PROCESS FOR THE PRODUCTION OF MELAMINE

Abstract

The present invention relates to a process for the production of melamine and the production plant including an off-gas condensation step carried out at a pressure of 70 to 220 bar by feeding gaseous and liquid streams at the same pressure, converting the liquid stream from the off-gas condensation step to a first stream, followed in succession by a high pressure decomposition step, a medium pressure decomposition step, a low pressure decomposition step, and urea concentration step.

Claims

1. A process for the production of melamine by means of a high-pressure urea pyrolysis reaction, comprising the following steps: a) carrying out an off-gas condensation step (OGC) at a pressure ranging from 70 to 220 bar, to which the following streams are fed: i) a gaseous stream of anhydrous off-gas comprising NH.sub.3 and CO.sub.2, coming from a scrubbing step of the off-gas (OGQ) with urea, a stream comprising NH.sub.3, CO.sub.2 and melamine vapours, produced as a gaseous effluent from the urea pyrolysis reaction in the reaction section (R) and the post-reaction and ammonia stripping section (PR) of the melamine synthesis process, ii) a liquid stream comprising NH.sub.3, CO.sub.2 and H.sub.2O, coming from an ammonia recovery step (AR); iii) a gaseous stream comprising NH.sub.3, CO.sub.2 and H.sub.2O coming from a process-water treatment step (PWT) and iv) a gaseous stream of off-gas comprising NH.sub.3, CO.sub.2 and H.sub.2O, coming from a high-pressure off-gas decomposition step (HPD); said streams i.-iv. being fed to the off-gas condensation step (OGC) at the same pressure at which the condensation step (OGC) is carried out, and obtaining a liquid stream comprising NH.sub.3, CO.sub.2 and H.sub.2O; b) carrying out a conversion step (OGR) to which said liquid stream comprising NH.sub.3, CO.sub.2 and H.sub.2O obtained at the end of said step a) is fed, obtaining a first stream comprising urea, ammonia, carbamate and H.sub.2O; c) carrying out a high-pressure decomposition step (HPD) to which the first stream comprising urea, ammonia, carbamate and H.sub.2O obtained at the end of the conversion step b) is fed, obtaining the stream iv) fed to the condensation step a) and a second stream comprising urea, carbamate and H.sub.2O; d) carrying out a medium-pressure decomposition step (MPD) to which the second stream comprising urea, carbamate and H.sub.2O obtained at the end of the high-pressure decomposition step c) (HPD) is fed, obtaining an off-gas stream comprising NH.sub.3, CO.sub.2 and water which is fed to the ammonia recovery step (AR), and a third stream comprising urea, carbamate and H.sub.2O; e) carrying out a low-pressure decomposition step (LPD) to which the third stream comprising urea, carbamate and H.sub.2O obtained at the end of the medium-pressure decomposition step d) (MPD) is fed, obtaining an off-gas stream comprising NH.sub.3, CO.sub.2 and water, which is fed to the process-water treatment step (PWT), and a fourth stream comprising urea and H.sub.2O; f) carrying out a urea concentration step (CON) to which the fourth stream comprising urea and H.sub.2O obtained at the end of the low-pressure decomposition step e) (LPD) is fed, obtaining a stream of concentrated urea which is fed to the off-gas scrubbing step (OGQ) with molten urea and a stream of H.sub.2O which is fed to the process-water treatment step (PWT).

2. The process according to claim 1, wherein the stream, comprising NH.sub.3, CO.sub.2 and melamine vapours, fed to the scrubbing step with urea (OGQ), is only composed of the gaseous stream, comprising NH.sub.3, CO.sub.2 and melamine vapours, obtained together with a liquid stream of raw melamine comprising melamine, unreacted urea, oxidized intermediate products of OAT pyrolysis and deammoniation and condensation products of melamine (polycondensates), from the separation of a biphasic liquid-gaseous effluent produced by the urea pyrolysis reaction (R), or said stream, comprising NH.sub.3, CO.sub.2 and melamine vapours, fed to the scrubbing step with urea OGQ, comprises said first stream of anhydrous off-gas (21) and a second stream of anhydrous off-gas comprising NH.sub.3, CO.sub.2 and melamine vapour, obtained by putting said liquid stream of raw melamine in contact, in a post-reaction and stripping with ammonia step (PR), with a stream of gaseous NH.sub.3, anhydrous and superheated under pressure, to form a liquid stream of raw melamine depleted of CO.sub.2 and said second stream of anhydrous off-gas.

3. The process according to claim 2, wherein said liquid stream of raw melamine depleted of CO.sub.2, before being subjected to a purification step (P) and a crystallization step (C) by cooling, is subjected to a quench-ammonolysis treatment (QAL) through contact with at least one aqueous ammonia stream, with the formation of an aqueous ammonia stream comprising purified melamine substantially free of polycondensates, said quench-ammonolysis treatment (QAL) being optionally carried out by contact with said aqueous ammonia stream coming from the process-water treatment step (PWT) and with a pure ammonia stream coming from the ammonia recovery step (AR).

