Method for providing a granulate suitable for combustion, granulate suitable for combustion, and use of same as a fuel

Abstract

The present invention relates to a method for providing a granulate suitable for combustion from the residue of distilling toluylene diisocyanate obtained by gas-phase phosgenation, to a granulate suitable for combustion obtained using the method according to the invention, and to the use of the granulate according to the invention as a fuel, in particular also as a substitute fuel for fuels such as coal, crude oil or natural gas, in a combustion plant.

Description

[0001] Method for providing granules suitable for combustion, granules suitable for combustion and use thereof as fuel

[0002] The present invention relates to a process for providing a granulate suitable for combustion from the residue of a distillation of toluene diisocyanate obtained by gas phase phosgenation, a granulate suitable for combustion obtainable by the process according to the invention and the use of the granulate according to the invention as fuel, in particular also as substitute fuel for fossil fuels such as coal, petroleum or natural gas, in an incineration plant.

[0003] During the large-scale production of isocyanates, distillation bottoms streams arise that require further processing. These distillation bottoms streams contain not only a distillation residue (also referred to as residue) consisting of compounds that are very difficult or impossible to evaporate undecomposed, but also portions of the isocyanate being produced. The isocyanate is deliberately not completely removed from the distillation bottoms during distillation because this would lead to a significant increase in viscosity and possibly even to solidification of the distillation bottoms. This applies in particular to the production of toluene diisocyanate (TDI) by phosgenation of toluenediamine (TDA). To keep the distillation bottoms stream flowable, a certain minimum proportion of TDI is deliberately left in it.To improve yield, it is common practice in the art to further concentrate the distillation bottoms stream after discharge from the distillation column, i.e., to recover as much of the TDI as possible. For this purpose, dryers are used in which TDI (and any other vaporizable components present) are evaporated and the remaining portions of the distillation bottoms stream are concentrated to a free-flowing solid, often with the addition of auxiliaries such as bitumen.For example, EP 0 626 368 A1 describes a processing process for the product of a phosgenation carried out in solution, in which the residue obtained from the distillation process is continuously fed together with high-boiling hydrocarbons, preferably bitumen, which are inert under the distillation conditions, to a heated, product-circulating vacuum dryer with a horizontal shaft, wherein the proportion of isocyanates still present in the residue is continuously distilled off and the remaining residue is continuously discharged as free-flowing, non-dusting granules, optionally cooled and optionally fed to an incineration after grinding.Similarly, EP 0 548 685 A2 describes a process for producing pure distilled isocyanates by reacting the corresponding amines with phosgene in a suitable solvent and multi-stage distillative workup to obtain pure isocyanates, pure solvent, and a portion of residue. This residue is worked up in such a way that the residue obtained from the distillation process is fed into a stirred and heated vessel partially filled with high-boiling hydrocarbons, preferably bitumen, which are inert under the distillation conditions, the portion of free isocyanates still present in the residue is distilled off, and the remaining residue is discharged as a free-flowing solid, cooled, and subjected to combustion. It is further disclosed, but not experimentally proven, that the free-flowing solid can be subjected to grinding prior to combustion.

[0004] EP 0 017 972 A1 describes a process for the separation of toluene diisocyanate and / or higher-boiling solvents from distillation residues obtained in the reaction of toluene diamine with phosgene in the presence of solvents in the liquid phase and subsequent distillation of the reaction solution, in which the distillation residues are treated in a fluidized bed at temperatures of 140 °C to 280 °C.

[0005] WO 2018 / 114846 A1 describes a process that enables the efficient recovery of the proportion of isocyanate to be produced in a distillation bottoms stream that arises during the workup of the crude, liquid process product containing the isocyanate to be produced, which is formed in an isocyanate production process (phosgenation of the corresponding amine). Phosgenation can be carried out both in the liquid phase and in the gas phase. In particular, a drying step is described in which the isocyanate to be produced is recovered to form a solid that is largely or completely free of this isocyanate.This drying step is characterized by a minimum content of carbodiimide group-containing compounds of 15 mass%, based on the total mass of the distillation residue fed to the drying device used. This minimum content can also be adjusted by in s / tu carbodiimidization. The drying step is preferably carried out in a drying device selected from the group consisting of heated, product-circulating vacuum dryers with a horizontal shaft, rotary tubes, disc dryers, belt dryers, and granulating screws. The examples describe the use of kneader dryers. The use of so-called CF1 ("Combi-Fluidization Technology") dryers (see below for details) is not disclosed.Preferably, a release agent selected from the group consisting of bitumen, talc, chalk, inorganic pigments, and completely dried residue (from a previous drying process; according to this document, the recycled portion can be finely ground beforehand, for which, however, no experimental evidence is provided) is added in the drying step to facilitate the formation of non-sticky dried residue particles. Bitumen was used in the examples. It was generally assumed that the addition of bitumen was necessary to achieve the formation of a free-flowing solid (and not just pasty products that would stick to the drying device). The drying step of the described process yields a solid that contains the isocyanate to be produced at most in traces of preferably a maximum of 1.0% by mass, particularly preferably a maximum of 0.1% by mass.No other possible uses (i.e., other than as a release agent) for this residue are disclosed. In this context, the document merely discusses the prior art, in particular EP 0 548 685 A2 and EP 0 626 368 A1 (see above), both of which deal with liquid-phase phosgenations.

[0006] WO 2020 / 201277 A1 describes a drying device for evaporating volatile constituents from a starting material to be dried (including a distillation bottoms stream from an isocyanate production process, where the isocyanate production can be carried out by phosgenation of the corresponding amine in the liquid phase or in the gas phase), a process for producing an isocyanate using this drying device, and the use of the drying device for drying distillation bottoms streams, oil-containing waste, paint or varnish waste, sewage sludge, minerals contaminated with organic compounds, and coal sludge. When drying a distillation bottoms stream from an isocyanate production process, the process described produces a solid which contains the isocyanate to be produced at most in traces of preferably a maximum of 1.0% by mass, particularly preferably a maximum of 0.1% by mass.Possible uses for this residue are not disclosed. The document merely discusses the prior art in this context, in particular application EP 0 626 368 A1 (see above), which deals with liquid-phase phosgenation.

[0007] In the drying device described in WO 2020 / 201277 A1, the evaporated components (vapors) are fed into a condenser via a vapor dome and a vapor line. The drying device is characterized in that, during operation, partial condensation of the vapors in the vapor dome and / or vapor line is deliberately allowed or induced, and condensed vapor components are removed from the drying device via devices installed for this purpose in the vapor dome and / or vapor line.According to the teaching of this document, the drying device, regardless of the field of application and the exact process control, is preferably selected from the group consisting of product-circulating vacuum dryers (in particular kneader dryers, paddle dryers and paddle dryers, the latter also being able to be designed as CFT dryers - see below for details on this dryer type) with horizontal shaft, rotary tubes, disc dryers, belt dryers and granulating screws.

[0008] CN 114 247734 A describes a process for converting a solid residue resulting from the production of toluene diisocyanate by phosgenation of toluenediamine, in particular in the gas phase, into a slurry, comprising:

[0009] Conversion of the solid residue into solid particles with an average diameter of < 5 mm (in particular by grinding and / or grinding);

[0010] Separating the solid particles into a first fraction of particles with an average particle size of > 75 pm and a second fraction of particles with an average particle size of < 75 pm;

[0011] Mixing solid particles of the first fraction and solid particles of the second fraction in a weight ratio of 8 : 2 to 3 : 7 and

[0012] Addition of a solvent (especially water) to obtain the slurry.

[0013] The slurry can be fed into an incinerator.

