PROCESS FOR THE RECOVERY OF AMMONIA FROM A GASEOUS STREAM

• US 20120039787A1
• Filed: 01/11/2010
• Published: 02/16/2012
• Est. Priority Date: 01/13/2009
• Status: Active Grant

• Current Assignee: Saipem SPA (Saipem)
• Original Assignee: Saipem SPA (Saipem)
• Inventors: Gianazza, Alessandro, Miracca, Ivano, Casara, Paolo
• Granted Patent: US 8,758,485 B2
• Time in Patent Office: Days
• US Class Current: 423/352
• CPC Class Codes: B01D 2257/40 Nitrogen compounds
• CPC Class Codes: B01D 2257/406 Ammonia
• CPC Class Codes: B01D 2257/70 Organic compounds not provided for in groups B01D2257/00 - B01D2257/602
• CPC Class Codes: B01D 2325/38 Hydrophobic membranes
• CPC Class Codes: B01D 53/1425 Regeneration of liquid absorbents
• CPC Class Codes: B01D 53/1493 Selection of liquid materials for use as absorbents
• CPC Class Codes: B01D 53/58 Ammonia
• CPC Class Codes: B01D 61/3641 comprising multiple membrane distillation steps
• CPC Class Codes: C01C 1/12 Separation of ammonia from gases and vapours as part of the ammonia synthesis process C01C1/04

Abstract

A process for the recovery of ammonia contained in a gaseous stream is described, said process comprising the following phases: (a) subjecting the gaseous stream containing ammonia to a washing (S) with an aqueous washing solution (5a) having a pH lower than 7.0, with the formation of a purified gaseous stream (6) and an aqueous solution (7) containing an ammonium salt; (b) subjecting the aqueous solution containing the ammonium salt coming from phase (a) to a distillation process (MD) with a hydrophobic microporous membrane at a temperature ranging from 50 to 250° C. and a pressure ranging from 50 KPa to 4 MPa absolute with the formation of a regenerated washing solution (16) and a gaseous stream (18) comprising NH₃ and H₂O; (c) recycling said generated washing solution to phase (a). The equipment for carrying out the above process is also described.

Claims

1. A process for the recovery of ammonia contained in a gaseous stream, said process comprising the following phases:

(a) subjecting the gaseous stream containing ammonia to a washing with an aqueous washing solution having a pH lower than 7.0, with the formation of a purified gaseous stream and an aqueous solution containing an ammonium salt;

(b) subjecting the aqueous solution containing the ammonium salt coming from phase (a) to a distillation process with a hydrophobic microporous membrane at a temperature ranging from 50 to 250°

C. and a pressure ranging from 50 KPa to 4 MPa absolute with the formation of a regenerated washing solution and a gaseous stream comprising NH₃ and H₂O;

(c) recycling said regenerated washing solution to phase (a).

2. The process according to claim 1, characterized in that phase a) is carried out at a pH ranging from 5 to 6.5.

3. The process according to claim 1 or 2, characterized in that the gaseous stream fed to phase (a) is a gaseous purging stream coming from a synthesis process of urea, preferably from a solidification section, even more preferably from a granulation or prilling section of urea.

4. The process according to claim 3, characterized in that said gaseous purging stream consists of air contaminated by ammonia (about 50÷

250 mg/Nm³ air), urea (about 30÷

200 mg/Nm³ air) and traces of formaldehyde.

5. The process according to claim 3 or 4, characterized in that said gaseous purging stream is at a temperature of about 45-100°

C., and is subjected to a preliminary washing with water.

6. The process according to any of the claims from 1 to 5, characterized in that the aqueous washing solution used in phase (a) is a buffer solution consisting of a conjugate acid-base pair deriving from phthalic acid, oxalic acid, phosphoric acid, citric acid, aryl- and alkyl-phosphonic acids.

7. The process according to claim 6, wherein said buffer solution has a pH ranging from 5 to 6.5 and an overall molar concentration of the conjugate acid-base pair ranging from 0.5 M to 5 M, preferably from 1 M to 4 M.

8. The process according to claim 5, wherein the buffer solution is composed of the pair H₂PO4⁻

/HPO₄²⁻

in which the overall concentration of the H₂PO4⁻

/HPO₄²⁻

species varies from 20 to 40% by weight, preferably from 30 to 35% by weight.

9. The process according to any of the claims from 1 to 8, characterized in that, when the gaseous stream fed to phase (a) does not contain urea, phase (b) is carried our at a temperature ranging from 100 to 140°

C. and a pressure ranging from atmospheric pressure to 0.2 MPa absolute.

10. The process according to any of the claims from 1 to 8, characterized in that, when the gaseous stream fed to phase (a) also contains urea, phase (b) is carried our at a temperature ranging from 100 to 200°

C., preferably from 130 to 180°

C., and a pressure ranging from atmospheric pressure to 2 MPa absolute, preferably from 0.15 to 1.5 MPa absolute.

11. The process according to claim 10, characterized in that phase (b) is carried out at about 0.3 MPa and at a temperature of about 150°

C., with the formation of a gaseous stream comprising NH₃, H₂O and CO₂ in which the concentration of ammonia varies from 5 to 35% by weight, preferably from 10 to 25%.

12. The process according to any of the previous claims, characterized in that said purified gaseous stream which is formed in phase (a) is released into the atmosphere.

13. The process according to any of the previous claims, characterized in that said purified gaseous stream which is formed in phase (a) consists of air or another inert gas having an ammonia content ranging from 10 to 25 mg/Nm³_air and, possibly, a urea content which ranges from 5 to 30 mg/Nm³_air.

14. The process according to any of the previous claims, characterized in that said gaseous stream comprising NH₃ and H₂O which is formed in phase (b) is recycled to a synthesis process of urea or to a synthesis process of ammonia.

15. The process according to any of the previous claims, characterized in that phase (b) is carried out by supplying heat to phase (b) by means of electromagnetic waves, preferably microwaves.

16. The process according to any of the previous claims, characterized in that the quantity of water or acid, necessary for maintaining the desired concentration and pH, is added to the regenerated aqueous washing solution coming from phase (b).

17. The process according to any of the previous claims, characterized in that in the hydrophobic membrane distillation process of phase (b), an additional transport stream is used, preferably a stream of water vapour.

18. Equipment for carrying out the process according to claim 1, characterized in that it comprises:

a washing unit (scrubber) in which a gaseous stream containing ammonia is put in contact with an aqueous washing solution,

a unit for the distillation with a hydrophobic microporous membrane of an aqueous stream of an ammonium salt with the formation of a gaseous stream comprising ammonia and H₂O and a regenerated washing solution, said distillation unit being connected to the washing unit from which it receives the aqueous stream of an ammonium salt.

19. The equipment according to claim 18, also comprising heating means of the membrane distillation unit consisting of microwave generation devices.