Heterogeneous coke deposition

However, coke deposition does not usually occur uniformly in the reactor, which means that the amount of heat evolved in the combustion of the coke deposits may vary locally whithin the bed. The dynamics of propagation of the hot fronts generated also contribute to the heterogeneity of the thermal history of the catalyst particles in bed, and thus, those located near the exit region are usually exposed to comparatively higher temperatures (refs 7-9). It is therefore reasonable to expect a different activity loss by thermal degradation in catalyst particles located at different positions in the bed. In the present work, an experimental study has been carried out of the loss of catalytic activity at different positions in a fixed bed reactor, in order to assess the extent of the thermal degradation of the catalyst. 

The aging and combustion kinetics of coke deposited on an HZSM-5 zeolite-based catalyst in the MTG process have been studied. The kinetic study of coke combustion in air was carried out at 500-550°C in a differential scanning calorimeter, by following the evolution of the combustion products with on-line FTIR analysis. The results provide evidence for limitations on coke reactivity that can be attributed to the combined effects of several circumstances (e.g. bad oxygen-coke contact and heterogeneous distribution of coke within the zeolite crystal). The need is demonstrated for a thermal aging treatment which equilibrates reproducibly the coke prior to combustion. The aging of coke is also limited by a peculiar coke deposit in the microporous stmcture of the zeolite. 

Corella and coworkers model deactivation due to coke deposition by assuming a homogeneous surface and different numbers of sites for each reaction alternatively, an energetically heterogeneous surface with the same numbers of sites is used. The activity functions (p are taken to be exponential with coke level, or... 

The deposition of coke within alumina pellet is not imiform and depends markedly on the heterogeneities in the porosity of the catalyst. 

Surface heterogeneities described earlier often were important relative to coke formed or deposited on aluminized Incoloy 800 surfaces. Figures SB, 6B, and 10B show globular and cylindrical coke which resulted preferentially in the pitted areas of aluminized surfaces in several runs. Filamentous coke formed at 700 C on an aluminized Incoloy 800 coupon subjected to a 0.05 atm. acetylene feed is depicted in Figure 7D every filament observed was in or near a pitted area but interestingly not every pitted area contained filaments. 

Using transmission electron microscopy (TEM) and electron energy loss spectroscopy (EELS), it has been demonstrated that the coke is highly heterogenous, with three dimensional stmcture, even from the beginning of the deposition... 

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