Abstract
Drying of porous media is strictly governed by heat and mass transfer. However, contrary to the definition that drying is simultaneous transport mechanisms of heat and mass, most past and current models either account for temperature or concentration gradient effects on drying. Even though the complexity of computations of these processes varies with area of application, in most cases, the Dufour and Soret effects are neglected. This leads to deviations and uncertainties on the assumptions and interpretations of these and other relevant effects on drying. This paper covers the theoretical methods to derive the coupled transfer effects. In addition, this work proposes and formulates relevant heat and mass transfer equations, as well as the governing equations for drying processes with Dufour and Soret effects. The application of a numerical approach to solve the equations allows for studying of the influence of these effects on the design and operation of dryers. It is shown that the Soret effect can be highly relevant on drying operations with dynamic heating operation. While for drying processes where the steady state drying process predominates, the effect is deemed negligible.
Highlights
Drying is a very energy costly operation and accounts for up to 15% of industrial energy usage, while the thermal efficiency is only about 25–50% [1]
Linear nonequilibrium thermodynamics allow the derivation of coupled transport phenomena, assuming that the system is close to global equilibrium [4]
The coupled heat and mass transport flux equations that are derived from linear nonequilibrium thermodynamics in the method section, are tested with a numerical simulation of a drying system
Summary
Drying is a very energy costly operation and accounts for up to 15% of industrial energy usage, while the thermal efficiency is only about 25–50% [1]. Most applied heat and mass transport relations for drying processes neglect the Dufour and Soret effect These coupled heat and mass transport phenomena are derived in detailed drying process literature by, e.g., Keey [2] or Kowalski [3], but only mentioned as being possibly of relevance. In an earlier conference proceeding by the authors the relevance of the coupled heat and mass transport effects on drying was argued based on these works. In this contribution the relevance of the coupled heat and mass transport phenomena on drying processes is shown in detail with the derivation of first principle flux equations for a drying process, using linear nonequilibrium thermodynamics. Quantifying the coupled heat and mass transport phenomena for drying processes
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