Transcript No Slide Title

Pulping and Bleaching
PSE 476
Lecture #5
Continuous Reactors
Continuous Reactors
Agenda
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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.
Chips Liquor
Diffusion
-
HS
OH
-
OH
OH
I
Diffusion
-
HS
Wood Components
OH
Convection
-
HS
-
Degradation
Products
HS
HS
-
-
Degradation
Products
-
OH
Degradation
Products
Wood Components
II
Carbohydrates +
residual lignin
III
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
Pulping Mechanics:
Typical Pulping Schedule: Batch
170
cooking
heating
150
Pressure
relief
130
110
gas relief
90
blowing
Time (minutes)
22
0
20
0
18
0
16
0
14
0
12
0
10
0
80
60
40
20
70
0
Temperature (°C)
190
Basic Process Steps:
Batch Digester (Kraft)
1. Digester filled with chips and cooking liquor.
2. Digester sealed, heating begins (cooking).
Text
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
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.
Text
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
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.
Text 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
Continuous Reactors
Chip 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)
Continuous Reactors
Notes
Kamyr Digester: an example of a continuous digester
Kamyr Digester
Continuous Reactors
Chips and
Liquor
Chips
Steam
Impregnation
Zone
Chips
Silo
Steam
Screens
Liquor
White Liquor
High
Pressure
Feeder
Cooking
Zone
Liquor
Washing
Zone
Blow Line
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.
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.
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,
W L and
C hips In
Impr e gnat ion Zone
C ounte r -c ur r e nt
he ating/w ashing
(star t of bulk
de lignific ation)
Ext r ac tion
Liquor O ut
W hit e
Liquor
B ulk
D e lignific at ion
W ash
W ate r
C ounte r -c ur r e nt
he ating/w ashing
(r e sidual
de lignific ation)
W hit e
Liquor
W ash W at e r
P ulp O ut
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.
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.
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
W L and
C hips In
Impr e gnat ion Zone
C ounte r -c ur r e nt
he ating/w ashing
(star t of bulk
de lignific ation)
Ext r ac tion
Liquor O ut
W hit e
Liquor
B ulk
D e lignific at ion
W ash
W ate r
C ounte r -c ur r e nt
he ating/w ashing
(r e sidual
de lignific ation)
W hit e
Liquor
W ash W at e r
P ulp O ut
Process simulation
http://www.psl.bc.ca/equipment/digester/
Chips - 26.5 l/s
White Liquor - 40 l/s
Wash Discharge - 25 l/s
Wash Nozzles - 15 l/s
Impregnation
Zone
Wash
Zone
Quench Discharge - 4 l/s
Extraction Screens - 45 l/s
Cooking
Zone
Upper Cook Discharge - 85 l/s
Upper Cook Screens -85 l/s
Lower Cook Discharge - 120 l/s
Lower Cook Screens - 120 l/s
0.44 m
54.4 m
950 t/D
5.359 m
Brown Stock
Chips (26.5 l/s)
Liquor (40 l/s)
Chips (26.5 l/s)
Liquor (40 l/s)
Liquor
Temperature
Chip
Temperature
Upper Cook
Discharge
(85 l/s)
Upper
Cook
Screens
(85 l/s)
Lower Cook
Discharge
(120 l/s)
Lower Cook
Discharge
(120 l/s)
Ts
165
158
151
144
136
129
122
115
108
101
94
86
79
72
65
Lower
Cook
Screens
(120 l/s)
Quench
Discharge
(4 l/s)
Wash
Discharge
(25 l/s)
Upper Cook
Discharge
(85 l/s)
Upper
Cook
Screens
(85 l/s)
Extraction
Screens
(42 l/s)
Cold Blow
Nozzles
(15 l/s)
Blowline
Tf
165
158
151
144
136
129
122
115
108
101
94
86
79
72
65
Lower
Cook
Screens
(120 l/s)
Quench
Discharge
(4 l/s)
Wash
Discharge
(25 l/s)
Extraction
Screens
(42 l/s)
Cold Blow
Nozzles
(15 l/s)
Blowline
Chips (26.5 l/s)
Liquor (40 l/s)
Chips (26.5 l/s)
Liquor (40 l/s)
% Lignin
Upper Cook
Discharge
(85 l/s)
Upper
Cook
Screens
(85 l/s)
Wash
Discharge
(25 l/s)
Upper Cook
Discharge
(85 l/s)
Upper
Cook
Screens
(85 l/s)
Lower Cook
Discharge
(120 l/s)
Lower Cook
Discharge
(120 l/s)
Lign
27.2
25.5
23.8
22.1
20.4
18.7
16.9
15.2
13.5 Quench
11.8 Discharge
10.1 (4 l/s)
8.4
6.6
4.9
3.2
% Carbo
Lower
Cook
Screens
(120 l/s)
Extraction
Screens
(42 l/s)
Cold Blow
Nozzles
(15 l/s)
Blowline
Carb
65.2
63.6
61.9
60.3
58.7
57.1
55.4
53.8
52.2
50.6
49.0
47.3
45.7
44.1
42.5
Lower
Cook
Screens
(120 l/s)
Quench
Discharge
(4 l/s)
Wash
Discharge
(25 l/s)
Extraction
Screens
(42 l/s)
Cold Blow
Nozzles
(15 l/s)
Blowline
Chips (26.5 l/s)
Liquor (40 l/s)
Chips (26.5 l/s)
Liquor (40 l/s)
Kappa
Number
Upper Cook
Discharge
(85 l/s)
Upper
Cook
Screens
(85 l/s)
Wash
Discharge
(25 l/s)
Upper Cook
Discharge
(85 l/s)
Upper
Cook
Screens
(85 l/s)
Lower Cook
Discharge
(120 l/s)
Lower Cook
Discharge
(120 l/s)
200
187
175
162
150
137
125
112
100 Quench
87 Discharge
75 (4 l/s)
62
50
37
25
Slip
Velocity
Lower
Cook
Screens
(120 l/s)
Extraction
Screens
(42 l/s)
Cold Blow
Nozzles
(15 l/s)
Blowline
u-us
0.010
0.009
0.007
0.006
0.004
0.003
0.001
0.000
-0.001
-0.003 Quench
Discharge
-0.004 (4 l/s)
-0.006
-0.007
-0.009
-0.010
Wash
Discharge
(25 l/s)
Lower
Cook
Screens
(120 l/s)
Extraction
Screens
(42 l/s)
Cold Blow
Nozzles
(15 l/s)
Blowline
Batch versus Continuous
Batch
Continuous
• More Flexible:
• Lower Energy Costs
• Easier to control noncondensable gases: odor
» Grade Changes
» Fiber Source
• Maintenance Issues
» Less down time,
production loss
• Production Flexibility
» Add another digester
» Lower environmental
impact
• Compact (space
efficient)
• Steady-state flow rate