Poisons Catalyst Life

Innes has defined a promoter as a substance added during the preparation of a catalyst which improves activity or selectivity or stabilizes the catalytic agent so as to prolong its life. The promoter is present in a small amount and by itself has little activity. There are various types, depending on how they act to improve the catalyst. Perhaps the most xferirive tud ie df pfomritef Ti Teen inTmhn c iron catalysis 

An inhibitor is the opposite of a promoter. When added in small amounts during catalyst manufacture, it lessens activity, stability, or selectivity. Inhibitors are useful for reducing the activity of a catalyst for an undesirable side reaction. For example, silver supported on alumina is an excellent oxidation catalyst. In particular, it is used widely in the production of ethylene oxide from ethylene. However, at the same conditions complete oxidation to carbon dioxide and water also occurs, so that selectivity to C2H4O is poor. It has been found that adding halogen compounds to the catalyst inhibits the complete oxidation and results in satisfactory selectivity. 

Poisons can be differentiated in terms of the way in which they operate. Many summaries listing specific poisons and classifying groups of poisons 

CHEMISORBED POISONS Compounds of sulfur and other materials are frequently chemisorbed on nickel, copper, and platinum catalysts. The decline in activity stops when equilibrium is reached between the poison in the reactant stream and that on the catalyst surface. If the strength of the adsorption compound is low, the activity will be regained when the poison is removed from the reactants. If the adsorbed material is tightly held, the poisoning is more permanent. The mechanism appears to be one of covering the active sites, which could otherwise adsorb reactant molecules. 

SELECTIVITY POISONS The Selectivity of a solid surface for catalyzing one reaction with respect to another is not well understood. However, it is known that some materials in the reactant stream will adsorb on the surface and then catalyze other undesirable reactions, thus lowering the selectivity. The very small quantities of nickel, copper, iron, etc., in petroleum stocks may act as poisons in this way. When such stocks are cracked, the metals deposit on the catalyst and act as dehydrogenation catalysts. This results in increased yields of hydrogen and coke and lower yields of gasoline. 

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The effect of synthesis gas composition on conversion, catalyst life, carbon black formation, etc. was determined in numerous tests. Characteristic variables in the synthesis gas composition are the H2/CO ratio, residual C02 content, and content of trace components in the form of higher hydrocarbons and catalyst poisons. 

These tests were performed to establish the limits in flexibility and operability of a methanation scheme. The two demonstration plants have been operated in order to determine the optimum design parameters as well as the possible variation range which can be tolerated without an effect on catalyst life and SNG specification. Using a recycle methanation system, the requirements for the synthesis gas concerning H2/CO ratio, C02 content, and higher hydrocarbon content are not fixed to a small range only the content of poisons should be kept to a minimum. The catalyst has proved thermostability and resistance to high steam content with a resultant expected life of more than 16,000 hrs. 

For the most highly developed processes, maf coal conversion can be as high as 90 to 95 % with a C4+ distillate yield of 60 to 75 wt % and a hydrogen consumption of 5 to 7 wt %. When an external catalyst is used, it is typically some combination of cobalt, nickel, and molybdenum on a solid acid support, such as silica alumina. In slurry hydrogenation processes, catalyst life is typically fairly short because of the large number of potential catalyst poisons present in the system. 

Metals cause particular problems because they poison catalysts used for sulfur and nitrogen removal as well as other processes such as catalytic cracking (Chapter 5). Thus, serious attempts are being made to develop catalysts that can tolerate a high concentration of metals without serious loss of catalyst activity or catalyst life. 

To prevent the poisoning action of hydrogen peroxide and catalyst life increase [51], the reaction system and the catalyst are added to by substances, i.e. stabilizers of the catalyst operation. They were ionol, hydroquinone, diphenyl amine and diphenylguanidine. 

Overall, it can be concluded that zeolites, and more specifically MFI, are adequate catalysts for oligomerization of short chain olefins to produce gasoline and even diesel range fuels. Selectivity and catalyst life is strongly dependent on parameters such as crystallite size, Si/Al ratio, and poisoning of external surface sites. The introduction of some metals (Ni) can be helpful. 