4. The process according to claim 3, wherein said aqueous ammonia stream, comprising purified melamine substantially free of polycondensates, is fed to the purification step (P) from which a solution of purified melamine is obtained, fed to the cold crystallization step (C), from which a suspension of melamine is obtained, then fed to a separation step(S) from which a stream of pure melamine and a stream of mother liquor are obtained, said mother liquor stream comprising H.sub.2O, traces of melamine and OATs being partly fed to the process-water treatment step (PWT), where it is put in contact with the aqueous stream coming from step f) and with the off-gas stream comprising NH.sub.3, CO.sub.2 and water coming from the low-pressure decomposition (LPD) step e), obtaining at the end of the process-water treatment step (PWT) where melamine and OATs are decomposed by hydrolysis at a temperature ranging from 270 C. to 300 C., a stream of pure water, the liquid stream comprising ammonia and water which is sent to the quench-ammonolysis step (QAL), a liquid stream comprising water, ammonia and CO.sub.2 which is sent to the ammonia recovery step (AR) and the gaseous stream comprising water, ammonia and CO.sub.2 which is sent to the off-gas condensation section (OGC).

5. The process according claim 1, wherein said ammonia recovery step (AR) removes from the liquid stream comprising water, ammonia and CO.sub.2 coming from the process-water treatment step (PWT) and from the stream of off-gas coming from the medium-pressure decomposition step (MPD), at least a part of the ammonia contained therein, forming a liquid stream comprising NH.sub.3, CO.sub.2 and water which is sent to step a) for the condensation of the off-gas (OGC) and a stream of pure ammonia which is sent partly to the quench-ammonolysis step (QAL), partly to the pyrolysis reaction (R) and partly to the post-reaction and stripping with ammonia step (PR).

6. The process according to claim 1, wherein the urea fed to the off-gas scrubbing step (OGQ) is concentrated urea (33) coming from the urea concentration step f) (CON), optionally with the addition of a stream of molten urea from the battery limits, the two streams of anhydrous off-gas comprising NH.sub.3CO.sub.2 and melamine vapours coming from the reaction step (R) and from the post-reaction and stripping with ammonia step (PR), being fed, together or separately, to said off-gas scrubbing step (OGQ) with urea, obtaining a stream of scrubbed off-gas which is sent to step a) for the condensation of the off-gas (OGC) and a stream of urea comprising the melamine removed from the off-gas stream, which is recycled to the reaction step (R).

7. The process according to claim 1, wherein the molten urea fed to the off-gas scrubbing step (OGQ) is only concentrated urea coming from the urea concentration step f) (CON), a stream of liquid ammonia and a stream of gaseous CO.sub.2 being fed to the conversion step (OGR) to be put in contact with the liquid carbamate stream comprising NH.sub.3, CO.sub.2 and H.sub.2O coming from the off-gas condensation (OGC) step a) to obtain the first stream containing urea, ammonia, carbamate and water, in turn sent to the high-pressure decomposition (HPD) step c).

8. The process according to claim 1, wherein, in the process-water treatment step (PWT), the mother liquor stream, comprising H.sub.2O, traces of melamine and OATs, is subjected to a decomposition treatment by hydrolysis at a temperature ranging from 270 C. to 300 C., with the formation of the gaseous stream, comprising NH.sub.3, CO.sub.2 and H.sub.2O, and a liquid stream subjected to a stripping treatment in order to obtain a liquid stream of pure water (300).

