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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.
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