Patent GB-930213-A (GB)
Title: Process for the preparation of urea
IPC: C07C273/04
Filed: 1961-01-23
Abstract: <PICT:0930213/III/1> Urea is produced by reacting under heat and pressure ammonia and carbon dioxide to form urea, ammonium carbonate, NH3 and H2O which are heated to decompose the carbonate to NH3 and CO2 off-gas which is cooled by contact with an aqueous solution of NH3 and ammonium carbonate which is thereby heated while a portion of the off-gas is condensed and absorbed. The remaining gas is further cooled to recover a CO2-free NH3.  A portion, e.g. major of the carbonate solution, is cooled and recycled to the gas contacting stage and the remainder is recovered.  In Fig. 1, a vertical or horizontal reactor 3, at, e.g. 2,000-6,000 p.s.i.g. and 320 DEG -430 DEG F., is fed with CO2 at 1, NH3 at 2 and recycle ag. NH3-ammonium carbonate solution at 6.  The NH3 and CO2 (molar ratio, e.g. 2-5 to 1,) form ammonium carbonate, part of which forms urea, and are passed through valve 8 whereby pressure is reduced to 200-400 p.s.i.g. to shell and tube heat exchanger 10 fed with steam at 11. A major proportion of the ammonium carbonate forms NH3 and CO2 at 240 DEG -300 DEG F. in 10 and is fed to gas-liuqid separator 14 where an ag. solution of urea and carbamate leaves at 15, and NH3 and CO2 at 16. The liquid passes valve 17 whereby pressure is reduced to 5-50 p.s.i.g. and then via 18, at 150 DEG -230 DEG F., to vessel 19, similar to 10.  Heat is provided by gas from line 16 and solution from line 20 which are fed to 19 at 250 DEG -350 DEG F.  The carbonate is finally decomposed in 19.  Liquid leaves 19 via 23 to vessel 24, similar to 14, and the urea solution containing small amounts of NH3 and CO2 is passed to prilling unit 26, leaving at 27 as finished urea.  Gas leaves at 28, to which a stream 29, consisting of NH3 containing stream from 26, may be added.  The gas containing 40% NH3, 25% CO2 and 30% H2O is fed to condenser-stripper 31 which contains packed sections 32, 33, bubble caps 34 and separator 40. The gas is scrubbed in 32-34 with ag. NH3-carbamate solution whereby CO2 is removed and H2O vapour and NH3 are condensed, the gas is cooled to 125 DEG -140 DEG F. at 32.  At 31 the carbonate solution contains 32% NH3, 32% CO2 and 36% H2O at 130 DEG -145 DEG F. and leaves at 35, a portion being recycled via 36 and the remainder is passed via 37.  The stream in 36 is cooled to 125 DEG -140 DEG F. in 38 and is fed to 31.  Solution is withdrawn at 41 at below 120 DEG F., cooled in 42 DEG  to 115 DEG F. and returned via 43.  The CO2 content of the gas is lowered in 33 from 5% to 0,3-0,5%.  The top plate of 34 is cooled to 50 DEG F. or less, e.g. by coils or NH3. NH3 leaving 45 is at, e.g. 50 DEG -75 DEG F., and contains less than 100 p.p.m. CO2.  NH3 is compressed to 200-400 p.s.i.g. in 46 and joins more NH3 from 53 at 54.  Solution at 37 is compressed at 48 to 200-400 p.s.i.g. and is combined with gas at 16, leaving 19 via 22 at 250 DEG -300 DEG F. and is fed to condenser-stripper 49 at 200-400 p.s.i.g. which contains packed sections 50, 51, bubble caps 52 cooled by coils or NH3 and separator 76.  Solution withdrawn at 71 at 200 DEG -212 DEG F. is cooled in 72 and returns at 73 at 140 DEG -150 DEG F.  Pure NH3 at 110 DEG -120 DEG F. is removed at 53.  Gas from 54 is fed to condenser 55, coolant enters at 56 and leaves at 57, and liquid NH3 leaving at 58 is pumped by 60 to join fresh NH3 at 4. Inert gases leaving at 59 are refrigerated for NH3 recovery.  At 49, solution is removed at 61, a portion 62 being returned at 70 via cooler 69, and the remainder is passed via 63 to pump 64, steam heater 66 and then via 6 to reactor 3.  Fig. 2, (not shown) uses in place of a second condenser-stripper a gas-liquid scrubber containing a packed and a liquid reservoir section.ALSO:Urea is produced by reacting under heat and pressure ammonia and carbon dioxide to form urea, ammonium carbamate, NH3 and H2O which are heated to decompose the carbamate to NH3 and CO2 off-gas which is cooled by contact with an aqueous solution of NH3 and carbamate which is thereby heated while a portion of the off-gas is condensed and absorbed.  The remaining gas is further cooled to recover a CO2-free NH3.  A portion, e.g. major, of the carbamate solution is cooled and recycled to the gas contacting stage and the remainder is recovered.  The carbamate decomposition may be carried out in 2 stages at 2 pressure levels, the first stage being at high pressure and the first off-gas being cooled while in heat exchange with the residual process shown. The contact between off-gas and solution may be carried out in a plurality of stages using solutions of successively lower temperature and CO2 content. The aq. NH3-carbamate solution and final off-gas stream may be recycled to the urea synthesis.  The molar ratio of NH3:CO2 may be 2-5:1 and the initial reaction conditions may be 2,000-6,000 p.s.i.g. at 320-430 DEG F.  The first carbamate decomposition may be at 200-400 p.s.i.g.