Process and system to capture ammonia from a purge gas of a urea plant

Abstract

A process and system for removing ammonia from ammonia-containing purge gas of a urea plant, the process comprising: contacting said ammonia-containing purge gas with carbon dioxide at a low temperature, reaction of ammonia to form crystals of ammonium salts in a multiphase stream, and removal of the solid ammonium salts from the multiphase stream.

Claims

1. A process for removing ammonia from ammonia-containing purge gas of a urea plant, the process comprising: a) said ammonia-containing purge gas is contacted with a stream of carbon dioxide; b) at least one of: said ammonia-containing purge gas prior to contacting said stream of carbon dioxide, said stream of carbon dioxide prior to contacting the ammonia purge-gas, the mixture of purge gas and carbon dioxide resulting from the contacting step a), is refrigerated; c) in the so obtained refrigerated mixture of purge gas and carbon dioxide, at least some of the ammonia contained in the purge gas reacts to form one or more ammonium salts and a multiphase mixed stream containing the ammonium salts is obtained, and d) said ammonium salts are removed from said mixed stream.

2. The process according to claim 1, wherein said at least one of purge gas, carbon dioxide and mixture of purge gas and carbon dioxide is refrigerated to a temperature lower than the temperature of the input ammonia-containing purge gas.

3. The process according to claim 1, wherein said mixed stream has a temperature equal to or less than 10 C.

4. The process according to claim 3, wherein the ammonia-containing purge gas is contacted with said carbon dioxide at a pressure of 1 to 10 bar abs.

5. The process according to claim 4, wherein the ammonia-containing purge gas and the refrigerated carbon dioxide are contacted in a mixer or in a gas ejector.

6. The process according to claim 4, wherein the ammonia-containing purge gas is contacted with said carbon dioxide at a pressure of 1 to 2 bar abs.

7. The process according to claim 3, wherein said mixed stream has a temperature in the range of 10 C. to 30 C.

8. The process according to claim 1, comprising refrigeration of said carbon dioxide, wherein the temperature of refrigerated carbon dioxide is equal to or less than 10 C.

9. The process according to claim 8, wherein refrigerated carbon dioxide is obtained by: separating a secondary gaseous stream of carbon dioxide from an inter-refrigeration stage of a carbon dioxide feed compressor of said urea plant, and expanding said secondary stream of carbon dioxide to a lower pressure in order to lower its temperature.

10. The process according to claim 9, wherein said secondary stream of carbon dioxide has a pressure of 65 to 85 bar and a temperature after inter-refrigeration of 40 to 80 C., and is expanded to atmospheric pressure.

11. The process according to claim 8, comprising refrigeration of said carbon dioxide, wherein the temperature of refrigerated carbon dioxide is in the range 25 to 65 C.

12. The process according to claim 1, wherein during or after the contacting of ammonia-containing purge gas and carbon dioxide, a further refrigerant medium is added.

13. The process according to claim 12, comprising the steps of: contacting the ammonia-containing purge gas and the carbon dioxide in an absorber, and injecting said refrigerant medium into said absorber, so that the refrigerant medium comes into direct contact with the purge gas and carbon dioxide.

14. The process according to claim 12, wherein the refrigerant medium is at a temperature equal to or less than 10 C.

15. The process according to claim 1, wherein said ammonia-containing purge gas comprises a plurality of purge streams taken from different sections of said urea plant.

16. The process according to claim 1, wherein said ammonium salts include ammonium bicarbonate NH.sub.4HCO.sub.3 and ammonium carbamate NH.sub.4COONH.sub.2.

17. The process according to claim 1, wherein ammonium salts are removed from said mixed stream in a filter or by a scrubbing process.

18. The process according to claim 17, wherein said scrubbing process comprises water as a scrubbing medium.

19. The process according to claim 1, wherein the ammonia-containing purge gas is a purge gas as vented from the urea plant, or the process comprise at least one step of removing ammonia from the purge gas prior to said contacting with cold carbon dioxide.

