HK1122792A - Process for the preparation of isocyanates in the gas phase - Google Patents

Description

Method for producing isocyanates in the gas phase

Technical Field

The invention relates to a method for producing isocyanates by reacting primary amines with phosgene under adiabatic conditions, said method being carried out at the boiling point of the amine and at an average contact time of 0.05 to 15 seconds.

Background

Various processes for preparing isocyanates by reacting amines with phosgene in the gas phase are known from the prior art. EP-A593334 describes a process for preparing aromatic diisocyanates in the gas phase, in which the reaction of diamines with phosgene is carried out in a tubular reactor which has no dynamic components (parts) but whose wall narrows along the longitudinal axis of the tubular reactor. However, this method is problematic because the function of mixing the educt (educt) streams only through narrowing of the tube walls is not as good as using appropriate mixing elements (elements). Poor mixing often results in the formation of large amounts of undesirable solids.

EP-A-699657 describes ｃA process for preparing aromatic diisocyanates in the gas phase, wherein the reaction of the appropriate diamine with phosgene is carried out in ｃA two-zone reactor, the first zone representing from 20 to 80% of the total reactor volume with the desired mixing system and the second zone representing from 80 to 20% of the total reactor volume with plug flow. However, since at least 20% of the reaction volume is ideally back-mixed, the resulting residence time distribution is not uniform, which can lead to the formation of large amounts of undesirable solids.

EP-289840 describes the preparation of diisocyanates by phosgenation in the gas phase. In the process disclosed herein, the reaction is carried out in a turbulent flow at 200-600 ℃ in a cylindrical chamber without a dynamic component. The omission of the dynamic components reduces the risk of phosgene leakage. In addition to the flow elements near the wall of the pipe, the turbulence in the cylindrical chamber (pipe) allows ｃA better flow uniformity distribution and thus ｃA narrower residence time distribution in the pipe to be obtained, which reduces the solids formation, as described in EP- ｃA-570799.

EP-A-570799 describes ｃA process for preparing aromatic diisocyanates in which the reaction of ｃA suitable diamine with phosgene is carried out in ｃA tubular reactor above the boiling point of the diamine at an average contact time of from 0.5 to 5 seconds. As described in this specification, both too long and too short reaction times lead to the formation of unwanted solids, and therefore, a process is disclosed in which the average deviation of the average contact time is less than 6%. This contact time is observed by carrying out the reaction in a tubular flow characterized by a reynolds number greater than 4000 or a Bodenstein number greater than 100.

EP-A-749958 describes ｃA process for preparing triisocyanates by the gas-phase phosgenation of (cyclo) aliphatic triamines having three primary amine groups, the triamines being reacted continuously with phosgene in ｃA cylindrical reaction chamber heated to 200-600 ℃ at ｃA flow rate of at least 3 m/s.

EP-A-928785 describes the use of ｃA micro-structured mixer in the phosgenation of amines carried out in the gas phase. The disadvantages of using such micromixers are: even in the case of very small amounts of solids, the formation of which cannot be completely ruled out in the isocyanate synthesis can lead to blockage of the mixer and thus to a reduction in the time available for the phosgenation apparatus.

WO03/045900 describes in detail the preparation of isocyanates on an industrial scale by gas-phase phosgenation. There are two possible technical approaches for carrying out the known gas phase phosgenation processes using a cylindrical reaction chamber, as described in WO 03/045900. In the first method, the reaction can be carried out in a single long tube, the diameter of which must be matched to the capacity of the plant. A disadvantage of this design according to WO03/045900 is that for very large production plants it is no longer possible to precisely control the temperature of the reactant stream in the flow centre by heating the tube walls. The local temperature inhomogeneities can lead to (a) decomposition of the product if the temperature is too high or (b) insufficient conversion of the educts into the desired isocyanate if the temperature is too low.

