Sintering, catalyst lifetimes

Using fixed dolomite guard beds to lower the input tar concentration can extend Ni catalyst lifetimes. Adding various promoters and support modifiers has been demonstrated to improve catalyst lifetime by reducing catalyst deactivation by coke formation, sulfur and chlorine poisoning, and sintering. Several novel, Ni-based catalyst formulations have been developed that show excellent tar reforming activity, improved mechanical properties for fluidized-bed applications, and enhanced lifetimes. 

Improved catalyst lifetime due to the use of reaction heat for distillation and hence a reduction of hot spots and sintering of the catalyst. The reflux could also remove the foulant or coke precursors from the catalyst. It is also possible to eliminate the catalyst poisons or impurities by selecting a suitable feed position in the CD column. 

The oligomerization of olefins is an exothermic consecutive reaction, which benefits from the application of CD for enhanced selectivity to intermediate products. Catalytic distillation plays a particularly important role in enhancing the catalyst lifetime because in situ separation reduces the undesirable high-molecular-weight oligomers or polymers, which will form coke and deactivate the catalyst. The use of reaction heat for distillation also reduces the formation of hot spots and catalyst deactivation due to sintering. 

A catalyst used in industry is very rarely a pure element or compound. Most catalysts contain a complex mixture of chemical additives or modifiers that are essential ingredients for high activity and selectivity. Promoters are beneficial additives that increase activity, selectivity, or useful catalyst lifetime (stability). Structural promoters inhibit sintering of the active catalyst phase or present compound formation between the active component and the support. The most frequently used chemical promoters are electron donors such as the alkali metals or electron acceptors such as oxygen and chlorine. For example, in the petroleum industry, chlorine and oxygen are often added to commercial platinum catalysts used for reforming reactions by which aliphatic straight-chain hydrocarbons are converted to aromatic molecules (dehydrocyclization) and branched isomers (isomerization). 

It was also shown that tubular carbon supports can prolong the lifetime of FTS catalysts, which is the result of deposition of catalytic sites on the interior surfaces of the CNTs resulting in decreased sintering of the metal particles and therefore a more stable catalyst [145]. 

Cathode lifetime durability presents a special challenge in PEM fuel cells. The major problems associated with the catalysts are Pt agglomeration, sintering, dissolution, and redistribution. The cathode environment is highly oxidative and corrosive due to high voltage (e.g., 0.6 to 1.0 V), low pH, elevated temperature, and the presence... 

There are however concerns with durability or lifetime of the catalyst. There can be contaminants within the reformate, in particular sulfur compounds, that can render the catalyst completely inactive. In addition, if the reactor temperature operates too high or an unexpected over-temperature event occurs, the performance or durability of the catalyst can be seriously compromised. Additionally, the support of the catalyst can sinter over time on stream due to the presence of significant amounts of water. Fouling can be another issue, which impacts durability if very pure water or clean air is not used. While all these factors are present, catalyst formulations have been shown to operate for hundreds of hours with minimal to no degradation. 

Commercial Ni-based reforming catalysts exhibit high activity and selectivity for tar conversion into hydrogen-rich gas, but suffer from a number of severe limitations regarding their other properties such as mechanical fragility rapid deactivation, mostly due to sulphur, chlorine, alkali metals and coke formation metal sintering. The overall effect is a limited active lifetime [2]. 

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