EP2069292A1 - Method for producing isocyanates - Google Patents

Abstract

The invention relates to a multistage method for the continuous production of organic, distillable isocyanates, preferably diisocyanates, more preferred aliphatic or cycloaliphatic diisocyanates, by reacting the corresponding organic amines with urea and alcohols while releasing ammonia to give low-molecular monomeric urethanes, and the thermal decomposition thereof. The method is characterized in that at least part of the released ammonia is utilized.

Description

 Process for the preparation of isocyanates

description

The invention relates to a multistage process for the continuous preparation of organic, distillable isocyanates, preferably of diisocyanates, more preferably of aliphatic or cycloaliphatic diisocyanates, by reacting the corresponding organic amines with urea and alcohols with release of ammonia into low molecular weight monomeric urethanes and their ther- mixed cleavage, in which resulting by-products are at least partially recycled.

The technical processes for the preparation of organic isocyanates, e.g. of aromatic, aliphatic or cycloaliphatic isocyanates, based on the phosgenation of the corresponding organic amines to carbamoyl chlorides and their thermal cleavage to the isocyanates and hydrogen chloride. Apart from the severe environmental, disposal and safety problems associated with the use of phosgene, these processes suffer from other significant disadvantages. Thus, the preparation of aliphatic or cycloaliphatic isocyanates succeeds only with fairly moderate space-time yields due to the stronger basicity of the starting amines. Another disadvantage is the formation of undesirable by-products which, already present in traces, can lead to strong discoloration of the isocyanates. In hexamethylene diisocyanate 1, 6 (HDI) preparation, e.g. several by-products, of which the most important, 6-chlorohexyl isocyanate, also has the disadvantage that it can be separated from the HDI only with considerable distillate effort.

The problem with this procedure is in particular the high conversion of chlorine over phosgene and carbamoyl chloride in hydrogen chloride, the toxicity of the phosgene and the corrosivity of the reaction mixture, the lability of the solvents used in the rule and the formation of halogenated residues.

The thermal cleavage of (cyclo) aliphatic and in particular aromatic mono- and diurethanes into the corresponding isocyanates and alcohol has been known for a long time and can be carried out both in the gas phase at high temperatures and in the liquid phase at comparatively low temperatures.

In recent decades, therefore, many efforts have been made to overcome these disadvantages of the process by a simpler and improved process. Thus, for the production of aliphatic and / or cycloaliphatic di- and / or polyurethanes according to EP 18588 B1 or also as in EP 28338 B2 primary aliphatic and / or cycloaliphatic di- and / or polyamines with O-Alkylcarbamid- acid esters in the presence of alcohols Temperatures of 160 to 300 ⁰ C with and implemented without catalyst. The resulting di- and / or polyurethanes can be converted into the corresponding isocyanates. The ammonia formed during the reaction of the amines can be separated off (EP 18588 B1, page 4, lines 45-46 and page 5, lines 40-53, EP 28338 B2, page 6, lines 38-42 ).

A recovery of the separated ammonia is not disclosed.

Other publications are concerned with the partial substitution of urea and / or diamines with carbonyl group-containing compounds, such as e.g. Carbamic acid esters (e.g., EP 27952 B1 or EP 126299 B1). The phosgene-free process is described in detail, for example, in EP 566925 B1.

The ammonia formed during the reaction of the amines can be separated off (EP 27952 B1, p.7, Z. 44-45, EP126299 B1, p. 5, Z. 13-16, EP 566925 B1, P8.8, Z 38-47).

A recovery of the separated ammonia is not disclosed.

EP 1512680 A1 describes a process for the preparation of cycloaliphatic diisocyanates in which, after the reaction of the starting materials to form the urethanes, the excess ammonia and alcohol are distilled off together and then separated from one another in a pressure distillation.

A recovery of the separated ammonia is not disclosed.

The separation operations are limited in the cited documents to a selective distillation from the reaction mixture.

The object of the present invention was to produce distillable organic isocyanates, in particular aliphatic and cycloaliphatic diisocyanates, with high selectivity in improved space-time yields in a cost-effective manner in a simple manner with improved energetic and / or material utilization.

This object has been achieved by a process for the preparation of isocyanates by reacting amines with urea and at least one alcohol to give the corresponding urethanes with liberation of ammonia, followed by cleavage of the urethanes into the corresponding isocyanates in which the liberated ammonia is at least partially is recycled.

The invention further provides a multi-stage process for the continuous preparation of organic isocyanates by reacting the corresponding organic amines with urea and at least one alcohol in the corresponding Urethanes with liberation of ammonia in at least one reactor and thermal cleavage of the urethanes, comprising the following stages and in which

a) at least one organic amine is mixed with urea in the presence or preferably in the absence of at least one catalyst and in the absence or preferably in the presence of at least one alcohol,

b) reacting the mixture obtained from a) in at least one residence time reactor to the corresponding urethane

c) separating the resulting ammonia,

d) separating from the effluent from c) excess alcohol and further low-boiling secondary components,

e) at least partially feeding the urethane from (d) freed from the alcohol and low-boiling components to a distillation,

f) the urethanes in the distillate from (e) and the portion of (d) which may not have been fed to distillation (e) in a continuous splitter decompose into the corresponding isocyanate and alcohol,

g) purifying the crude isocyanate obtained from (f) in at least one distillation and recirculating distillation residues to cleavage (f) and / or converting them to urethanes with alcohol and feeding them to reaction unit (a) and / or (b),

h) the reaction product from (f), which contains a high proportion of urethanes and useful compounds, is again converted into urethanes by reaction with alcohols and,

i) in which the separated in c) ammonia is at least partially thermally utilized.

The inventive method has an improved material and / or energy recovery, as a mere separation of the ammonia, as known from the prior art.

In purely formal terms, the method according to the invention can be schematically represented by the following equation:

R- (NH ₂ ) _n + n H ₂ N (CO) NH ₂ + n R ¹ OH → R (NCO) _n + n R ¹ OH + 2n NH ₃ Amines of the formula R (NH 2) _n , in which R is a polyvalent, preferably bivalent organic radical, such as an optionally substituted, for example substituted by an alkyl group, aromatic or preferably a linear or are suitable for the preparation of the present invention as intermediates monomeric urethanes branched-chain, aliphatic or optionally substituted cycloaliphatic radical.

Examples of suitable aromatic amines are 2,4- and 2,6-toluylenediamine, naphthylenediamine, 4,4'-, 2,4'- and 2,2'-diaminodiphenylmethanes and the corresponding isomer mixtures, 4 , 4'-, 2,4'- and 2,2'-diamino-ditolylmethane and the corresponding isomer mixtures, benzidine (biphenyl -4,4'-diamine).

Suitable aliphatic or cycloaliphatic amines are, for example: butanediamine 1, 4, 2-ethylbutanediamine-1, 4, octanediamine-1, 8, decanediamine-1, 10, dodecanediamine-1, 12, cyclohexanediamine-1, 4,2-Methyl-, 4-methylcyclohexanediamine-1,3,3,1,3- and 1,4-bis (aminomethyl) cyclohexane. Preferably use find 2-methylpentanediamine-1, 5, 2,2,4- and 2,4,4-trimethylhexanediamine-1, 6, 3 (or 4), 8 (or 9) -Bis (aminomethyl) - tricyclo [5.2.1.0 ²⁶ ] decane isomer mixtures, 4,4'- or 2,4'-di (aminocyclohexyl) methane, tetramethylxylylenediamine, triaminononane and in particular hexanediamine-1, 6 and 3-aminomethyl-3,5,5 -trimethylcyclohexylamin.

Suitable alcohols are in principle all cycloaliphatic and preferably aliphatic alcohols. Preferably, however, one will select those whose boiling points are sufficiently far from the boiling point of the isocyanate obtained by the thermal cleavage, preferably diisocyanate, so that a possible quantitative separation of the cleavage products isocyanate, preferably diisocyanate and alcohol is possible.

