NOVELTY - Processing cellulosic biomass solids, comprises: providing a biomass conversion system (100); providing a cellulosic biomass in hydrothermal digestion unit (102); at least partially digesting cellulosic biomass in the digestion unit to form a hydrolysate comprising soluble carbohydrates and cellulosic fines within a liquor phase; flowing the liquor phase through solids separation unit (110); after removing at least a portion of the cellulosic fines, flowing the liquor phase to the catalytic reduction reactor unit and forming a reaction product; and recirculating portion of product.    USE - The method is useful for processing cellulosic biomass to produce a hydrolysate, where the biomass sources are e.g. corn stover, straw, bagasse, Miscanthus, Sorghum residue, switch grass, bamboo, water hyacinth, hardwood chips, hardwood pulp, and softwood.    ADVANTAGE - The method is cost effective and energy-efficient.    DETAILED DESCRIPTION - Processing cellulosic biomass solids comprises: providing a biomass conversion system (100) that comprises a fluid circulation loop (120) comprising a hydrothermal digestion unit (102), a solids separation unit (110) that is in fluid communication with an outlet of the hydrothermal digestion unit, where the solids separation unit comprises a centripetal force-based separation mechanism that comprises a fluid outlet and a solids outlet, and a catalytic reduction reactor unit i.e. in fluid communication with the fluid outlet of the centripetal force-based separation mechanism and an inlet of the hydrothermal digestion unit; providing a cellulosic biomass in the hydrothermal digestion unit; at least partially digesting the cellulosic biomass in the hydrothermal digestion unit to form a hydrolysate comprising soluble carbohydrates and cellulosic fines within a liquor phase; flowing the liquor phase through the solids separation unit to remove at least a portion of the cellulosic fines; after removing at least a portion of the cellulosic fines, flowing the liquor phase to the catalytic reduction reactor unit and forming a reaction product in the catalytic reduction reactor unit; and recirculating the portion of the reaction product to the hydrothermal digestion unit. An INDEPENDENT CLAIM is included for a biomass conversion system comprising: a fluid circulation loop comprising a hydrothermal digestion unit and a solids separation unit that is in fluid communication with an outlet of the hydrothermal digestion unit, where the solids separation unit comprises a centripetal force-based separation mechanism that comprises a fluid outlet and a solids outlet, and a catalytic reduction reactor unit that is in fluid communication with the fluid outlet of the centripetal force-based separation mechanism and an inlet of the hydrothermal digestion unit.    DESCRIPTION OF DRAWING(S) - The figure shows a schematic view of a biomass conversion system.    Biomass conversion system (100)    Hydrothermal digestion unit (102)    Solids separation unit (110)    Hydroclone (112)    Fluid circulation loop. (120)