The second possible technical approach, i.e. dividing the reaction mixture into separate partial streams which are then passed in parallel through smaller separate pipes, the temperature of which can be better controlled on the basis of the smaller diameter of the pipes, is also considered to be disadvantageous in WO 03/045900. A disadvantage of this variant of the method according to WO03/045900 is that it is more susceptible to clogging if the volumetric flow rate through each individual conduit is not controlled. WO03/045900 confirms this by the following explanation: when one of the tubes has a deposit accumulated at a certain point, the pressure loss of the flow through the tube increases and the reaction gas is automatically switched to the other tubes remarkably. The result is that less gas flows through the pipe containing the precipitate, so that the flow through the pipe experiences ｃA longer residence time, as described in EP- ｃA-570799, which leads to an increase in the formation of solids.

In summary, WO03/045900 explains that in industrial gas phase phosgenations the use of one large tube has the problem of temperature control of the overall flow and the use of many small tubes risks uneven flow through these tubes.

According to the teaching of WO03/045900, if the reaction is carried out in a non-cylindrical reaction chamber (preferably a plate reactor, highly preferably providing advantageous temperature control of the reactants and having a width of at least 2 times the height), the above-mentioned disadvantages can be avoided and the continuous phosgenation of the amine can be advantageously carried out in the gas phase, with a significant increase in the number of operating hours of the production plant. As described in WO03/045900, there is generally no limitation on the height of the reaction chamber and the reaction can be carried out in a reaction chamber having a height of, for example, 40 cm. However, if a better heat exchange with the reactor wall is to be achieved, WO03/045900 teaches that the reaction should be carried out in a reaction chamber of small height (e.g. only a few centimeters or a few millimeters), and therefore with such reactor dimensions, as indicated in WO03/045900 in the review of EP-928758: even with the smallest amounts of solids, their formation cannot be completely avoided in the synthesis of isocyanates and can lead to blockage of the reactor, thus reducing the time available for the phosgenation plant.

Disclosure of Invention

Surprisingly, it has now been found that: the reaction of suitable primary amines with phosgene to prepare isocyanates can be carried out in the gas phase by ensuring an average residence time in the reaction chamber of from 0.05 to 15 seconds under adiabatic conditions. It is thus possible, advantageously and independently of the geometry of the reactor, to avoid temperature control problems and to obtain isocyanates on an industrial scale, with high space/time yields and a significantly high number of operating hours of the production plant.

Detailed Description

The invention provides a process for preparing isocyanates by reacting suitable primary amines with phosgene, wherein the phosgene and the primary amine are reacted above the boiling point of the amine at an average contact time of from 0.05 to 15 seconds, the reaction being carried out under adiabatic conditions.

Preferably, the process of the invention comprises one or more of the following steps a) to d), particularly preferably all steps a) to d) are carried out. Steps a) to d) are as follows:

a) vaporized amine (vaporized amine), optionally diluted with an inert gas or vapor of an inert solvent, and phosgene are separately heated to 200-600 ℃ and continuously mixed.

b) The reaction mixture consisting of vaporized amine and phosgene is passed continuously through the reaction chamber, while avoiding back-mixing, and reacted therein under adiabatic conditions with an average contact time of 0.05 to 15 seconds,

c) cooling the gas mixture leaving the reaction chamber to condense the isocyanate formed, the temperature being maintained above the decomposition point of the carbamoyl chloride corresponding to the amine reacted, and

d) the uncondensed isocyanate is separated from the gaseous mixture by washing with a liquid.

Preferably, the reaction chamber used in step b) has a rotationally symmetric geometry with a constant or increasing flow area in the flow direction of the reaction mixture. Preferably, the reaction chamber used is a tubular reactor having a substantially constant or increasing flow area in the direction of flow of the reaction mixture. In another preferred embodiment, the reaction chamber, preferably a tubular reactor, has a constant and increasing cross-section/section (section) of flow area in the direction of flow.

Embodiments of the invention in which the reaction chamber has a rotationally symmetrical geometry and a stepwise and/or continuous change of the flow area in the flow direction have the advantage that the flow velocity along the axial direction of the reaction chamber can be adjusted. The constant flow area in the direction of the fluid results in an acceleration of the flow due to the volume increase during the phosgenation. By suitably widening the flow area in the flow direction, the flow rate of the reaction mixture can be kept constant with respect to the reactor length, and thus the effective (available) reaction time can be increased for the same reactor length. This advantage is particularly important when reacting relatively non-reactive aromatic amines.

