PROCESS FOR SYNTHESIS OF UREA AND A RELATED ARRANGEMENT FOR A REACTION SECTION OF A UREA PLANT

Abstract

A process for synthesis of urea and a related reaction section of a urea plant, where: ammonia and carbon dioxide are reacted in a liquid phase in a first reaction zone (S1) and heat (Q1) is withdrawn from said first reaction zone to promote the formation of ammonium carbamate, the liquid product (103) from said first reaction zone is then passed to a second reaction zone (S2) distinguished from said first reaction zone, and heat (Q2) is added to said second reaction zone to promote the decomposition of ammonium carbamate into urea and water, where the liquid phase in at least one of said first reaction zone and second reaction zone is kept in a stirred condition. A downflow reactor for carrying out the above process is also disclosed.

Claims

1. A process for synthesis of urea from reaction of ammonia and carbon dioxide, the process comprising the steps of: reacting ammonia and carbon dioxide in a liquid phase and in a first reaction zone (S1), and withdrawing heat (Q1) from said first reaction zone to promote the formation of ammonium carbamate, said first reaction zone producing a first liquid product mainly comprising ammonium carbamate, ammonia and water; and passing said first product to a second reaction zone distinguished from said first reaction zone, and adding heat to said second reaction zone to promote the decomposition of ammonium carbamate into urea and water, said second reaction zone producing a second liquid product containing urea, residual unconverted carbamate and excess ammonia, wherein the liquid phase in at least one of said first reaction zone and second reaction zone are kept in a stirred condition induced by mechanical stiffing means.

2. A process according to claim 1, said stirred condition being provided in a fully-baffled condition of the liquid phase.

3. A process according to claim 1, said second reaction zone having a temperature higher than temperature of said first reaction zone.

4. A process according to claim 1, said first reaction zone and said second reaction zone being physically separated.

5. A process according to claim 1, said first reaction zone and said second reaction zone being contained in a single vessel or being arranged in different vessels or compartments of vessels.

6. A process according to claim 1, further comprising a third reaction zone, or stripping zone, fed with said second liquid product obtained in the second zone, and where a carbamate contained in said second liquid product is decomposed by means of a heat supply and optionally by means of addition of a stripping medium, releasing ammonia and carbon dioxide, and the liquid phase in said third reaction zone being also kept in a stirred condition induced by mechanical stiffing means.

7. A process according to claim 6, where a gaseous stream comprising at least part of said ammonia and carbon dioxide released in the third reaction zone is fed directly in the gaseous state into said first reaction zone.

8. A reaction section of a urea plant, suitable for carrying out the process of claim 1, said reaction section comprising: a first reaction zone for conversion of ammonia and carbon dioxide into ammonium carbamate and a second reaction zone for decomposition of carbamate into urea, said second reaction zone being distinguished from said first reaction zone; means for feeding ammonia and carbon dioxide to said first reaction zone, and cooling means disposed in the first reaction zone and adapted to remove the heat of formation of ammonium carbamate, means for feeding a first product, mainly comprising ammonium carbamate, ammonia and water, from said first reaction zone to said second reaction zone; heating means disposed in said second reaction zone, adapted to provide heat for the decomposition of part of said carbamate into urea, and a flow line for removing a second product containing urea, residual unconverted carbamate and excess ammonia from said second reaction zone, and stirring means arranged in at least one of said first reaction zone and second reaction zone.

9. A reaction section according to claim 8, further comprising a third reaction zone, or stripping zone; means feeding a flow of said second liquid product from the second zone to said third zone; heating means for heating said third zone; optionally a line for addition of a stripping medium to said third zone; and stiffing means for keeping the liquid phase in said third reaction zone in a stirred condition.

10. A reaction section according to claim 9, further comprising a gas flow line for a direct connection between said third zone and first zone, arranged to recycle a gaseous flow of ammonia and carbon dioxide released in the third reaction zone into said first reaction zone.

11. A reaction section according to claim 10, said gas flow line being arranged to direct said gaseous flow close to stiffing means acting in the first reaction zone.

12. A reaction section according to claim 8, said first reaction zone and second reaction zone being hosted in a single vessel.

13. A reaction section according to claim 8, comprising: a first pressure vessel containing the first reaction zone, and comprising a heat exchanger for cooling the first reaction zone, and a first impeller for providing a stirred condition of the liquid phase in said first reaction zone, and a second pressure vessel containing the second reaction zone, and comprising at least one heat exchanger for heating the second reaction zone, and at least one second impeller for providing a stirred condition of the liquid phase in said second reaction zone.

