Investigation of factors affecting isolation of needle-shaped particles in a vacuum-agitated filter drier through non-invasive measurements by Raman spectrometry

The effects of pressure filtration and vacuum agitated drying on cellobiose octaacetate (COA) particles in methanol slurries were studied by making Raman measurements through the glass wall at the side of a filter drier beneath the oil jacket. The change in intensity of methanol peaks in the spectra allowed the removal of the solvent from the particle bed to be monitored. Also, drying curves for COA generated from the Raman measurements gave an indication of the changing physical status of the particle bed during continuous or intermittent agitation. The intensity of the Raman signal for COA depended on the bulk density of the particle bed, which changed due to aggregation and attrition that occurred during solvent removal and particle motion induced by agitation during vacuum drying. Loss on drying (LOD) measurements of samples removed at the end of the pressure filtration and vacuum agitated drying stages established the degree of wetness and confirmed the end point of drying (<0.5% w/w solvent), respectively. Dynamic image analysis confirmed that minimum attrition of COA was achieved when (a) the majority of the methanol was removed during pressure filtration at 0.5bar N₂ and (b) intermittent agitation was applied during the vacuum drying stage.