STEAM REFORMING

Abstract

A process for steam reforming a hydrocarbon feedstock containing one or more nitrogen compounds, including passing a mixture of the hydrocarbon feedstock and steam through a catalyst bed of one or more nickel steam reforming catalysts disposed within a plurality of externally heated tubes in a tubular steam reformer, each tube having an inlet to which the mixture of hydrocarbon and steam is fed, an outlet from which a reformed gas containing hydrogen, carbon monoxide, carbon dioxide, steam, ammonia and methane is recovered. The steam reforming catalyst at least at the outlet of the tubes comprises nickel dispersed over a porous metal oxide surface present as a coating on a non-porous metal or ceramic structure. The nickel content of the metal oxide coating is in the range of 5 to 50% by weight and the thickness of the coating is in the range of 5 to 150 micrometres.

Claims

1. A process for steam reforming a hydrocarbon feedstock containing one or more nitrogen compounds, comprising passing a mixture of the hydrocarbon feedstock and steam through a catalyst bed consisting of one more nickel steam reforming catalysts disposed within a plurality of externally heated tubes in a tubular steam reformer, wherein each tube has an inlet to which the mixture of hydrocarbon and steam is fed, an outlet from which a reformed gas containing hydrogen, carbon monoxide, carbon dioxide, steam, ammonia and methane is recovered, and the steam reforming catalyst at least at the outlet of the tubes is a structured steam reforming catalyst comprising nickel dispersed over the surface of a porous metal oxide present as a coating on a non-porous metal or ceramic structure, wherein the nickel content of the metal oxide coating is in the range of 5 to 50% by weight and the thickness of the coating is in the range of 5 to 150 micrometres.

2. A process according to claim 1, wherein the thickness of the porous metal oxide coating containing the nickel on the non-porous structure is in the range of 10 to 100 micrometres.

3. A process according to claim 1, wherein the nickel content of the metal oxide coating is in the range of 10 to 30% by weight.

4. A process according to claim 1, wherein a platinum group metal promoter selected from platinum, palladium, rhodium or ruthenium, or a mixture thereof, is included in the coating.

5. A process according to claim 4, wherein the platinum group metal promoter is present in the coating in amounts in the range of 0.05 to 1% by weight.

6. A process according to claim 1, wherein the porous metal oxide over which the nickel is dispersed is a refractory oxide, comprising alumina, titania, zirconia, zinc oxide, magnesia, ceria, praseodymium oxide, yttria, and lanthana.

7. A process according to claim 1 wherein the amount of coating on the non-porous support structure is in the range of 10 to 150 g/m.sup.2.

8. A process according to claim 1 wherein the structured catalyst comprises metal or ceramic structures having a plurality of passages through which a process fluid may pass in ordered, non-random directions.

9. A process according to claim 8, wherein the structured catalyst comprises cylindrical units with a diameter complimentary to the tubes in which they are placed, comprising a plurality of passages through which a process fluid may pass in ordered, non-random directions.

10. A process according to claim 1 wherein the hydrocarbon feedstock comprises methane and a pre-reformed gas, an associated gas or natural gas.

11. A process according to claim 1, wherein the feedstock is compressed to a pressure in the range 10-100 bar abs.

12. A process according to claim 1, wherein the one or more nitrogen compounds comprises nitrogen gas, N.sub.2.

13. A process according to claim 12, wherein the nitrogen gas content of the hydrocarbon feedstock is in the range of 0.1 to 25% by volume.

14. A process according to claim 1, wherein the mixture of hydrocarbon feedstock and steam has a steam to carbon ratio in the range 1.8:1 to 5:1.

15. A process according to claim 1 wherein the mixture of hydrocarbon feedstock is fed to the inlets of the tubes at an inlet temperature in the range 300-650° C.

16. A process according to claim 1, wherein the tubular steam reformer contains a plurality of tubes through which the mixture of the hydrocarbon feedstock and steam is passed, and to which heat is transferred by means of a hot gas comprising a combustion gas or a synthesis gas, flowing around the tubes.

17. A process according to claim 1, wherein the catalyst bed consists of one, two, three or more layers of steam reforming catalyst wherein in each case the layer of steam reforming catalyst adjacent the outlets of the tubes is the structured catalyst.

18. A process according to claim 17, wherein there are two or more layers of steam reforming catalyst within the tubes and the structured catalyst layer comprises 95% to 5% of the volume of the bed.

19. A process according to claim 1, wherein the methane content of the reformed gas is less than 15% by volume on a dry gas basis.

20. A process according to claim 1, wherein the ammonia content of the reformed gas is below 200 ppmv on a dry gas basis.

