Scaling with radiation absorption

SCALING-UP OF A PHOTOCATALYTIC WALL REACTOR WITH RADIATION ABSORPTION AND REFLECTION... 

SCALING-UP OF A HOMOGENEOUS PHOTOCHEMICAL REACTOR WITH RADIATION ABSORPTION... 

Fig. 8-65 Therma-radiation spectrums for the sun and earth with primary absorption bands indicated by shaded areas. Note different scales 1 erg/cm2-s = 1 mW/m2...

Data were collected in the a /20 scan mode (scan width (w) 1.10 -I- 0.34 tan 0), using graphite monochromated MoATa radiation. The intensity data were corrected for Lorentz and polarization eff ects and for long-time-scale variation. No absorption correction was applied. The structure was solved with MULTAN [29] and refined by... 

In all studies involving methods based on absorption or scattering of light, X rays, or neutrons, the characteristic time scales on which radiation interacts with the substance are many orders of magnitude shorter than those of atomic motions. Therefore, it is not the motions themselves but the disordering which arises due to molecular dynamics that should be investigated. 

K. They noted a decay over timescales 95 and < 35 ns, respectively, which was attributed to geminate ion-pair recombination (see Fig. 33). The decay of the optical absorption is independent of the dose of radiation received and continues for about lps. Rather than displaying a dependence on time as eqn. (153), i.e. at f 3/2, the experimental results are more nearly represented by either at f 1 decay to an optical density about one tenth of the maximum or by a decay as t 1/2 to zero absorption. These effects may be the recombination of ions within a spur (or cluster of ion-pairs), which is more nearly like a homogeneous reaction. The range of electrons in propane at 100 K is 10 nm [334] and the extrapolated diffusion coefficient is 10 11 m2 s 1 [320]. The timescale of recombination is 10 ps. The locally greater concentration of ions within a spur probably leads to a faster rate of reaction and is consistent with the time-scale of the reaction observed. Baxendale et al. [395] observed the decay of the infrared optical absorption of the solvated electron in methylcyclo-hexane at 160 K. They noted that the faster decay occurring over < 50 ns was independent of dose and depended on time as t 1/2, i.e. the reaction rate decays as t 3/2, see eqn. (153). It was attributed to recombination of... 

Reactions of organics are very rapid and occur in less than a second, thereby allowing a continuous flow-through system with good process flexibility at full scale when flows vary with time. The absorption dosage of radiation as a function of the penetration depth is shown in Figure 12.4. Clearly, the penetration depth increases with the applied voltage of electrons. Because an electron is able to penetrate as deep as 1 cm, a continuous flow reactor can be used. 

When a molecule is supplied with an amount of energy that exceeds some threshold energy, a unimolecular reaction can take place, that is, a dissociation or an isomerization. We distinguish between a true unimolecular reaction that can be initiated by absorption of electromagnetic radiation (photo-activation) and an apparent unimolecular reaction initiated by bimolecular collisions (thermal activation). For the apparent unimolecular reaction, the time scales for the activation and the subsequent reaction are well separated. When such a separation is possible, for true or apparent unimolecular reactions, the reaction is also referred to as an indirect reaction. We will discuss the following. 

As mentioned above, small-scale photoreactions are quite often carried out in quartz or Pyrex tubes, by external irradiation. However, this is certainly not an optimal solution for maximizing the exploitation of the emitted radiation. Internal irradiation is obviously better from the geometric point of view, but (relatively) large-scale preparations must take into account all of these factors and achieve optimal light and mass transfer. These elements are not taken into account in exploratory studies or small-scale syntheses, just as is the case for thermal reactions, where the optimization is considered at a later stage the essential requirement is that the explorative study is carried out under conditions where occurrence of the reaction is not prevented. Thus, it is important that the source is matched with the reagent absorption, the vessel is of the correct material, and the solvent does not absorb competitively (unless it acts also as the sensitizer). Figure 1.7 and Table 1.1 may help in this choice, in conjunction with the U V spectra of all of the compounds used (it is recommended that the spectra are measured on the actual samples used, in comparison with those taken from the literature, in order to check for absorption by impurities). 

Figure 16 shows the absorption spectrum obtained by additive-free polyethylene [67], At ambient temperature the absorption observed on nanosecond time-scale increased continuously from 500 to 200 nm without showing any maximum. The absorption in UV is similar to that obtained by y-irradiation. Considering the results obtained by liquid alkanes, the absorption seems to be comprised of several different free radicals. At 95 K additional absorption due to the trapped electron was observed at wavelengths longer than 600 nm the band was observable even at ambient temperature in the picosecond time-domain [96]. The electron decays presumably by the hole-electron recombination. The decay of the trapped electron was independent of the presence of carbon tetrachloride, suggesting that the additives reacted with a mobile electron but not with the trapped electron. On adding naphthalene, the radiation-induced spectrum showed the bands due to the first excited triplet state and the radical... 

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