Catalysts deactivation by sintering

The heat released from the CO—H2 reaction must be removed from the system to prevent excessive temperatures, catalyst deactivation by sintering, and carbon deposition. Several reactor configurations have been developed to achieve this (47). 

Nevertheless efforts to understand, treat and model sintering/thermal-deactivation phenomena are easily justified. Indeed deactivation considerations greatly influence research development, design and operation of commercial processes. While catalyst deactivation by sintering is inevitable for many processes, some of its immediate drastic consequences may be avoided or postponed. If sintering rates and mechanisms are known even approximately, it may be possible to find conditions or catalyst formulations that minimize thermal deactivation. Moreover it may be possible under selected circumstances to reverse the sintering process through redispersion (the increase in catalytic surface area due to crystallite division or vapor transport followed by redeposition). 

Pure iron(iii) oxide performs rather poorly as a WGS catalyst, due to rapid catalyst deactivation by sintering. Traditional iron catalysts typically consist of iron(iii) oxide (80-90% by mass), chromium(iii) oxide (8-10% by mass) and small amounts of other stabilisers and promoters such as copper(ii) oxide, aluminium oxide, alkali metals, zinc oxide and magnesium oxide. The small fraction of chromium(iii) oxide acts to prevent catalyst sintering, and also promotes the catalytic activity of iron. Catalyst deactivation is typically caused by poisons in the feedstock gases and by deposition of solids on the catalyst surface. 

In some cases, the temperature inside the reactor has to be limited for example, to prevent overheating of the catalyst (deactivation by sintering) or to avoid cracking of thermally sensitive reactants and products. Other reasons are limitations of the conversion and yield in the case of exothermic reversible reactions (thermodynamic constraints), or the occurrence of unwanted side reactions at elevated temperatures (kinetic constraints). 

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. 

Due to the lower operating temperature (< 250 C). prccipitalcd iron catalysts show little sensitivity towards carbon deposition. However, these catalysts are deactivated by sintering which lead to a reduction of the surface area from about 200 m /g for a freshly-conditioned catalyst to about 50 m /g for a used catalyst. 

Usually the sintering of the catalytic components causes deactivation of catalysts. For example the automotive three-way catalysts deactivate by the sintering of fine particles of precious metals and alumina supports as well as CeOj when the catalysts are subjected at high temperatures for a long time. Sintering of Ce02 facilitates the... 

The influence of internal and interfacial diffusion on catalyst deactivation by simultaneous sintering and poisoning is examined. The study focuses on the copper catalyst used in the water gas shift reaction ( WGSR). It is found that catalyst life increases when internal and external poison diffusional resistance increases. Temperature reduces the total average activity but this effect is partially neutralized by the diffusional effects undergone by the reactants inside the pellet. 

Deactivation by Sintering and Coking of Sol-Gel NiO-Al203-Ti02 Hydrogenation Catalysts... 

The highly exothermic combustion reaction at the top of the catalytic bed produces hot spots in the supported catalysts that increase the problem of deactivation by sintering, but also produce some results... 

The deactivation process can therefore be described as the sum of two simultaneous processes deactivation by sintering following second order kinetics, and deactivation due to catalyst reduction. A mathematical description of this deactivation mechanism was not available, but the examination of the first phase of the deactivation curves seemed to indicate an exponential dependency. Therefore the following equation was proposed to describe the... 

The results presented in Figures 6.11.17-6.11.21 were calculated with the commercial program Presto-Kinetics (solver for differential equations, www.cit-wulkow. de). The maximum temperature of the ARGE catalyst is about 260 °C as the Fe catalyst then starts to deactivate by sintering, which substantially lowers the internal surface area (Kuntze, 1991). Hence, to be on the safe side, 250 °C was chosen as the maximum allowable temperature. 

The behavior of the catalyst is also a consideration. Particularly in high-temperature petrochemical processes, catalysts are deactivated by sintering, coking, and similar processes. 

Hydrogenation of the oxides of carbon to methane according to the above reactions is sometimes referred to as the Sabatier reactions. Because of the high exothermicity of the methanization reactions, adequate and precise cooling is necessary in order to avoid catalyst deactivation, sintering, and carbon deposition by thermal cracking. 

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