Back to results
US 4,053,508 AGranted
Process and installation for preparing urea from ammonia and carbon dioxide
AssigneeStamicarbon B.V.
InventorMathieu Bongard
Life of this patent
Estimating the term…
Loading patent…
Loading patent…
AssigneeStamicarbon B.V.
InventorMathieu Bongard
Estimating the term…
A urea synthesis process is disclosed, wherein carbon dioxide and ammonium carbamate are reacted at elevated pressure to form urea, the resulting process stream is heated at reduced pressure to decompose ammonium carbamate, the off-gas from the ammonium carbamate decomposition is separated from the residual process stream (containing produce urea), and the off-gas is condensed to form an ammonium carbamate solution which is recycled to urea synthesis. A minimum amount of water in the condensate is recycled to the urea synthesis by taking an expanded gas-liquid mixture from a pressure reduction stage and directly separating the gas and liquid phases in such mixture without the application of heat thereto, thereafter heating the separated liquid phase to vaporize part of the liquid therein, and subjecting the resultant gas-mixture to separation, and condensing the gas phases obtained from both gas-liquid separations as condensate to be recycled to urea synthesis.
In a process for the preparation of urea from ammonia and carbon dioxide in a urea synthesis zone at a pressure of at least 160 atmospheres and a temperature of at least 150.degree. C. wherein ammonia and carbon dioxide which have not been converted into urea are separated from the reaction product and recirculated back to the urea synthesis in the form of ammonium carbamate by steps including discharging the reaction mixture from the synthesis zone, expanding the thus discharged reaction mixture in a plurality of pressure stages operated at sequentially lower pressures and separating the gas and liquid phases after the expansion in each of said stages to yield a urea solution which is practically free of unreacted ammonia and carbon dioxide in the last stage, condensing the gas comprising ammonia, carbon dioxide and water from each pressure stage, the condensate formed in a lower pressure stage serving as condensation agent for the gas from a preceding pressure stage, and feeding back the condensate obtained in the first pressure stage to the urea synthesis, the improvement whereby a minimum amount of water is included in the condensate returned to said synthesis, said improvement comprising the steps of taking the expanded gas-liquid mixture from a pressure stage and directly separating the gas and liquid phases (A) and (B), respectively, in said mixture without any application of heat thereto, thereafter heating the separated liquid phase (B) to vaporize part of the liquid therein, subjecting the resulting gas-liquid mixture to gas-liquid separation thereby obtaining gas phase (C) and liquid phase (D) and condensing the gas phases (A) and (C) obtained from both of said separations.
Dependencies are inferred from the claim text.
Drawings are temporarily unavailable. Open the PDF at USPTO
From the US filing history. Foreign family members aren’t listed.
Of urea, its salts, complexes or addition compounds: from carbon dioxide and ammonia
Acyclic or carbocyclic compounds
Term ended 11 Oct 1994 (estimated)
USPTO file history isn’t available for this patent, so only indexed dates are shown.
The present invention relates to a process and an installation for preparing urea from ammonia and carbon dioxide, and more particularly to an improvement for reducing the amount of water in starting materials recirculated to the vessel in which urea is formed.
In this preparation, which is usually carried out by introducing liquid ammonia and carbon dioxide into an autoclave in which the reaction mixture is kept at a pressure of at least 160 atm. and at a temperature of at least 150.degree. C., the reaction components first react to form ammonium carbamate. After that the ammonium carbamate is converted into urea and water, which conversion, however, does not proceed to completion; dependent on temperature and pressure a state of equilibrium is reached at a degree of conversion of 45 to 55%. This equilibrium can be shifted to the side of the urea formation by adding to the urea synthesis an amount of NH.sub.3 exceeding the stoichimetric amount, in other words by employing a molar NH.sub.3 --CO.sub.2 -ratio greater than 2.
Also when carried out in this way the conversion of ammonium carbamate into urea and water is far from complete, so that the reaction mixture leaving the synthesis autoclave still contains ammonium carbamate in addition to water and urea, and, if a reaction mixture has a NH.sub.3 -CO.sub.2 -ratio greater than 2, free NH.sub.3.
FIG. 1 is a diagram of this process; in the process shown in this figure the urea synthesis-reaction mixture is expanded in two pressure stages; in the first stage the reaction mixture issuing from the autoclave 1 under synthesis pressure is expanded via the expansion valve 2 to, for instance, 18 atm.; after this the mixture is heated in heater 3, the gas and liquid phases are separated in separator 4, and the liquid phase is expanded to the second pressure stage of, for instance, 3 atm. via expansion valve 9. Under this pressure the liquid-gas mixture obtained after expansion is passed through a heater 6 and after that separated in the liquid-gas separator 7. The resulting liquid phase, a urea solution which is practically free of NH.sub.3 and CO.sub.2 , can be discharged via valve 12 to be evaporated in vacuo for the recovery of crystals or prills.
The gases issuing from the gas-liquid separators 4 and 7 are condensed in condensers 5 and 8, respectively, the condensate formed in condenser 8 being pumped into condenser 5, in which a higher pessure prevails, by means of pump 10, while the condensate formed in condenser 5 is pumped back into the urea synthesis reactor by pump 11. NH.sub.3 and CO.sub.2 are admitted to the urea synthesis reactor via the conduits 13 and 14, respectively.
When this system is applied, a concentrated ammoniacal ammonium carbamate solution is ultimately fed back into the urea autoclave. As it is required that the amount of water in the autoclave is as small as possible in order that a good conversion yield may be obtained, it is advantageous if the water content in the ammonium carbamate solution or suspension to be returned is kept as low as possible.