Pulping and Bleaching PSE 476

1. Pulping and BleachingPSE 476 Lecture #5 Continuous Reactors

2. Continuous ReactorsAgenda • Last lecture • Kamyr Digester • Impregnation and Heating • Cooking • Washing • Batch versus Continuous

3. Chip Impregnation: Diffusion • This process involves the diffusion of ions through liquid present in the chip internals (free water, cell walls). • Occurs wherever water is present. • Diffusion is optimum in water saturated wood. • Diffusion is effective only over short distances. • Diffusion is effective across the grain. • There are only small differences between different woods. • Diffusion is affected by wood density. • Diffusion is strongly affected by liquor composition.

4. Diffusion

5. Basic Process Steps:Batch Digester (Kraft) 1. Digester filled with chips and cooking liquor. 2.Digester sealed, heating begins (cooking). a. Liquor is penetrating the chips (during filling and heating) b. Air and non condensable gasses vented as pressure builds. c. ~90 minutes to reach cooking temperature (~175°C) 4. Cooking temperature maintained for ~ 20 - 45 min a. Cooking proceeds till desired endpoint (kappa) 5. Contents of digester are discharged to blow tank Text

6. Pulping Mechanics: Typical Pulping Schedule: Batch cooking heating Pressure relief blowing gas relief

7. Basic Process Steps:Batch Digester (Kraft) 1. Digester filled with chips and cooking liquor. 2.Digester sealed, heating begins (cooking). a. Liquor is penetrating the chips (during filling and heating) b. Air and non condensable gasses vented as pressure builds. c. ~90 minutes to reach cooking temperature (~175°C) 4. Cooking temperature maintained for ~ 20 - 45 min a. Cooking proceeds till desired endpoint (kappa) 5. Contents of digester are discharged to blow tank Text

8. Basic Process Steps:Batch Digester (Kraft) 1. Digester filled with chips and cooking liquor. 2.Digester sealed, heating begins (cooking). a. Liquor is penetrating the chips (during filling and heating) b. Air and non condensable gasses vented as pressure builds. c. ~90 minutes to reach cooking temperature (~175°C) 4. Cooking temperature maintained for ~ 20 - 45 min a. Cooking proceeds till desired endpoint (kappa) 5. Contents of digester are discharged to blow tank Text

9. Basic Process Steps:Batch Digester (Kraft) 1. Digester filled with chips and cooking liquor. 2.Digester sealed, heating begins (cooking). a. Liquor is penetrating the chips (during filling and heating) b. Air and non condensable gasses vented as pressure builds. c. ~90 minutes to reach cooking temperature (~175°C) 4. Cooking temperature maintained for ~ 20 - 45 min a. Cooking proceeds till desired endpoint (kappa) 5. Contents of digester are discharged to blow tank Text

10. Continuous ReactorsChip and Liquor Charging • It is very important that the chips are added to continuous digesters in a consistent and accurate fashion. A chip meter is used to accomplish this. • This is a rotating wheel which meters chips. • A presteaming vessel is used to heat the chips and remove air and volatiles. • The chips are steamed for 2-3 minutes at 103-124 kPa. Chip Feeding System (blow the slide up to see better)

11. Continuous Reactors Notes Kamyr Digester: an example of a continuous digester

12. Kamyr Digester

13. Continuous Reactors Chips and Chips Steam Liquor Impregnation Chips Zone Silo Steam Liquor s n Cooking e e Zone r c S White Liquor High Liquor Pressure Washing Feeder Zone Blow Line

14. High Pressure Feeder • Because the digester is under pressure, a high pressure feeder is used to get the chips into the digester. • Chips are mixed with white liquor and fed through the top of the digester. • In kraft cooking, white liquor is the cooking liquor. It is mostly a combination of NaOH and NaHS. • Impregnation of the chips begins at this point. • Excess liquor must be used to convey the chips.

15. Chip and Liquor Charging • At the top of the digester, the chips and the liquor are added via a screw surrounded by strainer. • The excess liquor needed to convey the chips is removed at this point. • Chips form a plug which moves through down the digester. • Additional new white liquor is added to the chips at the top of the plug.

16. Impregnation Zone • The top zone in the digester is the impregnation zone. • It takes approximately 45 minutes to pass through this zone. • The temperature of this section is between 105°C to 130°C. Like in batch cooking, it is important that good impregnation is obtained before rapid delignification begins. • Most modern digesters have separate vessels for impregnation,

17. Heating Zone • Heating Zone • After liquor impregnation, the chips are rapidly heated to cooking temperature (160°C-170 ° C) • This is accomplished by removing the liquor in this section through screens, pumping the liquor through a heat exchanger, and returning the liquor to the digester. This is done twice in this section. • Heating is accomplished in a small section of the digester.

18. Cooking Zone • The chips are cooked for 1-2 hours @ 160°C - 170°C. • Unlike a batch digester, the liquor composition in a continuous digester is controllable. Liquor is removed through screens and new liquor pumped back into the system.

19. Washing Zone • The chips are cooled rapidly in the washing zone with fresh washing liquor to (130°C) using a countercurrent process to quench degrading reactions. • The liquor use to wash is the black liquor from cooking*. • Chemicals are removed through a diffusion controlled process. • Washing takes about 1.5 hours. • Pulp is washed at bottom with fresh water prior to removal. *Notes

20. Process simulationhttp://www.psl.bc.ca/equipment/digester/

24. Batch More Flexible: Grade Changes Fiber Source Maintenance Issues Less down time, production loss Production Flexibility Add another digester Continuous Lower Energy Costs Easier to control non-condensable gases: odor Lower environmental impact Compact (space efficient) Steady-state flow rate Batch versus Continuous