Process and plant for the synthesis of urea and melamine

Abstract

An integrated process for the synthesis of urea and melamine, wherein: urea is synthesized with a stripping process in a high-pressure synthesis loop comprising a reactor, a stripper and a carbamate condenser, and the urea solution leaving said stripper is sent to a recovery section to produce a concentrated urea product and a recovered carbamate solution; at least part of said urea product is converted to melamine, and the off-gas from the synthesis of melamine are recycled to the urea synthesis by mixing with the gas phase from the stripper and with said recovered carbamate solution, thus forming a mixed flow which is then condensed in said carbamate condenser, and the condensate is eventually directed to the reactor.

Claims

1. A process for the combined synthesis of urea and melamine, wherein: urea is synthesized from ammonia and carbon dioxide with a stripping process, said stripping process including at least the steps of reacting ammonia and carbon dioxide in a reaction section, to form an aqueous solution comprising urea, ammonium carbamate and unconverted ammonia, and treating said solution in a stripping section, obtaining a urea solution and a gas phase containing ammonia and carbon dioxide, also including a step of condensation in a condensation section, at least a portion of synthesized urea is used to produce melamine in a tied-in melamine plant, obtaining also a flow of melamine off gas which contain ammonia and carbon dioxide, and said flow of melamine off gas is recycled back to said process for the synthesis of urea, either in a gaseous state or in a liquid state after condensation, wherein a first portion of said gaseous phase obtained from the stripping process is fed directly to said reaction section in a gaseous state, and wherein said urea solution leaving the stripping section is further processed in at least one recovery section obtaining a liquid carbamate solution and a more concentrated solution, and a first portion of said concentrated solution is used to produce melamine, and wherein the melamine off gas are recycled to the process for synthesis of urea by: mixing said flow of melamine off gas with a second portion of said gaseous phase from the stripping treatment, and with a second portion of said liquid carbamate solution coming from said recovery section, thus obtaining a gaseous-liquid mixed flow, condensing said mixed flow in said condensation section, obtaining a condensate, and feeding said condensate to said reaction section.

2. The process according to claim 1, wherein a portion of said gaseous phase from the stripping section is fed directly to said reaction section, and a remaining portion of said gaseous phase is sent to said condensation section.

3. The process according to claim 1, wherein said condensate is further separated into a gaseous phase comprising vapors and non-condensable inert gases and a liquid phase, said gas phase is directed to a process of scrubbing and further condensation, and said liquid phase is fed to said reaction section.

4. The process according to claim 1, said flow of melamine off gas having a pressure of 80 bar or greater, and being substantially free of water.

5. The process according to claim 1, said flow of melamine off gas being released at a medium pressure of no more than 30 bar, and said off gas being recycled to the urea synthesis in a liquid state after condensation.

6. The process according to claim 1, said stripping process for the synthesis of urea being a self-stripping or ammonia stripping process.

7. The process according to claim 1, said stripping process for the synthesis of urea being a CO2-stripping process.

8. The process according to claim 1, wherein the condensation step in said carbamate condensation section is a substantially total condensation.

9. The process according to claim 1, wherein at least 50% of the synthesized urea is used to produce melamine.

10. The process according to claim 9, wherein all the synthesized urea is used to produce melamine.

11. The process according to claim 1, wherein a portion of synthesized urea is used to produce melamine, and a remaining part of the synthesized urea is exported as such or sent to a finishing section.

12. The plant for the synthesis of urea and melamine according to the process of claim 1, the plant comprising a urea synthesis section and a melamine synthesis section, wherein: said urea synthesis section includes a synthesis loop which in turn comprises at least a reactor, a stripper, and a condenser, the urea section comprises a flow line to feed at least a portion of said gaseous phase leaving said stripper directly to said reactor, and the urea synthesis section comprises at least one recovery section where the urea solution leaving the stripper is further processed, a liquid carbamate solution and a more concentrated urea solution being obtained in said at least one recovery section, said melamine section receives at least a portion of the urea produced in the urea synthesis section, and produces melamine and a flow of melamine off gas containing ammonia and carbon dioxide, the plant comprises flow lines for recycling said flow of melamine off gas back to the urea section, the flow lines arranged to mix said flow of melamine off gas with a first portion of a gaseous phase emerging from said stripper, and with a portion of said liquid carbamate solution coming from said at least one recovery section, thus obtaining a gaseous-liquid mixed flow which is condensed in said condenser of the urea section.

