What is monomer residue

The present invention comprises a method for the purification of isocyanate-containing residues.

In the production of diisocyanates, a predominantly polymeric residue is produced industrially, which can still contain significant proportions of usable product which is to be separated off in order to improve the yield of the process. In addition, the monomeric diisocyanates are mostly toxic, so that a content of monomeric diisocyanate in the residue can make them dangerous to handle, for example for disposal.

Depending on the nature of the isocyanate, the residue is solid or forms a viscous, sticky and consequently difficult to pump mass.

Processes for separating monomeric diisocyanate from residues from the phosgene process are known in principle.

It was therefore an object of the present invention to provide a process for separating monomers from diisocyanate residues with which the residues can be treated in a simple manner in terms of process engineering.

The object was achieved by a process for separating monomers from a diisocyanate-containing residue, in at least one apparatus in which the residue is a highly viscous liquid and during the entire residence time in the apparatus at a temperature between 210 and 330 ° C and a pressure below 300 hPa / or forms a non-embrittling solid and is discharged from this apparatus by means of a forced conveyance for non-solid media.

It is an advantage of the present invention that, with the selected apparatus, the discharge of the monomer-depleted residue from the apparatus is possible in a simplified manner and, furthermore, it is not necessary for the residue to form a brittle solid.

Diisocyanates, the residues of which can be treated in the process according to the invention, are preferably (cyclo) aliphatic diisocyanates, particularly preferably diisocyanates having 4 to 20 carbon atoms. Examples of customary diisocyanates are aliphatic diisocyanates such as tetramethylene diisocyanate, hexamethylene diisocyanate (1,6-diisocyanatohexane), 1,8-octamethylene diisocyanate, 1,10-decamethylene diisocyanate, 1,12-dodecamethylene diisocyanate, 1,14-tetradecamethylene diisocyanate, derivatives of lysine diisocyanate, tetramethylene diisocyanate, derivatives of lysine diisocyanate, tetradecamethylene diisocyanate or tetramethylhexane diisocyanate, as well as cycloaliphatic diisocyanates such as 1,4-, 1,3- or 1,2-diisocyanatocyclohexane, 4,4'- or 2,4'-di (isocyanatocyclohexyl) methane, 1-isocyanato-3,3,5- trimethyl-5- (isocyanatomethyl) cyclohexane (isophorone diisocyanate), 1,3- or 1,4-bis (isocyanatomethyl) cyclohexane, 2,4- or 2,6-diisocyanato-1-methylcyclohexane and 3 (or 4), 8 (or 9) -bis (isocyanatomethyl) -tricyclo [5.2.1.02.6] decane isomer mixtures.

Preferred among these are 1,6-diisocyanatohexane, 4,4'- or 2,4'-di (isocyanatocyclohexyl) methane and 1-isocyanato-3,3,5-trimethyl-5- (isocyanatomethyl) cyclohexane, 1 being particularly preferred , 6-diisocyanatohexane.

Cycloaliphatic isocyanates are those which contain at least one cycloaliphatic ring system.

Aliphatic isocyanates are those which exclusively contain isocyanate groups which are bound to straight or branched chains.

In the context of this application, (cyclo) aliphatic isocyanates stands for cycloaliphatic and / or aliphatic isocyanates.

The residues which can be used as monomer-containing residues in the process according to the invention are the residues of those diisocyanates whose monomer-containing residues form a highly viscous liquid and / or a non-embrittling solid during the entire residence time within the apparatus under the conditions prevailing there.

“Highly viscous” is understood here to mean a viscosity of more than 1000 mPas according to DIN EN ISO 3219 under the conditions prevailing in the apparatus.

"Pasty" is understood here to mean a liquid or highly viscous medium that contains solid components but is rheologically flowable under the conditions prevailing in the apparatus but is not flowable under the sole influence of the acceleration of gravity.

