Investigation of sheet molding during through air drying of tissue paper: A comparative numerical and experimental study of the solid content

Through Air Drying (TAD) technology enhances high-quality tissue-grade paper production by shaping a wet paper web on a structured fabric using vacuum or molding boxes, followed by hot air displacement drying over TAD cylinders. This study uses CFD modeling with COMSOL Multiphysics to understand drying rate during the molding process better. Two-dimensional models of paper sheets estimated solid content over vacuum time. Tissue samples were generated using COMSOL’s interface with MATLAB, enabling a random fiber distribution. Fluid flow and moisture transport were simulated by coupling the Navier-Stokes and advection-diffusion equations, while Darcy’s law with inertial correction described fluid migration within fibers, considering equilibrium between moist air and liquid water. Simulations examined the impact of refining levels and fiber compositions for basis weights from 15 to 30 g/m². Material properties, including porosity and permeability, were calibrated with laboratory tests and validated in pilot experiments. Results showed that increased refining of softwood pulp significantly affects porosity and initial dryness, achieving reasonable agreement between predicted and actual tissue dryness. The calibrated models could be a first step to improve energy efficiency in TAD processes, paving the way for more sustainable tissue-making methods. • Outgoing dryness of the tissue is well predicted by the CFD models • Validation accuracy strongly depends on permeability change during molding • Including web deformation during molding would improve permeability representation • Poroelastic theory could accurately predict deformation and drying during molding