Unusual drying behaviour of droplets containing organic and inorganic solutes in superheated steam
Introduction
Hot air is the typical standard medium used in most drying related processes. For specific applications and product requirements, the usage of different drying mediums were also reported (Mujumdar et al., 2010). Some of the reported studies include the use of humid air to control the evaporation rate in the dehydration process (Ebrahimi and Langrish, 2015; Islam et al., 2010). There were also reports focusing on the use of carbon dioxide and ethanol vapour to enhance product properties and induce crystallization and precipitation of particles (Brown et al., 2008; Mansouri et al., 2012; Mansouri et al., 2013; Tan et al., 2014; Tan et al., 2015). The use of nitrogen as a dehydration medium is also widely adopted to prevent oxidation and as a safety blanket during the dehydration of volatiles (Mujumdar, 2006). The premise of this report is on the application of superheated steam on the dehydration process, specifically on liquid droplets.
The use of superheated steam (SHS) in drying processes has been gaining much research attention in the past decades. To date, the application of SHS in various drying processes has covered a diverse range of products, predominantly in food processing, timber processing as well as in the waste treatment industry (Pang and Pearson, 2004; Shiravi et al., 2007; Speckhahn et al., 2010; Van Deventer and Heijmans, 2001; Yamsaengsung and Sattho, 2008; Zielinska et al., 2015; Devahastin et al., 2004). Studies have shown that the substitution of hot air with SHS have resulted in the enhancement of either product quality or process efficiency and thus is able to counteract some of the shortcomings in conventional hot air-drying process. A study on SHS drying of parawood produced a significant decrease in drying time, from 8 to 16 days to less than 35 h and less energy usage while maintaining the same dried wood quality (Bovornsethanan and Wongwises, 2007). Another study on beef drying under both hot air and SHS condition showed that SHS drying resulted in a shorten drying time as well as the reduction of lipid oxidation reactions which has minimized undesirable quality changes (Speckhahn et al., 2010). Investigations on the use of SHS in the drying of waste sludge revealed several advantages such as simultaneous disinfection during drying which extends the storage life of biomass, eliminate odour as well as extensive energy recovery as high as 80% (Hoadley et al., 2015; Jarrett et al, 1996). A drying study on reconstituted whole milk also showed minimisation of lipid oxidations and greater nutritional retention in milk powders as a result of SHS drying (Ghazwan Mahdy, 2014). Apart from the potential to improve product quality, investigations on the energy efficiency of a SHS pilot study also revealed the possible recovering nearly 85% of the input energy (Robert, 1985). SHS drying models and trials have also shown positive energy savings with one case study involving SHS impinging drying of paddy rice study, reporting approximately 25–30% energy reduction as compared to air drying (Li et al., 2016; Romdhana et al., 2015).
While most of these reports mainly focused on the drying of solids or suspensions, there are currently minimal reports on investigating the fundamental use of SHS for liquid drying such as those in spray drying. The potential application of SHS in spray drying was first discovered by Gauvin. W.H (Gauvin, 1981, 1983). Subsequent reports in the literature on this area, which were rare, focused mainly on measuring or understanding quantitatively the heat and mass transfer of droplets dried under superheated steam conditions. These studies ranged from SHS single droplet studies (Trommelen and Crosby, 1970) to large scale SHS computational fluid dynamics studies (Frydman et al., 1998, 1999); Ducept et al., 2002. Although these reports examined the mass and temperature changes droplets with dissolved solute, the reports neglected detailed investigation into the entire solidification process of droplets. The direct effect of SHS on the solid formation from a solute containing droplet and the final particle morphology have yet to be fundamentally examined. Therefore, this presents a gap in the literature in which this report aimed to fill.
In a previous publication, using the single droplet drying technique, Lum et al. (2018) investigated in detail how single milk droplet is solidified under SHS conditions. It was found that SHS produced single particles which were more hydrophilic and displayed improved wettability characteristics when compared to hot air drying. Milk is a complex system constituting carbohydrates, fats, proteins and minerals. In this work, we extended the same experimental technique to investigate in a more fundamental approach on individual classes of materials, namely droplets containing: slow to crystallize organic solutes (lactose), fast to crystallize organics solutes (mannitol), inorganic solutes (salt) and dissolved protein (whey isolate). The aim here was so that a more general conclusion can be made on how SHS affects the different types of materials typically found in food and pharmaceutical spray dried products.
Section snippets
Sample preparation drying
Lactose, mannitol, protein and salt solutions with 10 wt% were prepared with a minimal stirring of 4 h for lactose (α lactose monohydrate, Sigma Aldrich) and 15 min for mannitol (D-Mannitol, Sigma Aldrich). Whey protein isolate (WPI) was used as the model protein material (100% Whey Protein Isolate, Bulk Nutrients Pure Supplements) and table salt was used (Natural Rock Salt 99.8% NaCl, SAXA).
Single droplet drying experiments
A detailed schematic of the experimental setup is given in the previous report (Lum et al., 2018). In
Organic material: lactose and mannitol
Comparison of lactose droplets experiments with hot air and SHS at the same temperature of 110 °C revealed a delay in shrinkage or size reduction when the lactose droplets were dried under SHS with an average percentage volume shrinkage per second of 1.45 ± 0.64% and 3.78 ± 0.71% for SHS and air, respectively. This can be further observed in Fig. 1 by comparing the size of the droplet throughout the shrinking phased of the droplet drying. Similar reduced SHS evaporation rates, particularly for
Conclusion
The effect of SHS on the drying and particle formation behaviour of organic and inorganic materials were studied. Within the range of temperatures evaluated in this study, SHS was found to have significantly lower moisture removal potential for the latter stages of evaporation when compared to hot air. Organic materials with hygroscopic behaviour such as lactose and mannitol was shown to have a greater propensity to withheld moisture compared to WPI and NaCl, hence resulting in a final sticky
Acknowledgments
The authors would like to thank Monash University and the Monash Institute of Graduate Research for the research scholarship.
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