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A typical example is the transformation of aluminium

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Unformatted text preview: the processes for the production of syngas described above. A typical example is the transformation of γ-aluminium oxide to α-aluminium oxide at high temperatures which is accompanied with a decrease of the surface area from ~200 m2 for γ-aluminium oxide to ~2 m2 for α-aluminium oxide upon heating to t~1100°C [72]. Such phase changes can result in the collapse of pores and the encapsulation of the active phase within the new lattice or the coalescence of active sites. Another example is the formation of NiAl2O4 from NiO on Al2O3 if steaming is carried out in the absence of H 2 above 700°C. The spinel is not active in the steam reforming process and more difficult to reduce. The most common and often most important cause for the deactivation of the catalysts is coking. Coking can result in the mechanical destruction of the catalyst, its deactivation by blockage of the active sites or the blockage of the reactor. Even partial blockage of the reactor is deteriorating as it results in uneven flow distributions causing overheating, which causes irreversible damage to the reactor. 1.4.1. Coking The terms ‘coke’ and ‘coking’ are generic descriptions for a variety of carbonaceous deposits. Coke is formed at relatively high temperatures and its chemical nature can vary widely from graphitic carbon to condensed polymers and metal carbides [17, 18, 71, 72, 90, 91, 92]. Which type of coke is formed depends greatly on the type of the 29 1. Introduction ytrtrghrhrhhrhrhhffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffff ff reaction, e.g. gas phase or liquid phase process, and the presence of catalytic elements, e.g. metallic surfaces or acidic centres. The properties of common forms of coke are summarised in Table 5. Tab. 5: Various types of coke [72] Type atomic carbon adsorbed on Temperature of C/H formation [°C ] Ca Origin Gasification ratio >200 a easy surfaces Cb polymeric amorphous film >300 »8 possible whisker whisker carbon usually with >300 »8 reasonably a metal particle on top carbides m etal carbides easy >300 reasonably easy surface deposited on non catalytic >300 » 50 difficult or catalytic surfaces gas phase formed in the gas phase >500 »8 difficult liquid phase deposited from heavy oils ~400 »1 difficult >1000 high difficult in the liquid phase graphit...
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