In the process of the present invention, primary amines may be used as starting materials. Preference is given to using primary amines which can be converted into the gas phase without decomposition. Particularly suitable amines, especially diamines, are those based on aliphatic or cycloaliphatic hydrocarbons having from 1 to 15 carbon atoms. Examples of preferred amines are 1, 6-diaminohexane, 1-amino-3, 3, 5-trimethyl-5-aminomethylcyclohexane (IPDA) and 4, 4' -diaminodicyclohexylamine. Particular preference is given to using 1, 6-diaminohexane (HDA).

Aromatic amines, preferably those which can be converted into the gas phase without decomposition, can also be used as starting materials for the process of the invention. Examples of preferred aromatic amines are Toluenediamine (TDA), in particular 2, 4-TDA and 2, 6-TDA and mixtures thereof; diaminobenzene; naphthalene Diamine (NDA); and 2, 2 ' -, 2, 4 ' -or 4, 4 ' -Methylenediphenyldiamine (MDA) or mixtures of these isomers. Tolylenediamine (TDA) is particularly preferred, especially 2, 4-TDA and 2, 6-TDA and mixtures thereof. Prior to carrying out the process of the present invention, the starting amine is typically heated to 200 ℃ to 600 ℃, preferably 201 ℃ to 500 ℃, most preferably 250 ℃ to 450 ℃ in a vaporized state, and optionally with an inert gas such as N₂He or Ar or an inert solvent such as an optionally halogenated aromatic hydrocarbon, for example chlorobenzene or o-dichlorobenzene, and introduced into the reaction chamber.

The vaporization of the starting amine can be carried out in known vaporization apparatuses. Preferred evaporation systems are those in which a small amount of operating hold-up is passed through a falling-film evaporator with a high circulation rate, wherein, in order to minimize the thermal stress of the starting amine, the evaporation process is optionally maintained as described above by feeding in an inert gas or a vapour of an inert solvent. The vaporized amine may also contain droplets of unvaporized amine (aerosol). Preferably, however, the vaporized amine is substantially free of droplets of unvaporized amine (i.e., up to 0.5 wt.% of the amine, more preferably no more than 0.05 wt.% of the amine, based on the total weight of the amine, is present as unvaporized droplets, and the remainder of the amine is present as a vapor). Most preferably, the vaporized amine is free of droplets of unvaporized amine. Preferably, after vaporization, the vaporized amine (optionally diluted with an inert gas or inert solvent vapor) is brought to the desired feed temperature by a post-heater.

In a preferred embodiment of the invention, the vaporization and superheating of the starting amine is carried out in several stages (stages) in order to avoid droplets of unvaporized amine in the vaporized amine stream. It is particularly preferred to use a multistage evaporation step, wherein a droplet separator is added between the evaporation and superheating system and/or the evaporation device also acts as a droplet separator. Suitable droplet separators are described, for example, in the following documents: "Droplet Separation", A.B. Turkholz, VCH Verlagsgesellschaft, Weinheim-New York-Basle-Cambridge, 1989. Particularly preferred droplet separators are those that result in low pressure losses. Most preferably, the vaporized amine is brought to the desired feed temperature by additionally acting as an afterheater for the droplet separator. Such an afterheater preferably has a liquid outlet for continuous emptying of the separator. The reactor operating time is significantly increased by making the vaporized feed amine stream substantially free of liquid droplets prior to its entry into the reactor.

In the process of the present invention, it is advantageous to use an excess of phosgene relative to amino groups, the molar ratio of phosgene to amino groups generally being from 1.1: 1 to 20: 1, preferably from 1.2: 1 to 5: 1. Phosgene is also heated to a temperature of 200 ℃ to 600 ℃ and optionally with an inert gas such as N₂He or Ar or an inert solvent such as an optionally halogenated aromatic hydrocarbon such as chlorobenzene or o-dichlorobenzene, which is then introduced into the reaction chamber.

The process of the invention is carried out in such a way that: the heated reactants are separately introduced into at least one reaction chamber, mixed and reacted under adiabatic conditions by observing a suitable reaction time. The isocyanate is then condensed by cooling the gas stream to a temperature above the decomposition point of the corresponding carbamoyl chloride (i.e. for example, toluenediaminyl chloride for TDA).

