Process analytical technology: application to particle sizing in spray drying

Abstract

The purpose of this research was to explore the possibility of employing PAT for particle sizing during spray drying with the use of an in-line and at-line laser diffraction system. Microspheres were made using maltodextrin and modified starch as wall material and size results obtained using PAT compared with those determined with off-line laser diffraction and light microscopy. Median particle size results were highest for in-line laser diffraction, followed by at-line and off-line laser diffraction and finally light microscopy. This was due to the presence of agglomerates which were measured as discrete microspheres in the in-line set-up. At-line and off-line laser diffraction gave results more closely correlated with individual microsphere sizes due to agglomerate breakdown during the measurement process. Light microscopy allowed direct observation of the particle morphology, however, its use for particle sizing was tedious and sample size was much smaller compared to laser diffraction. Although PAT was found to be an efficient and convenient tool, careful data interpretation was needed taking into account the cohesiveness of the material measured. The at-line set-up appeared to be more suitable in this particular application.

Fig. 1

Layout of the spray dryer with the in-line and at-line laser diffraction set-up

Fig. 2

Representative particle size distribution curves for microspheres obtained from in-line laser diffraction (open symbols, microspheres produced using modified starch as wall material; filled symbols, microspheres produced using maltodextrin as wall material)

Fig. 3

Particle size history during process start-up obtained from in-line laser diffraction

Fig. 4

Effect of eductor air flow on Dv(50) and Span for at-line sizing (open symbols, microspheres produced using modified starch as wall material; filled symbols, microspheres produced using maltodextrin as wall material)

Fig. 5

Representative particle size distribution curves for microspheres obtained from at-line laser diffraction (open symbols, microspheres produced using modified starch as wall material; filled symbols, microspheres produced using maltodextrin as wall material)

Fig. 6

Size distribution curves obtained by off-line laser diffraction for maltodextrin microspheres produced at filled circle, low and filled square, high wheel speed