Understanding mechanisms of drying of a cellulose slurry by magnetic resonance imaging

Abstract

This study concerns the spatial evolution of the internal components of a macroscopic sample of cellulose slurry during its convective drying. In a first stage, some water is extracted which induces a shrinkage of the structure, the sample remaining saturated, i.e. the solid porous structure made of fibers in contact remains filled with water. In a second stage, when capillary effects are unable to induce further shrinkage, the porous structure formed by the cellulose network homogeneously desaturates and all bulk water is extracted. All along these two stages the drying rate remains constant. Confined water (i.e. likely between macrofibrils) and possibly bound water start to be extracted in a third final stage, during which the sample somewhat further shrinks. In that case a dry front tends to develop from the free surface of the sample, but the water content still decreases homogeneously in the wet region, which suggests that a connected hydraulic network for the confined and bound water persists. This network in fact already ensured the full extraction of bulk water at the end of the second stage keeping a constant drying rate, in contrast with simple porous media.Graphic abstract