13. A modification of a urea plant operating according to a stripping process, said urea plant including at least a reactor, a stripper and a condenser, the urea plant further comprising at least one recovery section where a urea solution leaving the stripper is further processed, a liquid carbamate solution and a more concentrated urea solution being obtained in said at least one recovery section, and the modification including at least the steps of: adding a tied-in melamine plant which converts into melamine a portion of the urea synthesized by said urea plant; recycling the off gas containing ammonia and carbon dioxide withdrawn from of said melamine plant to the urea plant, and directing a first portion of the gaseous phase separated in withdrawn from separated in the stripper of the urea plant, containing ammonia and carbon dioxide, to the reactor of the urea plant, the modification further including: mixing said off gas withdrawn from the melamine plant with a second portion of said gaseous phase from the stripper, and with a portion of said liquid carbamate solution obtained from said at least one recovery section, thus forming a gaseous-liquid mixed flow which is condensed in said condenser of the urea plant.

14. The process according to claim 1, said flow of melamine off gas being released at a medium pressure of around 20 bar, and said off gas being recycled to the urea synthesis in a liquid state after condensation.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a simplified block scheme of a urea-melamine plant according to a first embodiment of the invention, with a urea section operating according to the CO2-stripping process.

(2) FIG. 2 is is a simplified block scheme of a urea-melamine plant according to a second embodiment of the invention, with a urea section operating according to the ammonia-stripping or self-stripping process.

DESCRIPTION OF PREFERRED EMBODIMENTS

(3) FIG. 1 is a general scheme of a plant for the synthesis of urea and melamine. The plant comprises a urea section which is generally denoted by 1, and a melamine section denoted by block 2.

(4) In the urea section 1, urea is synthesized from ammonia input 3 and carbon dioxide input 4. The urea section 1 produces a urea-containing stream in the form of a concentrated aqueous solution 14. At least a portion 16 of said urea solution 14 is used in the melamine section 2 to produce a melamine product 18. A remaining portion 17 may be exported in some embodiments, or sent to a finishing section for the production of a solid urea product. The melamine section 2 includes an evaporation section suitable for bringing the input solution 16 to the desired purity.

(5) The melamine section 2 releases melamine off gas 19 which are recycled back to the urea section 1.

(6) The urea section 1 operates with a stripping process. More in detail, said section 1 comprises a high-pressure loop which includes a reactor 5, a stripper 6, a condenser 7 and possibly a high-pressure scrubber 27. Preferably the stripper 6, the condenser 7 and the scrubber 27 are shell-and-tube heat exchangers. The urea section 1, more preferably, comprises at least one recovery section 8 operating at a pressure lower than pressure of said loop, for example a medium pressure recovery section and/or a low-pressure recovery section.

(7) According to the invention, a portion 13 of the gaseous phase 11 emerging from the stripper 6 is fed directly to the reactor 5. The amount of gaseous carbon dioxide fed to the reactor 5 is regulated in such a way to obtain the desired heat balance, i.e. to provide at least the necessary heat for dehydration of carbamate. In particular, the amount of said portion 13 depends on the amount of urea which is used for the synthesis of melamine compared to the total urea which is synthesized, for example in FIG. 1 the amount of the gaseous portion 13 will depend on the amount of solution 14 directed to the melamine section (line 16). In some embodiments, the portion 16 directed to the melamine block 2 is at least 50% or all of the solution 14.

(8) The synthesis of melamine requires urea of a high purity, typically 99.7%. Referring to the example of FIG. 1 said purity may be reached by subjecting the stream 16 to evaporation. In other embodiments, a high purity urea melt obtained in the urea section and suitable for the synthesis of melamine may be directed to the melamine section.

(9) FIG. 1 illustrates a preferred embodiment with a CO.sub.2 stripping urea process, were gaseous carbon dioxide input 4 is fed to the bottom of said stripper 6.

(10) Referring more specifically to FIG. 1, ammonia is directly fed to the reactor 5, while the CO2 feed reaches the reactor 5, after being used in the stripper 6. Optionally, a part of ammonia 3a, instead of being fed to the reactor 5, is sent to the condenser 7. The effluent 9 of the reactor 5 is an aqueous solution of urea containing some carbamate and unconverted ammonia. Said effluent 9 is fed to the stripper 6 for decomposition of the unreacted ammonium carbamate and stripping of ammonia, producing a stripped solution of urea 10 and a gas phase 11 containing ammonia and carbon dioxide. Stripping of the solution 9 is promoted by the feed of gaseous carbon dioxide 4, and heat is furnished for example by hot steam admitted into the shell side of the stripper 6.