14. A reaction section according to claim 13, said second pressure vessel comprising a cascade of compartments, each compartment being a respective portion of said second reaction zone and having a respective heat exchanger and impeller.

15. A reaction section according to claim 8, comprising: a first pressure vessel containing the first reaction zone, and comprising a heat exchanger for cooling the first reaction zone, and a first impeller for providing a stirred condition of the liquid phase in said first reaction zone, and a plurality of second pressure vessels arranged in a cascade, each of said second vessels containing a respective portion of said second reaction zone and having a respective heat exchanger and impeller.

16. A vertical reactor for the synthesis of urea from ammonia and carbon dioxide with the process of claim 1, comprising a vertical pressure vessel, where: the pressure vessel hosts a plurality of reaction zones, including at least a first reaction zone and a second reaction zone; the reactor comprises stiffing means arranged in at least one of said first reaction zone and second reaction zone; the reactor also comprises first heat exchange means arranged to remove heat from said first reaction zone, and second heat exchange means arranged to furnish heat to the second reaction zone; said reaction zones are arranged vertically and one above the other in the pressure vessel, the first reaction zone being the highest, and are in fluid communication so that a liquid effluent from a reaction zone can flow by gravity to a reaction zone below; the reactor comprises a fresh liquid ammonia input line arranged to feed liquid ammonia directly in the first reaction zone, and an output for withdrawing a liquid urea effluent which is located below the second or a lower reaction zone, the reactor being then structured to operate with a liquid phase which traverses the pressure vessel downwards.

17. A reactor according to claim 16, the pressure vessel comprising a further reaction zone which: is the lowest reaction zone in the pressure vessel; comprises dedicated stiffing means and heating means, and act substantially as a stripping zone.

18. A reactor according to claim 17, comprising a recovery line arranged for directing a gaseous stream comprising ammonia and carbon dioxide to flow upwards in the vessel from said stripping zone to the first and upper reaction zone.

19. A reactor according claim 16, said ammonia input line being arranged to direct the fresh liquid ammonia input in proximity of said stiffing means.

20. A reactor according to claim 19, comprising also a carbon dioxide input line, arranged to feed carbon dioxide in said first region of the pressure vessel, and preferably in proximity of said stiffing means.

21. A reactor according to claim 16, the stiffing means being in the form of bladed rotors and the heating means being in the form of heating coils.

22. A reactor according to claim 21, said rotors being associated to a common shaft extending all along the pressure vessel.

23. A reactor according to claim 16, comprising a plurality of compartments inside the pressure vessel, the compartments being arranged vertically one above the other and divided by horizontal baffles, wherein each of said reaction zones is formed by one or more of said compartments.

24. A reactor according to claim 23, each compartment having dedicated stiffing means and heating means.

25. A reactor according to claim 24, comprising: an upper compartment which delimits the first reaction zone; a plurality of intermediate compartments which delimit the second reaction zone; a lower compartment which delimits the stripping zone.

26. A method for the modernization of a vertical reactor for the synthesis of urea from ammonia and carbon dioxide, said reactor comprising a vertical pressure vessel, the method comprising the following steps: dividing the pressure vessel in a plurality of reaction zones, including at least a first reaction zone and a second reaction zone, wherein said reaction zones are arranged vertically and one above the other in the pressure vessel, the first reaction zone being the highest, and said reaction zones being in fluid communication so that a liquid effluent from a reaction zone can flow by gravity to a reaction zone below; arranging stiffing means in at least one of said first reaction zone and second reaction zone; arranging first heat exchange means are arranged to remove heat from said first reaction zone, and arranging second heat exchange means are arranged to furnish heat to the second reaction zone; and arranging a fresh liquid ammonia input line in order to feed liquid ammonia directly in the first reaction zone, and arranging an output for withdrawing a liquid urea effluent which is located below the second or a lower reaction zone, wherein the modified reactor is structured to operate with a liquid phase which traverses the pressure vessel downwards.

27. The process according to claim 3, wherein said second reaction zone having substantially the same pressure of said first reaction zone

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0067] FIG. 1 is a block scheme of a process according to a preferred embodiment of the invention.

[0068] FIG. 2 is a scheme of an equipment for carrying out the process, in accordance with a single-vessel embodiment.

