Dynamic compression and liquid transport in fibre systems under press nip conditions
Time: Thu 2025-12-11 10.00
Location: D3 / 4333, Lindstedtsvägen 5
Video link: https://kth-se.zoom.us/j/66656634355
Language: English
Subject area: Fibre and Polymer Science
Doctoral student: Patrick Wegele , Fiberprocesser
Opponent: Professor Mark Martinez, University of British Columbia, Kanada
Supervisor: Professor Daniel Söderberg, Wallenberg Wood Science Center, Teknisk mekanik, Fiberprocesser
QC 20251118
Abstract
Paper, as a bio-based product, is a key material in advancing a sustainable circular economy. In papermaking, energy-intensive drying is required to remove residual water from the cellulose fibre network. Wet pressing is therefore a crucial step, reducing the water that must be evaporated in the dryer section and significantly lowering overall energy demand.
In wet pressing, the paper web enters a nip formed by two loaded rolls while supported by a press felt. The applied load drives water from the web through the felt into the voids of roll covers. Industrial observations suggest that the compression behaviour and saturation of both components strongly affect dewatering efficiency, yet their response under realistic press nip conditions remains insufficiently understood. This thesis aims to investigate the compressibility and liquid distribution of fibre systems under such conditions.
The work combines laboratory-scale experimental rigs, X-ray imaging techniques and calculation models derived from physical laws. This enables the quantification of dynamic compressibility and void volume loss in grooved polyurethane roll covers, a key factor in roll cover design. Studies of stress variations at the press felt–roll cover interface show that dewatering improves when high-permeability felt regions are created by the groove structure. The liquid distribution in press felts is characterised as a function of load and saturation, showing out-of-plane redistribution during compression due to the nonwoven morphology. Higher felt saturation enhances dynamic liquid transport, linking relative permeability to improved dewatering once nip saturation is reached. Finally, X-ray multi-projection imaging (XMPI) is shown to resolve pore-scale liquid transport mechanisms, enabling future studies of rewetting between the press felt and paper web.