The residence time required to react the amine groups with phosgene to give the product isocyanate is from 0.05 to 15 seconds, depending on the type of amine used, the starting temperature, the increase in adiabatic temperature in the reaction chamber, the molar ratio of starting amine to phosgene and the extent of any dilution of the reactants with inert gas.

If, for a particular system (starting temperature, increase in adiabatic temperature, molar ratio of reactants, dilution gas, starting amine), the predetermined minimum residence time for complete reaction exceeds less than 20%, preferably less than 10%, the formation of secondary reaction products such as isocyanurates and carbodiimides can be largely avoided (extensivelly).

Within this very narrow range of contact times (spectra) for chemical reactions, the reactants must be mixed as homogeneously as possible and subsequent reactions must take place. The subsequent reaction is preferably carried out without back-mixing, which would have the effect of widening the contact period and thus increasing the formation of undesired by-products and secondary products.

When the process is carried out in practice, there may be deviations from the average contact time due to the time required to mix the reactants. If the reactants are not yet uniformly mixed, the reactor still contains an unmixed or partially mixed volume of gas, wherein there is still no or yet incomplete contact between the reactants. The reactants should therefore preferably be mixed up to at least 10 in a time of 0.01-0.3 seconds^-1Degree of separation (segregateddegree). The degree of separation is a measure of incomplete mixing (see, e.g., chem. -Ing. -Techn.44(1972), p 1051 and below; appl. Sci. Res. (The Hague) A3(1953), p 279).

Methods to achieve shorter mixing times are known in theory. Examples of suitable mixing devices include mixing units or mixing zones with dynamic or static mixing elements (elements) or nozzles. Static mixers such as those described in, for example, EP-A-1362847, EP-A-1526129 or EP-A-1555258 are preferred.

After the reaction components are mixed, the reaction mixture flows through the reaction chamber. Neither the mixing zone nor the adjoining reaction chamber have heating surfaces, which can cause thermal stress leading to secondary reactions such as the formation of isocyanurates and carbodiimides; there is also no cooling surface which can cause condensation resulting in deposition. The components are reacted under adiabatic conditions, the adiabatic temperature increase in the reactor being adjusted individually by the temperature, composition and relative proportions of the educt streams and by the residence time in the reactor.

The flow through the reaction chamber should preferably be in the form of about 90% plug flow, so that all parts of the flow volume have about the same flow time, thereby minimizing any further broadening of the contact time distribution between the reactants. The degree of achievement of an ideal plug flow (mean deviation from mean residence time of 0) is described in the flow technology by the Bowden number Bo (Fitzer, Techn. Chemie, Springer 1989, pp 288-295). Preferably, the Bowden number of the process according to the invention should be at least 10, preferably more than 100 and most preferably more than 250.

In step c), after the phosgenation has taken place in the reaction chamber, the gaseous mixture, which preferably contains at least one isocyanate, phosgene and hydrogen chloride and is free of isocyanate formed, continuously leaves the reaction chamber. This can be achieved in ｃA single stage by selective condensation, for example in an inert solvent, as has been recommended for other gas-phase phosgenations (EP-A-0749958).

Preferably, however, condensation is achieved by spraying one or more suitable liquid streams (quench liquids) into the gaseous mixture leaving the reaction chamber. This provides ｃA rapid cooling of the gaseous mixture without the use of cold surfaces, as described in EP- ｃA-1403248. Regardless of the type of cooling, however, the temperature of the cooling zone is preferably selected such that it is above the decomposition point of the carbamoyl chloride corresponding to the isocyanate and such that the isocyanate, and optionally the solvent concomitantly used as diluent in the amine vapor stream and/or the phosgene stream, is condensed or dissolved in this solvent, while excess phosgene, hydrogen chloride and any inert gases concomitantly used as diluent are passed through a condensation or quenching stage. Solvents kept at a temperature of 80 to 200 c, preferably 80 to 180 c, such as chlorobenzene and/or dichlorobenzene, or isocyanates kept at this temperature range, or mixtures of isocyanates with chlorobenzene and/or dichlorobenzene, are particularly suitable for selectively obtaining isocyanates from gaseous mixtures leaving the reaction chamber.

