Electroassisted Filtration of Microfibrillated Cellulose: Insights Gained from Experimental and Simulation Studies

Abstract

An electroassisted filtration technique has been employed to improve dewatering of a suspension of microfibrillated cellulose (MFC) produced via 2,2,6,6-tetramethylpiperidinyl-1-oxyl (TEMPO)-mediated oxidation. In addition, all-atom molecular dynamic (MD) simulations were performed to deepen the understanding of the complicated dewatering mechanism on a molecular level. Both the experimental and the simulation results implied that the dewatering rate was not only improved when electroassisted filtration was used but also found to be proportional to the strength of the electric field. A channeled dewatered structure was observed for these experiments and may have contributed to enhanced dewatering by providing high overall permeability. The MD simulations revealed that the electric field had a significant impact on the fibril movement, whereas the impact of pressure was limited. The simulations also suggested that the increased filtrate flow upon the application of an electric field was not only due to electroosmotic flow but also due to electrophoretic movement of the fibrils toward the anode that led to the release of water that had been trapped between the fibrils, allowing it to be pressed out together with the rest of the bulk water. This study shows that electroassisted filtration has the potential to improve the dewatering of TEMPO-MFC, and the MD simulations provide further insights into the dewatering mechanism.

Structure of Filter Cakes during the Electroassisted Filtration of Microfibrillated Cellulose

“…The unoxidized pulp had a carbohydrate composition according to Table 1 and a carboxylate content of 0.025 ± 0.002 mmol/g pulp. 28 The pulp was subjected to TEMPO-mediated oxidation (see the TEMPO-Mediated Oxidation section) during which the chemicals used were NaClO (10−15% available chlorine) from VWR and TEMPO (98%) and NaBr (>99.5%) from Sigma-Aldrich. NaOH (purity >98%, Sigma-Aldrich) was used to regulate the pH during the oxidation process and absolute ethanol (≥99.8%, VWR) to quench the reaction.…”

“…The procedure employed is based on work by Brodin and Theliander 40 and explained further by Karna et al 28 In brief, NaBr (1 mmol/g pulp) and catalytic amounts of TEMPO (0.1 mmol/g pulp) were dissolved in water and added to a pulp suspension in a baffled vessel (⌀ = 0.19 m) under stirring with a four-pitch blade impeller (⌀ = 0.09 m). The reaction was carried out at 1 wt % and at room temperature, being initialized by the dropwise addition of NaClO (5 mmol/g pulp).…”

“…The present study aims at investigating two observations sprung from the previous study on the electroassisted filtration of MFC 28 that are related to understanding the structure of the filter cake. The first is related to the hypothesis regarding the potential alignment of cellulose fibrils in the electric field and is evaluated both qualitatively and quantitatively using scanning electron microscopy (SEM) and wide-angle X-ray scattering (WAXS), respectively.…”

“…This technique has been used to dewater various kinds of materials that are hard to dewater, e.g., sludges 24−26 Electroassisted filtration has been used to successfully dewater MFC suspensions and is reported in a patent from 2014, 27 as well as in a recent study based on experimental work and molecular dynamics simulations. 28 The latter employed a filter press with an electric field applied parallel to the flow across the lower part of the filter cell. An interesting observation was made in these particular experiments regarding the structure of the filter cake: the cellulose fibrils were assembled in pillars, and there were channels corresponding to the openings of the anode mesh.…”

“…Electroassisted filtration has been used to successfully dewater MFC suspensions and is reported in a patent from 2014, as well as in a recent study based on experimental work and molecular dynamics simulations . The latter employed a filter press with an electric field applied parallel to the flow across the lower part of the filter cell.…”