching zone are arranged in a common technical device, namely a reactor.

[0034] The quenching liquid used contains at least one organic solvent, preferably aliphatic hydrocarbons, non-halogenated aromatic hydrocarbons, halogenated aromatic hydrocarbons, and mixtures of the above solvents. Preferably, the at least one organic solvent contained in the quenching liquid is chlorobenzene, p-dichlorobenzene, o-dichlorobenzene, chlorotoluene, and chloronaphthalene, or a mixture of the above solvents. More preferably, the at least one organic solvent contained in the quenching liquid is chlorobenzene and o-dichlorobenzene, or a mixture of the above solvents. Very particularly preferably, the at least one organic solvent contained in the quenching liquid is chlorobenzene. Preferably, the quenching liquid contains not only the solvent but also the isocyanate generated in the reaction, because preferably, at least a portion of the quenching liquid is derived from the obtained crude isocyanate product. In this case, to avoid solid deposits in pipes, valves, nozzles, or other quenching devices, it is recommended to separate the optional solid particles contained in the quenching liquid. Measures known to those skilled in the art for carrying out such solid separation are preferably filtration of at least a portion of the quenching liquid.

[0035] To achieve a solvent content of 68 to 88% by weight in the resulting crude product solution according to the invention, it is preferable that the solvent content of the quenching liquid itself is higher than this target value. The solvent content of the quenching liquid is preferably at least 70% by weight, more preferably at least 72% by weight, and even more preferably at least 75% by weight. If two or more stages of quenching with potentially different quenching liquids are used, the calculated solvent content of all quenching liquids used (i.e., the sum of all quenching liquid streams) is crucial. If the quenching liquid contains more than one solvent, the solvent content involves the total amount of solvents.

[0036] In a preferred embodiment of the invention, the flow rate and solvent content of the quenching liquid are set such that the solvent content of the resulting crude product solution is 70 to 86% by weight, preferably 72 to 85% by weight, more preferably 74 to 84% by weight. A higher content here is advantageous for resulting in particularly low chloroalkyl isocyanate content in the crude product. On the other hand, excessively high solvent content leads to increased costs for solvent separation and recycling, therefore excessive dilution of the crude product should be avoided.

[0037] The resulting crude product can be further diluted with a further solvent in a collection tank, but given the already high solvent content of the crude product, this is generally neither necessary nor beneficial. On the contrary, it negatively impacts the overall process because these additional solvents need to be separated from the isocyanate again afterward. An exception is a method where at least a portion of the crude product solution is removed from the collection tank and used as the quenching liquid itself. In this case, the additional solvent added to the collection tank increases the solvent content in the quenching medium, allowing the solvent content in the resulting crude product to be influenced and adjusted by adding further solvent to the collection tank. If it is not possible to sample the resulting crude product before further altering its composition, or simply to reduce the cost of process analysis, those skilled in the art can also determine the solvent content in the resulting crude product by calculation. This calculation can also be advantageously used to set the required quenching liquid flow rate and its solvent content leading to the quenching zone.

[0038] In a preferred embodiment of the method, the flow rate and / or solvent content of the quench liquid are set in a computer-implemented manner, wherein the expected solvent content in the crude product solution is calculated using operating parameters and converted into at least one control variable of at least one actuator that affects the flow rate and / or solvent content of the quench liquid. Examples of such actuators include pumps or valves, preferably regulating valves, which regulate the flow rate of the quench liquid or regulate the input amount of fresh or post-treated solvent into the quench liquid, thereby affecting the solvent content of the quench liquid and thus also the solvent content in the crude product. Another subject of the invention is a computer-implemented method for setting the flow rate introduced according to the invention, characterized in that the crude product concentration in the process of preparing aliphatic diisocyanates by gas-phase phosgenation of the corresponding aliphatic diamine is determined by calculation using data and parameters.

[0039] The quenching zone can be configured to add quench liquid at only one location along the longitudinal direction of the reactor, or at two or more locations along the longitudinal direction of the reactor. These different locations for adding quench liquid along the longitudinal direction of the reactor are hereinafter referred to as quenching stages. Each quenching stage preferably has multiple nozzles distributed along the circumference of the reactor. Preferably, the quenching zone comprises two or more quenching stages, more preferably exactly two quenching stages. In this configuration, different quenching stages can be injected with different quench liquids, i.e., quench liquids with different compositions and therefore different solvent contents. Preferably, in a vertically arranged tubular reactor, the quench liquid with the lowest solvent content is added at the most downstream position (i.e., the lowest position), while the quench liquid with a higher solvent content is used at the upstream positions.