[0069] FIG. 3 is a scheme of an equipment according to a multiple-vessel embodiment, including two stirred-tank reactors and a stripper.

[0070] FIG. 4 is a scheme of an embodiment including a cascade of stirred-tank reactors, and a stripper.

[0071] FIG. 5 is a scheme of an embodiment alternative to FIG. 4, wherein the cascade of reactors for the second reaction zone is replaced by a horizontal reactor, provided with internal, mechanically stirred compartments.

[0072] FIG. 6 is a scheme of a single-vessel vertical reactor according to another embodiment of the invention, providing two reaction zones and a final stripping zone.

[0073] FIG. 7 is a cross section of the reactor of FIG. 6.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

[0074] Referring to the block scheme of FIG. 1, the high-pressure conversion of carbon dioxide and ammonia into urea is carried out with a first step in a first reaction zone S1, followed by a second step in a second reaction zone S2.

[0075] A gaseous stream 100 of carbon dioxide and a liquid stream 101 containing ammonia make-up and some carbamate recycle are added to said reaction zone S1, where a liquid phase is maintained in agitation by a suitable mixer M1. A strong heat flow is released by the fast, exothermal conversion of ammonia and carbon dioxide into ammonium carbamate, and heat Q1 is removed from said reaction zone S1 to maintain the desired reaction temperature for the formation of ammonium carbamate. Heat Q1 is removed by appropriate means, e.g. by a heat exchanger crossed by a cooling medium.

[0076] The liquid phase is taken from reaction zone S1 and passed to the subsequent reaction zone S2 via line 103. The temperature of the liquid phase in the reaction zone S2 is similar or preferably higher than temperature of the liquid phase in zone S1, thus favouring the endothermic decomposition of ammonium carbamate into urea and water. This is achieved by supplying heat Q2 to zone S2 by appropriate means, e.g. a heat exchanger crossed by a heating medium.

[0077] The pressure in the second zone S2 may be substantially the same as in the first zone S1. Preferably said pressure is in the range 120 to 250 bar, more preferably around 160 bar. The liquid phase in said second zone S2 is kept in agitation by a suitable mixer M2, enhancing the transfer of heat Q2 to the liquid mass.

[0078] A concentrated aqueous solution of urea, with residual non-converted carbamate, is obtained at line 105, while a gaseous phase, mainly consisting of ammonia, carbon dioxide, water vapour and inert gases, is vented out from zones S1 and S2 via the line 104. Said line 104 may be throttled for the purpose of pressure control of the whole system.

[0079] A third reaction zone, or stripping zone, S3 is dedicated to the removal of unconverted carbamate and excess NH.sub.3 from the reaction product 105 (urea solution), via thermal decomposition and gas stripping process. Optional addition of a stripping medium such an inert gas stream, or carbon dioxide, is indicated by line 106. The gaseous products leave said third zone S3 through the line 102, and are redirected to the first reaction zone S1, where they are partially recovered as reactants. Heat is supplied to zone S3 by appropriate means, e.g. a heat exchanger crossed by a heating medium, preferably reaching temperatures exceeding 200 C. A more concentrated urea aqueous solution is delivered by line 107. In some embodiments the redirection of gaseous products from the third zone to the first zone may require a gas compressor or a blower (not shown in the figures).

[0080] Each of the zones S1, S2 or S3 can be implemented with one or more reactor vessels. In particular, the zones S1 and S2 may be implemented with a cascade of reactors or partitioned reactors. Some preferred embodiments of the outlined technology are presented below, with reference to FIGS. 2, 3 and 4.

First Embodiment

[0081] In a first embodiment of the invention, the reaction zones S1 and S2 are respectively the upper part and the mid part of a down-flow vertical reactor.

[0082] FIG. 2 shows a first implementation where the reactor is contained in a vertical, elongated pressure vessel 211 and includes: a top mixing turbine 217 and an upper heat exchange coil 219; another heat exchange coil 229 in the mid-part; perforated trays 230 and a line 231 for recovery of gaseous reactants; a bottom mixing turbine 237 and a bottom heat exchange coil 239. Baffles 218 are extended to the whole height of the vessel 211 to realize a fully baffled condition as explained above. The impeller 217 has a driving motor 217a and a shaft 217b extending inside the vessel 211. The mixer is preferably a magnetically-driven machine, eliminating the problem of sealing the driving shaft.

