Patent ID: 8734610

Claim:
A method to produce pulp from cellulosic material comprising: adding a liquid to a cellulosic material to form a slurry of the cellulosic material, wherein the liquid is at least one of water and a liquid extracted from an upper inlet of a first reactor vessel, and the liquid to the cellulosic material is substantially devoid of an acid; pumping the slurry of cellulosic material to the upper inlet in the first reactor vessel; hydrolyzing the cellulosic material in an upper region of the first reactor vessel by adding pressure and heat energy to the vessel, wherein the hydrolysis is conducted at a temperature of at least 150 degrees Celsius; extracting hydrolysate from the cellulosic material through an extraction screen below the upper region and in the first reactor vessel; introducing a wash liquid to a lower region of the first reactor vessel where the wash liquid suppresses hydrolysis of the cellulosic material in the lower region and said wash liquid flows upward through the cellulosic material to the extraction screen, wherein the wash liquid as introduced to the lower region has an amount of alkali chemicals of no more than five percent of the amount of cellulosic material in the first reactor vessel; discharging the cellulosic material from a lower outlet of the first reactor vessel; introducing the discharged cellulosic material to a second reactor vessel; introducing cooking liquor into an upper region of the second reactor vessel to digest the cellulosic material to produce pulp; extracting liquor from the second reactor vessel and introducing the extracted liquor into the lower region of the first reactor vessel, and monitoring the pH of the extracted liquor and increasing a flow of wash liquid to the lower region if the monitored pH is above a predetermined pH level.