For these reasons, therefore, alcohols such as e.g. Methanol, ethanol, n-propanol, n-butanol, iso-butanol, n-pentanol, iso-pentanol, n-hexanol, isohexanols, cyclopentanol, cyclohexanol, 2-ethylhexanol, decanol or mixtures of said alcohols, but especially n-butanol and / or iso-butanol use.

The individual stages of the process are described below:

a) mixing of the reaction components

The mixing of the educt streams can be carried out in any apparatus which is known per se to the person skilled in the art. The mixing in step (a) can also be carried out together with the reaction in step (b). Preferably, the mixing can be carried out in a suitable separate mixing device, more preferably in a special mixing device, which is characterized by low mixing times. Separate mixing devices are, for example, mixing circuits, stirred tanks, stirred tank cascades, tubes with static mixers or mixing pumps. It is possible to separate or combine step (a) (mixing) and (b) (urethane formation). In most cases, depending on the reaction conditions, the formation of urethane already starts with the mixing of the educts.

To prepare the urethanes in reaction step (a), the amines are reacted with urea and at least one, preferably exactly one, alcohol in a molar ratio of amine, urea and alcohol such as 1: 2 to 20: 5 to 40 at temperatures of 50-300 ^° C and in particular at 180 - 220 ⁰ C under a pressure of 0.1 to 30 bar, preferably 5 - 20 bar reacted. Under these reaction conditions, average reaction times of fractions of seconds to minutes are obtained for the process according to the invention.

In order to prevent or reduce the significant onset of urethane formation already during the mixing of the components, it is usually sufficient to mix the component at a temperature below 50 ⁰ C.

The reaction in the reaction step (a) may be carried out in the presence of dialkyl carbonates, suitably in an amount of 0.1 to 30 mol%, preferably 1 to 10 mol% or Carbamidsäurealkylestern expediently in an amount of 1 to 20 mol%, preferably from 5 to 15 mol%, based on the amine, preferably diamine, are performed. In particular, mixtures of dialkyl carbonates and alkyl carbamates are used in the stated proportions. Preferred dialkyl carbonates and / or carbamic acid esters are those whose alkyl radicals correspond to the alkyl radical of the alcohol used.

As already stated, the reaction in reaction stage (a) can also be carried out in the presence of catalysts. These are expediently used in amounts of from 0.001 to 20% by weight, preferably from 0.001 to 5% by weight, in particular from 0.01 to 0.1% by weight, based on the weight of the amine.

Suitable catalysts are inorganic or organic compounds containing one or more cations, preferably a cation of metals of group IA, IB, IIA, IIB, HIB, IVA, IVB, VA, VB, VIB, VIIB, VIIIB of the Periodic Table of the Elements as defined in Handbook of Chemistry and Physics 14th Edition, published by Chemical Rubber Publishing Co., 23 Superior Ave. N.E., Cleveland, Ohio. The cations of the following metals may be mentioned by way of example: lithium, sodium, potassium, magnesium, calcium, aluminum, gallium, tin, lead, bismuth, antimony, copper, silver, gold, zinc, mercury, cerium, titanium, vanadium, chromium, molybdenum, Manganese, iron and cobalt.

The catalyst may further contain at least one anion, for example halides, such as chlorides and bromides, sulfates, phosphates, nitrates, borates, alcoholates, phenates, sulfonates, oxides, oxide hydrates, hydroxides, carboxylates, chelates, carbonates and thio- or dithiocarbamates. The catalysts can also be used in the form of their hydrates or ammoniaates without noticeable significant disadvantages.

Examples of typical catalysts are: lithium methoxide, lithium ethanolate, lithium propoxide, lithium butanolate, sodium methoxide, potassium tert-butoxide, magnesium methoxide, calcium methoxide, tin (II) chloride, tin (IV) chloride, Lead acetate, lead phosphate, antimony (III) chloride, antimony (V) chloride, aluminum acetylacetonate, aluminum isobutoxide, aluminum trichloride, bismuth (III) chloride, copper (II) acetate, copper (II) sulfate, copper (II) nitrate, bis (triphenylphosphine oxido) copper (II) chloride, copper molybdate, silver acetate, gold acetate, zinc oxide, zinc chloride, zinc acetate, zinc acetonyl acetate, zinc octoate, zinc oxalate, zinc hexoxide, Zinc benzoate, zinc undecylenate, cerium (IV) oxide, uranyl acetate, titanium tetrabutoxide, titanium tetrachloride, titanium tetraphenolate, titanium naphthenate, vanadium (III) chloride, vanadium acetylacetonate, chromium (III) chloride, molybdenum (VI) oxide, molybdenum acetylacetonate, tungsten (VI) oxide, manganese (II) chloride, manganese (II) acetate , Manganese (III) acetate, iron (II) acetate, iron (III) acetate, iron phosphate, iron oxalate, iron (III) chloride, iron (III) bromide, cobalt acetate, cobalt chloride, cobalt sulfate , Cobalt naphthenate, nickel chloride, nickel acetate and nickel naphthenate and mixtures thereof.

Examples of preferred catalysts are the following compounds: lithium butanolate, aluminum acetylacetonate, zinc acetylacetonate, titanium tetrabutoxide and zirconium tetrabutoxide.

The mixing of the educt streams is carried out in the process according to the invention in a suitable special mixing device, which is characterized by low mixing times.

The mixing time in special mixing devices with a short mixing time is usually from 0.0001 s to 2 s, preferably from 0.0005 to 1 s, particularly preferably from 0.001 to 0.5 s, very particularly preferably from 0.005 to 0.2 s and in particular from 0.007 to 0.1 s. The mixing time is understood to be the time which elapses from the start of the mixing operation until 97.5% of the fluid elements of the resulting mixture have a mixing fraction less, based on the value of the theoretical final value of the mixture fracture of the resulting mixture upon reaching the state of perfect mixing than 2.5% deviates from this final value of the mixture fraction (for the concept of mixture fracture, see, for example, J. Warnatz, U. Maas, RW Dibble: Verbrennungs, Springer Verlag, Berlin Heidelberg New York, 1997, 2nd edition, p. 134.) , The mixing device used is preferably a mixing circuit, a stirring vessel, a mixing pump or a nozzle mixing device, for example coaxial mixing nozzles, Y or T mixers, or a vortex impinging jet mixing configuration, preferably a mixing circuit, a stirring vessel, a mixing pump or a nozzle mixing device. When using a mixing circuit or a mixing vessel as a mixing device, it is important that the amine solution is injected at high speed. Usually, the speeds are between 10 and 100 m / s, preferably between 20 and 80 m / s.

Preferably, a mixing nozzle and a mixing pump is used as a mixing device. Particularly preferably, a mixing nozzle is used as the mixing device. It is important that both the alcohol and amine flow are introduced at high speed into the mixing nozzle. The speeds are between 10 and 100 m / s, preferably between 20 and 80 m / s.

The pressure in the feed lines to the nozzle is considerably higher than in the outlet of the mixing nozzle, but usually not higher than 110 bar abs, preferably not higher than 100 bar abs, more preferably the pressure of 5 to 95 bar abs, most preferably of 10 to 50 bar abs and in particular from 10 to 30 bar abs.

The pressure at the outlet of the mixing device is generally above the reaction pressure in stage b), for example between 5 and 100 bar, preferably between 10 and 80 bar, more preferably between 10 and 50 bar.

The temperature of the discharge from the mixing device is generally between 25 and 240 ^° C., preferably 30-190 and particularly preferably 40-180 ^° C.

The discharge from the mixing device can be brought to the desired temperature there before being introduced into the step b) with the aid of a heat exchanger.

The conversion based on amino groups in the amine used to form urethane groups in the step a) is generally not more than 10%, preferably not more than 5%, particularly preferably not more than 2%.

