Membrane Filtration of Wood Components − Investigation of Fouling Layer Characteristics using Fluid Dynamic Gauging

Licentiate thesis, 2022

As process industries transition from using fossil-based to bio-based resources, energy-efficient and highly selective operations will be required to process lignocellulosic biomass. Membrane separation, as a highly efficient operation, can be employed in the fractionation and dewatering of streams containing lignocellulosic biomass. The application of membrane separation in these processes has nevertheless proved to be challenging due to membrane fouling: the process by which suspended or dissolved substances are deposited on the surface or within the pores of a membrane. An in-depth mechanistic understanding of such membrane fouling behavior is therefore necessary in order to devise appropriate strategies to address fouling.

In this work, the in situ monitoring technique fluid dynamic gauging (FDG), was used to investigate the fouling behavior of two feed streams containing wood components: a liquor obtained when wood was treated using steam explosion (STEX) and a microcrystalline cellulose (MCC) suspension. The cross-flow ultrafiltration (UF) of STEX liquors was performed at 200 kPa transmembrane pressure (TMP) using 10 kDa polysulfone membranes, whereas
the cross-flow microfiltration (MF) of MCC suspensions was operated at 40 kPa TMP using 0.45 μm polyethersulfone membranes.

Experimental results show that FDG provides valuable information regarding membrane fouling in both cross-flow MF and UF. The FDG profiles obtained during the STEX liquor fouling experiments revealed that an initial thick fouling layer of high resistance was formed rapidly. After this initial phase, the resistance increased further but only gradually, which may be due to pore blocking and/or rearrangement of the structure of the fouling layer. These results also indicate that the highest resistance of the fouling layer is close to the membrane. In the MCC fouling experiments, highly resilient layers were formed close to the membrane surface, as supported by the presence of attractive energy regions at the close interparticle separations identified via molecular dynamics (MD) simulations. These results highlight how FDG, when complemented with MD simulations, can provide a better mechanistic understanding of the fouling behavior of streams containing wood components during cross-flow filtration.