1. 1 Pulping and Bleaching PSE 476/Chem E 471 Lecture #15 The Kraft Recovery Process Lecture #15 The Kraft Recovery Process

2. 2 Agenda Overview of process Black Liquor Evaporation Recovery Furnace Smelt Causticizing Lime Kiln Overview of process Black Liquor Evaporation Recovery Furnace Smelt Causticizing Lime Kiln

3. 3 Basics of the Kraft Recovery System This diagram (taken from “Kraft Pulping: A Compilation of Notes” shows what will be covered in this lecture. The object of this system is to regenerate the cooking chemicals and to generate heat = steam & power. Cooking & Washing Evaporation & Burning Causticizing BLACK LIQUOR Alkali Lignin Hydrolysis Salts Sulfonation Products GREEN LIQUOR Na 2 CO 3 Na 2 S WHITE LIQUOR NaOH Na 2 S ChipsPulp CaCO 3 CaO Makeup Chemicals HeatWater

4. 4 Black liquor separation

5. 5 Basics of the Kraft Recovery System II

6. 6 Black Liquor Digester Chips NaOH Na 2 S Pulp Black Liquor 14-18% Solids pH 12+ 65% Organics 35% Inorganics Inorganics NaOH (7%) Na 2 S (19%) Na 2 CO 3 (36%) Na 2 SO 3 (9%) Na 2 SO 4 (13%) Na 2 S 2 O 3 (16%) Notes

7. 7 Evaporation I In order to burn the black liquor, the solids content needs to be raised to over 60%. »This means that over 80% of the water needs to be removed. This is accomplished through the use of multiple effect evaporators. In order to burn the black liquor, the solids content needs to be raised to over 60%. »This means that over 80% of the water needs to be removed. This is accomplished through the use of multiple effect evaporators.

8. 8 Evaporation II Evaporation is accomplished through: »Direst contact with flue gasses (old) »Indirect contact with steam -Falling film (new) -Raising film (old) With indirect contact, steam is applied countercurrent to the liquor ( high temperature/pressure steam is applied to thickest liquor). The steam produced from the liquor is used to heat the next effect (evaporator) at a lower temperature and pressure. This continues to the first effect (most dilute liquor) which is run under vacuum. (Please read the reading) Evaporation is accomplished through: »Direst contact with flue gasses (old) »Indirect contact with steam -Falling film (new) -Raising film (old) With indirect contact, steam is applied countercurrent to the liquor ( high temperature/pressure steam is applied to thickest liquor). The steam produced from the liquor is used to heat the next effect (evaporator) at a lower temperature and pressure. This continues to the first effect (most dilute liquor) which is run under vacuum. (Please read the reading)

9. 9 Evaporation III

10. 10 Recovery Furnace Concentrated black liquor is burned in a recovery furnace. The functions of the recovery furnace are: »Remove remaining water from black liquor. »Burn the black liquor organic compounds -Solves disposal issues. -Produces steam (for heating and energy). »Reduce sulfur species to sulfide. »Generate Na 2 CO 3 which will later be converted to NaOH

11. 11 Recovery Furnace III Smelt (molten Na 2 CO 3, Na 2 S, etc) Liquor Air System for recovery of heat to generate power Reduction Zone Combustion Zone Gasses from Pyrolysis (A) Notes (B) (C) (D)

12. 12 Recovery Furnace IV Liquor is sprayed to char bed (droplet size 2-3mm) Particles should be dry before landing the char bed Char is required in bed for effective reduction of sulfates

13. 13 Reduction of Inorganics Formation of sodium carbonate (Na 2 CO 3 ) »Combustion of organics results in the formation of CO 2 and CO. Sodium compounds are converted into Na 2 O. »Na 2 O + CO 2  Na 2 CO 3 »Na 2 S + H 2 O + CO 2  Na 2 CO 3 + H 2 S Reductive reactions in the char results in the formation of sodium sulfide (Na 2 S) »Na 2 SO 4 + 2C  Na 2 S + 2CO 2 »Na 2 SO 4 + 4C  Na 2 S + 4CO Formation of sodium carbonate (Na 2 CO 3 ) »Combustion of organics results in the formation of CO 2 and CO. Sodium compounds are converted into Na 2 O. »Na 2 O + CO 2  Na 2 CO 3 »Na 2 S + H 2 O + CO 2  Na 2 CO 3 + H 2 S Reductive reactions in the char results in the formation of sodium sulfide (Na 2 S) »Na 2 SO 4 + 2C  Na 2 S + 2CO 2 »Na 2 SO 4 + 4C  Na 2 S + 4CO

