Transfer Processes as a Physical Foundation of Spray Pyrolysis of Micron-Sized Droplets of Solutions
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Journal of Engineering Physics and Thermophysics, Vol. 93, No. 6, November, 2020
TRANSFER PROCESSES AS A PHYSICAL FOUNDATION OF SPRAY PYROLYSIS OF MICRON-SIZED DROPLETS OF SOLUTIONS S. P. Fisenkoa and Yu. A. Khodykob
UDC 541.18.04:536.423.4:533.723
The efficiency of the process of spray pyrolysis allowing one to obtain nanoparticles from micron-sized droplets of solutions is determined by the processes of mass transfer in a droplet and of heat and mass exchange in a vapor–gas mixture. It is shown that the slowest and most important process of transfer is evaporation of the droplet. Nucleation of nanoparticles in a supersaturated solution and their growth proceed much more rapidly. The lifetime of the droplet in an aerosol reactor determines the morphology of an ensemble of nanoparticles by coalescence or Brownian diffusion of nanoparticles. Keywords: evaporative cooling, supersaturated solution, nucleation, coalescence, Brownian diffusion. Introduction. Spray pyrolysis is a nonequilibrium physicochemical process of obtaining nanoparticles in evaporation of micron-sized droplets of solutions during their passage together with the carrying gas flow through an aerosol reactor [1, 2]. The time of passage through the reactor is usually slightly less than 1 s. From the engineering point of view, the attractive sides of the spray pyrolysis are the simplicity of the construction of the aerosol reactor, continuous character of the production of nanoparticles, and the ample scope for reactor scaling to increase the output. Despite the external simplicity, from the physical point of view the spray pyrolysis represents a sequence of relatively independent processes of transfer at micro- and nanoscales [3]. The aim of the work is to show the influence of the basic processes of transfer on the efficiency of obtaining nanoparticles in spray pyrolysis. The basic nonequilibrium physical process in spray pyrolysis is evaporation of micron-sized droplets of salt solutions [4, 5]. Formation of a flux of micron-sized (femtoliter) droplets is usually carried out with the aid of an ultrasonic nebulizer. In this case, the smaller the coefficient of surface tension of a solution, the smaller the mean size of the droplets obtainable with the aid of the nebulizer [6]. In particular, addition of ethyl alcohol to a water solution substantially decreases the surface tension of the solution. As is known, finer droplets evaporate more quickly than large ones [4, 5]. Removal of relatively large droplets from the flow with the aid of separators also substantially increases the efficiency of the spray pyrolysis [7]. Evaporation of a solvent inside droplets leads unavoidably to origination of a supersaturated salt solution. The rate of droplet evaporation depends not only on the droplet radius, but also on gas pressure and wall temperature in an aerosol reactor. The formed supersaturated solution inside the droplet disintegrates by the nucleation mechanism. Solid clusters with a characteristic dimension of about 1 nm first grow, move inside the dr
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