"Brittle" is understood here to mean the property of substances to break or tear under stress. The brittleness generally occurs below a transition temperature T.Ü above which the fabric behaves tough. Brittle substances are those that show a steep Hooke's straight line in the stress-strain diagram that is obtained when a sample is loaded with a tensile force F and the resulting change in length ΔL is applied, which characterizes the proportional range of elongation and stress, where Hooke's straight line ends with the break.

In general, diisocyanates can be obtained industrially either essentially by phosgenation of diamines, for example by processes based on the processes described in DE-PS 20 05 309 and DE-OS 2 404 773, or by phosgene-free processes (cleavage of biurethanes), as described, for example in EP-B 126 299 (US-A-4 596 678), EP-B 126 300 (US-A-4 596 679), EP-A 355 443 (US-A-5 087 739), EP 566 925 A2 as well as in EP-A 568,782.

Production by a phosgene process takes place by phosgenation of the corresponding diamines and thermal cleavage of the dicarbamic acid chlorides formed as intermediates. Isocyanates resulting from a phosgenation process generally have a total chlorine content of 100–700 mg / kg. The main high-boiling impurities are chlorine-containing secondary components.

Production by the phosgene-free process, also known as the urea process, is preferably carried out by thermal cleavage of the corresponding carbamates. This cleavage is carried out at temperatures from 150 to 300 ° C., usually using catalysts. The diisocyanates and alcohols formed during the cleavage are removed from the reaction mixture and purified, mostly by distillation.

Diisocyanate obtained by the latter process have a total chlorine content of less than 80 mg / kg, preferably less than 60, particularly preferably less than 40, very particularly preferably less than 20, in particular less than 10 mg / kg and especially 0 mg / kg.

In principle, it is possible to use diisocyanate-containing residues in the process according to the invention which originate from a phosgene process or from a phosgene-free process. In a preferred embodiment of the present invention, diisocyanate-containing residues are used which originate from a phosgene-free process.

If residues containing diisocyanate are used which originate from a phosgene process, it can be useful to manufacture the apparatus at least partially from stainless steel at the thermally stressed points where the apparatus is exposed to the chlorine-containing residue. DIN-EN 10088-1 in the version of August 1995 defines stainless steels as those that contain at least 10.5% chromium and a maximum of 1.2% carbon. In the process according to the invention, preference is then given to using apparatuses which are at least partially manufactured from austenitic and / or austenitic-ferritic steels.

Austenitic steels are those with an austenitic lattice type (γ phase) at 20 ° C. They preferably have a Cr content of 16 to 28% and a Ni content of 3.5 to 32%, and optionally proportions of S (up to 0.35%), P (up to 0.045%), Mo ( up to 7%), Si (up to 4.5%), Cu (up to 4%), N (up to 0.25%) and / or Mn (up to 10.5%), and possibly Ti ( up to 0.7%) and / or Nb (up to 1%). The carbon content is usually below 0.15%. Of these, the high-alloy austenitic 18/8 chrome-nickel steels are particularly preferred.

Austenitic-ferritic steels have a two-phase structure of ferrite and austenite with a ferrite content of approx. 60%. The proportion of Cr is usually 19–28%, Ni 3.5–8%, Mo up to 4.5% and, if necessary, proportions of Mn (up to 2%), Cu (up to 2.5%), N ( up to 0.35%), W (up to 1%), S (up to 0.015%), Si (up to 1%) and / or P (up to 0.035%). The carbon content is usually below 0.05%.

