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Effects of Dual Porosity during the Consolidation of Fibrous Media

Bandaru V. Ramarao, Empire State Paper Research Institute, State University of New York, 1 Forestry Drive, Syracuse, NY 13210-2278

Fibrous materials such as paper, board and nonwovens are manufactured by dewatering a suspension containing fibers along with other components. Since ultimately, the fibrous material is dried using thermal energy, it is necessary to obtain the highest amount of dewatering by mechanical action to limit thermal energy requirements. Well validated mathematical models which can describe the physics of dewatering adequately can enable the optimization of filtration, pressing and consolidation operations.

Current models of filtration and expression model the solid phase as an elastic medium with non-linear behavior [1-3]. However, many materials such as wood pulp fibers are substantially porous in themselves giving rise to void spaces within the fibrous medium on two distinct scales. For instance, the cell walls of papermaking pulp fibers are known to contain nanoscale pores of sizes in the range of 10-50 nm in the radial direction and micron sized pores in the axial direction [4, 5, 6]. This is in addition to the interfiber pores within the fibrous mat which are in the range of 2-50 microns in size. Since the fibers are compressible, they expel water into the interfiber pore space under compression. The interaction of the expression phenomenon at the micro (or nano) scale with the overall consolidation of the medium determines the response of the fibrous media and its final structure.

In this paper, we develop a mathematical model to describe consolidation on this dual scale with suitable 'averaged' consolidation coefficients or diffusivities. The model is then solved numerically to obain predictions of dewatering rates and fibrous medium porosity distributions under different loading conditions.

We compared our results with experimental data obtained by consolidation under specific loading conditions applied to a fibrous material using an Instron machine.

1. Ramarao, B.V., Tien, C. and Satyadev, C. N. “Determination of Constitutive Relationhip for Filter Cakes in Cake Filtration Using the Analogy between Filtration and Diffusion” in "Transport Processes in Bubbles, Drops and Particles”, D. DeKee and R.P. Chhabra, Eds., 2nd Ed., Taylor and Francis, 2002.

2. Shirato, M., Murase, T., Negawa, M. and Senda, T., J. Chem. Eng. Japan, 1970, 3: 105-116.

3. Tien, C., and Bai, R., Chem. Eng. Sci., 2003; 58: 1325-1336.

4. Li, T. Q., U. Henriksson and L. Odberg. J. Coll. Int. Sci., 169, 376-379 (1995). 5. Maloney, T. C. and H. Paulapuro. Tappi J., 82, 150-154 (1999). 6. Maloney, T. C., Paulapuro, H., Stenius, P., Nord. Pulp Pap. Res. J., 13, 31-36 (1998).