A reactor vessel system including: a first reactor vessel having a hydrolysate and liquid extraction screen, a first region above the extraction screen that is maintained at conditions promoting a hydrolysis reaction in the cellulosic material, a second region below the extraction screen in which the hydrolysis is substantially suppressed and a wash liquid inlet below the extraction screen providing wash liquid at a temperature below a hydrolysis temperature; a transport pipe having an inlet coupled to the first reactor vessel and an outlet coupled to a second reactor vessel, and the second reactor vessel includes a liquid discharge that extracts a portion of liquid from the second reactor vessel and directs the portion of liquid to the first reactor vessel or to the transport pipe.