Process and device for producing isocyanates

Abstract

The invention relates to a process for producing isocyanates by phosgenation of amines with phosgene, to the use thereof and to a phosgenation facility for carrying out the process.

Description

[0001] Process and apparatus for producing isocyanates

[0002] The invention relates to a process for producing isocyanates by phosgenation of amines in a phosgenation plant, in which problems resulting from the formation of solid amine deposits in amine-bearing plant components, particularly during planned or unplanned plant shutdowns, are reduced or completely avoided. Furthermore, the invention relates to a use and a phosgenation plant for carrying out the process according to the invention.

[0003] The large-scale production of isocyanates by reacting the corresponding amines with phosgene has long been known in the art. The reaction can take place continuously or discontinuously in the gas or liquid phase. Of particular importance are di- or polyisocyanates, which are suitable for the production of polymers such as polyurethanes, polythiourethanes, polyisocyanurates, or polyureas. Both aromatic isocyanates such as methylenediphenyl diisocyanate and its higher homologues, toluene diisocyanate, and aliphatic isocyanates such as hexamethylene diisocyanate or isophorone diisocyanate are used. During industrial-scale production, planned shutdowns or unplanned process failures naturally occur.The reasons for this are varied and range from the failure of equipment or control systems to disruptions in the supply of raw materials to economic reasons such as weak demand for the product.

[0004] Unplanned outages in particular pose problems because they often require additional safety measures to bring the plant into a so-called safe state. This includes, for example, shutting down energy sources. Since many technically relevant diamines have melting points between 5 °C and 105 °C, they tend to solidify in such cases. This applies not only, but especially, to the use of the amines in outdoor plants where ambient temperatures are low in winter. Once solidified, the melting of the diamine in equipment and pipelines causes considerable additional effort when restarting a plant after a shutdown or when cleaning the corresponding equipment.WO2015 / 144658 A1 specifically addresses deviations from normal operation in the phosgenation of amines and provides guidance on how, in the case of such deviations, the risk of deposits in the mixing zone for mixing amine and phosgene, or in the reaction zone arranged downstream of it in terms of flow, can be minimized. For this purpose, it is recommended to always ensure an excess of phosgene compared to the amine. For example, when starting up continuous production, the reactor is first heated with solvent and phosgene and only then starts the amine feed. Or, when shutting down continuous production, the amine feed is first interrupted and only then the phosgene feed.However, in the equipment upstream of the mixing zone and in the reaction zone, which is used for the preparation of the amine, for example its temperature control, transport, evaporation or superheating, there are also the problems mentioned above in the event of a plant shutdown, which are not discussed in this document.

[0005] WO2010 / 100221 A1 describes a process for producing isocyanates by reacting the corresponding amines in the gas phase. Problems caused by solid deposits between the evaporator and the mixing device, i.e., along the path through which the amine passes in gaseous form, are avoided by keeping the temperature of surfaces in contact with the gaseous amine above the dew point of the amine-containing gas stream. Problems caused by solid deposits in pipelines and apparatus further upstream, through which the amine passes in liquid form, are not addressed, and the measures described are also not suitable for remedying them.

[0006] There is therefore a need for a process and apparatus for producing isocyanates that avoid the problems mentioned as far as possible.

[0007] Taking this need into account, the present invention provides a process for the preparation of isocyanates by phosgenation of amines with phosgene, in which the amine is provided in liquid form in an amine receiver which is connected to a reactor via at least one pipeline, wherein one or more conveying devices, sensors, actuators, heat exchangers and / or mixing devices are optionally assigned to the pipeline as further apparatus, and the amine is transferred from the amine receiver through the pipeline and optionally apparatuses assigned to this pipeline into the reactor and is reacted in the reactor with an excess of phosgene to form the isocyanate, characterized in that the temperature of surfaces of the amine receiver, the pipeline, the conveying device and any apparatus present which are in contact with the liquid amine is maintained at least 5 K above the melting temperature of the amine during normal operation.wherein at least one of the following features is fulfilled: a) heating the amine receiver b) heating the amine c) heating the pipeline and / or any equipment associated therewith d) insulating the amine receiver e) insulating the pipeline and / or any equipment associated therewith.

[0008] In the context of this invention, the word "eitr" is to be understood as an indefinite article in connection with countable quantities and only as a numeral if this is explicitly stated, for example by the addition "exactly one." Expressions such as "one heat exchanger" or "one amine stream" therefore do not exclude the possibility of the presence of additional heat exchangers or amine streams.

[0009] According to the invention, the expressions “comprising” or “containing” preferably mean “consisting essentially of” and particularly preferably “consisting of”.