The generation of a flow of the gaseous reaction mixture, essentially in the form of a plug flow, essentially without back-mixing from the mixing zone through the reaction chamber, which is essential for the process of the invention, is ensured by the pressure difference between the educt feed line of the mixing zone and the outlet of the condensation or quenching section. Typically, the absolute pressure in the educt feed line of the mixing zone is 200-. However, the maintenance of the pressure difference is basically only for the purpose of ensuring a directional flow.

In step d), the gaseous mixture leaving the condensation or quenching section does not contain residual isocyanate with suitable scrubbing liquids in the downstream gas scrubber and thus does not contain excess phosgene in the known manner. This can be achieved by cold traps, by absorption in inert solvents (e.g. chlorobenzene or dichlorobenzene) or by adsorption on activated carbon and hydrolysis. The hydrogen chloride gas which passes through the phosgene recovery stage can be recirculated in a known manner in order to recover the chlorine required for the phosgene synthesis. The washing liquid obtained in step d) after use in the gas scrubber is then preferably used in step c) as quench liquid to cool the gaseous mixture leaving the tubular reactor.

The isocyanate is then purified, preferably by distillative work-up of the solution or mixture from the condensation or quenching section.

Examples

Example 1 "non-adiabatic phosgenation of TDA" (comparative example)

At 400 ℃ 20kg/h of a mixture of 2, 4-and 2, 6-toluenediamine in a weight ratio of 80% to 20% were vaporized and introduced into the tubular reactor in gaseous form. At the same time, in a parallel operation, 100kg/h of gaseous phosgene were heated to 310 ℃ and likewise introduced into the tube reactor. The streams are injected through nozzles into a mixing zone and mixed before entering the reaction chamber. The mixing zone is thermally insulated to prevent heat loss before and during mixing. The reaction chamber is not thermally insulated and is cooled by thermal radiation. The reaction conditions are therefore non-adiabatic. The final temperature of the gaseous mixture leaving the tubular reactor after 2.2 seconds was 380 ℃ and cooled by injection of o-dichlorobenzene. The isocyanate formed is condensed, washed and then worked up distillatively by known methods. The pressure difference between the TDA feed line and the condenser section was 200mbar in order to achieve a directional flow of gas between the feed line and the condenser section of the mixing zone. After 96 hours of reaction, the pressure in the TDA feed line increased sharply due to the formation of precipitates in the reaction which narrowed the reaction chamber of the tubular reactor at the tube wall. The formation of precipitates can be attributed to increased formation of by-products. The experiment therefore had to be terminated.

Example 2 "adiabatic phosgenation of TDA" (inventive)

20.5kg/h of a mixture of 2, 4-and 2, 6-toluenediamine in a weight ratio of 80% to 20% were vaporized at 320 ℃ together with 500kg/h of nitrogen and introduced into the tubular reactor in gaseous form. At the same time, in a parallel operation, 182kmol/h of gaseous phosgene and 1000kg/h of o-dichlorobenzene were heated to 360 ℃ and likewise introduced into the tube reactor. The streams are injected through nozzles into a mixing zone and mixed before entering the reaction chamber. The mixing zone and the reaction chamber are thermally insulated so that no additional heat input is generated by heating, nor is heat dissipation due to external cooling or heat radiation. The reaction is therefore carried out under adiabatic conditions. At the outlet of the reaction chamber, the final temperature measured using a surface thermometer was 405 ℃. The gaseous mixture leaving the reaction chamber after 5.5 seconds was cooled by injection of o-dichlorobenzene and the isocyanate formed was condensed, washed and then worked up distillatively by known methods. The pressure difference between the TDA feed line and the condenser section was 60mbar in order to achieve a directional flow of gas between the feed line and the condenser section of the mixing zone. Even when measured after 720 hours of reaction, the pressure difference did not increase, indicating that no precipitate was formed in the reaction. Examination of the reaction chamber also did not indicate the formation of residues.

