Transcript No Slide Title

Pulping and Bleaching
PSE 476
Lecture #8
Kraft Pulping: Early Reactions and
Kraft Pulping Lignin Reactions
1
Agenda
• Basic Chemical Pulping Discussion
• Loss of Components During Kraft Pulping
• Reactions in the Early Portion of the Cook
» Saponification
» Neutralization of Extractives
• Initial Lignin Discussion
• Kraft Pulping Lignin Reactions
2
Wood Chemistry
• For the students who
do not recognize this
molecule (did not take
PSE 406), there is a
short appendix at the
end of this lecture to
help you. Additionally,
the class notes are
available for review.
3
Pulping
• The goal of kraft pulping
is to remove the majority
of lignin from chips (or
other biomass) while
minimizing carbohydrate
loss and degradation.
• Removal of lignin is
accomplished through
treatment of raw material
with NaOH and Na2S at
elevated temperatures.
4
The Goal of Lignin Reactions
in Kraft Pulping
H2COH
CH2
CH2
H2COH
CH
CH
OH
OCH3
6
16
H3CO
OH
H COH
HC
CH2OH
OH C CH CH2OH
O
H3CO
OCH3
H3CO
5
7
HC
HC
HC
H2C
O
HC
4
OH
HOH2C
O
OCH3
HOCH
3
HC
H2C
O
O
O
H3CO
HC
27
OHC CH CH2OH
O
CH2OH
H3CO
HC
HC
H3CO
1
O
C O
CH
CH2
O
OH
O
CH2OH
CH H3CO
CH
O
HC
HCOH
CH
H2COH
HOCH
O
HC
CHO
12
Kraft Pulping
O
23
OCH3
O
17
O
9
O
HOCH
CH
H2COH
HC
H2COH
H3CO
HC
Soluble
Fragments
CHO
CH
CH
22
OCH3
O
18
Carbohydrates are also
degraded during pulping
21
H3CO
HO
During kraft pulping, the
large insoluble lignin
molecules are converted
into small alkali soluble
fragments.
24
H2 COH
CH2OH
10
H2 COH
H2COH
HC
CH
H3CO
13
CH2
H3CO
CH
CH
O
14
CH
C O
CH
CH2OH
OCH3
CH
O
OH
HCOH
HCOH
H2C O
CH
CH
HC O
25
H3CO
2
O
O
OCH3
CH2OH
8
CH
H2 COH
CH
CH
H C O Carbohydrate
15
O
CH2OH
28
HO
26
H3CO
CH3
CH3O
19
HC
O
CH
H2COH
CH2
HC
H COH
OCH3
O
11
20
H3CO
OCH3
OH
OH
5
Yield of Wood Components
After Kraft Pulping
Notes
Pine
Birch
After*
Before*
After*
Before*
Cellulose
35
39
34
40
Glucomannan
4
17
1
3
Xylan
5
8
16
30
Other Carb.
-
5
-
4
Lignin
3
27
2
20
0.5
4
0.5
3
Extractives
* Yields = % of wood (pulp) components
6
Initial Reactions:
Low Temperature
• Carbohydrates
» Alkaline hydrolysis of acetyl groups on xylan (see next
slide).
» Removal of certain soluble carbohydrates.
- Certain galactoglucomannans.
- Arabinogalactans.
• Extractives
» Alkaline hydrolysis of fats (saponification), waxes, and other
esters.
» Neutralization of extractives.
- There are a number of acidic extractives which consume NaOH.
7
Alkaline Hydrolysis:
Example Using Acetyl Groups
CH2OH
OH
HO
OH
CH2OH
O
OH
-
HO
OH
CH2OH
OH
HO
O C CH3
O
OH
O
OH
O C CH3
O
O
HO
+
OH
HO C CH3
O
• Esters are cleaved in alkaline solutions through hydrolysis
reactions forming carboxylic acids and alcohols.
• Hydrolysis of acetyl groups occurs readily in alkaline solutions.
» Reaction occurs rapidly even at room temperature.
