(315a) 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.