properties of spray dried particles

The atomization process followed by rapid evaporation in a spray dryer is responsible for the exceptional quality and economics of spray drying but it is also responsible for many of the physical properties of the dried particles. Some product developers may find these characteristics to be undesirable for their products while others will find them highly desirable. The truth is, many of the properties can be modified with good formulation or post-processing steps. Give us a call to see what we can do. Below is a discussion about the properties that our clients ask about the most:

Shape: As shown in the slideshow about particle morphology, spray dried particles tend to be very spherical. This is caused by the atomization process and rapid drying in a fast moving air stream.

Particle size distribution: Different atomization technologies and dryer configurations will produce different particle size distributions. Atomization technology and dryer configurations are a topic for a different discussion (coming soon) but in general, the counter-current configuration produces larger particles and the co-current configuration produces smaller particles. Two-fluid atomizers produce smaller particles with a wider distribution and high-pressure atomizers produce larger particles with a narrower distribution. Rotary atomizers are somewhere in the middle for both size and distribution width. Nearly all spray dried particles regardless of atomization technology will be between 1-100 um in diameter. Operational parameters of the dryer can also be adjusted to affect particle size distribution, usually the concentration of the feed material has the greatest effect on particle size distribution - higher concentrations giving larger particles.

Bulk density: Even though spray dried particles tend to be spherical, they don't necessarily pack together well. This is because they tend to form agglomerates with each other. The agglomerates are basically irregular shaped clusters of spheres and because of the irregularity of shape they don't pack together in a uniform way. Its more like trying to stack popcorn kernels than oranges - the irregularity in shape causes a lot of void spaces and the particles don't pack as neatly as they would if they were all spheres of identical diameter. Overall the effect of this is a pretty low bulk density. Most spray dried materials will have a loose bulk density in the range of 0.1-0.8 g/ml without post-processing. For some applications, a low bulk density is a good thing but for others it is not. If you'r process demands a higher bulk density then you may want to talk to us about agglomeration and densification methods.

Particle density: Even if you can prevent particle agglomeration and produce uniform spheres in a spray dryer, the particles still tend to be hollow and have a lot of void spaces throughout the particle. This is because as the water is quickly evaporated a solid shell forms around pockets of evaporated water which must then break through the shell. You can see evidence of this unique mass transfer process by looking at the particles through a scanning electron microscope. You will see a lot of hollow spheres, broken shells, and "blow holes" where the water broke through the shell.

Dissolution rate and solubility: The high surface area to mass ratio of spray dried particles gives them good solubility properties. But dissolution is a complicated process. Particles dissolve fastest if they are discretely dispersed in water with good agitation. This allows the particles to be "wetted" before they clump together. powdered products with good "wet-ability" will dissolve faster than particles with poor wet-ability even if the solubility is lower. Spray dried particles don't always have the best wet-ability because the density is so low that they tend to float on the top of a glass of water forming a "lilly pad" of dry powder that is hard to break up with regular stirring. The industry has solved this problem with the advancement of "instantization technology". This is the technology that is responsible for fast dissolving protein shakes, hot chocolate, and powdered milk just to name a few. If rapid dissolution is a must for your product then ask us about how we can help you with instantization. Usually spray drying is so economical that even if you have to follow-up with an instantization step it is still cheaper than other drying technologies that don't require instantization (such as crystalization).

hygroscopicity: Because of the high surface area to volume ratio of spray dried particles, they also tend to be on the hygroscopic side. Proper packaging will be important. Give us a call and we can help you with packaging solutions