The transfer of the reaction output from stage a) into the subsequent stage can advantageously take place via pressure holding valves, wherein the pressure at the outlet of stage a) by at least 1 bar, preferably at least 2 bar, more preferably at least 3 bar above the prevailing in stage b) Pressure should be.

b) Reaction of the mixture from a)

The liquid phase leaving the mixing device is then fed to at least one, preferably exactly one biphasic (gaseous / liquid) operated reactor. This may be a non-backmixed, for example, stirred tank, or preferably a non-mixed or little backmixed reactor, for example tubular reactors or stirred tank cascades. Preferably, the mixture is fed to a tubular reactor or a plurality of reactors, the one of their residence time Resemble tubular reactor in which the gas phase with the liquid phase are conducted in cocurrent.

The tubular reactor should preferably be largely backmixing free. This is achieved for example by the ratio of the diameter of the tubular reactor to its length or by internals such as perforated plates, slot floors or static mixer. Preferably, the backmixing freedom is achieved by the ratio of length to diameter of the tubular reactor.

Suitable tubular reactors are, for example, those tubes whose length to diameter ratio is greater than 5, preferably greater than 6, particularly preferably greater than 10.

The Bodensteinzahl of the tubular reactor should be greater than 5, preferably greater than 6, more preferably greater than 10, most preferably from 10 to 600 and in particular from 10 to 100.

One aspect that contributes significantly to the invention is the presence of a flow, ideally a plug-flow, which in reality is to be approximated as much as necessary. For this purpose, the axial mixing, so a mixing along the flow direction through the reactor, as possible reduced and the flow is ideally turbulent.

This is achieved in practice by high flow velocities and small cross-sectional areas, for example in flow tubes.

The tubular reactor can have any orientation in space. Preferably, it is constructed as a vertical tube reactor, which is particularly preferably flowed through from bottom to top.

The tubular reactor can be made isothermal or preferably tempered. A tempering can be done by a jacket heating or by internal pipes or plates. The heating is preferably carried out by the jacket.

Of course, the tubular reactor can also consist of several serially connected pieces of pipe, as long as the backmixing freedom is ensured. If necessary, phase separators for separating liquid and gaseous phases can optionally be provided in the course of the tubular reactor, for example between such pieces of pipe, in which ammonia formed during the reaction can be separated so that the equilibrium of the reaction is shifted. To increase the production capacity, several tubular reactors can also be connected in parallel according to the invention.

Optionally, in the tubular reactor, as stated above, at one or more points, for example at the beginning and in the middle of the tubular reactor, still urea and / or alcohol or preferably amine are metered.

The average residence time in the tube reactor is generally 10 seconds to 5 hours, preferably 20 seconds to 20 minutes, more preferably 30 seconds to 10 minutes.

In order to keep the gas load for the subsequent stage low, the discharge from the tubular reactor can be fed to a phase separator in a preferred embodiment, and the liquid phase taken from the phase separator can then be fed to the subsequent stage.

Such a phase separator is a container in which the phase separation between gas and liquid phase is achieved by the calming of the two-phase, emerging from the DC reactor flow.

The phase separator can be carried out isothermally or preferably heated to prevent the precipitation of poorly soluble by-products. The heating can be done for example via the jacket or via a circuit with an external heat exchanger. When using an external heat exchanger, normal insulation of the heat exchanger is sufficient.

The temperature in the tubular reactor and in optional phase separator is in general between 50 ⁰ C and 300 ⁰ C, preferably between 180 ⁰ C and 220 ⁰ C.

The pressure in stage b) is generally between 0.1 bar abs and 30 bar abs and preferably between 5 and 20 bar abs.

The transfer of the reaction output from stage b) into the subsequent stage can advantageously take place via pressure-holding valves, wherein the pressure in stage b) should generally be at least 0.1 bar above the pressure prevailing in stage c). If this is not the case, transferring e.g. with the help of a pump or barometric.

The residence time in stage b) is selected so that the conversion, based on amino groups in the amine used to urethane groups, after leaving the (tubular) reactor at least 95%, preferably at least 98%, particularly preferably at least 99%, most preferably at least 99.5% and especially at least 99.8. The aim is reaction conditions which lead to a complete conversion.

Usually, the total residence time in stage a) and b) together is less than 5 hours, preferably less than 4 hours and more preferably less than 3 hours.

The discharge of the reaction mixture from (b) can be fed directly to the ammonia separation (c) at complete conversion of the amine groups to the urethane or it is supplied to achieve a complete conversion to another reactor or reactor system. As reactors, further tubular reactors, mixed reactor cascades or columns with the necessary average residence time can be used.

If the conversion, based on amino groups in the amine used to form urethane groups, is not complete after leaving the tubular reactor and is, for example, less than 95%, then the discharge can be further reacted.

For this purpose, the reaction mixture can be allowed to react to complete the conversion in a further tubular reactor or else in a back-mixed reactor, preferably until the conversion is 98% or more.

Under backmixed reactor system is understood here that the Bodensteinzahl the reactor system is less than 5, preferably less than 4, is.

c) Ammonia separation

For the separation of the ammonia columns are advantageously used, preferably the ammonia is separated by distillation. This creates a good separation between the alcohol and ammonia. Usually, the separation takes place in a pressure range of 0.01 to 20 bar, preferably 0.04 to 15 bar. The necessary temperatures depend on the alcohol or alcohol mixture used. For n-butanol, the temperature is for example at 60-150 ⁰ C, preferably at 80 to 140 ⁰ C.

It has proved to be advantageous to separate the resulting ammonia immediately from the reaction mixture, so that an occupancy by ammonium carbamate, which is formed in minimal amounts of ammonia and carbon dioxide by decomposition of urea, can be avoided.

This distillation unit is of a known type and has the usual installations. In principle, all standard installations are suitable as column internals, for example trays, packings and / or fillings. From the floors are Bubble trays, sieve trays, valve trays, Thormann trays and / or dual-flow trays are preferred; of the trays are those with rings, spirals, calipers, Raschig, Intos or Pall rings, Barrel or Intalox saddles, Top-Pak etc or braids preferred. Preferably, packs are used.

The distillation column preferably has 10 to 20 theoretical plates.

The distillative separation can be assisted by a gas which is inert under the reaction conditions (stripping gas). Such stripping gases are, for example, those having an oxygen content of less than 2% by volume, preferably less than 1% by volume, more preferably less than 0.5% by volume, nitrogen, argon, helium, nitrogen-noble gas mixtures are preferred, and nitrogen is particularly preferred.

The mixture taken off as vapors contains as the main constituent, for example more than 50% by weight, preferably more than 65% by weight, more preferably more than 75% by weight, very preferably more than 85% by weight and in particular more than 90% by weight % Ammonia. Further constituents of the vapor stream are the alcohol used, the ether formed therefrom, the dialkyl carbonate or carbamates formed from the alcohol, and further gaseous constituents such as carbon dioxide, nitrogen and oxygen. This stream can then be utilized, preferably burned, as described below in step (i).

d) removal of the excess alcohol

Alcohol, dialkyl carbonates, if they are formed or present in the reaction mixture, or alkyl carbamates or mixtures of at least two of these components are then removed from the resulting ammonia-depleted reaction mixture and preferably recycled to the reaction stage (a) and / or (b).

To separate off the components, the reaction mixture is advantageously expanded from the pressure level of reaction stage (b) to a pressure in the range from 1 to 500 mbar, preferably from 10 to 100 mbar. This gives gaseous vapors (dι_) containing the predominant amount of alcohol and 0 to 30 wt.%, Preferably 1 to 10 wt.% Dialkyl carbonate and / or 1 to 50 wt.%, Preferably 1 to 20 wt.% Carbamidsäurealkylester, and a liquid discharge, which consists essentially of the monomeric urethane, preferably diurethane, and optionally contains oligourea-polyurethanes and high-boiling oligomers.

The resulting vapors (dι_) are separated in subsequent suitably distillative purification stages, preferably by rectification, and the product of value isolated here alcohol and Carbamidsäurealkylester, individually or as a mixture, preferably recycled to the reaction step (a) to form the monomeric urethanes.

This distillation unit is of a known type and has the usual built-in. In principle, all standard installations are suitable as column internals, for example trays, packings and / or fillings. Of the trays, bubble-cap trays, sieve trays, valve trays, Thormann trays and / or dual-flow trays are preferred; of the trays are those with rings, coils, calipers, Raschig, Intos or Pall rings, Barrel or Intalox saddles, Top-Pak etc. or braids preferred. Preferably, packs are used.

