Organic compounds -- part of the class 532-570 series – Organic compounds – Isocyanate esters
Reexamination Certificate
1999-04-29
2001-08-07
Killos, Paul J. (Department: 1621)
Organic compounds -- part of the class 532-570 series
Organic compounds
Isocyanate esters
C560S336000
Reexamination Certificate
active
06271414
ABSTRACT:
The present invention relates to a process for separating isocyanate isomers by suspension crystallization. During its production, tolylene diisocyanate (TDI) is generally obtained in the form of a mixture of 80% by weight of 2,4-tolylene diisocyanate (2,4-TDI) and 20% by weight of 2,6-tolylene diisocyanate (2,6-TDI).
Since the properties of the isomers are different, and in particular their reaction rate in the presence of polyols during the polyaddition reaction for the production of polyurethanes, both pure 2,4-TDI (T100) and an isomer mixture with a reduced content of 2,4-TDI of 65 to 68% by weight (T65) are used industrially in addition to the use of the isomer mixture (T80) resulting from the production process.
Since the molecular structures of the two isomers differ in geometry, the phase diagram displays a considerable decrease in the freezing point on the 2,4-TDI side. The freezing point of the 80:20 isomer mixture is 13.6° C. and that of the 65:35 isomer mixture is only 5° C. In the entire mixing range between 100 and 65% by weight of 2,4-TDI more than 99% pure T100 always crystallizes on slowly cooling the mixtures to below the liquidus temperature.
A similar situation applies to diphenylmethane diisocyanate (MDI). This is obtained in the form of a mixture of 4,4′-MDI and 2,4′-MDI in a ratio of between 70:30 and 90:10. The melting point of 4,4-MDI is 40° C. and that of a mixture of 60% of 4,4-MDI and 40% of 2,4-MDI is between 21 and 23° C., depending on the additional content of up to 2% of 2.2-MDI in the mixture.
Crystallization from the corresponding mixtures is therefore a suitable process for separating the isomers.
The static crystallization process is the most commonly used isomer separation process, in which the isomer to be isolated is deposited on stationary cooled surfaces, subsquently separated from the “mother liquor” and remelted.
Suspension crystallization processes can generally be carried out in an energetically more favourable manner and with a more efficient separation effect than static crystallization processes, if so-called wash columns according to U.S. Pat. Nos. 3,777,892 and 3,872,009 are used for separating and melting the crystal phase.
Such suspension crystallization processes are basically known in particular for aqueous systems, such as for example for the removal of water from foodstuff liquids, as described in U.S. Pat. Nos. 4,004,886, 4,316,368, 4,430,104 and 4,459,144. In addition, U.S. Pat. No. 4,787,985 proposes suspension crystallization processes for the purification of chemicals.
A common feature of all these processes is that they comprise (per stage in multi-stage crystallization processes) a crystallization tank in which crystals are frozen out by the removal of heat, a recrystallization tank in which larger crystals grow at the expense of smaller crystals, and a filter in which the crystals are separated from the mother liquor. At least the filter of the last stage is in the form of a wash column.
Due to the exceedingly low solid phase solubility of 2,6-TDI in 2,4-TDI and of 2,4-MDI in 4,4-MDI, single-stage suspension crystallization processes are in principle suitable for the separation of isomers.
Problems do however arise during recrystallization in the recrystallization tank, i.e. during crystal enlargement, due to the low diffusion rate of the molecules in the liquid phase. Since the mass transfer at the crystal surface is determined by the diffusion rate in the liquid interfacial layer, uneconomically long recrystallization times would be necessary together with correspondingly large recrystallization tanks.
Problems are also caused by the longitudinal, almost acicular, 2,4-TDI crystals, which not only produce a major reduction in the recrystallization rate but are also difficult to separate in the wash column.
It has now been found that the suspension crystallization process can be advantageously used for the separation of isocyanate isomers, and in particular TDI and MDI isomers, if prior to the introduction of the crystal slurry into the wash column, 5 to 40, preferably 10 to 40, and most preferably 15 to 30% by weight of the isomer crystals to be isolated are remelted while simultaneously increasing the temperature of the crystal slurry by 4.5 to 9° C.
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Drope Rüdiger
Grenner Dieter
Hetzel Hartmut
Schal Hans-Peter
Wegener Gerhard
Bayer Aktiengesellschaft
Gil Joseph C.
Killos Paul J.
Oh Taylor V
Whalen Lyndanne M.
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