(538b) Flocculant-Aided Solid-Liquid Separation of Biomass Slurries

Authors: 
Stickel, J. J., National Renewable Energy Laboratory
Crawford, N. C., National Renewable Energy Laboratory
Lischeske, J. J., National Renewable Energy Laboratory
Patton, C. M., National Bioenergy Center
Sievers, D. A., National Renewable Energy Laboratory

As biochemical conversion strategies transition away from biomass-to-ethanol pathways and into bio-based hydrocarbon fuels, the removal of residual biomass solids after saccharification (pretreatment and enzymatic hydrolysis) has become imperative. The residual solids are known to inhibit the growth and utility of fermentation media, limit oxygen transfer during aerobic fermentation, poison catalysts, and disrupt flow pathways within catalyst beds. In addition, continuous microorganism-driven sugar-to-lipid conversion often necessitates a solids-free, highly concentrated syrup-like sugar stream (for example, with Lipomyces starkeyi). As a result, removal of insoluble solids from hydrolyzate slurries is required prior to downstream purification, concentration, and upgrading. However, these solids are commonly deformable, porous, and asymmetric, have a density similar to water, and have a wide particle size distribution, making them extremely difficult to remove from solution.

Here we utilize polyelectrolyte-flocculating agents to enhance the solid-liquid separation of enzymatically-hydrolyzed (EH) corn stover slurries. The EH corn stover slurries are a two-phase system with 5–10 wt% insoluble solids (typically rich in lignin) suspended in an aqueous phase at pH 4–5. The undigested corn stover particles (23 μm mean diameter) displayed a negative zeta potential over a wide pH range (1–13), and were easily flocculated with low doses of polymeric cationic flocculants. Initial studies evaluated flocculant dosing and separation effectiveness using centrifugation. However, economics for a centrifugation process are not expected to be favorable. Therefore, the focus of this work was vacuum-driven bulk filtration aided by flocculation. Without flocculation, the solids form a compressible cake with low permeability. However, adding low doses of a cationic polyacrylamide flocculant generated large flocs (1 mm target size), which formed highly permeable cakes under low applied differential pressures. The use of flocculation increased the overall capacity of vacuum filtration (for the slurries tested) by ~50-fold. Unfortunately, this increase in filtration capacity came at the expense of increased wash water, where washing is used to remove the residual sugar retained in the solids cake bed.