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Diffusion-controlled processes, pressure effects

Inert gas pressure, temperature, and conversion were selected as these are the critical variables that disclose the nature of the basic rate controlling process. The effect of temperature gives an estimate for the energy of activation. For a catalytic process, this is expected to be about 90 to 100 kJ/mol or 20-25 kcal/mol. It is higher for higher temperature processes, so a better estimate is that of the Arrhenius number, y = E/RT which is about 20. If it is more, a homogeneous reaction can interfere. If it is significantly less, pore diffusion can interact. [Pg.110]

The important effect of increasing pressure on the kinetics of chemical reactions has been noted since the hrst chemical experiments at high pressure. The simplest expectation derives from the observation that in liquids the viscosity rapidly increases with pressure. As a result, in strongly compressed liquids, and hnally in glasses, diffusion-controlled processes can be retarded. In contrast, however, other reaction pathways can be substantially accelerated. In general, the evolution of a reaction at high pressure can be heavily controlled by kinetic aspects, and these deeply involve intermolecular effects. [Pg.148]

However, the majority of experimental studies have shown that the rate of volatilization from molten glasses decreases after a longer period of time according to a parabolic lime dependence. There is evidence that this is due to reduced concentration of the volatile component on the melt surface this brings about a decrease in vapour pressure at the interface and diffusion of the volatile component in melt becomes the controlling process. This effect does not arise when the melt has the... [Pg.69]

Complications caused by secondary decomposition of AIBN need not be considered. Its initiating effectiveness, /, is practically independent of temperature, but it increases with pressure. The combination rate of the radicals (CH3)2 C (CN) in the toluene cage also increases with pressure the process therefore is evidently diffusion-controlled [55]. AIBN decomposition can be accelerated by the addition of Lewis acids. In this respect some formal similarity therefore exists between the behaviour of peroxy and azo initiators. [Pg.86]

The analysis of the effect of hydrostatic pressure and electron irradiation on the diffusion-controlled crystallization process of amorphous alloys of the metal-metalloid and metal-metal types in [6.48,49] leads to the following conclusions. [Pg.228]

The large positive AV values observed for the quenching by B and TMB are due to the diffusion limit that applies, such that the change in viscosity of the solvent with pressure leads to decreased kq. In the activation-controlled limit, two terms contribute to the observed value of AK, namely, the volume change for the association of the precursor and that associated with the electron-transfer process. The latter contributions can partially cancel each other and account for the rather small pressure effects sometimes observed under such conditions. A more detailed analysis revealed that changes in the dielectric constant of the medium can account for the observed effects in the case of activation-controlled electron transfer [62],... [Pg.122]

In the frequency response method, first applied to the study of zeolitic diffusion by Yasuda [29] and further developed by Rees and coworkers [2,30-33], the volume of a system containing a widely dispersed sample of adsorbent, under a known pressure of sorbate, is subjected to a periodic (usually sinusoidal) perturbation. If there is no mass transfer or if mass transfer is infinitely rapid so that gas-solid mass-transfer equilibrium is always maintained, the pressure in the system should follow the volume perturbation with no phase difference. The effect of a finite resistance to mass transfer is to cause a phase shift so that the pressure response lags behind the volume perturbation. Measuring the in-phase and out-of-phase responses over a range of frequencies yields the characteristic frequency response spectrum, which may be matched to the spectrum derived from the theoretical model in order to determine the time constant of the mass-transfer process. As with other methods the response may be influenced by heat-transfer resistance, so to obtain reliable results, it is essential to carry out sufficient experimental checks to eliminate such effects or to allow for them in the theoretical model. The form of the frequency response spectrum depends on the nature of the dominant mass-transfer resistance and can therefore be helpful in distinguishing between diffusion-controlled and surface-resistance-controlled processes. [Pg.57]

The rate of diffusion controlled reaction is typically given by the Smoluchowski/Stokes-Einstein (S/SE) expression (see Brownian Dynamics), in which the effect of the solvent on the rate constant k appears as an inverse dependence on the bulk viscosity r), i.e., k oc (1// ). A number of experimental studies of radical recombination reactions in SCFs have found that these reactions exhibit no unusual behavior in SCFs. That is, if the variation in the bulk viscosity of the SCF solvent with temperature and pressure is taken into accounL the observed reaction rates are well described by S/SE theory. However, these studies were conducted at densities greater than the critical density, and, in fact, the data is inconclusive very near to the critical density. Additionally, Randolph and Carlier have examined a case in which the observed diffusion controlled, free radical spin exchange rates are up to three times faster than predicted by S/SE theory, with the deviations becoming most pronounced near the critical point. This deviation was attributed to some sort of solvent-solute clustering effect. It is presently unclear why this system is observed to behave differently from those which were observed to follow S/SE behavior. Possible candidates are differences in thermodynamic conditions or molecular interactions, or even misinterpretation of the data arising from other possible processes not considered. [Pg.2837]

Variation in the pressure of the reacting gas can affect corrosion processes in two ways. In the cases more usually met with in practice, in which the corrosion rate is controlled by diffusion processes in the surface film of corrosion product, the influence of gas pressure on corrosion rate is slight. If, however, the dissociation pressure of the oxide or of a constituent of the scale lies within the range involved, the stability of the corrosion product will be critically dependent on the pressure. The effect of temperature is, however, far more critical and thus, in practical cases, pressure variations rarely decide the stability of corrosion products. [Pg.954]


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Control effect

Control effectiveness

Diffusion control

Diffusion controlled

Diffusion effective

Diffusion effects diffusivity

Diffusion process

Diffusion-controlled process

Diffusivity pressure effect

Effective diffusivities

Effective diffusivity

Pressure control

Pressure process

Pressures processing

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