How is spray drying being used in the pharma industry?
Spray drying has played a significant role in the pharmaceutical industry for many years in the drying of excipients, APIs
and final drug formulations. To a large extent, interest in spray drying has been fuelled by the fact that it is in an 'enabling
technology'; for example, using spray drying to produce amorphous solid dispersions can improve the performance of poorly
soluble APIs. Also, processes that are based on organic solvents are ideally suited for spray drying and produce results that
generally surpass those of alternative technologies.
Interest has also spread to other areas where spray drying may be more economical or where the particle engineering possibilities
are better than the traditional methods of production. Examples could be to obtain controlled release or to produce very fine
powders for inhalation.
When designed for aseptic production, spray drying can be used to produce aseptic products, such as vaccines, antibiotics
and other parenterals.
Spray drying is a simple process where droplets/particles are dried while suspended in a drying gas. This turns a liquid formulation
into a dry powder in a single, continuous process. The basic spray drying process has three essential steps:
- Atomisation, where the droplets are formed.
- Drying gas and droplet contact, where the liquid is evaporated.
- Powder recovery, where the dried particles are separated from the drying gas stream.
The spray dryer is a suspended particle dryer. Drying particles while suspended requires drying to be very fast and droplet
trajectories must also be kept away from the drying chamber wall for as long as possible
Fast drying is achieved by effective and uniform atomisation of the liquid, which creates a very large surface area and
by ensuring effective mixing of the droplets with the drying gas
Because of the relatively short residence time (a function of the size of the drying chamber and the flow pattern of the drying
gas), the process requires small droplets in order to work properly. The smaller the drying chamber, the smaller the maximum
particle size that can be successfully dried. Hence, the spray drying process is most limited at the small scale, where as
scale-up provides 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 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 production size.
Spray drying also offers a range of particle engineering possibilities. By altering the process parameters (and/or spray dryer
configuration), spray drying can produce complex particulates that meet exact powder properties in terms of particle size
and shape, bulk density, dispersability, polymorphism, flow properties and so on. A good example is the manufacture of a polymer-stabilised,
amorphous 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 formulation that contains the necessary components of the final tablet in solution or suspension. The fast drying
at low temperatures (below the glass transition temperature) 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.
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 the technology worth considering even when drying is not otherwise
required. Spray dryers 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 and for agglomeration with integrated fluid
beds and more.