20. The process according to claim 1, wherein said mixed stream contains a residual amount of free gaseous ammonia of less than 10 ppm.

21. An ammonia capture system for removing ammonia contained in a purge gas of a urea plant, comprising: a mixing device wherein said ammonia-containing purge gas and a stream of carbon dioxide are introduced, at least one of a refrigerator for the ammonia-containing purge gas, a refrigerator for said stream of carbon dioxide, means to add a refrigerant medium to said mixing device, the mixing device delivering a multiphase mixed stream containing solid ammonium salts, as a result of mixing and refrigeration of the purge gas and carbon dioxide, and a phase-separation device disposed to receive said mixed stream from the mixing device and adapted to remove said ammonium salts from said mixed stream.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a scheme of a urea plant including an ammonia capture system according to a first embodiment of the invention.

(2) FIG. 2 is a scheme of a second embodiment of the invention.

DETAILED DESCRIPTION

(3) Referring to FIG. 1, a carbon dioxide feed 1 is elevated to urea synthesis pressure in a multi-stage compressor 2 and the resulting compressed CO2 stream 3 is fed to a urea plant 4.

(4) The urea plant 4 includes a synthesis section 20, a recovery section 21, an evaporation section 22 and a finishing section 23. The urea plant 4 also receives an ammonia feed 24 and delivers a urea product U. Some water 25 is discharged by the evaporation section 22.

(5) Purge gas streams 5a, 5b and 5c are vented from said sections 20, 21 and 22 respectively and are collected in a stream 5. The purge gas 5 contains inert gases such as nitrogen, oxygen, methane, argon and hydrogen, and further contains some ammonia.

(6) The ammonia-containing purge gas 5 is blended in a mixer 11 with a cold gaseous carbon dioxide 26 at a suitable low temperature resulting in the formation of a mixed stream 12 containing a solid phase of crystals of ammonium salts dispersed in a gaseous phase. Said crystals are mainly formed due to the change in the equilibrium of reactions (1) and (2) and (3) mentioned above, which is a consequence of the intimate mixing with the cold CO2. In some embodiments the stream 26 may contain a small liquid fraction.

(7) The solid ammonia salts are removed from the stream 12 in a solid scrubber 13 wherein clean water 15 is injected, preferably cyclically (semi-continuous operation). The ammonia salts are dissolved in water and leave the scrubber 13 with an aqueous stream 16. The aqueous stream 16 can be recycled to the urea plant 4, for example to the recovery section 21.

(8) The gas 14 which emerges from the scrubber 13 is substantially ammonia-free. As stated above, only few ppm of free ammonia remain in the mixed stream 12 and therefore, after removal of the solid ammonia salts, a gaseous phase practically free of ammonia is obtained. For example said gas 14 contains less than 10 ppm ammonia and preferably around 5 ppm.

(9) Recycle of the aqueous stream 16 to the urea plant is advantageous since it reduces the total clean water amount needed for the ammonia scrubbing. Also, the amount of water introduced in the urea plant 4 is smaller compared to water washing of the prior art. In some embodiments the solid ammonium salts can be displaced and disposed safely with no recycle to the urea process.

(10) The scrubber 13 can be replaced in other embodiments by another phase-separation device suitable to separate the solid phase and gaseous phase of the mixed stream 12. In some embodiments said phase-separation device may include a filter.

(11) The generation of the cold carbon dioxide 26 is now described in accordance with a preferred embodiment.

(12) Said cold carbon dioxide 26 is obtained from an inter-refrigeration stage 27 of the compressor 2. Preferably the inter-refrigeration stage 27 is between the last two stages of compression.

(13) The inter-refrigeration stage 27 comprises essentially a refrigerator 7 and a phase separator 8. A carbon dioxide 6 taken from an intermediate stage of the compressor 2 passes through the refrigerator 7 and then the separator 8.

