Unusual drying behaviour of droplets containing organic and inorganic solutes in superheated steam

Role of Steam as a Medium for Droplet Crystallization

“…Therefore, the relatively high droplet temperature (corresponding to the saturation temperature of the steam) during the constant rate period would have then been the only factor increasing the nucleation rate during the dehydration process leading to the formation of the hopper structure. In view that similar observations were also obtained in the SHS single droplet drying experiments with salt droplets 15 (single droplet drying time scale in the order of minutes, while the current drying time scale is in the order of seconds or fraction of a second), this high nucleation phenomenon in SHS is not rate or process time scale dependent. From the discussion above, this is because the key to such high nucleating conditions is the relatively high steam saturation (boiling) temperature which is an inherent physical characteristic of steam.…”

“…It is noteworthy that the formation of the bipyramidal salt crystals via spray drying has not been reported before in the literature, apart from our earlier report utilizing the SHS single droplet drying technique to convectively dry salt droplets to mimic SHS spray drying. 15 When the temperature of the SHS was reduced to 140 °C, a similar observation was obtained where a mixture of particle morphologies was produced similar to those in Figure 5 (not shown here for brevity−refer to the Supporting Information). The main difference when compared to the high temperature runs was that the cubic crystalline structure became less distinct and the hollow bipyramidal structures had less indentation (less hollow).…”

“…These will be discussed in detail later in the manuscript. It is noteworthy that the formation of the bipyramidal salt crystals via spray drying has not been reported before in the literature, apart from our earlier report utilizing the SHS single droplet drying technique to convectively dry salt droplets to mimic SHS spray drying …”

“…14 Recently, in further exploration of superheated steam (SHS) as a drying medium for convective droplet drying, unexpectedly, the use of SHS led to a unique salt crystallization behavior. 15 In that earlier report, SHS was found to be able to lead to relatively high nucleation rates during droplet solidification when compared to hot air drying at the same drying temperature. This was despite SHS exhibiting lower drying rates when compared to hot air drying (2.94 × 10 −5 vs 1.08 × 10 −4 kg m −1 s −1 , respectively).…”

“…Woo et al proposed the use of ethanol vapor or steam as the drying medium as the antisolvent vapor to precipitate water-soluble and insoluble solute from within droplets in spray drying . Recently, in further exploration of superheated steam (SHS) as a drying medium for convective droplet drying, unexpectedly, the use of SHS led to a unique salt crystallization behavior . In that earlier report, SHS was found to be able to lead to relatively high nucleation rates during droplet solidification when compared to hot air drying at the same drying temperature.…”