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.
Figure 1. Total surface area of on liter of feed atomized into different droplet 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.
- Higher viscosity makes atomization more difficult. An upper limit for the viscosity of an atomizable feed cannot be given as this is influenced by the feed specific rheological behavior but feeds with viscosities above 500 mPa-s can most often not be atomized. Often feeds show shear thinning behavior, which means that the apparent viscosity can be lowered by stirring. Increasing the temperature of the feed can be done to lower viscosity, making atomization easier. However, beware of age thickening of various dairy products at higher temperatures and solid content. Dilution is another way to reduce viscosity, however, this leaves more solvent to be evaporated.
- Surface tension is the force that has to be overcome in the generating of the larger surface area of the droplets. Due to the surface tension (or surface energy), a considerable energy input is required to create the surfaces. This energy is supplied through the atomization process. Surface tension of a feed is largely influenced by the solvent (water or organic solvents) and to a minor extent the solutes. Organic solvents have lower surface tension than water, making feed based on organic solvents easier to atomize. Surface tension decreases with increasing temperature making atomization easier at higher temperatures.
- Suspended materials‘ size, shape, and physical behavior affects atomization considerably, but this effect is not easily quantified. However, it is evident that drops should preferably be bigger than the largest dimensions of the suspended materials. Otherwise, the drops become distorted and will not form a spherical drop.
The three most common atomization methods used in spray drying are:
- Pressure nozzle atomization
- Pneumatic nozzle atomization
- Rotary atomization
A rough indication of the different atomization methods operating range in terms of droplet size is shown in Figure 2.
Figure 2. Typical droplet size ranges produced by three different atomization devices. Text in the bar indicates how droplet sizes are controlled. ALR stands for Air to Liquid Ratio.
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.
Spray generated from a rotary atomizer
In the rotary atomizer, the feed is introduced to the centre of a rotating wheel. The feed is centrifugally accelerated in the wheel to a high velocity and spread out over the rotating surface as a thin film. The thin film of liquid is shred into droplets at the edge of the rotating wheel. A picture of a rotary atomizer with a cloud of droplets is shown right. Droplet size of the spray is controlled by varying the rotating speed of the wheel.
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.