Very particularly preferred materials are the austenitic and austenitic-ferritic materials listed in DIN-EN 10088-1 and particularly preferred are the materials 1.4539 (Falk steel), 1.4541, 1.4571 and 1.4462, as well as Hastelloy A and C and zirconium. The mentioned materials according to DIN-EN 10088 correspond approximately to the following materials according to AISI (American Iron and Steel institute), UNS (Unified Numbering System), SS (Swedish Standard), AFNOR (Association Francaise de Normalization), BS (British Standard) and JIS (Japanese Industrial Standards):
1.4462 (X 2 CrNiMoN 22 5 3): UNS: S 31803, SS: 2377, AFNOR: Z 5 CNDU 21.08, JIS: SUS 329 J3L
1.4539 (X 1 NiCrMoCuN 25 20 5): UNS: N 08904, SS: 2562, AFNOR: Z 1 NCDU 25.20
1.4541 (X 6 CrNiTi 18 10): AISI: 321, UNS: S 32100, SS: 2337, AFNOR: Z 6 CNT 18.10, BS: 321 S 31, JIS: SUS 321
1.4571 (X 6 CrNiMoTi 17 12 2): AISI: 316 Ti, UNS: S 31635, SS: 2350, AFNOR: Z 6 CNDT 17.12, BS: 320 S 31, JIS: SUS 316 Ti

Among the materials listed, those with increased chromium, copper, molybdenum and / or nickel proportions are advantageous.

In a preferred embodiment of the present invention, the diisocyanate-containing residue comes from a phosgene-free process.

The simple transfer of residue work-ups from phosgene processes is not possible, since product streams in processes produced without phosgene have a completely different spectrum of by-products and consequently also entail other separation problems.

Despite the changed by-product spectrum, it is not only possible with the process according to the invention to recover monomeric diisocyanate contained in the residue, but also to split back mono- and / or diurethane contained therein and other usable products, so that the effective yield of monomeric diisocyanate with the Processes according to the invention are higher for the residues from a phosgene-free process than for residues from a phosgene process.

A typical phosgene-free production is described, for example, in EP 566925 A2, Column 6, line 31 to column 8, line 58, which is hereby incorporated by reference.

For production (urethanization), the diamines corresponding to the diisocyanates are generally mixed with urea and an alcohol, optionally in the presence of dialkyl carbonates and / or carbamic acid esters and optionally in the presence of catalysts at temperatures from 150 to 300 ° C. under a pressure of 0.1 to 60 bar brought to reaction within 0.5 to 50 hours. The alcohols used are preferably methanol, ethanol or n-butanol. In this reaction, the diurethanes are ideally formed, which are split into the diisocyanates in a subsequent stage.

The resulting diurethane-containing reaction mixture is then continuously thermally cleaved in a suitable device in the presence or preferably in the absence of solvent in the liquid phase, preferably in the presence of catalysts at temperatures from 200 to 300 ° C. and under reduced pressure from 0.1 to 200 mbar. The conversion of diurethane to diisocyanate in the device for thermal cleavage can be largely freely selected depending on the urethane used and is expediently in a range from 10 to 95% by weight of the amount of urethane supplied.

The unsplit portion of the reaction mixture, which contains unreacted diurethanes, oligourea-diurethanes, high-boiling oligomers and other recyclable and unusable by-products, is separated off, continuously discharged from the splitting device and used in the process according to the invention directly or, if necessary, after reaction with alcohol. Such a conversion (re-urethanization) can take place, for example, as in DE 10338511 A1, Paragraph 0033, the full content of which is hereby incorporated by reference.

The cleavage products formed in the thermal cleavage, which are composed primarily of alcohol, diisocyanate and partially cleaved urethanes, are then advantageously with the help of one or more distillation columns, preferably by rectification at temperatures of 100 to 220 ° C and a pressure of 1 to 200 mbar in alcohol and a crude diisocyanate mixture with a diisocyanate content of 85 to 99% by weight separated (distillative purification). The distillation columns generally each have 1 to 50 theoretical plates and are of a type known per se. The higher-boiling by-products obtained in the separation by distillation and in particular the uncleaved and partially cleaved polyurethanes can likewise be used in the process according to the invention.

