Transcript Pulping and Bleaching PSE 476/Chem E 471

Pulping
and
Bleaching
Pulping and Bleaching
PSE 476/Chem E 471
Lecture #11
Kraft Pulping Kinetics
PSE 476: Lecture 11
1
Pulping
and
Bleaching
Agenda
Effect of Variables on Kraft Pulping
» Wood Species
» Effective Alkali
» Sulfidity
• Kraft Cooking Models
PSE 476: Lecture 11
2
Pulping
and
Bleaching
Kinetics?
• It is important as a pulping engineer to understand
how to produce pulp with the desired properties in
the most economical fashion.
• In order to do this, the kinetics of the pulping
reactions need to be understood.
• Pulping variables:
» Wood: species, source, etc (discussed previously)
» Chemical charge (EA, sulfidity, liquor/wood, etc)
» Temperature
PSE 476: Lecture 11
3
Kraft Pulping:
Pulping
and
Bleaching
Effect of Wood Species
200
100
150
80
Aspen
Pine
60
100
40
50
20
0
Temperature (C)
Lignin Loss (%)
120
0
0
Notes
2
4
6
8
Time (Hours)
PSE 476: Lecture 11
4
Pulping
and
Bleaching
Effect of Effective Alkali on
Kraft Pulping (1)
• Simple story: if there is enough alkali present (with a
certain level of sulfidity), pulp can be made.
• Increasing EA will increase rate and degree of
delignification.
• Two ways of changing EA charge:
» Increasing concentration of EA in white liquor
» Increasing volume of White Liquor charged to digester
• Increasing EA using either method will increase the rate of
delignification and change pulp properties.
» Concentration change may have a larger effect than White
Liquor volume.
Notes
PSE 476: Lecture 11
5
Pulping
and
Bleaching
Lignin Removal:
Effect of Effective Alkali (2)
200
100
150
80
60
100
40
15.8% EA
50
25% EA
20
Temperature (C)
Lignin Yield (%)
120
Temperature
0
0
50
100
150
200
250
0
300
Time (minutes)
PSE 476: Lecture 11
6
Lignin in Pulp (%)
Pulping
and
Bleaching
Lignin Removal:
Effect of Effective Alkali (3)
Higher levels of EA
results in more lignin
removal but also lower
screened pulp yield.
16
14
12
10
8
6
4
2
0
8
12
16
20
24
Initial Effective Alkali %
PSE 476: Lecture 11
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Pulping
and
Bleaching
Lignin Removal:
Effect of Effective Alkali (4)
Cooking Time (min)
200
35 KAPPA
40 KAPPA
150
45 KAPPA
100
50
0
12
14
16
18
20
Effective Alkali % as Na20
PSE 476: Lecture 11
8
Pulping
and
Bleaching
Effect of Increasing Effective
Alkali on Pulp Properties.
• In the kraft pulping of Scandinavian Pine, and
increase in EA from 13.6% to 15.5% had the
following effects:
» Cooking time to yield cut in half.
» Pulp yield at given kappa reduced.
» Xylan content of pulp reduced while glucomannan content
was increased.
- Brightness increased.
- Longer beating time to reached CSF.
- Pulp strength properties (tear, breaking length,etc). changed.
PSE 476: Lecture 11
9
Removal of Glucomannans:
Effect of Effective Alkali
120
200
100
150
80
25% EA
60
100
15.8% EA
40
50
20
0
Temperature (C)
Glucomannan Yield (%)
Pulping
and
Bleaching
0
0
50
100
150
200
250
300
Time (minutes)
Why doesn’t EA affect glucomannan removal?
PSE 476: Lecture 11
10
Removal of Xylans:
Effect of Effective Alkali
Xylan Yield (%)
120
200
100
150
80
60
100
40
25% EA
15.8% EA
20
50
0
0
50
100
150
200
250
Temperature (C)
Pulping
and
Bleaching
0
300
Time (minutes)
Why does EA affect xylan removal?
PSE 476: Lecture 11
11
Lignin Removal:
Pulping
and
Bleaching
Effect of Sulfidity
• Sulfidity effect zero order in initial delignification (this
means that changing sulfidity has no effect on initial rate)
» Sulfidity is important during bulk delignification
» Effects final lignin/carbohydrate ratio
- HS- does not react with carbohydrates
40
KAPPA
30
20
10
0
0
10
20
30
40
50
60
Sulfidity
PSE 476: Lecture 11
12
Kraft Pulping Kinetics
Lignin - Carbohydrate Ratio
Pulping
and
Bleaching
Lignin Removal: Effect of Sulfidity
0.4
0.3
0% Sulfide
3.75% Sulfide
5% Sulfide
0.2
0.1
0
0
1
2
3
Hours at Temperature
PSE 476: Lecture 11
13
Pulping
and
Bleaching
Kraft Pulping Kinetics
Kinetic Models
• Reaction models have been developed which
simulate the pulping process.
• Models range from simple to quite complex.
» Typically use the following reaction variables:
- Time, Temperature, Alkali, Sulfidity, Liquor/Wood, Etc
• Models roughly derived from the following
expression:
dL = k[Lignin]a[OH-]b[HS-]c[other variables]d
dt
PSE 476: Lecture 11
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