Deactivation, of catalysts

Testing of catalyst poisoning is best done in CSTRs since then all of the catalyst is exposed to the same concentration of impurity and the temperature is uniform. 

Activity may depend on time on stream. One index of activity ratio of the rate at time t to the rate with fresh catalyst, 

The rate of gradual destruction of active sites and pore structure can be expressed as a mass action relation, for instance a second order reaction, 

The constant is expected to have an Arrhenius dependence on temperature. Deactivation by coke deposition in cracking operations apparently has this kind of correlation. 

Assumption of a first order rate law gives rise to a = exp(k.[-k2t) (7.55) 

7 Mechanism of catalyst deactivation during SR of sulphur containing liquid fuel (Lakhapatri and Abraham, 2011). 

Coke formation is the most well-known deactivation mechanism in the membrane catalysed reactions (Reitz ef a/., 2000).The three reversible reactions that contribute to the coke formation and form the basis for removal of carbon in the regeneration step by gasification (Domok, 2007 Trimm, 1997,1999) are as follows  

It has been shown that a small trace of sulphur on the inlet stream can reduce coke formation on the nickel catalyst. Deactivated sulphur-nickel sites inhibit the active carbon (Ca) polymerisation/isomerisation to less active carbon (Cj3) due to the lack of free sites. In other words, SR needs three to four nickel sites while carbon formation requires six to seven nickel sites. Thus, the SR catalyst loses some of its activity, but coke formation is minimal (Trimm, 1997,1999). 

When fuel contains heavier hydrocarbons than methane, or it is biofuel, or contains alcohols, the feedstock often contains additional compounds such as sulphur and phosphorus, that is, fertiliser impurities. In the petrochemical industry, gas-borne reactive spedes (i.e., sulphur, arsenic, chlorine, mercury, zinc, etc.) or unsaturated hydrocarbons (i.e., acetylene, ethylene, propylene and butylene) may act as contaminating agents (Deshmukh et al, 2007). These impurities cause catalyst deactivation by poisoning. The effect of a poison on an active surface is seen as site blockage or atomic surface structure transformation (Babita et a/., 2011). Therefore, it is important to choose poisoning-resistant catalyst materials. For example, nickel is not the most effective MSR catalyst although it is widely used in industry due to its low market price compared to ruthenium and rhodium. Both Ru and Rh are more effective in MSR and less carbon is formed in these systems, than in the case of Ni. However, due to the cost and availability of precious metals, these are not widely used in industrial applications. 

In MSR, the catalysts used are based on Cu, Pd, Ru, Ni, Zn or on a combination of these compounds (Basile et a/., 2008b).The choice of the active metal in the process is dependent on the reaction temperature. The Cu-based catalyst can be a good solution at low temperatures T 300°C for MSR membrane reactors. However, Cu catalysts have been found to suffer thermal deactivation above the 300-350°C range, mainly due to sintering of Cu particles. (Twigg and Spencer, 2001). 

During the operational lifetime of most catalysts, their activity decreases. Interestingly, the time period of economic operation can be very different even for commercial catalysts and ranges from a couple of seconds to many years. Table 2.3.8 gives an overview of some important heterogeneous catalyzed reactions, their reaction conditions, deactivation mechanism, possible regeneration options, and lifetime. It can be seen from the table that there is no direct correlation between thermal stress and lifetime. 

The deactivation of a catalyst with activity Ucat (here dimensionsless, that is, relative to the initial activity) occurs over time. To obtain manageable equations to describe in a quantitative manner catalyst deactivation it is useful to define a rate of deactivation rjejct (dimensionless, i.e., the ratio of the actual reaction rate to the initial rate) that may be equivalent to the reduction of the number of active sites Nad (relative to the initial number) over time, Eq. (2.3.7). Note that F is the time on stream of the catalyst, which may be different to the reaction time  

In most cases rjeact will depend on the temperature T, the concentration of deactivating components Cjeact. and/or on the activity Ucat itself (with kdeact. feodeact. and l A,deact as rate constant, pre-exponential factor and activation energy, respectively, of the deactivation). Consequently, rjeact may be expressed in quantitative terms as  

In the least complicated case, a power law rate expression can be applied  

For example, in a pure sintering process, no deactivating component is considered (m = 0) and the deactivation is first order with regard to the (actual) remaining activity. Thus we obtain after integration for a at ( 0  

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Thermal Degradation and Sintering Thermally iaduced deactivation of catalysts may result from redispersion, ie, loss of catalytic surface area because of crystal growth ia the catalyst phase (21,24,33) or from sintering, ie, loss of catalyst-support area because of support coUapse (18). Sintering processes generally take... 

Deactivation of catalysts is an important and complex phenomenon, which can have many causes. It can be kept to a minimum by carefully purifying the feed and by keeping process conditions optimal, which often implies a temperature as low as possible. 

For some processes, though they would not be classified as batch processes, the period of continuous production will be limited by gradual changes in process conditions such as, the deactivation of catalysts or the fouling of heat-exchange surfaces. Production will be lost during the periods when the plant is shut down for catalyst renewal or equipment clean-up, and, as with batch process, there will be an optimum cycle time to give the minimum production cost. 

However, it should not necessarily be concluded that the reactor should be operated at low temperature, as the rate of reaction has yet to be considered. Also, catalysts and the deactivation of catalysts have yet to be considered. 

Carbon Deposition. The processing of hydrocarbons always has the potential to form coke (soot). If the fuel processor is not properly designed or operated, coking is likely to occur. Carbon deposition not only represents a loss of carbon for the reaction but more importantly also results in deactivation of catalysts in the processor and the fuel cell, due to deposition at the active sites. 

Hydrothermal Deactivation of Catalyst Impregnated with Different Levels of Metal... 

Deactivation of catalysts in the reforming of liquid fuels is caused principally by two processes the formation of carbon-containing deposits and sulfur poisoning. This section examines the thermodynamics and the literature dealing with these processes. 

Reaction Studies. There have been a significant number of studies on the deactivation of catalysts in the reforming of liquid fuels, especially the formation of elemental carbon and coke. Collectively, these studies show that the... 

MAS NMR experiments characterizing catalysts in reaction environments in flow systems may be carried out under conditions close to those of industrial processes. The formation of catalytically active surface species and the cause of the deactivation of catalysts can be characterized best under flow conditions. When flow techniques are used for the investigation of reactions under steady-state conditions, a continuous formation and transformation of intermediates occurs. The lifetime of the species under study must be of the order of the length of the free-induction decay, which is ca. 100 ms for " C MAS NMR spectroscopy. 

In Section V, deactivation of catalyst pellets and reactor beds during residuum hydroprocessing is considered. The chemical nature of the metal deposits is described, including a discussion of the physical distribution of these poisons in aged catalysts and reactor beds. Models to predict... 

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