The present study is a novel approach that uses the analogy of regimes defined in heterogeneous reactions (gas−liquid and gas−solid). A different perspective of drying can be achieved by defining regimes as they provide a different outlook and by considering the uniqueness of the type of dryer used or the d rying air patterns used therein. By defining regimes, we are capturing the internal characteristics of the material as the mathematical expressions incorporating the necessary modifications under the prevailing drying conditions. This is one of the key characteristics of the proposed "regime-based model" that is different from the remaining models that are reported in the literature. Starting with the formulation and modeling and applications of the regime-based approach, it goes on to detail the use of the heterogeneous reaction concepts to describe external mass transfer and internal diffusion during the drying process. In the present study, three primary regimes are defined in such a way, Regime 1 states that the ratecontrolling step is due to internal diffusion, whereas Regime 2 is controlled by external mass transfer and in Regime 3, internal diffusion and external mass transfer rate are comparable and each plays a prominent role in the overall drying kinetics. The Model equation consists of terms that are related to external drying conditions and intrinsic material property of a sample material (Apple) and these terms are evaluated using the drying model. A simplified approach is used that defines a parameter r' which compares actual drying rate with external mass transfer rate and it is used to distinguish between drying regimes 1 or 3 and regime 2. The activation energy required for regime 3 is seen to be much larger as compared to regime 2 and regime 1 which indicates that to overcome the controlling resistances of regime 3 i.e., internal diffusion resistance and external mass transfer resistance requires much higher activation energy. The present work allows us to shift the regimes by varying the operational parameters such as drying air velocity, relative humidity, and temperature.
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