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 application_number US 201414278579 A description in a two reactor vessel system steam is introduced to the top of both vessels for heating and pressurizing purposes hydrolysis occurs above extraction screens in the top of the first reactor vessel the extraction screens in the first reactor vessel remove hydrolysate as the wood chips or other cellulosic or fiber material collectively referred to cellulosic material introduced at the top of the first vessel progress through the vessel and to a lower extraction port of that vessel the cellulosic material is washed in the first reactor vessel below the extraction screens wash liquid is introduced at the bottom of the first reactor vessel and flows upwards to the extraction screens the wash liquid may be water only or water mixed with one or more chemicals such as sodium hydroxide naoh and essentially sulfur free white liquor the diameter of the first vessel may be uniform above and below the extraction screen the cellulosic material discharged from the extraction port of the.