Process for revamping an ammonia plant

Abstract

A method is described for revamping an ammonia production facility said ammonia production facility having a front end comprising one or more reformers fed with a hydrocarbon feedstock at a hydrocarbon feed stock feed rate and a high-temperature shift reactor fed with a reformed gas obtained from said one or more reformers and containing a fixed bed of iron-containing water-gas shift catalyst, said front end operating at a first steam-to-carbon ratio and a first pressure drop, said method comprising the steps of (i) replacing the iron-containing water-gas shift catalyst with a low-steam water-gas shift catalyst to form a modified front end, (ii) operating the modified front end at a second steam-to-carbon ratio and a second pressure drop, wherein the second steam-to-carbon ratio is at least 0.2 less than the first steam-to-carbon ratio and the second pressure drop is less than the first pressure drop, and (iii) increasing the hydrocarbon feed stock feed rate to said one or more reformers.

Claims

1. A method for revamping an ammonia production facility, said ammonia production facility having a front end comprising: (a) one or more reformers fed with a hydrocarbon feedstock at a hydrocarbon feed stock feed rate; and (b) a high-temperature shift reactor fed with a reformed gas obtained from said one or more reformers, the high-temperature shift reactor having an inlet temperature in a range of from 300° C. to 450° C. and containing a fixed bed of iron-containing a high-temperature water-gas shift catalyst, said front end operating at a first steam-to-carbon ratio at or above 1.5 and a first pressure drop at or above 5 barg, said method comprising the steps of: (i) replacing the iron-containing high-temperature water-gas shift catalyst with a low-steam water-gas shift catalyst to form a modified front end, wherein the low steam water gas shift catalyst is an enhanced iron-containing high temperature shift catalyst that is a precipitated iron-containing catalyst with an iron oxide content, expressed as Fe.sub.2O.sub.3, of 60 to 95% by weight, having a BET surface area in the range of from 20 m.sup.2/g to 40 m.sup.2/g, or an iron-free high temperature shift catalyst comprising a zinc-aluminate spinel or oxides of zinc and aluminum and one or more promoters that is Na, K, Rb, Cs, Cu, Ti, Zr, a rare earth element or a mixture thereof, (ii) configuring the modified front end to operate at a second steam-to-carbon ratio and a second pressure drop, wherein the second steam-to-carbon ratio is at least 0.2 less than the first steam-to-carbon ratio and the second pressure drop is less than the first pressure drop, and (iii) increasing the hydrocarbon feed stock feed rate to said one or more reformers; such that the high-temperature shift reactor remains configured to operate under high-temperature water-gas shift conditions.

2. The method of claim 1, wherein the ammonia production facility front end comprises a fired steam reformer and optionally a secondary reformer.

3. The method of claim 1, wherein the high temperature shift reactor is operated at an inlet temperature in the range of from 310 to 380° C. and at a pressure in the range of from 1 to 100 bar abs.

4. The method of claim 1, wherein the second steam-to-carbon ratio is at least 0.3 less than the first steam-to-carbon ratio.

5. The method of claim 1 wherein the steam to dry gas ratio at the inlet to the high temperature shift reactor is reduced to 0.45:1 or less after replacement of the iron-containing high-temperature water-gas shift catalyst with the low-steam water-gas shift catalyst.

6. The method of claim 1, wherein the second pressure drop through the front end is at least 1 barg lower than the first pressure drop through the front end.

7. The method of claim 1 wherein the low steam water gas shift catalyst is the iron-free high temperature shift catalyst comprising the zinc-aluminate spinel or oxides of zinc and aluminum and one or more promoters that is Na, K, Rb, Cs, Cu, Ti, Zr, a rare earth element or a mixture thereof.

8. The method of claim 1, wherein the low steam water gas shift catalyst is the enhanced iron-containing water gas shift catalyst that is the precipitated iron-containing catalyst with an iron oxide content, expressed as Fe.sub.2O.sub.3, of 60 to 95% by weight, having a BET surface area in the range of from 20 m.sup.2/g to 34 m.sup.2/g.

9. The method of claim 1, wherein the low steam water gas shift catalyst is the enhanced iron-containing water gas shift catalyst is in the form of a cylindrical pellet having a length C and diameter D, wherein the surface of the cylindrical pellet has two or more flutes running along its length, said cylinder having domed ends of lengths A and B such that (A+B+C)/D is in the range of from 0.25 to 1.25, and (A+B)/C is in the range of from 0.03 to 0.3.

10. The method of claim 1, wherein the low steam water gas shift catalyst is the enhanced iron-containing water gas shift catalyst comprising one or more iron oxides stabilized with chromia, acicular iron oxide particles, and one or more copper compounds.

