An apparatus and process for 
solvent pulping of 
cellulose-containing 
biomass utilizes at least one 
steaming vessel, a plug screw feeder or 
compression screw device, at least one super-atmospheric impregnation vessel, a 
solvent delignification reactor capable of operating at a pressure of 350 psig or more, and a 
solvent containing line for introducing solvent-containing liquor at the plug screw feeder outlet or 
compression screw device outlet. The process and 
system can also include at least one series connected pressure diffuser and optionally a retention tube downstream of each pressure diffuser to provide sufficient 
retention time to substantially preclude re-deposition of 
lignin on the 
cellulose fibers of the 
biomass, a blow tank connected to the last of the pressure diffusers and retention tubes, and vessels for multistage 
alcohol washing. The method steams the 
biomass and impregnates it with solvent to produce an aqueous 
slurry of biomass and solvent, delignifies the particulate biomass in the 
slurry, removes solvent while continuing delignification of the biomass in the 
slurry and while substantially precluding re-deposition of 
lignin on the 
cellulose of the biomass, reduces the pressure of the slurry; and then washes the slurry.