Pulping and Bleaching PSE 476 Lecture #5 Continuous Reactors Lecture #5 Continuous Reactors.

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Pulping and BleachingPSE 476

Lecture #5Continuous Reactors

Continuous ReactorsAgenda

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

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.

• 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.

DiffusionC

W oo d C om p o n en ts

D eg ra d a tio nP ro d u cts

Convec tion

D iffu sion

C a rb o h y d ra tes + resid u a l lig n in

D eg ra d a tio nP ro d u cts

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

5. Contents of digester are discharged to blow tankText

Pulping Mechanics: Typical Pulping Schedule: Batch

0 20 40 60 80 100

Time (minutes)

0 20 40 60 80 100

Time (minutes)

gas relief

cooking

Pressurerelief

blowing

heating

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.

• 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)

Continuous Reactors

Kamyr Digester: an example of a continuous digester

Kamyr Digester

Continuous Reactors

Chips and Liquor

Liquor

Cooking Zone

WashingZone

Blow Line

Impregnation

High PressureFeeder

ChipsSilo Zone

White Liquor

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.

• 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.

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.

• 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.

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,

• 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,

ExtractionLiquor Out

Pulp OutWash Water

WhiteLiquor

WashWater

WhiteLiquor

WL andChips In

Impregnation Zone

Counter-currentheating/washing(start of bulkdelignification)

Bulk Delignification

Counter-currentheating/washing(residualdelignification)

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.

• 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.

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.

• 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.

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.

• 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.

ExtractionLiquor Out

Pulp OutWash Water

WhiteLiquor

WashWater

WhiteLiquor

WL andChips In

Impregnation Zone

Counter-currentheating/washing(start of bulkdelignification)

Bulk Delignification

Counter-currentheating/washing(residualdelignification)

*Notes

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

5 .3 59 m

U p p er C o o k S creen s -8 5 l/sU p p er C o o k D isch a rg e - 8 5 l/s

9 5 0 t/D

L o w er C o o k D isch a rg e - 12 0 l/sL o w er C o o k S creen s - 1 2 0 l/s

Q u en ch D isch a rg e - 4 l/s

W a sh D isch a rg e - 2 5 l/s

E x tra ctio n S creen s - 4 5 l/s

W a sh N o zz les - 1 5 l/s

C h ip s - 2 6 .5 l/s W h ite L iq u o r - 4 0 l/s

B ro w n Sto ck

Ts1651581511441361291221151081019486797265

ChipTemperature

Cold BlowNozzles(15 l/s)

WashDischarge(25 l/s)

Upper CookDischarge(85 l/s)

QuenchDischarge(4 l/s)

ExtractionScreens(42 l/s)

LowerCookScreens(120 l/s)

UpperCookScreens(85 l/s)

Lower CookDischarge(120 l/s)

Blowline

Chips (26.5 l/s)Liquor (40 l/s)

Tf1651581511441361291221151081019486797265

LiquorTemperature

Blowline

Lign27.225.523.822.120.418.716.915.213.511.810.18.46.64.93.2

% Lignin

Blowline

Carb65.263.661.960.358.757.155.453.852.250.649.047.345.744.142.5

% Carbo

Blowline

200187175162150137125112100877562503725

KappaNumber

Blowline

u-us0.0100.0090.0070.0060.0040.0030.0010.000

-0.001-0.003-0.004-0.006-0.007-0.009-0.010

SlipVelocity

Blowline

Batch versus Continuous

Batch• More Flexible:

» Grade Changes» Fiber Source

• Maintenance Issues» Less down time,

production loss

• Production Flexibility» Add another digester

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

Continuous• Lower Energy Costs• Easier to control non-

condensable gases: odor» Lower environmental

impact

• Compact (space efficient)

• Steady-state flow rate

Pulping and Bleaching PSE 476 Lecture #5 Continuous Reactors Lecture #5 Continuous Reactors.

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