[0014] In an environmental impact report prepared by Fujian Jinhuang Environmental Protection Technology Co., Ltd., dated June 2021 for ancillary facilities of Wanhua Chemical Fujian Isocyanate Co., Ltd. (linked at http: / / www.fuzhou.gov.cn / zgfzzt / shbj / zz / xxgk / spgs / 202106 / t20210628_4129694.htm; downloaded on June 2, 2023), Chapter 3.6.8.2 discloses the processing of so-called "TDI tar particles." The diameter of these TDI tar particles is approximately 1000 to 5000 pm. The tar particles are conveyed from a solids silo through a screw conveyor into a tar particle grinding and screening device, where they are ground and screened. The ground and sieved tar particles, which now have a diameter of < 200 pm, are conveyed into the incineration plant by blowing the fine particles remaining after grinding away from the upper part of the grinder by a conveying air blower.The conveying air laden with tar particles is separated by a special cyclone separator and a dust collector and fed into the tar powder buffer silo. The small particles in the tar powder buffer silo are conveyed by a screw conveyor and a rotary valve into the tar powder injection device, where the tar powder is conveyed to the incinerator by a nitrogen gas stream. Unburned tar particles are collected and returned to the incinerator. This is done by a tar backflushing device installed near the ash silo, which conveys the unburned tar particles back to the incinerator by a nitrogen gas stream. Wanhua is known to produce TDI by phosgenating TDA in the liquid phase (see, for example, the Environmental Impact Statement of Wanhua Chemical (Fujian) Co., Ltd. regarding [the German text].]the expansion of a TDI plant to a production capacity of 250,000 tons per year from September 2021 in Chapter 4.8.1.6; available at https: / / www.fuzhou.gov.cn / zgfzzt / shbj / zz / xxgk / spgs / 202109 / P020210923330526893459.pdf; downloaded on June 13, 2023). Therefore, the report does not address the specifics of the distillation residue obtained in a gas-phase phosgenation process.

[0015] As can be seen from the aforementioned publications, it is common practice to incinerate the solid residue obtained after liquid-phase phosgenation and subsequent drying. To date, a meaningful material recycling of this dried residue has not been permanently established. Previous attempts to use the solid residue in road construction as an additive to asphalt ultimately failed to meet expectations and therefore did not gain traction. Landfilling the solid residue would ultimately be a waste of a potentially valuable raw material.The combustion of the solid residue with energetic utilization of the released combustion heat, particularly for the generation of high-pressure steam, which can then be widely used in the TDI plant and replace steam produced elsewhere (particularly by the combustion of natural gas, petroleum, or coal), is therefore currently a proven procedure from both an economic and ecological perspective. It is naturally desirable to also apply this procedure to the processing of the product of a gas-phase phosgenation. However, the production of a slurry described for this purpose in CN 114 247 734 A is not without disadvantages. Firstly, the process described therein is multi-stage and therefore comparatively complex; secondly, the calorific value of the resulting slurry is lower than that of the pure solid.

[0016] In this context, the aim is to produce a solid residue that is free of TDI except for insignificant traces, has a high calorific value and exhibits mechanical properties that allow it to be easily conveyed to an incineration plant. The solid residue should also, if possible, be of a nature that allows it to be incinerated outside the TDI production plant in which it was originally produced, for example in energy-intensive industrial plants such as cement works or in the steel industry. To this end, the solid residue should also be easy to convey and transportable outside of an isocyanate production plant, i.e. in public spaces, if necessary over long distances. Finally, it is desirable for the solid residue to be suitable for incineration in as many types of incineration plants as possible, even those that are actually designed for the combustion of liquids (so-calledLWI - liquefied waste incinerators) as completely as possible (i.e. with the least possible formation of ash and slag) in order to open up the widest possible range of applications.

[0017] The present invention therefore provides:

[0018] In a first aspect, the present invention is directed to a method for providing a granulate suitable for combustion, the method comprising the following steps:

[0019] (A) phosgenating toluenediamine to toluene diisocyanate (in particular meta-toluenediamine to meta-toluene diisocyanate) in the gas phase to obtain a gaseous product mixture and cooling the gaseous product mixture to obtain a liquid crude product;

[0020] (B) processing the liquid crude product to obtain (in particular meta-)tolylene diisocyanate, wherein the processing comprises (at least) one distillation step in which a liquid distillation bottoms stream comprising (in particular meta-)tolylene diisocyanate and secondary components boiling higher than (in particular meta-)tolylene diisocyanate is obtained;

[0021] (C) introducing the liquid distillation bottoms stream via an inlet opening into a dryer, in which the liquid distillation bottoms stream is dried to obtain a solid residue - preferably to a mass fraction of toluene diisocyanate (determined by means of the method described in the description [see further below for details]) based on the total mass of the solid residue of 0% to 0.100%, preferably 0% to 0.060%, particularly preferably 0% to 0.010%, very particularly preferably 0% to 0.002% and extraordinarily very particularly preferably 0% to 0.001% - and the solid residue is discharged from the dryer via a discharge opening, wherein the dryer is a paddle dryer having an interior space in which a rotor shaft is arranged that can be driven to rotate about its axis,which distributes the distillation bottoms stream during the drying process onto a solid material swirled up by rotor blades arranged on the rotor shaft and conveys it from the inlet opening towards the discharge opening; and,

[0022] (D) grinding the solid residue to give granules having a volume-related particle size distribution (determined by laser diffraction according to the Fraunhofer method in the wet dispersion process according to the method described in the description [see below]) in which dgo is in the range from 55 pm to 500 pm, preferably 60 pm to 300 pm, particularly preferably 100 pm to 250 pm, very particularly preferably 100 pm to 215 pm and extraordinarily very particularly preferably 100 pm to 180 pm.

[0023] In a second aspect, the present invention is directed to a granulate suitable for combustion obtainable by, in particular obtained by, the process according to the first aspect of the invention.

[0024] In a third aspect, the present invention is directed to the use of the granulate according to the invention as fuel, in particular also as a substitute fuel for fossil fuels such as coal, crude oil or natural gas, in an incineration plant.

[0025] Surprisingly, it has now been found that a solid residue that meets or at least comes close to the above-mentioned requirements can be provided by phosgenation in the gas phase and drying in special dryers, so-called CFT dryers (see the following description for details) in conjunction with grinding to specific grain sizes.

[0026] Before the invention is described in more detail below, some terms will first be explained:

[0027] In the terminology of the present invention, the term distillation is used for both "simple" distillation and distillation with separating internals under reflux (rectification). The same applies to derived terms such as distillation step or distillation column. A distillation bottoms stream is understood to be the stream obtained in a distillation which has the highest boiling point. Such streams are obtained from the so-called "bottoms" (the lowest part in which high-boiling constituents accumulate) of a distillation column or an evaporator (the evaporator of the distillation column or the evaporator of the optional preconcentration step described in more detail below).

[0028] All pressure specifications refer to absolute pressures.

[0029] The term meta-toluene diisocyanate (meta-TDI) refers to the relative position of the isocyanate groups; these are in the meta position. In the context of the present invention, meta-TDI therefore refers in particular to a mixture of the industrially relevant isomers 2,4-TDI and 2,6-TDI. However, the term also includes so-called T 100, which consists almost entirely of 2,4-TDI.

[0030] In the context of the present invention, the particle size distribution of the granules according to the invention is specified by percentiles, namely at least by the dgo percentile (which indicates the particle size below which 90 mass% of the particles lie, based on the total mass of the particles), in preferred embodiments also by the dio percentile (which indicates the particle size below which 10 mass% of the particles lie, based on the total mass of the particles).

[0031] "Start-up" of the dryer is understood to mean the period from the start of a drying process starting from a dryer that is out of operation until a steady state is reached in which liquid distillation bottoms stream is continuously introduced into the dryer and solid residue is continuously removed from the dryer. "Continuous operation" of the dryer is understood to mean the drying period after start-up until the completion of the drying process.

[0032] The following is a brief summary of various possible embodiments of the invention:

[0033] In a first embodiment of the method according to the invention, which can be combined with all other embodiments, step (A) comprises the following:

[0034] (Al) providing a gaseous (in particular meta-)toluenediamine stream;

[0035] (A.ll) providing a gaseous phosgene stream;

[0036] (A.lll) Mixing the gaseous (in particular meta-)toluenediamine stream and the gaseous phosgene stream in a mixing zone and converting them into the gaseous product mixture in a reaction zone downstream of the mixing zone;

[0037] (A.IV) cooling the gaseous product mixture by contacting it with a quench liquid in a quench zone to obtain a mixture of reaction product mixture and quench liquid; and

[0038] (AV) Passing the mixture of reaction product mixture and quench liquid into a collection zone, phase separation into the liquid crude product and a gaseous crude process product and separate removal of both from the collection zone.