(14) A major portion 9 of the carbon dioxide 28 emerging from the separator 8 is reintroduced to the next stage of the compressor 2. A secondary stream 10 is separated and expanded to a much lower pressure (e.g. atmospheric pressure) through a let-down valve 29. The temperature of the gaseous CO2 drops substantially, due to the free expansion through said valve 29, thus obtaining the cold carbon dioxide 26.

(15) In a variant embodiment the secondary stream 10 can be separated after the refrigerator 7 but before the phase separator 8.

(16) The valve 29 is preferably close to the inlet of the mixer 11 to reduce the risk of ice formation due to residual moisture in the secondary stream 10.

(17) Preferably the flow rate of said secondary stream 10 is regulated to keep the temperature of the mixed stream 12 within a target range. In FIG. 1 the flow rate through the valve 29 is controlled via a flow controller 30, a temperature probe 31 of the mixed stream 12 and a flow meter 32 of the carbon dioxide stream 10.

(18) The flow controller 30 receives signals 33 and 34 from said temperature probe 31 and flow meter 32, corresponding to the temperature of the stream 12 and current flow rate of cold carbon dioxide directed to the mixer 11. Based on these signals, the flow controller 30 adjusts the flow rate of the carbon dioxide directed to the mixer 11, in order to keep the temperature of the mixed stream 12 within a desired range suitable for the de-sublimation of ammonia.

(19) FIG. 2 illustrates a second embodiment of the invention.

(20) The ammonia containing purge gas 5 and carbon dioxide 41 enter an absorber 40. Preferably the absorber 40 is a packed column, equipped with suitable packing 50 to enhance the heat and mass transfer among the phases.

(21) The carbon dioxide 41 can be taken from the suction side of the compressor 2. The carbon dioxide 41 may be at atmospheric pressure.

(22) A solvent 48 at low temperature (i.e. 10 C.), for instance an oil for low temperature service, is injected in the top of the absorber 40, e.g. with a sprayer 51, and contacts the mixture of purge gas 5 and carbon dioxide 41 in a counter-current arrangement.

(23) The ammonia contained in the purge gas 5 and the carbon dioxide 1a react at low temperature forming ammonium salts. A multi-phase stream 42 comprising the ammonium salts dispersed in a solid-liquid slurry is withdrawn from bottom of the absorber 40. Said multi-phase stream 42 may also contain ice crystals originated by water contained in the streams 5 and 41.

(24) The multi-phase stream 42 is fed by a suitable pump 52 to a filter 43 where ammonium salts are separated from the solvent. The ammonium salts including some ice crystals are safely disposed as stream 44.

(25) The solvent is recycled via line 45 through a heat exchanger 46, resulting again in the cold solvent 48. In the heat exchanger 46, the temperature of the solvent is dropped by means of a suitable refrigerant 47 provided by a cryostat 49.

(26) As in FIG. 1, a substantially ammonia-free gas 14 is withdrawn from top of the absorber 40.

EXAMPLES

(27) Referring to FIG. 1, a total of 47 kmol/h of purge gas 5 are vented from a urea plant having a capacity of 1000 to 2000 MTD. Said gas 5 has a temperature of 30 C., a pressure of 1 bar, and contains (in volume) 93.2% of inert gas, 2.1% ammonia, 0.4% carbon dioxide, 4.3% water.

(28) The CO2 stream 28 at 65 to 85 bar and 50 C. after refrigeration. The CO2 stream 10 is expanded to atmospheric pressure obtaining the cold carbon dioxide 26.

(29) After mixing with 79 kmol/h of said cold carbon dioxide 26, the multi-phase stream 12 has a temperature of 5 C., a pressure of 1 bar, the gas phase contains: 62.9% CO2, 34.7% inert gas, 1.6% water and 0.8% nitrogen, ammonia negligible. All the ammonia is in the solid phase as ammonium salts.

(30) The embodiment of FIG. 2 has generally a smaller flow rate than the previous embodiment. For example 20 kmol/h of carbon dioxide are enough to be added to 47 kmol/h of purge gas containing 2.1% of ammonia.