Particle formation reaction controlled processes

Each stage of particle formation is controlled variously by the type of reactor, i.e. gas-liquid contacting apparatus. Gas-liquid mass transfer phenomena determine the level of solute supersaturation and its spatial distribution in the liquid phase the counterpart role in liquid-liquid reaction systems may be played by micromixing phenomena. The agglomeration and subsequent ageing processes are likely to be affected by the flow dynamics such as motion of the suspension of solids and the fluid shear stress distribution. Thus, the choice of reactor is of substantial importance for the tailoring of product quality as well as for production efficiency. 

Particle formation processes can further be subdivided into transport-and reaction-controlled processes. Typically one assumes that the synthesis in the gas phase is not controlled by any mass transfer limitations, whereas mass transfer issues are very common in liquid phase synthesis. Prior discussing mixing and reaction-controUed systems in more detail, it needs to be mentioned that perfect stabilization is assumed for the time being. Effects of coagulation would completely change the PSD. 

One of the areas critical to the MCVD process was understanding the chemistry of the oxidation reactions. It was necessary to control the incorporation of Ge02 while minimizing OH formation. Additionally, understanding the mechanism of particle formation and deposition was critical to further scale-up of the process. 

This chapter focuses on silica synthesis via the microemulsion-mediated alkoxide sol-gel process. The discussion begins with a brief introduction to the general principles underlying microemulsion-mediated silica synthesis. This is followed by a consideration of the main microemulsion characteristics believed to control particle formation. Included here is the influence of reactants and reaction products on the stability of the single-phase water-in-oil microemulsion region. This is an important issue since microemulsion-mediated synthesis relies on the availability of surfactant/ oil/water formulations that give stable microemulsions. Next is presented a survey of the available experimental results, with emphasis on synthesis protocols and particle characteristics. The kinetics of alkoxide hydrolysis in the microemulsion environment is then examined and its relationship to silica-particle formation mechanisms is discussed. Finally, some brief comments are offered concerning future directions of the microemulsion-based alkoxide sol-gel process for silica. 

Alternative processes. In the case of a transport-controlled process, the overall reduction rate can be significantly increased if a continuous renewal of the gas in contact with the solid oxide is readily achieved, i.e., when the oxide particles are not in close contact with each other, as, for example, in a fluidized bed, or a laminar flow reactor, where the reaction is virtually instantaneous. Under these conditions, however, the change in the water removal rate will have a strong impact on the powder properties, in particular on the average grain size. It is very unlikely that under such conditions the production of coarse powder (i.e., the formation of small W single crystals of size >10 pm) can be successful. Thus the inherent ability of the powder bed to retard the water vapor within the layer and to build up locally humid conditions is an important aspect in industrial powder manufacture. 

The chemical reaction taking place is a very important characteristic in all CVD processes. Flow rate, gas composition, deposition temperature, pressure and chamber geometry are the process variables by which deposition is controlled. As CNTs are grown on the particles in the CVD process, the formation of these fine catalytic metal particles is the most important step. A variety of CNTs in large quantities can be produced by changing different reaction parameters and hydrocarbon sources by thermal CVD method. In addition, the process seems adequate for commercial production of CNTs by virtue of comparatively lower operating costs. However, an effective control of all the experimental parameters is required to obtain better results. 

Organic reactions can be understood in terms of a large toolbox with well-known reaction processes, better control in the particle formation by monitoring the reaction rates taking place along with their high-crystalline shape at moderate temperature with high chemical purity. 

Of particular interest to silver particle formation is the effect of changing solution conditions during the particle formation process. Surface potentials are controlled by adsorption of species from solution and can be temperature saisitive. Borohydride ions adsorb strongly to the silver particle surface providing a negative charge. Over the course of the reaction, borohydride oxidizes water such that sevraal hours after initiation ofthe reaction no borohydride is left in solution. Therefore, early in the reaction, borohydride adsorbs... 

Supercritical fluids have already been shown to be useful in the production of particles, including those of metal oxides and other metal species. The near-critical and supercritical fluid environment therefore offers a means for the control of size and composition of mixed oxide nanoparticles. Supercritical fluids also have the potential for providing facile separation of impurities from desired species, such as CoFc204. The role of processing variables such as temperature, pressure, pH, residence time, and relative solubility has been discussed briefly in this work. However, a complete understanding of the reaction and subsequent particle formation will require much further work. Nevertheless, the examples discussed in this work clearly demonstrate the potential of near-critical and supercritical fluids in this area. 

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