Fractionating and Purifying Softwood Kraft Lignin with Aqueous Renewable Solvents: Liquid–Liquid Equilibrium for the Lignin–Ethanol–Water System

Abstract

Hot ethanol–water solutions can be used to simultaneously fractionate and purify softwood Kraft lignin through the Aqueous Lignin Purification with Hot Agents (ALPHA) process, using the regions of liquid–liquid equilibrium (LLE) that form at selected temperatures and solvent‐to‐lignin feed (S/F) ratios. Lignin, ethanol, and water compositions are measured for the solvent‐rich (SR) and lignin‐rich (LR) liquid phases in mutual equilibrium, as well as the lignin and metals mass distributions between the two phases. As depicted in quasi‐ternary diagrams for clarity, both temperature and S/F ratio can be used to grow, merge, and even split the regions of LLE, giving significant control over both molecular weight (MW) and lignin purity. For example, a solvent comprising 45:55 EtOH/H2O at 75 °C and an S/F ratio of 6:1 enables recovery of an ultrapure (95 % of Na removed), higher MW (Mn=8400 Da) lignin fraction in the LR phase. On the other hand, 95:5 EtOH/H2O at 45 °C and S/F=3:1 enables recovery of an ultrapure, low MW (1500 Da) lignin in the SR phase.

Techno-Economic Analysis and Life Cycle Assessment of Waste Lignin Fractionation and Valorization Using the ALPHA Process

“…Thus, we incorporated those ALPHA processing conditions into our process model. Recently, LLE for lignin with ethanol–water solutions was measured and was found to exhibit phase behavior analogous to that for acetic acid–water solutions. However, applications research with the latter ALPHA solvent system has only recently been initiated, so results are not yet available.…”

“…To address the challenges of lignin quality, Thies and co-workers , have developed a novel separations process, referred to as Aqueous Lignin Purification with Hot Agents (ALPHA; originally referred to as “Hot Acids” , ). The ALPHA process exploits the novel liquid–liquid equilibrium that exists between lignin and hot, one-phase solutions of aqueous renewable solvents to simultaneously purify and fractionate raw bulk lignin.…”

“…For example, applying ALPHA to Kraft lignin via a concentrated acetic acid–water solution has been shown to isolate a high MW (30000 Da) lignin fraction that can be processed into carbon fibers with dramatically improved properties . The contact of raw bulk lignin with a dilute ethanol–water solution has been used to isolate a low MW fraction suitable as a replacement for polyol in polyurethane foams. Recent work , indicates that the remaining, medium-MW bulk lignin can be converted to activated carbon.…”

“…The solvent recovery is modeled using a distillation column with the constraints of ≥99% solvent recovery at ≥95% purity. It is important to note that highly pure solvent is not desired as the distillate product, as our work 12,13 has shown that water significantly enhances the solvent power of the organic solvent for lignin.…”

Liquefying Lignins: Determining Phase-Transition Temperatures in the Presence of Aqueous Organic Solvents

“…A wide range of solvents, including alcohols, ketones, ethers, and alkanes, exhibit this conventional solid–liquid phase behavior and have been used to practice sequential lignin fractionation. − In contrast, the formation of a liquid-lignin phase is unusual, and the temperatures at which the system undergoes a transformation from a solid–liquid equilibrium to liquid–liquid equilibrium, as a function of solvent composition, have been reported only for AcOH–water solutions . Preliminary estimates of a few phase-transition temperatures have been derived from measured acetone–water and ethanol–water isotherms with lignin. , Of equal importance is the fact that these solid–liquid to liquid–liquid equilibrium (SL–LL) phase transitions have been investigated for only one type of lignin, namely, a softwood Kraft lignin.…”