Ultrasonic atomizers, spray drying

Ultrasonic Atomization Nebulizers 1-5 (55kHz, 0.12 1/min) 30-60 (50 kHz) l-200[88] Medical spray. Humidification. Spray drying. Acid etching. Printing circuit. Combustion Very fine and uniform droplets, Low spray rates Incapable of handling high liquid flow rates... 

Bittner, B., and Kissel, T. (1999), Ultrasonic atomization for spray drying A versatile technique for the preparation of protein loaded bio-degradable microspheres,/. Micro-encapsul., 16, 325-341. 

FIGURE 4 Impact of atomization on spray dried panicle size and distribution (A) ultrasonic atomizer, span = 1.0 (B) twin-fluid atomizer, span = 1.9. 

Since the choice of the atomizer is very crncial, it is important to note the key advantages and limitations of different atomizers (centrifngal, pressnre, and pneumatic atomizers). Other atomizers, e.g., ultrasonic atomizer, can also be nsed in spray dryers (Bittern and Kissel 1999) but they are expensive and have rather low capacity. Although different atomizers can be used to dry the same feedstock, the final product properties (bulk density, particle size, flowability, etc.) are quite different and hence a proper selection is necessary. 

Berger, H. L. 1998. Ultrasonic Liquid Atomization Theory and Application. New York Partridge Hill Pubhshers. Bird, R. B., Stewart, W. E., and Lightfoot, E. N. 1960. Transport Phenomena. New York John WUey Sons Inc. Bittner, B. and Kissel, T. 1999. Ultrasonic atomization for spray drying a versatile technique for the preparation of protein loaded biodegradable microspheres. J. Microencapsul. 16(3) 325-341. 

Spray drying is an example of powder production from solution. It is used widely for preparing ferrites, titanates, and other electrical ceramics. Fine droplets produced by an atomizer are sprayed into a drying chamber and the powder is collected (Figure 20.3). There are different types of atomizers. One uses ultrasonic atomization in which the solution is passed over a rapidly vibrating piezoelectric membrane. Droplet sizes in the range of 10 pm to over 100 pm can be produced. 

Spray drying Slurry Powder Monodisperse product using sonic/ ultrasonic atomizers built-in filters in chamber new chamber designs and air handling systems multi-stage operations... 

Essentially, three major types of atomizers are used in practice (1) rotary wheel (or disk) atomizers, (2) pressnre nozzle, and (3) two-fluid nozzle. Figure 23.20 shows some typical atomizer designs. Ultrasonic and electrostatic atomizers can also be used for special applications to produce monodisperse sprays, but they are very expensive and low capacity. Most spray dryers operate at slight negative pressnre. New designs with low-pressure chambers enhance drying rates at lower temperatnres to dry heat-sensitive products. 

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