The distillation column preferably has 10 to 20 theoretical plates.

For distillative removal of the alcohol or the alcohol mixture, a so-called flash is preferably used. This apparatus may be a container or a combination of container and column, preferably a column, wherein in the head of the alcohol or the alcohol mixture and in the bottom, the urethane can be withdrawn. In the top of the column, in addition to the alcohol, more easily than the urethane boiling substances may be included. The separation takes place in a pressure range of 0.001 to 2 bar, preferably 0.02 to 0.5 bar.

In a preferred embodiment of the present invention is dispensed as complete as possible separation of ammonia and alcohol, as described above under c). For this purpose, the stages c) and d) are combined in one stage, so that the number of separation stages in such a combined stage is less than the sum of the separation stages in stages c) and d).

It is accepted that the vapor stream in addition to ammonia may still contain alcohol and optionally other low boilers, for example, up to 5% by weight, dialkyl carbonates or carbamates, which then contribute positively to the energy balance in the thermal utilization (stage i)) by combustion. The alcohol is obtained in the separation with a purity of more than 98% by weight, so that it can be readily used again in the reaction, ie in step a) and / or b).

Another advantage is that the separation apparatus can be made simpler in such a combined step than in two separate stages, e) urethane purification

The liquid reaction mixture (d) containing the monomeric urethanes, preferably diurethanes, and optionally oligourea-polyurethanes and high-boiling oligomers, in the reaction step (d) after removal of the vapors, can be led either completely into the subsequent stage or is preferably divided into two partial streams, the weight ratio of the partial amounts 5 to 50:95 to 50 parts by weight, preferably 10 to 30:90 to 70 parts by weight.

The equal or preferably smaller subset is separated by distillation by means of a conventional distillation unit, preferably a thin film evaporator, at a temperature of 170 to 240 ⁰ C, preferably from 180 to 230 ⁰ C and under a pressure of 0.001 - 1 bar, preferably 0.002 - , 01 bar, in a desired product which contains the urethanes, preferably diurethanes and lower-boiling by-products (eι_), and non-distillable by-products (a), which are separated from the manufacturing process and usually discarded as non-recyclable residue. This can be performed in a preferred embodiment in the thermal utilization. The desired product (distillate) is combined with the same large or preferably larger other subset and fed the combined urethane, preferably diurethane-containing reaction mixture of the thermal cleavage (f).

By this process measure in the reaction stage (e), the proportion of non-distillable by-products in the reaction mixture, which form in the successive partial reactions and would constantly accumulate through the recycling recyclable feedstocks in the reaction cycle, to a content of 3 to 30 wt.% , Preferably 5 to 20 wt. Limited%, thereby ensuring a high selectivity trouble-free reaction.

Thin-film evaporators or short-path evaporators can be used as distillation devices. The urethane is distilled at pressures of 0.001-1 bar, preferably in the range of 0.002-0.01 bar. The distillate (eι_) is the cleavage (f) supplied.

The high boiler-containing bottoms (a) is preferably discarded or, less preferably, partially fed to the reurethanization (h). The high-boiling marsh (a) can be performed in a preferred embodiment in the thermal utilization.

f) urethane cleavage

The reaction mixture obtained in the reaction step (e), preferably diurethanes containing reaction mixture is in a suitable device, preferably solvent-free in the liquid phase in the presence of catalysts at temperatures of 200 to 300 ° C, preferably 220 to 280 ⁰ C and under reduced pressure of 0.01 - 0.6 bar, preferably thermally split in the range of 0.02 - 0.1 bar. The conversion of urethane to isocyanate, preferably of diurethane to diisocyanate, in the thermal cleavage device can be chosen largely freely depending on the urethane used and is expediently in a range from 10 to 98% by weight, preferably from 40 to 90% by weight. the amount of urethane supplied. The uncleaved portion of the reaction mixture containing unreacted urethanes, ON-urea-urea-polyurethanes, high-boiling oligomers and other recyclable and unworkable by-products is separated, continuously discharged from the cleavage apparatus (fa) and directly or optionally after reaction with alcohol in the Reurethanization (h) in the reaction stage (a) and / or (b) recycled.

As catalysts for the chemical cleavage of the urethanes find e.g. the aforementioned urethane formation catalyzing inorganic and organic compounds use.

Particularly useful and therefore preferably used are dibutyltin dilaurate, iron (III) acetylacetonate, cobalt (II) acetylacetonate, zinc acetylacetonate, zirconium tetra-n-butoxide and tin (II) dioctoate.

As cleavage devices are, for example, cylindrical cleavage reactors, such. Tubular ovens or, preferably, evaporators, for example thin-film or bulk evaporators, e.g. Robert evaporator, Herbert evaporator, caddle-type evaporator, Plattenspalter and preferably Heizkerzenverdampfer.

The separation of the cleavage products takes place in a column in which usually the isocyanate in the side (fivi) and the alcohol (fι_) are taken off at the top.

g) isocyanate purification

The crude isocyanate mixture is freed in a subsequent distillation of recombination products, by-products and, if present, the solvent. The by-products are preferably recycled to the thermal cleavage. A part can also be removed.

The cleavage products formed in the thermal cleavage, which are composed primarily of alcohol, isocyanate, preferably diisocyanate, and partially cleaved urethanes, are then advantageously by means of one or more distillation columns, preferably by rectification at temperatures of 100 to 220 ⁰ C, preferably 120 to 170 ⁰ C and a pressure of 1 to 200 mbar, preferably 5 to 50 mbar, in low boilers and especially alcohol (gι_) and a crude Isocyana- natmischung (gu) with an isocyanate content of 85 to 99 wt.%, Preferably from 95 to 99% by weight separately. The higher-boiling by-products (gH) obtained in the distillative separation and in particular the uncleaved and partially split urethanes are preferably fed into the cleavage apparatus (f) and / or reurethanization (h).

The subscript "L" is used to denote low-boiling streams of the individual stages, with the index "H" high-boiling and "M" medium-boiling. The crude isocyanate mixture (gu), preferably obtained by rectification, is purified by distillation at a temperature of from 100 to 180 ^° C. and under a pressure of from 1 to 50 mbar, the individual fractions being recycled or isolated as a pure product. As has already been stated, in the case of the preferred purifying distillation, the top fraction, which preferably consists of isocyanate, in particular diisocyanate, optionally after reaction of the free isocyanate groups with alcohol, is recycled to reaction stage (a) and / or (b), which Side fraction consisting of pure isocyanate, in particular diisocyanate, preferably having a purity of at least 98 wt.%, In particular more than 99 wt.% Is derived and fed to storage and the bottom fraction containing as essential components the partially split urethanes and isocyanates is preferably recycled to the cleavage device for thermal cleavage.

However, according to other process variants, the sump fraction (giπ) can also be recycled to the distillation column (d) for the separation of crude isocyanate and alcohol or into the reaction stage (a) and / or (b), the urethane formation. It is also possible to divide the sump fraction into 2 or 3 product streams, these preferably being recycled in the urethane formation (a) and / or the cleavage apparatus (f) and optionally in the distillation column (g) and / or in the reurethanization (h).

h) reurethanization

The reaction effluent (fa) from f) and / or distillation residues (gH) from (g) are preferably recycled to the process. In this case, the isocyanate groups contained in this mixture and / or allophanates and / or ureas or other reactive constituents are converted to urethanes with alcohol. It is possible, these reactions in separate reactors such. B. mixed reactors or flow tubes or in (b) perform.

For the alcoholysis of the residues temperatures of 100 - 250 ⁰ C, preferably 150 - 220 ⁰ C are required. The mean residence times are in the range of a few minutes to hours. In general, the reaction is preferably carried out in a single phase in the liquid phase. The pressure during the reaction is of no particular importance except that it should be sufficient to keep the reaction mixture in the liquid phase.

