Reaction front movement

The ammonia oxidation reaction proceeds in the first part of the catalyst bed [Fig. 16(a)]. This part is subsequently deactivated, mainly by nitrogen species. The high activity of the catalyst is maintained due to the movement of the reaction front to the next positions in the catalyst bed. When [ Nj-NH3 is injected at the moment that the reaction was already 20 seconds on-stream, labelled N species adsorb further on in the catalyst bed. Thus, in time to come, the deactivation front moves to the end of the catalyst bed. When this front reaches the end of the bed, the catalyst is covered with reaction species and the deactivation is observed in the concentration of the products. An experiment with half an amount of the catalyst also supports this reaction front movement. This experiment showed the formation and concentration of the products in the same manner, however, the catalyst remained active for half the time of the normally applied catalyst bed. Thus, below 413 K, the catalyst remains initially active because the reaction zone moves to the next bed positions, after the previous positions became fully covered with the adsorbed reaction species. Injection of a [ N]-NH3 or [ 0]-02 pulse after the initial deactivation, confirmed that the platinum surface is fully covered and that conversion of ammonia and oxygen is low. No significant amount of nitrogen or oxygen species remains adsorbed at the catalyst surface. 

Modeling of Reaction Front Movement in Combustion Synthesis for Catalyst Preparation... 

Example 11.14 Model the movement of the reaction front in an ion-exchange column. 

OS 90] [R 31] [P 70] At weak electrical field, the propagation velocity of a reaction front in a capillary-flow reactor was increased or decreased depending on the mutual orientation of the electrical field and the reaction zone propagation [68]. The movement of two reaction fronts was given by optical images in [68]. 

The explosion of a dust or gas (either as a deflagration or a detonation) results in a reaction front moving outward from the ignition source preceded by a shock wave or pressure front. After the combustible material is consumed, the reaction front terminates, but the pressure wave continues its outward movement. A blast wave is composed of the pressure wave and subsequent wind. It is the blast wave that causes most of the damage. 

On the equations for the movement and deformation of a reaction front (with R.H. Knapp). Arch. Ration. Mech. Anal. 44, 165-177 (1972). 

Empirical Methods. The grcphical deactivation plot is a very useful empirical method for prediction of the catalyst performance and for estimation of catalyst lifetime (18,19). The deactivation plot shows the length of the reaction front as a function of time. This illustrates the movement of the temperature profile caused by the progressive deactivation of the catalyst. The method is illustrated in Figure 3. The temperature increase over the catalyst bed is calculated as AT = Texii - Twet and a certain percentage hereof, e.g. 90% (AT90) is calculated. The axial distance in the... 

The model showed that for larger drops the reaction front penetration was small, while for smaller drops the movement of the reaction front was very fast and most ofthe nickel is consumed in the early stage (see Fig. 4.4). Afterward there was virtually simple physical absorption of nickel and these smaller drops were no longer effective in extracting the solute, resulting in a loss of effective interfacial area for solute transport. 

FIGURE 18.15 The movement of the reaction front across a catalytic bed with increase in the residence time the point of maximum flux moves from the end of the bed facing the bulk solution toward the end facing the current collector and is maximum at the surface of the current collector toward the end of the residence time. 

The partial differential equations that describe the movement of the reaction front shown in Figure 10-28 are derived and solved in an example in the CD-ROMAVeb Summary Notes for Chapter 10. 

Chemical attack by solutions containing sulfates proceeds by an inward movement of a reaction front. In this way a surface region is produced whose thickness increases with time, and in which the reactions that take place first are those taking place in the greatest depth. 

Klueh, R.L. and Mullins, W.W., Periodic precipitation (Liesegang phenomenon) in solid silver II. Modification of Wagner s mathematical analysis, Acto Met, 17,69,1969. Knapp, R. and Aris, R., On the equations for the movement and deformation of a reaction front. Arch. Rat Mech. Anal, 44, 165, 1972. 

While the geometry of the systems depicted in Fig. 7.10 differs quite appreciably, the actual movement of the reaction front will be identical, provided... 

Noncatalytic gas-solid reactions in mixed bed systems usually involve the movement of a reaction front in the direction of the flow and radial gradients of concentration are usually not very signfiicant. It follows that radial dispersion usually plays an insignificant role in mass transfer problems. However, radial eddy diffusion of heat (eddy thermal conductivity) may play an important role in reactors that are heated or cooled through the bounding walls. An interesting example of this type has been presented by Amundson [20]. 

When a hydrophobic polymer with a physically dispersed acidic excipient is placed into an aqueous environment, water will diffuse into the polymer, dissolving the acidic excipient, and consequently the lowered pH will accelerate hydrolysis of the ortho ester bonds. The process is shown schematically in Fig. 6 (18). It is clear that the erosional behavior of the device will be determined by the relative movements of the hydration front Vj and that of the erosion front V2- If Vj > V2, the thickness of the reaction zone will gradually increase and at some point the matrix will be completely permeated with water, thus leading to an eventual bulk erosion process. On the other hand, if V2 = Vj, a surface erosion process wiU take place, and the rate of polymer erosion will be completely determined by the rate at which water intrudes into the matrix. 

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