Spray drying granulation is a common granulation technology that offers many advantages not found in other granulation methods. However, not all products are suitable for processing using this method. Understanding the mechanism of spray drying granulation will help you determine whether your product is suitable for this processing method.
What Is Spray Drying Granulation?
Spray drying granulation is a process that converts liquid materials into solid granules. It combines drying and granulation into a single and continuous step, thereby simplifying the production process.
The granules produced by spray drying granulators have good flowability and uniform particle size. This process is suitable for granulating various materials, including ceramic materials, pharmaceuticals, and food additives.
The Mechanism of Spray Drying Granulation
The core process of spray drying granulation can be divided into three basic stages. Atomization of the feed solution into droplets, drying of the droplets through contact with hot air, and agglomeration to form granules.
Atomization
Atomization is the first step in spray drying granulation. Its purpose is to disperse the feed solution into fine droplets, creating favorable conditions for rapid drying in subsequent steps. There are three methods of atomization.
Centrifugal Atomization
Centrifugal atomization uses a rotary atomizer, also known as a centrifugal nozzle. Its core component is a high-speed rotating atomizing disc.
Principle
The feed liquid is fed into the center of a high-speed rotating atomizing disc and is flung outward by centrifugal force. Under the combined action of centrifugal force and air drag, the feed liquid breaks up into fine droplets. The rotational speed of the atomizing disc can reach 3,000–20,000 r/min. In industrial applications, the disc’s peripheral speed is generally controlled at 75–170 m/s.
Characteristics
Since the liquid is flung out by centrifugal force, this atomization method carries virtually no risk of clogging the atomizer. You can control the droplet size by adjusting the rotational speed. The droplets produced by centrifugal atomization are uniform in size.
Pressure Atomization
Pressure nozzles conduct pressure atomization. They are also called hydraulic nozzles. They consist primarily of an orifice disc and a swirl chamber.
Principle
After the feed liquid is pressurized (typically 1.5–30 MPa in industrial applications), it is ejected through a small orifice in the nozzle. What makes this process unique is that the liquid does not enter straight on but rather enters at an angle. The liquid rotates very fast and forms a hollow conical liquid film inside the nozzle. The liquid film instantly breaks into fine droplets as it is ejected from the orifice.
You can control the droplet size by adjusting the pressure and the orifice size. Higher pressure results in finer droplets, and a smaller orifice also produces finer droplets.
Characteristics
Pressure atomization is best suited for liquids with low viscosity. The nozzles may be clogged if the liquid has high viscosity. Pressure nozzles have a simple design and are easy to assemble and disassemble.
Pneumatic Atomization / Air Atomization
Pneumatic atomization primarily uses two-fluid or three-fluid nozzle atomizers. This atomization method uses the velocity difference between compressed air (or steam) and the feed liquid to atomize the material into fine droplets.
Principle
Compressed air is ejected at high speed from the air ring of the atomizer. When the feed liquid comes into contact with the high-speed air stream at the nozzle outlet, the high relative velocity generates significant friction, which breaks the feed liquid into fine droplets.
Characteristics
If you’re looking for a way to atomize high-viscosity liquids, then congratulations. Pneumatic atomization is very suitable for handling liquids with high viscosity. It can also produce finer droplets. However, this method consumes a significant amount of power and is better suited for small-scale equipment.
Drying
The atomized droplets enter the drying tower, where they come into full contact with high-temperature air, causing the moisture on their surfaces to evaporate rapidly. As the droplets travel through the drying chamber, moisture evaporates rapidly, and solid particles are formed. The entire drying process takes only a few seconds or a few dozen seconds.
Agglomeration
As the droplets gradually dry into particles, they collide with one another and coalesce into larger micro-nuclei. At the same time, tiny particles of dust floating in the surrounding air are adsorbed onto them, causing them to grow larger layer by layer, like a snowball.
These processes must be completed before the moisture in the particles has completely evaporated. Once the surface of the particles is completely dry, it becomes difficult for other fine powders to adhere to them.
FAQ
Why Are the Particles Produced by the Spray Drying Granulator Finer Than Expected?
There are many reasons. It could be due to inappropriate atomization parameters, such as excessive centrifugal atomization speed or excessive pressure in a pressure nozzle. It could also be due to a low solid content in the feed liquid, resulting in insufficient solids remaining after drying. If you want to produce coarse particles, we recommend maintaining the solid content between 30% and 50%.
Other possible causes include excessively high inlet air temperature, poor material viscosity (making agglomeration difficult), or excessive airflow velocity (which impedes small particles from adhering).
What Is the Difference between Spray Drying Granulation and Fluid Bed Granulation?
The raw materials used in spray drying granulation are liquids. In contrast, the raw materials used in fluidized bed granulation are solid particles or powder. In fluidized bed granulation, hot air blows the solid particles upward, suspending them in a fluidized state. This is what makes fluid bed granulation unique.
Can Feed Liquid Containing Sensitive Compounds Be Granulated Using Spray Drying?
Sure. For heat-sensitive substances, you can process them at a lower drying temperature. You can also choose excipients that help protect the active ingredients during spray drying granulation processing.
Why Is Atomization Uneven During Spray Drying Granulation?
Uneven atomization during spray drying granulation is typically caused by wear on the atomizing disc or nozzle, abnormal material viscosity, or unstable rotational speed. You can replace worn parts, adjust the feed liquid concentration to 30%–50%, and calibrate the rotational speed sensor to keep the error within ±2%.
Why Do Particles Become Hollow or Concave During the Spray Drying Granulation Process?
This is a typical drying defect. When water on the surface of a droplet evaporates rapidly, it forms a hard shell. The vapor pressure generated by the continued evaporation of water inside the shell pushes against the outer shell, causing it to bulge and form a hollow particle. If the shell is not strong enough, the particle will collapse into a concave shape once the vapor escapes.
If this happens, you can adjust the solids content (which should not be too low) or properly lower the drying temperature. You can also change to an appropriate type of binder.
Final Thoughts
If you plan to order spray dryers or spray granulators, just contact us. Pilotech is a professional spray dryer manufacturer. We have over twenty years of manufacturing experience. We have process validation laboratories that can provide you with spray drying and granulation process validation services for a variety of materials.