Scale-up of continuous microcapsule production

Abstract

The objective of this work is to establish an experimental approach to scale up the production of microcapsules in an inline rotor stator mixer (RSM), ensuring product size distributions as developed in batch. The emulsification step for a melamine formaldehyde microencapsulation process is therefore studied in a lab scale batch and an inline pilot scale RSM. The inline RSM is operated in CSTR mode, seldom reported in literature, allowing any feed flow rate to be implemented ranging from lab scale to (pilot) production scale (4-330 ml/min). Parameters including the Weber number, tip speed and energy density are investigated to correlate the mean capsule size of the batch and flow process. As a model system, silicone oil is dispersed in melamine formaldehyde prepolymer by the RSM at a high oil-to-water ratio of 46%. Rotational speeds range from 3000-26000 rpm. Volume based capsule size distributions presented no common correlating parameter, however the dimensionless maximum diameter correlated well with the Weber number to the power -0.4, for both devices operated at the same residence time. The difference in mean diameter and covariance between batch and flow at similar weber numbers is attributed to differences in droplet break-up mechanism due to different rotor stator geometries. Volume and number based mean diameters showed little influence of the feed flow rate of the CSTR operated inline RSM, making it an ideal tool to experiment on lab scale and increase production by increasing the feed flow rate, while maintaining rotor-stator geometry.