Kontinuerlig kokning av massa med nettomedströmsflöde i kokarens bottenparti

摘要

The invention relates to a process for continuous cooking of wood chips at elevated pressure and temperature in a vertical digester (1) for production of chemically dissolved pulp. Fibre material and cooking liquor are introduced at the top of the digester and pulp is discharged from the bottom of the digester (1), via a line (20) in which the pulp is maintained at substantially the same pressure level, to a pressurized wash (7). More than 50% of the used cooking liquor (black liquor) which is extracted from the system in total is extracted from the wash filtrate of the pressurized wash. At the same time a small portion of the wash filtrate is also to be recirculated to the bottom of the digester as dilution liquid. The extraction is regulated so that a net co-current flow is established at the bottom of the digester. The invention permits increased production in overloaded digesters where substantial extractions of cooking liquor from the digester are made difficult by the increased pulp speed down through the digester.