Patent ID: 7285179
Filing Date: 2007-10-23
Classification: D21C

Abstract:
1. A process for operating a continuous digester for manufacturing chemical pulp, comprising: providing the digester with a first treatment zone having a first liquid-to-wood ratio, a subsequent second treatment zone having a second liquid-to-wood ratio and a third treatment zone being subsequent to the second treatment zone, the third treatment zone having a third liquid-to-wood ratio, the first liquid-to-wood ratio being greater than the second liquid-to-wood ratio, the second liquid-to-wood ratio being greater than the third liquid-to-wood ratio, the first treatment zone being above the second treatment zone, the second treatment zone being above the third treatment zone in the digester; providing the digester with a first extraction position between the first and second treatment zone and a second extraction position between the second and third treatment zone, the first extraction position being above the second extraction position; feeding a mixture of chips and cooking liquid to a top of the digester; passing the chips through the first treatment zone; extracting a first portion of the cooking liquid at the first extraction position and re-circulating the first portion to a reintroduction position in the digester above the first treatment zone to maintain the first liquid-to-wood ratio in the first treatment zone; passing the chips through the second treatment zone; extracting a second portion of the cooking liquid at the second extraction position and re-circulating the second portion to the reintroduction position to maintain the second liquid-to-wood ratio in the second treatment zone; passing the chips through the third treatment zone; mixing the first and second portions of cooking liquid with the chips at the top of the digester before the first treatment zone; the dwell time for the chips in the treatment zones lies within the interval 10-120 minutes between the extraction positions, and dissolving the chips to pulp in treatment zones of the digester and feeding out pulp from a bottom of the digester.