Filtration of Cellulosic material - the impact of ionic strength and electric field

It is crucial that dewatering technology is considered if an economically feasible production of cellulosic materials is to be achieved. Cellulosic materials are often produced in dilute streams and therefore require great amounts of water to be removed to allow for reasonable transportation costs and/or further modifications. Although various types of drying techniques are suitable for this purpose, they are rather energy intensive. One way of reducing the total energy demand is to add an efficient mechanical dewatering step prior to the drying step: filtration is the technique most commonly used. However, micro/nano cellulosic materials have large surface areas, resulting in an extensive filtration resistance and thus requiring large filters. It therefore becomes important to evaluate the impact of external factors, and ionic strength in particular, as this is known to affect electrostatic interactions between cellulose particles and may thereby impact the filtration behaviour. Another alternative is to use an electro-assisted method where an electric field, applied across the filter chamber, introduces several electrokinetic phenomena that can be beneficial to dewatering. The work presented in this thesis examines the filtration of cellulosic material in the form of microcrystalline cellulose (MCC) and microfibrillated cellulose (MFC). The influence of ionic strength on the dead-end filtration of MCC was investigated by the addition of NaCl in the range of 0-1 g/L. It was concluded that increasing the ionic strength improved the filtration rate: the surface charges of the MCC particles were shielded which, in turn, promoted agglomeration and reduced the total surface area subjected to the liquid flow. This confirms the importance of electrostatic interactions between MCC particles during dead-end filtration.The MFC was produced via 2,2,6,6-tetramethylpiperidinyl-1-oxyl (TEMPO)-mediated oxidation and an electro-assisted filtration technique was employed to dewater the suspension. Compared to conventional dead-end filtration, electro-assisted filtration significantly improved the dewatering rate due to the electrokinetic phenomena it introduced. Three different levels of electric field were used, and it was observed that the dewatering rate increased proportionally to the strength of the electric field. In addition, molecular dynamic (MD) simulations were performed to obtain an understanding of the dewatering mechanism on a molecular level

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

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

Dewatering Microfibrillated Cellulose: the use of electro-assisted filtration

Winning presentation in Gunnar Sundblad session, presented in the format of Pecha Kucha (20 slides \ue0 20 s

Structure of Filter Cakes during the Electroassisted Filtration of Microfibrillated Cellulose

Microfibrillated cellulose (MFC) is a biobased material with unique properties that can be used in a multitude of applications. Water removal from the dilute product streams is, however, challenging and hinders its commercial attractiveness. One possible method of improving dewatering is the use of electroassisted filtration, in which an electric field is applied across part of the filter chamber. In this work, a bench-scale dead-end filter press, modified to allow for electroassisted filtration, was used to dewater a suspension of MFC produced via 2,2,6,6-tetramethylpiperidinyl-1-oxyl (TEMPO)-mediated oxidation. A filter cake was produced with a channeled structure related to the design of the anode mesh, indicating that the cellulose microfibrils were aligned in the direction of the electric field. This was investigated, qualitatively and quantitively, using scanning electron microscopy and wide-angle X-ray scattering, which showed a preferred orientation on a microscopic level but only a partial orientation on a molecular level (fc between 0.49 and 0.57). The influence of the density of the anode mesh, in terms of the structure/permeability of the filter cake and dewatering rate, was also evaluated using two different anode mesh densities (5 x 5 and 10 x 10 mm). It was not, however, found to have any major impact on the dewatering rate

Dewatering microcrystalline cellulose: The influence of ionic strength

This study investigates the influence of the ionic strength on the dead-end filtration of microcrystalline cellulose (MCC) suspensions in the range of 0.1–1 g/L NaCl, in altering the electrostatic interactions between particles. The formation of larger agglomerates of increasing ionic concentration was observed using Focused Beam Reflectance Measurement (FBRM\uae). Local filtration properties were investigated as the experimental set-up allowed for measurements of local hydrostatic pressure and solidosity to be made. The results show that the addition of ions decreases both the average and local filtration resistance. The formation of a resistant skin layer was observed for the suspension without the addition of NaCl but was counteracted when ions were added. Furthermore, the ionic strength did not seem to have any notable effect on the structure of the cake in the range 0.15–1.0 g/L NaCl. However, the pressure dependency of the solidosity at lower ionic concentration was higher. The local filtration properties were fitted to semi-empirical relations, which indicated the formation of moderately to highly compressible cakes when NaCl was added

Wettability of cellulose surfaces under the influence of an external electric field

Hypothesis: Interfacial tensions play an important role in dewatering of hydrophilic materials like nanofibrillated cellulose, and are affected by the molecular organization of water at the interface. Application of an electric field influences the orientation of water molecules along the field direction. Hence, it should be possible to alter the interfacial free energies to tune the wettability of cellulose surface through application of an external electric field thus, aiding the dewatering process. Simulations: Molecular dynamics simulations of cellulose surface in contact with water under the influence of an external electric field have been conducted with GLYCAM-06 forcefield. The effect of variation in electric field intensity and directions on the spreading coefficient has been addressed via orientational preference of water molecules and interfacial free energy analyses. Findings: The application of electric field influences the interfacial free energy difference at the cellulose-water interface. The spreading coefficient increases with the electric field directed parallel to the cellulose-water interface while it decreases in the perpendicular electric field. Variation in interfacial free energies seems to explain the change in contact angle adequately in presence of an electric field. The wettability of cellulose surface can be tuned by the application of an external electric field