Process for making ammonia

Abstract

A process for production of ammonia includes: providing a reaction stream including carbon monoxide and hydrogen; passing the reaction stream and steam over a water gas shift catalyst in a catalytic shift reactor, forming a shifted gas mixture depleted in carbon monoxide and enriched in hydrogen; passing the shifted gas mixture with an oxygen-containing gas over a selective oxidation catalyst at 175 C., forming a selectively oxidized gas stream with a portion of the carbon monoxide converted to carbon dioxide; removing some of the carbon dioxide from the selectively oxidized gas stream in a carbon dioxide removal unit; passing the carbon dioxide depleted stream over a methanation catalyst in a methanator to form a methanated gas stream, optionally adjusting its hydrogen:nitrogen molar ratio to form an ammonia synthesis gas; and passing the ammonia synthesis gas over an ammonia synthesis catalyst in an ammonia converter to form ammonia.

Claims

1. A process for producing ammonia, comprising the steps of: (a) passing a reaction stream comprising (i) carbon monoxide, (ii) hydrogen, and (iii) steam over a copper-based water gas shift catalyst in a catalytic shift reactor to form a shifted gas mixture containing methanol; (b) without steps of cooling to condense steam to water and separating the condensed water, passing the shifted gas mixture with an oxygen-containing gas over a selective oxidation catalyst at an inlet temperature in a range of from 175 C. to 250 C. to form a selectively oxidised gas stream; (c) removing at least a portion of the carbon dioxide and steam from the selectively oxidised gas stream in a carbon dioxide removal unit; (d) passing the carbon dioxide depleted stream over a methanation catalyst in a methanator to form a methanated gas stream, and (e) passing the methanated gas stream over an ammonia synthesis catalyst in an ammonia converter to form ammonia.

2. The process of claim 1, wherein the inlet temperature is in the range of from 180 C. to 220 C.

3. The process of claim 1, wherein step (b) is performed adiabatically in the range of from about 175 C. to about 350 C.

4. The process of claim 1, wherein step (b) is performed isothermally.

5. The process of claim 1, wherein step (b) is carried out at a pressure in the range of from 10 to 80 bar absolute.

6. The process of claim 1, wherein the selective oxidation catalyst is a supported platinum group metal catalyst.

7. The process of claim 6, wherein the selective oxidation catalyst comprises 1 to 5% wt platinum and 0.1 to 1.0% wt iron, expressed as Fe.sub.2O.sub.3.

8. The process of claim 1, wherein the selective oxidation and the catalytic water-gas shift catalysts are disposed within one vessel.

9. The process of claim 1, wherein the water gas shift catalyst is an alkali-promoted copper-zinc oxide alumina water gas shift catalyst.

10. The process of claim 1, wherein the reaction stream in step (a) is formed by pre-reforming and/or primary steam reforming, and secondary or autothermal reforming a hydrocarbon feedstock with oxygen, air or oxygen-enriched air.

11. The process of claim 1, wherein steam is added to the reaction stream before it is subjected to the catalytic water-gas shift conversion.

12. The process of claim 1, wherein the carbon dioxide removal unit is an absorption unit or a pressure swing adsorption unit.

13. The process of claim 1, wherein carbon dioxide removed in the carbon dioxide removal unit is reacted with product ammonia to form urea.

14. The process of claim 1, further comprising adjusting the hydrogen:nitrogen molar ratio of the methanated gas stream to form an ammonia synthesis gas and passing the ammonia synthesis gas over an ammonia synthesis catalyst in an ammonia converter to form the ammonia.

Description

(1) The present invention will now be described by way of example with reference to the following drawings, in which:

(2) FIG. 1 is a schematic representation of one embodiment of the present invention; and

(3) FIG. 2 illustrates an integrated water-gas shift converter and selective oxidation unit.

(4) It will be understood by those skilled in the art that the drawings are diagrammatic and that further items of equipment such as reflux drums, pumps, vacuum pumps, temperature sensors, pressure sensors, pressure relief valves, control valves, flow controllers, level controllers, holding tanks, storage tanks, and the like may be required in a commercial plant. The provision of such ancillary items of equipment forms no part of the present invention and is in accordance with conventional chemical engineering practice.

(5) As illustrated in FIG. 1, a reaction gas stream comprising hydrogen, carbon monoxide and nitrogen produced by primary reforming of natural gas and secondary reforming of the primary reformed gas mixture with air, is fed in line 1 to a catalytic water-gas shift reactor 3 containing a water-gas shift catalyst with steam added in line 2. In the embodiment of FIG. 1, the steam 2 is combined with the gas 1 before it enters the catalytic shift reactor 3. This stage is depicted as a single step, but in this embodiment is performed by a high-temperature shift stage and a subsequent low-temperature shift stage at the appropriate inlet temperatures over the appropriate catalysts. A portion of the carbon monoxide present in the reaction gas is converted to carbon dioxide over the water-gas shift catalysts to form a shifted gas mixture depleted in carbon monoxide and enriched in hydrogen. The shifted gas mixture 4 is recovered from the catalytic shift reactor 3. Air is added to the shifted gas mixture in line 5 and the combined stream is passed to the selective oxidation reactor 6. The gas fed to the selective oxidation reactor will be at a temperature of from 175 C. In the selective oxidation reactor 6, the gas mixture is passed over a selective oxidation catalyst comprising 1-5% wt platinum and 0.1-1.0% wt iron, expressed as Fe.sub.2O.sub.3. At least a portion of the remaining carbon monoxide is converted into carbon dioxide, forming a carbon monoxide depleted stream. The carbon monoxide depleted stream is then passed in line 7 into the carbon dioxide removal unit 8, in which the carbon dioxide is removed using an absorbent. Carbon dioxide is removed from carbon dioxide removal unit 8 in line 9. This may be stored for reaction with the product ammonia to form urea. The exhaust gas from the carbon dioxide removal unit 8 is then passed in line 10 to the methanator 11, which converts any residual carbon monoxide by reacting it with hydrogen to form methane. The stream removed from the methanator 11 in line 12 has a hydrogen:nitrogen ratio of about 3 and is passed into the ammonia converter 13, where it is used to create ammonia which is recovered in line 14.