9. A plant for the production of melamine, comprising: a condensation section (OGC), carried out at a pressure ranging from 70 to 220 bar, suitable for condensing: i. a gaseous stream of anhydrous off-gas comprising NH.sub.3 and CO.sub.2, coming via a first line from an off-gas scrubbing section (OGQ) with urea, a stream comprising NH.sub.3, CO.sub.2 and melamine vapours, produced as a gaseous effluent from the urea pyrolysis reaction and coming via a second line from the reaction section (R) and from the post-reaction and ammonia stripping section (PR), ii. a liquid stream comprising NH.sub.3, CO.sub.2 and H.sub.2O, coming via a third line from an ammonia recovery section (AR); iii. a gaseous stream comprising NH.sub.3, CO.sub.2 and H.sub.2O coming via a fourth line from a process water treatment section (PWT) and iv. a gaseous stream of off-gas comprising NH.sub.3, CO.sub.2 and H.sub.2O, coming via a fifth line from a high-pressure off-gas decomposition section (HPD); said condensation section (OGC) being connected via said first, third, fourth and fifth supply lines to the scrubbing (OGQ), ammonia recovery (AR), process-water treatment (PWT) and high-pressure decomposition (HPD) sections, from which it receives streams i.-iv.) at the same pressure present in the condensation section (OGC); said condensation section (OGC) being connected via a sixth line to a conversion section (OGR) to which it feeds a liquid stream comprising NH.sub.3, CO.sub.2 and H.sub.2O; the conversion section (OGR) to which said liquid stream comprising NH.sub.3, CO.sub.2 and H.sub.2O coming from the condensation section (OGC) is fed via the sixth line, is connected to a high-pressure decomposition section (HPD) to which it feeds via a seventh line a first stream comprising urea, ammonia, carbamate and H.sub.2O; the high-pressure decomposition section (HPD) to which the first stream comprising urea, ammonia, carbamate and H.sub.2O is fed via the seventh line, is connected via the fifth line to the condensation section (OGC) and via an eighth line to a medium-pressure decomposition section (MPD) to which it feeds, via the eighth line, a second stream comprising urea, carbamate and H.sub.2O; the medium-pressure decomposition section (MPD) to which the second stream comprising urea, carbamate and H.sub.2O is fed via the eighth line, is connected via a ninth line to the ammonia recovery section (AR) to which it feeds an off-gas stream comprising NH.sub.3, CO.sub.2 and H.sub.2O, and via a tenth line to a low-pressure decomposition section (LPD) to which it feeds a third stream comprising urea, carbamate and H.sub.2O; the low-pressure decomposition section (LPD) to which the third stream comprising urea, carbamate and H.sub.2O from the medium-pressure decomposition section (MPD) is fed, via the tenth line, is connected via an eleventh line to the process-water treatment section (PWT) to which an off-gas stream comprising NH.sub.3, CO.sub.2 and H.sub.2O is fed, and via a twelfth line to the urea concentration section (CON) to which a fourth stream comprising urea and H.sub.2O is fed; the urea concentration section (CON) to which the fourth stream comprising urea and H.sub.2O is fed, via the twelfth line, from the low-pressure decomposition section (LPD), is connected via a thirteenth line to the off-gas scrubbing section (OGQ) to which it feeds a stream of concentrated urea and via a fourteenth line to the process-water treatment section (PWT) to which it feeds a stream of H.sub.2O.

10. The plant according to claim 9, wherein the urea scrubbing section (OGQ) is connected via the second line, to the reaction section (R) and post-reaction and ammonia stripping section (PR) from which it receives, via fifteenth and sixteenth lines, a stream of anhydrous off-gas comprising NH.sub.3, CO.sub.2 and melamine vapors, said reaction section (R) being connected via a seventeenth line to the post-reaction section (PR) to which it feeds a liquid stream of raw melamine comprising melamine, unreacted urea, oxidized intermediate products of pyrolysis (OATs) and deammoniation and condensation products of melamine (polycondensates), said reaction (R) and post-reaction and stripping sections with ammonia (PR) being respectively connected, via eighteenth and nineteenth lines, to the ammonia recovery section (AR) from which they receive a stream of gaseous, anhydrous and superheated ammonia under pressure via the eighteenth line and a stream of gaseous ammonia, anhydrous and superheated under pressure via the nineteenth line, said post-reaction section (PR) being connected via a twentieth line to the quench-ammonolysis section (QAL) to which it feeds, via the twentieth line, a liquid stream of raw melamine depleted of CO.sub.2, said quench-ammonolysis section (QAL) being connected, via twenty-first, twenty-second and twenty-third lines, to the ammonia recovery section (AR) from which it receives a stream of pure ammonia, to the process-water treatment section (PWT) from which it receives an aqueous ammonia solution, to a separation section(S) from which it receives a stream of mother liquor, said quench-ammonolysis section (QAL) being finally connected, via a twenty-fourth line, to a purification section (P) to which it feeds an aqueous ammonia stream comprising purified melamine substantially free of polycondensates, said purification section (P) being connected via a twenty-fifth line to the cold crystallization section (C) to which it feeds a purified melamine solution, said cold crystallization section (C) being connected via a twenty-sixth line to a separation section(S) to which a melamine suspension is fed, said separation section(S) being connected via a twenty-seventh line to the process-water treatment section (PWT) to which it feeds a first part of the mother liquor comprising H.sub.2O, traces of melamine and OATs to be thermally decomposed at a temperature ranging from 270 C. to 300 C., and via a twenty-eighth line to the quench-ammonolysis section (QAL) to which it feeds a second part of the mother liquor comprising H.sub.2O, traces of melamine and OATs, and, via a twenty-ninth line, to the outside of the plant where a stream of pure melamine is sent.