14. 14 Smelt The molten inorganics at the bottom of the recovery furnace are known as the smelt. This material at 1500°F leaves the furnace and is added to water in the smelt tank. »This results in the formation of green liquor »This is also a dangerous operation. The extremely hot smelt causes the water to instantly vaporize in a violent reaction. »Green liquor composition: NaOH (8%), Na 2 S (20%), Na 2 CO 3 (60%), Na 2 SO 3 ( 3%), Na 2 SO 4 (6%), Na 2 S 2 O 3 (3%). The molten inorganics at the bottom of the recovery furnace are known as the smelt. This material at 1500°F leaves the furnace and is added to water in the smelt tank. »This results in the formation of green liquor »This is also a dangerous operation. The extremely hot smelt causes the water to instantly vaporize in a violent reaction. »Green liquor composition: NaOH (8%), Na 2 S (20%), Na 2 CO 3 (60%), Na 2 SO 3 ( 3%), Na 2 SO 4 (6%), Na 2 S 2 O 3 (3%).

15. 15 Caustizing Caustizing is the process in which sodium carbonate is converted into sodium hydroxide. In the first step, green liquor is clarified to remove insoluble materials. »Calcium compounds »Unburned carbon »Metals Clarified green liquor is sent to a “slaker” where lime (CaO/Ca(OH) 2 ) is added. Caustizing is the process in which sodium carbonate is converted into sodium hydroxide. In the first step, green liquor is clarified to remove insoluble materials. »Calcium compounds »Unburned carbon »Metals Clarified green liquor is sent to a “slaker” where lime (CaO/Ca(OH) 2 ) is added.

16. 16 Slaker (Causticizing) Chemistry Calcium carbonate (CaCO 3 ) has extremely low solubility in water while sodium carbonate (Na 2 CO 3 ) has fairly high solubility. When lime (CaO) is added to a Na 2 CO 3 solution, the following reaction occurs: (important to note that CaO + H 2 O  Ca(OH) 2 this is the slaking reaction). »Ca(OH) 2 + Na 2 CO 3  CaCO 3 + 2 NaOH »The calcium carbonate precipitates out of solution driving the reaction to the right side of the above equation thus forming sodium hydroxide. »Calcium carbonate is removed through clarification forming white liquor. This step completes the circle as the white liquor is returned to the digester for pulping. Calcium carbonate (CaCO 3 ) has extremely low solubility in water while sodium carbonate (Na 2 CO 3 ) has fairly high solubility. When lime (CaO) is added to a Na 2 CO 3 solution, the following reaction occurs: (important to note that CaO + H 2 O  Ca(OH) 2 this is the slaking reaction). »Ca(OH) 2 + Na 2 CO 3  CaCO 3 + 2 NaOH »The calcium carbonate precipitates out of solution driving the reaction to the right side of the above equation thus forming sodium hydroxide. »Calcium carbonate is removed through clarification forming white liquor. This step completes the circle as the white liquor is returned to the digester for pulping.

17. 17 Lime Kiln The wet calcium carbonate slurry is treated in a lime kiln. »This is a very long, refractory brick lined, slightly tilted, rotating tube which is extremely hot (1500 to 2100°F). »The calcium carbonate slurry is dried in the first section of the kiln and then the calcium carbonate is converted to CaO: CaCO 3 + heat  CaO + CO 2 The wet calcium carbonate slurry is treated in a lime kiln. »This is a very long, refractory brick lined, slightly tilted, rotating tube which is extremely hot (1500 to 2100°F). »The calcium carbonate slurry is dried in the first section of the kiln and then the calcium carbonate is converted to CaO: CaCO 3 + heat  CaO + CO 2

18. 18 Black versus green liquor