Plant for the synthesis of melamine with offgas recovery in a tied-in urea plant

Abstract

Plant for the synthesis of melamine starting from urea, wherein the stream of offgas containing NH3 and CO2 produced by the synthesis of melamine is converted into urea in a dedicated urea plant.

Claims

1. A melamine-urea plant comprising: a melamine plant (MEL) for the production of melamine starting from urea, obtaining a product stream containing melamine and a stream of offgas containing NH3 and CO2 in gaseous form; a offgas-fed urea plant (UR) dedicated to the synthesis of urea starting from said stream of offgas, said offgas-fed urea plant being apt to produce urea starting from ammonia and CO2, and comprising at least a high-pressure synthesis section running at a synthesis pressure and a recovery section running at a recovery pressure lower than said synthesis pressure, wherein said stream of offgas provides at least 90% of ammonia and CO2 supply of said offgas-fed urea plant.

2. The plant according to claim 1, wherein said stream of offgas provides the sole supply of ammonia and CO2 of said offgas-fed urea plant.

3. The plant according to claim 1, wherein all or part of the urea produced by said offgas-fed urea plant supplies said melamine plant.

4. The plant according to claim 1, wherein the offgas-fed urea plant further includes a concentration section arranged to receive a urea solution effluent from the recovery section and to produce a concentrated solution or a urea melt containing at least 95% urea.

5. The plant according to claim 4, wherein the offgas-fed urea plant further includes a finishing section to produce solid urea.

6. The plant according to claim 1, comprising: a first urea plant, which is a main urea plant and comprises at least a high-pressure synthesis section running at a synthesis pressure and a recovery section running at a recovery pressure lower than said synthesis pressure, a second urea plant, which is said offgas-fed urea plant, wherein a urea supply of said melamine plant is produced at least in part by said main urea plant.

7. The plant according to claim 6, wherein the urea supply of said melamine plant is produced in part by said main urea plant and in part by said second offgas-fed urea plant.

8. The plant according to claim 6, wherein a part of the urea produced by said second urea plant is exported from the plant.

9. The plant according to claim 1, wherein the urea product of said off-gas urea plant (UR) is any of: an aqueous urea solution comprising at least 65% urea in weight; a molten urea comprising at least 95% urea, a solid urea product.

10. The method for revamping a urea plant comprising: adding a melamine synthesis section to said urea plant, wherein at least part of the urea produced in the existing urea plant is intended to supply said melamine synthesis section; adding a second urea plant which is an offgas-fed urea plant for the production of urea starting from the melamine offgas withdrawn from said melamine plant, wherein a stream of offgas extracted from the melamine plant provides at least 90% of ammonia and CO2 supply of said second urea plant, wherein said second urea plant comprises at least a high-pressure synthesis section running at a synthesis pressure and a recovery section running at a recovery pressure lower than said synthesis pressure.

11. The method according to claim 10, wherein the added urea plant is suitable to produce a urea product which is any of: an aqueous urea solution comprising at least 65% urea in weight; a molten urea comprising at least 95% urea, a solid urea product.

12. A method for revamping a urea/melamine plant, wherein the plant comprises a melamine plant and a tied-in urea plant, the method comprising: adding a second urea plant specifically dedicated to the synthesis of urea from offgas coming from the melamine plant wherein a stream of offgas extracted from the melamine plant provides at least 90% of ammonia and CO2 supply of said second urea plant, and said second urea plant comprises at least a high-pressure synthesis section running at a synthesis pressure and a recovery section running at a recovery pressure lower than said synthesis pressure.

13. The method according to claim 12, wherein the second urea plant is suitable to produce a urea product which is any of: an aqueous urea solution comprising at least 65% urea in weight; a molten urea comprising at least 95% urea, a solid urea product.

14. The plant according to claim 1, wherein said synthesis pressure is at least 80 bar.

15. The plant according to claim 1, wherein said synthesis pressure is in the range of 100 to 200 bar.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is an illustration of a combined urea/melamine plant of a first embodiment of the invention.

(2) FIG. 2 is an illustration of a urea-melamine plant which does not comprise the urea synthesis plant UP of a second embodiment of the invention.

DESCRIPTION OF PREFERRED EMBODIMENTS

(3) FIG. 1 shows in schematic form a combined urea/melamine plant comprising:

(4) a first urea plant UP (main urea plant),

(5) a melamine plant MEL,

(6) a second urea plant UR which is dedicated to the synthesis of urea from the offgas coming from said melamine plant MEL.

(7) The main urea plant UP is supplied with a stream 1 of ammonia and with a stream 2 of CO2.

(8) A part U1 of the urea produced by the main urea plant UP supplies the melamine plant MEL. Another part U2 is exported or sent to another use.

(9) The urea U1 is preferably in the form of a urea melt or solid urea. The urea U2 may be in a similar form or in the form of an aqueous solution.

(10) The melamine plant MEL produces melamine M and a stream G of offgas mainly containing NH3 and CO2.

(11) Said stream G supplies the second urea plant UR which is dedicated to the recovery of the reagents contained in the offgas, by producing additional urea.

(12) In the example, said stream G represents the sole supply of reagents, i.e. ammonia and CO2, of the second urea plant UR. Said urea plant UR does not have other inputs of ammonia and/or CO2. In some variants, if necessary, small additional inputs of ammonia and CO2 may be provided.

(13) The use of melamine offgas to produce urea is also termed offgas recovery. The offgas stream G may be recovered in the urea plant UR for example by supplying the offgas in the form of an anhydrous gas or in an aqueous solution.

(14) Recovery in an anhydrous form involves for example the introduction of the offgas into a high-pressure condenser of the urea plant UR, as described in EP 2 940 006.

(15) Recovery in an aqueous solution involves for example a condensation step with water so as to form an aqueous solution containing ammonia and CO2 and the introduction of said solution in the urea reactor of the urea plant UR.

(16) Said urea plant UR produces urea U3 which advantageously provides for the supply of the melamine plant MEL.

(17) FIG. 2 shows an embodiment of a urea-melamine plant which does not comprise the main urea synthesis plant UP. The only urea synthesis plant in this embodiment is the offgas-fed urea plant UR.

(18) In FIG. 2 the melamine plant MEL is supplied with urea U4 coming from an external source. The urea U3 produced in the urea plant UR is advantageously also returned to the melamine plant MEL.