Emulsions combustion

FIGURE 3.13 lypes of emulsions discussed at symposium on the use of processes, US Department of Transportation, Cambridge, MA. (Reproduced from Zwillenberg, M.L. etal., Water-Oil Emulsion Combustion in Boilers and Gas Turbines. Engineering Foundation Conference, New England College, Henniker, NH, July 26, 1977. With permission.)... 

The fundamental equations governing the process [10] are given in Chapter 40, Emulsion Combustion Method, where the basic phenomena involved in evaporation of solution droplets and evolution of particles are described mathematically. It should be noted that, especially for industrial SD process, commercial packages are available to simulate the process but only from a macroscopic point of view. These packages do not consider the detail of the molecular phenomena involved and just provide general information of the overall process. 

Particle Production via Emulsion Combustion Spray Method... 

Takatori and his collaborators in Toyota Research Center were one of the first who developed and systematically studied the emulsion combustion method (ECM) [3]. This method is basically a combination of the microemulsion wet chemistry and the flame spray pyrolysis methods. In ECM, an aqueous solution of a metal salt is mixed with a fuel such as kerosene and a small amount of an emulsifier or surfactant to obtain a water-in-oil (W/O) type of emulsion. Using a spray nozzle, the solution is then atomized to produce a spray. The size of the mother emulsion droplets depends on the atomizer type and the atomization conditimis, and is on the order of 10 pm for air-assist nozzles. The size of the dispersed micro-solution droplets depends on the string process and surfactant, and is about 1 pm [3]. Figure 40.1 shows a schematic diagram of the ECM. 

Fig. 40.1 Schematic diagram of the Emulsion Combustion Method. (Reprinted from [2] with permission. Copyright 2009 of Bentham Science Publishers)...

Fig. 40.2 Possible scenarios for particle formation via Emulsion Combustion Method. Right branch, each micro-solution droplet produces one particle. Left branch, shrinkage of the emulsion droplet is faster than that of micro-solution droplets, and therefore, some of the micro-solution droplets merge to form larger particles. Middle branch. microexplosion of the emulsion droplet and the formation of smaller emulsion droplets. (Reprinted from [2] with permission. Copyright 2009 of Bentham Science Publishers)...

K. Takatori, T. Tani, N. Watanabe, N. Kamiya Preparation and characterization of nano-structured ceramic powders synthesized by emulsion combustion method, Journal of Nanoparticle Research, 1(2), 197-204 (1999). 

Tani T, Watanabe N, Takatori K, Pratsinis SE. Morphology of oxide particles made by the emulsion combustion method. J Am Ceram Soc 2003 86 898-904. 

A recent development in the conversion of emulsion droplets into solid particles is the so-called emulsion combustion method [172,173]. In this process, the W/O emulsion is atomized, the oil part in the atomized droplet is fired in a burner and the salts in the water part are oxidized. Further discussion on this will be made at appropriate places of this Chapter. 

An emulsion combustion method was developed by Tani et al. [172] for synthesis of alumina and other oxides. The method consists of three steps (i) atomization of a W/O emulsion, (ii) firing the oil phase in the atomized emulsion (SOO C), and (iii) oxidation of the precursors into the metal oxide. In the specific example of aluminum oxide (see BaTiOj for another example), the oil phase was kerosene and the aqueous phase, a solution of Al-nitrate. The surfactants were SUNSOFT 818H, hexa(2-hydroxy-l,3-propyleneglycol)diricinoleate and NP-6, hexaethyleneglycol nonyl phenyl ether. The three components were varied in ratio... 

Considering that the conventional gathering of nanoparticles from microemulsions is difficult, tedious and not remunerative due to limited quantity, Bonini etal. [230] developed a flame spraying technique for microemulsions with gold nanoparticles. The process is reminiscent of the emulsion combustion technique of Tani et al. [172, 173], but has been used for deposition of nanostructured coatings on substrates. 

Unlike microemulsion-mediated particle synthesis, the macroemulsion method is relatively simple and mature. The main reasons behind this are that the properties of the final products are rather easily controlled by (a) the water phase properties like viscosity and (b) the engineering aspects mentioned in Section 6.2.1. The process, thus, is stabilized for bulk production of powders. However, some new developments on the process have been reported in recent times. One of them is the emulsion combustion method described in Chapter 5 [172, 173]. In this process, atomization and firing (800"-850 C) of the emulsion leads to the formation of dry and crystalline powders that can be directly collected in a bag filter. This is also a facile procedure for obtaining hollow particles, useful for thermal insulation and various other purposes [178]. 

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