• Reaction consumes alkali.
8
Saponification of Fats
(Review slide from PSE 406)
• Treatment of fats with alkali converts them to fatty acids
and glycerol through saponification.
OH
O
H2C O C
HOCH2CHCH2OH
R
-
OH
Glycerol (glycerine)
-
O
H2C O C
OH
R
H2O
Once again this reaction
consumes part of the alkali
charge.
O
H2C OH
-
O C
R
9
Acidic Extractive Species
Lignans
Resin Acids
Monoterpenoids
O
CH 3O
O
COOH
HO
COOH
OCH 3
OH
10
Consumption of Alkali
Residual NaOH
(moles/kg wood)
4
3
2
1
0
0 Impregnation
zone
50
100
150
Time (minutes)
11
Where Does All the Alkali
Go?
• Spruce wood was soda pulped at a NaOH
concentration of 19% (as Na2O).
• 12.5% (or 66% of alkali) consumed to lower
lignin content of wood to 2.8%.
» 2.3-3% used in dissolution of lignin.
» 1.3% for hydrolysis of acetyl and formyl groups.
» 8.2-8.9% for neutralization of acidic products
- Some extractives
- Mostly carbohydrate degradation products (discussed
later).
12
Lignin Removal during Kraft
Pulping
• This chart shows the lignin removal rate during a kraft cook. It
is important to note that the rate of lignin removal is
temperature dependent. What does this fact tell us about of
lignin removal in this slide?
200
80
150
60
100
40
Lignin
20
50
Temperature (C)
Lignin Yield (%)
100
Temperature
0
0
0
50
100
150
200
250
Time (minutes)
13
Lignin Removal
• In the last slide, the rate of lignin removal appears to
be linear over a large portion of the cook; even as the
temperature increases.
• This means that lignin removal in the first portion of
the cook is easier than as the cook proceeds.
• Lignin removal has been broken into three sections:
» Initial Phase (fast lignin removal reactions)
» Bulk Phase (slow lignin removal reactions)
» Residual Phase (really slow lignin removal)
14
Kraft Pulping:
Effective Alkali (g/l NaOH)
Reaction Phases of Lignin Removal
60
70°C
70°C
137°C
50
40
30
Impregnation zone
170° C
20
Initial Phase
Bulk Phase
Residual Phase
10
0
0
Notes
5
10
15
20
25
30
Yield of Lignin (%)
15
Kraft Pulping Lignin Reactions
16
Dissolution of Lignin
• In review the goal in kraft pulping is the cleavage of
lignin into alkali soluble fragments.
• Cleavage is affected by the following factors:
»
»
»
»
»
Type of linkage
Presence of free phenolic hydroxyl group
Functional groups (benzyl hydroxyl, carbonyl)
Type and amount of nucleophiles (OH-, HS-)
Reaction temperature
• We are going to first look at the chemical
mechanisms of the reactions and then the kinetics.
17
Sites for Nucleophilic Attack
• The cooking chemicals
used in kraft cooking
(NaOH and Na2S: OHand HS-) both act as
nucleophiles* because of
their free pair of
electrons.
• Sites for nucelophilic
attack in lignin are those
areas of reduced
electron density
(partially positive sites).
Alkaline Media
HC  
HC R1
-
- R1