[0010] In this case, “normal operation” means that the device is in production mode, i.e. a reaction of amine with phosgene takes place in the reactor and the device has not been put into a safe state due to a malfunction or shut down for other reasons, for example for maintenance purposes or due to a campaign or product change.

[0011] If the temperature of surfaces in contact with the liquid amine is not or cannot be measured directly, it can be assumed in this case that the temperature of the surface corresponds to the temperature of the amine in contact with the surface. This is a useful and sufficiently accurate approximation, especially for pipelines or vessels through which flow occurs and in which the amine is circulated. In general, when carrying out the process according to the invention, it is advisable to avoid dead spaces in which the amine is not moved. This can be achieved, for example, by equipping the feed or intermediate vessels with an agitator or a pumping device and, for example, by avoiding branch lines wherever possible.

[0012] The “melting temperature of the amine” in this case is the temperature above which the amine exists as a homogeneous liquid phase and below which it begins to solidify. For pure substances or eutectic mixtures, this temperature is identical to the temperature at the melting point. If the amine has a melting interval (also referred to as the melting range or solidification interval), for example because it exists as a mixture of isomers, the liquidus temperature of the mixture, i.e. the upper limit of the melting interval, should preferably be used as the “melting temperature of the amine”. This is the temperature above which the amine or amine mixture exists as a homogeneous liquid phase and below which the amine or amine mixture begins to solidify from a homogeneous liquid phase.

[0013] For most amines, reliable information on their melting points is available in the literature. In cases of doubt, the "melting point of the amine" can be determined with sufficient accuracy within the scope of the present invention, preferably using a capillary method. Corresponding procedures are familiar to those skilled in the art. The substance, which can be frozen and pulverized if necessary, is filled to a height of 2 to 5 mm into a melting point tube and compacted, e.g. by letting the melting point tube fall through a glass tube onto a solid surface. For hygroscopic or sublimating substances, the tube is sealed at the upper end. In the range of the expected melting temperature, which can be derived from literature data or a rough determination carried out beforehand, a low heating rate of no more than 1 K per minute is used. The temperature at which the last solid particle transforms into the liquid phase is read off.

[0014] "Insulating" or "insulating" refers here to the thermal insulation of equipment or piping, i.e., reducing heat transfer by applying insulating materials such as rubber, foam, or mineral wool. Mineral wool is generally preferred due to its high temperature resistance and low fire risk. The insulation usually also includes a metallic sheath on the outside, for example, made of galvanized sheet metal or aluminum foil. Applying insulation minimizes heat loss, so that the cooling of the amine occurs more slowly due to sometimes unavoidable thermal bridges or lack of heating. This allows the temperature difference to the melting point to be smaller, or, with the same distance, more time is available before problems arise.

[0015] The present invention relates to a process for the preparation of isocyanates, wherein the amine preferably has a melting temperature above 5 °C. Preferably, the amine is a diamine, particularly preferably selected from the group consisting of 1,6-diaminohexane (HDA), l-amino-3,5,5-trimethyl-5-aminomethylcyclohexane (IPDA), 1,5-diaminopentane (PDA), 1,3-bis(aminomethyl)benzene (m-XDA), 2,4-diaminotoluene (2,4-TDA), 2,6-diaminotoluene (2,6-TDA), 4,4'-diaminodiphenylmethane (4,4'-MDA), 2,4'-diaminodiphenylmethane (2,4'-MDA), 2,2'-diaminodiphenylmethane (2,2'-MDA), 4,4'-diaminodicyclohexylmethane (PACM), 1,4-diaminobenzene (pPDA), 3,3'-dimethyl-4,4'-4,4'-biphenyldiamine and 1,5-Diaminonaphthalene (NDA). Mixtures of such amines can also be used to produce the corresponding isocyanates.Most preferably, the amine is selected from the group consisting of 1,6-diaminohexane (HDA), 2,4-diaminotoluene (2,4-TDA), 2,6-diaminotoluene (2,6-TDA), 4,4'-diaminodiphenylmethane (4,4'-MDA), 2, 4'-diaminodiphenylmethane (2,4'-MDA), 2, 2'-diaminodiphenylmethane (2,2'-MDA), 4,4'-diaminodicyclohexylmethane (PACM), 1,4-diaminobenzene (pPDA), 3,3'-dimethyl-4,4'-4,4'-biphenyldiamine and 1,5-diaminonaphthalene (NDA).

[0016] The isocyanates can be prepared by phosgenating amines using various prior art processes known to those skilled in the art. Phosgenation can be carried out continuously, semicontinuously, or batchwise. Phosgenation is preferably carried out as a continuous process.