[0039] In a second embodiment of the process according to the invention, which is a particular embodiment of the first embodiment, the quenching liquid comprises an organic solvent, (in particular meta-)tolylene diisocyanate or a mixture of an organic solvent and (in particular meta-)tolylene diisocyanate.

[0040] In a third embodiment of the process according to the invention, which is a particular embodiment of the second and third embodiments, step (B), (B1) optionally comprises separating hydrogen chloride and phosgene from the liquid crude product to obtain a liquid product depleted in hydrogen chloride and phosgene;

[0041] (Bl I ) optionally, separating quench liquid from the liquid crude product or from the liquid product depleted in hydrogen chloride and phosgene to obtain a liquid product depleted in hydrogen chloride, phosgene and quench liquid; and

[0042] (B.lll) distillative processing of the liquid crude product or of the liquid product depleted in hydrogen chloride and phosgene or of the liquid product depleted in hydrogen chloride, phosgene and quench liquid, comprising the (at least one) distillation step to obtain the liquid

[0043] Distillation bottoms stream containing (in particular meta-)toluene diisocyanate and higher than (in particular meta-)toluene diisocyanate boiling

[0044] Minor components.

[0045] In a fourth embodiment of the process according to the invention, which is an alternative to the fifth embodiment described below, but can otherwise be combined with all other embodiments, the (at least one) distillation step for obtaining the liquid distillation bottoms stream comprising (in particular meta-)tolylene diisocyanate and secondary components boiling higher than (in particular meta-)tolylene diisocyanate is a distillation in a dividing wall column in which a top stream, a side stream and a bottoms stream are obtained, wherein the side stream comprises (in particular meta-)tolylene diisocyanate and the bottoms stream is the liquid distillation bottoms stream comprising (in particular meta-)tolylene diisocyanate and secondary components boiling higher than (in particular meta-)tolylene diisocyanate.

[0046] In a fifth embodiment of the process according to the invention, which is an alternative to the fourth embodiment described above, but can otherwise be combined with all other embodiments, the (at least one) distillation step for obtaining the liquid distillation bottoms stream comprising (in particular meta-)tolylene diisocyanate and secondary components boiling higher than (in particular meta-)tolylene diisocyanate is a distillation in a dividing wall column in which a top stream, a side stream and a bottoms stream are obtained, wherein the side stream comprises (in particular meta-)tolylene diisocyanate and the bottoms stream is subjected to preconcentration in an evaporator downstream of the dividing wall column, wherein the liquid distillation bottoms stream comprising (in particular meta-)tolylene diisocyanate and secondary components boiling higher than (in particular meta-)tolylene diisocyanate is obtained.In a sixth embodiment of the process according to the invention, which is a particular embodiment of the fifth embodiment, the preconcentration takes place at a temperature in the range from 120 °C to 180 °C and at a pressure in the range from 10 mbar to 60 mbar, preferably at a temperature in the range from 130 °C to 175 °C and at a pressure in the range from 25 mbar to 45 mbar.

[0047] In a seventh embodiment of the process according to the invention, which is a particular embodiment of the fourth to sixth embodiments, the distillation in the dividing wall column is carried out at a temperature in the range from 160 °C to 200 °C and at a pressure in the range from 50 mbar to 100 mbar (both figures referring to the bottom of the dividing wall column).

[0048] In an eighth embodiment of the process according to the invention, which can be combined with all other embodiments, in step (C) for commissioning the dryer, solid residue from a previous drying process, granules from a previous drying and grinding process, and / or an (otherwise provided) inert solid, such as, in particular, aluminum oxide, is used as the solid material. "Commissioning" is understood to mean the period from the start of a drying process, starting from a dryer that is out of operation, until a steady state is reached in which liquid distillation bottoms stream is continuously introduced into the dryer and solid residue is continuously removed from the dryer.

[0049] In a ninth embodiment of the process according to the invention, which can be combined with all other embodiments, the solid material during ongoing operation of the dryer comprises, and in particular consists of, a portion of the solid residue formed during ongoing operation. "Continuous operation" of the dryer is understood to mean the drying period after commissioning until the completion of the drying process.

[0050] In a tenth embodiment of the process according to the invention, which can be combined with all other embodiments, the drying of the distillation bottoms stream in step (C) is carried out at a temperature in the range from 150 °C to 500 °C and at a pressure in the range from 20 mbar to 200 mbar, preferably at a temperature in the range from 185 °C to 320 °C and at a pressure in the range from 50 mbar to 180 mbar, particularly preferably at a temperature in the range from 250 °C to 310 °C and at a pressure in the range from 80 mbar to 150 mbar.

[0051] In an eleventh embodiment of the process according to the invention, which can be combined with all other embodiments, a rotor mill is used for grinding in step (D). In a twelfth embodiment of the process according to the invention, which can be combined with all other embodiments, the following applies to the particle size distribution of the granules obtained in step (D): d0 is from 2.5 pm to 30 pm, preferably from 2.5 pm to 20 pm. The individual preferred levels of the value ranges for d00 and d00 can in principle be combined with one another as desired.

[0052] In a thirteenth embodiment of the process according to the invention, which can be combined with all other embodiments, the granulate is freed from fractions having a diameter of more than 500 pm, preferably of more than 300 pm, particularly preferably of more than 250 pm, very particularly preferably of more than 215 pm and extraordinarily very particularly preferably of more than 180 pm by sieving.

[0053] In a first embodiment of the granulate according to the invention, it has a mass fraction of toluene diisocyanate (sum of all isomers) based on its total mass (determined by means of the method described in the description [see further below for details]) of 0% to 0.100%, preferably of 0% to 0.060%, particularly preferably of 0% to 0.010%, very particularly preferably of 0% to 0.002% and extraordinarily very particularly preferably of 0% to 0.001%.

[0054] In a first embodiment of the use according to the invention, the incineration plant is used to provide heat for the production of cement, for the production of steel, for the processing of steel in foundries or for the production of chemicals.

[0055] In a second embodiment of the use according to the invention, the incineration plant is part of an isocyanate production plant and is used to provide heat required in isocyanate production.

[0056] In a third embodiment of the use according to the invention, which can be combined with all other embodiments, the fuel is combusted at a combustion temperature of 900 °C or more, preferably 1000 °C or more, particularly preferably 1100 °C or more, wherein the combustion temperature in each case preferably does not exceed 2000 °C, in particular 1500 °C.

[0057] In a fourth embodiment of the use according to the invention, which is a particular embodiment of the third embodiment, the fuel is exposed to the combustion temperature for a residence time of 2 seconds or more, preferably from 2 seconds to 10 seconds, particularly preferably from 2 seconds to 5 seconds.

[0058] In a fifth embodiment of the use according to the invention, which can be combined with all other embodiments, the use comprises the features of one or more of the previously described embodiments of the method according to the invention.

[0059] The embodiments briefly described above and other possible configurations of the invention are explained in more detail below. All of the embodiments described above and the other configurations of the invention described below can be combined with one another as desired, unless the context clearly indicates otherwise to a person skilled in the art or unless expressly stated otherwise.

[0060] In a first aspect, as already mentioned, the invention relates to a process for providing a granulate suitable for combustion, comprising the above-mentioned steps (A) to (D). Detailed aspects of the process according to the invention are described below. It goes without saying that all of these detailed aspects are also applicable mutatis mutandis to the other two aspects of the invention—the granulate suitable for combustion and its use as fuel in an incineration plant—without necessarily being referred to in each case.

[0061] Step (A) of the process according to the invention, the phosgenation of toluenediamine to toluene diisocyanate in the gas phase, is known per se from the prior art. In a preferred embodiment of step (A), the procedure is as follows:

[0062] In one step (A1), a gaseous stream of TDA is provided. Suitable methods for this are generally known to those skilled in the art. Preferred embodiments are described below.

