Hydrodynamic reaction zone

Catalytic reactors can roughly be classified as random and structured reactors. In random reactors, catalyst particles are located in a chaotic way in the reaction zone, no matter how carefully they are packed. It is not surprising that this results in a nonuniform fiow over the cross-section of the reaction zone, leading to a nonuniform access of reactants to the outer catalyst surface and, as a consequence, undesired concentration and temperature profiles. Not surprisingly, this leads, in general, to lower yield and selectivity. In structured reactors, the catalyst is of a well-defined spatial structure, which can be designed in more detail. The hydrodynamics can be simplified to essentially laminar, well-behaved uniform fiow, enabling full access of reactants to the catalytic surface at a low pressure drop. 

Extension of the hydrodynamic theory to explain the variation of detonation velocity with cartridge diameter takes place in two stages. First, the structure of the reaction zone is studied to allow for the fact that the chemical reaction takes place in a finite time secondly, the effect of lateral losses on these reactions is studied. A simplified case neglecting the effects of heat conduction or diffusion and of viscosity is shown in Fig. 2.5. The Rankine-Hugoniot curves for the unreacted explosive and for the detonation products are shown, together with the Raleigh line. In the reaction zone the explosive is suddenly compressed from its initial state at... 

Cowperthwaite and Adams (Ref 10) show from details of the energy distribution in the reactive wave at various times, how the propagation bf a reactive shock depends on the flow conditions behind it. Soloukhin (Ref 9) conducted a study of the hydrodynamic structure of an exothermic reaction zone behind a nearly one-dimensional shock. Measurements were made in shock... 

The question considered is a description of the conditions which must be met by a localized initiator if a spherical detonation wave is to be formed. The first problem is a determination of the possibility of the existence of such a wave. Taylor analyzed the dynamics of spherical deton from a point, assuming a wave of zero-reaction zone thickness at which the Chapman-Jouguet condition applies. He inquired into the hydrodynamic conditions which permit the existence of a flow for which u2 +c2 = U at a sphere which expands with radial velocity U (Here U = vel of wave with respect to observer u2 = material velocity in X direction and c -= sound vel subscript 2 signifies state where fraction of reaction completed e = 1). Taylor demonstrated theoretically the existence of a spherical deton wave with constant U and pressure p2equal to the values for the plane wave, but with radial distribution of material velocity and pressure behind the wave different from plane wave... 

The propagation velocity of the TM does not exceed 85-87% of the theoretical detonation velocity DT. Calculation of DT is carried out under the assumption of a chemical reaction which runs after compression by the shock wave without any thermal or hydrodynamic losses. In the case of the TM, meanwhile, the very possibility of propagation of a fast flame with the velocity of the shock wave depends on a velocity redistribution as a result of braking of the layers adjacent to the wall. In constructing the equations for the motion as a whole, braking plays the role of a loss which reduces the velocity. In fact, the velocity will be even smaller than the value cited besides the losses in the hydrodynamic preparation zone (the zone of velocity redistribution between the shock wave front and the forward point of the flame front, zone I-II in Fig. 19) we must add the losses in the combustion zone (from the forward point of the flame front to the cross-section in which combustion has ended, zone II-III in Fig. 19). 

The hydrodynamics of the electrochemical system and temperature are also important because these control the rate of transport of reactants and products to and from the electrochemical reaction zone. This in turn determines the polymerization efficiency. Hydrodynamics are also important in determining the form of the PPy produced. For example, using a flow-through cell and in appropriate chemical envi-... 

The transport of reactants and products to and from the reaction zone is extremely dependent on the cell hydrodynamics and on whether the solution is stationary or moving. Products generated at the auxiliary electrode may also be critical, and for this reason, the auxiliary electrode should be separated or placed downstream. 

The normal detonation velocity Dq is then computed from the perturbed hydrodynamic equations. With an expression for the ideal detonation velocity D, it is then possible to relate the ratio D jDg to the reaction-zone length and the charge radius. The final formulation of this theory by Parlin and Robinson employed the a(v) equation of state. The final results of this formulation were as follows ... 

Diffusional transport and location of the reaction zone are strongly influenced by the hydrodynamic conditions. The relative roles of diffusional and kinetic phenomena may be illustrated by considering and idealised exchanger tube... 

A recent review of detonation theory is given elsewhere [12]. Models of the phenomenon envisage a detonation wave propagating into unreacted material with a sharp discontinuity in temperature and pressure at the detonation front. A reaction zone of a millimeter or smaller dimensions and yielding the equilibrium quantities of reaction products at high temperature and pressure abuts the up-stream side of the front. Using macroscopic hydrodynamic-thermodynamic theory, the energy released, and an equation of state for the assumed products, detonation velocities, pressures, and temperatures may be calculated in certain cases. 

The hydrodynamic approaches assume instantaneous reaction, and apart from a dependence on density, the simplest theories assume detonation parameters to be invariant for a substance and applicable to propagation in infinite, homogeneous (isotropic) media. They give no information on the effect of size or crystal orientation, or on the detailed mechanism by which a detonation propagates. Several theories developed by Jones in the United Kingdom and by Eyring, Wood, and Cook in the United States related detonation velocity to reaction-zone length and explosive diameter, but experimental problems severely limited their validation and application to azides. 

The measuring of flow speed in any point of industrial apparatus is very complicated, and sometimes practically impossible. So, one can determine hydrodynamic structure of reaction mixture movement indirectly, in particular by studying of fluid particles distribution by their residence times in reaction zone. For measuring of aleatory variable, i.e. particle residence time in apparatus, this particle should be marked in some way allowing to register its entering and coming out of apparatus and to receive concentration curve of flow at the outlet. This curve is called output curve or response curve [14, 15]. 

Cellular and diffusion models are usually used for estimation of longitudinal mixing (turbulence) in reaction zone and consequently for evaluation of deviation degree of fluids hydrodynamic structure from ideal displacement and mixing regimes [3, 7, 19-21]. 

Correlation between eeometn of reaction zone with kinetic and hydrodynamic parameters... 

The method of reception of curves of response to indicator introduction (see 2.2.8) is useful for studying of reagents residence times distribution in reaction zone and estimation of hydrodynamic regime of tubular turbulent apparatus operation with different canal s geometry (cylindrical and divergent-convergent) and reagents introduction way (coaxial and radial). 

The hydrodynamic action increases the content of fraction II in C-1, but its activity in isoprene polymerization does not increase (Fig. 3.2). For C-2 the analogous change in the fractional composition is accompanied by an increased activity of fraction II (method 2). The addition of piperylene to C-3 results in a stronger effect on the activity of fraction II under the hydrodynamic action. The change of the hydrodynamic regime in the reaction zone does not affect the activity of fine particles of catalyst fraction III. Isoprene polymerization in the presence of fraction III always has a low rate and a cis-l,4-polyisoprene yield not exceeding 7-12%. 

Relationship between a Reaction Zone Size and Kinetic and Hydrodynamic Parameters... 

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