Comprehensive analysis of intermittent drying. A theoretical approach

Abstract

Intermittent drying is a promising technique that may improve drying performance, when compared to stationary drying. An intermittent drying is composed of a succession of cycles, each of them consisting in an active drying period followed by a tempering period. It is typically characterized by two parameters: the duration of the cycles tc and the ratio of the duration of the drying period of a cycle to the total duration of the cycle χ. In this work, we derive two general models of the intermittent drying of a solid material, corresponding to different physical situations: a continuous model, to describe a situation where liquid water is continuously distributed through the material, and a shrinking core model, to describe a situation where the material is divided into a humid core and a dry zone. With these models, we provide a thorough analysis of the influence of the parameters characterizing the intermittent drying of a material on the dynamics of this drying, over wide ranges of values of these parameters. The results allows the identification of three possible regimes in the dynamics of an intermittent drying, when it is compared to a stationary one, what is summarized into an operational diagram that could be useful to select the best parameters for an intermittent drying process. Notably, the results show that one of these regions (with small values of tc and χ) corresponds to high energy efficiency, without significantly increasing the total drying time compared to continuous drying, if the external resistance to mass transfer is negligible. The models also exhibit that the external mass transfer Biot number, Bim, has a strong influence on the dynamics of an intermittent drying; the influence of tc and χ on drying performance is mitigated as Bim decreases. Finally, the models are used to shed new light on experimental data from the literature, regarding the intermittent drying of several products, for different operating conditions. The investigation of the influence of the operating conditions and the product properties on the drying performance shows that strategies such as varying the intermittency ratio or using inherently intermittent dryer can enhance the drying kinetics.