CONVERSION PROCESS OF ANHYDROUS OFF-GAS STREAM COMING FROM MELAMINE SYNTHESIS PLANTS INTO UREA

Abstract

A process for converting an anhydrous off-gas stream from melamine synthesis plants to urea and the relative plant for the direct conversion of an anhydrous off-gas stream from melamine synthesis plants into urea, are described.

Claims

1. A conversion process of an anhydrous off-gas stream coming from melamine synthesis plants, at a pressure lower than 120 bars, into urea, comprising the following steps: a) a first isobar cooling step of the off-gas stream; b) transforming the cooled off-gas stream coming from step a) into ammonium carbamate and NH3; c) an isothermal pressurizing of the ammonium carbamate obtained in the previous step; d) an isobar heating of the product coming from step c); e) converting the ammonium carbamate coming from step d) into urea.

2. The process according to claim 1, wherein step a) is carried out on an anhydrous off-gas stream coming from the melamine synthesis reactor or from the post-reactor optionally possibly present in the melamine plant, or from both.

3. The process according to claim 1, wherein step a) further comprises a subsequent purifying step of the cooled off-gas stream from the melamine residues contained therein.

4. The process according to claim 1, wherein step b) consists of condensing or desublimating the off-gas stream coming from step a).

5. The process according to claim 4, wherein step b) comprises condensing the off-gas stream to give ammonium carbamate and NH3 in the form of a liquid solution.

6. The process according to claim 4, wherein step b) comprises desublimating the off-gas stream to give ammonium carbamate and NH3.

7. The process according to claim 5, wherein the excess ammonia produced in step b) is recycled to the melamine synthesis plant or to a urea synthesis plant.

8. The process according to claim 1, wherein step c) consists of an isothermal pressurizing wherein the anhydrous, liquid or solid, ammonium carbamate, obtained in the previous step b), is pressurized at a pressure ranging from 145 to 220 bars, in continuous or batchwise.

9. The process according to claim 1, wherein step d) consists of an isobar heating wherein the anhydrous ammonium carbamate, coming from step c), is heated to a reaction temperature T ranging from 180 to 200° C.

10. The process according to claim 1, wherein step e) consists of converting the ammonium carbamate coming from step d) into urea wherein the anhydrous ammonium carbamate is converted into urea with a conversion ranging from 75 to 85%.

11. The process according to claim 1, wherein the anhydrous off-gases, coming from melamine synthesis plants, have a pressure lower than 120 bars.

12. A plant for the direct conversion of an anhydrous off-gas stream coming from melamine synthesis plants, at a pressure lower than 120 bars, into urea which comprises: a cooling device suitable for cooling an off-gas stream coming from the melamine synthesis plant; a condenser or at least a pair of desublimators, respectively suitable for condensing or desublimating and pressurizing an off-gas stream coming from the cooling device into ammonium carbamate and ammonia; a pump suitable for pressurizing the ammonium carbamate coming from the condenser at a pressure ranging from 145 to 220 bars and sending said ammonium carbamate to a heating element or a pump suitable for sending the ammonium carbamate coming from the desublimator to said heating element; a heating element suitable for heating the pressurized ammonium carbamate to a reaction temperature T ranging from 180 to 200° C.; a reactor suitable for converting the ammonium carbamate into urea.

13. The plant according to claim 12, which comprises the pair of desublimators which send the ammonium carbamate directly to the heating element, without a pump.

14. The process according to claim 6, wherein the excess ammonia produced in step b) is recycled to the melamine synthesis plant or to a urea synthesis plant.

15. The process according to claim 11, wherein the anhydrous off-gases, coming from melamine synthesis plants, have a pressure lower than 100 bars.

16. The process according to claim 11, wherein the anhydrous off-gases, coming from melamine synthesis plants, have a pressure within the range of 60-100 bars.

