Transcript No Slide Title

Agenda
• Chemical Description of Wood
» Carbohydrates
» Extractives
» Lignin
• Loss of Components During Kraft Pulping
• Reactions in the Early Portion of the Cook
1
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
2
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

OH
CH2OH

O
O
CH2OH
O
HO
OH
HO
OH
O

CH2OH
O
CH2OH

O
HO
OH
O
O

Cellulose
3
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.
4
Xylan Structure

4--D-Xly-14--D-Xly-14--D-Xly-14--D-Xly4--D-Xly




4-O-Me--D-Glc 

O
O
HO
O
OH
HO
O
H3CO
CO2H
O
O
O
O
OH
HO


-L-Araf
O
HO
O
OH
OH
O
O
O
HOH2C
OH
OH
5
Glucomannan Structure


4--D-Glc-14--D-Man-14--D-Man-14--D-Man-1
6
2,3
1
-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.
6
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
7
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
8
odor of wood.
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.
9
Five Steps in Kraft Pulping
Process
• Transport of ions from the liquor to the exterior
surface of the chip
• Diffusion of the ions to the interior of the chip
• Chemical reactions between the ions and the wood
components
• Diffusion of the reaction components to the the
chip exterior
• Transport of the reaction products into the liquor
10
Yield of Wood Components
After Kraft Pulping
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
11
Initial Reactions:
Low Temperature
• Carbohydrates
» Saponification of acetyl groups on xylan (see next slide).
» Reactions with easily removable carbohydrates.
- Galactoglucomannans.
- Arabinogalactans.
• Extractives
» Saponification of fats.
» Neutralization of extractives.
- There are a number of acidic extractives which consume NaOH.
12
Saponification:
Example Using Acetyl Groups
CH2OH
OH
HO
OH
CH2OH
O
OH
-
HO
O C CH3
O
OH
CH2OH
OH
O
OH
HO
OH
O C CH3
O
O
HO
+
OH
HO C CH3
O
• Saponification is the basic hydrolysis of esters.
• Saponification of acetyl happens readily in alkaline
solutions.
» Reaction occurs rapidly even at room temperature.
13
Acidic Extractive
Species/Saponifiables
COOH
COOH
Levopimaric Acid
Abietic Acid
Fats
Waxes
H2C O CO R1
H2C O CO R1
H2C O CO R2
H2C OH
H2C O CO R3
H2C OH
Triglyceride
CH3 (CH2)n O CO (CH2)m CH3
Monoglyceride
COOH
C20H41OH
Arachidic Alcohol
C24H49OH
Lignoceric Alcohol
Oleic Acid
COOH
Linoleic Acid
14
Consumption of Alkali
Rapid initial
drop in alkali
concentration
that must be
replenished by
diffusion of
chemicals into
the chip
Residual NaOH
(moles/kg wood)
4
3
2
1
0
0
50
100
150
Time (minutes)
15