Multiphysics simulation and experimental validation of convective drying of paper as a hygroscopic porous biomaterial

While recent modeling efforts have largely focused on lower basis weight paper sheets, detailed multi-physics models for convective drying of thicker paper sheets remain scarce, despite growing industrial interest in higher-basis-weight packaging products. The present study addresses the same by: (i) developing a physics-based non-conjugate multiphase porous-media model in 1D and 2D domains for accurately simulating convective drying treating paper as a hygroscopic porous material, and (ii) investigates via experimental validation of the model, the necessity of using heat and mass transfer coefficients as functions of sample moisture content. A coupled experimental-computational framework is used to demonstrate predictive accuracy of the model using paper samples across three grammages (125, 210, and 300 GSM). Experiments are carried out at a set point air temperature of 100 °C and velocity of 5 m/s. Continuous, in-situ measurements in a tunnel dryer combine high-resolution TDLAS humidity sensing with infrared thermography to obtain bulk moisture histories, drying rates, and spatial surface-temperature fields. The heat and mass transfer coefficients are obtained as functions of bulk sample moisture, as moisture is removed. An empirical approach is devised to obtain the local distribution of the transfer coefficients. The numerical model is developed using COMSOL, incorporating hygroscopic porous biomaterial characteristics including capillary pressure, relative permeability, a full range moisture sorption-driven vapor-pressure reduction, and effective gas diffusivity as functions of liquid saturation, porosity and temperature. Both in 1D and 2D simulations, moisture-dependent coefficients yield the best agreement with experimental data when compared with time-invariant coefficients, capturing warm-up, constant-rate, and falling-rate drying regimes. Spatial distribution of liquid saturation shows that the saturation levels are lower on and near the surface compared to the center of the sheet. The liquid and vapor fluxes were found to be higher near the exposed surfaces and at the receding, drying front. Drying of hygroscopic porous biomaterials is a complex interplay of various time-dependent and moisture-dependent external and internal heat and mass transfer mechanisms. They need to be considered to accurately describe the complex multi-physics involved.