For this purpose, for example, the streams (fa) and / or (gH) and optionally a portion of the stream {eπ) are combined with alcohol, wherein the molar ratio of NCO groups or their equivalents, ie, for example urethane, to hydroxy groups from the alcohol to to 1: 100, preferably up to 1: 20, more preferably up to 1: 10. The alcohol may be, for example, the low-boiling stream (dι_) from stage (d) and / or the alkoho-containing stream (H) from the urethane cleavage (f) and / or fresh alcohol.

If a catalyst should be used, it is preferably the same catalyst as in the step of urethane formation (b).

The reaction mixture is for reurethanization in the presence or absence of catalysts within 1 to 150 minutes, preferably 3 to 60 minutes at a temperature of 20 to 200 ⁰ C, preferably 50 to 170 ⁰ C at a pressure of 0.5 to 20 bar, preferred 1 to 15 bar implemented.

The reaction can take place in a continuous boiler cascade or in a tube reactor.

Suitable catalysts are in principle all compounds which promote the reaction of NCO- with OH groups or the cleavage (alcoholysis) of high molecular weight compounds. Examples include tin octoate, dibutyltin dilaurate, tin chloride, zinc dichloride, tin (II) dioctoate and triethylamine.

(i) recovery

According to the invention, the ammonia separated off in stage c) or the stage combined from stages c) and d) is at least partially utilized, preferably thermally utilized.

One possibility of non-thermal utilization is to carry the separated ammonia into a plant for the production of ammonia by-products, preferably the production of urea or melamine by processes known per se. If the ammonia is passed into the production of urea, Thus, with regard to the process according to the invention, the material cycle is closed, since the urea produced in this way can be recycled to a process for the preparation of isocyanates, for example by the process according to the invention.

It is advantageous if the content of organic material contained in the separated ammonia is separated off in advance in order to avoid side reactions in the subsequent stage. This is preferably done by absorption on a suitable material, for example activated carbon, alumina, silica gel or titanium dioxide. This material can be regenerated after loading, for example by desorption or by flaming. Preferably, however, the thermal utilization of the separated ammonia, in the ammonia with oxygen or an oxygen-containing gas, preferably air, is burned.

For this purpose, the reactions known to the expert for the combustion of ammonia are suitable, the reaction (M) is preferred.

4NH ₃ + 5O ₂ → 4NO + 6H ₂ O

as the first stage of the Ostwald process for the production of nitric acid. This reaction is usually followed by further oxidation according to 2 NO + O ₂ → 2 NO ₂ ≠N ₂ O ₄

and absorption of the resulting nitrogen dioxide in water under nitrogen monoxide release.

It is not crucial according to the invention in which form the Ostwald process is carried out.

If the ammonia is oxidized to nitrogen oxides according to Equation (M), according to the invention preferably no further residues from the process for the preparation of the isocyanates, for example the stream (eπ) or (fa), are passed into the combustion.

The Ostwald process can be carried out, for example, as a monoprinting process in which ammonia combustion and NO _x absorption take place essentially in the same pressure range. This can be medium pressure (230 - 600 kPa) or high pressure (700 - 1 100 kPa).

The method can also be practiced as a dual-pressure method. As a rule, combustion takes place at 400-600 kPa and absorption at 900-1400 kPa.

The temperatures for the oxidation generally be between 700 and 1000 ⁰ C, aeration vorzugt 800 to 900 ⁰ C and more particularly 830-850 ⁰ C.

The ammonia-air mixture fed into the ammonia combustion should have an ammonia content of not more than 1 1% (under high-pressure combustion) or not more than 13.5% (under medium-pressure combustion).

As catalysts known contacts are used. Preference is given to precious metals, particularly preferably platinum catalysts, which may optionally be incorporated with 5-10% rhodium and / or 5% palladium. The catalysts are usually used in the form of nets, sieves, braids or wires or moldings molded therefrom, or as noble metal-coated carrier, such as ceramic lattice.

The wires used to construct the screens generally have a diameter of 0.06-0.076 mm. The sieves are very fine-meshed and can have 500 to 2000 openings / cm ² , preferably 800 to 1500 and particularly preferably around 1000 openings / cm ² .

However, the reaction (i2) is particularly preferred.

4NH ₃ + 3O ₂ → 2N ₂ + 6H ₂ O

in which ammonia is burned to molecular nitrogen (N ₂ ).

Optionally, this reaction can be combined with nitric oxide (i3) decompproportionation.

4NH ₃ + 6NO → 5N ₂ + 6H ₂ O

to molecular nitrogen.

Alternatively, an ammonia decomposition according to (i4) is also conceivable:

2 NH ₃ → N ₂ + 3 H ₂

Reactions (M) to (i3) are exothermic (M: -904 kJ / mol, i2: -1268 kJ / mol, i3: -1808 kJ / mol), so that the energy liberated in these reactions is preferably used in the process for the preparation of the isocyanates can be fed, for example for the production of steam for the evaporator of distillations or particularly preferably for the cleavage (f).

The ammonia in the exhaust gas can preferably be burned according to reaction (i3). This can be done as known in the art, for example, K. Steinbrunner, R. Becker, H. Seifert, "combustion of NH ₃ -containing process gases", Chem.-Ing.-Tech. 67 (1995) No. 2, 199-202.

This usually high temperatures of 800 ⁰ C and more are required, in which a part of the ammonia are converted to nitrogen oxides. In addition to a residual content of a few percent of ammonia, part of the ammonia is further oxidized to nitrogen oxides (NO _x ), which in turn are environmental pollutants. Nitrogen oxides (NO _x ) are understood to mean exclusively compounds of any stoichiometry consisting of nitrogen and oxygen, for example N ₄ O, N ₂ O, NO, N ₂ O ₃ , NO 2, N ₂ O 5 and NO 3 (according to Römpp-Online, keyword nitrogen oxides, document RD-19-04121 , March 2002).

The reaction can be carried out on catalysts in which ammonia is oxidized in an excess of oxygen to nitrogen. (See: EP 686423 A2, EP 706816 A1)

These may, for example, be catalysts which contain silver, beryllium, magnesium, zinc or aluminum in the form of the metals or titanium, vanadium, iron, tungsten, nickel, cobalt, copper, chromium, uranium, molybdenum or tin or mixtures thereof in the form of their oxides and / or sulfides, preferably copper, cobalt, iron, chromium, nickel, vanadium or manganese or mixtures thereof n form their oxides.

Suitable supports are aluminum oxide, silicon dioxide and titanium dioxide in the form of any desired shaped bodies, for example rings, strands or tablets for full contacts or honeycomb structures for coated catalysts.

Particularly suitable are catalysts based on platinum.

During the thermal combustion of ammonia, nitrogen is produced in the first place, but nitrogen oxides (NO _x ) are also formed.

This happens on the one hand via the reaction (M), but also through the reactions

4NH ₃ + 7O ₂ → 4NO ₂ + 6H ₂ O

and or

2NH ₃ + 2O ₂ → N ₂ O + 3H ₂ O

In addition, a residual amount of ammonia can remain unburned.

By selecting the reaction conditions, the partial conversion of ammonia converts the reacted ammonia completely and without substantial formation of nitrogen oxides into nitrogen. However, part of the ammonia remains unreacted.

If, on the other hand, a higher conversion is set, for example by increasing the temperature of the catalyst, nitrogen oxides are generally produced as a by-product, so that the exhaust gas contains not only nitrogen but also amounts of ammonia and nitrogen oxides. It is then preferred to pass the exhaust gas of the combustion of ammonia into a catalytic reduction plant, in which ammonia and nitrogen oxides are converted into nitrogen, for example, according to reaction (i3).

Since the NO _x content increases, the higher the ammonia conversion is set, the residual amount of ammonia which is not reacted in the ammonia incineration plant should stoichiometrically equal to the amount of NO _x as possible, for example at most 15%, based on the ammonia content in the supplied exhaust gas.

Since the nitrogen oxides can then be reduced to nitrogen in a catalytic reduction plant, the residual amount of ammonia, which remains unburned, should correspond in mole percent according to equation (i3) at most to the amount of nitrogen oxides formed.