(11) A first portion 12 of said gas phase 11 is directed to the condenser 7 and a remaining second portion 13 of said gaseous phase 11 is directed to the reactor 5. The first portion 12, before admission into the condenser 7, is mixed with the off gas 19 coming from the melamine section 2, and with a liquid carbamate solution 15b coming from the recovery section 8.

(12) The recovery section 8 produces the urea product 14, which is urea solution to be concentrated depending on its use, and a liquid carbamate solution 15. The liquid carbamate solution 15 is preferably at a medium pressure, for example being produced in a medium-pressure condenser of said section 8. Preferably, most of the effluent 15 from the recovery section 8, indicated with 15a, is fed directly to the scrubber 27, while the remaining portion 15b, which is less than 50%, is mixed with said first portion 12 of the gas phase 11 from the stripper 6.

(13) The melamine section 2 operates preferably according to the non-catalytic high pressure melamine process.

(14) The melamine section 2 produces the melamine product 18 and the current of off-gas 19 containing ammonia and carbon dioxide.

(15) In the embodiment of FIG. 1, said current 19 is discharged by the melamine section 2 at a high pressure, preferably around 110 bar, and is substantially free of water. Accordingly, the off gas can be introduced in the condenser 7 of the urea synthesis loop.

(16) More preferably, said current 19 is mixed with the first portion 12 of the gas phase 11 emerging from the stripper 6, and also with at least a portion of the liquid carbamate solution 15, namely 15b. The pressure of the carbamate solution 15, to this purpose, is raised with a pump 21. The remaining portion 15a is preferably sent to the scrubber 27 in order to condense vapors from the reactor 5 and receiver 23.

(17) Mixing of the off gas current 19 with said solution 15b and said gas 12 forms a two-phase mixed flow 20 which is admitted to the condenser 7. Mixing the off gas 19 with the liquid carbamate solution 15b has the double advantage of a better condensation of vapors and reduced precipitation of carbamate in the condenser 7.

(18) The condensate flow 22 from said condenser 7 is recycled to the reactor 5, preferably via the carbamate receiver 23. Said carbamate receiver 23 separates a liquid carbamate solution 24 and a gas phase 25 containing non-condensed gas and inerts. The liquid solution 24 is pumped to the reactor 5; the gas phase 25 is sent to the high pressure scrubber 27 for further condensation together with the overhead vapors 26 of the reactor 5. Inerts 31 are vented from the scrubber 27.

(19) Preferably, the condensation process in the condenser 7 is a total condensation, which means that the inlet gases are fully condensed, apart from the unavoidable small fraction of non-condensed gas and inert gas, i.e. the condensate flow 22 is liquid.

(20) The heat content of the currents 19 and 12 can be recovered for example by producing hot steam in the shell side of the condenser 7.

(21) The invention reaches the above stated aims. The current 19 of melamine off gas is recycled to the urea synthesis section in an efficient manner, reducing the consumptions of fresh reagents and the consumption of energy. The CO2-containing gaseous feed 13 directed to the reactor prevents the cooling down of the reactor 5 even if all of the urea solution 14 is used to produce melamine.

(22) In the embodiment of FIG. 2, the references have the same meaning as in FIG. 1 and are not explained in detail. However, in this case the CO2 feed 4 is directed to the reactor 5, while the NH3 feed 3 enters the recovery section 8 and reaches the reactor 5 by means of ejector 30. More in detail, the condensate flow 22 from the condenser 7 is recycled to the reactor 5 by means of said ejector 30, forming the reactor input stream 32. Line 31 denotes the inert gas which are vented from said condenser 7.

(23) Also in this embodiment, the gas phase 11 emerging from the stripper 6 is split into two currents, namely a first current 12 directed to the condenser 7 and a second current 13 is fed directly to the reactor 5.

(24) A further aspect of the invention is modification of a urea plant operating according to a stripping process, said urea plant including at least a reactor and a stripper. Said urea plant may run for example the self-stripping process or the CO2 stripping process.

(25) The modification includes adding a tied-in melamine plant which converts into melamine a portion of the urea synthesized by said urea plant. Preferably a major portion and more preferably all of the urea can be used to produce melamine. The off gas of said melamine plant to the urea plant are recycled to the urea plant, and a portion of the gaseous phase separated in the stripper of the urea plant, containing ammonia and carbon dioxide, are directed to the reactor.