Dependence of in situ precipitate deposition on flow characteristics in multi-permeability porous media

Previous studies have shown that displacement washing of kraft pulp pads can be improved by the addition of cationic polymer to the wash water. The washing efficiency improvements are due to in situ precipitate formations which develop and deposit in the high permeability zones of the pad and consequently divert washing fluids to the lower permeability zones. To determine whether there is a relationship between the fluid flow profile and the location of precipitate formation, a number of multi-permeability bed configurations were studied with tracer and displacement washing experiments. These results, with bed property parameters determined in column experiments using homogeneous permeability bead packs, lead to the development of a continuum model that describes the fluid mechanics of various multi-permeability porous media. The multi-permeability simulation model predictions for residence time distribution and the displacement experiments indicate that precipitates develop in the location where the residence time gradient is the steepest and where the direction of the dominant fluid flow is opposite to this gradient.