Method and apparatus for digesting cellulose material without screening

Abstract

Apparatus and method for digestion of cellulose material in a continuous digestion process, that do not require screens in the outlet of the digesting zone. Cellulose chips and digesting liquid at relatively low temperature are introduced into a relatively long and thin cylindrical impregnation vessel, having a first diameter. The chips from the impregnation vessel pass into a digestion vessel having a second diameter substantially greater than the first diameter, the outlet for the impregnation zone into the digestion vessel being disposed well into the digestion vessel and below an outlet(s) disposed in the top of the digestion vessel, and form a chips column in the digestion vessel. Digesting liquid at a relatively high temperature is introduced into contact with the chips, at a point a significant distance below the impregnation vessel outlet, and the quantity of digesting liquid being introduced and the first and second diameters are so arranged that the velocity of liquid flowing to the digestion vessel top outlet is substantially less than the velocity of liquid that would entrain significant numbers of particles therein, whereby a clarifying section is provided between the chips column and digestion vessel outlet so that the need for screens at the outlet is eliminated. A pair of vertically arranged screens which are alternately connected to a suction source may be provided in the clarifying section between the top of the chips column and the outlet to remove any particles that might be entrained in the liquid therein from the liquid.