[0063] The conversion of TDA into the gas phase can be carried out in any evaporation apparatus known from the state of the art, in particular in a falling-film evaporator. Evaporation apparatuses in which a small working volume is passed through a falling-film evaporator with a high circulation rate are preferred.In order to minimize the thermal load on the TDA, it is preferred, regardless of the precise design of the evaporation apparatus, to support the evaporation process by feeding in an inert gas such as N2, He, Ar (in particular N2) or the vapors of an inert solvent, preferably selected from aliphatic hydrocarbons [preferably decahydronaphthalene], aromatic hydrocarbons without halogen substitution [preferably toluene or xylene, in particular toluene], aromatic hydrocarbons with halogen substitution [preferably chlorobenzene, para-dichlorobenzene, ortho-dichlorobenzene, chlorotoluene or chloronaphthalene, in particular ortho-dichlorobenzene] or mixtures of the aforementioned organic solvents.Furthermore, the evaporation – and, if necessary, superheating – of the TDA (in particular to a temperature in the range of 200°C to 430°C, preferably 250°C to 420°C, particularly preferably 250°C to 400°C) is preferably carried out in multiple stages to avoid unevaporated droplets in the gaseous amine stream. Particular preference is given to multi-stage evaporation and superheating steps in which droplet separators are installed between the evaporation and superheating systems and / or the evaporation apparatus also functions as a droplet separator. Suitable droplet separators are known to those skilled in the art.

[0064] In one step (A.11), a gaseous phosgene stream is provided. A molar ratio of phosgene to primary amine groups of 1.1:1 to 20:1, particularly preferably 1.2:1 to 5.0:1, is preferably set.The phosgene is also preferably heated, as previously described for the TDA, to a temperature in the range from 200 °C to 430 °C, preferably 250 °C to 420 °C, particularly preferably 250 °C to 400 °C, and optionally diluted with an inert gas such as N2, He, Ar (in particular N2) or with the vapors of an inert solvent, preferably selected from aliphatic hydrocarbons [preferably decahydronaphthalene], aromatic hydrocarbons without halogen substitution [preferably toluene or xylene, in particular toluene], aromatic hydrocarbons with halogen substitution [preferably chlorobenzene, para-dichlorobenzene, ortho-dichlorobenzene, chlorotoluene or chloronaphthalene, in particular ortho-dichlorobenzene] or mixtures of the aforementioned organic solvents.

[0065] In one step (A.111), the reactants TDA and phosgene are mixed in a mixing zone and reacted in a subsequent reaction zone. The separately heated reactants TDA and phosgene are preferably fed to the mixing and reaction via a nozzle arrangement. Possible configurations for such nozzle arrangements are known to those skilled in the art.

[0066] In addition to the already mentioned possibility of diluting the gaseous stream of TDA and the gaseous phosgene stream, a separate diluent gas stream (an inert gas such as N2, He, Ar (in particular N2) or the vapors of an inert solvent, preferably selected from aliphatic hydrocarbons [preferably decahydronaphthalene], aromatic hydrocarbons without halogen substitution [preferably toluene or xylene, in particular toluene], aromatic hydrocarbons with halogen substitution [preferably chlorobenzene, para-dichlorobenzene, ortho-dichlorobenzene, chlorotoluene or chloronaphthalene, in particular ortho-dichlorobenzene] or mixtures of the aforementioned organic solvents) can also be fed directly into the mixture in step (A.III). In this case, this diluent gas stream is preferably heated to a temperature in the range of 100 °C to 500 °C, preferably 150 °C to 450 °C, particularly preferably 150 °C to 400 °C.

[0067] The further conversion of the reactants TDA and phosgene mixed in the mixing zone in the reaction zone preferably takes place adiabatically. Adiabatic conversion means that there is no targeted removal of the reaction heat generated by a heat transfer medium. Therefore, the reaction enthalpy - apart from unavoidable heat losses - is quantitatively reflected in the temperature difference between the product and reactant gas streams. In particular, the invention also relates to a process in which step (A. III) is carried out adiabatically and wherein the composition and temperature of the gaseous stream of TDA in step (A1) and of the phosgene stream in step (A1 I) are each selected such that in step (A. III) a temperature in the range from 250 °C to 450 °C, preferably in the range from 270 °C to 425 °C, particularly preferably in the range from 280 °C to 420 °C is established in the mixing zone and in the reaction zone.This means that the temperature at every point in the mixing zone and the reaction zone is within this range.

[0068] The mixing zone and reaction zone are preferably arranged in a common technical device for carrying out chemical reactions, the reactor. In this arrangement, the mixing and reaction zones generally merge seamlessly into one another, without a strict separation between the two being possible—as is the case when using separate mixing equipment, which is also possible in principle. The reaction zone after mixing the reactants serves to provide a residence time to ensure the most complete conversion possible. Details on the construction of suitable phosgenation reactors are known to those skilled in the art.

[0069] In the reaction zone, TDA and phosgene are rapidly converted to TDI, preferably adiabatically as described. The reaction is preferably conducted such that the TDA is completely converted before entering the quench zone, which is described in more detail below.

[0070] In a step (A.IV), the resulting gaseous product mixture containing TDI is rapidly cooled and liquefied (except for trace amounts remaining in the gas phase) by contacting it with a quench liquid in a quench zone. Suitable quench liquids include (organic) solvents, TDI, and mixtures of TDI and an (organic) solvent, in particular solvents and mixtures of TDI and an (organic) solvent. Solvents for the quench are preferably selected from chlorobenzene, ortho-dichlorobenzene, para-dichlorobenzene, the isomers of trichlorobenzene, toluene, the isomers of xylene, and mixtures of the aforementioned solvents. Particularly preferred solvents are chlorobenzene and dichlorobenzene, with ortho-dichlorobenzene being especially preferred. Contacting is preferably carried out by injecting the quench liquid into the stream of the gaseous product mixture.

[0071] Possibilities for the construction and operation of a quench zone are known in principle from the prior art. The apparatus and methods of the prior art can also be used in the context of the present invention. Possible configurations of the quench zone are disclosed, for example, in EP 1403 248 A1 and EP 1 935 875 A1.

[0072] The temperature of the quench liquid used in step (A.IV) is preferably selected such that, on the one hand, it is high enough to split the TDI carbamoyl chloride back into TDI and hydrogen chloride. (It is by no means certain whether the intermediate product carbamoyl chloride, known from liquid-phase phosgenation, is also formed in gas-phase phosgenation. However, since it is conceivable, regardless of this, that TDI liquefied in the quench partially reacts with the hydrogen chloride gas present to form TDI carbamoyl chloride, the temperature of the quench liquid should be high enough to suppress this reaction.) On the other hand, TDI and, if appropriate, the solvent used as diluent in the gaseous TDA stream and / or gaseous phosgene stream should be largely condensed ordissolve largely in the solvent, while excess phosgene, hydrogen chloride, and any inert gas used as a diluent pass through the quench zone largely uncondensed or undissolved, so the temperature of the quench liquid must not be too high. Quench liquids maintained at a temperature of 50 °C to 200 °C, preferably 80 °C to 180 °C, are particularly suitable for the selective recovery of TDI from the gaseous reaction mixture.

[0073] Based on the physical data, at a given temperature, pressure, and composition, it is easy for a person skilled in the art to predict which mass fraction of the TDI will condense in the quench or pass through it uncondensed. It is also easy to predict which mass fraction of the excess phosgene, hydrogen chloride, any solvent, and any inert gas used as diluent will pass through the quench uncondensed or dissolve in the quench liquid. The mixture of reaction product mixture and quench liquid thus obtained in the quench zone therefore contains gaseous and liquid components, i.e., it is two-phase.

[0074] In a step (AV), the two-phase mixture of reaction product mixture and quench liquid obtained in step (A.IV) is fed into a collection zone for phase separation.

