Local filtration properties of microcrystalline cellulose: Influence of an electric field
Introduction
Dewatering is an important step in many industrial processes, e.g. in the forest industry, mineral processing and wastewater treatment. Thermal drying is often preceded by a mechanical dewatering technique, such as filtration, in order to achieve an energy-efficient solid–liquid separation. Filtration can however be challenging for materials that form highly compressible filter cakes due to the high filtration resistances involved. The difficulties associated with mechanical dewatering on an industrial scale are also largely dependent on the external specific surface area of the solid material: an increase in external surface area increases the filtration resistance. The size of the filtration equipment, as well as the filtration time required, could therefore become unfeasible. Both the compressibility of the filter cake and high specific surface areas are expected to contribute to the challenges of filtration in the development of biorefineries aimed at producing materials such as e.g. microfibrillated cellulose.
The use of assisted filtration techniques could play a role in improving the energy efficiency of the production of this type of material by increasing the rate and extent of mechanical dewatering. Depending on the application, assisted filtration techniques using either thermal effects (Clayton et al., 2006), acoustic fields (Muralidhara et al., 1985, Smythe and Wakeman, 2000), magnetic fields (Stolarski et al., 2006) or electric fields (Iwata et al., 2013, Mahmoud et al., 2010) have been suggested. Pressure filtration in an electric field (i.e. electrofiltration) has been shown to have the potential of improving the filtration rate of hard-to-filter materials ranging from wastewater sludge (Citeau et al., 2012, Mahmoud et al., 2011, Olivier et al., 2015) to biopolymers (Gözke and Posten, 2010, Hofmann et al., 2006, Hofmann and Posten, 2003) through a combination of electrophoretic and electroosmotic actions. Electrofiltration has therefore been shown to have the potential of decreasing the energy demand of solid–liquid separation by reducing the need for thermal drying (Larue et al., 2006, Loginov et al., 2013, Mahmoud et al., 2011).
The local conditions in the filter cell vary significantly during the electrofiltration of materials that form compressible filter cakes. Depending on the electrical conductivity of the fluid and solid phases, the strength of the electric field may vary greatly as a result of the solid content of the filter cake and/or the suspension. In spite of this, few studies have measured local filtration properties during electrofiltration (Saveyn et al., 2006). This study investigates the local filtration properties of a cellulosic material during one-sided dead-end electrofiltration. A mechanically-modified microcrystalline cellulose was used as a model material for cellulosic materials with high specific surface areas. The filtration behaviour was studied at different strengths of the electric field and the influence of electrokinetic effects, ohmic heating and electrolysis reactions is described using an electrofiltration model based on average filtration properties. The validity of the model is discussed using experimental measurements of the local hydrostatic pressure in the filter cake and the local solidosity of the filter cake.
Section snippets
Electrofiltration
Electrofiltration utilises the charge of particle surfaces to improve the filtration operation. In an electric field colloidal particles move through electrophoresis, thereby influencing (in this case) the growth of the filter cake (Moulik, 1971). The electric field also gives an electroosmotic flow, thereby resulting in an additional driving force for separation (Kobayashi et al., 1979, Yukawa et al., 1971, Yukawa et al., 1976).
In addition to electrophoresis and electroosmosis the electric
Filtration equipment
The filtration experiments were performed using a bench-scale filter press designed to measure local filtration properties (Johansson and Theliander, 2003). The equipment was modified for electro-assisted filtration as shown in the schematic representation given in Fig. 1. Photographs of the filtration equipment are included in the supplementary material. The filter cell is cylindrical, with a height of 175 mm and an internal diameter of 60 mm. The lower section of the filter cell consists of a
Characterisation
The surface charge of the microcrystalline cellulose particles used in this study was measured by titration with a linear polyDADMAC. Its dependence on pH is shown in Fig. 2, where the charge is seen to be negative at neutral and alkaline conditions and approach zero at pH 3. This surface charge arises from acidic groups that are either native to the wood material or introduced during the production of the microcrystalline cellulose particles through acid hydrolysis (Araki et al., 1999).
The
Conclusions
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The formation of the filter cake was influenced by the electric field during electrofiltration of mechanically-treated microcrystalline cellulose. This behaviour shows that electrofiltration has the potential for being used to prevent cake formation during dewatering of cellulosic materials with high specific filtration resistances.
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The filtrate flow rate of the cellulosic material could be increased more by the application of an electric field than by modifying the pH of the suspension. This
Acknowledgements
This study was performed within the framework of the Wallenberg Wood Science Center. The financial support of the Knut and Alice Wallenberg Foundation is gratefully acknowledged.
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