Pulping and Bleaching PSE 476/Chem E 471

1
Pulping and BleachingPSE 476/Chem E 471

• Lecture 15
• The Kraft Recovery Process

2
Agenda

• Overview of process
• Black Liquor
• Evaporation
• Recovery Furnace
• Smelt
• Causticizing
• Lime Kiln

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.

4
Black liquor separation
5
Basics of the Kraft Recovery System II
6
Black Liquor
Inorganics NaOH (7) Na2S (19) Na2CO3
(36) Na2SO3 (9) Na2SO4 (13) Na2S2O3 (16)
14-18 Solids pH 12 65 Organics 35 Inorganics
Notes
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.

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)

9
Evaporation III
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 Na2CO3 which will later be converted to
  NaOH

11
Recovery Furnace III
System for recovery of heat to generate power
(A)
Combustion Zone
(C)
Liquor
(B)
Reduction Zone
Gasses from Pyrolysis
(D)
Smelt (molten Na2CO3, Na2S, etc)
Notes
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
Reduction of Inorganics

• Formation of sodium carbonate (Na2CO3)
• Combustion of organics results in the formation
  of CO2 and CO. Sodium compounds are converted
  into Na2O.
• Na2O CO2 ? Na2CO3
• Na2S H2O CO2 ? Na2CO3 H2S
• Reductive reactions in the char results in the
  formation of sodium sulfide (Na2S)
• Na2SO4 2C ? Na2S 2CO2
• Na2SO4 4C ? Na2S 4CO

14
Smelt

• The molten inorganics at the bottom of the
  recovery furnace are known as the smelt. This
  material at 1500F 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), Na2S (20),
  Na2CO3 (60), Na2SO3 ( 3), Na2SO4 (6), Na2S2O3
  (3).

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 .

16
Slaker (Causticizing) Chemistry

• Calcium carbonate (CaCO3) has extremely low
  solubility in water while sodium carbonate
  (Na2CO3) has fairly high solubility. When lime
  (CaO) is added to a Na2CO3 solution, the
  following reaction occurs (important to note
  that CaO H2O ? Ca(OH)2 this is the slaking
  reaction).
• Ca(OH)2 Na2CO3 ? CaCO3 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
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 2100F).
• The calcium carbonate slurry is dried in the
  first section of the kiln and then the calcium
  carbonate is converted to CaO CaCO3 heat ? CaO
  CO2

18
Black versus green liquor