Hydrated silica escaping from the catalyst would

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Unformatted text preview: ree because SiO2 is volatile under the reaction conditions. Hydrated silica escaping from the catalyst would deposit downstream the reactor damaging the equipment [17]. Coke formation is the main problem in the STR process [10, 11, 17, 18, 72, 73, 74, 75, 79]. Coking may not only lead to the deactivation or the mechanical destruction of the catalyst, but may also cause partial or total blockage of the reactor. The tendency for coking of the used feedstock increases in the order methane < light paraffins < higher paraffins < naphtha. In order to avoid excessive carbon formation high steam to carbon ratios are used that facilitate gasification of carbon deposits [17, 72, 74]. Addition of basic elements such as alkali and alkaline earth elements are well known to minimise carbon formation by enhancing the gasification of carbon deposits with steam. As shown by Borowiecki et al. also a high Ni dispersion of the catalyst reduces the carbon formation in the STR process [74, 75]. The Topsoe SPARG process has focused on minimising coke formation in the STR of methane by ensemble size effect on the Ni surface [17, 18, 72, 76, 77]. In this process traces of sulphur are added to the feed. Sulphur adsorbs reversibly on the surface in a regular array blocking part of the Ni sites. It was suggested that 3-4 Ni atoms are needed for the steam reforming reaction, but 6-7 for the formation of coke. Tests on industrial scale showed that using this process carbon formation was essentially eliminated. As the adsorption of sulphur is reversible it has to be added constantly to the feed. Very careful control of the sulphur levels in the feed is essential, as too small amounts do not prevent coking and too large amounts lead to the total deactivation of 1. Introduction 25 ytrtrghrhrhhrhrhhffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffff the catalyst. A further disadvantage is that the adsorption of sulphur does not only decrease the rate o...
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