The residues obtained by phosgene-free processes do not contain any chlorine compounds as by-products and therefore have a fundamentally different range of by-products due to the production process. In particular, the residue contains mono- and / or diurethanes, as well as allophanates, biurets and / or uretdiones as by-products, which according to the invention can be at least partially recovered by thermal cleavage to diisocyanates in the apparatus.

An additional advantage of the process according to the invention for residues obtained from a phosgene-free process is that, as a rule, there are no special requirements to be placed on corrosion-resistant materials, so that the materials from which the apparatus is made are not, as is the case with residues, which originate from a phosgene process, stainless steels should be used, but the apparatus can be made in addition to stainless materials and preferably completely from normal steel.

The residue which is fed to the apparatus can be, for example, a high-boiling distillation or rectification residue from the reaction discharge of the diisocyanate formation. Distillation devices are usually operated at 1 to 80 mbar and a bottom temperature of 100 to 240 ° C. The residues are usually obtained as a sump discharge.

The residues from phosgene-free processes usually contain not only monomeric diisocyanate but also its polyisocyanates, in particular polyisocyanate containing uretdione, biuret and / or isocyanurate groups. It is a peculiarity of diisocyanate, which has been produced by a phosgene-free process, that the residue also contains mono- or diurethanes, allophanates and / or ureas which, under the separation conditions according to the invention, can at least partially be cleaved back product of value, in particular to monomeric diisocyanate.

The content of monomers in the residue depends on the previous separation of the diisocyanate. The content of monomers can be up to 90% by weight, preferably up to 80, particularly preferably up to 70 and very particularly preferably up to 60% by weight.

The monomer-containing residue which is fed into the apparatus for monomer recovery is referred to in this document as “residue” for simplicity, whereas the discharge leaving the apparatus is referred to as “monomer-depleted residue” to distinguish it.

The term "apparatus" is used here to define a process-engineering apparatus in which the monomer can be separated off from the residue and then discharged.

The monomer is separated from the residue by a distillation and / or stripping process, and the residue is mixed in the apparatus.

According to the invention, the temperature inside the apparatus is between 210 and 330 ° C, preferably between 225 and 305 ° C, particularly preferably from 235 to 290 ° C, very particularly preferably from 245 to 275 ° C and in particular from 255 to 265 ° C.

In the case of certain compositions, the residue can tend to foam, with foam formation starting at a certain lower temperature limit and decreasing again at an upper temperature limit. If this is the case, it is an advantageous embodiment of the invention to feed the residue into the apparatus at a temperature which is above the temperature limit at which foam formation is reduced again.

The temperature at which the residue is passed into the apparatus is preferably more than 120 ° C. and less than 240 ° C., particularly preferably more than 150 ° C. and less than 220 ° C., very particularly preferably more than 180 ° C. and less than 210 ° C.

According to the invention, the pressure within the apparatus is below 300 hPa, preferably below 200 hPa and particularly preferably below 100 hPa.

The upper residence time limit within the apparatus is generally up to 5 hours, preferably up to 3.5 hours, particularly preferably up to 2.5, very particularly preferably up to 2 and especially up to 1.5 hours.

According to the invention, the lower residence time limit within the apparatus is at least 5 minutes, preferably at least 10 minutes, particularly preferably at least 15 and very particularly preferably at least 20 minutes.

According to the invention, the apparatus is forcibly discharging and / or has a conveying gradient for the discharge whose forced discharge conveying acceleration is greater than the acceleration due to gravity (> 10 m / s2).

In addition, it makes sense to design the device with forced delivery and / or with a delivery gradient for conveying the residue within the device.

In the context of the present invention, the term "forced conveying" means that the residue introduced into the apparatus is moved through the apparatus by the introduction of mechanical energy.

The conveyance through the apparatus takes place with little or no backmixing. This delivery characteristic is characterized by a Bodenstein number of at least 3, preferably at least 5, particularly preferably at least 7.

