liquid e g either simply water or a mixture of water and chemicals preferably has a temperature of 10 u00b0 c to 70 u00b0 c cooler than the hydrolysis temperature more preferably 20 u00b0 c to 50 u00b0 c cooler and most preferably 25 u00b0 c to 35 u00b0 c cooler the ph of the wash liquid is preferably 3 to 7 and more preferably 4 to 5 the wash liquid is supplied to upper elevations of the vessel such as zone b from a wash liquid source 36 and by recirculating liquor extracted from the wash extraction screen 33 the wash liquid and the recirculating liquor are sufficient to create an upward flow of fluids through zone b to the upper extraction screen 26 preferably most of the washing of the cellulosic material occurs in zone b the wash liquid in source 36 may be simply wash water or a combination of wash water and chemicals such as one or more of sodium hydroxide naoh and essentially sulfur free white liquor preferably essentially sulfur free white liquor has no more than 0 10 parts per million ppm of sulfur compounds for example the amount of chemicals added to the wash water may be 0 01 to 5 preferably 0 1 to 1 of the amount of cellulosic material e g wood in the slurry flowing through the vessel the chemicals are provided from a chemical source 53 and flow through pipe 57 to mix with wash water 34.