Steam reforming

Abstract

A process is described for steam reforming a hydrocarbon feedstock, comprising passing a mixture of the hydrocarbon feedstock and steam through a catalyst bed comprising a particulate nickel steam reforming catalyst and a structured nickel steam reforming catalyst 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 and methane is recovered, and the steam reforming catalyst at the outlet of the tubes is the structured steam reforming catalyst, wherein the particulate steam reforming catalyst comprises 5 to 30% by weight nickel, and the structured steam reforming catalyst comprises nickel dispersed over the surface of a porous metal oxide present as a coating on a non-porous metal or ceramic structure.

Claims

1. A process for steam reforming a hydrocarbon feedstock, comprising passing a mixture of the hydrocarbon feedstock and steam through a catalyst bed comprising a particulate steam reforming catalyst and a structured steam reforming catalyst disposed within a plurality of externally heated tubes in a tubular steam reformer, wherein each tube has an inlet to which the mixture of the hydrocarbon feedstock and the steam is fed, an outlet from which a reformed gas containing hydrogen, carbon monoxide, carbon dioxide, steam and methane is recovered, and the steam reforming catalyst at the outlet of the tubes is the structured steam reforming catalyst, wherein the particulate steam reforming catalyst comprises 5 to 30% by weight nickel, expressed as NiO, and has a largest dimension that is at most 15% of the internal diameter of the plurality of tubes, and the structured steam reforming catalyst comprises 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 coating of the porous metal oxide, as expressed as NiO, is in the range of 5 to 50% by weight and the thickness of the coating of the porous metal oxide is in the range of 5 to 150 micrometres, wherein the plurality of the externally heated tubes consists of three layers of nickel steam reforming catalyst: a structured steam reforming catalyst layer at inlets of the plurality of the externally heated tubes, another structured steam reforming catalyst layer at outlets of the plurality of the externally heated tubes, and a particulate steam reforming catalyst layer located in between these two structured stream reforming catalyst layers, and a structured steam reforming catalyst layer at outlets of the plurality of the externally heated tubes.

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

3. The process according to claim 1, wherein the tubular steam reformer contains the plurality of the externally heated 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 plurality of the externally heated tubes.

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

5. The process according to claim 1, wherein the structured steam reforming catalyst comprises cylindrical units with a diameter complimentary to the plurality of the externally heated tubes in which they are placed, comprising a plurality of passages through which a process fluid may pass in ordered, non-random directions.

6. The process according to claim 1, wherein the particulate steam reforming catalyst comprises an alkali metal oxide, in an amount in the range of 0.5 to 7.0% by weight on the particulate steam reforming catalyst.

7. The process according to claim 1, wherein the largest dimension of the particulate steam reforming catalyst is in the range of 3 to 15% of the internal diameter of the plurality of the externally heated tubes in which it is placed.

8. The process according to claim 1, wherein the particulate steam reforming catalyst is a pelleted steam reforming catalyst.

9. The process according to claim 1, wherein the particulate steam reforming catalyst is cylindrical with a diameter in the range 3 to 10 mm or 5 to 25 mm and a length in the range 2 to 15 mm or 5 to 25 mm and an aspect ratio in the range of 1:0.5 to 2:1.

10. The process according to claim 1, wherein the particulate steam reforming catalyst comprises 3 to 10 through holes and optionally 3 to 12 flutes or lobes around the periphery of the pellet.

11. The process according to claim 9, wherein the particulate steam reforming catalyst is cylindrical with an aspect ratio in the range of 0.75 to 1.0 and the ratio of the internal diameter of the plurality of the externally heated tubes to the diameter of the particulate steam reforming catalyst (Dt/dp) is in the range 5:1 to 50:1.

12. The process according to claim 1, wherein the particulate steam reforming catalyst has an equivalent spherical diameter, expressed as a sphere of the same specific surface area, in the range of 3 to 5 mm.

13. The process according to claim 1, wherein the particulate steam reforming catalyst has an external geometric surface area in the range of 200 to 2000 m.sup.2/m.sup.3.

14. The process according to claim 1, wherein the particulate catalyst is contained within a gas permeable container or containers located between the structured steam reforming catalyst layers.