11. The plant according to claim 9, wherein the process-water treatment section (PWT) is connected via the eleventh line, to the low-pressure decomposition section (LPD) from which it receives, via said eleventh line, an off-gas stream comprising NH.sub.3, CO.sub.2 and H.sub.2O, via the fourteenth line, to the urea concentration section (CON) from which it receives an aqueous stream, and via a thirtieth line, to the outside of the plant where a stream of pure water is sent, said process-water treatment section (PWT) also being connected via the twenty-second line, to the quench-ammonolysis section (QAL) to which it feeds a liquid stream comprising ammonia and water, via a thirty-first line, to the ammonia recovery section (AR) to which it feeds a liquid stream comprising water, ammonia and CO.sub.2 and, via a thirty-second line, to the off-gas condensation section (OGC) to which it feeds a gaseous stream comprising NH.sub.3, CO.sub.2 and H.sub.2O.

12. The plant according to claim 9, wherein the off-gas scrubbing section (OGQ) is connected via said thirteenth line, to the urea concentration section (CON) from which it receives a stream of concentrated urea and, via a thirty-third line, with the outside of the plant from which it receives a stream of molten urea from the battery limits, said off-gas scrubbing section (OGQ) being then connected via a thirty-fourth line, with the reaction section (R) to which it feeds a urea recycling stream comprising melamine removed from the off-gas stream from the second line.

13. The plant according to claim 9, wherein the off-gas scrubbing section (OGQ) is connected, via the thirteenth line, to the urea concentration section (CON) from which it receives a stream of concentrated urea and wherein the conversion section (OGR), via thirty-fifth and thirty-sixth lines, with the outside of the plant from which it receives a stream of liquid ammonia and a stream of gaseous CO.sub.2.

14. The plant according to claim 9, wherein the urea concentration section (CON) is connected, via a thirty-seventh line, with the outside of the plant from which it receives a stream of urea solution from the battery limits, and, via the thirteenth line, to the off-gas scrubbing section (OGQ) to which it feeds a stream of concentrated urea, said off-gas scrubbing section (OGQ) being then connected, via the thirty-fourth line, to the reaction section (R) to which it feeds a urea recycling stream comprising the melamine removed from the off-gas stream from the second line.

15. The process for the production of melamine according to claim 1 wherein the off-gas condensation step (OGC) is carried out at a pressure ranging from 80-150 bar.

16. The plant for the production of melamine according to claim 9 wherein the condensation section (OGC) is carried out at a pressure ranging from 80-150 bar.

Description

DESCRIPTION OF THE DRAWINGS

[0051] The present invention is described hereunder with reference to the following schematic figures which are illustrative and non-limiting with respect to the protection scope defined by the enclosed claims.

[0052] In the figures attached to the present invention,

[0053] FIG. 1 is a block diagram of the melamine production process according to a first embodiment of the present invention;

[0054] FIGS. 2-7 are block diagrams of the melamine production process according to further embodiments of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0055] In the following description, for illustrating the figures, identical reference numbers are used for indicating elements with the same function. Furthermore, for clarity of illustration, some numerical references may not be repeated in all the figures.

[0056] The different characteristics in the different embodiments can be combined together as desired according to the previous description, should the advantages resulting specifically from a particular combination be used.

[0057] A first embodiment of the Stand-Alone Melamine Process, object of the present invention, is illustrated in the block diagram of FIG. 1 and comprises:

a scrubbing section of the off-gas OGQ for recovering the melamine vapour contained in the anhydrous off-gas coming from the Reaction section R and the Post-Reaction section PR. The anhydrous off-gas streams 21 and 22 coming from the sections R and PR are subjected, together or separately, to a melamine recovery treatment by scrubbing with urea, in a single off-gas scrubbing section OGQ or in separate scrubbing sections (not shown in FIG. 1).

[0058] The second stream 22 of anhydrous off-gas coming from the Post-Reaction section is preferably combined with the first stream 21 of anhydrous off-gas coming from the reaction section, forming a single stream 23 of anhydrous off-gas which is subjected to the same treatment in a single scrubbing section of the off-gas with urea OGQ. The section OGQ operates at approximately the same pressure as the Reaction R and Post-Reaction PR sections, whereas the scrubbing temperature varies within the range of 140 C. to 250 C. In the scrubbing with urea, the melamine contained in the anhydrous off-gas is removed and brought into solution/suspension in the urea. The stream of urea 1 fed to the off-gas scrubbing section OGQ is obtained from the combination of the molten urea stream from the battery limits 100 and the recycled concentrated urea stream 33, coming from the urea concentration section CON. The urea stream 2 exiting from the off-gas scrubbing section OGQ is fed to the Reaction section R, whereas the anhydrous off-gas stream 24 is fed to the off-gas condensation section OGC.

