Uniformly reacting solids

To attain adequate surface contact between the reacting solids it is necessary to use the ingredients in fine condition and to bring them into contact by pressure. To maintain both solid contact and thermal conductivity correct, the density of the column is controlled at a uniform value. The manufacture of satisfactory delay compositions, therefore, entails the provision of suitably sized ingredients followed by adequate mixing and accurate pressing. 

The second class of solid reactions involves situations where the solid does not disappear or appear but rather transforms from one solid phase into another as the reaction proceeds, as shown in Figure 9-6. For transformations of solids there are several models that may be appropriate, depending on the microstmcture of the reacting solid. Limiting cases of concentration profiles within the solid are (1) uniform reaction and (2) film formation. Concentration profiles within the solid for these situations are shown in Figure 9-7. 

Sampling and analysis of products can be difficult. Precaulions need to be taken to keep catalyst or reacting solid uniformly distributed in the reactor or to separate them immediately after they leave the reactor otherwise, instantaneous quenching of the product stream will be required. 

We notice that Vth ioes not depend on the concentration of ions in the S E (which for YSZ, for example, is fixed and uniform to a good approximation), px can be fixed by a single phase, for example, O2 gas with a given P02 or Ag metal or it can be fixed by a reaction in the electrode. For example, P02 may be fixed by the couple of reacting solids Fe/FeO when in equilibrium. /x(Na) can be fixed by Sn02/Na2Sn03/02 under a P02 value for which Sn02 is stable or it... 

Fluidized-bed catalytic reactors. In fluidized-bed reactors, solid material in the form of fine particles is held in suspension by the upward flow of the reacting fluid. The effect of the rapid motion of the particles is good heat transfer and temperature uniformity. This prevents the formation of the hot spots that can occur with fixed-bed reactors. 

If it slow, then nucleation is likely to be due solely to proximity. Model D is an example of volame nucleation idiere decomposition of a solid is involved whereas Model E is that involving gas or liquid nucleation of the solid. Note that if nucleation does not occur, the solid reacts uniformly throughout its whole volume (Model F). However, this mode is rare and the nucleation stages are more likely to occur. We wUl not dwell upon how these nucleation models were derived and will only present the results here. One is referred to Appendix I wherein one can study the mathematics used to obtain the net-result. 

The reactant solid B is porous and the reaction occurs in a diffuse zone. If the rate of the chemical reaction is much slower compared to the rate of diffusion in the pores, the concentration of the fluid reactant would be uniform throughout the pellet and the reaction would occur at a uniform rate. On the other hand, if the chemical reaction rate is much faster than the pore diffusion rate, the reaction occurs in a thin layer between the unreacted and the completely reacted regions. The thickness of the completely reacted layer would increase with the progress of the reaction and this layer would grow towards the interior of the pellet). 

Figure 22.2 Schematic representation of fluid (A) + solid (B) reacting system with solid in BMF and fluid uniform...

A schematic representation of this reactor model is shown in Figure 22.2. Particles of solid reactant B are in BMF, and fluid reactant A is uniform in composition, regardless of its flow pattern. The solid product, consisting of reacted and/or partially reacted particles of B, leaves in only one exit stream as indicated. That is, we assume that no solid particles leave in the exit fluid stream (no elutriation or entrainment of solid). This assumption, together with the assumption, as in the SCM, that particle size does not change with reaction, has an important implication for any particle-size distribution, represented by P(R). The implication is that P(R) must be the same for both the solid feed and the solid exit stream, since there is no accumulation in the vessel in continuous operation. Furthermore, in BMF, the exit-stream properties are the same as those in the vessel Thus, P(R) is the same throughout the system ... 

Calcium reacts with phosphine in an analogous manner as the alkali metals. In liquid ammonia, solid Ca(PH2)2 nNHs is formed with hydrogen evolution 128,280) -pjjg corresponding reaction with a solution of elemental strontium in liquid ammonia does not lead to a uniform product. ... 

As sketched for spheres and films, the processes involve either a totally reacted external region and a totally umeacted core or a spatially uniform solid in which the transformation occurs microscopically (perhaps in each single crystal grain) such that macroscopicaUy the solid appears to be spatially uniform at any degree of conversion. 

A systematic study of Cu(II)-Y(III) and Cu(II)-La(llI) systems has shown that uniform spheres can be produced by aging the corresponding metal nitrate solutions in the presence of urea (30). In both cases the internal composition of the resulting particles varied with their size in the course of their growth (Fig. 7.1.6). Once the entire precipitation process was completed, the molar ratios of both metals in the solids corresponded to the same ratio initially introduced into the reacting solutions. 

Mercuric chloride in methanol also reacts with compounds 8 (in dichloro-methane), forming colorless mercury complexes, which can in turn be reconverted to the cyanines 8. Such addition compounds are stable only as solids, decomposing rather quickly in solution. Mercuric acetate in methanol reacts rapidly with the formation of elemental mercury, where by the phosphamethin-cyanines are destroyed uniform products from this reaction have not as yet been isolated. 

Chemical vapor deposition (CVD) of thin solid films from gaseous reactants is reviewed. General process considerations such as film thickness, uniformity, and structure are discussed, along with chemical vapor deposition reactor systems. Fundamental issues related to nucleation, thermodynamics, gas-phase chemistry, and surface chemistry are reviewed. Transport phenomena in low-pressure and atmospheric-pressure chemical vapor deposition systems are described and compared with those in other chemically reacting systems. Finally, modeling approaches to the different types of chemical vapor deposition reactors are outlined and illustrated with examples. 

In our simple coke burning model, the gaseous reactant (oxygen) is assumed to be present uniformly throughout the catalyst particle at a constant concentration without diffusional resistance. The oxygen reacts with the solid reactant B, consisting of coke deposited on the catalyst according to the rate equation... 

In the KI/C12 reaction, an interesting effect of high defect concentration in preventing growth of nuclei of a new solid phase was disclosed [103]. Solid samples differed widely in their initial ionic conductivity and hence in their initial concentration of cation vacancies. The ones with high vacancy concentration reacted very slowly with chlorine and the colour of iodine was seen uniformly as a pale coloration of the whole solid, which had reacted to the extent of only a few percent after several days. Samples with a low vacancy concentration reacted more rapidly and a sharp boundary developed between an outer, iodine-blackened completely reacted region and a pale-coloured inner region. In the un-reactive samples, electronic (presumably positive hole) conductivity appeared transiently for a few hours and then decayed. In the reactive... 

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