Description

(1) The invention is further described by reference to the following Examples and FIGS. 1 to 3, in which:

(2) FIG. 1 is a comparison of the R-Factor over tube length for different catalyst arrangements;

(3) FIG. 2 is a comparison of pressure drop over tube length for different catalyst arrangements; and

(4) FIG. 3 is a comparison of relative tube wall temperature margin over tube length for different catalyst arrangements.

EXAMPLES

(5) Steam reformer modelling software was used to examine the difference between various catalyst configurations in order to quantify benefits of using commercially available pelleted steam reforming catalyst over CATACEL SSR structured steam reforming catalysts in a steam reformer comprising a plurality of tubes each having an internal diameter of 127 mm. The properties of the particulate catalysts were as follows:

(6) TABLE-US-00001 Nickel Potash content content Catalyst (NiO % wt) (K.sub.2O % wt) Shape KATALCO 57-4Q 16 0 4-fluted, 4-hole cylinder Diameter 13 mm Length 17 mm KATALCO 25-4MQ 18 1.8 4-fluted, 4-hole cylinder Diameter 10.5 mm Length 13 mm

(7) The following arrangements were modelled:

(8) TABLE-US-00002 Example Catalyst arrangement by volume (inlet to outlet) Comparative Example 1 100% KATALCO 57-4Q Comparative Example 2 40%/60% KATALCO 25-4MQ/KATALCO 57-4Q Comparative Example 3 100% CATACEL SSR Example 4 40%/60% KATALCO 25-4MQ/CATACEL SSR

(9) The model was run with fixed methane slip and fixed exit pressure in each case under operating conditions where carbon formation is possible.

(10) Key performance indicators used in this assessment are shown in the Table below. The R-factor represents a ratio between carbon gasification rate and carbon formation rate. A R-factor greater than 1.0 indicates a tendency for carbon formation in the reformer. The tube wall temperature margin is a difference between a design tube temperature and the operating temperature and indicates potential for tube failure if this margin is less than 25 C. Pressure drop indicates the resistance to flow though the tubes in the reformer.

(11) TABLE-US-00003 Relative minimum Pressure tube wall drop temperature margin Catalyst arrangement (dP), (To Comparative (inlet to outlet) bar R-factor Example 1) Comparative Example 1 1.6 1.223 1.000 Comparative Example 2 2.0 0.607 1.077 Comparative Example 3 1.1 0.739 2.000 Example 4 1.8 0.598 1.846

(12) Comparative Example 1 represents a conventional pelleted catalyst use. The pressure drop for the larger pellets is modest, but the R-factor indicates that under the reaction conditions carbon formation is likely.

(13) Comparative Example 2 replaces a portion of the pelleted catalyst with a smaller potassium-doped pelleted catalyst. The R-factor is lower and the tube wall temperature margin is larger, but the pressure drop is much higher.

(14) Comparative Example 3 uses only the structured catalyst. The pressure drop is very low and the minimum tube wall temperature margin is higher, but the R-factor is increased over Comparative Example 2.

(15) Example 4, which uses a small pellet with a largest dimension about 10% of the internal diameter of the plurality of tubes upstream of the structured catalyst, indicates that under the same conditions, the carbon formation is reduced to below that of Comparative Example 2 but that the pressure drop is improved. Versus Comparative Example 3 the R-factor is also lower, indicating a reduced tendency for carbon formation in the reformer from the claimed combination and the tube wall temperature margin is relatively high.

(16) A comparison of R-factor, pressure drop and minimum tube wall temperature margin is depicted FIGS. 1-3. These illustrate that the use of hybrid configuration of particulate catalysts with the structured catalyst offers an optimal solution in terms of reliable plant operation and increased plant efficiency because: (i) An increased carbon margin enables operation at lower steam to carbon ratios, which means improved reliability and reduced sensitivity to steam flow fluctuations. It also improves feedstock flexibility whereby gas with increased content of higher hydrocarbons can be processed; (ii) Lower tube wall temperatures (TWTs) enable safer, more reliable and more efficient furnace operation, by reducing likelihood of tube failure and reduced fuel usage; and (iii) Reduced pressure drop reduces demand on compression power and allows increase in plant throughput for the same pressure drop, without additional power demand.