Poison distribution

Two kinds of poison distributions must be distinguished. One distribution is that along the catalyst bed, the other one is within the porous system of the catalyst. It may be reasonably anticipated that under most conditions there will be a gradient of contaminant concentration which decreases in the direction from inlet to outlet also that there will be a decreasing concentration of contaminants from the outer confines of each separate catalyst body inwards into the pore system. The contaminant distribution will, however, differ for different types of catalysts and contaminants. 

Not surprisingly, all the data pertaining to axial distribution of contaminants in the bed were obtained for monolithic catalysts, where such determination is performed simply by successive sectioning (see Fig. 3) and analysis of each separate section. In pelleted catalysts there is considerable spatial mixing of the pellets during operation, and the sampling is also difficult. 

From the numerous examples of the axial gradient of poisons we have 

With increasing volatility of the poison compounds, i.e., with increasing temperature, the axial concentration gradient can be flattened or even slightly reversed, as shown for lead in Fig. 6. Temperatures of the order of 850°C or above are quite uncommon, and the gradients in automotive catalysts employed under realistic conditions will be usually like those depicted in Fig. 4. 

An important factor that influences the lead distribution, especially in the channeled monolith bodies, is the character of flow. There is strong evidence that an induced change of the flow from laminar to turbulent augments the deposition of lead. An example is given in Fig. 7 by the deposition pattern of lead in a dual catalyst. Within each of the catalysts, the deposit pattern is in agreement with those shown in Figs. 4 and 5. However, the lead deposit on the inlet of the oxidation catalyst exceeds con- 

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Poisoning curves for porous catalysts. Curve A is for a porous catalyst with hT very small and poison distributed homogeneously. Curve B is for large hT with the poison distributed homogeneously. Curves C and D correspond to preferential adsorption of poison near the pore mouths. For curve C, hT = 5, and for curve D, hT = 20. 

The description is familiar, but we are not writing of America in 1978, but China in 1838, on the eve of the first Opium War, when Great Britain landed troops to compel China to ingest the poison distributed by British merchants. 

There was marked radial and axial distribution of lead and phosphorus within the catalyst bed. The radial poison distribution was almost... 

The quantitative assessment of axial poison distribution (when the first order rate constant and percentage poison pickup are known accurately), together with knowledge of radial distributions, will allow optimization of catalyst design for poison resistance. 

Hegedus and Baron evaluated this model numerically and solved the equations for the poison distribution in the bed. They calculated the total poison content of the bed as ... 

The equation for poisoning kinetics and the conservation equation giving the poison distribution in the bed are... 

Analytical solution in terms of the diffusion-free forms is possible as p 0, but in general numerical solution will be required. If the poison distribution is determined as a function of time and position in the bed from equations (9-171) and (9-172), then the corresponding bed activity is... 

By use of the Poison distribution, Avrami derived the famous Avrami phenomenological equation to treat a kinetic process (Avrami 1939, 1940, 1941). Kolmogorov first discussed the formulation of this equation (Kolmogorov 1937). Johnson and Mehl also made similar derivation independently (Johnson and Mehl 1939). Evans proposed a very concise derivation as introduced below (Evans 1945). 

These are made of boron carbide ia a matrix of aluminum oxide clad with Zircaloy. As the uranium is depleted, ie, burned up, the boron is also burned up to maintain the chain reaction. This is another intrinsic control feature. The chemical shim and burnable poison controls reduce the number of control rods needed and provide more uniform power distributions. 

Cardiac steroids occur ia small amounts ia various plants with a wide geographical distribution. The purple foxglove Di talispurpurus has been used for centuries as both a dmg and a poison. Isolation and characterization of the various cardiac steroids have been reviewed (122,123). 

This tree occurs widely distributed in Africa and the bark is known under a variety of native names, e.g., sassy bark in West Africa, where it was formerly used as an ordeal poison in East and Central Africa it is said to have been an ingredient in arrow-poisons. The bark was first examined by GaUois and Hardy who isolated a toxic alkaloid, erythrophleine, which- was examined by Hamack and Zabrocki and later by Hamack, whose results differed from those of Gallois and Hardy and were generally confirmed by Power and Salway. Recently interest... 

Proton Exchange Membrane 0-85 Can operate at ambient temperature High power density Sensitive to CO-poisoning Need for humidification Transportation Distributed Power... 

Th ese incidents compelled the U.S. Surgeon General to investigate the health effects of TEL. The industry itself moved rapidly to deal with the crisis by instituting a series of safety measures. Now, ethyl fluid was blended at distribution centers and not at seiMce stations (it had been done on the spot and increased the chances of lead poisoning to seiMce sta-... 

Oxidation kinetics over platinum proceeds at a negative first order at high concentrations of CO, and reverts to a first-order dependency at very low concentrations. As the CO concentration falls towards the center of a porous catalyst, the rate of reaction increases in a reciprocal fashion, so that the effectiveness factor may be greater than one. This effectiveness factor has been discussed by Roberts and Satterfield (106), and in a paper to be published by Wei and Becker. A reversal of the conventional wisdom is sometimes warranted. When the reaction kinetics has a negative order, and when the catalyst poisons are deposited in a thin layer near the surface, the optimum distribution of active catalytic material is away from the surface to form an egg yolk catalyst. 

In contrast to the controlled use of these compounds in the neighborhood of farms and human habitation, they have sometimes been used in a less controlled way against rodents and vertebrate predators, which causes problems in conserved areas. In a number of conserved islands in New Zealand, for example, bait containing brodiphacoum has been used for rodent control, both at bait stations and by aerial distribution (Eason et al. 2002). In the latter case, poisoned bait is freely available, and herbivores and omnivores, as well as predators and scavengers are at high risk. This problem will be discussed further in Section 11.6. 

A map of the electron density distribution around these atoms provides important information. It tells us to what distance from the adatom the surface is perturbed or, in catalytic terms, how many adsorption sites are promoted or poisoned by the adatom. The charge density contours in Fig. 6.27 are lines of constant electron density. 

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