The exhaust gas obtained from the regenerative ammonia combustion plant, containing a small residual amount of ammonia and nitrogen oxides, is then fed to a catalytic reduction, a so-called SCR plant (selective catalytic nitrogen oxide reduction).

While the temperature in the combustion chamber of the ammonia combustion plant for the combustion of ammonia 700 ⁰ C to 1000 ⁰ C, preferably 800 ⁰ C to 900 ⁰ C, the temperature of the catalyst of the catalytic reduction plant is preferably between 180 ⁰ C and 500 ⁰ C, in particular between 200 ° C and 380 ⁰ C. For the SCR system can be used commercially available honeycomb DENOX catalysts, preferably based on titanium dioxide, as they are largely insensitive to catalyst poisons and sulfur compounds. Typical residence times are 0.2 - 0.3 seconds.

If the stoichiometry of ammonia to nitrogen oxides is not yet sufficient, then a small partial flow of the raw, highly loaded with ammonia exhaust gas of the SCR system is fed directly regulated.

The temperature required for the implementation of the ammonia of z. B. 800 ° C to 900 ° C in the combustion chamber of the ammonia combustion plant may possibly arise solely by the combustion of ammonia. At concentrations above 3 g / m ^{3, there} is excess energy that can be used to heat the pre-purified gas in the SCR unit.

However, this released energy is preferably fed into the process for producing the isocyanates, for example for producing steam for the evaporators of distillations or particularly preferably for the cleavage (f). However, if the ammonia concentration is low, the combustion chamber temperature is maintained at the desired operating temperature with a burner or similar heater.

For this purpose, additional heating material can be metered from the outside, for example natural gas, petroleum or naphtha and / or preferably waste streams from the process for producing the isocyanates.

A portion, for example, 5 to 25% by volume of the hot combustion chamber air can be diverted from the combustion chamber directly to heat the catalyst of the SCR system.

The exhaust gas, which exits from the ammonia combustion plant with a temperature of, for example, 40 ⁰ C to 100 ⁰ C, is passed to further heating by a heat exchanger, which is heated by the exiting the SCR system clean gas.

In order to compensate for the heat exchanger loss, the hot combustion chamber air is now supplied to the pre-purified exhaust gas leaving the heat exchanger.

The addition of the crude, ammonia-rich, exhaust gas to reduce NO _x in the exhaust gas pre-cleaned in the regenerative thermal post-combustion unit is adjusted so that the NCv content in the clean gas exiting the SCR unit is below the allowable limit, which is typically below 500 mg / m ³ , preferably below 350 mg / m ³ , more preferably below 200 mg / m ³ , most preferably below 150, especially below 100, especially below 75 and even below 50 mg / m ³ iN (dry, in the normal state, as NO2 calculated) amounts to. However, a low, although well below this limit NCv content is left in the clean gas, otherwise there is a risk that a portion of the ammonia, which is fed to the SCR system breaks through. It should be noted that usually no ammonia should escape into the environment. In general, it is possible to adjust the content of ammonia in the exhaust gas to not more than 5 ppm, preferably not more than 4 ppm, more preferably not more than 3 ppm, most preferably not more than 2 ppm and especially not more than 1 ppm.

Accordingly, the addition of the crude, ammonia-rich exhaust gas to the pre-purified exhaust gas from the ammonia combustion plant is controlled by the NO _x concentration in the clean gas exiting the SCR system. For this purpose, the clean gas line of the SCR system is provided with a NCV measuring device, which controls a valve provided in the bypass line which connects the raw gas line with the line for the pre-cleaned exiting from the regenerative thermal afterburner exhaust gas. At a low concentration of, for example, less than 1 g / m ³ remaining traces of ammonia in the exhaust gas can be removed by an acid wash.

The ammonia can also be burned together with organic material. This may be, for example, natural gas, petroleum or naphtha and / or preferably waste and / or by-product streams from the process for producing the isocyanates. The ammonia separated off from the preparation of the isocyanates and incinerated can, as described above, contain a proportion of separated alcohol and other by-products of the process, which can also be burned with it. However, co-combustion with organic material can lead to increased NO _x levels.

Strong NO _x formation can be significantly counteracted by lowering the flame peak temperatures, reducing the available oxygen in the reaction zone, uniform and rapid mixing of the reactants in the flame, reduction of the residence time at high temperatures and reduction of already formed nitrogen oxides at the flame end. For this purpose, numerous devices and methods, such as internal and external flue gas recirculation or water and steam addition have been developed. The internal and external flue gas recirculation is a process in which the flue gas is cooled either in a sufficiently large combustion chamber or by elimination from the combustion chamber and recirculated at a suitable point to reduce the flame peak temperature. Particular importance is attached to the so-called internal recirculation, which involves intensive mixing of the flame gases with relatively cold furnace gases.

Also significant for reducing the NO _x - content in the exhaust gas and also in nitrogenous fuels is effective two-stage combustion with a so-called stepped addition of air. In this case, only so much combustion air is supplied to the fuel in the first combustion stage that there is no excess of oxygen, so that the nitrogen-containing component contained in the fuel is degraded by the substantially reducing conditions to molecular nitrogen. In the second combustion stage then takes place combustion with excess oxygen, in which, however, only the thermal NO _x formation, which plays a lesser role compared to the formation of NO _x from nitrogen-containing components contained in the fuel, takes place.

A similar solution is also based on the so-called three-stage combustion, wherein initially by combustion with excess oxygen, a nitrogen-containing fuel is selectively implemented under NO _x formation, while in a second

Combustion stage by an excess of organic material and lack of oxygen by reducing conditions, the NO _{x formed is} again largely reduced to molecular nitrogen. In the subsequent third Combustion stage takes place again only thermal NO _x formation, which plays a minor role in the total NO _x formation.

In reversal of the formation equation for ammonia generally known to those skilled in the art from the components nitrogen and hydrogen, which usually takes place at high pressures and high temperatures, ammonia can be prepared according to equation (i4) at low to normal pressure and elevated temperatures according to equation (i4). at least partially decomposed into its starting components nitrogen and hydrogen.

These gases obtained from the cleavage can then preferably be thermally utilized in a subsequent combustion.

This reverse reaction proceeds particularly advantageously and almost quantitatively in the presence of catalysts, in particular in the presence of metals or metal oxides. Advantageously, therefore, the decomposition of the ammonia-containing gas stream according to one embodiment of the method according to the invention is catalyzed by metals or metal oxides. In this document, the term "catalyst" is used synonymously both for catalytically active metals and for metal oxides.

If the ammonia-containing gas stream is catalytically cleaved using catalysts, preferably in a cleavage reactor, a split gas containing substantially only ISb, H2, and optionally CO and / or CO2 and / or H2O, is obtained, taking into account the necessary stoichiometric ratios. if organic components are contained in the ammonia-containing gas stream. Such a gas mixture is thus largely free of ammonia and NO _x and can then be burned by known methods to avoid thermal and prompt NO _x formation. By the energy released during this combustion, in particular by the combustion of hydrogen, the energy required to split the ammonia can be compensated, so that the process can be made energy efficient.

For catalytic cleavage of the ammonia-containing gas component, preference is given to using a metal and / or metal oxide-containing catalyst. When using metals as catalysts for cleaving the ammonia-containing gas stream, it is advantageous if at least the metal used for the catalytic cleavage contains sub-group elements such as chromium, titanium, niobium, molybdenum, nickel, vanadium or iron, for example in the form of a mixture or alloy ,

Optionally, the individual metals can also be used in pure form. The use of stainless steel as a catalyst has proven to be particularly advantageous and therefore preferred, with the stainless steel containing the elements listed being mixed in varying compositions and different amounts.

