Numerical analysis of in-flight freezing droplets: Application to novel particle engineering technology

• Published in: Food and bioproducts processing Vol. 116; pp. 30 - 40
• Main Authors: Tait, Andrew, Lee, Jonathan G.M., Williams, Bruce R., Montague, Gary A.
• Format: Journal Article
• Language: English
• Published: Rugby Elsevier B.V 01.07.2019; Elsevier Science Ltd
• Subjects: Ambient temperature; Coalescence; Coalescing; Computer simulation; Continuous spray freeze drying; Cooling; Design; Distance; Droplets; Falling; Flow rates; Flow velocity; Freeze drying; Freeze time; Freezing; Freezing transitions; Lyophilisation; Mathematical models; Numerical analysis; Numerical modelling; Numerical models; Optimization; Rivers; Spray freezing; Supercooling; Temperature; Temperature effects; Terminal velocity; Velocity; cSFD; Freeze drying; Spray freezing; Numerical modelling; Freezing; ICH; NDF; NMR; Freezing transitions; Lyophilisation; Continuous spray freeze drying; NDS; VSVO
• ISSN: 0960-3085; 1744-3571
• DOI: 10.1016/j.fbp.2019.04.009

[Display omitted] •The rate of supercooling was predicted in the range from 48 to 830 °C s−1.•Droplet coalescence was predicted at 0.05 Lh−1 for 100 μm diameter droplets.•A separation distance of 23 μm preventing coalescence was maintained at 0.02 Lh−1. The freezing of a stream of free-falling monodispersed droplets was simulated through the development of numerical models in this work. Prediction of the freezing time and temperature transition of a single droplet is beneficial for optimisation of novel continuous spray freeze drying (cSFD) processes. Estimations of the vertical free-falling distance of the droplets in a slip stream and predictions of the chances of droplet coalescence greatly enhance process understanding and can be leveraged to direct equipment design and process development. A design space of droplet diameters in the range from 100 μm to 400 μm and ambient temperatures from −120 °C to −40 °C was explored. The rate of supercooling within the design space was predicted to range from 48 to 830 °C s−1 depending on the ambient temperature and droplet size. A comparison of the vertical free-falling distances of solitary droplets and streams of droplets at different temperatures showed that the terminal velocity of a vertically falling stream of droplets is always in excess of the terminal velocity of a solitary droplet of the same size. A difference of 1.35 m was predicted for the free-falling distance of a 400 μm droplet compared to a stream of droplets at −42 °C. A comparison between flow rates for consecutively generated 100 μm droplets showed that droplet coalescence was predicted at 0.05 Lh−1, whilst at 0.02 Lh−1 a separation distance of 23 μm was maintained thus preventing coalescence.