The specific size and surface area of droplets produced by atomization is the most critical step in the spray dryer drying process. The degree of atomization, under a set of spray drying conditions, controls the drying rate, and thus the required particle residence time, and therefore, the size of the spray dryer. All atomization techniques can give good average particle size control, but there are major differences in the particle size distribution produced. The most commonly used atomization techniques for spray dryers are:
The spray from pressure nozzles is produced by forcing fluid through an orifice. The energy required to overcome the pressure drop is supplied by the spray dryer feed pump. The narrowest particle size distributions are possible using this technique. Minimizing "fines" is important when using products. The average particle size produced, for a given feed, is a function of the flow rate and injection pressure for each nozzle. Injection pressure depends on feed characteristics and desired particle size, and can range from 300 to 3000 psig. The most energy-efficient atomization technology. Requires a positive displacement, high pressure feed pump such as a plunger or piston/diaphragm pump. Routine changes are required in the nozzle internals, usually made of tungsten carbide. The change schedule depends on the application. A feed rate of at least about 0.10 GPM is required, depending on the size of the undissolved particles in the dryer feed, due to potential clogging of the small pores required. With a multi-nozzle spray dryer, the problem of having one nozzle is not shutting down the operation anymore. The spray dryer wall buildup control can be achieved by changing the spray angle.
The spray from a two-fluid nozzle is produced by contacting two fluids, feed and compressed gas. Atomization energy is provided by a compressed gas, usually air. The contact can be internal or external to the nozzle. Generate a wider particle size distribution. The average particle size produced for a given spray dryer feed is primarily a function of the feed flow rate per nozzle, and the compressed gas velocity and pressure. Minimal energy efficient atomization technology. Can be used to make very fine particles (10-30 microns) due to relatively high abrasion resistance. At the same time, it is usually in the test machine for small flow. Periodic changes of air and liquid caps are required. Generally any type of spray dryer feed pump can be used. Control of the spray angle is limited.
centrifugal atomization
Centrifugal atomization sprays are created by passing a fluid across or across a rotating wheel or disk. The energy required for atomization is supplied by the atomizer motor. Typically produces a broader particle size distribution. The average particle size for most products is limited to 100 microns due to wall build-up issues. The average particle size for a given feed is primarily a function of wheel diameter and RPM. Relatively high gas inlet velocities are required to control wall buildup, which can increase the amount of fines produced. Generally can run for longer periods of time without routine maintenance. Typically, wear resistant. Periodic changes are required for wheel inserts, usually made of tungsten carbide. Control of wall packing is minimal due to the orientation of the spray (horizontal) and wide particle size distribution forcing the dryer to be relatively large in diameter. The capital cost of atomizers is usually high. Larger spray drying tower diameters increase capital costs. As with any high-speed rotating machinery, maintenance costs are high. The design of the top and atomizer support adds to the manufacturing cost. Problems with the nebulizer will shut down the spray drying operation.
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