[0059] The Reaction section R comprises a pyrolysis reactor that operates continuously and in which a suitable heating system provides the reagent system with the necessary reaction calories, maintaining the temperature within the range of 360 C. to 420 C. The reaction pressure is kept at a value higher than 70 bar, preferably within the range of 70 to 220 bar, more preferably within the range of 80 to 150 bar. A stream of gaseous NH.sub.3, anhydrous and superheated under pressure 16R, coming from the ammonia recovery section AR is also preferably introduced into the reactor together, with the urea.

[0060] The product of the urea pyrolysis reaction, consisting of a two-phase effluent comprising a gaseous phase 21 and a liquid phase 3, is subjected to subsequent treatments. The liquid phase 3, exiting from the Pyrolysis Reactor R, containing melamine, unreacted urea, oxidized intermediate products of OAT pyrolysis and melamine deammoniation and condensation products (polycondensates), is sent to a subsequent reaction step in a Post-Reactor PR which operates under temperature and pressure conditions substantially the same as those of the Pyrolysis Reactor R and into which a stream of gaseous, anhydrous and superheated NH.sub.3 under pressure 16P is fed, in order to eliminate the dissolved CO.sub.2 and complete the urea pyrolysis reaction and transform most of the OATs into melamine, obtaining a liquid phase of melamine 4 and a second gaseous phase 22.

[0061] The liquid phase of melamine exiting from the Post-Reactor 4 is brought into aqueous solution in a quench-ammonolysis section QAL, in the presence of NH.sub.3, fed as pure NH.sub.3 by means of a stream 15 coming from the ammonia recovery section AR and as ammonia in solution by means of a stream 12 coming from the water treatment section PWT, the whole system being kept under controlled conditions of temperature and residence time, preferably at a temperature ranging from 160 C. to 180 C., more preferably from 170 C. to 172 C., at a pressure ranging from 25 to 45 bar, more preferably equal to 30 bar, for a time ranging from 30 to 60 minutes, more preferably equal to 45 minutes, obtaining a solution substantially free of polycondensates 5 which is brought to a purification section P to obtain a solution free of impurities 6 which is then subjected to crystallization in the crystallization section C, to obtain a melamine suspension 7 which is then fed to the separation section S to give high-purity melamine 200. A part of the mother liquor 10 containing melamine and small quantities of OATs is recycled from the section S, without any treatment, to the quench-ammonolysis section QAL.

[0062] The remaining part of the mother liquor 9 is treated in a process-water treatment section PWT. Said section PWT also receives an off gas stream 31 from the low-pressure decomposition section LPD and a liquid stream 34 of process water coming from the urea concentration section CON. A liquid stream containing water and ammonia 12 is obtained, through decomposition and distillation, from the process-water treatment section PWT, which is sent to the quench-ammonolysis section QAL, together with a liquid stream 13 containing water, NH.sub.3 and CO.sub.2 which is sent to the ammonia recovery AR section, a gas stream 14 containing NH.sub.3, CO.sub.2 and water sent to the off-gas condensation section OGC and a stream of purified process water 300 sent to the battery limits.

[0063] The ammonia recovery section AR, in addition to receiving the liquid stream 13 from the process-water treatment section PWT, also receives the off-gas gaseous stream 29 from the medium-pressure decomposition section MPD. In the ammonia recovery section AR, a stream of pure ammonia is obtained, by distillation, partly recycled to the quench-ammonolysis section QAL as stream 15, partly recycled to the reaction section R as stream 16R and partly recycled to the Post-reactor RP as stream 16P. A second liquid stream 17 is obtained from the ammonia recovery section AR, at the outlet, containing NH.sub.3, CO.sub.2 and water sent to the off-gas condensation section OGC.

[0064] Step a) of the process according to the present invention is carried out in the off-gas condensation section OGC which has an operating pressure equal to or slightly lower than the off-gas scrubbing section OGQ, which in turn has an operating pressure equal to or slightly lower than the reaction section R, and has an operating temperature such as to obtain the total condensation of the incoming gaseous streams. The off-gas condensation section OGC receives three gaseous streams, the off-gas stream 24 from the off-gas scrubbing section OGQ, the stream 14, comprising NH.sub.3, CO.sub.2 and water, from the process-water treatment section PWT and the off-gas stream 27 from the high-pressure decomposition section. Furthermore, the section OGC receives a liquid stream 17, comprising NH.sub.3, CO.sub.2 and water, from the ammonia recovery section AR. The condensation heat of the three gaseous streams is recovered by generating steam which is used within the Stand-Alone Melamine Process. The liquid stream 25 exiting from the section OGC comprising NH.sub.3, CO.sub.2 and water is pumped to the conversion section OGR, where step b) of the process according to the present invention takes place. The liquid stream 25 can be possibly preheated to improve the conversion of the off-gas in step b) of the process according to the present invention.