21. A process according to claim 1, wherein the process further comprises cooling the reformed gas to below the dew point to condense steam and separating the liquid condensate to form a synthesis gas from the reformed gas.

22. A process according to claim 21, wherein the ammonia content of the liquid condensate is below 400 mg/Litre.

23. A process according to claim 21, wherein at least a portion of the condensate is recycled and used to generate steam used in the steam reforming process.

24. (canceled)

25. A process according to claim 12, wherein the nitrogen gas content of the hydrocarbon feedstock is in the range of 0.5-25% by volume.

26. A process according to claim 12, wherein the nitrogen gas content of the hydrocarbon feedstock is in the range of 1-10% by volume.

27. A process according to claim 12, wherein the nitrogen gas content of the hydrocarbon feedstock is in the range of 1-5% by volume.

28. A process according to claim 17, wherein there are two or more layers of steam reforming catalyst within the tubes and the structured catalyst layer comprises 80% to 20% of the volume of the bed.

29. A process according to claim 17, wherein there are two or more layers of steam reforming catalyst within the tubes and the structured catalyst layer comprises 75% to 25% of the volume of the bed.

30. A process according to claim 1, wherein the methane content of the reformed gas is less than 10% by volume on a dry gas basis.

31. A process according to claim 1, wherein the methane content of the reformed gas is less than 5% by volume on a dry gas basis.

32. A process according to claim 1, wherein the ammonia content of the reformed gas is below 100 ppmv on a dry gas basis.

33. A process according to claim 1, wherein the ammonia content of the reformed gas is below 50 ppmv on a dry gas basis.

34. A process according to claim 1, wherein the ammonia content of the reformed gas is below 10 ppmv on a dry gas basis.

35. A process according to claim 21, wherein the ammonia content of the liquid condensate is below 200 mg/Litre.

36. A process according to claim 21, wherein the ammonia content of the liquid condensate is below 100 mg/Litre.

37. A process according to claim 21, wherein the ammonia content of the liquid condensate is below 50 mg/Litre.

38. A process according to claim 21, wherein the ammonia content of the liquid condensate is below 20 mg/Litre.

Description

[0043] The invention is further described by reference to the following Examples and FIGS. 1 to 6, in which:

[0044] FIG. 1 is a graph depicting the ammonia produced per second versus %mol ethane conversion in tests using a reformer feed containing 2% vol N.sub.2;

[0045] FIG. 2 is a graph depicting the ammonia produced per second versus %mol ethane conversion in tests using a reformer feed containing 5% vol N.sub.2;

[0046] FIG. 3 is a graph depicting the ammonia produced per second versus %mol ethane conversion in tests using a reformer feed containing 8% vol N.sub.2;

[0047] FIG. 4 is a graph depicting %mol ethane conversion for catalysts versus ammonia produced per second per m.sup.2 of Ni in the catalysts in tests using a reformer feed containing 2% vol N.sub.2;

[0048] FIG. 5 is a graph depicting %mol ethane conversion for catalysts versus ammonia produced per second per m.sup.2 of Ni in the catalysts in tests using a reformer feed containing 5% vol N.sub.2; and

[0049] FIG. 6 is a graph depicting %mol ethane conversion for catalysts versus ammonia produced per second per m.sup.2 of Ni in the catalysts in tests using a reformer feed containing 8% vol N.sub.2;

EXAMPLE 1

[0050] Testing was carried out on a conventional cylindrical pelleted steam reforming catalysts containing 17.6% wt nickel or 7.2% wt nickel and on a structured catalyst comprising a catalyst coating containing 13% wt nickel and 0.25% wt rhodium on a stabilised aluminium oxide, applied as a wash-coat to stainless steel pellets (3.3 × 3.3 mm cylinders). The catalyst coating loading was 23 mg/in.sup.2. The thickness of the catalyst coating was about 30 micrometres.

TABLE-US-00001 Catalyst Shape Catalytic metal Support NiO content Amount 1(a) Comparative 3.3 × 3.3 mm cylinder Ni Ca-aluminate 17.6% wt 24.94 g 1(b) Comparative 3.3 × 3.3 mm cylinder Ni Ca-aluminate 7.2% wt 22.85 g 1(c) Structured 3.3 × 3.3 mm cylinder Ni/Rh Stainless steel 0.07% wt 86.48 g

[0051] The catalysts were tested in a laboratory scale steam reformer with a single electrically heated reformer tube with an internal diameter of about 25 mm and a length of about 2100 mm. The reactor operated on an up-flow basis. Water for generating steam was supplied to the rig via a variable stroke pump and was fed to the bottom of the reactor where it was vaporised. Natural gas was fed through a separate desulphurisation vessel before being delivered to the reactor via a thermal mass flow controller. Nitrogen and hydrogen were also be supplied to the reactor via independent mass flow controllers if required. The water and gases all entered the reactor via the same inlet pipe. The product gas exited the reactor via an outlet from the tube and was cooled to ambient temperature to condense the steam which was then collected in a catch-pot. A small volume of dry exit gas was fed to a Varian CP490 quad-channel micro GC analyser. This gas then returned to the exit gas meter to allow for a full mass balance from the reformer to be calculated.

