Modeling of filler retention in compressible fibrous media

Abstract

Abstract Filler retention is an important issue at the wet-end of the paper machine. Filler is added to improve the printing and mechanical properties of paper. Filler retention in sheets has long been studied using elements of depth filtration theory ignoring the compressibility of the media. The objective of this work is to develop a theoretical model for depth filtration that accounts for compressibility of fibrous media. The model development is based on principles of volume averaged continuum theory. Material parameters as required by the model are estimated from experimental data of filtration experiments. Filtration experiments are done with Prince Albert softwood pulp fibers and titanium dioxide as filler. For the depth filtration study the fiber bed is formed first, conditioned and then used as media for retaining filler particles. All filtration experiments are performed at constant flow rate conditions. Numerical simulation of the model predicts the cake compression and filler retention profiles within the fibrous mat. Porosity profiles indicate that the reduction in volume fraction at the end of filler filtration is mostly due to compression of the cake, than due to the physical volume of the retained filler. Retention of filler is highest at the entrance of the cake and lowest at the exit. This is due to high concentration of filler in slurry at the top of the cake, which decays exponentially towards the bottom due to retention on fibers. Filler concentration in the fluid phase at the exit of the bed follows a typical `S' shaped breakthrough curve.