MXPA99004140A - Process for the isomeric separation of isocyanates - Google Patents

Abstract

The invention concerns a process for obtaining pure isocyanate isomers from an isocyanate isomer mixture by crystallization in suspension and separation of isomerically pure crystals. The process is characterized in that, before separation of the crystals, between 5 and 40 wt%of the crystals produced are melted again whilst the temperature of the crystalline substance is simultaneously increased by between 4.5 and 9°C.

Description

PROCEDURE FOR THE SEPARATION OF ISOCYANATES ISOMERS Field of the Invention The present invention relates to a process for the separation of isocyanate isomers by suspension crystallization.

Description of the Invention Toluylene diisocyanate (TDI) is obtained in its preparation in general as a mixture of 80% by weight of toluylene- (2,) -diisocyanate (2,4-TDI) and 20% by weight of toluylene- (2) , 6) -diisocyanate (2,6-TDI). Due to the different properties, especially the reaction rate against polyols, the polyaddition reaction for the preparation of polyurethanes is used industrially in addition to the mixture of isomers (T80) resulting from the preparation process both 2,4-TDI pure (T100) as well as a mixture of isomers depleted in 2,4-TDI with a content of 2,4-TDI from 65 to 68% (T65). Due to the different geometry of the molecular structures of both isomers, the phase diagram shows a clear decrease of the freezing point by the 2,4-TDI. The freezing point of the mixture REF.:29971 isomers 80:20 reaches 13.6 ° C and that of the mixture of isomers 65:35 only reaches 5 ° C. In the entire mixing range between 100 and 65% by weight, 2,4-TDI crystallizes on cooling-slowly at the temperature of the liquid in each case with more than 99% T100. Similar relationships occur in diphenylmethane diisocyanate (MDI). It is obtained as a mixture of 4,4'-MDI and 2,4'-MDI in a ratio between 70:30 and 90:10. The melting point of 4,4-MDI is 40 ° C, that of a mixture of 60% of 4,4-MDI and 40% of 2,4-MDI of 21 to 23 ° C according to the respective additional content of the mixture of 2,2-MDI in the range of up to 2%. The crystallization from the corresponding mixtures is therefore a suitable process for the separation of isomers. As the usual method of separating isomers, the static crystallization process has been carried out in which the isomer to be isolated is separated on cooled fixed surfaces, then separated from the "mother liquors" and remelted. It is more suitable energetically and with a better separation effect as a static crystallization process to carry it out in general as a suspension crystallization process if for the separation and the melting of the crystalline phase the so-called washing columns are used according to US Pat. A 3 777 892 and US-A 3 872 009. Fundamentally such slurry crystallization processes are known for aqueous systems, such as, for example, the elimination of water from food liquids, by US-A 4 004 886, US Pat. A 4 316 368, US-A 4 430 104 and US-A 4 459 144. In addition, suspension crystallization processes for the purification of chemical products have been proposed according to US-A 4 787 985. It is common to all these presenting procedures. { in crystallization of several steps in each step) a crystallization vessel in which crystals have been segregated by freezing crystals by removing water, a recrystallization vessel in which larger crystals grow at the expense of the smaller ones, and a filter, in which the crystals separate from mother liquors. At least the filter of the last step is structured as a washing column. Due to the only minimal solid phase solubility of 2,6-TDI and 2,4-MDI in 4,4-MDI, suspension crystallization methods for the separation of isomers are suitable in themselves.

However, recrystallization in the recrystallization vessel causes problems, that is, the increase in size of the crystals, due to the small diffusion rate of the molecules in the liquid phase. Since the exchange of matter on the surface of the crystals is determined by the rate of diffusion at the interface of the liquid, uneconomically long recrystallization times with correspondingly large recrystallization vessels would be required. further, the elongated, almost needle-like, crystals of 2,4-TDI cause problems, which on the one hand cause a greatly reduced recrystallization rate and on the other hand can only be separated in the washing column with difficulty. It has now been found that the suspension crystallization process for the separation of isomers of isocyanates, in particular of TDI and MDI, can be advantageously used if, before introducing the mass of crystals in the washing column, they melt again from 5 to 40, preferably from 10 to 40, with particular preference from 15 to 30% by weight of the crystals of the isomers to be separated by simultaneously raising the temperature of the crystal mass from 4.5 to 9 ° C.

