41 catalytic performance of ni10c900 for different

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Unformatted text preview: 3· h-1 g-1] · Fig. 42 Coking and deactivation of Ni10C900 for different GHSVs, 800°C, 25% O2 surplus, TOS 240 min 4. Results and discussion - catalytic partial oxidation of propane 132 The results show that the reaction is only slightly sensitive to changes in the GHSV in the investigated range. However, formation of unsaturated hydrocarbons in the product mixture observed only at the lowest GHSV might be somewhat unexpected. The formed alkenes are supposed to be more reactive than C3H8. Therefore they might be expected to be rather found at higher GHSV (shorter contact time), as it may be more likely that they pass the catalyst unreacted. However, as the yield of CH4 also decreases, this could be a sign of a decrease in cracking with increasing GHSV. Especially CH4 and C2H4 are typical products of the thermal cracking of C3H8 and C3H6 is a typical product of the oxidative or thermal cracking. A decrease in cracking may be due to the shorter residence time of the substrate in the reactor, which could suppress homogeneous gas phase reaction such as cracking. Another possibility for this finding may be that with increasing GHSV a higher amount of hot spots is created due to the increase in the turnover rate. As the oxidation reactions, partial as well as total, are exothermic this might lead to a higher local catalyst temperature than measured, although the temperature rise due to the reaction was compensated by decreased heating power of the furnace. This would also explain the increase in the yield of H2 at the highest GHSV and the increase in the C3H8 conversion at medium GHSV. The decrease in the C3H8 conversion at the highest GHSV may be explained here by the shortening of the contact time. The differences in the catalyst deactivation and the tendency towards coking can be explained by both concepts. Due to the possible increase of the catalyst temperature with increasing GHSV less coke is formed. At temperatures below 800°C also before slight deactivation was observed. It could also be that by the decrease i...
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