Patent ID: 6605180
Filing Date: 2003-08-12
Classification: D21C

Abstract:
A process for a continuous cooking of wood chips at elevated pressure and temperature in a vertical digester for production of chemically dissolved pulp, comprising:providing the digester with a top and a bottom and a digester screen arrangement disposed between the top and the bottom; introducing the wood chips and a cooking liquor at the top of the digester to form a pulp slurry; maintaining a mean cooking temperature in a cooking zone at a temperature between 140Â° C. and 180Â° C.; extracting used cooking liquor from the cooking zone of the digester by using the digester screen arrangement; conveying a first quantity (Q1) of the extracted used cooking liquor to a recovery unit for liquor recovery; establishing a net co-current flow of liquid at the bottom of the digester; discharging the pulp slurry from the bottom of the digester into a line at a temperature that is not more than 20Â° C. below the cooking temperature in the cooking zone; feeding the pulp slurry in the line, at a pressure level that does not induce cooking of the pulp slurry, to a pressure diffuser; feeding a wash liquid into the pressure diffuser; withdrawing at least a portion of a used cooking liquor from the pulp slurry to an inside chamber defined in the pressure diffuser by using the wash liquid to displace the used cooking liquor in the pulp slurry to obtain a wash filtrate; in a buffer, extracting used cooking liquor from the wash filtrate and conveying a second quantity (Q2) of the used cooking liquor extracted from the wash filtrate to the recovery unit for liquor recovery, the first quantity (Q1) and the second quantity (Q2) representing a total quantity of used cooking liquor extracted from the process; conveying a first portion of the wash filtrate back to the bottom of the digester; and regulating a ratio of between the first quantity (Q1) and the second quantity (Q2) of the used cooking liquor so that (Q2)>(Q1).