In its basic form, spray drying is a very simple process where droplets/particles are dried while suspended in the drying gas, turning a liquid feed into a dry powder in a single continuous process step. The basic spray drying process has three essential steps: atomisation, where the droplets are formed; drying gas and droplet contact, where the liquid feed is turned into droplets; and finally powder recovery, where the dried particles are separated from the drying gas stream.
The source of the excellent performance of spray dryers is also the source of their main limitation. A spray dryer is a suspended particle dryer, where the droplets/particles need to be dried before they hit the interior wall of the drying chamber to avoid deposits of wet and sticky particles. Drying the particles while suspended requires that drying is very fast and that droplet trajectories are kept away from the drying chamber wall for as long as possible.Fast drying is achieved through two means: effective and uniform atomisation of the feed, thereby creating a very large feed surface area; and by ensuring effective mixing of the droplets with the drying gas. The available drying time is a result of the size of the drying chamber and the flow pattern. Additionally, the process requires small droplets to work. The smaller the dryer, the smaller the maximum droplets that can be dried, and drying times are short (at least for spray drying in its basic form), which sets a limit to how low a residual solvent level can be achieved in the final product. Consequently, the spray drying process is most limited at the small-scale. Scale up, however, offers more opportunities and improved process robustness.
The fast drying, the short residence time and predictable temperature exposure — caused by the effective mixing of the droplets with the drying gas — results in a lenient drying process that can be adjusted for a broad range of process conditions, including the drying of heat sensitive materials such as live vaccines and complex proteins. Just as importantly, spray drying is a highly reproducible process that can be predictably scaled up to nearly any scale of production — spray drying processes can be designed for batches of a few grams of solids, all the way up to very large instruments that can continuously produce more than 10 metric tons of powder per hour.
Apart from drying, spray drying offers a range of particle engineering possibilities. By altering the process parameters (and/or spray dryer configuration), spray drying can produce complex powders that meet exact powder properties in terms of particle size and shape, bulk density, dispersability, polymorphism, flow properties and so on. An example could be the manufacture of a polymer-stabilised, amorphous and solid dispersion of an API for direct compression into tablets, without an intermediate granulation or mixing stage. In this case, the process conditions are chosen to produce a homogenous, free flowing and non-dusty powder of good density from a liquid feed containing the necessary components of the final tablet in solution and/or suspension. The fast drying at low temperatures (below the glass transition temperature of the solids) will intentionally favour the amorphous form of the API/polymer mixture. In other cases, the requirement may be completely different; for example, powders for inhalation must have a small aerodynamic size, resulting from the small geometric size and low density.
Modern spray can be built for operation with a wide range of (flammable) solvents as well as water, for congealing of melts, for contained processing of potent compounds, for aseptic processing, for agglomeration with integrated fluid beds and more.
In short, spray drying is a very versatile lenient drying process that turns a liquid into a powder in a single process step at any scale with particle engineering possibilities that makes spray drying worth considering even when drying is not otherwise required.