Process for the continuous digestion of cellulosic fiber material

Claim 1

A process for the continuous digestion of cellulosic fiber material using a closed impregnation system having a concurrent flow zone followed by a countercurrent flow zone, and a digester system, comprising the steps of impregnating the fiber material with liquid in the closed impregnation system, withdrawing a predetermined amount of liquid from the impregnation system at a point located between the concurrent flow zone and the countercurrent flow zone, and supplying to the concurrent flow zone black liquid, and to the countercurrent flow zone white liquor and a predetermined amount of black liquor; wherein the liquids withdrawn from the impregnation system and the digester system are transferred to at least two serially connected flash cyclones for heat recovery, the black liquor supplied to the countercurrent flow zone of the impregnation system comprising substantially the effluent from the first one of the flash cyclones which receives liquid withdrawn from the digester between a digester concurrent flow zone and a digester countercurrent flow zone, and the black liquor supplied to the concurrent flow zone of the impregnation system consisting of the effluent from a subsequent flash cyclone which receives liquid withdrawn from the digester via said first flash cyclone and liquid withdrawn from the impregnation system.