1412 coking in gas solid catalytic reactions the

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Unformatted text preview: o increase the surface to volume ratio in order to retard gas phase radical reactions. 1.4.1.2. Coking in gas-solid catalytic reactions The catalysts themselves may also be centres of coke formation. Here coking may occur on metal particles and acid centres [17, 71, 72, 91, 92]. The polymerisation of alkenes or alkynes is suspected to be the main reaction involved on acidic catalysts. If not present in the feed, alkenes and alkynes can be readily produced especially in high temperature processes by thermal cracking. 1. Introduction 32 On metal surfaces various types of coke can be formed. For example on Ni surfaces in the steam reforming process catalytic results from the dissociative adsorption of hydrocarbons (CxHy → x C + y/2 H2) and disproportionation of CO by the Boudouard reaction (2 CO → C + CO2) both resulting in reactive Cα coke. If Cα is gasified, for example by steam in the steam reforming process, then the desired CO and H2 are the products. However, Cα may react further either by polymerisation to form Cβ or by dissolution in the metal [17, 71, 72, 90, 92]. It is uncertain whether only Cα or also Cβ dissolve in the metal. Once the carbon is dissolved it may form metal carbide and diffuse through the metal particle to precipitate out a grain boundary. Continuation of the process leads to the growth of a filament, a so-called whisker, with a metal particle on the top of the whisker. As a result the metal particle is still accessible to the gas and the catalytic activity is generally not decreased [72, 90]. Though the formation of carbon whiskers does not necessarily decrease the catalytic activity, it is nevertheless deteriorating for the process because it can cause partial or total blockage of the reactor. This results in uneven flow distributions causing overheating, which finally results in irreversible reactor damage [10, 17, 18]. Polymerisation of C α to Cβ followed by possible ordering of the carbon can lead to the accumulation of the surface deposits formin...
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