Reaction-diffusion system apparatus

Fig. 5.1. Schematic apparatus for reaction-diffusion system in one spatial dimension. The boxes 1 through N are separated from a constant-pressure reservoir of A by a membrane permeable only to A, and similarly for the reservoir of B. From [1]...

Fig. 5.2. Schematic apparatus for determining relative stability of two stable stationary states of a reaction-diffusion system. For description see text. Prom [1]...

These intriguing situations, which are similar to the so-called "diffusion falsification" regime of fluid-porous catalytic solid systems (5), can be successfully handled by the "theory of mass transfer with chemical reaction". Indeed, they can be deployed to obtain kinetics of exceedingly fast reactions in simple apparatuses, which in the normal investigations in homogeneous systems would have required sophisticated and expensive equipment. Further, it is possible, under certain conditions, to obtain values of rate constants without knowing the solubility and diffusivity. In addition, simple experiments yield diffusivity and solubility of reactive species which would otherwise have been - indeed, if possible - extremely difficult. 

When conducting a reaction or distillation under a continuous flow of nitrogen or argon rather than in a closed system, a suitable outlet valve must be used in order to prevent back-diffusion of oxygen into the apparatus from the surrounding air. In the simplest cases, it is sufficient to use a Bunsen valve (a rubber tube with two lengthwise slits, closed at one end with a glass rod), or a paraffin oil valve. 

The gas is then passed into the reaction system at a slow rate. If a rapid rate of flow is used, the eflSciency of the purifying and drying tubes is reduced, and a considerable amount of solvent will be lost by evaporation. With a reasonably tight apparatus to prevent diffusion into the system and to prevent air from entering if the solution is cooled, only a very slow flow of gas is required. In any case, it is important to minimize diffusion by using a drying tube on the apparatus which will both decrease the rate of diffusion and prevent the entry of water vapor. 

W. Wakao and J. M. Smith [Chem. Eng. ScL, 17, 825 (1962)] thoroughly analyzed the diffusion data of Rothfeld. These data were obtained in an apparatus of the type shown in Fig. 11-1. For the butane-helium system this means that A He/ c is 3.80. Diffusion is far from equimolal, suggesting that Eqs. (11-26) and (11-27) for D values are not exact. For this particular case Eq. (11-2) should be used. In most reaction systems the counterdififusion of reactants and products is much closer to equimolal, so that Eqs. (11-26) and (11-27) are better approximations. 

Phase diagrams of ternary systems usually contain two-phase regions in the solid state (see e.g., [2] and Figure 10.1). The diffusion mass transfer in ternary and multicomponent systems is essentially different from the case of a binary system in quasiequilibrium as there exists the possibility of two-phase zone formation in the diffusion process. Though two-phase formation is connected with the thermodynamic disadvantage of interphase boundaries formation, there are cases when any other diffusion mode is impossible. Formation of two-phase regions may also proceed at high reaction rates at interfaces, that is, the assumption of quasiequilibrium of the interdiffusion process is imposed. Subsequently, we can apply the apparatus of hnear thermodynamics for irreversible processes [3-5]. 

Droplets distribution of the dispersed phase in size to the formation of fine homogeneous systems in the confiisor-diffuser channels is narrowed by increasing speed of immiscible fluid streams. Increase in volumetric flow velocity co and the number of diffuser confused sections 1 to 4 leads to reduction of the volume-surface diameter of droplets of the dispersed phase and, consequently, to increase in the specific surface of the interface, which in the case of fast chemical reactions intensify flie total process. Inadvisability of using the apparatus with the number of diffuser sections iV confused over 5 1, making these devices simple and inexpensive to manufacture and operate as well as compact, for example, length does not exceed 8-10 caliber (L/d ). 

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