Filtration of Microcrystalline Cellulose: Electro-assisted filtration and the influence of surface properties

University dissertation from Chalmers University of Technology

Abstract: One of the main requirements for the sustainable usage of materials is the use of renewable resources. This presents the Swedish forestry industry with the promising opportunity of using wood to produce not only traditional paper products but also a range of new materials of high value. The production of new types of materials introduces new challenges: a main one that faces large-scale production processes based on bio-based resources is solid-liquid separation.

Thermal drying is often preceded by a mechanical dewatering technique, such as filtration, in order to achieve an energy-efficient solid-liquid separation. However, conventional pressure filtration operations can be impaired when employed for materials with high specific surface areas due to the operation time and/or equipment size required. This is especially so for materials that form compressible filter cakes. The use of assisted filtration techniques to increase the dewatering rate could therefore play an important role for these types of materials. The application of an electric field during pressure filtration, electro-assisted filtration, is one option to improve the filtration behaviour of materials with charged surfaces.

In this thesis, the filtration behaviour of cellulosic materials with high specific surface areas was studied, and the use of electro-assisted filtration as a method for mechanical dewatering was investigated. The ways in which the surface properties, structure and charge of the solid material affected the filtration behaviour were studied using experimental measurements of the local filtration properties. The local filtration properties were also used to model the pressure filtration operation.

Electro-assisted filtration was shown to increase the filtration rate of the cellulosic material when compared to pressure filtration. The contributions made by electroosmosis and electrophoresis on the filtration operation were described by an electrofiltration model. The specific surface area of the cellulosic material had a large influence on the specific filtration resistance, but was not found to have a major influence on the rate of electroosmotic dewatering. The improvement obtained by electro-assisted filtration thus increased with increasing specific surface area of the solid material.