[0075] In a preferred embodiment, the mixing zone, reaction zone, quench zone, and collection zone are arranged from top to bottom in the stated order in an upright, particularly conical or cylindrical or conical-cylindrical, reactor. In this embodiment, the mixture of reaction product mixture and quench liquid obtained in step (A.IV) flows into the collection zone by gravity (i.e., "automatically"). With a different arrangement of the collection zone, it may be necessary to pump the mixture of reaction product mixture and quench liquid into the collection zone.

[0076] In the collection zone, the mixture of reaction product mixture and quench liquid obtained in step (A.IV) is separated into a liquid crude process product and a gaseous crude process product. The liquid crude process product contains at least TDI and secondary components boiling higher than TDI (as well as any solvent used as quench liquid, any by-products boiling lower than TDI or unreacted impurities introduced via the reactants, any dissolved phosgene used in excess of stoichiometric amounts, and any dissolved hydrogen chloride). The gaseous crude process product contains at least the reaction co-product hydrogen chloride (as well as any phosgene used in excess of stoichiometric amounts, any evaporated solvent, any inert gases, and any non-liquefied TDI).The liquid and gaseous phases are preferably removed continuously from the collection zone. In this embodiment, the liquid phase thus obtained is the starting material for the processing to be carried out in step (B), i.e., this liquid phase is the liquid crude product.

[0077] The liquid crude product obtained in step (A) is subjected to a workup in step (B) to recover the resulting toluene diisocyanate. This workup is also known per se from the prior art.

[0078] The crude isocyanate can be processed by generally known methods. Optionally, dissolved phosgene and dissolved hydrogen chloride are first separated from the liquid crude product obtained in step (A) in a separate step. separated. Step (B1) can in principle be carried out in any manner known to the person skilled in the art, in particular by distillation, absorption, or a combination of both.

[0079] Following step (B1)—or, preferably, immediately following step (A), solvent can be removed in a separate step (B1 I). Step (B.II) can be carried out in any manner known to the person skilled in the art, in particular by distillation. In step (B.III), TDI is isolated by distillation. This can, in principle, be carried out in any manner known to the person skilled in the art for this purpose.

[0080] Various embodiments are possible for the detailed design of the processing according to step (B). Preferred variants are described below:

[0081] Variant 1

[0082] Variant 1 is fundamentally described in Chem Systems' PERP Report for TDI / MDI (Chem Systems, Process Evaluation Research Planning TDI / MDI 98 / 99 S8, Tarrytown, NY, USA: Chem Systems 1999, pp. 27 to 32). It is preferably used when the liquid reaction mixture, after distillative separation of hydrogen chloride and phosgene (corresponding to step (B1) in the terminology of the present invention), still contains a solvent content of > 50 mass %, preferably 51 mass % to 85 mass %, particularly preferably 55 mass % to 65 mass %, based on its total mass. This mixture is fed to a solvent separation step (corresponding to step (B.II) in the terminology of the present invention), wherein a solvent-TDI mixture is first distilled off in a pre-evaporator in a solvent distillation column. In the solvent distillation column, solvent is distilled off and returned to the process.The bottoms stream of this solvent distillation contains, based on its total mass, in addition to TDI, particularly preferably 15% to 25% by mass of solvent. This stream is passed into a so-called intermediate column, in which further solvent is distilled off, and the bottoms product, freed from solvent, is fed to a final distillation column for purification of the TDI. This column is operated under reduced pressure and yields the purified, saleable isocyanate TDI as a distillate stream (corresponding to step (B.III) in the terminology of the present invention). A portion of the TDI remains in the distillation bottoms stream of this final distillation column.The functions of the intermediate column and the distillation column for TDI purification can also be combined in a dividing wall column, yielding a vapor stream consisting of low-boiling components and solvent, a fraction of pure TDI as the distillate stream removed in the dividing wall region, and a product stream containing TDI and higher-boiling components (distillation residue) as the distillation bottoms stream. The distillation bottoms stream of the distillation column for TDI purification or of the dividing wall column combining the intermediate and TDI purification columns is processed to recover TDI. For this purpose, it is possible to feed this stream into the pre-evaporator of the solvent distillation, as shown in Figure ILA.5 of the aforementioned PERP system report. The bottoms product of this pre-evaporator is then sent to a processing stage to recover the TDI it contains.Of course, it is also possible to dispense with feeding the distillation bottoms stream from step (B. III) into the pre-evaporator and instead to feed this distillation bottoms stream directly to a processing plant to recover the TDI contained therein.

[0083] Variant 2

[0084] In contrast to variant 1, in this embodiment, the liquid reaction mixture, after distillative separation of hydrogen chloride and phosgene, still contains a solvent content of only < 50.0 mass% based on its total mass. This mixture is fed to a pre-evaporator, from which a solvent-TDI mixture is distilled into a distillation column. In this variant, the TDI is already freed from the solvent in the latter distillation column, so that the bottoms stream of this distillation column can be fed into the TDI purification column. Thus, in this variant, there is one column less than in variant 1. The TDI purification column is operated under reduced pressure and delivers the purified, saleable isocyanate TDI as a distillate stream.The functions of the TDI purification column and the distillation column upstream of it can also be combined in a dividing wall column, yielding a vapor stream comprising low-boiling components and solvent, a fraction of pure TDI as the distillate stream withdrawn in the region of the dividing wall, and a product stream containing TDI and higher-boiling components (distillation residue) as the distillation bottoms stream. The distillation bottoms stream of the TDI purification column or the distillation bottoms stream of the dividing wall column combining the TDI purification column and the distillation column upstream of it is processed to recover TDI. For this purpose, it is possible to feed this stream into the aforementioned pre-evaporator. The bottom product of this pre-evaporator is then passed to the processing stage to recover any TDI contained therein. Of course, it is also possible to dispense with the feed of the distillation bottoms stream from step (B).lll) in the pre-evaporator and instead feed this distillation bottoms stream directly to the processing for the recovery of the TDI contained therein.

[0085] Variant 3

[0086] Variant 3 comprises the distillation sequences described in variants 2 and 1, but without the respective pre-evaporator. In this case, the proportion of distillation residue in the described distillation sequences is carried along via the liquid streams to the respective last TDI purification column. This process is also known in principle (EP 1 717 223 A2). In this case, the entire distillation residue is discharged via the distillation bottoms stream of the last distillation column (which, in the terminology of the present invention, is to be assigned to step (B.III)).

[0087] Variant 4

[0088] This variant is preferred. Since step (A) in the process according to the invention is carried out in the gas phase and gas-phase phosgenation generally yields a liquid crude product which contains dissolved phosgene and dissolved hydrogen chloride at most in relatively small amounts (i.e., compared with liquid-phase phosgenation), separate separation of hydrogen chloride and phosgene in step (B1) is omitted in this variant. The liquid crude product is either fed directly to a solvent separation (corresponding to step (B1 I)), in which solvent and any dissolved hydrogen chloride and any dissolved phosgene are removed via distillation overhead, or, if the solvent content is sufficiently low - possible implementations for this are described in EP 3 634 947 B1 - it is fed directly to a TDI purification column.In both cases, the TDI purification column is preferably designed as a dividing-wall column. Low boilers (i.e., by-products boiling lower than TDI, any remaining hydrogen chloride and phosgene, any remaining solvent, and any inert gases) are removed overhead as vapors. The purified TDI is removed as a distillate stream in the region of the dividing wall. The resulting distillation bottoms stream contains the distillation residue and a certain amount of TDI, which is not distilled off to keep the distillation bottoms stream processable, as well as any trace amounts of solvent. Instead of a dividing-wall column, two distillation columns without a dividing wall connected in series can, of course, also be used.

[0089] In this variant, the solvent removal according to step (B. II) - if carried out - is preferably carried out at a temperature in the range from 160 °C to 200 °C and at a pressure in the range from 160 mbar to 220 mbar (where both figures refer to the bottom of the distillation column used). In this way, a bottom stream is obtained which, based on its total mass, preferably contains 9 mass % to 20 mass % solvent, 79 mass % to 90 mass % TDI, and 1 to 5 mass % of compounds boiling higher than TDI.

