Patent GB-926781-A (GB)
Title: Dehydration of urea melts
Inventor: SUMMERVILLE ROBERT NATHAN
IPC: C05C9/00
Filed: 1959-06-02
Abstract: A urea melt containing about 10-30% by weight of water and having a temperature of about 200-290 DEG F. is dehydrated to a water content of not more than 1,5% by weight by evaporating the melt in such manner that the water vapour pressure over the urea melt is greater than the water vapour pressure which would be in equilibrium with the urea melt at its crystallization temperature, but not more than 20 mm. Hg greater when the water concentration in the melt is less than 10% by weight; biuret formation is thereby minimized and the formation of urea crystals during the evaporation avoided. Centrifugal thin-film evaporators, provided with heating jackets and rotating plates, are suitably used.  In embodiments of the invention: (i) The urea melt is first evaporated at a pressure of 2,71-3,68 p.s.i.a. to provide a melt containing 4-6% of water and having a temperature of 245 DEG -260 DEG F., and the latter is then evaporated at less than 1,36 p.s.i.a. to provide a melt having \sO 1,5% water and at 270 DEG -285 DEG F.  (ii) The first evaporation is at 2,71-3,68 p.s.i.a. to provide a melt containing 4-6% water at 245 DEG -260 DEG F., and the latter is evaporated at 4-19,7 p.s.i.a. while flowing countercurrent to a stream of inert gas to provide a melt containing \sO 1,5% of water at 270 DEG -285 DEG F.  (iii) The melt is evaporated at 4-19,7 p.s.i.a. while flowing countercurrent to a stream of inert gas; the resulting melt is optionally dehydrated further at 5-13 mm. Hg to provide a melt containing \sO 0,20% water at 272 DEG -285 DEG F.  A table is given showing the partial pressure of water over urea melts containing 4-30% by weight of water.  The stream of inert gas is preferably preheated, e.g. to about 280 DEG F.ALSO:A urea melt containing about 10-30% by weight of water and having a temperature of about 200-290 DEG F. is dehydrated to a water content of not more than 1.5% by weight by evaporating the melt in such manner that the water vapour pressure over the urea melt is greater than the water vapour pressure which would be in equilibrium with the urea melt at its crystallisation temperature but not more than 20 mm. Hg greater when the water concentration in the melt is less than 10% by weight; biuret formation is thereby minimized and the formation of urea crystals during the evaporation avoided.  The resulting urea melt may be flaked, pelletized, granulated, or formed into prills.  In embodiments of the invention:-(i) The urea melt is first evaporated at a pressure of 2.71-3.68 psia to provide a melt containing 4-6% of water and having a temperature of 245-260 DEG F., and the latter is then evaporated at less than 1.36 psia to provide a melt having not greater than 1.5% water and at 270-285 DEG F. (ii) The first evaporation is at 2.71-3.68 psia to provide a melt containing 4-6% of water at 245-260 DEG F., and the latter evaporated at 4-19.7 psia while flowing countercurrent to a stream of inert gas to provide a melt containing not greater than 1.5% water at 270-285 DEG F. (iii) The melt is evaporated at 4-19.7 psia while flowing countercurrent to a stream of inert gas; the resulting melt is optionally dehydrated further at 5-13 mm. Hg to provide a melt containing not greater than 0.20% of water at 272-285 DEG F.  A table is given showing the partial pressure of water in psia over urea melts of various water contents from 4 to 30% by weight.  The stream of inert gas is preferably preheated, e.g. to about 280 DEG F.