A Model Describing Liquid Phase Migration Within an Extruding Microcrystalline Cellulose Paste

Abstract

A quantitative, one-dimensional model has been developed to predict liquid phase migration, and hence the redistribution of water, within a microcrystalline cellulose (MCC) paste undergoing ram extrusion. The process features an initial compaction stage, followed by paste convection after the onset of flow. Compaction is treated similarly to the consolidation of soils, with Darcy-type liquid flow behaviour and stresses estimated using the Janssen–Walker analysis. The forces required in the convection stage are calculated using the modified Benbow–Bridgwater model, incorporating water-content-dependent pseudo-plastic and shear deformation terms. The model estimates extrusion pressure–time and liquid content distributions. Factors affecting liquid phase migration, such as the extrusion rate and die geometry, have been considered. The model shows reasonable comparison with experiment and mimics the observed trends. A dimensionless number has been defined, being the ratio of the maximum initial liquid phase velocity relative to the solids phase, to the absolute solids velocity, which suggests a criterion for the occurrence of significant liquid phase migration. The model does not incorporate pore suction pressure effects, which are likely to be important in other paste systems.