[0090] The TDI purification according to step (B.III), particularly when carried out in a dividing-wall column, is preferably carried out at a temperature in the range from 160°C to 200°C and at a pressure in the range from 50 mbar to 100 mbar, both of which refer to the bottom of the distillation column used. In this way, a distillation bottoms stream is obtained which, based on its total mass, preferably contains 0.00 mass% to 1.00 mass% of solvent, 80.0 mass% to 95.0 mass% of TDI, and 4.00 to 20.0 mass% of compounds boiling higher than TDI.

[0091] All four variants mentioned above therefore produce a distillation bottoms stream which, in addition to distillation residue, also contains significant amounts of TDI and which is processed to obtain this TDI. For this purpose, this distillation bottoms stream, optionally after preconcentration in an evaporator, is fed to the drying process in step (C) described in more detail below. Preconcentration can be useful if the distillation bottoms stream obtained as described above can be further concentrated in an evaporator without solidification occurring. In this embodiment of the invention, the distillation bottoms stream to be fed to step (C) is therefore the bottoms stream of the preconcentration evaporator. This preconcentration by distilling off TDI (and any other distillable constituents still present) can in principle be carried out in all evaporators known to those skilled in the art.The preconcentration is particularly preferably carried out in an evaporator selected from thin-film evaporators, climbing evaporators, falling-film evaporators, long-tube evaporators, spiral-tube evaporators, forced circulation flash evaporators, or a combination of these apparatuses. Falling-film evaporators are particularly preferred. It is also possible to connect several evaporators in series. The preconcentration is preferably carried out at a temperature in the range of 120°C to 180°C and at a pressure in the range of 10 mbar to 60 mbar, particularly preferably at a temperature in the range of 130°C to 175°C and at a pressure in the range of 25 mbar to 45 mbar. The preconcentration can be carried out continuously or batchwise. Continuous operation is preferred.

[0092] In step (C) of the process according to the invention, the distillation bottoms stream obtained in step (B) containing toluene diisocyanate and secondary components boiling higher than toluene diisocyanate is dried. According to the invention, special paddle dryers are used for this purpose. In these paddle dryers, a solid material is brought into a state of motion by a particularly rapidly rotating paddle system that at least approximates a fluidized bed (or ideally and preferably represents a fluidized bed). The starting material to be dried, i.e. in this case the distillation bottoms stream from step (B), is conveyed onto this quasi-fluidized bed (or actual fluidized bed). The starting material to be dried is thus applied to an at least partially (ideally and preferably actually) fluidized bed.Such dryers, which are particularly suitable for drying sticky products, are known as Combi-Fluidization Technology (CFT) dryers. In contrast to convective fluidized-bed dryers, the fluidization is achieved purely mechanically. CFT dryers are described, for example, in WO 2012 / 159736 A1 and EP 2 540702 A2. These dryers are characterized by the fact that a rotor shaft, which can be driven to rotate about its axis, is arranged in the interior of the dryer. This rotor shaft is designed to distribute the starting material during the drying process onto a solid material swirled up by rotor blades arranged on the rotor shaft and to convey it from the inlet opening toward the outlet opening.During operation of these dryers, it is intended that a bed of fluidized material is already present when starting material to be dried is first introduced into the dryer (i.e. during commissioning from a dryer that is not in operation). For this purpose, (1) already dried material, in this case solid residue from a previous drying process (i.e. obtained by steps (A) to (C) according to the invention) or - less preferably - granules from a previous drying and grinding process (i.e. obtained by steps (A) to (D) according to the invention), or (2) a solid that is inert under the prevailing conditions (preferably inorganic spherical particles such as, in particular, aluminum oxide spheres) is introduced into the dryer and set in motion by the rotor shaft.After a start-up phase, a steady-state operating state is established in which the starting material solidifies through drying and is partially discharged as a solid, while partially remaining as a fluidized bed of solid particles for the subsequent drying process. Further addition of already dried material or inert solids is then no longer necessary; this only occurs during commissioning of the dryer.

[0093] It is also preferable to use solid residue from a previous drying process or granules from a previous drying and grinding process as the solid material during commissioning, as this avoids dilution of the combustible granules obtained in step (D) with non-combustible inert substances. For this purpose, a portion of the solid residue obtained in step (C) or the granules obtained in step (D) is simply set aside and not incinerated. Such solid residue or granules are naturally not available during initial commissioning of a dryer. If it is not possible to use solid residue / granules from other dryers / grinding devices (e.g., from other production lines in which the process according to the invention is also carried out, or possibly also from other isocyanate production facilities), inert solids are used in such cases.It should be noted that for a limited period after such initial startup, a solid residue diluted with the inert solid will be produced, which may have to be blended with later-produced solid residue (or, in extreme cases, discarded) in order not to excessively reduce the calorific value of the granules obtained in step (D) or to prevent interference with the operation of the grinding device. In the process according to the invention, it is therefore preferable to use solid residue from a previous drying process or granules from a previous drying and grinding process as the solid material, and to use an inert solid provided from another source, such as, in particular, aluminum oxide (see above), only during the initial startup of the dryer.

[0094] The drying of the distillation bottoms stream is preferably carried out at a temperature in the range from 150 °C to 500 °C and at a pressure in the range from 20 mbar to 200 mbar, particularly preferably at a temperature in the range from 185 °C to 320 °C and at a pressure in the range from 50 mbar to 180 mbar, very particularly preferably at a temperature in the range from 250 °C to 310 °C and at a pressure in the range from 80 mbar to 150 mbar.

[0095] During drying, TDI is evaporated, removed, and liquefied in a condenser, thereby largely or completely recovering the TDI originally present in the stream to be dried. A solid residue remains, which consists almost exclusively of distillation residue and contains only insignificant traces of TDI isomers. The solid residue generally occurs in the form of macroscopic particles with grain sizes in the range of a few mm, especially up to 5.0 mm.

[0096] In step (D), the solid residue obtained in step (C) is ground to form granules with a volume-related particle size distribution (determined by laser diffraction according to the Fraunhofer method in the wet dispersion process according to the method described in more detail below), in which dgo lies in the range from 40 pm to 500 pm, preferably 40 pm to 300 pm, particularly preferably 40 pm to 250 pm, very particularly preferably 45 pm to 215 pm and extraordinarily very particularly preferably 50 pm to 200 pm. Particles that are too fine can have a detrimental effect on conveying; moreover, it cannot be ruled out that particles that are too fine tend to agglomerate again into particles that are too large. Therefore, in a preferred embodiment, dgo lies in the range from 2.5 pm to 30 pm, preferably 2.5 pm to 20 pm. The individual preferred levels of the value ranges for dgo and dio can in principle be combined with one another as desired. Method for using

[0097] The measurement is carried out according to the Fraunhofer method. This method considers the portion of the light deflection that is caused exclusively by diffraction. The determined particle size corresponds to the equivalent diameter of the sphere with the same diffraction pattern. The measurement is carried out using the wet dispersion method, with water as the dispersing liquid. To stabilize the dispersion, the surfactant "TWEEN®80" (polyoxyethylene(20) sorbitan monooleate) is used as a dispersing aid, and ultrasound (60 s) is applied. The quality requirements of DIN ISO 13320:2022-12 must be observed. For the purposes of the present invention, the result is expressed as a dg0 percentile, or if necessary also as a dw percentile, which indicates the equivalent diameter below which 90 mass% (dg0) or 10 mass% (dw) of the particles, respectively, fall short, based on the total mass of the particles.

[0098] For grinding, state-of-the-art grinding methods suitable for fine grinding of solids can be used.

[0099] Initial results were demonstrated in a Retsch ZM200 ultracentrifugal mill. In this mill, the incoming solids stream is set in rotation and centrifugal force conveys it to a rotating sieve, where the particles to be ground are sheared and discharged through the sieve. Examples of suitable speeds (specified in revolutions per minute - rpm) are 6,000 rpm to 18,000 rpm. Examples of suitable sieve mesh sizes are 120 pm or 250 pm.

