Improving Lignin Recovery From Paper Mill and Biorefinery Waste Streams via Liquid-Phase Splitting

Lignin is an abundant biopolymer with significant promise due to its aromaticity. It has been targeted as a replacement for a number of petroleum-based products including adhesives, coatings, polyurethane foams, activated carbon, and carbon fibers. However, commercially available bulk lignins are too polydisperse, and contain too many residual metals from the pulping process that are detrimental to the properties of the final product. The Sequential Liquid-lignin Recovery and Purification (SLRP) process was developed by Michael Lake and John Blackburn, in collaboration with Clemson, with the intention of creating a continuous method for recovering lignin from paper-mill black liquors. Thies and co-workers have developed the Aqueous Lignin Purification with Hot Agents (ALPHA) process for taking bulk lignins (including lignin from the SLRP process) and purifying and fractionating that lignin based on molecular weight. The key advantage to both processes is that they can be carried out continuously and, therefore, can be scaled up to meet demand. This work starts with the development of a technique (under patent application) that links the liquid-phase recovery of lignin from black liquor (SLRP) to the simultaneous fractionation and purification of said lignin (ALPHA). This was accomplished by incorporating renewable organic solvents into the SLRP process, after precipitation of the liquid-lignin phase, which resulted in the formation of a second liquid–liquid equilibrium. This allowed for lignin to be continuously separated. This lignin was also shown to be an order of magnitude higher in purity than the SLRP process practiced alone. In addition to this, conditions for using the SLRP process on agricultural black liquors from a lignocellulosic biorefinery were outlined, demonstrating that SLRP can be used for a range of biomass feed streams and is not limited to Kraft black liquor from paper mills. The other byproduct from a biorefinery, lignin cake, was also investigated to see if useful lignins could be recovered from it using organic solvents. We found that high-purity lignins could be recovered in appreciable quantities, and these lignins had the potential to be converted into value-added products. Lastly, a redesign of the existing SLRP pilot plant at Clemson University was undertaken with the goal of converting the unit into a research facility, such that the effect of processing variables could be investigated in a cost-effective manner. The existing reactors were redesigned and refabricated, and a complete overhaul of the process controls was necessary to improve the automation and reliability of the pilot plant. These changes, as well as the design considerations and technical challenges encountered, are described