Hybrid purification of Isocyanate mixtures with melt
crystallization
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Isocyanates are compounds containing the group (-NCO). They
react with compounds containing alcohol groups (-OH) to produce polyurethane
polymers, which are components of flexible and rigid foams, insulation,
thermoplastic elastomers, spandex fibers, industrial adhesives and polyurethane
paints. Diisocyanates (a –NCO group at each end of a hydrocarbon) are the raw
materials that make up most typical polyurethane products. The most common
diisocyanates used today are toluene diisocyanate (TDI) and methylene biphenyl
diisocyanate (MDI). The production process for these feedstocks generates
undesirable impurities in the form of other isomers and reaction byproducts
that must be separated from the final product. Distillation is typically used
to remove most of the impurities; however, the isomers are generally difficult
to remove due to their similarities with the main product.
In the case
of MDI the main isomer is 4,4’ MDI with 2,4’ MDI and 2,6’ MDI as the main
impurities. The ratio of the 4,4’ MDI and 2,4’ MDI isomers in the diisocyanate
solution will affect the properties of the final polyurethane and mixtures of
the isomers as well as pure 4,4’ MDI are commercially desirable products.
Suspension-based melt crystallization is an ideal separation process for this
mixture. Continuous operation at ambient temperature and pressure provides the
lowest energy requirements of any other separation process. The high
selectivity of crystallization produces pure 4,4’ MDI as one product in a
single step from most industrial feedstock. The reject from the crystallization
process is also a common second product. Hybrid crystallization systems coupled
with distillation provide a highly efficient purification process for both MDI
and TDI that is applicable in grassroots units or as upgrade to existing
distillation based units.