Rotary atomization of non-Newtonian liquids for Spray drying applications

Abstract

Food wastage poses a significant threat to the nation’s food supply and safety due to inadequate storage infrastructure and logistics facilities. This study explores the potential of spray drying as a viable solution to address the food wastage in India’s agricultural sector. By converting the fruit and vegetables, in their juice form, into dried powders, spray drying is proposed as a method to preserve the nutritional value of food products and extend their shelf life. The present study investigates the atomization and evaporation characteristics of surrogate solutions that mimic the rheological and chemical properties of fruit and vegetable juices, with the goal of optimizing the design of a compact, portable spray-dryer prototype. The atomization study utilizes a rotary atomizer to determine the spray characteristics of surrogate solutions. The results indicate that rotary atomization is effective in producing small size droplets with wider droplet size distributions oriented more towards smaller diameters, essential for efficient spray drying. For the spray characterization study, xanthan gum (XG) based surrogate solutions demonstrate negligible atomization across all polymer concentrations at rotational speeds below 15,000 revolutions per minute (RPM). This behavior is attributed to the viscoelastic nature of these solutions, which inhibits droplet formation at lower inertial forces. However, when the rotational speed exceeds 15,000 RPM, the inertial forces counteract the viscoelastic effects, leading to droplet formation. Furthermore, as the rotational speed increases beyond 25,000 RPM, the negligible reduction in the Sauter mean diameter (SMD) values indicates that inertial forces completely dominate the viscoelastic forces, achieving the best possible atomization under the chosen conditions. The study also examines the effect of liquid temperature on the spray characteristics of XG solutions. The temperature sweep tests reveal that the viscosity of XG solutions decreases with increasing temperature, enhancing the atomization process. The findings highlight the importance of controlling the liquid temperature to optimize the spray drying process. Upon increasing the liquid temperature from 25 ^0C to 75 ^0C, the viscosity of water as a solvent decrease significantly, and similar trends are observed for XG solutions. The reduction in the shear viscosity at higher temperatures results in smaller SMD values for 0.1 wt. % XG solutions, indicating more efficient atomization. However, for highly concentrated 1.0 wt. % XG solutions, the SMD increases due to solvent evaporation from the liquid structures, which delays the breakup into smaller droplets. This suggests that heating the liquid solutions to a high temperature enhances atomization for lower polymer concentration solutions but is not recommended for higher-concentration solutions. The single droplet evaporation study further investigates the drying kinetics of the surrogate solutions. The results indicate that the evaporation rate is significantly influenced by the initial droplet size and the airflow temperature. Smaller droplets exhibit faster evaporation rates due to their larger surface area-to-volume ratio and higher airflow temperatures accelerate the evaporation process. These insights are crucial for optimizing the spray drying process for agricultural food products. For the evaporation study, it is crucial to identify the constituents in fruit and vegetable juices that significantly influence their evaporation timescales to achieve dry powder. Water constitutes approximately 90% of these juices, with the remaining portion comprising sugars, fibers, and proteins. Fructose, glucose, and sucrose are the primary sugars in food products. Due to their hygroscopic nature and tendency to retain moisture, these sugars largely determine the residence time required for droplets to dry completely in a spray dryer. Consequently, a surrogate solution consisting of 16 wt.% fructose, 11.2 wt.% glucose, and 4.8 wt.% sucrose in water (FGS) has been identified. The evaporation characteristics of these surrogate solution droplets are investigated using a single pendant droplet evaporation setup. The results show deviations from the classical D2- law due to solute particles in the solution. This behavior is modeled using a Two-stage single droplet evaporation model. This model is used to predict the evaporation timescales corresponding to droplet sizes obtained from the atomization experiments, providing the droplet residence times required for complete drying. The findings from this study have significant implications for the agricultural sector in India. By adopting spray drying techniques, farmers can reduce post-harvest losses and improve the shelf life of their produce. This not only enhances food security but also provides economic benefits by reducing wastage and increasing the marketability of agricultural products. The development of portable, compact spray dryers tailored for agricultural use can facilitate the adoption of this technology in rural areas. Furthermore, the use of surrogate solutions to study the atomization and evaporation characteristics of fruit and vegetable juices provides a cost-effective and efficient method for optimizing the spray drying process. Future research should focus on refining the design of spray dryers, exploring the use of alternative surrogate solutions, and investigating the long-term storage stability of spray-dried products. Additionally, the development of advanced atomization techniques and the integration of real-time monitoring systems could enhance the efficiency and effectiveness of the spray drying process. By addressing these challenges, spray drying can become a pivotal technology in reducing food wastage and ensuring food security in the country.