[0083] It may be noted that, in order to exploit efficiently the heat transfer conditions, in connection to the mechanical agitation, the coil assembly 219 must not prevent the liquid circulation imparted by the mixing turbine 217. Some expedients can be adopted to this purpose, as for instance by keeping the coil bank sufficiently away from the shell of the vessel 211, and by keeping a reasonable clearance between successive coils.

[0084] Ammonia, usually with some recycle of carbamate solution, is introduced via a liquid duct 213 at the top of vessel 211, in proximity of the upper face of the mixing turbine 217. Carbon dioxide is added via line 214 to the liquid phase in the vessel 211, preferably in proximity of the mixing turbine.

[0085] The product of reaction, mainly comprising carbamate, ammonia and water, flows downwards to cross the reaction zone S2. The liquid volume in S2 may be significantly larger than the volume of the first reaction zone S1, due to the relatively lower reaction rate. The heat supply from the coil 229 controls the temperature of the vessel content. The S2 zone is preferably equipped with the perforated plates 230, as used in the state of art technologies.

[0086] Finally, the liquid phase reaches the lowest part of the vessel 211 where, at higher temperature, CO.sub.2 is evolved, and possibly added through the line 234, in proximity of the lower face of the mixer 237, with the aim of stripping out the residual excess of dissolved ammonia. The resulting gaseous stream, comprising water-saturated CO.sub.2 and NH.sub.3, flows up in the direction of the mixer 217, carried by the line 231, to be recovered in the upper first reaction zone S1.

[0087] The urea aqueous solution constituting the final product is available at line 232. The outflow is controlled by the valve 236, actuated on the basis of the liquid level inside the vessel. A residual gas stream is discharged from top of reactor 211 through a line 215, where a manual or automatic valve 216 controls the pressure inside the reactor itself.

[0088] A second implementation is shown in FIG. 6. In this case, a vertical down-flow reactor is internally subdivided in a series of compartments by ring-shaped horizontal baffles 1230. One or more compartments form the reaction zones S1 or S2.

[0089] In the shown example, the first reaction zone S1 is substantially delimited by the upper compartment of the vessel 1211, above the top baffle 1230. This zone S1 is fitted with a first mixing turbine 1217 and a heat exchange coil 1219. In use, a cooling medium is circulated in said coil 1219, so that the reaction zone S1 is dedicated mainly to formation of ammonium carbamate.

[0090] The second reaction zone S2 is delimited by a series of compartments below said upper compartment. Each compartment has a respective mixing turbine and heat exchanger. In the figure, the second zone S2 comprises four compartments, with the respective mixing turbines 1227a to 1227d, and heat exchange coils 1229. In use, said coils 1229 are fed with a heating medium, in order to promote the formation of urea in said zone S2. FIG. 7 shows a coil 1229 and one of said mixing turbines denoted with 1227.

[0091] The optional third reaction zone S3 is delimited by the lower compartment and is equipped with a mixing turbine 1237 and a heat exchange coil 1239. Line 1234 is an optional feed of carbon dioxide, for use as stripping medium. Preferably, said line 1234 ends in proximity of the lower face of the mixer 1237, so that the additional carbon dioxide is delivered near the blades of said mixer.

[0092] The mechanical agitation system dedicated to the full reactor comprises the driving motor 1217a and a power shaft 1217b carrying the above mentioned turbines and extending all along the vertical axis of the vessel 1211, down to a final support located at the lower end. Preferably the reactor includes longitudinal baffles 1218, extended to the whole height of the vessel, which are appropriate to realize the intensive mixing action, known as fully baffled condition.

[0093] Ammonia, usually with some recycle of carbamate solution, is introduced in the first zone S1 via the liquid duct 1213 from top of the vessel 1211. The end of said duct 1213 delivers the ammonia feed in proximity of the upper face of the mixing turbine 1217, operating in the upper compartment. Carbon dioxide is added via line 1214 in proximity of the lower face of the same mixing turbine. A residual gas stream is discharged through the line 1215, where the manual or automatic valve 1216 controls the pressure inside the reactor.

[0094] The products of the ammonia and carbon dioxide condensation reaction, mainly comprising carbamate, ammonia and water, obtained in the upper compartments, flow downwards to cross the compartments of the reaction zone S2 below. It should be noted that the liquid volume in S2 may be significantly larger than the volume of the first reaction zone S1, due to the relatively lower reaction rate. Coils 1229 control the temperature of the various vessel compartments in said zone S2.