Example 3 "adiabatic phosgenation of IPDA" (inventive method)

17.6kmol/h of isophoronediamine together with 42kg/h of nitrogen are vaporized, superheated to a temperature of 300 ℃ and introduced into the tube reactor in gaseous form. At the same time, in a parallel operation, 64kmol/h of gaseous phosgene were heated to 300 ℃ and likewise introduced into the tube reactor. The streams were mixed and entered the reaction chamber with a mixing time of 0.02 seconds. The mixing zone and the reaction chamber are thermally insulated so that no additional heat input is generated by heating, nor is heat dissipation due to external cooling or heat radiation. The reaction is therefore carried out under adiabatic conditions. At the outlet of the reaction chamber, the final temperature measured using a surface thermometer was 450 ℃. The gaseous mixture leaving the reaction chamber after 0.1 seconds is cooled by injection of monochlorobenzene and the isocyanate formed is condensed, washed and then worked up distillatively by known methods. The pressure difference between the IPDA feed line and the condenser section was 200mbar and the pressure difference between the phosgene feed line and the condenser section was 40mbar, in order to achieve a directional flow of gas between the feed line and the condenser section in the mixing zone. Even after 1000 hours of reaction, no pressure increase was observed. No significant residual precipitate was found in subsequent examination of the reaction chamber.

Although the invention has been described in detail in the foregoing for the purpose of illustration, it is to be understood that such detail is solely for that purpose and that variations can be made therein by those skilled in the art without departing from the spirit and scope of the invention except as it may be limited by the claims.

Claims (11)

1. A process for preparing isocyanates by reacting a primary amine with phosgene under adiabatic conditions in the gas phase at a temperature above the boiling point of the amine and at an average contact time of from 0.05 to 15 seconds.

2. The process of claim 1 further comprising (i) condensing the isocyanate in the primary amine and phosgene reaction mixture by cooling the isocyanate-containing gas stream to a temperature above the decomposition point of the corresponding carbamoyl chloride, (ii) removing excess phosgene from the primary amine and phosgene reaction mixture, and (iii) recycling the hydrogen chloride gas to recover the chlorine used in the phosgene synthesis.

3. The process according to claim 1, wherein prior to the reaction with phosgene the amine is vaporized and optionally diluted with an inert gas or a vapour of an inert solvent and heated to 200-600 ℃ to form vaporized amine substantially free of droplets of unvaporized amine.

4. The method of claim 1, further comprising:

a) separately heating amine gas optionally diluted with an inert gas or steam of an inert solvent and phosgene to a temperature of 200-600 ℃ and continuously mixing the amine and phosgene to prepare a gaseous reaction mixture,

b) continuously passing the gaseous mixture prepared in step a) through a reaction chamber without back-mixing, reacting therein an amine and phosgene under adiabatic conditions with an average contact time of 0.05 to 15 seconds, to form a gas stream containing isocyanate,

c) cooling the isocyanate-containing gas stream leaving the reaction chamber to a temperature above the decomposition point of the corresponding carbamoyl chloride of said amine to condense the isocyanate, and

d) the uncondensed isocyanate is separated from the gas stream by washing with a liquid.

5. The method according to claim 4, wherein the reaction chamber has a rotationally symmetric geometry with a constant or increasing flow area in the flow direction of the reaction mixture.

6. A method according to claim 4, wherein the reaction chamber has a cross-section with a constant and increasing flow area in the flow direction.

7. A process according to claim 4, wherein the gaseous mixture leaving the reaction chamber comprises at least one isocyanate, phosgene and hydrogen chloride and is cooled in step c) by injecting at least one liquid stream therein.

8. The process according to claim 7, wherein at least part of the scrubbing liquid obtained in step d) after use in the gas scrubber is used in step c) to cool the gaseous mixture leaving the reaction chamber.

9. The process according to claim 7, wherein at least part of the mixture obtained after condensation in step c) is used in step c) to cool the gaseous mixture leaving the reaction chamber.

10. The process according to claim 1, wherein the isocyanate is toluene diisocyanate, methylene diphenyl diisocyanate, dicyclohexylmethane diisocyanate, hexamethylene diisocyanate and/or isophorone diisocyanate.

11. The process according to claim 3, wherein the vaporized amine is free of any droplets of unvaporized amine.