Mass Transfer “Regime” Approach to Drying

Abstract

The current article introduces a new tool to model drying kinetics. The perspective described here involves application of regimes originally used for heterogeneous reactions to quantify the reaction rate against the diffusional transfer of reactants. This method helps to determine the controlling steps in the falling and constant rate periods of drying and can be used as a base for advanced mathematical models. The proposed method can be used for solid/semisolid materials of varying dimensions. It addresses the research gap of the current unavailability of a universal technique to describe drying kinetics. The effect of parameters such as temperature, humidity, and the dominant mode of drying has been discussed with the help of a sample material (tomato). The effect of using different regimes for the variation of relative excess activation energy against the moisture content is also discussed. This approach showcases the "internal" character of the material and the kinetic model can be applied to different drying processes. The conditions under which transition between regimes may occur are also discussed and elaborated in terms of dimensionless numbers, such as the Hatta number and the Biot number.

NMR Relaxometry Studies on the Drying Kinetics of Cellulose Nanofibers

“…In this study, empirical equations referred to as thin-layer drying models were used to emulate the drying behavior of nanocellulose following the literature precedent for several materials, including wood, pharmaceuticals, solid waste, and food products. − The dimensionless moisture content ( X n ) relates the moisture content (g/g) in the sample at a given time ( X ) with the initial ( X 0 ) and equilibrium ( X eq ) moisture contents, as shown in eq . Here, X eq was determined when differences in the sample mass were not observed over extended periods. The simplest empirical drying model, the Lewis equation, states that the drying rate is proportional to X n through an exponential function, with the assumption of a homogeneous population of water molecules and negligible shrinking during drying, as shown in eq . ,,− , where k symbolizes a drying constant parameter and t represents the experimental time. The use of model-based parameters supports the correlation between the obtained results with process conditions .…”

“…The simplest empirical drying model, the Lewis equation, states that the drying rate is proportional to X n through an exponential function, with the assumption of a homogeneous population of water molecules and negligible shrinking during drying, as shown in eq 2. 37,38,[40][41][42]45 = − X e n kt (2) where k symbolizes a drying constant parameter and t represents the experimental time. The use of model-based parameters supports the correlation between the obtained results with process conditions.…”

Mass Transfer Regime Approach for Drying of Apple

“…When it comes to the drying of thick-layered materials, certain resistances add up that control the drying rate, and the reaction engineering approach (REA) model, for instance, cannot in many cases explain the overall drying rate. It is also pertinent here that when the diffusion rate and the drying rate are comparable, the regime-based approach can be more effective in quantifying the overall process of drying [7].…”

“…The above Regime 1 equation is obtained from [7], the Regime 1 equation is modified since it represents dependency of rate on the transfer of moisture from within the material which is given by 𝜌 𝑙,𝑐𝑜𝑟𝑒 hence the equation is reduced to terms containing the moisture concentration inside the material and the liquid holdup. If the rate of the drying process in kg.s -1 is proportional to the moisture concentration in the core of the material, which is increased with the moisture of the sample in the order of (n), the subscript l and core represents the liquid phase and the core of the material respectively.…”

“…The internal characteristics of the material include density, porosity, and geometry of the material. The regime-based approach is considered to be more effective in quantifying the overall process of drying [7].…”