[0052] For each of the catalysts, pellet dimensions were measured to determine how many pellets were required to result in a geometric surface area (GSA) of 21080 mm.sup.2. 363 coated pellets were charged for the structured catalyst and 389 pellets were charged for the comparative catalysts. The amounts of nickel charged to the reaction tube were 0.07 g for the structured catalyst and 3.45 g and 1.30 g for the comparative catalysts 1(a) and 1(b) respectively. The pellets were diluted to 100 mL with 3.35 to 4.00 mm alumina chips and the mixture charged to the reformer tube near the outlet. The remainder of the reformer tube was charged with 3.35 to 4.75 mm alumina chips.

[0053] The catalysts were reduced using 50 vol% H.sub.2 in N.sub.2 at 600° C. for 2 hours.

[0054] Reforming was then carried out at a pressure of 27 barg using bed inlet temperatures in the range of 510 to 800° C. with a steam to carbon ratio of 3:1. Catalyst conditioning of the comparative catalysts was first performed by operating the reformer at inlet temperatures of 610° C., 685° C., 735° C., 800° C., and 735° C., each for at least 8 hours. Catalyst conditioning of the structured catalyst was performed by operating the reformer at inlet temperatures of 510° C., 580° C., 610° C., 685° C., 735° C., 800° C., 735° C., 685° C., 610° C., 580° C. and 510° C. and each for at least 8 hours, followed by treatment with H.sub.2 again at 800° C. for 16.5 hours to ensure all of the nickel was in active reduced form. This additional conditioning was to ensure the catalyst was fully reduced and is not believed to effect the ammonia formation in the subsequent testing.

[0055] After conditioning, tests were performed on each of the catalysts at inlet temperatures of 685° C., 735° C. and 800° C.

[0056] The nitrogen content of the feed was adjusted to provide N.sub.2 in the feed gas mixture at the inlet of the catalyst of 2, 5 and 8% by volume on a wet gas basis.

[0057] Reformed gases were collected from the reformer and cooled to below the dew point to condense the steam and form condensates containing ammonia. The amount of ammonia in the condensates is proportional to the ammonia formed by the catalysts in the steam reformer. Condensate samples (250 ml) were collected over a period of 5 minutes at the end of the 8-hour test periods and analysed for their ammonia contents.

[0058] The ammonia concentrations in the condensates recovered from the reformed gases were measured using a calibrated Ion Selective Electrode (ISE). Standard solutions of 0.1, 1 and 10 ppm (w/v) ammonia were prepared. A sodium hydroxide buffer solution was added to the sample to liberate the ammonia. When the ISE voltage measurement was stable, the reading was used to generate a linear calibration curve of ISE voltage reading against log10 ammonia concentration. The ammonia concentrations of the condensates were analysed in the same way, using the ISE measured voltage reading to determine the ammonia concentration by derivation from the calibration curve.

[0059] The tests were repeated for each catalyst using feed gases containing different amounts of nitrogen. This was carried out by introducing nitrogen via a nitrogen supply line at various flows to provide the desired level in the feed gas fed to the reformer tube.

[0060] Tables showing the results of the ammonia produced in the condensates for the different catalysts for the different nitrogen contents in the feed gas are set out below.

TABLE-US-00002 Comparative Catalyst 1(a) Bed inlet temperature °C [N.sub.2], vol.% [NH.sub.3], mg/L 685 2 0.102 685 5 0.226 685 8 0.451 735 2 0.209 735 5 0.515 735 8 0.901 800 2 0.501 800 5 1.100 800 8 1.800

TABLE-US-00003 Comparative Catalyst 1(b) Bed inlet temperature °C [N.sub.2], vol.% [NH.sub.3], mg/L 685 2 0.050 685 5 0.081 685 8 0.144 735 2 0.071 735 5 0.210 735 8 0.420 800 2 0.166 800 5 0.435 800 8 0.807

TABLE-US-00004 Structured catalyst 1(c) Bed inlet temperature °C [N.sub.2], vol.% [NH.sub.3], mg/L 685 2 0.015 685 5 0.018 685 8 0.019 735 2 0.014 735 5 0.020 735 8 0.026 800 2 0.031 800 5 0.060 800 8 0.081

[0061] Over the range of inlet temperatures, the structured catalyst produces lower amounts of ammonia than the comparative examples. However, the catalysts contain differing amounts of nickel, have different nickel surface areas and have different activities. If a catalyst is more active, the amount of steam consumed will be greater than that for a less active catalyst. When this unreacted steam is condensed, it will affect the ammonia concentration. To account for this, a molar flow of water was calculated based on an oxygen balance derived from a knowledge of the feed gas composition and rate and gas-chromatography data on the reformed gas obtained using a GC system coupled to the steam reformer. The difference in the amount of oxygen entering and exiting the system can be used to determine the amount of ammonia produced per second.

