Process stability optimization of the twin-screw extrusion adapted for concentrated cellulose fibrillation

The twin-screw extrusion (TSE) process has recently been investigated for the production of cellulose nanofibrils (CNF). The main advantage of this process is the production of CNF at high solid content (around 20 wt% suspensions) and lower energy consumption. Typically, studies focus on the quality of the extruded material (i.e., CNF) by comparing different pretreatments, biomass sources or combinations of processes. Limited information is available on the TSE process itself and its stability. This work proposes to analyze some key parameters such as torque, in situ temperature and flow of extruded material. All together they reflect a certain stability of the TSE process. The process is considered stable when the torque and temperatures plateau and when the mass flow is continuous and constant. These parameters vary depending on the different pretreatments and the solid content of the cellulose suspensions introduced in the TSE. Morphologies of extruded CNF are characterized at different scales with optical microscopy, SEM-FEG and TEM images. A simplified quality index was calculated to compare extruded CNF. A high and fluctuating torque, rising temperatures and discontinuous mass flow were recorded for the enzymatically pretreated pulp and refined pulp with a long dwell time of the fibers in the extruder. On the contrary, a low and very stable torque, stabilized temperatures and continuous mass flow were reported for the pulp obtained after a combination of enzymatic hydrolysis and refining. Graphical abstract