[0040] The temperature of the quenching liquid is preferably selected to be higher than the decomposition temperature of carbamoyl chloride corresponding to the diisocyanate, but lower than the condensation temperature of the diisocyanate under the process conditions in the quenching zone. If solvent vapor is added as a diluent inert gas, the temperature of the quenching liquid is also preferably lower than the condensation temperature of the solvent, so that the diisocyanate and co-solvent condense or dissolve in the quenching liquid, while excess phosgene, hydrogen chloride, and optionally a portion of the solvent and evaporated quenching liquid leave the quenching zone in gaseous form. For selectively obtaining the diisocyanate from the mixture leaving the reaction zone in gaseous form, a quenching liquid maintained at a temperature of 60 to 200°C, preferably 90 to 170°C, is particularly suitable.

[0041] The portion of the reaction mixture that leaves the quenching zone in gaseous form is then degassed of excess phosgene using known methods. This can be achieved by cold trapping, absorption in an inert solvent (e.g., chlorobenzene MCB or dichlorobenzene ODB) maintained at -10°C to 8°C, or by adsorption and hydrolysis on activated carbon. The hydrogen chloride gas flowing through the phosgene recovery stage can be recycled using methods known per se to recover chlorine required for phosgene synthesis.

[0042] The purification of diisocyanate is preferably achieved by distillation of the crude isocyanate solution.

[0043] The advantages of the method of the present invention are: a) Fewer byproducts are generated, resulting in low levels of chlorinated byproducts (especially chloroalkyl isocyanates) even in the crude product. The concentration of chloroalkyl isocyanates in the crude product solution is less than 0.25% by weight, regardless of the solvent. This keeps subsequent distillation costs low.

[0044] b) Avoid solid deposits on the reactor walls and during rapid cooling.

[0045] Another subject of the present invention is an apparatus for preparing aliphatic diisocyanates by gas-phase phosgenation according to the method of the present invention, the apparatus comprising: A reactor having at least one reaction zone and at least one quenching zone, the reaction zone being configured to react at least one aliphatic diamine with phosgene, and the quenching zone being configured to contact the reaction mixture with a quenching liquid containing at least one organic solvent. • At least one actuator configured to influence the flow rate and / or solvent content of the quenching fluid. • An interface unit configured to continuously or periodically read at least one operating parameter of the reactor. • A processor configured to process the at least one continuously or periodically read operating parameters, calculate the theoretical solvent content of the crude product therefrom, and output a prompt on a display or transmit a new control variable to the at least one actuator when the solvent content deviates from a predetermined target value, thereby adjusting the solvent content in the crude product.

[0046] Another subject of the invention is the use of a quenching liquid, the flow rate and solvent content of which are configured to reduce the chloroalkyl isocyanate content in aliphatic diisocyanates. Furthermore, the subject of the invention is also the use of the apparatus of the invention for reducing the chloroalkyl isocyanate content in aliphatic diisocyanates.

[0047] The present invention will now be described in more detail through embodiments, but is not limited to these embodiments.

[0048] Example: GC method for crude product analysis: Gas chromatograph: Agilent (formerly Hewlett Packard), 7890, A or B series (6890 A or B series is also acceptable). Separation column: RXI 17 (Restek), fused silica, 30 m in length, 0.32 mm in inner diameter, 1.0 µm in film thickness. Temperature: Injector 250℃, Detector (FID) 350℃ Oven: Initial temperature 80℃, holding time 0 minutes. Heating rate 10°K / min to 140°C, holding time 7.5 minutes. Heating rate 20°K / min to 250°C, holding time 5.0 minutes. Running time: 24 minutes. Carrier gas: Hydrogen The gas is set to a constant flow rate rather than a constant pressure. The column pressure analysis begins with an absolute pressure of approximately 0.4 bar. The column flow rate is approximately 100 mL / min at a constant flow rate. Split output flow rate: 100 mL / min Ratio 50:1 The diaphragm is flushed at a rate of approximately 3 mL / min.

[0049] Quantitative assessment was performed using the standardized area percentage method. The response factor for each solvent within the expected concentration range was determined using external standard calibration, and this factor was taken into account when determining the solvent content in the crude product solution. For chloroalkyl isocyanates and diisocyanates, a response factor of 1 was assumed to be used to determine the standardized area percentage.

[0050] Comparative Example 1: PDA is evaporated and superheated to 310°C, providing a gaseous PDA flow rate of 8.46 kg / h. Phosgene at 310°C is also provided at a flow rate of 45 kg / h, and nitrogen at 310°C at a flow rate of 1.48 kg / h. These streams are fed into a vertically arranged conical reactor comprising a mixing zone, a reaction zone, a quenching zone, and a collection tank. The addition occurs at the top of the reactor, with PDA added through a central tube coaxially extending from a separation gap nozzle, and nitrogen added through the separation gap of the nozzle. Phosgene flows through the nozzle via the remaining annular space between the nozzle and the reactor wall. Thus, at the nozzle outlet, the nitrogen stream is positioned between the amine and phosgene streams. Downstream, the streams spontaneously mix, and the reaction mixture flows through the reaction zone to form isocyanates. The resulting reaction mixture is cooled further downstream in a two-stage quencher by injecting quenching liquid, and a portion of the reaction mixture condenses and flows freely into a collection tank located below the quencher. The unprecipitated portion of the reaction mixture exits the reactor in gaseous form and isocyanate residues are removed in a scrubbing tower operated with chlorobenzene. The resulting scrubbing liquid contains approximately 98% by weight chlorobenzene, along with trace amounts of phosgene and PDI.

