Process for the preparation of isocyanates
View Patent ↗An isocyanate is produced by reacting an amine with a stoichiometric excess of phosgene in the gas phase. This reaction is carried out at a temperature above the amine's boiling point to obtain a liquid stream containing the isocyanate and a gas stream containing hydrogen chloride and phosgene. The gas stream containing hydrogen chloride and phosgene thus produced is separated into a gas stream containing hydrogen chloride and a liquid stream containing phosgene. At least part of the liquid stream containing phosgene is then converted to a gas stream containing phosgene which gas stream is then recycled. The gaseous phosgene stream is maintained at a higher pressure than the liquid phosgene stream.
1. A process for the production of an isocyanate comprising:
a) reacting an amine with a stoichiometric excess of phosgene in a gas phase in a reactor at a temperature above the amine's boiling point to obtain a liquid stream containing the isocyanate and a gas stream containing hydrogen chloride and phosgene,
b) separating the gas stream containing hydrogen chloride and phosgene produced in step a) into a gas stream containing hydrogen chloride and a liquid stream containing phosgene,
c) converting at least part of the liquid stream containing phosgene produced in step b) to a gas stream containing phosgene,
d) recycling the gas stream containing phosgene produced in step c) into step a), and
e) maintaining the gas stream containing phosgene from step d) at a pressure higher than that of the liquid stream containing phosgene produced in step b).
2. The process of claim 1 in which step d) is carried out without using pressure-raising elements.
3. The process of claim 1 in which temperature of the liquid stream containing phosgene produced in step b) is raised from 0.5 to 220° C. before step c).
4. The process of claim 1 in which the gas stream containing phosgene produced in step c) contains from 80 to 100% by weight of phosgene, based on total weight of the gas stream.
5. The process of claim 1 in which the liquid stream containing phosgene produced in step b) contains from 30 to 90% by weight of phosgene, based on total weight of the liquid stream.
6. The process of claim 1 in which the pressure difference in step e) is at least 50 mbar.
7. The process of claim 1 in which the gas stream containing hydrogen chloride and phosgene obtained in step a) is not contacted with any pressure-raising element prior to separation in step b).
8. The process of claim 1 in which the liquid stream containing phosgene from step b) is pumped into step c).