[0062] Moreover, the structured catalyst was able to produce a reformed gas with a high conversion of the hydrocarbons in the natural gas.

[0063] The reformed gas after condensate removal was analysed by gas chromatography to establish the conversion of hydrocarbons to hydrogen and carbon oxides. The conversion of the ethane in the natural gas is a better measurement of overall catalyst activity than methane conversion, which is reversible.

[0064] Plotting the ammonia concentration/second versus the ethane conversion illustrates the effectiveness of the catalyst in terms of activity and ammonia production. FIGS. 1-3 depict the ammonia made/second versus the percentage ethane conversion. The results are set out below:

TABLE-US-00005 Catalyst Comparative Catalyst 1(a) Inlet Temperature Ethane conversion (%) 2% N.sub.2 [NH.sub.3]/(×10.sup.-9) mols.sup.-1 5% N.sub.2 [NH.sub.3]/(×10.sup.-9) mols.sup.-1 8% N.sub.2 [NH.sub.3]/(×10.sup.-9) mols.sup.-1 685° C. 52.19 5.1 11.3 22.6 735° C. 59.50 10.1 25.0 43.7 800° C. 74.12 23.1 50.8 83.1

TABLE-US-00006 Catalyst Comparative Catalyst 1(b) Inlet Temperature Ethane conversion (%) 2% N.sub.2 [NH.sub.3]/ (×10.sup.-9) mols.sup.-1 5% N.sub.2 [NH.sub.3]/ (×10.sup.-9) mols.sup.-1 8% N.sub.2 [NH.sub.3]/ (×10.sup.-9) mols.sup.-1 685° C. 32.71 2.6 4.1 7.4 735° C. 39.64 3.5 10.4 20.8 800° C. 57.14 7.9 20.6 38.2

TABLE-US-00007 Catalyst Structured Catalyst 1(c) Inlet Temperature Ethane conversion (%) 2% N.sub.2 [NH.sub.3]/ (×10.sup.-9) mols.sup.-1 5% N.sub.2 [NH.sub.3]/ (×10.sup.-9) mols.sup.-1 8% N.sub.2 [NH.sub.3]/ (×10.sup.-9) mols.sup.-1 685° C. 9.2 0.8 1.0 1.1 735° C. 26.9 0.7 1.1 1.4 800° C. 59.0 1.5 2.9 3.9

[0065] The ethane conversion for the structured catalysts at the temperatures tested start off at a lower level than the comparative catalysts, but it can be seen that at 800° C., the structured catalyst provides a higher ethane conversion than comparative catalyst 1(b) but with a fraction of the ammonia produced, whether the N.sub.2 content of the feed gas was 2, 5 or 8% vol. Comparative catalyst 1 (a) at 735° C. gives an ethane conversion comparable with the structured catalyst at 800° C., but the latter contains a fraction of the nickel content. Testing at higher inlet temperatures for the structured catalyst could improve the ethane conversion further and maintain a low ammonia concentration.

[0066] Whereas the activity of the structured catalyst is below that of standard pelleted catalyst for a given inlet temperature, it is useful to consider the ammonia made when the catalyst is operating at the same hydrocarbon conversion to better reflect the operation that would be expected in service. Furthermore, in the structured catalyst, the nickel crystallites are an order of magnitude smaller than those in the pelleted catalyst giving rise to different nickel surface areas (0.5 m.sup.2/g for Comparative catalyst 1(a) and 8 m.sup.2/g for the structured catalyst). Taking this into account, the differences between the performance of the structured catalyst and the pelleted catalysts are even more clearly depicted. FIGS. 4, 5 and 6 illustrate the percentage ethane conversion versus the ammonia produced as a function of the nickel surface area. The Figures illustrate that the amount of ammonia produced with the structured catalyst is significantly lower than with conventional pelleted catalysts. Furthermore, it is possible to obtain comparable activity to the pelleted catalyst while using significantly less nickel and producing significantly less ammonia. This has been achieved by dispersing the nickel in a thin coating on the non-porous support. These results illustrate that ammonia formation can be reduced by utilising the structured catalyst near the reformer exit, without impacting overall reforming performance.