Spray drying is a drying method that was firstly described more than 140 years ago as an improvement in drying and concentrating liquids1. But it was not until the beginning of the 20th century that the level of sophistication and knowledge of the process allowed its industrial use. The production of milk powder was the first commercial application and still remains one of the most important uses of the technology.
What is Spray Drying?
Spray drying is a method for producing a dry powder directly from a solution or slurry. In a continuous process, the liquid feedstock is dispersed as a mist of fine droplets into a stream of hot drying gas, all within a cylindrical drying chamber. In aqueous processes, the drying gas is typically air. For solutions composed of flammable solvents such as ethanol and acetone, the inert drying gas is nitrogen and the system operated in a closed cycle as illustrated below. In addition, an inert closed cycle is also useful for drying chemical and biologic materials that are sensitive to oxygen. Because the kinetics of drying are so rapid, materials are dried in a fraction of a second which helps to make the process viable for thermal sensitive materials such as foods, extracts and biologics.
In terms of particle size control and consistency, spray drying is the most dependable process for producing dry powders from any type of liquid solution or slurry feed.
In a basic spray dryer, the feed is dispersed into a mist of fine droplets either by means of a rotary disc atomizer or a spray nozzle. This spray mist is distributed into a cylindrical drying chamber along with a stream of hot gas and typically blown dry within a split second.
After remaining in the chamber for a certain residence time – for further reduction of the residual moisture, the generated dry powders are separated from the gas flow either directly at the base point of the chamber or via downstream aero cyclones and bag filters.
The Principles of Spray Drying
The basic principle of spray drying hinges on water removal, but how you remove that water can make a significant difference on the product’s overall quality.
Many factors must be considered when selecting the best method to produce your dry powder. A primary starting point would be to determine product moisture- and particle-size- requirements, as this will impact the atomizer choice.
Another factor to consider is the type of dryer you are using. If the dryer is a tall form dryer, then you have more surface area for drying, which will impact how product specs are achieved.
Upstream processing is as crucial to the spray drying process as the actual spray drying. Order of addition of raw materials will impact the viability of the emulsion or solution to be spray dried. Slurry temperature can also impact the viscosity of the material to be pumped. Too fast or two slow a pump rate can also impact how well the product dries in the spray dryer. If the emulsion is homogenized, the size of the micelle can also impact spray drying, along with shelf life in the future.
These are just some of the basic variables to look at when evaluating spray drying as a means to produce your powder.
Applications of spray drying
Today, spray drying is widely used in all sorts of industries. It has especially found applications in food or pharma. Indeed, products like instant milk or instant coffee are produced thanks to this process. The possibility to avoid degradation during drying makes it also a process of choice for pharma.
Examples of applications for spray drying are given below :
– Instant milk
– Whey powder
– Instant coffee
– Non dairy creamer
– Detergents, soaps. surface active agents
– Antibiotics, vaccine, vitamins, yeast
The list is only partial but already very long. Many industries use spray drying because it offers a continuous drying technique, with a very short residence time in temperature, thus allowing, if the spray drying system is well tuned, to dry heat sensitive components.