The object of the present invention is therefore a process for obtaining isomers of pure isocyanates, especially 2,4-TDI and 4,4-MDI, from a corresponding mixture of isomers by crystallization in suspension and separation of the pure crystals, which is characterized in that before the separation of the crystals, 5 to 40% by weight of the crystals produced are melted again, simultaneously raising the temperature of the crystal mass from 4.5 to 9 ° C. Preferably, the new melting of the crystals is carried out in a mixing vessel which is disposed in front of the washing column and in which a temperature of 4.5 to 9 ° C higher than the temperature of the crystallizer is maintained. By introducing the mass of crystals into the mixing vessel at a higher temperature a special thermal stress is generated which causes the dissolution of the smaller crystals in particular and thus compensates for the small recrystallization rate of the isocyanate crystals pure isomers . The mixing vessel in which the mixture of starting isomers is preferably also introduced thus replaces the recrystallization vessel used in the usual suspension crystallization process.

Depending on the size of the mixing vessel and consequently on the residence time in the mixing vessel, an increase in the size of the crystals can also be achieved by recrystallization. But it is preferable according to the invention to provide a mixing container with a small volume and a relatively short residence time. In particular, it is preferable to dimension the mixing vessel so that the residence time of the glass mass in the mixing vessel reaches between 1 and 15 minutes, particularly preferably between 5 and 10 minutes.

It is essential according to the invention to supply heat to the mixing vessel to maintain the elevated temperature and to melt the crystals. The heat can be supplied indirectly by means of an enclosing heating of the mixing vessel, or, preferably, by the corresponding thermal conditioning of the mixture of starting isomers which is likewise introduced into the mixing vessel. The exclusion of the impoverished mother liquor in the pure isomer isocyanate is carried out by means of a filter arranged between the crystallizer and the mixing vessel.

Preferably a mixture of 80% by weight of 2,4-TDI and 20% by weight of 2,6-TDI is used as a mixture of starting isomers. The mixture of starting isomers is fed to the mixing vessel preferably at a temperature between 17 and 30 ° C, particularly preferably between 18 and 24 ° C.

The liquid phase depleted in 2,4-TDI still contains 65 to 69%, preferably 66 to 68% by weight of 2,4-TDI. A temperature between 5 and 7 ° C is maintained in the crystallizer. At the head of the washing column, 2,4-TDI is removed with a concentration higher than 99%, preferably higher than 99.8%, more preferably higher than 99.9% at a temperature of at least 22 ° C. The mother liquor separated in the washing column is returned to the crystallizer. In the case of the separation of the MDI isomers, a mixture of starting isomers of 70 to 90% of 4,4'-MDI and 30 to 10% of 2,4'-MDI is preferably used. The mixture of starting isomers is fed into the mixing vessel preferably at a temperature of 30 to 45 ° C. The depleted liquid in 4,4'-MDI preferably still contains 50 to 60% of 4,4'-MDI. The temperature in the crystallizer is preferably between 15 and 21 ° C. According to another formulation of the invention, this relates to a process for the separation of isomers of isocyanates, in particular TDI and MDI, by crystallization in suspension and separation of isomerically pure crystals, which is characterized in that the suspension of crystals after the exit of the crystallizer is mixed with the mixture of isomers, feeding the mixture of starting isomers at a temperature such that the mother liquor which is formed has a relative sub-saturation of 4 to 20%, preferably of 7 to 17%. In this respect, it should be understood by mother liquors that the mixture of mother liquors fed from the crystallizer having the crystallizer temperature and a residual concentration of the isomers obtained as crystals and the mixture of starting isomers fed with higher concentration are formed. This mixture must have a concentration that is 4 to 20% lower than the equilibrium concentration corresponding to the temperature of the mixture according to the liquid state curve of the phase diagram. The crystals at the lowest temperature of the crystalliser are still first present in the mixing vessel by mixing the produced mother liquors. The smaller crystals dissolve in the subsaturated mother liquors. The larger crystals dissolve in the first place so that the concentration of mother liquors grows at the interface. Simultaneously, the larger crystals subtract to the mother liquors that surround them so much heat that they reach the subsaturation interval. The larger crystals grow more. The greater the subsaturation, the fewer small crystals dissolve; the greater the subsaturation, the more intensely the larger crystals grow. The optimal subsaturation depends on the size distribution of the crystals that are produced in the crystallizer. This can be determined by varying the temperature of the mixture of isomers added as a function of the separation performance of the scrubber by simple preliminary tests.