For this purpose, in particular such stainless steels in question, as described for example in DIN 17440 or in DIN 17441. Particularly suitable is the DIN 17440 with the short name XΘCrNiTi 18 10 designated stainless steel. In this case, it is generally sufficient if the cleavage reactor itself or at least its inner surface coming into contact with the ammonia or the ammonia-containing gas mixture consist of the catalytically active catalyst metal or contain this at least proportionally. An increase in surface area, for example by grinding or roughening, can be carried out as required, or by use in the form of shaped articles, for example rings, tubes, chips, nets, sieves, wires, fabrics, braids and packages, whereby the catalytically active surface is increased. If necessary, the catalyst itself can be made more compact.

Also preferred and regarded as equivalent in terms of efficiency in the catalytic cleavage of ammonia-containing gas components is the use of metal oxides. Basically suitable for this purpose are the oxides of the abovementioned metals, wherein the use of nickel oxide as catalyst is particularly preferred. In the case of the use of metal oxide as catalyst, the catalyst is usually present in powder form in a catalyst bed in the cleavage reactor, but it is also conceivable a fluidized bed. Likewise, other catalyst geometries can be used, for example in the form of honeycomb bodies or the like.

In practice, in the pyrolysis, ie a thermal decomposition, of nitrogen-containing organic compounds or by admixtures of organic material in ammonia-containing streams NO _x , HCN and NH3 arise. The combustion of such gas streams containing these substances then leads to significant NOx contents of the combustion exhaust gases. The NO _x , HCN and NH 3 components in the combustion exhaust gas must therefore be suppressed with great technical effort either during the combustion process (primary measures), or subsequently removed from the exhaust gas (secondary measures).

The cleavage of the ammonia-containing gas stream expediently takes place in a tubular, catalyst-containing or catalyst metal-containing or catalytic metal reactor having circular, oval, rectangular or square cross-sections, where appropriate other forms such as training as a star or n-corner may be useful , The tubular reactor is also referred to below as the "fission reactor". Optionally, an external source of energy may be used to heat the cleavage reactor, but it is particularly advantageous if the desired catalyst temperature is at least substantially maintained by the energy available from the burner firing output (see below). For this purpose, it is particularly preferable to use the residues from the preparation process of the isocyanates, of which the streams {eπ), (fa) and / or (gH) are particularly preferred.

Advantageously, the cracking gas formed in the decomposition and intended for combustion contains only the components ISb, Hb and optionally CO, CO2 and H2O, since this ensures optimum combustion having low NO _x emissions. Optionally, a carbonization (in oxygen deficiency) or strong NO _x formation (in excess of oxygen) occur and interfere with the cleavage process sensitive. The presence of optionally formed during the cleavage of water in the cleavage gas stream does not interfere with the combustion. A particularly high degree of effectiveness is exerted by the catalysts used in the cleavage when they are brought to an elevated temperature. Satisfactory results can be achieved in a temperature range of about 200 ⁰ C to about 1200 ⁰ C, wherein it is particularly advantageous if the catalyst has a working temperature of about 500 ° C to about 1000 ⁰ C, preferably about 650 ° C to 1000 ° C. , reached.

An external energy source can be used to heat the cleavage reactor, but it is particularly advantageous if the desired catalyst temperature is at least largely, preferably completely, set and / or maintained by the energy available from the burner's combustion output.

Optionally, for certain mixtures of ingredients in the pyrolysis gas, a highly exothermic reaction may occur (for example, in the presence of large amounts of NO _x with the simultaneous presence of H 2) which increases the catalyst temperature above the temperature range required for ideal reaction conditions. In this case, water can be added for cooling the reaction mixture, or external cooling of the cleavage reactor can take place by addition of water into the combustion chamber or through heat exchangers along the cleavage reactor.

It is useful, but not necessarily provided, that the gas fed into the cleavage reactor is exclusively or predominantly ammonia. It is equally possible that the ammonia is mixed with a gas stream originating from another source which is to be used, for example, as a carrier gas. The proportion of ammonia in the gas fed into the cleavage can therefore vary within the scope of the process within wide limits. Thus, proportions of ammonia of less than about 1 wt .-% of the total amount of gas possible, the upper limit is 100 wt.%. Below a proportion of about 1 wt .-%, for example at about 0.5% by weight or less, a implementation of the method is indeed possible, but no longer economically useful. However, between the limits of about 1 wt% and about 100 wt%, any amount of ammonia content is processable with the process. Thus, portions of for example, 10 wt .-%, 20 wt .-%, wt .-% 40, 60 wt .-% or 80 wt .-% of ammonia can be cleaved, and then NO _x -emissionsarm be burned in a Verbrennungsvor- direction.

The combustion can in principle be carried out in one stage, two stages or three stages. During combustion, internal and external flue gas circulation (see above) may be used, preferably internal flue gas circulation and additionally or instead water or steam may be added to lower the flame peak temperatures.

To ensure the lowest possible NOx emissions by using a single-stage combustion process, the cleavage of the ammonia-containing gas stream in the cleavage reactor should be as complete as possible before entering the combustion chamber, ie, the cleavage gas should only the components ISb, Hb and possibly CO and / or contain CO2 and / or H2O. How far the pyrolysis gas splitting should be carried out in the cleavage reactor, depends on the loading of the gas fed with ammonia. Contains the exhaust gas only small amounts of ammonia, then may be sufficient in the cleavage of, for example, 30, 40 or 50% conversion. In general, however, it will be so that the gas fed is loaded with larger amounts of ammonia. Since it is generally assumed that the entering into the combustion chamber ammonia is converted to at least 80% in nitrogen oxides, should contain only more than 200 ppm, advantageously highest about 150 ppm and more preferably at most about 100 ppm of ammonia in the cracking gas usually be. This means that the gap yield of the catalyst, for example based on an ammonia concentration of 100% before entering the cleavage reactor, should be at least 99.98%. Advantageously, the gap yield is at least 99.99% or more. Of course, higher fission yields result in even further reduction of NOx emissions.

At lower concentrations of ammonia, of course, a lower NO _x emission value can also be achieved with a lower cleavage efficiency of the cleavage reactors. Thus, for example, at a concentration of 10% ammonia before entering the cleavage reactor a gap performance of 99.9% is sufficient, for example, the

Concentration of 100 ppm in the gas to be fed into the combustion chamber. A further reduction of the ammonia content before entry into the cleavage reactor to, for example, 1% requires only a 99% slit yield of the catalyst. The corresponding Leistungserfordenisse for other ammonia concentrations are very easy to calculate by the expert himself. An "at least substantial" cleavage is therefore understood as meaning a gap yield which leaves at most 300 ppm, especially less than 250 ppm and in particular 100 ppm or less ammonia, in the gas flowing out of the cleavage reactor. Another possibility for regulation The cleavage performance of the catalyst consists in the suitable choice of the residence time of the gas stream to be cleaved in the cleavage reactor.

As a rule, the residence time should be about 0.1 to 10 seconds, with retention times of 0.2 to 5 seconds and in particular 0.3 to 3 seconds being particularly preferred. As a rule, with residence times in the range of about 0.5 to 2.5 seconds, very good results can be achieved. Examples of suitable parameters for the choice of residence time are: material of the catalyst, geometry of the catalyst, pressure drop over the catalyst, degree of turbulence, etc.

The residence time of the ammonia-containing gas to be split in the cleavage reactor is also dependent on its temperature. In general, it will be so that with increasing temperature of the cleavage reactor, a shorter residence time for at least extensive cleavage is necessary. While for the cleavage reactor of about 700 to 900 ⁰ C, the residence time of the gas in the cleavage reactor should be about 0.5 to 5 seconds, can be with an increase in the temperature necessary for at least substantial cleavage time significantly reduced. Thus, depending on the catalyst efficiency, for example, at cleavage reactor temperatures of 900 to about 1000 ^° C. with residence times of less than 0.5, preferably less than 0.4 and particularly preferably less than 0.3 seconds, an at least substantial cleavage of ammonia is possible to reach the exhaust.

The gas streams which can be disposed of by the process optionally contain, in addition to the ammonia, also components which may have a disturbing effect on the catalyst operation.

