Pulping lignocellulose in continuous pressurized batch digesters

Claim 1

The process for the batch production of cellulosic pulps by the chemical digestion of pieces of lignocellulose which comprises: (a) providing a pressurizeable batch reaction vessel defining a vertical path along which the lignocellulose progresses from top to bottom, (b) pressurizing the vessel with steam to substantially the predetermined digestion pressure and temperature, (c) while maintaining the vessel at substantially digestion pressure and temperature charging the vessel with flows of presteamed lignocellulose pieces and both hot fresh and hot spent digestion liquors for a predetermined charging period, (d) the fresh liquor flow containing digestion chemicals in the predetermined amount being charged to the vessel at a substantially constant rate commensurate with the rate of charging the lignocellulose pieces, (e) the spent liquor flow being charged to the vessel at a variable rate sufficient to maintain during the charging period a substantially constant rate of liquor rise within the vessel and to keep the lignocellulose pieces substantially submerged in the liquor, thereby compensating for variations in water extraneously introduced into the digester from steam condensation and lignocellulose water content, (f) digesting the lignocellulose in the vessel for the predetermined digestion period at the digestion pressure and temperature, (g) when the lignocellulose in the lower part of the digester has been digested for said digestion period, continuously withdrawing the resulting pulp from the bottom of the vessel over a withdrawing period substantially equal to the vessel-charging period, thereby insuring subjection of the top and bottom portions of the lignocellulose charge to equal durations of pulping treatment within the vessel, and (h) while withdrawing the pulp from the vessel, diffusing cold spent digestion liquor from a stock thereof into a plurality of vertically and radially spaced diffusion zones in the bottom of the vessel at a diffusion rate predetermined to pass the displacement liquor radially through the zones during a time interval substantially equal to the time required for vertical travel of the pulp through the vertical zones.