In order to narrow the dwell time distribution in the paddle dryer, the interior space carrying the product is preferably separated into different segments with diaphragm-like disks. At least two disks are particularly preferably used.

In a preferred embodiment, the axial transport through the apparatus takes place through the arrangement of conveying, kneading and / or mixing elements, for example disk elements, shafts, screws, blades, wipers or rotors.

In the apparatus, a mechanical energy input of 5 W / kg or more is usually sufficient, preferably 10 or more W / kg, particularly preferably 20 or more, very particularly preferably 40 or more, in particular 80 or more and especially 100 W / kg or more more. As a rule, an energy input of more than 200 W / kg does not bring any advantages. The specified specific power entry is here as the entered power per amount of residue in the apparatus.

It is preferred that the paddle dryer has a forced delivery in the axial direction. The forced conveyance is achieved, for example, by inclining the surfaces of the conveying elements.

It is also advantageous that the paddle dryer has a forced cleaning of the inner surfaces in contact with the product of at least 50%, preferably at least 60%, very particularly preferably of at least 70% and in particular at least 80% of these inner surfaces in contact with the product. The forced cleaning is ensured by the proximity of the conveyor elements to the outer wall or by the proximity of cleaning hooks to the conveyor elements.

In addition, the process according to the invention not only recovers monomeric diisocyanates, but also their secondary products, insofar as they can be separated off and / or cleaved back.

In this document, the term "monomers" is used to denote not only monomeric diisocyanate, but also its monourethane, diurethane, uretdione, biuret and / or allophanate.

The residual content of monomers in the monomer-depleted residue after the separation is generally less than 20, preferably less than 15, particularly preferably less than 10, very particularly preferably less than 5 and in particular less than 1% by weight.

According to the invention, it can be useful not to separate off the monomer as completely as it would be possible in terms of apparatus if this made the discharge solid. Instead, in this case it makes sense according to the invention to leave part of the monomer in the monomer-depleted residue discharged from the apparatus if this does not become solid as a result. This is particularly useful if the savings due to the simplified apparatus overcompensate for the loss of monomers.

In this case, the residual content of monomers after the separation is generally, for example, 5 to 25% by weight and preferably 10 to 20% by weight.

In order to separate off at least some of the monomer present in the bottom discharge before the apparatus, it can optionally be useful to subject the residue to a distillation before it is introduced into the apparatus.

Such a, preferably single-stage, distillation is preferably carried out in a falling film evaporator, climbing evaporator, thin film evaporator, long tube evaporator or helical tube evaporator, particularly preferably in a falling film evaporator.

Such a single-stage distillation usually takes place at 80-320 ° C., preferably 100-300 ° C. and a pressure of 0.1-40 mbar, preferably 0.5-20 mbar.

The low boiler discharge containing diisocyanate from such a single-stage distillation can then preferably be fed into the urethanization and / or purification by distillation.

The residue fed to the apparatus is generally liquid, highly viscous or pasty, preferably liquid or highly viscous, and often has a viscosity of up to 500 mPas in accordance with DIN EN ISO 3219 at a temperature of 150.degree.

According to the invention, the method is carried out in such a way that the added residue forms a highly viscous liquid and / or a non-embrittling solid during its entire residence time in the apparatus.

According to the invention, the method is carried out in such a way that the residue leaving the apparatus after depletion of monomer without cooling as a highly viscous liquid, as a non-brittle, non-free-flowing, non-free-flowing, cohesive, dough-like compacting under pressure and / or plastically deformable solid with a flow limit greater than 1 N / m2, preferably greater than 100 N / m2 particularly preferably greater than 500 N / m2 is carried out.

The advantages of the process according to the invention are that the discharge does not form any dusts even without the need to add high-boiling hydrocarbons and, in the process according to the invention, phases of the residue that are not brittle can also be discharged from the apparatus, so that a cooling zone within the Apparatus can be dispensed with.

