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Atomization is the first and very important step of the spray drying process. Spray drying is a fast and efficient drying process and allows only a short time for evaporation to occur. A condition for successful spray drying is that the feed can adequately be atomized into a spray of droplets. This enables fast drying of the droplets as evaporation occurs from the large surface area generated by the atomizer.
The surface area resulting from atomization of a feed is considerable. Figure 1 shows the surface area of feed after being atomized into droplets of different sizes.
Viscosity, surface tension and size, shape, and physical behavior of suspended components are intrinsic factors that determine how well feed atomizes. As a rule of thumb: if the feed creates a drop as it leaves a syringe, it is likely that atomization will be efficient. However, if it does not break up into drops, it may prove to be difficult or impossible to atomize.
The three most common atomization methods used in spray drying are:
A rough indication of the different atomization methods operating range in terms of droplet size is shown in Figure 2.
Pressure nozzle atomization utilizes a high pressure pump to increase the static pressure of the liquid. As the liquid passes the nozzle, the static pressure is converted into kinetic energy causing the liquid to break up into droplets as it accelerates away from the nozzle and into the drying air. The resulting droplet size distribution and mean droplet size is controllable by varying the static pressure of the feed. The pressure nozzle normally applied produces a hollow spray cone with spray angles typically between 60 °C and 90 °C. The spray angle also influences the droplet size and distribution.
Pneumatic nozzles operate by exposing a liquid feed stream to a high velocity gas stream creating high frictional forces over the liquid surface which causes disintegration of the liquid into droplets. The pneumatic nozzles come in different designs where two fluid nozzles with internal or external mixing are the most frequently used. Droplet size is most commonly controlled by varying the flow rate of the high velocity gas stream or the liquid feed stream, but changing orifice size is also used.
Atomization is highly important for the overall drying process as the initial droplet size distribution very much determines the final particle size distribution of the powder and GEA Niro therefore also has the capability to measure the droplet size distribution for the various atomizers. Combined with detailed knowledge on atomization of an extensive range of products, GEA Niro can guide you in the selection of the best suited atomization technology and which type of spray dryer you should use for a given product.
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