(661a) Developing Improved Understanding of Spray Drying Through Process Modelling

Spray drying is often used in the pharmaceutical industry to
rapidly produce particles with narrow size distributions and with low moisture
contents.  In the spray drying process, a suspension or solution of a desirable
product in a volatile solvent is converted to a largely dry solid product by
contact with a drying medium. The process starts with an atomizer which creates
droplets from the feed suspension or solution. These droplets are then mixed
with a hot gas. Evaporation of the volatile solvent in the droplets takes place
and a dry solid product is thus obtained.

Significant work has been done in the past to look at how
individual droplets dry within a spray dryer. These models include detailed
mass and energy balances and predict the droplet temperature and radius profile
as drying occurs. In practice however, the atomisation process produces
droplets of a range of sizes. As one of the factors influencing the rate of
drying of a droplet is its initial droplet size, a model of a spray dryer
should consider the distribution of droplet sizes generated in the atomiser in
order to obtain a better prediction of product temperature and moisture
content.

A model has been developed that considers this distribution
of droplet sizes within the spray dryer. The model outputs include temperature
and droplet size histories of droplets of various sizes as well as the
evolution of the droplet size distribution within the spray dryer. The model
has been used in a pharmaceutical application to look at how the mathematical
modelling can aid in process development. The results obtained from this case
study are promising and indicate that process modelling has the potential to
significantly reduce process development times and costs.