The density of this discharge is mostly between 500 and 3000 g / l, preferably 600 to 1000, particularly preferably 700 to 900 and very particularly preferably 750 to 850 g / l.

The apparatuses which are used in the process according to the invention are apparatuses with a forced discharge.

For improved separation of the monomers when the residue passes through the apparatus, it is preferred that the latter has a Bodenstein number of at least 3, particularly preferably at least 5 and very particularly preferably at least 7.

Preferred embodiments of such apparatus are

  • a) paddle dryer without cooling zone with forced discharge elements,
  • b) Extruder with degassing option and
  • c) vertical thin-film processors with forced discharge organs.

a) Paddle dryer without cooling zone with forced discharge elements.

In order to counter the disadvantage of the prior art, the paddle dryers that can be used according to the invention do not have any separation into heating and cooling zones, i.e. a sudden change in temperature by far more than 100 ° C, as in FIG US 5962728 described, in the course of the passage of the residue through the apparatus, but at most a temperature gradient which does not change by more than 50 ° C during the passage of the residue through the apparatus, preferably increases, and particularly preferably no significant temperature change in the course of the passage of the residue through the apparatus, ie less than 20 and in particular less than 10 ° C. As a result, the energy requirement of the apparatus is significantly reduced compared to the prior art and there are no start-up problems due to different temperature zones.

Such paddle dryers are constructed essentially horizontally; the residue is usually conveyed via one or two mixing and kneading shafts inside the apparatus. In the specialist literature, these apparatuses are also referred to as particle bed reactors, kneading dryers or kneading reactors.

The heating is done through the wall and can be done in any way. The heating preferably takes place not only via the outer wall of the apparatus, but also via the built-in components such as cleaning hooks, segmenting disks and kneading shaft.

The thermal energy that is introduced into the residue via the walls is usually more than 120 kJ / kg residue and less than 2400 kJ / kg residue, preferably more than 220 kJ / kg residue and less than 1800 kJ / kg residue, in particular preferably more than 300 kJ / kg residue and less than 1400 kJ / kg residue and very particularly preferably more than 360 kJ / kg residue and less than 900 kJ / kg residue.

The heating section of the residue stream fed to the paddle dryer is preferably more than 10% and less than 70% of the total length of the paddle dryer, preferably more than 20% and less than 60%, particularly preferably more than 30% and less than 50% of the total length of the paddle dryer.

Apparatuses of this type are available, for example, from List AG, Arisdorf, Switzerland, under the trade name Discotherm® B or List-CRP or AP, as well as from Buss-SMS-Canzler GmbH, Butzbach, Germany under the name Reasol® or Reactotherm® offered.

For example, screws, preferably twin screws, can serve as discharge elements for the forced discharge of the solid, monomer-depleted residue that is highly viscous, liquid and / or non-embrittling, even after the monomer has been separated off.

Furthermore, the paddle dryer is preferably operated with a vapor condenser with which the separated monomer can be recovered.

It is a preferred embodiment of the present method to fill the usable volume of the paddle dryer only to 25 to 90%, preferably 30 to 80%, particularly preferably 40 to 75% and very particularly preferably 50 to 66% with residue.

This is advantageous in that such an incomplete degree of filling allows the residue in the apparatus to foam up to a certain extent.

b) Extruder with degassing options

Alternatively, the monomers can also be separated off in an extruder which is provided with at least one degassing facility, for example a degassing dome, via which the separated monomer can be discharged.

For this purpose, the residue is conveyed in a vacuum and at the specified temperature against, for example, a perforated or slotted screen. As a result of the kneading inside the extruder, the residue is thoroughly mixed in such a way that the monomer is expelled and removed from the extruder via the degassing facility. There it is then condensed and utilized in a manner known per se.

This alternative is particularly preferred when there are only small amounts of monomer to be separated off in the residue, for example 30% by weight or less.

c) vertical thin-film processors with forced discharge organs.