Description of the Figure The invention is illustrated below with the help of the accompanying Figure 1: From the washing column 1 through the pipe 2 by the pump 3 are introduced into the crystallizer 4, whose cooling jacket is fed with the cooling equipment 6, waters recirculated mothers. The mass of crystals is taken from the crystallizer 4 through the line 7 to the filter 8, where the impoverished mother liquor is excluded via the line 9. The mass of crystals correspondingly enriched in crystals is introduced through line 10. in the mixing vessel 11 in which a temperature higher than that of the crystallizer 4 reigns. In addition, in the mixing vessel 11, the mixture of starting isomers is introduced through line 12. This is at a temperature sufficient to Keep the temperature in the mixing vessel. Alternatively, the mixing vessel 11 can be equipped with an enclosing heater. The mother liquor leaving the mixing vessel 11, free of the smaller crystals fused, is carried through the pipe 20 to the washing column 20 above the perforated plate 13. The mother liquor passes through the perforated plate and is brought back to the crystallizer through the conduit 2. The perforated plate 13 'is periodically displaced by an alternative piston pump 15 against the glass squeegee, whereby the mother liquor is pressed out from the space above the perforated plate 13 and the crystals are compacted. The doctor blade 14 detaches from the compacted glass block both crystals and those rejected by the perforated plate. The crystals are melted in the heat exchanger 19 and are evacuated through the valve 16 and the conduit 17. The melt exiting the heat exchanger 19 and evacuated through the valve 16 and the conduit 17. The mass The melt exiting the heat exchanger 19 is partially fed back by the pump 21 to the head of the washing column, so that a temperature close to that of the melting point is maintained here. The valve 16 maintains above the perforated plate 13 at least one pressure such that a part of the molten crystals, ie the pure liquid, is pressed to wash washed mother liquids still adhered through the block of compacted crystals. A part of the mother liquor separated in the filter 8 can be fed back through the pipe 18 to the crystallizer 4 to guarantee the solids content in the crystallizer 4. Example In a pilot installation according to Fig. 1, they are introduced through the conduction 12, 100 parts per hour of TDI 80/20 at a temperature of 25 ° C. Through line 9, 60.6 parts of TDI 67/33 are fed per hour and 39.4 parts of 2,4-TDI of 99.9% are excluded via line 17. In the crystallizer 4 a temperature of 6.3 ° C is maintained. The content of crystals in crystallizer 4 is maintained by partial recirculation of the mother liquor separated in filter 8 through line 18 between 18 and 25% by weight. After a part of the mother liquor is separated in the filter 8, the mass of crystals, which has a temperature equal to 6.3 ° C., is passed to the mixing vessel 11. The mass of crystals leaving the mixing vessel 11 and fed to the washing column has a temperature also of 6.3 ° C, to the mixing vessel 11. The mass of crystals leaving the mixing vessel 11 and fed to the washing column has a temperature of 11.5 ° C and a concentration in the liquid phases of 76.5% by weight of 2,4-TDI. The residence time in the mixing vessel 11 amounts to 10 minutes, the waiting time in the crystallizer 4 to 120 minutes.

It is noted that in relation to this date, the best method known to the applicant to carry out the aforementioned invention, is that which is clear from the present description of the invention.

Having described the invention as above, property is claimed as contained in the following:

Claims (10)

R E I V I ND I C A C I O N S

1. Process for obtaining isomers of pure isocyanates from a mixture of isomeric isocyanates by crystallization in suspension and separation of the isomerically pure crystals, characterized in that before the separation of the crystals, 5 to 40% by weight of the crystals are melted again. crystals produced by simultaneously raising the temperature of the crystal mass from 4.5 to 9 ° C.

Method according to claim 1, characterized in that from 10 to 24% by weight of the crystals produced are melted.

Method according to claim 1 or 2, characterized in that the melting and the temperature rise of the crystal mass is carried out in a mixing vessel arranged between the crystallization vessel and the separation device to which heat is supplied. .

Process according to one of claims 1 to 3, characterized in that the starting mixture is fed into the mixing vessel.

5. Process for the separation of isomers of isocyanates, characterized in that the crystal suspension after the crystallizer exit is mixed with the mixture of isomers, feeding the mixture of starting isomers at a temperature such that the mixture of starting isomers and the liquid phase fed to the crystallizer is subsaturated without taking into account the crystals.

6. Procedure according to. one of claims 1 to 5, characterized in that between the crystallizer and the mixing vessel is arranged a filter with which the liquid phase impoverished in the pure isomer to be obtained is excluded.

Method according to one of claims 1 to 6, characterized in that the heat required for thermal conditioning of the feed mixture fed to the mixing vessel is supplied.

Method according to one of claims 1 to 7, characterized in that a mixture of 80% by weight of 2,4-TDI and 20% by weight of 2,6-TDI is used as the starting mixture.

9. Method according to one of claims 1 to 8, characterized in that the excluded liquid phase depleted in 2,4-TDI contains from 65 to 69% by weight of 2,4-TDI. Method according to one of claims 1 to 3, characterized in that the residence time in the mixing vessel reaches from 1 to 15 minutes.