As a result of the operation of the cleavage reactor, which generally proceeds under reducing conditions, it is possible for hydrocarbons or at least largely pollutants built up from hydrocarbons to lead to soot formation and thus to deactivation of the catalyst surface. In particular, hydrocarbons such as methane, ethane, propane and higher, unbranched or branched paraffins, unbranched or branched olefins, but also alcohols such as methanol, ethanol, propanol or butanol lead to soot formation in the cleavage reactor and thereby to an occupancy of the catalyst. As a result, as a result of the restriction of the cleavage, the NO _x values can generally rise above the desired or admissible limit value after only a short time.

Such soot formation can be prevented by adding to the gas stream fed in a small amount of oxygen, which is just so great that oxidation of the impurities to a mixture of predominantly carbon monoxide and hydrogen gas takes place. In general, such an oxidation can be achieved in that about half of the oxygen necessary for complete combustion of the impurities is added to the gas. By the oxygen addition is Normally, no increase in the NO _x values in the combustion exhaust gas is observed, but values that correspond to those without hydrocarbon-containing impurities in the exhaust gas can be achieved in this way.

The combustion of the gas obtained from the decomposition (cracking gas) is preferably carried out in combustion devices using so-called impulse burners. Pulsed or high-speed burners are characterized in that they have a nozzle outlet for the combustion air, which accounts for the majority of the combustion air mass flow. As a result of the combustion air flowing out of the inlet nozzle (s) with a high exit impulse, an injector effect occurs in the vicinity of the outlet nozzles, through which combustion exhaust gases are sucked out of the heating chamber and admixed with the combustion air. The occurrence of such an effect is strongly dependent on the exit or flow velocity of the combustion air escaping the nozzle.

Advantageously, the combustion of the resulting fission gas can be carried out with natural gas-fired pulse burners, wherein the feed of the fission gas is in the backflow region of the flue gases of the pulse burner. This form of injection causes the fission gas to be charged with an increasing O2 concentration profile, which contributes to lowering the flame tip temperature.

It is also possible, the air used for combustion to an elevated temperature preheat up to 1000 ⁰ C. Particularly good results are obtained, however, when the air temperature about 100 ⁰ C to about 600 ⁰ C and in particular about 200 ° C to about 400 ° C is.

In general, it is believed that in pulse burners discharge speeds of about 80 to about 120 m / sec are required. However, this value can be even higher if the combustion chamber is constructed appropriately. Thus, the execution of erfindungsgernäßen process is easily possible even at exit velocities of more than 130, 140 or 150 m / sec. Lower discharge velocities than 80 m / sec are also generally no obstacle to carrying out the process described here. Thus, a corresponding gas backflow can also be achieved at discharge velocities of less than 80 m / sec, such as, for example, 70 or 60 m / sec. sec, realize. Exit velocities of less than 60 m / sec require a special design of the combustion chamber to produce a sufficiently strong exhaust gas backflow.

For the purposes of this document, impulse burners are understood to mean burners in which the exit velocity of the combustion air is high enough to ensure a sufficient exhaust gas return flow. The flow of the combustion gases present in the combustion chamber in the direction of the inlet nozzles for the combustion air is also referred to below as "return flow". Such impulse burners, increasingly for The conversion of fuel energy into heat are usually operated in one stage.

It is inventively preferred that the cleavage reactors are heated to cleave the pyrolysis. It is particularly advantageous if the cleavage reactor can be heated by the energy resulting from the firing capacity of the burner. In a particular embodiment, this is achieved in that the cleavage reactor protrudes at least partially into the interior of the combustion chamber.

With the multistage process according to the invention for the continuous production of organic isocyanates with recirculation and removal of the by-products, distillable isocyanates, preferably diisocyanate, can be prepared in high yields with high selectivity.

The inventive method is particularly suitable for the preparation of aliphatic diisocyanates, such as 2-methylpentane diisocyanate-1, 5, isomeric aliphatic diisocyanates having 6 carbon atoms in the alkylene radical and mixtures thereof and preferably hexamethylene diisocyanate-1, 6 and cycloaliphatic diisocyanates, in particular 3-isocyanatomethyl-3,5,5-trimethylcyclohexylisocyanate 4,4'-di (aminocyclohexyl) methane by an economical method.

The isocyanates prepared are outstandingly suitable for the production of plastics containing urethane, isocyanurate, amide and / or urea groups by the polyisocyanate polyaddition process. They are also used for the preparation of urethane, biuret and / or isocyanurate-modified polyisocyanate mixtures. Such polyisocyanate mixtures of aliphatic or cycloaliphatic diisocyanates are used in particular for the production of light-resistant polyurethane coatings and coatings.

According to a preferred embodiment, the overhead fraction obtained in the distillative purification of the crude isocyanate (f) is recycled to the reaction stage (a), the side fraction consisting of substantially pure isocyanate is added to a vessel for storage and the bottoms fraction to the reaction stage (a ) or (d) or (a) and (d).

The following examples are intended to illustrate but not to limit the invention.

Examples

example 1 In a continuously operated apparatus, consisting of a 20 liter pressure autoclave with heating jacket, a rectification column mounted on the reactor and a condensation system were continuously 1.01 kg / h of 1, 6-hexamethylenediamine (HDA) and 1, 2 kg / h of liquid urea dosed. The necessary for the reaction n-butanol of about 4.3 kg / h, which also acts as a solvent, in addition to the necessary reflux for the separation column of 6.3 kg / h was added to the top. The pressure in the reactor was 10.5 bar. The temperature was adjusted by the heating medium to 220 ⁰ C. The reactor left about 4.8 kg / h of a reaction mixture whose main components 39 Ma% n-butanol and about 43 Ma% dibutylurethane are.

During the reaction of the HDA ammonia was formed. This left together with n-butanol (boiling temperature) the reactor and was largely separated from butanol in the rectification column. The vapor stream from the column with a temperature of about 192 ⁰ C was condensed in a glass condenser (water cooling) at 160 ⁰ C for the most part. The amount of condensate was 6.3 kg / h during the trial setting. The gas flow leaving the condenser of about 150 l / h according to analyzes contained about 65% by mass of n-butanol and 34.6% by mass of ammonia and was well suited for incineration.

Example 2

In the apparatus as already described in Example 1, the same dosing diamine, urea and n-butanol is used, with the difference that the column is removed and the gas stream from the reactor is condensed directly in the cooler. Instead, the n-butanol feed of 4.2 kg / h is metered directly into the reactor. The condensation temperature in the cooler is maintained at 160 ^° C. by the cooling medium. The pressure in the apparatus is set by a pressure maintenance to 10.5 bar. The mean residence time of the liquid in the reactor, the thermal power introduced by means of Marlotherm oil, the discharge rate of about 4.8 kg / h and the n-butanol and dibutylurethane concentrations are approximately the same as those of example 1.

The elimination of the column, the reactor temperature can be lowered to only 212-214 ⁰ C, with the result that in the product a lower content of by-products occurs and the apparatus requires less regulatory effort by eliminating the column. The composition and the amount of the ammonia gas stream is approximately comparable to that in Example 1.

Claims

claims

1. A process for the preparation of isocyanates by reacting amines with

Urea and at least one alcohol to the corresponding urethanes to release ammonia, followed by cleavage of the urethanes in the corresponding isocyanates, characterized in that the liberated ammonia is at least partially utilized.

2. The method according to claim 1, characterized in that the liberated ammonia is at least partially thermally utilized.

3. The method according to claim 2, characterized in that the energy released in the thermal utilization is used at least partially for the cleavage of the urethanes.

4. The method according to claim 2 or 3, characterized in that the thermal utilization of the ammonia consists essentially in a reaction to molecular nitrogen (N2).

5. The method according to claim 2 or 3, characterized in that the thermal utilization of the ammonia consists essentially in a reaction to nitric oxide (NO).

6. The method according to claim 1, characterized in that the utilization of the ammonia consists essentially in a reaction to urea or MeI- amine.

7. The method according to claim 4, characterized in that the exhaust gases of the

Method containing a content of nitrogen oxides (NO _x ) below 350 mg / m ³ .

8. The method according to claim 2 to 5 or 7, characterized in that residues from the process are conducted together with ammonia in the thermal utilization.