Catalyst poisoning is one of the most severe problems associated with the commercial application of catalysts. It is a phenomenon whose global behavior is studied extensively in industrial laboratories to allow adequate prediction of commercial catalyst life and commercial behavior. Yet, a quantitative understanding of the intrinsic rates and mechanisms of catalyst poisoning is generally lacking, partly because of the complexity of poisoning processes and partly because of the lack of sufficiently careful studies of these processes. 

What are the rates of sulfur adsorption and of sulfur poisoning and can they be predicted Can catalyst life in commercial catalyst applications be predicted based on poisoning mechanisms and models ... 

It should be emphasized here that even though sulfur tolerances (steady-state activities in the presence of sulfur impurities), of promoted and unpromoted nickel catalysts are extremely low, their sulfur resistances (rates of activity loss in the presence of sulfur impurities), can vary greatly with catalyst configuration, composition, and support. Thus it may be possible to extend significantly nickel catalyst life in the presence of sulfur poisons through addition of promoters or use of novel supports (23,99,100,113, 114, 161, 194, 225-227). This is in fact the basis of two recent patents (225, 226). 

Deactivation parameters obtained by plotting ln[(l — a) a)] versus time are listed in Table XIX for a number of nickel and nickel bimetallic catalysts. The fact that these plots were generally linear confirms that these data are fitted well by this deactivation model. These data, which include initial site densities for sulfur adsorption, deactivation rate constants, and breakthrough times for poisoning by 1-ppm H2S at a space velocity of 3000 hr-1 provide meaningful comparisons of sulfur resistance and catalyst life for both unsupported and supported catalysts. Table XIX shows that the... 

We have not discussed catalyst life as one of the determinants in the selection of a catalyst, although it obviously can be of critical importance. In the absence of poisoning, hydrogenation catalysts frequently last for several years in plant service, and catalyst costs are thus relatively minor in the overall product cost breakdown. Candidate catalysts that did not exhibit exceptional life in laboratory tests were simply excluded from consideration for our purposes. 

However, when we wish to prolong the catalyst life, it is necessary to pay attention not only to thermal deactivation but also to the deactivation by trace amounts of poisonous elements which are not problematie for automobiles due to the shorter life time required. 

For the present purpose, a high Rh surface area even after thermal deactivation is desirable. Therefore, one of the improved catalysts, in which a relatively high amount of Rh is loaded, GEC-01 (Pt = 3 g/1, Rh = 0.6 g/1), was selected as a long life catalyst which would have high poison resistance. Figure 6 shows the activity of model Pb-poisoned catalysts. The characteristics of the activity of Pb-poisoned catalyst are clearly observed on the conventional catalyst, but hardly observed on GEC-01. 

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. 

The effect of internal and interfacial diflRision on catalyst decay by both, poisoning and sintering, was analyzed. Catalyst life increases when internal diffiisional resistance for the poison increases ( high ki values ). Catalyst life also increases when interfedal difiusional resistance increases ( low Reynolds numbers ). Although this effect is less pronounced it might be taken into accoimt when industrial reactor design and its operation are studied. 

Environmental catalysis is required for cleaner air, soil, and water. Various catalysts are in use to improve and/or protect our environment. Catalysts are used in environmental technologies to convert environmentally hazardous materials into harmless compounds. Deactivation of the environmental catalysts occurs as a result of thermal aging, physical and chemical poisoning, and masking mechanisms. Regeneration procedures, which include thermal, physical, and chemical treatment have been developed in order to extend catalyst life. 

A typical nickel catalyst is made from nickel(15—25 wt%) that is finely dispersed onto support material. Unfortunately, the nickel catalyst is highly susceptible to poisoning by sulfur compounds. The catalyst life is dependent upon several factors such as sulfur poisoning, sintering, and carbon deposition. Therefore, sulfur compounds must first be removed from the natural gas feedstock to ensure catalyst life. 

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