Vapour-phase diffusion

In the first scheme the metal boiling point is less than the oxide boiling point and the model consists of a vaporising droplet of metal surrounded by a detached reaction zone where condensed oxides appear as fine droplets. The reaction rate is said to be controlled by the vapour phase diffusion of metal and atmospheric oxygen into the reaction zone as in Figure 5.6. 

Figure 5.6 Combustion mechanism for vapour phase diffusion flame.

DIFFUSION FLAMES The flames seen in pyrotechnics are classified as vapour phase diffusion flames. The principal characteristic is that the fuel and oxidiser are initially separate (as finely powdered components). Combustion takes place when fuel and oxidiser crystals are subjected to high temperatures (from an igniter flame) the finely powdered components decompose, giving off columns of gas which mix by diffusion. The diffusion flame takes place in the zone where the gases mix. 

There is a qualitative distinction between these two types of mass transfer. In the case of vapour phase transport, matter is subtracted from the exposed faces of the particles via dre gas phase at a rate determined by the vapour pressure of the solid, and deposited in the necks. In solid state sintering atoms are removed from the surface and the interior of the particles via the various diffusion vacancy-exchange mechanisms, and the centre-to-cenU e distance of two particles undergoing sintering decreases with time. 

Both these diffusion controlled and the vapour phase transport processes may be described by tire general equation... 

These compounds have been the subject of several theoretical [7,11,13,20)] and experimental[21] studies. Ward and Elliott [20] measured the dynamic y hyperpolarizability of butadiene and hexatriene in the vapour phase by means of the dc-SHG technique. Waite and Papadopoulos[7,ll] computed static y values, using a Mac Weeny type Coupled Hartree-Fock Perturbation Theory (CHFPT) in the CNDO approximation, and an extended basis set. Kurtz [15] evaluated by means of a finite perturbation technique at the MNDO level [17] and using the AMI [22] and PM3[23] parametrizations, the mean y values of a series of polyenes containing from 2 to 11 unit cells. At the ab initio level, Hurst et al. [13] and Chopra et al. [20] studied basis sets effects on and y. It appeared that diffuse orbitals must be included in the basis set in order to describe correctly the external part of the molecules which is the most sensitive to the electrical perturbation and to ensure the obtention of accurate values of the calculated properties. 

It should be noted that in a vapour phase the liquid layer on the surface of a sensitive element of the sensor (zinc oxide) must be sufficiently thin, so that it would not produce any influence on the diffusion flux of oxygen through this layer. Possible lack of the film continuity (the presence of voids) does not prevent determination of concentration of oxygen in the bulk of the cell by the vapour - gas method. In this case, one deals with a semi-dry method. On the contrary, the presence of a thick liquid layer causes considerable errors in measuring t, because of different distribution of oxygen in a system gas - liquid layer -semiconductor film (this distribution is close to that in the system semiconductor film - liquid), in addition to substantial slowing down of oxygen diffusion in such systems. 

Solvates and hydrates can be unstable when removed from solution, and are not usually desired as the solid form of the final API. The water or solvent molecules often lie along a crystal axis and can diffuse out of the crystal along these channels to achieve equilibrium with the surrounding vapour phase. In some instances this weakens the crystal structure and may cause fragmentation. 

The sequence of IR spectra demonstrates that the molecules of the preloaded component A (pyridine, benzene) are displaced by the ingoing component B (benzene, ethylbenzene) when the preloaded sample is contacted with the vapour phase of the second compound. The process is slow because in both cases component A is more strongly held by the sorbent than component B (vide infra). But these experiments showed, that in principle, it should be possible to monitor counter-diffusion in zeolites via the IR method. 

B. Quenching of fluorescence by a08. The oxygen quenching of fluorescence of aromatic hydrocarbons both in solution and in vapour phase is in general diffusion-controlled. 1 he rate constants for the 08-qucnching of excited singlet and triplet states are given in Table 8.2... 

For a formal kinetic description of vapour phase esterifications on inorganic catalysts (Table 21), Langmuir—Hinshelwood-type rate equations were applied in the majority of cases [405—408,410—412,414,415]. In some work, purely empirical equations [413] or second-order power law-type equations [401,409] were used. In the latter cases, the authors found that transport phenomena were important either pore diffusion [401] or diffusion of reactants through the gaseous film, as well as through the condensed liquid on the surface [409], were rate-controlling. 

For the liquid phase kinetic studies of esterification, with a few exceptions [402,435—437] only the standard (non-porous, see Sect. 1.2.5) ion exchangers were used. The macroreticular (porous) ion exchangers with a large inner surface area are prefered for vapour phase reactions, especially in more recent studies [436—443]. The authors claimed that diffusion was not the limiting process under their conditions. This observation cannot be generalised, however, and even with vapour phase reactions and macroreticular polymers, the possibility of transport limitations through the pores or the polymer mass cannot be excluded a priori. 

It is therefore suggested that provided the diffusion coefficient is rightly chosen the following simple theory of distillation will prove quite servicable. In the simplified theory we assume that the velocity of the vapour is everywhere constant and equal to v, that diffusion takes place in the direction of the vapour flow only with diffusion coefficient D and that concentrations are constant on planes perpendicular to the flow. If X is the mole fraction in the vapour phase of the more volatile component of a binary mixture and Y its mole fraction in the condensate, then at total reflux a mass balance over a section of a column gives... 

Apart from via the vapour phase the material can also be transported to the neck via the solid phase this is called diffusion. During this process the particles move through the solid, from their place in the lattice to an adjacent empty lattice place, a so-called vacancy. 

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