Reaction Engineering Approach in Drying

There are different ways to model drying processes, which is a necessary part for the development of innovative and energy efficient drying techniques. There are three different common approaches used for modeling the drying process. The concept of characteristic drying rate curve, use of empirical models: which are system specific and cannot be generalized as these does not involve any physical basis, the most famous model under this category is the Page model.

The third approach is mechanistic models: which are based on the drying phenomenon as well as the physics involved and can be generalized, mainly involves coupled heat and mass diffusion equations. However, under mechanistic models, there are different mechanisms proposed and generally these models involve high mathematical complexity and determination of too many parameters. It is necessary to have simple, accurate and robust mathematical model with minimum mathematical complexity to reduce the computational time.

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Chen, 1998 has proposed Reaction Engineering Approach to model the drying processes. Some of the constraints with characteristic drying rate curve approach are: one has to determine critical moisture content of the material which is a function of drying conditions, the experimental drying data is relatively scattered and the flux which is calculated using following equation is based on the wet bulb temperature (TWB) for gas phase and not based on the actual surface temperature (TS).

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Drying formula

𝑁 = 𝑘𝑚 �𝜌𝑣,𝑠𝑎𝑡(𝑇𝑆) − 𝜌𝑣,∞� ≈ 𝑘𝑚 �𝜌𝑣,𝑠𝑎𝑡(𝑇𝑤𝑏) − 𝜌𝑣,∞� (1.13) Where, km is the mass transfer coefficient However, according to REA model, which was intended particularly for small case of Biot number (< 0.1), the temperature of the sample air interface TS is considered equal to the product temperature TP. Where biot number is defined as 𝐵𝑖 = ℎ 𝐿𝐶 𝑘𝑠 (1.14) According to REA the drying rate can be written as 𝑚𝑆 𝑑𝑋� 𝑑𝑡 = 𝑘𝑚 𝐴 �𝜌𝑣,(𝑇𝑆) − 𝜌𝑣,∞� (1.15) ms is the dry mass of the solids and ρv,s is the vapor concentration at the solid-gas interface.

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Conclusion

This is a unknown parameter and changes as the drying proceeds, but which is al- Jangam, Mujumdar – Basic Concepts and Definition 24 Drying of Foods, Vegetables and Fruits ways less than ρv,∞. The surface area for mass transfer can also change as the drying proceeds.

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