Simulation of Through Air Drying in Tissue Paper in a Papermaking Process
Water removal during paper and board manufacturing is an intensive energy process. The dewatering process generally consists of four stages wherein the last stage is comprised by thermal drying in where remaining water in fibres are evaporated on steam-heated cylinders. The thermal drying section significantly exceeds the other mechanical drying stages in terms of energy demand. Hence, improving the mechanical dewatering processes prior to the thermal stage could reduce major production cost. The previous sections generally comprise gravitational dewatering, vacuum dewatering and pressing. A way to improve product performance of tissue grade paper products is to replace the press section with a through air drying (TAD) section which is a technique where paper sheets are molded into a structured fabric and transferred over one or more TAD cylinders with displacement drying with vacuum boxes.
This process is modelled with the COMSOL Multiphysics® software where the computational model is setup with a 2-dimensional representation of the paper sheet. The tissue sample with randomly distributed fibre positions is generated using a MATLAB® script written in the LiveLink™ for MATLAB® interface with COMSOL®. At the start of the simulation process, the pulp is saturated with water. As air flows through the paper sheet, the fibres are dried due to moisture mitigation from the core of the fibre to the surface through convection and capillary forces. At the surface, water is evaporated though forced convection and vapour diffusion. The process is simulated with the Moisture Flow multiphysics interface. The comprising physics interfaces are the Laminar Flow and the Moisture Transport in Air interfaces. Respectively, these interfaces calculate the velocity and pressure field of the moist air as well as the relative humidity, which is a rewrite of the concentration of water in air. The fibres in the sheet are modelled as porous media where the fibres contain both moist air and liquid water in equilibrium.
The modelling process considers tissue paper with various basis weights of 17 up to 37 g/m2. Different structures are generated and simulated in order to analyse how local heterogeneities affects the overall drying rate. The aim of the model is to estimate solid content in the paper sheet over vacuum time as well as energy demand and required airflow through various paper structures.