[0017] Phosgenation can be carried out in both the gas and liquid phases. In liquid-phase phosgenation, the amine can be reacted with phosgene as it is (base phosgenation), or it can be first converted into a salt, for example, by reaction with hydrogen chloride or carbon dioxide, and then reacted with phosgene in this form.

[0018] If the phosgenation takes place in the gas phase, the amine is removed from the amine receiver using the conveying device and is generally converted into the gas phase in a suitable evaporator before entering the reactor. This evaporation is preferably followed by superheating of the gaseous amine stream to a temperature that is at least 5 K above the condensation temperature of the amine under the prevailing conditions, in particular the prevailing pressure. This temperature is usually between 200 °C and 600 °C, preferably between 250 °C and 500 °C, and particularly preferably between 250 °C and 330 °C.

[0019] If phosgenation occurs in the liquid phase, it is usually carried out in the presence of a solvent, such as chlorobenzene, dichlorobenzene, or a mixture of these two solvents. As previously explained, the amine can be reacted directly with phosgene or, for example, first reacted with hydrogen chloride gas or carbon dioxide to form the corresponding salts and then phosgenated. This salt formation also usually occurs in an inert solvent, resulting in a suspension of the salt.

[0020] Regardless of the phosgenation process used, the amine in the process according to the invention is first provided in liquid form in an amine receiver. The term “liquid form” is to be understood to mean that the corresponding amine is in liquid form. Small amounts of suspended matter, for example due to impurities, cannot always be completely avoided and are included here. The amine receiver can be a stationary or mobile tank suitable for storing the respective amine. It is preferably a stationary tank. The amine receiver is connected to the reactor via a pipeline, through which the amine can be transferred from the amine receiver into the reactor.

[0021] If necessary, additional equipment can be assigned to the pipeline. These include, in particular, one or more conveying devices, such as pumps, which serve to convey the amine through the pipeline to the reactor. For precise dosing of the amine and monitoring and / or control of the process conditions, it is beneficial if the pipeline is equipped with sensors such as mass flow meters, pressure transducers and / or temperature sensors as well as actuators such as control valves. In addition, mixing devices such as static mixers or dynamic mixing units can be assigned to the pipeline. These can be used, for example, to mix the amine with other amines, solvents or auxiliary materials. Dynamic mixing units are usually arranged in mixing vessels, which can then also be assigned to the pipeline if necessary.

[0022] Particularly, but not only, in the case of gas-phase phosgenation, heat exchangers or electric heaters can be assigned to the pipeline to heat the amine to a desired temperature. The heat exchangers can be operated with steam or heat transfer oil, for example. Another attractive option is operation with a warm process stream to be cooled, from which heat is extracted and transferred to the amine. This type of energy integration can save heating and cooling energy. If, for example, the amine is evaporated in a heat exchanger for a subsequent gas-phase phosgenation, this is usually referred to as an evaporator. If the resulting gaseous amine is further heated, this is referred to as superheating, and corresponding heat exchangers are also called superheaters. With an appropriate design of the evaporator, the superheating can also take place directly in the evaporator.

[0023] Especially in continuous processes, there is a high probability that unplanned production interruptions will occur during extended operation. The usual measures in such a case include not only stopping the dosing of feedstocks into the process, but usually also switching off energy sources. This causes the amine in the amine reservoir and in the piping to the reactor to cool down. If the amine falls below the solidifying point, deposits of solid amine regularly form, which cause problems when the phosgenation plant is subsequently restarted. Pipe sections narrowed by deposits can prevent the required throughput from being achieved or can even lead to complete blockage of pipe sections. Deposits can also form in the amine reservoir, which may impair heat transfer or cause stirring units to jam.Melting such amine deposits and thus safely restarting the plant represents a significant additional expense for the plant operator. This is even more true if the blockages make it impossible to flow hot amine through the pipelines and thus melt the deposits. In some cases, thermal expansion during melting may even pose a safety risk.

[0024] To prevent these problems, the invention ensures that the temperature of surfaces in contact with the liquid amine, the amine receiver, the pipeline, and any apparatus associated with the pipeline, is maintained at least 5 K, preferably at least 10 K, more preferably at least 20 K, and most preferably at least 30 K above the melting temperature of the amine. It is not necessary for all of the aforementioned surfaces to have the same temperature. Thus, surface temperatures, and thus ultimately also amine temperatures, that increase in steps along the flow direction are preferred.In this way, the amine remains comparatively cool in the amine receiver, in which there is usually a long residence time, and is only heated to the actual starting temperature shortly before the reaction, so that the thermal load on the raw material remains low despite the temperatures being increased above the melting temperature according to the invention and, ultimately, energy losses to the outside due to large hot surfaces are also kept to a minimum.

