NOVELTY - Treating biomass involves treating biomass with biomass swelling agent within vessel to swell/rupture at least a portion of biomass; removing at least a portion of agent from first end of vessel while leaving majority of treated biomass within vessel and contacting treated biomass with steam passed into second end of vessel to further remove agent from vessel, where during the contact of steam and treated biomass, the vessel is at bed angle of not greater than 30 degrees from vertical; and the contact of steam/treated biomass in the vessel is carried out to provide Archimedes number of at least 4.    USE - For treating biomass (claimed), such as cellulosic biomass used as fuel or for industrial production.    ADVANTAGE - The process is highly effective and highly efficient in recovery and reuse of the swelling agent. The swelling agent can be recovered in a relatively pure form such that downstream processing and/or recovery of the swelling agent for recycle is minimal. The biomass is treated with the swelling agent in a manner that swells or ruptures the biomass, leaving a biomass that can be more easily digested by enzyme action, such as, the biomass can be treated with swelling agent to increase enzyme digestibility by at least 10% relative to that initially supplied to the vessel. Treating the biomass with the swelling agent increases the chemical and biological reactivity of biomass for subsequent processing. The process represents a significant improvement over known treatment processes by effectively recovering the swelling agent directly from the treatment vessel and recycling the swelling agent for reuse. The swelling agent is recovered in substantially pure vapor form, i.e., little to no moisture or water content, when steam is used as a driving force to evacuate the treatment vessel of residual swelling agent following treatment of the biomass with the swelling agent to swell and/or rupture at least a portion of the biomass.    DETAILED DESCRIPTION - Treating biomass involves treating the biomass with biomass swelling agent within a vessel to swell or rupture at least a portion of the biomass; removing at least a portion of the swelling agent from a first end of the vessel, while leaving at least a majority of the treated biomass within the vessel; and contacting the treated biomass left within the vessel with steam introduced into a second end of the vessel different from the first end to further remove swelling agent from the vessel, where during the contact of the steam and the treated biomass in the vessel, the vessel is at a bed angle of not greater than 30 degrees from vertical; and the contact of the steam and the treated biomass in the vessel is carried out to provide an Archimedes number of at least 4.