Catalyst poisoning deactivation and

In the following, catalyst poisoning and deactivation through coking and metals deposition will be considered, but not fouling by salt deposition, etc. 

The questions of scale-up take us closer to industrial realities. A common problem for fixed-bed reactors is how to decrease the tendency of catalyst poisoning and deactivation. Since this seems to be unavoidable, one has to And methods to restore the catalytic activity. This problem affects all fixed-bed reactors, and consequently it will not degrade the cross-flow reactors compared to other fixed-bed reactors. 

First, sulfur is removed from the hydrocarbon stream (usually natural gas), in order to prevent catalyst poisoning and deactivation with the use of a guard bed. Steam is mixed in the main stream in a fixed steam to carbon molar basis. The steam reform reactor (SRR) is a multitubular catalyst filled furnace reactor where the hydrocarbon plus steam are converted into syngas at high temperatures (700°C - 850 C) according to the following reaction ... 

The typical industrial catalyst has both microscopic and macroscopic regions with different compositions and stmctures the surfaces of industrial catalysts are much more complex than those of the single crystals of metal investigated in ultrahigh vacuum experiments. Because surfaces of industrial catalysts are very difficult to characterize precisely and catalytic properties are sensitive to small stmctural details, it is usually not possible to identify the specific combinations of atoms on a surface, called catalytic sites or active sites, that are responsible for catalysis. Experiments with catalyst poisons, substances that bond strongly with catalyst surfaces and deactivate them, have shown that the catalytic sites are usually a small fraction of the catalyst surface. Most models of catalytic sites rest on rather shaky foundations. 

Many industrial processes are mass-transfer limited so that reaction kinetics are irrelevant or at least thoroughly disguised by the effects of mass and heat transfer. Questions of catalyst poisons and promoters, activation and deactivation, and heat management dominate most industrial processes. 

In the actual process (Figure 10-5), the natural gas feedstock must first be desulfurized in order to prevent catalyst poisoning or deactivation. The desulfurization step depends upon the nature of the sulfur-containing contaminants and can vary from the more simple ambient temperature adsorption of the sulfur-containing materials on activated charcoal to a more complex high-temperature reaction with zinc oxide to catalytic hydrogenation followed by zinc oxide treatment. 

We can also distinguish a few lines of approach regarding catalyst poisons which deactivate the catalyst by coke formation, that is by blocking of pores and the catalyst active sites,... 

Coke is a typical example of a reversible catalyst poison. The deactivation influence of coke depends very much on the nature of the coke, its structure and morphology and the exact location of its deposition on the catalyst surface [42, 43, 44]. 

The monolith catalysts are the least tested in pilot scale, however they have the advantage that they offer good mechanical strength and have high catalytic activity. On the other hand their cost is considerably higher and they arc more prone to poisoning and deactivation than dolomite and related catalysts. Because of their cost, the most important operational variable is the life of the catalyst. 

To improve the resistance of the catalyst towards poisoning and deactivation the tresh catalyst was immersed into a slurry of CaS04 x V2 H2O, 30 weight-% in H2O. The nets were blown clean with compressed air, and then left to harden. This procedure was repeated until 130 g/m of gypsum were deposited on the nets. 

Carbon forms play important roles as intermediates, catalyst additives and deactivating species in Fischer-Tropsch synthesis on iron catalysts. Deactivation may be due to poisoning or fouling of the surface by atomic carbidic carbon, graphitic carbon, inactive carbides or vermicular forms of carbon, all of which derive from carbidic carbon atoms formed during CO dissociation (ref. 5). While this part of the study did not focus on the carbon species responsible for deactivation, some important observations can be made to this end. 

In this paper we present our results on a study ofthe deactivation and characterisation of FCC catalysts, together with product yields at realistic coke levels (0.5 to 1.0%), that are typically found on FCC catalysts during industrial operation. In particular, the effect of quinoline and phenanthrene as additives to the n-hexadecane feedstock has been studied at two concentration levels and the relative roles ofthese additives as catalyst poison and coke inducer are discussed. A further aspect investigated is the influence of catalyst formulation. Pure zeolites are seldom used as FCC catalysts instead, catalysts comprise a number of components, which apart from the zeolite, may include matrix, binder and clay. In the present work, catalyst formulations ranging fi"om 100% matrix to 100% zeolite have been examined and the influence ofthe various catalyst compositions on product distribution and coke formation is assessed. 

Catalytic total oxidation of volatile organic compounds (VOC) is widely used to reduce emissions of air pollutants. Besides supported noble metals supported transition metal oxides (V, W, Cr, Mn, Cu, Fe) and oxidic compounds (perovskites) have been reported as suitable catalysts [1,2]. However, chlorinated hydrocarbons (CHC) in industrial exhaust gases lead to poisoning and deactivation of the catalysts [3]. Otherwise, catalysts for the catalytic combustion of VOCs and methane in natural gas burning turbines to avoid NO emissions should be stable at higher reaction temperatures and resists to thermal shocks [3]. Therefore, the development of chemically and thermally stable, low cost materials is of potential interest for the application as total oxidation catalysts. 

Two types of deactivation studies may be distinguished those in which catalysts are doped with the catalyst poison and those in which the catalyst is deactivated by compounds present in flue gases. 

All compounds were combusted with the required efficiency, with the exception of chlorinated hydrocarbons, which were seen to partially deactivate the catalyst. Chlorinated hydrocarbons are frequently difficult to destroy, with both chlorinated reagents and products acting as catalyst poisons and thus causing catalyst deactivation, resulting in a decrease in activity. A similar situation is seen for fluorinated VOCs. 

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