[0065] The off-gas conversion section operates at a pressure within the range of 120 bar to 250 bar, preferably at a pressure within the range of 140 bar to 220 bar. The operating temperature of the section OGR varies within the range of 150 C. to 250 C., preferably within the range of 170 C. to 220 C. and the molar ratio NH.sub.3/CO.sub.2 is equal to 2.2-5, preferably equal to 2.5-4.5 mole/mole.

[0066] The carbamate fed with stream 25, comprising NH.sub.3, CO.sub.2 and water, is partially converted into urea inside the section OGR. The stream 26 exiting from the section OGR contains urea, ammonia, unreacted carbamate and water. This stream 26 is sent to step c) of the process according to the present invention in the high-pressure decomposition section HPD where the liquid carbamate is decomposed into NH.sub.3 and CO.sub.2 in an exchanger to which heat is supplied by steam. The section HPD operates at the same pressure as the section OGC or slightly higher, and at an operating temperature within the range of 150 to 250 C., preferably 170 C. to 220 C. A gaseous stream 27 is formed at the outlet of the section HPD, which is recycled to the section OGC, together with a liquid stream 28 containing urea, water and also a part of non-decomposed carbamate, which is sent to step d) of the process according to the present invention in the medium-pressure decomposition section MPD. The section MPD operates at a pressure ranging from 8 to 30 bar, preferably at a pressure within the range of 12 to 24 bar and at an operating temperature within the range of 140 C. to 180 C., preferably 150 C. to 170 C. In the section MPD, part of the carbamate contained in stream 28 is decomposed, the necessary heat is supplied by steam, and a liquid stream 30 containing urea, water and a part of carbamate is obtained, which is sent to step e) of the process according to the present invention in the low-pressure decomposition section LPD together with an off-gas gaseous stream 29 sent to the ammonia recovery section AR. The section LPD operates at a pressure within the range of 1 to 8 bar, preferably at a pressure within the range of 2 to 6 bar and at an operating temperature within the range of 130 C. to 170 C., preferably 140 C. to 160 C. In the section LPD, the carbamate contained in the stream 30 is decomposed, the necessary heat is supplied by steam, a liquid stream 32 is obtained, mainly comprising urea and water, sent to step f) of the process according to the present invention in the urea concentration section CON and a gaseous stream 31 sent to the process-water treatment section PWT. The urea solution is concentrated in the section CON, obtaining a urea stream 33 which is recycled to the section OGQ and a process-water stream 34, mainly comprising water and a low concentration of urea, NH.sub.3 and CO.sub.2 which is sent and treated in the section PWT.

[0067] In the above-mentioned liquid stream 32, comprising mainly urea and water, the term prevalently means that the stream contains approximately 98-99% by weight of urea and water, the remaining percentage being composed of traces of ammonia and CO.sub.2. In the process-water stream 34, comprising mainly water and a low concentration of urea, NH.sub.3 and CO.sub.2, the term prevalently means that the stream contains approximately 98-99% by weight of water.

[0068] A second embodiment of the Stand-Alone Melamine Process, object of the present invention, is illustrated in the block diagram of FIG. 2 and differs from the first embodiment in that the urea fed to the off-gas scrubbing step is only the stream of recycled concentrated urea 33 coming from the urea concentration step f) which is effected in the concentration section CON, whereas a liquid ammonia stream 101 and a gaseous CO.sub.2 stream 102 are fed to the conversion step in the section OGR to be put in contact with the liquid stream comprising ammonia, CO.sub.2 and water coming from the off-gas condensation step a) in the section OGC to obtain the first stream containing urea, carbamate and water to be sent to step c) in the high-pressure decomposition section HPD.

[0069] In a third embodiment of the process according to the present invention, (represented in FIG. 3), the urea concentration step f) CON receives a stream 110 of urea solution from the battery limits from outside the system, and feeds a concentrated urea stream 33 to the off-gas scrubbing step OGQ, whereas a recycling stream of urea 2 comprising the melamine removed from the off-gas stream 23 coming from the off-gas scrubbing step OGQ is fed to the reaction step R.

[0070] The three embodiments illustrated in the block diagrams of FIGS. 1-3 can therefore be effected as alternatives or they can constitute embodiments of the process according to the present invention, combining them in all possible ways as represented in FIGS. 4-7.

[0071] The present invention as described above relates to a non-catalytic high-pressure melamine production process which does not need to be integrated with a urea process, as the off-gas produced from the synthesis of urea to melamine are converted back into urea within the same process for the production of melamine, thus solving the problems dependent on integration with a urea process.

[0072] The process for the production of melamine, object of the present invention, has an innovative element compared to the known art of being a scheme which uses some sections in common both in the steps of the melamine production process relating to the production, treatment and purification of the melamine streams and in the steps of the melamine production process according to the present invention relating to the treatment of the off-gas: these common sections have revealed surprising positive effects by solving the problems listed above.