[0100] A centrifugal mill is a special type of rotor mill, in which the material to be ground is reduced in size by a rotor using impact, shear, and / or impact forces. This grinding principle is also used in basket-type mills, which are particularly suitable for large-scale applications. These mills feature rapidly rotating screens (usually rotating in opposite directions).

[0101] Rotor mills with high rotation speeds of up to 100 m / s and above are preferred. The material to be ground is accelerated to these speeds in a rotating motion, sheared between the rotating arms and the inner wall of the mill, which is preferably ribbed, and then ground into fine particles. Discharge occurs, for example, with conveying air, with separators in the form of rapidly rotating turbine wheels being used to remove only the sufficiently ground particles from the mill.

[0102] Also suitable are so-called coal mills, such as those used for grinding coal in coal-fired power plants. Examples include tube mills (mills with a length-to-diameter ratio greater than 5, in which balls, cylinders, or rods are used as grinding media) or vertical mills (mills consisting of a rotating grinding table onto which the grinding tools are pressed by their own weight and often additional force from hydraulic cylinders; the grinding tools can be balls, cylindrical, conical, or spherical rollers).

[0103] Other mills available on the market can also be used, which generate high mechanical stress and avoid excessive heating of the product, e.g., by using cold conveying air that can absorb the dissipated grinding energy.

[0104] The previously described grinding of the solid residue proceeds without problems, which was by no means expected and is surprising to the expert, as it was feared that the polymeric compounds in the residue would become rubbery soft during the grinding process and could not be ground in the described manner. This, in turn, would have prevented complete combustion.

[0105] To reliably prevent the presence of excessively large particles, it may be advantageous to sieve the granulate obtained during grinding to remove any residual components of such particles. Particles with a diameter of more than 500 μm, preferably more than 300 μm, particularly preferably more than 250 μm, very particularly preferably more than 215 μm, and extremely particularly preferably more than 180 μm are preferably removed. The term "diameter" used in this context, in the terminology of the invention, refers to the largest dimension of imperfectly spherical particles and, for practical purposes, corresponds to the mesh size of the sieve used, i.e., to separate fractions with a diameter of more than 500 μm, a sieve with a mesh size of 500 μm is used, etc.

[0106] In a second aspect, as already mentioned, the invention relates to granules suitable for combustion, obtainable, in particular obtained, by the process according to the invention described above. The granules therefore have a volume-related particle size distribution (determined by means of laser diffraction as described above) in which dgo is in the range from 40 pm to 500 pm, preferably 40 pm to 300 pm, particularly preferably 40 pm to 250 pm, very particularly preferably 45 pm to 215 pm and extraordinarily very particularly preferably 50 pm to 200 pm. As already mentioned in the description of the process according to the invention, dio is preferably from 2.5 pm to 30 pm, particularly preferably from 2.5 pm to 20 pm. The individual preferred levels of the value ranges for dgo and dio can in principle be combined with one another as desired.Without wishing to be bound by any theory, it is assumed that the granules consist almost entirely of oligomeric compounds formed by the reaction of TDI isomers with each other and / or with by-products. Besides trace amounts of free TDI (sum of all isomers; in particular, a maximum of 0.100 mass% TDI, preferably a maximum of 0.060 mass% TDI, particularly preferably a maximum of 0.010 mass% TDI, very particularly preferably a maximum of 0.002 mass% TDI, and extraordinarily very particularly preferably a maximum of 0.001 mass% TDI, based on the total mass of the granules), at most minor amounts of chlorinated by-products are also present. The TDI content data refer to extractable TDI, determined using the method described below:

[0107] Method for determining the residual content of (extractable, i.e. not "chemically bound") TDI

[0108] The analysis is performed by gas chromatography using cumene as the internal standard. The gas chromatograph is equipped with a flame ionization detector (FID) and operates with hydrogen as the carrier gas. 5.0 g of the granules and 10 ml of a 0.1% solution of cumene in acetonitrile (purity > 99.0%; water content < 100 ppm) are mixed in an ultrasonic bath at room temperature (20 °C) for 10 minutes and then allowed to stand for another 10 minutes. The areas of TDI and any aminotolyl isocyanates present (formed from TDI and traces of water) are used to quantify the TDI content. Aminotolyl isocyanates are counted as TDI.

[0109] In a third aspect, as already mentioned, the invention relates to the use of the granulate according to the invention as fuel in an incineration plant, or, in other words, to a process for combusting a granulate provided by the inventive process of the first aspect in an incineration plant. The granulate according to the invention is characterized by the fact that it can be used in conventional incineration plants and can replace fossil fuels (coal, petroleum, or natural gas). This conserves these valuable raw materials and allows them to be used for more sustainable material applications.

[0110] If the incineration plant is one that is actually designed for the combustion of liquids (so-called LWI - liquid waste incinerator), it will generally be necessary to make structural adjustments to enable it to burn the granules according to the invention. However, such adjustments generally only affect the fuel supply. In LWI plants, the liquid to be burned is usually fed into the combustion chamber via lances. These lances may need to be replaced with a device for supplying solids with a particle size similar to that of the granules according to the invention. The same applies to incineration plants designed exclusively for the combustion of gases.

[0111] Such devices for supplying solids may, in particular, include suction via a Venturi nozzle, pneumatic conveying, and / or a screw conveyor. Such devices are known per se from the prior art and therefore require no further explanation here.

[0112] Combustion takes place at temperatures of preferably 900°C or more, more preferably 1000°C or more, and particularly preferably 1100°C or more. It is preferable not to exceed a combustion temperature of 2000°C, in particular 1500°C. The granules are preferably exposed to these combustion temperatures for a period of 2 seconds or more, preferably from 2 seconds to 10 seconds, more preferably from 2 seconds to 5 seconds, in order to ensure the most complete combustion possible. A residence time of the granules for a period of 2 seconds to 5 seconds at a combustion temperature of 1000°C to 1500°C is very particularly preferred.

[0113] To start combustion, the combustion chamber is preheated to the specified temperatures, preferably with an auxiliary fuel such as natural gas or diesel. As soon as combustion of the granulate according to the invention begins, combustion of the auxiliary fuel is stopped. The ratio of auxiliary fuel to granulate is preferably reduced gradually (i.e., not abruptly) to zero.

[0114] Since the granulate according to the invention contains nitrogen, it is recommended to take measures to avoid the formation of nitrogen oxides (NO X). Such measures are known from the state of the art. In particular, reference is made to so-called staged combustion, which is used, for example, in the cement industry. In such staged combustion, the fuel is treated in separate zones with a deficit of oxygen (X < 1; so-called reduction zone) and with an excess of oxygen (X > 1; so-called burnout zone). X denotes the so-called combustion air ratio (also called air ratio or air number), the ratio of the actual air volume flow to the minimum air volume flow (minimum air quantity required for the complete conversion of the fuel).

[0115] It does not necessarily have to be an incineration plant located in close proximity to the TDI production site. The granules according to the invention can also be used in incineration plants to provide heat for a wide variety of purposes, for example the production of cement, the production of steel, the processing of steel in foundries, or the production of a wide variety of chemicals. However, it is particularly preferred if the incineration plant is part of an isocyanate production plant and is used to provide the heat required in isocyanate production. Isocyanate production plants today are often plant complexes in which not only the phosgenation of a specific amine to a specific isocyanate is carried out, but which can also include sub-plants for the production of various isocyanates and their precursors (aromatic amines and the corresponding aromatic nitro compounds).In such cases, the energy recovery from the combustion heat is preferably achieved by first generating steam (especially water vapor) and feeding it into a steam network, from which several sub-units, in particular all sub-units, of the plant complex can be supplied, regardless of whether these sub-units are used for the phosgenation of TDA to TDI, the phosgenation of another amine to another isocyanate, or the production of one or more of the precursors of TDI or another isocyanate. All of this is encompassed by the term "for the provision of heat required in isocyanate production."

