Modelling compression of fiber suspension in 1-D pressing

Compression of fiber suspension is an important process in the pulp industry. Despite this, there is a lack of knowledge of the interaction between the different material parameters during compression. The purpose of this work was to increase the knowledge of the phenomena that arise in compression such as fiber bending, fiber repositioning, water flow through the fiber network, intra-fiber flow, and stratification. To fulfil this goal a mathematical model was developed and experiment in a 1-D press was also performed. In the model the flow through the fiber network is governed by Darcy’s law and the fiber bending and repositioning are governed by a solid pressure function determined from experimental work at Metso Paper in Sundsvall. In the model the fiber mat thickness was divided into several segments in order to be able to predict a non-uniform porosity profile of the fiber suspension.

Comparison between the simulations and the experiments showed that it was impossible to simulate the fiber compression at higher speeds with a static solid pressure and a hydraulic pressure. The reason for this is that the fiber network next to the piston is compressed too fast compared with the rest of the fiber mat, i.e. the effects from the stratification are too strong. When a dynamic part was added to the solid pressure this difference was less and the errors in the simulations decreased. The dynamic part in the solid pressure acts as a damper in the system and can be explained by intra-fiber fluid being squeezed out from the fibers. Simulations showed also that a rigid network (both with and without a dynamic solid stress) gives a lower total pressure. The reason for this is that the compression over the fiber network is more uniform, which leads to a lower hydraulic pressure.

In a continuation of this work the appearance of the dynamic part of the solid pressure could be improved until a closer agreement between the simulation and experiment is reached.

Author: Lindgren, Kristian

Source: Lulea University of Technology

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