Description

[0070] In order to better understand the features of the present invention, the following description refers to the following figures:

[0071] FIG. 1: a schematic representation of the steps of the process in relation to temperature and pressure based on a P-T state diagram of pure ammonium carbamate;

[0072] FIG. 2: simplified diagram of a first embodiment of the process according to the present invention;

[0073] FIG. 3: simplified diagram of a second embodiment of the process according to the present invention.

[0074] In accordance with FIG. 1, the variations in pressure and temperature are represented, of the steps of the process according to the present invention.

[0075] More specifically, the continuous line shown in FIG. 1 is representative of steps a)-c) and partially d) of the process according to the present invention, when the operating pressure of the melamine synthesis process is higher than the triple point pressure (in

[0076] FIG. 1 indicated with .square-solid.) of ammonium carbamate.

[0077] The dotted line is representative of steps a)-c), and partially d) of the process according to the present invention, identified in FIG. 1 with the letters a′)-c′) and partially d′, when the operating pressure of the melamine synthesis process is lower than the triple point pressure (.square-solid.) of ammonium carbamate.

[0078] In particular, during heating (Heating Mode), the pressurization and heating can be carried out partially (path c″ in FIG. 1) or completely (path c′ in FIG. 1) inside the desublimator. If the pressurization and heating are carried out only partially inside the desublimator (first section c′ and first section c″ respectively), the remaining part of pressurization and heating is carried out externally by means of a pump and a standard exchanger (second section c″ and d′).

[0079] The pressurization inside the desublimator can be advantageously carried out with the “liquid bottle” technique using compressed CO.sub.2 (and possibly NH.sub.3) and in this way the carbamate recycling pump (path c′) can be eliminated, or its prevalence (path c″) is greatly reduced.

[0080] Steps d) and e), represented by a continuous line, are common to both processes. The point (.circle-solid.) in FIG. 1 represents the urea synthesis pressure and temperature conditions.

[0081] In short, the solid line represents the solution:

[0082] off-gas pressure >triple point carbamate pressure (.square-solid.): path a-b-c-d

[0083] a) off-gas isobar cooling

[0084] b) off-gas condensation to liquid ammonium carbamate and liquid ammonia

[0085] c) carbamate+ammonia compression with pump

[0086] d) heating of liquid carbamate/ammonia up to the Urea synthesis conditions.

[0087] The dotted line represents the solution: off-gas pressure <triple point carbamate pressure (.square-solid.): path a′-b′-c′-d′

[0088] a) off-gas isobar cooling

[0089] b) off-gas desublimation/condensation to solid ammonium carbamate and liquid ammonia

[0090] c) carbamate +ammonia isothermal compression in confined volume

[0091] d) heating of carbamate/ammonia with carbamate melting up to the Urea synthesis conditions.

[0092] In FIG. 2, a simplified diagram of a first embodiment of the process according to the present invention is represented, namely the case in which, in step b) of the process according to the present invention, the off-gases are condensed in liquid phase in the form of a liquid solution of ammonium carbamate and NH.sub.3, the operating pressure of the melamine synthesis process being higher than the triple point pressure of the ammonium carbamate.

[0093] In accordance with FIG. 2, the anhydrous off-gas stream, at a pressure higher than the triple point pressure of the ammonium carbamate, and lower than 120 bars, coming from the melamine reactor (1), is sent to a gas cooler or off-gas cooling device (E-1). The off-gas stream thus cooled (stream 2) leaves the gas-cooler (E-1) and is sent to a carbamate condenser or surface condenser (E-2) where the off-gases are condensed in liquid phase in the form of a liquid solution of ammonium carbamate and NH.sub.3. The stream (3) of liquid carbamate is sent to a pressurization pump P-1, where it is pressurized at the urea synthesis pressure (145-220 bars), whereas the stream of excess ammonia (4) is recovered in gas phase and recycled in a position suitable for the melamine plant or urea plant. The liquid stream of pressurized ammonium carbamate (5) is then sent to a heating element (heater-E-3) where it is brought to a temperature suitable for the synthesis of urea (180-200° C.).