[0025] The inventive distance between the temperature of the surfaces in contact with the liquid amine and the melting temperature of the amine ensures that, in the event of an unplanned shutdown, sufficient time remains to take appropriate countermeasures before the temperature of the amine in the amine reservoir and / or in the pipeline or in the associated equipment falls below the melting temperature and problems arise due to solidified amine in the amine reservoir and / or in the pipeline or in the associated equipment. Suitable countermeasures can, for example, include correcting the fault that caused the unplanned shutdown so that the process can be restarted before further problems arise.Another possible countermeasure, in the event of a planned or unplanned shutdown of the phosgenation, is to at least partially remove the amine from one or more amine-bearing plant components. The removed amine is preferably transferred to a collection vessel. The collection vessel is preferably the amine receiver. Another possible countermeasure is the implementation of makeshift external heating of critical amine-bearing plant components or even circulating the amine, preferably introducing thermal energy into the amine to prevent the temperature from falling further below the amine's melting point.

[0026] Even in the event that only the temperature control of the amine fails, the process according to the invention offers the advantage that the process can be continued for a certain time, which can then be used to remedy the temperature control disturbance.

[0027] Since some organic amines are thermally sensitive substances that should not be kept at excessively high temperatures for extended periods, it is recommended, particularly for the initial amine charge, that the temperature of the surfaces in contact with the liquid amine does not exceed 180 °C, preferably 150 °C, and particularly preferably 120 °C. It goes without saying that maintaining such a maximum surface temperature in the process according to the invention is only possible for amines whose melting point is sufficiently far below this maximum surface temperature. For higher-melting amines, it is preferable to also allow higher surface temperatures, for example up to 200 °C or 220 °C, which are at least 5 K above the melting temperature of the amine.

[0028] In order to achieve the inventive aim of keeping the temperature of surfaces of the amine receiver, the pipeline, the conveying device and any equipment present that are in contact with the liquid amine at least 5 K above the melting temperature of the amine during normal operation, at least one of the following measures is implemented: a) heating the amine receiver b) heating the amine c) heating the pipeline and / or any equipment associated therewith d) insulating the amine receiver e) insulating the pipeline and / or any equipment associated therewith. In a preferred embodiment of the invention, two or more of these measures are implemented in any desired combination.

[0029] In a further preferred embodiment of the invention, at least one of the measures a) heating the amine receiver, b) heating the amine or c) heating the pipeline and / or any apparatus associated therewith is implemented in combination with at least one of the measures d) insulating the amine receiver or e) insulating the pipeline and / or any apparatus associated therewith.

[0030] In a further preferred embodiment of the invention, at least the measures a) and d) and / or the measures c) and e) are implemented in combination with one another, so that each apparatus or pipe section which is heated is also insulated.

[0031] In a particularly preferred embodiment of the invention, at least measures a), c), d) and e) are implemented.

[0032] The amine receiver can be heated in various ways. For example, a double-jacketed design of the amine receiver can be selected, so that heating can occur through the circulation of a heat transfer medium or by means of steam in the space between the double jacket. Furthermore, it is also possible to attach a heating coil, preferably a half-pipe heating coil, to the outside or inside of the amine receiver and to flow through it a suitably tempered heat transfer medium or another warm process medium. Another option is heating via an externally mounted electric heating jacket. Depending on local conditions, there are other options for heating the amine receiver, such as exposure to hot air or IR radiation, which, however, will not be discussed in detail here.It is only important that the heating is arranged in such a way that the surface temperature of the surfaces in contact with the liquid amine can be achieved according to the invention.

[0033] The amine can be heated in various ways. For example, it can be achieved using a heating register integrated into the amine reservoir, which is heated, for example, by a heat transfer medium, condensing steam, or electrically. Heating using microwave radiation is also possible. One advantageous method is to remove a portion of the amine from the reservoir, heat it using a heat exchanger, and then return it to the amine reservoir. This pumping process simultaneously mixes the amine. In this design, the amine can be heated in a very controlled manner and is particularly easy to optimize in terms of energy, for example, by integrating heat with other process streams.

[0034] Pipelines can be heated in a variety of ways. Trace heating is commonly used. This can be achieved, for example, using electric heating strips, preferably self-regulating electric heating strips that automatically adjust the power input depending on the surface temperature. Alternatively, fluid trace heating can be used. This involves hoses or small pipes that are heated, for example, using hot water, steam, or a heat transfer medium such as glycol. As an alternative to trace heating, pipes or pipe sections can also be designed with completely double walls and heated via the shell space. Trace heating offers many advantages, however. Installation and maintenance are relatively simple, and leaks in the actual pipeline can be easily detected and repaired.

