Simulation of Sheet Molding during Through Air Drying of Tissue Paper within a Papermaking Framework K. Rezk 1 , B. Sjöstrand 1 1. Pro2BE, the research environment for Processes and products for a circular forest-based bioeconomy, Department of engineering and chemical sciences, Karlstad University, Karlstad, Sweden Abstract 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 moulded into a structured fabric by vacuum boxes and transferred over one or more TAD cylinders with steam heated displacement drying. The process of sheet molding is modelled with Comsol Multiphysics 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 interface with Comsol. The process is simulated with the Moisture Flow multiphysics interface. The comprising physical modules are the Laminar Flow and the Moisture Transport in Air modules. Respectively, these modules 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. In this paper, a basis weight at roughly 20 g/m 2 is simulated and compared to laboratory data. 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 the structure.
Keywords: CFD, Dewatering, Moisture transport, tissue paper, through air dying.
These studies show that a prolonged dwell time would increase dryness to a certain level. To reach a higher dryness level, increased vacuum pressure is necessary. Numerous researchers have conducted theoretical and numerical analysis on various aspects on vacuum dewatering such as single-flow mechanics in fibrous porous media where permeation through compressible fiber beds, permeability in various representative fiber structures were studied as well as penetrated air volume calculated and compared to experimental data [26-30]. Some attempts on simulating the two-phase flow behavior during the early stage of dewatering of the unbound water where progression on estimating the dwell time and dry content with varying pressure pulses has been conducted in [16, 31, 32]. While there are improvements that need to me made on the estimation of the dewatering rate, the penetrated air volume showed excellent agreement with experimental data. Fluid inertia as well as spatial heterogeneity has been studied for moderate Reynolds number in various fiber arrangements by [33-37]. Moreover, the influence on the permeability based on sample size, boundary conditions, homogeneity and isotropy were studied. The relations are used to predict permeability for various fiber arrangements and porosity levels.
Introduction Water removal during paper and board manufacturing is an energy intensive process. The dewatering process in paper and board manufacturing generally consists of four stages with progressively high energy demand [1, 2]. The first stage comprises water removal with gravity or centrifugal forces where water escapes without additional energy input and then with help of static elements in the geometry of the paper machine, followed by low and high vacuum pressure. In the second stage of vacuum exposure, where higher levels of vacuum pressure are applied through suction boxes [2-8]. Most of the suspended water is removed during gravitational and vacuum dewatering [9]. The third stage is performed in the press section by compressing the paper in one or more press nips [10]. The last stage, which far exceeds the other stages in energy demand, is comprised by thermal drying where remaining water in fibres are evaporated on steam-heated cylinders [9, 11]. Hence, improving the mechanical dewatering processes prior to the thermal stage could reduce major production cost. A number of researchers have studied vacuum dewatering experimentally using laboratory equipment as well using pilot scale machines [2-8, 12-21]. Process parameters that influence sheet dryness such as applied vacuum, dwell time and basis weight have been analysed.[4, 6, 13, 22-25]
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