Morteza Eslamian Nasser Ashgriz 1 e-mail: ashgriz@mie.toronto.edu Department of Mechanical and Industrial Engineering, University of Toronto, 5 King’s College Road Toronto, ON, Canada M5S 3G8 Effect of Atomization Method on the Morphology of Spray- Generated Particles Effect of various atomization methods and solute concentration on the morphology of spray dried magnesium sulphate particles is investigated. Four types of atomizers are characterized and tested including (i) a vibrating mesh nebulizer, (ii) a splash plate nozzle, (iii) an air mist atomizer, and (iv) a pressure atomizer. Several types of particle morphologies are identified in this research. Spray characteristics, such as droplet num- ber density, droplet size, and velocity, and accompanying atomizing air have major in- fluence on the drying and morphology of the particles. High initial solute concentrations result in the formation of thick-walled particles, and this prevents the particles to burst. It is found to be difficult to obtain fully filled magnesium sulphate particles, even for saturated solutions at room temperature because the solution equilibrium saturation changes substantially with temperature. DOI: 10.1115/1.2400270 Keywords: spray drying, spray pyrolysis, powder morphology, atomization method, spray characteristics Introduction Various types of powders, including ceramic, metal, food, and pharmaceutical, have numerous applications in the areas of mate- rials science, electronics, catalysis, pharmaceutics, and analytical chemistry. Spray pyrolysis and drying are two methods of powder production, which exploit sprays to produce a variety of powders 1–21. In spray drying, a solution is atomized into fine droplets. Next, these solution droplets are introduced to a reactor, wherein they dry. In spray pyrolysis, the drying process is the same as that in spray drying. However, in spray pyrolysis, after drying the particles undergo a chemical reaction, and one or more elements, which are usually in gas form, separate from the original material. A schematic of the spray pyrolysis technique is shown in Fig. 1. During evaporation, a portion of the solvent is vaporized before the crust starts to form. The solute concentration on the droplet surface increases as more solvent evaporates. Simultaneously, the solute diffuses toward the center of the droplet and a concentra- tion gradient develops within the droplet. Once the solute concen- tration on the droplet surface reaches the critical supersaturation of the solution, the solute starts to precipitate on the droplet sur- face. This moment is called the onset of precipitation. If at the onset of precipitation the solute concentration everywhere within the droplet is higher than the equilibrium saturation of the solution at the droplet temperature, then usually the precipitation will be volumetric and a fully filled particle forms. Otherwise the particle is hollow. If this is the case, a crust forms around the droplet and the rest of the solution is trapped inside the droplet. Evaporation of this liquid may increase the pressure inside the particle. Whether this pressure buildup bursts the particle or not depends on several parameters, which will be discussed at the rest of this paper. A key process in spray drying and spray pyrolysis is atomiza- tion. Different atomization methods produce different sprays with different characteristics, such as droplet size and velocity distri- bution, droplet mean size and velocity, spray angle, droplet num- ber density, and breakup mechanism. For the synthesis of pow- ders, using spray drying or spray pyrolysis, different methods of atomization have been employed, including twin fluid atomization for the production of lead zirconate titanate 14, ultrasonic atomi- zation for the production of zirconia powders 1,19,21, air atomi- zation for the formation of yttrium barium cuprate films 18, and electrospray pyrolysis for the production of ZnS nanoparticles 3. In this paper, we present the results of our investigation on the influence of different atomization techniques on the morphology of spray dried particles of magnesium sulphate. Experimental Setup A schematic of the experimental apparatus, used in this study, is shown in Fig. 2. The experimental setup consists of i a chamber and its accessories, including fluid and power feedthroughs; ii a tubular reactor consisting of two semi-cylindrical radiant heaters; iii spray nozzles, which convert aqueous solution of magnesium sulfate into fine droplets; and iv several thermocouples to mea- sure the required temperatures. For all experiments, the tempera- ture at top of the reactor was set to 420°C and was controlled by a temperature controller Watlow, series 965. The chamber is 2 m in height and 1 m dia, and is made of stainless steel. The reactor is made of two semi-cylindrical radiant heaters. The inner diameter of each heater is 8 cm, and the length is 71 cm. The time required for drying of solution droplets is a function of droplet size, veloc- ity, heater temperature, etc. These parameters determine the length of the required heater. For example, low-velocity small droplets 1 Corresponding author. Contributed by the Materials Division of ASME for publication in the JOURNAL OF ENGINEERING MATERIALS AND TECHNOLOGY. Manuscript received September 24, 2005; final manuscript received September 8, 2006. Review conducted by Prof. Golam Newaz. Paper presented at the 2005 ASME International Mechanical Engineering Congress IMECE2005, November 5, 2005–November 11, 2005, Orlando, Florida, USA. Fig. 1 Basic processes of spray drying and pyrolysis. In spray pyrolysis after crust formation, if the reactor temperature is high enough, then a chemical decomposition may occur in the droplet. 130 / Vol. 129, JANUARY 2007 Copyright © 2007 by ASME Transactions of the ASME