[0035] It is not mandatory, but preferred, to heat the entire pipeline or to heat it entirely using the same method. Depending on local conditions, it may be more appropriate to leave certain sections of the pipeline unheated or to heat them using different methods. If there are unheated pipeline sections, it is advisable to insulate them to prevent local cold spots.

[0036] Heat losses can be minimized by insulating the amine reservoir, the pipeline, and / or any other equipment associated with the pipeline. In the event of a malfunction, this leads to slower cooling of the amine contained therein, thus making it possible to keep the necessary distance between the surface temperature and the melting temperature of the amine as small as possible. This minimizes decomposition of the amine on hot surfaces and the energy required for the process. A major advantage of insulation is that, unlike trace heating, it is not dependent on an external energy supply. Its function and thus the aforementioned advantages are therefore retained even in the event of a malfunction.

[0037] Suitable materials for insulating the amine reservoir, the pipeline, and / or any associated equipment include rubber, foam, or mineral wool. Mineral wool is generally preferred due to its high temperature resistance and low fire risk. The insulation usually includes a metallic sheath on the outside, for example, made of galvanized sheet metal or aluminum foil.

[0038] The process according to the invention ensures that, in the event of a production interruption, sufficient time remains to eliminate the cause of the interruption or to take other countermeasures suitable for minimizing or, if possible, avoiding subsequent problems caused by solidified amine in the plant components or pipelines. In the event of a production interruption, the procedure according to the invention preferably takes at least 2 minutes, more preferably at least 5 minutes, most preferably at least 15 minutes, and most preferably at least 30 minutes before the amine falls below its melting temperature and solidifies due to heat losses.

[0039] Another object of the invention is the use of at least one heating device and at least one insulation with at least one temperature control system or temperature regulation system for maintaining the liquid state of an amine in a process for producing isocyanates by phosgenation of amines for at least 2 minutes before the amine falls below its melting temperature due to heat losses and solidifies.

[0040] It is further preferred that the liquid state of the amine is maintained for at least 5 minutes, preferably at least 15 minutes, and particularly preferably at least 30 minutes. Alternatively or additionally, the use according to the invention is characterized in that the liquid state is maintained during a production interruption. In processes for preparing isocyanates in the liquid phase, a solution of the amine in an inert solvent such as, for example, chlorobenzene, dichlorobenzene, or a mixture of both is often prepared in a first step. This usually takes place in a mixing vessel, which, for the purposes of the present invention, is to be regarded as an apparatus assigned to the pipeline. The path of the amine from the mixing vessel to the reactor is then less critical in terms of temperature, since freezing of the amine and thus crystallization can be prevented by a suitable choice of solvent.It is therefore preferable, particularly in the amine receiver and / or in the pipeline section up to the mixing vessel in which the amine solution is prepared, to ensure that the temperature difference according to the invention between the surfaces in contact with the liquid amine and the melting temperature of the amine is maintained.

[0041] The invention further relates to a phosgenation plant for carrying out the process according to the invention, comprising or consisting of

[0042] I) at least one amine receiver for providing a liquid amine stream,

[0043] II) at least one phosgene stream device for providing a phosgene stream,

[0044] III) a reactor for mixing the streams from (I) and (II) and reacting the phosgene stream with the amine stream,

[0045] IV) wherein at least one pipeline for conveying the at least initially liquid amine to the reactor leads from the amine reservoir, characterized in that the phosgenation plant comprises at least one temperature control system or temperature regulation system which has one or more heating devices for directly and / or indirectly heating the liquid amine.

[0046] The one or more heating devices of the phosgenation plant according to the invention are preferably configured to maintain a temperature difference of at least 5 K, preferably at least 10 K, more preferably at least 20 K, and most preferably at least 30 K above the melting temperature of the amine. The amine receiver is, for example, a stationary or mobile tank suitable for storing the respective amine. It is preferably a stationary tank. The amine receiver preferably comprises a device for circulating the liquid amine, for example an agitator or a pumping device.The phosgene flow device may comprise additional equipment necessary for the operation of the phosgenation plant, such as heat exchangers, devices for recovering and processing excess phosgene, mixing and metering devices for producing and metering a mixed phosgene from recycled and fresh phosgene, and measuring devices for determining process parameters such as the pressure or temperature of the phosgene stream. The phosgene flow device is connected to the reactor via at least one feed line. The connection can be made, for example, via simple piping and flange connections.

