(526c) Sustainable Biolubricant Production from Lignin and Waste Cooking Oil

Lignin accounts for 15-30% of lignocellulosic biomass weight and 40% of the biomass heat energy. Today, lignin from lignocellulosic biorefineries is underutilized because of the absence of a high-value product from wet unhydrolyzed solids (UHS), a waste stream that has very little value. Finding ways to valorize this byproduct is crucial to enhance the economic viability and sustainability of biofuels production.

In this work, a process is developed to transform lignin, or UHS, into a sustainable biolubricant. The development of sustainable and environmentally friendly lubricants has in fact become an important challenge, and their market has increased significantly in recent years. Fatty acids and their corresponding methyl esters (or FAMEs) are good candidate as biolubricants, however those derived from plant oils do not have desirable oxidative stability due to the presence of C=C bonds that make them susceptible to autooxidation. The stability can be improved by chemically modifying the fatty acid molecule, utilizing the C=C bond’s reactivity to create linkages or new molecules with a branched structure.

The proposed process consists of three steps: (1) Hydrothermal liquefaction (HTL) of lignin (UHS), to depolymerize it into monomeric phenolic compounds, collected into a bio-oil; (2) Hydrodeoxygenation (HDO) of the bio-oil to replace the hydroxyl group of phenolic compounds with hydrogen, and produce aromatic hydrocarbons; (3) alkylation of unsaturated FAMEs (in particular from transesterification of yellow grease) with the aromatic hydrocarbons obtained from lignin degradation.

HTL was conducted at temperatures between 280-320°C, and reaction times between 1 - 15 min using an extremely efficient induction heating system that allowed reaching the reaction temperature within 2-3 min. The optimal operating conditions that allowed to maximize the bio-oil yield were found to be 320 °C at 1 min residence time, and the oil characterization revealed that in these conditions the selectivity of the phenol was highest at 52% of relative peak area with GCMS analysis. Furthermore, the fast HTL was found to increase the bio-oil yield and phenol content in it significantly compared to those conventional HTL which take pre-heating time of 60 minutes.

Hydrodeoxygenation was performed at 250°C and 600 psi of hydrogen gas. It was hypothesized that the polarity of the solvent affects the aromatic selectivity of the hydrodeoxygenation process. An alkane (i.e., n-hexadecane) and water were selected as nonpolar and polar solvent, respectively. Based on the experimental results using p-cresol as a model compound, aromatic selectivity was significantly higher when hexadecane was used as a solvent. Thus, hexadecane was used in the HDO of biooil obtained from HTL step. Alkylation of the aromatic hydrocarbons with unsaturated FAMEs was conducted using an acid-treated montmorillonite catalyst at 210°C for 4 hours under argon gas.

Finally, a techno-economic analysis was conducted based on the experimental data collected, to verify the scalability and economic viability of the proposed process when integrated in an already existing lignocellulosic biorefinery.