Schulzediss

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Unformatted text preview: e The tendency to form coke depends on the hydrocarbon feedstock. The tendency of hydrocarbon feedstock is generally characterised by their boiling point. Although certain hydrocarbon structures can have a particularly high coking effect depending on the catalyst, fractions with a higher boiling point are more prone to coking [24, 91]. For each hydrocarbon type, the boiling point increases with the molecular weight. Also the reaction temperature plays an important role in the formation of carbon deposits. 1. Introduction 30 ytrtrghrhrhhrhrhhffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffff Hydrocarbons are generally less stable at higher temperatures and low pressure with regard to their decompositions into the elements, whereas in reactions such as steam reforming or partial oxidation coke formation is less favoured at higher temperatures, because of the overall thermodynamics of these processes. The temperature and the reaction conditions also play a key role in the formation of the type of coke formed. As already discussed is coke formation the main cause of catalyst deactivation in the processes for the syngas and hydrogen. In these processes coke can be formed by gas phase reactions and by gas-solid reactions. 1.4.1.1. Coking in gas phase reactions Coke formation in the gas phase is complicated because of the interaction with surfaces, even if they are not catalytic. Reactions in the gas phase are free radical in nature and occur at relatively high temperatures [18, 71, 72]. The sequence of events is still unknown, but high molecular weight compounds (tars) are built up by a series of polymerisation-condensation reactions. Their vapour pressure is low, even at high temperatures, and they may nucleate to form a droplet in the gas phase or condense on any relatively cool surface. Subsequent dehydrogenation and ordering reactions can lead to carbon whether or not the surface is catalytic [72, 90]. Catalyst deact...
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This document was uploaded on 10/07/2013.

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