[0095] Finally, the liquid phase reaches the lowest part of the vessel 1211 where, under heat supply by the coils 1239, possible residual carbamate is decomposed. Carbon dioxide evolving from decomposition of carbamate, together with carbon dioxide added through the line 1234 (if provided) in proximity of the lower face of the mixer 1237, promote the stripping out of the residual excess of dissolved ammonia. The resulting gaseous stream, comprising water-saturated CO.sub.2 and NH.sub.3, rises up the full length of the vessel 1211, finally reaching top compartment, near the mixer 1217. Here, the carbon dioxide and ammonia in the uprising are recovered inside the reaction zone S1.

[0096] The urea aqueous solution constituting the product of the reactor is available at line 1232. The outflow is controlled by the valve 1236, actuated on the basis of the liquid level inside the vessel.

[0097] It can be noted that the embodiment of FIG. 6 has several intermediate stiffing means (mixing turbines) and has a better stage separation, compared e.g. to the simpler embodiment of FIG. 2; the latter however, might be preferred in some cases being less expensive.

Second Embodiment

[0098] Referring to FIG. 3, reaction zones S1 and S2 are now obtained with a first stirred vessel 311 and a second stirred vessel 321, connected by a transfer line 312. A third stirred vessel 331 provides the stripping zone S3.

[0099] Vessels 311, 321 and 331 have a similar structure. They are equipped with respective mixing turbines 317, 327 and 337. References 317a, 317b, 327a, 327b denote motors and shafts. Preferably the turbines are magnetically-driven. Full-length vertical baffles 318, 328 are to realize a fully baffled condition

[0100] The liquid volume in S2 may be significantly larger than the volume of the first reaction zone S1, due to the relatively lower reaction rate. Due to larger volume of liquid, the second vessel 321 is usually larger than the other ones, in particular than the first vessel 311. The turbine 327 may comprise several blade sections mounted on a shaft 327b, to keep uniform agitation in said vessel 321.

[0101] The vessels also contain respective heat exchangers. In particular, a coil 319 is arranged to remove heat from the first reaction zone S1 in vessel 311, while the coils 329 and 339 supply heat to the zones S2 and S3.

[0102] Ammonia, usually with some recycle of carbamate solution, is introduced via the liquid duct 313 into the agitated vessel 311, in proximity of the upper face of said mixing turbine 317. Carbon dioxide is added via line 314 to the liquid phase in the vessel 311, in proximity of the lower face of the mixing turbine. A residual gas stream is discharged by reactor 311 through the line 315, where the manual or automatic valve 316 controls the pressure inside the reactor itself.

[0103] The reactor product, mainly comprising carbamate, ammonia and water, is collected by line 312, and transferred to the second stirred vessel 321, wherein it is released by the pipe 323 in proximity of the upper face of a mixer 327. A coil 329, located inside the vessel 321, is devised to supply heat, controlling the temperature of the vessel content.

[0104] The reactor 321 is vented together with the reactor 311 through the line 325, joining the line 315 upstream the valve 316.

[0105] The final liquid product, mainly urea in aqueous solution, is obtained at line 322 and is transferred to vessel 331. The required stripping action, necessary for the recovery of the surplus ammonia, is granted by the carbon dioxide resulting from the unconverted carbamate decomposition, with optionally added extra carbon dioxide, injected by a pipe 334 below the mixer 337. The resulting gaseous stream, comprising water-saturated CO.sub.2 and NH.sub.3, is transferred by the line 335, to be recovered inside the first reaction zone S1.

[0106] The urea aqueous solution, constituting the final product, is available at line 332. The outflow is controlled by the valve 336, actuated on the basis of the liquid level inside the vessel 331.

Third Embodiment

[0107] In this embodiment, the second reaction zone is set up with multiple stirred reactors, arranged in cascade or in series. The advantage is that back-mixing phenomena are minimised, in comparison to the previous embodiments, increasing the achievable conversion rate.

[0108] Referring to FIG. 4, the first reaction zone S1 is formed by vessel 411, while the second reaction zone S2 is formed by three vessels in cascade, items 421A, 421B and 421C. The third zone or stripping zone is in a further vessel 431. Said vessels include mixing turbines and heat exchangers similarly to embodiments of FIGS. 1 to 3.

[0109] The liquid ammonia feed enters the first vessel 411 via the pipe 413, while CO.sub.2 is fed below the mixer of the same vessel via the pipe 414. The reaction heat removal is provided by banks of coils located inside the vessel, crossed by an adequate cooling fluid. The off gas is discharged by the pipe 415, and is used to control the system pressure by means of the valve 416.

