Process for the recovery of ammonia from a gaseous stream

• US 8,758,485 B2
• Filed: 01/11/2010
• Issued: 06/24/2014
• 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
• Primary Examiner(s): Smith, Duane
• Assistant Examiner(s): SHAO, PHILLIP Y
• Application Number:
  US13/143,988
• Publication Number:
  US 20120039787A1
• Time in Patent Office: 1,625 Days
• Field of Search: None
• US Class Current: 95/232
• 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 removing ammonia from a gaseous stream, the process comprising:

(a) washing a gaseous stream comprising ammonia with an aqueous washing solution having a pH of lower than 7.0 and comprising a conjugate acid-base pair deriving from phthalic acid, oxalic acid, phosphoric acid, citric acid, an aryl-phosphonic acid, and an alkyl-phosphonic acid, to obtain a purified gaseous stream and an aqueous solution comprising an ammonium salt;

(b) distilling the aqueous solution comprising the ammonium salt from (a) 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, to obtain a regenerated washing solution and a gaseous stream comprising NH3 and H20,

(c) recycling the regenerated washing solution into (a).

2. The process of claim 1, wherein the gaseous stream in (a) is a gaseous purging stream from a synthesis process of urea.

3. The process of claim 2, wherein the gaseous purging stream comprises air contaminated by ammonia in an amount of from about 50 to 250 mg/Nm³ air, urea in an amount of from about 30 to 200 mg/Nm³ air, and traces of formaldehyde.

4. The process of claim 2, wherein the gaseous purging stream is at a temperature of about 45-100°

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

5. The process of claim 1, wherein the 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.

6. The process of claim 1, wherein the distilling (b) is carried out at a temperature ranging from 130 to 180°

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

7. The process of claim 6, wherein the distilling (b) is carried out at about 0.3 MPa and at a temperature of about 150°

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

8. The process of claim 1, wherein purified gaseous stream obtained in (a) is released into the atmosphere.

9. The process of claim 1, wherein the purified gaseous stream obtained in (a) comprises air or another inert gas having an ammonia content ranging from 10 to 25 mg/Nm³_air and, optionally, a urea content ranging from 5 to 30 mg/Nm³_air.

10. The process of claim 1, wherein gaseous stream comprising NH₃ and H₂O obtained in (b) is recycled to a synthesis process of urea or to a synthesis process of ammonia.

11. The process of claim 1, wherein the distilling (b) is carried out by supplying heat with electromagnetic waves.

12. The process of claim 1, wherein the quantity of water or acid, necessary for maintaining the desired concentration and pH, is added to the regenerated aqueous washing solution from (b).

13. The process of claim 1, wherein in the hydrophobic membrane distillation in (b), an additional transport stream is employed.

14. The process of claim 1, wherein, during the distilling, the urea present in the aqueous solution is at least partially hydrolyzed to form NH₃ and CO₂.

15. The process of claim 1, wherein washing is carried out by contacting the gaseous stream with the aqueous washing solution in a scrubber.

16. A process for removing ammonia from a gaseous stream, the process comprising:

(a) washing a gaseous stream comprising ammonia with an aqueous washing solution having a pH lower than 7.0 and comprising the conjugate aci-base pair H₂PO4⁻

/HPO₄²⁻

in which the overall concentration of the H₂PO4³¹ /HPO₄²⁻

species varies from 20 to 40% by weight, to obtain a purified gaseous stream and an aqueous solution comprising an ammonium salt;

(b) distilling the aqueous solution comprising the ammonium salt from (a) 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, to obtain a regenerated washing solution and a gaseous stream comprising NH₃ and H₂O;

(c) recycling the regenerated washing solution into (a).