The microstructure (internal distribution of components and porosity) and morphology (external particle shape) of multi-component granules are key product attributes as they determine, among other things, the bulk powder properties such as packing density, vapour permeability, or effective thermal conductivity (Kohout et al., 2004), as well as end-use behaviour such as the effective rate of dissolution (Stepanek, 2004). The effect of processing parameters – namely temperature – on the morphology and to a lesser extent also microstructure of spray-dried particles has been experimentally demonstrated by the work of Walton and Mumford (1999). Qualitative ‘maps' relating process parameters to the particle shape that is most likely to form, have been constructed. However, quantitative predictive models of granule microstructure evolution during spray drying, that would also allow the quantification of the effect of material properties on microstructure and thus facilitate formulation development, are not yet available.

We have recently developed a computer simulation method allowing the prediction of particle microstructure in the case of wet granulation (Stepanek, 2004; Stepanek and Ansari, 2005). The agglomeration of primary solid particles of defined size and shape distribution, and the spreading and solidification of binder droplets on the growing agglomerate are explicitly simulated. A population of three-dimensional ‘virtual granules' that bear full information about the microstructure and morphology, is obtained as output from the simulations. Functional dependence of granule porosity on dimensionless groups comprising physical parameters of the binder (viscosity, contact angle on primary particles) and process parameters (solidification rate, collision frequency) has been constructed by computer simulations.

In this contribution we will present a continuation of the work, which now allows us to explicitly simulate particle microstructure evolution also during spray drying of slurry droplets. Starting from a droplet with randomly dispersed primary solid particles, whose volume fraction is defined by the slurry density and solubility of the solids, the evaporation of solvent from free liquid interfaces with simultaneous liquid phase rearrangement within the evolving structure and growth of solid phase as function of local super-saturation is simulated using the Volume-of-Fluid (VoF) method. The result is a population of particles whose microstructure (porosity, distribution of solid phases for multi-component mixtures) is the result of the initial conditions (slurry density), the rate of solvent evaporation (controlled by temperature and relative gas-particle velocity) and the material properties (solubility and density of the solid phase). We will present correlations between microstructure parameters (porosity, correlation length for each phase), slurry composition, and solidification rate, which can then be used for formulation design and process parameter optimisation.

REFERENCES

Stepanek, F., (2004), Computer-aided product design: granule dissolution, Chemical Engineering Research and Design, 82, A11, pp. 1458-1466.

Stepanek, F. and Ansari, M.A., (2005), Computer simulation of granule microstructure formation, Chemical Engineering Science, 60, pp. 4019-4029.

Walton, D.E., and Mumford, C.J., (1999), The morphology of spray-dried particles: the effect of process variables upon the morphology of spray-dried particles, Transactions of IChemE Part A, 77, pp. 442-459.