Process for continuous cooking of pulp

• US 6,605,180 B2
• Filed: 09/28/2001
• Issued: 08/12/2003
• Est. Priority Date: 11/03/2000
• Status: Expired due to Term

• Current Assignee: Metso Paper Sweden Aktiebolag
• Original Assignee: Kvaerner Pulping AB (Akastor ASA)
• Inventors: Snekkenes, Vidar, Gustavsson, Lennart, Olsson, Sven-Erik
• Primary Examiner(s): ALVO, MARC S
• Application Number:
  US09/967,346
• Publication Number:
  US 20020079070A1
• Time in Patent Office: 683 Days
• Field of Search: 162/60, 162/19, 162/17, 162/29, 162/30.1, 162/30.11, 162/47, 162/37, 162/39, 162/40
• US Class Current: 162/17
• CPC Class Codes: D21C 3/24 Continuous processes
• CPC Class Codes: D21C 9/04 in diffusers ; Washing of pulp of fluid consistency without substantially thickening

Abstract

Fiber material and cooking liquor are introduced at the top of the digester and pulp is discharged from the bottom of the digester, via a line in which the pulp is maintained at substantially the same pressure level, to a pressurized wash. 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 pressurized wash is regulated so that a high temperature is maintained in the wash filtrate. The extraction is regulated so that a net co-current flow is established at the bottom of the digester.

Claims

1. 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 140°

C. and 180°

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 20°

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).

2. The process according to claim 1 wherein the step of establishing at the net co-current flow segment comprises physically extending the net co-current flow segment from the bottom of the digester to an extraction screen disposed in the digester.

3. The process according to claim 1 wherein the method further comprises establishing a net co-current flow segment through essentially the entire digester.

4. The process according to claim 1 wherein the method further comprises establishing a net co-current flow that extends over at least 80% of a height of the digester.

5. The process according to claim 1 wherein the method further comprises establishing a short net co-current flow segment that extends over less than 80% of a height of the digester and the short net co-current flow segment is disposed at an upper part of the digester.

6. The process according to claim 5 wherein the method further comprises arranging the short net co-current flow segment within an upper two thirds of a height of the digester.

7. The process according to claim 1 wherein the method further comprises providing a net co-current flow with a sufficient flow to establish a net co-current flow of the liquid disposed at the bottom of the digester.

8. The process according to claim 1 wherein the method further comprises maintaining the mean cooking temperature in the cooking zone at a temperature between 150°

C. and 155°

C. when hard wood is used.

9. The process according to claim 1 wherein the method further comprises maintaining the mean cooking temperature in the cooking zone at a temperature between 150°

C. and 155°

C. when soft wood is used.

10. The process according to claim 1 wherein the method further comprises maintaining mean cooking temperature for a wood chip dwell time of at least 120 minutes.