Membrane Filtration of Lignocellulosic Materials: In situ Monitoring of Membrane Fouling Using Fluid Dynamic Gauging

Doctoral thesis, 2023

The transition of process industries from utilizing fossil-based resources to bio-based ones necessitates energy-efficient and selective operations. One practical operation that can be employed for processing streams containing lignocellulosic biomass, such as forest and agricultural residues, is membrane separation. However, the persistent challenge that limits the application of membrane separation in such processes is the deposition of suspended or dissolved substances on the surface or within the pores of a membrane, a phenomenon referred to asmembrane fouling. An in-depth understanding of membrane fouling is therefore necessary to develop appropriate antifouling strategies.

In this work, fluid dynamic gauging (FDG) was employed as an in situ and real-time technique for monitoring the fouling characteristics of microcrystalline cellulose (MCC), Kraft lignin, and steam explosion (STEX) liquors on flat-sheet polymeric membranes. Cross-flow microfiltration (MF) was performed for the MCC and Kraft lignin suspensions, whereas cross-flow ultrafiltration was carried out for the STEX liquors. Furthermore, physical and chemical cleaning were performed to the fouled membranes after the cross-flowMF of MCC and Kraft lignin suspensions, respectively, to restore their separation performance.

The thickness and strength properties of the fouling layers formed were investigated using FDG. The FDG profiles revealed that the build-up of fouling layers was significantly influenced by the feed characteristics and operating conditions. The thickness of the cake layers varied with changes in process conditions, while the cohesive strength of fouling layers increased towards the membrane due to higher compressive pressures exerted on foulants deposited near the surface. Observations from refouling and membrane cleaning also provided significant insights into the fouling behavior,
showing changes in the membrane resistance and flux recovery. These results highlight how FDG can serve as a valuable tool in gaining a better mechanistic understanding of the fouling behavior of streams containing wood components during cross-flow filtration. Such knowledge is essential,
especially in developing membrane separation processes for lignocellulosic materials.