[0047] Reactors that can be used to react the phosgene stream with the amine stream are known to those skilled in the art. Residence-time reactors with an upstream or integrated mixing device are suitable. Suitable residence-time reactors include, for example, tubular reactors or stirred-tank cascades, preferably tubular reactors. In tubular reactors, the two streams are preferably mixed using smooth-jet nozzles or annular gap nozzles. Particularly preferably, the nozzle is arranged within the reactor and connected to the pipeline through which the amine is supplied from the amine initial charge.

[0048] A temperature control system in the present case is a system that is designed to directly or indirectly influence the temperature of the amine. The temperature control system has an open operating sequence, i.e. there is no feedback to the control unit, so temperature changes can be caused by disturbances. This can be taken into account when carrying out the method according to the invention, for example, by targeting a larger temperature difference or by monitoring possible disturbances themselves and taking them into account when determining a control value for the heating device. However, the use of a temperature control system in which the controlled variable is measured and flows into a closed control loop is preferred.

[0049] In a preferred embodiment, the phosgenation plant accordingly has a temperature control system which comprises at least one temperature sensor for measuring the temperature of the amine in the amine receiver and at least one control unit which is connected to the temperature sensor and one or more heating devices, wherein the control unit is designed to control the temperature of the liquid amine in the amine receiver, preferably by controlling the power of the heating devices.

[0050] Suitable heating devices for the various possible applications are known to those skilled in the art. The amine receiver can be designed with a double jacket, so that heating can be achieved by a suitable heating medium via the space between the double jacket. The temperature control system can then act, for example, on the mass flow of heating medium or the temperature of the heating medium, usually steam or a heat transfer oil. Instead of the double jacket, the heating device can also have a heating coil applied to the outside or inside of the amine receiver, preferably a half-pipe heating coil, which in turn is designed to transfer heat from a suitably tempered heat transfer medium or another warm process medium to the amine. Further embodiments of the heating device include, for example, oil baths, IR radiators, electric heating bands or jackets, electric heating rods, gas or oil burners.A combination of circulation and heating can be achieved if the heating device is integrated into a pumping device of the amine feeder. For this purpose, the amine feeder can, for example, have a ring line comprising a conveying device and at least one heat exchanger. The heat exchanger can then be designed, for example, as a shell-and-tube heat exchanger, a double-walled tube, or a plate heat exchanger. Suitable heating devices for the pipeline include, for example, electric heating bands, preferably self-regulating electric heating bands. Also possible are fluid trace heating systems, comprising hoses positioned on the pipeline or small pipes that are designed to be heated, for example, by hot water, steam, or a heat transfer medium such as glycol.

[0051] Even and especially in pipelines, constrictions and even blockages can quickly occur due to unfavorable surface-to-volume ratios when the melting point of the amine is undershot. Critical areas in this regard include, in particular, points with flange connections or those where the pipeline is secured by clamps or other supports, as these frequently cause additional heat losses. In a further preferred embodiment, the phosgenation plant therefore has at least one heating device associated with the pipeline. The heating device runs along a substantial portion of the pipeline and is designed to heat the pipeline and thus indirectly the liquid amine inside the pipeline.In the present case, a “substantial part of the pipeline” is understood to mean at least 50%, preferably at least 80%, particularly preferably at least 90% and very particularly preferably at least 95% of the pipeline section between the amine feed and the reactor.

[0052] In order to delay problems caused by crystallizing amine even in the event of a malfunction of the heating device, it is preferable to insulate a substantial portion of the pipeline. Here, too, a "substantial portion of the pipeline" is understood to mean at least 50%, preferably at least 80%, more preferably at least 90%, and most preferably at least 95% of the pipeline section between the amine reservoir and the reactor. In particular, it is recommended to insulate areas containing brackets, flanges, sampling points, or other potential thermal bridges. Suitable insulating materials that can be applied to the pipeline include rubber, foam, or mineral wool. Mineral wool is generally preferred due to its high temperature resistance and low fire risk.It is advisable to cover the insulation on the outside with a layer of plastic or metal.

[0053] If the reaction of the amine with the phosgene to form the isocyanate is to take place in the gas phase, a phosgenation plant according to one of the previously described embodiments is preferably suitable for this purpose, characterized in that the phosgene stream device provides a gaseous phosgene stream, optionally comprising an inert substance in the phosgene stream in addition to phosgene, and the device comprises at least one amine stream device for converting the liquid amine stream into a gaseous amine stream, optionally comprising an inert substance in the amine stream in addition to amine, and optionally comprises at least one inert substance stream device for providing an inert substance stream.