11. The method of claim 1, wherein the low steam water gas shift catalyst is the iron-free high temperature shift catalyst comprising the zinc-aluminate spinel.

12. The method of claim 1, wherein the low steam water gas shift catalyst comprises a mixture of zinc alumina spinel and zinc oxide in combination with an alkali metal that is Na, K, Rb, Cs, or a mixture thereof.

13. The method of claim 1, wherein the low steam water gas shift catalyst is the iron-free high temperature shift catalyst comprising the oxides of zinc and aluminum and one or more promoters that is Na, K, Rb, Cs, Cu, Ti, Zr, a rare earth element, or a mixture thereof.

14. The method of claim 1, wherein the front-end pressure drop is increased by the increase in hydrocarbon feedstock feed rate in step (iii) to 90-100% of the first front-end pressure drop.

15. The method of claim 1, wherein the second steam-to-carbon ratio is at least 0.4 less than the first steam-to-carbon ratio.

16. The method of claim 1, wherein the steam to dry gas ratio at the inlet to the high temperature shift reactor is reduced after replacement of the iron-containing water-gas shift catalyst to ≤0.42:1.

17. The method of claim 1, wherein the low steam water gas shift catalyst is the enhanced iron-containing water gas shift catalyst having a BET surface area in a range of from 20 m.sup.2/g to 34 m.sup.2/g.

Description

EXAMPLE 1

(1) A large scale ammonia process was modelled using Aspen HYSYS, to ascertain the effects of changes to the steam ratio. The unit operations of the process are as follows; purification, primary and secondary steam reforming, high temperature and low temperature shift, CO.sub.2 removal, methanation, compression and ammonia synthesis. The process operated with a conventional iron-based high temperature shift catalyst operates at a steam to dry gas molar ratio of 0.48 at the inlet to the high temperature shift (HTS) unit. Replacement of the conventional iron-based high temperature shift catalyst with the low steam water gas shift catalyst, KATALCO™ 71-6, enabled the steam to dry gas ratio to be reduced to 0.40. Correspondingly, the steam-to-carbon ratio at the inlet of the primary reformer was reduced from 2.9 to 2.5.

(2) The enhanced iron containing high temperature shift catalyst used in this example, Katalco™ 71-6, is a co-precipitated iron chromia high temperature shift catalyst with a BET surface area in the range 20-30 m.sup.2/g. Both the replaced conventional iron-based catalyst and the KATALCO™ 71-6 catalyst were cylindrical pellets with lengths in the range 4.8-4.9 mm and diameters in the range 8.3 to 8.5 mm.

(3) TABLE-US-00001 Nitrogen Physisorption BET Surface Area (m.sup.2g.sup.−1) min max Katalco ™ 71-16 20 30

(4) The reduction in steam ratio enabled a 6% increase in throughput for an equivalent pressure drop over the front end of the plant, where the steam/water and process air are increased in step with the process gas flow. This increased plant throughput corresponds to a 4.6% increase in molar flow to syngas compression and a 3.4% increase in terms of ammonia production.

(5) When throughput is constrained by the syngas compressor, the reduction in steam to dry gas ratio increases the supply pressure to the inlet of the compressor, decreasing the pressure ratio and thus enabling increased flowrate. For a typical compressor operating close to its optimum efficiency at 100% speed, the gradient of the compressor performance curve is such that the reduction in pressure ratio would enable a 4% increase in throughput in this case (corresponding to a 1% drop in pressure ratio). This increased plant throughput corresponds to a 2.8% increase in molar flow to syngas compression and a 2% increase in terms of Ammonia production.

(6) TABLE-US-00002 Comparative Replaced HTS Catalyst No increase in Replaced HTS Catalyst hydrocarbon Increased hydrocarbon Comparative feedrate feed Original HTS Throughput Throughput Throughput catalyst increase increase increase Initial 0% 4% 6% Pressure 32.00 34.20 32.81 32.46 Inlet 1st stage of syngas compression (barg) Molar flow 12100 11988 12436 12659 Inlet 1st stage of syngas compression (kmolh.sup.−1) Increase in molar flow — −0.009 0.028 0.046 Mass flow 106.8 107.0 111.2 113.3 Inlet 1st stage of syngas compression (teh.sup.−1) Increase in mass flow 0 0.003 0.042 0.061 Pressure drop 15.30 13.10 14.49 14.84 (bar) Compression ratio 1.84 1.80 1.83 1.83 Compressor available flow 1.000 1.087 1.043 — ratio

(7) TABLE-US-00003 Comparative Throughput increase Initial 4% 6% Ammonia Production 2206.9 2250.4 2282.8 (teday.sup.−1) % of base-case production 100.0 102.0 103.4