OCH3





OCH3
O
O -
R1 = OH, OAr or OAlk
* Notes
18
Formation of Quinone Methide
HC OH
HC
OCH3
OCH3
OThese arrows indicate that a pair
of electrons are moving
Nucleophillic
attack
site!
O
Quinone Methide
(very reactive)
19
Formation of Nucleophilic Attack
Sites
• A free phenolic hydroxyl
group is needed for the
formation of a quinone
methide.
• The oxygen of the quinone
group (carbonyl) attracts the
electron density on the
double bond thus making
the carbon more positive.
This in turn shifts the
electron densities of the
other bonds on this
conjugated system.
HC 



OCH3
O
20
Two Additional Examples of
Nucleophilic Addition Sites
H2C 
H2C R1
HC
HC
OCH3
O-
R
HC
HC  
-
- R1
H2C



O

C
C

OCH3
Coniferaldehyde type structures
R
+ C
O
Notes
C
O
- R1

R1 = OH, OAr or OAlk
H2C +
R1
OCH 3
O
OCH 3
O
R = OAr, Ar or Alk, R1 = OH, OAr, or OAlk
This structure contains an a-keto
group. Notice that a free phenolic
hydroxyl groups is not needed!
21
Important Issues!!!!
• When learning about alkaline pulping
mechanisms, remember to ask yourselves
these questions!
» Which reactant are we concerned with: OH- or HS-?
» Does the lignin structure have a free phenolic
hydroxyl group or is it etherified?
» Which linkage are you hoping to cleave?
» Is there an a-carbonyl or benzyl hydroxyl?
22
Reactions of α-O-4 Linkage
Phenolic and Etherified
• In kraft pulping, α-O-4
linkages do not react
with HS• Reaction with OH» Phenolic Units: α -O-4 are
very rapidly cleaved by
alkali. This is the fastest of
the lignin degradation
reactions. (Will occur at
low temperatures)
» Etherified Units: α -O-4
linkages are stable (no
reaction).
» Please work out reaction
mechanism.
R
HC
R
HC
O
(-)
(-)
O
OH
CH3O
O
CH3O
(-)
O
R
HC
O
OH
(-)
No Reaction
CH3O
OR
23
Reactions of b-O-4 Linkages:
Free Phenolic Hydroxyl/Benzyl Hydroxyl
• Reaction with OH- alone
» The ether linkage is not
cleaved; a vinyl ether
structures is formed.
» Vinyl ether linkages are
difficult to cleave.
• Reaction with HS- (OHpresent)
» HS- is a very strong
nucleophile which cleaves
the β-O-4 linkage.
» Reaction is very rapid
even at lower
temperatures.
HC
CH3O
CH3O
H2COH
HC
O
O
HC
HC O R
-
OH
O-
OCH3
O-
OCH3
Vinyl Ether Linkage
CH3O
H2COH
HC
H2COH
O
HC
HC O R
-
O-
OCH3
CH3O
HC
+ -O
OH
HS
O-
OCH3
* Mechanisms on following pages
24
Kraft Reactions of b-O-4 Linkage
(Free Phenolic Hydroxyl)
HO
CH3O
H2COH
H C
H2COH
O
C
HC
CH3O
CH3O
O
Formaldehyde
-
HC
H2COH
HC
O
HC O R
OCH3
-
OH
O
-
HO
OCH3
O-
Notice that the
b-O-4 bond is
not cleaved.
Notes
OCH3O
CH3O
H2CO H
HC
OCH3
O
HC
O
HC
HC
+
OCH3
O
O-
HCHO
OCH3
Vinyl Ether
25
Appendix
Basic Wood Chemistry
26
What is the Chemical
Makeup of Wood?
60
50
40
Cellulose*
Hemicellulose*
Lignin*
Extractives
% 30
20
10
0
Douglas Redwood
Fir
Yellow Balsam Fir
Pine
* Data for Cellulose, Hemicellulose & Lignin on extractive free wood basis
27
Cellulose
• Very long straight chain polymer of glucose (a sugar):
approximately 10,000 in a row in wood. Cotton is
nearly pure cellulose.
» Think about a very long string of beads with each
bead being a glucose molecule.
• Cellulose molecules link up in bundles and bundles
of bundles and bundles of bundles of bundles to
make fibers.
• Uncolored polymer.
Cellobiose Unit
HO
O
b
OH
CH2OH
b
O
O
CH2OH
O
HO
OH
HO
OH
O
b
Cellulose
CH2OH
O
CH2OH
b
O
HO
OH
O
O
b
28
Hemicelluloses
• Branched little uncolored sugar polymers (~
50 to 300 sugar units)
» Composition varies between wood species.
-
5 carbon sugars: xylose, arabinose.
6 carbon sugars: mannose, galactose, glucose.
Uronic Acids: galacturonic acid, glucuronic acid.
Acetyl and methoxyl groups (acetic acid & methanol).
• Major hemicelluloses:
» Xylans - big in hardwoods
» Glucomannans: big in softwoods
• Minor hemicelluloses: pectins, others.
29
Xylan Structure

