Increasing attention has been given to spray drying because of its popularity in the manufacturing of various powders with prerequisite quality. A large number of theoretical and experimental studies have been published during the last five decades to study spray‐drying operations. Despite this progress, there is still lack of information on the selection of an accurate and simple drying kinetics model to characterize the droplet drying process. This article has particularly focused on the comparison of two simple and effective drying kinetics models: the characteristic drying‐rate curve and reaction‐engineering approach (REA) models. This article reported mathematical models that can be used to design the spray‐drying operation and to predict the particle's characteristics for realistic dryer‐wide situations. Both the physical and biochemical quality attributes were investigated regarding their sensitivity to the model parameters used in the two drying kinetics models. The REA model was found to deliver more realistic predictions during the modeling of the droplet drying process for all the sets of conditions tested.

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in Wiley InterScience (www.interscience.wiley.com).An assumption of uniform temperature is frequently used when evaluating average temperature-time and average moisture content-time profiles for the convective drying of small droplets or thin-layer materials. In most studies, the assumption of uniform temperature was justified by estimating the heat-transfer Biot number at the beginning and end of the drying process. However, the conventional Biot number analysis performed in the literature does not reflect the evaporative effect. In this article, we have examined the temperature uniformity and the heat-transfer Biot number during the drying of skim milk droplets under laboratory drying conditions following the entire drying process. Surface-centre temperature differences and conventional and drying-based Biot numbers are calculated during the drying of skim milk droplets. A simple procedure is outlined to estimate the extent of temperature nonuniformity within the droplet. Results demonstrated that temperature nonuniformity within the skim milk droplets under drying conditions examined is very small, thus the uniform temperature assumption is more likely to be a reliable approach to model heat and mass transfer processes in industrial spray dryers. The analyses provided in this study help in understanding a few assumptions used in literature, and offer a framework that may be used in the future. 2008 American Institute of Chemical Engineers AIChE J, 54: 3273-3290, 2008 Keywords: biot number, spray drying, droplet drying, modeling, temperature distribution IntroductionSpray drying is a widely used industrial operation that involves evaporation of moisture from droplets until the desired particle moisture content is achieved. A mathematical analysis to characterize the drying of individual droplets is important because it provides a more intelligent and robust control over the product quality and the plant operation. A simple approach to achieve the expected product quality during spray drying can be formulated by recognizing what the droplet/particle experiences during its transit in the drying chamber.1 The physical and biochemical properties of the final dried product are directly influenced by the droplet's temperature and moisture content history within the dryer. Hence, predicting the droplet's temperature-time and moisture concentration-time profiles following the entire drying operation is crucial for manufacturing the best-quality product and optimizing the operation. During spray drying, the evaporation from droplets is facilitated by mixing hot drying gas with a spray of small droplets. Droplets receive the heat from drying gas, and subsequently moisture is transferred from the droplets to the bulk gas in the form of vapor. The driving force for these heat and mass transfer processes may be the liquid moisture concentration difference, the vapor pressure difference, or the temperature difference within and outside the droplet. The transport of heat and moisture may lead to nonuniformity of temperatu...

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