Methods and systems for processing lignin during hydrothermal digestion of cellulosic biomass solids

A method comprising:

• providing cellulosic biomass solids in a digestion solvent to a hydrothermal digestion unit;
• heating the cellulosic biomass solids and the digestion solvent in the presence of molecular hydrogen and a slurry catalyst capable of activating molecular hydrogen, thereby forming a phenolics liquid phase comprising lignin, an aqueous phase comprising an alcoholic component derived from the cellulosic biomass solids, and an optional light organics phase, in the hydrothermal digestion unit, wherein the slurry catalyst being at least partially distributed in the cellulosic biomass solids using upwardly directed fluid flow and at least a portion of the slurry catalyst accumulating in the phenolics liquid phase as it forms;
• forming a combined phase comprising at least a portion of the phenolics liquid phase and at least a portion of the aqueous phase by combining at least the phenolics liquid phase and the aqueous phase with one another;
• upwardly circulating at least a portion of the combined phase through the cellulosic biomass solids in the hydrothermal digestion unit to provide at least a portion of the upwardly directed fluid flow;
• and
• separating at least a portion of the alcoholic component from at least a portion of the combined phase external to the hydrothermal digestion unit.