4-b-D-Xly-14-b-D-Xly-14-b-D-Xly-14-b-D-Xly4-b-D-Xly




4-O-Me-a-D-Glc 

O
O
HO
O
OH
HO
O
H3CO
CO2H
O
O
O
O
OH
HO


a-L-Araf
O
HO
O
OH
OH
O
O
O
HOH2C
OH
OH
30
Glucomannan Structure


4-b-D-Glc-14-b-D-Man-14-b-D-Man-14-b-D-Man-1
6
2,3
1
a-D-Gal
Acetyl
• There are different structured glucomannans in
hardwoods and softwoods (and within softwoods)
• Glucomannans are mostly straight chained polymers
with a slight amount of branching. The higher the
branching, the higher the water solubility.
31
Lignin
• Phenolic polymer the glue that holds
the fibers together.
• Lignin is a very
complex polymer
which is connected
through a variety
of different types of
linkages.
• Colored material.
H2COH
CH2
CH2
H2COH
CH
CH
OH
OCH3
6
16
H3CO
OH
H COH
HC
CH2OH
OH C CH CH2OH
O
H3CO
OCH3
H3CO
5
7
HC
HC
HC
H2C
O
HC
4
OH
HOH2C
O
OCH3
HOCH
3
HC
H2C
O
O
O
H3CO
HC
27
OHC CH CH2OH
O
CH2OH
H3CO
HC
HC
H3CO
1
O
C O
CH
CH2
O
OH
O
CH2OH
CH H3CO
CH
O
24
H2 COH
HC
HCOH
CH2OH
CH
H2COH
HOCH
O
HC
CHO
12
O
23
OCH3
O
17
O
9
O
10
H2 COH
H2COH
HC
CH
H3CO
13
CH2
H3CO
CH
CH
O
14
CH
C O
CH
CH2OH
OCH3
CH
O
HOCH
CH
H2COH
HC
H2COH
H3CO
OH
HCOH
HCOH
H2C O
CH
CH
HC O
25
H3CO
2
O
O
OCH3
CH2OH
8
CH
26
CH
H C O Carbohydrate
15
O
CH2OH
28
HO
H2 COH
CH
H3CO
CH3
CH3O
HC
CHO
CH
CH
22
OCH3
O
18
21
H3CO
HO
19
HC
O
CH
H2COH
CH2
HC
H COH
OCH3
O
11
20
H3CO
OCH3
OH
OH
32
Lignin Nomenclature

C
b
C
a
C
Side Chain
1
6
2
3
5
4
OCH3
OH
Met hoxyl Group
Phenolic Hydroxyl
Notes
Phenylpropane Unit
C9
}
Common Names
33
Lignin Reactions:
Linkage Frequencies
Linkage
b-O-4
Softwood
%
50
Hardwood
%
60
a-O-4
2-8
7
b-5
9-12
6
5-5
4-0-5
10-11
5
4
7
b-1
7
7
b-b
2
3
C
C
C
C
O
C
C
C
O
O
b-O-4
a-O-4
C
O
C
C
O
C
O
b-1
C
C
C
C
C
C
C
C
C
C
C
C
C
C
C
C
C
C
C
C
C
O
O
O
O
C
O
O
b-b
5-5
O
O
4-O-5
b-5
Notes
34
Extractives
• The term extractives refers to a group of unique chemical
compounds which can be removed from plant materials
through extraction with various solvents.
• Typically these chemicals constitute only a small portion of
the tree (<5%).
» In some tropical species this can be as high as
25%.
• Extractives are produced by plants for a variety of uses.
» The most common use by plants is protection.
• Extractives can cause serious problems for processing.
• Pitch is a term which is often used when describing some
groups of extractives.
• Extractives are responsible for the characteristic color and
35
odor of wood.