[0054] The present invention is discussed below using exemplary embodiments, but is not limited to these embodiments. Examples

[0055] The process is carried out in a phosgenation plant according to the invention for producing isocyanates by phosgenating corresponding amines. This device comprises a vertically arranged tubular reactor (phosgenation reactor) for reacting the amine with phosgene in the gas phase. Furthermore, the device comprises an amine receiver for the amine, which is fluidly connected to the tubular reactor via a pipeline. This receiver is double-walled, with a temperature-controlled heat transfer oil circulating through the space between the reactor and heating the amine receiver.

[0056] For the examples according to the invention, the device is supplemented by an externally applied insulation of the amine reservoir as well as trace heating and insulation of the pipeline.

[0057] The heat transfer oil itself can be tempered using an electric heating element. The temperature within the amine reservoir is measured using a thermocouple inserted into the reservoir and transmitted to a control unit, which calculates a control value for the heating element from the measured data and transmits it to the control unit, allowing the temperature in the amine reservoir to be regulated to a desired target value.

[0058] A pipeline is connected to an outlet of the amine feeder, which connects the amine feeder to the tubular reactor via a feed pump for the amine, a control valve for regulating the mass flow, an evaporator, and finally a superheater. There, the pipeline opens into a nozzle from which the vaporized amine emerges, mixes with phosgene, and is subsequently converted to isocyanate.

[0059] Comparative Example 1 (Preparation of toluene-2,4-diisocyanate)

[0060] Molten 2,4-diaminotoluene is placed in the amine receiver of the previously described device, and the temperature in the receiver is regulated to 100 °C. To start up the phosgenation, the tubular reactor is first charged with phosgene preheated to 330 °C until a constant temperature is established. The reaction is then started by commencing the metering of amine. The amine is fed into the evaporator using the feed pump through the pipeline connected to the amine receiver, where it is vaporized and superheated, and then fed into the reactor as superheated steam through a nozzle. The required mass flow of amine is adjusted using a control valve in the pipeline between the feed pump and the evaporator. After a running time of approximately 1 hour, the reaction is interrupted for approximately 15 minutes by stopping the amine feed and the power supply to the heating element.After 15 minutes, it is not possible to restart the amine supply and adjust it to the desired target throughput.

[0061] Comparative example 2 (PACM)

[0062] Molten 4,4'-diaminodicyclohexylmethane is placed in the amine receiver of the previously described apparatus, and the temperature in the receiver is regulated to 66 °C. To start the phosgenation, the tubular reactor is first charged with phosgene preheated to 360 °C until a constant temperature is established. The reaction is then initiated by commencing the metering of amine. The amine is fed into the evaporator with the aid of the feed pump through the pipeline connected to the amine receiver, where it is vaporized and superheated, and then fed into the reactor as superheated steam through a nozzle. The required mass flow of amine is adjusted using a control valve in the pipeline between the feed pump and the evaporator. After a running time of approximately 1 hour, the reaction is interrupted for approximately 15 minutes by stopping the amine feed and the power supply to the heating element.In this example, too, it is not possible to restart the amine supply and adjust it to the desired target throughput after 15 minutes.

[0063] Example 1 (Preparation of toluene-2,4-diisocyanate)

[0064] The device described above is supplemented by externally applied insulation for the amine reservoir. In addition, trace heating and insulation are attached to the pipeline. Molten 2,4-diaminotoluene is placed in the amine reservoir of the modified device and the temperature in the reservoir is regulated to 123 °C. To start up the phosgenation, the tubular reactor is first charged with phosgene preheated to 340 °C until a constant temperature is established. The reaction is then started by commencing the metering of amine. The amine is fed into the evaporator with the help of the feed pump through the pipeline connected to the amine reservoir, where it is evaporated and then fed into the reactor as steam via a nozzle. The required mass flow of amine is set using a control valve in the pipeline between the feed pump and the evaporator. After a running time of approx. 1 hour, the reaction is stopped for approx.The reaction is interrupted for 15 minutes by stopping the amine supply, the power supply to the heating element, and the power supply to the trace heating. After 15 minutes, the reaction can be easily restarted by quickly restarting the amine supply, followed by the power supply to the heating element and trace heating. The target amine flow rate can be quickly adjusted again.

[0065] Example 2 (2.4 TDI long interruption)

[0066] The experiment from Example 1 is repeated. This time, during the interruption, the amine is forced from the pipeline back into the amine reservoir using nitrogen, thus emptying the pipeline. Only the following day is the amine in the amine reservoir initially heated to 123 °C, thereby melting it. The reaction can be started as usual. The amine supply through the pipeline is not subject to any disruptive flow restrictions.