[0116] 1: Grinding test

[0117] In a Retsch ZM200 ultracentrifugal mill, a solid residue prepared according to steps (A) to (C) of the inventive process was milled at a speed of 6000 rpm and using a sieve with a nominal mesh size of 250 μm (step (D)). The particle size distribution was investigated by laser diffraction using the wet dispersion method, with the values ​​d0 o = 17.30 μm and d0 o = 212.00 μm being determined. The granules obtained in this way are perfectly suitable for incineration. at the of residues of different

[0118] In both examples, a solid residue prepared according to steps (A) to (C) of the process according to the invention was milled with an ultra-rotor air vortex mill and sieved through a sieve with a nominal mesh size of 180 pm (step (D)). The residue originated from a "start-up material" of a CFT dryer and therefore contained small amounts of aluminum oxide (approx. 0.1 mass%). The following values ​​for d io and d o were determined by laser diffraction using the wet dispersion method:

[0119] Both samples were burned:

[0120] A vertically arranged, essentially cylindrical combustion chamber was used. At the beginning of each test, the combustion chamber was heated by burning natural gas. The natural gas was then gradually replaced with the ground residue. The ground residue was swirled into a stream of hot air by a metering screw with a rotary valve and conveyed from above into the combustion chamber (target mass flow gradually increased to 9.0 kg / h). A second stream of hot air was introduced from the side at the inlet point. The flue gas produced was removed from the lower section of the combustion chamber and extracted by a fan, cooled, and passed through a bag filter to separate entrained fly ash. The particles collected here are only exposed to the combustion temperatures of approximately 900 to 1000 °C for a very short time (approx. 0.9 s).The short residence time is due to the test setup and facilitates the assessment of the influence of particle size on combustion. Part of the flue gas thus freed of solids was recycled by combining it with the side-inlet air stream. During the test, the composition of the flue gas was analyzed for O2, CO2, CO, and NOx.

[0121] The following observations were made:

[0122] In Example 2 (comparison), problems occurred with the fuel supply. The maximum temperature after complete replacement of the natural gas with the ground residue was 982 °C. A rather unstable flame appearance was observed, combined with fluctuating flue gas composition (O2, CO, NO) and incomplete combustion (45 to 50% unburned carbon was found in the fly ash collected in the bag filter). A solid, predominantly inorganic (due to the aluminum oxide content) combustion residue containing 2.2 to 5.2 mass% unburned carbon was found in the lower part of the combustion chamber.

[0123] In Example 3, there were no problems with the fuel supply. The maximum temperature after complete replacement of the natural gas with the ground residue was 1046 °C. A stable flame pattern with a uniform flue gas composition was observed. Combustion was almost complete. The fly ash collected in the bag filter contained 25 to 30% unburned carbon. In the lower part of the combustion chamber, a solid, predominantly inorganic (due to the aluminum oxide content) combustion residue containing 0.9 to 1.1 mass% unburned carbon was found.

Claims

1. A process for providing a granulate suitable for incineration comprising the steps: (A) phosgenating toluenediamine to toluene diisocyanate in the gas phase to obtain a gaseous product mixture and cooling the gaseous product mixture to obtain a liquid crude product; (B) working up the liquid crude product to obtain toluene diisocyanate, wherein the workup comprises a distillation step in which a liquid distillation bottoms stream containing toluene diisocyanate and secondary components boiling higher than toluene diisocyanate is obtained; (C) introducing the liquid distillation bottoms stream via an inlet opening into a dryer, in which the liquid distillation bottoms stream is dried to obtain a solid residue and the solid residue is discharged from the dryer via a discharge opening, wherein the dryer is a paddle dryer having an interior space in which a rotor shaft is arranged which is rotatably driven about its axis and which distributes the distillation bottoms stream during the drying process onto a solid material which is swirled up by means of rotor blades arranged on the rotor shaft and conveys it from the inlet opening towards the discharge opening; and (D) Grinding the solid residue to form granules having a volume-related particle size distribution in which dgo is in the range of 55 pm to 500 pm.

2. The method of claim 1, wherein step (A) comprises: (Al) providing a gaseous toluenediamine stream; (A.ll) providing a gaseous phosgene stream; (Al II) mixing the gaseous toluenediamine stream and the gaseous phosgene stream in a mixing zone and converting them into the gaseous product mixture in a reaction zone downstream of the mixing zone; (A.IV) cooling the gaseous product mixture by contacting it with a quench liquid in a quench zone to obtain a mixture of reaction product mixture and quench liquid; and (AV) Passing the mixture of reaction product mixture and quench liquid into a collection zone, phase separation into the liquid crude product and a gaseous crude process product and separate removal of both from the collection zone.

3. The method of claim 2, wherein step (B) comprises: (Bl) optionally, separating hydrogen chloride and phosgene from the liquid crude product to obtain a liquid product depleted in hydrogen chloride and phosgene; (B.ll) optionally, separating quench liquid from the liquid crude product or from the liquid product depleted in hydrogen chloride and phosgene to obtain a liquid product depleted in hydrogen chloride, phosgene and quench liquid; and (B.lll) distillative workup of the liquid crude product or of the liquid product depleted in hydrogen chloride and phosgene or of the liquid product depleted in hydrogen chloride, phosgene and quench liquid, comprising the distillation step to obtain the liquid distillation bottoms stream comprising toluene diisocyanate and secondary components boiling higher than toluene diisocyanate.

4. A process according to any one of claims 1 to 3, wherein the distillation step for obtaining the liquid distillation bottoms stream comprising tolylene diisocyanate and secondary components boiling higher than tolylene diisocyanate is a distillation in a dividing wall column in which a top stream, a side stream and a bottoms stream are obtained, the side stream comprising tolylene diisocyanate and the bottoms stream being the liquid distillation bottoms stream comprising tolylene diisocyanate and secondary components boiling higher than tolylene diisocyanate.

5. The process according to any one of claims 1 to 3, wherein the distillation step for obtaining the liquid distillation bottoms stream comprising tolylene diisocyanate and secondary components boiling higher than tolylene diisocyanate is a distillation in a dividing wall column in which a top stream, a side stream and a bottoms stream are obtained, the side stream comprising tolylene diisocyanate and the bottoms stream being subjected to preconcentration in an evaporator downstream of the dividing wall column, the liquid distillation bottoms stream comprising tolylene diisocyanate and secondary components boiling higher than tolylene diisocyanate being obtained.

6. The process according to claim 5, wherein the preconcentration is carried out at a temperature in the range of 120 °C to 180 °C and at a pressure in the range of 10 mbar to 60 mbar.

7. A process according to any one of claims 4 to 6, wherein the distillation in the dividing wall column is carried out at a temperature in the range from 160 °C to 200 °C and at a pressure in the range from 50 mbar to 100 mbar.

8. A process according to any one of the preceding claims, wherein in step (C) for commissioning the dryer, solid residue from a previous drying process, granules from a previous drying and grinding process and / or an inert solid is used as the solid material.

9. A method according to any one of the preceding claims, wherein during operation of the dryer the solid material comprises a portion of the solid residue formed during operation.

10. A process according to any one of the preceding claims, wherein the drying of the distillation bottoms stream in step (C) is carried out at a temperature in the range from 150 °C to 500 °C and at a pressure in the range from 20 mbar to 200 mbar.

11. Process according to one of the preceding claims, in which the particle size distribution of the granules obtained in step (D) is in the range from 2.5 pm to 30 pm.

12. A process according to any one of the preceding claims, wherein the granules are freed from particles having a diameter of 500 pm or more by sieving.

13. Granules suitable for incineration, obtainable by a process according to any one of the preceding claims.

14. Granules according to claim 13, having a mass fraction of toluene diisocyanate based on its total mass of 0% to 0.100%.

15. Use of a granulate according to one of claims 13 or 14 as fuel in an incineration plant.

16. Use according to claim 15, wherein the fuel is combusted at a combustion temperature of 900°C or more.

17. Use according to claim 16, wherein the fuel is exposed to the combustion temperature for a residence time of 2 seconds or more.

18. Use according to any one of claims 15 to 17, wherein the incineration plant is used to provide heat for the production of cement, for the production of steel, for the processing of steel in foundries or for the production of chemicals, or where the incineration plant is part of an isocyanate production plant and is used to provide the heat required in isocyanate production.

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