[0073] In the process scheme according to the present invention, in fact, the process-water treatment section PWT, the ammonia recovery section AR and the off-gas condensation section OGC are common sections, i.e. they receive and return streams to the process sections for the production of melamine and the sections of the process for the treatment of off-gas and the production of urea.

[0074] The off-gas condensation section OGC has the purpose of condensing the high-pressure off-gas gaseous streams coming from the different steps of the process for the production of melamine according to the present invention and using the condensation heat for generating steam, in turn used in the melamine production process. The section OGC receives the anhydrous off-gas from the section OGQ, it receives the off-gas generated by the decomposition of the carbamate (a stream containing NH.sub.3, CO.sub.2 and H.sub.2O) from the section HPD, it receives an off-gas stream 14 from the common section PWT and a liquid stream of carbamate 17 from the common section AR. The section OGC returns a liquid stream of carbamate fed to the conversion section OGR to convert some of the carbamate back to urea.

[0075] The ammonia recovery section AR has the purpose of recovering pure ammonia and recycling it to the appropriate sections. The section AR receives the off-gas from the section MPD, it receives a liquid carbamate stream 13 from the section PWT and returns a liquid carbamate stream 17 to the section OGC and a stream of pure ammonia sent partly as stream 15 to the section QAL and partly as streams 16R, 16P to the reaction section R and to the post-reaction and ammonia stripping section PR.

[0076] The process-water treatment section PWT has the purpose of treating all the process water and recycling it to the appropriate sections. The section PWT receives the mother liquor from the section S with stream 9, the process water from the urea concentration CON with stream 34, the off-gas from the low-pressure decomposition section with stream 31 and returns a high-pressure off-gas stream 14 to the section OGC, a liquid stream 12 of NH.sub.3 and water to the section QAL and a liquid stream of carbamate 13 to the section AR.

[0077] It should also be pointed out that, unlike the sections HPD, MPD and LPD of the integrated melamine/urea processes of the state of the art, each of which comprises a decomposer (heater) and a condenser, the sections HPD, MPD and LPD of the process for the production of melamine according to the present invention require only the presence of a heater with considerable advantages in terms of installation costs and consumption. Condensers are not necessary thanks to the particular process scheme according to the present invention which involves the use of sections OGC, AR and PWT in common for different steps of the process.

[0078] A further advantage of the process according to the present invention and the relative plant is that the entire system is managed by a single operating control unit.

[0079] The following example represents a non-limiting embodiment of the present invention.

Example 1

[0080] In a melamine plant with a nominal capacity of 40,000 t/y, 16.0 t/h of molten urea are sent to the off-gas scrubbing section OGQ to effect the scrubbing of the off-gas stream of 13.0 t/h, coming from the reaction sections R and PR, in order to obtain a stream of anhydrous off-gas, free of melamine; the section OGQ operates at the same pressure as the reaction sections R and PR, 110 bar and at a temperature of 215 C.

[0081] The molten urea containing the melamine recovered from the section OGQ and a stream of gaseous NH.sub.3 of 1.0 t/h are fed to the melamine reactor R; the reactor operates at 380 C. and 110 bar.

[0082] 11.8 t/h of anhydrous off-gas comprising melamine steam and 5.7 t/h of raw liquid melamine are separated from the reactor. This raw liquid melamine is treated under the same reactor conditions in a post reactor PR with 1.1 t/h of anhydrous gaseous NH.sub.3 and superheated under pressure, obtaining 5.6 t/h of liquid melamine comprising only 0.02% by mass of dissolved CO.sub.2; this stream also contains 3.4% by mass of polycondensates, 0.6% by mass of OATs and is substantially free of unreacted urea.

[0083] 1.2 t/h of anhydrous off-gas comprising melamine steam is separated from the post-reactor PR, which combined with the off-gas exiting from the reactor R form a stream of 13.0 t/h comprising 3.8% by mass of melamine steam which, as indicated above, is sent to the off-gas scrubbing section OGQ.

[0084] The liquid melamine exiting from the PR is sent to the quench-ammonolyser QAL for purification in aqueous solution with NH.sub.3, under the following operating conditions: 172 C., 30 bar and NH.sub.3 concentration 14% by mass. The aqueous solution present in the quench-ammonolyser is obtained by recycling to QAL an aqueous stream coming from the section PWT of 53.9 t/h having a concentration of NH.sub.3 at 15% by mass and a stream coming from the section AR of 0.2 t/h of pure ammonia.

[0085] 67.1 t/h of an aqueous solution containing 8% by mass of melamine, 14% by mass of NH.sub.3 and comprising approximately 4,000 ppm by mass of OATs and approximately 500 ppm by mass of polycondensates, leave the QAL quench-ammonolyser.