[0067] Example 3 (PACM)

[0068] The experiment from Example 1 is repeated using 4,4'-diaminodicyclohexylmethane as the amine. The temperature in the amine receiver is controlled at 90 °C for this experiment, and the phosgene stream is heated to 360 °C. Otherwise, the procedure is as in Example 1. In this configuration, too, the reaction can be restarted easily after an interruption by quickly restarting the amine feed, followed by the power supply to the heating element and trace heating. The target amine throughput can be quickly adjusted again.

Claims

Claims 1. A process for the preparation of isocyanates by phosgenation of amines with phosgene, in which the amine is provided in liquid form in an amine receiver which is connected to a reactor via at least one pipeline, wherein one or more conveying devices, sensors, actuators, heat exchangers and / or mixing devices are optionally assigned to the pipeline as further apparatus, and the amine is transferred from the amine receiver through the pipeline and optionally apparatuses assigned to this pipeline into the reactor and is reacted in the reactor with an excess of phosgene to form the isocyanate, characterized in that the temperature of surfaces of the amine receiver, the pipeline and any apparatuses assigned to the pipeline which are in contact with the liquid amine is kept at least 5 K above the melting temperature of the amine,wherein at least one of the following features is fulfilled: a) heating the amine receiver b) heating the amine c) heating the pipeline and / or any equipment associated therewith d) insulating the amine receiver e) insulating the pipeline and / or any equipment associated therewith.

2. Process according to claim 1, characterized in that the amine has a melting temperature above 5 °C.

3. Process according to one of claims 1 or 2, characterized in that the phosgenation is carried out continuously, semi-continuously or discontinuously.

4. A process according to any one of claims 1 or 2, characterized in that the phosgenation takes place in the gas phase or in the liquid phase, wherein in the gas phase it is preferably carried out continuously or semi-continuously and in the liquid phase it is preferably carried out continuously, semi-continuously or discontinuously.

5. Process according to one of claims 1 to 4, characterized in that the temperature of surfaces of the amine receiver, the pipeline and any apparatus associated with the pipeline which are in contact with the liquid amine is kept at least 10 K, preferably at least 20 K and particularly preferably at least 30 K above the melting temperature of the amine.

6. Process according to one of claims 1 to 5, characterized in that in the case of a planned or unplanned shutdown of the phosgenation, the amine is at least partially removed from one or more amine-bearing plant sections, the removed amine preferably being transferred to a collecting container.

7. Process according to one of claims 1 to 6, characterized in that the temperature of the surfaces in contact with the liquid amine does not exceed 180 °C, preferably 150 °C and particularly preferably 120 °C.

8. Phosgenation plant for carrying out the process according to claims 1 to 7, comprising or consisting of I) at least one amine receiver for providing a liquid amine stream, II) at least one phosgene stream device for providing a phosgene stream, III) a reactor for mixing the streams from (I) and (II) and reacting the phosgene stream with the amine stream, IV) wherein at least one pipeline for conveying the at least initially liquid amine to the reactor leads from the amine reservoir, characterized in that the phosgenation plant comprises at least one temperature control system or temperature regulation system which has one or more heating devices for directly and / or indirectly heating the liquid amine.

9. Device according to claim 8, characterized in that the phosgenation plant comprises a temperature control system comprising at least one temperature sensor for measuring the temperature of the amine in the amine receiver and at least one control unit which is connected to the temperature sensor and one or more heating devices, wherein the control unit is arranged to control the temperature of the liquid amine in the amine receiver, preferably by controlling the power of the heating devices.

10. Device according to claim 8 or 9, characterized in that at least one heating device is assigned to the pipeline, wherein the heating device runs along a substantial part of the pipeline and is designed to heat the pipeline and thus indirectly the liquid amine in the interior of the pipeline.

11. Device according to one of claims 8 to 10, characterized in that a substantial part of the pipeline is provided with insulation.

12. Device according to one of claims 8 to 11, characterized in that the phosgene stream device provides a gaseous phosgene stream, optionally comprising an inert substance in the phosgene stream in addition to phosgene, and the device comprises at least one amine stream device for converting the liquid amine stream into a gaseous amine stream, optionally comprising an inert substance in the amine stream in addition to amine, and optionally comprises at least one inert substance stream device for providing an inert substance stream.

13. Use of at least one heating device and at least one insulation with at least one temperature control system or temperature regulation system for maintaining the liquid state of an amine in a process for producing isocyanates by phosgenation of amines for at least 2 minutes before the amine falls below its melting temperature due to heat losses and solidifies.

14. Use according to claim 13, characterized in that the liquid state of the amine is maintained for at least 5 minutes, preferably at least 15 minutes and particularly preferably at least 30 minutes.

15. Use according to claim 13 or 14, characterized in that the preservation of the liquid state takes place during a production interruption.