Activity with time, variation

CA 68,61112 (1968) [The authors report on their mathematical analysis of the expl decompn of mixts of H azide with w. From kinetic data and IR spectra of the decompn products a series of reactions were developed to describe the initiation, growth, and completion of the decompn (deton) process. From these reactions the authors derived a series of differential equations based on the variation of the energy of activation with time. They found that the limiting partial pressure of H azide ign is dependent upon the partial pressure of the diluent (w)l... 

Figure 3, Cyclic activation analysis. Variation of induced activity with time and cycle number. Key T, cycle period tj, time of irradiation U, time between end of irradiation and start of count tc, time of counting and t ,, time between end of counting and start of irradiation. The detected radiations at each counting period are summed to give a cumulative detector response, Dc, over the total experimental time. Dc is maximized when = t , and tf = tc = T/2 ( See reference 17...

The deactivation model allows calculation of the rate of deactivation given the temperature and the activity of the catalyst (via site concentrations). Integration of the rate permits computation of the variation in activity with time. The model is incorporated into a reformer code by making the quasi-steady state assumption the rate of deactivation is slow so that the species and temperature profiles in the reformer are determined by the existing activity profile of the catalyst. 

Some approaches to the modeling of the kinetics of catalyst deactivation are also suggested by the results given in Table 3.5. We see there observations of the variations of catalyst activity with time, which in some cases are linear, exponential, or hyperbolic. Hopefully we remember from Chapter 1 that these types of temporal variations are the fingerprints of zero-, first-, and second-order reactions, respectively, so the suggestion is that catalyst deactivation kinetics may be represented by simple power-law forms. Now, how might this variation be incorporated into the rate law for a surface reaction Let us reconsider the isomerization scheme of (XXV) on an ideal surface but with the surface reaction step slow and rate-controlling ... 

Integrating Equation 19.6, one obtains the variation of activity with time at a constant temperature, i.e. ... 

Half-life values can be determined from decay curves showing the variation of activity with time these values must be correlated with the types of radiation found. Once this correlation has been made, appropriate tables can be used for nuclide identification. 

Akers and Camp(1955) studied the reaction between steam and natural gas within the temperature range of 340-640°C at 1 atmosphere using 3mm diameter pellets of nickel produced by the reduction of nickel oxide supported on kieselguhr. There was little reaction at temperatures less than 600°C and they found first order dependence of the rate of disappearance of methane upon the partial pressure of methane at 640 C. They reported that there was no significant variation of catalyst activity with time. 

If the catalyst active centers are nonuniform, a time variation of the average value of Kp may be caused by the change of the proportion between the centers with various reactivity during polymerization. However, in the case of chromium oxide catalysts the experimental data show that the... 

Adsorption of various organic compounds (e.g., cyclohexanol, adamantanol-1, and camphor) has been studied at a renewed Sn + Pb alloy/electrolyte interface.820-824 The time variation of the surface composition depends on the solution composition, the nature and concentration of the surface-active substance, and on E. The " of cyclohexanol for just-renewed Sn + Pb alloys shifts toward more negative E with time, i.e., as the amount of Pb at the Sn + Pb alloy surface increases. 

Another activation treatment, suitable for most celluloses (although with great variation of the time required, 1 to 48 h) is polar solvent displacement at room temperature. The polymer is treated with a series of solvents, ending with the one that will be employed in the derivatization step. Thus, cellulose is treated with the following sequence of solvents, before it is dissolved in LiCl/DMAc water, methanol, and DMAc [37,45-48]. This method, however, is both laborious, needs ca. one day for micro crystalline cellulose, and expensive, since 25 mL of water 64 mb of methanol, and 80 mb of DMAc are required to activate one gram of cellulose. Its use may be reserved for special cases, e.g., where cellulose dissolution with almost no degradation is relatively important [49]. 

Fig. IB shows that at all temperatures the rate of CH4 oxidation by O2 alone is lower than the rate of CH4 oxidation during the SCR reaction, e.g., at 400 C with CoZSM-5 catalyst the difference between these rates is about 10 times. With increasing temperature this difference diminishes due to the different activation energies of these reactions (Fig. 2). At high temperatures these rates become comparable (in considering Figs. IB and 2 recall that the rate of CH4 oxidation during the SCR process includes a contribution from the rate of CH4 oxidation by O2 alone). These data suggest that below 500 C O2 does not compete effectively with NO, for CH4, but that at high temperatures such a competition must exist. The data of Table I support this view. At ADO C an increase in 62 concentration results in an increase in conversions of both NO into N2 and CH4 into CO2. At the same time, variation of Oj concentration by a factor of 13 has practically no effect on the...

In fluidized beds, the temperature is uniform within a few degrees even in the largest vessels, but variation of comnposition is appreciable in large vessels, and is not well correlated for design purposes. One currently successful moving bed process is the UOP "Stacked Reactor" platforming where the catalyst is transported and regenerated in a separate zone. When the activity of the catalyst declines fairly rapidly, its variation with time and position must be taken into account by the mathematical formulation. 

McCoy. H. N. and Ross, W. H. The specific radioactivity of thorium and the variation of the activity with chemical treatment and with time. J. Am. Chem. Soc. 29, 1709-1718 (1907). 

Figure 8 shows the relationship between the hydrogenolytic behaviors and reduction time (52). The Mo(V) in the reduced catalyst is related neither to the catalytic activity nor to the hydrogenolytic behaviors. The electron spin resonance signal reaches a maximum within a very short reduction period, then drops and reaches a constant with continued reduction. This variation of Mo(V) concentration is compatible with the data obtained by Seshadri and Petrakis (67) and Massoth (55). The changes in the bja ratio and the catalytic activity with the time of reduction agree with the amount of Mo(IV) species reported by Massoth (55), as quoted in Fig. 8. 

Another high-latitude area where substantial ozone concentrations have been observed is London. Derwent and Steward reported ozone concentrations in excess of 0.1 ppm. These were recorded at Harwell, Brookshire, on July 3 and July 7, 1971, by Atkins et al The main subject of Derwent and Steward s paper, however, deals with measurements in central London. Figure 4-10 (from Derwent and Steward ) shows the time variation of photochemically active pollutants for 3 days in central... 

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