Such thin-film processors are oriented essentially vertically, so that here the earth's gravity field acts as a conveying gradient for the residue within the apparatus, as long as the residue is flowable under the conditions within the apparatus.

By means of suitable mechanical devices, for example wiper blades, the residue is applied to the heated surface as a thin product film and distributed so that the volatile monomer can be separated off.

If necessary, the separation can additionally be assisted by passing through a gas which is inert under the separation conditions, preferably nitrogen.

The vapor can be taken off at the top (in countercurrent) or at the foot (in cocurrent) of the thin-film processor and is then condensed and utilized in a manner known per se.

With vertical thin-film processors, the residue can be processed, for example, up to a viscosity of up to 15,000, preferably up to 10,000 Pas according to DIN EN ISO 3219.

The conveyance should preferably be supported by rotors with optimized shear, so that the wiper blades (rotors) not only produce a thin film of product on the heated surface, but also forcibly convey the residue within the apparatus. This can be done, for example, by wiper blades inclined in the conveying direction.

When the highly viscous, liquid and / or non-embrittling solid, monomer-depleted residue has arrived at the lower end of the thin-film processor after the monomer has been separated off, the natural as well as the additional conveying gradient is usually no longer sufficient for conveying. For this purpose, the discharge is then discharged at the foot by means of suitable forced discharge systems, for example screws or shafts.

Such thin-film processors are for example from Buss-SMS-Canzler GmbH, Butzbach, Germany under the name Filmtruder® or Viscon® offered.

This alternative is particularly preferred if there are still larger amounts of monomer to be separated off in the residue, for example 40% by weight or more.

All alternatives have in common that the separated monomer, after removal from the apparatus and condensation, can preferably be fed into the purification of the diisocyanate. For this purpose, it is combined with a stream from the production of the diisocyanate or a stream in the distillative purification, which has a composition that is as similar as possible.

Alternatively, in phosgene-free processes, the separated monomer can also be fed back into the urethanization and / or carbamate formation, if necessary after previous reurethanization or recarbamate formation, i.e. reaction of the separated monomer with alcohol. Another possibility is to recycle the separated monomer into the carbamate or urethane cleavage. In phosgene-free processes, it is therefore preferred according to the invention to quench the gaseous vapors separated from the apparatus with an alcohol, preferably the alcohol with which the monomer is urethanized in the phosgene-free process. For this purpose, a sufficient amount of alcohol is used to bring the vapor into solution or at least to suspend it.

Regardless of the preparation of the diisocyanate, it can also be useful to recycle the vapors in gaseous form. For this purpose, it makes sense to heat the return circuit in order to avoid undesired condensation. The temperature is chosen so that it is less than or equal to the temperature of the residue in the apparatus according to the invention and is greater than or equal to the boiling temperature at the selected operating pressure in the gas space of the isocyanate to be recovered.

In an alternative, albeit less preferred embodiment, it is possible according to the invention to add at least one additional substance to the residue before or during the treatment in the apparatus which facilitates distilling off the monomer and / or leads to a higher viscosity of the discharge from the apparatus.

These can be, for example, diphenyl ether-biphenyl mixtures (so-called diphyl), N-methylpyrrolidone, tetradecaline, high-boiling hydrocarbon mixtures, in particular aromatic hydrocarbon mixtures, for example Solvesso® 200 from ExxonMobil Chemical, Kristallöl 60: CAS no. 64742-82-1), heavy solvent naphtha (boiling range approx. 225-300 ° C) or washing oil. The process is preferably carried out without the addition of additional substances.

After being discharged from the apparatus, the monomer-depleted residue can be disposed of, for example disposed of in a landfill or incinerated. The consistency of the monomer-depleted residue outside the apparatus is insignificant, i.e. solidification and / or embrittlement outside the apparatus is possible without this being disadvantageous according to the invention, as long as the residue does not become brittle inside the apparatus.