[0110] The liquid product from the first vessel 411 is transferred by pipe 412 to the reactor 421A, namely the first reactor of the cascade, and carried in proximity of the mixer thereto, as indicated by the end of flow line 412, to be evenly distributed inside the vessel.

[0111] The liquid phase, which main component is ammonium carbamate, crosses in series the cascade of stirred vessels 421A, 421B and 421C, where the decomposition of the carbamate gives out progressively urea and water. A heating medium is supplied by the pipe 429 to the coil banks of the reactors, to compensate for the required endothermic heat. The final product, aqueous urea solution with excess ammonia, is discharged from the last reactor of the cascade, say 421C, to the next stripping vessel 431. Vent lines from the cascade join the line 415, as shown.

[0112] The vessel 431 has the same duty and operating conditions as 331 in FIG. 3.

Fourth Embodiment

[0113] In this further embodiment, depicted in FIG. 5, the second reaction zone S2 is realised by a single, multi-compartmented, horizontal vessel. A cylindrical, horizontal vessel 521 is partitioned in consecutive chambers or compartments as 522A, 522B and 522C, separated by frames 523A,523B and 523C, allowing the liquid phase to overflow from each chamber to the next one.

[0114] The first reactor vessel 511 is similar to reactors 311, 411 of the previously described embodiments. Each of the compartments in the vessel 521 has a mixing turbine and a heat exchanger.

[0115] The liquid phase from the first reactor 511, coming from line 512, crosses in series the three compartments inside the vessel 521, where urea and water are progressively obtained from the decomposition of the carbamate. A heating medium is supplied by the pipe 529 to the coil banks in the compartments 522A, 522B and 522C, to compensate for the required endothermic heat. The final product, aqueous urea solution, is discharged from the last compartment to the stripping vessel 531, as in the preceding embodiments.

EXAMPLE

[0116] In a commercial unit, taken as a reference, producing 1000 MTPD (metric tons per day) of urea, NH.sub.3 and CO.sub.2 feeds, together with a carbamate recycle stream containing water, are fed into the bottom section of cylindrical, vertical reactor of 75 m.sup.3 internal volume, provided with specially perforated trays. The operating pressure is 160 bar, measured at reactor bottom section, where ammonia and recycle carbamate solution, plus gaseous CO.sub.2, are introduced.

[0117] Under steady state conditions the reactor effluent leaves the reactor in the top section at 188 C. Said effluent is analysed in this example. The reactor material balance, based on reactor feeds, carbamate recycle solution analysis, and net urea produced, is checked as follows: [0118] urea formed in reactor 34.2% [0119] CO.sub.2 as unreacted carbamate 14.7% [0120] free NH.sub.3 plus NH.sub.3 in carbamate 31.3% [0121] total water 19.8%
wherefrom: [0122] total CO.sub.2 in reactor 39.8% [0123] total NH.sub.3 in reactor 50.7% [0124] net water fed to reactor 9.5%
and therefore: [0125] NH3/CO2 molar ratio 3.30 [0126] H2O/CO2 molar ratio 0.58 [0127] Conversion rate 63%

[0128] In comparison to this commercial set up, a pilot reactor system according to a single-vessel embodiment, similar to FIG. 2 has been operated at 150 bar and 170 C. in the first reaction zone S1.

[0129] In this zone S1, heat is removed to maintain the above temperature, by circulation of pressurised water, generating low pressure steam in a separate drum. The liquid phase containing the carbamate is proceeding downwards to the zone S2, where urea is formed in practically isothermal conditions, and finally to the lower reactor end (zone S3), wherein the residual carbamate is decomposed at higher temperature (>200 C.). The released CO.sub.2 is stripping out some ammonia excess, this gaseous phase travelling upwards to the zone S1.

The resulting mass balance is as follows: [0130] urea formed in reactor 43.5% [0131] CO2 as unreacted carbamate 6.7% [0132] free NH.sub.3 plus NH.sub.3 in carbamate 24.8% [0133] total water 27.0%
wherefrom: [0134] total CO.sub.2 in reactor 38.6% [0135] total NH.sub.3 in reactor 49.5% [0136] net water fed to reactor 13.9%
therefore: [0137] NH3/CO2 molar ratio 3.32 [0138] H2O/CO2 molar ratio 0.88 [0139] Conversion rate 82.6