[0086] This solution is fed to the purification section P, comprising filters, from which a solution containing less than 100 ppm by mass of polycondensates exits; the purification section P operates under the same conditions as the section QAL (pressure 30 bar and temperature 172 C.). The solution exiting from the section P is then sent to the crystallizer C, which operates at a temperature of 45 C., a pressure of 0.5 bar and a pH of approximately 11.5.

[0087] The suspension exiting from the crystallizer C is sent to a solid/liquid separator (centrifuge) S, which separates the crystallization mother liquor from the high-purity melamine (titre over 99.9% by mass on dry matter).

[0088] The approximately 62.1 t/h of mother liquor exiting from S is divided into two streams. A stream of 7.4 t/h is recycled directly to the quench-ammonolysis section QAL, without undergoing any treatment; the remaining 54.7 t/h are sent to the process-water treatment section PWT.

[0089] The section PWT, in addition to receiving the stream of mother liquor, receives a stream of process water from the urea concentration section CON and an off-gas stream from the low-pressure decomposition section LPD. The purpose of the section PWT is to purify the process water by thermally decomposing urea, OATs and melamine contained in the incoming streams obtaining NH.sub.3 and CO.sub.2, together with pure water to be sent to the battery limit. A stream of purified process water of 2.1 t/h sent to the battery limit, a liquid stream of water and ammonia of 53.9 t/h recycled to the section QAL, a liquid stream of carbamate (NH.sub.3, CO.sub.2 and H.sub.2O) of 3.7 t/h recycled to the section AR and a stream of gaseous off-gas of 0.9 t/h recycled to the section OGC, exit from the section PWT.

[0090] The section PWT comprises equipment for the thermal decomposition of urea, for the thermal decomposition of OATs and melamine at a temperature of 290 C. and equipment for the distillation and recovery of ammonia which is thus recycled internally.

[0091] The ammonia recovery section AR has the purpose of distilling pure ammonia from the incoming streams and recycling it within the process. The ammonia recovery section AR, in addition to the carbamate stream from the section PWT, also receives an off-gas stream of 4.9 t/h from the medium-pressure decomposition section MPD. At the outlet of the section AR, there is a liquid stream of 2.3 t/h of pure ammonia and a liquid stream of 6.3 t/h of carbamate recycled to the section OGC.

[0092] The off-gas condensation section OGC has the purpose of condensing the incoming gaseous off-gas streams and using the condensation heat for generating steam and therefore recovering energy. The section OGC, in addition to receiving the off-gas stream from the section PWT and the liquid carbamate stream from the section AR, receives the gaseous off-gas stream of 10.1 t/h from the high-pressure decomposition section HPD and receives the gaseous off-gas stream of 12.5 t/h from the section OGQ. The section OGC has an operating pressure slightly lower than the sections OGQ and HPD from which it receives the off-gas, its operating conditions are 105 bar and 155 C. At the outlet of the section OGC, a stream of liquid carbamate of 29.8 t/h is obtained which is preheated and fed to the conversion section OGR where the carbamate is converted into urea according to reaction (3).

[0093] The section OGR operates at a pressure of 200 bar and a temperature of 190 C. and a liquid stream is obtained at the outlet, having a concentration of 30% by mass of urea, containing unreacted carbamate and water. This stream is treated in the section HPD where part of the carbamate contained in the stream exiting from the section OGR is thermally decomposed at a pressure of 105 bar and at a temperature of 200 C. obtaining a liquid stream of 19.7 t/h having a concentration of 45% urea by mass. The stream is further concentrated in the section MPD at a pressure of 18 bar and a temperature of 160 C., obtaining a liquid stream of 14.8 t/h at the outlet, containing 60% urea by mass. A further decomposition step at low pressure at 4 bar and a temperature of 145 C. allows a liquid solution of 13.6 t/h to be obtained, containing water and urea with a mass concentration of 65% urea. This stream is concentrated in the concentration section CON operating at a pressure lower than atmospheric pressure and at a temperature of 136 C., from which a stream of concentrated urea of 8.9 t/h and a stream of process water of 4.7 t/h are obtained, sent to the section PWT. The stream of concentrated urea from the section CON is combined with a stream of molten urea of 7.1 t/h from the battery limit to form a stream of urea of 16 t/h fed to the off-gas scrubber section OGQ.

[0094] The example describes a melamine plant with a nominal capacity of 40,000 t/y which receives from the battery limit only a stream of molten urea of 7.1 t/h and which has, at the outlet, a stream of pure melamine of 5 t/h and a stream of pure water of 2.1 t/h.

[0095] The mass percentages indicated in this example are calculated with respect to the total mass of the stream or solution considered.