Modelling of a Spray Drying Process

Spray drying is a process in which 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.

Several models are available in the literature with varying levels of fidelity. The levels of fidelity differ both with respect to the modelling of the drying process as well with respect to the flow of the droplets and hot gas.

In a recent Ph.D. thesis, Jakob Sloth developed a model of enzyme spray drying. The model considers the drying of a single particle containing a dissolved enzyme together with additives and predicts the change in droplet size, temperature, moisture content and enzyme inactivation as drying progresses.

In this paper, the work of Sloth is (i) implemented in gSOLIDS, a commercial flowsheeting environment for solids processes and (ii) extended to describe spray drying of droplet size distributions as they flow through an industrial scale unit.

The extended model captures the following phenomena:

1.     the evolution of the droplet size distribution as constant-rate drying takes place;

2.     the evolution of the distribution of temperature with respect to droplet size; and

3.     the effect of backmixing of droplets and gas.

This model is then used to construct a flowsheet that also contains models of downstream fluid bed drying and classification processes to determine the effects of key operating parameters on the overall performance of the process in steady state as well as transient periods.

References

J. Sloth (2007) Formation of enzyme containing particles by spray drying. Ph.D. thesis, Technical University of Denmark.