[0094] The stream (6) of anhydrous ammonium carbamate thus obtained in liquid phase and under pressure and temperature conditions suitable for the efficient formation of urea (T=180-200° C., P=145-220 bars), is thus introduced into a plug-flow reactor (R-1), preferably fed from above (down-flow) with a residence time of a few tens of minutes in order to reach equilibrium.

[0095] The product obtained from the urea synthesis reactor (7) is sent to a decomposer (stripper E-4) where the urea is separated from most of the unreacted CO.sub.2 and NH.sub.3 which are recycled (stream 9), after being joined with stream (2), to the condenser E-2, whereas the bottom stream (8) of the decomposer, consisting of urea (about 73% wt) and synthesis water (about 22% wt), with small quantities of NH.sub.3 and CO.sub.2 (<5%) is sent to a normal Urea concentration section, not shown in FIG. 2.

[0096] Alternatively, the stream (10) of unreacted reagents separated at the head of the decomposer E-4 can be recycled to the Urea plant in the form of an anhydrous gas mixture of NH.sub.3 and CO.sub.2, having a composition similar to that of the starting off-gases, but in significantly reduced quantities (25-15%).

[0097] In FIG. 3, a simplified diagram of a second embodiment of the process according to the present invention is represented, namely the case in which, in step b) of the process according to the present invention, CO.sub.2 and NH.sub.3 desublimate in solid phase in the form of solid ammonium carbamate:

[0098] (2+x) NH.sub.3(g)+CO.sub.2(g).fwdarw.NH.sub.2CONH.sub.4(l,$)+x NH.sub.3(g)

the operating pressure of the melamine synthesis process being lower than the triple point pressure of the ammonium carbamate.

[0099] In accordance with FIG. 3, the anhydrous off-gas stream coming from the melamine reactor (1′) is sent to a gas-cooler or off-gas cooling device (E-1). The off-gas stream thus cooled (stream 2′) leaves the gas-cooler (E-1) and is sent to a pair of desublimators (E-2-AB), which operate alternately in the desublimation phase (Cooling-Mode) and pressurization/melting (Heating-Mode), where CO.sub.2 and NH.sub.3 desublimate in solid phase in the form of solid ammonium carbamate and the excess ammonia is recovered in gas phase (4′) and recycled in a position suitable for the melamine plant or urea plant. The stream (3′) of liquid ammonium carbamate is pressurized at the urea synthesis pressure (>150 bars) with a discontinuous process by pressurization with CO.sub.2 and NH.sub.3 in the desublimator itself. The stream (3′) of pressurized carbamate is therefore a liquid carbamate stream (5′) which is sent by means of a pump P-2 to a heating element (heater-E-3) where it is brought to a temperature suitable for the synthesis of urea ranging from 180 to 200° C.

[0100] The stream (6′) of anhydrous ammonium carbamate thus obtained in liquid phase and under pressure and temperature conditions suitable for the efficient formation of urea (T=180-200° C., P=145-220 bars), is thus introduced into a plug-flow reactor (R-1), preferably fed from above (down-flow) with a residence time of a few tens of minutes in order to reach equilibrium.

[0101] The product obtained from the urea synthesis reactor (7′) is sent to a decomposer (stripper E-4) where the urea is separated from most of the unreacted CO.sub.2 and NH.sub.3 which are recycled (stream 9′), after being joined with stream (2′), to the condenser E-2, whereas the bottom stream (8′) of the decomposer, consisting of urea (about 73% wt) and synthesis water (about 22% wt), with small quantities of NH.sub.3 and CO.sub.2 (<5%) is sent to a normal Urea concentration section, not shown in FIG. 3.

[0102] Alternatively, the stream (10′) of unreacted reagents separated at the head of the decomposer E-4 can be recycled to the Urea plant in the form of an anhydrous gas mixture of NH.sub.3 and CO.sub.2, having a composition similar to that of the starting off-gases, but in significantly reduced quantities (25-15%).