Time-resolved near-infrared spectroscopy for controlled lignin esterification and scale-up in reactive extrusion

Reactive extrusion (REx) is emerging as a powerful technology for the continuous and solventless production of modified lignins. However, the optimization of REx-based processes for modifying lignin relies on offline lignin analytics, which are time-consuming and heavily influenced by sample preparation. This study integrated near-infrared (NIR) spectroscopy into a twin-screw extruder to monitor in real time the modification of softwood kraft lignin via esterification with octenyl succinic anhydride (OSA). The NIR data was processed by means of chemometric methods. Temperature and screw configuration were found to influence the esterification of lignin. Combining these results with offline lignin analytics, 120 °C was selected as the optimal temperature in conjunction with the integration of kneading elements into the screw profile, to yield OSA-lignin esters with ≥ 50% degree of modification. The product output was successfully scaled up sevenfold while using NIR spectroscopy to monitor the extrusion. In addition, the broader applicability of this inline monitoring method was demonstrated by using a biorefinery lignin from hardwood. The target degree of modification was achieved with minimal recalibration of process parameters. A space-time yield of up to 7 x 10⁴kg∙m^-3∙day^-1 was realized, indicating the potential of this REx process for industrial adoption. Overall, this work provides a foundation for the development of a process analytical technology for monitoring lignin modification during REx that can be expanded to other lignin chemistries, thus providing scalable and adaptable solutions for adding value to lignin.