[0051] The quenching liquid in the upper first quenching stage is taken from the washing liquid, while in the lower second quenching stage, the crude product stream taken from the collection tank is injected into the quenching zone as quenching liquid, so that the crude isocyanate product is eventually circulated through the collection tank and the lower second quenching stage.

[0052] Adjust the flow rate of the quench liquid in the first quench stage so that the chlorobenzene content of the crude product generated in the collection tank is 65% by weight.

[0053] GC analysis of the obtained crude solution showed that the CPI content based on the crude product (excluding solvent) was 0.48 wt%.

[0054] Example 1: Phosgenesis was carried out as in Comparative Example 1, but the flow rate of the first quench stage was increased so that the chlorobenzene content in the crude product was 68% by weight.

[0055] GC analysis of the obtained crude solution showed that the CPI content based on the crude product (excluding solvent) was 0.22% by weight.

[0056] Example 2: Phosgenesis was carried out as in Comparative Example 1, but the chlorobenzene content in the crude product (and therefore the chlorobenzene content in the quench liquid of the second quench stage below) was adjusted to 70% by weight by adding chlorobenzene to the collection tank.

[0057] GC analysis of the obtained crude solution showed that the CPI content based on the crude product (excluding solvent) was 0.16 wt%.

[0058] Example 3: Phosgenesis was carried out as in Comparative Example 1, but the flow rate of the first quench stage was increased so that the chlorobenzene content in the crude product was 74.5% by weight.

[0059] GC analysis of the obtained crude solution showed that the CPI content based on the crude product (excluding solvent) was 0.09 wt%.

[0060] Example 4: The phosgenation in Example 2 was repeated, except that instead of PDA, HDA was used for phosgenation at a flow rate of 9.62 kg / h. The solvent content of the crude product solution was adjusted to 70% by weight as in Example 2.

[0061] GC analysis of the obtained crude solution showed that the CHI content was 0.17 wt% based on the crude product (excluding solvent).

Claims

1. A method for preparing aliphatic diisocyanates by gas-phase phosgenation of the corresponding aliphatic diamine. • In this process, at least one aliphatic diamine in a certain flow rate reacts with phosgene to form a reaction mixture, and • The reaction mixture is contacted, at least partially, in a quenching zone with a quenched liquid containing at least one organic solvent to obtain a crude product solution. Its features are, The quenching liquid is introduced into the quenching zone at such a flow rate and solvent content that the solvent content of the crude product solution is 68 to 88 by weight.

2. The method as described in claim 1, characterized in that, The aliphatic diisocyanate is pentane-1,5-diisocyanate.

3. The method as described in claim 1 or 2, characterized in that, The at least one aliphatic diamine is 1,5-pentanediamine, preferably 1,5-pentanediamine, and is derived from bio-based production and / or sources.

4. The method according to any one of claims 1 to 3, characterized in that, The at least one solvent is chlorobenzene, p-dichlorobenzene, o-dichlorobenzene, chlorotoluene, chloronaphthalene, or a mixture of the above solvents.

5. The method according to any one of claims 1 to 4, characterized in that, The solvent content of the quenching liquid is at least 70% by weight, preferably at least 72% by weight, and more preferably at least 75% by weight.

6. The method according to any one of claims 1 to 5, characterized in that, The flow rate and solvent content of the quenching liquid are set such that the solvent content of the resulting crude product solution is 70 to 86% by weight, preferably 72 to 85% by weight, and more preferably 74 to 84% by weight.

7. The method according to any one of claims 1 to 6, characterized in that, The quench zone may include two or more, preferably two quench stages.

8. The method according to any one of claims 1 to 7, characterized in that, The flow rate and / or solvent content of the quenching liquid are set in a computer-implemented manner, wherein the expected solvent content in the crude product solution is calculated with the aid of operating parameters and converted into at least one control variable of at least one actuator that affects the flow rate and / or solvent content of the quenching liquid.

9. An apparatus for preparing aliphatic diisocyanates by gas-phase phosgenation as described in any one of claims 1 to 7, comprising: A reactor having at least one reaction zone and at least one quenching zone, the reaction zone being configured to react at least one aliphatic diamine with phosgene, and the quenching zone being configured to contact the reaction mixture with a quenching liquid containing at least one organic solvent. • At least one actuator configured to influence the flow rate and / or solvent content of the quenching fluid. • An interface unit configured to continuously or periodically read at least one operating parameter of the reactor. • A processor configured to process the at least one continuously or periodically read operating parameters, calculate the theoretical solvent content of the crude product therefrom, and output a prompt on a display or transmit a new control variable to the at least one actuator when the solvent content deviates from a predetermined target value, thereby adjusting the solvent content in the crude product.