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

Licentiate thesis, 2021

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.