Initial rate regime

The radiation yield depends on the temperature of oxidation and the initiation rate, i.e., the intensity of radiation IT [233], Radoxidation occurs as an initiated chain reaction at an elevated temperature when peroxyl radicals react more rapidly with hydrocarbon RH than disproportionate, kp(2kt) [RH]2 > (see Chapter 2)]. Radoxidation proceeds as a nonchain reaction at low temperatures when peroxyl radicals disproportionate more rapidly than react with hydrocarbon. The temperature boundary Tv between these two regimes of oxidation depends on the value of radiation intensity 7r. The values of Tv for irradiated heptane oxidation is as follows [233] ... 

The FTIR cure profiles of some I mixtures using the overlap and the acrylate bands (Figure 2) show a fast initial rate followed by a slow down with extended irradiation times. The change in cure regime takes place at... 

Existence of a steady-state regime in which the global oxidation rate does not depend on the initiation rate constant. 

Another potential dark source of in the atmosphere, more particularly in the boundary layer, is from the reactions between ozone and alkenes. The ozonolysis of alkenes can lead to the direct production of the OH radical at varying yields (between 7 and 100%) depending on the structure of the alkene, normally accompanied by the co-production of an (organic) peroxy radical. As compared to both the reactions of OH and NO3 with alkenes the initial rate of the reaction of ozone with an alkene is relatively slow, this can be olfset under regimes where there are high concentrations of alkenes and/or ozone. For example, under typical rural conditions the atmospheric lifetimes for the reaction of ethene with OH, O3 and NO3 are 20 h, 9.7 days and 5.2 months, respectively in contrast, for the same reactants with 2-methyl-2-butene the atmospheric lifetimes are 2.0 h, 0.9 h and 0.09 h. 

Analysis of the kinetic data shows that the chain initiation rate is constant at kj = 6.25 lO Mrad" [G = 1.2 (100 eV) ]. Therefore, the sudden increase of the overall reaction rate in the rapid reaction regime must be attributed to an increase of the kinetic chain length. 

The above result suggests that the dynamical fluctuations possess a nontrivial effect on the kinetics of consumption of solute A in the initial time regime. Exphcitly, the perturbative correction to the consumption rate J evolves as in contrast to the behavior embodied in 7o(O [cf. Eq. (34)]. 

The selective hydrogenation of crotonaldehyde to n-butyraldehyde was studied using Pd/C catalyst. The initial rate of hydrogenation was analysed mainly to assess the importance of various mass transfer effects from which it was found that all the rate data under the conditions of the present work were in the kinetic regime. A Langmuir - Hinshelwood type rate model has been derived and the rate parameters were evaluated by using concentration-time data. The agreement of the predicted results with the experimental data was found to be excellent. 

Predicted and Experimental Rates at 25 0. On the basis of previous work ( 3) in which the same nitric acid concentration was employed, a sulphuric acid strength of 79 8 should be well inside the fast reaction diffusional regime. The initial rate according to surface renewal theory (8) should therefore be given... 

It is well known that at extremely low shear rates the slope of the r/y curve (Figure 3.26) is constant and that there exists some very low but finite threshold shear rate beyond which deviation from linearity commences. The slope of the initial linear portion of the curve is known as the limiting viscosity, the zero shear viscosity, or the Newtonian viscosity. Beyond this low shear rate region (initial Newtonian regime) the material is shear-softened (i.e., becomes pseudoplastic), a phenomenon which has its counterpart in the solid state where it is known as strain-softening. 

Figure 23 A-C illustrates the kinetie eurves of major products formation - CHP, DMPC and OZ. The rate of CHP formation for sample 5 with Mo V =1 1 ratio amounts to 1.6.10 M.s C The values of W jjp for samples Nos 7 and 8 are similar to that value. In case of sample Nol the initial rate of CHP formation is to 0.4.10 M.s and is the same as that for the noncatalysed reaction (curve K). The untreated SiO (sample 8) exerts also some catalytic properties. The integral areas below (under) the kinetic curves corresponding to the amount of the products formed vary in the presence and in the absence of the catalyst. For example, the area below the kinetic curve of CHP formation in the catalyzed reaction is greater as compared to that of the noncatalyzed one. The stationary concentration of CHP depends on the type of the catalyst and in relation to their catalytic activity the sample shows the following order 5>8>7>1>K. Curve K is characterized by a S-shape form whereby three periods are distinguished (1) the start of the product accumulation (2) stationary regime in relation to the product (3) autocatalytic process of product formation (Fig. 23, A).

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