(6) FIG. 2 shows an integrated water-gas shift converter and a selective oxidation unit within a single vessel. A water-gas shift section 23 is disposed above an oxidation unit 25 within the vessel and is separated from it by a plate 27. A gas stream 21 comprising hydrogen and carbon monoxide and steam enters the top of the water-gas shift section 23. The carbon monoxide is partially converted over a suitable water-gas shift catalyst before being removed from the water-gas shift section 23 section via line 22. Air 24 is introduced into line 22, before the gas stream is sent back into selective oxidation unit 25. The residual carbon monoxide in the gas stream is further oxidised to form carbon dioxide in the selective oxidation unit 25, before passing out of the integrated apparatus by line 26. The plate 27 is welded in the middle of the apparatus to separate the water-gas shift section 23 and a selective oxidation unit 25, so that the air 24 does not oxidise the catalyst in the water-gas shift section 23.

(7) An ammonia process according to FIG. 1 was modelled to determine the effects of including the selective oxidation process as claimed on a 1200 mtpd ammonia plant fed with a natural gas feed subjected to conventional primary and secondary steam reforming, wherein the water-gas conversion stage was effected by including both high-temperature and low-temperature water gas shift converters without further steam addition.

(8) TABLE-US-00001 HTS LTS Stream 1 exit exit 5 7 9 10 12 14 Temp C. 996 442 228 170 261 35 70 340 Pressure bar abs 34.0 32.6 31.6 31.6 31.2 1.5 29.4 29.4 Flow kmol/hr 10030.4 10030.4 10030.4 98.5 10108.3 3803.7 6304.6 6287.5 3184.0 Composition % vol CH.sub.4 0.32 0.32 0.32 0.00 0.31 0.00 0.50 0.64 0.10 CO.sub.2 4.78 11.36 13.73 3.00 13.83 36.59 0.10 0.00 0.00 CO 9.18 2.60 0.23 0.00 0.02 0.00 0.04 0.00 0.00 Ar 0.18 0.18 0.18 0.93 0.19 0.00 0.30 0.30 0.05 H.sub.2 37.23 43.80 46.17 0.00 45.62 0.25 72.99 72.68 0.00 N.sub.2 15.10 15.10 15.10 78.08 15.75 0.05 25.22 25.29 0.05 O.sub.2 0.00 0.00 0.00 20.96 0.00 0.00 0.00 0.00 0.00 H.sub.2O 33.21 26.64 24.27 0.00 24.27 63.10 0.85 1.09 0.05 NH.sub.3 99.75
Gain in Production

(9) TABLE-US-00002 N.sub.2 H.sub.2 NH.sub.3 CO.sub.2 Without selective kmol/hr 1591.9 4722.1 3148.1 1197.5 oxidation With selective oxidation kmol/hr 1604.9 4764.0 3176.0 1208.1 Gain in NH.sub.3 kmol/hr 27.9 Gain in CO.sub.2/urea kmol/hr 10.6

(10) The process requires an increase in air consumption of 16.5 kmol/hr or 0.83% to maintain the hydrogen:nitrogen ratio of 2.97 in the loop.

(11) The process requires an increase in fuel to the primary reformer to make up for the reduced amount available from the ammonia loop purge of 12.1 kmol/hr and increases demand on the carbon dioxide removal unit by 0.88% for CO.sub.2.

(12) The invention may be illustrated by reference to the following examples.

Example 1. Selective Oxidation Catalyst

(13) A solution was prepared using iron (Ill) nitrate nonahydrate (Fe(NO.sub.3).sub.3.9H.sub.2O) and platinum nitrate. The required quantities were mixed together in a citric acid solution. The solution was added to and mixed with a gamma alumina support (SCFa140 available from Sasol) in a volume sufficient to fill the total pore volume of the support. The impregnated support was oven dried and then calcined at 500 C. The calcined catalyst comprised; 3% wt platinum and 0.3% wt iron.

Example 2. Catalyst Testing

(14) 0.01 g catalyst powder, ground to 250-355 m, was mixed with 0.09 g cordierite of the same size distribution. Quartz wool was used to contain the mixture in a quartz reactor tube with a thermocouple monitoring the bed temperature. The following shifted gas composition was used for testing.

(15) TABLE-US-00003 CO.sub.2 17.5% H.sub.2 41.5% CO 0.6% O.sub.2 0.6% N.sub.2 39.8%

(16) Gas chromatography was used to monitor gas composition.

(17) The results were as follows;

(18) TABLE-US-00004 Temperature ( C.) CO Conversion (%) Selectivity (%) 139.7 62.8 52.0 150.7 72.9 51.6 160.5 82.1 51.6 171.6 90.3 51.2 183.0 95.4 50.6 192.6 97.5 50.2 201.4 98.4 50.2 211.2 98.4 49.9 220.2 98.7 50.2 230.4 95.6 48.4 242.5 93.3 47.3 251.3 91.1 46.1 259.9 88.2 44.6 271.4 83.7 42.4

(19) There is a clear optimum in terms of carbon monoxide conversion in the region of 200-220 C. whilst maintaining a reasonable selectivity towards carbon monoxide.