Spray drying processes can deliver particles with precisely controlled size and shape, making the processes extremely valuable for the manufacture of a wide range of products. Selecting, designing and optimizing equipment to meet application-specific needs is crucial for success.
Preci Co., Ltd., Tokyo — a process engineering company with cutting-edge spray drying technology that supplies customers in chemical, pharmaceutical and other industries, and also does toll manufacturing — uses particle size and shape data to optimize its spray drying processes. Preci relies on measurements from a Morphologi G3 automated imaging system from Malvern Instruments, Malvern, U.K. Case study data we’ll discuss show how imaging can facilitate tuning process variables to meet requirements and ensure exemplary quality control.
The Merits Of Spray Drying
In the technique, liquid is sprayed or atomized into a drying chamber in which air (or an inert gas) drives off the solvent, typically water. The resultant dry material is then classified to give a product of closely defined particle size distribution.
Spray drying processes:
• make highly spherical particles with excellent flow properties (fluidity);
• operate continuously; and
• deliver particles/granules with a sharp, stable and closely defined particle size distribution.
These benefits have led manufacturers in many industries to adopt spray drying to make a wide variety of products. Ensuring the desired properties of these materials requires choosing the appropriate atomization technology and optimizing a range of process parameters.
Preci offers various types of state-of-the-art atomization technology. Core designs include:
Rotary atomizer. A rotating disc applies shear to break up the liquid solution. Droplet size is controlled by the shape and design of the disc and, once that design is fixed, by varying the speed of rotation. This technology especially suits making particles in the 20–200-µm size range; compared with alternatives, it produces particles with higher sphericity and narrower particle size distributions.
Two-fluid nozzle. Here, the energy for atomization comes from the flow of compressed gas, which is used to drive two-phase flow through and from the nozzle. Manipulating compressed gas pressure (flow rate) controls droplet size. Two-fluid nozzles can produce very fine particles, from around 1 to 20 µm, and offer a relatively small footprint process. However, they require a ready supply of large quantities of compressed gas for operation.
Pressure nozzle. Forcing the liquid through one or more orifices under high pressure causes atomization. This simple design features low operating costs and provides significant scope, via orifice selection, to vary the size of granules produced.
Besides atomization technique, the specifics of the liquid solution/slurry determine process performance and the properties of the resulting granules. Key characteristics here include concentration, viscosity, specific gravity, additive content/inclusion (composition) and primary particle size distribution (the dispersion/aggregation state of particle in the feed solution). In addition, adjusting the inlet and outlet temperature of the dryer, the feed rate of the liquid and the airflow rate through the dryer can alter granule properties. Timely and informative particle characterization drives the comprehensive optimization of this web of interacting process parameters.
The shape of granules produced by spray drying plays a vital role in defining their value. These particles generally are more spherical in shape — and thus flow more easily — than those of equivalent size produced by alternative means. As a result, the optimization of spray drying processes calls for analytical technology that offers both size and shape measurement.